Modifying Carbon Supports for Improved Ion-Exchange Capacity in Proton-Exchange Membrane Fuel Cells
Date of Award
Master of Science (MS)
Stephen E. Creager
Joseph S. Thrasher
Sulfonated perfluorocyclobutyl phosphonate (s-PFCB-PO3) polymers are studied extensively in proton-exchange membrane (PEM) fuel-cell applications for their favorable ionic-conducting properties. Inspired by their high chemical, mechanical and thermal stability, PFCB polymers have been used by several groups as grafting compounds onto zirconia-decorated carbon black (ZrC) catalyst supports. The trifluorovinyl ether (TFVE) moiety of the aryl bis-TFVE monomer provides a template for polymerization by step-growth [2+2] cycloaddition to afford PFCB polymers which upon subsequent sulfonation can have high ion-exchange capacity and high proton conductivity. Aryl phosphonic acid substitution onto the terminal TFVE groups provides a route to anchoring the ionic polymers onto the surface of ZrC supports by binding of phosphonates onto zirconia particles. Electrodes prepared from such materials are expected to be mixed ionic and electronic conducting (MIEC) composites, which allows for their use in a PEM fuel cell without diminishing proton transport due to water exposure.
Herein, a sulfonated PFCB polymer is synthesized from a precursor PFCB polymer and characterized by ion-exchange capacity (IEC) and NMR spectroscopy measurements. The molecular weight and average number of repeat units per PFCB polymer chain are calculated from 19F-NMR spectroscopy data. An IEC of 2.35 meq/g is expected upon mono-sulfonation of each repeat unit in the polymer. Neutralization titration reveals a much higher 3.94 meq/g IEC, which may be due to a combination of impurities from the sulfonation and/or hydrolysis reactions, and from some polymer repeat units having more than one sulfonation site.
Electronic resistance measurements were made using an electrochemical hydrogen-pumping device as a diagnostic tool to calculate the effective ionic conductivity of composite materials made from the s-PFCB-PO3 polymer and the ZrC material. The ZrC and s-PFCB-PO3-ZrC (the MIEC) catalyst inks are used to make s-PFCB-PO3-ZrC layers that are placed in a sandwich configuration between Nafion membranes in an electrochemical hydrogen-pump cell to isolate the ionic resistance contributions from the immobilized PFCB polymer.
A composite consisting of 25 weight percent Nafion® (1,100 equivalent weight) and 75 weight percent s-PFCB-PO3-ZrC has an ionic conductivity of 66.0 mS/cm, whereas a control composite containing 100 weight percent ZrC has an ionic conductivity of 57.9 mS/cm. These results provide new insight into the proton transport properties of composites made from ionomer polymers and carbon-based electrocatalyst supports.
Bill, Bryan, "Modifying Carbon Supports for Improved Ion-Exchange Capacity in Proton-Exchange Membrane Fuel Cells" (2019). All Theses. 3208.