Synthesis Of Transition Metal Borides Via Room-Temperature Solid-Phase Reaction Method And Their Electrocatalytic Performance For Water Spiltting

To meet increasing global energy demand,fossil fuels have been quickly consumed,which results in the greenhouse gases and seriously environment pollution.As a consequence,it is urgently to exploit clean and renewable alternative energy.Due to its advantages of environmentally-friendly,secure,abundant sources,and wide application range,hydrogen emerged as an ideal substitute for fossil fuel.Among various methods for hydrogen production,electrochemical water splitting（EWS）is one of the cleanest methods to produce hydrogen（H₂）.EWS includes two half reactions,that is,hydrogen evolution reaction（HER）at the cathode and oxygen evolution reaction（OER）at the anode.All of them require efficient electrocatalysts to accelerate the reaction kinetics.However,the scarcity and high cost of noble metal catalysts such as Pt and Ru O₂seriously hindered the large scale commercialization application.Thus,it is urgent to develop earth-abundant,stable and efficient EWS catalysts to promote the practical applications.Transition metal borides have emerged as promising bifunctional electrocatalysts for EWS due to their simple preparation,low cost,and good conductivity.At present,common preparation methods include High temperature solid-state reaction method（HTSR）and solution reaction method（SR）.By the former,metal borides are prepared at high temperature using mixture of metals and boron,but the method requires high reaction temperature and excessive boron,resulting in high preparation costs.While the latter requires excessive consumption of sodium borohydride and also generates a large amount of wastewater,increasing preparation costs and resulting in environmental pollution.To address these issues,this study presents a new synthetic approach:room-temperature solid-phase reaction method（RTSR）to prepare metal borides.Using cobalt sulfate,iron sulfate,and sodium borohydride as precursors,two kinds of transition metal boride electrocatalysts are synthesized by grinding.Both cobalt borate and iron cobalt borate exhibit excellent electrocatalytic activities for HER,OER,and full EWS.This study is mainly divided into the following two parts:（1）:Using cobalt sulfate and sodium borohydride as precursors with different molar ratios of Co and B,Co-B_x（s）was prepared by RTSR.When the molar ratio of Co and B is 1:4,the prepared sample Co-B₄（s）,displays the best electrochemical performance.Physical and electrochemical performance characterizations were carried out using XRD,XPS,SEM,LSV,EIS,and other techniques.In 1 mol L^-1 KOH(containing 0.05 mol L^-1 Na₂B₄O₇)electrolyte solution at a current density of 100 mA cm^-2,Co-B₄（s）prepared by RTSR showed lower HER overpotential（457 mV）than Co-B（aq）prepared by SR（480 mV）.Co-B₄（s）exhibited more excellent electrocatalytic performance,which can be attributed to the higher boron content in Co-B₄（s）than Co-B（aq）.At a voltage of 1.8 V for fully EWS,Co-B₄（s）,as a bifunctional electrocatalysts（HER,OER）,reaches a current density of 49 mA cm^-2and stability for 120 h.（2）:Based on the above work,cobalt sulfate,iron sulfate,and sodium borohydride were utilized as precursors to prepare the Fe-doped cobalt borate catalyst by RTSR.When the molar ratio of Fe and Co is 1:6,the prepared sample Fe-Co₆-B（s）,displays the best electrochemical performance.In 1 mol L^-1 KOH(containing 0.05 mol L^-1Na₂B₄O₇)electrolyte solution,the overpotential of HER and OER was 445 m V and 490m V at the current density at 100 mA cm^-2 for Fe-Co₆-B（s）.The incorporation of Fe resulted in improved performance of the electrocatalysts.