Modification Of Electronic Structure Enhances Electrocatalyst Activity

This thesis aims to develop new and effective methods for the electronic structure modification of electrocatalysts,and reveal the roles of electron in electrochemical conversion,thus to provide new ideas to further enhance the intrinsic activity and application availability of noble metal-free electrocatalysts.In this thesis,various methods for the electronic structure modification of electrocatalysts were developed,including dilute electron doping,electron transfer over Schottky interface,introduction of strongly coupled twin vacancies,etc.,and the intrinsic activities of transition metal oxides,transition metal phosphides,transition metal borides,etc.were enhanced.In addition,the structure-performance relationships between the electronic structure and electrochemical activity of the active sites of electrocatalysts were discovered by advanced characterization techniques and theoretical computations,and the new understanding of the selective dependence of the intrinsic activity of active sites on three degrees of freedom of electron was proposed.The specific research contents of this thesis include:(1)The strongly correlated electron spin finely modulated by dilute electron doping optimizes the intrinsic ORR activity of perovskite oxides.A ferromagnetic La2CoMnO6 doped by very dilute nonmagnetic Mo6+was used as a model catalyst,which retains the original lattice ordering and ferromagnetism.This dilute Mo6+doping generates additional high-spin Mn3+(t2g3eg1)active sites and induces Co2+(eg2)-O-Mn3+(eg1)double exchange coupling,leading to five-fold increase in intrinsic ORR activity.The computation results show that the interaction between?-symmetry eg orbitals of Mn3+active sites and ?*orbitals of O2,which are of similar energy,enables an electron transfer from Mn3+(t2g3egl)to O2(3?g-[?g1,?g1,?u4]),forming a thermally favorable Mn4+-·O2-bond.Besides,the computations confirm that the dilute Mo6+doping contributes to the ORR process both thermodynamically and kinetically.(2)The introduction of a semiconductor with suitable majority-carrier type and band structure to build a metal/semiconductor Schottky junction increases the surface electron density of metal electrocatalysts,leading to an intrinsic HER activity enhancement.Metallic MoB and n-type semiconductor g-C3N4 were used as our research model.The spectroscopy characterizations show that the Schottky contact between MoB and g-C3N4 leads to an obvious charge density redistribution,in which the surface electron density of MoB is greatly increased and more Mo active sites in a reduced state are generated.Meanwhile,the MoB crystal structure without introducing additional defects guarantees efficient electron transport within bulk.The MoB/g-C3N4 Schottky electrocatalyst exhibits excellent and stable intrinsic HER activity with a 46 mV dec-1 Tafel slope and an exchange current density of 17 ?A cm-2,which is far better than those of MoB.The computations confirm that the Schottky junction greatly reduces the activation energies and kinetic barriers of hydrogen adsorption and reduction.(3)The intrinsic HER activity of Ni2P is significantly enhanced by the introduction of twin Ni/P vacancies,in which their mutual local strong coupling optimizes the electron density of surface active sites.Twin Ni/P vacancies exhibit characteristics which differ from those of traditional point defects such as heteroatom doping or monovacancies.Ni vacancies act as electron acceptors and P vacancies are electron donors,and the charge transfer between them leads to an increase in electron density of neighboring Ni atoms and a decrease for P atoms.According to the electrochemical measurement results,the Ar-plasma treated Ni2P with abundant twin Ni/P vacancies exhibits excellent and stable intrinsic HER activity with a Tafel slope of 38 mV dec-1 and an exchange current density of 42 ?A cm-2,which is far better than those of Ni2P without vacancies and Ni2P with Ni or P monovacancies.The computation results show that the strong coupling between twin Ni/P vacancies contributes to optimizing the hydrogen adsorption and reduction ability of the neighboring Ni active sites.(4)The introduction of abundant edges and topological defects induces the charge density redistribution on the surface of graphene,leading to an intrinsic CRR activity enhancement.Due to high surface fluidity and mobility of vaporous Ga catalysts at high temperatures,the CN sources are converted into defect-rich graphene tubes(DGTs)with abundant surface wrinkles during the carbonization and graphitization.The electron microscope images and spectroscopy characterizations show that the DGTs are composed of discontinuous graphitic carbon layers containing a large number of dislocations and stacking faults with a N-doping concentration of only?2 wt.%,in which edge and topological defects are the main defect types.The DGTs exhibit excellent intrinsic CRR selectivity towards CO formation with a FEco of 98.7%,and their FEco and current density remain unchanged after a continuous electrolysis of 10 h,which exceeds those of other carbon-based electrocatalysts previously reported.The computations show that the defect types covering armchair edge,zigzag edge,and heptagon-pentagon pair greatly contribute to CO2 activation,intermediate*COOH formation,and CO desorption.(5)MoSi2 comprising electron-donating silicene subsunits exhibits an anomalous HER behavior.Under continuous activation in an alkaline medium,the HER activity of MoSi2 shows a more than one order of magnitude improvement in current density from 1.1 to 21.5 mA cm-2 at 0.40 V overpotential.Meanwhile,this anomalous behavior is highly reversible.The electrochemical measurements of a series of transition metal silicides(WSi2,CrSi2,and TaSi2)were performed,and the results show that only WSi2 which has the same silicene subunits as MoSi2 exhibits a similar anomalous HER behavior.A thin amorphous shell of MoSi2 is observed after reaction,with which the chemical valence of Si remains partially oxidized.The computations show that the coverage of-OH and Si vacancy concentration at the edge of silicene subsunits of MoSi2 can influence the intrinsic HER activity of Mo active sites.Such an anomalous behavior can be explained as the synergy of HER on Mo and partial oxidation of silicene subunits.