TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Components: The inlet comprises a fuel inlet. The cathodes of the fuel cells are accessible from an outside of the first flexible sheet. The anodes of the fuel cells are accessible from within the plenum. The plenum is defined between the first flexible sheet and a second flexible sheet sealed to the first flexible sheet by a seal extending around a periphery of the plenum. The areas of the first and second flexible sheets are different. A distance between first and second opposed portions of the seal measured along the first flexible sheet on a cross section of the plenum is different from a distance between the first and second seal portions measured along the second flexible sheet. The distance between the first and second seal portions measured along the first flexible sheet is shorter or longer than the distance between the first and second seal portions measured along the second flexible sheet. The plenum is rectangular, or round in a plane of the first flexible sheet. The inlet comprises a tube sealed between the first and second flexible sheets. The inlet comprises an aperture in one of the first and second flexible sheets. The fuel cell includes a gas permeable porous medium (21) disposed between the first flexible sheet and the external support structure. The external support structure comprises a gas-permeable porous medium. The porous medium is hydrophobic or porous poly tetrafluoro ethylene (PTFE) material. The external support structure comprises spaced-apart projections. The projections limit expansion of the plenum. The projections comprise ribs. The external support structure comprises openings to permit air to pass through it. The external support structure comprises a portion of an outer housing of a portable device. The portable device is telephones; music players, video players, cameras, video cameras, radios, navigation devices, lights, electronic games, computers, or personal digital assistants.