Incorporation of Graphene and Graphene-Based Materials into Membranes As an Alternative Electrolyte Configuration for Low and High Temperature Polymeric Electrolyte Membranes Fuel Cells.

Abstract

The electrolyte plays an important role in the Membrane Electrode Assembly (MEA) of Polymeric Electrolyte Membranes Fuel Cells, (PEMFCs), having three main functions, to act as ion conductor, electronic insulator and a separator for the reactant gases [1]. The electrolyte is a Proton Exchange Membrane and Nafion is the most widely used polymeric membrane in this technology. However, because its particular structure it requires a good hydration to lead a high proton conductivity, which limit the operating temperature to lower than 100 oC. Poly[2,2′-m-(phenylene)-5,5′-benzimidazole (PBI) membrane doped with Phosphoric Acid, has been presented as an alternative to Nafion membranes allowing higher operating temperatures, 200oC, which enhance the kinetics of the reactions. However, during the operation of High Temperature Fuel Cells (HT-PEMFCs) these membranes suffer Phosphoric Acid leaching leading to a low proton conductivity and degradation of the fuel cell [2].The 2D honeycomb lattice of Graphene provides this material particular properties such as high surface area, exceptionally high electrical conductivity, good chemical and thermal stability and excellent barrier properties, which make it a potential material for a large number of applications. In addition to this [1], Hu. et al. [3] have demonstrated that Single Layer Graphene present a high proton conductivity being at the same time a barrier for other elements, which makes this material a potential electrolyte for PEMFC. However, SLG has some drawbacks notably due to the complex process to produce it and its high cost. These limitations leave an open window to Graphene based materials such as Graphene Oxide, which shows very attractive properties to act as filler in Nafion and PBI membranes due to its chemical functionalization and its non-porous structure, which provides additional resistance for mass transport and increases the tortuosity of the permeation pathways leading to higher selectivity [1].Here we will describe the work carried out at University of Manchester incorporating SLG and GO composite membranes into the MEA of both PEM Fuel Cells, Low Temperature (LT-PEMFC) and HT-PEMFC. The addition of the SLG produced by CVD into the MEA of LT-PEMFC has demonstrated no change in the proton conductivity, supporting the hypothesis that SLG is a good proton conductor. In addition, for a particular LT-PEMFC, Direct Methanol Fuel Cells (DMFC), a decreasing of the methanol crossover has found leading a significant enhancement of the power density [4]. SLG has been also incorporated into the MEA of HT-PEMFC, which use PBI membrane instead of Nafion, to evaluate its proton conductivity and barrier properties, this time to stop the Phosphoric Acid leaching.Although positive results have been reported by adding SLG into the MEAs, a different approach has been conducted by the development of GO-composite membranes also for both type of PEMFC. Exfoliated Graphene Oxide has been synthesized by electrochemical exfoliation of graphite and has been incorporated as a filler into two different of matrix polymer, Nafion and PBI, to evaluate their performance in both PEMFC, high and low temperature.