Book/Dissertation / PhD Thesis

FZJ-2026-02769

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Semiclassical Thermodynamics of Metal-Solution Interfaces: A Density-Potential Functional Theory Study

Zhang, Z. (Corresponding author)FZJ*

2026
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-95806-947-3

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Please use a persistent id in citations: urn:nbn:de:0001-2608051428361.303455331779  doi:10.34734/FZJ-2026-02769

Abstract: Electrified metal-electrolyte interfaces play a central role in electrochemical energy conversion and storage technologies. Despite extensive experimental and theoretical efforts, a comprehensive understanding of electrical double layers (EDL) remains elusive in several key aspects. In particular, existing models struggle to describe (i) the structure and thermodynamic properties of EDL at highly charged states, i.e., at potentials, far from the potential of zero charge (PZC), where many important electrocatalytic reactions take place; (ii) the surface stress and mechanical response of solid-liquid interfaces, for which classical electrocapillary concepts based on surface tension are thermodynamically inconsistent; and (iii) the EDL structure and thermodynamic driving forces at stepped metal surfaces, where atomic-scale heterogeneity strongly influences both catalytic activity and morphological stability. These limitations largely stem from the reliance on classical continuum theories that neglect the quantummechanical response of metal electrons and fail to consistently couple interfacial electrostatics with mechanics under constant-potential conditions. In response to these challenges, this thesis addresses three interrelated research directions: first, the development of an improved theoretical description of EDLs at highly charged metal-solution interfacesacross a wide range of electrolyte compositions; second, the establishment of a thermodynamically consistent framework for quantifying surface stress at solid-liquid interfaces under constant-potential conditions; and third, the elucidation of the thermodynamic origins of EDL formation and morphology evolution at stepped metal electrodes. To achieve these objectives, a semiclassical density-potential functional theoretical (DPFT) framework is developed and applied, which unifies the quantum-mechanical electronic response of the metal, including electron spillover, with a statistical thermodynamic description of the electrolyte. This approach enables a computationally efficient and physically consistent investigation of metal-solution interfaces under realistic electrochemical conditions. In the first part of this work, the classical Gouy–Chapman–Stern (GCS) model is critically assessed against experimental differential double-layer capacitance data for mercury electrodes over a wide potential range and across diverse electrolyte compositions. While GCS-based descriptions perform adequatelynear the potential of zero charge, they fail to capture key experimental features at highly charged states, including electrolyte-dependent shifts in the potential of zero charge, asymmetric capacitance profiles, and solvent-specific effects. To overcome these limitations, the DPFT framework is refined by introducing a physically motivated description of interfacial permittivity that distinguishes free solvent molecules from those bound in ionic solvation shells, as well as by incorporating potential-dependent metal-solvent short range interactions and partial ion desolvation at highly charged surfaces. The resulting model quantitatively reproduces experimental capacitance profiles across different cations, anions, solvents, and concentrations, providing a robust description of EDL structure under far-from-zero-charge conditions. In the second part, the DPFT framework is extended to investigate the thermodynamics and mechanics of solid metal-solution interfaces. By identifying surface stress, rather than surface tension, as the fundamental thermodynamic quantity for solid-liquid interfaces and by explicitly applying the Shuttleworth equation, a thermodynamically consistent methodology is established to compute potential-dependent surface stress. Within this framework, a new interfacial descriptor—the potential of zero stress (PZS)—is defined to characterize the stress-neutral state and discuss its relationship with the potential of zero free charge (PZFC). The results show that the potential dependence of surface stress is governed by the strain sensitivity of the potential of zero charge, leading to either monotonic or nonmonotonic behavior with electrode potential. By further incorporating strain-dependent metal– solvent interactions, the model reveals the key role of interfacial water in controlling the sign, magnitude, and tunability of surface stress. In the final part of this thesis, the DPFT framework is applied to stepped metal surfaces to elucidate the thermodynamic origins of structural evolution at electrified interfaces. Using vicinal Au and Ag electrodes as model systems, the theory successfully reproduces experimentally observed trends in the PZFC, including its systematic decrease with increasing step density. Beyond global quantities such as PZFC, local potentials of zero charge are introduced to capture the intrinsic heterogeneity of the EDL at stepped surfaces. By connecting these step-induced shifts to variations in interfacial free energy, the modeldemonstrates that step bunching at elevated electrode potentials is thermodynamically driven at more positive potentials and strongly modulated by electrolyte composition. Overall, this thesis provides a unified and computationally efficient theoretical framework for describinginterfacial thermodynamics at electrified metal-solution interfaces under constant-potential conditions. Key advances include the identification of partial ion desolvation as a governing mechanism at highly charged surfaces, the introduction of the PZS as a fundamental mechanical descriptor for solid-liquid interfaces, and the establishment of a thermodynamic mechanism for step bunching driven by interfacial free-energy minimization. The insights gained bridge microscopic electronic structure with macroscopic electro-mechano-chemical behavior and provide a foundation for the rational design and control of stable and efficient electrocatalytic interfaces through electrolyte selection, potential control, and surface morphology engineering.

Note: Dissertation, RWTH Aachen University, 2026

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Contributing Institute(s):

1. IET-3 (IET-3)

Research Program(s):

1. 899 - ohne Topic (POF4-899) (POF4-899)

Appears in the scientific report 2026

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