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A molecular-level mechanistic framework for interfacial proton-coupled electron transfer kinetics

Nature Chemistry volume 16, pages 343–352 (2024)Cite this article

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Abstract

Electrochemical proton-coupled electron transfer (PCET) reactions can proceed via an outer-sphere electron transfer to solution (OS-PCET) or through an inner-sphere mechanism by interfacial polarization of surface-bound active sites (I-PCET). Although OS-PCET has been extensively studied with molecular insight, the inherent heterogeneity of surfaces impedes molecular-level understanding of I-PCET. Herein we employ graphite-conjugated carboxylic acids (GC-COOH) as molecularly well-defined hosts of I-PCET to isolate the intrinsic kinetics of I-PCET. We measure I-PCET rates across the entire pH range, uncovering a V-shaped pH-dependence that lacks the pH-independent regions characteristic of OS-PCET. Accordingly, we develop a mechanistic model for I-PCET that invokes concerted PCET involving hydronium/water or water/hydroxide donor/acceptor pairs, capturing the entire dataset with only four adjustable parameters. We find that I-PCET is fourfold faster with hydronium/water than water/hydroxide, while both reactions display similarly high charge transfer coefficients, indicating late proton transfer transition states. These studies highlight the key mechanistic distinctions between I-PCET and OS-PCET, providing a framework for understanding and modelling more complex multistep I-PCET reactions critical to energy conversion and catalysis.

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Fig. 1: Differences between outer-sphere and interfacial proton-coupled electron transfer.
Fig. 2: Synthesis, structure and reactivity of GC-COOH.
Fig. 3: Representative equilibrium cyclic voltammogram of GC-COOH and pH dependence of I-PCET thermochemistry.
Fig. 4: Fast-scan-rate cyclic voltammogram of GC-COOH and representative trumpet plots for GC-COOH.
Fig. 5: Apparent rate constant as a function of pH.
Fig. 6: Fitting model to experimental data.

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Data availability

All data used to support the claims of the main text have been included with this manuscript as a .zip file. Data used to support claims discussed in the Supporting Information can be made available on reasonable request. Source Data are provided with this paper.

Code availability

All scripts used in CV simulation and data fitting are included in a compressed .zip file in the Supplementary Code section.

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Acknowledgements

We gratefully acknowledge C. Costentin for fruitful discussions. We thank the entire Surendranath laboratory for their support and scientific discussions, with particular acknowledgment towards B. Tang, N. Razdan, S. Weng, A. Chu, M. Pegis and H. Wang for their helpful discussions. This research was supported by the US Department of Energy, Office of Basic Energy Sciences, under award no. DE-SC0020973 (N.B.L. and Y.S.).

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Authors and Affiliations

  1. Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA, USA

    Noah B. Lewis, Ryan P. Bisbey & Yogesh Surendranath

  2. Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA

    Karl S. Westendorff & Yogesh Surendranath

  3. Department of Chemistry, Yale University, New Haven, CT, USA

    Alexander V. Soudackov

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Contributions

N.B.L. and Y.S. conceived the research and developed experiments. N.B.L conducted all experiments and data analysis. N.B.L, Y.S., R.P.B. and A.V.S. developed I-PCET kinetic model. N.B.L., R.P.B. and K.S.W developed scripts for data simulation and fitting. N.B.L. and Y.S. wrote the manuscript with input from all authors.

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Correspondence to Yogesh Surendranath.

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Nature Chemistry thanks Zhongbin Zhuang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Notes 1–8, Datasets 1–6, Figs. 1–15, and Tables 1 and 2.

Supplementary Data 1

A .zip file containing tabulated source data for Figs. 1–6, as well as raw CVs used to create Pourbaix Diagram in Fig. 3 and to create all Trumpet plots comprising the data in Figs. 4–6.

Supplementary Code 1

MATLAB scripts used to simulate and fit trumpet plots.

Source data

Source Data Fig. 3

Panel a: slow scan rate cyclic voltammogram of GC-COOH in pH 14. Panel b: equilibrium potential of GC-COOH I-PCET as a function of solution pH.

Source Data Fig. 4

Panel a: fast-scan-rate cyclic voltammogram of GC-COOH in pH 14. Panel b: trumpet plots for GC-COOH I-PCET at pHs 0, 7 and 14, with simulated fits.

Source Data Fig. 5

Apparent rate constant data of I-PCET at GC-COOH extracted from trumpet plots as a function of pH.

Source Data Fig. 6

Apparent rate constant data of I-PCET at GC-COOH extracted from trumpet plots as a function of pH with fits from mechanistic model for the ‘acid’ and ‘base’ reactions and their overall sum.

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Lewis, N.B., Bisbey, R.P., Westendorff, K.S. et al. A molecular-level mechanistic framework for interfacial proton-coupled electron transfer kinetics. Nat. Chem. 16, 343–352 (2024). https://doi.org/10.1038/s41557-023-01400-0

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