| Hydrogen production from water electrolysis is a key research direction for future hydrogen energy development,and the energy efficiency of which is controlled by anodic oxygen evolution reaction(OER).The design of high-performance electrocatalysts is crucial in overcoming the kinetic barriers of OER and reducing the energy consumption of water electrolysis.Herein,we take transition metal catalytic materials with low price as the research objects,especially focusing on metal-organic frameworks(MOFs)and layered double hydroxides(LDHs).By means of defect introduction methods,interface construction ways and element chemical doping strategies,and preceeding from composition matching optimization,morphology regulation and microstrucutre adjustment,we systematically research the structure-activity relationship of catalytic materials guided by the amount of doped elements and propose a novel method to construct heterogeneous core-shell materials with unique characteristic oxygen vacancies as well as special hybrid crystal structure.Based on the rational design of control experiments,comprehensive structural characterization,performance testings and theoretical calculation simulation,the micro-mechanism of optimizing the electronic structure by chemical doping,defect engineering and interface engineering is revealed,realizing the significant improvement of water electrolysis performance for transition metal electrocatalytic materials.(1)In view of element-controllable doping strategy,a kind of three-dimensional hydrangea-like material formed by hierarchical self-assembly of two-dimensional Ni2V-MOFs nanosheets can be grown in situ on the surface of nickel foam via a two-step method containing ultrasonic and solvothermal treatments.According to a series of control experiments and characterization results,the effects of the chemical composition,geometric morphology,and electronic structure of prepared materials on the catalytic performance are clarified:i)the special morphology design endows Ni2V-MOFs with extremely high specific surface area and increases the abundance of active sites;ii)The introduction of multivalent and highly variable V ions modulates the electronic structure of Ni species in nickel-based MOFs,forming intensive electronic coupling between Ni and V,thereby improving the electron transfer ability of Ni2V-MOFs.iii)The introduction of vanadium also narrows the energy barrier of rate-determining step in OER process and improves the adsorption of H2O and hydrogen intermediates,which enhances intrinsic catalytic activity of Ni2V-MOFs.This work might contribute to providing supplementary insight and guidance for the design of high-performance bifunctional MOFs catalysts toward water electrolysis and the exploration of catalytic reaction mechanisms.(2)To further address the problem of increased system complexity caused by the introduction of other elements via element doping strategy,we select CoFe-LDH with potential OER and HER performance as the research object,accompanied with electronic optimization of Co2+and Fe3+through introducing multivalent and highly variable V ions.CoFeV-LDH nanosheets can be grown in situ on the surface of carbon cloth by a simple one-step hydrothermal method.The descriptive relationship between the electrochemical performance of CoFeV-LDHs and the molar ratio of metal elements is established by finely quantifying the introduced amount of doping element.Relying on mathematical"limit theory",it can be further confirmed that the molar ratios of M2+:M3+(M represents metal sites)and Fe3+:V3+are the dominant factors determining the expression of catalytic activity and kinetic properties.In this particular system,V3+tending to a low valence state acts as a pivot for electronic structure optimization to expand accelerated electron transfer channels,which significantly narrows the kinetic barriers in OER and HER processes.Under the induction of V3+,Co2+becomes the secondary active center for OER process(Fe3+is considered as the primary active center),producing great contribution to OER with quantitative advantage of active sites.For the HER,Co2+with optimized electronic structure dominates in promoting the generation of hydrogen intermediates while the presence of Fe3+effectively reduces the reaction energy barrier of rate-determining step.Three metals perform their respective functions and coordinate with each other.Such the ’symbiotic relationship’ is different from pure doping or co-doping effect,which facilitates the excellent electrocatalytic performance of ternary CoFeV-LDH.This work might give an important perspective to comprehend the practical contribution of complex composition induced by elemental doping strategy in terms of water electrolysis perfromance.(3)On the basis of defect engineering and interface engineering,taking cheap iron,cobalt,and nickel transition metal elements as raw materials,a step-by-step process of hydrothermal-pyrolysis-electrodeposition is used to construct heterogeneous core-shell materials with regularly distributed oxygen vacancies and crystalline/amorphous hybrid structure,which is composed of one-dimensional nanowires(Co3O4)and two-dimensional nanosheets(NiFe-LDH).Owing to the modification effect of fluorine substitution in Co3O4 precursor,oxygen vacancies obtained during the subsequent pyrolysis process at a specific temperature reach a higher concentration and a regularized distribution,significantly optimizing the electronic structure of Co3O4 and reducing the dynamics for OER.Afterwards,electrochemical deposition with ultra-short synthesis time and mild rection conditions is beneficial to acquire NiFe-LDH shells with crystalline/amorphous hybrid structure.With the construction of hetero-interface dominated by Ni species,the enhanced interaction between Co species in Co3O4 and Fe species in NiFe-LDH accelerates the electron transfer process.Simultaneously,the interaction between abundant regularly distributed oxygen vacancies in Co3O4 and coordinatively unsaturated Fe species in the amorphous region of NiFe-LDH via electrostatic force assists in the electronic backtracking,further optimizing the electronic structure of the entire core-shell system.Finally,the enhancement of electrocatalytic activity and kinetic properties are synchronously realized.This work may provide a feasible idea for the design of special heterogeneous core-shell materials with high-performance dominated by defect engineering and interface engineering. |
