Construction Of Self-supporting Cobalt-based(Hydro) Oxides Electrodes For Electrocatalysis

Following the national dual-carbon strategic demand of"carbon peaking and carbon neutrality",the development of green energy and energy storage and conversion of high efficiency are the bottlenecks restricting the transformation of the energy structure.The electrocatalytic water splitting for hydrogen and battery devices provide effective ways to solve the above two problems.In the electrochemical system,the oxygen evolution reaction（OER）,which couples many cathodic reactions to complete the energy storage conversion,has slow kinetics due to multiple electron-proton transfer and intermediate transition states,and becomes the half-reaction with high energy consumption.The current high-efficiency oxygen evolution catalysts are noble metal-based oxides,and their scarcity and high cost restrict the popularization and application of electrocatalysis technology.Cobalt-based（hydro）oxides are a class of oxygen evolution catalysts with high catalytic activity potential,but their catalytic performance cannot meet the needs of practical applications.Therefore,this paper focuses on cobalt-based（hydrogen）oxides,and designs and constructs self-supporting hierarchical structure or multi-active-site electrodes for its slow OER kinetics,insufficient electrical conductivity,and limited catalytic active sites.Optimize and improve the intrinsic catalytic activity and reaction kinetics of materials.The specific research contents are as follows:1.Taking advantage of the oxophilicity and variable valence of the rare earth metal element Ce,which was introduced into the Co₃O₄ lattice structure by a simple hydrothermal-calcination method,and the Ce_xCo_3-xO₄ electrode with unique microflowers/nanoneedles morphology was prepared.The incorporated Ce heteroatom can effectively tune the local electronic structure of the Co metal active site to form oxygen vacancies.By changing the amount of Ce doping,the regulation of oxygen vacancy content and the hierarchical morphology of microflowers/nanoneedles can be achieved,and the latter will affect the water/air affinity/phobic properties of the surface and interface of the Ce_xCo_3-xO₄ electrode.Finally,the optimized Ce_0.12Co_2.88O₄electrode has high oxygen vacancy content（37.05%）and favorable OER surface properties of superhydrophilic/superaerophobic.The unique hierarchical structure and surface physicochemical properties of the electrode synergistically optimize the conductivity,intrinsic catalytic activity,and interfacial mass transfer kinetics of Co₃O₄.The test results show that the overpotential（282 mV）of the Ce_0.12Co_2.88O₄ electrode is70 mV lower than that of Co₃O₄,and the Tafel slope is also reduced from 103.0 mV dec^-1 to 81.4 mV dec^-1;at high current density(100 mA cm^-2),the OER activity of the Ce_0.12Co_2.88O₄ electrode is even better than that of the benchmark catalyst RuO₂.This work provides a new idea for optimizing the intrinsic catalytic activity and surface properties of metal oxides to improve OER charge transport and mass transport.2.Aiming at the insufficient in-plane site activity and electrical conductivity of CoO nanosheets,an etching-self-assembly strategy at room temperature was used to anchor CoFe/CoFe₂O₄ on the surface of CoO nanosheets to form the hierarchical morphology of nanocubes-on-nanosheets.Structured CoFe/CoFe₂O₄/CoO electrode material.The study found that CoFe/CoFe₂O₄/CoO has abundant heterointerfaces,which can effectively activate the in-plane activity of CoO nanosheets by inducing the generation of high-valence Co³⁺species through Fe–O–Co bridge on the heterointerfaces,thereby enhancing the intrinsic activity.The thickness of CoO nanosheets prepared by solvothermal method with MoO₄^2-anion intercalation is only3.8 nm,which is beneficial to expose more catalytic active sites.The coexistence of CoFe alloys with CoFe₂O₄ and CoO metal oxides and the unique nanocubes-on-nanosheets hierarchical structure increase the in-plane roughness of CoO nanosheets,endow the CoFe/CoFe₂O₄/CoO electrodes with favorable superhydrophilicity and superaerophobicity surface.Finally,the optimized CoFe/CoFe₂O₄/CoO exhibited excellent OER performance superior to the RuO₂ benchmark catalyst.At 10 mA cm^-2,the overpotential is as low as 256 mV,the Tafel slope is as low as 34.8 mV dec^-1,and the ultra-high stability is maintained for up to 110 h in the alkaline medium performance test.3.A facile and low-energy-consumption method for etching Co-MOF self-templates by MoO₄^2-anion was proposed.MoO₄^2-anion intercalation can effectively stabilize theα-Co（OH）₂ crystal phase structure with high activity,high conductivity and metastable state.The ultrathinα-Co（OH）₂ nanosheets can fully expose the catalytic active sites,and the Moirésuperlattice effect existing in the structure has a positive effect on improving the electrocatalytic performance and electrical conductivity.The optimized MoO₄^2--Co（OH）₂ electrode exhibits excellent HER and OER bifunctional activities,which can reach a current density of 10 mA cm^-2 with only 96 mV and 280mV overpotentials for HER and OER,respectively;The HER and OER overpotentials are as low as 405 mV and 526 mV to achieve industrial 500 mA cm^-2 current density.Electrolyzer assembled with MoO₄^2--Co（OH）₂ electrodes can achieve current densities of 10 mA cm^-2 and 100 mA cm^-2 with only 1.61 V and 1.90 V,and exhibits excellent durability.4.The carbon fiber membrane electrode material of Co₃Mo and CoO dual-active-sites anchored S,N co-doped carbon fiber was prepared by electrospinning technology and applied to the energy storage device of zinc-air battery.The construction of Co₃Mo and CoO dual active sites can effectively optimize the electronic structure of single CoO site and induce some Co²⁺in the structure to generate highly active Co³⁺species to improve the OER performance.By adding thiourea to the spinning solution,the doped S and N heteroatoms into the carbon material is beneficial to improve its electrical conductivity,and the coordination environment and electronic structure of the metal site are regulated,which can limit the Co₃Mo and CoO nanoparticle sizes during the pyrolysis process.Finally,the optimally prepared Co₃Mo/CoO@SNC electrode exhibits excellent ORR/OER bifunctional catalytic performance（ΔE=0.714V）.Co₃Mo/CoO@SNC exhibits higher ORR half-wave potential（0.884 V）and limiting diffusion current density(5.00 mA cm^-2),superior to noble metal Pt/C catalysts.Co₃Mo/CoO@SNC exhibited lower OER overpotential（368 mV）and Tafel slope(111.7 mV dec^-1).In addition,the open-circuit voltages of liquid and flexible all-solid-state Zn-air batteries assembled with Co₃Mo/CoO@SNC are 1.45 V and 1.34 V,and the peak power densities are 103.5 mW cm^-2 and 55.9 mW cm^-2,respectively.The discharge specific capacity of liquid zinc-air battery is 778 mAh g^-1 at 50 mA cm^-2,and the battery device still maintains good energy storage performance after a 70 h charge/discharge cycle test.