1,721,126 research outputs found
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Confirmation of Photoacidic Quantum Dots Using Electrochemical Impedance Spectroscopy
Get PDFThis thesis describes the first demonstration of a quantum dot photoacid. Photoacids are a class of molecules with a protic bond that is weakened upon photo-excitation. CdSe quantum dots were synthesized and were made to have mixed-ligand surfaces with a majority of water-solubilizing sulfonate functional groups and fewer acidic electron-donor functional groups, notably 4-mercaptophenol (MPh). Sulfonate groups are very acidic and therefore remain charged over a wide range of pH values. Through judicious choice of synthesis and ligand-exchange conditions, water-solubilizing 2-mercaptoethanesulfonate (MES) was introduced in addition to an equal concentration of protic and dipolar MPh groups. The electronic states of surface-bound MPh groups are such that they can serve as electron-transfer donors to excited-state CdSe quantum dots. The challenge to characterize the photoacidity of this class of chromophores is that the charge-separated CdSe– and MPh+ state does not efficiently emit photons. Therefore, a recently developed electrochemical impedance spectroscopy technique was studied in great detail to understand the cause of observed changes in impedance and then utilized to definitively demonstrate that this new class of quantum dot dyes were photoacidic. This light-driven energy conversion process is uniquely suited to convert light into ionic power, which can be used for direct desalination of saltwater
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PEROVSKITE AND PEROVSKITE-LIKE MATERIALS FOR SOLUTION-PROCESSED THIN-FILM SOLAR CELLS
Get PDFLead–halide-based hybrid organic–inorganic perovskite materials (APbX3) have recently garnered increased attention among researchers worldwide as promising thin-film photovoltaic materials. Laboratory-scale solar cells incorporating APbX3 materials as the light absorber have demonstrated impressive efficiencies of > 20%, but commercialization of solar cells based on solution-cast materials and with only low-temperature processing steps is in-part limited by toxicity and instability of the photoactive materials even under ambient conditions. Specifically, APbX3 contains toxic lead and is not stable in ambient conditions in part because AX dissociates into two water-soluble, low-boiling point species. Therefore, the goal of my doctoral thesis project was to replace the organic monocationic salts in perovskite materials with non-volatile and less water-soluble analogs in the form of organic dicationic salts. Using this rationale, I have demonstrated the use of novel perovskite-like bismuth–halide and copper–halide materials as the photoactive layer in solar cells, which contain hexanediammonium dications (HDA2+) that serve as the organic crystal fastener. I have elucidated structure–property relationships of these and related materials with different organic linkers through measurement of crystal structure, powder and thin-film characterization of the materials, and long-term thermal and moisture stability. I have also studied these materials using ultrafast spectroscopy techniques to elucidate energy-transport and charge-transport dynamics of these materials as thin films. Efficient photovoltaic devices featuring a dicationic metal–halide material that does not contain lead may offer a more environmentally-friendly, stable alternative to the APbX3 photovoltaic devices that currently dominate emerging photovoltaic technology research
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Aqueous Ionic Photovoltaics from Photoacid-Sensitized Ion-Selective Polymers
Get PDFThis Thesis provides the seminal work towards the development of new technologies for ionic power generation and solar saltwater desalination. Central to the approach is the transformation of optically-inactive ion-exchange membranes used in electrochemical technologies into optically-active power-producing membranes. Chapter 1 provides a brief description of anthropogenic emissions and human induced global warming and climate change. It is desired that these human induced calamities can be mitigated by replacing fossil fuel energies with solar energy conversion technologies. Chapter 2 presents studies on a model system of optically-active membranes. Included is the synthesis of an exemplary visible-light absorbing photoacid dye and its covalent attachment to a nanoporous membrane. Basic materials characterization and more specialized photophysical techniques were used to characterize the dyes dissolved in solution and the dyes covalently bound to membranes. The study suggests that ion transport initiated by photoacids bonded to mesoporous materials will require careful molecular engineering to enable efficient light-to-ionic energy conversion.Chapter 3 shows the first demonstration of photovoltaic action from a covalently sensitized ion-exchange membrane. Photoelectrochemical measurements quantified the photocurrent and photovoltage production and control studies suggested that the power production is a result of photo-generated ions produced from the photoacid dye. Results were consistent with protons being transported against a pH gradient opposite to the thermodynamically favored direction; albeit, no net power was generated because of a rapid and continuous ion crossover across the membrane due to use of a single monopolar ion-exchange membrane with a large pH gradient across it. The arrangement of these optically-active cation-exchange membranes adjacent to anion-exchange membranes is the focus of Chapter 4. The bipolar membranes showed enhanced photovoltaic efficiencies as compared to the monolithic counterpart and analogies from the physics of solar cells was used to elucidate the mechanism of photovoltaic action. The obtained photovoltages are more than half that needed to desalinate the sodium chloride in sea water to potable water and all that needed to convert moderately saline brackish water to potable water. The Thesis concludes with Chapter 5, which presents the ongoing solar-energy conversion optimization tactics for these devices
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Synthesis and Characterization of Infrared-Absorbing Osmium-Polypyridyl Dyes for Use in Dye-Sensitized Solar Cells
Get PDFDye-sensitized solar cells (DSSCs) are a potentially low-cost alternative to silicon solar cells, however DSSCs have not been largely commercialized due to their poor solar to electric energy conversion efficiency, ~14%, compared to silicon, ~25%. One reason for their poor efficiency is that current record holding DSSCs employ dyes that absorb a less-than-ideal portion of the solar spectrum as dictated by the Shockley-Queisser Limit. To remedy this, infrared-absorbing osmium polypyridyl dyes were synthesized and their basic parameters characterized by ultraviolet-visible absorption spectroscopy, stepwise potential step spectroelectrochemistry, and cyclic voltammetry. The dyes appeared to have close-to-ideal optical pseudo-bandgaps and excited state energy levels, however, their performance in DSSCs employing the commonly used iodide-triodide redox shuttle was very poor. To better understand the behavior of these dyes in a working environment apparent diffusion coefficients of later self-exchange electron transfer between oxidized and reduced dyes bound to mesoporous titanium dioxide films was studied using several different techniques which produced different results. The different techniques were able to be harmonized by correct use of mathematical assumptions and measurement of non-ideal Nernstian behavior of bound dyes. Foot-of-the-wave analysis was modified for use on dyes bound to mesoporous films with substrate in solution. This revealed that several infrared absorbing osmium polypyridyl dyes performed very slow electron transfer from iodide with maximum second order electron transfer rate constants of 78 M−1 s−1, which could explain their poor performance in DSSCs using the iodide triiodide redox shuttle. Rates with 1,1’-dimethyl ferrocene were much faster, with rate constants as high as 74,000 M−1s−1. Nanosecond transient absorption spectroscopy and nanosecond transient microwave conductivity measurements were performed on DSSCs containing working benchmark dye N3 and infrared-absorbing osmium polypyridyl dyes. Osmium polypyidyl dyes were observed to successfully inject electrons into the TiO2 film with electron in TiO2 to oxidized dye recombination rates comparable to N3. Oxidized N3 rapidly regenerated in the presence of iodide, resulting in charge separated states that lasted for almost a second, but the infrared-absorbing osmium dyed did not. Several catalysts were fabricated to speed up iodide oxidation, but none worked in a solar cell
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Control of the Hydrogen Evolution Reaction on Metal Oxides for Energy Storage
Get PDFHydrogen can be used as a replacement for fossil fuels in many applications. However,commercially relevant scales of hydrogen production utilize either electrolysis or a combination of steam methane reforming and the water gas shift reaction which are energy intensive and non-renewable. Here I report on a portion of work toward understanding the fundamental physical and chemical phenomena that constrain the maximum solar-to-hydrogen (STH) conversion efficiency for photoelectrochemical particle-slurry reactors. Using Rh-doped strontium titanate (SrTiO 3 :Rh), a variable power light source, and an inline-mass spectrometry the rate of hydrogen production can be controlled and optimized.In the second part of my work, I report on the spectro-electrochemical behavior of crystalline titanium dioxide (TiO2) mesoporous films as a model system for aqueous redox flow battery charging/discharging. The effects of potential determining ions on the density of states in the materials are discussed
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Stochastic Models of Photoexcitation and Charge Accumulation for Solar Energy Conversion
Get PDFA stochastic model has been created to efficiently explore the parameter space surrounding schemes for a new dye-sensitized solar cell and photoelectrochemical constructs. Models of a variety of semiconductor configurations that are populated with surface-anchored light-absorbing dye molecules and electrocatalysts are simulated under various illumination conditions. Absorption of light by dyes results charge transfer with the semiconductor scaffold to create mobile opposite charges in the dye layer that hop by self-exchange electron transfer across the semiconductor surface and ultimately accumulate on electrocatalysts or recombine. Additionally, simulations of transient absorption pulsed-laser experiments are compared to those of continuous illumination conditions to determine if pulsed-laser experiments can be used as an analog for real-world sunlight illumination conditions. Results from these simulations help to expand the current knowledgebase for these systems via rapid analysis of a wide range of parameter sets in order to better understand the limitations and possibilities for these and similar systems. It is our hope that these results are used by researchers to better focus their efforts in developing cost-competitive dye-sensitized solar energy conversion technologies
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Understanding and Controlling Ion Transport in Aqueous Ion-Exchange Membranes of Relevance to Energy Conversion and Storage
No full textTo obtain the desired function from most electrochemical technologies it is important to understand and control transport of charged species. While many researchers think that the premier complementary charge carriers are electrons and holes in solids, ions have an equal capacity to carry charge and are subject to the same forces of drift and diffusion as dictated by the Nernst-Planck equation and additional reactivity as dictated by the continuity of mass equations. This fact motivated my study of two different electrochemical systems, each dominated by the transport of ions. The first system is a ratchet-based ion pump, which takes advantage of an electrostatic asymmetry of the charging behavior of poly-crystalline surfaces in a nanoporous ceramic material to achieve net directional ion transport even under conditions where the electrochemical driving force from rapidly alternating electric potential on metal surfaces that span the membrane is symmetric in voltage and time. Transport selectivity arises from the sign of their charge and their hydrated size. Gold and chromium were deposited on each side of porous alumina to create electrical contacts that were charged in an oscillatory manner to opposite electric potentials, modulating electric fields within the nanopores. Although the time average of this electric field is zero, a difference in the way that each side charges results in a net flow of ionic current. We report this ability to charge a concentration gradient of ions across the porous material that depends on the charge of the ion.The second system is a bipolar membrane electrode assembly which is thought to operate based on well-known solid-state semiconductor diode behavior. It consists of oppositely charged ion-exchange membranes in intimate contact. Expanding upon the idea that ions have the same capacity to carry charge as electrons and holes, when solving for the electrostatic properties a pn junction, the resulting charge, electric field, and electric potential profiles are agnostic to the type of charge. The underlying diode physics of the system do not change when changing from electronic to ionic current and the analysis shown uses characterization techniques typical of solid-state diodes on these water-based protonic diodes. The next several chapters focused on this. First, an analytical model of a BPM junction was created to determine suitable photoacid dyes that could be ionically associated in the membrane to photosensitize the BPM for light-to-electrical energy conversion. Optically transparent membranes do not absorb light to create positive/negative charge pairs as do semiconductors with a specific bandgap, so a photoacid is used to absorb photons and undergo the photoprotolytic (Förster) cycle to generate mobile proton (H+) and hydroxide (OH-) pairs. This model uses kinetic rate law equations to determine, for a photoacid with a specific pKa, the possible observed photovoltage response under illumination. To further mimic properties of solid-state diodes, we developed a technique to diffusion-dope singular ion exchange membranes (IEM) into a single-layer BPM. The homogeneity of the junction’s interface determines the profile of the fixed charge across the junction and influences the quality of the diode. We hypothesize that diffusion doping, using a substitution reaction to exchange the fixed-charge group on the backbone of the ion-exchange membrane with one of the opposite sign, would make a more homogeneous interface of positive and negative fixed-charge domains. We observe an increase in conductivity for the diffusion-doped BPMs, as opposed to the previously reported fabrication methods, and address further optimizations that can be made to the system.To understand the characteristics of the composite BPM involves gaining knowledge of the individual ion-exchange membrane properties. For this purpose, the polymer|electrolyte interface was analyzed as a semiconductor|electrolyte junction. Motivated by patterns in experimental data as well as common misconceptions about properties of the junction, we created an analytically approximate model that solves for capacitances and potentials across individual components to decompose the effects of each on the overall behavior of the system. The model was then used to identify input parameters such as dopant density, relative permittivities, and interfacial dipoles that allow for congruence with experimental capacitance data gathered using electrochemical impedance spectroscopy
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Electrochemical Evaluation of Bipolar Ion-Exchange Membranes for Solar Fuels
Get PDFPhotoelectrochemical cells that utilize bipolar ion-exchange membranes (BPMs) can support and maintain a pH difference across the membrane. Redox-inactive salts are often also incorporated in these cells and it is unclear if salts have an effect on the electrostatics within the cell and perceived thermodynamics of the overall redox chemistry. In this thesis, commercial BPMs wetted by various electrolytes were investigated, e.g. in the presence of acid, base, and/or salt. The electrochemical behavior of the BPMs was assessed by measuring the current density vs potential behavior. The results suggest that potential differences measured across BPMs may not always reflect the pH of the bulk electrolyte and therefore may require additional information in order to justify the overall free energy required for the redox reactions.The second half of this thesis was dedicated to the development of an artificial light-driven ion pump using BPM materials. An activated Nafion precursor membrane was functionalized with newly developed photoacid molecules and sandwiched between an anion-exchange membrane and a cation-exchange membrane. The photoelectrochemical properties of the resulting photoacid-functionalized BPM were measured and a photovoltaic response was observed. These initial findings provide a solid foundation for future research into customized BPMs to be used as artificial light-driven ion pumps
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Detailed Balance Modeling of Novel Solar Fuels Designs
Get PDFModels of solar fuels devices that are completely electrochemically-mediated and consist of ensembles of optically thin light absorbers were used to calculate their maximum theoretical solar-to-fuel efficiencies. These models are based on the thermodynamic principles of detailed balance and blackbody radiation, semiconductor device physics, and simple catalysis. The maximum efficiency of an electrochemically-mediated tandem water splitting device was calculated and the crucial dependence of this efficiency on the redox shuttle thermodynamic potential was elucidated. A novel model of a stack of optically thin, radiatively coupled light-absorbers that each independently perform solar fuels reactions was developed to demonstrate the substantial gains in solar-to-fuel efficiency that are possible when using ensembles of small light-absorbers. The results presented herein can be used by researchers to develop high-efficiency, low-cost solar fuels materials and devices
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Perturbation of Equilibrium Chemical Systems: Excited-State Proton Transfer with Reversible Photoacids and Structural Evolution of Pyrochlore Ceramics
No full textThis Dissertation deduces the thermodynamic and kinetic properties of reversible excited-state photoacids to understand what governs proton-transfer dynamics under different nonequilibrium conditions. The investigation of pyrochlore ceramic oxide crystallinity upon different synthetic methods and He2+ ions irradiation is also explored.Chapter 1 introduces photo-induced proton transfer processes by reversible excited-state photoacids dissolved in dilute and high ionic strength aqueous media. Steady-state and time-resolved approaches to understanding thermodynamic and kinetic properties of excited-state proton transfer are discussed. A separate research topic involving understanding of pyrochlore ceramic oxide crystallinity changes upon different synthetic routes and He2+ ions irradiation is also discussed for the advancement of nuclear fuel and nuclear waste form.Chapter 2 focuses on the observation of changes in the extent of photoacid ground-state proton transfer (GSPT) and excited-state proton transfer (ESPT) due to addition of inert salt. The addition of inert salt perturbs the reaction equilibrium due to changes in pH, pKa, and pH2O that are observable in absorption and photoluminescence spectra. Unlike typical studies that assume pKa and pH2O to be constant, here it is shown that it is important to include effects due to changes in pKa and pH2O in the presence of high concentrations of protic or inert salt.Chapter 3 focuses on investigation of the ESPT behavior of reversible photoacids, 9-hydroxyphenanthrene-3-sulfonate (HPhenMS) and 9-hydroxyphenanthrene-3,10-disulfonate (HPhenDS). Results indicate surprisingly large reorganization energies of ~0.22 eV, which is significantly larger than generally observed for proton-transfer reactions, which typically exhibit near-zero reorganization energy (i.e. < 0.1 eV). This behavior is unequivocal, where instead of observing a typical linear free energy relationship there is a parabolic dependence of the logarithm of the reaction rate constant on thermodynamic driving force, consistent with semiclassical Marcus theory. My data is consistent with extensive electronic delocalization in the electronic excited state.Chapter 4 focuses on the synthetic control of crystalline disorder, mainly in relation to valence in pyrochlores, as achieved via various synthesis routes. This includes the more common examples currently in literature, such as the pyrochlores (A2B2O7) with an A cation of valency 3+ and a B cation of valency 4+, as well as the disordered defect pyrochlores such as those composed of B5+ cations and the highly disordered high entropy pyrochlores that are currently gaining interest. Synthetic methods discussed include the solid-state method, coprecipitation method, sol-gel synthesis, Pechini method, hydrothermal method, molten salt synthesis, and combustion method.Chapter 5 focuses on understanding the mechanism of the order-disorder transitions of zirconate and titanate pyrochlores under He2+ ions radiation damage upon long-term waste storage conditions. The titanate pyrochlore yielded an expected result of transitioning into an amorphous state. For the zirconate pyrochlore, results exhibited a formation of two different pyrochlore phases rather than the defect fluorite phase. It is explained that the built-up tensile stress without detectable strain from radiation damage caused the grains to break into smaller fractions, thus coherently creating an appearance of defect fluorite from the diffraction pattern
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