Development of advanced porous transport electrodes for water electrolysis

“Green hydrogen” is expected to play a pivotal role in decarbonizing various industrial sectors, including transportation, metal production, and chemical processes. However, as of 2024, the relatively high cost of green hydrogen ($5–$7 per kilogram) remains the primary barrier to its large-scale commercialization. Reducing its price significantly depends on lowering capital costs and enhancing the performance of green hydrogen generators, specifically proton exchange membrane water electrolyzer cells (PEMECs). My dissertation addresses the cost and performance challenges of PEMECs by investigating mass transport phenomena and developing advanced porous transport electrodes. This dissertation starts with the mechanism investigation of mass transport in PEMECs. With the help of a high-speed and micro-scale visualization system (HMVS), the bubble dynamics in various PTLs and operation conditions were clearly captured and analyzed. It is found that the PTL structure and bubble release efficiency have a significant dependance. PTLs with relatively large and low-tortuosity pores benefit the bubble release. Notably, the bubble release efficiency is changeable depending on the evolution of catalyst surface during the operation. More importantly, an interface-visible characterization cell (IVCC) was successfully developed, which can directly observe the mass transport in the electrode/electrolyte interface for the first time. The local bubble blockage and dehydration were unveiled for the first time. To understand the observed phenomena, a 3D multiphysics model was developed. It is found that the local bubble blockage and electrode pattern will cause significant local current crowding and dehydration, which degrades the performance of PEMECs. Based on these discoveries, the flow-enhanced liquid/gas diffusion layer (FELGDL) and reaction enhanced liquid/gas diffusion layer (RELGDL) were successfully developed, and their advances were successfully demonstrated in PEMECs. Finally, the integration methods between catalyst layers and porous transport layers are investigated. A novel electrodeposition-based ink-free integrated dual electrode assembly (IDEA) is developed serving as high-performing and durable electrode assembly for “green hydrogen” production. We believe the comprehensive mechanism investigation, numerical analysis, and innovative design/manufacturing approaches developed in this dissertation can serve as a guideline for the next-generation electrodes for affordable “green hydrogen” production

Investigation of Mass Transport Phenomena in Polymer Electrolyte Membrane Water Electrolysers

Polymer Electrolyte Membrane Water Electrolysers (PEMWEs) are considered a promising candidate for large-scale renewable energy storage and green hydrogen production. To improve efficiency and minimize cost for large-scale deployment, operation at high current densities is necessary. However, a consequence of high current density operation is increased mass transport hindrance which degrades performance. Two components are critical to mass transport in PEMWEs, namely the porous transport layer (PTL) and the flow-field plates. Both are expected to transport liquid water, product gases, electrons, and heat with minimal fluidic, thermal and voltage losses. However, the influence of morphology and configuration of both these components and operating conditions on cell performance are not well understood. This research investigates the mass transport phenomena in the PTL and in the flow-field channels in relation to performance in PEMWEs. The influence of flow-field configuration and two-phase flow characteristics in the flow channels on performance was studied by combined high-speed optical imaging and electrochemical characterization at various operating conditions. Results showed a strong correlation of performance with the flow path length and flow regime. Further, a correlative ex-situ X-ray tomography and in-situ electrochemical characterization approach was used to investigate the influence of PTL microstructural parameters such as mean pore diameter, pore size distribution, porosity, tortuosity, and porosity distribution on performance. Results indicated that minimizing contact resistance is most beneficial for improved performance over the range of current density studied. The influence of flow channel depth on performance was investigated by electrochemical impedance spectroscopy and a design of experiment (DoE) approach was employed to investigate the relative importance and interaction effects of mass transport factors on cell performance. Results showed the water feed rate and two-way interaction between the flow-field and PTL are most significant. This study provides enhanced understanding of the mass transport characteristics in PEMWEs for optimized design and improved performance

Computational investigation of polymer electrolyte membrane fuel cell with nature-inspired Fibonacci spiral flow field

Polymer electrolyte membrane fuel cells (PEMFC) are promising clean energy devices. The flow field design has crucial role in PEMFC performance for effective distribution of reactants and removal of products. Several nature-inspired flow field designs have recently been proposed in the literature. Common characteristics of these designs were sudden changes in the flow direction through sharp bends and flow field geometries restrained to areas having corners. In this thesis, Fibonacci spiral configuration, which is found in the nature from hurricanes to seashells, was considered for flow field pattern of a PEMFC. Contrary to the bio-inspired designs proposed in previous studies, continuous smooth change in the flow direction through curved spiral channel and flow field geometry restrained to the rounded area was attained. Computational studies for the PEMFC performance with Fibonacci spiral flow channel were conducted by solving the governing electrochemical equations using the Ansys Fluent software. In addition to the Fibonacci spiral geometry, a novel rectangular spiral design and the conventional parallel design were also simulated for performance comparisons. Polarization, power density, and fuel cell power output per required compressor power curves were computed in addition to distribution contours of pressure, velocity, reactant concentrations, and water mass fractions for all three flow field designs. Fibonacci spiral design exhibited uniform reactant distribution, improved water management, and extremely low-pressure drop compared to the rectangular spiral and conventional parallel designs --Abstract, page iii

STRUCTURE, SURFACES, AND COMPOSITION OF CATALYTIC NANOPARTICLES FROM QUANTITATIVE ABERRATION CORRECTED TRANSMISSION ELECTRON MICROSCOPY

Proton exchange membrane fuel cells (PEMFC) are a technology of high interest for the automotive and power generation industry. The catalyst layer plays a critical role in fuel cells as it is responsible for catalyzing hydrogen oxidation and oxygen reduction to generate electricity. The current challenge in catalyst development is to produce highly active and economical catalysts. This challenge cannot be overcome without an accurate understanding of catalyst surfaces and morphology since the catalytic reactions occur on the surface active sites. Transmission electron microscopy (TEM) is an excellent tool to understand the structures of the nanoparticles down to the atomic level in determining the relationship with the catalyst’s performance in fuel cell applications. Platinum (Pt) is one of the best commercially available catalysts for PEMFC due to its highly active, inert, and relatively stable properties. However, Pt is a rare precious metal due to its low abundance and high demand. Further research is aimed at developing highly active and more economical catalysts in order to mass produce PEMFC. A strategic approach is to use platinum bimetallic alloys, which greatly reduce the platinum loading as they enhance the oxygen reduction reaction. A detailed understanding of the nanoparticle surface is critical as the catalyst surface strongly determines its catalytic activity. Furthermore, another challenge in utilizing fuel cells is the life-time of the catalysts. It is known that electrochemical cycling affects Pt alloys. As a result, the understanding of the effect of electrochemical treatments on the catalyst’s v morphology and composition is key to improving the fuel cell’s performance and durability. This thesis demonstrates that through the use of TEM, useful insights regarding the morphology, surfaces, and compositions of the catalysts can be gained and contribute to the improvement in catalyst development for next generation fuel cells.Master of Applied Science (MASc

Ejector refrigeration: A comprehensive review

The increasing need for thermal comfort has led to a rapid increase in the use of cooling systems and, consequently, electricity demand for air-conditioning systems in buildings. Heat-driven ejector refrigeration systems appear to be a promising alternative to the traditional compressor-based refrigeration technologies for energy consumption reduction. This paper presents a comprehensive literature review on ejector refrigeration systems and working fluids. It deeply analyzes ejector technology and behavior, refrigerant properties and their influence over ejector performance and all of the ejector refrigeration technologies, with a focus on past, present and future trends. The review is structured in four parts. In the first part, ejector technology is described. In the second part, a detailed description of the refrigerant properties and their influence over ejector performance is presented. In the third part, a review focused on the main jet refrigeration cycles is proposed, and the ejector refrigeration systems are reported and categorized. Finally, an overview over all ejector technologies, the relationship among the working fluids and the ejector performance, with a focus on past, present and future trends, is presented. (C) 2015 Elsevier Ltd. All rights reserved

Catalytic Hollow Fibre Membrane Micro-reactors for Energy Applications

An asymmetric ceramic hollow fibre is proposed as a substrate for the development of a catalytic hollow fibre microreactor (CHFMR) and a catalytic hollow fibre membrane microreactor (CHFMMR). The ceramic substrate that is prepared using the phase inversion and sintering technique has a finger-like structure and a sponge-like region in the inner region and the outer surface respectively. The finger-like structure consists of thousands of conical microchannels distributed perpendicularly to the lumen of ceramic hollow fibres onto which a catalyst is impregnated using the sol-gel Pechini method to improve a catalytic reaction. To further enhance the catalytic reaction, a membrane has been incorporated on the outer layer of ceramic hollow fibre. This study focuses on the use of palladium (Pd) and palladium/silver (Pd/Ag) membranes to separate hydrogen from reaction zones in the water-gas shift (WGS) reactions and the ethanol steam reforming (ESR) respectively. In the development of CHFMMR, the fabrication of Pd and Pd/Ag membranes is carried out prior to the catalyst impregnation process to avoid the dissolution of catalyst into the plating solution due to the presence of ammonia and ethylenediaminetetraacetic acid (EDTA). The catalytic activity tests show that the CHFMR, that does not have the Pd membrane on its outer surface, improves the carbon monoxide (CO) conversion compared with its fixed-bed counterpart. The presence of conical microchannels is expected to enhance the activities of the catalyst in the substrate. The incorporations of Pd and Pd/Ag membranes on the outer layer of ceramic hollow fibres enable pure hydrogen to be produced in the shell-side for both the WGS reaction and the ESR. The CHFMMR is used to remove one of the products enabling the WGS reaction to favour the formation of product. It also facilitates the small amount of catalyst to be used to produce significant amount of hydrogen in the ESR

Two-phase flow behaviour and performance of polymer electrolyte membrane electrolysers: Electrochemical and optical characterisation

Understanding gas evolution and two-phase flow behaviour are critical for performance optimization of polymer electrolyte membrane water electrolysers (PEMWEs), particularly at high current densities. This study investigates the gas-bubble dynamics and two-phase flow behaviour in the anode flow-field of a PEMWE under different operating conditions using high-speed optical imaging and relates the results to the electrochemical performance. Two types of anode flow-field designs were investigated, the single serpentine flow-field (SSFF) and parallel flow-field (PFF). The results show that the PFF design yielded a higher cell performance than the SSFF design at identical operating conditions. Optical visualization shows a strong relationship between the flow path length and the length of gas slugs produced, which in turn influences the flow regime of operation. Longer flow path length in the SSFF results in annular flow regime at a high current density which degrades cell performance. The annular flow regime was absent in the PFF design. It was found the effect of flow rate on performance depends strongly on operating temperature in both flow patterns. Results of this study indicate that long channel length promotes gas accumulation and channel-blocking which degrades performance in PEMWEs

Modeling and optimization of reactant gas transport in a PEM fuel cell with a transverse pin fin insert in channel flow

A proton exchange membrane (PEM) fuel cell has many distinctive features which make it an attractive alternative clean energy source. Some of those features are low start-up, high power density, high efficiency and remote applications. In the present study, a numerical investigation was conducted to analyse the flow field and reactant gas distribution in a PEM fuel cell channel with transversely inserted pin fins in the channel flow aimed at improving reactant gas distribution. A fin configuration of small hydraulic diameter was employed to minimise the additional pressure drop. The influence of the pin fin parameters, the flow Reynolds number, the gas diffusion layer (GDL) porosity on the reactant gas transport and the pressure drop across the channel length were explored. The parameters examined were optimized using a mathematical optimization code integrated with a commercial computational fluid dynamics code. The results obtained indicate that a pin fin insert in the channel flow considerably improves fuel cell performance and that optimal pin fin geometries exist for minimized pressure drop along the fuel channel for the fuel cell model considered. The results obtained provide a novel approach for improving the design of fuel cells for optimal performance.The University of Pretoria, NRF, TESP, EEDSM Hub, CSIR, and the Solar Hub of the University of Pretoria and Stellenbosch University.http://www.elsevier.com/locate/heai201