Molecular cathode and photocathode materials for hydrogen evolution in photoelectrochemical devices

• Molecular-engineered electrodes are alternatives to the use of Pt for H2 evolution.
• Inorganic and organic semi-conductors allow developing H2-forming photocathodes.
• Direct solar-to-H2 conversion is demonstrated in photoelectrochemical (PEC) cells.
• Fully molecular dye-sensitized PEC cells are a promising solar fuels technology.

Storage of solar energy in the form of readily available easy-to-handle fuels is the main bottleneck toward the development of a carbon-neutral alternative energy. Taking inspiration from natural systems, artificial photosynthesis is a technology to be for efficiently converting the tremendous solar energy received every day on Earth into chemical energy, i.e. fuels. In particular, hydrogen production through light-driven water splitting is the subject of numerous investigations. We focus here on the construction of electrodes and photoelectrodes achieving H2 evolution, as components of photoelectrochemical (PEC) cells. In such devices, H2 evolution at the cathode or photocathode is combined with water oxidation to oxygen at the photoanode or anode. We review here the various molecular-based materials developed in this context with emphasis on those specifically exploiting the properties of Earth-abundant elements.