Ab Initio Atomistic Thermodynamics for Fuel Cell Catalysis - Fuel Cell Catalysis: A Surface Science Approach - page 128

& CHAPTER 5
Ab Initio Atomistic Thermodynamics
for Fuel Cell Catalysis
TIMO JACOB
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4 - 6, Berlin, D-14915 Germany
5.1 INTRODUCTION
In recent years, there has been increasing interest in electrochemistry. Here, fuel cells
have played an important role, triggered, for example, by the desire for ecologically
sound and economically efficient use of energy resources. However, electrochemical
reactions occur in a multicomponent environment (including, e.g., oxygen, water, and
impurities) and under conditions of finite temperature, pressure, and electrode poten-
tial, leading to extremely complex systems. In particular, the presence of an electrode
potential, which results in the formation of an electric double layer, influences the
structure of the electrode/electrolyte interface. All of these factors make experimental
studies and ab initio calculations most challenging.
Regarding the electrode/electrolyte interface, it is important to distinguish between
two types of electrochemical systems: thermodynamically closed (and in equilibrium)
and open systems. While the former can be understood by knowing the equilibrium
atomic structure of the interface and the electrochemical potentials of all components,
open systems require more information, since the electrochemical potentials within the
interface are not necessarily constant. Variations could be caused by electrocatalytic
reactions locally changing the concentration of the various species. In this chapter,
we will focus on the former situation, i.e., interfaces in equilibrium with a bulk elec-
trode and a multicomponent bulk electrolyte, which are both influenced by tempera-
ture and pressures/activities, and constrained by a finite voltage between electrode and
electrolyte.
Experimentally, different structure- and surface-sensitive techniques such as in situ
scanning tunnelling microscopy (STM), in situ X-ray diffraction (XRD), transition
electron microscopy (TEM), and in situ infrared (IR) spectroscopy have been