Fuel cell device and system

A fuel cell system comprising:

• a hot zone chamber having a chamber wall with a thickness T;
• one or more fuel cell devices each comprising an elongate rectangular or tubular ceramic substrate having a length that is the greatest dimension whereby the elongate rectangular or tubular ceramic substrate exhibits thermal expansion along a dominant axis that is coextensive with the length, a reaction zone along a first portion of the length positioned in the hot zone chamber for exposure to an operating reaction temperature, at least one cold zone along a second portion of the length extending outside the hot zone chamber and shielded by the chamber wall so as to remain at a temperature below the operating reaction temperature, an electrolyte disposed between an anode and a cathode in the reaction zone to form an active cell, wherein the active cell resides within and is substantially surrounded by the ceramic substrate, and a third portion of the length equal to thickness T positioned within the chamber wall, the third portion having a maximum dimension L in a plane transverse to the direction of the length wherein T≧ 1/2 L;
• and
• a heat source coupled to the hot zone chamber and adapted to heat the reaction zone to the operating reaction temperature within the hot zone chamber.