We have developed new composite membranes of sulfonated polyimide containing triazole-groups (SPI-8) as a matrix ionomer and SiO2 nanoparticles. The incorporation of SiO2 nanoparticles remarkably improved the fuel cell performances during low humidity operation at 53% RH and 80 degrees C. Among the cells with SPI-8 membranes with uniformly dispersed SiO2 from 0 to 15 wt%, the single cell with 10 wt% SiO2/SPI-8 was found to exhibit the highest I - E performance, with the highest mass activity at 0.85 V and the smallest oxygen-transport overpotential (O-2-gain) as well as the lowest ohmic resistance. This strongly indicates that SiO2 nanoparticles were able to promote the back-diffusion of water produced in the cathode catalyst layer to the anode catalyst layer, maintaining high Water content in the membrane during the operation. It was found that the cell with a bilayer SPI-8 membrane having 10 wt% SiO2 in the anode-side layer and 3 wt% SiO2 in the cathode-side layer exhibited performance superior to that with a uniform dispersion of 10 wt%SiO2, especially in the higher current density region at low RH, which can be ascribed with certainty to the fact that the concentration gradient of SiO2 in the SPI-8 led to enhancement of the back-diffusion of water through the membrane (C) 2014 Elsevier Ltd. All rights reserved.