Chemists at the University of Calgary have created two new metal-organic frameworks (MOFs) that demonstrate some of the best performance yet in proton transport. Such advances could serve to create better hydrogen fuel cells.

Recent research into MOFs has focused largely on their ability to capture gases like CO2, but they could one day provide an alternative to the expensive polymer electrolyte membranes (PEMs) used to transport H+ ions in fuel cells. “MOFs are crystalline and ordered, so they’re very good for determining structure-activity relationships,” says George Shimizu, co-author of two papers which recently appeared in the Journal of the American Chemical Society. “This allows you to improve your design in a rational way, which is harder to do with amorphous PEMs.” Moreover, at higher temperatures, ion conductivity tends to decrease in PEMs but can increase in MOFs.

The drawback of MOFs is that due to their similarity to salts, many are unstable in water. That’s not the case with Shimizu’s team’s latest, called PCMOF5. “You can take this material and put it in boiling water, and it retains the same conductivity,” says Shimizu. PCMOF5’s conductivity exceeded 10-3 Siemens per centimetre, but even better performance was obtained with another material, called PCMOF2½ because of the way it combines features of two previous creations, PCMOF2 and PCMOF3.

Its mix of sulfonate and hydrogen phosphonate groups allowed it to reach a maximum ion conductivity of 2.1 × 10-2 Siemens per centimetre, the best yet measured for any MOF. Still, this conductivity value is at least an order of magnitude below the best PEMs currently available.

Shimizu speculates that even better performance could be obtained by design or by combining PEMs and MOFs in a composite. In the meantime, it’s the insight into structure-activity relationships that will prove most valuable. “This is not computational modelling, it’s actual data from a material with a known structure that has the lowest activation energy seen in a MOF for proton transfer,” he says.