Engineering a Kesterite-Based Photocathode for Photoelectrochemical Ammonia Synthesis from NOx Reduction

Abstract

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 (NOxRR) 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, selective ammonia synthesis from PEC NOxRR by a kesterite (Cu2ZnSnS4 [CZTS]) photocathode through loading defect-engineered TiOx cocatalyst on a CdS/CZTS photocathode (TiOx/CdS/CZTS) is demonstrated. The uniquely designed photocathode enables selective ammonia production from NOxRR, yielding up to 89.1% Faradaic efficiency (FE) (0.1 V vs reversible hydrogen electrode (RHE)) with a remarkable positive onset potential (0.38 V vs 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. Further attempts on PEC ammonia synthesis from simulated wastewater show good FE of 64.9%, unveiling the potential of using the kesterite-based photocathode for sustainably restoring ammonia from nitrate-rich wastewater.