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.
Highlights
Nowadays, graphene oxide (GO) is considered among the most attractive graphenerelated materials
Pristine GO was characterized by three main weight losses at an increasing temperature
The first one, which occurred below 100 ◦ C and was about 15%, was associated with the removal of physically adsorbed water on the hydrophilic groups of GO planes [18,26]
Summary
Graphene oxide (GO) is considered among the most attractive graphenerelated materials. It can be described with good approximation as a one-atom-thick twodimensional sheet of carbon atoms arranged in a texture similar to a honeycomb. The carbon atoms can be sp2 - or sp3 -hybridized and form chemical bonds with oxygenated functionalities. The random distribution of these moieties causes a nonstoichiometric atomic composition; a univocal molecular structure of GO is still under debate inasmuch as several preparation factors can directly influence it, such as the graphite source, the oxidizing agent and the reaction conditions. The presence of oxygenated functionalities endows GO with an amphiphilic nature that explains its dispersibility in aqueous media
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