Session K16: Aqueous Solutions, Solvated Interfaces, and Ionic Polarization I

Non-equilibrium efects at solvated interfaces under an applied external bias

Understanding the local structure of water at the interfaces of metallic electrodes is a key issue in aqueous-based electrochemistry. Nevertheless a realistic simulation of such a setup is challenging, particularly when the electrodes are maintained at different potentials. To correctly compute the effect of an external bias potential applied to truly semi-infinite surfaces, we combine Density Functional Theory and Non-Equilibrium Green’s Function methods. This framework allows for the out-of-equilibrium calculation of forces and dynamics, and directly correlates to the chemical potential of the electrodes, which is introduced experimentally. In this work, I will discuss this frame work and some applications to water molecules at the interface with metallic surfaces.   

Toward first-principles modelling of charged solid-electrolyte interfaces

Oxide-electrolyte interfaces are universally present in energy storage device, nanofluidic chemical processor, drug delivery nanoparticles and containments treatment in ground water. The surface charge of all these interfaces is controlled by the pH of the electrolyte solution and this leads to the formation of the electric double layer (EDL) by deprotonation of adsorbed water molecules or protonation of the oxide surfaces. Despite of the rapid development of experimental techniques, the missing of microscopic understanding imposes a knowledge gap. In this regard, modelling and simulation of EDL can provide complementary information of the structure, dynamics and energetics of charged interfaces. Here, I will report our current methodological progress on the atomistic modelling of dielectric properties of charged oxide-electrolyte interfaces.

[1] Zhang C. and Sprik M. Phys. Rev. B, 2016, 93: 144201.
[2] Zhang C., Hutter J. and Sprik M. J. Phys. Chem. Lett., 2016, 7: 2696.
[3] Zhang C. and Sprik M. Phys. Rev. B, 2016, 94: 245309 (Editors’ Suggestion).
[4] Sayer, T., Zhang, C. and Sprik M. J. Chem. Phys., 2017, 147: 104702.
[5] Zhang C. J. Chem. Phys., 2018, 149: 031103 (Communication).
[6] Sayer, T., Sprik, M. and Zhang C. J. Chem. Phys., 2018, arXiv: 1809.00892 (Invited article).   

First-principles modeling of electrochemical reactions at the metal-water interface under applied voltages

Ab initio modelling using density functional theory (DFT) is a powerful technique to study reactivity at electrochemical interfaces. We have recently developed an approach that allows to describe electrochemical systems under applied voltages and that can be easily implemented in existing DFT codes [1]. We apply this method to study Mg aqueous corrosion at anodic conditions, for which unusually high H₂ evolution rates are observed. The DFT based molecular dynamics simulations we perform for the Mg(0001)/H₂O system under increasing anodic polarization show dissociation events, proton transfer as well as H₂ evolution at the Mg-water interface. A detailed analysis of our calculations reveals a novel and hitherto unconsidered reaction mechanism for the H₂ evolution reaction, caused by an unusual adsorption phenomenon triggering a reaction resembling the cathodic Heyrovsky reaction.

[1] S. Surendralal, M. Todorova, M. W. Finnis, and J. Neugebauer, Phys. Rev. Lett. 120, 246801 (2018).   

DFT Characterization of Solvated NaCl Potentials of Mean Force and Energetics

Potentials of mean force have long been a standard of measurement to characterize the energetics of the ion solvation process. In particular, classical molecular dynamics simulations frequently predict deeper minima than DFT simulations for the different solvated states. Moreover, different exchange-correlation functionals often don't yield consistent potentials within DFT, some even predicting more stable second minima (solvent-separated ion pairs) than the usual deepest first minimum (contact ion pair). In this work, we investigate the electronic structure of solvated NaCl and the effects the charge localization has on nearby water molecules. We further investigate the errors in the system arising from the functional approximations and from the self-consistent ground-state Kohn-Sham electron density, and how the errors contribute to different stability characteristics.   

Energetics of the adsorption of iodide ion at the air-water interface

Ion adsorption at air-water (a/w) interfaces has been extensively probed due to its
significance in a large number of phenomena in environmental science, chemical
physics, biophysics, etc. Recently, our group has investigated the ion-capillary wave
interaction and its impact on the energetics of ion adsorption at an a/w interface
considering generic ion. However, the specific ion adsorption behavior is more
important, especially for those ions having a favorable energy of adsorption at the
air/water interface. Therefore, we employed molecular dynamics method to study
the iodine ion-capillary wave interaction. We reconstruct the potential of mean
force using the distance between ion and the Gibbs dividing surface as the reaction
coordinate. Furthermore, by analyzing the response of the system to the presence of
iodine at the interface, e.g., the energetic change, the change in the overall
fluctuations of surface and interfacial water, etc. we shed light to the understanding
of the iodine ion adsorption at air/water interface.   

First-principles based quantification of charged species redistribution at electrochemical interfaces: Model system of zirconium oxide

Modeling local distribution of charged ions and ionic defects at electrochemical interfaces is key to understanding related electrochemical processes. Based on the grand canonical approach which defines the electrochemical potential of individual charged species, a unified treatment of defects in solid oxide and ions on water side can be established. In this work, we apply this framework to the system of ZrO₂/water interface. Density functional theory calculations are performed to obtain defect formation energy in the oxide materials and ab initio molecular dynamics is used to assess the formation free energy of H⁺ and OH^- ions in water at different distances from the ZrO₂ surface. The results are fed into a continuum model which produces the equilibrated distribution of these charged species. The continuum model considers explicitly both ion adsorption and defect segregation in the vicinity of the interface, and the diffuse layer and space charge layer in the extended area. Such a unified description reveals the influence of interfacial chemistry on oxide defect chemistry, and vice versa. This framework based on the grand canonical approach allows easy inclusion of additional charged species into the system and offer a general tool for studying electrochemical interfaces.   

Polarization induced ion adsorption on aqueous interfaces: Solvent effect on image charge interactions

Understanding electrostatic interactions near interfaces is of critical importance in broad fields of science. Continuum electrostatics expects ions to be attracted to metal electrodes but repelled from low dielectric surfaces, while recent studies found certain 'chaotropic' ions are adsorbed on both air/water and graphene/water interfaces. Here we systematically study the effect of polarization of electrode, solvent and solute molecules on the adsorption of ions near interfaces with molecular dynamics simulation. An efficient method is developed to treat an electrolyte system between two surfaces, exploiting the mirror-expanded symmetry of the exact image charge solution. With neutral surfaces, the image interaction induced by the solvent dipoles and ions largely cancel each other, resulting in no significant net differences in the ion adsorption profile regardless of the interface. Under external electric field, the adsorption of ions are enhanced by the polarizing electrode. Explicit polarization of the electrolyte is necessary to capture the increased propensity of big halide ions near the electrode seen in experiments. Our analysis shows inclusion of both electrode and electrolyte polarization is of critical importance to model the electrolyte behavior with interfaces.   

Intercalation behavior of MXenes from ab initio molecular dynamics

MXenes are a large family of two-dimensional materials, primarily transition metal carbides and highly conductive, displaying exceptional performance for supercapacitors, batteries, and water purification. MXenes can host a wide range of species between their layers. Predictions and understanding are complicated by the presence of surface functional groups, such as O, OH, and F. Intercalation with aqueous ions such as Li, K, or Mg results in distinct structural and electrochemical responses. Here we present density functional ab intio molecular dynamics calculations for the behavior of ions confined in wetted MXene Ti₃C₂ layers. In this confined aqueous environment, ions display distinct behavior relating to their size, preferred solvation shell, and interaction with the surface groups, consistent with recent scanning probe microscopy data. This suggests that, besides the choice of electrolyte, careful selection of the surface groups and choice of MXene surface metal atoms are routes to optimize supercapacitor and sensing performance.   

Detection of Amino Acids and Other Biochemical Molecules with GaAs Schottky Diodes

The sensitivity of the GaAs surface makes it a good candidate for sensing various molecular species of interest in biochemistry. Simultaneously, the growth in nanofluidics applications calls for detailed understanding of liquid-solid interfaces for a variety of interfaces’ combinations. The 1V operation range for GaAs Schottky sensor is shown to be sufficient to detect H₂O, D₂O, -OH, as well as PBS solutions of various concentrations at 300K. We show how interfacial density of states changes with each compound probed, and how this density changes with the applied electric field. Inorganic bonds, such as Ga-H, Ga-O (and related As-based ones), are differentiated through correlation of transport and FTIR data of liquid-solid interfaced. The same GaAs Schottky diode can be used to distinguish between different types of amino acids (non-polar, polar, and basic) through the specific examples of Gly, His, and Cys, respectively. Finally, the built-in potential and the interfacial density of states are determined as a function of the pH factor of solutions tested (4 < pH < 9). These results offer perspective on developing GaAs based system for micro- and nanofluidics based detection and differentiation of peptides.   

Polarizability, Infrared and Raman Spectra of Water from First-Principles Simulations Using Recent Exchange-Correlation Functionals

We present the results of a series of uncorrelated first-principles molecular dynamics simulations of liquid water obtained with the recently proposed SCAN density functional[1]. Results are compared with those obtained using the PBE density functional, and known experimental results. Estimates of the polarizability, infrared spectra, and Raman spectra are calculated. These results, in addition to previous structural analysis performed by us and others, are used to further evaluate the accuracy of the SCAN functional when applied to simulations of water.

[1] J. Sun, A. Ruzsinszky, J. P. Perdew, Phys. Rev. Lett. 115, 036402 (2015).
[2] http://qboxcode.org