Impact of Substrates on Water Uptake Behavior of Fluorocarbon-based Ionomers under Confinement
Ashley Miller
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04/05/2021
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Video Poster Presentation 2021
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- [00:00:00.840]Hello, my name is Ashley Miller and I'm a senior in chemical engineering.
- [00:00:04.680]I conducted my research with the help of graduate student, Seefat Farzin,
- [00:00:07.920]and under the guidance of Dr. Dishari
- [00:00:09.290]of the department of chemical and biomolecular engineering.
- [00:00:12.750]I will be going over my poster on the impact of substrates on water uptake
- [00:00:16.320]behavior of fluorocarbon-based ionomers under confinement.
- [00:00:21.000]The purpose of this research
- [00:00:22.140]was to understand the behavior of hydration environment as a function of
- [00:00:25.350]relative humidity, thickness,
- [00:00:27.030]substrate, and ionomer structure of multi acid side chain ion conducting polymers,
- [00:00:31.680]or ionomers, under confinement for proton exchange membrane fuel cells.
- [00:00:36.810]To begin with some background information,
- [00:00:38.400]proton exchange membrane fuel cells are a type of fuel cell that utilizes a
- [00:00:41.850]special proton conducting polymer electrolyte membrane. In proton exchange
- [00:00:45.840]membrane fuel cells, the ionomer membrane serves many functions.
- [00:00:49.560]It transfers protons from anode to cathode,
- [00:00:51.840]separate the electrodes, and prevents electrons from passing through.
- [00:00:55.950]One of the purposes of the ionomer thin-film is to bind to the catalyst.
- [00:01:00.150]A commonly used catalyst is platinum,
- [00:01:02.040]which is one of the substrates I studied. The ionomer thin film
- [00:01:05.430]also promotes protons in the catalyst interface for the oxygen reduction
- [00:01:09.300]reaction of the fuel cell. The ionomer thin film experiences
- [00:01:12.750]severe confinement in the catalyst interface.
- [00:01:15.720]Below is a figure of a proton exchange membrane fuel cell with a close up image
- [00:01:19.410]of the ionomer-catalyst interface.
- [00:01:22.470]There are two mechanisms for proton conduction,
- [00:01:24.570]and they depend on how well connected the water domains are.
- [00:01:27.870]The Grotthus mechanism occurs when the water domains are well-connected,
- [00:01:31.530]and so the protons are able to jump between adjacent water molecules through
- [00:01:34.800]the formation and breaking of hydrogen bonds.
- [00:01:38.160]The vehicle mechanism occurs when the water domains are poorly connected, so
- [00:01:41.930]water molecules act as a vehicle for transporting the protons. For materials
- [00:01:46.920]and methods, Nafion and 3M PFIA
- [00:01:49.740]were the ionomers studied and their chemical structure is shown in the figure.
- [00:01:53.370]Nafion and 3M PFIA differ from each other because 3M PFIA is
- [00:01:57.870]multiacid with a primary side chain, and there's a longer side chain
- [00:02:01.230]than Nafion. Nafion is single acid with primary and secondary side chains.
- [00:02:05.820]This research was conducted by measuring the frequency of the deposited film on
- [00:02:09.480]quartz crystal. Then using Sauerbrey analysis, the water absorbed per unit area
- [00:02:14.400]was able to be calculated using the change in frequency.
- [00:02:18.450]The Sauerbrey equation I used is shown below where delta f is the change in
- [00:02:21.900]frequency, f naught is the resonant frequency of
- [00:02:24.650]the crystal, delta m is the change in mass, which is equal to the water
- [00:02:28.290]absorbed, A is the active area of the crystal,
- [00:02:31.860]Mu q is the shear modulus of the crystal,
- [00:02:34.470]and rho q is the density of quartz. The hydration number equation is also shown,
- [00:02:38.670]which was used to convert the mass absorbed to hydration number. In the equation,
- [00:02:43.320]lambda w is the hydration number, m RH is the sample mass
- [00:02:47.040]certain relative humidity, m naught is the
- [00:02:49.520]mass of the dry sample,
- [00:02:51.240]m H2O is the molecular mass of water,
- [00:02:54.180]and IEC is the ion exchange capacity
- [00:02:56.100]of the ionomer. In the results section
- [00:02:58.500]there are figures displaying the hydration
- [00:03:00.460]number as a function
- [00:03:01.360]of percent relative humidity for three different film thicknesses.
- [00:03:05.740]There are figures for each ionomer, 3M PFIA and Nafion, on each substrate,
- [00:03:10.330]Silicon dioxide crystals
- [00:03:11.710]and platinum coated crystals. Comparing Nafion and 3M PFIA, PFIA
- [00:03:16.300]generally had a higher hydration number and absorbed more water than Nafion across
- [00:03:20.680]the different thicknesses studied.
- [00:03:22.600]This is due to the differences between the structure of Nafion and PFIA. In the
- [00:03:27.330]discussion section
- [00:03:28.270]a figure is used to describe the differences between the structure of Nafion,
- [00:03:31.990]which has a very similar structure to PFSA, compared to PFIA.
- [00:03:36.520]As you can see, more water is able to be absorbed within a PFIA film.
- [00:03:41.230]The hydration numbers of the ionomers on platinum, remain constant irrespective of
- [00:03:45.640]film thicknesses due to the controlling interaction of the substrate on
- [00:03:49.580]the nanostructure of the thin films. In conclusion,
- [00:03:53.290]ionomer thin films absorb a consistently higher
- [00:03:55.690]amount of water deposited on platinum coated substrates,
- [00:03:59.020]irrespective of ionomer structure and thickness.
- [00:04:02.500]The future work for this project will be to measure water uptake of films that
- [00:04:05.860]are less than a hundred nanometers thick and simultaneously measure proton
- [00:04:10.030]conductivity on platinum coated substrates.
- [00:04:12.970]I would like to thank professor Dishari for her continuous guidance, Seefat Farzin for
- [00:04:17.260]mentoring in the experiments and the department of chemical and biomolecular
- [00:04:20.830]engineering for funding this research.
- [00:04:23.130]Thank you.
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