TABLE OF CONTENTS Durability, in the present context, is a concept that relates how fuel cells and fuel cell systems are made and operated to how long they last. The science and methodology of this concept are the subject of this chapter. The approach we will discuss has its roots in the history of technical development of such concepts for composite material systems in general [1]. It is appropriate as a starting point for fuel cells, since fuel cells are functional composite material systems.
The definition of the concept for fuel cells and fuel cell systems is illustrated in Figure 3.1. The foundation of the concept is a "performance metric." The performance metric is defined as a function or functional that relates operating conditions (inputs, or "extensive variables") to cell, stack, or system properties and characteristics, to predict (calculate) an output that defines performance. The output, for a fuel cell, may be voltage, current, power, power density, or any other single or multiple output that defines the performance. One may write an expression such as
As Figure 3.1 shows, if we ask what is the (maximum) performance available from our fuel cell (or system), we can expect a level defined by how the fuel cell is made. If we have mature and robust representations (balance equations and constitutive equations) of the multiphysics processes that define that performance, it can be calculated.
