Evaluating Perfluorinated Acid Electrolytes for High-Temperature Proton Exchange Membrane Fuel Cell

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) based on phosphoric acid doped polybenzimidazole (H₃PO₄/PBI) membrane operate at elevated temperatures between 150 ºC and 180 ºC. The HT-PEMFC stacks have much simpler water and thermal management than lower-temperature PEM fuel cells and can operate on reformat gas.  These advantages make this type of fuel cells an interesting candidate for auxiliary and stationary power units. At this moment, however, no commercial HT-PEMFCs have been developed to meet the reliability and cost requirements.

One of the main reasons that limit the wide application of HT-PEMFCs is the sluggish oxygen reduction reaction (ORR) rate in concentrated phosphoric acid. Despite good physical and chemical properties, phosphoric acid is a rather poor fuel cell electrolyte. The slow ORR kinetics in concentrated phosphoric acid is still not completely understood, and it is believed to be related to strong adsorption of phosphoric acid species on the surface of the platinum catalyst.

Fluorinated acids as alternative electrolytes might hold the key to overcoming this obstacle. They seem to adsorb less strongly on platinum compared with phosphoric acid, which may increase the ORR rate and lead to high-performance fuel cells. In addition, these novel acids are ideal model systems to study the influence of adsorbates on ORR.

Appleby and Baker [1] were among the first who proposed trifluoromethane sulfonic acid (CF₃SO₃H) as an alternative electrolyte for phosphoric acid fuel cells. Later, Yeager and his co-workers adopted the idea and conducted a more extensive research on fluorinated acids such as monofluorophosphoric acid [FPO(OH)₂] and trifluoromethane sulfonic acid. 

The quest for novel electrolyte compounds that have the physical and chemical properties to meet the HT-PEMFC requirements remains challenging. An ideal candidate material should be both electrochemically and thermally stable, and it should have a very low vapour pressure over the operating temperature range of HT-PEMFCs. Recently, a number of perfluoroalkyl- phosphonic and phosphinic acids with different chain length have been successfully synthesized. These compounds might have the desired properties to serve as electrolytes for HT-PEMFCs. Besides their higher O₂solubility they show significant advantages over phosphoric acid, such as a high protonic conductivity under unhydrous conditions [2]. Furthermore, these strong acids tend not to have specific adsorption on the platinum surface.

Here, we report cyclic voltammetry (CV) and rotating disc electrode (RDE) experiments with small amounts of phosphoric acid and novel perfluorinated electrolyte candidates added into the perchloric acid electrolyte. Noticeable differences in the voltammograms (Fig. 1) and significant impact on the RDE kinetic current (Fig. 2) were observed. For a more thorough analysis of the adsorption mechanism, electrochemical impedance spectroscopy (EIS) measurements were carried out. Evaluation of the charge transfer resistance for the ORR on platinum was found to be in good agreement with the RDE measurements.

Additionally, preliminary experiments were performed to assess the effectiveness of such alternative electrolytes for practical fuel cell applications. Perfluorinated acids were added into the catalyst layer of a HT-PEMFC. First results on the influence on membrane electrode assembly (MEA) performance will be presented.

References

[1]        A.J. Appleby and B.S. Baker. Oxygen reduction on platinum in trifluoromethane sulfonic acid. Journal of the Electrochemical Society, 125(3):404-406, 1978.

[2]        M. Herath et al. Perfluoroalkyl phosphonic and phosphinic acids as proton conductors for anhydrous proton-exchange membranes. ChemPhysChem, 11(13):2871-2878, 2010.