Enhancement in the Charge-Transfer Kinetics of Pseudocapacitive Iridium-Doped Layered Manganese Oxide

Birnessite manganese oxide is a promising candidate as an electrode material for aqueous supercapacitors owing to its pseudocapacitance associated with fast redox processes. While manganese oxides are semiconductive, the conductivity is much lower than that of typical materials used for capacitive electrodes such as activated carbon or ruthenium oxide. In an attempt to increase the electronic conductivity of birnessite, a new solid solution phase, K(Mn_1-Ir)O₂, was synthesized, and the electrochemical charge storage capability of Ir-doped birnessite was studied in aqueous Li₂SO₄. Structural characterization revealed that the single-phase K(Mn_1-Ir)O₂ could be synthesized up to x = 0.1. An increase in the pseudocapacitive charge was observed with the increase in Ir content. In addition to the increase in the pseudocapacitive charge, an unusual change in the peak potential was observed. The peak-to-peak difference for the Mn⁴⁺/Mn³⁺ redox decreased with increasing Ir content, indicating an increase in the reversibility of the pseudocapacitive process. The decrease in peak-to-peak difference was observed only by Ir substitution and was not observed for physical mixtures of K_0.28MnO₂ and IrO₂, suggesting a strong electronic interaction between the host Mn ion and the substituting Ir ion.