The articles in this issue of MRS Bulletin highlight the enormous potential of fuel cells for generating electricity using multiple fuels and crossing a wide range of applications. Fuel cells convert chemical energy directly into electrical energy, and as a powergeneration module, they can be viewed as a continuously operating battery.They take in air (or pure oxygen, for aerospace or undersea applications) and hydrocarbon or hydrogen fuel to produce direct current at various outputs. The electrical output can be converted and then connected to motors to generate much cleaner and more fuelefficient power than is possible from internal combustion engines, even when combined with electrical generators in today's hybrid engines. The commercialization of these fuel cell technologies is contingent upon additional advances in materials science that will suit the aggressive electrochemical environment of fuel cells (i.e., both reducing an oxidizing) and provide ionic and electrical conductance for thousands of hours of operation.

The concept of using an ion-exchange membrane as an electrolyte separator for polymer electrolyte membrane (PEM) fuel cells was first reported by General Electric in 1955. However, a real breakthrough in PEM fuel cell technology occurred in the mid-1960s after DuPont introduced Nafion®, a perfluorosulfonic acid membrane. Due to their inherent chemical, thermal, and oxidative stability, perfluorosulfonic acid membranes displaced unstable polystyrene sulfonic acid membranes.Today, Nafion® and other related perfluorosulfonic acid membranes are considered to be the state of the art for PEM fuel cell technology. Although perfluorosulfonic acid membrane structures are preferred today, structural improvements are still needed to accommodate the increasing demands of fuel cell systems for specific applications. Higher performance, lower cost, greater durability, better water management, the ability to perform at higher temperatures, and flexibility in operating with a wide range of fuels are some of the challenges that need to be overcome before widespread commercial adoption of the technology can be realized. The present article will highlight the membrane properties relevant to PEM fuel cell systems, the development history of perfluorosulfonic acid membranes, and the current status of R&D activities in PEM technology.

Three solid-oxide fuel cell (SOFC) electrolytes, yttria-stabilized zirconia (YSZ), rare-earth–doped ceria (REDC), and lanthanum strontium gallium magnesium oxide (LSGM), are reviewed on their electrical properties, materials compatibility, and mass transport properties in relation to their use in SOFCs. For the fluorite-type oxides (zirconia and ceria), electrical properties and thermodynamic stability are discussed in relation to their valence stability and the size of the host and dopant ions. Materials compatibility with electrodes is examined in terms of physicochemical features and their relationship to the electrochemical reactions. The application of secondary ion mass spectrometry (SIMS) to detect interface reactivity is demonstrated. The usefulness of doped ceria is discussed as an interlayer to prevent chemical reactions at the electrode–electrolyte interfaces and also as an oxide component in Ni–cermet anodes to avoid carbon deposition on nickel surfaces. Finally, the importance of cation diffusivity in LSGM is discussed, with an emphasis on the grain-boundary effects.

This article summarizes a unique approach in which all-ceramic interconnects are used in place of metal interconnects in solid-oxide fuel cell (SOFC) stacks. The approach combines advanced SOFC materials with the manufacturing technology and infrastructure established for multilayer ceramic (MLC) packaging for the microelectronics industry. The MLC interconnect is fabricated using multiple layers of yttria-stabilized zirconia (YSZ) tape, with each layer containing conductive vias to provide for electrical current flow through the interconnect. The all-ceramic interconnect design facilitates uniform distribution of air and fuel gas to the respective electrodes of adjacent cells. The multilayer interconnects are fabricated using traditional MLC manufacturing processes. A detailed description of the processes for fabricating the all-ceramic interconnect is presented.To aid in moving from prototype fabrication to commercialization of these fuel cell systems, a detailed cost model has been used as a roadmap for commercial stack development. Cost model projections are presented for three different interconnect footprint sizes. These projections show an SOFC stack cost of less than $150 per kilowatt for the optimized SOFC stack design produced at high volume.

High-temperature carbonate fuel cells are recognized as the cleanest and most efficient power generation option for commercial and industrial customers. The firstgeneration carbonate fuel cell plants have shown an electrical efficiency of 45–48%.The electrolyte in this fuel cell is a mixture of alkali carbonates, and it operates at a highenough temperature that the heat by-product can be used for cogeneration applications such as district heating, hot water, process steam, and absorption chilling for air conditioning. Alternatively, the heat by-product can be used with an unfired gas turbine for additional electrical generation. Depending on location, application, and load size, carbonate fuel cells are expected to achieve an overall energy efficiency of 65–80% in cogeneration and combined cycle applications. The cell hardware uses commonly available stainless steels. Electrode materials are nickel-based. Furthermore, standard, well-established manufacturing processes are employed. Therefore, carbonate fuel cells are well positioned to be cost-competitive with alternative technologies. Significant progress has been made in the development, manufacturing, product engineering, and field operation of carbonate fuel cell technology. Megawatt and submegawatt units are operating worldwide. A comprehensive review of carbonate fuel cell technology and materials are presented in this article.