The Vagelos Institute for Energy Science and Technology at the University of Pennsylvania gave me the opportunity to meet some of the most talented scientists I have had the pleasure of working with. Myself, Dr. Ranadeb Ball, and Lizhu Zhang explored the fundamentals of ion transport in nanopores, using the lyotropic mesophase system that had been the focus of my Ph.D.
The first of these papers was published in ACS Nano, and explored the differences in morphology and hydration between different nanoporous morphologies made of the same chemical units. We went in with the hypothesis that the gyroids (GYR), with a larger pore size and minimal increase to tortuosity, would exhibit a higher relative conductivity. As shown by the image below, that was not the case. The highly constrictive hexagonally packed cylinders (HEX) were 3-4 times higher in conductivity at the same relative humidity, which was surprising.
The key phrase there is “relative humidity.” The hydration behavior of each mesophase is different, and largely determines the conductivity. The HEX mesophase is able to fit in more water at higher relative humidities, and is able to conduct much more readily at similar conditions. If we correct by the number of waters available per charge site (represented as a lambda), we get the following result:
From the article: (a) Vapor sorption data show water content in the form of hydration number as a function of RH at 25 °C (open symbols) and 70 °C (closed symbols). (b) Relative change in pore size dlimit/dlimit,0 as a function of hydration number from humidity-dependent (in situ) SAXS measurements. (c, d) Concentration normalized conductivity for bromide and hydroxide anions as a function of hydration number at 25 °C (open symbols) and 70 °C (closed symbols). (e) Activation energy in HEX and GYR for bromide and hydroxide ions determined at a fixed hydration number λ = 3.6.
Shown in this image, we see that the differences between our mesophases collapse for bromide conductivity! Arguably, for hydroxide conductivity, we could see that our initial hypothesis had the potential to be correct at high temperatures, but we can’t make statistically significant claims about that. What this tells us about the design of nanoporous polymers for ion separations is that if you are including a morphology in your material, don’t worry about the geometric parameters – focus on getting as much solvent as possible into the material. For lyotropic materials, the interactions of the ion at the nanopore interfaces will be very similar regardless of curvature, tortuousity, or pore size. For some mesophases, the amount of water available is much greater.
I am excited in the next year or so to show the next set of results from these materials, where we explore ion-identity and pore-size specific phenomena.
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