Preparation And Oxygen Evolution Performance Of Cobalt-based Electrocatalysts

Hydrogen,as a kind of high-density energy carrier,has attracted much attention due to its characteristics of clean,pollution-free and wide sources,etc.Photo-electrochemical and electrochemical water splitting are regarded as the most promising techniques to generate hydrogen sources,because they can provide high-purity H₂ and O₂ without any other products.Compared with photo-electrochemical water splitting,electrochemical water splitting has attracted more attention due to its higher flexibility and practicability.Generally,electrochemical overall water splitting process involves two half reactions:the cathodic hydrogen evolution reaction?HER?and the anodic oxygen evolution reaction?OER?.And the anodic OER has the sluggish kinetics since its complex electrode reaction accompanying four electrons transfer.As a result,the overall water splitting rate is significantly restricted by OER process.Consequently,the preparation of highly efficient and stable non-noble electrocatalysts with a lower OER overpotential is an effective way to reduce the energy consumption and accelerate the rate of the overall water splitting.Therefore,in this paper,a series of cobalt-based oxide,phosphide and sulfide OER electrocatalysts were synthesized by doping,heterostructure construction,which can optimize the intrinsic activity of each component and increase the active sites of electrode materials.The OER performances of these prepared electrocatalysts were systematically studied.The details are as follows:1.Co₃O₄/Fe₂O₃ nanosheets supported on nickel foam?NF??Co₃O₄/Fe₂O₃@NF?were prepared employing hydrothermal method and subsequent oxidation process.The as-prepared Co₃O₄/Fe₂O₃@NF presents a unique thin-walled cell structure,which can be directly used as a working electrode and demonstrates excellent electrocatalytic activity for OER in 1.0 M KOH.The overpotential is 254 mV at the 10 mA cm^-2 current density,the Tafel slope is only 33 mV dec^-1,and the electrode also demonstrates good stability of 24 h at the current density of 20 mA cm^-2.Compared to pristine Co₃O₄/Fe₂O₃(?₁₀=493 mV),Co₃O₄@NF(?₁₀=350 mV)and Fe₂O₃@NF(?₁₀=378mV),the Co₃O₄/Fe₂O₃@NF electrode displays obviously enhanced electrocatalytic performance.2.The Fe-doped cobalt phosphate nanosheets directly grown on NF substrate is fabricated?denoted as Fe-Co₃?PO₄?₂/NF?through hydrothermal process and subsequent phosphorization at low temperature.Compared to pristine Co₃?PO₄?₂/NF,Fe doping obviously improves the catalytic performance of the material.The results display the Fe-Co₃?PO₄?₂/NF has a lower OER overpotential of 274 mV at a current density of 10mA cm^-2 in 1.0 M KOH and a smaller Tafel slope of 33.9 mV dec^-1,which is far lower than the overpotential of 331 mV and the Tafel slope of 56.7 mV dec^-1 of the Co₃?PO₄?₂/NF.The Fe-Co₃?PO₄?₂/NF also displays a remarkable stability of 24 h at a current density of 45 mA cm^-2 for the OER in 1.0 M KOH electrolyte,indicating excellent catalytic performance and stability.It can conclude that Fe doping can effectively improve the performance of catalyst,providing a useful strategy to further improve the performance of the non-noble metal catalysts.3.The Co₃?PO₄?₂-Ni₂P-Ni₃S₂@NF composite material was prepared by a hydrothermal process and low-temperature phosphorization method.A series of Co₃?PO₄?₂-Ni₂P-Ni₃S₂@NF-x catalysts were prepared by adjusting the amount of thiourea in reaction process.The influence of doping amount of S on material performance was investigated systematically.The materials as the oxygen evolution electrocatalyst show good catalytic activity,indicating appropriate amount of thiourea was added in reactants for S element doping can significantly improves the performance of the composite catalyst.The optimized Co₃?PO₄?₂-Ni₂P-Ni₃S₂@NF-4 electrode shows the overpotentials of 306 mV(?₁₀=306 mV)and 331 mV(?₂₀=331 mV)at the current density of 10 mA cm^-2 and 20 mA cm^-2 in 1.0 M KOH respectively,and the Tafel slope is 76.8 mV dec^-1.Moreover,there is only 11 mV decay of overpotential at the current density of 20 mA cm^-2 after the continuous test of 12 h at the current density of 10 mA cm^-2,showing a good electrocatalytic OER activity.