Hydrogen is a promising energy carrier, but also a valuable chemical used in a variety of industrial processes. Hydrogen is typically produced from natural gas by steam reforming, a process which requires high temperatures and generates CO2. More sustainable routes for H2 production exist. Water electrolysis, for example, is carried out at room temperature, H2 and O2 are the sole products of the reaction, and photovoltaics or wind turbines can supply the current. Unfortunately, several challenges remain.