Session G45: Modeling the electrochemical interface and aqueous solutions II

First-principles electrochemistry with grand-canonical DFT and continuum-solvation methods

First-principles calculations combining density-functional theory (DFT) and continuum solvation models have been highly successful in enabling the theoretical design of liquid-phase catalyst materials. The application of such methods to heterogeneous catalysis in electrochemical solid-liquid interfaces has however been far more challenging for two reasons. First, conventional continuum solvation methods designed for finite molecular systems are not readily applicable to solid-liquid interfaces with metal or oxide surfaces. Second, processes at these interfaces continuously exchange electrons with the electrode, which makes the charge states of the surface difficult to determine.

I will present recent developments in continuum solvation methods that enable simultaneously accurate treatment of solid surfaces and molecules in electrochemical interfaces. Additionally, by automatically equilibrating charge at a fixed electrochemical potential, new 'grand-canonical DFT' algorithms and software (JDFTx) facilitate realistic description of processes in these interfaces. Together, these methods enable first-principles prediction of electro- and photo-catalysis mechanisms for energy conversion, and of the surface structures and dynamics in battery materials. With clean prototypical electrochemical measurements as a benchmark, I will outline the present capabilities, pending challenges and necessary future developments in first-principles electrochemistry.   

The nanoscopic structure of Pt-water electrified interface under applied potential

In this talk I will introduce some issues connected with the simulation of electrified interfaces at the nanoscale focusing in particular on modelling the effect of an applied potential to an electrochemical cell. I will present a new methodology to model charged electrodes and highlight some recent progress in the simulation of the double layer of the fundamental Pt-water interface and its response to changes of potential applied to the cell. We reveal that the metal/surface charging cannot be described using a traditional simple capacitor model and that the double layer nanoscopic structure, mass density and charge distribution, strongly depend on the applied potential.   

Probing electrochemical processes at surfaces and interfaces by a combination of ab-initio simulations and in-situ characterizations

Electrochemical processes are ubiquitously seen in industrial applications, and it is often playing crucial roles in the device functionalities, yet, obtaining precise microscopic information of such processes are still very challenging due to the complex nature of the problems.

In this presentation, we will discuss how a combination of ab-initio simulations and in-situ characterizations could be used to access to such information and how one could reduce uncertainty in interpreting the experimental data. The key is to use ab-initio simulations as link between multiple observables that are not linearly dependent each other, which may be used to avoid a misinterpretation due to accidental agreement on one observable. We will show the examples based on oxidation of III-V semiconductor surfaces induced by water and oxygen.   

Activating 2D materials for hydrogen evolution reaction (HER) by electron doping

The two-dimensional materials have emerged as an effective electrocatalysts for Hydrogen evolution reaction in the recent few decades.[1] However, the database of such materials is limited so far mostly to family of transition metal dichalcogenides and MXenes. Moreover, majority of these need prior activation by defect engineering or elemental doping.
In this presentation, we show an electron doping pathway to enhance the electro-catalytic efficiency of two-dimensional material surfaces towards HER. A high-throughput computational scheme has been employed, starting from a large computational database of the easily exfoliable compounds.[2] By exploiting continuum embedded density functional theory and Grand Potential Simulation techniques, we studied the H⁺ electrosorption reaction thermodynamics and the aqueous stability of the 2D materials at different applied potentials and pH.[3,4] Our results allow to identify some promising materials, which show potential catalytic activity and considerable stability at reducing potentials and acidic pH.

References
[1] Turner, J. A. Science 2004, 305, 972– 974.1
[2] Mounet et. al. Nat. Nanotech. 2018, 13, 246–252.
[3] Andreussi et. al. J. Chem. Phys. 2012, 136, 064102.
[4] Hörmann et. al. J. Chem. Phys. 2019, 150, 04173   

Probing Pseudocapacitive response of MXene electrodes for energy storage from first principles

Pseudocapacitive devices are able to store and release electrical energy by virtue of the rapid and reversible redox reactions which occur at the electrode surfaces. The family of transition metal carbides and nitrides, MXenes, show promise as pseudocapacitive electrodes. As the performance, power and energy densities, of the pseudocapacitive device is predicated on the quantity of lithium-ion adsorption onto the surface of the MXene, we are interested in the effects of MXene composition and solution properties on this phenomenon. We perform voltage-dependent cluster expansions to study reversible lithium-ion adsorption in realistic environments. The cluster expansions are fit with semi-local density functional calculations in implicit solvent. Preliminary results show that lithium-ion pseudocapacitance is sensitive to the dielectric constant of the solution and extent of surface electrification through electrochemical double layer capacitance. Our results also suggest that solvents with higher lithium reduction potentials that stabilize the lithium-ions in solution can extend the potential window of the pseudocapacitive electrode.   

Understanding water/silicon carbide interfaces using first principle simulations

Interfaces between water and solid surfaces are key to many technological applications, including electrochemical devices, the fabrication of membranes for water purification and sensors for biomedical devices. Here we consider a bio-compatible material, SiC, of interest for biosensing and electrochemistry applications. We investigated the interaction of water with its hydrophobic and hydrophilic interfaces using first principles molecular dynamics and the Qbox code (http://qboxcode.org/). We report results on band offsets between the solid and the liquid and their dependence on the atomistic structure of the interface, which in turn is related to the hydrophobic or hydrophilic character of the solid termination. In addition, we will discuss the effect of an external applied electric field at the interface, to understand how the interface properties are modified under typical electrode working conditions.   

Continuum models to handle electrolyte solutions effects in first-principles simulations of materials

Continuum models of solvation have played a crucial role in quantum chemistry simulations and are now starting to be popular for the computational characterization of solvated, possibly electrified, interfaces. Recent advances in the field opened the possibility of modeling heterogeneous catalysis and electrochemistry in a first-principles-based framework, where the multiscale nature of the developed approaches provides a significant reduction of the computational burden while retaining a good accuracy. Nonetheless, extending continuum approaches to condensed-matter simulations present non-trivial issues, related to the complexity of the electrostatic problem in charged 2D interfaces and to the open structure of many crystalline substrates. Here we will present some of our recently proposed approaches to overcome these limitations, in particular focusing on a hierarchy of methods to describe the electrochemical diffuse layer. Moreover, handling environment effects through continuum embedding allows us to exploit a rigorous grand canonical approach to study the thermodynamic properties of electrochemical interfaces, thus overcoming some limitations of the computational-hydrogen electrode technique. Applications to noble metal (electro-)catalysis and beyond will be presented.   

Engineering Trimetallic Core-shell Nanoclusters for CO2 Electro-reduction at Low Overpotentials

Storing energy in chemical bonds and finding an electrochemical catalyst to reduce CO₂ to hydrocarbon fuels such as CH₄ would provide an ideal solution for discontinuous renewable energy sources. Commercially used pure copper catalysts are know to possess the best Faradaic yield capacity (upyo 50%) for CO₂ conversion to CH₄, however these require large overpotentials to perform this transformation. Using density functional theory (DFT), we tailor TM_xNi_13−x@Cu₄₂(TM = 3d transitionmetals; x = 3, 6, and9) nanoclusters to catalyze CO₂ electro-reduction to CH₄ with lower overpotentials than commercial catalysts. Among these, Sc₅₍₆₎Ni₈₍₇₎@Cu₄₂ possess unprecedented low overpotentials, ∼0.17 V below standard potential value (SPV). The given core compositions ensure the optimal position of d-band center of Cu, which is required for better interaction with π orbital of CHO than with CO. This results in better stabilization of CHO, giving exergonic CO reduction. Estimated statistical coverage of CHO exceeds CO by 60% at ambient conditions on Sc₅Ni₈@Cu₄₂, ensuring CO₂ conversion to high end fuel CH₄ without CO poisoning. The energetics of the reaction pathway and the overpotential values are relatively unchanged with the inclusion of implicit aqueous solvent.   

Adsorption of small gas molecules on a single Pt atom supported by pristine graphene: diffusion Monte Carlo study

We have used diffusion Monte Carlo (DMC) method to study adsorption of small gas molecules on a single Pt atom supported by pristine graphene. Firstly, we obtain Pt-graphene binding energy curves as a function of the vertical distance between Pt and graphene for three different adsorption sites (bridge, on-top, hollow), from which the equilibrium binding energies and distances are determined. Our DMC results are compared with DFT results based on several different density functionals, which reveals that PBE and rVV10 results show good agreement with our DMC results for both equilibrium energies and distances. For molecular adsorption on the Pt-graphene complex, we observe significant overestimation in DFT binding energies compared to the DMC ones. In the case of O₂ adsorption, the spin degrees of freedom are considered along with geometries. While DFT predicts the lowest-energy structure of a spin-triplet side-on configuration where the molecular axis of O₂ is parallel to the graphene sheet, DMC finds that the spin-singlet side-on configuration with the molecular axis being slightly tilted toward graphene is the lowest-energy state. The DMC calculations are currently in progress to examine diffusion path of O₂ and catalytic performance of Pt-graphene complexes.   

Morphological stability of electrodeposition through a viscoelastic coating

Lithium metal anodes, the key component of several emerging lightweight, energy-dense battery chemistries, continue to suffer from serious safety and cycle life concerns due to dendritic lithium growths. A promising approach toward dendrite suppression is to insert a viscoelastic polymeric interphase between the anode and the separator. Good coating layers have been empirically found to exhibit instantaneous elastic response and adaptability to volume change, but no mechanistic understanding was established. We develop a continuum model to study the effects of a polymer coating on the morphological dendritic instability by explicitly incorporating the viscoelastic response and the dielectric permittivity of the polymer. The effects of charging current density and overpotential during cycling are explored, and the coating properties and battery operation conditions for stable, uniform lithium growth and long cycle life are discussed.   

Field-Aware Interfaces in Continuum Solvation

The continuum embedding approach has seen a rapid influx of interest from the condensed matter community in recent years. We present here a number of additions to the modelling of continuum interfaces that aim to provide an implicit consideration of charged species and compounds with highly polarized regions in an effort to further expand on the capabilities of continuum models, especially in the treatment of electrochemical interfaces. These advances follow recent approaches of using the electric field as an effective proxy for the localized charge surrounding a specific region. This “field-aware” approach is applied to the recently proposed soft-sphere continuum solvation (SSCS) method, wherein the radius of each soft-sphere composing the interface is readjusted as a function of the field flux through its surface, as well as the self-consistent continuum solvation (SCCS) method, whose interface definition is modified by the normal component of the electric field. In both cases, a complex dependence of the interface function on both the electronic and ionic degrees of freedom of the solute is introduced. Analytic derivatives of the new interface are thus implemented during optimization procedures (SCF and geometry optimization).