Study of Proton Permeability and Ion-permselectivity of Ionomer Membranes for the Application of Electrochemical Devices
Nate Wagner
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08/03/2021
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This video discusses the research I have completed over the course of summer 2021. Most of it involves testing ionomer membranes for proton permeability and ion-permselectivity with the ultimate goal of finding comparable or better results to nafion, the standard type of membrane in proton exchange membrane fuel cells (PEMFCs).
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- [00:00:00.568]Hi I’m Nate Wagner, & I'll be presenting
- [00:00:02.549]my research for the 2021 Nebraska Summer
- [00:00:05.625]Research Symposium. I am studying proton
- [00:00:08.263]permeability & ion-permselectivity of
- [00:00:10.863]ionomer membranes for the application of
- [00:00:13.283]electrochemical devices.
- [00:00:15.974]Electrochemical devices I am most familiar
- [00:00:18.159]with are called proton exchange membrane
- [00:00:20.588]fuel cells or (PEMFCs), which produce
- [00:00:23.608]electricity, are efficient & good for the
- [00:00:25.838]environment, and therefore produce only
- [00:00:27.881]water and heat as byproducts.
- [00:00:29.991]The ionomer membranes in PEMFCs, shown in
- [00:00:33.424]green in the diagram, have many functions:
- [00:00:36.455]separating the anode from the cathode,
- [00:00:38.265]transferring protons from the anode to the
- [00:00:40.264]cathode, & preventing electrons from
- [00:00:42.639]passing through. The diagram on the
- [00:00:45.439]right displays the process of a PEMFC:
- [00:00:50.784]hydrogen gas goes into the anode side of
- [00:00:53.168]the fuel cell. With the help of a
- [00:00:55.418]catalyst, the chemical reaction creates
- [00:00:58.245]H+ and electrons, shown by the first
- [00:01:01.755]chemical reaction. Electrons are moved
- [00:01:05.272]through a wire connecting the two
- [00:01:07.000]electrodes & allowing the load to
- [00:01:09.490]function. The H+ ions move through the
- [00:01:14.418]ionomer membrane to the cathode side of
- [00:01:16.758]the fuel cell, where air is allowed
- [00:01:18.838]inside, & with the help of a catalyst, a
- [00:01:23.084]chemical reaction occurs to form water as
- [00:01:25.334]a byproduct, & this process is shown by
- [00:01:28.537]this chemical reaction.
- [00:01:32.177]My main research objective focuses on the
- [00:01:34.817]membrane of the fuel cell. I observe the
- [00:01:37.347]membranes ability to allow H+ ions to pass
- [00:01:40.216]through it & its tendency to allow
- [00:01:42.116]cations & anions to pass through it.
- [00:01:44.666]These processes are known as proton
- [00:01:46.406]permeability & ion-permselectivity.
- [00:01:50.406]The two membranes I tested for proton
- [00:01:52.392]permeability were Nafion & sulfonated
- [00:01:54.755]polysulfone, or SPS. Nafion is a
- [00:01:57.834]state-of-the-art ionomer because it is
- [00:01:59.693]very efficient. It is currently the
- [00:02:01.670]standard membrane used in PEMFCs but there
- [00:02:04.540]are reasons why it should be replaced.
- [00:02:06.700]Namely, it's very expensive & contains
- [00:02:08.810]fluorocarbons, which are not eco-friendly.
- [00:02:11.442]You can see in the structure of Nafion to
- [00:02:13.392]left that it contains long fluorocarbon
- [00:02:15.702]chains. The reason I am testing Nafion
- [00:02:19.227]for proton permeability is because it acts
- [00:02:21.549]as a baseline for the other membranes. I
- [00:02:23.969]can compare my Nafion results to other
- [00:02:26.229]membranes to better analyze the data.
- [00:02:29.479]My other material, SPS, has a proton
- [00:02:32.315]conductivity similar to Nafion at specific
- [00:02:35.137]conditions, 120 C & 100% relative
- [00:02:39.137]humidity. These values are about 0.16
- [00:02:43.137]siemens per centimeter. SPS also has the
- [00:02:46.973]added bonus of being inexpensive, & it
- [00:02:49.296]contains no fluorocarbons, which is shown
- [00:02:51.716]in its structure. I studied SPS at three
- [00:02:55.086]different ion exchange capacity values:
- [00:02:58.196]1.0, 1.4, & 1.6.
- [00:03:01.856]The procedure for proton permeability
- [00:03:04.135]experiments involves adding NaCl solution
- [00:03:07.655]into the H-shaped chamber & placing the
- [00:03:10.121]calibrated pH probe into the NaCl
- [00:03:12.743]solution. In the diagram the NaCl solution
- [00:03:16.333]is right here, & the pH probe is held
- [00:03:19.633]inside it. HCl solution is added into the
- [00:03:23.523]other side of the H-shaped chamber & a
- [00:03:25.573]timer's immediately started. I then record
- [00:03:28.290]the pH at specific times in order to
- [00:03:30.420]collect my data. Over time, H+ ions from
- [00:03:34.420]the HCl solution permeate the membrane &
- [00:03:37.570]decrease the pH of the NaCl solution. This
- [00:03:41.120]is important to test because it's the main
- [00:03:43.400]function of the membrane in fuel cells so
- [00:03:46.030]its proton permeability needs to be high.
- [00:03:49.389]After performing all proton permeability
- [00:03:51.659]experiments, I've found that SPS membranes
- [00:03:54.229]are IEC-dependent on proton permeability,
- [00:03:57.459]meaning that as the IEC value increases,
- [00:04:00.139]so does the rate of concentration of H+
- [00:04:02.319]ions. Still, the rate of concentration is
- [00:04:06.319]over 7x higher in Nafion than SPS 1.6.
- [00:04:10.849]This can be explained by differences in
- [00:04:12.753]microstructure & acidity of sulfonic
- [00:04:15.129]groups between Nafion & SPS. The
- [00:04:18.559]picture on the bottom-right compares the
- [00:04:20.669]microstructures of Nafion & an ionomer
- [00:04:23.039]called s-radel. Please note that S-radel
- [00:04:26.207]is a polymer group & polysulfone is one of
- [00:04:28.737]the polymers in that group. The fact that
- [00:04:31.637]Nafion has wide & open channels allows
- [00:04:34.320]H+ ions to easily navigate through them,
- [00:04:37.300]whereas the narrow channels of SPS
- [00:04:40.112]restrict the movement of H+ ions.
- [00:04:43.922]Now for the membranes I studied for
- [00:04:45.636]ion-permselectivity. In this experiment,
- [00:04:48.456]Nafion is coated with polyethylenimine, or
- [00:04:51.546]PEI, & porin-type molecules as a method of
- [00:04:55.096]improving permselectivity. On the top left
- [00:04:58.126]is a ribbon diagram for Porin, the
- [00:05:00.446]biological ion channel, & directly to the
- [00:05:02.946]right is a diagram of how the porin-type
- [00:05:05.606]molecules will be assembled in the
- [00:05:07.466]membrane. Similar to this structure of
- [00:05:10.456]porin, Dr. Dishari’s lab was inspired to
- [00:05:13.166]develop a few monomers and polymers that
- [00:05:15.736]have both a similar structure &
- [00:05:17.506]self-assembling property. I have performed
- [00:05:19.956]experiments for monomer a1, a2 & a3, but I
- [00:05:23.956]will only be showing the data for monomer
- [00:05:26.355]a3 since it has a similar structure to the
- [00:05:28.565]other monomers and is therefore
- [00:05:30.499]representative of the rest of the data.
- [00:05:33.299]This means the membranes I will be
- [00:05:34.978]showing you are Nafion, Nafion-PEI, &
- [00:05:39.362]Nafion-PEI-monomer a3. The schematic on
- [00:05:42.892]the bottom left explains the coating
- [00:05:45.002]procedure for the membranes tested. In
- [00:05:47.952]addition to these membranes, I will also
- [00:05:50.942]be testing the ion-permselectivity of SPS
- [00:05:53.992]compared to Nafion.
- [00:05:56.632]The procedure for ion-permselectivity
- [00:05:58.451]involves adding KCl solution into both
- [00:06:01.585]sides of the H-shaped chamber separated
- [00:06:03.816]by the membrane, then placing electrodes
- [00:06:06.326]connected to a Sourcemeter into each side
- [00:06:08.884]of the solution. During this lab we want
- [00:06:11.834]the slope to ideally increase for positive
- [00:06:14.134]bias over negative bias because it
- [00:06:16.383]demonstrates the membrane’s ability to
- [00:06:18.323]allow cations to pass through more often
- [00:06:20.551]than anions. This significant & sudden
- [00:06:23.441]change in the slope of the graph is known
- [00:06:25.556]as a gating property.
- [00:06:28.576]After performing all trials of
- [00:06:30.202]ion-permselectivity, I have found that the
- [00:06:32.582]Nafion-PEI-porin-type molecule series
- [00:06:36.582]produced high current flows in forward
- [00:06:38.682]bias & low current flows in reverse bias,
- [00:06:42.242]which reveals a gating property. As you
- [00:06:44.672]can see, the slope increases when the
- [00:06:47.282]voltages are positive over the negative
- [00:06:49.832]voltages. The SPS trials produced similar
- [00:06:54.236]current flows in forward and reverse bias,
- [00:06:56.946]no gating properties, & very little
- [00:06:58.994]difference between the Nafion trial.
- [00:07:02.534]Regarding the Ion-permselectivity of the
- [00:07:05.074]Nafion-PEI-monomer membrane, these
- [00:07:07.724]lab-synthesized monomers have sulfonic
- [00:07:09.922]acid groups on one side of the membrane &
- [00:07:12.332]hydroxyl groups on the other side, so in
- [00:07:14.912]forward bias, the SO3- on one side allowed
- [00:07:18.060]K+ to move through membrane, whereas Cl-
- [00:07:21.520]ions were transported in the reverse
- [00:07:23.480]direction through the membrane. In reverse
- [00:07:26.570]bias, K+ moved through membrane, whereas
- [00:07:29.730]the anionic group SO3- prevented Cl- ions
- [00:07:34.531]from being transported. This indicates
- [00:07:37.141]that these nature inspired monomers can
- [00:07:39.571]create unique & efficient proton
- [00:07:41.697]conduction pathways across their cavities.
- [00:07:45.057]Getting towards the end of my research has
- [00:07:47.147]allowed me to make a few conclusions. SPS
- [00:07:50.309]& Nafion membranes showed very little to
- [00:07:52.569]no gating property, but SPS showed
- [00:07:55.102]IEC-dependent proton permeability behavior
- [00:07:58.040]even though the rate of concentrations was
- [00:08:00.540]much lower than Nafion. Also, additional
- [00:08:05.030]layers of PEI & monomers on Nafion
- [00:08:08.218]membranes allowed ions to be transported
- [00:08:10.842]selectively, which means that through the
- [00:08:13.762]gating property significantly more cations
- [00:08:16.710]than anions were transported while in
- [00:08:18.779]reverse bias. My future research would
- [00:08:22.929]involve studying proton permeability &
- [00:08:25.059]I-V behavior of SPS membranes with higher
- [00:08:27.971]IEC levels, like 1.8 or 2.0, especially
- [00:08:31.971]since SPS showed IEC-dependence. I could
- [00:08:35.971]also work with other plant-based ionomer
- [00:08:38.223]membranes, such as cellulose or lignin.
- [00:08:41.533]Lastly, I want to acknowledge those who
- [00:08:43.443]have been helping me throughout my
- [00:08:45.113]research. My research mentor, Professor
- [00:08:47.333]Dishari, & my graduate student mentors,
- [00:08:50.453]Seefat & Oghenetega, have helped me with
- [00:08:53.623]lab work, as well as understanding the
- [00:08:55.473]basic principles of my research. I also
- [00:08:57.973]want to acknowledge the Department of
- [00:08:59.813]Chemical and Biomolecular Engineering at
- [00:09:01.943]UNL, who helped fund my research. Thank
- [00:09:05.203]you for listening to my presentation.
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