SIPN Systems Used as Membranes for Fuel Cell
The development of new membranes in order to replacement of Nafion ®, with greater thermal stability, chemical, electrochemical, and high conductivity is one of the key to reducing the manufacturing costs of PEMFC. Many studies were found in literature working with composites and nanocomposites based on Nafion®, which does not eliminate the polymer dependence . Several studies involve commercial polymers sulfonation  and others employ basic polymers, among them poliimidazol and polibenzimidazol . From these basic polymers are formulated complexes with strong acids, particularly H3PO4, which act as a source of protons to membrane charge transport. New polymeric materials have been explored, such as interpenetrating polymer networks (IPN) . Using this semi-IPN systems allow to control the free volume and dimensional mechanical stability. The semi-IPN (SIPN) is defined as “a polymer comprising one or more networks linear or branched polymer(s) characterized by the penetration on a molecular scale of at least one of the networks by at least some of the linear or branched macromolecule .
In current work the SIPN are based in monomer diglycidyl ether of bisphenol A (DGEBA), using curing agent4,4' diaminodiphenyl-sulphone (DDS) and the linear polymer polyethyleneimine (PEI) as described in previous work by Loureiro .
The membrane SIPN 47% of PEI was doped with phosphoric acid in varying concentrations in 5, 10 and 15, 20 and 30% (w/w). For comparison the same sample was sulfonated (SIPN47-SO3H) procedure adapted from Rocco et.al.  by employing nominal degree of sulfonation of 1:4, 1:2, 1:1 and 2:1. After the sulfonation procedure, membranes were kept in a desiccator under vacuum.
Samples are sulfonated at different ratios and were characterized by FTIR, TGA, DSC and Electrochemical Impedance Spectroscopy (EIS).
The thermal study by TGA and DSC to the SIPN47 show the high stability of membrane. The TGA and DTG curves evidence an initial weight loss at approximately 80-85oC, associated with elimination of adsorbed water. SIPN membrane exhibited 5.2 % weight losses, indicating the amount of water retained in the samples. Since the samples were dried prior to the analysis, the amount of water eliminated was probably rapidly adsorbed during the sample handling, evidencing a highly hygroscopic behavior. SIPN samples degradation starts at approximately 280oC, which corresponds to the decomposition temperature. The SIPN membranes exhibit water elimination in a broad temperature range, up to 200oC. This is a very promising characteristic of the studied system, since water retention above 100 oC is one of the limiting features of polymer membranes for fuel cells, which, operating at temperatures higher than 120 oC, avoid the catalyst poisoning by CO. The DSC study show the higher stability of SIPN sample, which show one only Tg at 67oC, and there aren’t another behavior of degradation, in order the only one Tg indicates that the system produced is fully miscible.
The Impedance study show the dependence of the conductivity with higher values of acid doped achieving high proton conductivity values 8,29x10-2W-1cm-1 at 80oC. On the other hand sulfonated SIPN samples do not show the same behavior, there aren’t significant increase with increasing degrees of sulfonated in the SIPN samples. Table I and II lists the conductivity values at 20 and 80 oC, as well as log(A0) and activation energy values for the SIPN/H3PO4 and SIPN-SO3H membranes at differents sulfonated degrees.
Table I: Conductivity, log(A0) and activation energy values obtained for IPN/H3PO4 membranes.
Table II: Conductivity, log(A0) and activation energy values obtained for IPN-SO3H membranes.
The conductivity increase with the temperature is a consequence of different factors, including: higher mobility of water molecules, higher acid dissociation and homogenization of the membranes nanostructure, which allows the proton transport among different hydrated nanodomains inside the membrane.
The high thermal stability exhibited by the SIPN membranes, above 270oC, is sufficient for their application in PEMFC for intermediate temperature operation and the SIPN produce is fully miscible. The membranes doped with H3PO4 at 20% shows higher values of conductivity 0,08 W-1cm-1 than the best sulfonated sample degree 10-3 W-1cm-1.
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