Transport Properties in Polymer Nanocomposite Membranes Cataldo Simari

Abstract

The aim of this thesis has been to prepare and characterize innovative composite membranes for polymer electrolyte fuel cells (PEMFCs) applications. Among the different energy conversion devices based on polymer electrolytes, PEMFCs, both hydrogen (DHFC) and direct methanol (DMFC), seems to be one of the most promising clean energy technologies. As electrochemical devices able to directly convert the chemical energy of a fuel into electrical energy, PEMFCs offer interesting advantages in vehicular or portable applications , as the quick start, the high energy conversion efficiency (~ 50%), the reduced environmental impact for the low CO2 emissions (zero in the case where the primary fuel is hydrogen) and the flexibility respect to the fuel, in fact, besides hydrogen (DHFC), they can be fed for example with methanol (DMFC). However, considerable efforts are still needed to be able to achieve satisfactory performance in terms of efficiency, durability and cost for mass deployment of such technology. It is necessary to deal with some problems that concern the electrolyte membrane, such as the degradation of the materials, the low proton conductivity at low relative humidity (RH) and poor mechanical properties at temperature higher than 130 °C. Therefore, the development of high-performance proton conducting polymer electrolyte membranes is critical for the optimal power density and efficiency a PEMFC can achieve because membrane ohmic loss is the major cause of overpotential in the operational current range of the fuel cell. In recent years, increasing interest has been devoted to the development of high temperature proton conducting polymer electrolyte fuel cell systems. In fact, most of the shortcomings associated with the lowtemperature PEMFC technology based on perfluorosulfonic acid (PFSA) membranes can be solved or avoided by developing alternative membranes with suitable ionic conductivity and stability up to 150 °C. The increasing the operational temperature would result in increased performance of the cell because of easier and more efficient water management, higher reaction rates to the electrodes, improved CO tolerance by the anode electro-catalysts, faster heat rejection rates and better systems integration. It has been mentioned the possibility to feed PEMFCs systems with other fuel respect to hydrogen. In particular, direct methanol fuel cells (DMFCs) combine the merits of polymer electrolyte fuel cells fueled by H2 with the advantages of a liquid fuel, such as easy handling and high energy density. However, despite these advantages, also regard this devices there are still technical barriers to overcome for their widespread commercialization such as methanol crossover from anode to cathode through the proton exchange membrane. From the above, it is thus highly important to enhance the proton conductivity of the electrolyte membrane under low RH in order to accomplish higher PEMFCs performance. On the other hand, is essential to develop polymer electrolytes with reduced methanol cross-over for DMFC. The work presented in this thesis is the result of a Ph.D. project carried out during a period of about three years from 2012 – 2015, in the Physical Chemistry Soft Matter Laboratory “Mario Terenzi” (PC_SM Mario Terenzi) at the Department of Chemistry and Chemical Technologies in the University of Calabria. The thesis was written as part of the requirements for obtaining the doctor of philosophy degree. The overall objective of this doctoral thesis was to design, synthesize and evaluate innovative composite electrolytes with specific properties suitable for PEM fuel cells that operate at high temperatures (above 100 ° C ) and low RH and/or with low methanol permeability. To this purpose, three main classes of materials have been explored as nanoadditives to create nanocomposite membranes: (i) organo-modified TiO2 nanoparticles, (ii) layered materials based on clays (anionic and cationic) and graphene oxide and (iii) hybrids clays-carbon nanotubes. While, as concern the ionomers, perfluorosulfonic acid (Nafion®) and polyaromatic polymers (sulfonated Polyether Ether Ketone and Polybenzimidazole) have been evaluated. In my doctoral porject an attempt was made to conjugate an intense basic research in order to understand the molecular mechanisms at the basis of ionic conduction in such complex systems, and the design, synthesis and more comprehensive characterization of new nanocomposites with opportune requisites. For this purpose an deep study of the transport properties of the water confined within the electrolyte membranes has been performed by NMR spectrocopy (diffusometry, relaxometry and 1H spectral analysis) together to a wide physico-chemical, mechanical and electrochemical characterization in order to achieve a systematic understanding at a fundamental level of the effects of dimensionality, architecture and organization of these nanofillers on the properties of the ionomers and to exploit this knowledge for the preparation of high performance electrolytes. Some of the electrolytes membranes investigated during my PhD thesis were prepared and studied in the framework of the PRIN Project: NAMED-PEM “Advanced nanocomposite membranes and innovative electrocatalysts for durable polymer electrolyte membrane fuel cells”. The last part of this thesis concerns a research work arisen from a collaboration with ITM-CNR of the University of Calabria, on the Ion Exchange Membranes for Reverse Electrodialysis (RED) process. Here, the NMR techniques were used to study the water dynamics in anion- and cation- exchange membranes (AEMs and CEMs) in order to achieved additional important insights about the effect of the electrolyte solution, on membrane microstructure and its transport and electrical properties. The results of this research have been published in scientific international Journals and reported in appendix to the end of the thesis. During these years I have spent two stages periods abroad: 1) in the “Department of Materials Science and Engineering of the University of Ioannina, Ioannina (Greece)”, where I worked under the supervision of Prof. D. Gournis, my research has been focused on the synthesis of novel carbon-based materials as additives for nanocomposite membranes; 2) in Department of Physics & Astronomy of the Hunter College, New York (USA), where I worked under the supervision of Prof. S. Greenbaumn, I performed the High Pressure NMR investigation of water and methanol transport properties in sPEEK-based nanocomposite electrolytes. Two scientific papers, based on the results obtained during these stages, have been recently submitted and also reported in appendix