(490a) Electrolyte Engineering for Efficient Electrochemical Nitrate Reduction to Value-Added Products

Nitrate contamination of bodies of water is both a human health concern and disrupts fragile aquatic ecosystems. Such contamination is largely attributed to the application of nitrate-based fertilizers to crops, upon which excess nitrates become incorporated into agricultural runoff. Such nitrate-based fertilizers are produced via the Ostwald process, which requires temperatures up to 900 ˚C and emits more N₂O globally than any other industrial process. Furthermore, the process most commonly uses Haber-Bosch ammonia, which is additionally cost- and energy-intensive to produce and is responsible for 1% of global CO₂ emissions. While many studies have focused on electrochemical reduction of nitrates to environmentally-benign N₂, direct electrochemical conversion of nitrates to ammonia could close this gap in the nitrogen cycle and recycle expensive pollutants (NO₃^-), to form value-added products.

To this end, a titanium-based cathode was used to electrochemically reduce NO₃^- to ammonia. Applied potential, electrolyte pH, and nitrate concentration were varied to map selectivity toward ammonia under a variety of electrode/electrolyte conditions. Selectivity varied depending on proton and nitrate anion concentration, with a maximum Faradaic efficiency (FE) toward NH₃ of 82% occurring at a pH of 1 and 0.4 M NO₃^- under an applied potential of -1 V vs RHE and current density of -22 mA/cm². Additionally, the cathode was stable over 8 hours while demonstrating a FE toward ammonia greater than 50%. Analysis of the cathode during and after operation suggests that titanium hydride (TiH_x) may play an important role in the reduction to NH₃. We have recently expanded our efforts to develop new electrochemical syntheses involving such environmental pollutants (NO₃^-), with an aim toward more sustainable resource cycling strategies that can be coupled to renewable sources of electricity.