Synthesis and characterization of non-fluorinated membrane for fuel cell applications

In the challenging path toward the energy transition, the World is facing the urgent need to reduce greenhouse gas emissions and gradually move away from fossil fuels. In this context, fuel cells are emerging as one of the most promising technologies for producing clean and sustainable energy, as they can directly convert the chemical energy of fuels, such as hydrogen, into electricity. The versatility of fuel cells enables a wide range of applications, from hydrogen-powered vehicles to the generation of electricity and heat for residential, commercial, and industrial uses. In a global energy scenario increasingly focused on sustainability, efficiency, and security of supply, fuel cells represent not only an advanced technological solution but also a key element in the future energy mix. In line with the sustainability goals defined by European and international standards, this Thesis focuses on the development of new polymeric membranes to be used as solid electrolytes in proton-exchange membrane fuel cells (PEMFC) that are more environmentally friendly, cost-effective, and, at the same time, high-performing and durable compared to those currently used. This ambitious objective was pursued through a comprehensive research plan including the design and synthesis of new non-fluorinated polymers, followed by their detailed thermal, mechanical, and morphological characterization, and finally by their electrochemical testing—carried out, in some cases, directly on electrochemical devices assembled with the newly developed membranes. Specifically, this work focuses on the development of polymer membranes obtained through chemical (Chapters I and V) and morphological-structural (Chapter IV) modifications of commercial, widely available polymers such as poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) and poly[2,2′-m-(phenylene)-5,5′-bibenzimidazole] (m-PBI), suitable for low and high-temperature fuel cell applications, respectively. Chapter I provides an overview of the global energy context and the fundamentals of fuel cell technology, emphasizing the need for non-fluorinated polymer membranes that meet modern environmental and performance standards. Chapter II and III describes in detail the chemical, physical, mechanical, and electrochemical characterization techniques used throughout the work. Chapter IV presents the synthesis and characterization of sulfonated poly(2,6-dimethyl-1,4-phenylene)oxide (sPPO) membranes, obtained by sulfonating PPO to impart key properties such as hydrophilicity, water uptake, and proton conductivity. Structural analyses, including FTIR spectroscopy, wide-angle X-ray diffraction (WAXD), as well as thermal, mechanical, and electrochemical tests, allowed the evaluation of proton conductivity as a function of temperature and humidity. The results are discussed in comparison with commercial benchmarks such as Nafion® and Pemion®, highlighting the main strengths and weaknesses of these different materials. Chapter V focuses on PBI-based membranes designed for high-temperature PEM applications. In particular, it explores innovative production techniques such as electrospinning and examines in detail the influence of fibrous morphology on the chemical, physical, and electrochemical properties of the membranes. Finally, Chapter VI focuses on the synthesis and characterization of a new aminated derivative of PPO to be used in high temperature PEM applications. Overall, this Thesis contributes to the challenging goal of developing fuel cell components that combine high performance with environmental and economic sustainability. The results obtained, though preliminary, are very promising and are intended to serve as a foundation for future studies and as a stimulus for the scientific community to continue exploring innovative polymer materials through chemical and structural modifications.