Preliminary Study On The Development Of Sulfonated Graphene Oxide Membranes As Potential Novel Electrolytes For PEM Fuel Cells

Abstract

In this work, preliminary results about the use of a graphene oxide-based membrane as a possible alternative electrolyte for proton exchange membrane fuel cells (PEMFCs) are reported. Currently, one of the key components of a PEMFC is the ionomer electrolyte, typically the perfluorosulfonic acid membrane known as Nafion, due to its high proton conductivity in standard operating conditions (temperature range: 60-80 °C and relative humidity of 60-100 %), good mechanical properties and its effectiveness as gases separator. However, Nafion limits fuel cell maximum operating temperature, reduces its conductivity upon dehydration and suffers shrinkage and swelling if water content changes significantly [1]. An interesting alternative aiming to enhance FCs efficiency and to overcome drawbacks related to Nafion employment could be represented by graphene oxide (GO). GO is obtained by chemical functionalization and exfoliation of graphite and is attracting attention of the scientific community due to its easy dispersion in water, electronically insulating properties and gas barrier properties. Moreover, GO self-standing membranes have been found impermeable to many gases, whilst allowing permeation of water and minimizing fuel crossover [2]. Some works have reported the use of GO in composite membranes with known proton conductors such as Nafion, S-PEEK (sulfonated poly ether ether ketone), PBI (polybenzimidazole) and PVA (polyvinyl alcohol) [3]. Our research group has developed and characterized a pure GO membrane as a self-standing electrolytic membrane. In a typical experiment, a GO aqueous solution was prepared, then it was vacuum filtered and the cake was dried at 40 °C for 6 hours. A uniform self-assembled membrane was obtained and morphologically characterized by SEM, TGA, XRD and FT-IR analysis. A homogeneous surface and the presence of functional groups, i.e. hydroxyl, epoxy oxygen and carboxyl groups, which should be able in ensuring proper proton conductivity [4], have been achieved. A thickness of around 2 μm was measured. Since thickness is strongly related to initial GO solution concentration, other experiments are going to be done in order to increase it. After proper activation in a sulfuric acid solution, proton conductivity of the prepared membranes was determined by using impedance spectroscopy in the frequency range 1 MHz-1 Hz with an AC amplitude of 500 mV. Preliminary measurements were performed at 25 °C and variable relative humidities (RHs), ranging from anhydrous conditions to 100 %. RH was controlled by using sealed-off glass cell specifically designed to be immersed in a thermostatic bath with saturated aqueous solutions of different salts. A value of 8 mS/cm was found at 25 °C and anhydrous conditions. This is a very promising result since at the same operating conditions Nafion 212 has shown a conductivity of almost one magnitude lower. Conductivity increased upon increasing RH but such effect was less sharp than the one observed for Nafion and needs further tests and detailed investigation. Measurements at different temperatures, as well as in-situ electrical tests, are in progress and will be shown during the conference.