One of the most promising types of fuel cells that could be used as power sources for cars and houses are polymer electrolyte fuel cells possessing a high energy conversion efficiency. Especially, high temperature polymer electrolyte fuel cells (HT-PEFC) are gaining increased attention due to several advantages compared to systems that work at lower temperature: high efficiency, avoiding cooling system, reduced CO poisoning of the platinum electrodes, and improved reactivity.
Protic liquids such as phosphoric acid work as the proton conduction medium in these systems. The major drawback however is the leaching of phosphoric acid within the membrane electrode assembly leading to an inhomogeneous distribution and a deterioration of the HT-PEFC during long-term operation.
In order to understand how the phosphoric acid is affecting the battery interior a deeper understanding of its wetting on the fuel cell materials at the process temperatures is required. Few studies focus on the wetting behaviours of phosphoric acid in HT-PEFCs. Halter et al. have recently studied the wetting mechanisms and found that phosphoric acid concentration and the temperature both play a role in the wetting behaviour of HT-PEFC. To this end, contact angle measurements were done on two different surfaces: polytetrafluoroethylene (PTFE) and highly oriented pyrolytic graphite (HOPG).
When the temperature increased the contact angle of 85 wt% phosphoric acid on PTFE increased from 104° at 25 °C over 110° at 100 °C to 112° at 160 °C. When the phosphoric acid concentration was increased from 0 to 85 wt% at 25 °C, the contact angle on PTFE decreased from 107° to 104°. So a slightly higher contact angle is observed when the temperature increases while increasing the phosphoric acid concentration slightly decreases the contact angle. Both trends are similarly strong resulting in a comparable wetting behaviour of concentrated phosphoric acid at 160 °C and water at room temperature.
On the highly oriented pyrolytic graphite the contact angle of 85 wt% phosphoric acid increased from 40° at 25 °C over 53° at 100 °C to 54° at 160 °C and decreased from 64° to 40° with increasing phosphoric acid content. HOPG compared to PTFE shows a much stronger wettability under process conditions. While the HOPG surface possessed a contact angle of 40° for phosphoric acid which is a spontaneous wetting behaviour, PTFE with a contact angle of 104° for phosphoric acid was non-wetting so the leached phosphoric acid will show a strong tendency to wet the HOPG parts in the fuel cell.