Manganese–cobalt geomimetic materials for supercapacitor electrode

Abstract

A manganese–cobalt asbolane material synthesized by low-temperature cationic exchange from birnessite in cobalt nitrate solution has been comprehensively characterized and tested for the first time as a massive (with high active mass loading) positive electrode material for to asymmetric aqueous supercapacitors. The structure of this Mn-rich material, which is homologous to the natural asbolanes well known by mineralogists, consists of MnO₂-type slabs with partial substitution of Co³⁺ for Mn; the slabs alternate with Co(OH)₂ islands located in the interlayer spacing. This structural arrangement was confirmed through in-depth electronic transmission microscopy analyses, which reveal two interlocking hexagonal sublattices with distinct a lattice-cell parameters but identical c parameters. The electrochemical performance of this geomimetic phase in alkaline electrolytes is highly promising, with specific capacitance of up to 180 F g⁻¹ at moderate current densities and 94 F g⁻¹ at 10 A g⁻¹. Investigation into the charge storage mechanisms indicates effective synergy between the pseudocapacitive properties of the MnO₂ slabs and the Co(OH)₂ islands, in which protonic conduction is suspected to play a key role. Additionally, long-term cycling and calendar aging tests suggest that the interlayer cobalt gradually migrates to the metal oxide layer upon cycling while maintaining excellent energy storage performance. This study clearly underscores the value of exploring geomimetic minerals as potential electrode materials for energy storage applications.