Construction And Performance Of Molybdenum-based Electrocatalyst Derived From Polyoxometalate

Electrochemical decomposition of water is considered a promising method for the production of clean and sustainable hydrogen fuel.However,the lack of high performance and low-cost hydrogen evolution（HER）and oxygen evolution（OER）reaction electrocatalysts hinders their large-scale application.Transition metal-based materials have become an important type of HER and OER electrocatalysts because of their low-cost,rich composition and structure,and excellent electron transfer ability.Polyoxometalate（POM）is a promising catalytic component for the preparation of pre-transition metal-based materials（especially Mo and W）for electrocatalytic hydrogen and oxygen evolution reactions due to its tunable composition and diverse structure.By combining POM with metal-organic frameworks（MOFs）,the target POM/MOF will promote the dispersion of POM and increase the composition and structural complexity of the derived materials.As a new type of porous materials with adjustable structure and composition,metal-organic framework is considered as an ideal material for the synthesis of various functional materials.Based on the above discussion,the POM（NH₄）₄[（HPO₃）₂Mo₅O₁₅]·4H₂O,（NH₄）₄[Ni Mo₆O₂₄H₆]of high yield element Mo was selected as the precursor in this paper.Co Mo P,Mo S₂/Co₉S₈,Mo₃P/C catalysts were prepared by etching and other methods for electrocatalytic hydrogen and oxygen production reactions,and their electrochemical performance in acid,alkaline,PBS（neutral buffer solution）and seawater medium was investigated.The results are as follows:（1）The Strandberg-type POM（NH₄）₄[（HPO₃）₂Mo₅O₁₅]·4H₂O（P₂Mo₅）was used to etch ZIF-67 to form P₂Mo₅@ZIF-67 hybrid material,which was pyrolyzed at high temperature to obtain Co Mo P catalyst.The catalyst with the best performance was prepared by optimizing the etching time and pyrolysis temperature,and its composition and structure were analyzed by IR,XRD,SEM,TEM and XPS.Using carbon rod as the counter electrode and glassy carbon electrode loaded with Co Mo P catalyst as the working electrode,the OER performance of the catalyst was investigated.The results showed that the Co Mo P catalyst had the best electrocatalytic oxygen evolution performance when the etching time was 6 h and the pyrolysis temperature was 800℃.This was attributed to the effective binding of P₂Mo₅ with ZIF-67 and the bimetallic active sites of the Co Mo P catalyst.（2）Using Anderson-type POM（NH₄）₄[Ni Mo₆O₂₄H₆]（Ni Mo₆）as precursor,Ni Mo₆etched ZIF-67 to form Ni Mo₆@ZIF-67 hybrid material,and then sulfurized in H₂S/H₂ to prepare Mo S₂/Co₉S₈ catalyst.The etching time and curing temperature were optimized,and various characterization methods（IR,XRD,SEM,TEM,XPS）were used to analyze the material.The glassy carbon electrode supported by Mo S₂/Co₉S₈ catalyst was used as the working electrode and the carbon rod as the counter electrode.HER performance was investigated in acid,alkaline,PBS and seawater respectively.The results showed that Mo S₂/Co₉S₈-1 catalyst exhibited excellent hydrogen evolution performance.At a voltage of396 m V,the current reached 1558 m A·cm^-2,which was attributed to the marginal active site of low Gibbs free energy of Mo S₂ and the synergistic effect of Mo S₂ and Co₉S₈.（3）Using Strandberg-type POM P₂Mo₅ with different P sources（phosphite,phosphate,phenylphosphonic acid）as precursors and loaded on conductive carbon black,the Mo₃P/C catalyst was prepared by optimizing the pyrolysis temperature with a simple one-step method,which overcame the traditional complicated multi-step synthesis method.The structure and component of the catalyst were analyzed by IR,XRD,SEM,TEM,Mapping and XPS.Three different catalysts were prepared as working electrodes,with carbon rod as counter electrode.The electrochemical properties of the catalysts were investigated in alkaline,neutral and seawater media.The results showed that the catalyst Mo₃P/C-1 exhibited a higher hydrogen evolution current density(2597 m A·cm^-2)under the over-potential of 412 m V in alkaline medium.It also has good hydrogen evolution performance under neutral condition,which is better than precious metal Pt.This is attributed to the better internal electron transfer capability of Mo₃P and the superconductivity of conducting carbon black.