Chao Zhang

associate senior lecturer at Department of Chemistry - Ångström Laboratory, Structural Chemistry

Email:
chao.zhang[AT-sign]kemi.uu.se
Telephone:
+4618-471 3721
Visiting address:
Lägerhyddsvägen 1

Postal address:
Box 538
751 21 Uppsala

Short presentation

According to the two-volume "Modern Electrochemistry" written by Bockris and Reddy, there are two kinds of electrochemistry. The first one is "The physical chemistry of ionically conducting solutions" and the second one is "The physical chemistry of electrically charged interfaces". I am a computational physical chemist working on these topics in energy storage/conversion and nanochemistry.

Drop me a line if you are interested in a thesis work (Master/PhD) or a research project (Postdoc).

Keywords: density functional theory molecular dynamics dielectrics electrolyte solid-electrolyte interfaces solid-state batteries

Also available at

My courses

Research

Density functional theory based molecular dynamics (aka Car-Parrinello MD)

A respectable senior colleague once told me that a person doing computational electrochemistry needs to know four things (five to include Machine Learning): Electronic structure theory, statistical mechanics, classical electrodynamics and chemical thermodynamics. This puts the density functional theory based molecular dynamics (DFTMD) as the method of choice. To myself, DFTMD is not just a method but a spirit (or a bridge) which connects the "hard" world (solid state/surface science community) and "soft" world (soft matter/liquid state theory community).

Finite-field molecular dynamics simulations

My recent practice following DFTMD spirit is to explore the constant electric displacement D Hamiltonian in modelling charged solid-liquid interfaces. Constant D Hamiltonian was designed by Stengel, Spaldin and Vanderbilt (SSV) for treating spontaneous polarization in groundstate ferroelectric systems [1].

The macroscopic electric field E and electric displacement D are conjugate variables in macroscopic Maxwell theory. Either E or D can be used as fixed thermodynamic boundary conditions. From classical electrodynamics, we know that the electric displacement D is continuous at a dielectric interface but not the electric field E. That is the reason why it is convenient to use the electric displacement D as the fundamental variable to simulate an interface.

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 dielectric properties of charged solid-electrolyte interfaces accessible to DFTMD (in theory).

Dielectric properties at charged solid-electrolyte interfaces

We showed that the simulation of finite temperature polarization fluctuations and dielectric constant in polar liquids are now doable in DFTMD [2,3]. The advantage of constant D simulations is not only to speed up simulations but also to eliminate the finite size effect for modelling the electric double layer due to the periodic boundary condition [4]. Now the Helmholtz capacitance of charged solid-liquid interface can be calculated from the supercell polarization [5, 6]. This methodology was further extended to treat the charge compensation between polar surfaces and the electrolyte solution [7, 8]. Its DFTMD implementation is available in one of our community codes CP2K (www.cp2k.org).

The dead-layer at water interfaces

A recent spin-off of constant D simulations is to study dielectric properties under confinement. We showed that the low dielectric constant of nanoconfined water found in molecular dynamics simulations can be largely explained by the so-called dielectric dead-layer effect using a simple capacitor model [9]. Later, this proposed dead-layer effect at water interfaces is confirmed by experiments [10].

[1] Stengel, M., Spaldin, N. A. Vanderbilt, D. Electric Displacement as the Fundamental Variable in Electronic Structure Calculations. Nat. Phys., 2009, 5: 304.

[2] Zhang, C. and Sprik, M. Computing the Dielectric Constant of Liquid Water at Constant Dielectric Displacement. Phys. Rev. B, 2016, 93: 144201.

[3] Zhang, C., Hutter, J. and Sprik, M. 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.

[4] Zhang, C. and Sprik, M. Finite Field Methods for the Supercell Modeling of Charged Insulator-Electrolyte Interfaces, Phys. Rev. B, 2016, 94: 245309 (Editors’ Suggestion).

[5] Zhang, C. Computing the Helmholtz Capacitance of Charged Insulator-Electrolyte Interfaces from the Supercell Polarization, J. Chem. Phys., 2018, 149: 031103 (Communication).

[6] Zhang, C., Hutter, J. and Sprik, M. Coupling of Surface Chemistry and Electric Double Layer at TiO2 Electrochemical Interfaces, J. Phys. Chem. Lett., 2019, 10: 3871.

[7] Sayer, T., Zhang, C. and Sprik, M. Charge Compensation at the Interface between the Polar NaCl(111) Surface and a NaCl Aqueous Solution, J. Chem. Phys., 2017, 147: 104702.

[8] Sayer, T., Sprik, M. and Zhang, C. Finite electric displacement simulations of polar ionic solid-electrolyte interfaces: Application to NaCl(111)/aqueous NaCl solution, J. Chem. Phys., 2019, 150: 041716 (Editor's pick).

[9] Zhang, C. Note: On the Dielectric Constant of Nanoconfined Water. J. Chem. Phys., 2018, 148: 156101.

[10] Fumagalli, L. et al., Anomalously low Dielectric Constant of Confined Water. Science, 2018, 360: 1339.

Publications

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