Uptake of protic electrolytes by polybenzimidazole-type polymers: absorption isotherms and electrolyte/polymer interactions

Abstract

Polybenzimidazole (PBI) membranes doped with phosphoric acid are used as proton-conducting membrane in high-temperature polymer electrolyte fuel cells. There is no general model for the thermodynamics of the absorption process over the whole accessible doping range for all published data. The interactions between H 3 PO 4 and polymer chains, the polycondensation equilibria of H 3 PO 4 and the implications on proton conductivity are still largely unknown. In this study, we demonstrate that the uptake of a protic electrolyte by a polymer with basic moieties, i.e. a PBI-type polymer, can be described satisfactorily with a BET-like absorption isotherm. The absorption equilibria of the uptake process are analysed using literature data on the H 3 PO 4 , H 2 SO 4 and HClO 4 uptake of non-crosslinked m-PBI and AB-PBI as well as using our own investigations on the H 3 PO 4 uptake of a commercial crosslinked PBI derivative, Fumapem AM-55. In addition to the thermodynamic data of the absorption process, Raman data on m-PBI taken from the literature and our own Raman investigations on Fumapem AM-55 are taken into account. It is possible to correlate domains in the absorption isotherms with specific features in the Raman spectra. Two stages for the uptake of a protic electrolyte can be distinguished: (i) the protonation of the polymer chains and thus coulombic interactions with the electrolyte anions, (ii) the formation of H bonds directly with the chains and with electrolyte molecules that are still absorbed.

PBI‐type Polymers and Acidic Proton Conducting Ionic Liquids – Conductivity and Molecular Interactions

“…The Fick's diffusion coefficientD [TfO] of [TfO] in m-PBI at a temperature of 100°C has a value of about 1.9 Á 10 -10 cm 2 s -1 . This is 3 orders of magnitude lower compared to the analogue doping process with H 3 PO 4 [12]. This may be caused by the apparent bigger molecular size of [TfO] and a spatial steric hindrance.…”

“…PBI is widely used in HT-PEFCs as a proton-conducting electrolyte matrix, because of its high decomposition temperature and excellent thermal and chemical stability. The basic imidazole moieties of the PBI chains are protonated by H 3 anions interact with the polymer chains by strong coulomb forces and the additional H 3 PO 4 molecules by H-bonds [6]. Mobile charge carriers for protons are formed by a strong autoprotolysis, i.e., H 4 PO 4 + and H 2 PO 4 -, hence providing a high proton conductivity at low water concentrations.…”

Fractal diffusion in high temperature polymer electrolyte fuel cell membranes

“…1759 1111) 41 was obtained from Fumatech BWT GmbH, Bietigheim-Bissingen, Germany in membrane sheets with a thickness of about 40 µm. It is the same material used in earlier studies 42 . After drying them in an ultra high vacuum, to get rid of unwanted synthesis wasteproducts, they were cut into pieces of desired size in inert gas atmosphere.…”

“…It has also to be noted that the composition of the phosphoric acid in the membrane is not identical to that in the doping bath 42 . According to ref.…”

“…According to ref. 42, at 386 wt% mass uptake, the concentration of the imbibed phosphoric acid would be 96-97%. As a caveat, we note that our way of preparation was not identical to the one used in that publication.…”