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.