Dottorato in ENERGIA E AMBIENTE

Titolo della tesi: Development of graphene oxide/Nafion polymeric membranes toward the improvement of Direct Methanol Fuel Cell membranes

Nowadays the direct methanol fuel cell (DMFC) technology attracts a lot of interest because it represents a low temperature, high efficiency power source that can be advantageously used for mobile and portable applications and thus, it can be a valid alternative to the use of lithium battery technology. The DMFC is an electrochemical system that produces energy by oxidizing the liquid fuel (a mixture of water and methanol) without auxiliary devices. The use of DMFC has several advantages: easy fuel storage, low cost of methanol, operation at low temperature and pressure, small system size and low weight. However, several issues hinder the spreading of this technology. Firstly, the fuel cell membrane is generally composed by Nafion, a perfluorosulfonic polymer. Such membrane is prone to allow the passage of methanol, the so called cross-over, that strongly reduces DMFC performance. Furthermore, high cost and limited operating temperature range represent a strong limit to the commercialization of that technology. Research activities are focused on developing new polymer electrolyte membrane (PEM) materials aiming at overcoming the crossover. Among several types of material, graphene oxide (GO) has been considered as a promising element that offers great results in terms of water uptake and mechanical properties. Several authors have reported that GO contributes to reduce methanol permeability because it acts as a barrier, due to its higher tortuosity, while proton conductivity shows an opposite trend. In addition, it is well known that temperature, methanol concentration as well as flow rate affect proton conductivity and methanol crossover and, consequently, DMFC performance. It is therefore necessary to investigate what are the best conditions to make better use of this innovative material. The objective of this study is to assess the potential of GO in improving DMFC performance. To this end, several composite Nafion/GO membranes with different GO loading were manufactured using casting method. The internship at the School of Chemical Engineering at the University of Birmingham, allowed me to learn the methodology to prepare and characterize polymeric membranes. The composite membranes demonstrated higher mechanical strength, enhanced water uptake but lower proton conductivity than recast Nafion. Once the optimum loading was estimated, the performance of the DMFC, in a passive configuration, was analysed through the analysis of the polarization and power curves. It was revealed that the DMFC performance was enhanced by increasing the temperature. The DMFC performance increases when using GO membranes when increasing methanol concentration and flow rate. However, it is necessary to use the appropriate range of methanol concentration and anode flow rate. Extending the anode flow rate and methanol concentration has a dual effect: increasing the flow of the reactant allows to obtain higher performance despite enhancing the methanol crossover and losses. At one point, the loss will be no longer counterbalanced, and performance starts decreasing. Comparing the results with those of recast Nafion, it was demonstrated that by utilising GO-Nafion composite membranes, an increase in the maximum power density, open circuit condition and operating range, at all operating conditions, can be achieved. So, the detriment of proton conductivity was counterbalanced by the reduction of fuel cell crossover.