Morphology and Phase-Controlled Synthesis of Defect-Rich Layered Cobalt Hydroxide Nanostructures for the Oxygen Evolution Reaction

The morphology of nanomaterials plays an important role in catalysis and other applications. Herein, we demonstrate that not only the morphology but also the crystal phase of the nanomaterial plays significant roles. A single-step synthesis of alpha- and beta-Co(OH)(2) with a unique morphology is demonstrated, and their oxygen evolution reaction (OER) activity is presented. Nanocone-like alpha-Co(OH)(2) and hexagonal plate-like beta-Co(OH)(2) nanostructures are obtained by a facile reflux method using a structure-regulating reagent to control the morphology. alpha-Co(OH)(2) exhibits a small overpotential of 307 mV, which is 50 mV lower than that of beta-Co(OH)(2) to achieve a current density of 10 mA cm(-2) for OER. Tafel slopes of 56 and 69 mV dec(-1) with electrochemical active surface areas of 25 and 3.25 cm(2) are estimated with alpha- and beta-Co(OH)(2), respectively. The superior OER activity of alpha-Co(OH)(2) nanocones is attributed to their unique morphology and oxygen vacancy defects. The overall water splitting is demonstrated in a two-electrode system in an alkaline medium using alpha-Co(OH)(2) as an OER electrocatalyst and Pt/C as a hydrogen evolution electrocatalyst. An overall voltage of 1.58 V at 10 mA cm(-2) is obtained with the two-electrode system, with excellent durability for the water-splitting application.