Characterization of Interfacial Properties in Electrochemical Nitrate Reduction to Optimize Ammonia Production

Abstract

Nitrate pollution of wastewater from agricultural runoff and industrial waste streams is common around the globe. Nitrate negatively impacts the environment through harmful algal blooms and can be dangerous for human consumption. Rather than treat nitrate as a waste, we electrochemically reduce nitrate (NO3RR) to ammonia (NO3 - + 9H+ + 8e- → NH3 + 3H2O), a commonly used fertilizer and clean fuel. Electrochemical treatment can enable on-site, modular treatment powered by renewable energy. As an inner-sphere reaction involving multiple hydrogenation and electron-transfer steps, the activity and selectivity of electrochemical NO3RR to NH3 are strongly influenced by properties at the electrode–electrolyte interface. When potential is applied, the electric double layer (EDL) forms that comprises ordered molecular layers extending up to 10 Å from the surface. This EDL is the primary environment where the heterogeneous NO3RR occurs and can impact the reaction in several ways, including blocking catalytic sites and stabilizing reactants and reaction intermediates. To understand the molecular mechanisms of these impacts, the EDL structure is studied with x-ray reflectivity (XRR), which provides atomic-level resolution of the near-surface electron density profile and reveals the number of ordered layers, the distance of each layer from the surface, the ion surface coverage, and the degree of hydration of ions in each layer. We complement these investigations with operando attenuated total reflectance–surface-enhanced infrared absorption spectroscopy (ATR–SEIRAS) under similar reaction conditions to investigate the adsorbed reactants, intermediates, and local pH. Furthermore, composition and structure of the EDL can change with varying bulk electrolyte compositions and mass transport regimes, which also provide levers to control reaction activity and selectivity. We use a continuum model (generalized modified Poisson–Nernst–Planck, GMPNP) to develop the spatial profiles of the electric field, reactant nitrate concentration, cation concentrations, and pH outside the EDL and how they change as functions of applied potential, bulk electrolyte composition, and flow rate. Taken together, our results relate 1) bulk electrolyte properties with interfacial EDL properties and 2) interfacial properties with reaction activity and selectivity, informing the choice of operating parameters such as electrolyte composition and flow rate to optimize ammonia production in electrochemical NO3RR.