NOVELTY - Fuel composition (I) comprises a polymer comprising silicon, and a fuel comprising alcohols or hydrocarbons, where (I) has a hardness of at least 2 g peak force as determined by a penetration test using a texture analyzer.    USE - (I) is useful in fuel cell system (claimed), such as a direct methanol fuel cell system.    ADVANTAGE - (I) is capable of delivering a vapor phase fuel, such as methanol vapor, to fuel cell stack. (I) is capable of delivering fuel vapor in low concentration and at a controlled rate without the need for mechanical assistance (such as a pump or a valve) or a pervaporation membrane to reduce cost and parasitic loss caused by power consumption of mechanical components, and increase energy density of the fuel cell system since more volume is made available for energy producing components. The controlled fuel delivery can also reduce migration of the fuel from the anode to the cathode (e.g. methanol crossover), which can cause parasitic loss (and reduced runtime) and mixed potentials at the cathode (and reduced output power). (I) can be conveniently filled into a fuel container (e.g. a fuel cartridge) to conform to the volume available for fuel in the fuel container to enhance (e.g. maximizing) fuel capacity. (I) is essentially leak proof (enhancing safety), and can deliver fuel consistently, regardless of the particular orientation of the fuel container and/or fuel cell system.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a fuel cell system (20) comprising: a fuel cell comprising an anode (34), a cathode (36), and an electrolyte (38) between the anode and the cathode; and a fuel source (24) in fluid communication with the anode, where the fuel source comprises the rigid composition comprising the polymer and the fuel, and when the fuel cell is in operation the composition delivers the fuel in vapor form to the fuel cell at a controlled rate without use of mechanical assistance or a pervaporation membrane.    DESCRIPTION OF DRAWING(S) - The figure shows a fuel cell system.    Fuel cell system (20)    Fuel source (24)    Anode (34)    Cathode (36)    Electrolyte. (38)