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Electrocatalysts for Acid Proton Exchange Membrane (PEM) Fuel Cells – an Overview

Michael Bron

1.1 Introduction

Fuel cells are devices that directly convert chemical energy stored in a fuel into electricity. The main components of fuel cells are the electrodes (anode and cathode), which are separated by an electrolyte. Several of these electrode–electrolyte units may be connected in series to give a so-called fuel cell stack. In a fuel cell, the fuel (typically hydrogen, but also others like methanol) is oxidized at the anode, and the electrons released during oxidation are conducted to the cathode, where the oxidant (typically oxygen, either pure or as air) is reduced. The driving force for this process is the negative Gibbs free energy of the overall reaction (see Section 1.2.2). The first description of the fuel cell principle dates back to the year 1839/1842, when Sir William Grove described his gaseous voltaic battery based on Schönbeins findings. Since then, different types of fuel cells have been developed [1–3], which differ in the electrodes and the electrolyte used, their operation temperature, and the fuel used. The main types of fuel cells are the low-temperature fuel cells, namely, the “alkaline fuel cell” (AFC) and “proton exchange membrane fuel cell” (PEMFC), including the “direct methanol fuel cell” (DMFC); the medium-temperature fuel cell, namely, “phosphoric acid fuel cell” (PAFC); and the high-temperature types, namely, the “molten carbonate fuel cell” (MCFC) and “solid oxide fuel cell” (SOFC). These fuel cells are labeled according to the electrolyte used; however, the DMFC, which is a proton exchange membrane (PEM) type FC, is an exception. Details on the design and operation of these fuel cells can be found in the cited literature. Other types of fuel cells have also been described, for example, biofuel cells, which use enzymes or even microorganisms to catalyze reactions, and borohydride fuel cells.

The aim of this chapter is to give an overview of the PEMFC, its design and operation, and to discuss the basics of its cathode reaction, namely, electrocatalytic oxygen reduction (ORR, oxygen reduction reaction). As will become clear, the ORR is a major challenge in current fuel cell research both from the fundamental as well as from the applications point of view. This chapter is intended to provide the background for more specialized chapters that follow.

1.2 Acid PEM Fuel Cell Background and Fundamentals

1.2.1 Acid PEM Fuel Cell Overview – History, Status, and Advantages

As mentioned above, the first description of the fuel cell principle dates back to Grove's gaseous voltaic battery. It is amazing that this first system already used hydrogen and oxygen, which were converted at Pt electrodes in an acid electrolyte. However, it took a long time and significant efforts to progress from Grove's Pt sheet electrodes to today's advanced catalysts, from the acid liquid electrolyte to today's proton-conducting membranes [3]. At that time, further development of the “gaseous voltaic battery” was impeded by two main factors. The first was that larger amounts of hydrogen were not easily available at that time and hydrogen was produced by laboratory techniques, for example, by the dissolution of Zn. The second issue was the development of the dynamo by Siemens in 1866/1867, which made available electrical energy on a larger scale, thus there was no technical need to further develop the gaseous voltaic battery. It thus took more than a century until the development of fuel cells gained momentum again, albeit research and development activities have been reported on during that time [1]. The development of fuel cells was boosted in the 1950s, with focus on the AFC, which found early application in spaceflight, where sources of electrical energy with high energy density were needed and cost was not an issue. Despite having certain advantages such as high efficiency, the necessity to use high-purity gases and the highly corrosive liquid electrolyte posed a challenge at that time. The next impulse for further developing the fuel cell technology was the oil crisis in the early 1970s. During these periods of technical progress, the above types of fuel cells were developed to an advanced state. However, in all these fuel cells, issues occurred that hindered their commercialization and it was only during the last 20 years that commercialization of fuel cells for a mass market came within reach. More details on the history and the state-of-the-art of fuel cells can be found in the literature [2–4].

The same issues hold true for the PEMFC, which is in the focus of this book. The prerequisite for the development of the PEM fuel cells was the development of proton-conducting polymer electrolytes that could be processed into thin membranes. Early materials have been described that led to the development of the PEM fuel cell in the 1960s; however, the PEMs had a limited lifetime. A breakthrough was the development of Nafion® by DuPont in 1967. It is still a state-of-the-art material although other proton-conducting membranes have been described.

While the most important step in the development of acid PEMFCs was the membrane, one of the major driving forces was the automobile industry, which was driven by the necessity to provide mobility solutions for the upcoming area of depleted fossil resources as well as to contribute toward more environmentally benign mobility. During the 1980s and 1990s, several large automotive companies announced research and development activities in the fuel cell car sector with the NECAR (new electric car) by Daimler being the most popular one. Later, various national hydrogen initiatives were announced and significant public funding for fuel cell research was available, for example, in Germany, Japan, or the United States.

In today's PEMFCs, the anode and the cathode are separated by the PEM, which serves as electrolyte. The PEM conducts protons from the anode, where they are formed by oxidation of hydrogen, to the cathode, where they are involved in the reduction of oxygen preferably to water. In order to keep Ohmic losses as small as possible, the membrane should be thin (depending on the fuel cell, below 100 μm) and at the same time be electronically isolating, mechanically stable, and impermeable to the reactive gases. The membrane is sandwiched between two gas diffusion electrodes (“GDEs”; i.e., the cathode and the anode), which themselves consist of at least two layers: a catalyst layer, about 5–30 μm in thickness, and a gas diffusion layer of 100–300 μm. While the former typically consists of carbon-supported Pt particles, the latter is usually a porous carbon paper or woven carbon cloth that should help to evenly distribute and transport the reactants to the catalyst layer and provide conductivity paths for the electrons. Often these two layers are separated by a microporous carbon layer. The requirements for GDEs are an optimum porosity, high electric and thermal conductivity, and chemical and mechanical stability. Equally important is an optimum hydrophobicity to avoid flooding of the electrodes (mainly the cathode) with the liquid water that is produced and help maintain a balanced water management. Furthermore there has to be intimate contact between the GDE and the membrane to form an extended three-phase boundary where electrolyte, electrode, and gas/reactants are in close contact. This three-phase boundary is of paramount importance as it is the actual zone in which the reaction takes place.

The “sandwich” of GDEs is called a membrane electrode assembly (MEA). Figure 1.1 schematically visualizes an MEA and the processes occurring in the fuel cell. Typically, several of such MEAs are arranged in series, separated by bipolar plates made of graphite, stainless steel, or other conductive composite materials (not shown in the figure). Such an array of MEAs separated by bipolar plates is usually called a stack. The electrical circuit certainly will not include electrical connections at the gas diffusion layers, as schematically visualized in Figure 1.1.

Figure 1.1 Schematic representation of a membrane electrode assembly and the principal processes occurring in fuel cells. The hydrogen side is the anode, whereas oxygen is reduced to water at the cathode.

The fuel cell stack is completed by endplates but the overall system needs a further periphery, for example, a system to provide the reactants, a cooling system, and electrical equipment.

Fuel cells, including PEMFCs, in general provide a wealth of advantages compared to other means of energy conversion. Fuel cells convert chemical energy to electrical energy at high efficiency (as is discussed in Section 1.2.2), and thereby produce only water (and, depending on the type of fuel cell and the fuel used, CO2), thus making them environmentally benign devices. Furthermore, high efficiency is also provided with small devices and at reduced load, which is a clear advantage compared to combustion engines. The environmental aspect is amplified if instead of using nonrenewable energy carriers (e.g., hydrogen from steam reforming of natural gas), renewable ones are used. In the power-to-gas(-to-power) scenario, renewable electrical energy from the wind, sun, and water would be converted into hydrogen via electrolysis and this hydrogen could then be...