Diagonally Related and Electronically Identical 3d–4d Redox Pairs for Competing Double-Exchange/Super Exchange Interactions in Pseudocapacitive Energy Storage

Leveraging the similarities in redox potentials and ionic radii of Mn and Ru species by virtue of their exclusive diagonal relationship in the periodic table, Ru-doped K1.39Mn3O6 (KMnO2:Ru) was proposed as an electrode material, yielding rich synergistic redox chemistry for high-performance supercapacitor applications. The competing interplay between double-exchange (Mn3+/Mn4+) and super exchange (Mn3+/Ru4+) interactions among Mn and Ru species led to a notable electrochemical performance, exhibiting a specific capacitance of 475 F/g at 2 A/g within a potential window of 0 to 1.1 V (vs Hg/HgO) in a pH neutral aqueous electrolyte. The Distribution of Relaxation Times (DRT) tool was used for the analysis of time scales corresponding to the processes governing the ohmic, charge-transfer, and diffusion resistances at the electrode/electrolyte interface. A two-electrode system consisting of KMnO2:Ru as a cathode and activated carbon as an anode was fabricated, delivering a high energy density of 160 Wh/kg at a power density of 2.1 kW/kg over a potential window of 2.1 V. Density functional theory (DFT) was used to estimate the trends in quantum capacitance (CQ) and surface charge density (Q), within the potential between −1.2 V < Φ < 1.2 V, revealing that Ru doping alters the electronic structure by increasing the population density at the Fermi level, enhancing CQ and Q as a consequence of improved electronic conductivity and redox activity. This study demonstrates that Ru doping effectively enhances the charge storage properties of KMnO2, highlighting its potential as an efficient material for advanced energy storage applications.