Determination of Long-Term Stability and Quality of HT-PEM MEAs

Abstract

Durability and stability are the quality features for any type of product, including High Temperature (HT) Polymer Electrolyte Membrane (PEM) Fuel Cells. Compared with the low temperature (LT) representatives of this fuel cell type, the degradation rates of HT-PEM fuel cells are slightly higher [1-2]. Nevertheless, HT-PEM fuel cells are steadily improving [3-5], as shown by Søndergaard et al. with ultralow degradation rates of 1.4 µV/h [6], and are on the rise, as their higher CO tolerance represents a significant advantage over the LT-PEM fuel cells [7]. However, while stability and quality of HT-PEM MEAs is constantly improving, durability is still a challenge. For this reason, within the framework of the German project QUALIFIX funded by BMWi, the longevity of MEAs was investigated by means of long term tests (duration: 1 year) under constant load of 0.3 A/cm². In addition, qualities of MEAs are reviewed by carrying out small series evaluations under load cycling conditions at high current densities, start/stop and potential cycling conditions [8-9]. In the results from the annual tests, for which MEAs of FUMATECH BWT GmbH were used, were noticeable quality differences observed. The first test (AT 01) had to be stopped already after one third of the planned runtime, as End of Life (EoL=starting voltage value - 10%) was reached. By contrast, the second annual test (AT 02) ran longer than the estimated 8,760 hours (currently still in operation). The voltage curves of the two tests are shown in Figure 1 a) and the two graphs exhibit significant differences. In addition to the shortened life, the starting voltage value of the AT 01 is significantly lower (0.559 V), while the starting value of the second annual test is 13% higher (0.631 V). In addition to extensive electrochemical characterizations during operation time (BoL (Begin of Life)/EoT (End of Test)), the MEAs were comprehensively analyzed ante- and post-mortem. These include, among others, the phosphoric acid concentration determinations with the help of titration as well as optical imaging. With the help of micro-computed tomography, structural changes of the MEAs, which occur during the experimental procedure, can be visualized. As shown in Figure 1 b), one possible reason for the rapid performance drop of the AT 01-MEA could be found with this method: the destruction of the membrane. The different lifetimes can be due to the varied membranes (AT 01 = fumapem AM and AT 02 = fumapem AP). After completion of the second annual test, this comprehensive post-mortem will be performed, analyzed and compared with AT 01. In summary, it can already be observed that these MEAs of the same manufacturer perform very differently and the MEAs achieved a significantly higher quality standard in the course of the project. Still, the different performances make stack construction difficult because a single poorly performing MEA can adversely affect the performance of the entire stack. Therefore, a uniform quality management in the production of single cells is necessary to fix the quality – as already summarized in the acronym of the QUALIFIX project. Figure 1 a) Voltage as function of time for both long term tests and b) reconstructed 3D-µ-CT-images of MEA AT 01.