Fuel Cell Technology

A Solid Oxide Fuel Cell (SOFC) is typically composed of two porous electrodes, interposed between an electrolyte made of a particular solid oxide ceramic material. The system originates from the work of Nernst in the nineteenth century. In his patent [1], Nernst proposed that a solid electrolyte could be made to electrically conduct, using a heater; the system then “glowed” by the passage of an electric current. The systems originally studied by Nernst were based on simple metal oxides. In 1937, Bauer and Preis [2] operated the first ceramic fuel cell at 1000°C, showing that the so-called “Nernst Mass” (85% zirconia and 15% yttria), and other zirconia-based materials present a reasonable ionic conduction at high temperature (600–1000°C). These works were really the prelude to the modern SOFC.

PEM fuel cells use a proton conductive polymer membrane as electrolyte. PEM stands for Polymer Electrolyte Membrane or Proton Exchange Membrane. Sometimes they are also called polymer membrane fuel cells, or just membrane fuel cells. In the early days (1960s) they were known as Solid Polymer Electrolyte (SPE) fuel cells. This technology has drawn the most attention because of its simplicity, viability, quick start-up, and it has been demonstrated in almost any conceivable application, from powering a cell phone to a locomotive.

Durability, in the present context, is a concept that relates how fuel cells and fuel cell systems are made and operated to how long they last. The science and methodology of this concept are the subject of this chapter. The approach we will discuss has its roots in the history of technical development of such concepts for composite material systems in general [1]. It is appropriate as a starting point for fuel cells, since fuel cells are functional composite material systems.

Fuel cell technology, a core component in a hydrogen-based energy economy, has the advantages of high energy conversion efficiency, low pollution, and no dependency on depleting fossil resources. Significant progress has been made in the state-of-the-art in materials, design, and fabrication, however, the widespread commercialization of most fuel cells is still limited by issues such as high cost and low durability [1–4]. Mathematical models are effective tools in understanding and optimization of various transport and electrochemical processes, leading to cost reduction and improved performance and durability. The purpose of this chapter is to summarize the current status of fundamental models for fuel cells that have been actively studied in the past decade

H₂ is an ideal fuel for fuel cells (FCs) because of its high reactivity and zeroemission characteristics. Unfortunately, H₂ is not easily available, and neither its production nor distribution infrastructure are widely spread. Therefore, development of technologies for production of H₂ onboard and onsite from other sources such as natural gas, methanol, and gasoline is necessary. In the next section, different feedstocks that are suitable for H₂ production for fuel cell application are presented. Subsequent sections focus on reforming of hydrocarbons that are processed by a series of steps that include fuel desulfurization, reforming, water-gas shift reaction and carbon monoxide (CO) removal [1,2,3,4]. Figure 5.1 illustrates what is known as the fuel processing train with some options for the essential steps. This general fuel processing train is usually used for PEM fuel cells running on fossil fuels such as natural gas. The processing steps that the feedstock are subjected to depend on the type of the fuel and the fuel cell. For example, if methanol is the fuel for PAFC, the CO removal step might not be necessary. The last two steps, water-gas shift reaction and CO removal, are not necessary in the case of MCFC. If feedstocks heavier than methane are used as a feed to the SOFC, an additional process step known as prereforming might be used. Such variations are described in more detail in Section 5.3.

The mass production of PEMFC power generators requires a price reduction and, thus, a decrease in the amount of noble metals present in the cathode and anode catalyst layers. Automotive, residential, military, and small scale applications require PEMFC stacks with a Pt-specific power density of at least 0.2 gPt/kW at cell voltages of about 0.65 V. However, existing PEMFC performance corresponds to approximately 0.85-1.1 g Pt/kW. Thus, at least a five-fold reduction of the amount of noble metal in the PEMFC catalyst layer is required for large scale manufacturing [1].

The development of Fuel Cell technology, an alternative energy source, involves direct conversion of chemical energy to electrical energy through a controlled chemical reaction. This has attracted attention due to the following attributes:

Cellular life exists at the interface between electrochemical extremes. The energy of most living cells depends on the transfer of electrons from intracellular, electrically reduced biochemicals to oxidized extracellular acceptors. For almost one hundred years investigators have tried to tap into these processes in microbes for electrical power generation. Efforts have been made to use microbes as complex catalysts to oxidize relatively inexpensive organic and inorganic substrates as fuels in compact spaces in microbial fuel cells (MFCs). However, natural selection does not favor microbial metabolism under such conditions. Evolution has shaped microbes to use their growth substrate efficiently to reproduce under changing environmental conditions. Consequently, they are not optimized for use in MFCs, where electrons derived from substrate oxidation go only to the anode and not to cell growth