Hydrogen fuel for fuel cells

Fuel cells typically operate on hydrogen, though there are various ways to supply this fuel. Hydrogen can be used directly or derived through chemical reforming from other substances. Reformed fuels, such as liquid hydrocarbons, offer high volumetric energy densities and easier transport, though they require additional processing equipment.

Fuel Cell Fuel Comparison

Fuel cell fuels vary significantly in energy density and efficiency. Hydrogen in its gaseous and liquid forms, metal hydrides, chemical hydrides, and alcohols like methanol are among the options. A comparison of current and theoretical energy densities for selected fuels reveals trade-offs in gravimetric and volumetric efficiency.

Compressed Hydrogen Storage

Hydrogen gas storage is straightforward but requires high-pressure containment (typically 3600 psi) due to its low volumetric density. Containers are made from steel or advanced carbon-fiber composites for weight savings and safety. Compression involves an energy penalty of 5–10%, and cylinders must accommodate thermal changes during filling and use. Safety mechanisms like Pressure Release Devices (PRDs) mitigate fire risks by venting gas in controlled scenarios.

Liquid Hydrogen Storage

Liquid hydrogen is stored at cryogenic temperatures (20 K), requiring highly insulated tanks. This method achieves higher volumetric densities but consumes 33–40% of the hydrogen’s energy content in cooling and liquefaction. Boil-off from gradual warming and expansion is a key concern in long-term storage.

Hydrocarbon Reforming

Reforming hydrocarbons like methane or methanol provides hydrogen-rich gas via two main methods: partial oxidation and steam reforming. Partial oxidation is simpler, producing a mix of CO and H2, while steam reforming is more efficient and yields more hydrogen. The water-gas shift reaction further enhances hydrogen output. Reformers must handle fuel streams with varied hydrogen concentrations, affecting cell efficiency.

Ammonia Reforming

Ammonia reforming offers a clean combustion pathway and easy liquefaction. With a hydrogen content of 17.6% by weight, ammonia reformers like the A-Cracker convert it into H2 and N2. Operating at over 400 °C, these systems require external heat and additional scrubbing to remove residual ammonia from product gases.

Chemical Hydrides

Chemical hydrides such as lithium hydride (LiH), lithium aluminum hydride (LiAlH4), and sodium borohydride (NaBH4) generate hydrogen through exothermic reactions with water. These materials are lightweight and can release more hydrogen due to the water component in the reaction. However, they generate significant heat and require careful storage under inert atmospheres. Cost remains a challenge due to purity requirements and the need for reprocessing waste materials.

Chemical hydrides are stored in small, sealed units and offer a viable alternative for portable hydrogen generation. For example, to produce 250 grams of hydrogen using LiH, approximately 0.98 kg of hydride and 2.2 kg of water are needed, along with about 200 grams of control hardware. Safety, thermal management, and recycling infrastructure are essential to enable practical use.

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