Příprava, charakterizace a studium přenosu elektronu v samoorganizovaných strukturách na pevných elektrodách

Předkládaná dizertační práce se zaměřuje na elektrochemické, spektroelektrochemické, adsorpční, komplexační a vodivostní vlastnosti prodloužených viologenů různé délky (sloučeniny 1 až 6). Jejich molekuly by v budoucnosti mohli sloužit jako vodivé molekulární dráty v přístrojích molekulární elektroniky. Všechny studované sloučeniny vytvářejí na rozhraní rtuť/elektrolyt kompaktní monovrstvu paralelně ležících molekul s difúzně řízeným vznikem. Přítomnost adsorbovaných molekul na povrchu Au(111) byla potvrzena pomocí technik STM a PM IRRAS. Molekula prodlouženého viologenu obsahující jednu opakující se jednotku (sloučenina 1) reverzibilně přijímá čtyři elektrony, jako plně delokalizovaný systém. Molekula sloučeniny 2 však přijímá dva elektrony nezávisle a má proto dvě nekomunikující centra. Molekuly obsahující větší počet jednotek (n) přijímají v prvních dvěch redukčních krocích 2(n-1) elektronů (n-1 v každém). Chemická stabilita redukovaných forem sloučenin 1 až 6 byla potvrzena pomocí in-situ UV/VIS/NIR spektroelektrochemických technik. Vodivost elektrických spojů obsahujících molekuly prodloužených viologenů (měřena Taovou metodou) klesá exponencielně s délkou molekuly. V případě jiných typů prodloužených viologenů (sloučeniny 1' a 4') se zjistilo, že na jejich elektrochemické chování má výrazný...The presented thesis focuses on electrochemical, spectroelectrochemical, adsorption, complexation and conductivity properties of extended viologens of variable length (the compounds 1 to 6). In the future, their molecules could serve as conducting molecular wires in the devices of molecular electronics. At the mercury/electrolyte interface, all studied compounds were found to form a compact monolayer with flat-lying molecules with the diffusion as the rate-determining step. The presence of adsorbed molecules on Au(111) surface was confirmed by STM and PM IRRAS techniques. The extended viologen molecule consisting of one repeating unit (the compound 1) was found to reversibly transfer four electrons, behaving as a fully delocalized system. On the other hand, the molecule of the compound 2 transfers two electrons independently, having thus two non-communicating centers. The molecules containing higher number of repeating units (n) transfer 2(n-1) electrons in the first two closely-positioned reduction steps (with n-1 electrons being independently consumed in each of them). Chemical stability of reduced forms of the compounds 1 to 6 was confirmed by UV/VIS/NIR in-situ spectroelectrochemical techniques. Electric conductance of junctions containing extended viologen molecules (scrutinized by Tao's...Katedra fyzikální a makromol. chemieDepartment of Physical and Macromolecular ChemistryFaculty of SciencePřírodovědecká fakult

Probabilistic mapping of single molecule junction configurations as a tool to achieve the desired geometry of asymmetric tripodal molecules

Four molecules containing identical tripodal anchors and p-oligophenylene molecular wires of increasing length were used to demonstrate tuning of the asymmetric molecular junction to the desired geometry by probabilistic mapping of single molecule junction configurations in a scanning tunnelling microscopy break junction experiment

Cracks as Efficient Tools to Mitigate Flooding in Gas Diffusion Electrodes Used for the Electrochemical Reduction of Carbon Dioxide.

The advantage of employing gas diffusion electrodes (GDEs) in carbon dioxide reduction electrolyzers is that they allow CO2 to reach the catalyst in gaseous state, enabling current densities that are orders of magnitude larger than what is achievable in standard H-type cells. The gain in the reaction rate comes, however, at the cost of stability issues related to flooding that occurs when excess electrolyte permeates the micropores of the GDE, effectively blocking the access of CO2 to the catalyst. For electrolyzers operated with alkaline electrolytes, flooding leaves clear traces within the GDE in the form of precipitated potassium (hydrogen)carbonates. By analyzing the amount and distribution of precipitates, and by quantifying potassium salts transported through the GDE during operation (electrolyte perspiration), important information can be gained with regard to the extent and means of flooding. In this work, a novel combination of energy dispersive X-ray and inductively coupled plasma mass spectrometry based methods is employed to study flooding-related phenomena in GDEs differing in the abundance of cracks in the microporous layer. It is concluded that cracks play an important role in the electrolyte management of CO2 electrolyzers, and that electrolyte perspiration through cracks is paramount in avoiding flooding-related performance drops

Eliminating Flooding-related Issues in Electrochemical CO₂-to-CO Converters: Two Lines of Defense

By using silver (Ag) in nanostructured (nanowire, nanosphere, etc.) or thin-layer form as a catalyst for electrochemical CO2 reduction, very high CO-forming selectivity of almost 100% can be achieved. Supported by gas diffusion layers (GDLs),  the reactant CO2 in the gas phase can approach and potentially access active Ag sites, which allows current densities in the range of a few hundred mA cm–2 to be reached. Yet, the stability of gas diffusion electrode (GDE) based electrochemical CO2-to-CO converters is far from perfect, and the activity of GDE cathodes, especially when operated at high current densities, often significantly decays during electrolyses after no more than a few hours. The primary reason of stability losses in GDE-based CO2-to-CO electrolysers is flooding: that is, the excess wetting of the GDE that prevents CO2 from reaching Ag catalytic sites. In the past years, the authors of this paper at Empa and at the University of Bern, cooperating with other partners of the National Competence Center for Research (NCCR) on Catalysis, took different approaches to overcome flooding. While opinions differ with regard to where the first line of defense in protecting GDEs from flooding should lie, a comparison of the recent results of the two groups gives unique insight into the nature of processes occurring in GDE cathodes used for CO2 electrolysis

Preparation, characterization and study of electron transfer in self-organized structures on solid electrodes

The presented thesis focuses on electrochemical, spectroelectrochemical, adsorption, complexation and conductivity properties of extended viologens of variable length (the compounds 1 to 6). In the future, their molecules could serve as conducting molecular wires in the devices of molecular electronics. At the mercury/electrolyte interface, all studied compounds were found to form a compact monolayer with flat-lying molecules with the diffusion as the rate-determining step. The presence of adsorbed molecules on Au(111) surface was confirmed by STM and PM IRRAS techniques. The extended viologen molecule consisting of one repeating unit (the compound 1) was found to reversibly transfer four electrons, behaving as a fully delocalized system. On the other hand, the molecule of the compound 2 transfers two electrons independently, having thus two non-communicating centers. The molecules containing higher number of repeating units (n) transfer 2(n-1) electrons in the first two closely-positioned reduction steps (with n-1 electrons being independently consumed in each of them). Chemical stability of reduced forms of the compounds 1 to 6 was confirmed by UV/VIS/NIR in-situ spectroelectrochemical techniques. Electric conductance of junctions containing extended viologen molecules (scrutinized by Tao's..

Charge Transport through Molecular Towers Based on Tetraphenylmethane Tripods

Molecular platforms with multiple anchors were introduced recently to provide robust\ncoupling between molecular electronic components and metallic electrodes. In this work,\ncharge transport in tower-shaped single molecule conductors based on tetraphenylmethane\ntripod possessing three thiol anchors is investigated by scanning tunneling microscopy break\njunction technique. The effect of tripod substitution pattern and number of repeating units in\nmolecule is addressed. The substitution was found to influence conductance value in early\nstages of molecular junction evolution, where tower lays parallel to surface. In late stages, the\ncharge is transported through principal molecular axis and junction breaks thermally in tilted\nconfiguration

Investigation of Single Molecule Charge Transport Properties and Geometrical Arrangement in Terpyridine Architectures Supported by the Tetraphenylmethane Tripod

Tripodal platforms were engineered recently to realize a well-defined directional contact between metallic electrodes and molecular architectures dedicated to serve as working elements for electronic applications. In this work we employ cyclic voltammetry, scanning tunneling microscopy break junction technique and theoretical approaches based on the combination of density functional theory and non-equilibrium Green´s function to investigate the geometrical arrangement and single molecule charge transport in terpyridine-based architectures supported by tetraphenylmethane tripod. We demonstrate that this architecture adopts a favorable geometrical arrangement capable of forming highly conductive molecular junctions and is thus suitable to serve as a basis for working molecular switches