In the past two years, tremendous improvements in high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs)(1) and HT-PEM hydrogen pumps(2) using membrane electrode assemblies containing ion-pair HT-PEMs(3, 4) and electrode phosphonic acid ionomer binders. A challenge in using phosphonic acid ionomers at high temperatures and under anhydrous conditions is their propensity to form anhydrides that compromise ionic conductivity(5). Poly(tetraflurostyrene phosphonic acid-co-pentafluorostyrene) (PTFSPA) is a good electrode ionomer binder material as it is less susceptible to anhydride formation when compared to poly(vinyl phosphonic acid) because of the electron-withdrawing fluorine in the styrene ring that increases the acidity of the phosphonic acid moiety. However, PTFSPA can still form anhydrides. Recently, Kim and co-workers blended PTFSPA with Nafion, a perfluorosulfonic acid (PFSA) material, as a strategy to mitigate anhydride formation(1). The sulfonic acid in PFSA was shown to protonate the phosphonic acid group in PTFSPA to foster proton conductivity. However, much is still not known about how the PFSA and PTFSA polymer blends affect the electrochemical properties of porous electrodes used in HT-PEM electrochemical systems. It is posited that PFSA may promote redox reaction kinetics because it is superacid material and kinetic rates are enhanced at extreme pH values. PFSA is also known to have higher gas permeability rates when compared to other polymer materials (6). This talk will present our research examining the electrochemical properties, and other thin-film properties, of PTFSPA-PFSA blends. PFSA materials will consist of Nafion and other PFSA analogous that have shorter side chains and lower equivalent weight values (e.g., Aquivion). Preliminary data has shown that Aquivion, which has a shorter side chain than Nafion, is more effective for promoting thin film ionic conductivity on interdigitated electrode (IDE) substrates under anhydrous (i.e., 0% RH) and 100% RH in PTFSPA-PFSA blends. References: K. H. Lim, A. S. Lee, V. Atanasov, J. Kerres, E. J. Park, S. Adhikari, S. Maurya, L. D. Manriquez, J. Jung, C. Fujimoto, I. Matanovic, J. Jankovic, Z. Hu, H. Jia and Y. S. Kim, Nature Energy, 7, 248 (2022). G. Venugopalan, D. Bhattacharya, E. Andrews, L. Briceno-Mena, J. Romagnoli, J. Flake and C. G. Arges, ACS Energy Letters, 7, 1322 (2022). K.-S. Lee, J. S. Spendelow, Y.-K. Choe, C. Fujimoto and Y. S. Kim, Nature Energy, 1, 16120 (2016). G. Venugopalan, K. Chang, J. Nijoka, S. Livingston, G. M. Geise and C. G. Arges, ACS Applied Energy Materials, 3, 573 (2020). V. Atanasov, A. S. Lee, E. J. Park, S. Maurya, E. D. Baca, C. Fujimoto, M. Hibbs, I. Matanovic, J. Kerres and Y. S. Kim, Nature Materials, 20, 370 (2021). S. Sambandam, J. Parrondo and V. Ramani, Physical Chemistry Chemical Physics, 15, 14994 (2013). Figure: a.) A picture of fabricated IDEs. The IDEs are used for thin-film ionic conductivity measurements. b.) Chemical structures of the ionomer materials. c.) Ionic conductivity data of different ionomer materials and blended ionomer materials at 0% RH (dry nitrogen) and 100% RH (humidified nitrogen) at 25 °C. Figure 1

Ion-pair high-temperature polymer electrolyte membranes (HT-PEMs) paired with phosphonic acid ionomer electrode binders have substantially improved the performance of HT-PEM electrochemical hydrogen pumps (EHPs) and fuel cells. Recently, blending poly(pentafluorstyrene-co-tetrafluorostyrene phosphonic acid) (PTFSPA) with NafionTM, and using this blend as an electrode binder, improved proton conductivity in the electrode layer resulting in a 2 W.cm-2 peak power density of fuel cells at 240 °C (a HT-PEM fuel cell record). However, much is unknown about how phosphonic acid ionomers blended with perfluorosulfonic acid materials affect electrode kinetics and gas transport in porous electrodes. In this work, we studied the proton conductivity, electrode kinetics, and gas transport resistances of 3 types of phosphonic acid ionomers, poly(vinyl phosphonic acid), poly(vinyl benzyl phosphonic acid), and PTFSPA by themselves and when blended with Aquivion® (a perfluorosulfonic acid material). These studies were performed using EHP platforms. For all phosphoric acid ionomer types, the addition of Aquivion® promoted ionic conductivity, hydrogen oxidation/evolution reaction kinetics (HOR/HER), and hydrogen gas permeability. Solid-state 31P NMR revealed that the addition of Aquivion® eliminated or significantly reduced phosphate ester formation in phosphoric acid ionomers and this plays a vital role in enhancing ionomer blend conductivity. Using the best blend variant, PTFSPA-Aquivion®, an EHP performance of 5.1 A cm-2 at 0.4 V at T = 200 °C was attained. Density functional theory (DFT) calculations identified that phosphonic acids with electron-withdrawing moieties reduced the propensity of the phosphonic acid to specifically adsorb on platinum electrocatalyst surfaces. The relative adsorption affinity of the various phosphonic acid ionomers from DFT is consistent with an experimentally obtained charge transfer resistance. A voltage loss breakdown model revealed that the addition of Aquivion® reduced activation and concentration overpotentials in EHPs. Overall, a systematic experimental and modeling approach provided further insight as to how perfluorosulfonic acid ionomers blended with phosphoric acid ionomers affect ionic conductivity, reaction kinetics, and gas permeability in EHP platforms.

Ion-exchange membranes (IEMs) are used in several electrochemical systems for a wide variety of applications, including water purification, mineral ion recovery, and energy storage and conversion. These materials often dictate...