Synthesis And Catalytic Properties Of Cobalt Molybdate/Cobalt Nitride And Cobalt Molybdate/Cobalt Phosphide Electrocatalysts Towards Hydrogen Evolution Reaction

Hydrogen,as an efficient and clean energy carrier,provides an ideal solution to the global energy and environmental problems.Electrochemical water-splitting is a clean and simple hydrogen production technology,it involves two half reactions,namely cathode hydrogen evolution reaction（HER）and anode oxygen evolution reaction（OER）.Decreasing the overpotential of the two reactions,which means reducing the energy consumption of electrolysis is the key issue of developing electrochemical water-splitting technology.Noble metal platinum（Pt）is well known as a best-performing electrocatalyst towards the HER,but its extensive commercial applications were severely hindered by its prohibitively high cost and resource scarcity.In the past decades,researchers from various countries have been actively committed to the development of efficient noble-metal-free electrocatalytic materials,and have made positive progress on the development of material modification methods,expansion of research scope of catalytic materials,optimization of composition design and control of nanostructure.But as a whole,the existing non-precious metal catalysts still cannot meet the practical requirements in terms of the alkaline HER activity.The rational design under the guidance of lack of mechanism understanding is still a common problem that restricts the exploration of catalysts.Among these non-precious metal electrocatalysts,group VIII non-noble transition metal nitrides（TMNs）and transition metal phosphides（TMPs）have attracted special attention due to their Pt-like catalytic behavior as well as their great electrical conductivity for achieving excellent catalytic performance.Based on the current status of that research,this thesis work selects a cobalt foam（CF）-supported Co₂N/Co N/Co₂Mo₃O₈/CF and Co P₃/CoMoO_4-x heterostructured nanocatalyst as research objects,focusing on the scientific/technical issues such as design and preparation of the efficient,low-cost HER catalysts and the correlation of catalyst phase composition-structure-performance.The research has made major progress as follows.（1）The CoMoO₄·n H₂O precursor with 3D hierarchical nanostructure was grown on the cobalt foam surface by hydrothermal process,and then the precursor was nitridated to prepare Co₂N/Co N/Co₂Mo₃O₈/CF heterostructured electrocatalyst.The Co2N/Co N was dispersed on the oxygen-defective Co2Mo3O8 nanosheet substrate in the form of tiny nanoparticles.The combination of the Co2N/Co2Mo3O8 can synergistically catalyze alkaline HER.The direct growth of Co nitride nanoparticles with metal-like electrical conductivity on the Co-vacancy-rich Co₂Mo₃O₈ substrate endowed the catalyst with good electron transfer property.In addition,Co₂Mo₃O₈ substrate with nanoporous architecture could offer an abundance of accessible active sites and further improve the mass transfer performance of the catalyst.The experimental results showed that the Co₂N/Co N/Co₂Mo₃O₈/CF electrocatalyst exhibited an extraordinarily high activity and good stability towards the alkaline HER,outperforming most existing non-precious catalysts.In particular,it exhibited a comparable catalytic performance to the commercial Pt/C catalyst.For instance,It showed a small overpotential of 25 m V to reach 10 m A cm^-2 current density and an overpotential of 93 m V to afford 100?m A cm^-2 for the alkaline HER.And in the50 h of constant-current tests,the Co₂N/Co N/Co₂Mo₃O₈/CF catalyst exhibited a potential fluctuation of only±3 m V at a cathodic current density of 10 m A cm^-2.This work used a simple synthesis method to simultaneously achieve improved intrinsic activity,increased number of active sites and improved catalyst conductivity,and modifed accessibility,highlighting the importance of multi-element catalyst design for the development of high-performance catalysts（2）Comprehensive use of modification strategies such as synergistic catalytic,nano-engineering,and defect engineering,a 3D hierarchical nanostructured Co P₃/CoMoO_4-x/CF monolithic electrocatalyst supported on cobalt foam was designed and prepared using simple hydrothermal method and subsequent nitridation treatment.The combination of the Co P₃/CoMoO_4-x can synergistically catalyze alkaline HER,in which CoMoO_4-x was effective for promoting H₂O dissociation and Co P₃ was a good catalyst for the adsorption and recombination of H intermediates.Co P₃ was dispersed on the surface of the nanoporous architectured CoMoO_4-x nanosheet in the form of tiny nanoparticles with 3D hierarchical nanostructures could offer an abundance of accessible active sites and further improves the mass transfer performance of the catalyst.Additionally,the metal-like electrical conductivity of Co P₃in combination with the defective structure of oxide substrate facilitates the catalyst with good electron transfer property.The Co P₃/CoMoO_4-x/CF monolithic electrocatalyst exhibited an extraordinarily high activity and good stability towards the alkaline HER,it only requires an overpotential of 54 m V to achieve a current density of 10 m A cm^-2and the Tafel slope of Co P₃/CoMoO_4-x/CF was determined to be 52 m V·dec^-1.In the 48 h of constant-current tests,the Co P₃/CoMoO_4-x/CF catalyst exhibited a potential fluctuation of only±2 m V at a cathodic current density of 10 m A cm^-2.