Towards Highly Efficient Chalcopyrite Photocathodes for Water Splitting: The Use of Cocatalysts beyond Pt

Solar radiation is a renewable and clean energy source used in photoelectrochemical cells (PEC) to produce hydrogen gas as a powerful alternative to carbon‐based fuels. Semiconductors play a vital role in this approach, absorbing the incident solar photons and converting them into electrons and holes. The hydrogen evolution reaction (HER) occurs in the interface of the p‐type semiconductor that works as a photocathode in the PEC. Cu‐chalcopyrites such as Cu(In, Ga)(Se,S)2 (CIGS) and CuIn(Se,S)2 (CIS) present excellent semiconductor characteristics for this purpose, but drawbacks as charge recombination, deficient chemical stability, and slow charge transfer kinetics, demanding improvements like the use of n‐type buffer layer, a protective layer, and a cocatalyst material. Concerning the last one, platinum (Pt) is the most efficient and stable material, but the high price due to its scarcity imposes the search for inexpensive and abundant alternative cocatalyst. The present Minireview highlighted the use of metal alloys, transition metal chalcogenides, and inorganic carbon‐based nanostructures as efficient alternative cocatalysts for HER in PEC. Replacing Pt: Chalcopyrite‐based photocathodes containing Pt as cocatalysts are highly efficient to convert solar energy to hydrogen from water splitting. In light of the high cost and low availability of Pt in the Earth's crust, this Minireview highlights the recent advances to replace Pt as cocatalyst and summarizes the main classes of compounds (transition metal chalcogenides, alloys, and carbon nanostructures) to also obtain highly efficient and stable photocathodes for hydrogen generation.