Proton conducting membranes in fully anhydrous conditions at elevated temperature: Effect of Nitrilotris(methylenephosphonic acid) incorporation into Nafion- and poly(styrenesulfonic acid)

Nitrilotris(methylenephosphonic acid) (NTMPA) was incorporated in various proportions into Nafion and poly(styrenesulfonic acid) (PSSA) to make membranes with proton conductivity under fully anhydrous conditions. NTMPA is anchored to the polymer matrix by ionic bond between the basic nitrogen and the acidic sulfonic group of the polymer. The NTMPA molecule also possesses six phosphonic terminals with relevant hydrogen-bonding activity, enabling proton transport by a hopping mechanism even in the absence of water. Morphological and structural characteristics of these composite membranes were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Thermal stability was assessed by TGA. Proton-conduction up to 190°C was investigated by electrochemical impedance spectroscopy. Moreover, 31P solid-state nuclear magnetic resonance (NMR) spectroscopy was applied to investigate condensation of the phosphonic groups at elevated temperature. The Nafion/NTMPA composites were homogeneous at the molecular level and exhibited excellent thermal stability. These membranes were flexible, robust and translucent. For the PSSA/NTMPA composite a heterogeneous structure was observed with bulk, micrometer-size NTMPA particles dispersed in the polymer matrix. The membranes appeared opaque and fragile, but exhibited fairly good thermal resistance. For both classes of membranes, moderate condensation of the P-OH terminals leading to dimeric phosphonic and mixed sulfonic/phosphonic anhydrides was observed after prolonged thermal treatment at elevated temperature (>170°C) and zero humidity. However, this process is completely reversed by humidification. Both Nafion/NTMPA and PSSA/NTMPA composite membranes displayed good proton conductivity (>10-2Scm-1) under fully anhydrous conditions at elevated temperature. Importantly, the present work demonstrates a general and versatile concept to design water-free proton-conducting membranes rationally and straightforwardly. That is anchoring a compound with proton-solvent characteristics to a suitable matrix. These ideas are applicable utilizing vast classes of polymer matrices and proton conducting fillers.