Elucidating the Water and Methanol Dynamics in Sulfonated Polyether Ether Ketone Nanocomposite Membranes Bearing Layered Double Hydroxides

Abstract

Conventional Nafion membranes demonstrate a strong affinity for methanol, resulting in a high fuel crossover, poor mechanical stability, and thus poor performance in direct methanol fuel cells (DMFCs). This study involves the synthesis and physiochemical characterization of an alternative polymer electrolyte membrane for DMFCs based on sulfonated poly(ether ether ketone) and a layered double hydroxide (LDH) material. Nanocomposite membranes (sPL), with filler loading ranging between 1 wt% and 5 wt%, were prepared by simple solution intercalation and characterized by XRD, DMA, swelling tests, and EIS. For the first time, water and methanol mobility inside the hydrophilic channels of sPEEK-LDH membranes were characterized by NMR techniques. The introduction of LDH nanoplatelets improved the dimensional stability while having a detrimental effect on methanol mobility, with its self-diffusion coefficient almost two orders of magnitude lower than that of water. It is worth noting that anionic lamellae are directly involved in the proton transport mechanism, thus enabling the formation of highly interconnected paths for proton conduction. In this regard, sPL3 yielded a proton conductivity of 110 mS cm−1 at 120 °C and 90% RH, almost attaining the performance of the Nafion benchmark. The nanocomposite membrane also showed an excellent oxidative stability (over more than 24 h) during Fenton’s test at 80 °C. These preliminary results demonstrate that an sPL3 nanocomposite can be potentially and successfully applied in DMFCs.

Nanocomposite Membranes for PEM-FCs: Effect of LDH Introduction on the Physic-Chemical Performance of Various Polymer Matrices

“…All the nanocomposite membranes were prepared by simple solution intercalation method [ 51 , 54 , 55 , 56 ]. In the case of Nafion/LDH composite membrane, 1 g of commercial Nafion resin solution was heated at 60 °C until dried from the solvents and then re-dissolved in 10 mL of DMF until a clear solution was obtained.…”

“…According to the literature, 3 wt.% of loading has been demonstrated to be the optimal one in most of the nanocomposite systems and for various types of inorganic materials [ 45 , 46 , 47 , 48 , 49 ]. Additionally, during our previous works, we have demonstrated that the introduction of 3 wt.% of LDH material can ensure the maximum improvement in terms of transport properties, conductivity values, and mechanical properties either in the case of Nafion-based [ 50 ], sPEEK-based [ 51 ], and sPSU-based [ 46 ] composite membranes. Indeed, higher filler contents resulted in particle agglomeration, membrane inhomogeneity, and thus deterioration of the physicochemical and electrochemical performance.…”

NMR Investigation of Water Molecular Dynamics in Sulfonated Polysulfone/Layered Double Hydroxide Composite Membranes for Proton Exchange Membrane Fuel Cells

“…Thereafter, a different NMR tube containing an sPSU or sPL sample was placed in the probe and analyzed. This allowed for a more rapid evaluation of how the nanoconfinement effect and interaction with the functional groups of both the polymer and the filler particles affect the chemical shift of water molecules [ 55 , 56 ]. Signal deconvolution was carried out using Origin Software, and details on mathematical model are provided in the SI.…”