FUEL CELL : DIFFERENT FUEL CELL TYPES AND THEIR APPLICATION

FUEL CELL : DIFFERENT FUEL CELL TYPES AND THEIR APPLICATION

Introduction

 

Fuel cells are electrochemical devices that convert chemical energy in fuels into electrical energy directly, promising power generation with high efficiency and low environmental impact. Because the intermediate steps of producing heat and mechanical work typical of most conventional power generation methods are avoided, fuel cells are not limited by thermodynamic limitations of heat engines such as the Carnot efficiency. In addition, because combustion is avoided, fuel cells produce power with minimal pollutant. However, unlike batteries the reductant and oxidant in fuel cells must be continuously replenished to allow continuous operation. Fuel cells bear significant resemblance to electrolyzers. In fact, some fuel cells operate in reverse as electrolyzers, yielding a reversible fuel cell that can be used for energy storage. 

Though fuel cells could, in principle, process a wide variety of fuels and oxidants, of most interest today are those fuel cells that use common fuels (or their derivatives) or hydrogen as a reductant, and ambient air as the oxidant. 

Most fuel cell power systems comprise a number of components: 

Unit cells, in which the electrochemical reactions take place 

Stacks, in which individual cells are modularly combined by electrically connecting the cells to form units with the desired output capacity 

Balance of plant which comprises components that provide feedstream conditioning (including a fuel processor if needed), thermal management, and electric power conditioning among other ancillary and interface functions 

In the following, an overview of fuel cell technology is given according to each of these categories, followed by a brief review of key potential applications of fuel cells.

1.2 Unit Cells

 

1.2.1 Basic Structure

Unit cells form the core of a fuel cell. These devices convert the chemical energy contained in a fuel electrochemically into electrical energy. The basic physical structure, or building block, of a fuel cell consists of an electrolyte layer in contact with an anode and a cathode on either side. A schematic representation of a unit cell with the reactant/product gases and the ion conduction flow directions through the cell is shown in Figure 1-1.


Figure 1-1 Schematic of an Individual Fuel Cell

 

In a typical fuel cell, fuel is fed continuously to the anode (negative electrode) and an oxidant (often oxygen from air) is fed continuously to the cathode (positive electrode). The electrochemical reactions take place at the electrodes to produce an electric current through the electrolyte, while driving a complementary electric current that performs work on the load. Although a fuel cell is similar to a typical battery in many ways, it differs in several respects. The battery is an energy storage device in which all the energy available is stored within the battery itself (at least the reductant). The battery will cease to produce electrical energy when the chemical reactants are consumed (i.e., discharged). A fuel cell, on the other hand, is an energy conversion device to which fuel and oxidant are supplied continuously. In principle, the fuel cell produces power for as long as fuel is supplied. 

Fuel cells are classified according to the choice of electrolyte and fuel, which in turn determine the electrode reactions and the type of ions that carry the current across the electrolyte. Appleby and Foulkes (1) have noted that, in theory, any substance capable of chemical oxidation that can be supplied continuously (as a fluid) can be burned galvanically as fuel at the anode of a fuel cell. Similarly, the oxidant can be any fluid that can be reduced at a sufficient rate. Though the direct use of conventional fuels in fuel cells would be desirable, most fuel cells under development today use gaseous hydrogen, or a synthesis gas rich in hydrogen, as a fuel. Hydrogen has a high reactivity for anode reactions, and can be produced chemically from a wide range of fossil and renewable fuels, as well as via electrolysis. For similar practical reasons, the most common oxidant is gaseous oxygen, which is readily available from air. For space applications, both hydrogen and oxygen can be stored compactly in cryogenic form, while the reaction product is only water. 

1.2.2 Critical Functions of Cell Components 

A critical portion of most unit cells is often referred to as the three-phase interface. These mostly microscopic regions, in which the actual electrochemical reactions take place, are found where either electrode meets the electrolyte. For a site or area to be active, it must be exposed to the reactant, be in electrical contact with the electrode, be in ionic contact with the electrolyte, and contain sufficient electro-catalyst for the reaction to proceed at the desired rate. The density of these regions and the nature of these interfaces play a critical role in the electrochemical performance of both liquid and solid electrolyte fuel cells: 

In liquid electrolyte fuel cells, the reactant gases diffuse through a thin electrolyte film that wets portions of the porous electrode and react electrochemically on their respective electrode surface. If the porous electrode contains an excessive amount of electrolyte, the electrode may "flood" and restrict the transport of gaseous species in the electrolyte phase to the reaction sites. The consequence is a reduction in electrochemical performance of the porous electrode. Thus, a delicate balance must be maintained among the electrode, electrolyte, and gaseous phases in the porous electrode structure. 

In solid electrolyte fuel cells, the challenge is to engineer a large number of catalyst sites into the interface that are electrically and ionically connected to the electrode and the electrolyte, respectively, and that is efficiently exposed to the reactant gases. In most successful solid electrolyte fuel cells, a high-performance interface requires the use of an electrode which, in the zone near the catalyst, has mixed conductivity (i.e. it conducts both electrons and ions). 

Over the past twenty years, the unit cell performance of at least some of the fuel cell technologies has been dramatically improved. These developments resulted from improvements in the three-phase boundary, reducing the thickness of the electrolyte, and developing improved electrode and electrolyte materials which broaden the temperature range over which the cells can be operated. 

In addition to facilitating electrochemical reactions, each of the unit cell components have other critical functions. The electrolyte not only transports dissolved reactants to the electrode, but also conducts ionic charge between the electrodes, and thereby completes the cell electric circuit as illustrated in Figure 1 -1. It also provides a physical barrier to prevent the fuel and oxidant gas streams from directly mixing. 

The functions of porous electrodes in fuel cells, in addition to providing a surface for electrochemical reactions to take place, are to: conduct electrons away from or into the three-phase interface once they are formed (so an electrode must be made of materials that have good electrical conductance) and provide current collection and connection with either other cells or the load ensure that reactant gases are equally distributed over the cell ensure that reaction products are efficiently led away to the bulk gas phase As a consequence, the electrodes are typically porous and made of an electrically conductive material. At low temperatures, only a few relatively rare and expensive materials provide sufficient electro-catalytic activity, and so such catalysts are deposited in small quantities at the interface where they are needed. In high-temperature fuel cells, the electro-catalytic activity of the bulk electrode material is often sufficient. Though a wide range of fuel cell geometries has been considered, most fuel cells under development now are either planar (rectangular or circular) or tubular (either single- or double-ended and cylindrical or flattened). 

1.3 Fuel Cell Stacking 

For most practical fuel cell applications, unit cells must be combined in a modular fashion into a cell stack to achieve the voltage and power output level required for the application. Generally, the stacking involves connecting multiple unit cells in series via electrically conductive interconnects. Different stacking arrangements have been developed, which are described below. 

1.3.1 Planar-Bipolar Stacking 

The most common fuel cell stack design is the so-called planar-bipolar arrangement (Figure 1-2 depicts a PAFC). Individual unit cells are electrically connected with interconnects. Because of the configuration of a flat plate cell, the interconnect becomes a separator plate with two functions: to provide an electrical series connection between adjacent cells, specifically for flat plate cells, and to provide a gas barrier that separates the fuel and oxidant of adjacent cells. 

In many planar-bipolar designs, the interconnect also includes channels that distribute the gas flow over the cells. The planar-bipolar design is electrically simple and leads to short electronic current paths (which helps to minimize cell resistance). 


Figure 1-2 Expanded View of a Basic Fuel Cell Unit in a Fuel Cell Stack (1)

Planar-bipolar stacks can be further characterized according to arrangement of the gas flow:

 Cross-flow. Air and fuel flow perpendicular to each other 

Co-flow. Air and fuel flow parallel and in the same direction. In the case of circular cells, this means the gases flow radially outward 

Counter-flow. Air and fuel flow parallel but in opposite directions. Again, in the case of circular cells this means radial flow

 Serpentine flow. Air or fuel follow a zig-zag path

 Spiral flow. Applies to circular cells

 The choice of gas-flow arrangement depends on the type of fuel cell, the application, and other considerations. Finally, the manifolding of gas streams to the cells in bipolar stacks can be achieved in various ways:

 Internal: the manifolds run through the unit cells

 Integrated: the manifolds do not penetrate the unit cells but are integrated in the interconnects

 External: the manifold is completely external to the cell, much like a wind-box 

1.3.2 Stacks with Tubular Cells 

Especially for high-temperature fuel cells, stacks with tubular cells have been developed. Tubular cells have significant advantages in sealing and in the structural integrity of the cells. However, they represent a special geometric challenge to the stack designer when it comes to achieving high power density and short current paths. In one of the earliest tubular designs the current is conducted tangentially around the tube. Interconnects between the tubes are used to form rectangular arrays of tubes. Alternatively, the current can be conducted along the axis of the tube, in which case interconnection is done at the end of the tubes. To minimize the length of electronic conduction paths for individual cells, sequential series connected cells are being developed. The cell arrays can be connected in series or in parallel. For a more detailed description of the different stack types and pictorial descriptions, the reader is referred to Chapter 7 on SOFC (SOFC is the fuel cell type for which the widest range of cell and stack geometries is pursued). 

To avoid the packing density limitations associated with cylindrical cells, some tubular stack designs use flattened tubes. 

1.4 Fuel Cell Systems 

In addition to the stack, practical fuel cell systems require several other sub-systems and components; the so-called balance of plant (BoP). Together with the stack, the BoP forms the fuel cell system. The precise arrangement of the BoP depends heavily on the fuel cell type, the fuel choice, and the application. In addition, specific operating conditions and requirements of individual cell and stack designs determine the characteristics of the BoP. Still, most fuel cell systems contain:

Fuel preparation. Except when pure fuels (such as pure hydrogen) are used, some fuel preparation is required, usually involving the removal of impurities and thermal conditioning. In addition, many fuel cells that use fuels other than pure hydrogen require some fuel processing, such as reforming, in which the fuel is reacted with some oxidant (usually steam or air) to form a hydrogen-rich anode feed mixture. Air supply. In most practical fuel cell systems, this includes air compressors or blowers as well as air filters. 

Thermal management. All fuel cell systems require careful management of the fuel cell stack temperature. 

Water management. Water is needed in some parts of the fuel cell, while overall water is a reaction product. To avoid having to feed water in addition to fuel, and to ensure smooth operation, water management systems are required in most fuel cell systems. 

Electric power conditioning equipment. Since fuel cell stacks provide a variable DC voltage output that is typically not directly usable for the load, electric power conditioning is typically required. While perhaps not the focus of most development effort, the BoP represents a significant fraction of the weight, volume, and cost of most fuel cell systems. 

Figure 1-3 shows a simple rendition of a fuel cell power plant. Beginning with fuel processing, a conventional fuel (natural gas, other gaseous hydrocarbons, methanol, naphtha, or coal) is cleaned, then converted into a gas containing hydrogen. Energy conversion occurs when dc electricity is generated by means of individual fuel cells combined in stacks or bundles. A varying number of cells or stacks can be matched to a particular power application. Finally, power conditioning converts the electric power from dc into regulated dc or ac for consumer use. Section 8.1 describes the processes of a fuel cell power plant system.


Figure 1-3 Fuel Cell Power Plant Major Processes 

1.5 Fuel Cell Types 

A variety of fuel cells are in different stages of development. The most common classification of fuel cells is by the type of electrolyte used in the cells and includes

1) polymer electrolyte fuel cell (PEFC),

2) alkaline fuel cell (AFC),

3) phosphoric acid fuel cell (PAFC),

4) molten carbonate fuel cell (MCFC), and

5) solid oxide fuel cell (SOFC).

Broadly, the choice of electrolyte dictates the operating temperature range of the fuel cell. The operating temperature and useful life of a fuel cell dictate the physicochemical and thermomechanical properties of materials used in the cell components (i.e., electrodes, electrolyte, interconnect, current collector, etc.). Aqueous electrolytes are limited to temperatures of about 200 °C or lower because of their high vapor pressure and rapid degradation at higher temperatures. The operating temperature also plays an important role in dictating the degree of fuel processing required. In low-temperature fuel cells, all the fuel must be converted to hydrogen prior to entering the fuel cell. In addition, the anode catalyst in low-temperature fuel cells (mainly platinum) is strongly poisoned by CO. In high-temperature fuel cells, CO and even CH4 can be internally converted to hydrogen or even directly oxidized electrochemically. Table 1-1 provides an overview of the key characteristics of the main fuel cell types.

Table 1-1 Summary of Major Differences of the Fuel Cell Types

 In parallel with the classification by electrolyte, some fuel cells are classified by the type of fuel used:

Direct Alcohol Fuel Cells (DAFC). DAFC (or, more commonly, direct methanol fuel cells or DMFC) use alcohol without reforming. Mostly, this refers to a PEFC-type fuel cell in which methanol or another alcohol is used directly, mainly for portable applications. A more detailed description of the DMFC or DAFC is provided in Chapter 3;

Direct Carbon Fuel Cells (DCFC). In direct carbon fuel cells, solid carbon (presumably a fuel derived from coal, pet-coke or biomass) is used directly in the anode, without an intermediate gasification step. Concepts with solid oxide, molten carbonate, and alkaline electrolytes are all under development. The thermodynamics of the reactions in a DCFC allow very high efficiency conversion. Therefore, if the technology can be developed into practical systems, it could ultimately have a significant impact on coal-based power generation. 

A brief description of various electrolyte cells of interest follows. Detailed descriptions of these fuel cells may be found in References (1) and (2). 

1.5.1 Polymer Electrolyte Fuel Cell (PEFC)

 Polymer electrolyte membrane (PEM) fuel cells—also called proton exchange membrane fuel cells—deliver high power density and offer the advantages of low weight and volume compared with other fuel cells. PEM fuel cells use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum or platinum alloy catalyst. They need only hydrogen, oxygen from the air, and water to operate. They are typically fueled with pure hydrogen supplied from storage tanks or reformers.

PEM fuel cells operate at relatively low temperatures, around 80°C (176°F). Low-temperature operation allows them to start quickly (less warm-up time) and results in less wear on system components, resulting in better durability. However, it requires that a noble-metal catalyst (typically platinum) be used to separate the hydrogen's electrons and protons, adding to system cost. The platinum catalyst is also extremely sensitive to carbon monoxide poisoning, making it necessary to employ an additional reactor to reduce carbon monoxide in the fuel gas if the hydrogen is derived from a hydrocarbon fuel. This reactor also adds cost.

PEM fuel cells are used primarily for transportation applications and some stationary applications. PEM fuel cells are particularly suitable for use in vehicle applications, such as cars, buses, and heavy-duty trucks.

The electrolyte in this fuel cell is an ion exchange membrane (fluorinated sulfonic acid polymer or other similar polymer) that is an excellent proton conductor. The only liquid in this fuel cell is water; thus, corrosion problems are minimal. Typically, carbon electrodes with platinum electro-catalyst are used for both anode and cathode, and with either carbon or metal interconnects. 

Water management in the membrane is critical for efficient performance; the fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated. Because of the limitation on the operating temperature imposed by the polymer, usually less than 100 °C, but more typically around 60 to 80 °C. , and because of problems with water balance, a H2-rich gas with minimal or no CO (a poison at low temperature) is used. Higher catalyst loading (Pt in most cases) than that used in PAFCs is required for both the anode and cathode. Extensive fuel processing is required with other fuels, as the anode is easily poisoned by even trace levels of CO, sulfur species, and halogens. 

PEFCs are being pursued for a wide variety of applications, especially for prime power for fuel cell vehicles (FCVs). As a consequence of the high interest in FCVs and hydrogen, the investment in PEFC over the past decade easily surpasses all other types of fuel cells combined. Although significant development of PEFC for stationary applications has taken place, many developers now focus on automotive and portable applications.

It has a wide range of efficiency (25% to 58%). It warms up and begins generating electricity quickly, which makes it very useful in transportation vehicle applications.

General Electric developed the first proton exchange membrane fuel cells for the Gemini space missions in the early 1960s. PEMFCs were replaced by alkaline fuel cells in the Apollo program and in the space shuttle program.

Advantages: The PEFC has a solid electrolyte which provides excellent resistance to gas crossover. The PEFC’s low operating temperature allows rapid start-up and, with the absence of corrosive cell constituents, the use of the exotic materials required in other fuel cell types, both in stack construction and in the BoP is not required. Test results have demonstrated that PEFCs are capable of high current densities of over 2 kW/l and 2 W/cm2. The PEFC lends itself particularly to situations where pure hydrogen can be used as a fuel. 

Disadvantages: The low and narrow operating temperature range makes thermal management difficult, especially at very high current densities, and makes it difficult to use the rejected heat for cogeneration or in bottoming cycles. Water management is another significant challenge in PEFC design, as engineers must balance ensuring sufficient hydration of the electrolyte against flooding the electrolyte. In addition, PEFCs are quite sensitive to poisoning by trace levels of contaminants including CO, sulfur species, and ammonia. To some extent, some of these disadvantages can be counteracted by lowering operating current density and increasing electrode catalyst loading, but both increase cost of the system. If hydrocarbon fuels are used, the extensive fuel processing required negatively impacts system size, complexity, efficiency (typically in the mid thirties), and system cost. Finally, for hydrogen PEFC the need for a hydrogen infrastructure to be developed poses a barrier to commercialization. 

One disadvantage is that, if onboard hydrogen is used, it must be stored as a compressed gas in pressurized tanks. In this form, it is not possible to store enough hydrogen to travel over 300 miles in a vehicle, so this technology cannot compete with traditional gasoline- or diesel-fueled vehicles at this time.

If onboard reformers are used, higher-density liquid fuels, such as methanol, ethanol, natural gas, liquefied petroleum gas, and gasoline, can be used to provide the hydrogen.

The onboard reformer is expensive and has additional costs, including maintenance; however, it is used in a number of applications.

FACTS OF PEM

FUEL CELL TYPE : POLYMER ELECTROLYTE MEMBRANE (PEM)

COMMON ELECTROLYTE : PERFLUOROSULFONICACID

OPERATING TEMPERATURE : < 120 c

TYPICAL STACK SIZE : , 1kW - 100 kW

ELECTRICAL EFFICIENCY : 60% DIRECT H2 40% REFORMER FUEL

APPLICATIONS : • Backup power, • Portable power • Distributed generation • Transportation Specialty vehicles

ADVANTAGES : Solid electrolyte reduces corrosion & electrolyte management problems

• Low temperature • Quick start-up and load following

CHALLENGES : • Expensive catalysts • Sensitive to fuel impurities

1.5.2 Direct Methanol Fuel CELLS (DMFCS)

The DIRECT-METHANOL FUEL CELL (DMFC) is a subcategory of a proton exchange membrane fuel cell. This fuel cell was invented by college researchers in cooperation with the Jet Propulsion Laboratory (JPL) in 1990.

Figure 2 shows a diagram of the DMFC. It uses a polymer electrolyte similar to the one in the PEMFC. Methanol is used as the fuel, and hydrogen is separated by a steam reformer.

Methanol and water are supplied to the fuel cell, as shown at the bottom left of the figure, and they react with the anode.

Hydrogen is extracted from the methanol, and electrons from the hydrogen are free to flow as current out from the anode and return back through the cathode.

Oxygen is supplied through the air and is ionized at the cathode. Oxygen ions combine with hydrogen ions to make water.

 The operating temperature range of this fuel cell is about 50° C to 120° C (122° F to 248° F). Typical units have a power rating between 25 W and 5,000 W.

The direct methanol fuel cell can be used in vehicles because it uses methanol in liquid form to provide the hydrogen source. Lower operating temperatures mean that this fuel cell does not need a large, heavy heat shield.

A disadvantage is that the efficiency is low, so the DMFC is more suited for portable applications, where energy and power density are more important than efficiency.

Another disadvantage is that carbon dioxide is a by-product and it is released into the atmosphere, just as it would be from the combustion of methanol.


Figure 2 Operation of the Direct Methanol Fuel Cell (DMFC)

Direct-methanol fuel cells or DMFCs are a subcategory of PEM fuel cells where, the fuel, methanol, is not reformed, but fed directly to the fuel cell. Storage of methanol is much easier than that of hydrogen because it does not need to be done at high pressures or low temperatures, as methanol is a liquid from -142.6 °F to 148.5 °F). Additionally, the energy density of methanol is much higher than that of highly compressed hydrogen.

The efficiency of direct-methanol fuel cells is low due to the high permeation of methanol through the membrane, which is known as methanol crossover, and the dynamic behavior is sluggish. Other problems include the management of carbon dioxide created at the anode. Current DMFCs are limited in the power they can produce, but can still store a high amount of energy content in a small space. Basically, DMFC’s can produce a small amount of power over a long period of time. This makes them ill-suited for powering vehicles, but ideal for consumer goods that do not require high power and high storage such as cell phones, laptops, or digital cameras.

Methanol is toxic and flammable. However, the International Civil Aviation Organization's (ICAO) Dangerous Goods Panel (DGP) voted in November 2005 to allow passengers to carry and use micro fuel cells and methanol fuel cartridges when aboard airplanes to power laptop computers and other consumer electronic devices.

Most fuel cells are powered by hydrogen, which can be fed to the fuel cell system directly or can be generated within the fuel cell system by reforming hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. Direct methanol fuel cells (DMFCs), however, are powered by pure methanol, which is usually mixed with water and fed directly to the fuel cell anode.

Direct methanol fuel cells do not have many of the fuel storage problems typical of some fuel cell systems because methanol has a higher energy density than hydrogen—though less than gasoline or diesel fuel. Methanol is also easier to transport and supply to the public using our current infrastructure because it is a liquid, like gasoline. DMFCs are often used to provide power for portable fuel cell applications such as cell phones or laptop computers.

1.5.2 Alkaline Fuel Cell (AFC) 

The ALKALINE FUEL CELL (AFC) is a very efficient fuel cell that requires pure hydrogen fuel and pure oxygen. It uses an aqueous (water-based) electrolyte solution of potassium hydroxide (KOH) in a porous stabilized matrix.

 AFCs have been used in the US space program since the 1960s to produce electricity and fresh water for spacecraft. The Apollo missions and the space shuttle used alkaline fuel cells to produce electrical power and fresh drinking water.

The AFC is very susceptible to contamination, so it requires pure hydrogen and oxygen, which makes this fuel cell expensive to operate and limits its application.

Figure 3 shows a diagram of the AFC. Pure hydrogen is supplied to the fuel cell at the anode, and pure oxygen is supplied at the cathode.

 The anode and cathode are made of lower-cost, nonprecious metals such as nickel. The electrolyte is a solution of potassium hydroxide (KOH) in water, which ionizes to form potassium ions (K+) and hydroxyl (OH−) ions.

 In the AFC, hydrogen gas is oxidized to hydrogen ions and combines with the hydroxide ions, which produces water (H2O) and releases two electrons. The electrons flow through the external circuit and return to the cathode, where they reduce oxygen to form more hydroxide ions and water.

The AFC operates at temperatures of 100° C to 250° C (212° F to 482° F). More recent AFCs operate at temperatures of 23° C to 70° C (74° F to 158° F). This fuel cell has an operating efficiency of 60% to 70%.

Excess heat is removed from the fuel cell as a by-product, and it is hot enough to provide steam to power a steam turbine. The heat can also be used to heat buildings.


Figure 3 Operation of the Alkaline Fuel Cell (AFC)

The AFC fuel cell works well in spacecraft or in undersea locations where the atmosphere can be controlled.

A disadvantage is a requirement for pure oxygen and pure hydrogen, and both gases must be supplied continuously. The fuel cell is also poisoned easily by carbon dioxide (CO2), which affects the cell’s lifetime.

AFCs do not currently have lifetimes beyond about 8,000 operating hours, so they tend to be less cost-effective than other types.

Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed, and they were the first type widely used in the U.S. space program to produce electrical energy and water on-board spacecraft. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. In recent years, novel AFCs that use a polymer membrane as the electrolyte have been developed. These fuel cells are closely related to conventional PEM fuel cells, except that they use an alkaline membrane instead of an acid membrane. The high performance of AFCs is due to the rate at which electro-chemical reactions take place in the cell. They have also demonstrated efficiencies above 60% in space applications.

A key challenge for this fuel cell type is that it is susceptible to poisoning by carbon dioxide (CO2). In fact, even the small amount of CO2 in the air can dramatically affect cell performance and durability due to carbonate formation. Alkaline cells with liquid electrolytes can be run in a recirculating mode, which allows for electrolyte regeneration to help reduce the effects of carbonate formation in the electrolyte, but the recirculating mode introduces issues with shunt currents. The liquid electrolyte systems also suffer from additional concerns including wettability, increased corrosion, and difficulties handling differential pressures. Alkaline membrane fuel cells (AMFCs) address these concerns and have lower susceptibility to CO2 poisoning than liquid-electrolyte AFCs do. However, CO2 still affects performance, and performance and durability of the AMFCs still lag that of PEMFCs. AMFCs are being considered for applications in the W to kW scale. Challenges for AMFCs include tolerance to carbon dioxide, membrane conductivity and durability, higher temperature operation, water management, power density, and anode electrocatalysis.

The electrolyte in this fuel cell is concentrated (85 wt percent) KOH in fuel cells operated at high temperature (~250 °C), or less concentrated (35 to 50 wt percent) KOH for lower temperature (<120 °C) operation. The electrolyte is retained in a matrix (usually asbestos), and a wide range of electro -catalysts can be used (e.g., Ni, Ag, metal oxides, spinels, and noble metals). The fuel supply is limited to non-reactive constituents except for hydrogen. CO is a poison, and CO2 will react with the KOH to form K2CO3, thus altering the electrolyte. Even the small amount of CO2 in air must be considered a potential poison for the alkaline cell. Generally, hydrogen is considered as the preferred fuel for AFC, although some direct carbon fuel cells use (different) alkaline electrolytes. 

The AFC was one of the first modern fuel cells to be developed, beginning in 1960. The application at that time was to provide on-board electric power for the Apollo space vehicle. The AFC has enjoyed considerable success in space applications, but its terrestrial application has been challenged by its sensitivity to CO2. Still, some developers in the U.S. and Europe pursue AFC for mobile and closed-system (reversible fuel cell) applications. 

Advantages: Desirable attributes of the AFC include its excellent performance on hydrogen (H2) and oxygen (O2 ) compared to other candidate fuel cells due to its active O2 electrode kinetics and its flexibility to use a wide range of electro-catalysts. 

Disadvantages: The sensitivity of the electrolyte to CO2 requires the use of highly pure H2 as a fuel. As a consequence, the use of a reformer would require a highly effective CO and CO2 removal system. In addition, if ambient air is used as the oxidant, the CO2 in the air must be removed. While this is technically not challenging, it has a significant impact on the size and cost of the system. 

FACTS OF AFC

FUEL CELL TYPE : ALKALINE FUEL CELL (AFC)

COMMON ELECTROLYTE : Aqueous potassium hydroxide soaked in a porous matrix or alkaline polymer membrane

OPERATING TEMPERATURE : < 100 c

TYPICAL STACK SIZE : , 1kW - 100 kW

ELECTRICAL EFFICIENCY : 60%

APPLICATIONS : • Military • Space • Backup power • Transportation

ADVANTAGES : Wider range of stable materials allows lower cost components • Low temperature • Quick start-up

CHALLENGES : •• Sensitive to CO2 in fuel and air • Electrolyte management (aqueous)

• Electrolyte conductivity (polymer)


1.5.3 Phosphoric Acid Fuel Cell (PAFC) 

The PHOSPHORIC ACID FUEL CELL (PAFC) is equivalent in structure to the proton exchange membrane fuel cell (PEMFC), but it has liquid phosphoric acid as the electrolyte.

The electrolyte is contained in a Teflon-bonded, silicon carbide matrix. It uses an external reformer to separate hydrogen from a hydrocarbon fuel.

 PAFCs are typically used in small, stationary, power generation systems, but research is being conducted for application in larger vehicles such as buses.

The PAFC operates at around 150° C to 200° C (300° F to 400° F). This operating temperature is hot enough to provide external heat as well as electricity.

If gasoline or diesel is used as a basic fuel, sulfur must be removed from the fuel prior to use or it will damage the electrode catalyst.

PAFCs are more tolerant of impurities in fossil fuels that have been reformed into hydrogen than are PEMFCs.

The electrical efficiency for PAFCs is 40% to 50%, and when the energy produced by the waste heat is considered, the efficiency rises to about 80%. Today, PAFCs are used in commercial electrical production.

The PAFC can tolerate a concentration of carbon monoxide (CO) of about 1.5%, which is a larger concentration than can be tolerated by other types of fuel cells.

 Another advantage to the PAFC is that the phosphoric acid electrolyte can operate above the boiling point of water.

A disadvantage of the PAFC is that, when compared to other fuel cells of similar weight and volume, it produces less power.

Also, PAFCs are expensive because of the platinum catalyst and the need for corrosion-resistant materials (because of the acid).

Phosphoric acid fuel cells (PAFCs) use liquid phosphoric ac/id as an electrolyte—the acid is contained in a Teflon-bonded silicon carbide matrix—and porous carbon electrodes containing a platinum catalyst. The electro-chemical reactions that take place in the cell are shown in the diagram to the right.

The PAFC is considered the "first generation" of modern fuel cells. It is one of the most mature cell types and the first to be used commercially. This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses.

PAFCs are more tolerant of impurities in fossil fuels that have been reformed into hydrogen than PEM cells, which are easily "poisoned" by carbon monoxide because carbon monoxide binds to the platinum catalyst at the anode, decreasing the fuel cell's efficiency. PAFCs are more than 85% efficient when used for the co-generation of electricity and heat but they are less efficient at generating electricity alone (37%–42%). PAFC efficiency is only slightly more than that of combustion-based power plants, which typically operate at around 33% efficiency. PAFCs are also less powerful than other fuel cells, given the same weight and volume. As a result, these fuel cells are typically large and heavy. PAFCs are also expensive. They require much higher loadings of expensive platinum catalyst than other types of fuel cells do, which raises the cost.

Phosphoric acid, concentrated to 100 percent, is used as the electrolyte in this fuel cell, which typically operates at 150 to 220 °C. At lower temperatures, phosphoric acid is a poor ionic conductor, and CO poisoning of the Pt electro-catalyst in the anode becomes severe. The relative stability of concentrated phosphoric acid is high compared to other common acids; consequently the PAFC is capable of operating at the high end of the acid temperature range 

(100 to 220 °C). In addition, the use of concentrated acid (100 percent) minimizes the water vapor pressure so water management in the cell is not difficult. The matrix most commonly used to retain the acid is silicon carbide (1), and the electro-catalyst in both the anode and cathode is Pt. 

PAFCs are mostly developed for stationary applications. Both in the U.S. and Japan, hundreds of PAFC systems were produced, sold, and used in field tests and demonstrations. It is still one of the few fuel cell systems that are available for purchase. Development of PAFC had slowed

down in the past ten years, in favor of PEFCs that were thought to have better cost potential.

Advantages: PAFCs are much less sensitive to CO than PEFCs and AFCs: PAFCs tolerate about one percent of CO as a diluent. The operating temperature is still low enough to allow the use of common construction materials, at least in the BoP components. The operating temperature also provides considerable design flexibility for thermal management. PAFCs have demonstrated system efficiencies of 37 to 42 percent (based on LHV of natural gas fuel), which is higher than most PEFC systems could achieve (but lower than many of the SOFC and MCFC systems). In addition, the waste heat from PAFC can be readily used in most commercial and industrial cogeneration applications, and would technically allow the use of a bottoming cycle.

 Disadvantages: Cathode-side oxygen reduction is slower than in AFC, and requires the use of a Platinum catalyst. Although less complex than for PEFC, PAFCs still require extensive fuel processing, including typically a water gas shift reactor to achieve good performance. Finally, the highly corrosive nature of phosphoric acid requires the use of expensive materials in the stack (especially the graphite separator plates). 

FACTS OF PAFC

FUEL CELL TYPE : PHOSPHORIC ACID FUEL CELL (PAFC)

COMMON ELECTROLYTE : Phosphoric acid soaked in a porous matrix or imbibed in a polymer membrane

OPERATING TEMPERATURE : 150-200 c

TYPICAL STACK SIZE : , 5 - 400 kW, 100 kW module (liquid PAFC);<10 kW (polymer

membrane)

ELECTRICAL EFFICIENCY : 40%

APPLICATIONS : • • Distributed generation

ADVANTAGES : Suitable for CHP • Increased tolerance to fuel impurities

CHALLENGES : •• • Expensive catalysts • Long start-up time • Sulfur sensitivity


1.5.4 Molten Carbonate Fuel Cell (MCFC) 

The molten-carbonate fuel cell (MCFC) is similar to the solid oxide fuel cell (SOFC), but it uses carbonate ions as the charge carrier in a high-temperature liquid solution of lithium, potassium, or sodium carbonate as an electrolyte.

The MCFC operates at a temperature of 600° C to 700° C (1,112° F to 1,292° F), so it can generate steam that can be used to generate more power. It is best suited for large stationary power generators.

Because the MCFC has an operating temperature that is a bit lower than solid oxide fuel cells, it can use less exotic materials, which makes it a little less expensive to manufacture and operate.


The MCFC was first designed in the early 1930s along with the SOFC. Research on both devices continued to provide improvements until the 1950s when researchers found that fused molten-carbonate salts could be used as an electrolyte in the fuel cell.

Because MCFCs operate at extremely high temperatures, nonprecious metals such as nickel can be used as catalysts in the anode and cathode, rather than the platinum that is used in some other fuel cells.

Figure 6 shows a diagram of an MCFC. Hydrogen is ionized at the anode by the catalyst. After the carbonate ions (CO−23 pass through the electrolyte, they combine with the hydrogen ions (H+) to form water (H2O) and carbon dioxide (CO2), which is returned to the input.

The carbon dioxide and electrons from the external circuit combine with oxygen to supply more carbonate ions, and the process continues.

The net reaction is hydrogen plus oxygen, which produces water as in other fuel cells. High-temperature MCFCs can extract hydrogen from a variety of fuels, such as natural gas, diesel, or coal, and they are not subject to the contamination issues of other types of fuel cells.

Because the MCFC operates at very high temperatures, these fuels can be converted to hydrogen within the fuel cell itself without a separate reformer, which also reduces cost.


Figure 6 Operation of a Molten-Carbonate Fuel Cell (MCFC)

An important advantage of an MCFC is that it can be used as a large stationary power plant that can supply power to the main load or at peak times as needed.

In addition to its ability to use various fuels, these plants are clean and quiet. The fuel cell can be placed close to the point where it is used, so it allows electric companies to provide electricity without a large investment in transmission lines.

One other advantage of MCFCs is that the waste heat can be captured and used, thus raising the overall efficiency.

One major disadvantage of molten-carbonate technology is that it is more difficult working with a very hot liquid electrolyte rather than a solid electrolyte.

Another disadvantage is that the chemical reactions at the anode use carbonate ions from the electrolyte, making it necessary to inject carbon dioxide at the cathode and thus requiring a supply of carbon dioxide.

Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications. MCFCs are high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide matrix. Because they operate at high temperatures of 650°C (roughly 1,200°F), non-precious metals can be used as catalysts at the anode and cathode, reducing costs.

Improved efficiency is another reason MCFCs offer significant cost reductions over phosphoric acid fuel cells. Molten carbonate fuel cells, when coupled with a turbine, can reach efficiencies approaching 65%, considerably higher than the 37%–42% efficiencies of a phosphoric acid fuel cell plant. When the waste heat is captured and used, overall fuel efficiencies can be over 85%.

Unlike alkaline, phosphoric acid, and PEM fuel cells, MCFCs do not require an external reformer to convert fuels such as natural gas and biogas to hydrogen. At the high temperatures at which MCFCs operate, methane and other light hydrocarbons in these fuels are converted to hydrogen within the fuel cell itself by a process called internal reforming, which also reduces cost.

The primary disadvantage of current MCFC technology is durability. The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life. Scientists are currently exploring corrosion-resistant materials for components as well as fuel cell designs that double cell life from the current 40,000 hours (~5 years) without decreasing performance.

The electrolyte in this fuel cell is usually a combination of alkali carbonates, which is retained in a ceramic matrix of LiAlO2. The fuel cell operates at 600 to 700 °C where the alkali carbonates form a highly conductive molten salt, with carbonate ions providing ionic conduction. At the high operating temperatures in MCFCs, Ni (anode) and nickel oxide (cathode) are adequate to promote reaction. Noble metals are not required for operation, and many common hydrocarbon fuels can be reformed internally. 

The focus of MCFC development has been larger stationary and marine applications, where the relatively large size and weight of MCFC and slow start-up time are not an issue. MCFCs are under development for use with a wide range of conventional and renewable fuels. MCFC-like technology is also considered for DCFC. After the PAFC, MCFCs have been demonstrated most extensively in stationary applications, with dozens of demonstration projects either under way or completed. While the number of MCFC developers and the investment level are reduced compared to a decade ago, development and demonstrations continue. 

Advantages: The relatively high operating temperature of the MCFC (650 °C) results in several benefits: no expensive electro-catalysts are needed as the nickel electrodes provide sufficient activity, and both CO and certain hydrocarbons are fuels for the MCFC, as they are converted to hydrogen within the stack (on special reformer plates) simplifying the BoP and improving system efficiency to the high forties to low fifties. In addition, the high temperature waste heat allows the use of a bottoming cycle to further boost the system efficiency to the high fifties to low sixties. 

Disadvantages: The main challenge for MCFC developers stems from the very corrosive and mobile electrolyte, which requires use of nickel and high-grade stainless steel as the cell hardware (cheaper than graphite, but more expensive than ferritic steels). The higher temperatures promote material problems, impacting mechanical stability and stack life.

Also, a source of CO2 is required at the cathode (usually recycled from anode exhaust) to form the carbonate ion, representing additional BoP components. High contact resistances and cathode resistance limit power densities to around 100 – 200 mW/cm2 at practical operating voltages. 

FACTS OF MCFC

FUEL CELL TYPE : MOLTEN CARBONATE FUEL CELL (MCFC)

COMMON ELECTROLYTE : Molten lithium, sodium, and/or potassium carbonates, soaked in a

porous matrix

OPERATING TEMPERATURE : 600-700 c

TYPICAL STACK SIZE : ,300 kW - 3 MW,300 kW module

ELECTRICAL EFFICIENCY : 50%

APPLICATIONS : • • ELECTRIC UTILITY, Distributed generation

ADVANTAGES : • High efficiency • Fuel flexibility • Suitable for CHP

• Hybrid/gas turbine cycle

CHALLENGES : •• • High temperature corrosion and breakdown of cell components

Long start-up time • Low power density

1.5.5 Solid Oxide Fuel Cell (SOFC)

The SOLID OXIDE FUEL CELL (SOFC) is named for the solid oxide electrolyte used. The solid oxide electrolyte consists of Yttria-stabilized zirconia, a zirconium oxide–based ceramic.

 Figure 4 shows a diagram of a standard solid oxide fuel cell. Hydrogen fuel (H2) is applied to the anode, which is oxidized to hydrogen ions. The electrons are given off to an external circuit as electrical current.

The oxygen from air is reduced to oxygen ions that diffuse through the solid electrolyte to the porous anode. (The acronym AN OX and a RED CAT remind you that oxidation occurs at the anode and reduction occurs at the cathode.)

The electrolyte is a hard, nonporous solid oxide that allows only the oxygen ions (rather than hydrogen ions) to pass and blocks electrons.

 A traditional SOFC must be operated at high temperatures (800° C to 1,000° C) to achieve reasonable conduction of the oxygen ions.


Figure 4 Operation of a Solid Oxide Fuel Cell (SOFC)

In the traditional SOFC, air is taken in so that oxygen (O2) can diffuse through the cathode. The unused gas is routed out of the cell.

The fuel cell produces water, which must be taken out of the cell. The solid oxide fuel cell takes several minutes to come up to temperature before it can produce power continuously to the external circuit, so this type of fuel cell is not useful for applications where it needs to be turned on and off frequently.

 It also produces a large amount of heat in the electrochemical reaction, and this heat must be removed from the fuel cell to ensure that it operates safely.

The high operating temperature allows the SOFC to separate (reform) hydrogen from other fuels internally so that expensive reforming equipment is not needed.

Solid oxide fuel cells (SOFCs) use a hard, non-porous ceramic compound as the electrolyte. SOFCs are around 60% efficient at converting fuel to electricity. In applications designed to capture and utilize the system's waste heat (co-generation), overall fuel use efficiencies could top 85%.

SOFCs operate at very high temperatures—as high as 1,000°C (1,830°F). High-temperature operation removes the need for precious-metal catalyst, thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system.

SOFCs are also the most sulfur-resistant fuel cell type; they can tolerate several orders of magnitude more sulfur than other cell types can. In addition, they are not poisoned by carbon monoxide, which can even be used as fuel. This property allows SOFCs to use natural gas, biogas, and gases made from coal. High-temperature operation has disadvantages. It results in a slow startup and requires significant thermal shielding to retain heat and protect personnel, which may be acceptable for utility applications but not for transportation. The high operating temperatures also place stringent durability requirements on materials. The development of low-cost materials with high durability at cell operating temperatures is the key technical challenge facing this technology.

Scientists are currently exploring the potential for developing lower-temperature SOFCs operating at or below 700°C that have fewer durability problems and cost less. Lower-temperature SOFCs have not yet matched the performance of the higher temperature systems, however, and stack materials that will function in this lower temperature range are still under development.

 The electrolyte in this fuel cell is a solid, nonporous metal oxide, usually Y2O3-stabilized ZrO2. The cell operates at 600-1000 °C where ionic conduction by oxygen ions takes place. Typically, the anode is Co-ZrO2 or Ni-ZrO2 cermet, and the cathode is Sr-doped LaMnO3. 

Early on, the limited conductivity of solid electrolytes required cell operation at around 1000 °C, but more recently thin-electrolyte cells with improved cathodes have allowed a reduction in operating temperature to 650 – 850 °C. Some developers are attempting to push SOFC operating temperatures even lower. Over the past decade, this has allowed the development of compact and high-performance SOFC which utilized relatively low-cost construction materials. 

Concerted stack development efforts, especially through the U.S. DOE’s SECA program, have considerably advanced the knowledge and development of thin-electrolyte planar SOFC. As a consequence of the performance improvements, SOFCs are now considered for a wide range of applications, including stationary power generation, mobile power, auxiliary power for vehicles, and specialty applications. 

Advantages: The SOFC is the fuel cell with the longest continuous development period, starting in the late 1950s, several years before the AFC. Because the electrolyte is solid, the cell can be cast into various shapes, such as tubular, planar, or monolithic. The solid ceramic construction of the unit cell alleviates any corrosion problems in the cell. The solid electrolyte also allows precise engineering of the three-phase boundary and avoids electrolyte movement or flooding in the electrodes. The kinetics of the cell are relatively fast, and CO is a directly useable fuel as it is in the MCFC. There is no requirement for CO2 at the cathode as with the MCFC. The materials used in SOFC are modest in cost. Thin-electrolyte planar SOFC unit cells have been demonstrated to be cable of power densities close to those achieved with PEFC. As with the MCFC, the high operating temperature allows use of most of the waste heat for cogeneration or in bottoming cycles. Efficiencies ranging from around 40 percent (simple cycle small systems) to over 50 percent (hybrid systems) have been demonstrated, and the potential for 60 percent+ efficiency exists as it does for MCFC. 

Disadvantages: The high temperature of the SOFC has its drawbacks. There are thermal expansion mismatches among materials, and sealing between cells is difficult in the flat plate configurations. The high operating temperature places severe constraints on materials selection and results in difficult fabrication processes. Corrosion of metal stack components (such as the interconnects in some designs) is a challenge. These factors limit stack-level power density (though significantly higher than in PAFC and MCFC), and thermal cycling and stack life (though the latter is better than for MCFC and PEFC). 

FACTS OF SOFC

FUEL CELL TYPE : SOLID OXIDE FUEL CELL (SOFC)

COMMON ELECTROLYTE : Yttria stabilized zirconia

OPERATING TEMPERATURE : 500-1000 c

TYPICAL STACK SIZE : ,1 kW - 2 MW,

ELECTRICAL EFFICIENCY : 60%

APPLICATIONS : • • Auxiliary power • Electric utility • Distributed generation

ADVANTAGES : • High efficiency • Fuel flexibility • Solid electrolyte • Suitable for CHP

• Hybrid/gas turbine cycle

CHALLENGES : •• • High temperature corrosion and breakdown of cell components • Long start-up time • Limited number of shutdowns

1.5.6 Reversible Fuel Cell

Reversible fuel cells produce electricity from hydrogen and oxygen and generate heat and water as byproducts, just like other fuel cells. However, reversible fuel cell systems can also use electricity from solar power, wind power, or other sources to split water into oxygen and hydrogen fuel through a process called electrolysis. Reversible fuel cells can provide power when needed, but during times of high power production from other technologies (such as when high winds lead to an excess of available wind power), reversible fuel cells can store the excess energy in the form of hydrogen. This energy storage capability could be a key enabler for intermittent renewable energy technologies.

1.6 Characteristics 

The interest in terrestrial applications of fuel cells is driven primarily by their potential for high efficiency and very low environmental impact (virtually no acid gas or solid emissions).

Efficiencies of present fuel cell plants are in the range of 30 to 55 percent based on the lower heating value (LHV) of the fuel. Hybrid fuel cell/reheat gas turbine cycles that offer efficiencies greater than 70 percent LHV, using demonstrated cell performance, have been proposed. 

Figure 1 -4 illustrates demonstrated low emissions of installed PAFC units compared to the Los Angeles Basin (South Coast Air Quality Management District) requirements, the strictest requirements in the U.S. Measured emissions from the PAFC unit are < 1 ppm of NOX, 4 ppm of CO, and <1 ppm of reactive organic gases (non-methane) (5). In addition, fuel cells operate at a constant temperature, and the heat from the electrochemical reaction is available for cogeneration applications. Table summarizes the impact of the major constituents within fuel gases on the various fuel cells. The reader is referred to Sections 3 through 7 for detail on trace contaminants. 

Another key feature of fuel cells is that their performance and cost are less dependent on scale than other power technologies. Small fuel cell plants operate nearly as efficiently as large ones, with equally low emissions, and comparable cost.1 This opens up applications for fuel cells where conventional power technologies are not practical. In addition, fuel cell systems can be relatively quiet generators. 

To date, the major impediments to fuel cell commercialization have been insufficient longevity and reliability, unacceptably high cost, and lack of familiarity of markets with fuel cells. For fuel cells that require special fuels (such as hydrogen) the lack of a fuel infrastructure also limits commercialization.

Automotive Batteries Verses Fuel Cells

Vehicles powered by fuel cells combine the attractive advantages of battery-powered cars and the convenience of an internal combustion engine. Fuel cells operate quietly and are zero to low emissions, comparable to a battery-powered vehicle. Fuel cell powered vehicles offer the range, power, responsiveness and rapid fueling that the internal combustion engine provides. Unlike battery-powered cars, fuel cells do not require lengthy recharge times. The batteries used in automobiles are extremely heavy which limits the vehicles range and capacity. The batteries are also composed of toxic materials and have a limited lifetime and must be recycled. Fuel cell vehicles operating on pure hydrogen produce only water vapor and heat as emissions while fuel cells reforming fossil fuels into hydrogen would be classified as ultra-low emission vehicles.  

The Future of Fuel Cells

In the future, fuel cells could play an increasing roll in everyday life. Fuel cell powered cars and trucks will be available, emitting nothing more than harmless water vapor. Fuel cells will find their way into portable devices such as cell phones and laptop computers. Homes and office buildings may have a fuel cell that replaces a conventional furnace, providing heat and electricity free from the disruptions associated with the utility's electric grid. Most of the companies planning to manufacture fuel cells are still in the research and development stage of production. Once their systems satisfy the manufacturers' stringent requirements for performance and safety, the fuel cell systems will be available to the general public.  

Figure 1-4 Relative Emissions of PAFC Fuel Cell Power Plants 

Compared to Stringent Los Angeles Basin Requirements

 1. The fuel processor efficiency is size dependent; therefore, small fuel cell power plants using externally reformed hydrocarbon fuels would have a lower overall system efficiency.

 Other characteristics that fuel cells and fuel cell plants offer are:

 Direct energy conversion (no combustion)

 No moving parts in the energy converter

Quiet

Demonstrated high availability of lower temperature units

Siting ability

 Fuel flexibility

Demonstrated endurance/reliability of lower temperature units

Lood performance at off-design load operation

Modular installations to match load and increase reliability

Remote/unattended operation

Size flexibility 

Rapid load following capability

 

General negative features of fuel cells include

 

Market entry cost high; Nth cost goals not demonstrated.

Endurance/reliability of higher temperature units not demonstrated.

Unfamiliar technology to the power industry.

No infrastructure.

 

Table 1-2 Summary of Major Fuel Constituents Impact on PEFC, AFC, 

In reality, CO, with H2O, shifts to H2 and CO2, and CH4, with H2O, reforms to H2 and CO faster than reacting as a fuel at the electrode. A fuel in the internal reforming MCFC. 

1.7 Advantages/Disadvantages 

The fuel cell types addressed in this handbook have significantly different operating regimes. As a result, their materials of construction, fabrication techniques, and system requirements differ. These distinctions result in individual advantages and disadvantages that govern the potential of the various cells to be used for different applications. Developers use the advantages of fuel cells to identify early applications and address research and development issues to expand applications (see Sections 3 through 7).

 1.8 Applications, Demonstrations, and Status

 The characteristics, advantages, and disadvantages summarized in the previous section form the basis for selection of the candidate fuel cell types to respond to a variety of application needs. The major applications for fuel cells are as stationary electric power plants, including cogen-eration units; as motive power for vehicles, and as on-board electric power for space vehicles or other closed environments. Derivative applications will be summarized.

 1.8.1 Stationary Electric Power

 One characteristic of fuel cell systems is that their efficiency is nearly unaffected by size. This means that small, relatively high efficient power plants can be developed, thus avoiding the higher cost exposure associated with large plant development. As a result, initial stationary plant development has been focused on several hundred kW to low MW capacity plants. Smaller plants (several hundred kW to 1 to 2 MW) can be sited at the user’s facility and are suited for cogeneration operation, that is, the plants produce electricity and thermal energy. Larger, dis-persed plants (1 to 10 MW) are likely to be used for distributed generation. The plants are fueled primarily with natural gas. Once these plants are commercialized and price improvements mate-rialize, fuel cells will be considered for large base-load plants because of their high efficiency. The base-load plants could be fueled by natural gas or coal. The fuel product from a coal gasi-fier, once cleaned, is compatible for use with fuel cells. Systems integration studies show that high temperature fuel cells closely match coal gasifier operation. 

Operation of complete, self-contained, stationary plants continues to be demonstrated using PEFC, AFC, PAFC, MCFC, and SOFC technology. Demonstrations of these technologies that occurred before 2000 were addressed in previous editions of the Fuel Cell Handbook and in the literature of the period. U.S. manufacturer experience with these various fuel cell technologies has produced timely information. A case in point is the 200 kW PAFC on-site plant, the PC-25, that was the first to enter the commercial market (see Figure 1-5). 

Figure 1-5 PC-25 Fuel Cell 

The plant was developed by UTC Fuel Cells, a division of United Technologies Corporation (UTC). The plants are built by UTC Fuel Cells. The Toshiba Corporation of Japan and Ansaldo SpA of Italy are partners with UTC Fuel Cells. The on-site plant is proving to be an economic and beneficial addition to the operating systems of commercial buildings and industrial facilities because it is superior to conventional technologies in reliability, efficiency, environmental impact, and ease of siting. Because the PC-25 is the first available commercial unit, it serves as a model for fuel cell application. Because of its attributes, the PC-25 is being installed in various applications, such as hospitals, hotels, large office buildings, manufacturing sites, wastewater treatment plants, and institutions to meet the following requirements: 

On-site energy

 Continuous power – backup

Uninterrupted power supply

Premium power quality

Independent power source

 

Characteristics of the plant are as follows:

Power Capacity Voltage and Phasing

0 to 200 kW with natural gas fuel (-30 to 45 °C, up to 1500 m) 480/277 volts at 60 Hz ; 400/230 volts at 50 Hz

Thermal Energy

740,000 kJ/hour at 60°C (700,000 Btu/hour heat at 140 °F);

 (Cogeneration)

module provides 369,000 kJ/hour at 120°C (350,000Btu/hour at 250 °F) and 369,000 kJ/hour at 60 °C

Electric ConnectionGrid-connected for on-line service and grid-independent for 

on-site premium service

Power Factor

Adjustable between 0.85 to 1.0

Transient Overload

None

Grid Voltage Unbalance

1 percent

Grid Frequency Range

+/-3 percent

Voltage Harmonic Limits

<3 percent

Plant Dimensions

3 m (10 ft) wide by 3 m (10 ft) high by 5.5 m (18 ft) long, not 

including a small fan cooling module (5)

Plant Weight 17,230 kg (38,000 lb) 

UTC Fuel Cells: Results from the operating units as of August, 2002 are as follows: total fleet operation stands at more than 5.3 million hours. The plants achieve 40 percent LHV electric efficiency, and overall use of the fuel energy approaches 80 percent for cogeneration applications 

(6). Operations confirm that rejected heat from the initial PAFC plants can be used for heating water, space heating, and low pressure steam. One plant has completed over 50,000 hours of operation, and a number of plants have operated over 40,000 hours (6). Fourteen additional plants have operated over 35,000 hours. The longest continuous run stands at 9,500 hours for a unit purchased by Tokyo Gas for use in a Japanese office building (9). This plant ended its duration record because it had to be shut down because of mandated maintenance. It is estimated at this time that cell stacks can achieve a life of 5 to 7 years. The fleet has attained an average of over 95 percent availability. The latest model, the PC-25C, is expected to achieve over 96 percent. The plants have operated on natural gas, propane, butane, landfill gas (10,11), hydrogen (12), and gas from anaerobic digestors (13). Emissions are so low (see Figure 1-4) that the plant is exempt from air permitting in the South Coast and Bay Area (California) Air Quality Management Districts, which have the most stringent limits in the U.S. The sound pressure level is 62 dBA at 9 meters (30 feet) from the unit. The PC-25 has been subjected to ambient conditions varying from -32 °C to +49 °C and altitudes from sea level to 1600 meters (~1 mile). Impressive ramp rates result from the solid state electronics. The PC-25 can be ramped at 10 kW/sec up or down in the grid connected mode. The ramp rate for the grid independent mode is idle to full power in ~one cycle or essentially one-step instantaneous from idle to 200 kW. Following the initial ramp to full power, the unit can adjust at an 80 kW/sec ramp up or down in one cycle.

 

The fuel cell stacks are made and assembled into units at an 80,000 ft2 facility located in South Windsor, Connecticut, U.S. Low cost/high volume production depends on directly insertable sub-assemblies as complete units and highly automatic processes such as robotic component handling and assembly. The stack assembly is grouped in a modified spoke arrangement to allow for individual manufacturing requirements of each of the cell components while bringing them in a continuous flow to a central stacking elevator (14).

Ballard Generation Systems: Ballard Generation Systems, a subsidiary of Ballard Power Systems, produces a PEFC stationary on-site plant. It has these characteristics:

Power Capacity

250 kW with natural gas fuel

Electric Efficiency

40% LHV

Thermal Energy

854,600 kJ/hour at 74 °C (810,000 Btu/hour at 165 °F)

Plant Dimensions

2.4 m (8 ft) wide by 2.4 m (8 ft) high by 5.7 m (18.5 ft) long

Plant Weight

12,100 kg (26,700 lb)

Ballard completed 10- and 60-kW engineering prototype stationary fuel cell power generators in 2001. Ballard, Shell Hydrogen, and Westcoast Energy established a private capital joint venture to help build early stage fuel cell systems. Ballard launched the NexaTM, a portable 1.2 kW power module, in September 2001. Ballard is also selling carbon fiber products for gas diffusion layers for proton exchange membrane fuel cells. Highlights of Ballard’s fuel cell sales are shown below. 

FuelCell Energy (FCE): FCE reached 50 MW manufacturing capacity and plans to expand its manufacturing capacity to 400 MW in 2004. The focus of the utility demonstrations and FCE’s fuel cell development program is the commercialization of 300 kilowatt, 1.5 megawatt, and 3 megawatt MCFC plants.

Power Capacity

3.0 MW net AC

Electric efficiency

57% (LHV) on natural gas

Voltage and Phasing

Voltage is site dependent, 3 phase 60 Hz

Thermal energy

~4.2 million kJ/hour (~4 million Btu/hour)

Availability

95% 

Siemens Westinghouse Power Corporation (SWPC): The Siemens Westinghouse SOFC is planning two major product lines with a series of product designs in each line. The first product will be a 250 kW cogeneration system operating at atmospheric pressure. This will be followed by a pressurized SOFC/gas turbine hybrid of approximately 0.5 MW. After the initial production, larger systems are expected as well. Also, a system capable of separating CO2 from the exhaust is planned as an eventual option to other products. 

The commercialization plan is focused on an initial offering of a hybrid fuel cell/gas turbine plant. The fuel cell module replaces the combustion chamber of the gas turbine engine. Figure 1-6 shows the benefit behind this combined plant approach. Additional details are provided in Section 7. As a result of the hybrid approach, the 1 MW early commercial unit is expected to attain ~60% efficiency LHV when operating on natural gas. 

EFFICIENCY (%)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

High

 

 

 

 

 

 

Advanced

 

Gas Turbine/


 

 

 

 

 

 

 

Figure 1-6 Combining the SOFC with a Gas Turbine Engine to Improve Efficiency 

Siemens Westinghouse is planning a number of tests on power plants that are prototypes of future products. All systems employ the tubular SOFC concept and most are combined with gas turbines in a hybrid configuration. Capacities of these systems are 250 kilowatts atmospheric, 300 kilowatt class hybrid, and 1 megawatt class hybrid. They are to operate at various sites in the U.S., Canada, and Europe. 

An eventual market for fuel cells is the large (100 to 300 MW), base-loaded, stationary plants operating on coal or natural gas. Another related, early opportunity may be in re-powering older, existing plants with high-temperature fuel cells (19). MCFCs and SOFCs coupled with coal gasifiers have the best attributes to compete for the large, base load market. The rejected heat from the fuel cell system can be used to produce steam for the existing plant's turbines. Studies showing the potential of high-temperature fuel cells for plants of this size have been performed (see Section 8). These plants are expected to attain from 50 to 60% efficiency based on the HHV of the fuel. Coal gasifiers produce a fuel gas product requiring cleaning to the stringent require-ments of the fuel cells’ electrochemical environment, a costly process. The trend of environmen-tal regulations has also been towards more stringent cleanup. If this trend continues, coal-fired technologies will be subject to increased cleanup costs that may worsen process economics. This will improve the competitive position of plants based on the fuel cell approach. Fuel cell sys-tems will emit less than target emissions limits. U.S. developers have begun investigating the viability of coal gas fuel to MCFCs and SOFCs (20,21,22). An FCE 20 kW MCFC stack was tested for a total of 4,000 hours, of which 3,900 hours was conducted at the Plaquemine, LA, site on coal gas as well as pipeline gas. The test included 1,500 hours of operation using 9,142 kJ/m3 syngas from a slip stream of a 2,180 tonne/day Destec entrained gasifier. The fuel processing system incorporated cold gas cleanup for bulk removal of H2S and other contaminants, allowing the 21 kW MCFC stack to demonstrate that the FCE technology can operate on either natural gas or coal gas. 

A series of standards is being developed to facilitate the application of stationary fuel cell technology power plants. Standard development activities presently underway are

 • Fuel Cell Power Systems ANSI/CSA America FC1-2004 (published)

 ࿿16 Stationary Fuel Cell Power Systems

-Safety IEC TC 105 Working Group #3 

࿿࿿1 Stationary Fuel Cell Power Systems•

-Installation

IEC TC 105 Working Group #5

Interconnecting Distributed Resources

IEEE P1547.1, P1547.2, P1547.3, P1547.4

Test Method for the Performance of Stationary Fuel Cell Power Plants

IEC TC 105 Working Group #4 

1.8.2 Distributed Generation 

Distributed generation involves small, modular power systems that are sited at or near their point of use. The typical system is less than 30 MW, used for generation or storage, and extremely clean. Examples of technologies used in distributed generation include gas turbines and reciprocating engines, biomass-based generators, solar power and photovoltaic systems, fuel cells, wind turbines, micro-turbines, and flywheel storage devices. See Table 1-3 for size and efficiencies of selected systems.

 

Table 1-3 Attributes of Selected Distributed Generation Systems

 

Type

Size

Efficiency, %

 

 

 

Reciprocating Engines

50 kW – 6 MW

33–37

Micro turbines

10 kW – 300 kW

20–30

Phosphoric Acid Fuel Cell (PAFC)

50 kW – 1 MW

40

Solid Oxide Fuel Cell (SOFC)

5 kW – 3 MW

45–65

Proton Exchange Membrane Fuel Cell

<1 kW – 1 MW

34–36

(PEM)

 

 

Photovoltaics (PV)

1 kW – 1 MW

NA

Wind Turbines

150 kW – 500 kW

NA

Hybrid Renewable

<1 kW – 1 MW

40–50

 

The market for distributed generation is aimed at customers dependent on reliable energy, such as hospitals, manufacturing plants, grocery stores, restaurants, and banking facilities. There is currently over 15 GW of distributed power generation operating in the U.S. Over the next decade, the domestic market for distributed generation, in terms of installed capacity to meet the demand, is estimated to be 5-6 GW per year. The projected global market capacity increases are estimated to be 20 GW per year (23). Several factors have played a role in the rise in demand for distributed generation. Utility restructuring is one of the factors. Energy suppliers must now take on the financial risk of capacity additions. This leads to less capital-intensive projects and shorter construction periods. Also, energy suppliers are increasing capacity factors on existing plants rather than installing new capacity, which places pressure on reserve margins. This increases the possibility of forced outages, thereby increasing the concern for reliable service. There is also a demand for capacity additions that offer high efficiency and use of renewables as the pressure for enhanced environmental performance increases  

There are many applications for distributed generation systems. They include:

࿿࿿1 Peak shaving - Power costs fluctuate hour by hour depending upon demand and generation, therefore customers would select to use distributed generation during relatively high-cost, on-peak periods. 

࿿࿿1 Combined heat and power (CHP) (Cogeneration) –The thermal energy created while converting fuel to electricity would be utilized for heat in addition to electricity in remote areas, and electricity and heat for sites that have a 24 hour thermal/electric demand.

࿿࿿1 Grid support – Strategic placement of distributed generation can provide system benefits and preclude the need for expensive upgrades and provide electricity in regions where small increments of new baseload capacity is needed.

 

࿿࿿1 Standby power – Power during system outages is provided by a distributed generation system until service can be restored. This is used for customers that require reliable back-up power for health or safety reasons, companies with voltage-sensitive equipment, or where outage costs are unacceptably high. 

࿿࿿1 Remote/Standalone – The user is isolated from the grid either by choice or circumstance. The purpose is for remote applications and mobile units to supply electricity where needed. 

Distributed generation systems have small footprints, are modular and mobile making them very flexible in use. The systems provide benefits at the customer level and the supplier level, as well as the national level. Benefits to the customer include high power quality, improved reliability, and flexibility to react to electricity price spikes. Supplier benefits include avoiding investments in transmission and distribution (T&D) capacity upgrades by locating power where it is most needed and opening new markets in remote areas. At the national level, the market for distributed generation establishes a new industry, boosting the economy. The improved efficiencies also reduce greenhouse gas emissions. 

However, a number of barriers and obstacles must be overcome before distributed generation can become a mainstream service. These barriers include technical, economic, institutional, and regulatory issues. Many of the proposed technologies have not yet entered the market, and will need to meet performance and pricing targets before entry. Questions have also risen on requirements for connection to the grid. Lack of standardized procedures creates delays and discourages customer-owned projects. Siting, permitting, and environmental regulations can also delay and increase the costs of distributed generation projects. 

In 1998, the Department of Energy created a Distributed Power Program to focus on market barriers and other issues that have prohibited the growth of distributed generation systems. Under the leadership of the National Renewable Energy Laboratory (NREL), a collaboration of national laboratories and industry partners have been creating new standards and are identifying and removing regulatory barriers. The goals of the program include 1) strategic research, 2) system integration, and 3) mitigation of regulatory and institutional barriers . 

Fuel cells, one of the emerging technologies in distributed generation, have been hindered by high initial costs. However, costs are expected to decline as manufacturing capacity and capability increase and designs and integration improve. The fuel cell systems offer many potential benefits as a distributed generation system. They are small and modular, and capital costs are relatively insensitive to scale. This makes them ideal candidates for diverse applications where they can be matched to meet specific load requirements. The systems are unobtrusive, with very low noise levels and negligible air emissions. These qualities enable them to be placed close to the source of power demand. Fuel cells also offer higher efficiencies than conventional plants. The efficiencies can be enhanced by using the quality waste heat derived from the fuel cell reactions for combined heat and power and combined-cycle applications. 

Phosphoric acid fuel cells have successfully been commercialized. Second generation fuel cells include solid oxide fuel cells and molten carbonate fuel cells. Research is ongoing in areas such as fuel options and new ceramic materials. Different manufacturing techniques are also being sought to help reduce capital costs. Proton exchange membrane fuel cells are still in the development and testing phase.

 

1.8.3 Vehicle Motive Power 

Since the late 1980s, there has been a strong push to develop fuel cells for use in light-duty and heavy-duty vehicle propulsion. A major drive for this development is the need for clean, effi-cient cars, trucks, and buses that operate on conventional fuels (gasoline, diesel), as well as renewable and alternative fuels (hydrogen, methanol, ethanol, natural gas, and other hydro-carbons). With hydrogen as the on-board fuel, these would be zero-emission vehicles. With on-board fuels other than hydrogen, the fuel cell systems would use an appropriate fuel processor to convert the fuel to hydrogen, yielding vehicle power trains with very low acid gas emissions and high efficiencies. Further, such vehicles offer the advantages of electric drive and low maintenance because of few moving parts. This development is being sponsored by various governments in North America, Europe, and Japan, as well as by major automobile manufacturers worldwide. As of May 1998, several fuel cell-powered cars, vans, and buses operating on hydrogen and methanol have been demonstrated. 

In the early 1970s, K. Kordesch modified a 1961 Austin A-40 two-door, four-passenger sedan to an air-hydrogen fuel cell/battery hybrid car (23). This vehicle used a 6 -kW alkaline fuel cell in conjunction with lead acid batteries, and operated on hydrogen carried in compressed gas cylinders mounted on the roof. The car was operated on public roads for three years and about 21,000 km. 

In 1994 and 1995, H-Power (Belleville, New Jersey) headed a team that built three PAFC/battery hybrid transit buses (24,25). These 9 meter (30 foot), 25 seat (with space for two wheel chairs) buses used a 50 kW fuel cell and a 100 kW, 180 amp-hour nickel cadmium battery. 

The major activity in transportation fuel cell development has focused on the polymer electrolyte fuel cell (PEFC). In 1993, Ballard Power Systems (Burnaby, British Columbia, Canada) demonstrated a 10 m (32 foot) light-duty transit bus with a 120 kW fuel cell system, followed by a 200 kW, 12 meter (40 foot) heavy-duty transit bus in 1995 (26). These buses use no traction batteries. They operate on compressed hydrogen as the on-board fuel. In 1997, Ballard provided 205 kW (275 HP) PEFC units for a small fleet of hydrogen-fueled, full-size transit buses for demonstrations in Chicago, Illinois, and Vancouver, British Columbia. Working in collaboration with Ballard, Daimler-Benz built a series of PEFC-powered vehicles, ranging from passenger

cars to buses . The first such vehicles were hydrogen-fueled. A methanol-fueled PEFC A-class car unveiled by Daimler-Benz in 1997 had a 640 km (400 mile) range. Plans were to offer a commercial vehicle by 2004. A hydrogen-fueled (metal hydride for hydrogen storage), fuel cell/battery hybrid passenger car was built by Toyota in 1996, followed in 1997 by a methanol-fueled car built on the same (RAV4) platform, 

In February 2002, UTC Fuel Cells and Nissan signed an agreement to develop fuel cells and fuel cell components for vehicles. Renault, Nissan’s alliance partner, is also participating in the development projects. UTC Fuel Cells will provide proprietary ambient-pressure proton exchange membrane fuel cell technology.

 

Ballard’s fuel cell engine powered DaimlerChrysler’s NECAR 5 fuel cell vehicle in a 13-day, 3,000-mile endurance test across the United States. The drive provided Ballard and DaimlerChrysler with testing experience in a variety of conditions.

 

Toyota Motor Corp. and Honda Motor Co. announced they would advance their initial vehicle introduction plans for fuel cell vehicles to late in 2002 from 2003. Honda achieved a significant milestone for its product launch by receiving both CARB and EPA certification of its zero emission FCX-V4 automobile. This was the first vehicle to receive such certification. Ballard’s fuel cell powered this Honda vehicle.

 

Other major automobile manufacturers, including General Motors, Volkswagen, Volvo, Chrysler, Nissan, and Ford, have also announced plans to build prototype polymer electrolyte fuel cell vehicles operating on hydrogen, methanol, or gasoline (29). IFC and Plug Power in the U.S., and Ballard Power Systems of Canada (15), are involved in separate programs to build 50 to 100 kW fuel cell systems for vehicle motive power. Other fuel cell manufacturers are involved in similar vehicle programs. Some are developing fuel cell-powered utility vehicles, golf carts, etc. (30,31).

 

Fuel Cell Benefits: 5 Facts You Should Know

Hydrogen fuel cells, non-emitting electrochemical power generating devices combining hydrogen and oxygen to produce electricity, have played and will increasingly play a crucial role in the global energy transition in response to climate change.

With a variety of application, fuel cell technology enables electricity production for anything ranging from commercial vehicles and at ports, and from stationary back-up power for microgrids to hydrogen fuel cells-powered forklifts driving warehouse efficiencies.

Here are five quick takeaways drilling down on why fuel cells will fuel the future in a variety of venues.

Benefit 1: Zero-Emission Power

Specifically, Plug utilizes and manufactures proton exchange membrane (PEM) fuel cell devices, with mid-range temperature functionality, offering power production opportunities across a wide range of use cases and product offerings. For fuel cells 101, check out the video below. 

Superseding all fuel cell benefits are their trump card: They do not emit greenhouse gasses nor other co-pollutants into the atmosphere, but instead benign by-products such as heat and water. With governments, the private sector, investors and stakeholders all aiming for mid-century “net zero” emissions targets, fuel cells’ ability to yield non-emitting electricity in hard to decarbonize and energy-intensive sectors is of paramount importance. 

And unlike batteries, fuel cells do not require storage of toxics like battery acis, an increasingly large global environmental justice issue as that technology scales. As Plug CEO Andy Marsh said in July 2022 Senate testimony, “Environmental justice is a fundamental tenet” for Plug because those communities bear “the brunt of climate change, toxic pollution, [and] adverse health effects.”

Some of Plug’s zero-emission fuel cell applications in recent years have included backup power generation for Southern Linc’s telecommunications network and a major Northern California winery, materials handling equipment such as forklifts within warehouses, as well as drones, delivery fleet vehicles, ground support vehicles at airports and other related settings, among others. Plug’s fuel cell technology within the e-mobility segment can help to reduce “middle mile ' emissions, 57%of greenhouse gas emissions within U.S. transportation.

For more on the transportation emissions mitigation benefits fuel cell vehicles can usher in, check out this fact sheett published by the Fuel Cell & Hydrogen Energy Association.

Benefit 2: Robust Reliability

Plug’s fuel cells also have a proven operational track record within tough conditions.

This has occurred particularly within bone-chillingly cold environments like warehouses requiring freezer technology to ensure maximum product freshness for their customers. Unlike batteries, whose power capacity decays and eventually completely degrades as temperatures lower, our technology can still prosper under such duress.

For example, FreezPak — an industry leader for the pre-retail market storage of frozen foods and dry foods — uses Plug’s hydrogen fuel cell-powered forklifts amid temperatures hitting -22°F within its warehouses. With battery-powered devices facing speed diminishments of 14% during the last half of their charging lives, the amount of time saved from not having to charge combined with full-scale functionality even amid Arctic-like temperatures equals $900,000 in annual savings for the typical 200-forklift warehouse.

On the other extreme is humidity-causing hurricanes. Amid Hurricane Michael in 2018, we provided 49 fuel cells, swiftly providing electricity amid massive regional grid damage. In all, Plug provided over 2,390 run-hours of backup power and two sites ran continuously for over 15 days.

Beyond these weather extremes, Plug has also stood ready as a back-up power provider for utilities, railroad switchboards, and telecommunications providers, among others.

Benefit 3: Improved Efficiency

With energy efficiency levels running between 40% to 60%, fuel cells defeat incumbent combustion engines, which maintain 25% energy efficiency levels. When it comes to warehouses, fuel cells can boost productivity by 15% when replacing batteries. This all amounts to warehouse productivity for fuel cell forklifts and extended electric vehicles mileage ranges. As mentioned earlier, fuel cells’ durability also increases long term efficiency.

According to U.S. Department of Energy analysis, “Fuel cells are the most energy efficient devices for extracting power from fuels.”

But even as fuel cells boost efficiencies accelerate productivity as a by-product, they also enhance robustness, another vital fuel cell metric.

For robustness, essentially finding a technology’s operational sweet spot, Plug’s engineers have concluded that fuel cells operate best when they are actually optimized for slightly lower efficiency levels than that for which they are technically capable. That’s because, in veering too close to a thermodynamics red line, this can shorten fuel cells’ lifetimes. And that actually means reduced efficiency and productivity in the long haul.

Benefit 4: Scalable

Technologies lacking finesse cannot meet wide-ranging customer needs.

Bearing that in mind, Plug creates drop-and-play custom-made fuel cell and electrolyzer configurations designed in a modular fashion as circumstances necessitate. At the same time, Plug engineers multiple modular fuel cell technologies, offering a wide range of applicable uses for the same equipment.

An example of modularity, Plug's ProGen engines can empower van, heavy-haul trucks, and buses alike. And for custom-designed solutions cases in point, Plug has teamed up with robotics and drone producers to deploy more narrowly-tailored and individuated solutions.

Taken as a whole, Plug’s nimbleness with its fuel cells technology is its strongest asset, ensuring its smooth scaling both at the customer- and marketing-level. Whether it’s for on-road mobility, materials handling, or stationary power production, Plug designs fuel cells products possessing agility, ability, and scalability.

Benefit 5: Lower Operational Costs

Because fuel cells can proverbially “brave the elements,” as explained earlier, they foment inherent operational cost-savings and efficiencies. But compared to battery technology’s mobility modality, fuel cells also ax the charging time element, compounding those gains in further operational expenditure reductions.

Not only do fuel cell engines and devices top off fuel in three to four minutes, substantially reducing vehicle downtime, but they also perform at mileage ranges 22% higher on average compared to battery-powered ones. By contrast, battery-powered vehicles can take 20 minutes to an hour during a designated “fast charge,” or seven to 20 times that of their fuel cell vehicles counterparts.

Translated, this means more time doing productive work and less time refueling when using fuel cells technology. As the old adage goes: time is money.

The Future with Fuel Cells

These five differentiated benefits of hydrogen fuel cells provide the potential to remake the global energy landscape as we know it, with Goldman Sachs projecting that green hydrogen will provide 25% of global energy outut by 2050. Clearly then, fuel cells sit at the nexus of today’s fossil fuel-laden energy landscape and the greener and more sustainable future.

Types of Fuel Cells

Fuel cells are classified primarily by the kind of electrolyte they employ. This classification determines the kind of electro-chemical reactions that take place in the cell, the kind of catalysts required, the temperature range in which the cell operates, the fuel required, and other factors. These characteristics, in turn, affect the applications for which these cells are most suitable. There are several types of fuel cells currently under development, each with its own advantages, limitations, and potential applications. Learn more about the following types of fuel cells.

  • Polymer electrolyte membrane fuel cells

Polymer electrolyte membrane (PEM) fuel cells—also called proton exchange membrane fuel cells—deliver high power density and offer the advantages of low weight and volume compared with other fuel cells. PEM fuel cells use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum or platinum alloy catalyst. They need only hydrogen, oxygen from the air, and water to operate. They are typically fueled with pure hydrogen supplied from storage tanks or reformers.

PEM fuel cells operate at relatively low temperatures, around 80°C (176°F). Low-temperature operation allows them to start quickly (less warm-up time) and results in less wear on system components, resulting in better durability. However, it requires that a noble-metal catalyst (typically platinum) be used to separate the hydrogen's electrons and protons, adding to system cost. The platinum catalyst is also extremely sensitive to carbon monoxide poisoning, making it necessary to employ an additional reactor to reduce carbon monoxide in the fuel gas if the hydrogen is derived from a hydrocarbon fuel. This reactor also adds cost.

PEM fuel cells are used primarily for transportation applications and some stationary applications. PEM fuel cells are particularly suitable for use in vehicle applications, such as cars, buses, and heavy-duty trucks.

  • Direct methanol fuel cells

Most fuel cells are powered by hydrogen, which can be fed to the fuel cell system directly or can be generated within the fuel cell system by reforming hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. Direct methanol fuel cells (DMFCs), however, are powered by pure methanol, which is usually mixed with water and fed directly to the fuel cell anode.

Direct methanol fuel cells do not have many of the fuel storage problems typical of some fuel cell systems because methanol has a higher energy density than hydrogen—though less than gasoline or diesel fuel. Methanol is also easier to transport and supply to the public using our current infrastructure because it is a liquid, like gasoline. DMFCs are often used to provide power for portable fuel cell applications such as cell phones or laptop computers.

  • Alkaline fuel cells

Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed, and they were the first type widely used in the U.S. space program to produce electrical energy and water on-board spacecraft. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. In recent years, novel AFCs that use a polymer membrane as the electrolyte have been developed. These fuel cells are closely related to conventional PEM fuel cells, except that they use an alkaline membrane instead of an acid membrane. The high performance of AFCs is due to the rate at which electro-chemical reactions take place in the cell. They have also demonstrated efficiencies above 60% in space applications.

A key challenge for this fuel cell type is that it is susceptible to poisoning by carbon dioxide (CO2). In fact, even the small amount of CO2 in the air can dramatically affect cell performance and durability due to carbonate formation. Alkaline cells with liquid electrolytes can be run in a recirculating mode, which allows for electrolyte regeneration to help reduce the effects of carbonate formation in the electrolyte, but the recirculating mode introduces issues with shunt currents. The liquid electrolyte systems also suffer from additional concerns including wettability, increased corrosion, and difficulties handling differential pressures. Alkaline membrane fuel cells (AMFCs) address these concerns and have lower susceptibility to CO2 poisoning than liquid-electrolyte AFCs do. However, CO2 still affects performance, and performance and durability of the AMFCs still lag that of PEMFCs. AMFCs are being considered for applications in the W to kW scale. Challenges for AMFCs include tolerance to carbon dioxide, membrane conductivity and durability, higher temperature operation, water management, power density, and anode electrocatalysis.

  • Phosphoric acid fuel cells

Phosphoric acid fuel cells (PAFCs) use liquid phosphoric acid as an electrolyte—the acid is contained in a Teflon-bonded silicon carbide matrix—and porous carbon electrodes containing a platinum catalyst. The electro-chemical reactions that take place in the cell are shown in the diagram to the right.

The PAFC is considered the "first generation" of modern fuel cells. It is one of the most mature cell types and the first to be used commercially. This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses.

PAFCs are more tolerant of impurities in fossil fuels that have been reformed into hydrogen than PEM cells, which are easily "poisoned" by carbon monoxide because carbon monoxide binds to the platinum catalyst at the anode, decreasing the fuel cell's efficiency. PAFCs are more than 85% efficient when used for the co-generation of electricity and heat but they are less efficient at generating electricity alone (37%–42%). PAFC efficiency is only slightly more than that of combustion-based power plants, which typically operate at around 33% efficiency. PAFCs are also less powerful than other fuel cells, given the same weight and volume. As a result, these fuel cells are typically large and heavy. PAFCs are also expensive. They require much higher loadings of expensive platinum catalyst than other types of fuel cells do, which raises the cost.

  • Molten carbonate fuel cells

Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications. MCFCs are high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide matrix. Because they operate at high temperatures of 650°C (roughly 1,200°F), non-precious metals can be used as catalysts at the anode and cathode, reducing costs.

Improved efficiency is another reason MCFCs offer significant cost reductions over phosphoric acid fuel cells. Molten carbonate fuel cells, when coupled with a turbine, can reach efficiencies approaching 65%, considerably higher than the 37%–42% efficiencies of a phosphoric acid fuel cell plant. When the waste heat is captured and used, overall fuel efficiencies can be over 85%.

Unlike alkaline, phosphoric acid, and PEM fuel cells, MCFCs do not require an external reformer to convert fuels such as natural gas and biogas to hydrogen. At the high temperatures at which MCFCs operate, methane and other light hydrocarbons in these fuels are converted to hydrogen within the fuel cell itself by a process called internal reforming, which also reduces cost.

The primary disadvantage of current MCFC technology is durability. The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life. Scientists are currently exploring corrosion-resistant materials for components as well as fuel cell designs that double cell life from the current 40,000 hours (~5 years) without decreasing performance.

  • Solid oxide fuel cells

Solid oxide fuel cells (SOFCs) use a hard, non-porous ceramic compound as the electrolyte. SOFCs are around 60% efficient at converting fuel to electricity. In applications designed to capture and utilize the system's waste heat (co-generation), overall fuel use efficiencies could top 85%.

SOFCs operate at very high temperatures—as high as 1,000°C (1,830°F). High-temperature operation removes the need for precious-metal catalyst, thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system.

SOFCs are also the most sulfur-resistant fuel cell type; they can tolerate several orders of magnitude more sulfur than other cell types can. In addition, they are not poisoned by carbon monoxide, which can even be used as fuel. This property allows SOFCs to use natural gas, biogas, and gases made from coal. High-temperature operation has disadvantages. It results in a slow startup and requires significant thermal shielding to retain heat and protect personnel, which may be acceptable for utility applications but not for transportation. The high operating temperatures also place stringent durability requirements on materials. The development of low-cost materials with high durability at cell operating temperatures is the key technical challenge facing this technology.

Scientists are currently exploring the potential for developing lower-temperature SOFCs operating at or below 700°C that have fewer durability problems and cost less. Lower-temperature SOFCs have not yet matched the performance of the higher temperature systems, however, and stack materials that will function in this lower temperature range are still under development.

  • Reversible fuel cells

Reversible fuel cells produce electricity from hydrogen and oxygen and generate heat and water as byproducts, just like other fuel cells. However, reversible fuel cell systems can also use electricity from solar power, wind power, or other sources to split water into oxygen and hydrogen fuel through a process called electrolysis. Reversible fuel cells can provide power when needed, but during times of high power production from other technologies (such as when high winds lead to an excess of available wind power), reversible fuel cells can store the excess energy in the form of hydrogen. This energy storage capability could be a key enabler for intermittent renewable energy technologies.


Proton exchange membrane fuel cells (PEMFCs)

Low temperature cells

The proton exchange membrane (a.k.a. polymer electrolyte membrane) fuel cell uses a polymeric electrolyte. This proton-conducting polymer forms the heart of each cell and electrodes (usually made of porous carbon with catalytic platinum incorporated into them) are bonded to either side of it to form a one-piece membrane-electrode assembly (MEA). A quick overview of some key advantages that make PEMs such a promising technology for the automotive markets:

  • Low temperature operation, and hence
  • Quick start up
  • No corrosive liquids involved
  • Will work in any orientation (or zero g for that matter)
  • Thin Membrane-electrode assemblies allow compact cells

 

Brief history

The PEM fuel cell was developed in the 1960’s in General Electric’s labs. As with so many technologies, the space program and military funded research fast-forwarded it’s development. PEM membranes were first applied to a US Navy project and projects for the US Signal Corps. PEM cells were used in NASA’s Gemini program, which was to serve as a means of testing technology for the Apollo missions. Batteries were not suitable for a journey to the moon because of the extended flight duration. Early PEM systems were, however, unreliable and plagued with leakages and contamination. The systems installed in Gemini spaceships had an operational lifetime of just 500 hrs, although this was considered suitable. Another issue was the water management systems, which are required to keep the membrane hydrated to the correct extent. Apollo designers opted for the more mature technology of AFCs, as did the Space Shuttle designers in the 70's. Recently however, as part of NASA’s program of continuous upgrade on the Shuttles, PEM systems have replaced the aging AFC technology as the primary power source for the Shuttles’ systems.  GE decided to abandon their research on PEMFCs in the 70’s, probably due to the cost. At that time, the catalysis required 28 mg of Platinum per cm2 of electrode, compared to the current figure of 0.2 mg cm–2, or less.

Automobiles are arguably one of the most important consumer products on the planet. The finite fuel reserves, which they are chewing through, are not currently a limiting factor, but they will be soon. Much investment has been aimed at developing fuel cell technology for the automotive industry and the electrolyte of choice is the PEM. We’ll look at the problems which automotive companies need to overcome before fuel cell cars hit the street.

However, recent developments in PEMFCs have brought their current densities up to around 1 A cm–2 and cut the platinum requirement to 1% of what used to be needed. The scope of PEMFCs is, arguably, wider than that of any other power supply technology; with the potential to power a range of devices from mobile phones and laptops to busses, boats and houses.

 

Construction of the PEM cell


The PEMFC is constructed in layers of bipolar plates, electrodes and membranes:

PEMFC components

Each individual cell produces about 0.7 V EMF when operating in air, as calculated by the expressions outlined in the efficiency section. In order to produce a useful voltage, the electrodes of many cells must be linked in series. In addition to connecting the cells, we must ensure that reactant gases can still reach the electrodes and that the resistance of the electrodes has a minimal effect. If, for example, we were to simply wire up the edge of the anode of one cell to the cathode of another, electrons would have to flow across the face of the electrodes. Each cell only produces ~0.7 V, even a small reduction in this isn’t permissible, so cells aren’t normally wired up this way.

A Bipolar Plate is used to interconnect the anode of one cell to the cathode of the next. It must evenly distribute reactant gases over the surface of the anode, and oxygen/air over the cathode. Bipolar plates may also need to carry a cooling fluid, and in addition, need to keep all these gases and cooling fluids separate. Design considerations:

  • The electrical contacts should be as large as possible
  • The plate should be thin to minimise resistance
  • Gas needs to flow easily across the plate

Often these factors are antagonistic to each other, for instance, large contact area would reduce the width of the gas channels. A very simple bipolar plate might look like this:

A typical bipolar plate (Left) found in a plate-type PEM assembly (Right)



Reactant gases flow at right angles to each other. In a simple plate design as above, the channels extend right to the edge. The reactant gases would probably be supplied to the system via external manifolding in this case.

External manifolding



External manifolding is a very simple solution, and therefore carries out the job cheaply, but the technique has two major disadvantages. 1) The gaskets needed to seal the plates don’t form a tight seal where the channels come to the edge of the plate, leading to localised leaks of the reactant gases. 2)  Additional channels for cooling fluids are very difficult to incorporate into an externally manifolded system, so all the cooling must be done by the air flowing across the cathode.  This means more air than is necessary for the reaction must be pumped through the channels, which in turn means the channels must be wider, that the chance of leaks is increased and that some of the energy produced must be used to power blowers. Whilst simplicity is always a bonus, external manifolding is rarely used in modern systems.

In this image of a Ballard Nexus™ fuel cell system, the fan used to blow air through the stack for cooling is visible on the left of the stack.

Most modern bipolar plates make use of internal manifolding. The three examples below show how this might be achieved. In each case, the channels do not run to the edge of the plates so a gasket could be fitted here and a gas-tight seal would be more easily achieved.

Internal manifolding


  • The design on the left is a fairly simple parallel channels design; reactant gases would be blown into one end of the channel through one hole, and removed at the other hole. There are many different designs possible, and designers of bipolar plates are yet to reach an agreement on which type is best. In parallel designs, water or gas may build up along one of the channels causing a temporary blockage. In this case the reactants will happily continue to pass through the other channels and not clear the blockage.

  • The second design, a serpentine design, guarantees that if reactants are flowing at all, they’re flowing all along the channel and blockages are easily cleared. The problem in this case is that it takes more effort to push reactants through the long, winding path.

  • The third design is more of a compromise between the two and is the type of thing often seen in bipolar plate design. The channels are typically about 1 mm in width and depth. The pressure difference between the start and end of a channel must be engineered to overcome the surface tension of water droplets forming on the channel walls in order to clear blockages. Ballard, for example, achieve this pressure difference with rectangular plates in which the gases run across the long axis in a long parallel design.

The material properties of a bipolar plate, as summed up by Ruge and Büchi (2001), must take into account several important factors:

  • Electrical conductivity >10 S cm–1
  • Heat conductivity of 20 W m–1 K–1 if cooling fluid is integrated, 100 W m–1 K–1 if heat is removed from the edges.
  • Gas permeability < 10–7 mbar L s–1 cm–2
  • Resistant to corrosion in an environment of acidic electrolyte, hydrogen, oxygen, heat and humidity.
  • Reasonably high stiffness E > 25 MPa
  • As ever, it should cost as little as possible.

The plates must also be manufactured so that they are:

  • Thin for maximum stack volume
  • Light for minimum stack mass
  • Able to be produced quickly with a short cycle time

These various and difficult specifications which must be met, along with the fact that modern electrodes require very little catalytic platinum, mean that the bipolar plate is the most expensive part of a modern fuel cell.

 Materials for constructing bipolar plates

PEMFCs without bipolar plates

As discussed, bipolar plates may provide excellent contact between cells, but they are expensive and complex. Some manufacturers, often on the smaller industrial scale, choose different techniques to link their cells. Cells could be connected simply edge to edge, reducing the possibility of leakage. One manufacturer (Intelligent Energy) produces cells with stainless steel bases through which hydrogen channels pass. The cathode current collector is a porous metal and these individual cell units are simply stacked with a piece of corrugated stainless steel between them. It’s a simple solution which may gain popularity.

In conclusion, we should note that although a broad range of bipolar plates techniques exist, none of them fully meet the criteria set above. There is lots of development still to be done in this area before we meet a new industry standard.

Solid oxide fuel cells (SOFCs)

High temperature cells

In the late nineteenth century, conduction was not yet understood. Later, Nernst observed at the University of Göttingen, that stabilized zirconia (ZrO2 doped with Ca, Mg, Y) was an insulator at room temperature, an ionic conductor from 600–1000 °C and a mixed conductor (both electronic and ionic) at around 1500 °C. The main part of the solid oxide fuel cell was therefore discovered. The fuel cell concept was demonstrated by Baur and Preis in the 1930’s using zirconium oxide, but many improvements were necessary to make a competitive device. In the 1950’s, simple, straightforward design made cheaper manufacturing processes possible: the flat plate fuel cell.

Flat plate solid oxide fuel cell


There are a few problems with the flat plate design when used for larger devices: sealing, around the edges, thermal expansion mismatch and cracking (intrinsically brittle ceramics are used). Tubular designs have been developed to solve these problems (see animation below).


SOFCs are the most efficient devices yet invented, that can convert chemical energy into electrical energy. Both electrodes (cathode and anode) and the electrolyte are made of ceramic materials, since the high operating temperature prevent the use of cheaper metals. The big advantage of the SOFC over the MCFC is that the electrolyte is solid and there are no pumps required to circulate the hot electrolyte. The anode contains nickel, for better electron conduction and catalysis. The operating temperature is between 600 and 1000 °C, depending on the generation of the fuel cell (first, second and third, with decreasing operating temperature). However, thermal cycling can cause cracking of the brittle ceramic components. Both hydrogen and carbon monoxide serve as fuels. Common hydrocarbon fuels can be used in SOFC (diesel, natural gas, gasoline, alcohol etc).


 

Operation of a SOFC

The operation of the solid oxide fuel cell is straightforward: oxygen atoms are reduced on the porous cathode surface by electrons. The oxide ions diffuse through the electrolyte to the fuel rich and porous anode, where they react with the fuel (hydrogen) and give off electrons to an external circuit. A large amount of heat is produced by the electrochemical reaction, which can be used by an integrated heat management system. Since it takes a long time to reach its operating temperature, the best applications for SOFCs are ones that use both the heat and electricity generated: stationary power plants, auxiliary power supplies. Start-up time problems could be solved by using supercapacitor batteries for the first few minutes of operation in mobile applications.

Molten carbonate fuel cells (MCFCs)

High temperature cells

Molten Carbonate Fuel Cells (MCFCs) are another type of high temperature fuel cell. A molten mixture of salts: lithium, sodium, potassium carbonate is used as the electrolyte. These salts melt and conduct carbonate ions (CO32–) from the anode to the cathode when heated to about 600°C. Hydrocarbons have to be used as part of the fuel since the charge carriers in the electrolyte are carbonate ions. Hydrogen is also needed at the anode. It is gained by internal reforming of hydrocarbon based fuels. The electrodes should be resistant to poisoning by carbon. The high exhaust temperature makes cogeneration of electricity with turbines possible; hence the efficiency (60% without and 80% with hybrid technology) is relatively high compared to other fuel cell systems. MCFCs are mainly used for stationary power generation in the 50 kW to 5 MW range. Since it uses a liquid and high temperature electrolyte, it is rather unsuitable for mobile applications. The main problem with MCFC is the slow dissolution of the cathode in the electrolyte. Most of the research is therefore in the area of more durable materials and cathodes.

Molten Carbonate Fuel Cell


DoITPoMS standard terms of use


Historical summary

Both the solid oxide and the molten carbonate fuel cells are high temperature devices.  Their development followed similar lines until the late 1950's. First, E. Baur and H. Preis experimented with solid oxide electrolytes in Switzerland. The technical problems they encountered were again tackled by the Russian scientist O.K. Davtyan without success though. In the late 1950's, Dutch scientists G.H.J. Broers and J.A.A. Ketelaar focused on molten carbonate salts as electrolyte. By 1960, they reported the first MCFC prototype. In the mid-1960's, the US Army’s Mobility Equipment Research and Development Center (MERDC) tested several MCFCs made by Texas Instruments ranging from 100 to 1000 Watts. Ishikawjima Heavy Industries showed in Japan in the early 1990s that a 1000 Watt MCFC power generator can operate for 10000 hours continuously. Other large power plants with outputs of up to 3 megawatts are already planned.

M-C Power's molten carbonate fuel cell power plant in San Diego, California, 1997. Smithsonian Institution, from the Science Service Historical Images Collection, courtesy of National Energy Technology Laboratory.

The MCFC has been under development for 15 years as a stationary electric power plant. Although when most problems with the Solid Oxide Fuel Cell are solved, work on the MCFC might be stopped.

Conclusion

This review discusses different types of fuel cells and their history, fundamentals, and applications. These fuel cells are PEMFCs, DMFCs, SOFCs, PAFCs, AFCs, and MCFCs. Fuel cells can be evaluated based on their efficiency, fuel used, and performance characteristics in different temperature ranges. The selection of fuel cell types depends on the specific operational requirements, considering factors such as temperature, efficiency, power output, and application demands. Recent technical progress with fuel cell technology has made them appear prime for diverse applications, such as transportation, aerospace, and portable and stationary power.

PEMFCs are highly efficient and versatile energy sources that can be used in various applications and are preferred in transportation. These fuel cells are known for their low operating temperatures and quick startup times. DMFCs, on the other hand, are ideal for small-scale applications due to their portability and simplicity. They work by directly utilizing methanol but face challenges like methanol crossover and efficiency limitations. SOFCs excel in high-temperature operations and offer high efficiencies and fuel flexibility. However, they require robust materials and face challenges in thermal management. PAFCs are well-established for stationary power applications, boasting reliability and efficiency, albeit at the expense of higher operating temperatures and slower response times. AFCs exhibit high efficiency and durability, but their reliance on precious metals and susceptibility to CO2 contamination limit their widespread adoption. Lastly, MCFCs offer high efficiency and can utilize various fuels but require high operating temperatures and face material corrosion and system complexity challenges.

In comparing different fuel cell types, studies found that PEMFCs have a relatively high specific power above 1,000 W/kg, higher than SOFCs (less than 100 W/kg). PEMFCs, SOFCs, PAFCs, AFCs, MCFCs, and DMFCs have typical efficiencies of 30%–55%, 40%–60%, 40%–50%, up to 70%, up to 50%, and up to 40%, respectively. PEMFCs and AFCs are highly efficient and suitable for low-temperature applications (50−100°C). SOFCs and MCFCs are better options for high-temperature operations (800−1,000°C). SOFCs are notable for their robustness and suitability for high-power demands, while MCFCs also demonstrate advantages, especially when high power output is a priority. Lifetime assessments showed PEMFCs, SOFCs, PAFCs, AFCs, MCFCs, and DMFCs can last up to 3,000 hr, 1,000 hr, >50,000 hr, 8,000 hr, 7,000 hr–8,000 hr, and 1,000 hr, respectively.

Hydrogen fuel cells, specifically PEMFCs, are an attractive technology for transportation and aerospace. They can significantly reduce the environmental impact by only producing water at low temperatures. However, the widespread adoption of this technology faces challenges primarily due to the efficient production and storage of hydrogen and the need to enhance fuel cell power density further. Despite these obstacles, hydrogen fuel cells show great promise in revolutionizing clean energy solutions and mitigating the harmful effects of traditional fossil fuel-based systems on the environment.


 


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