How Plant-microbial Fuel Cells Work

Soil, as it turns out, is full of untapped (electrical) potential.

As green plants go about the business of photosynthesis -- converting energy from sunlight to chemical energy, then storing it in sugars like glucose -- they exude waste products through their roots into a soil layer known as the rhizosphere. There, bacteria chow down on plants' sloughed-off cells, along with proteins and sugars released by their roots [source: Ingham].

In PMFC terms, this means that, as long as the plant lives, the bacteria have a meal ticket and the fuel cell generates power. The first law of thermodynamics, which some translate as "there's no such thing as a free lunch," still applies because the system receives energy from an external source, namely the sun.

But how on Earth, or under it, do microbes generate electricity simply by consuming and metabolizing food? As with love or baking, it all comes down to chemistry.

Broadly speaking, MFCs work by separating two halves of an electro-biochemical process (metabolism) and wiring them together into an electrical circuit. To understand how, let's look at cell metabolism in detail.

In the textbook example that follows, glucose and oxygen react to produce carbon dioxide and water [sources: Bennetto; Rabaey and Verstraete].

C₆H₁₂O₆ + 6O₂ → 6C_O2 + 6H₂O

But within individual cells -- or single-celled organisms like bacteria -- this broad statement glosses over a series of intermediate steps. Some of these steps temporarily release electrons which, as we all know, are handy for generating electricity. So, instead of glucose and oxygen reacting to produce carbon dioxide and water, here glucose and water produce carbon dioxide, protons (positively charged hydrogen ions (H⁺)) and electrons (e^-) [sources: Bennetto; Rabaey and Verstraete].

C₆H₁₂O₆ + 6H₂O → 6CO₂ + 24H⁺ + 24e^-

In a PMFC, this half of the process defines one half of the fuel cell. This portion is located in the rhizosphere with the plant roots, waste and bacteria. The other half of the cell lies in oxygen-rich water on the opposite side of a permeable membrane. In a natural setting, this membrane is formed by the soil-water boundary [sources: Bennetto; Rabaey and Verstraete; Deng, Chen and Zhao].

In the second half of the cell, free protons and electrons combine with oxygen to produce water, like so:

6O₂ + 24H⁺ + 24e^- → 12H₂O

Protons reach this second half by flowing across the ion exchange membrane, creating a net positive charge -- and an electrical potential that induces electrons to flow along the external connecting wire. Voila! Electric current [sources: Bennetto; Rabaey and Verstraete; Deng, Chen and Zhao].

But how much?

As of 2012, PMFCs don't produce much energy and work only in aquatic environments, with plants like reed mannagrass (Glyceria maxima), rice, common cordgrass (Spartina anglica) and giant reed (Arundo donax) [sources: Deng, Chen and Zhao; PlantPower]. If you ran across a field of PMFCs, like the rooftop patch at the Netherlands Institute of Ecology in Wageningen, you'd never know it was anything more than a collection of plants, except for the colorful wiring trailing out from the soil [source: Williams].

Still, their potential applications in addressing other global sustainability problems, including the strain placed by biofuels on an already overburdened global food supply system, continues to inspire researchers and at least one exploratory venture, the 5.23-million-euro project PlantPower [sources: Deng, Chen and Zhao; PlantPower; Tenenbaum].

Because PMFCs already work on aquatic plants, farmers and villages need not dump their water-based rice crops in order to implement them. On a larger scale, communities could set up PMFCs in wetlands or areas of poor soil quality, avoiding land competition between energy and food production [source: Strik et al.]. Manufactured settings like greenhouses could produce energy throughout the year, but farmland electricity production would depend on the growth season [source: PlantPower].

Producing more energy locally could lower carbon emissions by reducing the demand for fuel shipping -- itself a major greenhouse gas contributor. But there's a catch, and it's a pretty significant one: Even if PMFCs become as efficient as possible, they still face a bottleneck -- the photosynthetic efficiency and waste production of the plant itself.

Plants are surprisingly inefficient at transforming solar energy into biomass. This conversion limit springs partly from quantum factors affecting photosynthesis and partly from the fact that chloroplasts only absorb light in the 400-700 nanometer band, which accounts for about 45 percent of incoming solar radiation [source: Miyamoto].

The two most prevalent types of photosynthesizing plants on Earth are known as C3 and C4, so named because of the number of carbon atoms in the first molecules they form during CO₂ breakdown [sources: Seegren, Cowcer and Romeo; SERC]. The theoretical conversion limit for C3 plants, which make up 95 percent of plants on Earth, including trees, tops out at a mere 4.6 percent, while C4 plants like sugar cane and corn climb nearer to 6 percent. In practice, however, each of these plant types generally achieve only 70 percent of these values [sources: Deng, Chen and Zhao; Miyamoto; SERC].

With PMFCs, as with any machine, some energy is lost in running the works -- or, in this case, in growing the plant. Of the biomass built by photosynthesis, only 20 percent reaches the rhizosphere, and only 30 percent of that becomes available to microbes as food [source: Deng, Chen and Zhao].

PMFCs recover around 9 percent of the energy from the resulting microbial metabolism as electricity. Altogether, that amounts to a PMFC solar-to-electrical conversion rate approaching 0.017 percent for C3 plants ((70 percent of the 4.6 percent conversion rate) x 20 percent x 30 percent x 9 percent) and 0.022 percent for C4 plants (0.70 x 6.0 x 0.20 x 0.30 x 0.09) [sources: Deng, Chen and Zhao; Miyamoto; SERC].

In fact, some researchers think those assumptions may underestimate the potential of PMFCs, which can only be good news for consumers.

Like any new technology, PMFCs face a number of challenges; for instance, they need a substrate that simultaneously favors plant growth and energy transfer -- two goals that are sometimes at odds. Differences in pH between the two cell halves, for example, can bring about loss of electrical potential, as ions "short" across the membrane to achieve chemical balance [source: Helder et al.].

If engineers can work out the kinks, though, PMFCs could hold both vast and varied potential. It all comes down to how much energy they can produce. According to a 2008 estimate, that magic number comes in at around 21 gigajoules (5,800 kilowatt-hours) per hectare (2.5 acres) each year [source: Strik et al.]. More recent research has estimated that number could go as high 1,000 gigajoules per hectare [source: Strik et al.]. A few more facts for perspective [sources: BP; European Commission]:

Based on these numbers, if 1 percent of U.S. and European farmlands were converted to PMFCs, they would yield a back-of-the-envelope estimate of 34.5 million gigajoules (9.58 billion kilowatt-hours) annually for Europe and 75.6 million gigajoules (20.9 billion kilowatt-hours) annually for America.

By comparison, the 27 European Union countries in 2010 consumed 1,759 million tons of oil equivalent (TOE) in energy, or 74.2 billion gigajoules (20.5 trillion kilowatt-hours). TOE is a standardized unit of international comparison, equal to the energy contained in one ton of petroleum [sources: European Commission; Universcience].

In this simplified scenario, PMFCs provide a drop in a very large energy bucket, but it's a pollution-free drop, and a drop generated from lush landscapes instead of smoke-belching power plants or bird-smashing wind farms.

Moreover, it's just the beginning. Researchers are already working on more efficient waste-gobbling bacteria and, between 2008 and 2012, advances in substrate chemistry more than doubled electrical production in some PMFCs. PlantPower argues that, once perfected, PMFCs could provide as much as 20 percent of Europe's primary energy -- that is, energy derived from untransformed natural resources [source: Øvergaard; PlantPower].

PMFCs must become cheaper and more efficient before they can enjoy wide implementation, but progress is under way. Already, many MFCs save money by manufacturing electrodes from highly conductive carbon cloth rather than precious metals or expensive graphite felt [sources: Deng, Chen and Zhao; Tweed]. As of 2012, it cost $70 to operate a one-cubic-meter setup under laboratory conditions.

When one considers their potential for removing pollutants and for reducing greenhouse gases, who knows? PMFCs could garner enough investor and government interest to become the power plants of the future -- or plant the seed for an even better idea [source: Deng, Chen and Zhao].