Design And Construction Of Cobalt-based Electrocatalyst And Its Performance In Electrocatalytic Water Splitting

The sustainable development of human society on is challenged by the depleted fossil fuels and deteriorated environment,which urges the development of sustainable energy system.Hydrogen is one of promising alternative energy carriers with the advantages of low carbon emissions,wide sources and high calorific value.Water electrolysis,which contains oxygen evolution reaction（OER）and hydrogen evolution reaction（HER）as two half reactions,is a feasible way for large scale hydrogen production.Due to the high energy barrier of these two reactions,active electrocatalysts are particularly important for high energy conversion efficiency water electrolysis.At present,precious metals are the benchmark electrocatalysts for water electrolysis,that is,OER uses Ru O₂/Ir O₂and HER uses Pt/C,but the expensive price and the limited sources restrict their practical application.To solve this problem,the academic community has made many efforts to develop efficient non-precious transition metal catalysts,where the cobalt-based electrocatalysts have attracted considerable research interest due to their promising electrocatalytic performance,but there is still large room to further improve their catalytic performance to compete with the precious metal-based electrocatalysts.This thesis thus focuses on cobalt-based electrocatalysts,and seeks highly active electrocatalysts and unveils their catalytic mechanism via rational synthesis,characterizations and electrochemical evaluation of the catalytic materials.The main research work is as follows:（1）Bimetal diselenide nanorods（Co Se₂/Fe Se₂@C）with rich heterostructures were synthesized as a highly active OER electrocatalyst.This material was synthesized via a simple solvothermal method to synthesize bimetallic MOF-74-Co/Fe,which was selenized and pyrolyzed as a precursor at high temperature to directly obtain Co Se₂/Fe Se₂@C.The ratio of mixed metals and selenization temperature were optimized in term of oxygen evolution performance.A detailed structural analysis of the synthesized Co Se₂/Fe Se₂@C diclosed that rich heterojunction structure and oxygen vacancies.At a current density of 10 m A cm^-2,itneeds an overpotential of only 219 m V and exhibits a Tafel slope of 62 m V dec^-1,both better than equivalent single metal selenides and commercial Ru O₂.The remarkable improvement of OER performance is attributed to its large reactive surface area,strong electron coupling in the heterojunction structure and the synergy of different components.This work provides a feasible route for the synthesis of high performance Co-based OER catalysts with rich heterojunctions.（2）Cobalt-based layered double metal hydroxide（Co-based LDH）has wonderful application prospects in HER electrocatalysis.However,its kinetics of volmer step in alkaline media is slow,and its conductivity is low,both limit its HER activity.In order to address this problem,Co V-LDH was doped by Ru with optimal adsorption energy of hydrogen and oxygen intermediates to promote the dissociation of water.A one-step hydrothermal method was used to grow the Co VRu-LDH array structure on the nickel foam.The designed Co VRu-LDH/NF showed a significant improvement in HER activity in alkaline media with overpotential of as low as 32 m V at 10 m A cm^-2.The low Tafel slope of 36.4 m V dec^-1indicates that ruthenium improves its volmer kinetics.Experimental results further confirm improved intristic HER activity due to the Ru-doping.In addition,the oxygen evolution performance of the material also showed a certain enhancement.When being applied as a bifunctional electrocatalyst in overall water hydrolysis,only a voltage of 1.50 V is required to reach an electrolytic current density of 10 m A cm^-2.In this part,Ru doping significantly improves the HER activity of Co VRu-LDH/NF,and this work will provide insights for the further optimization of cobalt-based LDH as a bifunctional electrocatalyst.（3）A Co P/Co Mo P/NF nanosheet array catalyst was prepared by hydrothermal method and subsequent phosphating process.As-prepared Co P/Co Mo P/NF has excellent performance under alkaline conditions.At a HER current density of 10 m A cm^-2,it needs an overpotential of as low as 51 m V with a Tafel slope of 44.5 m V dec^-1.The introduction of Mo assists Co P to exert its hydrogen evolution ability in alkaline media,resulting in a significant improvement in its HER performance.This work provides a feasible way for the design of phosphide HER catalyst with improved performance.