Sustainable nitrogen fixation by bubble discharge plasma: Performance optimization and mechanism

Sustainable nitrogen fixation driven by renewable energy sources under mild conditions has been widely sought to replace the industrial Haber-Bosch process. The fixation of nitrogen in the form of NO_x⁻ and NH₄⁺ into aqueous solutions using electricity-driven gas–liquid discharge plasma is considered a promising prescription. In this paper, a scalable bubble discharge excited by nanosecond pulse power is employed for nitrogen fixation in the liquid phase. The nitrogen fixation performance and the mechanisms are analyzed by varying the power supply parameters, working gas flow rate and composition. The results show that an increase in voltage and frequency can result in an enhanced NO₃⁻ yield. Increases in the gas flow rate can result in inadequate activation of the working gas, which together with more inefficient mass transfer efficiencies can reduce the yield. The addition of O₂ effectively elevates NO₃⁻ production while simultaneously inhibiting NH₄⁺ production. The addition of H₂O vapor increases the production of OH and H, thereby promoting the generation of reactive nitrogen and enhancing the yield of nitrogen fixation. However, the excessive addition of O₂ and H₂O vapor results in negative effect on the yield of nitrogen fixation, due to the significant weakening of the discharge intensity. The optimal nitrogen fixation yield was up to 16.5 μmol/min, while the optimal energy consumption was approximately 21.3 MJ/mol in this study. Finally, the mechanism related to nitrogen fixation is discussed through the optical emission spectral (OES) information in conjunction with the simulation of energy loss paths in the plasma by BOLSIG + . The work advances knowledge of the effect of parameter variations on nitrogen fixation by gas–liquid discharge for higher yield and energy production.