Perfluorinated sulfonic acid (PFSA) membranes have long been the standard for proton exchange membranes (PEMs), despite many efforts to improve upon their chemistry. The saturated conductivity of these PFSAs is impressive, but a need still exists for a membrane with similar proton conductivity at low relative humidities.¹ The motivation of this study was to optimize the loading of heteropolyacid (HPA) covalently bonded to the fluoroelastomer in order to increase low humidity conductivity while maintaining chemical and mechanical stability. HPAs are known to be strong Brønsted acids as well as strong oxidants and have high thermal stability in the solid state.² These qualities should serve to effect enhanced proton conductivity and chemical durability at high temperature ranges (80 - 120 ºC) and lower humidity ranges (less than saturated conditions). Specifically, a Keggin-type silica tungstic HPA was used for its comparative stability to hydrolysis in aqueous solutions.³

In order to classify membrane behavior as a function of weight percent polyHPA (reference figure 1), membranes were synthesized in 0 wt%, 25 wt%, 50 wt%, 75 wt%, and 100 wt% polyHPA and the balance PFSA. Electrochemical impedance spectroscopy (EIS) was used to characterize conductivity of the membranes at 80 ºC with a relative humidity sweep (50 - 90 %RH). Additionally, small-angle x-ray scattering (SAXS) was performed at 80 °C with a relative humidity sweep from 0 - 95 %RH, and FTIR was performed at 80 °C at 0 %RH and 100 %RH. These conditions were chosen specifically for the typical operating region for PEMFC applications. In this blend study, the addition of polyHPA was found to categorically decrease performance (conductivity) of standard 3M© 825EW PFSA membrane. Though improvement of the PFSA membrane was not achieved, this research will provide valuable information regarding the morphology of blended membranes, particularly in membranes exhibiting two-phase behavior; furthermore, results will provide some insight into choosing other viable additives for improved conductivity of PEMFCs.

Figure 1. Chemical structure of polyHPA, 60 wt% HPA (K₈SiW₁₁O₃₉)

References

1. Ramani, V., H.R. Kunz, and J.M. Fenton. "Investigation Of Nafion®/HPA Composite Membranes For High Temperature/Low Relative Humidity PEMFC Operation". Doi.org. N.p., 2017. Web. 24 Apr. 2017.

2. Kozhevnikov, I.V., and K.I. Matveev. "Homogeneous Catalysts Based On Heteropoly Acids (Review)". Applied Catalysis 5.2 (1983): 135-150. Web.

3. Tsigdinos, G.A., and G.H. Moh. "Aspects Of Molybdenum And Related Chemistry". Materials Chemistry 4.1 (1979): 111-112. Web.