Quantum–Continuum Modeling of Electrodeposition

Open Access
Author:
Weitzner, Stephen E
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
April 25, 2018
Committee Members:
  • Ismaila Dabo, Dissertation Advisor
  • Kwadwo Osseo-Asare, Committee Chair
  • Ralph Colby, Committee Member
  • Hojong Kim, Committee Member
  • Michael Janik, Outside Member
Keywords:
  • Electrochemistry
  • Density functional theory
  • DFT
  • Solvation
  • Quantum-Continuum
  • Interface
  • Electrodeposition
  • Underpotential deposition
  • Thermodynamics
  • Statistical Mechanics
  • Monte Carlo
  • Implicit solvent
  • Multiscale
  • Alloy
  • Electrode
  • Catalysis
  • Electrocatalysis
  • UPD
  • Cluster expansion
Abstract:
As the world transitions to clean renewable energy sources such as wind, solar, geothermal, and hydroelectric power, it is necessary to identify scalable and cost effective technological solutions for storing, distributing, and utilizing the generated electrical energy. One approach that is actively being investigated to meet these needs employs catalytic alloys to interconvert chemical and electrical energy, allowing for the renewable production of chemical fuels that can be used in fuel cells to derive electrical power. However, a persistent challenge in this area has been the identification and design of alloy catalysts that are simultaneously active and selective for desirable electrochemical reaction pathways and that are also able to withstand the harsh electrochemical environments of fuel cells and electrolysis cells. While recent progress has been made in the preparation of surface engineered multimetallic nanoparticles that exhibit enhanced catalytic performance, the durability of such catalysts is often limited by undesirable leaching and segregation events that occur under high current densities or when cells are cycled frequently. To address these challenges, first principles density functional theory (DFT) has been applied successfully to obtain atomistic insight into the environmental effects that limit catalyst durability. Much attention has been devoted to understanding the role of adsorption and finite temperature effects on the stability of alloy surfaces, providing an effective assessment of alloy catalyst durability under vacuum conditions. However, such stability analyses neglect solvation and surface electrification effects that are anticipated to play a critical role in determining the stability of multimetallic catalysts as evidenced by their poor cyclability. Thus, the aim of the present work is to extend conventional thermodynamic stability analyses based on first-principles calculations to rigorously consider the influence of electrochemical environments on the stability of model alloy catalysts. A quantum–continuum model of the electrode-solution interface based on first-principles DFT and a self-consistent continuum solvation model is developed and applied to study the interfacial electrochemistry of bimetallic gold alloy surfaces under applied voltage. In what follows, a brief summary of the results obtained in this work using the proposed methodology is presented. To assess the predictive quality of the quantum–continuum model, the electrochemical stability of an underpotentially deposited copper monolayer on the gold (100) surface was modeled. Previous work using conventional DFT methods led to the incorrect prediction that the copper monolayer is unstable at voltages where bulk copper films are observed to grow. Application of the quantum–continuum model using a planar ionic countercharge to represent the response of the electrolyte demonstrated that the finite surface charge that develops during electrode polarization can partially explain the stability of the copper monolayer. Thermal effects were additionally accounted for by fitting quantum–continuum results to a two-dimensional Ising model and performing Metropolis Monte Carlo simulations in the grand canonical ensemble across a range of voltages. The Monte Carlo simulations revealed that the copper monolayer desorption and adsorption process occurs as a first order phase transition on the surface, consistent with the single peak observed in voltammetric measurements. Tuning the surface charge by varying the distance of the layer of countercharges from the surface in a sensitivity analysis led to shifts in the position of the first order phase transition. Sulfate co-adsorption along the four-fold hollow sites of the copper monolayer was also considered, leading to a substantial improvement in monolayer stability at experimentally relevant values of the differential capacitance. This result suggests that the simultaneous consideration of thermal effects, surface charging effects, and co-adsorption effects may be necessary to obtain accurate models of alloy-electrolyte interfaces. The stability of an underpotentially deposited pseudomorphic silver monolayer was additionally studied on the gold (100) surface. Compared to copper, silver exhibits a more complicated monolayer formation process that occurs over three distinct peaks in voltammetry. To accommodate the additional complexity of the adsorption process, quantum–continuum results were used to fit a two-dimensional cluster expansion model of the surface. The cluster expansion was then used as an effective Hamiltonian to perform large scale Metropolis Monte Carlo simulations of the interface in the grand canonical ensemble to simulate the monolayer under applied voltage. In the absence of surface charging effects, silver is incorrectly predicted to form a monolayer in a single first order phase transition. The introduction of finite surface charge controlled via an applied voltage led to the appearance of a second peak in the adsorption isotherm, providing a qualitative enhancement in the theoretical description of the silver electrodeposition process. Additional work is required to assess the role of co-adsorption effects on the monolayer stability. Finally, the quantum–continuum model was applied to study a well-characterized palladium-gold surface alloy on the gold (111) surface. Palladium-gold surface alloys are known to exhibit ensemble effects due to the formation of dispersed palladium multimers along the surface giving rise to a distribution of different active sites. A sizable body of work exists examining the effects of temperature and adsorption on the stability of the surface alloy. We extend this work to consider the effects of voltage and finite temperature using the proposed quantum–continuum approach. A two-dimensional cluster expansion was fit to mixing enthalpy data and canonical Monte Carlo simulations of the alloy-solution interface were performed under applied voltage. We report the dominant type of palladium multimers present under a range of electrochemical conditions, finding good agreement with in situ scanning tunneling microscopy results. Additional work is required to study the combined influence of adsorption-induced segregation effects in the presence of excess surface charge. The results presented herein demonstrate the development of a comprehensive yet computationally tractable model of electrodeposition phenomena. Beyond the scope of transition metal electrodeposition, the proposed methodology may be used to study a wide array of interfacial electrochemical phenomena at general electrode-electrolyte interfaces.