An acetate bound cobalt oxide catalyst for water oxidation: role of monovalent anions and cations in lowering overpotential

A homogeneous solution of Co(II) in acetate buffer at pH 7 is found to be an efficient water oxidation catalyst (WOC) showing significantly greater current density than Co(II) in phosphate buffer (Co–Pi) under identical conditions owing to the higher solubility of the former. When electrodeposited on ITO/FTO electrodes it forms acetate bound cobalt(II)-oxide materials (Co-Ac-WOC) showing a catalytic current density of 0.1 mA cm−2 at 830 mV and 1 mA cm−2 at 1 V in a pH 7 buffer solution. The morphology of Co-Ac-WOC and its evolution with time and deposition potential is investigated with AFM, HR-TEM and SEM. The chemical composition of Co-Ac-WOC is investigated using XPS, EDX, ATR-FTIR and combustion analysis which indicate that this material has a CoO core with chloride and acetate anions bound to the Co center. Sodium is found to be integrated in the Co-Ac-WOC. The presence of the sodium and chloride ions lowers the onset potential for the oxygen evolution reaction (OER) by 240 mV relative to the classic Co–Pi at pH 7. The lower onset potential and higher OER current lowers the exchange current density to 10−6.7 A cm−2 in Co-Ac-WOC relative to 10−8–10−10 A cm−2 in Co–Pi and its derivatives.