Chapter 1
Fuel Cells

Fuel cells produce more electricity than batteries or combustion engines, with far fewer emissions. An introduction to the principles and practicalities behind fuel cell technology has been presented (1). Beginning with the underlying concepts, the discussion explores the thermodynamics of fuel cells, kinetics, transport, and modeling before moving onto the application side with guidance on system types and design, performance, costs, and environmental impact.

The latest technological advances and relevant calculations have been presented, along with enhanced chapters on advanced fuel cell design and electrochemical and hydrogen energy systems (1).

Fuel cells are commonly classified on the basis of their electrolyte according to which they can be divided into five main groups (2, 3):

1. Alkaline fuel cells (AFC),
2. Phosphoric acid fuel cells (PAFC),
3. Polymer electrolyte fuel cells (PEFC),
4. Molten carbonate fuel cell (MAFC), and
5. Solid oxide fuel cells (SOFC).

Polymer electrolyte fuel cells can be further subdivided into three general groups: The polymer electrolyte fuel cells feed on hydrogen, direct methanol fuel cells and direct ethanol fuel cells.

The basic issues of fuel cells have been collected in a way also suitable for beginners (2, 4).

Also, the issues of direct liquid fuel cells have been reviewed (5). Direct liquid fuel cells are one of the most promising types of fuel cells due to their high energy density, simple structure, small fuel cartridge, instant recharging, and ease of storage and transport. Alcohols such as methanol and ethanol are the most common types of fuel.

1.1 Conventional Fuel Cells

A schematic view of a polymer electrolyte membrane fuel cell is shown in Figure 1.1.

Figure 1.1 Polymer Electrolyte Membrane Fuel Cell (6).

1.1.1 Sealing Material for Solid Polymer Fuel Cell Separator

A sealing material for solid polymer fuel cells includes a silicone rubber composition and, compounded therewith, a layered double hydroxide has an excellent resistance to hydrofluoric acid (7).

The molecular structure of the organohydrogen poly(siloxane) may be a linear, cyclic, branched or three-dimensional network structure. Illustrative examples of the organohydrogen poly(siloxane) component are summarized in Table 1.1. Some of the components are shown in Figure 1.2.

Table 1.1 Organohydrogen poly(siloxane) (7).

1,1,3,3-Tetramethyldisiloxane

1,3,5,7-Tetramethylcyclotetrasiloxane

Tris(hydrogendimethylsiloxy)methylsilane

Tris(hydrogendimethylsiloxy)phenylsilane

Methylhydrogencyclo poly(siloxane)

Methylhydrogensiloxane-dimethylsiloxane cyclic copolymers

Methylhydrogen poly(siloxane) capped at both ends with trimethylsiloxy groups

Dimethylsiloxane-methylhydrogensiloxane copolymers capped at both ends with trimethylsiloxy groups

Dimethyl poly(siloxane) capped at both ends with dimethylhydrogensiloxy groups

Dimethylsiloxane-methylhydrogensiloxane copolymers capped at both ends with dimethylhydrogensiloxy groups

Methylhydrogensiloxane-diphenylsiloxane copolymers capped at both ends with trimethylsiloxy groups

Methylhydrogensiloxane-diphenylsiloxane-dimethylsiloxane copolymers capped at both ends with trimethylsiloxy groups

Methylhydrogensiloxane-methylphenylsiloxane-dimethylsiloxane copolymers capped at both ends with trimethylsiloxy groups

Methylhydrogensiloxane-dimethylsiloxane-diphenylsiloxane copolymers capped at both ends with dimethylhydrogensiloxy groups

Methylhydrogensiloxane-dimethylsiloxane-methylphenylsiloxane copolymers capped at both ends with dimethylhydrogensiloxy groups

Copolymers consisting of (CH3)2HsiO units and SiO units

Figure 1.2 Siloxanes (7).

1.1.2 Water Management in a Polymer Electrolyte Fuel Cell

Water management of polymer electrolyte fuel cell has been extensively studied because of its effect on the performance of a polymer electrolyte fuel cell system (8). The transport and congelation of water significantly affect the efficiency and durability of a polymer electrolyte fuel cell.

The electrochemical reaction in a polymer electrolyte fuel cell produces water, thereby dampening the electrolyte membrane. The electrochemical reaction at the anode is

(1.1)

and the reaction at the cathode is

(1.2)

Nafion®, c.f. Figure 1.3, is typically used as the electrolyte membrane. However, Nafion exhibits a proton conductivity only in the presence of water.

Figure 1.3 Nafion®.

Therefore, the reactive gases supplied to the fuel cell should be humidified in order to ensure an efficient transport of protons. Unfortunately, an excessive amount of accumulated water in the gas diffusion layer reduces the performance and the durability of a cell (9).

In contrast, operating with a high current density and back diffusion dehydrates the gas diffusion layer of the anode and the membrane (10).

An efficient water management is essential to maintain the performance of a polymer electrolyte fuel cell. Therefore, water in a polymer electrolyte fuel cell system should be accurately analyzed for understanding the water balance. Therefore, the water balance and the removal of water from a polymer electrolyte fuel cell system are the key parameters that govern its efficiency and durability (8).

Several empirical methods have been used to visualize the distribution of water in a polymer electrolyte fuel cell. These methods include (8):

1. Optical imaging (11-13),
2. Magnetic resonance imaging (MRI) (14),
3. Neutron radiography (15-18), and
4. X-ray imaging techniques.

Experimental studies using high-resolution imaging techniques have been conducted to reveal the unknown morphological aspects that reduce the performance of a polymer electrolyte fuel cell system.

The X-ray imaging technique is the preferred method over other imaging techniques because of its high spatial and temporal resolution. Recently, X-ray micro computed tomography has been introduced to better characterize the anisotropic structure of a gas diffusion layer by reconstructing its three-dimensional structure. Due to the development of advanced software and hardware, the X-ray imaging technique has become essential in the visualization of the water management in polymer electrolyte fuel cells (8).

In particular, X-ray imaging experiments have been detailed in order to visualize the water contents and water management in a polymer electrolyte fuel cell system (8).

A highly focused X-ray beam, a high-density scintillator, highly magnified optics, and a digital detector with small pixel size are used here.

The light intensity of the X-ray beam that passes through a test sample can be described by the Beer-Lambert law.

The complementarity properties of the neutron imaging method and synchrotron X-ray radiography have been shown (19). The synchrotron X-ray they employed had a spatial resolution of 3 µm and a temporal resolution of 5 s, whereas those of the neutron imaging system were 150 µm and 10 s, respectively. The field of view for the synchrotron X-ray radiography was only 7×7 mm2, whereas that of the neutron imaging was more than 100 mm2, which is sufficiently large to cover the entire active area of the cells.

The formation of liquid water was investigated as a function of the current density in the in-plane direction of polymer electrolyte fuel cells (20). The synchrotron X-ray imaging setup used in the study is similar to that in the study described in reference (19), with a spatial resolution of 3 µm and a temporal resolution of 5 s. The amount of water in the gas channels exhibited a cyclic eruption of water. The liquid water formation was mostly located beneath the rib, caused by the reduced porosity as a result of compression and the increase of electrical conductivity (8).

Also, the results of several other techniques have been detailed (8). In summary, X-ray radiography is a suitable method for studying the water management in a polymer electrolyte fuel cell, because it has a higher spatial and temporal resolution compared to other imaging techniques. X-ray µCT provides details of the water transport in a gas diffusion layer by adopting a tomograph.

1.1.3 Alkaline Fuel Cells

An alkaline fuel cell is also known as a Bacon fuel cell, named after its inventor, Francis Thomas Bacon (21, 22).

Alkaline fuel cells are among the most efficient fuel cells, with the potential to reach 70% (23).

This type of fuel cell produces electrical power by the following reactions. At the anode hydrogen is oxidized in the presence of alkali according to the following reaction:

(1.3)

Here water is produced and electrons are released. On the cathode, oxygen is reduced according to the following reaction:

(1.4)

Here, the hydroxide ions are given back and the electrons are consumed (23).

The anode and cathode are separated by a porous matrix saturated with an aqueous alkaline electrolyte solution. The...