NOVELTY - Fuel-cell system (100) includes a controller (170) including memory and processing circuitry. It is operable in a first mode to react a feedstock material (180) by a non-electricity-generating reaction to produce a product and in a second mode to react the feedstock material by an electricity-generating reaction to produce electricity; and the memory stores non-transitory instructions that, when executed by the controller using the processing circuitry, cause the system to switch between operating in the first mode and operating in the second mode in response to an input.    USE - The fuel-cell system is used to produce a product i.e. non-gaseous or gaseous product and electricity. The product includes silicon, which can be reacted with nitrogen to form silicon nitride (all claimed). The system is used for producing clean-burning chemical fuel (e.g. hydrogen), repurpose carbon, silicon, and/or other constituents of feedstock materials (e.g. for use in durable goods). Constituents of feedstock materials can be further processed into polymers, carbon composites, and/or other useful materials. The system is for use near a landfill for processing methane that would otherwise be flared or released into the atmosphere or underwater or on a floating or anchored platform for processing ocean biomass and/or methane hydrates from the ocean floor. It is used for processing stranded well gas at oil fields, methane hydrates from permafrost sources, and/or other feedstock materials that would otherwise be wasted or underutilized. Chemical reaction product(s) from operation of the system can be used to form dielectric materials for use in durable goods. The reaction products can be used to form polymers (e.g. polyimides, polyetherimides, Parylene (RTM: chemical vapor deposited poly(p-xylylene) polymers), or fluoropolymers) and/or inorganic dielectrics (e.g. silicon dioxide or silicon nitride) that can be incorporated into polymer-based nanodielectrics. The reaction products can be used to form thin films e.g. of inorganic amorphous carbon, silicon oxynitride or aluminum oxynitride.    ADVANTAGE - The fuel-cell system can be configured to be moved to new sources of feedstock material as old sources of feedstock material are depleted. It uses non-hydrogen feedstock materials, thus, reducing capital and operational costs of the system relative to conventional fuel-cell systems and facilitating operation of the fuel cell in multiple modes. Processing propane or propylene using the system can produce little or no harmful pollution (e.g. environmentally released carbon dioxide, oxides of nitrogen, or particulates) or less harmful pollution relative to simple combustion. Composites of inorganic and organic materials (one or both of which can be produced by operation of the system) can provide relatively high dielectric and mechanical strengths along with flexibility. Such materials can be well suited for use at a wide range of temperatures, such as temperatures ranging from cryogenic temperatures (e.g. greater than or equal to - 200 degrees C) to heat-engine exhaust temperatures (e.g. greater than or equal to 500 degrees C). The system can have dual-beam deposition and/or web-handling capabilities useful for processing suitable chemical reaction products (e.g. to form amorphous or crystalline carbon films).    DETAILED DESCRIPTION - Fuel-cell system (100) comprises a first electrode (115a) positioned in a first region of the system to receive a feedstock material (180); a second electrode (115b) positioned in a second region of the system; an ion-transport medium (117) positioned between the first and second regions; an electrical circuit (160) extending between the first and second electrodes; and a controller (170) including memory and processing circuitry. It is operable in a first mode to react the feedstock material by a non-electricity-generating reaction to produce a product and in a second mode to react the feedstock material by an electricity-generating reaction to produce electricity; and the memory stores non-transitory instructions that, when executed by the controller using the processing circuitry, cause the system to switch between operating in the first mode and operating in the second mode in response to an input corresponding to a change in demand for electricity, a change in demand for the product, or both. An INDEPENDENT CLAIM is included for a method of operating a fuel-cell system.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic block diagram of a fuel-cell system.    Fuel-cell system (100)    First electrode (115a)    Second electrode (115b)    Ion-transport medium (117)    Electrical circuit (160)    Controller (170)    Feedstock material (180)