Basic Parameters of Fuel Cells

Most electrical parameters of fuel-cell plants are analogous to those of conventional batteries described in Chapter 3. 

At each electrode j in contact with an electrolyte, a defined value of electrode potential Ej is set up. It can be measured only relative to the potential of another electrode. By convention, in electrochemistry the potential of any given electrode is referred to as the potential of the so-called standard hydrogen electrode (SHE), which in turn by convention is taken as zero. A practical realization of the SHE is an electrode made of platinized platinum dipped into an acid solution having a mean ionic activity of hydrogen ions of unity and is washed by gaseous hydrogen at a pressure of 1 bar. 

In our example, the potential of the hydrogen electrode to which, according to Equation 16.2, electrons are transferred from the hydrogen molecule is more negative 

The potentials of electrodes can be equilibrium (reversible ones) and nonequilibrium or irreversible. An electrode s equilibrium potential (which will be denoted as E in the following) reflects the thermodynamic properties of the electrode reaction occurring at it (thermodynamic potential). The hydrogen electrode is an example of an electrode at which the equilibrium potential is established. When supplying hydrogen to the gas-diffusion electrode mentioned above, a value of electrode potential  

An example of an electrode having a nonequilibrium value of potential is the oxygen electrode. The thermodynamic value of potential E. e. of an oxygen electrode at which Reaction (16.3) takes place is 1.229 V (relative to the SHE). When supplying oxygen to a gas-diffusion electrode, the potential actually established at is 0.8-1.0 V, (i.e., 0.3-0.4 V less positive than the thermodynamic value) (steady-state potential). 

---

A more detailed description of different types of batteries and other electric energy storage systems for electric vehicles can be found in Sect. 5.3, while a description of the main characteristics and properties of fuel cells for automotive application is given here, starting from some basic concepts of electrochemistry and thermodynamic, and focusing the attention on the operative parameters to be regulated to obtain the best performance in the specific application. 

The overall efficiency is a very important parameter for fuel-cell-based power plants, both the centralized plants of high-capacity and the medium- or small-capacity plants set up in large numbers in a distributed fashion. The basic goal of these semps is that of reducing the specific consumption of primary fuels for power generation. 

As a key module of the fuel cell system, the performance and reliability of the stack is crucial. As a world leader, Ballard Power Systems (Canada) has spent many years developing stacks. Table 6.7 lists the basic parameters of Ballard9 SSL stacks. At a rated power of 21.0 kW (current is 300 A), the specific and volumetric power densities of the stack are 1.24 kW kg and 1.52 kW H, respectively. 

In this chapter, the fundamentals of classical thermodynamics as it applies to the study of fuel cells is introduced. Although the reader is assumed to have a background in basic thermodynamics, this chapter includes a review of the physical meaning of several parameters used frequently in electrochemistry and how calculations of their values can be made. This chapter concludes by applying the thermodynamic concepts presented to determine the maximum expected thermodynamic efficiency and open-circuit voltage expected for a fuel cell at a given condition. 

As will be seen, the rate at which the potential is changed (i.e., the sweep rate) becomes veiy important. For complex reactions, it may have to be so slow (0.01 mV s 1) that cyclic voltammetry approaches a potentiostatic (rather than a potentiody-namic) technique. On the other hand, too large a sweep rate may yield parameters that are not those of the steady state and hence are difficult to fit into a mechanism of consecutive reactions in which the attainment of a steady state (d6/dt = 0) at each potential is a basic assumption. Thus, determining the mechanisms of reactions that are to function in steady-state devices such as fuel cells or reactors is more likely to... 

Although EIS offers many advantages for diagnosing fuel cell properties, clear difficulties exist for applying impedance methods and fitting the data to the model to extract the relevant electrochemical parameters. The limitations of the EIS technique derive from the several requirements required to obtain a valid impedance spectrum, because the accuracy of EIS measurement depends not only on the technical precision of the instrumentation but also on the operating procedures. Theoretically, there are three basic requirements for AC impedance measurements linearity, stability, and causality. 

The basic thermodynamic and electrochemical kinetic concepts involved in batteries and the parameters used to evaluate their performance are summarized in Section 2.2. The most widespread primary and rechargeable systems are described by highlighting the most recent advances in Section 2.3. Supercapacitors and fuel cells, whose importance in the field of energy conversion is growing, are also briefly treated in this section. The lithium-based rechargeable systems, the most advanced batteries with the highest performance, are discussed in detail in Section 2.4, with particular emphasis on the new materials on which these batteries are based. 

To design a cooling system for the fuel cell stack the thermal parameters of the stack and its components had to be determined accurately. For this reason a 3D FV (finite volume) thermal model was created. Special emphasis was placed on the thermal conduction of the various layers responsible for heat flow in all directions of the fuel cell stack. The basic parameters are given in Table 8-1. 

This chapter presents the simulation of a button cell data reported by Liu et al. [38]. The model parameters derived here are further used in the performance analysis presented in the later chapters. A schematic representation of button cell is given in Fig. 6.1. The configuration is basically a concentric cylindrical assembly intercepted by the membrane electrode assembly (MEA). The fuel and air inlet are through the inner cylindrical pipe, which reaches above the anode and cathode. The product gas outlet is through the concentric space between the inner and outer cylinder. 

Shah and Besser presented results from their development work that was aimed at a 20-Wd methanol fuel processor/fuel cell system [436]. The principle layout of the device consisted of a methanol steam reformer, preferential oxidation, a catalytic afterburner and an evaporator The basic process and design parameters are summarised in Table 9.3. Nevertheless, it is obvious that the size of the steam reformer exceeded by far the size of all other components. The weight hourly space... 

In this thesis study, a PEM fuel cell was modeled. Using the model, the most important performance parameters of the PEM fuel cell, namely operation temperature and pressure were investigated and their affects were determined on the PEM fuel cell performance. The performance is reflected primarily in the PEM fuel cell output voltage, electrical power output and efficiencies. The basic results of the PEM fuel cell modeling (Table VII. 1) are presented below ... 

---