Chemically Crosslinked Polymer Electrolyte Membranes from Fluorinated Liquid Precursors for Application in Fuel Cells
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2010-07-29
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Abstract
Chemical crosslinking of polymer electrolyte membrane (PEM) liquid precursors has the ability to support variable acid loading and create intricate structures that are highly effective in suppressing methanol crossover while maintaining reasonable conductivity in PEMs for direct methanol fuel cells (DMFCs). PEM fabrication from photocuring of liquid precursors is another advantage over traditional methods of melt-processing and solvent-casting in which high cost and sophisticated equipment are employed. Linear-chain PEMs have certain shortcomings for application in DMFCs because of methanol crossover from the anode to the cathode, limited conductivity and high cost from processing steps and conditions.
In our approach, photocuring of liquid precursors produces chemically crosslinked PEMs with good mechanical strength and dimensional stability. Styrene functionalization of perfluoropolyethers diols of six different molecular weights (MWs) between 1000-4000 g mol-1 yields crosslinkers that afford crosslink density corresponding to their MWs. Copolymerization of the fluorinated liquid crosslinker and a styrene sulfonate ester co-monomer via UV-light initiated free-radical bulk polymerization produces chemically crosslinked PEMs. Conversion of the ester into corresponding sulfonic acid through base/alcohol hydrolysis and ion-exchange in acid solution confers these PEMs excellent proton conductivity.
The variable crosslink density from different MWs of crosslinker provides wide window of compostions to form PEMs with ion-exchnage capacity (IEC) varying from 0.5 to 1.85 meq g-1. The higher end of acid loading is two times that of benchmark Nafion membrane at good mechanical strength and dimensional stability in these crosslinked PEMs. Such high-acid loading in linear-chain PEMs leads to dissolution in polar solvents. Combining low MW (1000 g mol-1) and high MW (4000 g mol-1) crosslinkers in these PEMs improves the mechanical strength further. The fluorinated chain in crosslinked structure from perfluoropolyethers provides thermal stability up to 260 °C that is sufficient for most practical applications of DMFCs.
Due to IEC of 1.85 meq g-1, these PEMs have shown conductivities of 220 to 340 mS cm-1 at 100% relative humidity and at 25 to 60 °C, respectively, that are 3-fold higher than that of Nafion 117 conductivity. For methanol crossover reduction, the nature of crosslinked structure has been exploited to obtain PEMs with good methanol barrier and proton conduction properties. An objective of this research was to optimize the composition of PEMs derived from crosslinker with six different MWs and comonomer resulting in low methanol permeability and reasonable conductivity. This combination of low permeability and reasonable conductivity has resulted in the selectivity of 1.36  105 S cm-1/cm2 sec-1 that is more than three times that of Nafion 117 selectivity.
Depending on the crosslinker MW and composition, these crosslinked PEMs have conductivities in the range of 10-4 to 10-1 S cm-1 and methanol permeabilities in the range of 10-9 to 10-6 cm2 sec-1. Under identical acid content, incorporation of low MW crosslinkers reduces the methanol permeability further by increasing the crosslink density. Nafion 117 has conductivity of 1.04  0.02 ï‚´ï€ 10-1 S cm-1 and methanol permeability of 2.19 ï‚±ï€ ï€°ï€®ï€¶ï€³ ï‚´ 10-6 cm2 sec-1 in liquid water and at room temperature.
In addition to crosslinked PEMs in acidic form, photocuring of mixture of liquid precursor and 4-vinylbenzyl trimethyl chloride dissolved in a solvent (3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octanol) yields a novel crosslinked alkaline anion exchnage membrane (AEMs). Without any optimization in composition, these crosslinked AEMs in hydroxyl form have shown good conductivity of 45 mS cm-1 in liquid water and at room temperature for IEC of 1.43 meq g-1. Analogous to crosslinked acidic PEMs, excellent opportunity exists to exploit the crosslinking approach and optimize the composition and processing condition to achieve maximum anionic ion-exchnage capacity, high conductivity and low methanol permeability in these crosslinked AEMs for application in alkaline fuel cells.
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Nafion Membrane, Alkaline AEMs, Methanol Permeability, Anion, Conductivity, Liquid Precursor, Membrane, Crosslinked PEMs, Crosslinked AEMs, DMFCs, PEM Fuel Cells
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Degree
PhD
Discipline
Chemical Engineering
