Transport Properties in Polymer Nanocomposite Membranes Cataldo Simari

The aim of this thesis has been to prepare and characterize innovative composite\nmembranes for polymer electrolyte fuel cells (PEMFCs) applications. Among the\ndifferent energy conversion devices based on polymer electrolytes, PEMFCs, both\nhydrogen (DHFC) and direct methanol (DMFC), seems to be one of the most promising\nclean energy technologies. As electrochemical devices able to directly convert the\nchemical energy of a fuel into electrical energy, PEMFCs offer interesting advantages in\nvehicular or portable applications , as the quick start, the high energy conversion\nefficiency (~ 50%), the reduced environmental impact for the low CO2 emissions (zero\nin the case where the primary fuel is hydrogen) and the flexibility respect to the fuel, in\nfact, besides hydrogen (DHFC), they can be fed for example with methanol (DMFC).\nHowever, considerable efforts are still needed to be able to achieve satisfactory\nperformance in terms of efficiency, durability and cost for mass deployment of such\ntechnology. It is necessary to deal with some problems that concern the electrolyte\nmembrane, such as the degradation of the materials, the low proton conductivity at low\nrelative humidity (RH) and poor mechanical properties at temperature higher than 130\n°C.\nTherefore, the development of high-performance proton conducting polymer electrolyte\nmembranes is critical for the optimal power density and efficiency a PEMFC can\nachieve because membrane ohmic loss is the major cause of overpotential in the\noperational current range of the fuel cell. In recent years, increasing interest has been\ndevoted to the development of high temperature proton conducting polymer\nelectrolyte fuel cell systems. In fact, most of the shortcomings associated with the lowtemperature\nPEMFC technology based on perfluorosulfonic acid (PFSA) membranes\ncan be solved or avoided by developing alternative membranes with suitable ionic\nconductivity and stability up to 150 °C. The increasing the operational temperature\nwould result in increased performance of the cell because of easier and more efficient\nwater management, higher reaction rates to the electrodes, improved CO tolerance by\nthe 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\nhydrogen. In particular, direct methanol fuel cells (DMFCs) combine the merits of\npolymer electrolyte fuel cells fueled by H2 with the advantages of a liquid fuel, such as\neasy handling and high energy density. However, despite these advantages, also regard\nthis devices there are still technical barriers to overcome for their widespread\ncommercialization such as methanol crossover from anode to cathode through the\nproton exchange membrane.\nFrom the above, it is thus highly important to enhance the proton conductivity of the\nelectrolyte membrane under low RH in order to accomplish higher PEMFCs\nperformance. On the other hand, is essential to develop polymer electrolytes with\nreduced methanol cross-over for DMFC.\nThe work presented in this thesis is the result of a Ph.D. project carried out during a\nperiod of about three years from 2012 – 2015, in the Physical Chemistry Soft Matter\nLaboratory “Mario Terenzi” (PC_SM Mario Terenzi) at the Department of Chemistry\nand Chemical Technologies in the University of Calabria. The thesis was written as part\nof the requirements for obtaining the doctor of philosophy degree.\nThe overall objective of this doctoral thesis was to design, synthesize and evaluate\ninnovative composite electrolytes with specific properties suitable for PEM fuel cells\nthat operate at high temperatures (above 100 ° C ) and low RH and/or with low\nmethanol permeability.\nTo this purpose, three main classes of materials have been explored as nanoadditives to\ncreate nanocomposite membranes: (i) organo-modified TiO2 nanoparticles, (ii) layered\nmaterials based on clays (anionic and cationic) and graphene oxide and (iii) hybrids\nclays-carbon nanotubes. While, as concern the ionomers, perfluorosulfonic acid\n(Nafion®) and polyaromatic polymers (sulfonated Polyether Ether Ketone and\nPolybenzimidazole) have been evaluated.\nIn my doctoral porject an attempt was made to conjugate an intense basic research in\norder to understand the molecular mechanisms at the basis of ionic conduction in such\ncomplex systems, and the design, synthesis and more comprehensive characterization of\nnew nanocomposites with opportune requisites. For this purpose an deep study of the\ntransport properties of the water confined within the electrolyte membranes has been\nperformed by NMR spectrocopy (diffusometry, relaxometry and 1H spectral analysis) together to a wide physico-chemical, mechanical and electrochemical characterization\nin order to achieve a systematic understanding at a fundamental level of the effects of\ndimensionality, architecture and organization of these nanofillers on the properties of\nthe ionomers and to exploit this knowledge for the preparation of high performance\nelectrolytes.\nSome of the electrolytes membranes investigated during my PhD thesis were prepared\nand studied in the framework of the PRIN Project: NAMED-PEM “Advanced\nnanocomposite membranes and innovative electrocatalysts for durable polymer\nelectrolyte membrane fuel cells”.\nThe last part of this thesis concerns a research work arisen from a collaboration with\nITM-CNR of the University of Calabria, on the Ion Exchange Membranes for Reverse\nElectrodialysis (RED) process. Here, the NMR techniques were used to study the water\ndynamics in anion- and cation- exchange membranes (AEMs and CEMs) in order to\nachieved additional important insights about the effect of the electrolyte solution, on\nmembrane microstructure and its transport and electrical properties.\nThe results of this research have been published in scientific international Journals and\nreported in appendix to the end of the thesis.\nDuring these years I have spent two stages periods abroad:\n1) in the “Department of Materials Science and Engineering of the University of\nIoannina, Ioannina (Greece)”, where I worked under the supervision of Prof. D.\nGournis, my research has been focused on the synthesis of novel carbon-based materials\nas additives for nanocomposite membranes;\n2) in Department of Physics & Astronomy of the Hunter College, New York (USA),\nwhere I worked under the supervision of Prof. S. Greenbaumn, I performed the High\nPressure NMR investigation of water and methanol transport properties in sPEEK-based\nnanocomposite electrolytes.\nTwo scientific papers, based on the results obtained during these stages, have been\nrecently submitted and also reported in appendix