Controllable synthesis of single-layer graphene over cobalt nanoparticles and insight into active sites for efficient oxygen evolution

Controllable synthesis of graphene-coated metal nanoparticles (NPs) presents a major challenge when considering the practical application of these catalysts. Herein, we use silica as a radical sieve to grow graphene over cobalt NPs via chemical vapor deposition. As-prepared single-layer graphene-coated cobalt NPs with and without N doping (Co@N-SG and Co@SG) exhibit noticeable oxygen evolution reaction (OER) activity. Furthermore, a magnet-assisted binder-free Co@N-SG electrode illustrates much improved OER activity and stability over conventional binder-assisted counterparts, suggesting this as an effective way to overcome the recognized issues of high electron transfer resistance and poor adhesion of binder-based electrodes in practical applications. Interestingly, the graphene shell possesses varying defects and major OER benefitting active sites are found around said defects in the shell, while separately isolated Co@SG with a defect-free shell, despite exhibiting a slightly lower initial activity, illustrates a much-improved durable OER performance. The underlying Co affects the electron density of the graphene shell through dipole interaction and the electron density is optimized for adsorption of reaction intermediates, hence accelerating OER performance. This work will provide new clues to design efficient and durable electrocatalysts with further enhanced OER performance.