Design of Highly Durable Electrocatalyst for High-Temperature Polymer Electrolyte Fuel Cell

Phosphoric acid-doped polybenzimidazoles (denoted PA-doped PBIs) have been considered as the most promising substitutive polyelectrolytes and the PEFC employing PA-doped PBIs not only in PEM but also in the catalyst layers (Cat-L) have been developed for high temperature operation. However, recent studies have revealed that leaching of liquid PA from PEM1) and Cat-L2) causes inhomogeneous PA distribution that results in deterioration of PEFC performance during long-term operation.

In this study, in order to prevent acid leaching from the high temperature PEFC system, we used poly(vinylphosphonic acid) (PVPA) in place of PA because PVPA is a polymeric acid and is stably bound to the PBIs via multipoint acid-base reactions. We fabricated a novel electrocatalyst for Cat-L assembled with carbon nanotubes (CNTs), PVPA-doped PBI and platinum (Pt) nanoparticles as an electron-conducting supporting material, electrolyte and metal catalyst, respectively. Bottom-up assembly of nanometer-thick PVPA-doped PBI layer around CNTs is expected to serve as an effective proton conduction pathway via the Grotthuss mechanism. The doping stability of PVPA in addition to the PEFC activity and durability were studied.

We tested the durability of the PVPA-doped MEA by following the protocol proposed by the Fuel Cell Commercialization Conference of Japan (FCCJ)3). As a comparison, the durability of a PA-non-doped MEA (MWNT/PyPBI/Pt electrocatalyst doped with leached PA) was tested. Dramatic drop of the cell voltage was observed for PA-non-doped MEA, while PVPA-doped MEA shows slight decrease (Fig. 1). Such high durability of PVPA-doped MEA shows an extended stability of PEM against degradation, and also shows a continuous connection of the proton path in the Cat-L4).

References

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