Tailoring NiO defectivity to boost the electrocatalytic activity toward nitrate reduction into ammonia

Ammonia is vital for global agriculture, yet its conventional synthesis via the Haber–Bosch process is energy-intensive and environmentally burdensome, contributing ∼2% of global CO2 emissions. Simultaneously, excessive use of ammonia-based fertilizers has led to nitrate pollution in water systems. Electrochemical nitrate reduction (E-NO3RR) offers a dual solution: mitigating nitrate contamination while enabling decentralized, sustainable ammonia production. Here, we explore nickel oxide (NiO) nanoparticles as efficient, low-cost electrocatalysts for E-NO3RR, capitalizing on their earth abundance and inherent ability to suppress competing hydrogen evolution. NiO is synthesized via a scalable precipitation method using different ethanol/water solvent ratios to modulate defect density, porosity, and crystallinity. Materials-related differences are probed by thermal, structural, and spectroscopy methods. Electrochemical tests reveal that increasing ethanol content during synthesis enhances defectiveness, correlating with improved Faradaic efficiency and ammonia production rates. This work underscores the critical role of synthetic parameters in tailoring catalytic performance and positions defect-engineered NiO as a promising platform for green ammonia generation via nitrate reduction.