Nb:TiO2 Protection Layers for FTO-based Cathodes for CO2 Reduction Under Neutral Conditions

With the rising anthropogenic CO2 concentrations in the atmosphere and the growing interest in converting solar energy into value-added products, interest in (photo)electrochemical reduction of CO2 is increasing. A key component for this process is an optically transparent and electrically conductive substrate, usually in the form of a transparent conducting oxide (TCO) film deposited on glass. However, most traditional TCO materials, such as fluorine-doped SnO2 (FTO) or tin-doped In2O3 (ITO), are not stable under cathodic conditions. TiO2 has been widely reported as an excellent protection layer material in photo-electrochemical (PEC) research, and niobium is one prospective dopant to improve its conductivity. In this work, we demonstrate that commercial FTO glass can be protected against (photo)corrosion under cathodic reaction conditions in pH-neutral electrolytes by direct-current sputtering of thin Nb-doped TiO2 (Nb:TiO2) overlayers. Combined with a pre-deposition FTO etching procedure, the Nb:TiO2-covered FTO electrode offers excellent electrochemical stability while maintaining good transparency and conductivity.