The
composition and structure are crucial for stabilizing an appropriate
electronic configuration (unit e
g electron
for example) in high-efficiency electrocatalysts for the oxygen evolution
reaction (OER). Here, an excellent platform to investigate the roles
of the composition and structure in tuning the electron configuration
for higher OER efficiency is provided by layered perovskite oxides
with subtle variations of composition and structure (doping with 0%,
50%, and 100% cobalt in the Bi7Fe3Ti3O21). The crystal structures were analyzed by X-ray diffraction
refinement, and the electronic structures were calculated based on
X-ray absorption spectroscopy and magnetization vs temperature plots
according to the Curie–Weiss law. The results indicate that
the elongation of oxygen octahedra along the c-axis
in layered perovskite could stabilize Co ions in the intermediate
spin (IS) (t
2g)5(e
g)1 state, resulting in dramatically enhanced
electronic conductivity and absorption capacity. Subsequently, the
OER efficiency of sample with 100% Co was found to be (incredibly)
100 times higher than that of the sample with 0% Co, with the current
density increased from 0.13 to 43 mA/cm2 (1.8 V vs reversible
hydrogen electrode); the Tafel slope was reduced from 656 to 87 mV/dec;
and double-layer capacity enhanced from 174 to 4193 μF/cm2. This work reveals that both the composition and structure
should be taken into account to stabilize a suitable electronic structure
such as IS Co ions with moderate absorption and benign electronic
conductivity for high-efficiency catalysis of the OER.
The size and shape of particles influence how effectively coarse angular aggregates of ballast interact. The aim of this study was to improve the characterisation of ballast particles using a threedimensional (3D) imaging method. Various size and shape indices, such as elongation ratio, sphericity and roundness, were determined from the scanned 3D images. A modified index called 'ellipsoidness' was proposed to capture adequately the shape of the 3D particles. Variation of these indices with particle size was studied. Comparison of the 3D true sphericity and the corresponding two-dimensional sphericity indicated that the latter would underestimate sphericity. A modified approach for transforming particle size distribution to constriction size distribution is proposed by capturing the size and shape effects of particles.
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