Ashok K. Vijh Hydro-Quebec Institute of Research Varennes, P.Q., Canada
Electrochemical Principles Involved in a Fuel Cell
T h e electrochemical energy converter, popularly known as "fuel cell," is an interesting modern modification of the primary cell (e.g., Leclanche cell) and it has had wide application in space exploration. Fuel cells were an important source of auxiliary power in the Gemini and Avollo svacecrafts. The fuel cell is also being studied very carefully for possible use for a type of electric automobile to partially or possibly completely replace the gasoline engine in order to minimize air pollution and noise. A fuel cell is thus related to the two major problems of today, namely, conquest of space and elimination of air vollution. The purpose of this article is to present an elementary discussion of the electrochemical principles involved in the overation of a fuel cell. Importance of fuel cells, and the scientific and technological problems involved in it, may be indicated by stating that the development of the fuel cell is one of the three major areas of energy research today, namely Direct conversion of fuel to electrical energy (viz. fuel cell) Direct conversion of solar energy to electrical energy Direct conversion of nuclear energy to atomic energy
Definition of a Fuel Cell
The term "fuel cell" usually refers to a family of devices used for generating electricity electrochemically. I n this sense, a fuel cell is very similar t o the conventional batteries. There are important differences, however. Firstly, a fuel cell is similar to a primary battery in that it consumes the chemical substances fed into it. On the other hand, the fuel cell differs from a primary battery in that it is not discarded after the chemical reactants ~ u int it initiallv have been consumed. Instead, so&e more chemicals are fed into it to produce more electricity and the process can be made continuous. A fuel cell hence is a continuous-feed primary battery which will, ideally, go on producing electricity as long as suitable reactants are being fed into it. The other major requirement is the continuous removal of products (wastes) which are produced as a result of electrochemical reactions within the fuel cells. The'similarity between a primary battery and a fuel cell may be illustrated by comparing the Leclanche cell with the hydrogen-oxygen fuel cell: in the former, the zinc provides the electrons for the outside circuit, while in the latter it is the hydrogen gas (fuel); again, in the former, the manganese dioxide accepts the electrons from the outside circuit, whereas in the latter, the oxygen gas is the electron acceptor. Fuel Cells:
Electrochemical Principles
To illustrate the electrochemical principles involved in a fuel cell is to discuss the simplest example, 680
/
lournal of Chemical Education
e.g., a hydrogen-oxygen fuel cell. I n this fuel cell, the process that results in the production of electricity is exactly the converse of the electrolysis of water. I n the electrolysis of water, we force the decomposition of water into hydrogen and oxygen by supplying electrical energy to the system, i.e., by passage of current. Since electrolysis of water into gaseous hydrogen and oxygen proceeds by a transfer of electrons from the OH- to the H+ to form H z and 0 2 as the electric current is forced through the solution, it is logical to assume that the reverse process, i.e., the combination of gaseous hydrogen and oxygen should result in the reverse transfer of electrons. This is indeed observed. Hence the combination of gaseous hydrogen and oxygen to produce water may also be carried out in an electrochemical cell and energy released may be tapped as the useful electrical energy. The device that accomplishes this is called a hydrogen-oxygen fuel cell. In its simprest form (see the figure), it consists of a container holding a suitable electrolyte solution (e.g., sulfuric acid, potassium hydroxide) and two inert electrodes, the anode (A) and the cathode (C). Gaseous hydrogen is admitted at the anode and gaseous oxygen is admitted a t the cathode. On joining the anode and cathode externally by means of a conducting wire, the current flows in the circuit, which may be made to do useful work, e.g., to light an electric bulb. The electrochemical reactions involved a t the two electrodes are as follows Anode Cathode
+
+ Hz
+
Anode
+ Hz0 + 26-
1/202
Hz
+
l/z02
+ 2H+ + 2eCathode + 20H-
(1)
+
(2)
"H20
(3)
If we combine the cathodic and anodic half cell re-
Hydrogek lnlet
)xygen lnlet
electrolyte ANODE (WHERE OXIDATION OCCURS)
H,O CATHODE (WHERE REDUCTION OCCURS)
A diogramatic representation of reactions involved in a simple hydrogenoxygen fuel cell, after Gregory (16).
actions, we obtain the total fuel cell reaction, namely, reaction (3). This reaction is simply the combination of gaseous hydrogen and oxygen to produce water and is accompanied by release of electrical energy. The anode and cathode in the fuel cell (Fig. 1) will assume, as a result of reaction (3), a difference of potential derived from the free energy change of the overall reaction (3). Both the intensity (reversible potential, E, in volts V) and the quantity (faradays, I?, or coulombs, C) of the total cell output are contained in the molar free energy, AG, of the overall cell reaction (3). This may be represented as
Here, n is the number of electrons Der molecule of the anodic fuel (hydrogen is the present example; hence n is two) that is being oxidized and I is the average current in amperes derived for t seconds. The total quantity of electricity ( I X t = C) derived from such a fuel cell (Fig. 1) is proportional to the weight of chemicals consumed. One Faraday or 26.8 A-hr/g-equivalent weight of the fuel is the electricity released assuming complete oxidation and absence of a variety of possible power losses. The nature of some of these power losses will be outlined in the following discussion. I t is important to note in above reactions (1)-(3) that anode and cathode are not consumed in the respective anodic oxidation (reaction (I)) and cathodic reduction (reaction (2)). What, then, is the function of these electrodes? The first and the most obvious function of these electrodes is that they act as source (anode) or sink (cathode) of electrons in the electrochemical reactions involved in the fuel cell. Since this function can be performed by any good metallic conductor, it follows that several of the metals or graphite material should be suitable as electrodes in the fuel cell. I n actual practice, it is observed, however, that among metals, platinum and related metals like iridium. rhodium, etc.. are the only ones which act as suitable electrode; or as suitable coating of electrodes in the fuel cells since they are needed as catalytic agents. The electrodes in a fuel cell, as in all electrochemical reactions. act not onlv as sources or as sinks of electrons, but also as specific catalysts for the two electrode reactions. Platinum is a good anode catalyst for reaction (1) in a fuel cell, because it provides a path of low activation energy for reaction (1) to occur, and the rate of reaction will be higher in the presence of platinum than without it. If platinum is replaced by a noncatalytic metal, reaction (1) would still occur; however, the activation energy for reaction (I) on a non-catalytic metal will be very high and the rate will be very low. Since heterogeneous catalysis in electrode reactions is accompanied by charge transfer a t the metal-electrolyte interface (e.g., in reaction (I)), platinum, which has been chosen as a n anodic catalyst in our example here, is called an electrocatalyst. An important requirement, therefore, for a suitable electrdde in a fuel cell is that it behave as a good electrocatalyst for the particular electrode reaction to be carried out on it. If one performs reaction (I), for example, on a poor electrocatalyst, the charge transfer in reaction (1) will assume significant rates only if it is driven by a high field a t the metal-solution interface. Application of such a field results in electrode polarization, i.e., the applied field forces the electrode away
from its reversible potential. I n terms of fuel cell terminology, this polarization amounts to a power loss. I n other words, in the absence of a good electrocatalyst, the fuel cell provides lesser amounts of useful power output since a part of the power is "wasted" on driving reactions (1) and (2) a t significant rates. The theoretical aspects of polarization a t the metal-solution interface and its relation to the rates of electrode reactions have been discussed in previous recent articles (1, 2) and will not be dealt with here. Other elementary (3, 4) or advanced accounts (5-8) of these matters are also available in the literature. From the foregoing introduction and Figure 1, it is obvious that the four most important parts of a fuel cell e.g., a hydrogen-oxygen fuel cell, are as follows Anodic electrocatalyst Cathodic electrocatalyst Anodic fuel, e.g., for the electron producing reaction ( I ) Cathodic fuel, e.g., for the electron consuming reaction (2)
As regards the question of a good anodic electrocatalyst, platinum and related metals are more or less satisfactory. On grounds of high cost and limited world supply, however, these metals must be excluded as attractive electrocatalysts for fuel cells meant to compete with other power sources (e.g., internal combustion engines, hydroelectric power, etc.) for a variety of terrestrial uses. For very restricted uses (e.g., supplying power to remote regions, military uses, etc.), fuel cells employing very small quantities of platinum metals are within the realm of commercial possibility. The situation for the case of cathodic electrocatalysts is less encouraging. Even though platinum metals are one of the best available electrocatalysts for reduction of oxygen (i.e., reaction (2)), the power losses are still excessive. Recently, it has been claimed that some phthalocyanines adsorbed on active charcoal and some tungsten bronzes approach platinum in electrocatalytic activity for reaction (2), i.e., reduction of oxygen gas to hydroxyl ions. These claims have yet to be corroborated under rigorous conditions of experimentation. The best claimed cathodic electrocatalvsts to date are far from being excellent since polarization (i.e., power loss) involved is still of appreciable magnitude. There is urgent need, therefore, for discovering a good cathodic electrocatalyst before fuel cells could become economically competitive sources of power. I t is also clear that some cheaper anodic electrocatalysts approaching platinum metals in their activity would be very desirable. Regarding fuel for the anodic reaction (I), hydrogen gas is, of course, very attractive from the point of view of ease of electrochemical oxidation. Because of hazard and high cost (at site) involved in the use of hydrogen gas, substitute sources of electrochemically available hydrogen are being explored, e.g., methanol, hydrazine, hydrocarbons, etc. For example, the electrons released in the anodic reaction (1) could also be obtained, instead, by reactions of the type Anode
+ CZHZ+ 4H20
-t
2CO2
+ 10H- + 10e-
(5)
I n other words, electrons may be released by oxidation of hydrogen-containing compounds. This approach has achieved some success even though an ideal fuel which meets all the scientific, technological, and Volume 47, Number 7 0, October 7 970
/
681
economic requirements for a commercially attractive fuel cell has yet to be found. The various requirements desired in a commercially attractive anodic fuel, briefly enumerated, are: low cost a t the site of fuel consum~tion:ease of electrochemical oxidation; high energy density (i.e., available electrons in a reaction of type (5) above) per unit weight of the fuel; adequate world reserves; ease of handling and transportation; lack of hazard and toxicity; chemical stability in a wide range of temperature, etc. It is believed that an ideal fuel cell would be a device which accepts conventional fuels such as coal, crude oil, or -gasoline as anodic fuels. These fuels, when oxidized directly, have very low electrochemical activity, however. To circumvent this problem, these fuels are sometimes first decomposed by a chemical-thermal treatment in a ('reformer," to yield hydrogen or hydrogen containing low molecular weight hydrocarbons, which are then fed to a fuel cell. This procedure is called indirect oxidation of these fuels. This. however. is not an entirely satisfactory procedure since it raises the cost of energy production by fuel cells and introduces other problems, e.g., catalyst poisoning by traces of CO present in the gases produced by the reformer (9-11). I t may be mentioned here that a great deal of electrochemical research effort has been devoted in recent vears to the ~ r o b l e mof elucidation of the mechanisms of anodic oxidation of organic compounds (12-15) with a view to understanding the nature of interfacial problems involved in the oxidation of fuels. The final problem involved in a fuel cell is that of a suitable cathodic fuel (reaction (2) in the foregoing discussion) and the solution is rather straightforward for the case of hydrogen-oxygen fuel cell. The oxygen which is contained in air, after suitable purification, is quite acceptable as a cathodic fuel. This is the source of oxygen in the present commercially used "air cell." I n applications where storage space and weight of a fuel are paramount considerations, pure oxygen in liquefied form is to be preferred, e.g., in the space missions in the USA. Other Types of Fuel Cells
The preceding discussion of fuel cells was centered around hydrogen-oxygen fuel cell. However, there are several other types of fuel cells (9-11) which essentially involve similar principles; i.e., the basic requirement in these cells is an electron producing reaction (i.e., reaction (1)) and electron consuming reaction (i.e., reaction (2)).l Since many of these fuel cells do not involve any completely novel principles, it is not necessary to discuss them here. The possible exception are the biological fuel cells, also referred to by the following names: bio-fuel cells; biochemical fuel cells; biobatteries; biosolar cells. The most important aspect of these fuel cells is involvement of living organisms, e.g., bacteria in extracting electricity from low cost or abundant fuels. Use of even household sewage has been suggested as the I One such cell that has been studied for electrical auto use is the sodium (anode)-sulfur (cathode) cell; another that has received some attention for an easily regenerative system is the Hz-lithium cell.
682
/
Journal of Chemical Educafion
possible fuel thus indicating that some fuel cells offer the possibility of achieving simultaneously two important goals, namely producing electricity and fighting water pollution. The role of these living organisms in the production of electricity may take one of the following forms 1) The bacteria may be used for breaking complex organic matter contained, e.g., in sewage or cellulose into simpler organic molecules (e.g., hydrogen sulfide, methane) that show electrochemical activity as anode fuels in conventional fuel cells. This type of bacterial decomposition is indeed frequently found in nature. 2 ) Some living organisms can convert complex organic wastes into oxygen which may then be used a t the cathode of a fuel cell. This particular phenomenon will have only limited application, however, since oxygen is already easily and cheaply available from the atmosphere. 3) Enzymes produced by some bacteria may act as catalysts or promotors for anodic oxidation of organic fuels.
Until now, the biochemical fuel cells (9-11) are nowhere near the stage of commercial feasibility. The possibilities, however, are enormous. For example, it has been suggested that biochemical fuel cells may provide the possibility of converting the entire Black Sea into a gigantic fuel cell. At the bottom of Black Sea, Disulfovibrio bacteria obtain oxygen from sulfates in water and in the process create large amounts of hydrogen sulfide which can, of course, be used as an anodic fuel. Sewage ponds in every community may be converted into huge fuel cells. These rather speculative possibilities have already been demonstrated to t the be valid a t the test tube level. At the ~ r e s e ntime, commercial exploitation of biochemical fuel cells is prohibited by, among other factors, the rather low concentrations of the electroactive materials ~roducedbv the bacterial decomposition. ~ o t w i t h s i a n d i n ~ thk lack of present commercial feasibility of biochemical fuel cells, the intellectual fascination of the problems involved cannot be disputed. Acknowledgment
The author is indebted to the reviewer of this paper for several suggestions for improvement of the manuscript. Literature Cited ( 1 ) CONWAY, B. E., AND SALOMON, M.,J. CHEM.EDUC.,44,554 (1967). ( 2 ) PARSONS, R., J. CHEM.EDUC.,45,390 (1968). ( 3 ) PARSONS, R., i n "Encyclopaedia of Electrochemistry," (Edztor: HAMPEL,C. A.), Reinhold Publishing Corp., New York, 1965. ( 4 ) POTTER, E. C . , "Electrochemistry," Cleaver-Hume Press Ltd., London, 1061
( 5 ) CONWAY, B. E., "Theory and Principles of Electrode Processes," Ronald Press, New York, 1965. ( 6 ) BOCKRIS, J. OIM., AND REDDY, A. K. N., "Essentials of Modern Electrochemistry," Plenum Press, New York, 1969, Vols. 1 and 2. ( 7 ) DELAHAY,P., "Double Layer and Electrode Kinetics," Interscience (division of John Wiley &Sons, Inc.), New York, 1965. ( 8 ) VETTER, K . J., "Electrochemical Kinetics," Academic Press, New York, 1967. ( 9 ) BERQER,C., "Handbook of Fuel Cell Technology," Prentice-Hall, Ennlewood Cliffs. New Jersev. 1968. ~", -~ - (10) AUSTFN,L. F., " ~ L e lcells," NASA Special Publication 120 (1967). NASA, Washington, D. C., 1967. (11) BACON. F. T.. Electrochim. Acta.. 14.569 - - , -11969). ~~ (12) GILEADI,E.,'AND PIERSMA,B., i n "MA&; Aspects of Electrochem, York, 1966, istry," (Editor: BOCKRIS,J. O'M.), Plenum P r e ~ s New Vol. 4. (13) GILEADI,E., "Ele~trosorption," Plenum Press, New York, 1966. (14) VIJH, A. K., AND CONWAY, B. E., Chem. Rev., 67, 623 (1967); J . Phl/s. Chem., 71,3637.3655 (1967). (15) CONWAY, B. E., i n "Progress in Reaction Kinetics," (Editor: PORTER, G . ) ,Pergamon Press Co., Oxford, 1967. (16) GREGORY, D . P., Endeavour, XXVIII, No. 103, January, 1969.
