1. Introduction CHAPTER What Is a Fuel Cell?

Transcription

1 CHAPTER 1 1. Introduction 1.1. What Is a Fuel Cell? A fuel cell is an electrochemical energy converter that converts chemical energy of fuel directly into DC electricity. Typically, a process of electricity generation from fuels involves several energy conversion steps, namely: 1. combustion of fuel converts chemical energy of fuel into heat, 2. this heat is then used to boil water and generate steam, 3. steam is used to run a turbine in a process that converts thermal energy into mechanical energy, and finally 4. mechanical energy is used to run a generator that generates electricity. A fuel cell circumvents all these processes and generates electricity in a single step without involving any moving parts (Figure 1-1). It is this simplicity that attracts attention. Such a device must be simpler, thus less expensive and far more efficient than the four-step process previously depicted. Is it really? Today not really! Or better, not yet. But fuel cells are still being developed. This book intends to provide a basis for engineering of fuel cell devices. It includes state-of-the-art designs and materials (as they exist at the time of this writing), which are likely to change in the future as this technology continues to develop (perhaps even sooner than the students using this book as a textbook get jobs in the fuel cell industry). However, the engineering basis will not change, at least not dramatically and not so quickly. The knowledge of engineering principles will allow future fuel cell engineers to adopt these new designs and new materials and, we hope, come up with even newer designs and materials. This is what the purpose of an engineering education should be. This book will not teach the principles of thermodynamics, catalysis, electrochemistry, heat transfer, fluid mechanics, or electricity conduction, but it will apply those engineering disciplines in the engineering of a fuel cell as an energy conversion device. 1

2 2 PEM Fuel Cells: Theory and Practice hydrogen electrode electrolyte FUEL CELL oxygen electrode waste heat water FIGURE 1-1. Fuel cell generates DC electricity from fuel in one step. The efficiency of an energy conversion process is one of the most important aspects of that conversion. Some typical questions are usually tackled by engineering textbooks: How much energy of one kind is required to generate one unit of energy of another kind? What is the theoretical limit? How close we can come to that limit in practical applications? This last question is where most engineering textbooks fail in providing practical results and real not theoretical efficiencies. As an example, let us examine the Carnot process or the Carnot engine. Every engineering student should know that the Carnot process is the most efficient process to operate between given temperatures. The fact that such an engine cannot be made, and even if it could be made it would have to operate infinitesimally slow to allow the heat transfer processes to happen with no losses, is maybe mentioned in some textbooks. But the fact that such an engine would be very efficient at generating no power is never emphasized enough. Yes, the famous Carnot engine would have to operate at the efficiencies lower than the famous Carnot efficiency in order to generate useful power. This book emphasizes the efficiency. But not only the theoretical efficiency, but the efficiency of practical, power-generating devices. That is why there is a subsection on efficiency in almost every chapter. The chapter on Fuel Cell Thermodynamics deals with theoretical fuel cell efficiencies. As important as it is to learn about the Carnot efficiency, it is equally important to learn about theoretical limits in fuel cells. The chapter on Fuel Cell Electrochemistry introduces various losses that are unavoidable because of the physical properties of the materials involved. These losses obviously have an effect on the efficiency of energy conversion. The chapter on Fuel Cell Systems discusses various supporting devices that are needed to get the fuel cell going. Most of those devices need power, which means that some of the power produced by a fuel cell would be used to run those supporting devices, and therefore less net power would actually be delivered by the fuel cell system. This means that the practical efficiency will

3 Introduction 3 be somewhat lower than the theoretical one. How much lower? That would depend on the system configuration, design, and selection of auxiliary components. And finally, the efficiency of an energy conversion device in a practical application will probably depend on how that device is being used. Does it run all the time at constant power output or does the power output vary? If it varies, how much and how often? These are the reasons why the efficiency is discussed in almost every chapter of this book. Another important aspect of an energy conversion process is the cost. It is the cost of produced energy that matters in practical applications. Obviously, this cost depends greatly on the efficiency of the energy conversion process and the cost of the consumed (or thermodynamically, the more correct term would be converted ) energy. The cost of the energy conversion device itself must also be taken into account. The cost of any device depends on the cost of the materials and the efforts (labor) involved to process those materials and make the components, and finally to assemble those components into a working device. Unfortunately, there is not enough information available on fuel cell costs, either materials or labor. One of the reasons the fuel cells are expensive is that they are not being mass produced. And one of the reasons they are not being mass produced is that their markets are limited because they are expensive. This chicken and egg problem is typical for any new technology. This book will interweave the theory and practice of the fuel cell and fuel cell system design, engineering, and applications. This book will not provide a recipe on how to build the best possible fuel cell, but it will give an engineering student an understanding of the basic processes and materials inside a fuel cell. It will also supply enough tools and instructions on how to use them, to design a fuel cell or a fuel cell system, or how to select a fuel cell for a particular application. This book does not provide a direct answer to all fuel cell-related questions, but it provides the engineering tools needed to find those answers. A fuel cell is in some aspects similar to a battery. It has an electrolyte, and negative and positive electrodes (Figure 1-2), and it generates DC electricity through electrochemical reactions. However, unlike a battery, a fuel cell requires a constant supply of fuel and oxidant. Also, unlike in a battery, the electrodes in a fuel cell do not undergo chemical changes. Batteries generate electricity by the electrochemical reactions that involve the materials that are already in batteries. Because of this, a battery may be discharged, which happens when the materials that participate in the electrochemical reactions are depleted. Some batteries are rechargeable, which means that the electrochemical reactions may proceed in reverse when external electricity is applied a process of recharging the battery. A fuel cell cannot be discharged as long as the reactants fuel and oxidant are supplied. Typical reactants for fuel cells are hydrogen and oxygen; however, neither has to be in its pure form. Hydrogen may be present either in a mixture with other gases (such as CO 2, N 2, CO), or in hydrocarbons such

4 4 PEM Fuel Cells: Theory and Practice hydrogen electrode electrolyte oxygen electrode waste heat water FIGURE 1-2. A fuel cell is similar to a battery in that it has electrodes and an electrolyte, but it needs a fuel and oxidant supply and it generates waste heat and water. as natural gas, CH 4, or even in liquid hydrocarbons such as methanol, CH 3 OH. Ambient air contains enough oxygen to be used in fuel cells. Yet another difference between a fuel cell and a battery is that a fuel cell generates by-products: waste heat and water, and the system is required to manage those (a battery also generates some heat but at a much lower rate that usually does not require any special or additional equipment) A Very Brief History of Fuel Cells The timeline of fuel cell development history is shown in Figure 1-3. The discovery of the fuel cell operating principle the gaseous fuels that generate electricity, is attributed to Sir William Grove in 1839 [1], although it appears that a Swiss scientist Christian F. Shoenbein independently discovered the very same effect at about the same time (or even a year before) [2]. However, in spite of sporadic attempts to make a practical device, the fuel cell, or the gaseous voltaic battery as it was called by Grove [3], remained nothing more than a scientific curiosity for almost a century. E. Chen, in Fuel Cells Technology Handbook [4], provides a very detailed description of these early fuel cell developments. It was another Englishman, Francis T. Bacon, who started working on practical fuel cells in 1937, and he developed a 6 kw fuel cell by the end of the 1950s. However, the first practical fuel cell applications were in the U.S. Space Program. General Electric developed the first polymer membrane fuel cells that were used in the Gemini Program in the early 1960s. This was followed by the Apollo Space Program, which used the fuel cells to generate electricity for life support, guidance, and communications. These fuel cells were built by Pratt and Whitney based on license taken on Bacon s patents (Figure 1-4).

5 Introduction 5 Invention of fuel cell Scientific curiosity Reinvention Industrial curiosity Space program Entrepreneural phase Birth of new industry s 1990 s FIGURE 1-3. Fuel cell history timeline. FIGURE 1-4. Apollo fuel cells. (Courtesy of UTC Fuel Cells.) In the mid-1960s General Motors experimented with a fuel cell-powered van (these fuel cells were developed by Union Carbide). Although fuel cells have continued to be successfully used in the U.S. Space Program until today, they were again forgotten for terrestrial applications until the early 1990s. In 1989, Perry Energy Systems, a division of Perry Technologies, working with Ballard, a then emerging Canadian company, successfully demonstrated a polymer electrolyte membrane (PEM) fuel cell-powered submarine (Figure 1-5). In 1993, Ballard Power Systems demonstrated fuel cell-powered buses. Energy Partners, a successor of Perry Energy Systems,

6 P Ch01.qxd 5/14/05 6:00 PM Page 6 6 PEM Fuel Cells: Theory and Practice FIGURE 1-5. PC1401 by Perry Group powered by PEM fuel cells (1989). (Courtesy of Teledyne Energy Systems.) demonstrated the first passenger car running on PEM fuel cells in 1993 (Figure 1-6) [5]. The car companies, supported by the U.S. Department of Energy, picked up on this activity and by the end of the century almost every car manufacturer had built and demonstrated a fuel cell-powered vehicle. A new industry was born. The stocks of fuel cell companies, such as Ballard and PlugPower, soared in early 2000 (Figure 1-7), based on a promise of a new energy revolution (eventually in 2001 they came down with the rest of the market). The number of fuel cell-related patents worldwide, but primarily in the United States and Japan, is increasing dramatically (Figure 1-8) [6,7], showing continuous interest and involvement of the scientific and engineering community Types of Fuel Cells Fuel cells can be grouped by the type of electrolyte they use, namely: Alkaline fuel cells (AFC) use concentrated (85 wt%) KOH as the electrolyte for high temperature operation (250 C) and less concentrated (35 50 wt%) for lower temperature operation (<120 C). The

7 P Ch01.qxd 5/14/05 6:00 PM Page 7 Introduction 7 FIGURE 1-6. Energy Partners GreenCar, the first PEM fuel cell-powered passenger automobile, (Courtesy of Teledyne Energy Systems.) Aug 1997 Jan 1999 Jan 2000 Jan 2001 Aug 2002 FIGURE 1-7. The stocks of fuel cell companies soared in early 2000 (example of Ballard).

8 8 PEM Fuel Cells: Theory and Practice 3000 number of patent publications FIGURE 1-8. Fuel cell patent publications per year worldwide. (Adapted from [6] and [7].) electrolyte is retained in a matrix (usually asbestos), and a wide range of electrocatalysts can be used (such as Ni, Ag, metal oxides, and noble metals). This fuel cell is intolerant to CO 2 present in either fuel or oxidant. Alkaline fuel cells have been used in the space program (Apollo and Space Shuttle) since the 1960s. Polymer electrolyte membrane or proton exchange membrane fuel cells (PEMFC) use a thin ( 50 mm) proton conductive polymer membrane (such as perfluorosulfonated acid polymer) as the electrolyte. The catalyst is typically platinum supported on carbon with loadings of about 0.3 mg/cm 2, or, if the hydrogen feed contains minute amounts of CO, Pt-Ru alloys are used. Operating temperature is typically between 60 and 80 C. PEM fuel cells are a serious candidate for automotive applications, but also for small-scale distributed stationary power generation, and for portable power applications as well. Phosphoric acid fuel cells (PAFC) use concentrated phosphoric acid (~100%) as the electrolyte. The matrix used to retain the acid is usually SiC, and the electrocatalyst in both the anode and the cathode is platinum. Operating temperature is typically between 150 and 220 C. Phosphoric acid fuel cells are already semicommercially available in container packages (200 kw) for stationary electricity generation (UTC Fuel Cells). Hundreds of units have been installed all over the world.

9 Introduction 9 Molten carbonate fuel cells (MCFC) have the electrolyte composed of a combination of alkali (Li, Na, K) carbonates, which is retained in a ceramic matrix of LiAlO 2. Operating temperatures are between 600 and 700 C where the carbonates form a highly conductive molten salt, with carbonate ions providing ionic conduction. At such high operating temperatures, noble metal catalysts are typically not required. These fuel cells are in the precommercial/ demonstration stage for stationary power generation. Solid oxide fuel cells (SOFC) use a solid, nonporous metal oxide, usually Y 2 O 3 -stabilized ZrO 2 (YSZ) as the electrolyte. These cells operate at 800 to 1000 C where ionic conduction by oxygen ions takes place. Similar to MCFC, these fuel cells are in the precommercial/demonstration stage for stationary power generation, although smaller units are being developed for portable power and auxiliary power in automobiles. Figure 1-9 summarizes the basic principles and electrochemical reactions in various fuel cell types. e - load depleted fuel and product gases out depleted oxidant and product gases out H 2 OH - O 2 H 2 O H 2 O AFC C H 2 H + O 2 H 2 O PEMFC PAFC C 205 C (CO) H 2 = O 2 CO CO 3 2 CO 2 H 2 O MCFC 650 C (CO) (CH 4 ) H 2 O = O 2 H 2 O SOFC C fuel in oxidant in anode electrolyte cathode FIGURE 1-9. Types of fuel cells, their reactions and operating temperatures.

10 10 PEM Fuel Cells: Theory and Practice Sometimes, a direct methanol fuel cell (DMFC) is categorized as yet another type of fuel cell; however, according to the previous categorization (based on electrolyte), it is essentially a polymer membrane fuel cell that uses methanol instead of hydrogen as a fuel How Does a PEM Fuel Cell Work? Although some general engineering principles may be applicable to all fuel cell types, this book is about PEM fuel cells, their operation, design, and applications. PEM stands for polymer electrolyte membrane or proton exchange membrane. Sometimes, they are also called polymer membrane fuel cells, or just membrane fuel cells. In the early days (1960s) they were known as solid polymer electrolyte (SPE) fuel cells. This technology has drawn the most attention because of its simplicity, viability, quick startup, and the fact that it has been demonstrated in almost any conceivable application, as shown in the following sections. At the heart of a PEM fuel cell is a polymer membrane that has some unique capabilities. It is impermeable to gases but it conducts protons (hence the name, proton exchange membrane). The membrane that acts as the electrolyte is squeezed between the two porous, electrically conductive electrodes. These electrodes are typically made out of carbon cloth or carbon fiber paper. At the interface between the porous electrode and the polymer membrane there is a layer with catalyst particles, typically platinum supported on carbon. A schematic diagram of cell configuration and basic operating principles is shown in Figure Chapter 4 deals in greater detail with those major fuel cell components, their materials, and their properties. Electrochemical reactions happen at the surface of the catalyst at the interface between the electrolyte and the membrane. Hydrogen, which is fed on one side of the membrane, splits into its primary constituents protons and electrons. Each hydrogen atom consists of one electron and one proton. Protons travel through the membrane, whereas the electrons travel through electrically conductive electrodes, through current collectors, and through the outside circuit where they perform useful work and come back to the other side of the membrane. At the catalyst sites between the membrane and the other electrode they meet with the protons that went through the membrane and oxygen that is fed on that side of the membrane. Water is created in the electrochemical reaction, and then pushed out of the cell with excess flow of oxygen. The net result of these simultaneous reactions is current of electrons through an external circuit direct electrical current. The hydrogen side is negative and it is called the anode, whereas the oxygen side of the fuel cell is positive and it is called the cathode.

11 Introduction 11 external load collector plate electrode membrane electrode oxygen feed collector plate O 2 2e - H 2 Æ 2H + + 2e - 2H + 1/2O 2 + 2H + + 2e - Æ H 2 O Carbon support H 2 hydrogen feed H 2 O Porous electrode structure Membrane Platinum FIGURE The basic principle of operation of a PEM fuel cell. Chapters 4 and 5 explain in greater detail all the processes involved in making the fuel cell work. Because each cell generates about 1 V, as will be shown subsequently, more cells are needed in series to generate some practical voltages. Depending on application, the output voltage may be between 6 V and 200 V or even more. How the cells are stacked up, and what the issues are in stack design, is discussed in Chapter 6. A fuel cell stack needs a supporting system (as explained in Chapter 9) to: Handle the supply of reactant gases and their exhaust, including the products; Take care of waste heat and maintain the stack temperature; Regulate and condition power output; Monitor the stack vital parameters; and Control the start-up, operation, and shutdown of the stack and system components.

12 12 PEM Fuel Cells: Theory and Practice 1.5. Why Do We Need Fuel Cells? Fuel cells are a very promising energy technology with a myriad of possible applications as discussed next and in greater detail in Chapter 10. Fuel cells have many properties that make them attractive when compared with the existing, conventional energy conversion technologies, namely: Promise of high efficiency Because the fuel cell efficiency is much higher than the efficiency of internal combustion engines, fuel cells are attractive for automobile applications. Also, fuel cell efficiency is higher than the efficiency of conventional power plants, and therefore the fuel cells may be used for decentralized power generation. However, new energy conversion technologies, such as hybrid electric vehicles and combined cycle power plants, also have high conversion efficiencies. Promise of low or zero emissions Fuel cells operating on hydrogen generate zero emissions the only exhaust is unused air and water. This may be attractive not only for transportation but also for many indoor applications, as well as submarines. However, hydrogen is not a readily available fuel, and if a fuel cell is equipped with a fuel processor to generate hydrogen, or if methanol is used instead of hydrogen, some emissions are generated, including carbon dioxide. In general, these emissions are lower than those of comparable conventional energy conversion technologies. Issue of national security Fuel cells use hydrogen as fuel. Although hydrogen is not a readily available fuel it may be produced from indigenous sources, either by electrolysis of water or by reforming hydrocarbon fuels. Use of indigenous sources (renewable energy, nuclear, biomass, coal or natural gas) to generate hydrogen may significantly reduce dependence on foreign oil, which would have an impact on national security. However, widespread use of hydrogen would require establishing a hydrogen infrastructure or the so-called hydrogen economy, which will be discussed in Chapter 11. Simplicity and promise of low cost Fuel cells are extremely simple. They are made in layers of repetitive components, and they have no moving parts. Because of this, they have the potential to be mass produced at a cost comparable to that of existing energy conversion technologies or even lower. To date, the fuel cells are still expensive for either automotive or stationary power generation, primarily because of use of expensive materials, such as sulfonated fluoropolymers used as proton exchanged membrane, and noble metals, such as platinum or ruthenium, used as catalysts. Mass production techniques must still be developed for fabrication of fuel cell components and for the stack and system assembly.

13 Introduction 13 No moving parts and promise of long life Because a fuel cell does not have any moving parts, it may be expected to exhibit a long life. Current fuel cell technology may reach the lifetime acceptable for automotive applications ( hours), but their durability must be improved by an order of magnitude for use in stationary power generation (where the requirement is >40,000 80,000 hours). Modular Fuel cells are by their nature modular more power may be generated simply by adding more cells. Mass produced fuel cells may be significantly less expensive than traditional power plants. Instead of building big power plants, which must be planned well in advance, and whose permitting process may be extremely cumbersome, it may be cost-effective to gradually increase generation capacity by adding smaller fuel cells to the grid. Such a concept of distributed generation may not only be cost-effective but also may significantly improve reliability of the power supply. Quiet Fuel cells are inherently quiet, which may make them attractive for a variety of applications, such as portable power, backup power, and military applications. Size and weight Fuel cells may be made in a variety of sizes from microwatts to megawatts which makes them useful in a variety of applications, from powering electronic devices to powering entire buildings. The size and weight of automotive fuel cells approaches those of internal combustion engines, and the size and weight of small fuel cells may offer advantage over the competing technologies, such as batteries for electronic devices Fuel Cell Applications Because of their attractive properties, fuel cells have already been developed and demonstrated in the following applications (some of which are shown in Figure 1-11): Automobiles Almost every car manufacturer has already developed and demonstrated at least one prototype vehicle, and many have already gone through several generations of fuel cell vehicles. Some car manufacturers are working on their own fuel cell technology (General Motors, Toyota, Honda), and some buy fuel cell stacks and systems from fuel cell developers such as Ballard, UTC Fuel Cells, and DeNora (DaimlerChrysler, Ford, Nissan, Mazda, Hyundai, Fiat, Volkswagen). Scooters and bicycles Several companies (Palcan, Asian Pacific, Manhattan Scientific) have demonstrated fuel cell-powered scooters and bicycles using either hydrogen stored in metal hydrides or methanol in direct methanol fuel cells.

14 P Ch01.qxd 5/14/05 6:00 PM Page PEM Fuel Cells: Theory and Practice FIGURE Collage of fuel cell applications and demonstrations to date (mid-2004). (Courtesy of Toyota, Asian Pacific Fuel Cell Technologies, Schatz Energy Center Humboldt State University, Honda, Aprilia, Teledyne Energy Systems, Daimler-Chrysler, Ballard Power Systems, Fuel Cell Propulsion Institute, Plug Power, MTU, Teledyne Energy Systems, Proton Energy Systems, MTI Micro Fuel Cells and Smart Fuel Cells.)

15 Introduction 15 Golf carts Energy Partners demonstrated a fuel cell-powered golf cart in 1994 (it was used in Olympic Village at the 1996 Olympic Games in Atlanta). Schatz Energy Center developed fuel cell-powered golf carts to be used in the city of Palm Desert in California. Utility vehicles Energy Partners converted three John Deere Gator utility vehicles to fuel cell power [8] and demonstrated them in service at Palm Springs airport (1996). John Deere is working with Hydrogenics, Canada, on development of fuel cell-powered electric utility vehicles, including those for lawn maintenance. Distributed power generation Several companies are working on development of small (1 10 kw) fuel cell power systems intended to be used in homes. Some of them are combined with boilers to provide both electricity and heat (PlugPower with Vaillant, and Ballard with Ebara). Backup power Ballard announced plans to commercialize 1 kw backup power generators in cooperation with Coleman (2000), but then bought back the technology and continued to sell the units (2002). Proton Energy Systems demonstrated regenerative fuel cells combining its own PEM electrolyzer technology with Ballard s Nexa units [9]. A regenerative fuel cell generates its own hydrogen during periods when electricity is available. Portable power Many companies (MTI, Motorola, NEC, Fuji, Matsushita, Medis, Manhattan Scientific, Polyfuel) are developing miniature fuel cells as battery replacements for various consumer and military electronic devices. Because of fuel storage issues, most of them use methanol in either direct methanol fuel cells or through microreformer in regular PEM fuel cells. Space Fuel cells continue to be used in the U.S. Space Program, providing power on the space orbiters. Although this proven technology is of the alkaline type, NASA announced plans to use PEM fuel cells in the future. Airplanes In November 2001 Boeing announced that it was modifying a small single-engine airplane by replacing its engine with fuel cells and an electric motor that would turn a conventional propeller. Test flights are scheduled to begin in early 2004, and are being conducted with the intention of using fuel cells as auxiliary power units on jet airliners in the future. Locomotives Propulsion Research Institute started a consortium that demonstrated a fuel cell-powered locomotive for mining operations (the fuel cell was built by DeNora). Boats MTU Friedrichschaffen demonstrated a sailboat on lake Constanze (2004) powered by a 20 kw fuel cell, developed jointly with Ballard.

16 16 PEM Fuel Cells: Theory and Practice Underwater vehicles In 1989 Perry Technologies successfully tested the first commercial fuel cell-powered submarine, the twoperson observation submersible PC-1401, using Ballard s fuel cell [10]. Siemens has been successfully providing fuel cell engines for large submarines used by the German, Canadian, Italian, and Greek Navies. References 1. Grove, W. R., On Voltaic Series and the Combination of Gases by Platinum, London and Edinburgh Philosophical Magazine and Journal of Science, Series 3, 14, , Bossel, U., The Birth of the Fuel Cell (European Fuel Cell Forum, Oberrohrdorf, Switzerland, 2000). 3. Grove, W. R., On a Gaseous Voltaic Battery, London and Edinburgh Philosophical Magazine and Journal of Science, Series 3, 21, , Chen, E., History, in G. Hoogers (editor), Fuel Cell Technology Handbook (CRC Press, Boca Raton, FL, 2003). 5. Nadal, M. and F. Barbir, Development of a Hybrid Fuel Cell/Battery Powered Electric Vehicle, in D. L Block and T. N. Veziroglu (editors), Hydrogen Energy Progress X, Vol. 3 (International Association for Hydrogen Energy, Coral Gables, FL, 1994) pp Stone C. and A. E. Morrison, From Curiosity to Power to Change the World, Solid State Ionics, Vol , 2002, pp Fuel Cell Today, (accessed August 2004). 8. Barbir, F., M. Nadal and M. Fuchs, Fuel Cell Powered Utility Vehicles, Proc. Portable Fuel Cell Conference, (Ed.) F. Buchi, pp , Lucerne, Switzerland, June Barbir, F., S. Nomikos, and M. Lillis, Practical Experiences with Regenerative Fuel Cell Systems, in Proc Fuel Cell Seminar (Miami Beach, FL, 2003). 10. Blomen, L. J. M. J. and M. N. Mugerwa, Fuel Cell Systems (New York, Plenum Press, 1993).