Design,Preparation And Electrochemical Performance Of Co-based Composite For Water Oxidation Catalyst

Hydrogen is known as the most promising clean energy in the 21st century because of its renewability,high energy density,and pollution-free combustion.Electrolysis of water is one of the most effective ways to produce high purity hydrogen at low cost.Water splitting includes the hydrogen evolution reaction(HER)of the cathode and the oxygen evolution reaction(OER)of the anode.The efficiency of water splitting is greatly limited by the OER process,which involves multi proton-couple electron transfer steps.Therefore,designing and preparing efficient,stable,and cheap electrocatalysts for water oxidation is the most efficient way to improve the rate of water splitting.Cobalt(Co)based materials have received researchers' extensive attention due to their excellent OER performance.On the one hand,the metal Co has a plurality of variable valence states.The transition potential between the various oxidation states is very suitable for the progress of the OER.On the other hand,the metal Co is abundant,cheap,and stable.At present,Co-based materials mainly include oxides,hydroxides,phosphides,selenides,and sulfides etc.However,for Co-based catalysts,the substances that actually play a crucial role in the OER catalytic process have changed.In addition,the active sites and reaction mechanism of metal-doped Co-based catalysts are still controversial.The three-dimensional(3D)hierarchical Co hydroxide,two-dimensional(2D)CoFe-based layered double hydroxide(LDH)and CoFeCu alloy with dendritic structure were synthesized.The morphology and structure of these materials were studied.The OER performance of these material in alkaline solution was measured.The active sites and OER process of these materials were preliminarily investigated.The structure-activity relationship between material properties and OER catalytic activity was constructed.The main results are as follows:1.3D Co-based hierarchical materials constructed with low-dimensional(one-dimensional(1D)and 2D)substructures were successfully prepared by the hydrothermal method using hexamethylenetetramine(HMTA)and cobalt acetate as raw materials.The microstructure was analyzed by XRD,SEM,TEM and other characterization methods.The MFs-2 sample shows a 3D micro-flower structure,which is made up of 2D nanosheets.The electrochemical tests show that the MFs-2 sample has excellent OER electrocatalytic performance and long-term stability.The overpotential and Tafel slope of MFs-2 is 295 mV at the current density of 10 mA cm-2 and 66.6 mV·dec-1,respectively.The excellent OER performance of MFs-2 sample is attributed to the hierarchical structure,which combines the fast charge transfer ability of low-dimensional structures and the fast mass transfer ability of high-dimensional structures.2.Fe-doped CoFe bimetallic layered hydroxide Co1-xFex-LDH(x=0%,5%,10%,15%,and 20%)were prepared by the hydrothermal method.Through various characterization methods,the following conclusions can be drawn.The Fe element in the Co1-xFex-LDH shows a mixed valence state(+2 and+3).With the increase of Fe amount,the phase and morphology of samples did not change greatly.The specific surface area decreases with the doping of Fe compared to the Co-LDH sample.Electrochemical results show that the introduction of Fe can increase the electrochemical surface area of samples and the electron transfer rate and promote the conversion of Co2+?Co3+.Among these Co1-xFex-LDH materials,Co0.85Fe0.15-LDH exhibits the best OER catalytic activity with an overpotential of 278 mV reaching a current density of 10 mA cm-2 and a Tafel slope of 49 mV dec-1.Moreover,the catalytic activity of the Co0.85Fe0.15-LDH can be maintained after controlled potential electrocatalysis for 10 h in alkaline solution.3.The ternary CoFeCu alloy was synthesized by the electrodeposition on the nickel foam(NF).Multiple characterization methods,such as XRD,SEM,and TEM etc.,show that the CoFeCu alloy was successfully synthesized.The concentration of Cu2+has great influence on the morphology of these samples.When the amount of Cu2+is 20 mmol(concentration is 0.2 mol/L),the obtained CoFeCu-2 sample has a dendritic structure constructed with nanoparticles.Electrochelical tests show that the introduction of proper amount of Cu2+can increase the electrochemical surface area of samples and reduce the charge transfer resistance.The CoFeCu-2 sample exhibits the best OER catalytic performance with an overpotential of 202 mV at the current density of 10 mA cm-2.At the same voltage,the catalytic current density and the electrical double-layer capacitance is 7.0 and 16 times than that of CoFe sample,respectively.In addition,the synergistic effect between the three elements of Co,Fe and Cu is also beneficial to the improvement of OER performance.