Chapter 1
Introduction

1.1 Principles of Fuel Cells

Fuel cells are one of the oldest energy conversion methods known to man since the mid-nineteenth century. Since the beginning of the twentieth century, the conversion of chemical energy into electrical energy has become more important due to an increase in the use of electricity. One of the major factors that has influenced the development of fuel cells has been the increasing concern about the environmental consequences of fossil fuel use in the production of electricity and for the propulsion of vehicles. The dependence of the industrialized countries on oil became apparent in the oil shock. Fuel cells may help reduce our dependence on fossil fuels and diminish poisonous emissions into the atmosphere, since fuel cells have higher energy conversion efficiencies compared with heat engines. Using hydrogen and oxygen, fuel cells produce only water that can eliminate the emissions caused by other methods used now for electricity production. The share of renewable energy from wind, sun, and water may also eliminate the pollution. However, these sources are not suited to cover the electrical base load due to their irregular availability. The combination of these sources, however, to produce hydrogen in cooperation with fuel cells will be an option for future power generation [1-3].

Fuel cells are galvanic cells in which the free energy of a chemical reaction is converted into electrical energy. The Gibbs free energy change of a chemical reaction is related to the cell voltage, as shown in Eq. (1.1) [4]:

where n is the number of electrons involved in the reaction, F is the Faraday constant, and ?U0 is the voltage of the cell for thermodynamic equilibrium in the absence of a current flow. The anode reaction in fuel cells is either the direct oxidation of hydrogen or the oxidation of the hydrocarbon compounds like methanol. An indirect oxidation via a reforming step can also occur. The cathode reaction in fuel cells is the reduction of oxygen, in most cases from air.

For the case of a hydrogen/oxygen fuel cell, the principle is shown in Fig. 1.1. The overall reaction is

with an equilibrium cell voltage of ?U0 for standard conditions at 25 °C of ?U0 = 1.23 V. The equilibrium cell voltage is the difference of the equilibrium electrode potentials of cathode and anode that are determined by the electrochemical reaction taking place at the respective electrode:

Fig. 1.1 Schematic drawing of a hydrogen/oxygen fuel cell and its reactions based on polymer electrolyte membrane fuel cell (PEMFC).

The basic structure of all fuel cells is similar: The cell consists of two electrodes that are separated by the electrolyte and that are connected with an external circuit. The electrodes are exposed to gas or liquid flows to supply the electrodes with fuel or oxidant (e.g., hydrogen or oxygen). The electrodes have to be gas or liquid permeable and, therefore, possess a porous structure. The structure and content of the gas diffusion electrodes (GDEs) are quite complex and require considerable optimization for practical application. The electrolyte should possess gas permeability as low as possible. For fuel cells with a proton-conducting electrolyte, hydrogen is oxidized at the anode (according to Eq. (1.4)) and protons enter the electrolyte and are transported to the cathode:

At the cathode, the supplied oxygen reacts according to

Electrons flow in the external circuit during these reactions. The oxygen ions recombine with protons to form water:

The product of this reaction is water that is formed at the cathode in fuel cells with proton-conducting membranes. It can be formed at the anode, if an oxygen ion (or carbonate)-conducting electrolyte is used instead, as is the case for high-temperature fuel cells.

1.2 Types of Fuel Cells

Fuel cells are usually classified by the electrolyte employed in the cell. An exception to this classification is DMFC (direct methanol fuel cell) that is a fuel cell in which methanol is directly fed to the anode. The electrolyte of this cell does not determine the class. The operating temperature for each of the fuel cells can also determine the class. There are, thus, low- and high-temperature fuel cells. Low-temperature fuel cells are alkaline fuel cells (AFCs), polymer electrolyte membrane fuel cells (PEMFCs), DMFC, and phosphoric acid fuel cells (PAFCs). The high-temperature fuel cells operate at temperatures ~600-1000 °C and two different types have been developed, molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFCs). All types of fuel cells are presented in the following sections in order of increasing operating temperature. An overview of the fuel cell types is given in Table 1.1 [1,5-7].

Table 1.1 The Different Fuel Cells That Have Been Realized and Are Currently in Use and Development

1.2.1 AFC

AFC has the advantage of exhibiting the highest energy conversion efficiencies of all fuel cells, but it works properly only with very pure gases, which is considered a major restraint in most applications. The KOH electrolyte that is used in AFCs (usually in concentrations of 30-45 wt%) has an advantage over acid fuel cells, which is due to the fact that the oxygen reduction kinetics are much faster in alkaline electrolyte than in acid, making AFC a very attractive system for specific applications. AFC was one of the first fuel cells used in space. It was used in Apollo missions and the Space Shuttle program.

The first technological AFC (1950s) was developed by the group of Bacon at the University of Cambridge, provided 5 kW power, and used a Ni anode, a lithiated NiO cathode, and 30 wt% aqueous KOH. Its operating temperature and pressure were 200 °C and 5 MPa, respectively. For the Apollo program, a PC3A-2 model was used that employed an 85% KOH solution at operating temperatures of 200-230 °C. In Space Shuttle program, the fuel cells are used for producing energy, cooling of Shuttle compartments, and producing potable water. Three plant modules are used, each with a maximum power output of 12 kW. AFCs are now normally run at operating temperatures below 100 °C, as a higher temperature is not needed to improve oxygen reduction kinetics (although higher temperatures are still advantageous for the hydrogen oxidation kinetics).

AFC electrodes used to be Ni-based catalysts and were sometimes activated with Pt. Pt/C gas diffusion electrodes are now generally used for both the anode and the cathode (see PEM), although other possibilities are being pursued, for example, Pt-Co alloys have been suggested [8] and have proved to have a superior activity than Pt for oxygen reduction due to a higher exchange current density. A Pt-Pd anode was tested for stability characteristics in comparison with Raney Ni [9]. It is known that Raney Ni electrodes have a high activity for hydrogen oxidation, but due to the wettability of the inner pores and changes in chemical structure under operation conditions, a decay in performance occurs. The Pt/Pd activity was also seen to have a very rapid decay initially, but after a short time the decay stopped and the performance remained...