Earth-abundant photoelectrochemical systems for sustainable solar hydrogen production: materials, architectures, and carbon nanomaterial strategies

Solar-driven production of green hydrogen is currently dominated by photovoltaic electrolysis systems. An emerging alternative is the use of photoelectrochemical (PEC) cells, which integrate at least one photoelectrode that directly transforms solar energy into chemical energy at the electrode-electrolyte interface. Although PEC devices based on classical photovoltaic materials and tandem architectures have achieved high solar-to-hydrogen (STH) efficiencies, they typically depend on expensive and critical elements, limiting large-scale implementation and sustainability. This review analyzes recent advances in photoelectrodes, co-catalysts, and tandem PEC configurations containing exclusively Earth-abundant elements, which have achieved STH efficiencies in the range of approximately 1-3.5%. Particular emphasis is placed on the role of carbon nanomaterials. Carbon nanotubes, graphene, and carbon dots combine unique optoelectronic properties, chemical stability, and structural versatility, enabling improved charge transport, catalytic activity, and durability in PEC systems. Their integration as non-critical components provides a viable route toward cost-effective, recyclable, and environmentally sustainable PEC hydrogen production technologies.