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_x)-coated carbon fiber paper in electrochemical water splitting. For a uniform coating of CoO_x 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_x (ALD-CoO_x) 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^–2, which is competitive with the activity of state-of-the-art CoO_x 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_x film is dependent on two competing effects: the high oxidation state of cobalt ions in thicker CoO_x helps the oxygen evolution, whereas the introduction of a thick oxide coating decelerates the rate of charge transfer at the surface.