Assessing MOFs’ ability to store hydrogen gas
by Peter Angerman
Our group at the University of Bath was working on producing materials for alternative way of storing hydrogen gas in context of using hydrogen to fuel cars. The most interesting part of producing Metal Organic Frameworks (MOFs) was the analysis of the sample I synthesized. To measure the surface area of our samples we used BET model, which meant that in fact we weren’t measuring the absolute surface area but the relative surface area as we used the same method for each sample.
BET surface area calculation consists of covering the sample with a monolayer of nitrogen gas at very low temperature (77k). Low temperature allows Van Der Waals’ forces to hold nitrogen molecules at the surface of the sample due to low kinetic energy of nitrogen molecules. Since nitrogen will form a single layer on the surface of my MOF I was able to find the relative surface area by multiplying the number of nitrogen molecules by their cross-sectional area (which is a known constant).
Finally, I was able to find the amount of nitrogen molecules on the surface of my MOF by attaching a sealed container with my sample under vacuum to a machine with an identical container filled with nitrogen gas. Because I knew the initial and final pressure I was able to find how many moles of nitrogen gas had to be stored within my sample as a monolayer. In the end, I found the relative surface area of my MOF (ZIF-8) to be roughly 1500 m2g-1 which might sound like quite a lot but in reality it fell short of being applicable in hydrogen storage by about 1500-1600 m2g-1 and by about 8000 m2g-1 to be competitive with the most promising MOFs currently being tested.