Highly Active Oxygen Evolution on Carbon Fiber Paper Coated with Atomic-Layer-Deposited Cobalt Oxide

In this work, we evaluated the oxygen evolution performance of cobalt oxide (CoO<sub><i>x</i></sub>)-coated carbon fiber paper in electrochemical water splitting. For a uniform coating of CoO<sub><i>x</i></sub> layers along the carbon fiber paper, the atomic layer deposition (ALD) technique was applied. We achieved a uniform and conformal coating of atomic-layer-deposited CoO<sub><i>x</i></sub> (ALD-CoO<sub><i>x</i></sub>) on the carbon fiber paper. The overpotential for oxygen evolution measured for the optimized ALD-coated carbon fiber paper was as low as 343 mV at 10 mA cm<sup>–2</sup>, which is competitive with the activity of state-of-the-art CoO<sub><i>x</i></sub> prepared on electrodes with large surface areas. Oxygen evolution is not enhanced after a critical thickness, about 28 nm in our study, is reached. The optimal thickness of the ALD-CoO<sub><i>x</i></sub> film is dependent on two competing effects: the high oxidation state of cobalt ions in thicker CoO<sub><i>x</i></sub> helps the oxygen evolution, whereas the introduction of a thick oxide coating decelerates the rate of charge transfer at the surface.