Alkali Cation Engineered Chemical Self-Oxidation of Copper Oxide Nanowire-Based Photocathodes

Hydrogen energy production through photoelectrochemical (PEC) water splitting has great potential in the field of renewable energy. This study focuses on the hydration enthalpy difference of cations (Li⁺, Na⁺, and K⁺) in an aqueous solution for the chemical self-oxidation process without an external applied bias. The thickness of the cation/H₂O double layer is controlled. The starting materials are low-cost copper foil and the synthesis uses alkali cation-engineered chemical self-oxidation. Li⁺ ions are strongly attracted to water molecules. This forms a sufficient OH⁻ layer on the Cu foil surface. By accelerating the oxidation reaction, a large surface area of Cu(OH)_x nanowires (NWs) with high purity and a uniform shape are obtained. This optimal p-type Cu₂O NWs photocathode is CuO-free, has the highest conductivity, and is fabricated through phase transition using precise vacuum annealing. The other alkali cations produce the Cu₂O/CuO mixed or CuO phases that degrade the PEC performances with severe corrosive reactions. The Cu/Li : Cu₂O/AZO/TiO₂/Pt photocathode has a 50 h stability with a photocurrent density of 8.4 mA cm⁻² at 0 V_RHE. The fabricated photoelectrode did not structurally collapse after stability measurements during this period. The captured hydrogen production was in agreement with the calculated faradaic efficiency.