Electrocatalysis

Theoretical Foundations and Model Experiments

Richard C. Alkire, Dieter M. Kolb, Jacek Lipkowski

• Publisher: Wiley-VCH
• Year: 2013
• Print ISBN: 9783527332274
• eBook ISBN: 9783527680450
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physical & Theoretical Chemistry

Catalysts speed up a chemical reaction or allow for reactions to take place that would not otherwise occur. The chemical nature of a catalyst and its structure are crucial for interactions with reaction intermediates.
An electrocatalyst is used in an electrochemical reaction, for example in a fuel cell to produce electricity. In this case, reaction rates are also dependent on the electrode potential and the structure of the electrical double-layer.
This work provides a valuable overview of this rapidly developing field by focusing on the aspects that drive the research of today and tomorrow. Key topics are discussed by leading experts, making this book a must-have for many scientists of the field with backgrounds in different disciplines, including chemistry, physics, biochemistry, engineering as well as surface and materials science. This book is volume XIV in the series "Advances in Electrochemical Sciences and Engineering".

1

Multiscale Modeling of Electrochemical Systems

Jonathan E. Mueller, Donato Fantauzzi, and Timo Jacob

1.1 Introduction

As one of the classic branches of physical chemistry, electrochemistry enjoys a long history. Its relevance and vitality remain unabated as it not only finds numerous applications in traditional industries, but also provides the scientific impetus for a plethora of emerging technologies. Nevertheless, in spite of its venerability and the ubiquity of its applications, many of the fundamental processes, underlying some of the most basic electrochemical phenomena, are only now being brought to light.

Electrochemistry is concerned with the interconversion of electrical and chemical energy. This interconversion is facilitated by transferring an electron between two species involved in a chemical reaction, such that the chemical energy associated with the chemical reaction is converted into the electrical energy associated with transferring the electron from one species to the other. Taking advantage of the electrical energy associated with this electron transfer for experimental or technological purposes requires separating the complementary oxidation and reduction reactions of which every electron transfer is composed. Thus, an electrochemical system includes an electron conducting phase (a metal or semiconductor), an ion conducting phase (typically an electrolyte, with a selectively permeable barrier to provide the requisite chemical separation), and the interfaces between these phases at which the oxidation and reduction reactions take place.

Thus, the fundamental properties of an electrochemical system are the electric potentials across each phase and interface, the charge transport rates across the conducting phases, and the chemical concentrations and reaction rates at the oxidation and reduction interfaces. Traditional experimental techniques (e.g., cyclic voltammetry) measure one or more of these continuous observables in an effort to understand the interrelationships between purely electrochemical phenomena (e.g., electrode potential, current density). While these techniques often shed light on both fundamental (e.g., ionic charge) and statistical (e.g., diffusion rates) properties of the atoms and ions that make up an electrochemical system, they provide little insight into the detailed atomic structure of the system.

In contrast, modern surface science techniques (e.g., STM, XPS, SIMS, LEISS) typically probe the atomistic details of the interface regions, and support efforts to gain insight into the atomistic processes underlying electrochemical phenomena. Indeed, these methods have been applied to gas–solid interfaces with resounding success, elucidating the atomistic structures underlying macroscopic phenomena [1]. Unfortunately, the presence of the electrolyte at the electrode surface hampers the application of many of these surface science techniques. Because the resulting solid–electrolyte interface is an essential component of any electrochemical system, electrochemistry has not yet fully experienced the atomistic revolution enjoyed by other departments of surface science, although these techniques are increasingly making their way into electrochemistry [2].

The dramatic increases in computing power realized over the past decades coupled with improved algorithms and methodologies have enabled theorists to develop reliable, atomistic-level descriptions of surface structures and processes [3]. In particular, periodic density functional theory (DFT) now exhibits a degree of efficiency and accuracy which allows it not only to be used to explain, but also to predict experimental results, allowing theory to take a proactive, or even leading, role in surface science investigations. A prime example of this is the design of a new steam reforming catalyst based on a combination of theoretical and experimental fundamental research [4].

The application of DFT to electrochemical systems is not as straightforward as it is to the surface–vacuum interfaces of surface sciences. There have indeed been promising efforts in this direction [5–7], and there is a growing interest in theorectical electrochemistry [8–10]; however, proper treatments of the electrolyte and electrode potential provide novel challenges for which there are not yet universally agreed upon solutions. Nevertheless, there are already success stories, such as the theoretical prediction [11,12] and experimental confirmation [13] of the nonmonotonic dependence of the electrocatalytic activity of the hydrogen evolution reaction (HER) on the thickness of Pd overlayers on Au(111).

Common to both the experimental and theoretical approaches mentioned above is the existence of two regimes – the macroscopic and the atomistic – and the importance of relating these in order to obtain a comprehensive picture of an electrochemical system. Statistical mechanics provides the necessary framework for relating the discrete properties and atomistic structures of the atomistic regime to the continuous variable controlled or observed in the macroscopic regime. The fundamental assumption underlying this relationship is what Richard Feynman called the “atomic hypothesis”, which we rephrase in terms of electrochemistry as follows: “there is nothing that electrochemical systems do that cannot be understood from the point of view that they are made up of atoms acting according to the laws of physics” [14].

Modern computational methods, based on the principles of quantum mechanics, provide a means of probing the atomistic details of electrochemical systems, as do the techniques of modern surface science techniques. The concepts of statistical mechanics are critical for extending the results of these molecular scale models to macro-scale descriptions of electrochemical systems. Such a procedure creates a multiscale model of an electrochemical system, built up from the atomistic details of the quantum regime to a description of the electrochemical phenomena observed in macroscopic systems.

This chapter is intended to serve as an introduction to multiscale modeling for electrochemists with minimal background in the methods of modern computational chemistry. Thus, the fundamentals of some of the most important methods are presented within the framework of multiscale modeling, which integrates diverse methods into a single multiscale model, which then spans a wider range of time and length scales than is otherwise possible. The physical ideas underlying the methods and the conceptual framework used to ...

Table of contents

• Cover
• Advances in Electrochemical Science and Engineering
• Title Page
• Copyright
• In Memoriam
• Preface
• List of Contributors
• Chapter 1: Multiscale Modeling of Electrochemical Systems
• Chapter 2: Statistical Mechanics and Kinetic Modeling of Electrochemical Reactions on Single-Crystal Electrodes Using the Lattice-Gas Approximation
• Chapter 3: Single Molecular Electrochemistry within an STM
• Chapter 4: From Microbial Bioelectrocatalysis to Microbial Bioelectrochemical Systems
• Chapter 5: Electrocapillarity of Solids and its Impact on Heterogeneous Catalysis
• Chapter 6: Synthesis of Precious Metal Nanoparticles with High Surface Energy and High Electrocatalytic Activity
• Chapter 7: X-Ray Studies of Strained Catalytic Dealloyed Pt Surfaces
• Index