Jinfeng Wu

Techniques for PEM Fuel Cell Testing and Diagnosis

The structures and components of proton exchange membrane (PEM) fuel cells and the designs of all the components, including the membrane electrode assembly, single cell, and stack, have been described and discussed in Chapter 2 . For a practical PEM fuel cell, every feature of the components, key materials, and cell assembly should be achievable and optimized to achieve high performance. Because a fuel cell is a very complicated device, all the components should fully perform their individual roles and simultaneously function together synergistically. To investigate the individual functioning of each component and the synergistic effect, fuel cell testing and diagnosis have been recognized as the most popular and reliable ways to validate the designs of these components and of the fuel cell itself.

Fuel Cell Open Circuit Voltage

The OCV of a PEM fuel cell was addressed in this chapter. OCV can be decreased by increasing the operating temperature. At certain operating conditions, the OCV is mainly determined by the hydrogen crossover rate and the mixed potential formed by the electrochemical reactions of Pt surface oxidation and the ORR. OCV measurement can be a useful diagnostic tool in fuel cell operation or stack lifetime testing. An abnormal OCV usually indicates fuel cell failure, such as a broken membrane, a gasket leak, and/or bipolar plate failure. OCV measurement can also help in obtaining information from within the fuel cell and in localizing the position of broken MEAs. The OCV hold test can be used in accelerated fuel cell durability tests to understand the failure mechanisms that lead to fuel cell degradation at OCV.

Design and Fabrication of PEM Fuel Cell MEA, Single Cell, and Stack

The design and fabrication of proton exchange membrane (PEM) fuel cell components, single cells, and stacks are two of the most important processes in fuel cell technology development. In general, the design and assembly of a fuel cell can have a strong effect on its performance. Given the materials and components used in the fuel cell, design and fabrication have to be optimized with respect to the corresponding fuel cell power output to achieve the best performance. To date, designs and assembly methods have been optimized and validated using fuel cell testing as well as real operation in various application systems, such as portable power devices, stationary power generators, and automobiles. The major challenges still hindering their commercialization are high cost and insufficient durability. The basic components of H 2 /air (O 2 ) PEM fuel cells/stack have been briefly introduced in Chapter 1 . In this chapter, the designs of the key components of PEM fuel cells and the resultant effects on cell performance will be discussed in detail, including the fabrication of the PEM fuel cell membrane electrode assembly, single cell, and stack.

The Effects of Temperature on PEM Fuel Cell Kinetics and Performance

Temperature is one the most important operating conditions of PEM fuel cells, and it can significantly influence cell performance. Generally, an increase in the temperature can improve performance. In light of this fact, high-temperature PEM (HT-PEM) fuel cells operated above 95u2009°C (usually 95–200u2009°C) have recently been developed. This performance improvement at higher temperatures is mainly due to increased membrane proton conductivity, enhanced electrode kinetics for the oxygen reduction reaction (ORR) and the hydrogen oxidation reaction (HOR), and improved mass transfer of the reactants. In addition, increasing the temperature can also increase the tolerance of electrocatalysts to contaminants. However, higher operating temperatures can lead to membrane dehydration, increased hydrogen crossover rate, and the degradation of components such as electrocatalysts, gasket materials, and bipolar plates, resulting in a shortened fuel cell lifetime. Chapter 10 presents a detailed discussion of HT-PEM fuel cells. This chapter will focus on conventional (i.e. low-temperature) PEM fuel cells that use perfluorosulfonic acid membranes (e.g. Nafion ® membranes) and are usually operated below 95u2009°C (typically from room temperature to 80u2009°C). In general, fuel cell performance can be affected by several operating conditions, such as temperature, pressure, and relative humidity (RH). We will discuss the effects of RH and pressure in Chapters 8 and 9, respectively. In this chapter, only the temperature effects on the performance of PEM fuel cells will be discussed in detail.

PEM Fuel Cell Fundamentals

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 time, low sensitivity to orientation, and environmental friendliness. Figurexa01.1 shows the schematic of a typical single PEM fuel cell [1] , in which the anode and cathode compartments are separated by a piece of PEMs 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. H 2 ) 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 an H 2 /air PEM fuel cell produces electricity, water, and heat, without any polluting byproducts.

Relative Humidity (RH) Effects on PEM Fuel Cells

Conventional proton exchange membrane (PEM) fuel cells (typically operated at 80% RH) is required to achieve high PEM fuel cell performance because the proton conductivity of PFSA membranes depends on their water content. Therefore, the RH is one of the important factors to consider in PEM fuel cell performance. Both modeling and experimental results have shown that the RH can influence PEM fuel cell performance by affecting the proton conductivity of membranes, the proton activity in the catalyst layers, electrode reaction kinetics, and the mass transfer process. As a PEM fuel cell is a complicated system, the effect of RH is also related to other operating conditions, such as temperature, pressure, flow field design. This chapter will address the effect of RH on fuel cell performance through theoretical analysis and typical experimental examples.

Electrochemical Half-Cells for Evaluating PEM Fuel Cell Catalysts and Catalyst Layers

As described in Chapter 1 , the electrochemical reactions in a proton exchange membrane fuel cell include two half-cell reactions: the fuel oxidation reaction, such as the hydrogen oxidation reaction (HOR), occurs at the anode while the oxygen reduction reaction (ORR) proceeds at the cathode. When investigating one reaction and its associated catalysis mechanism, as well as the effects that operating conditions have on this reaction, possible interference from the other reaction is normally eliminated through a half-cell method. In addition, for quick downselection of electrode materials and components, such as the catalyst and its associated catalyst layer, an ex situ approach using a half-cell setup is the quickest and most cost-effective method. Half-cell testing is usually conducted in a three-electrode system containing working, counterelectrode and reference electrodes. Cyclic voltammetry, rotating disk electrode, rotating ring-disk electrode, and electrochemical impedance spectroscopy are the typical half-cell testing techniques to investigate a catalyst’s characteristics in terms of the HOR and ORR. Besides their utility for investigating these two reactions, some special half-cell designs also allow testing of other operating conditions, such as catalyst layer/membrane electrode assembly designs, temperature, pressure, humidity, as well as fuel and air-flow rates.

Fuel Cell Degradation and Failure Analysis

Through tremendous research efforts, significant progress has been achieved in PEM fuel cell technologies over the past decade, especially in the areas of increasing volumetric and/or gravimetric specific power densities as well as more effective use of materials. However, technical challenges still remain for the onboard storage of hydrogen fuel and the infrastructure for its widespread distribution, as well as for the fuel cell system itself. With regard to the fuel cell system, there are two major challenges: high cost and insufficient durability.

Membrane/Ionomer Proton Conductivity Measurements

The proton exchange membrane (PEM) is a key component of PEM fuel cells. It separates the anodic and cathodic compartments and at the same time acts as a proton conductor by transporting protons generated at the anode to the cathode. The protons in the membrane are the main charge carriers. Hence, the conductivity induced by this proton transport is called proton conductivity. Because protons can get transported in two directions, both across and through the membrane, there are two types of conductivity: in-plane and through-plane. The two types are theoretically different unless the membrane is isotopic in these two dimensions. In reality, the PEM is not an absolute electronic isolator. The electronic conductivity, normally much smaller than the proton conductivity, can also contribute to the overall measured conductivity and is difficult to distinguish from the proton-mediated contribution.

Pressure Effects on PEM Fuel Cell Performance

As one of the key operating conditions of PEM fuel cells, the operating pressure plays a significant role in determining PEM fuel cell performance. A fuel cell can be operated under a wide range of pressures, from ambient to 5 atm. Operating pressure can influence the fuel cell performance by affecting the fuel cell open circuit voltage, the partial pressures of the reactant gases, hydrogen crossover, exchange current densities, and mass transfer in the electrode reactions. Usually, the performance of a PEM fuel cell in terms of voltage and power density can be improved by increasing the operating pressure. However, pressurization of the PEM fuel cell system can bring about increased gas permeation (e.g. hydrogen or oxygen crossover), water management issues, increased cost, size, and weight, and parasitic energy loss. Kazim, for example, conducted a comprehensive exergoeconomic analysis of a 10-kW PEM fuel cell stack and concluded that the energy cost could be increased by operating at higher pressures. Boyer et al. indicated that operating a fuel cell under higher pressure could help overcome the oxygen transfer limitation but would decrease the fuel efficiency and increase the complexity and cost of the fuel cell system. Al-Baghdadi et al. showed that a higher operating pressure could generate a more even distribution of local current density due to high oxygen concentration in the catalyst layer. However, Sun et al. concluded that higher pressure could result in a nonuniform current distribution. Thus, the operation of a PEM fuel cell at high temperatures has both advantages and disadvantages. This chapter examines in detail, the effects of operating backpressure on fuel cell performance.