Acta Materialia 51 (2003) 5981–6000 www.actamat-journals.com Fuel cell materials and components  Sossina M. Haile * Department of Materials Science and of Chemical Engineering, California Institute of Technology, 138-78, Pasadena, CA, 91125, USA Accepted 31 August 2003 Abstract Fuel cells offer the possibility of zero-emissions electricity generation and increased energy security. We review here the current status of solid oxide (SOFC) and polymer electrolyte membrane (PEMFC) fuel cells. Such solid electrolyte systems obviate the need to contain corrosive liquids and are thus preferred by many developers over alkali, phosphoric acid or molten carbonate fuel cells. Dramatic improvements in power densities have been achieved in both SOFC and PEMFC systems through reduction of the electrolyte thickness and architectural control of the composite electrodes. Current efforts are aimed at reducing SOFC costs by lowering operating temperatures to 500–800 °C, and reducing PEMFC system complexity be developing ‘water-free’ membranes which can also be operated at temperatures slightly above 100 °C.  2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Fuel cells; Solid electrolytes; Electroceramics; Polymers; Platinum group metals 1. Introduction Because of their potential to reduce the environ- mental impact and geopolitical consequences of the use of fossil fuels, fuel cells have emerged as tanta- lizing alternatives to combustion engines. Like a combustion engine, a fuel cell uses some sort of chemical fuel as its energy source; but like a bat- tery, the chemical energy is directly converted to electrical energy, without an often messy and rela- tively inefficient combustion step. In addition to * Tel.: +1-626-395-2958; fax: +1-626-395-3933. E-mail address: smhaile@caltech.edu (S.M. Haile).  The Golden Jubilee Issue—Selected topics in Materials Science and Engineering: Past, Present and Future, edited by S. Suresh. 1359-6454/$30.00  2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2003.08.004 high efficiency and low emissions, fuel cells are attractive for their modular and distributed nature, and zero noise pollution. They will also play an essential role in any future hydrogen fuel economy. The primary components of a fuel cell are an ion conducting electrolyte, a cathode, and an anode, as shown schematically in Fig. 1. Together, these three are often referred to as the membrane-elec- trode assembly (MEA), or simply a single-cell fuel cell. In the simplest example, a fuel such as hydro- gen is brought into the anode compartment and an oxidant, typically oxygen, into the cathode com- partment. There is an overall chemical driving force for the oxygen and the hydrogen to react to produce water. Direct chemical combustion is pre- vented by the electrolyte that separates the fuel (H 2 ) from the oxidant (O 2 ). The electrolyte serves