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 massive positive electrode material dedicated to asymmetric aqueous supercapacitors. The structure of this Mn-rich material, homologous of natural asbolanes well known from mineralogists, consists of MnO2 type slabs, with partial substitution of Co 3+ for Mn, alternating with Co(OH)2 islands located in the interlayer spacing. This structural arrangement has been confirmed through in-depth electronic transmission microscopy analyses, revealing two interlocking hexagonal sublattices with distinct a lattice cell parameter but an identical c parameter. The electrochemical performance of this geomimetic phase exhibit great promise in alkaline electrolytes, with specific capacitance up to 180 F/g at moderate current densities and 94 F/g at 10 A/g. Investigation into the charge storage mechanisms puts in evidence an effective synergy between the pseudocapacitive properties of the MnO2 slabs and the Co(OH)2 islands, in which the protonic conduction is suspected to bring a key role. Additionally, long-term cycling and calendar aging tests suggest that the interlayer cobalt gradually migrates upon cycling to the metal oxide layer while maintainng excellent energy storage performance. The study clearly underscores the value of exploring geomimetic minerals as potential electrode materials for energy storage applications.