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

Abstract

Anthropogenic ammonia production has sustained exponential population growth but exacerbated wastewater nitrate pollution. Abundant nitrate can be refined to purified nitrogenous chemicals through the electrochemical nitrate reduction reaction (NO₃RR). However, the dilute and impure composition of nitrate-bearing wastewaters presents barriers to practical electrocatalytic systems. We address these barriers in our investigation of the model ammonia-selective homogeneous molecular NO₃RR catalyst Co(DIM) in real wastewater and reactive separations architectures. In this work, we elucidate catalysis inhibition mechanisms imposed by magnesium in real wastewaters that decrease nitrate conversion activity by 62.0%. These mechanisms informed our design of electrocatalyst-in-a-box (ECaB), a novel NO₃RR reactive separation that exhibits the lowest reported energy consumption for purified wastewater-derived ammonia production (90.0 ± 2.7 kW h kg N⁻¹). Engineering ECaB's subunit processes enhanced the rate of ammonia production by 20.4×. This work demonstrates a use-informed engineering approach that iterates between mechanistic insights and unit process-level performance of electrochemical wastewater refining systems in complex aqueous streams.