42 Hydrogen Fuel Cells Basic Principles

4.2.1 Electrode reactions

We have seen that the basic principle of the fuel cell is the release of energy following a chemical reaction between hydrogen and oxygen. The key difference between this and simply burning the gas is that the energy is released as an electric current, rather that heat. How is this electric current produced?

To understand this we need to consider the separate reactions taking place at each electrode. These important details vary for different types of fuel cell, but if we start with a cell based on an acid electrolyte, we shall consider the simplest and the most common type.

At the anode of an acid electrolyte fuel cell the hydrogen gas ionises, releasing electrons and creating H+ ions (or protons).

This reaction releases energy. At the cathode, oxygen reacts with electrons taken from the electrode, and H+ ions from the electrolyte, to form water.

Clearly, for both these reactions to proceed continuously, electrons produced at the anode must pass through an electrical circuit to the cathode. Also, H+ ions must pass through the electrolyte. An acid is a fluid with free H+ ions, and so serves this purpose very well. Certain polymers can also be made to contain mobile H+ ions. These materials are called 'proton exchange membranes', as an H+ ion is also a proton, and their construction is explained below in Section 4.5.

Comparing equations (4.2) and (4.3) we can see that two hydrogen molecules will be needed for each oxygen molecule if the system is to be kept in balance. This is shown in Figure 4.3. It should be noted that the electrolyte must allow only H+ ions to pass through it, and not electrons. Otherwise the electrons would go through the electrolyte, not round the external circuit, and all would be lost.

4.2.2 Different electrolytes

The reactions given above may seem simple enough, but they do not proceed rapidly in normal circumstances. Also, the fact that hydrogen has to be used as a fuel is a disadvantage. To solve these and other problems many different fuel cell types have been tried. The different types are usually distinguished by the electrolyte that is used, though there are always other important differences as well. Most of these fuel cells have

Hydrogen fuel

Anode 2H2 ^ 4H

+ 4e-

H+ ions through electrolyte

Cathode O2 + 4e- + 4H+ ^ 2H2O

Oxygen, usually from the air

Oxygen, usually from the air

LOAD

e.g. electric motor

Electrons flow round the external circuit

Figure 4.3 The reactions at the electrodes, and the electron movement, in a fuel cell with an acid electrolyte

Table 4.1

Data for different types of fuel cell

Fuel cell type

Mobile ion Operating temp.

Applications and notes

Alkaline (AFC)

OH-

50-200°C

Used in space vehicles, e.g. Apollo,

Shuttle.

Proton exchange

H+

30-100°C

Vehicles and mobile applications, and

membrane (PEMFC)

for lower power CHP systems

Direct methanol

H+

20-90°C

Suitable for portable electronic

(DMFC)

systems of low power, running for

long times

Phosphoric acid

H+

~220°C

Large numbers of 200 kW CHP

(PAFC)

systems in use

Molten carbonate

CO32-

~650°C

Suitable for medium to large scale

(MCFC)

CHP systems, up to MW capacity

Solid oxide (SOFC)

O2-

500-1000°C

Suitable for all sizes of CHP systems,

2kW to multi MW

somewhat different electrode reactions than those given above, however such details are given elsewhere (Larminie and Dicks 2003).

The situation now is that six classes of fuel cell have emerged as viable systems for the present and near future. Basic information about these systems is given in Table 4.1.

As well as facing up to different problems, the various fuel types also try to play to the strengths of fuel cells in different ways. The PEM fuel cell capitalises on the essential simplicity of the fuel cell. The electrolyte is a solid polymer, in which protons are mobile. The chemistry is the same as the acid electrolyte fuel cell of Figure 4.3 above. With a solid and immobile electrolyte, this type of cell is inherently simple; it is the type that shows by far the most promise for vehicles, and is the type used on all the most impressive demonstration fuel cell vehicles. This type of fuel cell is the main focus of this chapter.

PEM fuel cells run at quite low temperatures, so the problem of slow reaction rates has to be addressed by using sophisticated catalysts and electrodes. Platinum is the catalyst, but developments in recent years mean that only minute amounts are used, and the cost of the platinum is a small part of the total price of a PEM fuel cell. The problem of hydrogen supply is not really addressed; quite pure hydrogen must be used, though various ways of supplying this are possible, as is discussed in Chapter 5.

One theoretically very attractive solution to the hydrogen supply problem is to use methanol1 as a fuel instead. This can be done in the PEM fuel cell, and such cells are called direct methanol fuel cells. 'Direct' because they use the methanol as the fuel as it is, in liquid form, as opposed to extracting the hydrogen from the methanol using one of the methods described in Chapter 5. Unfortunately these cells have very low power, and for the foreseeable future at least their use will be restricted to applications requiring slow and steady generation of electricity over long periods. A demonstration DMFC powered go-kart has been built, but really the only likely application of this type of cell in the near future is in the rapidly growing area of portable electronics equipment.

1 A fairly readily available liquid fuel, formula CH3OH.

Although PEM fuel cells were used on the first manned spacecraft, the alkaline fuel cell was used on the Apollo and is used on the Shuttle Orbiter. The problem of slow reaction rate is overcome by using highly porous electrodes, with a platinum catalyst, and sometimes by operating at quite high pressures. Although some historically important alkaline fuel cells have operated at about 200°C, they more usually operate below 100°C. The alkaline fuel cell has been used by a few demonstration electric vehicles, always in hybrid systems with a battery. They can be made more cheaply than PEMFCs, but they are lower in power, and the electrolyte reacts with carbon dioxide in the air, which make terrestrial applications difficult.

The phosphoric acid fuel cell (PAFC) was the first to be produced in commercial quantity and enjoy widespread terrestrial use. Many 200 kW systems, manufactured by the International Fuel Cells Corporation, are installed in the USA and Europe, as well as systems produced by Japanese companies. However, they are not suitable for vehicles, as they operate at about 220°C, and do not react well to being cooled down and re-started; they are suited to applications requiring power all the time, day after day, month after month.

As is the way of things, each fuel cell type solves some problems, but brings new difficulties of its own. The solid oxide fuel cell (SOFC) operates in the region of 600 to 1000°C. This means that high reaction rates can be achieved without expensive catalysts, and that gases such as natural gas can be used directly, or 'internally reformed' within the fuel cell; they do not have to have a hydrogen supply. This fuel cell type thus addresses some of the problems and takes full advantage of the inherent simplicity of the fuel cell concept. Nevertheless, the ceramic materials that these cells are made from are difficult to handle, so they are expensive to manufacture, and there is still quite a large amount of extra equipment needed to make a full fuel cell system. This extra plant includes air and fuel pre-heaters, also the cooling system is more complex, and they are not easy to start up. No-one is developing these fuel cells as the motive power unit for vehicles, but some are developing smaller units to provide the electric power for air conditioning and other systems on modern 'conventional' engined vehicles, which have very high electric power demands these days. However, that is not the focus of this book.

Despite operating at temperatures of up to 1000°C, the SOFC always stays in the solid state. This is not true for the molten carbonate fuel cell (MCFC), which has the interesting feature that it needs the carbon dioxide in the air to work. The high temperature means that a good reaction rate is achieved using a comparatively inexpensive catalyst, nickel. The nickel also forms the electrical basis of the electrode. Like the SOFC it can use gases such as methane and coal gas (H2 and CO) as fuel. However, this simplicity is somewhat offset by the nature of the electrolyte, a hot and corrosive mixture of lithium, potassium and sodium carbonates. They are not suitable for vehicles,2 as they only work well as rather large systems, running all the time.

So, fuel cells are very varied devices, and have applications way beyond vehicles. For the rest of this chapter we will restrict ourselves to the PEM fuel cell, as it is by far the most important in this context.

2 Except ships.

4.2.3 Fuel cell electrodes

Figure 4.4 is another representation of a fuel cell. Hydrogen is fed to one electrode, and oxygen, usually as air, to the other. A load is connected between the two electrodes, and current flows. However, in practice a fuel cell is far more complex than this. Normally the rate of reaction of both hydrogen and oxygen is very slow, which results in a low current, and so a low power. The three main ways of dealing with the slow reaction rates are: the use of suitable catalysts on the electrode, raising the temperature, and increasing the electrode area.

The first two can be applied to any chemical reaction. However, the third is special to fuel cells and is very important. If we take a reaction such as that of equation (4.3), we see that oxygen gas, and H+ ions from the electrolyte, and electrons from the circuit are needed, all three together. This 'coming together' must take place on the surface of the electrode. Clearly, the larger the electrode area, the more scope there is for this to happen, and the greater the current. This is very important. Indeed, electrode area is such a vital issue that the performance of a fuel cell design is often quoted in terms of the current per cm2.

The structure of the electrode is also important. It is made highly porous so that the real surface area is much greater than the normal length x width.

As well as being of a large surface area, and highly porous, a fuel cell electrode must also be coated with a catalyst layer. In the case of the PEMFC this is platinum, which

Cathode Electrolyte Anode

Figure 4.4 Basic cathode-electrolyte-anode construction of a fuel cell. Note that the anode is the negative terminal, and the cathode the positive. This may seem counter to expectations, but is in fact true for all primary cell. The rule is that the cathode is the terminal that the electrons flow into. So, in electrolysis cells the cathode is the negative

Cathode Electrolyte Anode

Figure 4.4 Basic cathode-electrolyte-anode construction of a fuel cell. Note that the anode is the negative terminal, and the cathode the positive. This may seem counter to expectations, but is in fact true for all primary cell. The rule is that the cathode is the terminal that the electrons flow into. So, in electrolysis cells the cathode is the negative is highly expensive. The catalyst thus needs to be spread out as finely as possible. This is normally done by supporting very fine particles of the catalyst on carbon particles. Such a carbon-supported catalyst is shown for real in Figure 4.5, and in idealised form in Figure 4.6.

The reactants need to be brought into contact with the catalyst, and a good electrical contact needs to be made with the electrode surface. Also, in the case of the cathode, the product water needs to be removed. These tasks are performed by the 'gas diffusion layer', a porous and highly conductive material such as carbon felt or carbon paper, which is layered on the electrode surface.

Figure 4.5 Electron microscope image of some fuel cell catalyst. The black specks are the catalyst particles finely divided over larger carbon supporting particles (Reproduced by kind permission of Johnson Matthey plc.)
Figure 4.6 The structure, idealised, of carbon-supported catalyst

Gas diffusion layer Electrode

Figure 4.7 Simplified and idealised structure of a PEM fuel cell electrode

Gas diffusion layer Electrode

Figure 4.7 Simplified and idealised structure of a PEM fuel cell electrode

Finally, some of the electrode is allowed to permeate over the surface of the carbon supported catalyst to increase the contact between reactants. The resulting structure is shown, in somewhat idealised form, in Figure 4.7. All items shown in this diagram are in reality very thin. The electrolyte is about 0.05 to 0.1 mm thick, and each electrode is about 0.03 mm thick, with the gas diffusion layers each about 0.2 to 0.5 mm thick. The whole anode/electrolyte/cathode assembly, often called a membrane electrode assembly or MEA, is thus typically about 1 mm thick, including the gas diffusion layers.

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