Degradation of PEMFC at low loadings

Abstract

Our presentation focuses on investigations of durability and of degradation of fuel cells at low Pt loadings for automotive application. Major motivations are problems related to the need to reduce the amount of Pt in MEAs down to values of e.g. 0.2 mgPt/cm2 in order to make PEMFC more competitive. The particular challenge is to maintain sufficiently high performance and long-term durability. Both requirements, however, are conflicting with the reduction of Pt loading. In this context, our study regarding the influence of Pt loading on PEMFC performance and on durability shows that for cathode loadings below 0.2 mgPt/cm2 and for current densities exceeding 1 A/cm2 a sudden increase of mass transport resistance is observed. The same threshold value is found for the increase of irreversible voltage losses which lead to a dramatic reduction of PEMFC durability for cathodic loadings below 0.2 mgPt/cm2. Another durability issue at low cathodic Pt-loadings (<0.4 mgPt/cm2) is the acceleration of reversible degradation, which leads to a dramatic voltage drop at fuel cell operation (without a recovery interruption). Such an increased reversible degradation is less important for automotive application where regular refresh (recovery of reversible losses) is performed by shutting down the system. Nevertheless, the accelerated reversible voltage decay may be very significant and is, of course, related to irreversible material changes that should not be neglected. A further aspect of this presentation is the lack of common procedures to reliably determine voltage loss rates in durability tests of fuel cell. This issue leads to severe difficulties in the comparison of results obtained by different institutions or within different projects. Accordingly, special attention is devoted to the discrimination between so called reversible and irreversible voltage losses. The first are permanent and determine the lifetime of a fuel cell. The latter strongly depends on the chosen operation conditions and can be recovered by specific procedures.