Electrocatalysis of Oxygen Evolution by Metal Oxides through Atomically-Thin Carbon

The oxygen evolution reaction (OER) is slow at most electrocatalysts and the preparation of inexpensive, sustainable, stable, and high-performance OER electrocatalysts remains the largest obstacle in the development of new electrolyzers. Carbon-coated metal/metal oxide (nano)particles have been used in such applications but the role played by the carbon coatings has provoked debate in the field and is poorly understood. Here, we show that the coating itself acts as the active site in OER, with the underlying material activating the carbon through a charge-transfer mechanism. We demonstrate this phenomenon by encapsulating < 2 nm metal-oxide (MO x ) electrocatalyst nanoparticles within single-walled carbon nanotubes (SWNT) to form Co 3 O 4 @SWNT, RuO 2 @SWNT and IrO 2 @SWNT. Access of electrolyte to the encapsulated metal oxides is blocked by plugging the SWNTs with fullerenes, which shuts off redox processes inherent to the encapsulated metal oxide but does not affect catalytic activity, showing that the carbon surface is the active OER site. This conclusion is supported by Tafel analysis which demonstrates that the rate-determining step for the OER is the same at MO x @SWNT and pristine SWNTs but different to that at bare MO x . In-situ electrochemical Raman spectroscopy and computational analysis demonstrate that charge transfer from the carbon shell to the metal oxide occurs during the OER. This charge transfer results in a decrease in electron density on the carbon surface, which may facilitate binding of – OH to the carbon surface. This is the first step in the OER and may be key to the activity of the hybrid nanomaterials. This understanding of how carbon coatings play an active role in electrocatalytic reactions is crucial for the development of future electrocatalysts for the OER, and the proposed charge transfer-driven mechanism could be tuneable to both oxidative and reductive reactions. These insights indicate that the route to high performance, sustainable electrocatalysts may be through electronic modification of conductive carbons, potentially through an interaction with inexpensive redox-active material which could be directed towards the development of reduction or oxidation electrocatalysts.