Tuning the Intermolecular Interactions in a Proton Exchange Membrane via Salt in Solvent Systems: Preparation and Characterization

Brianna Ryan Author

04/05/2021 Added

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[00:00:01.860]Hi, I'm Brianna.
[00:00:02.940]And this is my presentation for the spring 2021 student research days,
[00:00:06.780]tuning the intermolecular interactions in a proton exchange membrane via salt,
[00:00:10.770]in solvent systems, preparation and characterization.
[00:00:15.240]So in this research, we look at membranes that are used in fuel cells,
[00:00:19.140]and part of the reason we're interested in fuel cells is because they're a
[00:00:22.260]promising alternative to fossil fuels
[00:00:26.160]which are known to cause greenhouse gas emissions and
[00:00:31.980]contribute to global warming
[00:00:34.200]fuel cells work by converting chemical energy to electricity. And,
[00:00:38.520]they work because of proton exchange membranes that can be found in the fuel
[00:00:42.870]cells. So, we work with proton exchange membranes in our research,
[00:00:47.730]proton exchange membranes also called PEMs are
[00:00:51.840]able to be used in fuel cells because they enable proton hopping,
[00:00:56.190]because of functional groups that can be found on the polymer membrane.
[00:01:00.300]We're working specifically with PBC or pentablock copolymer,
[00:01:03.630]which has a sulfuric acid group that allows that proton hopping to happen.
[00:01:09.330]and fuel cell performance can be influenced by a number of things,
[00:01:14.280]cations, temperature, humidity, stress,
[00:01:17.370]and we focus mostly on cations in this research.
[00:01:22.200]So.
[00:01:25.050]cations
[00:01:25.290]are able to form ionic interactions with the sulfuric acid group,
[00:01:29.880]which I have circled on the polymer below.
[00:01:33.750]and it can affect cell performance and durability because
[00:01:38.760]the sulfuric acid group is what is responsible for
[00:01:43.920]enabling proton hopping to happen
[00:01:46.800]which without proton hopping the membranes won't conduct and won't work in fuel
[00:01:51.690]cells. So,
[00:01:54.330]the proton hopping mechanism,
[00:01:56.820]which is we've got pictured up on the right corner,
[00:02:01.740]is literally just a proton that's able to hop from water to water molecule.
[00:02:07.910]And this is
[00:02:09.470]able to happen because the water molecules are lined up in a certain way,
[00:02:12.110]thanks to those sulfuric acid groups. But when cations are present,
[00:02:17.360]like lithium or aluminum there's
[00:02:21.740]the cations will kind of push the water molecules out of the way and,
[00:02:26.570]disrupt that proton hopping.
[00:02:29.930]and another thing that we consider with pro, uh, with cat ions,
[00:02:35.270]is whether they're monovalent or trivalent. So,
[00:02:40.130]or I guess just multivalent. Monovalent,
[00:02:44.030]cations are going to try and get really close to that sulfuric acid group and
[00:02:47.660]kind of push water molecules out of the way,
[00:02:50.750]which disrupts the proton hopping because then the water molecules aren't lined
[00:02:54.080]up the way we need them to be. And aluminum is going to be very similar,
[00:02:58.190]only it's going disrupt even more because it's trivalent and,
[00:03:04.090]it's really going to push water molecules out of the way and disrupt,
[00:03:08.890]proton hopping. So in the past,
[00:03:13.330]this has been studied by just soaking already formed
[00:03:18.310]membranes in salt solution. But in this study,
[00:03:23.140]we wanted to introduce cations
[00:03:26.740]directly into the proton exchange membrane as we were forming the proton
[00:03:30.880]exchange membrane.
[00:03:33.100]so our research objectives were to design a proton exchange
[00:03:37.660]membrane complex with salt in a solvent system.
[00:03:41.260]and we wanted to investigate the water mass uptake and decomposition
[00:03:46.240]of the membrane that we formed.
[00:03:50.260]so we prepared three different kinds of membranes. We perform, uh,
[00:03:55.120]prepared, 10 weight percent PBC in THF,
[00:03:59.980]10 weight percent PBC in 0.1 molar AlCl3,
[00:04:03.190]in THF and 10 weight percent PBC in 0.1
[00:04:07.780]Molar, lithium chloride, in THF.
[00:04:11.410]and the way we prepared those is we
[00:04:14.200]prepared a 0.1 molar salt solution in THF. So we make,
[00:04:17.830]we weighed out some salt and we added it to THF, let it dissolve.
[00:04:22.300]and once it was dissolved, we
[00:04:26.620]measured out the PBC that we needed to create a 10 weight percent solution and
[00:04:30.100]added it, let it stir, until it dissolved.
[00:04:34.930]And then we cast the polymer
[00:04:38.470]in a Teflon mold in the fume hood. We covered it,
[00:04:41.620]to kind of control the evaporation and we let it dry in the fume hood for about
[00:04:46.240]a day to get, um,
[00:04:48.490]once most of the THF had evaporated and to get
[00:04:53.350]any water or solvent left in the membrane out,
[00:04:58.150]we put it in the vacuum oven and we dried it at 35 degrees for
[00:05:02.770]about 21 hours.
[00:05:05.890]and then once we had formed our membranes,
[00:05:09.070]we analyzed them with TGA and water
[00:05:13.390]uptake studies, so results and discussion.
[00:05:19.090]So water uptake. So to get the water uptake, we,
[00:05:23.830]measured the
[00:05:27.070]mass and dimensions of a dry membrane.
[00:05:31.240]and then we soaked that membrane in water for a couple of days and took the
[00:05:34.720]dimensions and mass again.
[00:05:37.480]and you can see the equation that we use to get the numbers on the
[00:05:42.620]plot on the right. And,
[00:05:45.040]so we noticed that the presence of cations decreased water uptake, which,
[00:05:52.360]kind of tracks with
[00:05:56.350]how the presence of cations disrupts
[00:06:00.590]proton hopping and
[00:06:05.060]Yeah.
[00:06:05.510]So we also looked at TGA, which is thermogravimetric analysis,
[00:06:10.130]and, it helps us analyze the thermal stability of the membranes.
[00:06:14.840]The presence of cations increased the thermal stability,
[00:06:19.670]and they have higher decomposition temperatures, which are
[00:06:23.690]shown in the graph on the right.
[00:06:29.080]So some conclusions.
[00:06:33.640]Cations directly incorporated into PBC membranes, decreased water uptake,
[00:06:37.450]and likely conductivity.
[00:06:39.970]Cations directly incorporated into PBC membranes,
[00:06:43.000]increased thermal stability and have higher decomposition temperatures.
[00:06:47.680]and some of the things that we want to do in the future is we want to optimize
[00:06:50.560]that aluminum chloride membrane to improve homogeneity because we noticed that it
[00:06:54.970]wasn't completely homogenous when we cast it.
[00:06:58.420]we would like to find more salts soluble
[00:07:00.610]in THF to explore different cations.
[00:07:05.050]and we want to test the proton conductivity,
[00:07:09.160]of these membranes and compare the membranes
[00:07:14.110]soaked in salt solution
[00:07:16.060]to membranes where we directly add the salt to the
[00:07:20.650]THF when we formed the membrane
[00:07:23.920]I'd like to acknowledge the Nebraska Center for Energy Sciences Research.
[00:07:28.990]And thanks for watching.

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Tuning the Intermolecular Interactions in a Proton Exchange Membrane via Salt in Solvent Systems: Preparation and Characterization

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