An investigation of chemical and electrochemical conversion of SILAR grown Mn3O4 into MnO2 thin films

Manganese oxide is an interesting material for electrochemical properties. It is well known that Mn3O4 (spinel) can be electrochemically converted to MnO2 (birnessite) via the electrochemical route during cyclic voltammetry (CV) cycling in aqueous Na2SO4 solution. Herein, the novel way is represented for the growth of highly adherent and compact Mn3O4 thin films by using successive ionic layer adsorption and reaction (SILAR) method. As grown Mn3O4 thin films are converted into MnO2 after chemical treatment by hydrochloric acid (HCl) via a disproportionate reaction. Mn3O4 thin films are converted into MnO2 by both chemical and electrochemical paths. During chemical conversion, at acidic pH, the crystal water insertion (H3O+) in Mn3O4 crystal provides the necessary driving force to transform it into MnO2 crystal. During electrochemical transformation, the specific capacitance was found to increase from 72 (1st CV cycle) to 393 F/g (1600th CV cycle). On the other hand, the specific capacitance was increased from 72 to 258 F/g through chemical transformation. Electrochemical and chemical conversion leads to similar to 5.5 and similar to 3.5 fold, respectively, improved supercapacitive performance than pristine Mn3O4 thin films. Both chemical and electrochemical conversion routes are extremely useful to recycle battery waste for supercapacitor applications.