Theoretical and Practical Approach of Soft Template Synthesis for the Preparation of MnO2 Supercapacitor Electrode

Abstract

Mesoporous manganese dioxide was successfully synthesized by alkene oxidation with permanganate in soft template medium. Four linear and three cyclic alkenes have been used as reactants to show their impact on the MnO2 particles microstructure (specially the pore volume), on their agglomeration, and hence on their electrochemical performances as positive electrode materials in supercapacitors. The reaction proceeds via an intermediate product step, a manganate(V) cyclic diester, capable of producing aggregates. Aggregate macromolecular characteristics, such as intrinsic viscosity [η], molecular weight M, aggregation number N, and radius R, depend on the alkene employed. The role of CTAP as phase-transfer agent and as templating agent for MnO2 synthesis when associated with alkene is also confirmed. This first study allowed the proposition of a structure for the aggregates (called model B in the text). The electrochemical performances of the manganese oxides were subsequently determined with aqueous, environmentally friendly K2SO4 electrolyte. Suitable performances, in terms of high specific capacitances (>150 F·g–1) are provided by oxides prepared by employing bulky cyclic or long-chain linear alkenes. Asymmetric devices AC∥MnO2 with one of the mesoporous manganese oxide as the positive electrode, obtained from the oxidation of 1-octadecene, and activated carbon (AC) as the negative electrode yield suitable energy density of 18.2 Wh·kg–1 and a power density of 0.2 kW·kg–1 for electrochemical capacitor purposes. Finally, this particular device, cycled under galvanostatic regimes, shows a high-capacitance retention (81.2% after 10000 cycles). Therefore, this particular soft template method is suitable for the preparation of supercapacitor electrodes.