Beyond Interfacial Structure: Ion Transport Mediates Cation Effects in Electrochemical Nitrate Reduction

Electrochemical reduction of nitrate to ammonia integrates wastewater nitrate remediation with sustainable ammonia production. Electrolyte engineering has revealed that the rate of nitrate reduction is sensitive to the identity of the supporting cation, increasing along Li+ < Na+ < K+ < Cs+. However, atomistic-level analysis of interfacial structure and mass transport across different electrolytes remains limited, leaving open questions about the physical origins of these specific ion effects. To address this gap, here we use classical molecular dynamics simulations in the constant potential ensemble to model the electric double layer in nitrate-based electrolytes. Contrary to initial hypotheses emphasizing the importance of cation-specific interfacial structure for successful nitrate adsorption, we find that diffusion-layer mass transport mediates the specific-ion rate enhancement. While the interfacial structure of the electrolyte is indeed cation-dependent, the thermodynamics of nitrate adsorption are, surprisingly, weakly dependent on the cation identity, as the interfacial electrostatic potential is dominated by water polarization, rather than cation adsorption. In contrast, analysis of bulk ion transport reveals that larger ions promote a lower nitrate transference number, reducing the energetic penalty for nitrate migration against the electric field. Combined with increased electrolyte diffusivity, this effect enhances the flux of nitrate to the cathode, mirroring rate enhancements recorded in experiment. Our results demonstrate the central importance of ion transport for electrochemical nitrate reduction and demonstrate how electrolyte design can be strategically leveraged to control reactant transport in the electric double layer.