Thesis (M.S.)--West Virginia University, 2003. / Title from document title page. Document formatted into pages; contains ix, 82 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 75-78).

The primary goal of this research was to investigate the effects associated with the application of magnetic fields to single electrodes.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE / Includes bibliographical references. / by Michael Stoukides. / Ph.D.

This study has been mainly focused on high temperature solid oxide electrolysis cells (HT-SOECs) for steam electrolysis. The compositions, microstructures and metal catalysts for SOEC cathodes based on (La₀.₇₅Sr₀.₂₅)₀.₉₅Mn₀.₅Cr₀.₅O₃ (LSCM) have been investigated. Hydrogen production amounts from SOECs with LSCM cathodes have been detected and current-to-hydrogen efficiencies have been calculated. The effect of humidity on electrochemical performances from SOECs with cathodes based on LSCM has also been studied. LSCM has been applied as the main composite in HT-SOEC cathodes in this study. Cells were measured at temperatures up to 920°C with 3%steam/Ar/4%H₂ or 3%steam/Ar supplied to the steam/hydrogen electrode. SOECs with LSCM cathodes presented better stability and electrochemical performances in both atmospheres compared to cells with traditional Ni cermet cathodes. By mixing materials with higher ionic conductivity such as YSZ(Y₂O₃-stabilized ZrO₂ ) and CGO(Ce₀.₉Gd₀.₁O₁.₉₅ ) into LSCM cathodes, the cell performances have been improved due to the enlarged triple phase boundary (TPB). Metal catalysts such as Pd, Fe, Rh, Ni have been impregnated to LSCM/CGO cathodes in order to improve cell performances. Cells were measured at 900°C using 3%steam/Ar/4%H₂ or 3%steam/Ar and AC impedance data and I-V curves were collected. The addition of metal catalysts has successfully improved electrochemical performances from cells with LSCM/CGO cathodes. Improving SOEC microstructures is an alternative to improve cell performances. Cells with thinner electrolytes and/or better electrode microstructures were fabricated using techniques such as cutting, polishing, tape casting, impregnation, co-pressing and screen printing. Thinner electrolytes gave reduced ohmic resistances, while better electrode microstructures were observed to facilitate electrode processes. Hydrogen production amounts under external potentials from SOECs with LSCM/CGO cathodes were detected by gas chromatograph and current-to-hydrogen efficiencies were calculated according to the law of conservation of charge. Current-to-hydrogen efficiencies from these cells at 900°C were up to 80% in 3%steam/Ar and were close to 100% in 3%steam/Ar/4%H₂. The effect of humidity on SOEC performances with LSCM/CGO cathodes has been studied by testing the cell in cathode atmospheres with different steam contents (3%, 10%, 20% and 50% steam). There was no large influence on cell performances when steam content was increased, indicating that steam diffusion to cathode was not the main limiting process.

Solar energy has the potential to meet all of society’s energy demands, but challenges remain in storing it for times when the sun is not shining. Electrolysis is a promising means of energy storage which applies solar-derived electricity to drive the production of chemical fuels. These so-called solar fuels, such as hydrogen gas produced from water electrolysis, can be fed back to the grid for electricity generation or used directly as a fuel in the transportation sector. Solar fuels can be generated by coupling a photovoltaic (PV) cell to an electrolyzer, or by directly converting light to chemical energy using a photoelectrochemical cell (PEC). Presently, both PV-electrolyzers and PECs have prohibitively high capital costs which prevent them from generating hydrogen at competitive prices. This dissertation explores the design of membraneless electrolyzers and PECs in order to simplify their design and decrease their overall capital costs. A membraneless water electrolyzer can operate with as few as three components: A cathode for the hydrogen evolution reaction, an anode for the oxygen evolution reaction, and a chassis for managing the flows of a liquid electrolyte and the product gas streams. Absent from this device is an ionically conducting membrane, a key component in a conventional polymer electrolyte membrane (PEM) electrolyzer that typically serves as a physical barrier for separating product gases generated at the anode and cathode. These membranes can allow for compact and efficient electrolyzer designs, but are prone to degradation and failure if exposed to impurities in the electrolyte. A membraneless electrolyzer has the opportunity to reduce capital costs and operate in non-pristine environments, but little is known about the performance limitations and design rules that govern operation of membraneless electrolyzers. These design rules require a thorough understanding of the thermodynamics, kinetics, and transport processes in electrochemical systems. In Chapter 2, these concepts are reviewed and a framework is provided to guide the continuum scale modeling of the performance of membraneless electrochemical cells. Afterwards, three different studies are presented which combine experiment and theory to demonstrate the mechanisms of product transport and efficiency loss. Chapter 3 investigates the dynamics of hydrogen bubbles during operation of a membraneless electrolyzer, which can strongly affect the product purity of the collected hydrogen. High-speed video imaging was implemented to quantify the size and position of hydrogen gas bubbles as they detach from porous mesh electrodes. The total hydrogen detected was compared to the theoretical value predicted by Faraday’s law. This analysis confirmed that not all electrochemically generated hydrogen enters the gas phase at the cathode surface. In fact, significant quantities of hydrogen remain dissolved in solution, and can result in lower product collection efficiencies. Differences in bubble volume fraction evolved along the length of the cathode reflect differences in the local current densities, and were found to be in agreement with the primary current distribution. Overall, this study demonstrates the ability to use in-situ HSV to quantitatively evaluate key performance metrics of membraneless electrolyzers in a non-invasive manner. This technique can be of great value for future experiments, where statistical analysis of bubble sizes and positions can provide information on how to collect hydrogen at maximum purity. Chapter 4 presents an electrode design where selective placement of the electrocatalyst is shown to enhance the purity of hydrogen collected. These “asymmetric electrodes” were prepared by coating only one planar face of a porous titanium mesh electrode with platinum electrocatalyst. For an opposing pair of electrodes, the platinum coated surface faces outwards such that the electrochemically generated bubbles nucleate and grow on the outside while ions conduct through the void spacing in the mesh and across the inter-electrode gap. A key metric used in evaluating the performance of membraneless electrolyzers is the hydrogen cross-over percentage, which is defined as the fraction of electrochemically generated hydrogen that is collected in the headspace over the oxygen-evolving anode. When compared to the performance of symmetric electrodes – electrodes coated on both faces with platinum – the asymmetric electrodes demonstrated significantly lower rates of cross-over. With optimization, asymmetric electrodes were able to achieve hydrogen cross-over values as low as 1%. These electrodes were then incorporated into a floating photovoltaic electrolysis device for a direct demonstration of solar driven electrolysis. The assembled “solar fuels rig” was allowed to float in a reservoir of 0.5 M sulfuric acid under a light source calibrated to simulate sunlight, and a solar to hydrogen efficiency of 5.3% was observed. In Chapter 5, the design principles for membraneless electrolyzers were applied to a photoelectrochemical (PEC) cell. Whereas an electrolyzer is externally powered by electricity, a PEC cell can directly harvest light to drive an electrochemical reaction. The PEC reactor was based on a parallel plate design, where the current was demonstrated to be limited by the intensity of light and the concentration of the electrolyte. By increasing the average flow rate of the electrolyte, mass transport limitations could be alleviated. The limiting current density was compared to theoretical values based off of the solution to a convection-diffusion problem. This modeled solution was used to predict the limitations to PEC performance in scaled up designs, where solar concentration mirrors could increase the total current density. The mass transport limitations of a PEC flow cell are also highly relevant to the study of CO2 reduction, where the solubility limit of CO2 in aqueous electrolyte can also limit performance.

Porous carbons are versatile materials with applications in different fields. They are used in filtration, separation and sequestration of fluids and gases, as conductive additives in many energy storage materials, as coloring agents, as pharmaceutical and food additives, and in many other vital technologies. Porous carbons produced by pyrolysis and activation of organic precursors commonly suffer from poorly controlled morphology, microstructure, chemistry, and pore structure. In addition, the poorly controlled parameters of porous carbons make it challenging to elucidate the underlying key physical parameters controlling their performance in energy storage devices, including electrochemical capacitors (ECs) and lead-acid batteries (LABs). Zeolite-templated carbons (ZTCs) are a novel class of porous carbon materials with uniform and controllable pore size, microstructure, morphology, and chemistry. In spite of their attractive properties, they have never been explored for use in LABs and their studies for ECs have been very limited. Here I report a systematic study of ZTCs applications in ECs operating at temperatures as low as - 70 C and in LABs. Greatly improved power and energy performance, compared to state of the art devices, has been demonstrated in the investigated ECs. Moreover, the application of ZTCs in LABs has resulted in a dramatic enhancement of their cycle life and power and energy densities.

Thesis (Ph.D.) - University of St Andrews, April 2007.

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Reducing carbon emissions is a looming challenge that will be required to limit global warming. One approach is to replace energy from fossil fuels with renewable electricity that has low carbon footprints. The continuous decrease of renewable electricity prices makes electrochemical processes very promising for environmentally friendly production of fuels and chemicals. One of the mature electrochemical processes is hydrogen (H₂) production from water electrolysis. If only excess solar/wind electricity is used to power water electrolyzers to produce green H₂, the intermittency of the electricity supply will require low-cost electrolyzer technologies. Emerging membraneless water electrolyzers offer an attractive approach to lowering the cost of H₂ production by eliminating membranes or diaphragms that are used in conventional water electrolyzers. One aim of this dissertation is to understand the performance limits of membraneless water electrolyzers compared to the conventional designs. Another key electrochemical process to reduce carbon emissions is the conversion of carbon dioxide (CO₂) to value-added fuels and chemicals. CO₂ captured from air or flue gas needs to be extracted from carbon capture solution and pressurized before feeding to a conventional CO₂ electrolyzer. In order to avoid these energy intensive steps to make pressurized CO₂, there is a growing interest in developing membrane-based electrolyzers that can directly utilize the carbon capture solution to conduct electrochemical CO₂ conversion. This dissertation also explores a scalable membrane-free electrolyzer design that can convert carbon capture solution to syngas. In Chapter 2, a parallel plate membraneless electrolyzer is used as a model system to demonstrate a combined experimental and modeling approach to explore its performance limits. This modeling framework quantitatively describes the trade-offs between efficiency, current density, electrode size, and product purity. Central to this work is the use of in situ high-speed videography (HSV) to monitor the width of H₂ bubble plumes produced downstream of parallel plate electrodes as a function of current density, electrode separation distance, and the Reynolds number (Re) associated with flowing 0.5 M H₂SO₄ electrolyte. These measurements reveal that the HSV-derived dimensionless bubble plume width serves as an excellent descriptor for correlating the aforementioned operating conditions with H₂ crossover rates. These empirical relationships, combined with electrochemical engineering design principles, provide a valuable framework for exploring performance limits and guiding the design of optimized membraneless electrolyzers. This framework shows that the efficiencies and current densities of optimized parallel plate membraneless electrolyzers constrained to H₂ crossover rates of 1% can exceed those of conventional alkaline electrolyzers but are lower than the efficiencies and current densities achieved by zero-gap polymer electrolyte membrane (PEM) electrolyzers. Chapter 3 presents a packed bed membraneless electrolyzer (PBME) design for which liquid bicarbonate electrolyte flows sequentially through alternating porous flow-through anodes and cathodes. Within this design, hydrogen oxidation at porous anodes is used to produce protons that trigger in situ CO₂ release immediately upstream of porous cathodes, where electrochemical CO₂ reduction generates the desired product and returns the solution pH back towards its inlet value. By using the sequential flow-cell arrangement, the PBME offers the ability to mitigate large concentration overpotentials and non-uniform current distributions that naturally arise during scale-up of conventional membrane-based devices that rely on lateral flow of catholyte parallel to the surface of the electrodes. This study uses in situ colorimetric imaging to highlight the ability of PBME to rebalance pH across electrodes. In addition, results obtained with a multi-cell PBME “stack” demonstrate the scalability of this concept and reveal the ability to increase CO₂ utilization from 12.9% for a single-cell PBME up to 20.5% for a four-cell PBME operated under baseline conditions. Modeling results indicate that current utilization values >80% are theoretically possible for optimized multi-cell PBMEs operated at ambient pressure. Chapter 4 demonstrates a stacked PBME design and an elevated pressure system. Hydrogen oxidation at the anode and hydrogen evolution at the cathode are conducted in this device at pressures up to 5 atm. The pressure of the system can be held at a constant value by the use of a back pressure regulator, and product gases can be collected with a gas sampling bag that’s directly connected to the back pressure regulator. The stereolithography 3D printing technology is used to fabricate components of the PBME from clear resin, which is suitable for the elevated pressure operation. Post-processing of the components makes surfaces transparent for imaging bubble dynamics in the device. Within this system, HOR current density of 120 mA cm-2 can be achieved at different pressures. Finally, Chapter 5 provides concluding remarks and discusses future opportunities and challenges for membraneless electrolyzers for water electrolysis and electrochemical CO₂ conversion.