Fabrication Of Ni/Co-Based Electrode Materials And Their Pseudocapacitive Performances

The global fossil fuels crisis and environmental pollution drive the rapid development of novel energy technology. Sustainable exploration and use of energy resource set higher requirements for energy storage and conversion. As a kind of novel energy storage devices, supercapacitors bridge the gap between traditional capacitors (high power output) and batteries (high energy-storage), with high charging/discharging rate, environmental benignity, high reliability and wide working temperature range. Electrode materials have a key impact on the performances of supercapacitors, and developing electrode materials with excellent properties and low cost has strategic significance. Transition metal oxides are widely studied as promising alternative electrodes for supercapacitors, this is due to the multiple oxidation states of metal ions which facilitate redox reactions and higher charge storage within the potential range of water decomposition. However, high resistance, low rate capability and poor cycling stability have highly restricted their practical applications. This work combines the in-situ synthesis method with nanostructured morphology design aiming at solving the above problems and improving the performances of pseudocapacitive electrode materials, through enhancing the conductivity of electrons and ions. The electrochemical results of the as-prepared electrode materials demonstrate excellent capacitive performances and promising prospect in practical applications.1. Anodization driven synthesis of nickel oxalate nanostructures with excellent performance for asymmetric supercapacitors.Nickel oxalate nanodiamonds directly growing on the 3D skeleton of nickel foam are fabricated by means of a facile efficient in-situ anodization approach. This is the first time to systematically investigate nickel oxalate as electrode materials for supercapacitors. The unique structure of the as-prepared electrode free of any binder and conducting agent accounts for its outstanding pseudocapacitive performances. The electrode exhibits high specific capacitance of 813.5 F g-1 and excellent cycling performance with 92.5% capacitance retention after 10000 cycles. Moreover, an asymmetric supercapacitor constructed based upon the nickel oxalate directly growing on the nickel foam (positive electrode, binder/additive-free) and activated carbon (AC, negative electrode) shows high energy density/power density and long cycling stability. Most importantly, the in-series supercapacitors could light up light-emitting diode (LED) arrays and high power LED (1 W), charge a mobile phone and even power the mobile phone for several minutes.2. Nickel oxide nanopetals decorated 3D nickel network with enhanced pseudocapacitive properties.Metal oxides possess high theoretical specific capacitance, but their pseudocapacitive properties are restricted by the poor electronic conductivity. A strategy is presented to synthesize 3D binder/conducting agent-free nickel oxide (NiO) electrode through the combination of anodization with calcination. The NiO electrode is composed of 3D conductive nickel network decorated with nanopetal-like NiO arrays. The influence of calcination temperature has been investigated, with respect to the microstructure and pseudocapacitive properties of the NiO electrodes. The NiO electrode obtained through calcination at 480℃ demonstrates great electrochemical properties, especially remarkable rate capability (82% retention of the highest value for the 25-fold enhanced current density) and cycling stability (good capacitance retention after 30000 cycles). Moreover, an asymmetric supercapacitor has been assembled using NiO as the positive electrode and activated carbon (AC) as the negative electrode. The as-prepared supercapacitor presents excellent cycling stability (91.3% retention after 10000 cycles), and could power a mini fan as well as a commercial red LED for more than 270 min.3. Facile fabrication of cobalt oxalate nanostructures with superior specific capacitance and super-long cycling stability.Transition metal oxalate materials have shown huge competitive advantages for application in supercapacitors. Nanostructured cobalt oxalate supported on cobalt foils has been facilely fabricated by anodization, and could directly serve as additive/binder-free electrodes for supercapacitors. The as-prepared cobalt oxalate electrodes present superior specific capacitance of 1269 F g-1 at the current density of 6 A g"1 in the galvanostatic charge/discharge test. Moreover, the retained capacitance is as high as 87.2% as the current density increases from 6 A g-1 to 30 A g-1. More importantly, the specific capacitance of cobalt oxalate retains 91.9% even after super-long cycling of 100000 cycles. In addition, an asymmetric supercapacitor assembled with cobalt oxalate (positive electrode) and activated carbon (negative electrode) demonstrates excellent capacitive performance with high energy density and power density.