Nucleation and Growth of Porous MnO(2) Coatings Prepared on Nickel Foam and Evaluation of Their Electrochemical Performance

Porous MnO(2) was uniformly electrodeposited on nickel foam in MnSO(4) solution, which was applied as the electrode of supercapacitors. The nucleation/growth mechanisms of porous MnO(2) were investigated firstly. Then two kinds of electrochemical measuring technologies, corresponding to the cycle voltammetry (CV) and galvanostatic charge-discharge, were adopted to assess the electrochemical performance of MnO(2) electrodes. The results demonstrated that the deposition of MnO(2) on nickel foam included four stages. Prior to the deposition, an extremely short incubation period of about 2 s was observed (the first stage). Then the exposed nickel foam was instantly covered by a large number of MnO(2) crystal nuclei and crystal nuclei connected with each other in a very short time of about 3 s (the second stage). Nucleation predominated in the second stage. The sharply rise of current was caused by the increase in substrate surface area which due to nucleation of MnO(2). Grain boundaries grew preferentially due to their high energy, accompanied with a honeycomb-like structure with the higher surface area was formed. However, accompanied with the electrochemical reactions gradually diffusion-controlled, the current presented the decline trend with increasing the time (the third stage). When the electrochemical reactions were completely diffusion-controlled, the porous MnO(2) coating with an approximately constant surface area was formed (the fourth stage). MnO(2) coatings deposited for different time (30, 60, 120, 300 s) exhibited a similar specific capacitance (CV: about 224 F/g; galvanostatic charge-discharge: about 264 F/g). Comparatively speaking, the value of MnO(2) deposited for 600 s was highest (CV: 270 F/g; galvanostatic charge-discharge: 400 F/g).