In this article, we focused on the
comprehension of the surface
reactivity of layered β(III)-cobalt oxyhydroxide, β(III)-CoOOH,
by implementing a multiscale study associating both experimental,
surface characterization by X-ray photoemission spectroscopy (XPS)
and scanning electron microscopy and first-principles calculations.
The surface reactivity and the chemical properties of the surface
are key factors in the charge-storage mechanism, and β(III)-CoOOH
presents interesting characteristics to be used as pseudo-capacitive
electrode materials in supercapacitors thanks to its large surface
specific area (∼100 m2/gs) and its high
electronic conductivity (10–3 to 1 S cm–1). The surface reactivity (basic and redox character) of the synthesized
compounds, which consists in aggregates of particles with 60–100
nm length, has been explored from the adsorption of SO2 molecules followed by XPS analyses. A kinetic study of the reactivity
allowed us to identify three steps in the adsorption mechanism of
our β(III)-CoOOH samples. The coupling of XPS and computational
results allows us to establish a link between the surface reactivity
in the identified domains, the formation of sulfate and sulfite species,
and the cobalt Co3+ and Co4+ species of the
active sites along with the underlying electronic processes.
Nano oxides and hydroxides generate great interest as promising positive electrode materials for the development of high energy density supercapacitors. However, their usually limited ionic and electronic conductivities significantly decrease their energy storage performances when increasing the electrode's mass loading. Here, we report on a sonochemical approach to functionalize the surface of Co(OH)2 nanomaterials by EmimBF4 ionic liquid that greatly improves the stability and the electrochemical performances of high mass loading electrodes (13 mg/cm²). This surface functionalization boosts the transport properties and strongly enhances the capacity as well as the capacity retention at higher current densities compared to basic Co(OH)2 (e.g. 113.5 C/g vs. 59.2 C/g at 1 A/g). Additionally, the protective layer formed by the ionic liquid stabilizes the electrode material upon cycling in KOH aqueous electrolyte and protects the material from oxidation upon open-air storage.
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