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Synergistically integrated phosphonated poly(pentafluorostyrene) for fuel cells

Abstract

Modern electrochemical energy conversion devices require more advanced proton conductors for their broad applications. Phosphonated polymers have been proposed as anhydrous proton conductors for fuel cells. However, the anhydride formation of phosphonic acid functional groups lowers proton conductivity and this prevents the use of phosphonated polymers in fuel cell applications. Here, we report a poly(2,3,5,6-tetrafluorostyrene-4-phosphonic acid) that does not undergo anhydride formation and thus maintains protonic conductivity above 200 °C. We use the phosphonated polymer in fuel cell electrodes with an ion-pair coordinated membrane in a membrane electrode assembly. This synergistically integrated fuel cell reached peak power densities of 1,130 mW cm−2 at 160 °C and 1,740 mW cm−2 at 240 °C under H2/O2 conditions, substantially outperforming polybenzimidazole- and metal phosphate-based fuel cells. Our result indicates a pathway towards using phosphonated polymers in high-performance fuel cells under hot and dry operating conditions.

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Fig. 1: Anhydride formation of phosphonic acid.
Fig. 2: Property comparison of proton conductors.
Fig. 3: Property spider charts of materials to be used as membrane and electrode binder.
Fig. 4: H2/O2 fuel cell performance comparison for different MEAs.
Fig. 5: H2/air fuel cell durability comparison for different MEAs.

Data availability

All the data represented in Figs. 15 and Extended Data Figs. 29 are provided with the paper as source data. All other data that support results in this Article are available from the corresponding author upon reasonable request. Source data are provided with this paper.

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Acknowledgements

This work was supported by the US Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO) and the Advanced Research Project Agency-Energy (ARPA-E). This work was also partly supported from the Bundesministerium für Bildung und Forschung (BMBF) on account of the ‘HT-Linked’ project with Förderkennzeichen: 03SF0531C. Los Alamos National Laboratory is operated by Triad National Security, LLC under US Department of Energy contract number 89233218CNA000001. Sandia National Laboratories is a multi-programme laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the US Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. I.M. acknowledges the computational resources from the Tri-Lab computing resources of Los Alamos National Laboratory and University of New Mexico Center for Advanced Research Computing. We thank E. De Castro and C. Kreller for providing the commercial PA-PBI MEA and SnP2O7, respectively. We also thank A. Muenchinger and K.-D. Kreuer for providing PWN70 conductivity data and discussions. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the US Department of Energy of the United States Government.

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Contributions

Y.S.K. designed and directed the study. V.A. and J.K. prepared the phosphonated polymers. A.S.L. performed electrochemical measurements. E.J.P., E.D.B., C.F. and M.H. prepared the ion-pair polymers. A.S.L., V.A., E.J.P., S.M., E.D.B., C.F., M.H. and Y.S.K. characterized polymers. I.M. performed the first principles calculations. A.S.L. and Y.S.K. wrote the paper, with contributions from all co-authors.

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Correspondence to Jochen Kerres or Yu Seung Kim.

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Extended data

Extended Data Fig. 1 Anhydride formation of phosphonic acid.

Gibbs free energy diagrams for the anhydride formation at 240 °C: phosphoric (black), methylphosphonic (orange), and pentafluorophenylphosphonic acid (light blue). Source data

Extended Data Fig. 2 Property change of PWN70 during high-temperature treatment.

a, The conductivity change of PWN70 after exposure at 200 °C. The proton conductivity was measured in 5 wt.% DMSO solution at 80 °C. b, The solubility change of PWN70 after exposing at 240 °C. c, Electrochemical impedance analysis of PWN70 and PVPA MEAs at 160 °C. The impedance spectra were obtained at 0.8 V and a frequency range of 0.1 Hz to 1 MHz. Source data

Extended Data Fig. 3 Proton conductivity of PWN70 as a function of temperature.

The conductivity was measured in an open system in ambient air (RH: ~35% at room temperature). Source data

Extended Data Fig. 4 Thermal oxidative stability of PWN70 and polypentafluorostyrene.

a, TGA profiles (solid lines) and Gram Schmidt profiles (dashed lines) of PWN70 (black) and polypentafluorostyrene (red). b,c, FTIR spectra at selected temperatures for PWN70 (b) and for polypentafluorostyrene (c). Source data

Extended Data Fig. 5 FTIR spectra of proton conductors at selected temperatures.

a, PWN70 (red), PA-QASOH (green), PA-QAPOH (black) and PA-PBI (blue) and b, TPP/Nafion composite as a function of temperature. Source data

Extended Data Fig. 6 Electrochemical impedance analysis of MEAs at 160 °C as a function of partial water vapour pressure.

The impedance spectra were obtained at a frequency range of 0.1 Hz to 1 MHz. Source data

Extended Data Fig. 7 Comparison of H2/O2 fuel cell performance between Nafion-based LT-PEMFC and PWN70-based HT-PEMFC.

Nafion LT-PEMFC: cell temperature: 80 °C, membrane: Nafion 211 (25 μm thickness), Anode: Pt/C (0.6 mgPt cm−2), Cathode: Pt/C (0.6 mgPt cm−2), absolute backpressure varied from 78 to 285 kPa; PWN70-based HT-PEMFC: cell temperature: 240 °C, membrane: QAPOH−PA ion pair (40 μm thickness), Anode: PtRu/C (0.5 mgPt cm−2), Cathode: Pt/C (0.6 mgPt cm−2), absolute backpressure of 147 kPa. Source data

Extended Data Fig. 8 H2/air fuel cell performance of MEA4.

The performance measured at 160, 200 and 240 °C under backpressure of 147 kPa. Membrane: QAPOH-PA ion pair (40 μm thickness), Ionomeric binder: PWN70, Anode: PtRu/C (0.5 mgPt cm−2), Cathode: Pt/C (0.6 mgPt cm−2). Source data

Extended Data Fig. 9 Structural characterization of polymers.

a, 1H NMR spectrum of QASOH in DMSO-d6. b, 19F NMR spectrum of PWN70 in DMSO-d6, RT. The phosphonation degree calculated from 19F NMR: 66–69 mol% and the phosphonation degree calculated from titration: 50 mol% (equivalent IEC = 2.21 mequiv. g-1). 1H-NMR (400 MHz, DMSO-d6, ppm) δ = 8.36 (s, H), 4.33 (s, H), 3.77 (m, H), 2.94 (s, H), 2.78 (s, H), 1.95 (s, H), 1.02 (s, H) 19F-NMR (250 MHz, DMSO-d6, ppm) δ = −133.10 (bp 2 F), −143.14 (bp, 2 F). 31P-NMR (101.2 MHz, DMSO-d6, ppm) δ = -1.09 (bp,1 P). c, GPC profile of PWN70. Eluent: water, standard: PSSNa, detector: Shodex RI 101. Mn 97 kg mol−1, Mw 136 kg mol−1, PDI 1.40. Source data

Extended Data Fig. 10 Conductivity measurement cell.

a, Liquid cell. b, Window cells for polymer film. Left: window opening: 2 cm; Right: window opening 0.5 cm. Source data

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Conductivity cell picture

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Atanasov, V., Lee, A.S., Park, E.J. et al. Synergistically integrated phosphonated poly(pentafluorostyrene) for fuel cells. Nat. Mater. 20, 370–377 (2021). https://doi.org/10.1038/s41563-020-00841-z

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