Fuel cell components provide fuel cells on a flexible sheet that defines a wall of a flexible plenum. An external support structure limits expansion of the plenum in response to forces exerted by a pressurized reactant. The external support structure may comprise a portion of a housing of a portable device. Cathodes of the fuel cells may be accessible from an outside of the flexible sheet and exposed to ambient air while anodes of the fuel cell are accessible from an inside of the flexible sheet and exposed to a fuel, such as hydrogen gas.