Session W49: Modeling the Electrochemical Interface and Aqueous Solutions I

Electrodes in aqueous electrolyte: modeling challenges and surprises

The electrochemical interface is challenging to model solely from first principles because of the long length and time scales of interfacial processes.  Approximating the properties of the charged interface by extrapolation from the neutral interface can lead to qualitatively incorrect descriptions (e.g., of the adsorbate geometry).  Coupling ab initio methods with more scalable methods such as classical molecular dynamics or continuum modeling can make this charging problem tractable.  Here, I discuss the challenges of modeling charged electrochemical interfaces.  I then focus on three explicitly charged interfaces and the connections to experimental observables that can be accessed through charging, including geometry changes, the capacitance, and the potential of maximum entropy.    

Power generation in Janus nanopores: Insights from All-atoms MD simulations.

By means of large-scale all-atoms molecular dynamic simulation, this work explores the physical mechanism for ionic current rectification (ICR) and energy extraction in asymmetrically charged "Janus" membranes. The asymmetry in these membranes promotes a directional ionic conduction that can be exploited to generate current from a gradient in salinity.

The results we present in this work show excellent agreement with the experimental ICR and power extraction performances reported for membranes of the same size and charge. We found ICR correlates with an accumulation of ions inside the pore in the active state compared to the inactive state. In contrast, the electric potential was almost identical in all the tested conditions. These findings support the "accumulation-depletion" mechanism proposed for ICR.

In our simulations, water orients anisotropically, with a larger average dipole moment at the ionic depleted regions. Water molecules align their dipoles towards the charged walls so that the dipole-induced interactions dominate the physics at the pore's surface.

The results of this work confirm that ICR is not limited to pore radii below the Debye length, which opens membrane design possibilities to more accessible fabrication processes.   

Understanding photocatalytic water oxidation in SrTiO3 [001] surfaces

Perovskite oxides such as SrTiO₃ have garnered considerable interest due to their high photocatalytic efficiency under UV irradiation as well as favorable interfacial interactions with water, two important features that drive solar water splitting. In this work, we present a unified computational and experimental account of the photocatalytic activity at the SrO- and TiO₂- terminations of aqueous-solvated [001] SrTiO₃. Our experimental investigations show that the overall water splitting proceeds only when both SrO- and TiO₂- terminations are exposed to water. Using density functional theory-based simulations, we verify this observation and elucidate the underlying mechanism driving this process using a sequence of four hole transfers. We conclude that while the water-oxidation reaction is favorably catalyzed only at the SrO-surface, the TiO₂-surface provides the necessary band alignment for the redox reaction to proceed, thus proving that both terminations are crucial for photocatalytic water splitting.   

Molecular dynamics simulations of electrical double layer capacitance and polarizability.

Electrical double layers, which form in response to surface charge on a conductor in contact with electrolyte solution, influence the thermodynamics and kinetics of electrochemical processes. Here we use atomistic molecular dynamics simulations to investigate the electrode-electrolyte interface, in which we control the total excess charge on the electrode, and dynamically distribute charges to individual electrode atoms to screen electric fields inside the metal.  We measure the local dielectric constant in the electrolyte using a novel AC electric field method, which is substantially more efficient compared to traditional dipole fluctuation or static response methods. The dielectric response in the double layer is markedly anisotropic, enhanced in the in-plane direction (ε≈110) and suppressed in the out-of-plane direction (ε≈3). Finally, we measure the double layer differential capacitance, which we use together with the measured dielectric constants, to assess classical double layer theories (Gouy-Chapman and Stern models).   

Development of a quantitative down-selection descriptor for electrochemical oxidation

Thermodynamic phase diagrams serve as important tools for understanding complex aqueous processes, such as corrosion and passivation. Here, we expand upon the concept of predominance and aqueous thermodynamic stability to develop a quantitative parameter describing solid-phase formation via electrochemical oxidation. We demonstrate ease of calculability and data-sourcing from high-throughput density functional theory databases. We then use our descriptor to show how it predicts time-dependent (hydr)oxide formation consistent with experimentally grown Ni thin films subjected to variable pH and potential. Last, we propose strategies for which our parameter can be leveraged for high-throughput alloy design and compositional optimization.

 

This work relates to Department of Navy Award N00014-20-1-2368 issued by the Office of Naval Research. The United States Government has a royalty-free license throughout the world in all copyrightable material contained herein.   

Computational and Experimental Insights into the BiVO4 (010) surface and interface for water-splitting applications

Bismuth vanadate (BiVO₄) is a promising photoanode for water splitting that is frequently paired with an oxygen evolution catalyst (OEC). However, little is known about interfacial properties of BiVO₄ with water or with the OEC. In our earlier work [1], we showed that the photoelectrochemical performance of BiVO₄ is greatly impacted by its surface termination. We present our recent findings from an integrated experimental and computational effort aimed at an atomistic understanding of the BiVO₄ (010) interface with water and with the OEC for different surface terminations. We combined first-principles calculations using Quantum Espresso (https://www.quantum-espresso.org/) and Qbox (http://qboxcode.org/) with XPS and resonant PES in ambient conditions to probe how the presence of water influences charge localization. We identify structural moieties with adsorbed water that can explain the observed enhancement in the polaron signal. Finally, we discuss how surface termination impacts the interfacial properties with water based on ab-initio molecular dynamics simulations.

[1] D. Lee, W. Wang, C, Zhou, X. Tong, M. Liu, G. Galli, K.-S. Choi. “The impact of surface composition on the interfacial energetics and photoelectrochemical properties of BiVO4 .” Nature Energy. 6, 287 (2021).   

Polarization of disc and ring electrodes in high-conductivity electrolyte solutions

Studies of electrical double layers near charged surfaces in electrolyte solutions are important for a wide range of applications including biophysics, colloidal science, and micro/nanofluidics. We investigate the polarization of disc and ring electrodes immersed in an electrolyte solution and subjected to a small external AC voltage governed by the Debye−Falkenhagen equation (a linearization of the Nernst−Planck equations) and the Poisson equation. Based on integral transforms, analytical techniques are developed for predicting the space charge density and complex impedance of the system. The effect of electrode dimension on the impedance is examined and compared with experiments. These mathematical models enable uncovering detailed knowledge of the surface charge densities on the electrodes from experimental impedance spectroscopy measurements.   

Probing the electronic properties of the electrified silicon/water interface by combining simulations and experiments

Silicon (Si) is used in electrochemical and photoelectrochemical devices, and capacitive and Faradaic reactions at the Si/water interfaces are critical for signal transduction or noise generation. However, probing these interfaces at the microscopic level remains a challenging task. Here we focus on hydrogenated Si surfaces in contact with water, relevant to transient electronics and photoelectrochemical modulation of biological cells and tissues. We show that by carrying out first principles molecular dynamics simulations of the Si(100)/water interface in the presence of an electric field, we can realistically correlate the computed flat-band potential and tunneling current images at the interface with experimentally measured capacitive and Faradaic currents. Specifically, we validate our simulations in the presence of bias by performing pulsed chronoamperometry measurements on Si wafers in solution. Consistent with prior experiments, our measurements and simulations indicate the presence of voltage dependent capacitive currents at the interface. We also find that Faradaic currents are weakly dependent on the applied bias, and we relate these currents to surface defects present in newly prepared samples.   

Structural insights into sodium chloride solutions from state-of-the-art neural network potentials

Sodium chloride (NaCl) solutions, also known as saltwater, are ubiquitous in nature. Understanding the effect of the dissolved Na⁺ and Cl^- ions on the tetrahedral hydrogen-bond network of water is essential to uncover the mechanisms underlying various physical, chemical, biological, and geological processes. Although the effect of ions on water has long been the focus of scientific interest during the past century, the way and the extent of ionic effects on the H-bond network of water is still an unsolved problem. In this work, we study the effect of increasing NaCl solute concentration on the structure of solvent water and compare it with the effect of increasing pressure on pure water using deep potential molecular dynamics (DPMD). In particular, the deep neural network potential is trained with the density functional theory data based on the strongly constrained and appropriately normed functional. Therefore, our DPMD simulations preserve the quantum mechanics accuracy with computational costs comparable to that of empirical force fields, which enables efficient simulations of different concentrations with large simulation cells and long simulation time. The computed reciprocal-space structure factors agree quantitatively with experimental neutron diffraction data. The detailed analyses suggest that ion-induced modifications to the structure of water are mainly restricted to the ionic first solvation shells.   

Molecular dynamics simulations for the molecular polarization of salt-free and salt-containing liquids with Stockmayer fluids and ensemble neural networks

We develop our simulation method for the molecular dielectric response via a Stockmayer fluid (dipolar molecule), combined with neural networks using machine-learning techniques. We first show that despite the drastic simplification of polar molecules with a minimal set of molecular parameters, our coarse-grained molecular dynamics simulations are consistent with the experimental data of the dielectric constants of various salt-free and salt-containing organic solvents. To substantially reduce the computational cost due to the statistical analysis of the simulation data and the parametrization of the model parameters, we also construct surrogate models for the dielectric constant of the solvents using ensemble neural networks. We show that our models can predict the qualitative trends of the simulation results by taking up a small number of samples from a population of statistically-noisy simulation data.