Hydrogen fuel refers to hydrogen which is burned as fuel with oxygen. It is zero-carbon, provided that it is created in a process that does not involve carbon. It can be used in fuel cells or internal combustion engines (see HICEV). Regarding hydrogen vehicles, hydrogen has begun to be used in commercial fuel cell vehicles such as passenger cars, and has been used in fuel cell buses for many years. It is also used as a fuel for spacecraft propulsion and is being proposed for hydrogen-powered aircraft. Production Because pure hydrogen does not occur naturally on Earth in large quantities, it usually requires a primary energy input to be produced on an industrial scale. Hydrogen fuel can be produced from methane or by electrolysis of water. As of 2020, the majority of hydrogen (∼95%) is produced from fossil fuels by steam reforming or partial oxidation of methane and coal gasification with only a small quantity by other routes such as biomass gasification or electrolysis of water. Steam–methane reforming, the current leading technology for producing hydrogen in large quantities, extracts hydrogen from methane. However, this reaction releases fossil carbon dioxide and carbon monoxide into the atmosphere, which are greenhouse gases exogenous to the natural carbon cycle, and thus contribute to climate change. In electrolysis, electricity is run through water to separate the hydrogen and oxygen. This method can use wind, solar, geothermal, hydro, fossil fuels, biomass, nuclear, and many other energy sources. Obtaining hydrogen from this process is being studied as a viable way to produce domestically at a low cost. Researchers are also developing artificial leaves which integrate light absorbers with catalysts and can produce hydrogen directly from water. While this technology is still at an early stage, floating farms of lightweight devices may potentially supply remote communities. The world's largest facility for producing hydrogen fuel is claimed to be the Fukushima Hydrogen Energy Research Field (FH2R), a 10 MW-class hydrogen production unit, inaugurated in March 2020, in Namie, Fukushima Prefecture. The site occupies 180,000 m2 of land, much of which is occupied by a solar array; power from the grid is also used for electrolysis of water to produce hydrogen fuel. Production is usually classed in terms of colour labels; 'grey hydrogen' is produced as a by-product of an industrial process, 'blue hydrogen' is produced through a production process where is also produced then subsequently captured via CCS, and