Durability Enhancement of Anion Exchange Membrane Based Fuel Cells (AEMFCS) And Water Electrolyzers (AEMELs) By Understanding Degradation Mechanisms

The past several years have seen remarkable progress in renewable energy sources such as solar and wind, showing excellent efficiency and significant cost reduction. The intermittent nature of renewable resources requires them to be integrated with energy storage systems that can hold or convert this excess energy into other forms. Though batteries have been the most popular, it is also possible to store the energy as chemicals and fuels, such as hydrogen (H2). Hydrogen generation from water electrolysis has been receiving significant attention in recent years. Among the existing water electrolysis technologies, anion exchange membrane electrolyzers (AEMELs) are the most immature and have thus been receiving a high level of interest in the research community. Anion exchange membrane fuel cells (AEMFCs), which operate in the reverse direction to produce energy from hydrogen and oxygen, are also a nascent technology that suffers from lower efficiency and poor durability. For both AEMELs and AEMFCs, there is a growing body of literature pushing for the design of new materials to concomitantly increase performance and durability. This thesis describes an effort to address the limitations of AEMELs and AEMFCs. First, the effect of electrode design and cell operation on the performance and stability of AEMELS will be discussed, elaborating the current challenges in this technology and how to approach them in a scientific manner. It will be shown that the detailed understanding of device operation is essential before new materials can be fairly evaluated and their impact on the performance and durability elucidated. Specifically, highly durable AEMELs were created and operated over 500 hours at 1.0 A/cm2 with no significant degradation, both on dilute alkaline feed as well as pure water. The pros and cons of alkaline feed are also discussed. Secondly, device designs were conceived that increased the durability of AEMFCs. This thesis reports an optimized high performing AEMFC that was stably operated continuously for 3600 hours (150 days) with a very low degradation rate. Moreover, at the end of life, the cell was disassembled and subjected to a number of experiments probing the physical and chemical degradation that occurred during normal operation – including high resolution STEM imaging and chemical mapping. Well-described physical and electrochemical evolution of the fuel cell operation, degradation mechanisms and pathways leading to both reversible and irreversible performance loss were identified. Finally, advances in both AEMELs and AEMFCs were used to improve the efficiency and cycling durability of AEM unitized regenerative fuel cells (AEM-URFCs). Considering all of the work in this thesis, the main contribution of the work was the unveiling of the variables that limit both the performance and lifetime of anion-exchange membrane-based devices. In particular, new designs for electrodes were conceived and insights into the behavior of operating cells (particularly related to water) were attained. It is expected that the new learning in this document will allow future researchers to take active steps towards improving performance, durability, and commercial viability