Solar driven ammonia synthesis with Co-TiO_x and Ag nanowires enhanced Cu₂ZnSnS₄ photocathodes

Restoring ammonia from waste nitrate stands as a promising strategy for reducing reliance on the energy-intensive Haber-Bosch process and tackling environmental pollutants. Advancing the catalytic aspects of photoelectrochemical (PEC) ammonia synthesis via waste nitrate reduction is of great importance to enhance its viability for sustainable chemical production. However, this process still suffers from low ammonia faradaic efficiency (FE) with high operational potential due to its involvement in multi-electron reactions. Herein, we integrated a cobalt-doped TiO_x (Co-TiO_x) cocatalyst and Ag nanowires (NWs) electron extraction layer onto TiO_x/CdS/Cu₂ZnSnS₄ (CZTS) photocathode, achieving nearly 100 % ammonia FE and an onset potential of ∼0.49 V vs. RHE. Evidenced by the in-situ synchrotron-radiated FTIR (SR-FTIR) and theoretical calculations, the increased ratio of surface oxygen vacancy sites (Vo) induced by Co-TiO_x is crucial for the key reaction intermediates adsorption (i.e. *NO₃ and *NO₂) for subsequent ammonia production. Moreover, the transparent Ag NWs facilitates the electron extraction from TiO_x/CdS/CZTS to the surface catalytic sites. Powered by CZTS solar cells, a standalone solar-to-ammonia system has been demonstrated with outstanding activity and catalytic performance.