Preparation Of Transition Metal Boride Composites And Its Electrochemical Properties

As a clean energy,hydrogen(H₂)can not only solve the environmental pollution caused by fossil fuel combustion,but also meet the large-scale energy demand in the future.Electrocatalytic decomposition of water provides a promising way to accelerate the development of hydrogen energy.However,electrolysis of water to produce hydrogen,including hydrogen evolution reaction(HER)and oxygen evolution reaction(OER),requires high overpotential in the reaction process.Noble metals(Pt and Ru/Ir O₂)can effectively reduce the overpotential of the reaction,but their application in industrial production is hindered due to highly cost and lower reserves.Therefore,it is necessary to develop non-noble metal electrocatalysts with high efficiency and low cost to accelerate the reaction process and reduce overpotential.In this work,a series of transition metal borides and Co B/r GO composites were prepared by chemical reduction method.Co B nanoparticles and two-dimensional layered graphene with excellent electrical conductivity formed Co B/r GO composites,and combined with transition metal atoms in manganese oxide(Mn O₂)and iron oxide(Fe₂O₃)with excellent adsorption properties to form ternary Mn Co B and Fe Co B,constructed three kinds of catalyst used in electrolytic water on the electrochemical performance of test and characterization.The specific research contents are as follows:(1)The graphene oxide(GO)was synthesized by using modified Hummers method.Then,Co B and GO were reduced to Co B and r GO by adding Na BH₄.Co B/r GO electrocatalyst was prepared at 400?for two hours.Co B was a non-precious metal material with low price.The existence of r GO could enhance its conductivity,promoted the good dispersion of HER active sites and improved the catalytic performance.The morphology of Co B/r GO composite was that the nanoparticles with regular size and shape grew on the lamellar substrate.The experimental results showed that Co B/r GO electrocatalyst has better HER performance in alkaline condition,the overpotential of217 m V and Tafel slope of 122.6 m V dec^-1 when the current density arrived 10 m A cm^-2,at the same time with the surface of the lower charge transfer resistance and electrochemical active surface area.The HER performance almost remains the good cycle stability after 3000 times of cyclic voltammograms test.(2)A series of amorphous ternary Mn Co B electrocatalysts were prepared by one-step reduction method.The activity of the catalysts was improved by adding polyvinylpyrrolidone as surfactant and optimizing the experimental conditions.The combination of Mn and Co had synergistic effect on the electrocatalytic activity,surface area,conductivity and electrochemical stability of the electrocatalyst,which improved the OER performance.The results showed that Mn Co B-1 catalyst had good OER activity and long-term stability in 1.0 M KOH solution,which showed a lower overpotential(292 m V),and the Tafel slope was only 92.4 m V dec^-1 when the current density arrived 10 m A cm^-2.The results of cyclic voltammetry showed that Mn Co B-1had a large electrochemically active surface area(985 cm²).The current density of Mn Co B-1 didn't decrease after 12 h of continuous hydrogen production under constant voltage,which proved that Mn Co B-1 had good chemical stability.Density functional theory(DFT)calculations showed that the Gibbs free energy(?G)of the rate determination step of Mn Co B-1(2.605 e V)was less than Co B(3.188 e V),indicated that Mn doping was more favorable for OER thermodynamics.Compared with Co B,Mn showed higher density of states at Fermi level,which indicated that Mn Co B has good electronic conductivity.(3)The nano-Fe Co B electrocatalytic material was prepared by chemical reduction method.It can be used in HER and OER of electrolyzed water as an effective electrocatalyst.Fe Co B nanoparticles can provide more catalytic active sites and have good chemical stability in alkaline environment because the effect between Fe and Co to improve the electrocatalytic activity.The overpotential of HER was 307 m V,and the Tafel slope was 74.0 m V dec^-1,which the overpotential of OER was 311 m V and Tafel slope was 82.5 m V dec^-1 when the current density was 10 m A cm^-2 in 1 M KOH.Fe Co B catalyst system has a broad application prospect as a catalyst for future electrochemical water splitting devices,which has important significance for the development of efficient bifunctional electrocatalyst.DFT calculation showed that the?G of Fe Co B was closer to 0,indicated that the bond energy between Fe Co B catalytic surface and active intermediates was more moderate,and H^*was easier to be adsorbed and desorbed,indicated that Fe Co B has better HER catalytic activity;the?G value of Fe Co B rate determining step(2.925 e V)was less than Co B(3.188 e V),indicated that OER thermodynamics of Fe Co B was more favorable.In this work,three kinds of excellent electrocatalysts,including Co B/r GO,Mn Co B and Fe Co B were prepared by chemical reduction precipitation method with simple method and convenient operation,and used as hydrogen evolution catalyst,oxygen evolution catalyst and hydrogen and oxygen evolution bifunctional catalyst respectively.The electrocatalytic performance of Co B was effectively improved by compounding with reduced graphene oxide and adding Fe and Mn to form ternary boride,the prepared catalyst was characterized and tested by water electrolysis simulation experiment.The theoretical calculation provided a theoretical basis for the experimental results,which has potential application value for the actual industrial hydrogen production from electrolytic water.