Delafossite CuFeO ₂ photocathodes for photoelectrochemical water splitting: fundamental properties, synthesis, and modification strategies

Delafossite CuFeO₂ has an Earth-abundant composition with a 1.3-1.7 eV band gap that straddles the hydrogen-evolution potential, positioning it as a leading p-type photocathode. Yet photocurrents rarely exceed 15% of the theoretical 15 mA cm^-2 ceiling because small-polaron transport and Fermi-level pinning limit carrier collection. This review maps how modern synthesis-from equilibrium powders to non-equilibrium thin-film growth-controls phase purity, point defects and interfacial energetics and surveys bulk-doping, nanostructuring, heterojunction and cocatalyst strategies aimed at relieving those intrinsic bottlenecks. We conclude by outlining quantitative design rules and pressing research priorities for pushing CuFeO₂ beyond the 10 mA cm^-2 milestone and into scalable photoelectrochemical architectures. This approach not only guides the development of CuFeO₂ photocathodes, but also offers a new design paradigm for other similar materials in photoelectrochemical applications.