Understanding the apparent fractional charge of protons in the aqueous electrochemical double layer

Creators: Chen, Leanne D.; Bajdich, Michal; Martirez, J. Mark P.; Krauter, Caroline M.; Gauthier, Joseph A.; Carter, Emily A.; Luntz, Alan C.; Chan, Karen; Nørskov, Jens K.

Abstract

A detailed atomic-scale description of the electrochemical interface is essential to the understanding of electrochemical energy transformations. In this work, we investigate the charge of solvated protons at the Pt(111) | H_2O and Al(111) | H_2O interfaces. Using semi-local density-functional theory as well as hybrid functionals and embedded correlated wavefunction methods as higher-level benchmarks, we show that the effective charge of a solvated proton in the electrochemical double layer or outer Helmholtz plane at all levels of theory is fractional, when the solvated proton and solvent band edges are aligned correctly with the Fermi level of the metal (E_F). The observed fractional charge in the absence of frontier band misalignment arises from a significant overlap between the proton and the electron density from the metal surface, and results in an energetic difference between protons in bulk solution and those in the outer Helmholtz plane.

Additional Information

© The Author(s) 2018. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Received 06 September 2017; Accepted 11 May 2018; Published 10 August 2018. We gratefully acknowledge support from the Office of Basic Energy Sciences of the U.S. Department of Energy to the SUNCAT Center for Interface Science and Catalysis. E.A.C. would like to thank the Air Force Office of Scientific Research for financial support (Grant FA9550-14-1-0254), and the High Performance Computing Modernization Program (HPCMP) of the U.S. Department of Defense and Princeton University's Terascale Infrastructure for Groundbreaking Research in Engineering and Science (TIGRESS) for providing the computational resources. C.M.K. acknowledges support by a fellowship within the Postdoctoral Research Program of the German Academic Exchange Service (DAAD). These authors contributed equally: Leanne D. Chen, Michal Bajdich. Author Contributions: L.D.C. carried out the GGA-DFT calculations and prepared the manuscript. M.B. carried out the hybrid-DFT calculations. J.M.P.M. and C.M.K. carried out the DFET and embedded correlated wavefunction calculations. The density of states analyses were carried out by J.A.G. All authors contributed to the discussion and analyses of data and approved the manuscript. The authors declare no competing interests. Data availability: The data that support the findings of this study are available from the corresponding author on reasonable request.

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