Chemical Stability of Perfluorobis(sulfonyl)imide-Acid (PFIA) Ionomers in Open Circuit Voltage (OCV) Accelerated Test Conditions

Abstract

Membranes based on 3M's perfluoro imide acid (PFIA) ionomer have been studied using the open circuit voltage (OCV) hold accelerated stress test. For these samples, a decay in the OCV potential within the first 200 hours was observed along with an increase in cell resistance that rapidly grows near the end of life. A multi-membrane electrode assembly (MEA) technique was employed to study the origins of these phenomena. Effluent water collected at the beginning of life and later, during a recovery protocol, was analyzed using liquid chromatography -mass spectroscopy (LC-MS). A variety of small molecule fragments were detected that could be traced to the ionomer side chain. The center layer of a three-membrane MEA was separated at the end of life and analyzed by 19 F nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy. Peaks associated with the bis(sulfonyl)imide group of the PFIA ionomer were observed to have reduced in intensity and new peaks, assigned to a perfluorosulfonamide side chain, appeared. The combination of the fragments detected in the effluent water, along with the spectral changes in the membranes after aging, point to one or more degradation processes involving the PFIA ionomer side chain. This decomposition is likely analogous to reactions described for perfluorosulfonic acid (PFSA) ionomers but with new consequences owing to the greater number of potential fragments.

Chemical and Morphological Origins of Improved Ion Conductivity in Perfluoro Ionene Chain Extended Ionomers

“…There is a need to develop alternative ionomers that can meet the ever-growing demand for diverse functionalities in many devices. 1,2,[14][15][16] Development of next-generation membranes that have suitable conductivity under drier conditions requires fundamental understanding and control over their nano-morphology and ion-transport pathways. Conductivity in PFSAs increases with macroscopic swelling (e.g., humidification), which is driven by the molecular hydration in terms of water content per poly(anion) group and associated expansion of the mesoscale domainnetwork.…”

“…11 [ ] To meet these design goals, ionomers with modified chemistries have been developed to enhance conductivity at lower humidity levels. 13,15 3M and Solvay have developed PFSA ionomers with shorter side-chains relative to Nafion, resulting in lower EW. However, very low equivalent weights can inhibit crystallization of the backbone and lead to excessive swelling and reduced stability in liquid water.…”

“…1,15 3M has synthesized a series of alternative multi-acid side-chain (MASC) ionomers based on the medium side-chain 3M PFSA, where the number of protogenic groups on the side-chain is increased for the same backbone length (m T F E ). A promising chemistry belonging to this class is Perfluoro Ionene Chain Extended (PFICE) ionomers, 11,13,15 where the number of protogenic bis(sulfonyl)imide groups on the side-chain can be controlled between n = 1 and 3 (Figure 1). One particular chemistry (n = 1, PFICE-2) is perfluoro imide acids (PFIA), which shares the same fluorocarbon backbone as the PFSA, but contains an additional imide acid group providing the secondary protogenic group (Figure 1).…”

“…1,11 While increasing TFE repeat units enhances the ionomer chain's ability to form more crystalline domains and therefore improves stability, it also reduces the number of sulfonic acid groups per unit volume, or the charge density (IEC), thereby deteriorating transport functionality. 1,15 One route to circumvent this dichotomy in ionomers is to alter the side-chain chemistry, potentially expanding the feasibility limits of high-IEC PFSAs. 11 To meet these design goals, ionomers with modified chemistries have been developed to enhance conductivity at lower humidity levels.…”

“…Despite Nafion’s long research history, perfluorinated ionomers with different side-chain chemistries and varying EWs have begun to attract interest. There is a need to develop alternative ionomers that can meet the ever-growing demand for diverse functionalities in many energy devices. ,,− For PEFCs, development of next-generation membranes that have suitable conductivity under drier conditions requires fundamental understanding and control over their nanomorphology and ion-transport pathways. Conductivity in PFSAs increases with macroscopic swelling ( e.g.…”