Systematic Study of Durability of High Temperature PEM Fuel Cells at Selected Temperatures, Flow Rates and Loads
Authors: Jensen Jens Oluf , Jakobsen Mark Tonny Dalsgaard , Cleemann Lars Nilausen , Steenberg Thomas , Hjuler Hans A. , Li Qingfeng 
Published: February 2020

High temperature PEM fuel cell based on phosphoric acid doped polybenzimidazole have reached a quite mature state of development. However, durability is still a critical issue and the key to improving it is a better understanding of the degradation mechanisms. It can be expected that the cells have most, if not all, degradation modes of the low temperature PEM fuel cell (corrosion, particle growth, membrane thinning etc.), but additionally there are issues with the phosphoric acid that the membrane is filled with to provide ionic conductivity at high temperature. In terms of degradation, one could argue that the cell combines the challenges of the low temperature PEM fuel cell and the phosphoric acid fuel cell. Nevertheless, lifetimes on the order of 10.000 hours are demonstrated by several groups [1]

In the present study, a large number of cells manufactured by Danish Power Systems were tested with hydrogen in three in-house built multichannel test rigs over several years. The working temperature, the oxidant and fuel flow rates and the current load were varied between 160 and 200 °C, stoichiometry up to about 10 and current densities between 200 and 800 mA cm-2, respectively. The cells were characterized repeatedly by polarization curves and/or electrochemical impedance spectroscopy. After test, selected cells were subjected to post mortem analyses by means of cross section examination by microscopy, X-ray diffraction for platinum particle sized and acid titration.
All the major degradation mechanisms were seen (catalyst particle growth, membrane thinning and acid loss). The natural question to answer is which mechanisms dominate under which conditions. It was a general trend that degradation rates increased significantly with temperature and with current density. The temperature of the cell is normally measured outside the gas diffusion layers and even outside the channel plates. Since the produced heat is liberated within less than a hundred micrometers (membrane and catalyst layers) and since the transport away from that area is driven by the temperature gradient, one must expect a significantly higher temperature in the center of the cell than the temperature measured in the channel plates or end plates of the test cell. At higher current densities, the heat evolution rate is higher and thus a higher central temperature is expected. The temperature was measured inside the membrane, but the temperatures could not alone explain the increased degradation rate at higher current densities.
A stronger effect seems to origin from the reactant flow rates. Some test series were carried out at high flow rates to mimic an air-cooled stack and in those cases, degradation was significant even at moderate current densities. A combination of acid loss and acid dehydration is suggested as decisive, although dehydration should at the same time limit acid evaporation. The presentation will review the findings of the temperature/load/flow matrix studies and suggest conclusions on the main factors limiting durability of high temperature PEM fuel cells.

[1] M. T. Dalsgaard Jakobsen, J. O. Jensen, L. N. Cleemann and Q. Li. Chapter 22. Durability Issues and Status of PBI Based Fuel Cells. In Q. Li, D. Aili, H. A. Hjuler and J. O. Jensen (eds). High Temperature Polymer Electrolyte Membrane Fuel Cells- Approaches, Status and Perspectives. Springer-Verlag 2016

Figure 1