• Aili, David

Abstract

Proton transport is one of the most fundamental phenomena in nature and it also plays key role in proton exchange membrane (PEM) fuel cells and water electrolyzers. Conventionally, a well hydrated membrane of perfluorosulfonic acid (PFSA) such as Nafion® (DuPont) is used as proton conductor and electrode separator in both PEM fuel cells and water electrolyzers. The proton conductivity mechanism of Nafion® is strongly dependent on the presence of water within the membrane nanostructure, which limits the operating temperature to about 80 °C unless the system is pressurized in order to keep the membrane well hydrated. However, some of the main issues of the conventional PFSA based PEM fuel cells and water electrolyzers are directly or indirectly related to their relatively low operating temperature. An elevated operating temperature results in better electrode kinetics in general and improved impurity tolerance of the catalysts, especially for the fuel cell anode. From a system engineering point of view it also allows for simplified cooling and water management and better possibilities for heat recovery. Furthermore, a higher operating temperature is strongly beneficial when the cell is operated in electrolysis mode since it results in a decreased thermodynamic energy requirement for the water splitting reaction. The aiming for an elevated operating temperature has thus resulted in an increasing demand for novel anhydrous proton conducting polymeric materials for both PEM fuel cells and water electrolyzers. This thesis gives an overview of the principles and the current state-of-the-art technology of the hydrogen based electrochemical energy conversion technologies, with special emphasis on the PEM based water electrolyzers and fuel cells (Chapter 1). The fundamental thermodynamics of water electrolyzers and fuel cells is also explained. A detailed literature review is given that covers proton conducting polymeric materials and composite membrane concepts as well as the mechanisms of proton conduction in these types of structures. The experimental part of this thesis has focused on the development and characterization of polymer based proton conducting membranes for operation at temperatures above 100 °C. The most frequently recurring experimental methods and techniques are described in Chapter 2. For PEM steam and liquid water electrolysis attemperatures up to 130 °C (Chapter 3 and 4), the presented approaches in this work mainly include chemical modification of the conventional PFSA materials with phosphoric acid or inorganic proton conducting particles such as zirconium phosphate (ZrP) or boron phosphate (BP). A novel methodology for the preparation of homogenous Nafion®/poly[2,2´(m-phenylene)-5,5’-bibenzimidazole] (PBI) polymer blends was also developed. The miscibility behavior between Nafion® and PBI was investigated using different Nafion® counter cations. It was demonstrated that NH4⁺ could be used as compatibilizer in the Nafion®/PBI system to give homogenous solution cast blend membranes covering the whole composition range. The strong intermolecular coulomb interactions gave the polymer blends improved mechanical characteristics. However, it allowed for very limited volume swelling in phosphoric acid and therefore a very low acid uptake, which resulted in poor proton conductivity in the 10-5-10-3 S cm-1 range at 130 °C. The pristine phosphoric acid doped Nafion® and PBI membranes, on the other hand, exhibited proton conductivity in the 10-2 S cm-1 range at 130 °C, which was superior compared with that of the ZrP or BP composite materials based on Nafion®. Steam electrolysis tests were thus conducted using membrane electrode assemblies (MEAs) based on pristine phosphoric acid doped Nafion® and PBI. The PBI based MEAs suffered from severe durability limitations due to membrane degradation, which was most likely connected to the acid catalyzed hydrolysis of the polymer. The phosphoric acid doped Nafion® based MEAs, on the other hand, exhibited better durability but showed rather high ohmic resistance. The moderate steam electrolysis performance was most likely connected to the poor dimensional stability of Nafion® under the experimental conditions. The mechanical properties of the recast Nafion® membranes at elevated temperature could be slightly improved by annealing the membrane in order to increase its degree of crystallinity. Short side chain (SSC) PFSA membranes such as Aquivion™ (Solvey Solexis), on the other hand, are generally characterized by a considerably higher degree of crystallinity than Nafion®. The melting point of the large crystalline regions is also considerably higher than that of Nafion®, which allows for better mechanical characteristics at elevated operating temperatures. The low equivalent weight of Aquivion™ also resulted in high phosphoric acid uptake, which allowed for high proton conductivity. The steam electrolysis performance of the Aquivion™ based MEA was thus superi...

Topics

• impedance spectroscopy
• zirconium
• composite
• Hydrogen
• Boron
• annealing
• durability
• crystallinity
• polymer blend