Deconvoluting charge-transfer, mass transfer, and ohmic resistances in phosphonic acid-sulfonic acid ionomer binders used in electrochemical hydrogen pumps

02 June 2023, Version 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 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.


Supplementary materials

Deconvoluting charge-transfer, mass transfer, and ohmic resistances in phosphonic acid-sulfonic acid ionomer binders used in electrochemical hydrogen pumps
The supporting information contains NMR spectra of the phosphonated ionomers, thin film thickness data, IEC measurement procedure, thin film proton conductivity measurement procedure, and the data, SEM and SEM-EDX images, the procedure to prepare IDAs with decorated Pt electrocatalysts and EHP data with IDAs with Pt electrocatalysts, a chemical structure of the HT-PEM membrane used in the EHP system, additional EHP polarization results and modeling of polarization data, a comparison of EHP data in this report against data in the literature, IDA fabrication and nanoscale catalyst formation, details about modeling the EHP polarization data, and additional information about the DFT calculations for phosphate and sulfate anion adsorption on Pt (111) surface.


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