David Wilkinson

Although great effort has been devoted to the investigation of lithium metal batteries (LMBs) and numerous papers are published, the deposition and corrosion behavior of lithium is still not fully understood and requires further investigations. Moreover, the characterization and data interpretation in the literature can be very misleading or ambiguous. In this thesis, the deposition behavior of lithium at a carbon host, the decoupling of impedance of the solid-electrolyte interface (SEI) and plated lithium, and the impact of electrolytes on the lithium corrosion behavior are presented.Firstly, pure hollow core-carbon spheres (PHCCSs) are prepared and applied onto the Cu current collector, which is then used to investigate the deposition behavior of lithium. This type of structure is selected because the nano-carbon-based materials are widely reported as lithium host materials in LMBs, while contradictory claims are reported as to where the lithium plating occurs. It is demonstrated that the lithium shows some initial and limited intercalation into the PHCCSs and then plates on the external carbon walls and the top surface of the carbon coating during the charging process. No evidence of lithium plating inside the hollow core space is found.Secondly, a combined electrochemical quartz crystal microbalance (EQCM) and electrochemical impedance spectroscopy (EIS) study is conducted to decouple the various contributions to the impedance response of an anode free battery. This decoupling is achieved by a comparison of the evolution of EIS spectra in 2- and 3- electrode cells during the Li plating/striping processes. The observed high and low frequency loops of the impedance spectra can be unambiguously assigned to the charge transfer of the SEI and the plated lithium layers, respectively. Finally, inspired by the capacity performance in the lithium bis(fluorosulfonyl)imide- lithium bis(trifluoromethanesulfonyl)imide-dimethyl ether-dioxolane (LiFSI-LiTFSI-DME-DOL) electrolyte system, the impact of electrolytes on lithium corrosion behavior is investigated systematically. Results show the key point of minimizing capacity loss during open circuit voltage (OCV) is the coordination of insoluble particles and polymers in the SEI layer. A “less-soluble particle-polymer symbiosis” (LSPPS) concept is thus proposed and validated by the preparation and performance evaluation of modified electrolytes.

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The development of effective yet inexpensive technologies for wastewater treatment is critical to the safeguarding of water resources and the delivery of clean water for sanitation and drinking. Electrochemical technologies are of growing relevance in an increasingly decentralized, distributed, and electrified system. High oxidation potential (HOP) materials, so called for their ability to generate efficacious conditions for water treatment, are critical to their development. One such HOP, Magnéli phase titanium oxides (MPTOs), are being utilized to advance this domain; however, issues of its stability constrain the applicability of devices that employ them. Doping influences many physicochemical properties of MPTOs relevant to their use as electrode materials. The rational selection of dopants for MPTOs is explored using Hume-Rothery rules for solid substitutional solutions, selecting transition metals vanadium, chromium, and iron as potentially highly soluble dopants. Their thermal stability is investigated using thermogravimetric analysis (TGA) in air and a model for their thermal oxidation is developed from kinetic data. Electrochemical accelerated life testing (ALT) is conducted with doped MPTO electrodes and their influence on the time to failure is evaluated. These tests establish that vanadium doping provides a significant improvement to the stability of MPTOs in thermally and electrochemically oxidizing conditions. Porous transport layers (PTLs) of vanadium-doped MPTOs are prepared and incorporated into a compact electrolyzer, which is used to treat industrial wastewater. Vanadium-doped MPTO PTLs provide a significant improvement in performance over pristine materials in terms of volumetric energy consumption. Furthermore, the compact electrolyzer design places this approach in the top quartile of this class of electrochemical devices, which is attributed to the reduced interelectrode distance.

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In this thesis, novel electrochemical methods for characterizing the oxygen reduction reaction (ORR), the oxygen evolution reaction (OER), and alternative materials for the current collector and flow field for the polymer electrolyte membrane water electrolyzer (PEMWE) are presented. A novel modified rotating disk electrode (MRDE) apparatus used for characterizing catalyst coated membranes (CCMs) for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cell (PEMFC) application was demonstrated first. Cyclic voltammograms (CV) and ORR curves were obtained at room temperature for Pt foil and three commercial CCM samples. The mass activity obtained from the MRDE technique, compared to classical thin-film RDE, were in closer agreement to the PEMFC results. For the OER study, six different current collectors were tested using one commercial CCM in a half cell at 25, 40, 55, and 75°C. The MRDE was able to reach 2 A cm⁻² for commercial OER catalysts. This is a large improvement over traditional OER RDE, which seldom reaches 50 mA cm⁻². There is an improvement in performance with increasing triple contact point (TCP).A transparent visualization cell was developed to expand the channel adjacent to the anode catalyst. This cell was designed to operate current densities higher than 2 A cm⁻², where mass transport effects are more dominant. A commercial catalyst and four Ti current collector meshes were tested at room temperature. Rapid cycling CVs and polarization curves were performed to validate this setup and rank the mesh performance. Fourier transform and bubble-ratio analysis were used to identify characteristic bubble lifetimes. Current density increases with decreasing bubble ratio in the frequency range 0.03
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Two electrochemical approaches that simultaneously convert carbon dioxide and high salinity brines to desalinated water and value-added chemicals in the form of inorganic acids and carbonate salts were demonstrated. In the first method, a multi-compartment electrodialysis cell module using anion exchange and cation exchange membranes, and a Pt/Ir-coated Ti anode and Ti mesh cathode was used to produce HCl and NaHCO₃ products from a carbonic acid and sodium chloride solution. Under an applied voltage inorganic carbon salts and acids were produced. A mathematical model for this electrodialysis cell configuration was developed to better understand limitations within the cell which were not available from experimental data including concentration profiles within the intra-membrane channels.In the second method, a first of a kind 5-compartment electrochemical cell including an anode, cathode and three electro-dialytic compartments was used. Water was converted to oxygen and protons at the anode while gaseous O₂ and CO₂ were converted to bi(carbonate) or hydroxide ions at the cathode. The three central electrodialysis compartments combine the ions present in the saline water with the products of the anode and cathode to produce value-added chemicals and desalinated water. A custom-built electrochemical cell with an active area of 3.24 cm² containing an array of two anion and cation exchange membranes each, Pt/C catalyzed cathode and titanium anode electrodes were used. The cell was investigated to understand the different factors affecting cell performance. Increasing concentrations of hydrochloric acid and bi(carbonate)/hydroxide salts was observed in the respective compartments over 24-hour periods of testing. Current efficiencies of alkaline (bicarbonate, carbonate or hydroxide) ions and proton production were as high as 71% and 96%, respectively. Polarization losses across the different compartments of the electrochemical cell were mapped to determine the source of overpotentials with different gas compositions. The new electrochemical cell platform is scalable and was used to successfully desalinate and convert carbon dioxide to chemicals. The process shows considerable commercial potential and has been scaled-up to a commercial module for deployment with different industrial operations within the oil and gas, and mining sectors.

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Featuring low operational temperature and high-power density, polymer electrolyte membrane fuel cells (PEMFCs) have become the most researched and used fuel cell for the emerging automotive applications. To further promote the competitiveness of the fuel cell, improvement in operational flexibility to enable fuel cell to maintain its performance under various conditions is critical. The approach taken here was to modify the membrane electrode assembly (MEA) structure, particularly the interfaces of the cathode catalyst layer. Two interfaces were studied and modified, namely the membrane | cathode catalyst layer (CCL) and the CCL | microporous layer (MPL) interfaces. Firstly, the interface of the membrane and CCL was modified by addition of a thin, dense Pt layer in the membrane subsurface (
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The stability and electrochemical regeneration of activated carbon electrodes used for Capacitive Deionization have been investigated. To assess the electrode state, electrochemical properties such as capacitance and potential at the point of zero charge (EPZC) were monitored and complemented by elemental composition, wettability and bulk porosity analysis.Long-term cycling was performed with electrolytes containing dissolved organic matter (DOM) and iron (II) under anaerobic and aerobic conditions. It was found that DOM, at concentrations of up to 40 mg L-¹, had a marginal impact on capacitance loss and relocation of the EPZC. On the other hand, the scaling nature of iron was apparent from early cycling stages, in waters containing as little as 0.2 mg Fe²⁺ L-¹. In all cases performed, elemental composition analysis demonstrated that incorporation of oxygen to the surface occurred predominantly during the first 15 to 20 cycles. Specifically, the oxygen content increased by a factor of four. Over this time frame, the contact angle decreased from approx. 130˚ to 34˚, on average; which indicated an increase in electrode wettability. In addition, cycling experiments with simple electrolytes revealed improved electrode stability as cycling time increased and as pH decreased. Degradation tests conducted using a two-electrode cell demonstrated the effect of reaction coupling between the processes of carbon corrosion and oxygen reduction. As a result, electrode decay occurred at an accelerated rate.Electrochemical regeneration was proven successful at recovering the electrode capacitance but not at regressing the EPZC. Their corresponding response surfaces were mapped through a 32-factorial design of experiments and revealed a significant potential dependence. Further tests revealed a point of maximum recovery near a potential of -1.68 V vs RHE and 50 s of holding time. Furthermore, the contribution of a regenerative step to the long-term electrode stability was assessed. In general, an improved retention of capacitance was observed during the first 25 cycles. However, it was noted that most of the capacitance recovered was lost in the subsequent degradation cycles. Consequently, the apparent initial improvement did not translate to an extension of the electrode lifetime, deeming this approach inadequate to mitigate the effect of carbon corrosion.

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The complex interface between polymer electrolytes and nanostructured electrodes is key for the operation of many electrochemical devices. While the surface science of electrocatalysts have been extensively studied using simplified model systems, such as Pt single crystals, the extent to which lessons from these models apply to nanostructured interfaces remains poorly understood. In this work, the interface between Pt nanoparticles and solid polymer electrolytes is explored in terms of catalytic activity, morphology and durability. The nucleation and growth of nanoparticles is controlled to produce contiguous ultrathin catalyst layers in a solution processable fashion using electroless deposition. The reactivity and structure of the Pt-polymer surface was probed with advanced characterization techniques using synchrotron radiation. Direct imaging of the solid-electrolyte-interphase was accomplished with X-ray spectromicroscopy. Degradation of this interface was visible after a simulated aging protocol. Finally, the mechanism of surface oxidation and reduction on Pt nanoparticles was explored with diffraction, moving towards an atomistic understanding of fuel cell electrochemistry inside functional devices under relevant operational conditions.

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Widespread application of polymer electrolyte membrane fuel cells (PEMFC) and water electrolyzers (PEMWE) in renewable energy systems depend on their performance in high current density conditions. For both applications, transport of species to/from the oxygen electrode is crucial to the system operation. Recent in-situ imaging of both PEMFC and PEMWE systems have shown that the product material accumulates at the interface of the oxygen electrode with the adjacent porous transport layer (PTL), and negatively affects the performance. This interface is therefore worth scrutinizing in terms of mass transport and electrostatics. In this thesis, I investigated how the structural and electrostatic properties of the catalyst layer and the PTL in each system leads to the electrochemical performance of the system. For a PEMWE cell, I modeled the bubble nucleation, growth and stability inside a PTL pore. Then, I analyzed the performance stability of the PEMWE in a diagnostic cell. The results show the distribution of electrolytic bubbles lifetime and directly address their performance impact for the first time.For a PEMFC, I investigated two interfaces at the fuel cell catalyst layer and PTL to find out how the properties of the interface affect the distribution of species at the interface. For a promising PTL candidate material, I investigated how engineering the pore and particle sizes could maximize the cathode catalyst layer (CCL) utilization. I modeled how liquid water pressure build-up should be balanced with the distance from the PTL interstitial pores to allow for the liquid water to diffuse out of the CCL. The developed models can be used as applied material development tools with respect to the transport. In the catalyst layer, I investigated how the oxidation state of Pt affects the local distribution of H⁺ at the Pt-O-H₂O-H⁺ interface using classical molecular dynamics. The results showed for the first time that classical double layer theories do not properly address the effect of surface oxide dipoles on the ion interactions at the charged interface. The results from my thesis can guide the material design for the transport layers in PEMFC and PEMWE applications that improve the two-phase transport and efficiency.

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Portable electronic devices for the next generation demand a quick charging and long-lasting energy power system. Micro direct methanol fuel cells (µDMFCs) are considered as one of the appropriate alternatives to rechargeable battery technology for portable power devices. Although a significant amount of work has been done with µDMFCs, it is still a design challenge to miniaturize the fuel cell and to provide adequate power. The conventional bipolar fuel cell architecture contains a membrane electrode assembly sandwiched between two flow field plates. In this research, we present an approach to enhance the maximum power density of µDMFCs without affecting the total fuel cell volume by depositing extra anode catalyst on the fuel flow channel walls. An air-breathing µDMFC with extra anode catalyst deposited on the channel walls was developed, and the effects of key design parameters and operating conditions on the fuel cell performance were examined by measuring the overall cell and individual electrode polarization curves. The fuel cell with extra anode catalyst on the channel walls improved the maximum power density by 20% compared to the conventional design with only a catalyst coated membrane. The fuel cell design approach with catalyzed flow field channel walls was also demonstrated in an air-breathing micro Fe(II)/Fe(III) redox anode fuel cell (μRAFC). The μRAFC with graphite channel walls as an anode improved the maximum power density by 281% compared to the μRAFC with inactive channel walls. The impacts of key operating conditions on the cell performance were also evaluated.A 3D simplified model for the µDMFC design with catalyzed channel walls was developed and applied to evaluate the key parameters. It was found that the fuel cell performance was mainly limited by the kinetics of the methanol oxidation reaction. For the fuel cell with anode catalyst both on the membrane and the channel walls, increasing the anode catalyst loading on the channel walls improved the contribution of the anode on the wall to the total anodic current, and reducing the channel dimensions only slightly improved the cell performance.

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High efficiencies and reduced greenhouse gas emissions promote proton exchange membrane fuel cells (PEMFCs) as a promising energy conversion technology. However, its widespread commercial application is hampered by certain cost, performance and durability limitations. The interface between the MPL (microporous layer) and the cathode CL (catalyst layer) plays an important role in a PEMFC’s overall performance, since it houses the reaction sites for the oxygen reduction reaction. The interface may furthermore significantly affect mass transport behavior, ohmic contributions and the hydration state of the membrane at different humidities. The main objective of the study was therefore to advance PEMFC research through the development of alternative MPLs offering dual-functional improvements: enhanced interfacial characteristics and improved operational flexibility via suitability for low cathode humidity applications. Alternative MPLs were evaluated based on extensive material characterization and single cell performance testing. Graphene was demonstrated to be a promising alternative. The material displays beneficial interfacial characteristics (a stacked planar morphology, superior conductivity, adhesive behaviour, and improved electrical connectivity with the CL) and furthermore results in improvements in the kinetic and ohmic polarization regions, compared to the conventional CB (carbon black) MPL. Although graphene MPLs also suffer from mass transport limitations, the problem can be addressed through the addition of CB. The addition increases the MPL’s water permeability, which helps to establish a balance between water removal (for the prevention of flooding) and water retention (for membrane hydration) at high and low RH (relative humidity). For graphene tested under one-dimensional control, this results in synergistic performance enhancements, showing a 30% and 80% increase in the maximum power density at 100% and 20% cathode RH. In addition to increased water permeability, other common effects resulting from the creation of CB composites (also observed for reduced graphene oxide and graphite) include decreased surface wettability and through-plane resistance. For the application to low loaded CCMs (0.1 mg cm-²), the potential to improve performance with graphene-based MPLs appears restricted by the catalyst loading itself. Nevertheless, graphene helps to improve performance preservation of low loaded CCMs at low humidity conditions, as also demonstrated for conventionally loaded CCMs (0.4 mg cm-²).

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For applications that require small amounts of H₂O₂ or have economically difficult transportation means, an alternate, on-site H₂O₂ production method to the current industrial anthraquinone auto-oxidation process is needed. Thus far neutral production of H₂O₂ has been limited to bench-top laboratory scaled research with low yield of H₂O₂ [1]. To produce neutral H₂O₂ on-site and on-demand for drinking water purification, the electroreduction of oxygen at the cathode of a solid polymer electrolyte (SPE) cell could be a possible solution. The work presented here has utilized a SPE cell operating in either fuel cell mode (power generating) or electrolysis mode (power consuming) to produce H₂O₂. The SPE cell reactor is operated with a continuous flow of cathode carrier water flowing through the cathode to remove the product H₂O₂. Two catalysts were chosen for further study in this work, one is the inorganic cobalt-carbon composite catalyst, to be used in both fuel cell mode and electrolysis mode operation. The other is the riboflavin-anthraquinone-carbon composite catalyst, to be used in only the electrolysis mode operation. Through parametric experiments in both modes of operation, the Co-C catalyst was able to achieve peroxide production rate of ~200 μmol hr-¹ cm-² and 4 mW cm-² operating at a cell temperature of 60°C with a current density of 30 mA cm-² and 30% current efficiency in fuel cell mode operation. Long term recycle experiments over a period of 72 hours showed an accumulated H2O2 concentration of over 1400 ppm. Investigation of both catalysts in electrolysis mode operation showed that the AQ-C catalyst achieved maximum H₂O₂ production of 580 μmol hr-¹ cm-² operating at 40°C and a current density of 240 mA cm-² with an 8% current efficiency; while the Co-C catalyst had a maximum H₂O₂ production rate of 360 μmol hr-¹ cm-² operated at 240 mA cm-² with 8% current efficiency. Long term recycle study of both catalysts in electrolysis mode generated maximum H₂O₂ concentrations of over 3000 ppm in 72 hours. Water sample analysis showed no degradation of the catalysts in either mode of operation.

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Growing global energy demands and an increased environmental awareness have resulted in a demand for renewable energy sources. Photocatalytic water splitting has long been explored as a direct solar-to-chemical energy conversion method in the hopes of creating a sustainable, emissions-free hydrogen production process. In this thesis we present the first focused effort on hydrogen production via photocatalytic water splitting in a UV-irradiated fluidized bed reactor. This novel approach was taken to address the mass-transfer effects, poor radiation distribution, parasitic back-reaction and photocatalyst handling difficulties that limit the efficiency and scalability of existing water splitting systems.By fluidizing platinum-deposited TiO₂ spheres in a 2.2M Na₂CO₃ solution, steady hydrogen production rates of 211 μmol/hr with an apparent quantum efficiency of 1.33% were achieved upon UV-irradiation. This represents a marked 44% increase in efficiency when compared to results obtained by suspended slurry TiO2 photocatalysts in the same reactor. A mathematical model describing the performance of the fluidized bed water splitting system was derived and then employed to estimate several key parameters. From the model, it was found that high rates of mass transfer in the separator unit could minimize the negative effects of the parasitic back reaction and greatly improve the overall rate of hydrogen evolution. Indeed, it was demonstrated experimentally that slight modifications to the liquid-gas separator to improve mass transfer resulted in a 350% increase in the rate of hydrogen evolution. The application of the model to the design of fluidized bed water splitting systems is described.Advanced, fluidizable nanowire and nanorod photocatalysts that can withstand the rigors of fluidization are described here for the first time. We present two novel, scalable methods that allow for the growth of anatase nanowires or rutile nanorods on porous glass particles, whose deep surface features protect the nanostructured films from mechanical attrition. It was found that the photocatalytic activity of anatase nanowires grown via a chemical bath deposition process was over three times greater than that of hydrothermally grown rutile nanorods when employed for photocatalytic hydrogen production and degradation of a model contaminant (Rhodamine B). The factors controlling nanowire growth and performance are discussed.

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The issue of efficient, low-cost, sustainable hydrogen (H₂) production is one of the barriers to the adoption of a H₂ economy. In this thesis, the electrochemical production of H₂ from liquid methanol (CH₃OH) in acidic aqueous media was studied in a proton exchange membrane (PEM) electrolyser in the static mode at low temperatures. A baseline study showing the influence of CH₃OH concentration, catalyst, catalyst support, operating temperature and operating mode was established. A theoretical thermodynamic analysis of the system was carried out as a function of temperature, and the limiting current densities, kinetic parameters, including the Tafel slopes and current exchange density, and apparent activation energies were determined. The effect of electrochemical promotion (EP) was investigated to see if it can increase the efficiency and performance of H₂ production through electrochemical processes.The electrochemical promotion of electrocatalysis (EPOE) was investigated by carrying out the electrolysis in triode and tetrode operation. It was shown to improve the PEM electrolysis in the galvanostatic and potentiostatic modes. A decrease in electrolysis voltage or an increase in electrolysis current proportional to the current or potential imposed in the auxiliary circuit was observed when the auxiliary current or potential was opposite to the electrolyser circuit current or potential. The effect was observed using catalytic and non-catalytic non-precious electrolyser electrode materials. It was postulated that triode and tetrode operation enhanced the electro-oxidation rate through electrochemical pumping and spillover of protons. With this novel electrolysis configuration, electrolysis cost reduction may be achieved through the use of non-precious electrolyser anode materials and/or improving electrolyser performance. The electrochemical promotion of catalysis (EPOC) was also investigated for the catalytic reforming of CH₃OH at low temperature with Pt-Ru/C and Pt-Ru/TiO₂. The synthesized Pt-Ru/TiO₂ was characterized physico-chemically and electrochemically. Powder catalytic CH₃OH reforming tests showed that both catalysts can be used to generate H₂. EPOC experiments were conducted on gas diffusion electrodes (GDEs) in galvanostatic control. Under the experimental conditions, only supplying H⁺ to the catalyst working electrode surface resulted only in a Faradaic enhancement of the catalytic activity for the low temperature reforming of CH₃OH, which appears to be a purely electrophilic behaviour.

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This thesis explores two-phase flow phenomena relevant to water management in PEM fuel cells. Particularly, pressure drop hysteresis is explored in depth, which occurs when the gas and liquid flow rates are increased and decreased along a set path but exhibit different pressure drops. The hysteresis effect is explored here experimentally in three studies: non-operating cold model to study hydrodynamics, non-operating hot model at fuel cell operating conditions to study increasingly relevant hydrodynamics, and an operating study to explore pressure drop hysteresis in an active cell. This is the first time pressure drop hysteresis has been studied in a PEM fuel cell. A specially designed visualization fuel cell, allowing for observation into the cathode flow field channels, is utilized to further understand these results. The pressure drop hysteresis occurs because liquid water accumulates in the cathode flow channels during the descending approach. The cathode air stoichiometry and temperature play a major role, as lower stoichiometries and lower temperatures lead to more water accumulation in the channels, which increases the hysteresis problem. The gas diffusion layer is not a main parameter affecting pressure drop hysteresis. Additionally, several other variables are studied through the three experimental setups to understand the hysteresis behavior. This thesis then examines anode water removal (AWR) as a diagnostic tool to determine maximum fuel cell performance in the absence of mass transfer limitations on the cathode. By exacerbating cathode flooding and using a variety of cathode GDLs, large voltage increases occur through the AWR process when the cathode GDL is under flooding conditions. Larger voltage gains occur during the AWR process with the use of GDLs without an MPL when the cathode gas stream is fully humidified. Both studies, pressure drop hysteresis and AWR, improve overall fuel cell performance by better understanding water management in PEM fuel cells. Understanding the pressure drop hysteresis is important to limit the parasitic power losses associated with higher pressure drops, and AWR is a novel tool researchers can use to evaluate new GDLs in terms of their ability to prevent voltage losses due to flooding.

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Water management in PEM fuel cells has received extensive attention for its key role in fuel cell operation. Several water management issues have been identified that needed further investigation, i.e., droplet behaviour on the GDL surface, two-phase flow patterns in gas flow channels, impact of two-phase flow on PEM fuel cell performance, impact of flow mal-distribution on PEM fuel cell performance, and mitigation of flow maldistribution. In this work, those issues were investigated based on simulations using computational fluid dynamics (CFD) method.Using the Volume of Fluid (VOF) two-phase flow model, droplet behaviour and two-phase flow patterns in mini-channels were identified consistently in both simulations and experimental visualizations. The microstructure of the GDL was found to play a significant role in the formation of local two-phase flow patterns, and the wettability of both GDL and channel wall materials greatly impacted on the two-phase flow patterns. A novel 1+3D two-phase flow and reaction model was developed to study the impact of two-phase flow on PEM fuel cell performances. The existence of two-phase flow, especially the slug flow, in gas flow channels was found to be detrimental to the fuel cell performance and stability. Uneven liquid flow distribution into two parallel gas channels significantly reduces the fuel cell output voltage because of the induced severe non-uniform gas distribution, which should be avoided in the operation due to its negative effect on the fuel cell performance and durability. Finally, several maldistribution mitigation methods were tested in the simulation. It was found that utilizing narrow communication channels or adding gas inlet resistances could effectively reduce the gas flow maldistribution.

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A fuel cell that promotes the direct use of alcohol fuels such as methanol and ethanol is attractive because these fuels are friendlier than other fuels, such as gasoline, to the end-user and are renewable. Therefore, these fuel cells continue to receive much interest from academia and industry who actively seek alternative energy sources and comprehensive energy supply solutions. However, one of the barriers to the performance improvement of the alcohol fuel cell is the CO-like poisoning intermediates that hinder the alcohol electro-oxidations.This thesis project has validated several different advanced approaches to eliminate the CO-like intermediates from the catalyst surface. A 3-electrode electrochemical glass cell, a half-cell and a single fuel cell have been used to study the effects of these approaches (i.e., introduction of oxidant additives, increased operating temperature, electrochemical pulse techniques, and fuel starvation) on intermediates. A 3-way relationship between the onset potential for electro-oxidation of alcohols, the CO oxidizing potential, and temperatures was determined, and conditions required for a performance benefit were identified. A higher temperature Direct Alcohol Phosphoric Acid Fuel Cell (DAPAFC) using Phosphoric Acid/Silicon Carbide (SiC) as an electrolyte/separator was investigated. Parametric studies were conducted to determine the effects of factors such as higher temperature operation (120-180ºC), etc. A reduced performance gap between PtRu and Pt catalyst at higher temperatures ((>120°C) was shown. Comprehensive studies were also conducted to demonstrate the performance effects of the gas diffusion layer and the micro-porous layer. It was shown that the structure improvement of the phosphoric acid electrode assembly significantly improved the durability and could also improve the cell performance. A higher temperature Direct Alcohol Alkaline Fuel Cell (DAAFC) was also developed to demonstrate the effectiveness of the alcohol electro-oxidation in alkaline medium. An advantage for this system was the use of pure fuel operation which provides at least a 10% improvement in performance compared to dilute fuel operation.In general, the higher temperature direct alcohol vapor fed fuel cells show significantly improved performance using a simple inexpensive separator approach. It appears that this is a new approach which could have a number of advantages.

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Green hydrogen generation can serve as a solution to energy storage needs of the 21st century if coupled with a renewable energy source such as solar power. There is an advantage to performing in-situ hydrogen generation at a photocatalytic surface in order to reduce system efficiency losses. Investigation into the limitations and benefits of coupled electrolytic and solar process are investigated in this dissertation.Of chief examination were polybithiophene-titania composite films used as a photocathode in a Nafion 117 membrane based H₂SO₄ electrolytic system. Titania content inclusion up to 0.35 mg/cm² with a range of film thicknesses between 1-20 μm, were investigated in several architectural configurations. The placement of the composite films was directed either towards or away from the proton conducing membrane in a 2-D or 3-D configuration.The results indicated that the electrochemical benefits to titania inclusion and film thickness increase were counter to photonic energy collection. With an increase in titania there was an increase in electrochemical performance, but it led to worse photonic efficiency as suspected due to an increase in recombination from defect trap states. With an increase in film thickness there was an increase in photonic efficiency with increased photon absorption, but this was accompanied by an increase in cell resistance leading to worse electrochemical performance. Electrochemical performance was also enhanced by placing the catalytic film directly against the membrane, although photonic stimulation in this configuration was impossible in our current cell configuration. Photonic stimulation was enhanced by the deposition of the composite films onto a distributed 3-D carbon fibre substrate. Doping of the 3-D composite films loaded on carbon fibre substrates with a Nafion ionomer interconnect was tested in an effort to enhance the triple phase connectivity of the cell. It was found that the doping resulted in a deactivation of the substrate (both electrochemical and photonic) due to the deposition method used, but increases in photonic performance at higher current densities showed that with less catalyst encapsulation this strategy may be a viable method to enhance PEM based photoelectrochemical cells.

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Development of intermediate temperature (200-500°C) fuel cells could possiblyovercome many disadvantages of both the high temperature (600-1000°C) solid oxide fuelcells (SOFC) and the low temperature (70-100°C) proton exchange membrane fuel cells(PEMFC) in terms of materials durability, cost, application, and overall system structure. Achange in materials, especially the proton conductive electrolyte, is required to achieve this.However, to date, no solid proton conductors have been developed that work satisfactorily inthis temperature range.The goal of this thesis was to develop a ceramic proton-conducting material to be usedas a dense electrolyte, as well as within the anode structure of an intermediate temperaturefuel cell. Investigated ceramic materials were based on oxygen deficient ceramic oxides –undoped and Ce- and La-doped Ba₂In₂O₅, which were expected to show proton conductivitywithin the intermediate temperature range due to water and/or proton incorporation into theirdefect structure. Five different compositions of brownmillerite materials, Ba₂In₂-xyCexLayO₅+x/₂ (x=0.25 and 0.5; y=0.25 and 0.5) were synthesized via the solid-state reactionand the glycine-nitrate process, characterized and electrochemically investigated in order tofind a suitable proton-conductive electrolyte. The materials were characterized using X-raypowder diffraction (XRD), thermogravimetric analysis (TGA), differential scanningcalorimetry (DSC), particle size analysis (PSA), scanning electron microscopy (SEM),transmission electron microscopy (TEM), etc. The electrical conductivities of the ceramicswere determined using ac impedance spectroscopy. Among the tested materials, undopedBa₂In₂O₅ produced by the glycine-nitrate process was selected as the material with thehighest total conductivity (between 0.02 S/cm and 0.7 S/cm) and stability in hydrogeniiicontaining atmospheres and at temperatures between 300°C and 480°C. High proton transportnumbers (e.g., 0.84 at 300°C) and relatively high open circuit voltage values of the air,Pt Ba₂In₂O₅ Pt, 50%H₂/50%N₂ cell (e.g., 0.81 V at 300°C) confirmed the predominantproton conductivity of this material. Although highly proton conductive in a hydrogencontainingatmosphere, Ba₂In₂O₅ showed poor performance as an electrolyte in anintermediate temperature fuel cell due to the incorporation of oxygen on the cathode sidewith associated blocking of the proton conduction. Application of sintered porous Ba₂In₂O₅in a cermet with a metal catalyst in the anode structure was shown to be beneficial.

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Proton exchange membrane fuel cells (PEMFC) are a promising energy conversion alternative for a number of applications including automotive, small power generation, and micro applications. However, many issues, such as poor water management and voltage instability, still have to be addressed in order to remove technical barriers to commercialization. In this work, water management issues in PEM fuel cells were investigated in detail with the purpose of developing approaches to reduce the negative effect of liquid water inside the fuel cell. The performance of the PEM fuel cell deteriorates when operated at low humidity to dry conditions. It was demonstrated that the use of perforated sheets as water barrier layers improved the operational life of the fuel cell significantly (>3x) compared to a fuel cell with no additional layers. These sheets increase the water content in the cathode catalyst layer and membrane, via back-diffusion to the anode. In addition, these perforated sheets were also used as a diagnostic tool in order to further investigate the role of cathode and anode MPLs. It was shown that the cathode MPL decreases the water saturation in the catalyst layer and improves water removal via the cathode GDL. It was also shown that the anode MPL plays a role in reducing voltage stability at high flow rates and flooding conditions. Perforated sheets were further explored for use as an engineered gas diffusion layer. This type of approach has the advantage that it can be tailored to specific parameters and conditions.Finally, a new flow field design, used on the cathode side, in which the active area can be modified, is presented and proven to improve the cell voltage and power stability at low power levels. This method increases the effective flow rate inside the flow field by decreasing the active area, resulting in the removal of liquid water and improving the gas diffusion to the cathode catalyst layer. This novel design can also be used to improve cell-to-cell water and gas distribution in fuel cell stacks.

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A key objective of this work is to address a number of the central issues associated with the direct methanol fuel cell (DMFC) through the investigation of a redox flow battery (RFB) / DMFC hybrid fuel cell. The air cathode (Pt/carbon) of the DMFC is substituted by a Fe²⁺/Fe³⁺ redox couple cathode (carbon) with no platinum-group metal (PGM) catalyst. In this configuration, referred to as the direct liquid redox fuel cell (DLRFC), the Fe²⁺/Fe³⁺ redox couple cathode is selective to the redox couple reaction and fuel crossover does not cause cathode depolarization. A wide range of anolyte fuel concentrations were tested (2-24 M CH₃OH) and the best DLRFC performance was obtained at 16.7 M CH₃OH (equimolar CH₃OH / H₂O).A significant improvement in the DLRFC performance and catholyte charge density was obtained by switching from a sulfate-based iron salt to a perchlorate-based iron salt. This led to a greater than 150% increase in the solubility of the redox couple, greater than a 200 mV increase in the equilibrium half-cell potential of the redox couple and a greater than 200% improvement in the DLRFC peak power density (79 mW/cm² vs. 25 mW/cm²) relative to the sulfate-based system.The selective nature of the redox cathode enabled the demonstration and characterization of a novel mixed-reactant DLRFC (MR-DLRFC) where a mixed electrolyte containing methanol and the redox couple is fed to the cathode and fuel crossover is the mode of fuel supply. A non-optimized peak power density of 15 mW/cm² was obtained with this system.A novel in-situ redox couple regeneration approach was also demonstrated and characterized, which involved substituting the methanol anolyte by an air stream. This approach exploits the hybrid nature of the DLRFC and utilizes the PtRu catalyst at the DLRFC anode as an O₂ reduction cathode during regeneration.Eliminating the use of PGM catalysts at the fuel cell cathode and enabling the use of high fuel concentrations are decisive advantages of DLRFC technology. Furthermore, the ability to extend DLRFC technology to a mixed-reactant architecture where fuel crossover is desirable paves new ground for the future of fuel cell research.

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The convergence of multiple functions in portable electronics is resulting in greater power requirements and a reduced operation time. The incumbent battery technology is not projected to accommodate these requirements. An attractive alternative is the direct liquid fuel cell, in particular the polymer electrolyte membrane (PEM) based direct methanol fuel cell (DMFC), as it does not suffer from the disadvantages associated with conventional battery technology and has the potential for extended and continuous operation. However, the wide spread adoption of the DMFC is prevented by a significant number of barriers that include: fuel crossover, catalyst and fuel utilization, efficiency, overall cost and size. The research presented in this thesis aims to address these areas through the development of simplified cell architectures and operational methods.In a conventional membrane electrode assembly (MEA), a PEM is compressed between an anode and cathode electrode. In this research a new branch of simplified architectures that is unique from those that have been reported in literature has been developed by eliminating and/or integrating key components of a conventional MEA. The membraneless 3D anode approach was shown to be fuel independent and scaleable to a conventional bipolar fuel cell arrangement and exhibits comparable performance to a conventional passive DMFC at ambient conditions (25C, 1 atm). The single electrode supported DMFC was fabricated through a sequential deposition of an anode catalyst layer, an electrically insulating layer and a cathode catalyst layer onto a single carbon fibre paper substrate. This resulted in a 42% reduction in thickness and a 104% improvement in volumetric specific power density over a two electrode DMFC configuration.In addition, simple methods to control fuel crossover and power output were developed and characterized. A perforated graphitic diffusion barrier with engineered properties reduced fuel crossover in the range of ~73% to ~94%. The power output of the membraneless DMFC was controlled through a selective activation/deactivation of triple phase boundary regions on the electrode assembly with a physical guard. This method enabled the DMFC to operate at a single optimized condition where the voltage, current density, crossover and overall efficiency were constant at any power level.

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