The Study On CoO_x Supercapacitor For Photoelectric Storage

With the deterioration of global environmental pollution and energy shortage,exploiting substitutes that can replace fossil fuels has become the common goal of scientists around the world.Solar energy,without regional requirements and environmental pollution,is considered as the most promising renewable energy.However,the harnessing and application of solar energy are significantly impacted by weather,which poses challenges in terms of their unpredictability and instability.This makes it challenging to connect solar energy systems to the grid for efficient electricity generation.Therefore,new energy storage methods are urgently needed to store electric charge and deliver it to the users at an appropriate time.Supercapacitors are the most viable electrochemical devices for storing energy and powering portable systems due to their high power density and excellent cyclic stability.Cobalt oxide（CoO_x）,including Co₃O₄ and CoO,has the characteristics of high theoretical specific capacity,environmental friendliness,low cost,and easy processing.Therefore,the CoO_x is promising to combine high energy density with high power density.However,the theoretical specific capacity is difficult to be converted into the actual capacity due to the poor conductivity and the structure collapse during the redox reaction.To solve the above problems,CoO_x with various morphologies and CoO_x composites have been reported.It can be concluded from the morphological comparison that CoO_x nanowires exhibit superior pseudocapacitance performances due to their numerous contact sites,which facilitate efficient ion transport and electron conduction.However,the length and diameter of CoO_x nanowires in different systems are quite different,and the overall morphology formed by stacking nanowires is also various.Therefore,the relationship between the aspect ratio of CoO_xnanowires and the capacity performance still needs to be comprehensively studied.Compositing with conductive materials is an effective strategy to enhance the energy storage capability of pseudocapacitive materials.For example,compositing with low-dimensional materials（graphene,MXene,etc.）can ameliorate the inherent electric conductivity and volume changes in redox reactions of the CoO_x.Nonetheless,these two-dimensional materials usually have a low specific capacity,and the decrease of CoO_x content will inevitably result in the decrease of the actual specific capacity of active materials.In addition,during the electrode preparation,binders and conductive additives are usually used,which will introduce excess internal resistance,which leads to the reduction of specific capacity.In this paper,self-supported CoO_x nanowires are synthesized by facile hydrothermal reaction and annealing treatment.During this process,the pH value and stability of the hydrothermal reaction were affected by changing the amount of ammonium fluoride,thus changing the morphology of the nanowire.The temperature of the hydrothermal reaction was adjusted to affect the molecular thermal motion,thus changing the aspect ratio of the nanowire.The gas atmosphere during the annealing process changes the composition and pore size distribution of nanowires by influencing the oxygen/water content.Then the structure and electrochemical properties of the obtained cobalt oxide electrodes were characterized respectively.The Co₃O₄/CoO nanowire composite was characterized as the mesoporous distribution.CoO_x electrodes deliver a high mass-specific capacity of 1425 C g^-1and can operate at a high current density of 100 A g^-1.Finally,we assembled CoO_xnanowires with commercial activated carbon electrodes into a hybrid supercapacitor with a high specific capacity of 261.8 C g^-1and a stable operating window of 1.7 V,which has advantages of high energy density,power density,and cycle stability compared to other capacitors reported recently.The hybrid supercapacitors can be made into soft-pack supercapacitors,which can store solar energy under light conditions and release electric energy when there is no light,which has a variety of practical application potential.