Preparation Of Cobalt-containing Hydrotalcite Composites And Their Oxygen Evolution Reaction Electrocatalytic Performance

The design and development of layered double metal hydroxide?LDH?-based electrocatalysts is a frontier issue in the current electrocatalysis field of oxygen evolution reactions?OER?.Owing to the unique layered structure,adjustable chemical components,and high catalytic activity,LDH materials have been extensively investigated widely studied among as OER electrocatalyst.This thesis is mainly focused on the design,fabrication,OER performance and mechanism of LDH-based nanocomposites.Combining the research accumulation of LDH materials over the years by previous researchers and applying two major strategies,i.e.,nanostructure engineering and effective contact of active materials and conductive substrates,LDH nanosheets with different morphologies were first obtained using the solvent and ion induction method.These nanosheets exposed more surface active sites of the layered materials and were further hybridzed with highly conductive or electrochemical active materials,leading to the formation of three novel LDH-based composites.The structure,morphology,OER performance and corresponding mechanism of the obtained composites were thoroughly investigated.The main results are research contents summarized as follows:1.A feasible one-step solvothermal approach was developed to directly grow 3DCoNi-LDH flower-like architectures onto porous and conductive Ni foam?NF??denoted asCoNi-LDH@NF hereafter?.It was found that the solvent of ethanol was indispensable for the formation of flower-like superstructure.The in situ direct growth strategy ensures an intimate contact between the electroactiveCoNi-LDHs and NF substrate,and the flower-like hierarchical architecture ofCoNi-LDHs allows the exposure of more accessible active sites and enables the efficient mass diffusion.As a consequence,the resultant self-supported and binder-free electrode ofCoNi-LDH@NF delivers an excellent OER performance with a small overpotential of 370 m V at a current density of 50 m A·cm^-2,a low Tafel slope(87 m V·dec^-1)and a long term electrochemical durability?showing no obvious change after 1000 potential cycles?in 0.1 M KOH solution,holding great promise in practical water electrolysis.2.A new composite electrocatalyst of Al³⁺-doped LDH and carbon fiber cloth?CFC?is designed by using a one-step hydrothermal method,in which CFC is used as a conductive and strong substrate and disc-shaped nanosheets of LDH are used as high-performance electrocatalysts,and their synergistics effectly improve the overall efficiency of the anode to OER.The electrocatalytic activity of Ni/Co-LDHs is improved by Al³⁺doping,because the introduction of Al³⁺ions will greatly increase the formation of low-coordinate Ni³⁺and Co³⁺.In addition,the direct growth of catalytically active LDH nanosheets on CFC substrates can avoid the use of other conductive agents and adhesives,thereby ensuring good connection and improving the conductivity and electrochemical performance of the electrode.Therefore,the formed self-supporting and binderlessCoAl-LDH@CFC self-supporting binderless electrode exhibits an excellent OER performance,having a small overpotential of 260 m V at a current density of 10 m A·cm^-2,and a good long-term electrochemical durability in 0.1 M KOH solution?no significant change after 5000 CV cycles?.3.CoMn-LDH@GNS composite is successfully synthesized by a simple two-step approach in which graphene oxide?GO?was first reduced by using glucose as a reducing agent and subjected to ultrasonic peeling to obtain graphene nanosheets?GNS?,and then the CoMn-LDH nanosheets were successfully anchored on the reduced graphene nanosheets uniformly by low saturation coprecipitation method.CoMn-LDH nanosheets grow vertically on GNS by an in-situ growth route,and show an array-like structure.Due to the synergistic effects between the catalytically active CoMn-LDH and the highly conductive GNS,the obtained CoMn-LDH@GNS composite has a large active specific surface area and a rapid electron transfer rate,and can be used as an effective OER catalyst.The OER activity of the CoMn-LDH@GNS is slightly weaker than that of Ir O₂,and the overpotential is as low as 360m V(j=10 m A·cm^-2),but it shows a better durability than Ir O₂.The novel array-like structure and preparation method of CoMn-LDH@GNS can provide insights into the design and development of efficient and green OER catalysts in the field of energy storage and conversion.