Nernst voltage losses in planar fuel cells caused by changes in chemical composition: effects of operating parameters

Abstract

Conventionally, the reversible potential at inlet conditions is considered to be the ideal voltage for a fuel cell. However, the reversible potential in planar cells varies along the cell length due to changes in gas composition caused by consumption of fuel and oxidant. This contributes to voltage losses, referred to as Nernst losses, compared to the reversible potential at the inlet of the cell. In the conventional breakdown of cell voltage losses, the variation of reversible potential is not accounted for. To estimate the “true” losses, this needs to be accounted for by employing calculations of the reversible potential with changing composition. This paper quantifies such losses in the context of solid oxide fuel cells (SOFCs) for which they are found to contribute significantly, in the range of 25 to 33% of the total voltage losses. The Nernst equation for fuel cell operation is analyzed and analytical expressions for Nernst losses are derived. The latter allows prediction of the effects of operating parameters which are later verified with an isothermal model of the SOFC.