Investigation of Sulfonated Graphene Oxide as the Base Material for Novel Proton Exchange Membranes

Abstract

This work deals with the development of graphene oxide (GO)-based self-assembling membranes as possible innovative proton conductors to be used in polymer electrolyte membrane fuel cells (PEMFCs). Nowadays, the most adopted electrolyte is Chemours’ Nafion; however, it reveals significant deficiencies such as strong dehydration at high temperature and low humidity, which drastically reduces its proton conductivity. The presence of oxygenated moieties in the GO framework makes it suitable for functionalization, which is required to enhance the promising, but insufficient, proton-carrying features of GO. In this study, sulfonic acid groups (–SO3H) that should favor proton transport were introduced in the membrane structure via a reaction between GO and concentrated sulfuric acid. Six acid-to-GO molar ratios were adopted in the synthesis procedure, giving rise to final products with different sulfonation degrees. All the prepared samples were characterized by means of TGA, ATR-FTIR and Raman spectroscopy, temperature-dependent XRD, SEM and EDX, which pointed out morphological and microstructural changes resulting from the functionalization stage, confirming its effectiveness. Regarding functional features, electrochemical impedance spectroscopy (EIS) as well as measurements of ion exchange capacity (IEC) were carried out to describe the behavior of the various samples, with pristine GO and commercial Nafion® 212 used as reference. EIS tests were performed at five different temperatures (20, 40, 60, 80 and 100 °C) under high (95%) and medium (42%) relative humidity conditions. Compared to both GO and Nafion® 212, the sulfonated specimens demonstrate an increase in the number of ion-carrying groups, as proved by both IEC and EIS tests, which reveal the enhanced proton conductivity of these novel membranes. Specifically, an acid-to-GO molar ratio of 10 produces a six-fold improvement of IEC (4.23 meq g−1) with respect to pure GO (0.76 meq g−1), while a maximum eight-fold improvement (5.72 meq g−1) is achieved in SGO-15.

Study of Innovative GO/PBI Composites as Possible Proton Conducting Membranes for Electrochemical Devices

“…Pure PBI pure GO showed expected values of TS (23.0 ± 6.6 and 105.7 ± 11.9 MPa, respectively) YM (0.9 ± 0.3 and 8.6 ± 0.4 GPa, respectively) [22,30,32,33,39,50]. The main information In the end, the fourth stage (dark grey) started at approximately 500 • C. It was related to the breakdown of the GO framework [38], to which the contribution of the partial decomposition of the PBI carbonaceous backbone was added at 600 • C [16,29,49]. Pure PBI demonstrated the largest residual mass at 900 • C (70.5%), such that even the lowest polymer content in GO/PBI 3:1 permitted to have a 15%-heavier residue with respect to pure GO.…”

“…The second stage (light grey), detected roughly between 160 and 220 • C for the composites and pure GO, was more prominent than the first one. It was ascribed to the decomposition of the oxygenated functional groups within GO [37,38]. Larger mass drops were measured while the GO mass content increased in the composites.…”

“…Ion exchange capacity (IEC) is a parameter estimating the total active sites responsible for ion exchange per unit mass of a material, and it is typically measured in milliequivalents per gram (meq g −1 ). Acid-base back-titration is the main technique through which IEC can be assessed [38]. In this work, the methodology used for the determination of the IEC of the GO/PBI X:Y composite membranes involved at first a preliminary 1-h drying in oven at 60 • C of the samples, so as to assess their dry masses.…”

“…In the case of the GO/PBI X:Y samples, moisture retained by the GO paste obtained in step (2) of the designed procedure was supposed to be the mainly accountable factor. The removal of traces of residual solvent still present after step (5) could have partially occurred in this range as well [34,38], although its contribution could be considered as negligible. The compatibility between GO and PBI appeared to have a significant influence on the thermal properties of the composite membranes.…”

“…In the end, the fourth stage (dark grey) started at approximately 500 °C. It was rel to the breakdown of the GO framework [38], to which the contribution of the partial composition of the PBI carbonaceous backbone was added at 600 °C [16,29,49]. Pure demonstrated the largest residual mass at 900 °C (70.5%), such that even the lowest p mer content in GO/PBI 3:1 permitted to have a 15%-heavier residue with respect to p GO.…”