Enhanced proton transport properties of Nafion via functionalized halloysite nanotubes

The introduction of naturally occurring fibrillary structured halloysite clays (HNTs) into the Nafion matrix resulted in an enhanced proton conductivity and a reduced activation energy for proton transport. To assess the effect of the various states of the HNTs as a membrane additive, the sigma(+)(H) of the composite membranes (Nafion/HNTs, Nafion/HNTs-NH2, Nafion/HNTs-SO3H) was measured at 30 degrees C and 80 degrees C and at different levels of relative humidity (%RH). An overall improvement of the sigma(+)(H) of the Nafion was observed in the presence of the nanotubular additives (HNTs or HNTs-NH2) which can be attributed to the high specific surface area accommodating hydrophilic functional groups and therefore contributing to the water retention/management in the composite membrane. However, the Nafion/HNT-SO3H composites are distinguished in terms of enhanced performance in sigma(+)(H) which persist in the whole range of % RH levels and noted that the sigma(+)(H) values are almost 2 times higher than native Nafion in the low humidity region (30%-50%) and at 80 degrees C. In agreement with the sigma(+)(H) enhancement, the activation energy is lower than that of native Nafion indicating that the proton transport is facilitated in the presence of -SO3H modified HNTs, probably due to an improved connectivity and arrangement of ionic conducting domains. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.