Enhancing Nitrogen Fixation Efficiency by Reducing Cathode-Fall Voltage*

In plasma nitrogen fixation devices, discharge electrodes are crucial but vulnerable to oxidation and corrosion due to the high temperatures and oxygen content in plasma, which can alter discharge modes. This study evaluates the impact of different electrode materials—iron, chromium, nickel, copper, and 304 stainless steel—on nitrogen fixation efficiency in glow-like discharges driven by high-voltage DC power. Notably, iron and 304 stainless steel cathodes undergo a mode transition at increased currents, evidenced by plasma color shifts and significant voltage reductions. FTIR spectroscopy analyses reveal that these mode changes minimally affect nitrogen oxide production rates but lead to a notable decrease in energy consumption for nitrogen fixation by up to 40%. OES and SEM-EDS measurements suggest that iron oxide, with its higher secondary electron emission, replaces metal as the cathode material, facilitating mode transitions and maintaining discharge current at lower voltages. This voltage change is largely attributed to the cathode voltage drop, underscoring the minimal role of the cathode fall region in NOx synthesis. These findings highlight the potential for improving plasma nitrogen fixation energy efficiency by selecting suitable cathode materials to lower the cathode-fall voltage.