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

Abstract

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/Cu 2 ZnSnS 4 (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. *NO 3 and *NO 2 ) 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. • Co-TiO x and Ag NWs have been applied on CZTS photocathode for ammonia synthesis from nitrate. • Nearly 100 % ammonia faradaic efficiency and optimised onset potential have been obtained. • Co-TiO x exhibits increased oxygen vacancy sites for key intermediate adsorption. • Ag NWs enhance the electron extraction from the CZTS photocathode to the catalytic sites. • The reaction mechanism was unveiled via in-situ synchrotron-radiated FTIR spectroscopy.