nature of the engine means it will not be able to operate in city zero emission zones, unless part of a hybrid powertrain. Drawbacks Hydrogen has a high energy content per unit mass. However, at room temperature and atmospheric pressure, it has a very low energy content per unit volume compared to liquid fuels or even to natural gas. For this reason, it is usually either compressed or liquefied by lowering its temperature to under 33 K. High-pressure tanks weigh much more than the hydrogen they can hold. For example in the 2014 Toyota Mirai, a full tank contains only 5.7% by weight of hydrogen, the rest of its mass being that of the tank. Hydrogen fuel is hazardous because of its low ignition energy and high combustion energy, and because it tends to leak easily from tanks. Explosions at hydrogen filling stations have been reported. Hydrogen fueling stations, like petrol, generally receive deliveries of hydrogen by truck from hydrogen suppliers. Similar to all commodity distribution systems, an interruption at a supply facility can shut down multiple fueling stations. See also Fuel cell vehicle HCNG Hydrogen compressor Hydrogen safety Hydrogen storage Hydrogen technologies Hydrogen transport Hydrogen vehicle Oxyhydrogen flame Photoelectrolysis of water Photocatalytic water splitting Synthetic fuel References Notes Bibliography Hydrogen as the fuel of the future, report by the DLR Fuels Hydrogen technologies Hydrogen economy Industrial gases