Research On The Performance Of Cobalt-molybdenum Oxides-based Heterostructures As Energy Storage And Conversion Electrode Materials

As binary transition metal oxides,cobalt-molybdenum oxides have broad application prospects in the field of energy storage and conversion due to the abundant natural reserves and active chemical properties.In the area of energy storage,cobalt-molybdenum oxide is a kind of lithium ion battery material with high theoretical capacity,but the large volume expansion rate could lead to the destruction of the electrode structure,resulting in the macroscopic powder phenomenon.In addition,when applied in the area of energy conversion,cobalt-molybdenum oxides have both high catalytic activity of cobalt-based oxides and high conductivity of molybdenum-base oxides,so they are more suitable to be used as catalyst materials for electrocatalytic decomposition of water than mono transition metal oxides.However,there still exist some issues for cobalt-molybdenum oxides as catalyst materials,such as low catalytic activity,less active sites,low inherent conductivity and serious coarsening of structure during long-term charge and discharge.Aiming at the problems in the above two areas,appropriate micron/nanostructure design of cobalt-molybdenum oxides was carried out,and heterostructures with other substances on the basis of micro/nanostructure design were performed to obtain synergistic enhancement effect.The main contents are divided into three parts:（1）Carbon coated and sulfur modified cobalt-molybdenum oxides were used as anode materials with high specific capacity of lithium ion batteries.Lithium-ion battery is an important energy storage device.Herein,CoMoO₄and polydopamine were synthesized to form a precursor,and the precursor was then carbonized and vulcanized to obtain a sulfur-modified heterostructure electrode material（CoMoO₃/C-S）.As a good conductive network,the carbon layer can not only promote the charge transfer,but also serve as a rigid supporting structure to ensure the stability.Surface modification of sulfur not only produces a large number of ultra-thin MoS₂nanosheets,but also provides a certain electrolysis region,which is helpful for lithium storage.Due to the synergistic effect of the various components,the sulfur-modified CoMoO₃/C-S electrode is a highly efficient anode material for lithium-ion batteries.In addition,the capacity of the electrode will be further improved,and the capacity retention rate of the electrode is still 99.7%after 500cycles.This work shows that carbon coating and sulfur modification are effective means to optimize the structure of cobalt-molybdenum oxides,and it also reveals that more active sites can be formed through microstructural reconstruction to improve the capacity.（2）The synergistic effect of cobalt-molybdenum/rGO/Ag on the catalytic stability of oxygen evolution reaction（OER）Watter splitting is an efficient way of energy conversion.To improve the OER（half reaction of watter splitting）catalytic performance of cobalt-molybdenum oxides,the flower-shaped CoMoO₃nanosheets and the reduced graphite oxide（rGO）heterostructure modified by Ag nanoparticles（CoMoO₃/rGO@Ag）was synthesized.The outer rGO envelope increases the electron transport rate and ensures the structural integrity of the inner CoMoO₃nanosheets.Ag nanoparticles are distributed uniformly on the surface of CoMoO₃and rGO hybrid structures as the auxiliary phase,which makes the materials have a rich multiphase interface and can provide more active sites,significantly improving the catalytic activity.By adding rGO and adjusting the content of Ag,the synergistic enhancement between the heterogeneous components in the optimized CoMoO₃/rGO@Ag catalyst was achieved,resulting in an overpotential of only 209 m V at 10 m A cm^-2and excellent cycle durability.In addition,two identical CoMoO₃/rGO@Ag electrodes were matched and used for total water decomposition.The device exhibited excellent cycle stability,and the operating potential of the device increased by only 4 m V from 2 h to 12 h.These results indicate that the heterostructure possesses fast reaction kinetics and stable electrocatalytic ability.（3）Introducing oxygen vacancies into the heterostructure of cobalt-molybdenum oxide/Cu to reduce the initial OER overpotentialBased on the fact that oxygen vacancy can change the electronic structure and improve the OER performance,a large number of oxygen vacancies were introduced on the surface of cobalt-molybdenum oxides by heating treatment and reduction methods in this work.Meanwhile,Cu nanoparticles were selected as the auxiliary phase and reducing agent.By controlling the temperature to the critical condition for the reduction of CoMoO₄,a cobalt-molybdenum oxide-based heterostructure（CoMoO₄/rGO@Cu）with open structure,high conductivity and high oxygen vacancy was synthesized.Compared with commercial RuO₂and other similar OER catalysts,the initial overpotential of the catalyst is very low,which is only 158 m V at 10 m A cm^-2.The high catalytic activity of CoMoO₄/rGO@Cu is mainly attributed to the high concentration of active sites provided by a large number of heterogeneous interfaces and the regulation effect of sufficient oxygen vacancy on the electronic structure.