Fuel cells are electrochemical devices that convert chemical energy of fuel into electricity, with water and heat as reaction byproducts. They have many promising features like high power density, good fuel efficiency, and zero GHG emissions. All these features make fuel cells great potential to be a future replacement for the fossil fueled combustion engines. Water management and flooding phenomena however has been one of the most crucial performance limiting issues under high current density conditions in proton exchange membrane fuel cells. Various strategies have been developed to address this and it is continuing topic of research in the field.

In my study, I examined a fuel cell component known as the gas diffusion layer (GDL). The GDL is often treated with hydrophobic agent like PTFE, and often incorporated with an additional layer referred to as a microporous layer (MPL) for improving the cell performance through effective water management. Many studies have been published to investigate the effect of wetproofing and MPL. But there has not been any systematic study in the literature to directly quantify the effect of GDL wetproofing and MPL. Fuel cell performance, limiting current techniques, and material characterization tools were applied for this study. Adding PTFE wetproofing and a MPL increased gas diffusion resistance slightly and therefore did not have a significant adverse effect on cell performance under dry conditions. Under wet conditions, the PTFE wetproofing could delay, but not completely mitigate the liquid water flooding in the catalyst layer. On the other hand, adding the MPL allows for stable high current density operation under wet conditions.

To further investigate transport phenomena in a fuel cell, I performed a preliminary study on an alkaline exchange membrane fuel cell (AEMFC). The preliminary results indicate that the cell performance is sensitive to electrode structure and operating conditions, such as temperature and relative humidity. These results identify key research areas for my future study

Proton exchange membrane fuel cell (PEMFC) is clean electrochemical energy generation device that generates electricity by electrochemical reactions between hydrogen and oxygen and the only byproducts of this reaction are heat and water. PEMFCs are considered to be the most promising technology to replace internal combustion engines for automotive application however for the commercialization of PEMFCs many technical challenges need to be overcome including decreasing Pt loading, increasing high current density performance, better water and thermal management and cell durability. Understanding the transport inside the PEMFC components are crucial for solving the technical challenges. In PEMFCs, most of the cell components are of nano to micro meter length scale which makes experimental characterization and diagnostics extremely difficult. Numerical models built on fundamental physics are critical for further development of this technology.

In my doctoral research I took a comprehensive simulation approach by first building a single-phase model, followed by expanding the physics towards two-phase simulation, and completed by a full down-the-channel model.At first a single phase steady state 1-D modeling framework is developed which accounts accurate membrane water balance considering electro-osmotic drag and back diffusion, non-isothermal heat transfer inside thin PEMFC components, oxide coverage dependent ORR kinetics, proton transport loss in electrode with respect to I/C ratio, convective and diffusive gas transport, shorting and cross over current to accurately capture open circuit voltage, effects of land/channel geometry on transport and non-fickian resistance in electrode and microporous layer.

Next to expand the modeling capability in high humidity or high current density operations two-phase water transport in the GDL and in electrode is added to the 1-D modeling framework. To model liquid water transport in the gas diffusion media, a novel approach has been proposed that only requires the GDL porosity and tortuosity to formulate. To capture the effects of liquid water condensation in the electrode an empirical approach is adopted which correlates local water activity with catalyst utilization factor. The developed 1-D model establishes a robust modeling framework and it is validated with polarization and limiting current experimental results.

Finally, the 1-D two-phase model is expanded to a down-the-channel performance model to study the effects of stoichiometric flow rate, flow orientation (co-flow and counter-flow), channel pressure drop and coolant temperature on cell performance. The developed down-the-channel model can simulate species and current density distribution in along-the-channel direction and the results from developed 1+1-D model has been validated with experimental measurements from literature. In addition, parametric numerical studies on asymmetric GDL thickness, membrane thickness and asymmetric channel RH have been conducted to study internal water circulation mechanism. Down-the-channel simulation results show that counter-flow operation at low stoichiometric conditions enables efficient internal water circulation inside the cell, which improves membrane humidification and cell performance under extremely dry conditions. The findings from the newly developed model provide critical insights on the interaction of coupled heat and mass transfer, charge transport and down-the-channel distribution at realistic fuel cell operating conditions.

Proton exchange membrane fuel cell (PEMFC) technology provides a sustainable power solution as it electrochemically converts the chemical energy stored in hydrogen molecules into electricity, heat, and water. The electrochemical reactions occurring in the catalyst layer (CL), however, requires the use of precious group metal (PGM) catalysts, such as platinum, which is a major factor contributing to the high cost of PEMFC technology. To reduce the cost, there have been extensive research efforts in developing low-PGM and PGM-free catalyst materials. Nevertheless, the CLs incorporating these novel materials are often found to suffer from severe mass transport resistance resulting in significant performance loss. Based on the literature, this undesired mass transport resistance is mainly attributed to the cathode oxygen transport due to the heterogeneous characteristics of the hierarchical microstructure of the CL. Although a standard CL only contains carbon supported catalyst and ionomer, understanding the formation of the structure and the origin of the structural characteristics have been very challenging due to its complicated preparation process. A CL is usually prepared by an ink casting method, which involves multiple steps. The catalyst ink consists of the CL constituent materials uniformly dispersed in a liquid medium forming a suspension, which has an opaque, heterogeneous, and highly time-sensitive nature making the experimental investigation on the particulate structure in the catalyst ink very challenging. Despite the long history of using the CLs, a principled framework for understanding the structure-property relationship of the CLs has not yet fully developed. To advance the design and development of low-cost and robust CLs, this research 1) conceptualized a physical process capturing the key aspects of the particulate structure formation with respect to the CL fabrication process, 2) based on the complex fluid nature of the catalyst ink, designed a holistic non-destructive hierarchical approach to investigate the particulate structure in the catalyst ink, and 3) qualitatively and experimentally established the formation-structure-process-functionality relationship for CLs. With the newly proposed framework, this research advances current understanding on the structure-property relationship of the CLs and provides novel and practical insights into the design of low-cost functionality-tailored CLs.

Limited resources of fossil fuels and climate change left human beings with no choicebut to seek alternate clean energy providers. Green hydrogen is one of the strong candidates toward electrification. Proton exchange membrane fuel cell (PEMFC) is a device that converts chemical energy stored in hydrogen to electricity with water and heat as the only byproducts. PEMFC is a multi-component system with all kinds of mass, heat, electron, and ion transport physics included in its operation. Diffusion through porous media, mainly when liquid water is generated at high current densities, is challenging. A porous carbon-based gas diffusion media (GDM) is in charge of reactant transfer to the catalyst layer to fulfill electrochemical reactions and electron generation. Precise control of the heterogeneous microstructure of the porous media is one of the major materials engineering challenges to enhance mass transport in the fuel cell. Two porous components in PEMFC with the most heterogeneous microstructures are built upon assembly of carbon or catalyst supported on carbon agglomerates. These components are the micro-porous layer (MPL) in the diffusion media and the catalyst layer. MPL and catalyst layers are fabricated by deposition of a thin film of their precursor slurry (ink), which is a complex fluid. Significant interactions occurring during the formulation, mixing, and deposition of the precursor materials dictate the final properties and microstructure of the porous structure. Regarding the catalyst layer, optimization of the microstructure through ink materials engineering could lead to more catalyst utilization. Hence, a lower amount of precious platinum (catalyst) is needed to reach a particular performance in the PEMFC. This study employs a bottom-up approach to study the microstructure of the carbon network formed in ink as a function formulation and its evolution during the high shear deposition process. Finally, the findings are correlated to the in-situ fuel cell performance and oxygen transport resistance of the components in the PEMFC. For MPL, optimum formulation parameters and novel porous media using additives are introduced. A new approach is used to elucidate interparticle interactions in catalyst ink from the simultaneous response of the structure to mechanical shear forces and small alternating current (AC) perturbation.

Fuel cells, specifically Proton Exchange Membrane Fuel Cells (PEMFC) and Anion Exchange Membrane Fuel Cells (AEMFC), are pivotal in reducing greenhouse gas emissions and promoting clean, renewable energy. These technologies are critical for the protection and sustainability of the planet's resources. Despite their distinct applications, PEMFC and AEMFC devices share many similarities in terms of their components and the underlying physics that govern their operation. This commonality allows research findings from one type of fuel cell to be adapted to the other with minimal modifications.

Like PEMFCs, AEMFCs are characterized by their complex nature and the highly coupled multiphysics involved in their operation. This complexity makes it challenging to draw comparisons and conclusions from various experimental studies. Additionally, local conditions and transport phenomena within the fuel cell are difficult to probe experimentally. In this context, modeling serves as a powerful tool that complements experimental studies, offering clear and direct relationships between operating parameters, material properties, and overall performance. In the work discussed in Chapter 3, a one-dimensional (1-D) analytical model of an AEMFC has been developed. This model is capable of simulating the fuel cell's performance under both dry and wet conditions. The modeling approach for handling flooding in the gas diffusion media and the catalyst layer involves a combination of semi-empirical models and multilayer discretization of domains. This robust approach can be extended to other cell models as well.

At the heart of both PEMFC and AEMFC devices are the electrodes or catalyst layers (CLs). These are complex, heterogeneous porous structures made up of catalyst particles, ion-conducting polymers, and void spaces. These components come together to form the triple phase boundary, which is essential for the electrochemical reactions that occur within the fuel cell. Specifically, Pt/C particles facilitate electron transport, ion-conducting polymers enable ion conduction, and void spaces are crucial for the transport of gases and water. Catalyst inks are typically used to fabricate these catalyst layers. Therefore, understanding how various parameters of the catalyst ink (e.g., ionomer chemistry and loading, solvent formulation, carbon support type, etc.) affect its properties, the interactions between the components, and ultimately the formation of the CL microstructure and cell performance, is vital for optimizing electrode design.

The studies described in Chapters 4 to 6 use a combination of rheological and electrical measurements to investigate the effects of catalyst ink parameters and shear on the properties of the ink. The simultaneous measurements will provide unique insights into the evolution of the microstructure of the catalyst layers. The proposed rheo-EIS measurement tool takes advantage of the dynamic nature of catalyst inks, where rheological and electrical behaviors are coupled to the material's microstructure. These coupled behaviors are typically difficult to measure using traditional optical or scattering techniques. This tool will be used to study catalyst inks for both PEMFC applications, providing detailed insights into their microstructural development and resulting electrode performance.

Clean and efficient energy technologies are in high demand to resolve the issues related to limited fossil fuels and the climate crisis. Recently, electrochemical conversion devices, such as fuel cells and electrolyzers, demonstrate a viable option for a sustainable energy system. Electrolyzers can generate green hydrogen through water electrolysis, which can then be used in fuel cells to directly convert chemical energy to electricity. However, there are still technical barriers that need to be addressed before reaching full commercialization in these emerging technologies. For fuel cells, especially in heavy-duty vehicle applications, durability is a critical concern to be competitive with internal combustion engines. One of the key degradation losses in fuel cells comes from the catalyst layer made of platinum nanoparticles dispersed on carbon support (Pt/C). Here, a fundamental study was conducted to investigate the degradation mechanism of Pt/C using accelerated durability testing protocols in acidic and alkaline media. It was found that the generation of carboxyl functional groups due to carbon corrosion in acid poisons the Pt active sites during oxygen reduction reaction (ORR). In alkaline, carbon dissolution happens that triggers the formation of large Pt agglomerates. For electrolyzers, hydrogen generation relies on an expensive and scarce iridium metal as a catalyst for the oxygen evolution reaction (OER). To lower the cost of this device, alternative materials are developed to reduce the iridium (Ir) loading. We proposed to enhance Ir utilization by alloying with cobalt (Co), being a less expensive and more available metal. Surfactant-assisted Adam’s fusion synthesis technique was developed as a scalable method to produce IrCo catalysts. The synthesized material outperforms commercial Ir baseline catalysts, in both acidic and alkaline media. In addition, the effects of the Ir/Co molar ratio, the use of surfactant, and acid etching were investigated to enhance OER performance. In this dissertation, the catalytic performance and degradation mechanisms of precious metals for ORR and OER in both acid and alkaline media were successfully studied using a half-cell electrochemical set-up and physicochemical characterization tools. The new findings provide insights into developing more efficient and durable fuel cells and electrolyzers to promote energy sustainability toward a decarbonized society.