Plasma-Electrocatalytic Nitrogen Reduction for Ammonia Synthesis

Electrochemical nitrogen reduction under ambient conditions is constrained by the kinetic inertness of N₂ and competition from the hydrogen evolution reaction. Here, we report an integrated plasma-electrolysis membrane-electrode assembly that couples a microstructured nonthermal surface discharge directly to a gas-diffusion cathode, enabling in situ generation and delivery of vibrationally excited N₂, N₂(ν), to the electrocatalytic interface. Among 14 metal catalysts screened, Ag provided the best balance of activity and selectivity, achieving an NH₃ production rate of 7.7 ± 0.8 nmol s^-1 cm^-2 with an electrochemical Faradaic efficiency of 86 ± 14% at -0.54 V versus RHE under atmospheric pressure and room temperature. Plasma/electrolysis on-off controls and ¹⁵N₂ isotope-labeling experiments establish that NH₃ formation requires simultaneous plasma activation and electrochemical polarization and originates from the supplied N₂. Plasma-kinetic modeling indicates strong vibrational excitation in the discharge, with a calculated effective vibrational temperature of approximately 4300 K and 11.8% of N₂ occupying levels (ν = 4-8). Transport modeling further suggests that these comparatively long-lived excited states persist to the membrane-catalyst region. In situ Raman spectroscopy reveals N-N-containing surface intermediates only during coupled operation, while density functional theory shows that vibrational excitation lowers the free-energy requirement for the initial hydrogenation of N₂ to *NNH and shifts the rate-determining step in a catalyst-dependent manner. These findings establish vibrational-state engineering as a strategy for coupling non-equilibrium molecular activation with electrocatalysis for distributed ammonia synthesis under ambient conditions.