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.