Engineering the Electrochemical Reaction Microenvironment to Valorize Nitrate-Polluted Wastewaters
Authors: Guo Jinyu , Brimley Paige , Liu Matthew , Corson Elizabeth R. , Smith Wilson , Tarpeh William Abraham 
Published: November 2022

Anthropogenic activities have imbalanced the global nitrogen cycle since the large-scale implementation of Haber-Bosch ammonia manufacturing. The removal of reactive nitrogen species (all inorganic forms besides dinitrogen) has fallen far behind their production, leading to heavy environmental burdens and continuous losses from the nitrogen economy. As the most prevalent waterborne reactive nitrogen pollutant, excessive nitrate jeopardizes both human and ecosystem health. Rather than treat nitrate as a waste, we electrochemically reduce nitrate (NO3RR) to ammonia, 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 is strongly influenced by properties at the electrode-electrolyte interface. Properties of the electrocatalyst (e.g., chemical composition, crystal structure) are crucial to intrinsic reaction activity and selectivity, while electrolyte properties (e.g., pH, species concentrations) also exert substantial impacts. Together, the electrode and interfacial electrolyte are referred to as the reaction microenvironment. Notably, the reaction environment is dynamically evolving during the reaction process: the electrocatalyst structure can undergo reconstruction, and the interfacial electrolyte properties usually deviate from the initial and bulk electrolyte properties. Titanium (Ti) has been identified as a robust NO3RR electrocatalyst, with appreciable NH3 selectivity and is suitable for real wastewater treatment due to its abundance, moderate cost, and corrosion resistance. We found that NO3RR introduces significant surface reconstruction and forms titanium hydride (TiHx, 0 < x ≤ 2). With a combination of ex situ grazing-incidence X-ray diffraction (GIXRD) and X-ray absorption spectroscopy (XAS), we demonstrated near-surface TiH2 enrichment with increasing NO3RR applied potential and duration. This quantitative relationship facilitated electrochemical treatment of Ti to form TiH2/Ti electrodes for use in NO3RR, thereby decoupling hydride formation from NO3RR performance. A wide range of NO3RR activity and selectivity on TiH2/Ti electrodes between −0.4 and −1.0 VRHE was observed and analyzed with density functional theory calculations. On the electrolyte side, mass transport modifies the interfacial electrolyte properties and thus regulates NO3RR activity and selectivity. In a representative flow-cell configuration with a Ti electrode, we systematically controlled mass transport conditions and demonstrated their impacts on NO3RR performance. With continuum model simulation and in situ infrared absorption spectroscopy, we characterized the interfacial environment under varied mass transport conditions and elucidated the impacts of interfacial electrolyte properties on NO3RR activity and selectivity. We found that diffusion layer thickness and interfacial cation concentration govern NO3RR activity, while interfacial pH steers NO3RR selectivity. In summary, our study underscores the importance of understanding the reaction microenvironment to inform rational catalyst design and electrolyte engineering to advance NO3RR performance.