In-Situ X-ray Absorption Spectroscopy Elucidating Mechanisms in HT-PEM Fuel Cells

The development of phosphoric acid (H₃PO₄) imbibed polybenzimidazole (PBI) membranes used in so-called high temperature polymer electrolyte membrane fuel cells (HT-PEMFC) has attracted increasing interest lately. Working temperatures of up to 180 °C can be utilized in this approach, which offers several advantages. For instance, it allows the use of less pure and therefore less expensive anode feeds due to a higher tolerance towards impurities, such as CO and H₂S, easier water management, simpler cooling systems and enhanced electrode kinetics. However, at these elevated temperatures degradation phenomena may play an important role and phosphate adsorption may poison electrode surfaces. These need to be understood in detail in order to develop both more active as well as more durable materials.

X-ray absorption spectroscopy (XAS) is a technique optimally suited to in-situ studies, as neither UHV conditions, nor long-range order in the sample are required. It can be applied to monitor the geometric structure of the catalytically active nanoparticles directly following particle oxidation and growth during operation.  With the delta µ XANES method and complementary theoretical FEFF8 calculations [1] it is possible to identify different adsorbates on small Pt nanoparticles and follow their change in coverage. Phosphoric acid [2], which is used as electrolyte, or impurities in the fuel gases may strongly adsorb and alter the reaction mechanisms.

In this work the coverage of different adsorbates and possible poisons on the anode and cathode side in H₃PO₄ imbibed PBI HT PEM fuel cells was investigated. At the cathode side, the adsorption of phosphoric acid and OH on the Pt surface was followed at different temperatures and potentials. At the fuel cell anode, the effect of CO and water was analysed.

1      Teliska M., O’Grady W. E., Ramaker D. E., J. Phys. Chem. B, 108, 2333 (2004); 109, 8076 (2005).

2      Zelenay P., Scharifker B. R., Bockris J.O´M., J. Electrochem. Soc., 133, 2262 (1986).