Speaker
Description
The oxygen reduction reaction (ORR) is a key reaction in fuel cells and Li-air batteries. Despite this, the fundamental reaction mechanism is still not understood. Frequently, kinetics are described with Butler-Volmer type theory, assuming that the electric bias is completely translated in reducing the activation enthalpy (activation energy). However, at solid-electrolyte interfaces, the bias can also change the activation entropy in the pre-exponential factor, either due to entropic changes on the surface or in the solvent.
Separating surface coverage and interfacial electrolyte effects has been a long-standing challenge in electrocatalysis. For example, Markovic and co-workers showed that the pre-exponential factor can be bias dependent, which was related to bias dependent coverage of competing OHad for the ORR in alkaline conditions (1). On the other hand, Feliu and co-workers reported on the influence of interfacial water on the stabilization of reaction intermediates (2), such as the proposed *OOH- in acid (3).
Here, we present our results on the impact of the bias and pressure on the transition state of the ORR on platin group metal nanoparticles in Nafion-based membrane electrode assemblies (4). We demonstrate that, both, surface coverage and electrolyte effects are key for ORR kinetics. At low bias, we observe a compensation effect between an increasing activation energy and pre-exponential factor with bias, which we interpret as fingerprints of the interfacial solvation kinetics that are impacted by the formation of a charged intermediate at the surface and entropic changes in the interfacial water, consistent with our recent results (5). At higher bias, we observe Butler-Volmer type behavior, although the coverage of reaction intermediates impacts the configurational entropy, and, thus, preexponential factor, too. Finally, by varying the O2 pressure, we probe the activation volume and how it is changing depending on the bias and rate. Taken together, our results substantially advance our understanding on the ORR and, more broadly, the impact of the bias in electrocatalysis. In particular, they highlight the pressing need to go beyond current simplified kinetic models to capture the true nature of complex transition states in electrocatalysis.
- Schmidt, T. J., Stamenkovic, V., Ross, Jr., P. N. & Markovic, N. M. Temperature dependent surface electrochemistry on Pt single crystals in alkaline electrolyte, Part 3: The oxygen reduction reaction. Phys. Chem. Chem. Phys. 5, 400–406 (2003).
- Briega-Martos, V., Herrero, E. & Feliu, J. M. Effect of pH and Water Structure on the Oxygen Reduction Reaction on platinum electrodes. Electrochimica Acta 241, 497–509 (2017).
- Gómez‐Marín, A. M. & Feliu, J. M. New Insights into the Oxygen Reduction Reaction Mechanism on Pt (111): A Detailed Electrochemical Study. ChemSusChem 6, 1091–1100 (2013).
- Silva Olaya, A.R., Gisbert-Gonzalez, J. M., Druce, J., Roldan Cuenya, B. & Oener, S.Z. The Impact of Pressure and Bias on the Transition State of the Oxygen Reduction Reaction. in preparation (2024).
- Rodellar, C. G., Gisbert-Gonzalez, J. M., Sarabia, F., Roldan Cuenya, B. & Oener, S. Z. Ion solvation kinetics in bipolar membranes and at electrolyte–metal interfaces. Nat Energy (2024) doi:10.1038/s41560-024-01484-z.
