Correlating Thickness and Phase of Single Co(OH)₂ Micro-Platelets to the Intrinsic Activity of Oxygen Evolution...

Morphology, crystal phase, and its transformation are important structures that frequently determine electrocatalytic activity, but the correlations of intrinsic activity with them are not completely understood. Herein, using Co(OH)₂ micro-platelets with well-defined structures (phase, thickness, area, and volume) as model electrocatalysts of oxygen evolution reaction, multiple in situ microscopy is combined to correlate the electrocatalytic activity with morphology, phase, and its transformation. Single-entity morphology and electrochemistry characterized by atomic force microscopy and scanning electrochemical cell microscopy reveal a thickness-dependent turnover frequency (TOF) of α-Co(OH)₂. The TOF (≈9.5 s^-1) of α-Co(OH)₂ with ≈14 nm thickness is ≈95-fold higher than that (≈0.1 s^-1) with ≈80 nm. Moreover, this thickness-dependent activity has a critical thickness of ≈30 nm, above which no thickness-dependence is observed. Contrarily, β-Co(OH)₂ reveals a lower TOF (≈0.1 s^-1) having no significant correlation with thickness. Combining single-entity electrochemistry with in situ Raman microspectroscopy, this thickness-dependent activity is explained by more reversible Co³⁺/Co²⁺ kinetics and larger ratio of active Co sites of thinner α-Co(OH)₂, accompanied with faster phase transformation and more extensive surface restructuration. The findings highlight the interactions among thickness, ratio of active sites, kinetics of active sites, and phase transformation, and offer new insights into structure-activity relationships at single-entity level.