Engineering a molecular electrocatalytic system for energy-efficient ammonia production from wastewater nitrate

Abstract

Haber-Bosch ammonia production and utilization has sustained exponential population growth but exacerbated wastewater nitrate pollution. Abundant nitrate pollutants could be refined to purified nitrogenous chemicals with the electrochemical nitrate reduction reaction (NO3RR). However, the dilute and impure composition of nitrate-rich wastewaters presents barriers to realizing practical electrocatalytic systems that to date operate most efficiently in concentrated, pure electrolytes. These barriers inform our investigation of the ammonia-selective homogeneous molecular NO3RR catalyst Co(DIM). In this work, we elucidate interfacial mechanisms of catalysis inhibition that describe Co(DIM)-mediated NO3RR performance in complex electrolyte compositions. These mechanisms inform design principles for a novel reactive separations platform, electrocatalyst-in-a-box (ECaB), that exhibits the lowest reported energy consumption for purified ammonia production from a real wastewater (90.0 ± 2.7 kWh kg-N–1). This work demonstrates a use-informed design approach that iterates between mutually informative mechanistic insights and performance of electrochemical wastewater refining systems in complex aqueous streams.

Flexible Electrochemical Stripping for Wastewater Ammonia Recovery with On-Demand Product Tunability

“…On days 11 and 13, we achieved higher Chamber 4 concentrations (up to 40.3 g/L TAN, 4.89% NH 3 (w/w)) and greater product distribution toward NH 3(aq) over (NH 4 ) 2 SO 4 by increasing the operation frequency of the second electrochemical cell (i.e., 1 h on versus 1 h off periods instead of 1 h on versus 6−12 h off periods) to limit diffusion from Chamber 4 to 3 (Figure S3- 11). Reactor modifications (e.g., using a thicker CEM in the second electrochemical cell, 68 alternative architectures like electrocatalyst-in-a-box (ECaB) 69 ) in combination with the demonstrated operational strategy could further enhance NH 3(aq) recovery.…”