Preparation And Performance Research Of Transition Metal Composites For Electrochemical Nitrogen Reduction

Ammonia is not only the raw material for the production of chemicals such as fertilizers,drugs,plastics and explosives,but also has a high energy density and is easy to transport and store in the existing facilities,which is an important chemical raw material and energy carrier.For the long-term use of ammonia,ammonia should be formed in an economical and carbon neutral way from abundant resources（i.e.,N₂ and H₂O）,rather than the traditional Harbor-Bosch（H-B）method through H₂ and N₂ at high temperature and pressure.In recent years,the electrochemical nitrogen reduction reaction（NRR）conducted through electric energy and water electrolysis tank has attracted wide attention because of its environmentally friendly and sustainable characteristics.However,due to the stable N≡N triple bond and the side reaction HER,the ammonia production rate and selectivity of ammonia synthesis are challenged.In the researchers’proposed measures to improve NRR performance,such as developing electrocatalysts and electrochemical equipment,good electrocatalysts are particularly critical.The transition metal because of its unique electronic structure,low cost become promising catalyst,and through the transition metal catalyst morphology regulation,alloy engineering and single atom design will achieve better catalytic effect,and with the development of technology,theoretical calculation become indispensable screening catalyst,predict an indispensable part of the reaction process.Therefore,in this paper,the transition metals are designed as alloys and single-atomic catalysts through experimental and theoretical calculations,and their NRR is studied.The main contents are shown as follows:（1）CoFe alloy was synthesized with cobalt nitrate ferric chloride by NaBH₄ and reduced with the modified Hummers method and loaded on GO（CoFe/GO）.In addition,a single metal was prepared（Co/GO,Fe/GO）without changing the metal amount,and compared with the CoFe alloy/GO.The results show that in 0.1 M Na₂SO₄ electrolyte,the highest ammonia yield at-0.4 V vs.RHE is 6.18μg h^-1 cm_cat.^-2,and the 38%Faraday efficiency at-0.3 V vs.RHE,where the ammonia yield is much higher than the prepared Co/GO,Fe/GO and GO catalysts.Through subsequent analysis,the high CoFe/GO performance may be due to the synergistic action of the elements to generate more active sites and promote charge transfer,thus promoting N₂ adsorption and activation.Moreover,the CoFe/GO catalyst prepared by this method maintains a good stability and selectivity.（2）Based on Co-loaded N-doped 555-777 graphene（Co-N₃/G）and active site Fe-N₄graphene（Fe-N₄/G）,a total of 16 multi-heteroatoms doped Single atom catalysts（SACs）were theoretically designed to study the stability,N₂ adsorption and activation,electrocatalytic properties and activation source of NRR of these SACs through a series of DFT simulations.Most of the SACs showed good stability and benefit to N₂ adsorption and activation.With solvent model correction,the four SACs,Co-B₁N₂/G,B is adjacent to the N atom in Co-BN₃/G,two P atoms are located in opposite positions in Fe-N₂P₂/G and Fe-N₃P/G,which favored N₂and N₂H activation and stabilization,were screened for promising NRR catalysts,and Fe-N₃P/G is the best candidate by enzymatic or continuous path,with only 0.41 eV energy.The high activity of the SACs results from the effective charge transfer between the NRR intermediate and the catalyst.Therefore,the defect type of graphene,the change of transition metal atoms and the introduction of heteroatoms can all affect the performance of the catalyst.