Carbons and nanocarbons play an important role as electrode materials in electrochemical conversion and electroanalysis. Effective control over interfacial properties via bulk or surface modifications is critical to achieve their desired performance as cathodes or anodes, and there is therefore significant interest in developing materials design principles to exploit their full potential in electrocatalysis. Modification via heteroatoms has emerged as a route to impart activity in important reactions such as the oxygen reduction reaction (ORR); nitrogenation in particular has been shown to both improve the activity of carbon-supported metal nanoparticles and to impart intrinsic electrocatalytic activity in the ORR. In this report we discuss our progress on the use of thin film electrodes as model systems to understand structure-activity relationships in carbon electrocatalysts. Reactive sputter deposition and thermal treatments were used to prepare smooth electrodes with controlled N/C concentration and type of nitrogen site to study the effect of N-site chemistry. These were used to study the electrocatalysis of the ORR over a wide pH range: results strongly indicate that both pyridinic and graphitic N-sites play important roles, while becoming the dominant site depending on the pH region investigated. We observed for the first time an inversion behaviour in N-doped catalysts at pH = 5 that is diagnostic of the role of N-site protonation under acidic conditions. Finally, in alkaline solutions, we discuss experimental and computational studies towards untangling cooperative effects of pyridinic and graphitic N-sites, demonstrating that a careful interplay of these is critical for ORR selectivity.

Thin film carbon electrodes via physical deposition as model systems for understanding electrocatalysis at metal-free carbon

ACS Spring Meeting 2021

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