Preparation,Characterization,and OER Performance Of N-doped Graphene Supported Nickel-iron Bimetallic Composite Catalysts

The depletion of fossil fuels necessitates the search for suitable alternative energy sources to meet global energy needs."Hydrogen"with a specific energy density of 142 k J mol^-1 plays an important role in promoting the green transformation of economic and social development.Electrocatalytic water splitting is a method of using electric energy to produce high-purity hydrogen from abundant water.Electrocatalytic water splitting consists of an oxygen evolution reaction（OER）at the anode and a hydrogen evolution reaction（HER）at the cathode.Compared with HER,OER requires a four-step electron transfer,which requires a higher reaction barrier and a more complex reaction process.Therefore,OER has a higher reaction overpotential in the actual electrolysis of water,which restricts the efficiency of overall water splitting.Noble metal oxides（RuO₂ and IrO₂）are considered the best electrocatalysts for OER.However,the high cost and low abundance of these noble metal-based electrocatalysts have driven the development of alternative electrocatalysts to enable large-scale industrial applications of hydrogen production by electrolysis of water.At present,various derivatives of transition metals have been found to be effective and low-cost OER catalysts for replacing precious metals.Among the many transition-metal electrocatalysts,the transition metal electrocatalyst of nickel has high OER activity due to the reasonable arrangement of nickel valence electrons.In order to further improve the catalytic activity of transition metal-based electrocatalysis,the researchers adopted vacancing,straining,doping,and alloying strategies.Nevertheless,their low conductivity and stability during reactions need to be improved.For a long time,graphene and its derivatives have had a large specific surface area,good electrical conductivity,and high electrochemical stability and have attracted much attention in the preparation of energy storage,conversion devices,and electrocatalysts.The use of graphene as a carrier of catalytically active substances will effectively improve the conductivity and durability of catalysts.However,graphene’s perfect hexagonal lattice and nanosheet-like structure limit the uniform dispersion of its surface-active species.Therefore,the functionalization of graphene can not only increase its structural defects and enhance its interaction with active substances but also help to stimulate its own catalytic activity.Based on the above discussion,we successfully prepared two functionalized N-doped reduced graphene oxide-supported transition-metal derivative composite catalysts by a simple in situ growth method and used them in efficient electrocatalytic OER.The phase composition and morphological structure characteristics of the catalyst were proven by XRD,XPS,Raman,BET,SEM,TEM,and other characterization methods.1.Preparation,characterization,and the study for OER performance of NrGO-supported Ni₃Fe electrocatalystSynthesis of graphene oxide（GO）dispersions by the modified Hummers method.Subsequently,a certain mass of GO was dissolved in ethylene glycol,and a uniformly dispersed GO dispersion was obtained by sonication.Finally,using urea as an N-dopant,nickel nitrate and iron nitrate as metal sources,an N-doped graphene-supported Ni₃Fe（Ni₃Fe/NrGO）electrocatalyst was obtained after one step of solvent-heat and subsequent high temperature reduction.A series of related Ni₃Fe/NrGO catalysts were obtained by varying the temperature of high-temperature reduction and the ratio of graphene to the metal source.The electrochemical OER test of the relevant catalyst was carried out in 1M KOH using the standard three-electrode system,and the test results showed that the Ni₃Fe/NrGO catalyst with the appropriate load obtained by reduction at a high temperature of 500°C showed the highest OER catalytic performance.At a current density of 10m A cm^-2,the overpotential is as low as 277 m V,and the Tafel slope is 89 m V dec^-1.In addition,the Ni₃Fe/NrGO catalyst also has good stability,and the change of overpotential remains almost unchanged after 14 h of continuous reaction.In order to determine the composition and morphology of Ni₃Fe/NrGO with the best performance,it was characterized by XRD,XPS,SEM,TEM,ET,and other characterization methods.2.Preparation,characterization,and the study for OER performance of NrGO-supported Fe-Ni S electrocatalystCompared with Ni₃Fe,transition metal sulfides can significantly promote the pre-oxidation reaction during the application of OER,promote the rapid formation of catalytically active species,and increase the kinetic rate of the OER reaction.Therefore,N-doped Ni S electrocatalysts（Fe-Ni S/NrGO）were prepared by solvothermal and high-temperature vulcanization to reduce graphene oxide-supported Fe-doped.The electrochemical test results show that the doping of Fe in Ni S/NrGO effectively improves the OER catalytic performance of the catalyst.To this end,we synthesized Fe-Ni S/NrGO catalysts with different Fe doping amounts by changing the Ni/Fe doping ratio.Among them,Fe-Ni S（2）/NrGO showed excellent OER catalytic activity in 1.0 M KOH,with an overpotential as low as 344 m V at a current density of 100 m A cm^-2 and a Tafel slope of 47 m V dec^-1,both lower than commercial RuO₂ catalysts.Finally,the structure and composition of Fe-Ni S（2）/NrGO were proved by XRD,XPS,SEM,and other tests.The characterization results show that Fe-doped nickel sulfide supported on NrGO is a single phase rather than a mixed phase and is uniformly dispersed on the NrGO surface.