Fe-modified Co2Mo3O8-promoted nitrate-cascade reduction reaction coupled with the oxygen evolution reaction for electrocatalytic ammonia synthesis

Abstract

Ammonia, as an important feedstock, is industrially produced through the Haber–Bosch process, which not only consumes massive amounts of energy, but also results in serious environmental problems. Electrochemically coupling the nitrate reduction reaction (NO₃RR) toward ammonia with the oxygen evolution reaction (OER) is both attractive for reducing environmental pollution and for facilitating energy efficient ammonia synthesis. However, the sluggish kinetics of the anodic OER and the overall high energy consumption limit the overall activity of the nitrate reduction system. The development of efficient electrocatalysts that promote both NO₃RR and OER processes remains challenging. In this work, a simple hydrothermal method followed by rapid Joule heating treatment was employed to synthesize Fe-modified Co₂Mo₃O₈ on nickel foam (FeCoMo/NF) as a bifunctional catalyst for the NO₃RR and OER. Notably, the introduction of Fe not only enhanced the adsorption of nitrate and various reaction intermediates on FeCoMo/NF, but also lowered the reaction energy barriers of the rate-determining step (NO₃⁻ → NO₂⁻). This greatly facilitated the cascade reaction of NO₃⁻ → NO₂⁻ → NH₃ and enhanced the electrocatalytic nitrate reduction activity. Meanwhile, FeCoMo/NF also demonstrated excellent OER activity, presenting a 318 mV overpotential at 40 mA cm⁻². In the two-electrode flow-cell system, FeCoMo/NF||FeCoMo/NF exhibited high electrochemical stability with an EPC of only 21.30 kW h kg_NH3⁻¹ and a combined overpotential of only 0.81 V, which was superior to that of an H-type cell. Therefore, a two-electrode NO₃RR and OER flow cell device structure was constructed based on the FeCoMo/NF catalytic system, which realized the effective utilization of energy and has great potential for practical application.