How Hydrogen Gas Powers Fuel Cells

Hydrogen fuel cells are an increasingly popular alternative to traditional combustion engines, particularly in the transport sector. Hydrogen is a clean fuel with many applications, and it is now being used in cars, buses, and trucks. However, hydrogen is difficult to store due to its gaseous state at room temperature and low ambient temperature density. This has led to the development of advanced storage methods, such as high-pressure tanks and liquid storage, which aim to increase the energy density of hydrogen. One of the more promising techniques is the storage of hydrogen in metal hydrides, which can absorb and expel hydrogen gas at a predictable rate under known conditions. The Hydrogen and Fuel Cell Technologies Office (HFTO) in the United States is also researching onboard automotive hydrogen storage systems that can provide a driving range of over 300 miles while meeting cost, safety, and performance requirements.

Hydrogen storage methods

Hydrogen can be stored physically as either a gas or a liquid. Gaseous hydrogen is typically stored in high-pressure tanks (350-700 bar tank pressure) to increase its storage density. However, this method of storage poses a challenge for the automotive industry, as high-pressure tanks weigh much more than the hydrogen they can hold. For instance, in the 2014 Toyota Mirai, a full tank contains only 5.7% hydrogen, the rest of the weight being the tank.

Liquid hydrogen requires cryogenic temperatures, as the boiling point of hydrogen at one atmosphere pressure is −252.8°C. This method of storage is technically complex and costly, requiring a technologically advanced freezer system.

Other hydrogen storage methods include:

• Geological hydrogen storage: Hydrogen is stored in salt caverns constructed deep underground.
• Materials-based storage: Hydrogen is stored on the surfaces of solids (by adsorption) or within solids (by absorption). This includes creating metal hydrides from elements such as palladium, magnesium, aluminum, and certain alloys.
• Compressed hydrogen storage: Hydrogen is compressed and stored in tanks, similar to any other gas.

Hydrogen purity

International standards have been published to specify the impurities and levels that must be identified for hydrogen to be a viable fuel in fuel cell electric vehicles (FCEVs). These standards include maximum concentration values for total sulphur compounds, total halogenated compounds, and particulates.

Hydrogen can be stored physically as either a gas or liquid, but typically requires high-pressure tanks (350-700 bar tank pressure) to store sufficient quantities. Another possibility is the chemical storage of hydrogen, where it is stored on the surface of solids (by adsorption) or within solids (by absorption). The automotive application utilizes physical storage.

Cold/cryo-compressed storage is a method where increased hydrogen density and insulated pressure vessels are used to store hydrogen. This method is being pursued by the Hydrogen and Fuel Cell Technologies Office (HFTO) as a long-term solution for hydrogen storage.

Hydrogen flammability

Hydrogen is a highly flammable gas. It has a very wide flammability range in air, between 4% and 75% compared to other fuels. Hydrogen also has the lowest ignition energy, which means that small sparks can easily ignite it.

Hydrogen is prone to leakage due to its low viscosity. In a confined space, leaking hydrogen can accumulate and reach flammable concentrations. In a closed environment, leaks of any size are a concern, as hydrogen is impossible for human senses to detect. However, hydrogen is also highly dissipative due to its low density and light weight. Flames will propagate through a flammable hydrogen-air cloud at several meters per second and, if in an open space, will burn out and move away from the structure. Hydrogen also burns with a nearly invisible flame, so special flame detectors are required.

Hydrogen fuel tanks are designed to withstand high pressures and store hydrogen without any leakage. Hydrogen tanks are equipped with pressure relief devices to prevent the pressure in the tanks from becoming too high. Hydrogen can be stored physically as a gas or a liquid. Gas storage typically requires high-pressure tanks (350-700 bar), while liquid storage requires cryogenic temperatures, as the boiling point of hydrogen at one atmosphere of pressure is −252.8°C.

Hydrogen is safer than conventional hydrocarbon fuels in some ways, but not in others. It is non-toxic, dissipates rapidly, and burns without leaving a residue. However, it is highly explosive and dangerous in closed environments. It also requires special engines and infrastructure due to being a pressurised gaseous fuel.

Hydrogen tanks

Hydrogen Tank Technology

Hydrogen can be stored physically as a gas or a liquid. Gas storage typically requires high-pressure tanks (350-700 bar), while liquid storage requires cryogenic temperatures as hydrogen boils at -252.8°C at standard pressure. Hydrogen can also be stored through chemical storage, where it is stored on or within solids through absorption or adsorption.

Hydrogen Tank Design and Materials

Hydrogen Tank Safety

Safety is a critical concern for hydrogen tanks due to hydrogen's high flammability and explosiveness. Hydrogen tanks are equipped with pressure relief devices to prevent excessive pressure build-up. Detection sensors are also installed to quickly identify leaks and minimise the risk of undetected flames. Hydrogen tanks have undergone rigorous safety tests and are considered safe when guidelines are followed.

Hydrogen Tank Innovations

Companies like Doosan Mobility Innovation (DMI) have developed ultralight Type 4 hydrogen tanks using carbon fibre composite materials. These tanks offer high storage efficiency and pressure, enabling more energy storage in the same volume. DMI's tanks have passed various safety tests, including drop tests from 150m and fire tests, demonstrating their safety for use in fuel cell technology.

Hydrogen production

Hydrogen can be produced from a variety of sources, including natural gas, nuclear power, biomass, and renewable power like solar and wind. The two most common methods for producing hydrogen are steam-methane reforming and electrolysis (splitting water with electricity).

Steam-Methane Reforming

This process involves separating hydrogen atoms from carbon atoms in methane (CH4) using high-temperature steam (1,300°F to 1,800°F) and pressure (3–25 bar). It accounts for nearly all commercially produced hydrogen in the US.

Electrolysis

Electrolysis uses electricity to split water into hydrogen and oxygen. It is commonly used in science classes to demonstrate chemical reactions. On a large, commercial scale, this process may be referred to as power-to-gas, where power is electricity and hydrogen is gas. Electrolysis produces no byproducts or emissions other than hydrogen and oxygen.

Other Methods

While the above methods are the most common, researchers are also exploring other pathways for hydrogen production, including:

• Thermochemical processes to convert biomass into gas or liquids and separate hydrogen.
• Photolytic (solar-driven) processes that use solar energy to split water into hydrogen and oxygen.
• Photoelectrochemical processes use specialized semiconductors to achieve this water-splitting.
• Solar thermochemical processes that use concentrated solar power to drive water-splitting reactions, often with other species like metal oxides.
• Biological processes that use microbes such as bacteria and microalgae to produce hydrogen through biological reactions.

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