Study On The Preparation Of Supercapacitor Eelectrode Material MnO₂ By Electrodeposition

The nucleation and growth mechanisms of porous MnO₂ coating deposited on graphite and nickel foam in Mn SO₄solution were investigated by cyclic voltammetry,chronoamperometry and scanning electron microscopy in detail.Honeycomb-like MnO₂with very high specific area was prepared on graphite and nickel foam and the electrochemical properties of MnO₂ deposited at different times were evaluated in Na₂SO₄solution by cycle voltammetry and galvanostatic charge-discharge measurement.The results indicated that MnO₂ both deposited on graphite and nickel foam was synthesized by the following three steps:9)²⁺?9)³⁺+0)^-;9)³⁺+2₂?9)+3⁺;MnOOH?Mn₂+⁺+0)^-.The deposition process of MnO₂ which deposited on graphite was divided into four stages.Prior to nucleation,a very short incubation period?about 1.5 s?was observed,in which the current was reduced rapidly with increasing the time.The decreasing trend of the current obviously slowed down with the increasing in time due to nucleation and growth of MnO₂ in the second stage.A huge number of nuclei were formed on almost all graphite flakes by instantaneous nucleation mode,grew and connected in an exceedingly short time of about 0.5 s.Some zones?especially on the edges of flakes?composed of fine MnO₂nanocrystals?about 20 nm?will preferentially grow into coarse spherical or strip-like particles.In the third stage,the initial morphology of the graphite electrode was gradually transformed into that of the MnO₂ deposit layer due to the gaps among initial graphite flake filled with MnO₂ deposits progressively.The reduction in surface area of the electrode caused the drastic decrease in current with the time.In the fourth stage,the graphite electrode was covered completely with a thick and dense layer of MnO₂ deposits with a comparatively flat surface.The deposit layer possessed a very high electric resistance due to its large thickness and very poor conductivity of MnO₂,which significantly reduced the deposition rate of MnO₂.The deposition process was not controlled by the diffusion of active ions(Mn²⁺).As a result,the current tended to be stable and the morphology and thickness of MnO₂ layer exhibited no obvious changes.With the increase in applied potential,the duration of the second and third stages became shorter due to the higher nucleation and growth rate in a higher potential.All the MnO₂ electrodes at different deposition time?1,2,5,10 min?had almost the same specific capacitance of about 186 F/g,which indicated the specific capacitance of electrodes had no relationship with the deposition time.When nickel foam was used as the substrate of MnO₂ electrode,the deposition process could also be divided into four segments.Unlike the graphite substrate,the chronoamperometric method showed that the second stage current rose obviously when the deposition voltage was higher than 1.5 V which resulted in nucleation.The current rose in the second stage which was caused by the increase of the electrode surface area of the nucleation phenomenon.At this stage,the nickel foam skeleton had been covered with MnO₂,but many gaps still existed and the surface of the deposit had a honeycomb-like morphology.After the rapid nucleation,the nucleus began to grow and the gaps on the surface of nickel foam gradually disappeared.The villi-like surface gradually turned into a flaky network structure and the structure became more uniform and dense with time increasing.In the fourth stage,the surface of the nicke foam had been completely covered by the MnO₂.Cyclic voltammetry and galvanostatic charge discharge tests of MnO₂ electrodes with different deposition time?30,60,120,300 and 600 s?were also carried out.The specific capacitance values?CV:about 230 F/g;galvanostatic charge/discharge:about 290 F/g?.It has been found that when the foam nickel was used as the substrate of MnO₂ electrode the specific capacitance is greater.However,the cyclic voltammetry curves and galvanostatic charge discharge curves showed that the electrode with graphite substrate had better galvanic charge and discharge performance and reversible performance than that of nickel foam based electrode.