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1.1 Hydrogen Fuel Cells - Basic Principles
The basic operation of the hydrogen fuel cell is extremely simple. The first demonstration of a fuel cell was by lawyer and scientist William Grove in 1839, using an experiment along the lines of that shown in Figures 1.1a and 1.1b. In Figure 1.1a, water is being electrolysed into hydrogen and oxygen by passing an electric current through it. In Figure 1.1b, the power supply has been replaced with an ammeter, and a small current is flowing. The electrolysis is being reversed - the hydrogen and oxygen are recombining, and an electric current is being produced.
Another way of looking at the fuel cell is to say that the hydrogen fuel is being 'burnt' or combusted in the simple reaction
[1.1] 2[H.sub.2] + [O.sub.2] [right arrow] 2[H.sub.2]O
However, instead of heat energy being liberated, electrical energy is produced.
The experiment shown in Figures 1.1a and 1.1b makes a reasonable demonstration of the basic principle of the fuel cell, but the currents produced are very small. The main reasons for the small current are
the low 'contact area' between the gas, the electrode, and the electrolyte - basically just a small ring where the electrode emerges from theelectrolyte.
the large distance between the electrodes - the electrolyte resists the flow of electric current.
To overcome these problems, the electrodes are usually made flat, with a thin layer of electrolyte as in Figure 1.2. The structure of the electrode is porous so that both the electrolyte from one side and the gas from the other can penetrate it. This is to give the maximum possible contact between the electrode, the electrolyte, and the gas.
However, to understand how the reaction between hydrogen and oxygen produces an electric current, and where the electrons come from, we need to consider the separate reactions taking place at each electrode. These important details vary for different types of fuel cells, but if we start with a cell based around an acid electrolyte, as used by Grove, we shall start with the simplest and still the most common type.
At the anode of an acid electrolyte fuel cell, the hydrogen gas ionises, releasing electrons and creating H.sup.+ ions (or protons).
[1.2] 2[H.sub.2] [right arrow] 4[H.sup.+] + [4e.sup.-]
This reaction releases energy. At the cathode, oxygen reacts with electrons taken from the electrode, and [H.sup.+] ions from the electrolyte, to form water.
[1.3] [O.sub.2] + [4e.sup.-] + 4[H.sup.+] [right arrow] 2[H.sub.2]O
Clearly, for both these reactions to proceed continuously, electrons produced at the anode must pass through an electrical circuit to the cathode. Also, [H.sup.+] ions must pass through the electrolyte. An acid is a fluid with free [H.sup.+] ions, and so serves this purpose very well. Certain polymers can also be made to contain mobile [H.sup.+] ions. These materials are called proton exchange membranes, as an [H.sup.+] ion is also a proton.
Comparing equations 1.2 and 1.3 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 1.3. It should be noted that the electrolyte must only allow [H.sup.+] ions to pass through it, and not electrons. Otherwise, the electrons would go through the electrolyte, not a round the external circuit, and all would be lost.
In an alkaline electrolyte fuel cell the overall reaction is the same, but the reactions at each electrode are different. In an alkali, hydroxyl (O[H.sup.-]) ions are available and mobile. At the anode, these react with hydrogen, releasing energy and electrons, and producing water.
[1.4] 2[H.sub.2] + 4O[H.sup.-] [right arrow] 4[H.sub.2]O + [4e.sup.-]
At the cathode, oxygen reacts with electrons taken from the electrode, and water in the electrolyte, forming new O[H.sup.-] ions.
[1.5] [O.sub.2] + [4e.sup.-] + 2[H.sub.2]O [right arrow] 4O[H.sup.-]
For these reactions to proceed continuously, the O[H.sup.-] ions must be able to pass through the electrolyte, and there must be an electrical circuit for the electrons to go from the anode to the cathode. Also, comparing equations 1.4 and 1.5 we see that, as with the acid electrolyte, twice as much hydrogen is needed as oxygen. This is shown in Figure 1.4. Note that although water is consumed at the cathode, it is created twice as fast at the anode.
There are many different fuel cell types, with different electrolytes. The details of the anode and cathode reactions are different in each case. However, it is not appropriate to go over every example here. The most important other fuel cell chemistries are covered in Chapter 7 when we consider the solid oxide and molten carbonate fuel cells.
1.2 What Limits the Current?
At the anode, hydrogen reacts, releasing energy. However, just because energy is released, it does not mean that the reaction proceeds at an unlimited rate. The reaction has the 'classical' energy form shown in Figure 1.5.
Although energy is released, the 'activation energy' must be supplied to get over the 'energy hill'. If the probability of a molecule having enough energy is low, then the reaction will only proceed slowly. Except at very high temperatures, this is indeed the case for fuel cell reactions.
The three main ways of dealing with the slow reaction rates are
the use of catalysts,
raising the temperature,
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 1.4, we see that fuel gas and O[H.sup.-] ions from the electrolyte are needed, as well as the necessary activation energy. Furthermore, this 'coming together' of [H.sub.2] fuel and O[H.sup.-] ions must take place on the surface of the electrode, as the electrons produced must be removed.
This reaction, involving fuel or oxygen (usually a gas), with the electrolyte (solid or liquid) and the electrode, is sometimes called the three phase contact. The bringing together of these three things is a very important issue in fuel cell design.
Clearly, the rate at which the reaction happens will be proportional to the area of the electrode. 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 [cm.sup.2].
However, the straightforward area (length × width) is not the only issue. As has already been mentioned, the electrode is made highly porous. This has the effect of greatly increasing the effective surface area. Modern fuel cell electrodes have a microstructure that gives them surface areas that can be hundreds or even thousands of times their straightforward 'length × width' (See Figure 1.6.) The microstructural design and manufacture of a fuel cell electrode is thus a very important issue for practical fuel cells. In addition to these surface area considerations, the electrodes may have to incorporate a catalyst and endure high temperatures in a corrosive environment. The problems of reaction rates are dealt with in a more quantitative way in Chapter 3.
1.3 Connecting Cells in Series - the Bipolar Plate
For reasons explained in Chapters 2 and 3, the voltage of a fuel cell is quite small, about 0.7V when drawing a useful current. This means that to produce a useful voltage many cells have to be connected in series. Such a collection of fuel cells in series is known as a 'stack'. The most obvious way to do this is by simply connecting the edge of each anode to the cathode of the next cell, all along the line, as in Figure 1.7. (For simplicity, this diagram ignores the problem of supplying gas to the electrodes.)
The problem with this method is that the electrons have to flow across the face of the electrode to the current collection point at the edge. The electrodes might be quite good conductors, but if each cell is only operating at about 0.7V, even a small voltage drop is important. Unless the current flows are very low, and the electrode is a particularly good conductor, or very small, this method is not used.
A much better method of cell interconnection is to use a 'bipolar plate'. This makes connections all over the surface of one cathode and the anode of the next cell (hence 'bipolar'); at the same time, the bipolar plate serves as a means of feeding oxygen to the cathode and fuel gas to the anode. Although a good electrical connection must be made between the two electrodes, the two gas supplies must be strictly separated.
The method of connecting to a single cell, all over the electrode surfaces, while at the same time feeding hydrogen to the anode and oxygen to the cathode, is shown in Figure 1.8. The grooved plates are made of a good conductor such as graphite, or stainless steel.
To connect several cells in series, 'bipolar plates' are made. These plates - or cell interconnects - have channels cut in them so that the gases can flow over the face of the electrodes. At the same time, they are made in such a way that they make a good electrical contact with the surface of each alternate electrode. A simple design of a bipolar plate is shown in Figure 1.9.
To connect several cells in series, anode/electrolyte/cathode assemblies (as in Figure 1.2) need to be prepared. These are then 'stacked' together as shown in Figure 1.10. This 'stack' has vertical channels for feeding hydrogen over the anodes and horizontal channels for feeding oxygen (or air) over the cathodes. The result is a solid block in which the electric current passes efficiently, more or less straight through the cells rather than over the surface of each electrode one after the other. The electrodes are also well supported, and the whole structure is strong and robust. However, the design of the bipolar plate is not simple. If the electrical contact is to be optimised, the contact points should be as large as possible, but this would mitigate the good gas flow over the electrodes. If the contact points have to be small, at least they should be frequent. However, this makes the plate more complex, difficult, and expensive to manufacture, as well as fragile. Ideally the bipolar plate should be as thin as possible, to minimise electrical resistance and to make the fuel cells stack small. However, this makes the channels for the gas flow narrow, which means it is more difficult to pump the gas round the cell. This sometimes has to be done at a high rate, especially when using air instead of pure oxygen on the cathode. In the case of low-temperature fuel cells, the circulating air has to evaporate and carry away the product water. In addition, there usually have to be further channels through the bipolar plate to carry a cooling fluid. Some of the further complications for the bipolar plate are considered in the next section.
1.4 Gas Supply and Cooling
The arrangement shown in Figure 1.10 has been simplified to show the basic principle of the bipolar plate. However, the problem of gas supply and of preventing leaks means that in reality the design is somewhat more complex.
Because the electrodes must be porous (to allow the gas in), they would allow the gas to leak out of their edges. The result is that the edges of the electrodes must be sealed. Sometimes this is done by making the electrolyte somewhat larger than one or both of the electrodes and fitting a sealing gasket around each electrode, as shown in Figure 1.11. Such assemblies can then be made into a stack, as in Figures 1.10 and 1.12.
The fuel and oxygen can then be supplied to the electrodes using the manifolds as shown disassembled in Figure 1.12 and assembled in Figure 1.13. Because of the seals around the edge of the electrodes, the hydrogen should only come into contact with the anodes as it is fed vertically through the fuel cell stack. Similarly, the oxygen (or air) fed horizontally through the stack should only contact the cathodes, and not even the edges of the anodes. Such would not be the case in Figure 1.10.
The arrangement of Figures 1.12 and 1.13 is used in some systems. It is called external manifolding. It has the advantage of simplicity. However, it has two major disadvantages. The first is that it is difficult to cool the system. Fuel cells are far from 100% efficient, and considerable quantities of heat energy as well as electrical power are generated. (Chapter 3 gives the reasons.) It is clear from Figures 1.12 and 1.13 that it would be hard to supply a cooling fluid running through the cells. In practice, this type of cell has to be cooled by the reactant air passing over the cathodes. This means air has to be supplied at a higher rate than demanded by the cell chemistry; sometimes this is sufficient to cool the cell, but it is a waste of energy. The second disadvantage is that the gasket round the edge of the electrodes is not evenly pressed down - at the point where there is a channel, the gasket is not pressed firmly onto the electrode. This results in an increased probability of leakage of the reactant gases.
A more common arrangement requires a more complex bipolar plate and is shown in Figure 1.14. The plates are made larger relative to the electrodes and have extra channels running through the stack that feed the fuel and oxygen to the electrodes. Carefully placed holes feed the reactants into the channels that run over the surface of the electrodes. This type of arrangement is called internal manifolding. It results in a fuel cell stack that has the appearance of the solid block with the reactant gases fed in at the ends where the positive and negative connections are also made.
Such a fuel cell stack is shown under test in Figure 1.15. The end plate is quite complex, with several connections. The stack is a solid block. Electrical connections have been made to each of the approximately 60 cells in the stack for testing purposes. The typical form of a fuel cell as a solid block with connections at each end is also illustrated in Figure 4.1.
The bipolar plate with internal manifolding can be cooled in various ways. The simplest way is to make narrow channels up through the plates and to drive cooling air or water through them. Such an approach is used in several systems shown in Chapter 4. Alternatively, channels can be provided along the length of the cell, and there is provision for this in the system shown in Figure 1.14. The preferred cooling method varies greatly with the different fuel cell types, and is addressed in Chapters 4 to 7.
It should now be clear that the bipolar plate is usually quite a complex item in a fuel cell stack. In addition to being a fairly complex item to make, the question of its material is often difficult. Graphite, for example, is often used, but this is difficult to work with and is brittle.
Excerpted from Fuel Cell Systems Explained by James Larminie Andrew Dicks Copyright © 2003 by John Wiley & Sons, Ltd. Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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|Foreword to the first edition|
|2||Efficiency and Open Circuit Voltage||25|
|3||Operational Fuel Cell Voltages||45|
|4||Proton Exchange Membrane Fuel Cells||67|
|5||Alkaline Electrocyte Fuel Cells||121|
|6||Direct Methanol Fuel Cells||141|
|7||Medium and High Temperature Fuel Cells||163|
|8||Fuelling Fuel Cells||229|
|9||Compressors, Turbines, Ejectors, Fans, Blowers, and Pumps||309|
|10||Delivering Fuel Cell Power||331|
|11||Fuel Cell Systems Analysed||369|
|App. 1: Change in Molar Gibbs Free Energy Calculations||391|
|App. 2: Useful Fuel Cell Equations||395|
Posted February 14, 2002
This book gives a great overview of many different types of fuel cells in use today. It is a great introductory book for someone who knows a little about thermodynamics but very little about fuel cells. I used it to perform some experiments on an alkali fuel cell in an undergraduate advanced physics lab course. It was a great asset for me.Was this review helpful? Yes NoThank you for your feedback. Report this reviewThank you, this review has been flagged.
Posted May 21, 2001
In an era of blackouts and high fuel prices you will do well to be armed with the facts about alternate energy systems. As advertised 'Fuel Cell Systems Explained' provides just what it promises. In great detail different types and uses for fuel cells are laid out. Along with the well written narrative are the underlining equations for development and performance of current systems in use. So, armed with the information contained within, you'll not be mislead by the promise of easy solutions to the complex energy problem.Was this review helpful? Yes NoThank you for your feedback. Report this reviewThank you, this review has been flagged.