Nucleation and Growth of Porous MnO₂ Coatings Prepared on Nickel Foam and Evaluation of Their Electrochemical Performance.

Porous MnO₂ was uniformly electrodeposited on nickel foam in MnSO₄ solution, which was applied as the electrode of supercapacitors. The nucleation/growth mechanisms of porous MnO₂ 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 MnO₂ electrodes. The results demonstrated that the deposition of MnO₂ 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 MnO₂ 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 MnO₂. 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 MnO₂ coating with an approximately constant surface area was formed (the fourth stage). MnO₂ 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 MnO₂ deposited for 600 s was highest (CV: 270 F/g; galvanostatic charge-discharge: 400 F/g).