Synergetic effects of inorganic sepiolite filler and electroactive polyaniline on the physical and charge transport properties of polyaniline-layered phosphonated PVA–sepiolite membranes

Abstract

Thermomechanically stable and cation exchange membranes are desired in high-temperature polymer electrolyte fuel cell (PEMFC) applications. The currently used membranes, such as Nafion®, face stability problems and dehydrate at high temperatures in fuel cells thus losing their efficiency. To develop a thermally stable and highly cation-exchanging membrane, this study elucidates the influence of two structural modifications: the addition of inorganic filler (i.e., sepiolite (SEP)) incorporated in phosphonated polyvinyl alcohol (PVA) membranes and surface layering of electroactive polyaniline (PANI) at the surface. This research specifically elucidates how these modifications affect membranes' physical and charge transport properties. The SEP content was varied from 0.5 to 10%, whereas a thin PANI layer was deposited using in situ vapor-phase polymerization of aniline at the surface of crosslinked PPVA matrix membranes. These membranes were characterized using FTIR spectroscopy and dynamic mechanical analysis (DMA) and, ion exchange capacity (IEC), water uptake, and proton conductivity were also measured. The charge transport mechanism was evaluated using electrochemical impedance spectroscopy (EIS). The hydrophilic nature of PANI and SEP enhanced proton conductivity up to 96 mS cm⁻¹ at a water uptake of 81%, whereas IEC was measured up to 2.08 meq. g⁻¹ at 5% of sepiolite content. The storage modulus and loss tangent showed membrane stability up to 80 °C, with the storage modulus as high as 320 MPa. In the EIS, equivalent circuit modelling was used to study various resistive and capacitive processes at the electrode/membrane interface and through the membrane bulk. SEP content increased bulk membrane capacitive charging owing to higher water retention in the membranes. Surface PANI layering significantly improved the charge transport, reduced the overall resistance, and increased the capacitance by 2–3 orders of magnitude across both electrode and bulk phases. This work validates a strategy for developing robust PEM system for PEMFCs by precisely adjusting physical and charge transport properties.