Development of film nanocomposite membranes from nanocrystalline cellulose combined with reduced graphene oxide or graphene oxide: Evaluation of potential applications in proton exchange membranes

Abstract

Raw materials from renewable sources are actually of interest to design materials with functional properties for use in the proton exchange membranes (PEMs) field, and thus replace the costly based-Nafion PEMs. Corncob (CC), the by-product of harvesting grains, is rich in lignocellulose polymers, which can be extracted and processed through pretreatments and hydrolysis chemical reactions to remove hemicellulose (H), lignin (L) and amorphous cellulose (C) to obtain nanocrystalline cellulose (CNC). The electrochemical, mechanical and water up-take properties of CNC can be adjusted by adding reduced graphene oxide (rGO) or graphene oxide (GO) promoting a synergistic effect among the components to increase the proton conductivity and improve the operating temperature of PEMs. This work deals with analyzing the pretreatment and acid hydrolysis (1:100 ratio, 1 h) through monitoring structural phases after the elimination of H, L and amorphous C; and the effect of temperature (50, 60 and 70 °C) to obtain CNC on crystallinity and crystal/fiber sizes is studied. The hydrolysis time (1 and 1.5 h) at 60 °C was also considered. The Results indicate the coexistence of phases I-α, I-β and II after pretreatments and the disappearance of phase II after acid hydrolysis. The increase in temperature was key inducing high crystallinity percentages (from 52% to 68%), crystallite sizes (from 2 to 4 nm) and diameter of microfiber (from 18 to 25 nm). Hybrid PEMs based on optimal CNC samples (60 °C, 1 h and 1.5 h) with GO and rGO (0.5 and 1.5 wt%) were processed via casting knife to evaluate moisture uptake as well as structural, mechanical and electrochemical performance. A shorter hydrolysis time (CNC1.0 h) induces a higher hardness value compared to CNC1.5 h; the optimal sample was found with rGO (CNC1.0 h:rGO1.5 wt%). Hydrogen bonding between GO or rGO was found to be key improving the water adsorption capacity and dimensionality at 60 °C (CNC1 h:GO 0.5 wt% and CNC1.5 h:rGO1.5 wt%). The impedance evaluated at 50, 60, 65 and 70 °C indicates a decrease with temperature. CNC1.5 h, had better impedance values more than CNC1.0 h. The greatest proton conductivity was found for CNC1.5 h:rGO 0.5 wt%. Therefore, these proposed and ecofriendly CNC:rGO and CNC:GO materials can fulfil the required properties for PEMs applications to replace Nafion membranes.