Engineering a Kesterite-Based Photocathode for Photoelectrochemical Ammonia Synthesis from NO_x 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 NO_x reduction reaction (NO_xRR) 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 NO_xRR by a kesterite (Cu₂ZnSnS₄ [CZTS]) photocathode through loading defect-engineered TiO_x cocatalyst on a CdS/CZTS photocathode (TiO_x/CdS/CZTS) is demonstrated. The uniquely designed photocathode enables selective ammonia production from NO_xRR, 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 TiO_x 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.