Gas diffusion electrodes and operating conditions for electrochemical energy storage applications

The world today faces two contradictory challenges: climate change and energy security. Interestingly, both challenges can be potentially addressed by targeting atmospheric CO2. The global demand for energy has increased due to population growth and most of this demand is met by the dwindling resource of fossil fuels. Consequently, the concentration of atmospheric CO2 has risen past the upper safety limit of 350 ppm, and has even reached as high as 404 ppm. This is believed to be a major cause of several undesirable climate effects such as global warming and increased occurrence of erratic weather. Multifaceted efforts have been made to curb atmospheric CO2 levels and decrease reliance on fossil fuels by seeking out clean, affordable and reliable energy sources. Renewable energy sources (i.e., wind, tide, and solar) are increasingly competitive due to their natural, clean and carbon-free nature. However, renewable energy sources are intermittent, limited by geography and seasons, and often unpredictable. To overcome these limitations and supply energy generated by renewable energy sources more efficiently and continuously, a suitable form of large-scale storage for on-demand utilization is needed. This thesis researches two energy storage technologies that show promise to help address both challenges: electrochemical reduction of CO2 into useful feedstock chemicals or fuels, and lithium air battery. Electrochemical reduction of CO2 into value-added chemicals is expected to play an important role in reducing CO2 emissions and dependence on fossil fuels as well as in utilizing excess, otherwise wasted energy from intermittent renewable sources. However, to be a viable technology, performance levels of CO2 electrolyzers need to be raised by making electrodes and catalyst efficient enough for commercialization. This dissertation starts with discussing the interplay between cathode performance and CO2 concentration in the feed as well as electrolyte pH (Chapter 2). Use of diluted feed elevates the utilization of CO2 up to 31 % with high Faradaic efficiency for CO (>80%). This work highlights the importance of mass transport and indicates that the direct use of flue gas as a feed for electroreduction of CO2 has promise. This dissertation also reports a detailed investigation of the relationship between the physical structure of electrodes and electrochemical activity (Chapter 3) as well as further improvement of electrodes by incorporating carbon nanotubes for electroreduction of CO2 (Chapter 4). Optimized gas diffusion electrodes (GDEs) outperform commercially available GDEs and exhibit no decay in performance during continuous operation. In addition, micro-porous layers (MPLs) composed of carbon powder exhibit better durability leading to high cathode performance compared to MPLs composed of carbon nanotubes for electroreduction of CO2. Lithium air (Li-air) battery can be a promising candidate for effective storage of renewable energy and has applications ranging from portable electronics to electric vehicles because of its extremely high theoretical energy density. However, several fundamental challenges such as poor round-trip efficiency, unsatisfactory durability and safety must be overcome to realize the promise of Li-air battery. This dissertation employs the design and fabrication of a non-aqueous Li-air battery with flowing ionic liquid (Chapter 5). The flow configuration exhibits a substantial increase in discharge capacity compared with the non-flowing battery. Also, this dissertation reports experimental and computational investigations on optimizing the gas diffusion-based cathode to produce higher discharge current densities particularly for Li-air flow batteries (Chapter 6)

Gas diffusion electrodes and operating conditions for electrochemical energy storage applications

The world today faces two contradictory challenges: climate change and energy security. Interestingly, both challenges can be potentially addressed by targeting atmospheric CO2. The global demand for energy has increased due to population growth and most of this demand is met by the dwindling resource of fossil fuels. Consequently, the concentration of atmospheric CO2 has risen past the upper safety limit of 350 ppm, and has even reached as high as 404 ppm. This is believed to be a major cause of several undesirable climate effects such as global warming and increased occurrence of erratic weather. Multifaceted efforts have been made to curb atmospheric CO2 levels and decrease reliance on fossil fuels by seeking out clean, affordable and reliable energy sources. Renewable energy sources (i.e., wind, tide, and solar) are increasingly competitive due to their natural, clean and carbon-free nature. However, renewable energy sources are intermittent, limited by geography and seasons, and often unpredictable. To overcome these limitations and supply energy generated by renewable energy sources more efficiently and continuously, a suitable form of large-scale storage for on-demand utilization is needed. This thesis researches two energy storage technologies that show promise to help address both challenges: electrochemical reduction of CO2 into useful feedstock chemicals or fuels, and lithium air battery. Electrochemical reduction of CO2 into value-added chemicals is expected to play an important role in reducing CO2 emissions and dependence on fossil fuels as well as in utilizing excess, otherwise wasted energy from intermittent renewable sources. However, to be a viable technology, performance levels of CO2 electrolyzers need to be raised by making electrodes and catalyst efficient enough for commercialization. This dissertation starts with discussing the interplay between cathode performance and CO2 concentration in the feed as well as electrolyte pH (Chapter 2). Use of diluted feed elevates the utilization of CO2 up to 31 % with high Faradaic efficiency for CO (>80%). This work highlights the importance of mass transport and indicates that the direct use of flue gas as a feed for electroreduction of CO2 has promise. This dissertation also reports a detailed investigation of the relationship between the physical structure of electrodes and electrochemical activity (Chapter 3) as well as further improvement of electrodes by incorporating carbon nanotubes for electroreduction of CO2 (Chapter 4). Optimized gas diffusion electrodes (GDEs) outperform commercially available GDEs and exhibit no decay in performance during continuous operation. In addition, micro-porous layers (MPLs) composed of carbon powder exhibit better durability leading to high cathode performance compared to MPLs composed of carbon nanotubes for electroreduction of CO2. Lithium air (Li-air) battery can be a promising candidate for effective storage of renewable energy and has applications ranging from portable electronics to electric vehicles because of its extremely high theoretical energy density. However, several fundamental challenges such as poor round-trip efficiency, unsatisfactory durability and safety must be overcome to realize the promise of Li-air battery. This dissertation employs the design and fabrication of a non-aqueous Li-air battery with flowing ionic liquid (Chapter 5). The flow configuration exhibits a substantial increase in discharge capacity compared with the non-flowing battery. Also, this dissertation reports experimental and computational investigations on optimizing the gas diffusion-based cathode to produce higher discharge current densities particularly for Li-air flow batteries (Chapter 6).LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste

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A proto-type ESD generator for system immunity test of wearable devices

A proto-type ESD generator for system immunity test of wearable devices is proposed. A wearable device is charged up to an ESD test voltage together with the designed ESD generator, and discharged to the ESD current target. The proposed ESD generator for wearable devices is validated with a real ESD measurement

Strain-multiplexed meta-hologram using soft nanoimprint lithography

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Employing vanadium dioxide nanoparticles for flexible metasurfaces with switchable absorption properties at near-infrared frequencies

Using the simple interference interactions in a three-layer thin film structure, absorbers in the near infrared with aesthetically pleasing reflective colouration were designed, fabricated, and characterised. By implementing the phase change material, vanadium dioxide (VO2), with its remarkable phase change properties, the absorbers are able to be switched between lower and higher absorption states depending on the external temperature. Conventional fabrication methods involving VO(2)require an annealing process after deposition, but here, VO(2)nanoparticles dispersed in a polymer mixture were employed to allow the simple and scalable spin coating process to be used, without the need for annealing. This simultaneously opens up the possibility of using flexible substrates for bendable devices. At a temperature of around 68 degrees C, a change in absorption of around 30% is observed between 800-1600 nm, while the vivid subtractive colours are maintained with almost no observable difference, on both silicon and flexible polymer-based substrates. The fabricated sample is robust to 2500 bending cycles, proving the possibility for scalable VO(2)fabrication methods for practical applications.11Nsciescopu

Effect of ALD Processes on Physical and Electrical Properties of HfO2 Dielectrics for the Surface Passivation of a CMOS Image Sensor Application

11Nsciescopu

Metasurface Holography Reaching the Highest Efficiency Limit in the Visible via One-Step Nanoparticle-Embedded-Resin Printing

Metasurface holography, the reconstruction of holographic images by modulating the spatial amplitude and phase of light using metasurfaces, has emerged as a next-generation display technology. However, conventional fabrication techniques used to realize metaholograms are limited by their small patterning areas, high manufacturing costs, and low throughput, which hinder their practical use. Herein, a high efficiency hologram using a one-step nanomanufacturing method with a titanium dioxide nanoparticle-embedded-resin, allowing for high-throughput and low-cost fabrication is demonstrated. At a single wavelength, a record high theoretical efficiency of 96.9% is demonstrated with an experimentally measured conversion efficiency of 90.6% and zero-order diffraction of 7.3% producing an ultrahigh-efficiency, twin-image free hologram that can even be directly observed under ambient light conditions. Moreover, a broadband meta-atom with an average efficiency of 76.0% is designed, and a metahologram with an average efficiency of 62.4% at visible wavelengths from 450 to 650 nm is experimentally demonstrated.11Nsciescopu

Metasurface Holography over 90% Efficiency in the Visible via Nanoparticle-Embedded-Resin Printing

Metasurface holography, the reconstruction of holographic images by modulating the spatial amplitude and phase of light using metasurfaces, has emerged as a next-generation display technology. However, conventional fabrication techniques used to realize metaholograms are limited by their small patterning areas, high manufacturing costs, and low throughput, which hinder their practical use. Herein, we demonstrate a high efficiency hologram using a one-step nanomanufacturing method with a titanium dioxide nanoparticle-embedded-resin, allowing for high-throughput and low-cost fabrication. At a single wavelength, a record high 96.4% theoretical efficiency is demonstrated with an experimentally measured conversion efficiency of 90.6% and zero-order diffraction of 7.3% producing an ultrahigh-efficiency, twin-image free hologram, that can even be directly observed under ambient light conditions. Moreover, we design a broadband meta-atom with an average efficiency of 76.0% and experimentally demonstrate a metahologram with an average efficiency of 62.4% at visible wavelengths from 450 to 650 nm