Graphene oxide-based composite membranes as novel electrolytes for PEM fuel cells

This work presents an innovative method for the functionalization of graphene oxide (GO) with sulfonic acid groups (-SO3H) equivalent to those of Nafion®. The aim is to manufacture self-assembling sulfonated graphene oxide (SGO) membranes to be studied as a novel proton conductor for polymer electrolyte membrane (PEM) fuel cells. Sulfonation has been carried out by reacting a commercial aqueous dispersion of GO with sulfuric acid, studying the effect of different volumes of acid on the structure and the degree of functionalization of the membranes. Thus, an optimal acid-to-GO molar ratio has been identified, considering an empirical formula of GO derived, as a first approximation, from the elemental analysis of the dispersion. SGO membranes have been characterized by ATR-FTIR, XRD, SEM and EDX spectroscopies, thermogravimetric analysis, optical microscopy and static contact angle measurements. Their water uptake (WU), ion exchange capacity (IEC) and degree of sulfonation (DS) have been evaluated as well. Sulfonated samples showed an improved WU behaviour compared to both Nafion® and unfunctionalized GO, specifically at low temperature and humidity. An IEC value above 1 meq/g has also been measured, which is even higher than IEC determinations reported for Nafion®, and a correlation among acid-to-GO molar ratio, WU, IEC and DS has been established. Then, a preliminary test in a lab-scale hydrogen-fed fuel cell has been performed on the membrane corresponding to a 1:1 molar ratio. A low open circuit voltage (0.63 V) has been detected at 40 °C, which may be ascribed either to hydrogen crossover issues or to a partial reduction of the basal plane of GO, possibly causing an undesired increase in the electrical conductivity and a consequent internal current loss. Nonetheless, the membrane displayed an encouraging mechanical resistance, and it was verified that, after the fuel cell test, the active area presented no carbon residues left by the gas diffusion electrode (GDE), which are a typical problem in the case of Nafion®. These findings confirmed that freestanding SGO membranes are a potential candidate for the replacement of Nafion® in fuel cell systems, especially in conditions of high temperature and low humidity. In fact, such circumstances are the most severe for the operation of conventional ionomers, due to a dramatic drop in proton conductivity caused by dehydration. Further tests are ongoing and different routes are currently under study in order to enhance the structure and the properties of sulfonated membranes along with their energy generation efficiency.