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Parameter-Fitting-Free Continuum Modeling of Electric Double Layer in Aqueous Electrolyte
Lab Researchers: Masao Suzuki Shibata, Yu Morimoto and Iryna V. Zenyuk
Summary: Electric double layers (EDLs) play fundamental roles in various electrochemical processes. Despite the extensive history of EDL modeling, there remain challenges in the accurate prediction of its structure without expensive computation. Herein, we propose a predictive multiscale continuum model of EDL that eliminates the need for parameter fitting. This model computes the distribution of the electrostatic potential, electron density, and species’ concentrations by taking the extremum of the total grand potential of the system. The grand potential includes the microscopic interactions that are newly introduced in this work: polarization of solvation shells, electrostatic interaction in parallel plane toward the electrode, and ion-size-dependent entropy. The parameters that identify the electrode and electrolyte materials are obtained from independent experiments in the literature. The model reproduces the trends in the experimental differential capacitance with multiple electrode and nonadsorbing electrolyte materials (Ag(110) in NaF, Ag(110) in NaClO4, and Hg in NaF), which verifies the accuracy and predictiveness of the model and rationalizes the observed values to be due to changes in electron stability. However, our calculation on Pt(111) in KClO4 suggests the need for the incorporation of electrode/ion-specific interactions. Sensitivity analyses confirmed that effective ion radius, ion valence, the electrode’s Wigner–Seitz radius, and the bulk modulus of the electrode are significant material properties that control the EDL structure. Overall, the model framework and findings provide insights into EDL structures and predictive capability at low computational cost.
Publication: Journal of Chemical Theory and Computation, 2024
Observation of monotonic surface charging at polycrystalline platinum-electrolyte interface using streaming current method
Lab Researchers: Jesus Lopez Ochoa and Iryna Zenyuk
Summary: Surface charging characteristics at polycrystalline Pt-electrolyte interfaces were studied using a novel combination of electrochemical techniques and electrokinetic streaming current methods developed by our group [Saha et al., 2019]. Surface charge typically increases with increasing electric potential on a metal, and this trend is commonly referred to as monotonic charging. Based on theoretical models, it was suggested that the Pt-electrolyte interface might deviate from this trend and have negative charge above the potential of zero charge (PZC) due to the metal’s strong chemisorption properties [Huang et al., 2016]. We attempted to experimentally determine the validity of this claim using our method. In addition to the surface charging relations, we also quantified the effect of anion adsorption (oxide, chloride, sulfate) on surface charging. The experimental method also allowed us to observe the slow time-dependent growth of oxides on Pt surface. Since our first publication in 2019 [Saha et al., 2019], we were able to develop the setup to be more efficient that enabled us to apply higher potential on the metal, observe stronger ion adsorption effects on surface charging, and obtain more reliable data. Our recent experimental study shows that Pt charging remains monotonic and does not reverse due to chemisorption of oxides nor strong anion adsorption in the potential window where the charge reversal was theoretically shown to occur.
Publication: Electrochimica Acta, 2024