| To date,the Haber-Bosch(H-B)process,which converts N2 and H2 into NH3 under harsh conditions(i.e.high temperature and high pressure),is still the most important pathway for the production of ammonia artificially.H-B process not only consumes a lot of energy,but also emits a lot of CO2 during the production of hydrogen,which further aggravates the greenhouse gas effect(GHG).Therefore,it is of great significance to find an economic,green and sustainable way for ammonia synthesis.The electrochemical ammonia reduction reaction(ENRR),which can be spontaneously carried out at ambient conditions,has been proposed and is gaining more and more attention.This method takes water as the hydrogen source and electric energy as the driving force,so that the generation of CO2 can be greatly reduced or minimized.However,ENRR still faces two big challenges:1)the lack of efficient electrocatalyst for breaking N≡N bond,and 2)the competitive reaction of HER in aqueous electrolyte.Therefore,in order to improve the ammonia production rate and current efficiency of ENRR,transition metal catalysts with speicific structures were designed and prepared,including carbon loaded TiO2,nickel foam supported FeMo bimetallic nanosheets and N doped MoFe bimetallic nanoparticle catalysts.The physicochemical and electrochemical properties of the catalysts were characterized through the characterization and electrochemical activity tests.In this paper,we first attemped to improve the Faradaic efficiency of electrocatalysts by utilizing the characteristics of carbon materials in inhibiting the hydrogen evolution reaction(HER).A carbon material was prepared from the unused cigarette filters.The characterization results showed that TiO2 was detected in the carbon materials,which was proved to be the active component of the catalytic system through the relevant experimental design.It was found that,when the applied potential was-0.2 V vs.RHE,the maximum ammonia production rate of CF-HC was 5.0 μg mgcat.-1 h-1.When the potential was-0.1 V vs.RHE,a higher FE of 12.38%of CF-HC was achieved.Inspired by the fact that multi-metal catalysts can effectively regulate the electronic structure of active sites,a bimetallic FeMo catalyst was constructed.N-doped FeMo bimetallic nanosheets coated on the surface of nickel foam were obtained by a two-step method(hydrothermal synthesis followed by calcination).The characterization results indicated that the presence of Mo can significantly enhance the N doping,and there was a strong electrons interaction between Fe sites and Mo-N bonds.The catalyst Fe1Mo1-N exhibited a promising performance of NH3 yield rate 46.64 μg mgcat.-1 h-1 and a Faradaic efficiency(FE)1.43%at0.8 V vs.RHE.The XPS characterization and examination of NH3 yield at different N contents in M-N bond revealed that Fe-Mo-N species are the primary active centers for NRR,and the N content forming the M-N bond is closely related to the catalytic activity.To further improve the NRR activity of catalyst effectively,so that the double increase in ammonia production rate and FE can be achieved simultaneously,MoFe bimetallic particles were produced via the stirring-precipitation method at room temperature.Subsequently,the Mo(IV)group generated under the action of NH3 atmosphere would induce the electronic interactions between the surface nitrogen species and Fe metal groups,thus forming ternary active sites of Mo-Fe-N.The prepared ternary MoFe-N catalyst exhibited a remarkable FE of 33.26%and a NH3 yield rate of 33.31 μg mgcat.-1 h-1 for NRR.The X-ray photoelectron spectroscopy(XPS)analysis further revealed that it is the thermal processing under NH3 atmosphere that form the Mo(Ⅳ)active sites in Mo-N bond,which led to a significant suppression on HER activity.Finally,through the study of the surface hydrogenation mechanism(SHM),we concluded that the synergistic effect of the adsorbed H*and Mo active sites is the main reason for the improved performance of NRR. |
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