A High Potential Trapped State upon H2-Starvation of a Low Pt-Loaded PEMFC Anode
Authors: Herranz Juan , Welmers Tim , Heinritz Adrian , Schmidt Thomas J. 
Published: December 2024

The price of the proton exchange membrane fuel cell (PEMFC) systems used to power light-duty fuel cell electric vehicles (FCEVs) must be decreased to ≤ 30 US$∙kW−1 in order to allow the extended commercialization of this environmentally benign automotive technology.1 The Pt-based catalysts used to speed up the O2-reduction and H2-oxidation reactions (ORR, HOR) in PEMFC-cathodes and -anodes are a significant contributor to these devices’ price, and thus the cost reduction measures1 envisaged to attain the above target include a decrease of the PEMFCs’ Pt-loading to ≤ 125 μgPt∙cm−2. However, some studies2 have suggested that an even lower Pt-loading of ≤ 62.5 μgPt∙cm−2 would be more adequate, since it would minimize the effect of the nascent FCEV market on the supply and cost of this scarce metal. To reach this challenging objective while assuring acceptable performance losses stemming from the sluggish ORR-kinetics, 80 % of the above Pt-inventory (i.e., 50 μgPt∙cm−2) should be implemented at the PEMFC cathode. Conversely, the ultra-fast kinetics of the HOR on Pt3 would translate into close-to-negligible kinetic losses if the remaining amount of this noble metal (12.5 μgPt∙cm−2) were devoted to this anodic reaction. On the other hand, little is known regarding the durability of such low Pt-loaded anodes, specifically during the high-potential excursions that are triggered when the supply of H2 to one of the cells within an operative PEMFC-stack is interrupted (i.e., under so-called gross H2-starvation conditions).4 In this regard, we recently showed that when such H2-starvation events were mimicked in a Pt rotating disk electrode (RDE), the latter remained trapped at high potentials (>> 1.5 V vs. the reversible hydrogen electrode) even when the H2 supply was re-established.5

Following the above RDE study, in the current work we proceeded to verify whether this high-potential trapped state is also relevant for PEMFC-anodes with ≈ 10 μgPt∙cm−2. The choice of this Pt-loading was not only motivated by its relevance for FCEV-commercialization (vide supra), but also by our observation that in the PEMFC environment such electrodes feature a quasi-negligible HOR-activity at the potentials > 1.5 V reached upon gross H2-starvation. We then proceeded to perform a series of single cell PEMFC-tests at 40 or 80 °C and imposing anode currents of 20 or 200 mA∙cm−2; to mimic a short H2-starvation event, the supply of this gas to the cell’s anode was halted for 8 s by substituting the H2 with N2 (in both cases at 80 % relative humidity [RH]), to then re-establish the initial H2 flow. Similarly to our observations in RDE tests, the high PEMFC-anode potential reached during the initial H2-to-N2 switch did not decay when the H2 flow was re-established. However, while in the RDE tests this trapped state could only be reversed by setting the electrode back to the open circuit, in the PEMFC the potential did eventually decay back to ≈ 0 V, and the duration of the trapped state highly depended on the chosen measurement conditions (e.g., temperature, imposed current). Figure 1 showcases an example of this high-potential state, in this case following an 8 s starvation event at 80 °C and 20 mA∙cm−2.
In summary, this contribution will introduce our findings regarding the effect of several experimental parameters (e.g., temperature, imposed current, presence / absence of an OER-catalyst) on the magnitude and reversibility of this high potential trapped state, including its impact on the PEMFC-performance.

References

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Figure 1