The Role of Carbon Catalyst Coatings in the Electrochemical Water Splitting Reaction

The availability of efficient electrocatalysts is key to the development of next-generation electrolyzers for water splitting to produce so-called ‘green hydrogen’ for a future hydrogen economy. The oxygen-evolution reaction (OER) is one half of the water-splitting reaction and is slow at most electrocatalysts. Designing inexpensive, sustainable, stable, and high-performance OER electrocatalysts remains the largest obstacle hindering 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 in electrocatalysis has provoked debate in the field and is poorly understood. We show here that the carbon 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 using model catalysts consisting of < 2 nm metal-oxide electrocatalyst nanoparticles encapsulated within single-walled carbon nanotubes (SWNTs). Access of electrolyte to the encapsulated metal oxides is blocked by plugging the SWNTs with fullerenes, which shuts off redox processes inherent to the metal oxide but does not affect catalytic activity, showing that the carbon surface is the active OER site. This analysis is supported by electrochemical analysis, which shows that the rate-determining step for the OER is the same at SWNT with and without encapsuled metal oxides, but different to that at bare metal oxides. In-situ electrochemical Raman spectroscopy and computational analysis demonstrate that charge transfer from the carbon host 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, the first step in the OER and the RDS (as determined by electrochemical analysis) and may be key to the activity of carbon coated electrocatalysts. This understanding of how carbon coatings play an active role in electrocatalytic reactions is crucial for the development of future low cost, sustainable electrocatalysts for the OER, and the proposed charge transfer-driven mechanism could be tuneable to both oxidative and reductive reactions.