PEM Fuel Cell Testing and Diagnosis

By Jiujun Zhang, Jifeng Wu and Huamin Zhang

PEM Fuel Cell Testing and Diagnosis covers the recent advances in PEM (proton exchange membrane) fuel cell systems, focusing on instruments and techniques for testing and diagnosis, and the application of diagnostic techniques in practical tests and operation. This book is a unique source of electrochemical techniques for researchers, scientists and engineers working in the area of fuel cells.

Proton exchange membrane fuel cells are currently considered the most promising clean energy-converting devices for stationary, transportation, and micro-power applications due to their high energy density, high efficiency, and environmental friendliness. To advance research and development of this emerging technology, testing and diagnosis are an essential combined step. This book aids those efforts, addressing effects of humidity, temperature and pressure on fuel cells, degradation and failure analysis, and design and assembly of MEAs, single cells and stacks.

• Provides fundamental and theoretical principles for PEM fuel cell testing and diagnosis.
• Comprehensive source for selecting techniques, experimental designs and data analysis
• Analyzes PEM fuel cell degradation and failure mechanisms, and suggests failure mitigation strategies
• Provides principles for selecting PEM fuel cell key materials to improve durability

• Language: English
• Publisher: Elsevier Science
• Release date: Jan 22, 2013
• ISBN: 9780444536891

Jiujun Zhang

Dr. Jiujun Zhang is a Senior Research Officer and Catalysis Core Competency Leader at the National Research Council of Canada Institute for Fuel Cell Innovation (NRC-IFCI, now changed to Energy, Mining & Environment Portfolio (NRC-EME)). Dr. Zhang received his B.S. and M.Sc. in Electrochemistry from Peking University in 1982 and 1985, respectively, and his Ph.D. in Electrochemistry from Wuhan University in 1988. After completing his Ph.D., he took a position as an associate professor at the Huazhong Normal University for two years. Starting in 1990, he carried out three terms of postdoctoral research at the California Institute of Technology, York University, and the University of British Columbia. Dr. Zhang has over 30 years of R&D experience in theoretical and applied electrochemistry, including over fourteen years of fuel cell R&D (among these 6 years at Ballard Power Systems and 9 years at NRC-IFCI), and 3 years of electrochemical sensor experience. Dr. Zhang holds several adjunct professorships, including one at the University of Waterloo, one at the University of British Columbia and one at Peking University. Up to now, Dr. Zhang has co-authored 290 publications including 190 refereed journal papers with more than 5000 citations, 9 edited /co-authored books, 11 conferences proceeding papers, 12 book chapters, as well as 50 conference and invited oral presentations. He also holds over 10 US/EU/WO/JP/CA patents, 9 US patent publications, and produced in excess of eighty industrial technical reports. Dr. Zhang serves as the editor /editorial board member for several international journals as well as Chief-in-Editor for book series (Electrochemical Energy Storage and Conversion, CRC press). Dr. Zhang is an active member of The Electrochemical Society, the International Society of Electrochemistry, and the American Chemical Society.

Book preview

1

PEM Fuel Cell Fundamentals

Chapter Outline

1.1. Introduction

1.2. Electrochemical Reaction Thermodynamics in a H2/Air Fuel Cell

1.2.1. Thermodynamic Electrode Potential and Cell Voltage of a H2/Air Fuel Cell

1.2.2. Fuel Cell Electrical Work and Heat

1.2.3. Fuel Cell Electrical Energy Efficiency

1.3. Electrochemical Reaction Kinetics in a H2/Air Fuel Cell

1.3.1. Kinetics of the Hydrogen Oxidation Reaction

1.3.2. Kinetics of the Oxygen Reduction Reaction

1.4. PEM Fuel Cell Current–Voltage Expression

1.5. Fuel Cell Components

1.5.1. Fuel Cell Electrocatalysts

1.5.2. Catalyst Layers

1.5.3. Gas Diffusion Layer

1.5.4. Membrane (or Solid Electrolyte)

1.5.5. Membrane Electrode Assembly

1.5.6. Flow Field Plate/Bipolar Plate

1.5.7. Current Collectors

1.5.8. Other Components

1.6. Single Cell and Fuel Cell Stack Operation

1.7. Fuel Cell Performance

1.7.1. Fuel Cell Power Density

1.7.2. Fuel Crossover

1.7.3. Practical Electrical Energy Efficiency of Fuel Cells

1.8. Fuel Cell Operating Conditions

1.8.1. Operating Temperature

1.8.2. Operating Pressure

1.8.3. Relative Humidity

1.8.4. Gas Flow Rates and Stoichiometries

1.9. Chapter Summary

References

1.1 Introduction

Proton exchange membrane (PEM) fuel cells, which directly convert chemical energy to electrical energy, have attracted great attention due to their numerous advantages, such as high power density, high energy conversion efficiency, fast startup, low sensitivity to orientation, and environmental friendliness. Figure 1.1 shows the schematic of a typical single PEM fuel cell [1], in which the anode and cathode compartments are separated by a piece of PEM such as Nafion®. This Nafion® membrane serves as the electrolyte and helps conduct protons from the anode to the cathode and also separates the anode and the cathode. During fuel cell operation, the fuel (e.g. H2) is oxidized electrochemically within the anode catalyst layer (CL), and this produces both protons and electrons. The protons then get transported across the membrane to the cathode side, while the electrons move through the outer circuit and thereby also reach the cathode side. These protons and electrons electrochemically react with the oxidant (i.e. oxygen in the feed air) within the cathode CL and produce both water and heat. The whole process of a H2/air PEM fuel cell produces electricity, water, and heat, without any polluting byproducts.

FIGURE 1.1 Schematic of a typical H 2 /air PEM fuel cell. (For color version of this figure, the reader is referred to the online version of this book.) [1]

To better understand how a PEM fuel cell works, it is necessary to grasp the fundamentals of PEM fuel cells, including their cell structure and the thermodynamics and kinetics of fuel cell electrochemical reactions. In the following sections of this chapter, the fundamentals of H2/air PEM fuel cells will be discussed in detail.

Several other types of fuel cells also belong to the PEM fuel cell family; these include the direct methanol fuel cell, direct ethanol fuel cell, and direct formic acid fuel cell. However, the scope of this book is such that we will only focus on the H2/air PEM fuel cell.

1.2 Electrochemical Reaction Thermodynamics in a H2/Air Fuel Cell

1.2.1 Thermodynamic Electrode Potential and Cell Voltage of a H2/Air Fuel Cell

A H2/air PEM fuel cell converts chemical energy stored in the fuel (hydrogen) into electrical energy through electrochemical reactions between H2 and O2. These electrochemical reactions can be written as follows:

(1.I)

(1.II)

(1.III)

Note that the two-directional arrows in these reaction expressions indicate that all these reactions are chemically or electrochemically reversible, although they are not thermodynamically reversible due to their limited reaction rate in both reaction directions. Assuming that these reactions are in equilibrium states, the thermodynamic electrode potentials for the half-electrochemical Reactions (1.I) and (1.II) and the overall Reaction (1.III) can be expressed using the following Nernst equations:

(1.1)

(1.2)

In is the reversible anode potential (V) at temperature Tis the electrode potential of the H2/H+ redox couple under standard conditions (1.0 atm, 25 °C), which is defined as zero voltage; nH is the electron transfer number (= 2 for the H2/H+ redox couple); R is the universal gas constant (8.314 J K−1 mol−1); and F is the reversible cathode potential (V) at temperature Tis the electrode potential of the O2/H2O redox couple under standard conditions (1.0 atm, 25 °C), which is 1.229 V (vs. the standard hydrogen electrode (SHE)); n, which can be expressed as Eqn (1.3):

(1.3)

equals 1.229 V. Therefore, if Tare known, the theoretical cell voltage under different conditions can be calculated according to , respectively.

, is the fuel cell open circuit voltage (OCV). However, in practice, the measured OCV value is always lower than the theoretical value calculated by Eqn (1.3). This is because several factors can affect the OCV, including Pt/PtO catalyst mixed potential and hydrogen crossover, which will be discussed in Chapter 7.

It is worth mentioning that GG Gcell) for fuel cell Reaction (1.III) can be related to the fuel cell voltage by the following equation:

(1.4)

where 2 is the electron number when each H2 is oxidized, and F has the same meaning as in Eqn (1.3). Using this equation, if the change in Gibbs free energy is known, the corresponding fuel cell OCV can be calculated. Of course, if the fuel cell OCV is known, the change in the Gibbs free energy of a reaction can be calculated using Eqn (1.4). Note that the calculated fuel cell OCV is not the real OCV in a practical fuel cell and is normally lower than the theoretically expected value due to the catalyst mixed potential and hydrogen crossover; therefore, Eqn (1.4), a thermodynamic equation, may not be applicable to a real situation. However, as a very rough estimation, this equation may be usable, depending on the user’s own opinion.

1.2.2 Fuel Cell Electrical Work and Heat

H, could be obtained. However, if the same reaction was carried out in the same calorimeter by using the fuel cell device shown in HH by the following equation:

(1.5)

H Gcell is the maximum electrical work that the fuel cell can generate, and T S is the maximum heat the fuel cell can release; here, T S HGS for fuel cell Reaction (1.III) are −285.8 kJ mol−1, −237.2 kJ mol−1, and −48.7 J mol−1, respectively. These numbers exactly obey Eqn (1.5).

1.2.3 Fuel Cell Electrical Energy Efficiency

In Gcell) produced by the fuel cell is practically desirable, whereas the heat (T S) produced is not as useful as the electrical work. In practical fuel cell operations, this heat has to be removed through a cooling system, which places an extra burden on the fuel cell system.

of a fuel cell can be defined as follows:

(1.6)

GH = 285.8 kJ mol−1, indicating that of the overall energy generated by the fuel cell reaction, 83% is converted into electrical energy and the other 17% is released as heat. In addition, this fuel cell electrical efficiency is a function of temperature. For example, in the temperature range of 25–1000 °C, this electrical efficiency will be reduced almost linearly from 83 to 66% at a reduction rate of 0.0174% per degree centigrade [2]. Note that this fuel cell electrical efficiency is a thermodynamic concept or a predicted maximum efficiency. For practical fuel cell operation, where the fuel cell reaction drifts significantly from the ideal thermodynamic situation, one should be especially cautious about using Eqn (1.6) to evaluate the electrical energy efficiency of a fuel cell.

1.3 Electrochemical Reaction Kinetics in a H2/Air Fuel Cell

In the previous section, we discussed fuel cell thermodynamics. However, in reality, fuel cell operation with an external load is much more practical than in a thermodynamic state. When a H2/air PEM fuel cell outputs power, the half-electrochemical reactions will proceed simultaneously on both the anode and the cathode. The anode electrochemical reaction expressed by Reaction (1.I) will proceed from H2 to protons and electrons, while the oxygen from the air will be reduced at the cathode to water, as expressed by electrochemical Reaction (1.II). For these two reactions, although the hydrogen oxidation reaction (HOR) is much faster than the oxygen reduction reaction (ORR), both have limited reaction rates. Therefore, the kinetics of both the HOR and the ORR must be discussed to achieve a better understanding of the processes occurring in a PEM fuel cell.

1.3.1 Kinetics of the Hydrogen Oxidation Reaction

For a H2/air PEM fuel cell, the anode HOR described by Reaction (1.I) is an overall reaction expression, which contains several steps that form the HOR mechanism. When Pt particles are used as the catalyst in the anode CL, a generally recognized reaction mechanism can be expressed as follows [3–8]:

(1.IV)

(1.V)

(1.VI)

Reaction are dependent on the electrode potential and can be expressed as follows:

(1.7)

(1.8)

The concentrations of Pt-related surface species or surface reaction sites, such as Pt–H2, Pt–H, and Pt in Reactions , and θpt, respectively. They have the following relationship:

(1.9)

Assuming the fast Reaction (1.IV) is always in an equilibrium state, the following equation will apply:

(1.10)

is the concentration or partial pressure of hydrogen in the CL. Combining Eqns (1.9) and (1.10) yields

(1.11)

Because Reaction (1.V) is the slowest reaction and Reaction (1.VI) is a fast reaction, it is reasonable to assume that the surface coverage of the Pt–H site during the whole reaction process remains constant (steady-state assumption), that is, the amount of Pt–H produced is equal to the amount consumed. The following equation will then apply:

(1.12)

is the concentration of protons in the aqueous phase. Combining Eqns (1.11) and (1.12) yields

(1.13)

By substituting and θPt become functions of the hydrogen concentration:

(1.14)

(1.15)

Equations and θPt can be expressed as functions of the hydrogen and proton concentrations, which are practically controllable and measurable.

are electrode potential dependent and can be expressed as follows:

(1.16)

(1.17)

The reaction rates in the forward and backward directions for Reaction (1.VI) can be expressed as Eqns (1.18) and (1.19), respectively:

(1.18)

(1.19)

are the forward and backward reaction rate constants, which are independent of the electrode potential; A is the electrode surface area; αH and nαH are, respectively, the electron transfer coefficient and the electron transfer number (here equals to 2) in Reaction (1.VI); Ea is the anode potential; and R and T can be expressed as in Eqn (1.20):

(1.20)

:

(1.21)

where nH is the overall electron transfer number for H2 electro-oxidation to protons, which is 2. In electrochemistry, Eqn (1.21) is a form of the Butler–Volmer equation.

in . In this case, the electrode potential should reach the thermodynamic or reversible or equilibrium electrode potential (E). Then Eqn (1.21) becomes Eqn (1.22):

(1.22)

are the surface coverage of the Pt–H and Pt sites at the equilibrium electrode potential. Therefore, the exchange current density can be expressed as in Eqn (1.23):

(1.23)

This exchange current density can also be expressed in another form. For example, Eqn (1.23) can be rewritten as Eqn (1.24):

(1.24)

Combining Eqns (1.23) and (1.24), an alternative expression for the exchange current density can be obtained:

(1.25)

It can be seen that , which actually can be expressed as functions of measurable and controllable parameters such as the concentrations of hydrogen and protons. For example, if the electrode potential Ecan be alternatively expressed as Eqns (1.26) and (1.27), respectively:

(1.26)

(1.27)

into can be derived, which contains all reaction constants as well as the measurable and controllable concentrations of both hydrogen and protons.

In fact, this exchange current density can be directly measured by experiments. To do this, another alternative expression for Eqn (1.21) is very useful. First, let us introduce a concept called overpotential. The anode overpotential, ηa, is the difference between the anode potential and its equilibrium potential, and it can be expressed as in Eqn (1.28):

(1.28)

By combining Eqns (1.21), (1.23), and (1.28), one can obtain an alternative Butler–Volmer equation:

(1.29)

If the overpotential is large enough, for example, ηa > 60 mV, the second term on the right-hand side of Eqn (1.29) can be omitted when compared to the first term, and this equation then becomes

(1.30)

By taking the log of both sides of Eqn (1.30), we get

(1.31)

Equation (1.31) are also known as the Tafel equation. In experiments, the electrode potential (Evs. Ea can then be obtained, as shown in Fig. 1.2. From the slope of this plot

can be calculated according to , electron transfer coefficient (αH), and electron transfer number (nαH)—can be experimentally measured based on Tafel Eqn (1.31).

FIGURE 1.2 Tafel plot for the HOR on a Pt/C electrocatalyst at 23 °C.

An alternative way to obtain the exchange current density from Eqn (1.29) is to use the data within a very small ηa range near the open circuit potential. In this small ηa range, Eqn (1.29) can be approximately simplified as Eqn (1.32):

(1.32)

By rearranging Eqn (1.32), we can obtain an alternative expression:

(1.33)

is defined as the charge transfer resistance for the HOR, which can be measured using electrochemical impedance spectroscopy (EIS) at the open circuit potential. From and nαH are known, the exchange current density can be calculated. A detailed discussion of EIS measurements will be presented in Chapter 3.

and the electron transfer coefficient (αcan be expressed as the Arrhenius form:

(1.34)

vs. 1/T will yield both the reaction activation energy and the exchange current density at infinite temperature.

In the above section, all the equations we derived are based on pure electron transfer kinetics. Unfortunately, in reality, mass transfer (e.g. hydrogen diffusion inside a porous fuel cell CL) will have an effect on the overall reaction rate, and sometimes can become the rate-determining step. To address this mass transfer effect, we need to introduce another concept, called limiting diffusion current density, which can be expressed as in Eqns (1.35) and (1.36) [9]:

(1.35)

(1.36)

may be the average mass transfer coefficient inside the CL. To reflect this mass transfer effect of the reactant, Eqn (1.29) can be modified to Eqn