Polymer electrolyte membrane (PEM) fuel cells can be separated into two types, the high-temperature (HT) PEM and low-temperature (LT) PEM fuel cell. As main advantages operating at higher temperatures improved tolerances to impurities like carbon monoxide, easier heat rejection and simpler water management can be mentioned. In comparison to LT-PEM fuel cells where sulfuric acid is applied polybenzimidazole (PBI) membranes doped with phosphoric acid (PA) were used in HT-PEM fuel cells. Nevertheless, degradation and long term stability are still major challenges. Here, it is necessary to understand the degradation mechanisms regarding membrane and catalyst degradation, carbon corrosion as well as loss of phosphoric acid and degradation of bipolar plates (BPP). Titration investigations of carbon based bipolar plates after operation in PEM fuel cells showed that phosphoric acid can be found within the composite matrix [1, 2] which can lead to corrosion of the graphite material.

In this work, a variety of graphite-polymer composite bipolar plate materials for PEM fuel cells were tested for their stability and compared with different methods. The electrochemical stability was investigated by means of corrosion measurements within a three-electrode cell setup. The bipolar plate material was applied as working electrode and a platinum wire with an activated felt was served as counter electrode. Potentials are shown with respect to the standard hydrogen electrode (SHE). Two different methods of aging were used: chemical storage and potentiostatic treatment. For determination of stability and corrosion rates OCP, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were evaluated. Additionally, the bipolar plate materials were compared by electrical conductivity measurements (in-plane and through-plane), contact angle measurements as well as structural changes of the surface area. Here different imaging methods like scanning electron microscopy (SEM), micro-computed tomography (µ-CT), atomic force microscopy (AFM) and confocal microscopy were applied.

First results of two graphite composite bipolar plate materials with different types and amount of polymer binder show different electrochemical behavior after chemical aging for 3 and 7 days in 0.5 M H₂SO₄. The bipolar plate material with higher polymer content (BPP-01) reveals lower double layer capacitance and a lower amount of oxygen functional groups in the potential range between 0.5-0.7 V vs. SHE (Figure 1) in comparison to the bipolar plate with higher polymer content (BPP-02). The reason for this behavior could be the lower amount of electrochemical active graphite at the BPP/electrolyte interface from BPP-01 in comparison to BPP-02. In addition, conductivity measurements show that BPP01 has a higher sheet resistance (53.6 +/- 5.6 mOhm) compared to BPP-02 (46.2 +- 4.5 mOhm) in good agreement to the content of polymer and graphite in the composite materials.

The various methods can be used to identify the difference of graphite compound materials and correlate it to degradation mechanism. Further results will be given.

[1] C. Hartnig and T. J. Schmidt, Electrochim. Acta, 56 (2011), 4237-4242.
[2] N. Pilinski, M. Rastedt and P. Wagner, ECS Trans., 69 (2015), 323-335.