High Proton Conductivity of a Bismuth Phosphonate Metal–Organic Framework with Unusual Topology

Despite the interest in proton exchange membrane (PEM)
technologies
(fuel cells and electrolyzers) for energy applications, the low stability
of the electrolyte materials under working conditions (i.e., humidity
and temperature) is one of their major limitations. Metal–organic
frameworks (MOFs) have recently emerged as promising electrolytes
due to their higher stability compared with the currently applied
organic polymers, proton conductivity, and outstanding porosity. Here,
a novel robust Bi phosphonate MOF (branded as IEF-7) was successfully
synthesized and fully characterized, exhibiting an unusual topology
due to the irregular coordination geometry of the bismuth cations.
Furthermore, IEF-7 exhibited potential porosity, very high chemical
and thermal stability, and free −PO3H groups involved
in its ultrahigh proton conductivity, reaching 1.39 × 10–2 S cm–1 at 90 °C and 90% relative
humidity for, at least, 3 cycles. In order to improve the consolidation
and shaping of the powder for testing its ion conductivity properties,
a highly MOF-loaded composite (90 wt %) was prepared by adding a proton
conductive sulfonated polysulfone binder. The proton conductivity
of the resulting composite was in the same order of magnitude as the
compacted MOF powder, making this polymeric composite electrolyte
very promising for PEM technologies.