Air Starvation Accelerated Stress Tests in Polymer Electrolyte Fuel Cells
Authors: Bodner Merit , Schenk Alexander , Marius Bernhard , Rami Mija , Hacker Viktor 
Published: February 2020

Lifetime of polymer electrolyte fuel cells (PEFCs) is limited by degradation mechanisms, often caused by undesirable operating conditions. Depending on the application, the likeliness of these operation modes varies; e.g. mobile applications inevitably undergo more start-up/shut-down cycles and dynamic load and humidity changes than stationary systems [1]. On the other hand, in stationary applications with steady load conditions, other critical events, such as cathode starvation, become lifetime defining issues. In case of oxygen undersupply, protons are reduced on the cathode catalyst instead of oxygen, leading to hydrogen evolution. However, with oxygen still present near the cathode inlet, a strong current and temperature gradient is induced. The temperature rises near the inlet, whereas it remains low in the areas of proton reduction reaction. This is accompanied by voltage oscillation due to the low inlet flux and thus insufficient water removal [2,3].

In order to study this particular situation, accelerated stress tests (ASTs) are used. However, so far, most ASTs were not suitable for portraying real cathode supply failure, as they do not alter the cathode gas flow under stable load conditions. Therefore, in this work, the cathode gas flow rate was varied in order to minimise the effects of other operating modes, contributing to the degradation of fuel cells.

Multiple ASTs with short intervals of reduced cathode gas flow were conducted analogously to the AST displayed in Figure 1. All tests were performed in a single hydrogen fuel cell with a segmented S++ current scan shunt device on the cathode for a spatial detection of current and temperature in 10x10 and 5x5 segments, respectively (Figure 2). The fuel cell was electrochemically characterised in-situ via polarisation, electrochemical impedance, linear sweep and cyclic voltammetry measurements [4]. Additionally, the fluoride emission rate (FER) was determined by effluent water analysis.

While a mild cathode starvation did not lead to a fundamental voltage drop, the cell voltage decreased to values below 0 V for a more detrimental cathode undersupply. A very distinct gradient in current and temperature was detected for the latter tests. Both temperature and current exhibited peak values near the cathode inlet. The current did, however, not drop to values below zero at the outlet, indicating that a second cathode reaction is taking place in areas of oxygen starvation. Thus, protons were inevitably reduced instead of oxygen at the cathode outlet in order to maintain the set current. The change in local behaviour of current and temperature before and after 100 hours of AST at a set current density of 0.6 A cm-² is shown in Figure 3.

Acknowledgement

The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n° 621216.

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Figure 1