E-Mobility Engineering 016 l Aurora Powertrains eSled dossier l In Conversation: Thomas de Lange l Automated manufacturing focus l Torque sensing insight l Battery Show Europe 2022 report l Sodium batteries insight l User interfaces focus

O ne of the barriers to wider adoption of fuel cells in vehicles is storing their hydrogen fuel (writes Peter Donaldson). Its main attraction is that it contains nearly three times as much energy in mass terms as petrol – 120 MJ/kg compared with 44 MJ/kg – but even when liquefied it has only a quarter of the energy density in volume terms (8 MJ/litre versus 32 MJ/litre), so better ways of storing it are a major subject of r&d. The most mature technologies involve storing it as a compressed gas, a cooled compressed gas or as a cryogenic liquid. Hydrogen gas tanks must be able to withstand pressures as high as 900 atmospheres, while cryogenic tanks must be heavily insulated and come with the additional complexities of minimising and managing boil-off, as it boils at -252.9 °C at standard atmospheric pressure. Compression and cooling, particularly to cryogenic temperatures, also use a lot of energy. Fortunately though, there are alternatives that exploit hydrogen’s propensity to form chemical bonds, one of the most important being the use of metal hydrides. Typical metal hydrides are powders with particles only a few microns across that absorb hydrogen when the container they are in has hydrogen pumped into it at pressures of around 5 atmospheres. The hydrogen molecules stick to the metal’s surface, break down into monatomic hydrogen and penetrate into the metal crystal’s interior to form the solid hydride in a reaction that releases heat. Applying heat to the hydride powder reverses the reaction and releases the hydrogen. Storing hydrogen as a solid metal hydride uses half as much energy as compressing the same amount of the gas, and about one-sixth of that required to liquefy it, while the volumetric energy density is far higher. However, the metal powders and their storage tanks are relatively heavy, so the gravimetric energy density has been relatively low historically. The technology has therefore been limited to vehicles in which weight is not critical, such as air-independent propulsion systems for non-nuclear submarines, for example. The technology is advancing, however. In Germany for example, the H2HybridTank project seeks to apply ‘light metal’ hydrides to cars. These are described as nanostructured compounds that store hydrogen efficiently at low pressures, which in turn means that recyclable metal tanks instead of stronger composite ones can be used. But metal hydrides face competition from other means of storing hydrogen in a chemical compound, particularly ammonia. Consisting of one nitrogen atom and three hydrogen atoms, ammonia is a gas at standard atmospheric pressure, liquefies at moderate pressures and at 10 atmospheres has a boiling point of 25 °C. Also, liquid ammonia contains twice as much hydrogen by volume as liquid hydrogen, and is also plentiful, inexpensive and widely used in industry. The need to ‘crack’ ammonia – separating the nitrogen from the hydrogen and feeding the latter to the fuel cell – has been a limitation because of the high temperatures required: more than 500 °C. However, direct ammonia fuel cell technology is maturing and solid-oxide fuel cells crack the gas internally. As reported in The Grid section of this issue, Amogy began demonstrating a tractor converted to run on ammonia fed to a 100 kW hybrid fuel cell system that includes cracking modules, and the company is reportedly also working on systems for trucks and ships. Hydrides and ammonia both show promise, and the engineering challenges seem surmountable, but at the moment they are produced by energy-intensive industrial methods – steam reforming of methane, in the case of hydrogen, and the Haber-Bosch process for ammonia – so cleaner sources of both will be essential. PS | Metal hydrides for hydrogen storage Storing hydrogen as a metal hydride uses half the energy of compressing the same amount of gas 74 Winter 2022 | E-Mobility Engineering