45 Water Management in the PEM Fuel Cell

4.5.1 Introduction to the water problem

We see in Figure 4.3 the different electrode reactions in a fuel cell. Looking back at this diagram, you will see that the water product from the chemical reaction is made on the positive electrode, where air is supplied.

This is highly convenient. It means that air can be supplied to this electrode, and as it blows past it will supply the necessary oxygen, and also evaporate off the product water and carry it off, out of the fuel cell.

This is indeed what happens, in principle, in the PEM fuel cell. However, unfortunately the details are far more complex and much more difficult to manage. The reasons for this require that we understand in some detail the operation of the electrolyte of a PEM fuel cell.

4.5.2 The electrolyte of a PEM fuel cell

The different companies producing polymer electrolyte membranes have their own special tricks, mostly proprietary. However, a common theme is the use of sulphonated fluoropolymers, usually fluoroethylene. The most well known and well established of these is Nafion (® Dupont), which has been developed through several variants since the 1960s. This material is still the electrolyte against which others are judged, and is in a sense an 'industry standard'. Other polymer electrolytes function in a similar way.5

The construction of the electrolyte material is as follows. The starting point is the basic and simplest to understand man-made polymer, polyethylene. Based on ethylene, its molecular structure is shown in Figure 4.17.

This basic polymer is modified by substituting fluorine for the hydrogen. This process is applied to many other compounds, and is called 'perfluorination'. The 'mer' is

5 For a review of work with other types of proton exchange membrane, see Rozière and Jones (2001).

H H HHHHHHHHHHHHHHH

H H HHHHHHHHHHHHHHH

Ethylene Polyethylene (or polythene)

Figure 4.17 The structure of polyethylene

F F FFFFFFFFFFFF FFF

\ / i i i i i i i i i i i i i i i C = C -C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-

F F FFFFFFFFFFFF FFF

Tetrafluoroethylene Polytetrafluoroethylene (PTFE)

Figure 4.18 The structure of PTFE

called tetrafluoroethylene.6 The modified polymer, shown in Figure 4.18, is polytetrafluoroethylene, or PTFE. It is also sold as Teflon, the registered trademark of ICI. This remarkable material has been very important in the development of fuel cells. The strong bonds between the fluorine and the carbon make it highly resistant to chemical attack and durable. Another important property is that it is strongly hydrophobic, and so it is used in fuel cell electrodes to drive the product water out of the electrode, and thus prevent flooding. It is used in this way in phosphoric acid and alkali fuel cells, as well as PEMFCs. (The same property gives it a host of uses in outdoor clothing and footwear.)

However, to make an electrolyte, a further stage is needed. The basic PTFE polymer is 'sulphonated'; a side chain is added, ending with sulphonic acid HSO3. Sulphonation of complex molecules is a widely used technique in chemical processing. It is used, for example, in the manufacture of detergent. One possible side chain structure is shown in Figure 4.19; the details vary for different types of Nafion, and with different manufacturers of these membranes. The methods of creating and adding the side chains is proprietary, though one modern method is discussed by Kiefer et al. (1999).

The HSO3 group added is ionically bonded, and so the end of the side chain is actually an SO3— ion. The result of the presence of these SO3— and H+ ions is that there is a strong mutual attraction between the + and — ions from each molecule. The result is that the side chain molecules tend to cluster within the overall structure of the material. Now, a key property of sulphonic acid is that it is highly hydrophyllic, it attracts water. (This is why it is used in detergent; it makes one end of the molecule mix readily with water, while the other end attaches to the dirt.) In Nafion, this means we are creating hydrophyllic regions within a generally hydrophobic substance, which is bound to create interesting results.

The hydrophyllic regions around the clusters of sulphonated side chains can lead to the absorption of large quantities of water, increasing the dry weight of the material by up to 50%. Within these hydrated regions the H+ ions are relatively weakly attracted to the SO3 — group, and are able to move. This creates what is essentially a dilute acid. The resulting material has different phases, dilute acid regions within a tough and strong hydrophobic

6 'Tetra' indicates that all four hydrogens in each ethylene group have been replaced by fluorine.

FFFFFFFFFFFF FFF

—C—C—C-C—C—C—C—C—C—C—C—C—C—C—C—

FFFFFFFOFFFF FFF

Figure 4.19 Example structure of a sulphonated fluoroethylene, also called perfluorosulphonic acid PTFE copolymer

Figure 4.20 The structure of Nafion-type membrane materials. Long chain molecules containing hydrated regions around the sulphonated side chains structure. This is illustrated in Figure 4.20. Although the hydrated regions are somewhat separate, it is still possible for the H+ ions to move through the supporting long molecule structure. However, it is easy to see that for this to happen the hydrated regions must be as large as possible. In a well hydrated electrolyte there will be about 20 water molecules for each SO3- side chain. This will typically give a conductivity of about 0.1 Scm-1. As the water content falls, so the conductivity falls in a more or less linear fashion.

From the point of view of fuel cell use, the main features of Nafion and other fluoro-sulphonate ionomers are that:

• they are highly chemically resistant;

• they are mechanical strong, and so can be made into very thin films, down to 50 ^m;

Water collects around the clusters of hydrophylic sulphonate side chains

Figure 4.20 The structure of Nafion-type membrane materials. Long chain molecules containing hydrated regions around the sulphonated side chains

Water collects around the clusters of hydrophylic sulphonate side chains

• they can absorb large quantities of water;

• if they are well hydrated, then H+ ions can move quite freely within the material, so they are good proton conductors.

This material then is the basis of the proton exchange membrane (PEM) fuel cell. It is not cheap to manufacture, but costs could fall if production was on a really large scale. The key point to remember for the rest of this section is that for the electrolyte to work properly, it must be very well hydrated.

4.5.3 Keeping the PEM hydrated

It will be clear from the description of a proton exchange membrane given in the last section that there must be sufficient water content in the polymer electrolyte. The proton conductivity is directly proportional to the water content. However, there must not be so much water that the electrodes, which are bonded to the electrolyte, flood and block the pores in the electrodes or gas diffusion layer. A balance is therefore needed, which takes care to achieve.

In the PEMFC water forms at the cathode; revisit Figure 4.3 if you are not sure why. In an ideal world this water would keep the electrolyte at the correct level of hydration. Air would be blown over the cathode, and as well as supplying the necessary oxygen it would dry out any excess water. Because the membrane electrolyte is so thin, water would diffuse from the cathode side to the anode, and throughout the whole electrolyte a suitable state of hydration would be achieved without any special difficulty. This happy situation can sometimes be achieved, but needs good engineering design to bring to pass.

There are several complications. One is that during the operation of the cell the H+ ions moving from the anode to the cathode (see Figure 4.3) pull water molecules with them. This process is sometimes called 'electro-osmotic drag'. Typically between 1 and 5 water molecules are 'dragged' for each proton (Zawodzinski et al. 1993, Ren and Gottesfeld 2001). This means that, especially at high current densities, the anode side of the electrolyte can become dried out, even if the cathode is well hydrated. Another major problem is that the water balance in the electrolyte must be correct throughout the cell. In practice, some parts may be just right, others too dry, and others flooded. An obvious example of this can be seen if we think about the air as it passes through the fuel cell. It may enter the cell quite dry, but by the time it has passed over some of the electrodes it may be about right. However, by the time it has reached the exit it may be so saturated that it cannot dry off any more excess water. Obviously, this is more of a problem when designing larger cells and stacks.

Yet another complication is the drying effect of air at high temperatures. If the PEM fuel cell is operating at about 85°C, then it becomes very hard not to dry out the electrolyte. Indeed, it can be shown7 that at temperatures of over about 65°C the air will always dry out the electrodes faster than water is produced by the H2/O2 reaction. However, operation at temperatures of about 85°C or so is essential if enough power is to be extracted for automotive applications.

7 Buchi and Srinivasan (1997).

The only way to solve these problems is to humidify the air, the hydrogen or both, before they enter the fuel cell. This may seem bizarre, as it effectively adds by-product to the inputs to the process, and there cannot be many other processes where this is done. However, in the larger, warmer PEM fuel cells used in vehicles this is always needed.

This adds an important complication to a PEM fuel cell system. The technology is fairly straightforward, and there are many ways in which it can be done. Some methods are very similar to the injection of fuel into the air stream of IC engines. Others are described in fuel cell texts. However, it will certainly add significantly to the system size, complexity and cost.

The water that is added to the air or hydrogen must come from the air leaving the fuel cell, so an important feature of an automotive fuel cell system will be a method of condensing out some of the water carried out by the damp air leaving the cell.

A further impact that the problem of humidifying the reactant gases has on the design of a PEM fuel cell system is the question of operating pressure. In Section 4.2.4 it was pointed out that raising the system pressure increases fuel cell performance, but only rarely does the gain in power exceed the power required to compress the reactant air. However, the problem of humidifying the reactant gases, and of preventing the electrolyte drying out, becomes much less if the cell is pressurised. The precise details of this are proved elsewhere,8 but suffice to say here that if the air is compressed, then much less water needs to be added to raise the water vapour pressure to a point where the electrolyte remains well hydrated. Indeed there is some synergy between compressing the reactant gases and humidifying them, as compression (unless very slow) invariably results in heating. This rise in temperature both promotes the evaporation of water put into the gas stream, and the evaporation of the water cools that gas, and prevents it from entering the fuel cell too hot.

Hybrid Cars The Whole Truth Revealed

Hybrid Cars The Whole Truth Revealed

Hybrid Cars! Man! Is that a HOT topic right now! There are some good reasons why hybrids are so hot. If you’ve pulled your present car or SUV or truck up next to a gas pumpand inserted the nozzle, you know exactly what I mean! I written this book to give you some basic information on some things<br />you may have been wondering about.

Get My Free Ebook


Post a comment