Surface And Interface Engineering Of Transition Metal Phosphides For Electrocatalytic Application

The increasing demand and consumption of traditional energy is causing severe energy shortages and a range of environmental problems.Hydrogen energy is attracting attention as one of the most promising new energy sources.Electrolysis of water is an effective way to produce hydrogen on a large scale and consists of two half-reactions:hydrogen reaction evolution(HER)and oxygen reaction evolution(OER).However,the practical application of electrolytic water is severely hampered by the large overpotential and slow kinetics of HER and OER.Therefore,the design of highly efficient electrocatalysts and the development of anodic reactions that can occur at lower potentials to replace OER are crucial to reduce overpotentials and improve the efficiency of water splitting.Traditionally,precious metal platinum(Pt)and precious metal oxide ruthenium oxide(RuO2)are considered to be the most efficient HER and OER catalysts respectively.However,their high cost,low reserves and poor stability have hindered their commercial application.The development of low-cost,efficient and stable electrocatalysts needs to be addressed.This paper describes the use of surface and interface engineering to modulate the electronic structure of transition metal phosphides to achieve a significant improvement in their electrolytic hydrogen production,and to use their excellent performance in the urea oxidation reaction(UOR)instead of OER to reduce the reaction potential of fully dissolved water and improve the electrolysis efficiency.The main comtents are as follows:(1)In this thesis,surface-limited sulphur-doped cobalt phosphide(CoP)nanowire array structures are prepared by an ion-exchange strategy.The nanowire array structure is based on a titanium mesh with a conductive core(CoP)and a surface layer of controlled thickness containing defective structures such as doped atomic sulphur,phosphorus vacancies and amorphous domains.Excellent catalytic performance has been demonstrated in alkaline HER applications,with a current density of 100 mA cm-2 achieved with only 114 mV overpotential.Advanced experimental characterisation and theoretical calculations have demonstrated that the excellent catalytic performance is due to the increased number of active sites and the good charge transfer and transport.It is further demonstrated that the synergistic effect of doped sulphur and phosphorus vacancies leads to a change in the electronic structure of the catalyst,which in turn weakens the adsorption energy of H on the catalyst surface and enhances the adsorption energy of H2O.(2)This thesis develops a universal strategy to modulate the electronic structure of transition metal phosphide surfaces using a simple surface boronization method.Whereas boron is less electronegative than phosphorus and doping with atoms less electronegative than phosphorus has rarely been reported,one-step boronisation not only introduces boron atoms but also creates defective structures and forms amorphous regions as well as vacancies in the nanostructured surface layer of the transition metal phosphide.In the case of cobalt phosphide(CoP),a current density of 100 mA cm-2 was achieved with an overpotential of only 96 mV in alkaline HER applications,an increase of 56 mV over the unborated CoP.In order to verify the universality of the proposed method,the same boron treatment was applied to copper and iron phosphides,and the test results confirmed that the catalytic hydrogen precipitation performance was significantly improved,thus confirming the generality of the method.Using structural characterisation and theoretical calculations,it was found that the boronisation process introduces boron atoms with the formation of vacancies,resulting in a change in the electronic structure of the metal phosphide surface.It was also demonstrated that the doping of boron atoms and phosphorus vacancies together weakened the adsorption energy of H on the catalyst surface and improved the reaction kinetics,thereby enhancing the electrocatalytic performance.This study has significant implications for the development of universal strategies to enhance the electrocatalytic performance of transition metal phosphides.(3)In this thesis,interfacial sulfur-doped transition metal phosphide heterojunction nanoarray structures(S-Co2P@Ni2P/TM)were successfully prepared by a series of hydrothermal-ion exchange-water bath-phosphorization processes.Electrochemical test results show that the prepared S-Co2P@Ni2P/TM electrodes exhibit excellent bifunctional electrocatalytic activity towards HER/UOR under alkaline conditions,achieving current densities of 100 mA cm-2 at overpotentials of 103 mV and 80 mV,respectively,and excellent stability.The enhanced performance can be attributed to(ⅰ)the interfacial doping of the sulphur atom that alters the functional function of the phosphide,(ⅱ)the synergistic effect of the cobalt and nickel bimetals,and(ⅲ)the heterojunction that accelerates ion transport and increases the active site.These factors make the Gibbs free energy of S-Co2P@Ni2P/TM more towards 0 and improve the electrocatalytic activity.This work provides an experimental basis and theoretical rationale for interfacial doping of catalysts.