Ammonia Synthesis via Synergistic Plasma–Electrochemical Nitrogen Reduction

We report ambient-condition ammonia synthesis via plasma-mediated electrocatalysis that couples a nonthermal plasma (NTP) with an electrochemical cell. In this hybrid scheme, the plasma pregenerates long-lived, soluble nitrogen intermediates (predominantly NO₃⁻/NO₂⁻) from N₂/H₂O, which are subsequently electrochemically reduced to NH₃. Using a dual-chamber H-cell (2.5 kV NTP, ~6–7 mA plasma current; −0.5 V vs. Ag/AgCl), we find that Ar plasma doubles NH₃ production relative to electrochemistry alone, and N₂ plasma provides a further approximately 50% increase. Acidic electrolytes are critical: 0.5 M H₂SO₄ maximizes NH₃ yield, while tuning acid concentration balances NH₃ selectivity against HER. Humidifying the N₂ plasma increases nitrate from approximately 5400 to approximately 7100 ppm and boosts NH₃ by approximately 21%, consistent with enhanced formation of soluble nitrogen intermediates. Catalyst screening shows PtIr and Pd outperform Pt/C, NbN, and NbO₂, indicating that in the plasma-assisted regime the rate-limiting step shifts from nitrate activation to hydrogenation, favoring catalysts that efficiently supply surface hydrogen. Time-resolved measurements reveal steady NH₃ accumulation with nonmonotonic NO₃⁻ evolution, reflecting a competition between plasma-driven formation and electroreduction. Under optimized conditions (N₂ plasma; 0.3 M–0.5 M H₂SO₄), the Faradaic efficiency (FE) reaches up to 47%; total specific energy consumption for our present setup is approximately 73 kWh/kg NH₃, dominated by the plasma input, indicating clear opportunities for reduction via high-frequency operation and reactor optimization. These results establish plasma–electrochemical coupling as a tunable route to decentralized, ambient NH₃ synthesis and outline design principles for plasma-compatible electrocatalysts and reactors.