Nitrate pollution of wastewater from agricultural runoff and industrial waste streams is ubiquitous around the globe. Elevated nitrate concentrations in water causes 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 carbon dioxide emissions. Ammonia is an important chemical precursor with potential applications in sustainable energy as a fuel or hydrogen carrier, or can be used as a fertilizer in the form of ammonium sulfate. 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 the selectivity and efficiency challenges of nitrate reduction, we must understand the molecular mechanisms in the electric double layer (EDL) that forms at the electrode–electrolyte interface, where heterogeneous electrochemical reactions occur. We use in situ attenuated total reflectance–surface-enhanced infrared adsorption spectroscopy (ATR–SEIRAS) to quantify the transient, interfacial pH within 5–10 nm of the cathode surface under NO3RR conditions, building calibration curves from phosphate buffers in low, intermediate, and high pH regimes. The 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.

Probing the interfacial electrolyte environment during nitrate reduction

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