When used in distributed generation (DG) applications, fuel cells have the potential to save energy and reduce emissions depending on the fuel-cell technology used, and their inherent fuel-flexibility could help address energy shortage issues through energy diversity. In addition, fuel cells have the potential to be quieter, more reliable, and have lower maintenance costs than most technologies used for DG. Similar to batteries, fuel cells are electrochemical devices that convert chemical energy directly into electric power. Unlike batteries, both the cathode and anode reactants of a fuel cell are constantly replenished by air and fuel, respectively. Some of the fuel cell technologies under consideration for DG applications include solid oxide, molten carbonate, phosphoric acid, and polymer electrolyte membrane (PEM) fuel cells. This article addresses PEM fuel cells (PEMFC). Subsequent articles will discuss other fuel cell options. In a PEMFC both the anode and cathode are comprised of platinum-based electrocatalysts supported on carbon particles. The electrolyte is a polymeric membrane that conducts protons. PEMFCs operate at lower temperatures than other fuel cells (i.e., 60°C to 90°C [140°F to 194°F] vs. 200°C to 1000°C [392°F to 1,832°F] ), which limits their combined heat and power applications. PEMFCs likely will not be able to provide the high waste heat temperatures required for absorption cooling and, in most cases, space heating. On the other hand, their fast startup, simplicity of operation, zero emissions (when operating on hydrogen [H 2 ]), and potential for low capital and maintenance costs have attracted interest for DG applications.