Novel Ionic Liquids For Electrochemical Applications

This thesis is organized into three main sections, and is concerned with the design and synthesis of novel ionic liquids (ILs), and their associated electrochemical applications. In the first section, a general introduction to the field of ionic liquids is presented, followed by the synthesis and characterization details of all novel compounds used in the study. The second section summarizes work done on the development of more stable dye-sensitized solar cells (DSCs). An introduction to the field of photovoltaics and DSCs is first delivered in Chapter 3, which is followed by two chapters of original work on ionic liqiud-based electrolytes for DSC applications. In Chapter 4, it is shown that novel triazolium-based ionic liquids can be used to fabricate dye-sensitized solar cells, giving power conversion efficiencies of up to 6.00 %. The ionic liquid-based electrolytes are found to be compatible with a range of organic and inorganic photosensitizers, although the ruthenium-based dye (coded C106) exhibits the best performance. A device employing one of these newly-developed electrolytes is also shown to be very stable, retaining ca. 90% of its initial performance even after 1000 hours of continuous full sun illumination at 60 âŠC. In Chapter 5, we focus on the development of stable porphyrin-sensitized solar cells, using IL-based electrolytes. Our studies reveal that the long-term stability of porphyrin-based DSCs are highly dependent on additives in the electrolyte. In particular, it is found that the additive, guanidinium thiocyanate, is essential for the preparation of long-lived devices. It is also shown that the beneficial effect of this additive originates from the thiocyanate anion, rather than the guanidinium cation. The third section of this thesis deals with the ionic liquid-catalyzed electroï¿Œchemical reduction of carbon dioxide. Following a general introduction on carbon dioxide mitigation using ionic liquids (Chapter 6), the results of our work in this area are presented in Chapter 7. Our studies reveal that not all ionic liquids are stable at the potentials required for CO2 reduction. However, the imidazolium-based salts generally perform quite well. The catalytic effect of the IL is found to mainly stem from the cation, with the anion playing a somewhat secondary role. Our results also demonstrate that substitutions on the imidazolium ring can have a huge impact on catalysis. Product analysis shows that the best-performing catalyst produces carbon monoxide as the dominant product, with Faradaic efficiencies of between 70 to 80 %.

4,4' Functionalized 2,2'-Bipyridines

Ryan Dykeman

This thesis presents two applications of 2,2'-bipyridine (bipy) derivatives, the first involves their use as stabilizers for nanoparticle (NP) catalysts and the second is concerned with the use of bipy ligands for ruthenium dye molecules applied in dye sensitized solar cells (DSCs). Chapters 2 and 3 describe the work concerning the ability of bipy derivatives to stabilize catalytically active Rh NPs for ionic liquid based, biphasic arene hydrogenation, with cationic bipy derivatives being first evaluated with a series of monocyclic aromatic hydrocarbons. The recyclability of the cationic bipy stabilized Rh NP systems were evaluated in catalysis and analyzed further by X-ray spectroscopy. The stereoelectronics were examined with respect to catalytic activity using various neutral bipy ligands. The differences in catalytic activity were rationalized with steric and electronic arguments throughout. Chapter 4 concerns the application of a cationic bipy derivative as a modifier/stabilizer in the Pd NP catalyzed partial hydrogenation of hexyne to 1-hexene. Factors for the high selectivity have been studied, including the presence of a ligand and/or an IL. The Pd NPs were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. Leaching was investigated by atomic absorption spectroscopy and the kinetcis of the reaction analyzed using a Langmuir-Hinshelwood model. Chapter 5 describes three heteroleptic ruthenium complexes incorporating new ancillary bipy based ligands synthesized by sequential connection of different alkyl functionalities using a triazole linker, prepared via click chemistry. These complexes were effective dyes when applied in DSCs with both a volatile and non-volatile IL electrolyte. The complexes were characterized and compared using their photovoltaic conversion efficiencies as well as their photophysical and electrochemical properties. A stability study was conducted at 60 °C for 1000 h to determine the most stable substituent.

EPFL2011

Rational design of ionic compounds for electrocatalytic reduction of carbon dioxide

Dmitry Vasilyev

Electrochemical reduction of carbon dioxide is one of the plausible approaches towards renewable energy carriers. When coupled with electricity, which can be provided by sustainable technologies, it becomes a method of high importance with potential value for future energy challenges. However, the electrochemical version of the CO2 reduction reaction (CO2RR) proceeds through highly energetic intermediates, therefore efficient catalytic and co-catalytic systems are needed. Recently, considerable attention was dedicated to application of ionic liquids (ILs) as promising promoters for the CO2RR. While most efforts in this domain have been concentrated on spectroscopic and electrochemical investigation of existing IL-based systems and on the development of more active electrodes, there are only a limited number of studies discussing the structure/activity relationships of ILs in the CO2RR. Our aim was to fill this gap and to find new classes of ILs that are able to promote CO2RR. Another aim of our project was to delineate the basic trends in the structures of the active co-catalysts and provide descriptors for the promoting activity. In this work three new classes of ILs are evaluated for the CO2RR. Within each IL series the structures of the cationic core were varied, the dependencies of the stabilities and activities of the ILs on the structure are discussed. Additionally, a fundamentally new type of ionic systems based on deep eutectic solvents was applied to the CO2RR and found to be a highly active, cheap and non-toxic alternative for the conventional ILs. Based on the obtained results, the charge and its accessibility are suggested to be the main descriptors for the activity of the ionic catalysts in non-aqueous environments.

EPFL2019

Decreasing the Stability of Borohydride with Ionic Liquids for Hydrogen Storage and Carbon Dioxide Conversion

Loris Giovanni Lombardo

The world needs to move from a fossil fuel to a renewable energy based society. With the increasing electricity production from wind and PV, short and long term energy storage are becoming one of challenge of the 21st century. While batteries are the preferred method for short term storage, another technology is required for seasonal storage. Hydrogen is a valuable energy carrier owning its high energy density (122 kJ/g). However, compact and safe storage of H2 is challenging. The current solution is to compress hydrogen up to 700 bar for mobile application (fuel cell cars), and metal hydrides for stationary application. Material-based H2 storage avoids using high-pressure cylinder. Complex hydrides are especially attractive for solid-state H2 storage due to their high gravimetric and volumetric hydrogen density. Sodium borohydride (NaBH4) contains 10.6 mass% of H2, but high temperatures are needed to release H2 (505 Â°C) and the reaction is not reversible under moderate conditions. To apply complex hydrides in real applications, they must be modified to improve the kinetics and thermodynamics of H2 desorption. Several possibilities exist such as confinement into scaffolds, catalyst addition, or combination of several hydrides to modify the dehydrogenation pathway. The stability of complex hydrides is determined by the localization of the charge on the central atom of the complex. Therefore, in this thesis the interaction of highly polar or ionic compounds with complex hydrides were investigated. We combine NaBH4 with a special class of organic salts: ionic liquids (IL). Considering their chemical and thermal stability, IL are attractive additive. Using organic additives instead of expensive metal catalysts is another advantage. Two different techniques were employed to combine the IL and NaBH4. The first one consists of mechanochemically milling them to create an interaction between BH4- and IL. The second one is to directly synthesized IL borohydrides ([IL][BH4]) by metathesis reaction. Different ILs, such as vinylbenzyltrimethylammonium chloride ([VBTMA][Cl]), 1-butyl-3-methylimidazolium chloride ([bmim][Cl]), and 1-ethyl-1-methylpyrrolidinium bromide ([EMPY][Br]) were tested. [bmim][BH4] was able to release H2 below 100 Â°C. The hydrogen content ranges between 2.4 mass% and 2.9 mass%. The second challenge of this century is the CO2 problematic. Atmospheric CO2 concentration linearly increases, leading to greenhouse effect. Carbon capture and sequestration (CCS) and utilization (CCU) are primordial research topic to stabilize the CO2 emission. Hydrides are primary reducing agents, thus reduction of CO2 can be accomplished with them. Direct CO2 reduction at ambient conditions is challenging for classical borohydrides. With our [IL][BH4], gaseous CO2 can be captured and reduced under ambient conditions (1 bar, RT). Three CO2 bounds with boron to give triformatoborohydride ([HB(OCHO)3]â. Dilute CO2 (6 vol%) can be used as well. The reaction was followed in real time with a magnetic suspended balance (MSB). Further, CO2 reduction happened when air is used as a carbon dioxide source. Formic acid can be obtained by adding HCl. Thus, ionic liquid borohydrides have great potential as a CO2 absorber and reducer to alternative fuels. In short, we applied ionic liquid to destabilized borohydrides. The resulting materials exhibited potential for solid-state hydrogen storage, as well as CO2 capture and transformation.