Pseudocapacitive Mn-Co mixed oxides obtained by thermal decomposition of manganese hexacyanocobaltate in presence of carbon structures

Pseudocapacitive materials are attractive for energy storage systems due to their electrochemical signature alike to that of double-layer supercapacitors, which can store a high amount of charge through fast redox processes. In this study, we explored a new route to obtain pseudocapacitive Mn-Co mixed oxides from thermal decomposition of manganese hexacyanocobaltate (III) in presence of activated carbon, carbon nanotubes, or graphene oxide. The heat treatment employed to form the Mn-Co oxides provokes the decomposition of the carbon structures. However, their presence during the treatment causes variation for the formed crystalline phases, decreases the particle size, and enhances the accessible surface and pore volume, improving the electrochemical behavior of the synthesized Mn-Co mixed oxides. Besides the easy processing, here-obtained mixed Mn-Co oxides exhibit remarkable energy storage characteristics. Mn-Co mixed oxide obtained in presence of graphene oxide reached the best performance, displaying capacitances of 209 F g⁻¹ at 0.25 A g⁻¹ and 135 F g⁻¹ at 4 A g⁻¹, in a 3 M KOH electrolyte. The stability of the material was tested in a two-electrode asymmetric supercapacitor assembled using biomass-derived activated carbon obtained by solar pyrolysis. The device retained 75% of the initial capacitance after 5000 GCD cycles (i = 2.5 A g⁻¹), with a columbic efficiency close to 100% and an energetic efficiency above 70%. Moreover, the device delivered a specific energy of 39.8 W h kg⁻¹ at a specific power of 0.2 kW kg⁻¹, and 19.1 W h kg⁻¹ at 6.6 kW kg⁻¹. The degradation of the asymmetric supercapacitor was evaluated using EIS. We identified a 4-fold increment of the ESR with a slight diminution of capacitance during the long-term cycling.