Modeling of the electric double layer at metal oxide electrode/electrolyte interfaces with density functional theory based molecular dynamics
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
Final Report Abstract
Most earth-abundant metal oxides for electrocatalysis are operated in alkaline solution (pH 14), the surface of oxides is known to undergo hydroxylation in the presence of adsorbed water molecules. When deprotonated at high pH, acidic sites acquire a negative charge and form an electric double layer (EDL) by attracting hydrated counter ions in solution. Thus, modeling and simulation of at the metal oxide electrode/electrolyte interfaces is an integral part of optimizing the cost efficiency of energy conversion. In this project, by combining concepts of (single) electrode potentials in electrochemistry and Berry phase polarization in periodic systems for the first time, we were able to charge the double layer at fixed chemical composition similar to experiments and to have the macroscopic polarization as a new observable. According to the classical Debye theory, switching the electric boundary condition from constant potential (E) to constant charge (D) leads to a speed-up of the relaxation time of the macroscopic polarization by a factor comparable to the dielectric constant of the medium. This would be two orders of magnitude difference for aqueous solutions and makes electric properties of oxide/electrolyte interfaces accessible to density functional theory based molecular dynamics (DFTMD). Adapting constant electric displacement D method originally designed for treating spontaneous polarization in groundstate ferroelectric systems, we showed that the simulation of finite temperature polarization fluctuations and dielectric constant in polar liquids are now doable in DFTMD. The advantage of constant D method is not only to speed up simulations but also to eliminate the finite size effect for modeling EDL due to the periodic boundary condition. We have also shown that the computed Helmholtz capacitance is independent of the size of charged insulator slab using constant electric displacement simulations. This methodology was further extended to treat the charge compensation between polar surfaces and the electrolyte solution. This series of works lays down the methodological foundations for DFTMD modeling of structural, dynamical and dielectric properties at charged oxide-electrolyte interfaces. Follow-up works in this area are expected in the near future.
Publications
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Computing the Dielectric Constant of Liquid Water at Constant Dielectric Displacement. Phys. Rev. B, 2016, 93: 144201
Zhang C. and Sprik M.
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Computing the Kirkwood g-factor by Combining Constant Maxwell Electric Field and Electric Displacement Simulations: Application to the Dielectric Constant of Liquid Water, J. Phys. Chem. Lett., 2016, 7: 2696
Zhang C., Hutter J. and Sprik M.
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Finite Field Methods for the Supercell Modeling of Charged Insulator-Electrolyte Interfaces, Phys. Rev. B, 2016, 94: 245309
Zhang C. and Sprik M.
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Charge compensation at the interface between the polar NaCl(111) surface and a NaCl aqueous solution, J. Chem. Phys., 2017, 147: 104702 (2017)
Sayer, T., Zhang, C. and Sprik M.
