Research On Electrochemical Performance Of NiO Of Different Morphology,NiO/MnO₂ And CoMoO₄

Electrodes,as the key factor which can influence the electrochemical performance of energy storing devices,have been the scientific research focus.NiO is one of the most promising candidates due to its easy availability,stable physical and chemical properties,strong electroactive and cost effectiveness.Unfortunately,the real specific capacity of NiO is far below its theoretical value.In addition,serious volume effect and poor cycling stability also limit its practical applications.The morphology and dimensions of electrode materials are very important to the electrochemical properties,designing nanostructure electrodes can shorten the diffusion path of ions in electrolyte,reduce migration resistance and enhance ionic conductivity,which will finally improve the electrochemical performance.Recently,binary metal oxides have been reported to demonstrate better performance than single-component oxides due to their feasible oxidation state and relatively high electrical conductivity,CoMoO₄ has gradually been considered as promising alternative,owing to their low-cost,environment benignity and abundance.Thus,we first study the relationship between the structure and electrochemical performance of NiO.Then NiO/MnO₂ core/shell composite materials are prepared on the basis of porous net-like array NiO greatly improving the pseudocapacitive behavior and lithium ion battery performance.Also,CoMoO₄ nanoflake-assembling porous pillar array was synthesized through hydrothermal,the pseudocapacitance performance is better than the CoMoO₄ of other structures reported before.In this thesis,the following were mainly studied:Chemical bath deposition and calcination were respectively developed for the growth of NiO of Four different morphologies.The morphologies are ordered net-like structure,unordered net-like structure,stacked needle ball structure and stacked petals structure.The pseudocapacitances of the four electrodes are studied,and the ordered porous net-like array NiO showed the best performance.At the 1 A g^-1,ordered net-like array structured NiO electrode deliver the first discharge specific capacitance of 573 F g^-1,which is much higher than 230 F g^-1of stacked needle ball structured NiO and 230 F g^-1of stacked petals structured NiO.After1000 cycles,ordered net-like array structured NiO kept 538 F g^-1,higher than 435?183?160 F g^-1of the others.When the current densities was increased form 1 A g^-1to 20 A g^-1and then decreased back to 1 A g^-1,the retention rate of specific capacitance of ordered porous net-like structured NiO is 95%,while the others only showed 85%,86% and 87%.The improvement of ordered net-like array structured NiO is due to the improvement specific surface and the contact between active materials and electrolyte,which promote the Faradic reactions.The improvement of cycle and rate performance are attributed to the stability of ordered net-like structure,which ensure the integrity of structure during the long cycles.On the basis of net-like array structured NiO,the NiO/MnO₂ nanocomposite were successfully prepared through another hydrothermal method.After hydrothermal process,the MnO₂ deposited on the both sides of NiO nanosheets,.The pseudocapacitance of pure NiO and MnO₂ are greatly improved by surface coating MnO₂ to form core/shell NiO/MnO₂ structure.T The first discharge specific capacitance of NiO/MnO₂ reached 1360 F g^-1 at 2 A g^-1,and the average capacitance of NiO/MnO₂ during 1000 cycles is 1323 F g^-1which is 179% and 73%improved when compared to pure NiO and MnO₂,respectively.The rate performance is also highly improved,when the current decrease back to 1 A g^-1,the retention rate of specific capacitance of NiO/MnO₂ is 98%.In addition,the core/shell strcuture helped to improve the cycle performance.After 4000 cycles,the specific capacitance of NiO/MnO₂ only decay 8%,however,the pure NiO and pure MnO₂ decay 31% and 36% after 1000 cycles.The reson of the improvement of specific capacitance and rate performance of NiO/MnO₂ is that the core/shell structure improve the electrochemical activity.The CV curves show that NiO and MnO₂ both participate in the Faradic reactions,and the area of CV curve is enlarged after coating.The result of EIS indicated that the Rct of NiO/MnO₂ is decreased,because the synergistic effect can improve the conductivity and lower the resistance of transfer of electrons and ions.The cycle performance was better after coating,which is due to the enhanced structural strength of nanocomposite.The electrochemical performance of NiO/MnO₂ core/shell nanocomposite was evaluated as the binder-free materials for lithium-ion batteries.The NiO/MnO₂ core/shell nanocomposite exhibited high specific capacity,excellent cyclic stability and desirable rate capability.Aftercomposite,the conductivity and electrochemical activity are improved,specific surface area is enlarged,electroactive positions are increased,extraction and insertion of Li⁺ are happened more frequently,all of these promoted the improvement of specific capacity.For example at 100 m A g^-1,the first discharge specific capacity showed 1926 m Ah g^-1 and the coulombic efficiency was81%.Core/shell structure can enhance bonding strength of materials,ensuring the integrality of structure and exhibiting good cycle performance.For example,the average discharge specific capacity during 100 cycles kept at 1050 m Ah g^-1,which was twice higher than that of pure NiO electrode,and the average coulombic efficiency reached 99%,telling more outstanding charge-discharge and cycle performance of NiO/MnO₂ core/shell nanocomposite.In the rate tests,the discharge specific capacity of NiO/MnO₂ core/shell nanocomposite at 2000 m A g^-1 was twice more as high as that of pure NiO and pure MnO₂ electrodes,when the current came back to 100 m A g^-1,the retention rate was 81% during 10 cycles,which is much higher than 65% and 49% of the other two electrodes,exhibiting improved rate and cycle performance.CoMoO₄ nanoflake-assembling porous pillar array,as electrodes for supercapacitors,was synthesized through a facile hydrothermal approach.The structure of the CoMoO₄ was investigated using FESEM and HRTEM,demonstrating that CoMoO₄ nanoflakes like flower petal assemble to porous pillar.The CV curves show two pair of redox peaks,and the charge-discharge curves are also non-linear,demonstrating the pseudocapacitance nature of the synthesized CoMoO₄.A maximum areal specific capacitance of 1.92 F cm^-2 is obtained at a constant current density of 4 m A cm^-2,owing to easier diffusion of the electrolyte and larger surface contact between CoMoO₄ with the electrolyte brought about by the structure of nanoflake-assembling porous pillar as well as higher electrochemical activity induced by nanoflake itself.The charge-discharge cycle test reveals that areal specific capacitance of CoMoO₄ pillar array still reaches 0.92 F cm^-2 after 4000 cycles at 10 m A cm^-2,suggesting a relatively good stability of CoMoO₄ pillar array.EIS indicates that charge-transfer resistance increases and the electrolyte diffusion slows down slightly after 4000 cycles.