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

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

doi:10.1038/s41467-018-05511-y

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

Nat Commun. 2018 Aug 10;9(1):3202. doi: 10.1038/s41467-018-05511-y.

Authors

Leanne D Chen^{1

2}, Michal Bajdich³, J Mark P Martirez⁴, Caroline M Krauter^{4

5}, Joseph A Gauthier¹, Emily A Carter⁶, Alan C Luntz³, Karen Chan³, Jens K Nørskov^{7

8

9}

Affiliations

¹ SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, Stanford, CA, 94305, USA.
² Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, 91125, USA.
³ SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, CA, 94025, USA.
⁴ Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, 08544, USA.
⁵ Schrödinger GmbH, Dynamostr. 13, D-68165, Mannheim, Germany.
⁶ School of Engineering and Applied Science, Princeton University, Princeton, NJ, 08544, USA.
⁷ SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, Stanford, CA, 94305, USA. [email protected].
⁸ SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, CA, 94025, USA. [email protected].
⁹ Department of Physics, Technical University of Denmark, 2800, Kongens Lyngby, Denmark. [email protected].

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) | H₂O and Al(111) | H₂O 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.

Publication types

Research Support, Non-U.S. Gov't
Research Support, U.S. Gov't, Non-P.H.S.