Despite the recent advances in fuel cell technology, lifetime is still a hindrance for broad commercialisation. One of the limiting factors in this regard is membrane degradation, which is either accelerated by mechanical stress, leading to creep and pinhole formation or by chemical degradation. As has been discussed previously [1], the decomposition of the polymer electrolyte results in the release of HF, which can be quantified by the use of a fluoride ion selective electrode (F-ISE). However, polymer fragments can be released in significant quantities as well and thus the fluoride emission rate (FER) is an insufficient parameter to quantify electrolyte degradation. A method to determine the total fluorine emission rate (tFER) is developed. The thermal decomposition of the ionomer is made use of and by adding different alkalis, released fluoride is captured.

A procedure that had been developed for Nafion® decomposition was adjusted to suit the requirements of effluent water samples in order to evaluate the tFER [2]. Accelerated stress tests were conducted in a 25 cm² single cell. The segmented cell was equipped with a membrane electrode assembly for stationary applications (IRD A/S, Odense, Denmark) and operated at 65 °C with a relative humidity of 80% at a reference current of 0.5 A cm^-2 and stoichiometries of 1.5 and 2 on anode and cathode, respectively. Each AST consisted of 100 cycles, during which the cathode stoichiometry was reduced either to 0.9, 0.7 or 0.0 once an hour for 10 seconds. In-between testing, the fuel cell was stored under nitrogen atmosphere at room temperature overnight [3].

The anode off-gas was washed by bubbling through a 1 M aqueous KOH solution to gather the effluent water and capture any volatile compound in the gas stream. The solution was changed and effluent water samples were taken every 100 hours of testing and after complete shutdown of the fuel cell, including the characterisation and purging [1].

From the solution, 100 ml samples are heated until dry, followed by thermal treatment at 500 °C for 7 h in lidded Ni-crucibles. After treatment, the samples were dissolved in deionised water, the pH adjusted to 7 with concentrated HCl and mixed with TISAB II in a ratio of 1:1. In untreated samples, the pH was also adjusted to 7 and mixed with TISAB II for F-ISE measurements [1,2].

As is shown in Figure 1 a, the total fluorine emission rate vastly exceeds the fluoride emission rate without previous treatment. This is very much expected from the literature [1,2,4]. Subtracting the FER from the tFER gives the fluorine emission rate alone (FnER, Figure 1 b). This represents a better visualisation of the emission of oligomeric fragments from the ionomer. The distinction of the different fluorine sources is fundamental for assessing the underlying degradation mechanism.

Thermal treatment of the samples makes the fluorine, bound in oligomeric residues, accessible for quantification. The thereby determined total fluorine emission rate is a more reliable indicator for electrolyte decomposition. With the knowledge of both FER and tFER, the fluorine emission rate can be obtained. This represents the emission of polymer fragments only. Therefore, a more detailed understanding is possible.

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.

References

[1] Bodner M, Rami M, Marius B, Schenk A, Hacker V. Determining Membrane Degradation in Polymer Electrolyte Fuel Cells by Effluent Water Analysis. ECS Trans 2016;75:703–6. doi:10.1149/07514.0703ecst.

[2] Rami M. Effluent Water Analysis in Polymer Electrolyte Fuel Cells. Graz University of Technology, 2017

[3] Bodner M, Schenk A, Marius B, Rami M, Hacker V. Air Starvation Accelerated Stress Tests in Polymer Electrolyte Fuel Cells. ECS Trans 2016;75:769–76. doi:10.1149/07514.0769ecst.

[4] Aarhaug TA. Assessment of PEMFC Durability by Effluent Analysis. Norwegian University of Science and Technology, 2011