Electrochemical nitrogen reduction reaction(eNRR) is a promising strategy for ammonia production because of its mild reaction conditions, high theoretical conversion rate, and green pollution. However, the strong stability of N N triple bond and competitive hydrogen evolution reaction (HER) limit the Faraday efficiency and ammonia conversion of eNRR. In recent years, the structural design of catalysts combined with external field enhancement to enhance the AC magnetic field electrocatalytic reaction has received extensive attention. In this study, an eNRR catalyst ( x Fe-M-TiO 2 ) with a high concentration of oxygen vacancies was constructed by the defect engineering strategy. The ammonia conversion rate of eNRR of 1.5Fe-M-TiO 2 was 7.35 μg h −1 mg cat −1 , and the Faraday efficiency was 22.69 %, which was 2 % higher than that before doping. The spin–orbit coupling effect induced by the AC magnetic field further improved the eNRR activity, in which the Faraday efficiency increased to 52.14 % (increased by 150 %), and the ammonia yield increased to 13.37 μg h −1 mg cat −1 (increased by 5 %). After 80 h of eNRR, the current density of the catalyst became 95 % of the initial current density, and the stability was good. Density functional theory calculation showed that Fe doping did not change the reaction mechanism of eNRR but increased the concentration of oxygen vacancy, resulting in a 0.62 eV decrease in the energy barrier of the first hydrogenation of eNRR, which was conducive to the occurrence of eNRR.
As nitrate contamination causes serious environmental problems, it is necessary to develop stable and efficient electrocatalysts for efficient electrochemical nitrate reduction reaction (ENRR). Here, a nonprecious Co3O4/carbon felt (CF) electrode with a 3D structure was prepared by integrating electrodeposition with calcination methods. This 3D structured Co3O4/CF electrode exhibits a high-rate constant of 1.18 × 10–4 s–1 cm–2 for the ENRR, surpassing other Co3O4 electrodes in previous literature. Moreover, it also has an excellent stability with a decrease of 6.4% after 10 cycles. Density functional theory calculations, electron spin resonance analysis, and cyclic voltammetry were performed to study the mechanism of the ENRR on the Co3O4/CF electrode, proving that atomic H* (indirect pathway) plays a prominent role in NO3– reduction and clarifying the synergistic effect of Co(III) and Co(II) in the Co(II)–Co(III)–Co(II) redox cycle for the ENRR: Co(III) prefers the adsorption of NO3– and Co(II) favors the production of H*. Based on this synergy, a relatively large amounts of Co(II) on the surface of the Co3O4/CF electrode (1.3 Co(II)/Co(III) ratio) was maintained by controlling the temperature of calcination to 200 °C with a lower energy barrier of H* formation of 0.46 eV than other ratios, which is beneficial for forming H* and enhancing the performance of the ENRR. Thus, this study suggests that building 3D structure and optimizing Co(II)/Co(III) ratio are important for designing efficient Co3O4 electrocatalyst for ENRR.
Electrocatalytic nitrogen reduction reaction (ENRR) is considered as a potential pathway for green synthesis of ammonia. However, the kinetic barriers of ENRR remain a tough challenge, which is imperative to design high effective electrocatalysts for accelerated activation of N N. Herein, the FeS/ZnS/ZIF-8-L with rich heterogeneous interfaces was successfully fabricated by governing the ordered lasso arrangement of Fe and Zn nodes in Fe-2MIM/ZIF-8-L precursor, where Fe and Zn were separately fixed in the confined space of ZIF-8-L and Fe-2MIM. Such intertwined Fe-2MIM/ZIF-8-L structure was beneficial to control the location and concentration of FeS and ZnS after sulfidation pyrolysis. The high-efficiency FeS/ZnS/ZIF-8-L exposed more catalytic active sites and possessed optimal electronic structure resulting in the lowered energy barrier of ENRR and the enhanced adsorption and activation of N 2 , which confirmed by experimental and theoretical characterization results. As a result, FeS/ZnS/ZIF-8-L displayed an optimal NH 3 yield rate of 89.52 μg h −1 mg −1 and Faradaic efficiency of 16.78 %, which surpassed most reported heterostructured ENRR catalysts. Our findings shed a new insight to govern the interfaces density of heterostructured electrocatalysts by predesigning the location of two metals in precursor, which can motivate an in-depth exploration to advanced catalysts for various electrocatalytic reactions.
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