| 1. |
- Dürr, Robin N.
(author)
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Potential Electrocatalysts for Water Splitting Devices : A Journey Through the Opportunities and Challenges of Catalyst Classes
- 2022
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Artistic work (other academic/artistic)abstract
- In this thesis work, different classes of catalysts and their suitability for integration into an electrolyzer cell has been investigated.Ruthenium based molecular catalysts have shown high activities and stabilities towards water oxidation in neutral pH. Especially the oligomeric catalysts exhibited a superior performance. The electrical conductivity of the electrode and the low loading of catalyst might impose limitations on reaching high current densities at reasonable potentials.Among the tested transition metal single atom catalysts, synthesized by pyrolyzing transition metal doped ZIF-8 structures, cobalt has shown the highest activity towards hydrogen evolution and a stable behaviour in acidic pH. The enhanced stability of single atomic sites compared to the corresponding nanoparticles was proposed. However, also for this class of catalyst, the low number of active sites seems to present a difficulty need to be overcome.With the novel method presented to fabricate a membrane electrode assembly, the usage of commonly used expensive membranes could possibly be avoided.Both nickel molybdate hydrate nanoparticle shapes have been proposed to transform in an electrochemical activation step into γ-NiOOH as active phase for the oxygen evolution reaction in alkaline pH. With the removal of molybdenum, a high electrochemical surface area with a large number of exposed nickel sites was indicated to be the origin behind the high catalytic activity of the nanoparticles. Molybdenum was suggested to only serve as structure and pore forming agent. Preliminary results indicated a higher activity for the rod structure towards the oxygen evolution reaction. An essential outcome is that it is uncertain if rods can be isolated synthesized on a nickel foam and hence the absence of the sheet structure should be verified, which could be done for example by selective molybdenum leaching combined with Raman spectroscopy. Furthermore, the two nanostructures are fundamentally different materials and characterized by various techniques.Among all different classes of catalysts investigated, the nanoparticle catalysts seem to be the most promising for a successful integration in a large scale electrolyzer cell for widespread use.
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| 2. |
- El-Zohry, Ahmed M.
(author)
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Exploring Organic Dyes for Grätzel Cells Using Time-Resolved Spectroscopy
- 2015
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Doctoral thesis (other academic/artistic)abstract
- Grätzel cells or Dye-Sensitized Solar Cells (DSSCs) are considered one of the most promising methods to convert the sun's energy into electricity due to their low cost and simple technology of production. The Grätzel cell is based on a photosensitizer adsorbed on a low band gap semiconductor. The photosensitizer can be a metal complex or an organic dye. Organic dyes can be produced on a large scale resulting in cheaper dyes than complexes based on rare elements. However, the performance of Grätzel cells based on metal-free, organic dyes is not high enough yet. The dye's performance depends primarily on the electron dynamics. The electron dynamics in Grätzel cells includes electron injection, recombination, and regeneration. Different deactivation processes affect the electron dynamics and the cells’ performance.In this thesis, the electron dynamics was explored by various time-resolved spectroscopic techniques, namely time-correlated single photon counting, streak camera, and femtosecond transient absorption. Using these techniques, new deactivation processes for organic dyes used in DSSCs were uncovered. These processes include photoisomerization, and quenching through complexation with the electrolyte. These deactivation processes affect the performance of organic dyes in Grätzel cells, and should be avoided. For instance, the photoisomerization can compete with the electron injection and produce isomers with unknown performance. Photoisomerization as a general phenomenon in DSSC dyes has not been shown before, but is shown to occur in several organic dyes, among them D149, D102, L0 and L0Br. In addition, D149 forms ground state complexes with the standard iodide/triiodide electrolyte, which directly affect the electron dynamics on TiO2. Also, new dyes were designed with the aim of using ferrocene(s) as intramolecular regenerators, and their dynamics was studied by transient absorption.This thesis provides deeper insights into some deactivation processes of organic dyes used in DSSCs. New rules for the design of organic dyes, based on these insights, can further improve the efficiency of DSSCs.
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| 3. |
- Karlsson, Susanne, 1980-
(author)
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Single and Accumulative Electron Transfer – Prerequisites for Artificial Photosynthesis
- 2010
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Doctoral thesis (other academic/artistic)abstract
- Photoinduced electron transfer is involved in a number of photochemical and photobiological processes. One example of this is photosynthesis, where the absorption of sunlight leads to the formation of charge-separated states by electron transfer. The redox equivalents built up by successive photoabsorption and electron transfer is further used for the oxidation of water and reduction of carbon dioxide to sugars. The work presented in this thesis is part of an interdisciplinary effort aiming at a functional mimic of photosynthesis. The goal of this project is to utilize sunlight to produce renewable fuels from sun and water. Specifically, this thesis concerns photoinduced electron transfer in donor(D)-photosensitizer(P)-acceptor(A) systems, in mimic of the primary events of photosynthesis. The absorption of a photon typically leads to transfer of a single electron, i.e., charge separation to produce a single electron-hole pair. This fundamental process was studied in several molecular systems. The purpose of these studies was optimization of single electron transfer as to obtain charge separation in high yields, with minimum losses to competing photoreactions such as energy transfer. Also, the lifetime of the charge separated state and the confinement of the electron and hole in three-dimensional space are important in practical applications. This led us to explore molecular motifs for linear arrays based on Ru(II)bis-tridentate and Ru(II)tris-bidentate complexes. The target multi-electron catalytic reactions of water-splitting and fuel production require a build-up of redox equivalents upon successive photoexcitation and electron transfer events. The possibilities and challenges associated with such processes in molecular systems were investigated. One of the studied systems was shown to accumulate two electrons and two holes upon two successive excitations, without sacrificial redox agents and with minimum yield losses. From these studies, we have gained better understanding of the obstacles associated with step-wise photoaccumulation of charge and how to overcome them.
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| 4. |
- Sandström, Niclas, 1973-
(author)
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Heavy-Core Staffanes : A Computational Study of Their Fundamental Properties of Interest for Molecular Electronics
- 2007
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Doctoral thesis (other academic/artistic)abstract
- The basic building blocks in molecular electronics often correspond to conjugated molecules. A compound class consisting of rigid rod-like staffane molecules with the heavier Group 14 elements Si, Ge, Sn and Pb at their bridgehead positions has now been investigated. Herein these oligomers are called heavy-core or Si-, Ge-, Sn- or Pb-core staffanes. These compounds benefit from interaction through their bicyclo[1.1.1]pentane monomer units. Quantum chemical calculations were performed to probe their geometries, stabilities and electronic properties associated with conjugation. The stabilities of the bicyclo[n.n.n]alkane and [n.n.n]propellanes (1 ≤ n ≤ 3) with C, Si, Ge and Sn at the bridgehead positions were studied by calculation of homodesmotic ring strain energies. The bicyclic compounds with n = 1 and Si, Ge or Sn at bridgehead positions have lower strain than the all-carbon compound. A gradually higher polarizability exaltation is found as the bridgehead element is changed from C to Si, Ge, Sn or Pb. The ratio between longitudinal and average polarizability also increases gradually as Group 14 is descended, consistent with enhanced conjugation in the heavier oligomers. The localization of polarons in C-, Si- and Sn-core staffane radical cations was calculated along with internal reorganization energies. The polaron is less localized in Si- and Sn-core than in C-core staffane radical cation. The reorganization energies are also lower for the heavier staffanes, facilitating hole mobility when compared to the C-core staffanes. The effect of the bicyclic structure on the low valence excitations in the UV-spectra of compounds with two connected disilyl segments was also investigated. MS-CASPT2 calculations of 1,4-disilyl- and 1,4-bis(trimethylsilyl)-1,4-disilabicyclo[2.2.1]heptanes and 1,4-disilyl- and 1,4-bis(trimethylsilyl)-1,4-disilabicyclo[2.1.1]hexanes revealed that although the bicyclic cage separates the two disilyl chromophores, there is a strong red-shift of the lowest valence excitations when compared to an isolated disilane.
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| 5. |
- Eilers, Gerriet, 1973-
(author)
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Molecular Approaches to Photochemical Solar Energy Conversion : Towards Synthetic Catalysts for Water Oxidation and Proton Reduction
- 2007
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Doctoral thesis (other academic/artistic)abstract
- A molecular system capable of photoinduced water splitting is an attractive approach to solar energy conversion. This thesis deals with the functional characterization of molecular building blocks for the three principal functions of such a molecular system: Photoinduced accumulative charge separation, catalytic water oxidation, and catalytic proton reduction. Systems combining a ruthenium-trisbipyridine photosensitizer with multi-electron donors in form of dinuclear ruthenium or manganese complexes were investigated in view of the rate constants of electron transfer and excited state quenching. The kinetics were studied in the different oxidation states of the donor unit by combination of electrochemistry and time resolved spectroscopy. The rapid excited state quenching by the multi-electron donors points to the importance of redox intermediates for efficient accumulative photooxidation of the terminal donor.The redox behavior of manganese complexes as mimics of the water oxidizing catalyst in the natural photosynthetic reaction center was studied by electrochemical and spectroscopic methods. For a dinuclear manganese complex ligand exchange reactions were studied in view of their importance for the accumulative oxidation of the complex and its reactivity towards water. With the binding of substrate water, multiple oxidation in a narrow potential range and concomitant deprotonation of the bound water it was demonstrated that the manganese complex is capable of mimicking multiple aspects of photosynthetic water oxidation.A dinuclear iron complex was investigated as biomimetic proton reduction catalyst. The complex structurally mimics the active site of the iron-only hydrogenase enzyme and was designed to hold a proton on the bridging ligand and a hydride on the iron centers. Thermodynamics and kinetics of the protonation reactions and the electrochemical behavior of the different protonation states were studied in view of their potential catalytic performance.
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| 6. |
- Lissau, Jonas Sandby, 1984-
(author)
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Non-Coherent Photon Upconversion on Dye-Sensitized Nanostructured ZrO2 Films for Efficient Solar Light Harvesting
- 2014
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Doctoral thesis (other academic/artistic)abstract
- Photon upconversion by sensitized triplet–triplet annihilation (UC-STTA) is a photophysical process that facilitates the conversion of two low-energy photons into a single high-energy photon. A low-energy photon is absorbed by a sensitizer molecule that produces a triplet excited state which is transferred to an emitter molecule. When two emitter triplet states encounter each other, TTA can take place to produce a singlet excited state which decays by emission of a high-energy (upconverted) photon. While traditional single-threshold dye-sensitized solar cells (DSSCs) have a maximum efficiency limit of ca. 30%, it has been predicted theoretically that implementation of UC-STTA in DSSCs could increase that efficiency to more than 40%.A possible way to implement UC-STTA into DSSCs, would be to replace the standard sensi- tized nanostructured TiO2 photoanodes by upconverting ones loaded with emitter molecules. Following TTA, the excited emitter molecule would be quenched by injection of a high-energy electron into the conduction band of the TiO2. To explore the practical aspects of this strategy for a highly efficient DSSC, in this thesis UC-STTA is studied in model systems based on nanostructured ZrO2 films. These ZrO2 films are a good proxy for the TiO2 films used in DSSCs, and allow for relatively easy optimization and study of UC-STTA by allowing measurements of the upconverted photons without the complications of electron injection into the film.Herein it is experimentally proven that UC-STTA is viable on nanostructured metal oxide films under non-coherent irradiation with intensities comparable to sunlight. Two different system architectures are studied, differing in the position of the molecular components involved in the UC-STTA mechanism. Both architectures have the emitter molecules adsorbed onto the ZrO2 surface, but the sensitizers are positioned either in solution around the nanostructure, or co-adsorbed with the emitters onto the ZrO2 surface. A set of challenges in the study and optimization of the UC-STTA process is identified for each type of system. Proposals are also given for how to further improve the understanding and UC-STTA optimization of these systems toward application in DSSCs to overcome the present solar energy conversion efficiency limit.
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| 7. |
- Sjödin, Martin, 1974-
(author)
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Regulation of Proton Coupled Electron Transfer from Amino Acids in Artificial Model Systems: A Mechanistic Study
- 2004
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Doctoral thesis (other academic/artistic)abstract
- Amino acid radicals are key redox intermediates in several natural enzymes including Cytochrome c peroxidase, DNA photolyase, ribonucletide reductase, cytochrome c oxidase and photosystem II. Electron transfer from amino acids is often coupled to deprotonation and this thesis concerns the coupling of electron transfer from tyrosine and tryptophan to trisbipyridineruthenium(III) with deprotonation in model complexes. Specifically the mechanisms for these proton coupled electron transfer reactions have been studied and the controlling parameters have been identified, the possible mechanisms being stepwise electron transfer followed by deprotonation and deprotonation followed by electron transfer or concerted electron transfer/deprotonation.Proton coupled electron transfer reactions have been studied using nano-second flash photolysis in water solution and the effect of pH, temperature, reaction driving force, deuteration and nature of the amino acid has been determined. I have shown that the rate constant for the concerted reaction depends intrinsically on the mixing entropy of the released proton and that the pH-dependence can be used as an experimental tool for mechanistic discrimination. Moreover I have shown that the concerted reaction inherently has a high reorganisation energy due to the coupling of the electron motion with deprotonation. Hydrogen bonding to the transferring proton however significantly reduces this reorganisation energy. The concerted reaction also has a relatively high driving force counteracting the high reorganisation energy in the competition between the concerted reaction and the stepwise electron transfer first reaction. The relative importance of the high reorganisation energy and the high driving force for the concerted reaction determines the mechanistic outcome of the reaction, the stepwise reaction being favoured by high over-all driving forces and the concerted reaction by high pH.By comparing my results from model complexes with tyrosineZ oxidation in photosystem II, I give strong evidence for a concerted electron transfer/deprotonation mechanism.
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