Introducing Fuel Cells COPYRIGHTED MATERIAL


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Introducing Fuel Cells
1.1 ­Historical Perspective
This book is an introduction to fuel‐cell systems; it aims to provide an understanding of the technology — what it is, how it works and what are its applications. Essentially, a fuel cell can be defined as a device that produces electrical power directly from a fuel via an electrochemical process. In some respects, this operation is similar to that of a conventional battery except that the reactants are stored outside the cell. Therefore, the performance of the device is limited only by the availability of the fuel and oxidant supply and not by the cell design. For this reason, fuel cells are rated by their power output (kW) rather than by their capacity (kWh).
Before addressing the technology in depth, it is necessary to understand that by virtue of being electrochemical, fuel cells have both chemical and electrical characteristics. Accordingly, their development has been inextricably linked with the development of electrochemistry as a distinct branch of physical chemistry.
At the beginning of the 19th century, it was recognized that an ‘electrochemical cell’ (nowadays, commonly called a ‘battery’) could be made by placing two dissimilar metals in an aqueous salt solution. This discovery was made by Alessandro Volta, the professor of experimental physics at Pavia University, who constructed a pile of alternating discs of copper (or silver or brass) and zinc (or tin) that were separated by pasteboard discs (or ‘any other spongy matter’) soaked in brine. When the top and bottom of the pile were connected by a wire, the assembly delivered, for the first time in history, a more or less steady flow of electricity. Volta introduced the terms ‘electric current’ and ‘electromotive force’, the latter to denote the physical phenomenon that causes the current to flow. In due course, he conveyed his findings in a letter dated 20 March 1800 to Joseph Banks, the then president of the Royal Society. Known as the ‘Volta (or Voltaic) pile’, this was the first ‘primary’ (or non‐rechargeable) power source, as opposed to a ‘secondary’ (or rechargeable) power source.
Sir Humphry Davy, who was working at the Royal Institution in London, soon recognized that the Volta pile produces electricity via chemical reactions at the metal–­ solution interfaces — hydrogen is evolved on the ‘positive’ copper disc, and zinc is consumed at the ‘negative’ disc. Indeed, this recognition of the relationship between chemical and electrical effects prompted Davy to coin the word ‘electrochemical’, from which sprang the science of ‘electrochemistry’. He gave warning that Volta’s work was ‘an alarm bell to experimenters all over Europe’. His prediction was soon to be verified.
Fuel Cell Systems Explained, Third Edition. Andrew L. Dicks and David A. J. Rand. © 2018 John Wiley & Sons Ltd. Published 2018 by John Wiley & Sons Ltd.

2 Fuel Cell Systems Explained
Volta had sent his letter to the Royal Society in two parts because he anticipated problems with its delivery given that correspondence from Italy had to pass through France, which was then at war with Britain. While waiting for the second part to arrive, Joseph Banks had shown the first few pages to Anthony Carlisle (a fashionable London surgeon) who, in turn, with the assistance William Nicholson (a competent amateur scientist) assembled on 30 April 1800 the first pile to be constructed in England. Almost immediately, on 2 May 1800, the two investigators found that the current from their device when passed through a dilute salt solution via two platinum wires was capable of decomposing water into its constituents — hydrogen at one wire and oxygen at the other. Details of the discovery were published in Nicholson’s own journal in July of the same year. Thus, the new technique of ‘molecular splitting’ — to be coined ‘electrolysis’ by Michael Faraday much later in 1834 and derived from the Greek ‘lysis’ = separation — was demonstrated before Volta’s own account of the pile was made public in September 1800. A schematic representation of the process is shown in Figure 1.1a.
It was left to Michael Faraday, Davy’s brilliant student, to identify the mechanisms of the processes that take place within ‘electrolytic’ cells and to give them a quantitative basis. In addition, he was also the guiding force behind the nomenclature that is still in use today. First, Faraday with the assistance of Whitlock Nicholl (his personal physician and accomplished linguist) devised the name ‘electrode’ to describe a solid substance at which an electrochemical reaction occurs and ‘electrolyte’ to describe the chemical compound that provides an electrically conductive medium between electrodes. (Note that in the case of dissolved materials, it is fundamentally incorrect to refer to the ‘electrolyte solution’ as the ‘electrolyte’; nevertheless, the latter terminology has become common practice.) To distinguish between the electrode by which conventional current (i.e., the reverse flow of electrons) enters an electrolytic cell and the electrode by which

(a) Electrolysis cell

Current

–+

e–

External

e–

power source

(b) Fuel cell

Current

External

e–

load

e–



+

– Anion

+ Cation

Cathode

Anode

Electrolyte solution



+

– Anion

+ Cation

Anode

Cathode

Electrolyte solution

Figure 1.1  Terminology employed in operation of (a) electrolysis cells and (b) fuel cells.

Introducing Fuel Cells 3
it leaves, Faraday sought the assistance of the polymath William Whewell, the Master of Trinity College at the University of Cambridge. In a letter dated 24 April 1834, he asked Whewell:
‘Can you help me out to two good names not depending upon the idea of a ­current in one direction only or upon positive or negative?’
In other words, he wanted terms that would be unaffected by any later change in the convention adopted for the direction of current. Eventually, they settled on calling the positive electrode an ‘anode’ and the negative electrode a ‘cathode’, which were coined from Greek ‘ano‐dos’ (‘upwards’–‘a way’) to represent the path of electrons from the positive electrode to the negative and ‘katho‐dos’ (‘downwards’–‘a way’) to represent the counter direction. For an electrolytic cell, then, the anode is where the current enters the electrolyte and the cathode is where the current leaves the electrolyte. Thus the positive electrode sustains an oxidation (or ‘anodic’) reaction with the liberation of electrons, while a reduction (or ‘cathodic’) reaction takes place at the negative electrode with the uptake of electrons.
With use of the Greek neutral present participle ‘ion’ — ‘a moving thing’ — to describe the migrating particles in electrolysis, two further terms were obtained, namely, ‘anion’, i.e., the negatively charged species that goes to the anode against the current (or with the flow of negative charge), and ‘cation’, i.e., the positively charged species that goes to the cathode with the current (or against the flow of negative charge). The operation of an electrolysis cell is shown in Figure 1.1a. It should be noted that the anode–cathode terminology for an ‘electrolytic cell’ applies to a ‘battery under charge’ (secondary system).
A fuel cell operates in the reverse manner to an electrolysis cell, i.e., it is a ‘galvanic’ cell that spontaneously produces a voltage (similar to a ‘battery under discharge’). The anode of the electrolysis cell now becomes the cathode and the cathode becomes the anode; see Figure 1.1b. Nevertheless, the directions of the migration of anions and cations with respect to current flow are unchanged such that the positive electrode remains a positive electrode and the negative electrode remains a negative electrode. Thus, in a fuel cell, the fuel is always oxidized at the anode (positive electrode), and the oxidant is reduced at the cathode (negative electrode).
There is some debate over who discovered the principle of the fuel cell. In a letter written in December 1838 and published on page 43 of the January issue of the January– June 1839 Volume XIV of The London and Edinburgh Philosophical Magazine and Journal of Science, the German scientist Christian Friedrich Schönbein described his investigations on fluids that were separated from each other by a membrane and connected to a galvanometer by means of platina wires. In the 10th of 14 reported tests, one compartment contained dilute sulfuric acid that held some hydrogen, whereas the other compartment contained dilute sulfuric acid that was exposed to air. Schönbein detected a current and concluded that this was caused ‘by the combination of hydrogen with (the) oxygen (contained dissolved in water)’. This discovery was largely overlooked, however, after the publication of a letter from William Robert Grove, a Welsh lawyer and a scientist at the Royal Institution; see Figure 1.2a. The letter, which was dated 14 December 1838, appeared on page 127 of the February issue of the aforementioned Volume XIV and described his evaluation of electrode and electrolyte materials for use

4 Fuel Cell Systems Explained (a)

(b) Ox Hy
Current
Ox Hy Ox Hy Ox Hy Ox Hy

Figure 1.2  (a) William Robert Grove (1811–1896) and (b) Grove’s sketch of four cells of his gaseous voltaic battery’ (1842). (Source: https://commons.wikimedia.org/w/index.php?curid=20390734.Used under CC BY‐SA 3.0; https://creativecommons.org/licenses/by‐sa/3.0/.)
in batteries. Unfortunately, the order in which these two letters had been written is unknown as Schönbein did not date his letter in full — he gave the month, but not the day. In fact, this chronology is of little importance given the following postscript that Grove had added to his letter in January 1839:
‘I should have pursued these experiments further, and with other metals, but was led aside by some experiments with different solutions separated by a diaphragm and connected by platinum plates; in many of these I have been anticipated.’
In the same postscript, Grove went on to speculate that by connecting such cells in series sufficient voltage could be created to decompose water (by electrolysis).
Grove carried out many experiments that demonstrated the principle of the fuel cell. In 1842, he realized that the reaction at the electrodes was dependent on an area of contact between the gas reactant and a layer of liquid that was sufficiently thin to allow the gas to diffuse to the solid electrode (today, this requirement is commonly related to the formation of a ‘three‐phase boundary’ or ‘triple‐point junction’ where gas, electrolyte and electrocatalyst come into simultaneous contact, v.i.). At that time, Grove was the professor of experimental chemistry at the London Institution in Finsbury Circus, and in the same communication he reported the invention of a ‘gaseous voltaic battery’. The device employed two platinized platinum electrodes (to increase the real surface area), and a series of fifty such pairs when semi‐immersed in dilute sulfuric acid solution was found ‘to whirl round’ the needle of a galvanometer, to give a painful shock to five persons joining hands, to give a brilliant spark between charcoal points, and to decompose hydrochloric acid, potassium iodide and acidulated water. An original sketch of four such cells is reproduced in Figure 1.2b. It was also found that 26 cells were the minimum number required to electrolyse water. Grove had indeed realized

Introducing Fuel Cells 5
the desire expressed in his 1839 postscript in that he had achieved the beautiful ­symmetry inherent in the ‘decomposition of water by means of its composition’.
The aforementioned apparatus became widely recognized as the first fuel cell and Grove was designated as the ‘Father of the Fuel Cell’. Historically, this title is not fully justified. More accurately, Schönbein should be credited with the discovery of the fuel‐ cell effect in 1838 and Grove with the invention of the first working prototype in 1842. Happily, such accreditations were of little concern to the two scientists and they became close friends. For almost 30 years, they exchanged ideas and developments via a dynamic correspondence and visited each other frequently.
It is interesting to note that many latter‐day authors have attributed the introduction of the term ‘fuel cell’ to Ludwig Mond and Charles Langer in their description of a new form of gas battery in 1889. Remarkably, however, there is no mention of ‘fuel cell’ in this communication. Other claims that William W. Jacques, in reporting his experiments to produce electricity from coal, coined the name are equally ill founded. A. J. Allmand in his book The Principles of Applied Electrochemistry, published in 1912, appears to attribute the appellation ‘fuel cell’ to the Nobel Laureate Friedrich Wilhelm Ostwald in 1894.
Grove concluded his short paper in 1842 with the following modest entreaty:
‘Many other notions crowd upon my mind, but I have occupied sufficient space and must leave them for the present, hoping that other experimenters will think the subject worth pursuing.’
Unfortunately, however, the invention of the first internal combustion engine to become commercially successful by Jean Joseph Étienne Lenoir in 1859, coupled ironically with Faraday’s earlier discovery of electromagnetic force, diverted the course of electricity generation from electrochemical to electromagnetic methods. As a result, the fuel cell became merely an object of scientific curiosity during much of the next half‐century. Meanwhile, knowledge of electrochemical conversion and storage of energy progressed largely through the development of battery technologies.
In 1894, a well‐documented criticism against heat engines was expressed by Friedrich Ostwald, who drew attention to the poor efficiency and polluting problems associated with producing electrical power via the combustion of fossil fuels rather than by direct electrochemical oxidation. A fuel cell is inherently a more thermodynamically efficient device since, unlike an engine in which heat is converted to mechanical work, the cell is not subject to the rules of the Carnot cycle. By virtue of this cycle, the efficiency of a thermal engine is always lowered to a value far below 100%, as determined by the difference between the temperature at which heat is taken in by the working fluid and the temperature at which it is rejected. On this basis, Ostwald advocated that:
‘The path which will help to solve this biggest technical problem of all, this path must be found by the electrochemistry. If we have a galvanic element which directly delivers electrical power from coal and oxygen, […] we are facing a technical revolution that must push back the one of the invention of the steam engine. Imagine how […] the appearance of our industrial places will change! No more smoke, no more soot, no more steam engine, even no more fire, […] since fire will now only be needed for the few processes that cannot be accomplished electrically, and those will daily diminish. […] Until this task shall be tackled, some time will pass by.’

6 Fuel Cell Systems Explained
Regrettably, Ostwald was proven to be correct as regards his closing prediction for although attempts were made at the turn of the century to develop fuel cells that could convert coal or carbon into electricity (for instance, the work of William W. Jacques in the United States), the need for an expensive platinum catalyst and its poisoning by carbon monoxide formed during the coal gasification limited cell affordability, usefulness and lifetime. Consequently, interest in such ‘direct carbon fuel cells’ dwindled.
In the 1930s, Emil Bauer and H. Preis in Switzerland experimented with solid oxide fuel cells (SOFCs). Given the limitations of solid oxides at that time (i.e., poor electrical conductivity and chemical stability), G.H.J. Broers and J.A.A. Ketelaar in the late 1950s turned to the use of fused salts as electrolytes. The work gave birth to the molten carbonate fuel cell (MCFC), which eventually became one of the main types of fuel cell in commercial production.
The renaissance of the fuel‐cell concept in the 20th century can be attributed largely to the work of Englishman F.T. (Tom) Bacon. He was an engineer by profession and thus appreciated the many potential advantages of the fuel cell over both the internal combustion engine and the steam turbine as a source of electrical power. His interest in fuel cells dated as far back as 1932, and he ploughed a lone furrow, with little support or backing, but showed enormous dedication to the challenge of developing practical cells. Early in his career, Bacon elected to study the alkaline‐ electrolyte fuel cell (AFC), which used nickel‐based electrodes, in the belief that platinum‐group electrocatalysts would never become commercially viable. In addition, it was known that the oxygen electrode is more readily reversible in alkaline solution than in acid. This choice of electrolyte and electrodes necessitated operating the cell at moderate temperatures (100–200°C) and high gas pressures. Bacon restricted himself to the use of pure hydrogen and oxygen as reactants. Eventually, in August 1959, he demonstrated the first workable fuel cell — a 40‐cell system that could produce about 6 kW of power, which was sufficient to run a forklift truck and to operate a welding machine as well as a circular saw.
A major opportunity to apply fuel cells arose in the early 1960s with the advent of space exploration. In the United States, fuel cells were first employed to provide spacecraft power during the fifth mission of Project Gemini. Batteries had been employed for this purpose in the four earlier flights, as well as in those conducted in the preceding Project Mercury. This switch in technology was undertaken because payload mass is a critical parameter for rocket‐launched satellites, and it was judged that fuel cells, complete with gas supplies, would weigh less than batteries. Moreover, the objective of Project Gemini was to evolve techniques for advanced space travel — notably, the extravehicular activity and the orbital manoeuvres (rendezvous, docking, etc.) required for the moon landing planned in the following Project Apollo. Thus, lunar flights demand a source of power of longer duration than that available from batteries.
A proton‐exchange membrane fuel cell (PEMFC) system manufactured by the General Electric Company was adopted for the Gemini missions (two modules, each with a maximum power of about 1 kW), but this was replaced in Project Apollo by an AFC of circulating electrolyte design, as pioneered by Bacon and developed by the Pratt and Whitney Aircraft Company (later the United Technologies Corporation). Both

Introducing Fuel Cells 7
types of system were fuelled by hydrogen and oxygen from cryogenic tanks. The AFC could supply 1.5 kW of continuous power, and its in‐flight performance during all 18 Apollo missions was exemplary. In the 1970s, International Fuel Cells (a division of United Technologies Corporation) produced an improved AFC for the Space Shuttle orbiter that delivered eight times more power than the Apollo version and weighed 18 kg less. The system provided all of the electricity, as well as drinking water, when the Space Shuttle was in flight.
The successful exploitation of fuel cells in the space programme drove research activity worldwide during the 1970s to develop systems that would generate power with high efficiency and low emissions for terrestrial applications. Research was stimulated further by the hiatus in the global oil supply in 1974. What followed was the emergence of various national initiatives on fuel‐cell development. In the United States, demonstrations of phosphoric acid fuel cell (PAFC) technology by the American Gas Association led to a Notice of Market Opportunities (NOMO) initiative. This activity, in turn, renewed interest in the MCFC by US researchers, and in the mid‐1980s, national research and development programmes were established in Japan and Europe. Renewed interest in the PEMFC was championed in the late 1980s by Geoffrey Ballard, a Canadian pioneer, who saw the potential for the technology to replace internal combustion engines. Since then, this system has been the subject of much advancement for a variety of applications, so much so that it merits two chapters in this book.

1.2 ­Fuel‐Cell Basics

To understand how the reaction between hydrogen and oxygen produces an electric current, and where the electrons are released, it is necessary to consider the reaction that takes place at each electrode. The reactions vary for different types of fuel cell, but it is convenient to start with a cell based around an acid electrolyte, not only because this system was used by Grove but also because it is the simplest and still the most chosen for commercial applications.
At the anode of an acid fuel cell, hydrogen is oxidized and thereby releases electrons and creates H+ ions, as expressed by:

2H2 4H 4e

(1.1)

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

O2 4e 4H 2H2O

(1.2)

Thus the overall cell reaction is:

2H2 O2 2H2O heat

(1.3)

8 Fuel Cell Systems Explained

Clearly, for both the electrode reactions to proceed continuously, electrons produced
at the negative electrode must pass through an electrical circuit to the positive. Also, H+ ions must pass through the electrolyte solution — an acid is a fluid with free H+ ions
and so serves this purpose very well. Certain polymers and ceramic materials can also be made to contain mobile H+ ions. These materials are commonly called ‘proton‐ exchange membranes’, as an H+ ion is also known as a proton. The PEMFC is examined
in detail in Chapter 4.
The cell reaction (1.3) shows that two hydrogen molecules will be needed for each
oxygen molecule if the system is to be kept in balance. The operating principle is
­illustrated in Figure 1.3.
In a fuel cell with an alkaline electrolyte (AFC), the overall reaction of hydrogen
oxidation is the same, but the reactions at each electrode are different. In an alkaline solution, hydroxyl (OH−) ions are available and mobile. At the anode, these ions react
with hydrogen to release electrons and energy (heat) together with the production
of water:

2H2 4OH 4H2O 4e

(1.4)

At the cathode, oxygen reacts with electrons taken from the electrode, and water in the electrolyte and thereby forms new OH− ions:

O2 4e 2H2O 4OH

(1.5)

Comparing equations (1.4) and (1.5) shows that, as with an acid electrolyte, twice as much hydrogen is required compared with oxygen. The operating principle of the AFC is presented in Figure 1.4.
There are many other types of fuel cell, each distinguished by its electrolyte and the reactions that take place on the electrodes. The different systems are described in detail in the following chapters.

Hydrogen fuel

– Anode

2H2

4H+ + 4e–

H+ Ions through electrolyte + Cathode O2 + 4e– + 4H+

2H2O

Load e.g., electric
motor

Oxygen, usually from the air

Electrons flow round the external circuit

Figure 1.3  Electrode reactions and charge flow for fuel cell with an acid electrolyte. Note that although the negative electrons flow from the anode to cathode, the ‘conventional positive current’ flows from cathode to anode.

Hydrogen fuel

Introducing Fuel Cells 9

– Anode 2H2 +

4OH–

4H2O + 4e–

OH– Ions through electrolyte
+ Cathode O2 + 4e– + 2H2O

4OH–

Load e.g., electric
motor

Oxygen, usually from the air

Electrons flow round the external circuit

Figure 1.4  Electrode reactions and charge flow for a fuel cell with an alkaline electrolyte. Electrons flow from negative anode to positive cathode, but ‘conventional positive current’ flows from cathode to anode.

1.3 ­Electrode Reaction Rates
The oxidation of hydrogen at the negative electrode liberates chemical energy. It does not follow, however, that the reaction proceeds at an unlimited rate; rather, it has the ‘classical’ energy form of most chemical reactions, as shown in Figure 1.5. The schematic represents the fact that some energy must be used to excite the atoms or ­molecules sufficiently to start the chemical reaction — the so‐called ‘activation energy’. This energy can be in the form of heat, electromagnetic radiation or electrical energy. In visual terms, the activation energy helps the reactant to overcome an ‘energy hill’,

Energy

Activation energy
Energy released

Stage of reaction Figure 1.5  Classical energy diagram for a simple exothermic chemical reaction.

10 Fuel Cell Systems Explained
and once the reaction starts, everything rolls downhill. Thus, if the probability of an atom or molecule having sufficient energy is low, then the reaction will only proceed slowly. This is indeed the case for fuel‐cell reactions, unless very high temperatures are employed.
The three main ways of dealing with the slow reaction rates are to (i) use catalysts, (ii) raise the temperature and (iii) increase the electrode area. Whereas the first two options can be applied to any chemical reaction, the electrode area has a special ­significance for electrochemical cells. The electrochemical reactions take place at the location where the gas molecules (hydrogen or oxygen) meet the solid electrode and the electrolyte (whether solid or liquid). The point at which this occurs is often referred to as the ‘three‐phase boundary/junction’ or the ‘triple‐phase boundary/ junction’ (v.s.).
Clearly, the rate at which either electrode reaction proceeds will be proportional to the area of the respective electrode. Indeed, electrode area is such an important issue that the performance of fuel cells is usually quoted in terms of the current per cm2. Nevertheless, the geometric area (length × width) is not the only issue. The electrode is made highly porous so as to provide a great increase in the ‘effective’ surface area for the electrochemical reactions. The surface area of electrodes in modern fuel cells, such as that shown in Figure 1.6, can be two to three orders of magnitude greater than the geometric area. The electrodes may also have to incorporate a catalyst and endure high temperatures in a corrosive environment; catalysts are discussed in Chapter 3.
Figure 1.6  Transmission electron microscope image of a fuel‐cell catalyst. The black spots are the catalyst particles that are finely divided over a carbon support. The structure clearly has a large surface area. (Source: Courtesy of Johnson Matthey Plc.)
75 nm

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Introducing Fuel Cells COPYRIGHTED MATERIAL