Unraveling Defect-Dependent Conductivity-Type Switching in CuFe₂O₄ for Enhanced Photoelectrocatalytic Reduction of...

Photoelectrocatalytic (PEC) reduction provides a sustainable route for upgrading biomass-derived feedstocks with reduced energy requirements, yet remains largely unexplored beyond hydrogen evolution and CO₂ reduction due to the scarcity of stable photocathodes. Here, we report a defect-engineered CuFe₂O₄ photocathode that enables directly quantified PEC reduction of benzaldehyde to benzyl alcohol using a single-component, earth-abundant oxide. By controlling annealing temperature and oxygen partial pressure, CuFe₂O₄ is systematically tuned from n-type to p-type conductivity. Electrochemical measurements, X-ray and ultraviolet photoelectron spectroscopy, and first-principles defect calculations collectively show that oxygen-rich annealing conditions suppress deep donor-type oxygen vacancies while stabilizing shallow acceptor-type copper vacancies, resulting in enhanced hole concentration and improved charge transport. In a mixed acetonitrile/water electrolyte employing 1,4-benzoquinone as a redox mediator, the optimized CuFe₂O₄ photocathode achieves stable photoelectrochemical operation over 18 h under continuous illumination with a benzyl alcohol production rate of 2.57 μmol/h at -0.50 V vs Ag/AgNO₃, corresponding to a Faradaic efficiency of 51.3%. This PEC approach lowers the required applied potential by ∼1 V compared to traditional electrocatalytic methods, offering a more energy-efficient route for carbonyl reduction. These findings establish CuFe₂O₄ as a viable photocathode platform for sustainable photoelectrocatalytic organic transformations under mild reaction conditions.