Entropy of Metal/Solution Interfaces: Thermodynamic Framework, Theoretical Models, and Roles of Interfacial Water

Authors

Zeng-Ming Zhang, Institute of Energy Technologies, IET-3: Theory and Computation of Energy Materials, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany; Faculty of Georesources and Materials Engineering, RWTH Aachen University, Aachen 52062, Germany
Zi-Xi Liang, Institute of Energy Technologies, IET-3: Theory and Computation of Energy Materials, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany; Institute for a Sustainable Hydrogen Economy, INW-1, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
Jun Huang, Institute of Energy Technologies, IET-3: Theory and Computation of Energy Materials, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany; Faculty of Georesources and Materials Engineering, RWTH Aachen University, Aachen 52062, GermanyFollow

Corresponding Author

Jun Huang (ju.huang@fz-juelich.de)

Abstract

Entropy is a basic thermodynamic property of the electrical double layer (EDL) at metal/solution interfaces, yet, its definition, measurement, and theoretical treatment are dispersed in the literature, and, in some cases, ambiguous. In this paper, we revisit the thermodynamic theory of EDL, from which two variants of entropy, excess entropy and formation entropy, are obtained and compared. In terms of the formation entropy, two calculation routes are validated in the context of a primitive EDL model, namely, the Gouy-Chapman (GC) model. After clarifying the concepts and calculation routes, we investigate interfacial water effects on the EDL entropy, using a refined Gouy-Chapman-Stern (GCS) model accounting for chemical potential difference between oxygen- and hydrogen-down water molecules, denoted δµ. The model-derived differential capacitance and entropy are compared with experimental data for the EDL at Au(111) in an aqueous electrolyte solution. The model reveals that the charge of maximum entropy (CME) is negative when water molecules have higher tendency to take oxygen-down configuration at the uncharged surface. Moreover, the formation entropy profile becomes asymmetric around the CME, when δµ is potential-dependent. However, the model fails to simultaneously reproduce capacitance and entropy measurements on the same system taken from two separate studies, indicating deficiencies of the model or experimental errors. Nevertheless, this work stresses the importance of measuring both capacitance and entropy of EDLs at the same time.

Graphical Abstract

Keywords

Electrical double layer, Excess entropy, Formation entropy, Interfacial water, Charge of maximum entropy

DOI Link

https://doi.org/10.61558/2993-074X.3601

Creative Commons License

This work is licensed under a Creative Commons Attribution 4.0 International License.

Publication Date

2026-03-28

Online Available Date

2025-12-24

Revised Date

2025-12-10

Received Date

2025-11-13

Recommended Citation

Zeng-Ming Zhang, Zi-Xi Liang, Jun Huang. Entropy of Metal/Solution Interfaces: Thermodynamic Framework, Theoretical Models, and Roles of Interfacial Water[J]. Journal of Electrochemistry, 2026, 32(3): 2515010.
DOI: 10.61558/2993-074X.3601
Available at: https://jelectrochem.xmu.edu.cn/journal/vol32/iss3/2