Electrolyte performance in proton exchange membrane fuel cells

Fernihough, Oliver Edward (2025). Electrolyte performance in proton exchange membrane fuel cells. University of Birmingham. Ph.D.

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Abstract

This thesis explores the production and performance of polymer electrolyte membranes patterned using Laser Induced Periodic Surface Structures (LIPSS). The study investigates how microscale patterning impacts the hydrophobicity, water management, and overall performance of the membranes in fuel cells. Patterns including flat, lotus, lines, and sharklet were created on steel moulds and transferred to Nafion membranes. Contact angle measurements and scanning electron microscopy (SEM) images confirmed that the patterning significantly increased the hydrophobicity of the membrane surfaces, with the sharklet pattern exhibiting the highest contact angle.

The bulk properties of the patterned membranes were assessed by evaluating ion exchange capacity (IEC), water uptake, and membrane swelling. The results indicated stable IEC across all patterns, while the sharklet pattern demonstrated the best water management, reducing dimensional change without affecting water uptake. Hydrogen crossover measurements revealed that patterns with greater depth exhibited higher crossover currents, suggesting thinner regions in the membrane.

Polarization curve analyses showed that the sharklet pattern achieved the highest power density, with a significant increase at both high and low relative humidity levels. This enhancement was attributed to the larger electrochemical surface area (ECSA) and improved water transport properties, confirmed through cyclic voltammetry and impedance spectroscopy. The sharklet pattern also maintained lower high-frequency resistance under varied humidity conditions, indicating better ionic conductivity and reduced ohmic losses.

In contrast, the lotus and lines patterns showed increased ohmic resistance and diminished performance under low humidity conditions, highlighting the importance of optimizing pattern feature size and depth. These findings suggest that while patterning can significantly enhance membrane performance, careful design is crucial to avoid adverse effects on functional properties.

In conclusion, this research demonstrates that microscale patterning, particularly the sharklet design, can significantly improve the mechanical and transport properties of polymer electrolyte membranes, leading to sustained fuel cell performance under diverse operational conditions. Future work should focus on refining the pattern designs, ensuring scalability, and assessing the long-term durability of the patterned membranes in practical fuel cell systems.

Additionally, this thesis investigates the performance and modelling of Polymer Electrolyte Fuel Cells (PEFCs) utilizing Nafion 211 membranes under various operational conditions, focusing on elevated temperatures (80°C, 100°C, and 120°C) and relative humidities (40-100%). The research aims to elucidate how these conditions affect fuel cell performance and develop robust models for predicting fuel cell behaviour. Experimental results indicate that while increasing temperature enhances mass transport and mitigates water flooding, it also leads to significant dehydration and reduced ionic conductivity, particularly beyond 100°C. At higher temperatures, membrane hydration decreases, causing a rise in ohmic resistance and a decline in overall performance.

High-pressure operation significantly enhanced performance by maintaining better water balance and ensuring adequate oxygen availability. The study extends empirical relationships for water content (lambda, \(\lambda\)) and ionic conductivity to higher temperatures, highlighting the necessity for accurate fitting parameters such as water activity. 1D modelling techniques were employed, showing good agreement with experimental results in the ohmic region of the polarization curve, thus validating their use for pre-emptive design adjustments and operational strategies.

The research also delves into the degradation mechanisms affecting the membranes, identifying the need for developing new materials or modifying existing perfluorosulfonic acid (PFSA) membranes to enhance thermal stability and water retention. The findings emphasize the complex interplay between thermal, mechanical, and chemical degradation pathways and the importance of robust membrane formulations.

The study concludes that while operating Nafion-based fuel cells at higher temperatures offers potential benefits, it also presents significant challenges. Higher operational pressures, necessary to maintain adequate humidity levels and oxygen availability, increase the energy requirements for air pumps and water vaporization. Despite these challenges, the research outlines a clear path forward for advancing PEFC technology through targeted improvements in membrane materials, operational strategies, and predictive modelling. The findings provide a foundation for developing more robust, efficient, and durable PEFC systems, marking a significant step forward in sustainable energy technologies. Further research into high-pressure operational scenarios could unlock new potentials for overcoming current performance limitations in fuel cell technologies.

Lastly, this thesis investigates the performance and modelling of composite graphene oxide Nafion (GO-Nafion) membranes for polymer electrolyte fuel cells (PEFCs) operating at intermediate temperatures (80-120°C). The experimental study involved fabricating GO-Nafion membranes via solution casting, incorporating 4~wt% graphene oxide. The performance of these membranes was evaluated using polarization curves and impedance spectra at 80°C, 100°C, and 120°C across various relative humidity levels (40-100%).

The results demonstrated that GO-Nafion membranes enhanced water retention and overall performance at elevated temperatures compared to pure Nafion. However, significant performance degradation occurred at lower relative humidity due to membrane dehydration. At 80°C, GO-Nafion membranes maintained performance across a wide humidity range, while at 100°C and 120°C, performance was more sensitive to humidity changes, with notable declines at lower humidity levels.

Two modelling approaches were developed to predict the performance of the GO-Nafion membranes: a polynomial model based on a rule-of-mixtures approach and a power-law model. The polynomial model accurately predicted performance at 80°C (R²>0.96) and 100°C (R²>0.84) but was less accurate at 120°C (R²<0.57) due to complex dehydration and reactant crossover effects. The power-law model showed reasonable fits across all temperatures but lacked a clear physical basis, limiting its application for full fuel cell modelling.

Incorporating the polynomial model into a single-cell fuel cell model revealed good agreement with experimental data at 80°C and reasonable fits at 100°C, though the model overpredicted mass transport resistance at high humidity. At 120°C, the model struggled to capture performance accurately, particularly at low relative humidity.

Overall, this thesis provides valuable insights into the potential of GO-Nafion membranes for intermediate temperature PEFCs and presents a comprehensive modelling framework. While the polynomial model shows promise for predicting water sorption behaviour and membrane performance, further refinement is needed for higher temperature predictions. Future work should explore advanced water sorption models and comprehensive mass transport effects to enhance predictive accuracy across all temperatures.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):