Synthesis And Electrocatalytic Performance Of Cobalt Phosphide And Cobalt Sulfide Composites

Transition metal phosphide and sulfide with good electrocatalytic oxygen evolution activity are considered as potential materials to replace precious metal electrocatalysts.However,it has been found that transition metal phosphide and sulfide are easily oxidized during the OER process,which reduces their catalytic activity.This dissertation focuses on the design and construction of transition metal phosphide and sulfide composites to improve the electrocatalytic OER performance and stability by integrating with carbon materials,stable metal oxides,and highly active HER catalysts.The details are as follows:Based on the problem that the cobalt phosphide is easily oxidized during the OER process,the 1D coordination polymer nanowire[Co（C₄H₇NO₄）]·x H₂O（Co-Asp,Asp=L-aspartic acid）was used as precursor and template to prepare CoP/N-doped carbon（CoP/NC）nanowire via phosphidation.Co-Asp acts as cobalt source,nitrogen source and carbon source at the same time,the nitrogen source and the carbon source can be directly converted into a nitrogen-doped carbon layer and coated with CoP nanoparticles during the phosphating process.This structure slows the corrosion of the CoP nanoparticles in the alkaline electrolyte and increases the conductivity of the catalyst.Electrochemical tests indicated that the CoP/NC-400 nanowire has excellent OER catalytic performance.The overpotential required to reach a current density of 10 m A cm^-2 is 320 m V with a minimum Tafel slope of 89 m V dec^-1.In addition,the CoP/NC-400 nanowire retain the morphology of the one-dimensional Co-Asp nanowire,and the CoP nanocrystals in the CoP/NC-400 nanowire are evenly distributed in the whole N-doped carbon nanowire,which makes the catalyst have a large specific surface area and more exposed active sites.Therefore,the synthesized CoP/NC-400 nanowire catalyst shows good OER performance.Based on the problem of the instability of the cobalt sulfide in alkaline solution,the Co-Asp was used as precursor to prepare hollow Co₃S₄@NiMoO₄ nanotube.Firstly,Co₄S₃ nanotube was prepared by anion exchange between Co-Asp nanowire and sodium sulfide under solvothermal condition.Then,Co₄S₃ nanotube was reacted with nickel acetate and sodium molybdate,NiMoO₄ nanosheets were grown directly on the surface of Co₄S₃ nanotube and Co₄S₃ was converted to Co₃S₄ in further hydrothermal reaction.Hollow Co₃S₄@NiMoO₄ nanotube exhibits good electrocatalytic OER performance in1.0 M KOH.The overpotential is only 320 m V at current density of 10 m A·cm^-2 and Tafel slope of 102 m V dec^-1.The Co₃S₄@NiMoO₄ nanotube showed good OER performance for the following reasons:（1）The hollow structure of Co₃S₄@NiMoO₄ allowed the electrolyte to freely diffuse and shortened the electron transport path;（2）NiMoO₄nanosheets increase the specific surface area and expose more active sites;（3）The deposition of NiMoO₄ nanosheets on the surface of Co₃S₄ nanotube can effectively retard the oxidation process,making Co₃S₄@NiMoO₄ nanotube has a stronger stability.Considering the problem that the cobalt sulfide is easily oxidized in the electrolyte and poor HER performance,combining the cobalt sulfide with a HER active material not only improve the stability of the cobalt sulfide but also construct bifunctional catalyst.The hollow Co₄S₃ nanotube severs as the substrate to react with sodium molybdate and thioacetamide（TAA）under hydrothermal reaction.MoS₂ nanosheets and Mo O₃ cubes uniformly grow on the surface of Co₄S₃ nanotube and Co₄S₃ is transformed into Co₃S₄and Co S₂ composites.Finally,hollow Co S_x@MoS₂-Mo O₃ nanotube was annealed under inert gas.Co S_x@MoS₂-Mo O₃ nanotube as a bifunctional electrocataly exhibited excellent performance with a low overpotential of 222 m V at current density of 10 m A·cm^-2 toward HER in 0.5 M H₂SO₄ and an overpotential of 340 m V at current density of 10 m A·cm^-2for OER in 1.0 KOH,respectively.