Engineering kesterite based photocathode for photoelectrochemical ammonia synthesis from NOx reduction

Ammonia is a key chemical feedstock for industry as well as future carbon-free fuel and transportable vector for renewable energy. Photoelectrochemical (PEC) ammonia synthesis from NOx reduction reaction (NOx RR) provides not only a promising alternative to the energy-intensive Haber-Bosch process through direct solar-to-ammonia conversion, but a sustainable solution for balancing the global nitrogen cycle by restoring ammonia from wastewater. In this work, we, for the first time, demonstrated selective ammonia synthesis from PEC NOx RR by kesterite (Cu2 ZnSnS4 (CZTS)) photocathode through loading defect-engineered TiOx cocatalyst on CdS/CZTS photocathode (TiOx /CdS/CZTS). The uniquely designed photocathode enables selective ammonia production from NOx RR, yielding up to 89.1% faradaic efficiency (0.1 V versus reversible hydrogen electrode (RHE)) with a remarkable positive onset potential (0.38 V versus RHE). By tailoring the amount of surface defective Ti3+ species, the adsorption of reactant NO3- and * NO2 intermediate is significantly promoted while the full coverage of TiOx also suppresses NO2- liberation as a by-product, contributing to high ammonia selectivity. Our further attempted PEC ammonia synthesis from simulated wastewater shows good faradaic efficiency of 64.9%, unveiling the potential of using kesterite based photocathode for sustainably restoring ammonia from nitrate-rich wastewater.