Probing the Electrochemical Behavior of High-Temperature Ionomer Blends

Abstract

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

Hydrocarbon Ionomeric Binders for Fuel Cells and Electrolyzers

“…[ 46b,47b,223 ] Arges and co‐workers elucidated that the proton conductivity of protonated phosphonic acids is improved due to the suppressed anhydride formation of phosphonic acids. [ 224 ] Due to the enhanced proton conductivity, the kinetic performance of HT‐PEMFC greatly improved (66 mA cm −2 for protonated phosphonic acid ionomer versus 19 mA cm −2 for non‐protonated phosphonic acid ionomer at 0.8 V, 160 °C under H 2 /air conditions). [ 47b ]…”

“…[ 52b ] The capability of secondary pore structures of aprotic solvents was investigated using protonated phosphonic acids. [ 224 ] It was found that solvents with low pK a , such as dimethylsulfoxide (DMSO) (pK a of protonated DMSO = −2.48) push the protons back to the polymer acids, thus keeping them to a more non‐dissociated state, and resulting in less‐porous films. Aprotic solvents with high pK a , such as DMAc, DMF, and NMP (pK a of protonated solvents = −0.17 to −0.19) can create porous structures to improve fuel cell performance.…”