Understanding the Role of Gas Diffusion Layer for Pressurised PEM Unitised Regenerative Fuel Cells

In this PhD thesis, the role of gas diffusion layer (GDL) in pressurised proton exchange membrane (PEM) unitised regenerative fuel cells (URFCs) is investigated. URFCs play a crucial role in the energy transition by efficiently integrating hydrogen production and electricity generation that is important for the applications that space utilisation is of great importance. The research aims to deepen the understanding of the oxygen-side GDL's role in PEM URFC performance, exploring necessary modifications to enhance this performance, particularly in the fuel cell mode of operation. In particular, the study sought to develop methods for estimating the electrical conductivity of metallic GDLs used in PEM URFCs and electrolysers, investigate the impact of GDL hydrophobisation on fuel cell-mode performance, and analyse the role of GDL alongside other parameters in hydrogen crossover in both electrolyser and fuel cell modes. The research methods implemented in this study included an extensive literature review, theoretical studies, and experimental investigations. The literature review involved critically examining a wide range of existing peer-reviewed research articles, conference proceedings, books, theses, and relevant technical publications. This phase was instrumental in establishing a detailed understanding of the current state of the art in GDL technology and identifying gaps in the field relevant to PEM URFCs. The theoretical aspect of this research utilised established models and approaches, including the rule of mixture, critical porosity, and tortuosity concepts, for estimating the electrical conductivity of sintered metal fibres. Darcy's law was applied for pressure drop estimations across porous media, and Fick’s law of diffusion was used in estimating the hydrogen crossover phenomenon during both fuel cell and electrolyser operation modes. The experimental component was conducted in RMIT University’s Sustainable Hydrogen Energy Laboratory (SHEL) by focusing on the effects of PTFE coating, i.e., as a means for GDL hydrophobisation, on the performance of PEM URFC. These experiments were crucial in translating small-scale single-cell PEM URFC findings to broader applications in larger-scale stacks, ensuring the scalability and practical application of the research. The small-scale PTFE coating of the GDLs was performed in-house and for the large-scale samples, external facilities were utilised. In these studies, the effect of PTFE coating was of interest at the lower temperature range, which has not received the needed attention in the published research. The research led to the development of innovative semi-empirical models for estimating the electrical conductivity of metallic GDLs, providing a substantial improvement over existing models. These new models particularly offer good accuracy for sintered metal fibres, a material mainly used in PEM URFCs as well as electrolysers due to their favourable properties, especially corrosion resistance. The theoretical models developed for hydrogen crossover in both fuel cell and electrolyser modes offered valuable insights into the complex interplay of operational parameters such as pressure, temperature, current density, and the physical properties of the GDL and catalyst layer, namely thickness and permeability. These models highlighted the critical role of the GDL in influencing hydrogen crossover, a phenomenon essential to the efficiency and safety of URFCs. It was found that GDL compression that affects the GDL permeability can have a marked effect on hydrogen supersaturation, a phenomenon that plays a major role in hydrogen crossover during water electrolysis. The study revealed experimentally that PTFE coating on GDLs had a considerable impact on fuel cell performance, particularly at lower operating temperatures. While low PTFE loading maintained electrical conductivity and did not have a negative impact on contact resistance, higher loadings resulted in decreased performance due to reduced porosity. These findings underlined the importance of carefully balancing hydrophobic properties depending on the desired operating conditions. The research effectively addressed critical questions and objectives. Through the comprehensive literature review, the study identified key attributes and modifications of the oxygen-side GDL that impact PEM URFC performance, including porosity, fibre diameter, and hydrophobisation techniques. The newly developed methodologies for estimating GDL bulk electrical conductivity eliminated the need for complex numerical models of GDL, which can be incorporated into numerical studies on PEM URFCs to study the effect of GDL porosity. The experimental investigations into the effects of PTFE coating revealed its significant role in enhancing fuel cell performance under varying operating conditions, providing valuable insights for optimising URFC design based on the desired operating conditions, especially practical operating temperature. The development of the models for hydrogen crossover enhanced the understanding of this phenomenon and provided the possibility of incorporating hydrogen crossover in 3D numerical models of both fuel cell and electrolyser operations. This enables a more accurate estimation of overall efficiency and the possible effects of hydrogen crossover on durability. Additionally, it assists in determining the safe operational conditions, especially the current density range to ensure hydrogen concentration in oxygen stays within the safe limits. This is a critical aspect of maximising the efficiency of the hydrogen generation and reducing the size of hydrogen production systems utilising water electrolysis, resulting in the reduction of both CAPEX and OPEX. Future research directions include further exploration into the mechanical properties of metallic GDLs and their behaviour during assembly. Additional experimental studies are required to explore the effects of PTFE coating under diverse operating conditions and hydrogen feed modes. Investigating the microporous layer for the oxygen side of PEM URFCs is crucial to understanding its impact on improving water management and durability. Moreover, considering the substantial impact of contact resistance on ohmic losses in electrochemical devices, investigating the effect of porosity and potentially fibre diameter on contact resistance are recommended. The inclusion of the developed hydrogen crossover models in the numerical modelling of PEM URFCs as well as fuel cells and electrolysers can provide further insight into this phenomenon and support investigating its effect on the durability of the membrane.