Highly efficient electrochemical reduction of nitrate to ammonia by a CuO-Co3O4 composite membrane electrode

Electrochemical ammonia synthesis via nitrate reduction reaction (NO 3 RR) offers a sustainable solution for mitigating nitrate pollution while recovering valuable nitrogen resources. Conventional flow-by/batch systems suffer from slow kinetics and low current efficiency due to a thick boundary layer at the electrode surface, which impairs NO 3 RR activity and selectivity. This study introduces a novel flow-through configuration employing a micron-porous CuO-Co 3 O 4 composite membrane electrode for enhanced NO 3 RR. Computational fluid dynamics simulations revealed a significantly thinner boundary layer (∼10 μm) in the flow-through mode compared to flow-by (>120 μm). Consequently, the flow-through mode exhibited a seven-fold increase in NO 3 RR kinetics, boosting the ammonia Faradaic efficiency (FE) from 7.2 % to 79.8 %. Optimization of the Co 3 O 4 membrane electrode, addressing parameters of Cu/Co loading and pore size, further enhanced performance. This achieved high ammonia selectivity and FE (up to 95.7 % and 88.1 %, respectively) across a broad nitrate concentration range (0.02–1.0 g N L −1 ). Mechanistic investigations, integrating scavenger-quenching experiments, electrochemical analyses, and electron paramagnetic resonance spectroscopy demonstrated that ammonia synthesis primarily followed the dominance of the direct electron transfer (DET) between the flow-through cathode and nitrate/nitrite, while hydrogen species (H*) served an auxiliary function. These results underscore the effectiveness of the flow-through membrane electrode configuration in facilitating efficient electrochemical ammonia synthesis, offering valuable insights for the advancement of electrochemical technologies for pollution control and resource recovery.