Hybrid inorganic–organic proton conducting membranes based on Nafion and 5 wt% of M xO y (M = Ti, Zr, Hf, Ta and W). Part II: Relaxation phenomena and conductivity mechanism

Abstract

In this report, we are presenting studies of the effect of M x O y nanopowders on the thermal, mechanical and electrical properties of [Nafion/(M x O y ) n ] membranes with M = Ti, Zr, Hf, Ta and W and n = 5 wt%. Five homogeneous membranes with thicknesses ranging from 170 to 350 μm were studied. The thermal transitions characterizing [Nafion/(M x O y ) n ] materials were investigated by modulated differential scanning calorimetry (MDSC). The mechanical parameters and relaxation processes were studied on temperature by dynamical mechanical analyses (DMA). Broadband dielectric spectroscopy (BDS) was used to study the conductivity mechanism and dielectric relaxation events in bulk materials. DMA investigations showed two distinct relaxation events. The first relaxation phenomenon, detected at about 19 °C, was attributed to the mechanical β-relaxation mode of Nafion. The second event, revealed in the temperature range 100–135 °C, was assigned to the mechanical α-relaxation mode of Nafion. The electric response of membranes was studied by BDS measurements in the frequency and temperature range respectively of 40 Hz–10 MHz and 5–135 °C. Real and imaginary components of permittivity ( ɛ*( ω) = ɛ′ ( ω) − i ɛ″( ω)) and conductivity spectra ( σ*( ω) = σ′( ω) + i σ″( ω)) were analyzed. Conductivity spectra allowed us to accurately determine the σ dc of membranes at 100% RH as a function of T. Two relaxation peaks were detected in the ɛ*( ω) profiles. The low frequency relaxation event was attributed to the α-relaxation mode of fluorocarbon chains of Nafion. The high frequency relaxation peak corresponds to the β-relaxation of acid side groups. The results allowed us to conclude that M x O y influences: (a) the relaxations of both the hydrophobic and the hydrophilic domains of Nafion polymer host; (b) the thermal stability range of conductivity (SRC) and the σ dc of membranes. In conclusion, nanofillers affect the macromolecular dynamics of Nafion-based polymer host owing to the formation of dynamic cross-links, R–SO 3H⋯M x O y ⋯HSO 3–R, in hydrophilic polar cages. The membranes doped with HfO 2 and WO 3 oxoclusters present a stability range of conductivity of 5 °C ≤ T ≤ 135 °C and give rise to σ dc values of respectively 2.8 × 10 −2 and 2.5 × 10 −2 S cm −1 at 135 °C and 100% RH. These latter conductivity values make the nanocomposite membranes based on HfO 2 and WO 3 oxoclusters very promising materials for the application in polymer electrolyte fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs).