Designed And Controlled Fabrication Of Cobalt-based Compounds And Their Efficient Application In Electrochemical Energy Storage And Electrocatalytic Conversion

With the continuously growing energy requirement in the development of society,fossil fuels would be facing with exhaustion.Meanwhile,climatic change and environmental pollution associated with the use of fossil fuels become increasingly serious.There is an urgent pursuit to change the traditional energy structure and develop renewable,green and clean energy sources.The natural energies,including solar energy,wind energy,hydroenergy,tidal energy,etc.,are inexhaustible clean resources.Unfortunately,owing to the intermittency,randomness,geographic restrictions and source dispersion,they cannot provide energy sustainably.Against that backdrop,it is necessary to reserve those intermittent clean resources by energy storage system or to convert them into other clean resource by energy conversion system,for example,hydrogen energy source.At present,lithium ion battery,supercapacitor and electrocatalytic hydrogen production are deemed as the most promising energy storage and conversion systems,which have potential applications in portable electronic products,electric vehicles,aviation and aerospace,and military fields.It is worth noting that the electrode materials have an important influence on the electrochemical performance of energy storage and consersion devices.Therefore,it is significant for energy storage and conversion systems to explore the Co-based transition metal compounds with high electrochemical performance as electrode materials.Based on the above considerations,various strategies,such as element doping,morphological control,surface modification,materials compositing and nanocrystallization,have been adopt to optimize the morphology,composition,conductivity and surface characterisitic of Co-based transition metal compounds.Meanwhile,their synergetic effect is beneficial to improving the electrochemical performance.A two-dimensional layered LiFe_0.2Co_0.8O₂ nanomesh has been successfully synthesized through a facile solvothermal route,followed by the thermal solid state reaction.The layered LiFe_0.2Co_0.8O₂ nanomesh owns porous characterisitic and high specific surface area(42.8 m² g^-1),which not only enlarge the contact area between the electrolyte and electrode active materials,improving the reversible specific capacity,but also shorten the diffusion pathway of Li⁺,enhancing the rate capability.Meanwhile,doping with Fe element is in favor of increasing the electroconductivity of materials and decreasing charge transfer resistance.When applied as cathode for lithium ion battery,the LiFe_0.2Co_0.8O₂ nanomesh exhibits the remarkable lithium-storage properties,including a high discharge capacity of 174 mAh g^-1 at 0.1 C,excellent cyclability with capacity retention of 92.5%after 200 cycles and superior rate capability(the high discharge capacity of 109 mAh g^-1at large current density of 10 C).Even so,the mediocre electroconductivity and stability still restrict the practical application of LiFe_0.2Co_0.8O₂ nanomesh.Hence morphological control and surface coating methods are conducted to further ameliorate the electrochemical performance of LiFe_0.2Co_0.8O₂ materials.Then,the sandwich-like two-layer graphene@LiFe_0.2Co_0.8O₂nanoparticles have been obtained through the solvothermal route followed by the solid state reaction with glucose as carbon source.In this specific morphology,the coating graphene could effectively prevent active particles from agglomeration,improving the stability.Besides,the excellent conductivity of graphene is beneficial to decrease the charge transfer resistance and enhance the rate capability.Therefore,compared to the LiFe_0.2Co_0.8O₂ nanomesh,the sandwich-like graphene@LiFe_0.2Co_0.8O₂ nanoparticles achieve further optimized electrochemical performances in terms of the high reversible capacity(175 mAh g^-1 at 0.1 C),outstanding rate capability(the high discharge capacity of 115 mAh g^-1at 10 C),prominent stability（capacity retention of 97.2%after 250cycles）and high coulombic efficiency（97.6%）.This work puts forward a simple template-free solvothermal method for the synthesis of dandelion-like NiCo₂O₄ microspheres@nanomeshes（NCO-M@N）.In the novel morphology,numerous nanoneedles radially grow on the surface to form a dandelion-like hollow microsphere and 2D porous nanomeshes develop on the inside of the hollow cavity,thus forming a 3D hybrid structure.Importantly,a reasonable synthesis mechanism has been put forward by tracing the morphology at different reaction stage.The NCO-M@N not only possesses excellent lithium-storage properties,but also owns the remarkable pseudocapacitive performances.Therefore,it can be used as bifunctional electrode materials for lithium ion batteries and supercapacitors.When applied as an anode for lithium ion batteries,NCO-M@N displays the superior rate capability(785 mAh g^-1 at big current density of 2 A g^-1)and excellent stability（capacity retention of 88%after 100 cycles）.When used as a working electrode for supercapacitors,it achieves the high specific capacitance(2184 F g^-1),outstanding cycling performance（94.2%retention after 4000 cycles）and excellent rate capability.Most importantly,when NCO-M@N is fabricated into an asymmetric supercapacitor,it exhibits high energy density(45.3 Wh kg^-1)and power density(533.3 W kg^-1).Then,in order to further optimize the electrochemical performance of dandelion-like NCO-M@N,Fe-doping method has been adopted.And experimental results demonstrate that the energy density and power density are raised to 46.68 Wh kg^-1 and 799.7 W kg^-1.Multiple factors could contribute to the high electrochemical performance.Firstly,the special morphology offers a large specific surface area for contacting between the electrolyte and electrode active materials,thus improving the specific capacity and capacitance.Secondly,the existence of pores in the nanoneedles and nanomeshes is advantageous to ion and electron transport due to the shorter transport path,benefiting the high rate performance.Thirdly,2D porous nanomeshes develop on the inside of the hollow microsphere,which is beneficial to relieve the strain induced by the volume expansion associated with the Li⁺intercalation/deintercalation process and the redox reactions,thereby guaranteeing the long-term cycling stability.Fourthly,the unique structure could avoid“dead”volume,enhancing the availability of material.Finally,Fe-doping could offer lots of electroactive sites for fast redox reactions,thus improving the reactivity.This paper displays the successful synthesis of Co₃O₄ nanocrystals with four morphologies（nanocube,nanobelt,nanooctahedron and nanosheet）exposed with four crystal planes including{001},{110},{111}and{112}facets,respectively.We explore the facet effect on electrochemical water splitting of Co₃O₄ nanocrystals through theoretical studies and experimental results,for the first time.And then,the correlation between Co₃O₄ crystal planes and catalytic activity is established as{111}>{112}>{110}>{001}.Beside,Density-functional theory（DFT）explains Co₃O₄{111}facet owns the biggest dangling bond density of Co³⁺,highest surface energy(4.24 J m^-2),lowest OER theoretical overpotential（0.72 V）,smallest|?G_H*|（0.166 e V）and enriched charge density leading to the enhanced electrocatalytic performance.It’s clear that selectively exposure of specific crystal planes opens up a new door to obtain the catalysts with high catalytic activity for overall water splitting.This paper offers a new strategy to synthesize a series of component-controllable,sandwich-like dianion transition metal dichalcogenides CoM_2xSe_2（1-x）（M=Te,S）graphitized carbon-based composites,and this is the first time to explore the effect of different dopants（S,Te）and doping content on the electrocatalytic performances of CoSe₂ towards overall water splitting.Interestingly,the experimental results confirm that the sandwich-like CoTe_2xSe_2（1-x）graphitized carbon-based composite is more beneficial to enhancing the activity and stability towards OER process,while sandwich-like CoS_2ySe_2（1-y）graphitized carbon-based composite is more conducive to the improvement of HER performance.After optimizing the composition,sandwich-like Co(Te_0.33Se_0.67)₂ displayed an admirable OER property with an early onset potential（1.483 V）,a small Tafel slope(44 mV dec^-1),a lowη₁₀（272 mV）and a striking stability for 50 h.And sandwich-like Co(S_0.72Se_0.28)₂ exhibited a charming HER performance with the small onset overpotential of 62 mV,minimum Tafel slope of 80 mV dec^-1,lowη₁₀ of 106 mV and outstanding durability for 50 h.In consideration of the results,an alkaline electrolyzer was fabricated using Co(Te_0.33Se_0.67)₂ as anode and Co(S_0.72Se_0.28)₂as cathode.This setup requires a potential of 1.62 V to generate the 10 mA cm^-2water-splitting current density and owns prominent stability for at least 40 h electrolysis,thereby displaying a glorious electrocatalytic performance for overall water splitting.