Preparation And Optimization Of High Surface Density Packed Manganese Dioxide Materials

Supercapacitors are promising electrochemical energy storage devices due to their fast charge/discharge rate and long cycle life,which mainly depend on the electrode materials used.Manganese dioxide（MnO₂）has been widely investigated as an electrode material.In recent years,it has been extensively studied in laboratory and a large number of good performance modified manganese dioxide have been reported.However,most studies measure its performance in thin films(less than 3 mg cm^-2)that do not represent commercial electrodes’high mass loading(more than 10 mg cm^-2)requirements.Increasing mass loading will lead to deterioration in performance.Therefore,the performance obtained in thin film is not representative of the performance under high mass loading.In order to suppress the load effect,it is very important to find out the rate-determining steps under different mass loading and optimize them.In this paper,MnO₂ electrode materials with different mass loading are prepared ongraphite paper by electrodeposition.The effects of different mass loading on electrode energy storage behavior are investigated.The results show that the rate-determining steps of electrode energy storage are different under different mass loading:surface control for low mass loading,diffusion control for medium mass loading and resistance control for high mass loading.High mass loading electrode is of great significance for commercial application,so this paper further determined the main source under high mass loading electrode resistance by combining with EIS impedance analysis,the main sources of electrode resistance are（1）solution resistance;（2）thickness of MnO₂ deposit;（3）MnO₂lamellar accumulation.In this paper,the resistance source of MnO₂ electrode under high mass loading is optimized,MnO₂ is combined with porous graphene.Under similar mass loading,the thickness of MnO₂ film is reduced,the lamellar accumulation of MnO₂ is inhibited,and the internal resistance of electrode is effectively reduced.As a result,the performance of the MnO₂ electrode which combined with graphene is greatly improved.The optimized electrode 8V-8min shows improved performance with the capacitance of 1.6 F cm^-2 at scan rate of 5 m V s^-1and 1.4 F cm^-2 of 2V-540min,and at higher scan rate of 50 m V s^-1,the capacitance improved from 0.2 F cm^-2 to 0.7 F cm^-2 at 2V-540min and 8V-8min,respectively.Finally,in order to verify the effectiveness of the electrode optimization strategy,symmetrical supercapacitors are assembled by using electrode materials before and after optimization,electrochemical tests are conducted.The results show that the capacitor with optimized capacitor 8V-8min//8V-8min can reach the energy density of 25.6 m Wh cm^-2 at the power density of 1,128.0 m W cm^-2,which is much better than the capacitor 2V-540min//2V-540min with the energy density of 3.5 m Wh cm^-2 at similar power density.