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.
ABSTRACT
This paper reports on an innovative and efficient approach based on the use of ionic liquids to govern the nano-structuration of HCoO2, in order to optimize the porosity and enhance the ionic diffusion through the electrode materials. In this work, we show that (1-pentyl-3-methyl-imidazolium bromide (PMIMBr) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4)) ionic liquids (ILs) used as templates during the synthesis orientate the nanoparticle aggregation which leads to increase of the porosity and the average pore size of the electrode material. It is also demonstrated that the ILs are strongly bonded to the HCoO2 surface, leading to surface functionalized HCoO2 materials, also called nanohybrids. This surface tailoring stabilizes the material upon cycling and shifts the oxidation potential linked to the Co(iii)/Co(iv) redox couple to lower voltage in an alkaline 5 M KOH electrolyte. The surface and porosity optimizations facilitate the ionic diffusion through the material, improve the electron transfer ability within the electrode and lead to greatly enhanced specific capacity in both alkaline 5 M-KOH and neutral 0.5 M-K2SO4 aqueous electrolytes (66.7 mA h g-1 and 47.5 mA h g-1 respectively for HCoO2-PMIMBr and HCoO2-EMIMBF4 compared to 18.1 mA h g-1 for bare HCoO2 in 5 M-KOH at 1 A g-1).