This paper develops a new thermodynamic model that predicts maximum fuel-cell efficiency and fuel utilization as a function of fuel composition and operating conditions. Interestingly, it is shown that maximum possible efficiency is independent of membrane-electrode assembly characteristics and internal polarization losses. However, because the power density depends on internal polarization losses, the cell size needed to achieve maximum efficiency depends greatly on cell architecture. A very detailed fuel-cell model is used to validate the thermodynamic model and to provide further insight on practical considerations such as power density. The model is illustrated using solid-oxide fuel cell examples. (c) 2006 Elsevier B.V. All rights reserved.