A fuel cell device comprising: an elongate ceramic substrate having an exterior surface, an interior structure, and a length that is the greatest dimension whereby the elongate ceramic substrate exhibits thermal expansion primarily along one dominant axis that is coextensive with the length, a reaction zone along a first portion of the length configured to be exposed to a heat source to heat the reaction zone to an operating reaction temperature, and at least one cold zone along a second portion of the length configured to remain at a temperature below the operating reaction temperature when the reaction zone is heated; one or more fuel passages within the interior structure in the elongate ceramic substrate each having a first fuel pathway extending from a fuel inlet in the at least one cold zone through the reaction zone and a second fuel pathway extending from the reaction zone to a fuel outlet, each first fuel pathway having an associated anode within the interior structure in the reaction zone; one or more oxidizer passages within the interior structure in the elongate substrate each having a first oxidizer pathway extending from an oxidizer inlet in the at least one cold zone through the reaction zone and a second oxidizer pathway extending from the reaction zone to an oxidizer outlet, each first oxidizer pathway having an associated cathode within the interior structure in the reaction zone positioned in opposing relation to a respective one of the associated anodes; and an electrolyte disposed between each opposing anode and cathode in the reaction zone, each opposing anode and cathode and electrolyte therebetween defining an active portion of the reaction zone, characterized in that each of the one or more fuel and oxidizer passages (14, 20) includes a neck-down point in the respective second fuel and oxidizer pathways downstream of the active portion of the reaction zone at which the cross-sectional area of the second fuel and oxidizer pathways becomes smaller than a cross-sectional area of the respective first fuel and oxidizer pathways extending through the reaction zone. an elongate ceramic substrate having an exterior surface, an interior structure, and a length that is the greatest dimension whereby the elongate ceramic substrate exhibits thermal expansion primarily along one dominant axis that is coextensive with the length, a reaction zone along a first portion of the length configured to be exposed to a heat source to heat the reaction zone to an operating reaction temperature, and at least one cold zone along a second portion of the length configured to remain at a temperature below the operating reaction temperature when the reaction zone is heated; one or more fuel passages within the interior structure in the elongate ceramic substrate each having a first fuel pathway extending from a fuel inlet in the at least one cold zone through the reaction zone and a second fuel pathway extending from the reaction zone to a fuel outlet, each first fuel pathway having an associated anode within the interior structure in the reaction zone; one or more oxidizer passages within the interior structure in the elongate substrate each having a first oxidizer pathway extending from an oxidizer inlet in the at least one cold zone through the reaction zone and a second oxidizer pathway extending from the reaction zone to an oxidizer outlet, each first oxidizer pathway having an associated cathode within the interior structure in the reaction zone positioned in opposing relation to a respective one of the associated anodes; and an electrolyte disposed between each opposing anode and cathode in the reaction zone, each opposing anode and cathode and electrolyte therebetween defining an active portion of the reaction zone, characterized in that each of the one or more fuel and oxidizer passages (14, 20) includes a neck-down point in the respective second fuel and oxidizer pathways downstream of the active portion of the reaction zone at which the cross-sectional area of the second fuel and oxidizer pathways becomes smaller than a cross-sectional area of the respective first fuel and oxidizer pathways extending through the reaction zone.