Fuel Cells

That energy can neither be created nor destroyed is a very well-known statement of the Principle of Conservation of Energy, and we all accept its truth although certain processes taking place in nuclear reactions appear to confound it: thus when we speak loosely of ‘energy production’ we do not mean its production from nothing but the conversion of one form of energy into another form more useful for our purpose. It is this conversion which lies at the heart of human life on earth today, most of the characteristics of what we see as civilisation depending entirely on forms of energy conversion, primarily from the chemical energy of fossil fuels.

The methods of chemical thermodynamics prove useful in the study of fuel cells despite the fact that normally only equilibrium properties can be discussed. There are two main avenues which can be explored. First, a study of the thermodynamics of the Carnot cycle for a perfectly reversible heat engine will show that there is a limiting efficiency which restricts the usefulness of any heat engine (as was mentioned in the previous chapter), and this can be compared with the possible efficiency of a fuel cell. Secondly, the electromotive force of a fuel cell (that is the maximum potential difference across the electrodes of a cell when no current is being taken from it) can be calculated by considering the electrochemical equilibrium set up when a fuel cell is studied potentiometrically.

In the previous chapter we have shown that the change in Gibbs function for a fuel cell reaction—or the reaction associated with any other galvanic cell—is equal to the electrical work done (though of opposite sign), which in turn depends on the electromotive force, or emf, of the cell when it is working reversibly under conditions where no current is taken from it. Although such conditions seem irrelevant to the actual working state of the cell (when large currents may well be taken from it), it is nevertheless worth considering the factors influencing the emf since this will determine the optimum efficiency, as we have seen earlier. It is usually thought most convenient to consider working potential differences in terms of their deviations from the emf, and this provides a further reason for looking further at the open circuit situation. These deviations arise from the effect of taking a current from the cell and are usually considered as kinetic effects. A discussion of them will be found in chapter 4.

The thermodynamics of galvanic cells as considered in the two previous chapters can, in general, only tell us of the feasibility of certain chemical reactions occurring at electrodes to give an electric potential; they can give no information about whether such processes will take place at a measurable rate. In other words, the characteristics of working under load of any kind of electrochemical cell (including a fuel cell) cannot be deduced from computation of the changes in thermodynamic parameters associated with the cell reaction.

It is, in many ways, convenient to classify fuel cells according to their temperature of operation although obviously some characteristics and properties will not vary with the temperature of operation of the system. Thus in this chapter hydrogen-oxygen cells working at temperatures below about 100 °C and at low pressures (up to about 5 atm) are described, and subsequent chapters deal with other low temperature systems, and with cells working at medium (100–500 °C) or high temperatures (above 500 °C). Certain rather special types of fuel cell, such as the biochemical cell, are dealt with separately.

We can conveniently make two main classifications of cells working at low temperatures other than the hydrogen-oxygen variety considered in the last chapter, although there may in fact be some overlap. Each type of cell involves the oxidation of materials containing elements other than hydrogen: one of fuels soluble in the electrolyte and the other of hydrocarbons fed to the cell as gases.

In this chapter we shall discuss those types of fuel cell operating at intermediate temperatures, that is to say, temperatures higher than those using aqueous electrolytes at atmospheric pressures but lower than those using solid electrolytes. The range is generally considered to be about 150–300 °C. Some kinds of cell operating in this temperature range have already been mentioned during consideration of aqueous electrolyte cells in chapters 5 and 6. In these particular cells, higher temperatures are possible with pressures only slightly greater than atmospheric because of the considerable boiling point elevation obtained with certain highly concentrated solutions, such as the caesium-rubidium carbonate-bicarbonate mixtures mentioned previously. This variety of cell will not be discussed further.

We have already seen that in almost all respects the application of increased temperatures to a fuel cell system increases its ease of operation, since almost all physical and chemical processes are speeded up by higher temperatures. In this chapter we shall consider types of cell which have been designed for operation at even higher temperatures than the high pressure aqueous electrolyte variety described in the last chapter. We shall not be surprised to learn that while fuel cell operation becomes easier at higher temperatures, there are vastly increased problems of construction and operation of peripheral equipment such as pumps, heaters and condensers.

Some mention was made in chapter 1 of the air depolarised cell; it is certainly worth considering in a book on fuel cells since, although the ‘fuel’ used—the metal electrode—is not at all a conventional fuel, there are nevertheless considerable resemblances. It was pointed out in chapter 1 that one of the important characteristics of a fuel cell was that the material used up in the operation of the cell was almost entirely supplied to it and was not built in at the time of construction, whereas other kinds of galvanic cells usually could only operate on the materials actually put into the cell during its manufacture. The air depolarised cell occupies an interesting position in between these extremes; the oxygen or air electrode clearly operates from material supplied during operation, while the fuel electrode is a metal one manufactured as part of the cell and converted to oxide or other products during its life. The simplest and most well-known air depolarised cell is the zinc-air cell with an alkaline electrolyte: which has a cell reaction the potassium zincate being of course an oxidation product of the zinc electrode.

In our survey of the possible applications of fuel cells to the provision of power for various operations, one way of conveniently dividing up the field would be to consider the different categories of power output requirements. For example, high power for industrial applications, medium power for domestic installations, and low power for certain kinds of vehicle and for use in space. Now, these last two classifications overlap to some extent and consequently it is proposed to deal with types of application—for example, transport—rather than classify installations wholly in terms of power output.

If we are to understand why fuel cells have been used only very little so far and to predict what will be their most favourable application in the future, then we must know something about the economics of their operation and compare this with the economics of other forms of energy conversion. In this chapter we shall try to evaluate the economic factors of fuel cell operation and make some attempt at comparison with its competitors, although this will not be very satisfactory since any costs chosen at the time of writing may be quite wrong by the time this book is read. In fact, of course, the whole field of economics is subject to this uncertainty and this must be borne in mind throughout the chapter.

Despite the numerous possible applications of fuel cell systems described in the last two chapters and despite the impressive advantages and superiority of fuel cells over other kinds of power generation, the only application that has passed beyond the strictly experimental stage is the supply of electrical power in space vehicles. The reasons for this are apparent from the economic considerations outlined in chapter 12 but are reinforced here.