Inorganic–organic membranes based on Nafion, [(ZrO2)·(HfO2)0.25] and [(SiO2)·(HfO2)0.28] nanoparticles. Part II: Relaxations and conductivity mechanism

Two classes of hybrid inorganic–organic proton-conducting membranes consisting of Nafion and either [(ZrO2)·(HfO2)0.25] or [(SiO2)·(HfO2)0.28] nanofiller are investigated to elucidate their relaxations and conductivity mechanism and are labeled [Nafion/(ZrHf)x] and [Nafion/(SiHf)x], respectively. The membranes are studied by dynamic mechanic analysis (DMA) and broadband electric spectroscopy (BES). The latter technique allows a determination of the direct current ionic conductivity (σDC) and the proton diffusion coefficient (DH+)(DH+). Pulse-field-gradient spin-echo nuclear magnetic resonance experiments (PFGSE-NMR) are carried out to determine the water self-diffusion coefficients (DH2O)(DH2O). DH+DH+ and DH2ODH2O are correlated to obtain insight on the conductivity mechanism of the proposed materials. Results indicate that the nanofiller particles play a major role in the proton conduction mechanism of the proposed materials. It is demonstrated that the basic [(ZrO2)·(HfO2)0.25] nanoparticles form Nafion–nanofiller dynamic cross-links with high ionic character. These cross-links improve the mechanical properties and enhance the overall proton conductivity of the membranes at low humidification levels owing to an efficient delocalization of the protons. In [Nafion/(SiHf)x] membranes, the dynamic cross-links occur due to dipole–dipole interactions between the side groups of the Nafion host polymer and the quasi-neutral [(SiO2)·(HfO2)0.28] nanoparticles. These cross-links significantly reduce the delocalization of the protons, which decreases the overall conductivity of materials.

► Two new “core–shell” nanofillers [(ZrO2)·(HfO2)0.25] and [(SiO2)·(HfO2)0.28] were prepared. ► The nanofillers were dispersed into Nafion to obtain two series of proton-conducting membranes. ► The membranes were investigated by DMA, BES and PFGSE-NMR to study their relaxations and conductivity mechanism. ► The introduction of the fillers significantly affected the structure and the molecular dynamics of the host matrix. ► The ionic character of the Nafion–nanofiller interactions proved crucial to modulate the proton conductivity.