Photoelectrochemical (PEC) water splitting is a promising approach to convert renewable solar energy to clean hydrogen (H2) fuels in one simple step. Although \u2162-\u2164 semiconductors are attractive candidates as light-absorbers in tandem solar-fuel devices, their long-term stability for the hydrogen-evolution reaction (HER) in either acidic or alkaline aqueous electrolytes needs to be established. Chapter 2-5 of this thesis first aims at revealing the underlying corrosion chemistry for a variety of \u2162-\u2164 semiconductors specifically under the HER conditions, offering a rational understanding towards the stability of semiconductor photoelectrode.
\r\n \r\nIn Chapter 2, we start from p-InP and reveal its susceptibility to cathodic photocorrosion forming metallic In0, which however can be completely mitigated by the presence of Pt catalyst due to kinetic stabilization. We also show that the resulting PEC performance of p-InP/Pt electrodes is sensitive to the changes in surface stoichiometry, whereas an InOx-rich surface developed in KOH caused a substantial degradation in the current density-potential (J-E) behavior. In Chapter 3, we discovered that a non-stoichiometric and As0-rich surface of p-GaAs, resulting from a galvanic corrosion by Pt, led to mid-gap surface states as well as a complete loss in photoactivity. In Chapter 4-5, we demonstrate similar kinetic stabilization applied to both p-InGaP2/Pt and pn+-InGaP2/Pt photocathodes for the HER at both pH 0 and pH 14. Additionally, we found that the corrosion of underlying GaAs substrates for the pn+-InGaP2/Pt photocathodes at positive potentials caused damage of structural integrity as well as instability in electrode performance. Altogether these works underscore the mutual dependence of the physical and electrochemical stability of semiconductor photoelectrodes during the HER, which also need to be considered separately. Moreover, both catalytic kinetics and surface stoichiometry are crucial factors for defining long-term corrosion chemistry for semiconductor photoelectrode.
\r\n \r\nIn Chapter 6-7, we further explore solar fuels beyond H2, namely electrochemical N2-to-NH3 conversion. We first establish a new analytical method to isotopically quantify the concentrations of 15NH3 in aqueous solutions with a high sensitivity and a low limit-of-detection of <1 \u03bcM. Further we applied this advanced method to rigorously verify the electrocatalytic activity of a CoMo electrode for reducing N2(g) to NH3. We show that the additional ammonia detected in electrolyte was instead attributed to the corrosion of N impurities present in the CoMo electrode under cathodic bias, thus giving false positive results. These works emphasize the importance of both rigorous product analysis and experiment design in further catalyst development.
" }, { "name": "Cheng, Wen-Hui (Sophia)", "degree": "PhD", "year": "2020", "title": "Towards High Solar to Fuel Efficiency: From Photonic Design, Interface Study, to Device Integration", "advisor": "Atwater, Harry Albert", "url": "https://resolver.caltech.edu/CaltechTHESIS:05282020-183139057", "creators": [ { "name": { "family": "Cheng", "given": "Wen-Hui (Sophia)" }, "id": "Cheng-Wen-Hui-Sophia", "orcid": "0000-0003-3233-4606", "display_name": "Cheng, Wen-Hui (Sophia)" } ], "advisors": [ { "name": { "family": "Atwater", "given": "Harry Albert" }, "id": "Atwater-H-A", "orcid": "0000-0001-9435-0201", "role": "advisor", "display_name": "Atwater, Harry Albert" } ], "committee": [ { "name": { "family": "Goddard", "given": "William A., III" }, "id": "Goddard-W-A-III", "orcid": "0000-0003-0097-5716", "role": "chair", "display_name": "Goddard, William A., III" }, { "name": { "family": "Atwater", "given": "Harry Albert" }, "id": "Atwater-H-A", "orcid": "0000-0001-9435-0201", "role": "member", "display_name": "Atwater, Harry Albert" }, { "name": { "family": "Johnson", "given": "William Lewis" }, "id": "Johnson-W-L", "role": "member", "display_name": "Johnson, William Lewis" }, { "name": { "family": "Houle", "given": "Frances A." }, "id": "Houle-Frances-A", "orcid": "0000-0001-5571-2548", "role": "member", "display_name": "Houle, Frances A." } ], "option_major": [ "matsci" ], "doi": "10.7907/kd6a-xt88", "abstract": "Efficient unassisted solar fuel generation, a pathway to storable renewable energy in the form of chemical bonds, requires optimization of a photoelectrochemical device based on photonic design and interface study. We first focused on enhancing absorption via nanophotonic design of light absorbers. Near-unity, broadband absorption in sparse InP nanowire arrays with multi-radii and tapered nanowire array designs are simulated and experimentally demonstrated. Later, a few strategies are introduced to achieved high solar-to-fuel efficiency.
\r\n\r\nOptically, photoelectrochemical device would require the catalyst ensembles to be highly transparent. We report a record solar-to-hydrogen efficiency by integrating Rh nanoparticle catalysts onto photocathodes with minimal parasitic absorption and reflection losses in the visible range. The other two light management strategies have been developed and experimentally verified to create highly active and effectively transparent catalyst structures: i) arrays of mesophotonic dielectric cone structures that serve as tapered waveguide light couplers to efficiently guide incident light through apertures in an opaque catalyst into the light absorber, and ii) an effectively transparent catalyst consisting of arrays of micron-scale triangular cross-sectional metal grid fingers, which are capable of redirecting the incoming light to the open areas of the PEC cell without shadow loss.
\r\n\r\nThe electronic properties of the surface films exposed to the electrolyte are also critical. The anatase TiO\u2082 protection layer on the photocathode creates a favorable internal band alignment for hydrogen evolution, promoting the transport of the excess electrons and inhibiting voltage drops. The interfacial conduction mechanism between the defected TiO\u2082 and metal catalysts is investigated. A combinatorial approach of electrochemistry, X-ray photoelectron spectroscopy, and resonant X-ray spectroscopy reveals the correlation between the interfacial quasi-metal phase with TiO\u2082 properties. By careful control of gas diffusion electrode assembling to maintain appropriate wetted catalyst interface, another record solar-to-CO efficiency with extended stability can be realized.
" }, { "name": "Moreno-Hernandez, Ivan A.", "degree": "PhD", "year": "2019", "title": "Earth-Abundant Metal Oxides for Anodic Reactions in Acidic Electrolytes", "advisor": "Lewis, Nathan Saul", "url": "https://resolver.caltech.edu/CaltechTHESIS:06042019-235050436", "creators": [ { "name": { "family": "Moreno-Hernandez", "given": "Ivan A." }, "id": "Moreno-Hernandez-Ivan-A", "orcid": "0000-0001-6461-9214", "display_name": "Moreno-Hernandez, Ivan A." } ], "advisors": [ { "name": { "family": "Lewis", "given": "Nathan Saul" }, "id": "Lewis-N-S", "orcid": "0000-0001-5245-0538", "role": "advisor", "display_name": "Lewis, Nathan Saul" } ], "committee": [ { "name": { "family": "Okumura", "given": "Mitchio" }, "id": "Okumura-M", "orcid": "0000-0001-6874-1137", "role": "chair", "display_name": "Okumura, Mitchio" }, { "name": { "family": "Lewis", "given": "Nathan Saul" }, "id": "Lewis-N-S", "orcid": "0000-0001-5245-0538", "role": "member", "display_name": "Lewis, Nathan Saul" }, { "name": { "family": "Gray", "given": "Harry B." }, "id": "Gray-H-B", "orcid": "0000-0002-7937-7876", "role": "member", "display_name": "Gray, Harry B." }, { "name": { "family": "See", "given": "Kimberly" }, "id": "See-Kimberly", "orcid": "0000-0002-0133-9693", "role": "member", "display_name": "See, Kimberly" } ], "option_major": [ "chemistry" ], "doi": "10.7907/XRN5-FV98", "abstract": "The development of electrochemical systems such as electrolyzers and photoelectrochemical devices in corrosive electrolytes has been limited by the lack of earth-abundant materials that are both stable in acidic electrolytes and efficiently utilize energy for electrochemical reactions. Chapter 1 introduces several of the challenges in developing earth-abundant materials for electrochemical systems in acidic electrolytes, such as electrocatalysts for the oxygen and the chlorine evolution reactions, and protective layers for photoanodes. Chapter 2 reports the electrochemical behavior of crystalline transition metal antimonates consisting of solid solutions of MnSb2O6 with NiSb2O6 for the oxygen evolution reaction in strongly acidic electrolytes. In Chapter 3, the crystalline transition metal antimonates NiSb2O6, CoSb2O6, and MnSb2O6 are investigated for the chlorine evolution reaction, and CoSb2O6 is found to exhibit activity and stability comparable to noble metal oxide electrocatalysts. Chapter 4 describes the development of earth-abundant SnOx coatings as protective heterojunctions for planar Si photoanodes in corrosive electrolytes. Chapter 5 focuses on the development of conformal SnOx coatings that form protective heterojunctions on Si microcone photoanodes. The work presented herein demonstrates several strategies towards the development of stable earth-abundant materials for efficient electrochemical and photoelectrochemical energy conversion in acidic electrolytes.
" }, { "name": "Jiang, Jingjing", "degree": "PhD", "year": "2018", "title": "Interfacial and Stability Studies of Photocathodes for Hydrogen Evolution", "advisor": "Lewis, Nathan Saul", "url": "https://resolver.caltech.edu/CaltechTHESIS:06112018-153323554", "creators": [ { "name": { "family": "Jiang", "given": "Jingjing" }, "id": "Jiang-Jingjing", "orcid": "0000-0002-3109-229X", "display_name": "Jiang, Jingjing" } ], "advisors": [ { "name": { "family": "Lewis", "given": "Nathan Saul" }, "id": "Lewis-N-S", "role": "advisor", "display_name": "Lewis, Nathan Saul" } ], "committee": [ { "name": { "family": "Lewis", "given": "Nathan Saul" }, "id": "Lewis-N-S", "role": "chair", "display_name": "Lewis, Nathan Saul" }, { "name": { "family": "Johnson", "given": "William Lewis" }, "id": "Johnson-W-L", "role": "member", "display_name": "Johnson, William Lewis" }, { "name": { "family": "Goddard", "given": "William A., III" }, "id": "Goddard-W-A-III", "role": "member", "display_name": "Goddard, William A., III" }, { "name": { "family": "Gray", "given": "Harry B." }, "id": "Gray-H-B", "role": "member", "display_name": "Gray, Harry B." } ], "option_major": [ "matsci" ], "doi": "10.7907/SBJ9-D062", "abstract": "Photoelectrochemical (PEC) water splitting is a promising way to generate clean hydrogen fuel from water and sunlight. The ideal photocathodes for hydrogen evolution reaction (HER) should have good electrical contact and mechanical adhesion on the interface between the semiconductor and the catalyst, and be stable during operation. However, the interfacial properties and the stability have not been intensively studied. We investigated the electrical and mechanical properties on the nanoscale of the interface of commonly used Si/Pt nanoparticles (Pt-NPs) electrodes with Pt-NPs as a catalyst, and showed that the Pt-NPs have a weaker adhesion in electrolyte than in air, and less than half of the Pt-NPs carry high currents, limiting the performance of the common Si/Pt-NPs electrodes. Furthermore, we explored the interfacial engineering of using TiO2 deposited by atomic layer deposition (ALD), and showed that annealed TiO2 led to higher open circuit voltages than the as grown ones by the possible formation of an interfacial Si-O-Ti mixture layer. Besides, the stability and corrosion behavior of CdTe electrodes for HER in the dark was studied in 1.0 M H2SO4(aq) and 1.0 M KOH(aq). The conditions studied herein include the electrochemical corrosion when biased at -100 mV vs. the reversible hydrogen electrode (RHE), the chemical corrosion when left at open circuit voltage (OCV), and the electrochemical corrosion with an active HER Pt catalyst overlayer when biased at -100 mV vs. RHE. The corrosion comes mostly from chemical corrosion and is reduced at negative bias in electrochemical condition. With a Pt catalyst overlayer at -100 mV vs. RHE, the corrosion rate is further reduced, indicating the promising utilization of CdTe for HER in PEC cells.
" }, { "name": "Lichterman, Michael Yang", "degree": "PhD", "year": "2018", "title": "Analysis of Metal-Oxide Protected Photoelectrochemical Systems for Water Splitting", "advisor": "Lewis, Nathan Saul; Gray, Harry B.", "url": "https://resolver.caltech.edu/CaltechTHESIS:06122018-090745040", "creators": [ { "name": { "family": "Lichterman", "given": "Michael Yang" }, "id": "Lichterman-Michael-Frankston-Yang", "orcid": "0000-0002-0710-7068", "display_name": "Lichterman, Michael Yang" } ], "advisors": [ { "name": { "family": "Lewis", "given": "Nathan Saul" }, "id": "Lewis-N-S", "role": "co-advisor", "display_name": "Lewis, Nathan Saul" }, { "name": { "family": "Gray", "given": "Harry B." }, "id": "Gray-H-B", "role": "co-advisor", "display_name": "Gray, Harry B." } ], "committee": [ { "name": { "family": "Grubbs", "given": "Robert H." }, "id": "Grubbs-R-H", "role": "chair", "display_name": "Grubbs, Robert H." }, { "name": { "family": "Weitekamp", "given": "Daniel P." }, "id": "Weitekamp-D-P", "role": "member", "display_name": "Weitekamp, Daniel P." }, { "name": { "family": "Brunschwig", "given": "Bruce S." }, "id": "Brunschwig-B-S", "role": "member", "display_name": "Brunschwig, Bruce S." }, { "name": { "family": "Lewis", "given": "Nathan Saul" }, "id": "Lewis-N-S", "role": "member", "display_name": "Lewis, Nathan Saul" }, { "name": { "family": "Gray", "given": "Harry B." }, "id": "Gray-H-B", "role": "member", "display_name": "Gray, Harry B." } ], "option_major": [ "chemistry" ], "doi": "10.7907/aq89-jv13", "abstract": "The photoelectrochemical splitting of water into oxygen and hydrogen gas is one pathway toward the renewable and economic generation of a fuel which is sufficiently scalable to power a large fraction, or even a majority, of the power requirements of\r\nmodern society. In order to make such a device economically promising, it must be sufficiently cheap, have sufficiently high efficiency, or some combination thereof. In this work, two primary routes toward such a device are discussed; the first is the use of a cheaply prepared photoanode material, BiVO4, the interactions of this material with cobalt oxide based catalysts, and the use of such structures in more extreme pH ranges than have previous been reported. The second route details the application of a protective layer, TiO2, on otherwise unstable materials such as GaP and CdTe when operated as\r\nphotoanodes in alkaline media. The further work herein applies operando ambientpressure x-ray photoelectron spectroscopy (AP-XPS) to understand the nature of the energetics which allow conduction in the aforementioned TiO2, as well as other\r\nenergetics in the electrochemical double layer in the adjacent electrolyte. Further experiments using Raman spectroscopy on associated III-V photoanode devices are also described." }, { "name": "Torelli, Daniel Anthony", "degree": "PhD", "year": "2018", "title": "The Discovery of Novel Materials for the Electrocatalytic Reduction of Carbon Dioxide", "advisor": "Lewis, Nathan Saul", "url": "https://resolver.caltech.edu/CaltechTHESIS:05292018-142628914", "creators": [ { "name": { "family": "Torelli", "given": "Daniel Anthony" }, "id": "Torelli-Daniel-Anthony", "orcid": "0000-0002-6222-817X", "display_name": "Torelli, Daniel Anthony" } ], "advisors": [ { "name": { "family": "Lewis", "given": "Nathan Saul" }, "id": "Lewis-N-S", "role": "advisor", "display_name": "Lewis, Nathan Saul" } ], "committee": [ { "name": { "family": "Gray", "given": "Harry B." }, "id": "Gray-H-B", "role": "chair", "display_name": "Gray, Harry B." }, { "name": { "family": "Okumura", "given": "Mitchio" }, "id": "Okumura-M", "role": "member", "display_name": "Okumura, Mitchio" }, { "name": { "family": "Goddard", "given": "William A., III" }, "id": "Goddard-W-A-III", "role": "member", "display_name": "Goddard, William A., III" }, { "name": { "family": "Lewis", "given": "Nathan Saul" }, "id": "Lewis-N-S", "role": "member", "display_name": "Lewis, Nathan Saul" } ], "option_major": [ "chemistry" ], "doi": "10.7907/RBD5-B141", "abstract": "The removal of atmospheric carbon dioxide is likely the only route to mitigating the effects of decades of increased fossil fuel combustion. Artificial photosynthesis presents one method for removal and conversion of problematic carbon dioxide into chemically useful products. By coupling electrochemical CO2 reduction (CO2R) to a renewable energy source atmospheric CO2 could be converted back into a fuel such as ethanol, or a commodity chemical such as ethylene. These products could then be consumed for energy or used to generate plastics effectively removing CO2 from the atmosphere. Significant advances in current electrocatalysts are needed in order for large scale CO2R to become a reality. Most known catalysts are only capable of transferring 2 electrons with needed protons to CO2 producing either carbon monoxide or formic acid. Copper is the only known metal capable of reducing CO2 to hydrocarbons at appreciable rates and low overpotentials. This work aims to find new materials that produce similar hydrocarbons, but at lower overpotentials with higher rates and greater selectivity than current copper catalysts. By implementing a cyclic process referred to as the Catalyst Discovery Cycle (CDC) iterations between predications, catalyst testing, and active site characterization allow for the rational design and discovery of new and improved catalysts. This methodology led to the discovery of nickel-gallium bimetallics as low overpotential catalysts for CO2R to methane, ethylene, and ethane. In addition, theoretical and experimental observations have determined a proposed active site and side reactions detrimental to their activity. " }, { "name": "Zhou, Xinghao", "degree": "PhD", "year": "2018", "title": "Performance and Stability Optimization of Solar Fuel Devices", "advisor": "Lewis, Nathan Saul", "url": "https://resolver.caltech.edu/CaltechTHESIS:05062018-164727269", "creators": [ { "name": { "family": "Zhou", "given": "Xinghao" }, "id": "Zhou-Xinghao", "orcid": "0000-0001-9229-7670", "display_name": "Zhou, Xinghao" } ], "advisors": [ { "name": { "family": "Lewis", "given": "Nathan Saul" }, "id": "Lewis-N-S", "role": "advisor", "display_name": "Lewis, Nathan Saul" } ], "committee": [ { "name": { "family": "Lewis", "given": "Nathan Saul" }, "id": "Lewis-N-S", "role": "chair", "display_name": "Lewis, Nathan Saul" }, { "name": { "family": "Faber", "given": "Katherine T." }, "id": "Faber-K-T", "role": "member", "display_name": "Faber, Katherine T." }, { "name": { "family": "Goddard", "given": "William A., III" }, "id": "Goddard-W-A-III", "role": "member", "display_name": "Goddard, William A., III" }, { "name": { "family": "Johnson", "given": "William Lewis" }, "id": "Johnson-W-L", "role": "member", "display_name": "Johnson, William Lewis" } ], "option_major": [ "matsci" ], "doi": "10.7907/SSNP-XW29", "abstract": "Fossil fuels enabled the Industrial Revolution, and have been the most important power for promoting the world's economic growth ever since. However, burning fossil fuels have also been causing severe air pollution, and global warming is also related to excessive use of fossil fuels. Solar energy is considered to be the largest renewable clean energy resource. The principal problems of solar energy are low energy concentration and intermittency. Storing solar energy in chemical bonds, similar to photosynthesis in nature, is a possible way to overcome these two problems. Carbon-free chemicals, like hydrogen gas produced by solar-driven water splitting, or carbon-neutral chemicals, like methane, ethylene, formic acid, carbon monoxide, etc. produced from solar-driven CO2 reduction, are all promising clean fuels for solar storage, as they feature high energy/power intensity, are easy and cheap to store and transport, and have direct interface with existing infrastructures.
\r\n\r\nIn this thesis, we focus on improving the efficiency and stability of the solar-driven fuel generation devices, which consist of (photo-)anode and (photo-)cathode. For the anode part, cobalt oxide Co3O4 ultrathin (2 nm) films by atomic layer deposition (ALD) were deposited onto silicon photoanode prior to deposition of thick nickel oxide (NiOx) layers. The photovoltage of the photoanode increased from 200 mV to 580 mV after including the interfacial Co3O4 layer, and the anode was stable in 1.0 M KOH(aq) for 1700 hours, which was equivalent to one year of operation in the field at a maximum photocurrent density of 30 mA/cm2 assuming a 20% solar capacity factor. Furthermore, the non-uniform sputtered (NiOx) layer of the n-Si/SiOx/Co3O4/NiOx photoanode was removed, and the 2 nm Co3O4 film was thickened to 50 nm, and the stability of n-Si/SiOx/50 nm-Co3O4 was improved to 2500 hours with lower efficiency decay rate. For the cathode part, an optimized Pd/C nanoparticle coated Ti mesh cathode exhibited < 100 mV overpotential at 8.5 mA/cm2 current density, and > 94% Faradaic efficiency for the reduction of 1 atm of CO2(g) to formate in 2.8 M KHCO3. A solar-driven CO2 reduction (CO2R) cell was constructed with this cathode, showing 10% solar-to-fuels conversion efficiency.
\r\n\r\nThis thesis can be divided into three parts. The first part discusses importance of solar fuels, as well as gives an introduction of solar-fuel generators. The second part includes Chapter II and Chapter III, which deal with performance improvement of silicon photoanode with ALD Co3O4 thin films. The third part is Chapter IV, in which we study the cathode for CO2 reduction to formate, and demonstrate a 10% efficiency solar-driven CO2 reduction cell with the cathode.
" }, { "name": "Saadi, Fadl Hussein", "degree": "PhD", "year": "2017", "title": "Acid-Stable Electrocatalysts for the Solar Production of Fuels", "advisor": "Lewis, Nathan Saul", "url": "https://resolver.caltech.edu/CaltechTHESIS:01132017-091611769", "creators": [ { "name": { "family": "Saadi", "given": "Fadl Hussein" }, "id": "Saadi-Fadl-Hussein", "orcid": "0000-0003-3941-0464", "display_name": "Saadi, Fadl Hussein" } ], "advisors": [ { "name": { "family": "Lewis", "given": "Nathan Saul" }, "id": "Lewis-N-S", "role": "advisor", "display_name": "Lewis, Nathan Saul" } ], "committee": [ { "name": { "family": "Lewis", "given": "Nathan Saul" }, "id": "Lewis-N-S", "role": "chair", "display_name": "Lewis, Nathan Saul" }, { "name": { "family": "Gray", "given": "Harry B." }, "id": "Gray-H-B", "role": "member", "display_name": "Gray, Harry B." }, { "name": { "family": "Goddard", "given": "William A., III" }, "id": "Goddard-W-A-III", "role": "member", "display_name": "Goddard, William A., III" }, { "name": { "family": "Atwater", "given": "Harry Albert" }, "id": "Atwater-H-A", "role": "member", "display_name": "Atwater, Harry Albert" }, { "name": { "family": "Soriaga", "given": "Manuel" }, "id": "Soriaga-M-P", "role": "member", "display_name": "Soriaga, Manuel" } ], "option_major": [ "matsci" ], "doi": "10.7907/Z9QF8QV4", "abstract": "Sunlight is one of the few renewable resources that can meet global energy demand. Unfortunately, while solar energy has grown in the past few years, several economic and scientific constraints have hindered mass adoption. One of the main obstacles solar energy faces is the lack of economically competitive storage technologies. Artificial photosynthesis is a potential solution in which solar energy is directly converted into energy dense chemical bonds that can be easily stored and transported.
\r\n\r\nOne impediment facing the commercialization of artificial photosynthesis is the use of expensive and rare precious metals as catalysts. This dissertation focuses on the achievements of the past five years in characterizing novel, earth-abundant, acid-stable hydrogen evolution catalysts. While nickel alloys have long been known as catalysts for the hydrogen evolution reaction in basic media, it has only been in the past decade that earth abundant catalysts that are stable in acidic media have been reported. These discoveries are critically important as the many proposed artificial photosynthetic devices require the use of acidic media.
\r\n\r\nIn this dissertation we examine two families of hydrogen evolution catalysts: transition metal chalcogenides (namely molybdenum and cobalt selenide) as well as transition metal phosphides (cobalt phosphide). In addition to the electrochemical characterization of these catalysts, spectroscopic characterizations were performed in order to carefully examine the chemical compositions of these catalysts before, after and during the hydrogen evolution reaction. This analysis elucidated both chemical, and structural changes that occurred after the catalysts had been subject to the hydrogen evolution reaction conditions.
\r\n\r\nThe final chapter in this thesis delves into the techno-economic realities of energy transportation via different fuels. Due to the strong interest in renewable energy, several future energy transportation scenarios, including 100% grid electrification and widespread installation of hydrogen pipelines, have been proposed. In order to get a fuller understanding of such potential infrastructure alternatives, we report their differing energy transportation costs.
" }, { "name": "Verlage, Erik A.", "degree": "PhD", "year": "2017", "title": "High-Efficiency Solar Fuel Devices: Protection and Light Management Utilizing TiO2", "advisor": "Atwater, Harry Albert", "url": "https://resolver.caltech.edu/CaltechTHESIS:06012017-152250262", "creators": [ { "name": { "family": "Verlage", "given": "Erik A." }, "id": "Verlage-Erik-A", "orcid": "0000-0001-5940-0859", "display_name": "Verlage, Erik A." } ], "advisors": [ { "name": { "family": "Atwater", "given": "Harry Albert" }, "id": "Atwater-H-A", "role": "advisor", "display_name": "Atwater, Harry Albert" } ], "committee": [ { "name": { "family": "Atwater", "given": "Harry Albert" }, "id": "Atwater-H-A", "role": "chair", "display_name": "Atwater, Harry Albert" }, { "name": { "family": "Lewis", "given": "Nathan Saul" }, "id": "Lewis-N-S", "role": "member", "display_name": "Lewis, Nathan Saul" }, { "name": { "family": "Goddard", "given": "William A., III" }, "id": "Goddard-W-A-III", "role": "member", "display_name": "Goddard, William A., III" }, { "name": { "family": "Gray", "given": "Harry B." }, "id": "Gray-H-B", "role": "member", "display_name": "Gray, Harry B." } ], "option_major": [ "matsci" ], "doi": "10.7907/Z9MC8X2P", "abstract": "Global climate change coupled with increasing global energy consumption drives the need for renewable and carbon-neutral alternatives to fossil fuels. Photoelectrochemical devices store solar energy in chemical bonds, and have the potential to provide cost-effective fuel for grid-scale energy storage as well as to serve as a feedstock for the production of carbon-neutral transportation fuels. A widely recognized goal is the demonstration of a monolithically-integrated solar-fuels system that is simultaneously efficient, stable, intrinsically safe, and scalably manufacturable. This thesis presents the development of three separate high-efficiency solar fuel devices protected by thin films of amorphous TiO2, and develops light management strategies to increase the performance of these devices.
\r\n\r\nFirst, high-efficiency monolithic cells were designed to perform solar water-splitting and CO2 reduction. These designs are driven by high-quality single-crystalline III-V semiconductors that are unstable when placed in direct contact with aqueous electrolytes but can be protected against corrosion by hole-conducting amorphous films. Experimental fabrication and characterization of this tandem device was realized in the form of a fully-integrated water-splitting prototype with a solar-to-hydrogen efficiency of 10% showing stability for over 80 hours of operation. This was followed by the demonstration of water-splitting and CO2 reduction devices enabled by bipolar membranes, which increased stability and alleviated materials-compatibility constraints by creating a pH difference between the anolyte and catholyte, maintained at steady-state. Finally, universal light management strategies were developed using high-aspect-ratio TiO2 nanocones, resulting in an increase in catalyst loading with ultrahigh broadband transmission.
" }, { "name": "Shaner, Matthew Reed", "degree": "PhD", "year": "2016", "title": "An Experimental and Technoeconomic Study of Silicon Microwire Arrays for Fuel Production Using Solar Energy", "advisor": "Lewis, Nathan Saul; Atwater, Harry Albert", "url": "https://resolver.caltech.edu/CaltechTHESIS:05312016-151045544", "creators": [ { "name": { "family": "Shaner", "given": "Matthew Reed" }, "id": "Shaner-Matthew-Reed", "orcid": "0000-0003-4682-9757", "display_name": "Shaner, Matthew Reed" } ], "advisors": [ { "name": { "family": "Lewis", "given": "Nathan Saul" }, "id": "Lewis-N-S", "role": "co-advisor", "display_name": "Lewis, Nathan Saul" }, { "name": { "family": "Atwater", "given": "Harry Albert" }, "id": "Atwater-H-A", "role": "co-advisor", "display_name": "Atwater, Harry Albert" } ], "committee": [ { "name": { "family": "Lewis", "given": "Nathan Saul" }, "id": "Lewis-N-S", "role": "chair", "display_name": "Lewis, Nathan Saul" }, { "name": { "family": "Atwater", "given": "Harry Albert" }, "id": "Atwater-H-A", "role": "member", "display_name": "Atwater, Harry Albert" }, { "name": { "family": "Flagan", "given": "Richard C." }, "id": "Flagan-R-C", "role": "member", "display_name": "Flagan, Richard C." }, { "name": { "family": "Davis", "given": "Mark E." }, "id": "Davis-M-E", "role": "member", "display_name": "Davis, Mark E." } ], "option_major": [ "chemeng" ], "doi": "10.7907/Z98C9T7Z", "abstract": "Direct solar energy conversion is one of few sustainable energy resources able to wholly satisfy global energy demand; however, utility scale adoption and reliance are currently limited by the lack of a cost effective energy storage technology. The production of fuel from sunlight (solar fuels) enables solar energy storage in chemical bonds, a volumetrically and gravimetrically dense form compatible with current infrastructure worldwide. Hydrogen production via water splitting is a first generation solar fuel targeted herein that is currently used for hydrocarbon up-grading and fertilizer production and could further be utilized in combustion cycles and/or fuel cells for electricity and heat production and transportation.
\r\n\r\nThis thesis presents achievements that form the foundation for Si microwire array based solar water splitting devices beginning with a tandem junction device design using Si microwire arrays as the architectural motif and one of many active components. Si microwire arrays have potential advantages over two dimensional planar device architectures such as minimized resistance losses, lower semiconductor material usage, and embedment in a polymeric membrane enabling a flexible device.
\r\n\r\nExperimental fabrication and characterization of this tandem junction device design was realized in the form of a np+-Si microwire array coated by either tungsten oxide (WO3) or titanium dioxide (TiO2) as the second tandem semiconductor. The Si/TiO2 device demonstrated the highest performance with an expected solar-to-hydrogen efficiency of 0.39%. To achieve these demonstrations new processing methods were needed and developed for formation of the np+-Si microwire array homojunction and formation of a low resistance contact between the p+-Si and second semiconductor using sputtered tin- doped indium oxide (ITO) and spray pyrolyzed fluorine-doped tin oxide (FTO).
\r\n\r\nAnother achievement includes demonstration of the longest known (>2200 hours) photoanode stability for water oxidation using a np+-Si microwire array coated with an in-house developed amorphous TiO2 protection layer and NiCrOx electrocatalyst. Additionally, the Si microwire array architecture was used to enable decoupling of semiconductor light absorption and catalytic activity, two performance metrics that ideally are maximized simultaneously. However, all previous demonstrations have shown anti-correlation between these performance metrics because planar architectures are subject to a trade-off where adding electrocatalyst increases catalytic activity, but decreases semiconductor light absorption and vice versa.
\r\n\r\nFinally, a techno-economic analysis of solar water splitting production facilities was performed to assess economic competitiveness because this is the ultimate metric by which all energy production technologies are currently evaluated. This analysis suggests that a hydrogen production facility that is cosmetically similar to current solar panel installations with hydrogen collection from distributed tilted panels is unlikely to achieve cost competitiveness with fossil fuel derived hydrogen due to the balance of systems costs alone. A cost of CO2 greater than ~$800 (ton CO2)-1 was estimated to be necessary for the least expensive base-case solar-to-hydrogen system to reach price parity with hydrogen derived from steam reforming of methane priced at $3 (MM BTU)-1 ($1.39 (kg H2)-1). Direct CO2 reduction systems were also explored and resulted in even larger challenges than hydrogen production. Accordingly, major facility wide breakthroughs are required to obtain viable economic costs for solar hydrogen production, but the barriers to achieve cost-competitiveness with existing large-scale thermochemical processes for CO2 reduction are even greater.
" }, { "name": "Fong, Henry", "degree": "PhD", "year": "2015", "title": "Metallaboratrane Facilitated E\u2012H Bond Activation and Hydrogenation Catalysis", "advisor": "Peters, Jonas C.", "url": "https://resolver.caltech.edu/CaltechTHESIS:12152014-181907516", "creators": [ { "name": { "family": "Fong", "given": "Henry" }, "id": "Fong-Henry", "display_name": "Fong, Henry" } ], "advisors": [ { "name": { "family": "Peters", "given": "Jonas C." }, "id": "Peters-J-C", "role": "advisor", "display_name": "Peters, Jonas C." } ], "committee": [ { "name": { "family": "Agapie", "given": "Theodor" }, "id": "Agapie-T", "role": "chair", "display_name": "Agapie, Theodor" }, { "name": { "family": "Gray", "given": "Harry B." }, "id": "Gray-H-B", "role": "member", "display_name": "Gray, Harry B." }, { "name": { "family": "Reisman", "given": "Sarah E." }, "id": "Reisman-S-E", "role": "member", "display_name": "Reisman, Sarah E." }, { "name": { "family": "Peters", "given": "Jonas C." }, "id": "Peters-J-C", "role": "member", "display_name": "Peters, Jonas C." } ], "option_major": [ "chemistry" ], "doi": "10.7907/Z9TX3C9P", "abstract": "The E\u2012H bond activation chemistry of tris-phosophino-iron and -cobalt metallaboratranes is discussed. The ferraboratrane complex (TPB)Fe(N2) heterolytically activates H\u2012H and the C\u2012H bonds of formaldehyde and arylacetylenes across an Fe\u2012B bond. In particular, H\u2012H bond cleavage at (TPB)Fe(N2) is reversible and affords the iron-hydride-borohydride complex (TPB)(\u03bc\u2012H)Fe(L)(H) (L = H2, N2). (TPB)(\u03bc\u2012H)Fe(L)(H) and (TPB)Fe(N2) are competent olefin and arylacetylene hydrogenation catalysts. Stoichiometric studies indicate that the B\u2012H unit is capable of acting as a hydride shuttle in the hydrogenation of olefin and arylacetylene substrates. The heterolytic cleavage of H2 by the (TPB)Fe system is distinct from the previously reported (TPB)Co(H2) complex, where H2 coordinates as a non-classical H2 adduct based on X-ray, spectroscopic, and reactivity data. The non-classical H2 ligand in (TPB)Co(H2) is confirmed in this work by single crystal neutron diffraction, which unequivocally shows an intact H\u2012H bond of 0.83 \u00c5 in the solid state. The neutron structure also shows that the H2 ligand is localized at two orientations on cobalt trans to the boron. This localization in the solid state contrasts with the results from ENDOR spectroscopy that show that the H2 ligand freely rotates about the Co\u2012H2 axis in frozen solution. Finally, the (TPB)Fe system, as well as related tris-phosphino-iron complexes that contain a different apical ligand unit (Si, PhB, C, and N) in place of the boron in (TPB)Fe, were studied for CO2 hydrogenation chemistry. The (TPB)Fe system is not catalytically competent, while the silicon, borate, carbon variants, (SiPR3)Fe, (PhBPiPr3)Fe, and (CPiPr3)Fe, respectively, are catalysts for the hydrogenation of CO2 to formate and methylformate. The hydricity of the CO2 reactive species in the silatrane system (SiPiPr3)Fe(N2)(H) has been experimentally estimated. " }, { "name": "Fountaine, Katherine Theresa", "degree": "PhD", "year": "2015", "title": "Mesoscale Optoelectronic Design of Wire-Based Photovoltaic and Photoelectrochemical Devices", "advisor": "Atwater, Harry Albert", "url": "https://resolver.caltech.edu/CaltechTHESIS:05292015-151831184", "creators": [ { "name": { "family": "Fountaine", "given": "Katherine Theresa" }, "id": "Fountaine-Katherine-Theresa", "orcid": "0000-0002-0414-8227", "display_name": "Fountaine, Katherine Theresa" } ], "advisors": [ { "name": { "family": "Atwater", "given": "Harry Albert" }, "id": "Atwater-H-A", "role": "advisor", "display_name": "Atwater, Harry Albert" } ], "committee": [ { "name": { "family": "Atwater", "given": "Harry Albert" }, "id": "Atwater-H-A", "role": "chair", "display_name": "Atwater, Harry Albert" }, { "name": { "family": "Brady", "given": "John F." }, "id": "Brady-J-F", "role": "member", "display_name": "Brady, John F." }, { "name": { "family": "Lewerenz", "given": "Hans Joachim" }, "id": "Lewerenz-H-J", "role": "member", "display_name": "Lewerenz, Hans Joachim" }, { "name": { "family": "Flagan", "given": "Richard C." }, "id": "Flagan-R-C", "role": "member", "display_name": "Flagan, Richard C." } ], "option_major": [ "chemeng" ], "doi": "10.7907/Z9P26W1K", "abstract": "The overarching theme of this thesis is mesoscale optical and optoelectronic design of photovoltaic and photoelectrochemical devices. In a photovoltaic device, light absorption and charge carrier transport are coupled together on the mesoscale, and in a photoelectrochemical device, light absorption, charge carrier transport, catalysis, and solution species transport are all coupled together on the mesoscale. The work discussed herein demonstrates that simulation-based mesoscale optical and optoelectronic modeling can lead to detailed understanding of the operation and performance of these complex mesostructured devices, serve as a powerful tool for device optimization, and efficiently guide device design and experimental fabrication efforts. In-depth studies of two mesoscale wire-based device designs illustrate these principles\u2014(i) an optoelectronic study of a tandem Si|WO3 microwire photoelectrochemical device, and (ii) an optical study of III-V nanowire arrays.
\r\n\r\nThe study of the monolithic, tandem, Si|WO3 microwire photoelectrochemical device begins with development and validation of an optoelectronic model with experiment. This study capitalizes on synergy between experiment and simulation to demonstrate the model\u2019s predictive power for extractable device voltage and light-limited current density. The developed model is then used to understand the limiting factors of the device and optimize its optoelectronic performance. The results of this work reveal that high fidelity modeling can facilitate unequivocal identification of limiting phenomena, such as parasitic absorption via excitation of a surface plasmon-polariton mode, and quick design optimization, achieving over a 300% enhancement in optoelectronic performance over a nominal design for this device architecture, which would be time-consuming and challenging to do via experiment.
\r\n\r\nThe work on III-V nanowire arrays also starts as a collaboration of experiment and simulation aimed at gaining understanding of unprecedented, experimentally observed absorption enhancements in sparse arrays of vertically-oriented GaAs nanowires. To explain this resonant absorption in periodic arrays of high index semiconductor nanowires, a unified framework that combines a leaky waveguide theory perspective and that of photonic crystals supporting Bloch modes is developed in the context of silicon, using both analytic theory and electromagnetic simulations. This detailed theoretical understanding is then applied to a simulation-based optimization of light absorption in sparse arrays of GaAs nanowires. Near-unity absorption in sparse, 5% fill fraction arrays is demonstrated via tapering of nanowires and multiple wire radii in a single array. Finally, experimental efforts are presented towards fabrication of the optimized array geometries. A hybrid self-catalyzed and selective area MOCVD growth method is used to establish morphology control of GaP nanowire arrays. Similarly, morphology and pattern control of nanowires is demonstrated with ICP-RIE of InP. Optical characterization of the InP nanowire arrays gives proof of principle that tapering and multiple wire radii can lead to near-unity absorption in sparse arrays of InP nanowires.
" }, { "name": "Narang, Prineha", "degree": "PhD", "year": "2015", "title": "Light-Matter Interactions in Semiconductors and Metals: From Nitride Optoelectronics to Quantum Plasmonics", "advisor": "Atwater, Harry Albert; Lewis, Nathan Saul", "url": "https://resolver.caltech.edu/CaltechTHESIS:06052015-164458210", "creators": [ { "name": { "family": "Narang", "given": "Prineha" }, "id": "Narang-Prineha", "orcid": "0000-0003-3956-4594", "display_name": "Narang, Prineha" } ], "advisors": [ { "name": { "family": "Atwater", "given": "Harry Albert" }, "id": "Atwater-H-A", "role": "advisor", "display_name": "Atwater, Harry Albert" }, { "name": { "family": "Lewis", "given": "Nathan Saul" }, "id": "Lewis-N-S", "role": "co-advisor", "display_name": "Lewis, Nathan Saul" } ], "committee": [ { "name": { "family": "Atwater", "given": "Harry Albert" }, "id": "Atwater-H-A", "role": "chair", "display_name": "Atwater, Harry Albert" }, { "name": { "family": "Goddard", "given": "William A., III" }, "id": "Goddard-W-A-III", "role": "member", "display_name": "Goddard, William A., III" }, { "name": { "family": "Refael", "given": "Gil" }, "id": "Refael-G", "role": "member", "display_name": "Refael, Gil" }, { "name": { "family": "Schwab", "given": "Keith C." }, "id": "Schwab-K-C", "role": "member", "display_name": "Schwab, Keith C." }, { "name": { "family": "Lewis", "given": "Nathan Saul" }, "id": "Lewis-N-S", "role": "member", "display_name": "Lewis, Nathan Saul" } ], "option_major": [ "appliedphys" ], "doi": "10.7907/Z9513W4S", "abstract": "This thesis puts forth a theory-directed approach coupled with spectroscopy aimed at the discovery and understanding of light-matter interactions in semiconductors and metals.
\r\n\r\nThe first part of the thesis presents the discovery and development of Zn-IV nitride materials.The commercial prominence in the optoelectronics industry of tunable semiconductor alloy materials based on nitride semiconductor devices, specifically InGaN, motivates the search for earth-abundant alternatives for use in efficient, high-quality optoelectronic devices. II-IV-N2 compounds, which are closely related to the wurtzite-structured III-N semiconductors, have similar electronic and optical properties to InGaN namely direct band gaps, high quantum efficiencies and large optical absorption coefficients. The choice of different group II and group IV elements provides chemical diversity that can be exploited to tune the structural and electronic properties through the series of alloys. The first theoretical and experimental investigation of the ZnSnxGe1\u2212xN2 series as a replacement for III-nitrides is discussed here.
\r\n\r\nThe second half of the thesis shows ab\u2212initio calculations for surface plasmons and plasmonic hot carrier dynamics. Surface plasmons, electromagnetic modes confined to the surface of a conductor-dielectric interface, have sparked renewed interest because of their quantum nature and their broad range of applications. The decay of surface plasmons is usually a detriment in the field of plasmonics, but the possibility to capture the energy normally lost to heat would open new opportunities in photon sensors, energy conversion devices and switching. A theoretical understanding of plasmon-driven hot carrier generation and relaxation dynamics in the ultrafast regime is presented here. Additionally calculations for plasmon-mediated upconversion as well as an energy-dependent transport model for these non-equilibrium carriers are shown.
\r\n\r\nFinally, this thesis gives an outlook on the potential of non-equilibrium phenomena in metals and semiconductors for future light-based technologies.
" }, { "name": "Leenheer, Andrew Jay", "degree": "PhD", "year": "2013", "title": "Light to Electrons to Bonds: Imaging Water Splitting and Collecting Photoexcited Electrons", "advisor": "Atwater, Harry Albert", "url": "https://resolver.caltech.edu/CaltechTHESIS:12102012-191527683", "creators": [ { "name": { "family": "Leenheer", "given": "Andrew Jay" }, "id": "Leenheer-Andrew-Jay", "display_name": "Leenheer, Andrew Jay" } ], "advisors": [ { "name": { "family": "Atwater", "given": "Harry Albert" }, "id": "Atwater-H-A", "role": "advisor", "display_name": "Atwater, Harry Albert" } ], "committee": [ { "name": { "family": "Atwater", "given": "Harry Albert" }, "id": "Atwater-H-A", "role": "chair", "display_name": "Atwater, Harry Albert" }, { "name": { "family": "Johnson", "given": "William L." }, "id": "Johnson-W-L", "role": "member", "display_name": "Johnson, William L." }, { "name": { "family": "Fultz", "given": "Brent T." }, "id": "Fultz-B-T", "role": "member", "display_name": "Fultz, Brent T." }, { "name": { "family": "Lewis", "given": "Nathan S." }, "id": "Lewis-N-S", "role": "member", "display_name": "Lewis, Nathan S." } ], "option_major": [ "matsci" ], "doi": "10.7907/A8ZZ-Z189", "abstract": "Photoelectrochemical devices can store solar energy as chemical bonds in fuels, but more control over the materials involved is needed for economic feasibility. Both efficient capture of photon energy into electron energy and subsequent electron transfer and bond formation are necessary, and this thesis explores various steps of the process. To look at the electrochemical fuel formation step, the spatially-resolved reaction rate on a water-splitting electrode was imaged during operation at a few-micron scale using optical microscopy. One method involved localized excitation of a semiconductor photoanode and recording the growth rate of bubbles to determine the local reaction rate. A second method imaged the reactant profile with a pH-sensitive fluorophore in the electrolyte to determine the local three-dimensional pH profile at patterned electrocatalysts in a confocal microscope. These methods provide insight on surface features optimal for efficient electron transfer into fuel products.
\r\n\r\nA second set of studies examined the initial process of photoexcited electron transport and collection. An independent method to measure the minority carrier diffusion length in semiconductor photoelectrodes was developed, in which a wedge geometry is back illuminated with a small scanned spot. The diffusion length can be determined from the exponential decrease of photocurrent with thickness, and the method was demonstrated on solid-state silicon wedge diodes, as well as tungsten oxide thin-film wedge photoanodes. Finally, the possibility of absorbing and collecting sub-bandgap illumination via plasmon-enhanced hot carrier internal photoemission was modeled to predict the energy conversion efficiency. The effect of photon polarization on emission yield was experimentally tested using gold nanoantennas buried in silicon, and the correlation was found to be small.
" } ]