Strategic Synthesis And Electrocatalytic Performance Of Transition Metal Boride For Water Splitting

Hydrogen has attracted much attention because of its high combustion value,non-polluting products and various forms of utilization.It is an ideal clean energy carrier to replace fossil fuels.Electrocatalytic water splitting is a promising,green technology which uses electricity derived from renewable energy to decompose water to produce hydrogen.However,due to the slow reaction kinetics,most electrocatalytic reactions require precious metals as catalysts.Limited by the high price and low reserves of precious metals,there is an urgent need to develop new non-precious metal electrocatalysts.Despite some progress in related research,most electrocatalysts still have to suffer low catalytic activity and insufficient stability.Transition metal borides generally have high electrical conductivity,moreover,they have good corrosion resistance due to the incorporation of boron.It is a potential new type of water splitting electrocatalyst.Although transition metal borides have many benefits in electrocatalytic water splitting,their synthesis has always been difficult which requires complicated and expensive methods.In addition,the current lack of understanding of the catalytic mechanism of these materials has hindered their development to some extent.Aiming at the above problems,this thesis developed a series of transition metal boride electrocatalytic water splitting catalysts through pack boronizing,molten salt assistation and heterogeneous metal atom doping to achieve high intrinsic catalysis in water splitting reaction.The intrinsic relationship between special crystal structure,electronic structure and catalytic properties of transition metal borides is illustrated by the analysis of their catalytically active phase structures.The three main parts of this thesis including:Firstly,the OER is a kinetically sluggish reaction which makes the low energy conversion efficiency of the overall electrocatalytic reaction.Despite many efforts are made in OER electrocatalysts,as we know,almost all the electrocatalysts still have low intrinsic catalytic activity,and the development of high intrinsic active electrocatalysts remains a significant challenge.We use a simple and easy-to-operate pack boronizing synthesis strategy to successfully convert commercially metal sheets?such as Ni,Co,Fe,NiFe Alloy and SUS304?into efficient,stable and corrosion-resistant inexpensive electrocatalytic materials.The catalytic activity of the boronized metal sheets under alkaline conditions are about an order of magnitude higher than that of the corresponding metal sheets,while significantly enhancing their electrocatalytic stability.We reveal that these metal borides exhibit unique self-functionalized behaviour during electrocatalysis by in situ growth of ultrathin?2-5 nm?,metaborate-containing oxyhydroxide thin films on the surface as a highly active catalytic phase.In particularly,the boronized NiFe Alloy sheet exhibits an extremely high intrinsic catalytic activity?overpotential at³09 mV?,and at the same time,it can maintain stable operation for more than 3000 hours.More attractively,the boronized NiFe Alloy and SUS304 present greatly enhanced and excellent catalytic stability and corrosion resistance during electrocatalysis in 30%KOH solution at large current density which simulated commercial conditions.This work shows that pack boronizing as an effective metal boride synthesis strategy can not only improve the catalytic activity of the material,but also significantly enhance its catalytic stability and corrosion resistance.It also opens up a new dimension for the rational design of advanced catalysts.Secondly,synthesis of metal borides usually needs to conquer a high energy barrier,which generally requiring extreme conditions?high temperature,high pressure?or long reaction processes.The molten salt-assisted synthesis strategy can optimize the synthesis conditions of the metal boride.By adding molten salt,it makes the reaction possibly occur under relatively mild conditions.We successfully synthesized 11 transition metal diborides by molten salt-assisted method.The structure of a series of transition metal diborides has great similarity.Combining theoretical calculations and electrochemical measurements,we systematically studied the HER performance of 10 non-precious metal diborides which containing"borane"subunit structures.We found that the acidic HER activity of the non-precious metal diborides have a certain periodicity,e.g.,VI B>V B>IV B,which is consistent with the theoretical calculation results.At the same time,we measured the activity of a series of non-precious metal diboride catalysts under acidic and basic conditions,and determined that VB₂,MoB₂ and WB₂ are potential high-efficiency acidic electrocatalysts,while WB₂ behaves high-efficiency and stable HER performance in alkaline condition.This research has further deepened our understanding of metal diboride catalytic materials,and successfully screened potential HER electrocatalysts with excellent performance through reasonable methods.Thirdly,most electrocatalysts have lower activity in alkaline media than acidic,because it needs additional energy to dissociate water to provide more H⁺for subsequent reactions under alkaline condition.Heterogeneous metal atom doping is one of effective methods to adjust surface electronic structure,activate reaction barrier,and improve catalytic activity.We have controlled the surface electron and atomic structure of WB₂ through the synthesis strategy of Ni doping,and prepared a new HER electrocatalytic material with high intrinsic catalytic activity in alkaline media.When the current density reaches 10 mA cm^-2,the required overpotential is 98 mV and it can catalyze stably for 100 h.Compared with other metals or alloys,Ni-doped WB₂ effectively reduces the kinetic barrier of hydrolysis and promotes the dissociation adsorption rate of water.At the same time,the incorporation of Ni atoms can induce a large amount of lattice distortion,which makes the surface atoms have higher lattice energy,thereby increasing the intrinsic activity of the catalytic sites.Combined with theoretical calculations,it is shown that Ni reduces the charge transfer between B and metal atoms,which changes the local electronic structure of B-layer atom,optimizes the adsorption free energy of hydrogen,and improves the HER performance.