Paradoxical Observance of “Intrinsic” and “Geometric” Oxygen Evolution Electrocatalysis in Phase-Tuned Cobalt Oxide/Hydroxide Nanoparticles

Excellent activity of cobalt oxides/hydroxides towards electrocatalytic oxygen evolution reaction (OER) demands the development of a unified protocol to synthesize different phases of these materials. In this paper, a low-temperature (120 °C) solvothermal method was developed to synthesize pure phase Co₃O₄, Co₂(OH)₃Cl, and Co^II_1–xCo^III_x(OH)₂Cl_x·nH₂O [HT­(II,III)] and detailed analyses of their formation mechanism, intrinsic as well as geometric activity toward OER electrocatalysis, were carried out. This method employs a variety of amines to facilitate the Co^II to Co^III oxidation, which otherwise requires continuous gas (purified air/oxygen) flow or usage of oxidizing agents for reactions carried out below 150 °C. The choice of solvent (water/water–ethanol mixture) was also found to play a pivotal role in the reaction pathway, phase, and morphology of the resulting products due to changes in the pH of the reaction medium. In contrast to the geometric OER activity that follows the order HT­(II,III) > Co₂(OH)₃Cl > Co₃O₄, the intrinsic activity (from specific activity and turnover frequency) of Co₂(OH)₃Cl was found to be significantly higher compared to that of HT­(II,III). Underlying factors behind this observation were probed and found to be the (i) higher OER active site density, (ii) larger interlayer spacing, (iii) slitlike pore geometry, (iv) amorphous and turbostratic nature, and (v) lower charge-transfer resistance of HT­(II,III). While for HT­(II,III) and Co₂(OH)₃Cl, structural and morphological changes coupled with low charge-transfer resistance led to the rapid generation of active catalytic centers that promote the electrocatalytic process, high charge-transfer resistance and reluctance toward any structural change were found to be responsible for the poor OER electrocatalytic performance of Co₃O₄.