Highlights ► PEM Fuel Cell with Fermat Spiral as distributor. ► Causes of irreversibilities. ► A new dimensionless parameter to determine contribution of mass transfer in entropy generation.

A four-layer solid oxide fuel cell stack consisting of standard anode-supported cells was assembled to investigate long-term stability, but at higher current densities and/or fuel utilization compared to previous investigations. The stack was operated within a furnace temperature range of 700 °C–750 °C with hydrogen fuel at a current density of up to 1 A·cm⁻² and fuel utilization of up to 80% for more than 10,000 h. The average voltage degradation rate was approximately 0.6%kh⁻¹. Increases in the ohmic resistance and anode polarization dominated the degradation behavior. An increase in the current density and fuel utilization under current testing conditions did not fundamentally influence the degradation rate. However, the possible modification in the nickel structure by local higher fuel utilization may have had a long-term impact on the lifetime of the stack. The complexity of the degradation analysis of stacks resulting from an inhomogeneous contact inside the stack was analyzed with the support of impedance measurements and a post-mortem analysis.