Fuel Cell Modeling and Simulation: From Microscale to Macroscale

By Gholam Reza Molaeimanesh and Farschad Torabi

Fuel Cell Modeling and Simulation: From Micro-Scale to Macro-Scale provides a comprehensive guide to the numerical model and simulation of fuel cell systems and related devices, with easy-to-follow instructions to help optimize analysis, design and control. With a focus on commercialized PEM and solid-oxide fuel cells, the book provides decision-making tools for each stage of the modeling process, including required accuracy and available computational capacity. Readers are guided through the process of developing bespoke fuel cell models for their specific needs.

This book provides a step-by-step guide to the fundamentals of fuel cell modeling that is ideal for students, researchers and industry engineers working with fuel cell systems, but it will also be a great repository of knowledge for those involved with electric vehicles, batteries and computational fluid dynamics.

• Offers step-by-step guidance on the simulation of PEMFC and SOFC
• Provides an appendix of source codes for modeling, simulation and optimization algorithms
• Addresses the fundamental thermodynamics and reaction kinetics of fuel cells, fuel cell electric vehicles (FCEVs) and fuel cell power plant chapters

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 12, 2022
• ISBN: 9780323856416

Gholam Reza Molaeimanesh

Gholam Reza Molaeimanesh is Assistant Professor of Automotive Engineering in the Department of Powertrain Systems, University of Science and Technology, Iran. His background is mechanical engineering and automotive engineering and has been working on the modeling and simulation of CFD (specifically via advanced numerical methods such as LBM) for more than 15 years.

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Preface

Global warming and the depletion of fossil fuel resources compel extensive research and development of renewable energies. In the near future, various applications of renewable energies will be observed all over the world, forecasted by climate experts. In this renewable world, green or renewable hydrogen energy (i.e., hydrogen from renewable electricity of solar plants etc.), accompanied by fuel cell systems, will have a crucial role. Evidently, in all eight scenarios proposed by the European Commission for the net zero emissions of the world economy in 2050, hydrogen energy exists [1]. To better reveal this critical role, we suggest reading the report of Bloomberg New Energy Finance in March 2020 [2]. It states that the clean hydrogen can address the most challenging third of global greenhouse gas emissions by 2050. This clean energy can play several roles; e.g., it can be used to store the generated heat in a solar power plant by employing an electrolyzer and producing hydrogen fuel; the produced hydrogen can be used in a fuel cell system to deliver electricity to the grid for peak power shaving.

Hydrogen energy and fuel cell engineering is an active field in both the academic and industrial sectors. Most universities in the world teach courses such as fuel cell systems in different departments, including chemical engineering, energy systems engineering, mechanical engineering, etc. The present book deals with all aspects of fuel cell modeling and simulation, from microscale to macroscale, and can be a precious source for such courses. For the numerical simulation of fuel cells, the book is of special interest. More specifically, for higher education students, the topic is of more interest. In the industrial sector, many companies in the world are working on the installation of fuel cell systems. These companies use different software like GT-SUITE, AVL, ANSYS, COMSOL Multiphysics, etc., for fuel cell simulations, which all use a numerical model or several numerical models.

A key feature of this book is the classified insight provided for the reader. This can effectively help the reader to decide which numerical model or simulation technique is appropriate for him/her according to the required accuracy and available computational capacity. This book includes seven chapters and five appendixes. Chapter 1 talks about fuel cell fundamentals concisely as a prestep for fuel cell modeling. However, those familiar with fuel cell fundamentals can directly study the other applied six chapters. These six chapters are about the modeling and simulation of several independent applied topics, from PEMFCs, SOFCs, and hydrogen storage, to FCEVs, fuel cell power plants, and fuel cell-based CHPs. Chapter 2, the largest and most important, discusses PEMFCs. After a brief introduction about PEMFCs and the transport phenomena in different parts of them, the microscale simulation techniques such as LBM, PNM, and VOF are presented. After that, single- and multiphase macroscale CFD models and 1D models for the simulation at both cell and stack levels are presented. In all these simulation techniques, the governing equations and the solution procedures are presented. This chapter consists of several examples and problem as well as several Fortran codes for the pore-scale simulation of PEMFC electrodes by LBM in Appendix A. Chapter 3 talks about SOFCs in a similar structure. It introduces transport phenomena, micro- and macroscale models. In Chapter 4, different methods of hydrogen storage and their simulation and modeling are presented. Special focus of this chapter is on the hydrogen storage in modern absorbers such as metal hydride and advanced carbon materials; a UDF for the simulation of an MH tank in ANSYS Fluent is provided in Appendix D. Chapter 5 is quite interesting for vehicle engineers; it explains vehicle dynamics concisely and then a 1D model of a vehicular powertrain, which can be used for the design and analysis of FCEVs. The MATLAB® code of the model is also provided in Appendix E. The topics of the last two chapters are about the modeling and simulation of power plants and CHP systems based on fuel cells. Since optimization is an important factor in design phase, its concept is explained in Appendix C, and different codes are provided to implement different optimization methods.

Finally, we sincerely hope that the goals of this book are achieved and welcome any feedback, such as comments, suggestions, queries, finding a typo, providing better explanations and discussing other important undiscussed concepts. Please help us to improve the next edition of the book with your precious feedback via sending emails to [e-mail] or [e-mail].

References

[1] European Commission, A Clean Planet for all. A European long-term strategic vision for a prosperous, modern, competitive and climate neutral economy, Depth Analysis in Support of the Commission; Communication COM 2018;773:2018.

[2] BloombergNEF, Hydrogen Economy Outlook: Key Messages, New York, USA, 2020.

Chapter 1: Fuel cell fundamentals

Abstract

The present book focuses on the simulation of different fuel cells. The core of the simulation method is based on the lattice-Boltzmann method or LBM. Before any simulation, we need to have fundamental knowledge about the involved physics and chemistry of the system. The present chapter talks about the fundamentals of fuel cells. Then it introduces different fuel cell types and technologies. After that, the basic thermodynamic relations important in fuel cell modeling are explained. The important transfer phenomena such as mass, electron, and heat transfer in a fuel cell are considered in more detail. Finally, the characteristic curve that determines the outcome of all the involved phenomena is discussed. The chapter contains lots of practical and theoretical examples for a better understanding.

Keywords

thermodynamics of fuel cells; fuel cell technologies; mass transfer in fuel cells; heat transfer in fuel cells; charge transfer in fuel cells; characteristic curve of fuel cells

Chapter Outline

1.1  Introduction

1.1.1  Fuel cell perspective

1.1.1.1  Roadmap of Japan

1.1.1.2  Roadmap of EU

1.1.1.3  Roadmap of the United States

1.1.1.4  Roadmap conclusions

1.1.2  Fuel cell operation

1.1.3  Fuel cell types

1.1.3.1  Proton-exchange membrane fuel cell (PEMFC)

1.1.3.2  Direct methanol fuel cells (DMFCs)

1.1.3.3  Alkaline fuel cells (AFCs)

1.1.3.4  Phosphoric acid fuel cells (PAFCs)

1.1.3.5  Solid-oxide fuel cell (SOFCs)

1.1.3.6  Molten-carbonate fuel cell (MCFC)

1.1.3.7  Other technologies

1.2  Thermodynamics

1.2.1  Gibbs free energy

1.2.2  Second law of thermodynamics and fuel cells

1.2.3  Fuel cell efficiency

1.2.4  Role of effective factors

1.2.4.1  Effect of temperature

1.2.4.2  Effect of pressure

1.3  Electrochemical reaction kinetics

1.3.1  Exchange current density

1.3.2  Butler–Volmer equation

1.3.3  Role of effective factors

1.4  Charge transfer

1.4.1  Electronic resistance

1.4.2  Ionic resistance

1.4.3  Role of effective factors

1.5  Mass transport

1.5.1  Convective mass transfer from flow channel to GDL

1.5.2  Diffusive mass transfer

1.5.3  Role of effective factors

1.6  Characteristic curve of a fuel cell

1.7  Summary

1.8  Problems

References

1.1 Introduction

A fuel cell is an electrochemical device in which a fuel is oxidized and generates electricity, heat, water, and in some cases, other materials such as carbon dioxide. For most cases, the fuel is hydrogen, and the oxidant is the oxygen available in atmosphere. In this view, fuel cells are nothing than the conventional electrochemical batteries. The only difference is that the available energy in batteries is limited to the amount of active mass that is confined in the battery, but a fuel cell does not store any energy. In fact, as long as the fuel and oxygen are fed into the device, they produce electricity. In other words, a fuel cell is just a reactor whose reactants (fuel and oxygen) are fed from external storage devices.

Although fuel cells are recently considered as a promising source of electricity generation, its invention dates back to the 19th century when the renown British physicist, Sir William Grove found out that steam can be dissociated into hydrogen and oxygen in a reversible reaction. In other words, the hydrogen and oxygen can be combined through an electrochemical reaction to produce water. After the invention, fuel cells did not get much attention until the 20th century where alkaline fuel cells were used in space programs. However, at that time, the cost of cells was too high to become interesting for residential applications. One of the main reasons that contributed to their high cost was the usage of platinum (Pt), which is too expensive. Many efforts were carried on to reduce the amount of Pt to reduce the cost. The result was a significant reduction in cost, which in turn resulted in more economic cells.

Nowadays, fuel cells are well known all over the world, and many factories and manufacturers produce different cell types. However, they are still costly devices, and their usage is still under debate. Although many efforts have been done to reduce the amount of Pt, the cost of devices is not still as low as it should be. One of the reasons is that the other parts of the cell are also expensive. For example, in many fuel cells the separator is made of Nafion, and in some others, it is made of a solid ceramic material. Both materials are still expensive, which increases the cost of the cells.

Recent researches show that although the fuel cells are still expensive, if we consider the whole life cycle assessment, they may be much better devices than the present engines. Note that the fuel cells have no moving part, are noiseless, and have a long service life and low maintenance. These factors make them quite attractive in comparison with the present generators such as gas engines or turbomachines.

In addition to the above, fuel cells are a part of hydrogen economy, which many scientists believe would be the future of human energy source. Without doubt, the fossil fuels will come to an end, and human has no choice but to rely on natural sources of energy such as solar energy, wind, biomass, ocean energy, and so on. These sources of energy have their own benefits and drawbacks. The good news about them is that they are renewable and will not come to an end. The bad news is that they may not be available in places where they are needed. For example, the solar or wind energy may not be available while we need to drive a car. Therefore we have to store their energy for our needs such as transportation and so. There are different ways of energy storage, one of which is storing the energy in chemical compounds. For example, we may use the renewable energies to dissociate water into hydrogen and store hydrogen as a fuel in specific tanks. These tanks may be used wherever we need including transportation, home applications, power plants, etc. The stored hydrogen can be burnt in conventional burners and engines, but as we know, the thermodynamic efficiency of these burners is very low comparing to fuel cells. In practice the efficiency of fuel cells may reach more than twice the efficiency of a conventional Otto engine. Therefore, from the energy point of view, it would be much better to use fuel cells instead of the present engines and burners.

1.1.1 Fuel cell perspective

To have a better understanding about the perspective of the fuel cells, a good idea is to study the programs and roadmaps of the pioneers in this field. In this section, we study the roadmaps of Japan, the European Union (EU), and the United States to see the targets of their roadmaps and their plans to achieve the hydrogen economy.

1.1.1.1 Roadmap of Japan

The leading countries such as Japan have solid programs for hydrogen economy or what is called the hydrogen world. In Japan, there are two main targets on FC economy, one for residential sector and the other for fuel cell vehicles (FCVs). The residential program focuses on the usage of solid oxide fuel cells (SOFCs), because these devices produce both electricity and heat. The working temperature of a SOFC is something between 500 and 1000 ∘C, which is quite suitable for combined heat and power (CHP) installation. By this architecture an SOFC produces both the required heat and electric power for a residential building. The benefit of this design is that the overall efficiency reaches above 80%.

Fig. 1.1 shows Japan's roadmap for hydrogen technology in residential sector. This roadmap started around 2005 and, as is shown in the figure, will continue till 2030. The program has three distinct phases:

Large-scale validation, in which the concept was going to be validated till the production of a commercializing product.

Market creation with policy support was the commercialization phase in which SOFC packages were to penetrate the market. Since at the time the price was so high, the penetration was supported by support.

Establishment of self-sustained market is the current stage in which the price of the packages is affordable by individuals, and the market is becoming more an more sustained.

The roadmap predicts that by the end of the present decade, 5.3 million units will be sold, and the price of each unit reaches from less than 500 to 600 JPY.

Figure 1.1 Japan's program for residential sector.

Japan's program on FCVs is based on proton exchange membrane fuel cells (PEMFCs). The roadmap is shown in Fig. 1.2 and started from around 2005 and will continue up to 2030. This program is demonstrated in four different phases:

Technology Demonstration, in which the technology was to be developed.

Technology and Market Demonstration is the continuation of the first phase with market consideration. At the end of the phase the first commercialized cars including Toyota Mirai (2014), Honda Clarity (2016), and Nissan X-TRAIL FCV (2017) were introduced.

Early Commercialization is the current stage. Early FEVs are produced and sent to market. However, the infrastructures such as production, hydrogen filling stations, and other facilities should be expanded.

Full Commercialization is a continuation of the previous phase in which the prices should be reduced and the number of FEVs in the market should be increased.

Figure 1.2 Japan's program for FC vehicles sector.

Japan's program on hydrogen and FC revised and extended for beyond 2040 as shown in Fig. 1.3. The revised roadmap focuses on three major phases:

Installation of Fuel Cell focuses on establishment of fuel cell devices on both residential and mobile sectors.

Power Plant and Mass Supply Chain starts around 2030 and focuses on production. At the early stage, hydrogen is imported from overseas, and the program aims to make full-scale power plants.

-free Hydrogen starts around 2040 and aims to produce hydrogen from renewable sources of energy by water splitting.

Figure 1.3 Japan's revised program.

1.1.1.2 Roadmap of EU

The EU program on hydrogen and fuel cell is shown in Fig. 1.4. The program has two parallel rails, one on hydrogen production and the other on fuel cell development. From the figure we can conclude the following:

Hydrogen Production starts by reformation of natural gas that has been developed and its infrastructure already exists. Hydrogen production technologies continue to be developed until will become fully available from water dissociation using natural or green energy sources around 2050.

Fuel Cell Development focuses on different purposes including usage of SOFCs and molten carbonate fuel cells (MCFCs) for stationary sectors such as power plants and PEMFCs for car industries. The technology aims to provide the energy required for aviation systems in about 2050.

Figure 1.4 EU's hydrogen and FC program.

1.1.1.3 Roadmap of the United States

The United States also have a similar roadmap on hydrogen technology, but the program gives a whole guide line up to 2040. At the early stages the government supports the technology, but in the second phase, the commercialization is given to private sectors. This roadmap is shown in Fig. 1.5, and as we can see, it has four distinct phases:

1.  Technology Development

2.  Initial Market Penetration

3.  Expansion of Markets and Infrastructures

4.  Fully developed Markets and Infrastructures

Figure 1.5 The US hydrogen and FC program.

1.1.1.4 Roadmap conclusions

Comparing the hydrogen program of the above three pioneers in the field reveals that hydrogen and fuel cell will play important roles in the near future. In addition to the above communities, other famous countries such as China, Canada, India, Brazil, and many others have their own roadmaps toward a hydrogen world concept in which becomes the main source of energy. This study shows the importance of fuel cells and their role in human life.

1.1.2 Fuel cell operation

As mentioned above, a fuel cell is an electrochemical device in which fuel and oxidant are combined to produce electricity. In addition to electricity, unavoidable products such as water and heat should be removed from the system. Note that the fuel can be oxidized in a chemical reaction in which the heat and water are produced. For example, the burning of hydrogen is expressed by

(1.1)

The burning reaction is considered to be chemical. The same reaction may also take place through a electrochemical reaction. So, what is the difference between the two? The answer is:

•  The electrochemical reactions take place at a solid surface. In other words, electrochemical reactions are surface phenomena. In contrast, chemical reactions can be done in the bulk.

•  The redox reactions of an electrochemical process take place at different locations. For example, the fuel is oxidized at the anode, and the oxygen is reduced at the cathode. However, in a chemical reaction, both redox reactions take place at the same location.

•  The electron transfer in an electrochemical reaction is performed via an external circuit, whereas for a chemical reaction, the electron is transferred directly from one species to another.

By these characteristics burning of hydrogen is considered a chemical reaction because it first happens in a bulk. Second, the reaction takes place at a single location, in which a hydrogen molecule meets an oxygen molecule. Finally, the electron transfer does not happen through an external circuit.

In contrast to burning process, the reaction of an FC is considered electrochemical because it meets all the above-mentioned characteristics. The process can be better understood by studying Fig. 1.6. The cell contains a negative electrode, which is separated by an electrolyte from a positive electrode. Each electrode is made of at least two layers, one of which is a porous substrate, and the other is a catalyst layer. Usually, the catalyst is coated on the substrate by different methods. The material used for making an electrode differs from technology to technology. Each technology uses a specific material for making the substrate and a different type of catalyst. In some FC types, there may be more layers such as sublayers for catalyst or other materials for enhancing the performance.

Figure 1.6 Schematic representation of a fuel cell and definitions.

The substrate is used for 1) backing the catalyst layer and 2) providing a uniform material concentration at the catalyst layer. Since in most of the FC types the fuel is in gas phase, the substrate is called a gas diffusion layer or GDL. From the name it is clear that GDL should be porous to let the fuel or oxygen diffuse through it and reach the catalyst layer (Cat). The materials used for making GDLs are electrically conductive, but it is worth noting that the reactions do not occur at GDL. The catalyst layer is where the electrochemical reactions occur and the electrons are produced or consumed. The electrons, however, can move through the GDL as well, since GDL is a conductive medium.

In studying Fig. 1.6 the negative electrode is named anode, and the positive electrode is named cathode. In practice and in many papers, books, handbooks, and related media, the words anode and cathode are used, but it should be noted that these terms are not always scientifically true. By definition an anode is a place where electrons are released or produced, and a cathode is a place at which electrons are consumed. In a normal behavior of an FC, the anode and cathode are quite equivalent to the negative and positive electrodes, respectively. However, in regenerative FCs, in which the electrochemical reactions are reversed by applying an external voltage (just like when a battery is recharged), the negative and positive electrodes remain the same, but the anode and cathode change their positions. In the charging process the anode is the positive electrode, and the cathode is the negative electrode.

Although the negative and positive electrodes are a better choice for distinguishing the electrodes (since they do not change during charge and discharge), and since FCs are mostly used as generators and are rarely used as a regenerative device, the terms anode and cathode are used in all the available literature. With this fact in mind, in this book, to be consistent with all the other literature, we also use anode instead of the negative electrode and cathode instead of the positive electrode.

The electrolyte shown in Fig. 1.6 acts as a separator as well. It is a separator because it separates the electrodes so that they should not make a short circuit. In addition, it provides a medium through which the ion transfer occurs. All the available FCs can be categorized in two different types.

1.  The first types are those in which the electrical charge is carried by cations through the electrolyte. In these types, as shown in the figure, the cations are generated at the anode catalyst layer and move from the anode to the cathode.

2.  The second types are those in which the electrical charge is carried by anions through the electrolyte. In these types the anions are generated at the catalyst layer of the cathode and, as shown in the figure, move from the cathode to the anode.

In a first-type FC, the fuel is fed to the anode as shown in Fig. 1.6. A part of the fuel is consumed in the cell, and the excess is returned back and recirculated. The fuel is dissociated by the anode catalyst via the following electrochemical reaction:

(1.2)

where F stands for fuel. Since in this reaction the electron is directly removed from the fuel, advanced and expensive catalysts should be used. The produced electrons cannot pass through the separator because it is a nonconductive material. Therefore the electrons move from an external circuit and pass through the load and reach the cathode. Meanwhile, the cations move through the electrolyte, and reach the cathode. At the cathode catalyst layer, the oxygen which is fed through the cathode GDL will react with the cations and electrons via the following reaction:

(1.3)

When using Eq. (1.3), depending on the fuel material, proper reaction balance should be carried on. The overall reaction is obtained by summing Eqs. (1.2) and (1.3):

(1.4)

For a second-type FC, reactions completely differ from those of the first type. At the anode catalyst layer the fuel reacts by the anions that come from the cathode through the following reaction:

(1.5)

In this reaction, is the moving anion, and M is an intermediate material. Note that since in reaction (1.5), the fuel electrons should not be directly removed, less expensive catalysts can also be used. Again, just like in the first-type FCs, the electrons move from the external circuit, pass through the load, and reach the cathode. At the cathode the oxygen is reduced on the catalyst layer via the following reaction:

(1.6)

The overall reaction is obtained by summing Eqs. (1.5) and (1.6):

(1.7)

The overall reactions of both types show that the final result of the processes is the burning of the fuel, but in an electrochemical manner. We reemphasize that in working with Eqs. (1.2) to (1.7), the reaction should be balanced according to the fuel, cation, anion, intermediate material, and oxygen. In the following subsections, we give the details for each FC technology.

1.1.3 Fuel cell types

There are many different fuel cells available, which are categorized in different perspectives including the choice of separator, operating temperature, and fuel. Among the above-mentioned factors, the choice of their separator is the most important one, and most of the time the fuel cells are named after it, but in some types, their names are given after their fuel or operation. These cells can provide a very large range of power from microwatts to kilo- or megawatts depending on the type and application. Large-scale devices are manufactured by assembling a number of single cells in series since a single cell is not able to produce a large amount of power. Typically, the voltage of a cell in operation is around 0.7 Volts, which is not sufficient for producing large powers.

Regardless of the type, in all the fuel cells, a fuel is oxidized at the anode, and the oxygen is reduced at the cathode. The oxidation and reduction take place on catalyst layers as mentioned before. For the redox reactions to become successful, the temperature should be kept within a specific limit. In this regard, all the cell types are categorized into three different levels:

Low-Temperature FCs operate below 100 ∘C including PEMFCs, direct methanol fuel cells (DMFCs), and microbial fuel cells (MFCs).

Medium-Temperature FCs operate at higher temperatures around 150 to 250 ∘C including AFC, PAFC, and HT-PEMFC.

High-Temperature FCs work in very high temperature over 500 ∘C and even may reach above 1000 ∘C. For this category, we can name SOFC and MCFC, which are used in CHP power plants.

There are lots of different technologies available both in the market and the labs. Many of these types are quite mature and commercially available. However, many different types are still under development. Here we introduce some important types, for which we discuss the main reactions, operating temperatures, and technology readiness.

1.1.3.1 Proton-exchange membrane fuel cell (PEMFC)

This technology is the most used FC, able to draw attention because of its unique characteristics. First of all, it is a low-temperature fuel cell whose operating temperature is less than 100 ∘C (normally, around 90 ∘C). Secondly, it uses a solid-state polymeric membrane, and hence its electrolyte does not suffer from leakage. Thirdly, its energy density is quite reasonable, which has made it a perfect choice for many portable high-power devices such as electrical vehicles. Finally, PEMFCs use hydrogen as fuel and combine it with oxygen taken from atmosphere to produce electricity as the main output and heat and water as by–products. Therefore they do not produce any hazardous materials such as acids, bases, or toxic gases. Consequently, their good characteristics have made them the best choice among the other low-temperature cells.

The name of the PEMFCs is taken from their polymeric membrane made of perfluorosulfonic acid, which when absorbs water, becomes a good conductor of hydrogen protons, , whereas it is an insulator for electron. Many efforts have been made to produce different material for these cells, but at the present time the only commercially available membrane is called Nafion, which is a brand name for the membrane discovered in the late 1960s by Walther Grot of DuPont. In some texts, PEMFC stands for Polymeric Electrolyte Membrane fuel cell as well. In other works, PEMFCs may be called solid-state membrane fuel cells or SSMFC. All these names refer to the material of the membrane.

As a low-temperature device, PEMFCs have some advantages and suffer some disadvantages. The main advantage is that a PEMFC quickly becomes operational since its operational temperature is close to the ambient. The main disadvantage of being a low-temperature device is that to accelerate the reactions, expensive catalysts are required. Usually, is a very expensive choice. Moreover, Pt becomes poisoned when combined with carbon monoxide or CO. Therefore, for industrial scales, proper facilities should be mounted to remove the CO to recover the poisoned Pt.

Fig. 1.7 shows the main components and reactions of a PEMFC. The fuel is hydrogen, which in practice is not completely pure. Fuel enters the anode and diffuses on the Pt catalyst through the anode GDL at which the platinum removes the hydrogen electron through the following reaction:

(1.8)

The electrons move toward the cathode via the external circuit, where it converts into work by the external load. Meanwhile, the protons or hydrogen ions move through the electrolyte to the cathode catalyst layer. At the cathode catalyst layer, the protons, electrons, and oxygen combine to produce water via the following reaction:

(1.9)

Thus the overall reaction of PEMFC is

(1.10)

Needless to say, PEMFCs are of the first type since hydrogen directly converts into ions by means of catalyst.

Figure 1.7 PEMFC configuration and reactions.

1.1.3.2 Direct methanol fuel cells (DMFCs)

Direct methanol fuel cells are another low-temperature FC technology whose construction is very similar to PEMFCs. They operate at temperature range less than 100∘ and use graphite as their electrode. The membrane is also made of Nafion, just like an PEMFC. There are mainly two differences between a PEMFC and a DMFC. First, in DMFC the primary fuel is methanol instead of hydrogen. Second, in DMFCs, Pt is not sufficient for dissociating methanol to electron and ion. Therefore, in addition to Pt, some other catalysts such as ruthenium, Ru, should be added.

In contrast to many other FC types, the name of DMFCs is not after its membrane type, but it is the name of its fuel. The most advantage of a DMFC is that there is no need for hydrogen storage or reformer. Liquid methanol mixed with steam is directly fed into the FC anode. It is quite clear that storage of methanol is not a matter at all. A simple plastic tank can store as much as methanol as needed. Moreover, the methanol production facilities are quite mature, and the infrastructure already exists. The methanol factories can produce pure methanol out of coal, natural gas, oil, and, more importantly, from biomass.

DMFCs are promising candidates for medium-power devices, especially for mobile applications such as cell phones and laptops. The most benefit of DMFCs is that they can be recharged by filling a small tank by methanol. This is quite cheap and handy. In addition to small-scale devices, DMFCs are enhancing for becoming competitive to PEMFCs in car industries. The choice of fuel is its most important factor. Storage of methanol, as mentioned, is not a matter at all.

DMFC is also a first-type FC, in which cations are moving from the anode to cathode. The configuration of a DMFC is shown in Fig. 1.8. In contrast to PEMFC, the fuel is not a gas but is a liquid consisting of liquid methanol mixed with water. Methanol reaches the anode catalyst and converts to proton according to the following reaction:

(1.11)

In this FC, six moles of electrons are released for each mole of fuel. Moreover, carbon monoxide is released as a byproduct. The produced is solved in the fuel until it reaches the solubility limit. Then the excess creates gaseous bubbles, which in turn increases the pressure drop, blocks the alcohol mass transfer to the catalyst, and, in general, reduces the performance of the cell. For DMFCs, special facilities should be designed to remove the gas from the mixture before recirculating.

Figure 1.8 Configuration of a DMFC.

At the cathode catalyst layer, the protons, the electrons, and the oxygen combine to produce water via the following reaction:

(1.12)

Thus the overall reaction of DMFC is

(1.13)

Note that carbon monoxide is not balanced and should be vented from the cell.

1.1.3.3 Alkaline fuel cells (AFCs)

Alkaline fuel cells or AFCs are one of the first industrialized FCs and were invented by Francis Thomas Bacon in 1959. AFC was the first technology that was used in actual programs such as Apollo space program, in which they were used as the main source of electrical energy. These cells are categorized as medium-temperature types since they operate at temperature levels between 100∘ to 250∘. The high operating temperature makes them work even if nonprecious metals are used as catalyst. Therefore, in construction of an AFC, other inexpensive catalysts can be used, such as Ag, CoO, etc. Although to improve the efficiency, Pt can also be used if the price is not a matter.

The name of AFC comes from the material used for construing the membrane. In this type the membrane is made of concentrated solution of an alkali metal such as NaOH or KOH. Alkaline solutions have suitable ionic conductivity in any temperature, whereas the temperature rise increases the conductivity even more. Therefore it becomes a good choice for the cell electrolyte or membrane. Recent advances on AFC introduced the low-temperature cells capable of working at from 25∘ to 70∘.

Since there is no need for precious metals in construction of AFC and their membrane is not very expensive, they are the cheapest FC technology at the present time. However, they suffer from electrolyte poisoning when carbon dioxide is solved in it. The formation of carbonate dramatically reduces the electrolyte conductivity, which is referred to as electrolyte poisoning. To overcome electrolyte poisoning, the KOH solution should be either refreshed or purified using carbon dioxide scrubbers. These limitations make them not quite applicable for large power plants, but efforts are being made to overcome the problems and reduce the