|SEM images of the MOF- material [β-PCMOF2(Tz)0.45], illustrating a rod-shape morphology. Source: Hurd et al. (2009), Supplementary Material. Click to enlarge.|
Researchers at the University of Calgary have developed a new membrane based on metal-organic frameworks (MOFs) (earlier post) that enables a polymer electrolyte membrane (PEM) fuel cell to operate at higher temperatures—an important step in terms of increasing the efficiency and decreasing the cost of PEM fuel cells.
A paper on the development by George Shimizu, Jeff Hurd, Ramanathan Vaidhyanathan and Venkataraman Thangadurai of the University of Calgary, and Christopher Ratcliffe and Igor Moudrakovski of the Steacie Institute for Molecular Sciences, National Research Council, was published online 18 October in the journal Nature Chemistry. Shimizu filed a patent with the US patent office last year.
MOFs—compounds consisting of metal-oxide clusters connected by organic linkers—are exciting materials that couple porosity, diversity and crystallinity. However, most work on MOF applications has focused on properties intrinsic to the empty frameworks—e.g., for storage of hydrogen, as one example (e.g., earlier post). Shimizu and his colleagues instead focused on the use of guest molecules within the framework to control other functions.
This research will alter the way researchers have to this point perceived candidate materials for fuel cell applications.
They report on Na3(2,4,6-trihydroxy-1,3,5-benzenetrisulfonate) (named β-PCMOF2), a MOF that conducts protons in regular one-dimensional pores lined with sulfonate groups. To confirm its potential as a gas separator membrane, the partially loaded MOF (β-PCMOF2(Tz)0.45) was incorporated into a H2/air membrane electrode assembly. The resulting membrane proved to be gas tight, and gave an open circuit voltage of 1.18 V at 100 °C.
Currently, PEM fuel cells can produce energy from hydrogen below 90 °C, just under the boiling point of water. With Shimizu’s material, energy can be produced at a higher temperature, up to 150 °C. This could ultimately make the fuel cell cheaper to produce because at a higher temperature less expensive metals can be used to convert hydrogen into energy. Reactions at a higher temperature would also be faster thus increasing efficiency.
Ours is an entirely new approach that strikes a balance between having a regular molecular structure and mobile components all while showing genuine promise of application.Jeff Hurd
Kevin Colbow, director of research and development at hydrogen fuel cell manufacturer Ballard Power Systems, calls the work significant. “We believe that further improvement on conductivity and robustness of these materials could provide next generation membranes for PEM fuel cells.”
Jeff A. Hurd, Ramanathan Vaidhyanathan, Venkataraman Thangadurai, Christopher I. Ratcliffe, Igor L. Moudrakovski & George K. H. Shimizu (2009) Anhydrous proton conduction at 150 °C in a crystalline metal–organic framework. Nature Chemistry doi: 10.1038/nchem.402