Efficiently unbiased solar-to-ammonia conversion by photoelectrochemical Cu/C/Si-TiO2 tandems

Photoelectrochemical nitrate reduction reaction (PEC NO3RR) is of interest as a promising route to directly realizing the solar-to-ammonia (NH3) conversion but the limited efficiency and high applied bias voltage hamper its commercial prospects. Here, we report a bias-free photoelectrochemical cell for PEC NO3RR in aqueous conditions, achieving a substantial NH3 yield rate of 13.1 μmol·h−1·cm−2, high faradaic efficiency of 93.8%, and recorded solar-to-NH3 conversion of ∼1.5% under 1 sun illumination. A hierarchical-structured Si-based photocathode with Cu+/Cu2+-containing Cu nanoparticles cocatalysts achieves a highly efficient PEC NO3RR with NH3 yield rate of 115.3 μmol·h−1·cm−2 in a three-electrode system. Integrating operando characterizations and systematic PEC measurements, the formation of Lewis acid sites on Cu nanoparticles by accepting the photoinduced electrons is the dominant factor for facilitating the absorption and hydrogenation of nitrate. This work will guide the development of a robust, high-performance, and unbiased PEC device for sustainable solar-to-NH3/other fuels conversion.