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/H2 O 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 Cu2 O 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 Cu2 O/CuO mixed or CuO phases that degrade the PEC performances with severe corrosive reactions. The Cu/Li : Cu2 O/AZO/TiO2 /Pt photocathode has a 50 h stability with a photocurrent density of 8.4 mA cm-2 at 0 VRHE . 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.