Controlled N₂/O₂ injection strategy for enhancing nitrate formation and reactive species generation in plasma electrolysis-based nitrogen fixation

Plasma-assisted nitrogen fixation is an alternative route for sustainable nitrate production. The influence of controlled N 2 /O 2 availability on the evolution of reactive species and nitrate selectivity remains insufficiently understood. This study introduces a controlled gas composition strategy to elucidate the relationship between plasma-generated reactive species and nitrogen fixation pathways in aqueous electrolyte systems. The K 2 SO 4 concentration significantly influenced the breakdown and discharge behaviors. Based on the balance between discharge initiation and power requirement, 0.02 M K 2 SO 4 was selected for subsequent nitrogen fixation experiments. The gas composition strongly affected the formation of nitrogen species. The N 2 /O 2 composition of 79/21 resulted in the highest nitrate concentration (1889 mg L⁻¹), followed by N 2 /O 2 = 50/50 (1702 mg L⁻¹), whereas pure O 2 yielded the lowest nitrate concentration (181 ± 3.00 mg L⁻¹). The optimized N 2 /O 2 ratio of 79/21 achieved the highest nitrate concentration of 1889 mg L⁻¹, demonstrating that balanced nitrogen and oxygen availability, rather than excessive oxygen supply, governs nitrate formation. These findings provide novel mechanistic insights into plasma-driven nitrogen fixation and offer a potential pathway for decentralized nitrate fertilizer production using electricity-driven processes. At approximately 400 W for 60 min, the optimum condition required 0.400 kWh of electrical energy and achieved an apparent nitrate energy yield of 5.67 g NO₃⁻ kWh⁻¹. These results demonstrate plasma-assisted aqueous nitrogen formation, enabling sustainable nitrate production, which aligns with SDG 2 (Zero Hunger).