Enhanced supercapacitive properties of hydrohausmannite by in-situ polymerization of polypyrrole

Abstract

The main limitations of manganese oxides as supercapacitor electrodes in negative potential (-0.5~0.5 vs saturated calomel electrode (SCE)/V) are irreversible redox reaction, phase transformation and low conductivity. Phase transformation to birnessite induced by irreversible Na+ intercalation was found in hydrohausmannite (abbreviated as MO) electrode, accompanied by Mn2+ dissolution and structural collapse. Polypyrrole (PPy) film was synthesized on the surface of MO by a simple and timesaving in-situ polymerization method with the assistance of H+ and Mn4+. The PPy/MO electrode shows a promoted specific capacitance (183 F g−1 at 0.5 A g−1), enhanced rate capability (91 F g−1 at 10 A g−1, about 10 times of MO), and excellent cycling stability (93.7% capacitance retention after 1000 cycles at 10 A g−1). The performance enhancement mechanism after coated by PPy film is investigated. Firstly, PPy films with appropriate polymerization time (or thickness) can allow H+ to pass and block Na+, so the irreversible phase transformation of MO is hindered and the structure and morphology of electrode is protected. Secondly, PPy film can also prevent Mn2+ from dissolving into electrolyte, so the disproportionation reaction is suppressed and cycling stability is promoted. Thirdly, PPy film can improve the conductivity of the electrode, which greatly accelerates the charge transfer process and enhances the rate capability. Fourthly, besides cations (H+ and Na+), anions (SO42− and P-toluene sulfonic (p-TS−)) also participate in energy storage process after coated by PPy, so the specific capacitance is also promoted.