Effect of Synthesis Conditions, Morphology and Crystallographic structure of MnO2 on its Electrochemical Behavior

Supercapacitors are a new generation of green energy storage devices with a higher capacitance than conventional capacitors and higher output power and longer life than lithium-ion batteries. The performance of supercapacitors mainly depends on the electrode material. Manganese dioxide has emerged as a very promising electrode material due to its high theoretical specific capacitance, high electrochemical activity and environmental friendliness. However, its low conductivity limits its application in energy storage systems. To avoid the mentioned problems, various methods are applied to obtain porous electrode materials with improved electrical conductivity and specific surface area. In this respect, carbonaceous materials are the ideal choice for mixing with MnO2 due to their low electrical resistance, significant thermal stability, large specific surface area and porosity. The present study examines the relationship between the morphology and crystallographic structure of manganese dioxide obtained by different methods (hydrothermal synthesis and co-precipitation method), synthesis conditions, and their electrochemical properties as electrode materials in supercapacitors. The role of activated carbon in the composite electrode is also shown. The electrodes were prepared by spreading a slurry containing the active materials (carbon xerogel and/or MnO2), carbon black, graphite and poly vinylidene difluoride (PVDF) binder over a glass plate. The layer formed was thermally treated to improve its mechanical stability. Various asymmetric solid-state supercapacitors were assembled with MnO2 based positive electrodes, carbon xerogel negative electrodes, and Aquivion® E87-05S membrane (Solvay Specialty Polymers, previously soaked in 1M Na2SO4) as a polymer electrolyte and separator. Some parameters, such as specific capacitance, energy and power density, energy efficiency and lifetime have been calculated. Based on the obtained results, the relationship between the structure and morphology of MnO2, the composition of the composite electrode, and the capacitance characteristics of the solid-state supercapacitor were discussed. ACKNOWLEDGMENTS Acknowledgements: This research is supported by the award No DO1-286/07.10.2020 granted from by the Ministry of Education and Science of Bulgaria. Authors gratefully acknowledge.