1
The Evolution of Fuel Cells and Their Components

Thomas A. Zawodzinski1, Zhijiang Tang2, and Nelly Cantillo3

1 Department of Chemistry and Biochemistry, University of Tennessee at Knoxville, Knoxville, TN, USA

2 Oak Ridge National Laboratory, Oak Ridge, TN, USA

3 Department of Chemical and Biomolecular Engineering, University of Tennessee at Knoxville, Knoxville, TN, USA

1.1 Overview: A Personal Perspective of Recent Developments

At the outset of this narrative, we note that many aspects of the recent history of proton exchange membrane fuel cells (PEMFCs) are presented through the lens of the perspective of the senior author. For this reason, there is a strong emphasis on events in North America, and a number of personal anecdotes are added. The emphasis on specific activities, contributors, and fields of work is that of the authors' and is not intended to be completely inclusive, but rather to provide a sketch of major activities during the period of intense development, leading to the introduction of fuel cell (FC) vehicles for lease or sale. Our hope is to provide a brief overview with sufficient references and links to key information in order to facilitate the reader in developing a thorough appreciation of the substantial body of work on FCs.

The history of FCs goes back ~170?years to Grove and Schonbein, and each claimed to be the "father" of FCs [1]. However these devices are rather unrepresentative compared with any present-day FC. Recognizable versions of the cell probably can be dated to the Bacon cell discussed in the succeeding text [2]. Alkaline cells based on liquid NaOH in a matrix operating on pure oxygen and hydrogen were sufficiently advanced by the 1960s and used on US spacecrafts as sources of both electricity and water [3]. PEMFCs were first described in the 1950s, employing polystyrene sulfonic acid as the primary membrane [4]. Phosphoric acid, molten carbonate, and solid oxide cells for use in stationary power applications have been developed for decades as well [5-7]. Direct methanol FCs were extensively developed for portable power applications in the 1990s, incorporating some of the advances from proton exchange membrane (PEM) technology [8].

United Technologies FC operations carried out much research in phosphoric acid technologies, which later found use in PEM-based FCs [9]. The next era of significant development in PEMFCs began with the establishment of the Department of Energy (DOE) program, most notably at Los Alamos National Laboratory (LANL), championed in the late 1970s by Byron McCormick [10]. There was a significant "induction period" before major discoveries emerged from LANL and from other laboratories. A major series of events followed the appointment by Ross Lemons of Shimshon Gottesfeld to the leadership of the program. At that point, the LANL program was split managerially and geographically into (loosely) scientific and engineering programs led by Gottesfeld and Nick Vanderborgh, respectively. The latter worked closely with the GM program, part of which took up residence in a closed facility at LANL. Meanwhile, S. Srinivasan left LANL and started a new and ultimately highly successful group at Texas A&M. In this way, a single appointment spawned three of the strongest groups shaping the PEMFC landscape during a major growth period in the 1990s.

Critical technical steps forward came with the development of significantly lowered Pt loadings, first in 1986 by Raistrick [11] and then in 1990 by Wilson [12]. From then until the present, a continuous series of "issues du jour" have been tackled by the US DOE program. Meanwhile, Ballard in Canada began to build systems with installations in place in the 1990 [13].

The leadership of the DOE FCs for transportation program was passed to Steve Chalk and JoAnn Milliken in 1992 [14]. This team set the tone for the development of PEMFCs and related technologies and had a dramatic positive effect on FC development. The later merger of the "hydrogen program" into this group added some emphasis on stationary systems. This was later augmented by extensive work on codes and standards and on demonstration projects. The introduction of the Annual Merit Review process shined light on activities in an open manner and was central to the progress made through this program [15]. The office relentlessly reviewed the program and sought outside review to ensure that all useful commentary was received, digested, and passed on for action by researchers. A formal and open program review was implemented for the first time in the mid-1990s. In addition, the industry was invited to participate in many ways, guiding the program and receiving funding, especially at the component level.

Some researchers referred to the approach of attacking a sequence of problems as "peeling the onion"; while perfectly feasible, it was unclear which layer of the problem would make us cry. Thus, we worked on membranes and water management, decreasing Pt content, reformate operation (CO tolerance, diluted gas streams), returning to hydrogen, impurities, water management at a more global level, and high temperature membranes, and back to the cathode (catalysts and structure) for even lower Pt loadings and non-Pt group metal catalysts. Two important threads throughout the work were the use of mathematical modeling at even more sophisticated levels and the need to demonstrate adequate durability.

The work featured the interplay of science and engineering approaches, with gradual "replacement" of science with engineering or development-style work as the technology matures. Indeed, the present DOE FC program is far more into the development and engineering (which still includes science but in a very directed and less overtly creative mode) approaches than into more innovative scientific discovery. This is a natural outcome of the maturation of the field.

Based on the differences in fundamental chemistry and electrolyte, there are six major types of FCs: PEMFCs, alkaline or anion exchange membrane fuel cells (alkaline fuel cell (AFC) or AEMFC), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFC), and solid acid fuel cells (SAFC). These are sorted by the featured ion-conducting electrolyte. The charge-carrying ions can be either protons (in PEMFC, PAFC, and SAFC) or anions (hydroxide in AFC, carbonate in MCFC, and oxide ion in SOFC). Sorted by the operating temperature, the FC types can be divided into three categories: low temperature (PEMFC and AFC), intermediate temperature (PAFC and SAFC), and high temperature (MCFC and SOFC). It should be noted that these FC types also differ in the physical nature of the electrolyte: solid electrolytes are used in SOFCs, SAFCs, PEMFCs, and AEMFCs, while AFCs, PAFCs, and MCFCs all use liquids electrolytes in an inert matrix. This is an important distinction since liquid electrolyte handling is more complex because of sealing and corrosion issues. We now consider these FC types, with very brief descriptions of all but PEMFCs and AFC/AEMFCs, given the emphasis of this book.

The detailed study of component and cell durability has many elements. The primary theme of this book is the degradation of polymer electrolytes used in FCs. However, each component must stand up to test protocols defined as part of the DOE FC targets [16], and the non-membrane components have their pathways to performance loss. These will be touched on later in the context of a description of the specific components and cell types.

1.2 Basics of Fuel Cell Operation

A schematic of the individual unit of an FC is shown in Figure 1.1. In accord with the subject of the book, the description will focus on low temperature FCs. The unit cell consists of a multilayer sandwich with a central layer of electrolyte, serving the functions of ion conductor, separator, and electronic insulator, between two electrocatalytically active electrodes, typically porous in nature. These are the anode and cathode catalyst layers (CLs), which are in turn nested between layers that serve to distribute gases across the CL, variously referred to as a gas diffusion layer (GDL) or mass transport layer (MTL). Each GDL or equivalent contacts a flow field that transports gases into the cell. The flow field is usually a feature machined, stamped, or otherwise impressed into a conductive plate, which may contact another conductive "current collector." The conductive plate is bipolar, conducting oxidant on one side and reducing agent on the other. Flow fields that are common include simple serpentine patterns, common in single-cell test fixtures, as well as interdigitated arrangements in which flow is forced to pass through the GDL. The unit cells are stacked as a series of flat plates with various seals and gaskets around them. Sealing of stacks is often the focus of significant work and design of materials and structures in the real world.

Figure 1.1 Schematic of fuel cell single unit cell with an overview of processes in PEM single cell.

The function of this cell is to take in a stream of fuel (e.g., hydrogen) and oxidant (e.g., oxygen in air) and harvest the current resulting from the electrochemical "burning" of the fuel, the direct conversion of chemical to electrical energy. Redox half-cell processes occur in a membrane-separated cell. At the cathode, oxygen reduction takes place. This is generally a rather sluggish reaction, and the...