Proton C 1 Onductive Membranes Based on 2 Carboxylated Cellulose Nanofibres and Their Proton 3 Exchange Membrane Fuel Cell Performance

The performance of carboxylated cellulose nanofibers (CNF) membranes has been measured in-situ as a function of CNF surface charge (600 and 1550 µmol g-1), membrane thickness and fuel cell relative humidity (RH 55 to 95 %). The structural evolution of the membrane as a function of RH has been measured by Small Angle X-ray scattering, showing that water channels are formed above 75 % RH. The amount of absorbed water depends on the membrane surface charge and counter ions (Na+ or H+). The high affinity of CNF for water and the high aspect ratio of the nanofibers, together with a well-defined and homogenous membrane structure, ensures a proton conductivity exciding 1 mS cm-1 at 30 °C between 65 and 95 % RH, around two orders of magnitude larger than previously reported values and only one order of magnitude lower than Nafion 212. Moreover, the CNF membranes are characterized by a lower hydrogen crossover than Nafion, despise been ≈ 30 % thinner. Thanks to their environmental compatibility and promising fuel cell performance the CNF membranes should be considered for new generation PEMFCs.