MnO2 for Supercapacitors

Abstract

MnO2 have attracted large attention as a supercapacitor material due to its high theoretical specific capacitance, low cost, natural abundance, easy preparation and environmental friendliness. Among the various polymorphs of MnO2, the 2D layered birnessite has shown promising charge storage capability1–3. In this work, Na-doped birnessite and hausmannite were synthesized by a redox reaction between Mn2+ and MnO4 - in a NaOH solution. The resulting precipitate was aged for one or seven days. The powders were characterized by XRD, BET and SEM. Electrodes were composed of MnO2, activated carbon (AB) and PTFE with the weight ratio 75:15:10. The MnO4 -/Mn2+-ratio and the synthesis temperature were found to determine the composition (i.e. the relative amounts of birnessite and hausmannite) and the ageing time determined the crystallinity. The XRD (see Fig. 1a) shows that the material denoted Mn3O4 is very well crystallized hausmannite. MnO2-A is a birnessite with low crystallinity, while MnO2-B consists of crystalline birnessite with some hausmannite. MnO2-C is more crystalline than MnO2-B. Both MnO2-A and MnO2-B have a surface area of 42.5 m2/g. Cyclic voltammograms measured at 5 mV/s between -0.05 and 0.85 V vs. Ag/AgCl in 1M Na2SO4 are shown in Fig. 1b. The specific capacitance of MnO2-A was 169 F/g at 5 mV/s with a capacitance retention of 92.5 % at 5 mV/s after 2000 cycles at 100 mV/s. The specific capacitance of MnO2-B was found to be 118 F/g at 5 mV/s with an increase to 129 F/g at 5 mV/s (109 % of the original capacitance) after 2000 cycles at 100 mV/s. The capacitance increase could be due to dissolution and redeposition of MnO2 4. The capacitance of MnO2-A is higher than the capacitance of MnO2-B despite equal surface areas. The CV curves show interesting redox waves and peaks. The shape of the CV curve for MnO2-A resembles CV curves reported for poorly crystallized birnessite1. The redox waves were assigned to cation and proton deintercalation upon oxidation and intercalation upon reduction. The CV curve for MnO2-B exhibit pronounced redox peaks, which are previously observed for well crystallized birnessites5, 6. Two anodic peaks and two cathodic peaks were reported. These peaks were related to proton and cation intercalation and deintercalation at two different sites in the interlayer space of the birnessite5– 7. The peaks have also been attributed to redox transitions between different valence states of Mn8. The reaction mechanisms and the degradation mechanisms will be further investigated by EQCM and EIS. The amount of Na+ in the materials will be determined by ICP. The most crystalline material (MnO2-C) and Mn3O4 will be tested. In addition, the electrochemical performance of the materials in different electrolytes (Li2SO4, K2SO4, MgSO4, LiOH) will be studied.