| Electrochemical nitrogen reduction is a highly promising technique for synthesizing ammonia,and it has the potential to displace the conventional Haber-Bosch process.In comparison with the Haber-Bosch approach,electrochemical nitrogen reduction has the following advantages.Electrochemical nitrogen fixation has the advantages of low energy consumption,operation at ambient conditions,simplicity and safety of operation,and easy access to raw materials such as nitrogen and water.Therefore,electrochemical reduction of nitrogen is projected to emerge as an environmentally friendly and sustainable technology for the production of ammonia in the future.However,the key challenge for this technology is the lack of efficient electrocatalysts.Molybdenum(Mo)is one of the most important elements in natural nitrogenase,and theoretical calculations have shown that Mo-based materials can be potential electrocatalysts for nitrogen reduction reaction(NRR).Therefore,it is crucial to design advanced Mo-based catalysts with advanced structures for efficient electrocatalytic N2-to-NH3 conversion.In this study,iron(cobalt)atom-doped Mo-based and sulfur-doped Mo-based catalysts with anionic vacancies were synthesized through vacancy,doping,and interface engineering.Their physicochemical structures were characterized,and their electrocatalytic performance for ammonia synthesis was evaluated.Firstly,Fe-doped MoO2/C heterogeneous catalysts were prepared by using ammonium molybdate as the molybdenum source,pectin as the carbon source,and FeCl3 as the doping agent,followed by a heat treatment process with a water bath and a hydrothermal treatment process.The optimal sample was treated with a sodium borohydride-ethanol solution to create oxygen vacancies.The effects of the Fe source dosage and the sodium borohydride-ethanol solution treatment on the morphology,electronic structure,and electrocatalytic nitrogen reduction performance of MoO2/C were studied.The results showed that the electrocatalytic ammonia synthesis performance reached the highest when the Fe source-to-Mo source ratio was 1:50.The introduction of oxygen vacancies not only reduces the energy barrier of RDS but also promotes the first step of nitrogen hydrogenation and ammonia desorption,further improving the ammonia production rate and Faraday efficiency.According to the DFT calculation results,the active center for electrocatalytic nitrogen reduction reaction is the molybdenum atom between the iron atom and the O vacancy,indicating that the synergistic effect of oxygen vacancy and Fe element doping can improve the electrocatalytic nitrogen reduction performance of the catalyst.Next,Co-doped MoS2/C catalyst was synthesized using ammonium molybdate as the molybdenum source,thiourea as the sulfur source,pectin as the carbon source,and Co(NO3)2 as the doping agent by hydrothermal method followed by thermal treatment.The catalyst was treated with a sodium borohydride-ethanol solution to introduce sulfur vacancies.The effects of different thermal treatment time,temperature,and sodium borohydride-ethanol solution treatment on the morphology,electronic structure,and electrocatalytic nitrogen reduction performance of the catalyst were investigated.The results showed that the crystallinity of the catalyst gradually increased with increasing thermal treatment time and temperature.The MoS2 nanosheets became thicker,and the nano-flower structure composed of MoS2 nanosheets became denser.Treatment with NaBH4-ethanol solution effectively introduced abundant sulfur vacancies,resulting in a significant improvement in the electrocatalytic nitrogen reduction performance.The ammonia production rate and FE reached a maximum of 21.4 ± 0.3 μg h-1 mg-1 and 11.5%,respectively,for Vs-Co-MoS2/C(700℃,3h)catalyst in 0.1 M Na2SO4 electrolyte during electrocatalytic nitrogen reduction tests.Finally,using ammonium molybdate as the source of molybdenum,thiourea as the source of sulfur,and pectin as the source of carbon,Fe-doped MoS2/C catalysts were synthesized using a hydrothermal and heat treatment method with FeCl3 as the doping agent.The effects of different annealing times,annealing temperatures,and treatment with sodium borohydride ethanol solution on the morphology,surface electronic structure,and electrocatalytic nitrogen reduction performance of the catalysts were studied.The results showed that the increase in annealing time and temperature caused an increase in the crystallinity of MoS2 in the catalyst,while the crystallinity of carbon decreased gradually with the increase in annealing time and temperature.The electrocatalytic nitrogen reduction performance of the catalyst increased first and then decreased with the increase in annealing temperature,and gradually improved with the increase in annealing time.The optimal sample,Vs-Fe-MoS2/C(annealed at 700℃ for 3 hours),had an ammonia production rate of 23.5 ± 0.4 μg h-1 mg-1 and the highest faradaic efficiency was 12.5%. |
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