Preparation Of Transition Metal Borides And Their Electrocatalytic Performance For Water Splitting

The rapid development of modern industry based on fossil energy has brought a series of ecological and environmental problems and fossil energy crisis,so there is an urgent need to develop clean and renewable energy.Hydrogen with high combustion calorific value and storability is a promising renewable clean energy carrier.At present,there are many methods for preparing hydrogen.Among them,electrolysis of water is the most possible to achieve large-scale commercial application.However,electrolysis of water for hydrogen production still has the disadvantage of high overpotential（namely high energy consumption）,and electrocatalysts can effectively reduce the overpotential.Electrolysis of water consists of two half-reactions,the hydrogen evolution reaction（HER）and the oxygen evolution reaction（OER）.To date,the best-performing HER and OER electrocatalysts are noble metal-based electrocatalysts.However,they are expensive and rare,and are not suitable for large-scale applications.Therefore,earth-abundant and inexpensive transition metal-based electrocatalysts have received extensive attention.Transition metal borides display good electrocatalytic HER and OER activities,and are potential bifunctional electrocatalysts for electrocatalytic total water splitting.In this paper,a low-temperature solid-phase reaction method was used to synthesize three kinds of transition metal boride electrocatalysts:iron boride,nickel boride and iron nickel boride.All of them exhibit good electrocatalytic activities for hydrogen evolution,oxygen evolution and total water splitting.The thesis is mainly divided into the following two parts:（1）In an Ar atmosphere glove box,the iron boride catalyst Fe-B（s）was synthesized by solid-phase reaction of ferrous sulfate with sodium borohydride at room temperature,and the catalyst Fe-B（s）was heated at 600°C for 1 h to obtain Fe-B（s）^Δ.Fe-B（s）^Δexhibits excellent electrocatalytic activities for HER,OER and total water splitting compared with Fe-B（aq）^Δwhich was prepared by a traditional solution method.In 1 mol L^-1 KOH(containing 0.05 mol L^-1 Na₂B₄O₇)electrolyte solution at a current density of 100 m A cm^-2,the HER and OER overpotentials of Fe-B（s）^Δare 303 and 409 m V,respectively.The excellent electrocatalytic HER performance is attributed to the fact that the content of B in Fe-B（s）^Δis higher than that of Fe-B（aq）^Δ,and the crystallinity of Fe-B（s）^Δis higher than that of Fe-B（aq）^Δ.The higher content of B is beneficial to improve the electrocatalytic activity of the catalyst Fe-B（s）^Δ,and high crystallinity of Fe-B（s）^Δenhances the electrical conductivity of the electrode to decrease overpotential.At the same time,Fe-B（s）^Δhas higher electrochemically active surface area.These factors make it exhibit excellent electrocatalytic performance.Fe-B（s）^Δcan run stably for 125 h for electrocatalytic total water splitting at a voltage of 1.8 V,which indicates its excellent electrocatalytic stability.（2）Similarly,Ni-B catalyst was synthesized with nickel sulfate and sodium borohydride at room temperature.In addition,ferrous sulfate and nickel sulfate were dissolved to form a mixed solution,and the solution was evaporated to obtain a precursor powder.And then,using the precursor and sodium borohydride,Fe-Ni-B catalyst was similarly obtained by the room temperature solid-phase method.Both Ni-B and Fe-Ni-B catalysts display good electrocatalytic activities for hydrogen evolution and oxygen evolution.In 1 mol L^-1 KOH(containing 0.05 mol L^-1 Na₂B₄O₇)electrolyte solution,Ni-B has a hydrogen evolution overpotential of 285 m V and an oxygen evolution overpotential of 467 m V at a current density of 100 m A cm^-2;Fe-Ni-B has a hydrogen evolution overpotential of 321 m V and an oxygen evolution overpotential of 390 m V at a current density of 100 m A cm^-2.The synthesis of Ni-B and Fe-Ni-B catalysts shows that this room temperature solid-phase method has certain universality for the preparation of transition metal borides,which provides a new strategy for the large-scale preparation of transition metal boride electrocatalysts.