Mediated Electrochemical Processes for Energy Conversion, and Relationship to Direct Electrocatalysis

Shannon Stahl^a

^aUniversity of Wisconsin-Madison, Department of Chemistry, 1101 University Ave, Madison, United States

Materials for Sustainable Development Conference (MATSUS)
Proceedings of nanoGe Spring Meeting 2022 (NSM22)

#ElectroCat22. Electrocatalysis for the Production of Fuels and Chemicals

Online, Spain, 2022 March 7th - 11th

Organizers: Julio Lloret Fillol and James Durrant

Invited Speaker, Shannon Stahl, presentation 299
DOI: https://doi.org/10.29363/nanoge.nsm.2022.299
Publication date: 7th February 2022

Mediated electrochemistry complements conventional electrocatalysis and introduces flexibility in catalyst design by enabling electrode-driven reactions to take place off-electrode under more conventional thermochemical conditions.[1] This talk will highlight the development and application of electron-proton transfer mediators that support redox processes relevant to fuel cells, emphasizing the oxygen reduction reaction (ORR).[2-4] Off-electrode mediated oxygen reduction proceeds effectively with non-precious-metal M-N-C (M = Fe, Co) catalysts that are similar or identical to those commonly used for direct electrocatalytic ORR. Mechanistic studies have been conducted to probe the mechanism of the thermochemical ORR with a hydroquinone mediator. In principle, this mediated ORR could proceed on the catalyst surface via the coupling of two independent half-reactions (IHR), consisting of hydroquinone oxidation and O₂ reduction, or by direct inner-sphere reaction (ISR) between O₂ and the hydroquinone. Data have been obtained that support the inner-sphere reaction pathway,[5] and these results have important implications for mediated electrochemical processes. Different mechanisms have different linear-free energy relationships (i.e., Tafel slopes) that offer potential advantages in overpotentials and/or rates of (net) electrochemical reactions.

References:

[1] Anson, C. W.; Stahl, S. S. Mediated Fuel Cells: Soluble Redox Mediators and Their Applications to Electrochemical Reduction of O2 and Oxidation of H2, Alcohols, Biomass, and Complex Fuels. Chem. Rev. 2020, 120, 3749-3786.

[2] Preger, Y.; Gerken, J. B.; Biswas, S.; Anson, C. W.; Johnson, M. R.; Root, T. W.; Stahl, S. S. Quinone-Mediated Electrochemical O2 Reduction Accessing High Power Density with an Off-Electrode Co-N/C Catalyst. Joule 2018, 2, 2722-2731.

[3] Gerken, J. B.; Stamoulis, A. G.; Suh, S.-E.; Fischer, N. D.; Kim, Y.-J.; Guzei, I. A.; Stahl, S. S. Efficient Electrochemical Synthesis of Robust, Densely Functionalized Water Soluble Quinones. Chem. Commun. 2020, 56, 1199-1202.

[4] Preger, Y.; Johnson, M. R.; Biswas, S.; Anson, C. W.; Root, T. W.; Stahl, S. S. Anthraquinone-Mediated Fuel Cell Anode with an Off-Electrode Heterogeneous Catalyst Accessing High Power Density when Paired with a Mediated Cathode. ACS Energy Lett. 2020, 5, 1407-1412.

[5] Bates, J. S.; Biswas, S.; Suh, S.-E.; Johnson, M. R.; Mondal, B.; Root, T. W.; Stahl, S. S. Chemical and Electrochemical O2 Reduction on Earth-Abundant M-N-C Catalysts and Implications for Mediated Electrolysis. J. Am. Chem. Soc. 2022 ASAP.

Acknowledgements:

This work was funded by the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences

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