Hydrogen storage challenges: Materials-based solid hydrogen storage by metal hydrides

This post is about hydrogen storage as solid using metal hydrides as sorbents.

The overarching technical challenge for hydrogen storage in transportation is how to store the amount of hydrogen required for a conventional driving range (>300 miles) while adhering to vehicle constraints such as weight, volume, efficiency, safety, and cost. The durability of these systems over their performance lifetimes must also be verified and validated, as must be acceptable refueling times. Off-board bulk storage requirements are generally less stringent than on-board bulk storage requirements.

The issue lies with the fact that Hydrogen has the highest energy density per mass of any fuel; however, because of its low ambient temperature density, it has a low energy density per unit volume, necessitating the development of advanced storage methods with the potential for higher energy density.

How does Hydrogen Storage work? Technologies

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Hydrogen can be physically stored as a gas or a liquid. High-pressure tanks (350-700 bar [5,000-10,000 psi] tank pressure) are typically required for hydrogen storage as a gas. Because the boiling point of hydrogen at one-atmosphere pressure is 252.8°C, storing it as a liquid necessitates cryogenic temperatures. Hydrogen can also be stored on solid surfaces (via adsorption) or within solids (by absorption).

Details

High-density hydrogen storage remains a significant challenge for stationary and portable applications, as well as transportation applications. Current storage options typically necessitate large-volume systems that store hydrogen in gaseous form. This is less of a problem in stationary applications, where the footprint of compressed gas tanks is less important.

However, fuel-cell vehicles require enough hydrogen to provide a driving range of more than 300 miles, as well as the ability to refuel the vehicle quickly and easily.

Hydrogen has nearly three times the energy content of gasoline in terms of mass-120 MJ/kg for hydrogen versus 44 MJ/kg for gasoline. However, on a volume basis, the situation is reversed; liquid hydrogen has an energy density of 8 MJ/L, whereas gasoline has an energy density of 32 MJ/L. To meet the driving range requirements for the entire range of light-duty vehicle platforms, onboard hydrogen storage capacities of 5-13 kg hydrogen will be required.

To address these challenges, HFTO [Hydrogen and fuel cell technologies office] is pursuing two strategic paths, focusing on both short-term and long-term solutions. The short-term path focuses on compressed gas storage, with advanced pressure vessels made of fibre reinforced composites capable of reaching 700 bar pressure and a strong emphasis on system cost reduction. The long-term path focuses on both (1) cold or cryo-compressed hydrogen storage, where increased hydrogen density and insulated pressure vessels may allow U.S. Department of Energy [DOE] targets to be met, and (2) materials-based hydrogen storage technologies, such as sorbents, chemical hydrogen storage materials, and metal hydrides, which have properties that may allow DOE hydrogen storage targets to be met.

Briefly, there are many H2 storage issues

 Volume and weight:

 Currently, the weight and volume of hydrogen storage systems are too large, resulting in insufficient vehicle range when compared to conventional petroleum-fueled vehicles.

Efficiency:

All hydrogen storage methods face energy efficiency challenges. The energy required to move hydrogen in and out of reversible solid-state materials is an issue. For chemical hydride storage, where the byproduct is regenerated off-board, life-cycle energy efficiency is a challenge.

Durability:

The durability of hydrogen storage systems is insufficient. Materials and components for hydrogen storage systems with a lifetime of 1500 cycles are required.

It's time to refuel. Refueling times are excessively long. It is necessary to develop hydrogen storage systems with refueling times of less than three minutes over the system's lifetime.

Cost:

On-board hydrogen storage systems are prohibitively expensive, especially when compared to conventional petroleum fuel storage systems.

Metal hydride sorbents

SOLID-H hydrogen storage containers are filled with metal powders that absorb and release hydrogen (metal hydrides).

At room temperature, the most popular SOLID-H containers provide a few atmospheres of hydrogen gas pressure. This is the most secure method of storing flammable hydrogen gas. SOLID-H immediately releases a small fraction of its stored hydrogen if your hydrogen system develops a leak. The remainder will be released over the course of several hours. All SOLID-H containers are equipped with a pressure relief valve for safety and a quick connect coupling for ease of connection to your system. SOLID-H is commonly used to supply hydrogen to gas chromatographs and to store the fuel for hydrogen engines or fuel cells.

Metal hydrides are the most compact form of hydrogen storage (they are denser than liquid).

SOLID-H containers typically hold 18, 20, 30, 120, 220, or 740 standard litres of hydrogen gas. These are on the shelf or available on short delivery schedules.

How to use solid -H metal hydride?

A fitting that you specify when ordering connects a SOLID-H container to your system. Typically, this is a quick connect for ease of installation and removal.

When ordering, you can specify the hydrogen pressure to be used when charging or discharging a SOLID-H container.

Connect your SOLID-H container to a source of clean dry hydrogen gas at the pressure specified in your SOLID-H manual to recharge it. The container will become hot to the touch. When it becomes cool again, the recharging process is complete.

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How Hydrogen Storage Works: Technologies