Resumen de Development of proton exchange membranes based on synergistic combination of polybenzimidazole, ionic liquids and zeolites/zeotypes for high temperature pem fuel cells
This thesis is based on a compilation of scientific papers, which addresses the development of multicomponent electrolyte membrane for High Temperature Polymer Electrolyte Membrane Fuel Cells (HT PEMFC). The general objective of this thesis is the exploration and validation of new membranes components for HT-PEMFCs based on synergic combination of micro-porous zeolite type materials, room temperature ionic liquids (RTILs) and polybenzimidazole (PBI) polymer. Therefore the specific objectives of this work refer to the synthesis and characterization of the different materials with adequate properties for further assembly.
The motivation of this work is the current movement towards sustainable and more efficient power production has shifted the bias from the conventional fuels and internal combustion engines and increased the interest on alternative fuels and power sources. Fuel cells (FC) are electrochemical devices that convert chemical energy of the reactants directly into electricity and heat with high efficiency. Electrochemical processes in fuel cells are not governed by Carnot’s law and therefore high operating temperature is not necessary for achieving high efficiency. Furthermore, contrary to internal combustion engines, the efficiency of fuel cells is not strongly dependent on operating power. High efficiency makes fuel cells an attractive option for a wide range of applications, including road vehicle power sources, distributed electricity and heat production, and even portable and mobile systems, for example consumer electronics. Fuel cells are generally classified according to the nature of the electrolyte, each type requiring particular materials and fuel. Each fuel cell type also has its own operational characteristics, offering advantages to particular applications.
In this work Polymer Electrolyte Membrane (PEM) is studied, because of its versatility and superior advantages. Nevertheless, there are still many challenges to achieve; among these the related with fuel cell proton exchange membranes (PEMs), which constitutes the core of this thesis. By working at higher temperatures (120 – 200 ºC) the electrochemical process becomes more efficient and therefore cheaper. In particular, the Fuel Cell operation at high temperature is desirable since at temperatures over 120 ºC overcome most of the functional problems currently associated with PEMFCs such as catalyst CO poisoning, water management, efficiency (polarization effects and electrochemical reaction rates), and cogeneration possibilities are greatly improved. In this high temperature scenario, the most important challenges are related to the electrolyte performance and durability, and also to the fuel utility (cross-over phenomena).Therefore, the scope of this work is the High Temperature Polymer Electrolyte Membrane Fuel Cell (HT PEMFC), which is considered one of the most promising alternative power sources especially for sub-megawatt scale applications. Fuel cells compared with liquid electrolytes, can efficiently produce energy from different fuels other than oil, including hydrogen and methanol. There are a variety of technological processes that can be used in hydrogen production (chemical, biological, electrolytic, photolytic, thermochemical, etc.) either from fossil fuels (natural gas or coal) or renewable energy sources (biomass, solar, wind, hydro and tidal). Each technology has a different level of development, each offering different opportunities and benefits. In the case of methanol, is mainly produced by controlled fermentation of sugars and other carbohydrates, therefore, is also considered a renewable energy source.
This thesis has been carried out by the financial support from the European Commission through the FP7 project ZEOCELL (Nanostructured electrolyte membranes based on polymer/ionic liquids/zeolite composites for high temperature PEMFCs; http://ina.unizar.es/zeocell) Grant Agreement 209481. This project puts forward an innovative and alternative concept to overcome the current limitations of commercial available Polymer Electrolyte Membrane Fuel Cells (PEMFCs), based on the use of multifunctional materials, capable to withstand temperatures in the range 120 – 200 ºC with the following properties: i) high ionic conductivity, ~ 100 mS cm-1 at temperatures above 150 °C and dry conditions; ii) high chemical, mechanical and heat stability up to 200 °C; iii) lower fuel crossover effect and iv) cost competitive. Within the context of this thesis, three types of electrolyte membrane compositions have been studied. All of the electrolyte membranes comprise at least one of the following basic materials: polybenzimidazole (PBI), room temperature ionic liquids (RTILs) and inorganic nanocrystals (zeolites/zeotypes). This new approach is based on the special properties of each single material. Therefore, by the synergic combination of these materials, a multicomponent electrolyte membrane with advanced properties should be achieved.
As result, three different membranes emerged from this work as potential electrolytes for high temperature PEMFCs under non humidified conditions: i) nanostructured membrane based on PBI-RTIL–H3PO4–ETS10 combination; ii) cross-linked poly [1-(3H-imidazolium) ethylene] bis (trifluoromethanesulfonyl) imide membrane from copolymerization with 2.5 % mol of divinylbenzene; iii) supported poly [1-(3H-imidazolium) ethylene] bis (trifluoromethanesulfonyl) imide –PBI membrane. Therefore, the feasibility of use of zeolites and zeotypes as raw materials for solid electrolyte membrane fabrication in Fuel Cell applications has been demonstrated within this thesis. Particularly, the incorporation of ETS10 to polymer electrolyte membranes for high-temperature PEMFCs has been reported here for the first time. The development of nanostructured membrane based on PBI, microcrystals and RITLs proved the efficiency of ionic liquids as ion conductors for PEMs. In order to find the most suitable ionic liquid for Fuel Cells applications, the conductivity performance of six commercial available room-temperature ionic liquids (RTILs) was evaluated from room conditions up to 200 ºC as a function of relative humidity for their potential application in high temperature PEMs. In particular, ammonium and imidazolium based ionic liquids with different counterions and substituents were investigated. The photo-polymerization of protic monomer ionic liquids (MILs) with vinyl polymerizable functionalities upon exposure to UV irradiation in presence of the photo initiator was successfully carried out. The new polymeric ionic liquid (PIL) films demonstrated high potentialities as electrolyte membranes in high temperature PEMs. Finally, two different approaches to enhance the durability of polymeric ionic liquid (PIL) membranes were attempted in this work. Thereby, highly conductive membranes, in absence of H2O and H3PO4 proton carriers, were prepared by using ultraviolet radiation-induced polymerization of ionic liquids either in presence of divinylbenze as cross-linker agent or upon infiltration on a preexisting highly porous (80% in porosity) PBI container (PIL-PBI). Following these strategies, the fluidity properties of the PEM are notably improved for high temperature applications, but the conduction performance was negatively affected: 1139 mS cm-1 for pure PIL vs. 371 and 309 mS cm-1 at 200 ºC for crosslinked PIL, respectively. Durability tests for more than 40 days yield to steady state conductivity values above 250 mS cm-1 at 200 ºC. The chemical resistance of PIL based membranes to methanol vapors is notable improved by PIL infiltration on the porous PBI due to the hydrogen bonding type interactions between the reactive benzimidazole group from the PBI and the ionic moieties from the PIL network.
MEAs prepared from prepared membranes discussed above were validated in H2/O2 single cell from room conditions up to 180 °C under non humidified conditions. Considering the polarization curves of dense PBI membranes under identical conditions, the “proof of concept” for the proposed electrolyte membranes could be considered as demonstrated. Furthermore, these results would be clearly amenable for improvement by tailoring MEA assembly due to the surface roughness of the nanostructured electrolyte membrane.
