(619f) Probing the Reaction Microenvironment during Electrochemical Nitrate Reduction

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. Electrochemical reduction of nitrate (NO₃RR) to ammonia can remove nitrate from water and is an alternative to Haber-Bosch (HB) ammonia production, a significant source of global CO₂ emissions. Ammonia is an important chemical precursor with potential applications in sustainable energy as a fuel or H₂ carrier, or can be used as a fertilizer in the form of ammonium sulfate. This research addresses the water­­–energy nexus by converting nitrate to a valuable product, off-setting the cost of water treatment, and reducing demand for HB.

NO₃RR processes must exhibit selectivity toward ammonia to avoid the formation of other undesired products and occur at a low overpotential to minimize operating costs from electricity. To address these challenges, we must understand the molecular mechanisms in the electric double layer (EDL) that forms at the electrode–electrolyte interface, where heterogeneous electrochemical reactions occur. The structure of the EDL can impact the reaction in several ways, including blocking catalytic sites and stabilizing reactants and reaction intermediates. We study the EDL with X-ray reflectivity (XRR), which provides atomic-level resolution of the near-surface electron density profile. We complement these investigations with in situ attenuated total reflectance–surface-enhanced infrared adsorption spectroscopy (ATR–SEIRAS) to investigate the adsorbed reactants, intermediates, and local pH. The XRR and ATR–SEIRAS results are correlated with electrochemical experiments measuring NO₃RR selectivity, activity, and efficiency. NO₃RR products are measured by ion chromatography and gas chromatography. Our results relate bulk electrolyte properties with interfacial EDL properties, and interfacial properties with reaction activity and selectivity. This understanding could lead to the development of electrolyte engineering strategies to optimize ammonia production in electrochemical NO₃RR.