Composite membranes added with graphene oxide and functionalized graphene oxide for anion and proton exchange membrane water electrolyzers

In recent years, interest in electrolysis for hydrogen production has increased, particularly as a means of utilizing surplus electricity from intermittent renewable sources. A promising approach to reduce CO2 emission is, in fact, the integration of hydrogen—produced using carbon-free electricity through advanced conversion technologies —into gas, chemicals, and fuels. Electrolysis, in particular, offers an environmentally friendly method of producing pure hydrogen as an energy carrier, utilizing electricity from renewable sources. In this regard Proton Exchange Membrane Water Electrolyzers (PEMWEs) and Anion Exchange Membrane Water Electrolyzers (AEMWEs) are among the most promising devices, but they still face some problems related to the stability and conductivity of the polymer electrolyte, that hinder their widespread use. Here in this thesis work we have focused on developing composite membranes in order to mitigate these problems. To do so, different amounts of Graphene Oxide (GO) and functionalized GO were added to the commercial Fumion ionomer (for AEMWE) and Nafion ionomer (for PEMWE) to obtain composites membranes with increased stability and conductivity. In the case of Anion Exchange Membranes (AEMs), bare GO, synthesized using the Modified Hummers Method, was first investigated to have information about the effect of the additive inside the polymeric matrix. Then GO was functionalized with quaternary ammonium groups using three different innovative and simple syntheses and the resulting composite AEMs were studied to investigate the effect of the GO quaternization. In the case of Proton Exchange Membranes (PEMs), GO was functionalized with phosphonate groups using the Arbuzov reaction. The composite PEMs prepared with this additive were characterized and the effect of the functionalization and quantity of the additive was studied. All the additives were characterized using different techniques like TGA, SEM, ATR-FTIR, Raman Spectroscopy, XRD and XPS in order to obtain information on the morphology, structure and the chemical nature of the samples. In the same way all the Ionic Exchange Membranes were characterized both at a fundamental and applicative level, using TGA, SEM, ATR-FTIR, Ionic Exchange Capacity (I.E.C.), Water Uptake (W.U.), alkaline stability (for Fumion-GO), stress-strain curves and conductivity, as well as electrolysis cell tests to evaluate the materials performances and stability. The first chapter of the thesis is dedicated to the scientific context (Chapter 1), in which significant attention is given to the explanation of the degradation phenomena, since these studies have been explored in depth and are included in a review that is currently under publication. The following chapter focuses on the materials and experimental methods (Chapter 2) and is followed by three chapters dedicated to the different types of composite membranes, namely AEMs added with Graphene Oxide (Chapter 3), AEMs added with Quaternized Graphene Oxide (Chapter 4), and PEMs added with Phosphonated Graphene Oxide (Chapter 5). Finally, the last chapter (Chapter 6) presents the conclusions and possible future perspectives.