Atom-by-atom electrodeposition of single isolated cobalt oxide molecules and clusters for studying the oxygen evolution reaction

We report an electrodeposition protocol for preparing isolated cobalt oxide single molecules (Co1Ox) and clusters (ConOy) on a carbon fiber nanoelectrode. The as-prepared deposits are able to produce well-defined steady-state voltammograms for the oxygen evolution reaction (OER) in alkaline media, where the equivalent radius (r(d)) is estimated by the limiting current of hydroxide oxidation in accordance with the electrocatalytic amplification model. The size of isolated clusters obtained from the femtomolar Co2+ solution through an atom-by-atom technique can reach as small as 0.21 nm (r(d)) which is approximately the length of Co-O bond in cobalt oxide. Therefore, the deposit was close to that of a Co1Ox single molecule with only one cobalt ion, the minimum unit of the cobalt-based oxygen-evolving catalyst. Additionally, the size-dependent catalysis of the OER on ConOy deposits shows a faster relative rate on the smaller cluster in terms of the potential at a given current density, implying the single molecular catalyst shows a superior OER activity.