Proton Conductivity Behavior of a H3PO4 Doped Membrane Employing a Semi-Interpenetrating Polymer Network

Research on renewable and less-polluting energy sources became essential for the development and sustainable use of natural resources. Devices such as fuel cells (FC) are pointed as an environmentally attractive alternative to conventional energy sources. Low-temperature fuel cells employ a solid polymeric electrolyte, typically Nafion, which shows decreasing in the conductivity with increasing temperature. Several polymeric systems have been intensively studied as possible Nafion’s substitutes, including interpenetrating polymer networking and, in a very low extension, semi-interpenetrating polymer network (SIPN) [1].

In the present work, semi-interpenetrating polymer network (SIPN) membranes based on the diglycidyl ether of bisphenol-A (DGEBA) crosslinked with 4,4’diaminodiphenyl-sulphone (DDS) and poly(styrene-co-allyl alcohol) (PSAA) were obtained and characterized by electrochemical impedance spectroscopy. These membranes were previously obtained and characterized at 100% relative humidity, keeping the electrode connected cell immersed in water [2] and the conductivity values stayed beyond 10^-5 and 10^-6 S.cm^-1 for membranes doped in H3PO4 5% aqueous solutions. It was observed that membranes of certain compositions showed loss of its dimensional stability.

The aim of this work was to verify the membranes conductivity behavior with the increase in charge carriers using a different experimental array keeping 70 % of relative humidity during the conductivity analysis.

     Cure reactions in solution were carried out in order to obtain Semi-IPN membranes from the DGEBA, DDS and PSAA. Semi-IPN membranes containing PSAA in mass ratios of 33, 38, 41, 44, 47 and 50% (w/w) were obtained. DGEBA and PSAA were dissolved in ethanol at 70 °C and a solution containing DDS was slowly added under constant stirring. The solutions were then heated to 130 °C under reflux until the reaction was completed, transferred to Petry dishes and dried under vacuum. Semi-IPN membranes were doped in H3PO4 20% aqueous solutions during 24 h and dried for proton conductivity determinations. 0.707 cm² membranes were sandwiched between stainless steel electrodes for Electrochemical Impedance Spectroscopy (EIS) studies, using an Autolab PGSTAT30/FRA, with frequency between 1 MHz and 10 mHz at 20, 30, 40, 60 and 80 °C, at 70 % of relative humidity. All EIS determinations were performed in triplicate, so the conductive values represent the average conductivity for each sample. The proton conduction resistance (R) values were determined at the intercept of the EIS spectra with the real axis and the conductivity (σ) was calculated from: σ = L / A × R , in which L is the membrane thickness (in cm) and A its geometrical area.

            The characteristic ion-conductive behavior was observed in all spectra recorded, regardless of the temperature. In the high frequency region, a semicircle associated with the ion transport resistance is observed and, in the low frequency region, a straight line associated with the electrode/electrolyte interface capacitive effects. The utilization of stainless steel blocking electrodes induces a polarization phenomenon inside the membrane, contributing to the capacitive effects mentioned.

Membranes containing 33 and 50 % PSAA exhibited conductivity values between 10–5 to 10-3 S.cm–1, from 20 to 80 ^oC, and a tendency of increasing conductivity with temperature, evidencing a thermally activated process.

Maximum conductivity values were obtained for the Semi-IPN membrane containing 50 % PSAA at 60 ^oC, (4.72 ± 0.31)×10^-3, and 33 % PSAA at 40 ^oC, (4.85 ± 1.49)×10^-3.

These values, are greater than the others described in the previous work in which it was employed H₃PO₄ 5 % aqueous solution for doping, showing the effect of an increased number of charge carriers (H3O+ from dissociated acid) on the conductivity.

            For the other samples it was observed conductivity values in the order of 10^-4 and 10^-5 S.cm^-1, and a decrease in the conductivity values at higher temperatures.

From this work it was observed that conductivity values can be modified according to the doping degree in the SIPN studied. Additionally, it was indicated that different macromolecular arrangements arising from changes in the SIPN composition have influence on the conductive behavior. In certain samples, it was verified loss of the dimensional stability with temperature increase. This behavior was previously observed in the analyses of EIS at 100 % of relative humidity and is probably originated on the different macromolecular arrangements, as early discussed, which can promote the release of PSAA macromolecules, the comercial polymer component used in the SIPN membrane.

[1] L. Chikh, V. Delhorbe, O. Fichet, Journal of Membrane Science. 368 (2011) 1.

[2] F.A.M. Loureiro, E.S. de Marins, G.D.C. dos Anjos, R.P. Pereira and A.M. Rocco, Polímeros. 24 (2014) 49.

Acknowledgements:

Authors would like to thank FAPERJ and CNPq for support.