On the Influence of Cations in Non-Aqueous Electrochemistry

Abstract

In this thesis, different aspects of the electrochemistry in non-aqueous electrolytes are presented. Electrochemistry in non-aqueous electrolytes plays a key role in the transition from conventional to renewable energy sources. Compact electrochemical energy storage systems, such as the non-aqueous lithium-ion battery, are indispensable for application in mobile devices, such as smartphones or laptops, and their further development is essential for the inevitable replacement of the conventional combustion engine by electrical engines. To overcome the limitations of the state-of-the-art-lithium-ion batteries, electrochemists all around the world seek for new technologies, enabling higher energy densities and a longer lifetime. One key technology currently discussed are so-called metal–air batteries, in which a metal-anode is combined with an oxygen cathode. However, while electrochemistry in aqueous electrolytes has a very long tradition and is well understood, we know much less about reaction mechanisms and the interface between electrode and electrolyte in non-aqueous electrolytes. <br /> The main focus of this work lies on the oxygen reduction reaction (ORR) and evolution reaction (OER) in dimethyl sulfoxide (DMSO)-based electrolytes. Employing operando mass spectrometry and classical electrochemical measurements, various mechanistic aspects have been elucidated. By comparison between the ORR in presence of different alkali and alkaline earth metal ions it was shown that the cation significantly affects the product distribution and the mechanism of the ORR. The extensive interaction and ion pair formation between the cation of the conducting salt and reduced oxygen species has been exemplarily studied in the K–O2 system. For all the cations under investigation, the main products are superoxide and peroxide species, while the oxide has not been found. Comparing the results for different alkali cations, it appears that a higher charge density of the cation fosters the formation of the corresponding, insoluble alkali peroxide. However, if the alkaline earth metals are also included the relation between charge density and product distribution is more complex. For instance, calcium mainly fosters superoxide formation at platinum or glassy carbon electrodes despite its relatively high charge density. Introducing another descriptor of the cation's effect, namely its acceptor number, a better correlation was obtained, although reasons for this behaviour remain elusive. <br /> Another important aspect of the electrochemistry in non-aqueous electrolyte apart from the effect of the conducting salt is the effect of the electrode material. While the special behaviour of gold electrodes towards the ORR and OER in non-aqueous electrolytes has already been highlighted, in the present work mechanistic aspect have been further elucidated and the overall mechanistic picture has been refined. For example, in Li⁺-containing electrolytes a direct, surface-confined reduction step from oxygen to peroxide has been identified at gold electrodes via the use of the rotating ring-disk electrode. Regarding divalent cations, gold seems to foster the peroxide formation regardless of the cation, while in the case of alkali cations a distinct, potential-dependent transition from the one- to the two-electron process was observed. This transition again was correlated with the charge density of the monovalent cations. Interestingly, this transition from superoxide to peroxide formation is followed by a second transition, where the product distribution changes back from peroxide to superoxide due to the deactivation of the surface. This second transition has also been observed in the presence of the divalent cations and shows that the main difference between the cations is their ability to foster peroxide formation. <br /> 'The effect of the partial pressure of oxygen on the ORR has also been investigated as it is an important parameter of a future, real-world battery, which will either be fed with air or with pure oxygen at variable pressures. While in Li⁺-containing electrolytes the expected electrochemical reaction order of 1 with respect to the oxygen concentration was obtained, the reaction order was significantly lower in the presence of Mg²⁺, implying the contribution of an adsorption process. Employing K⁺-containing electrolyte we were able to show that an increase of the oxygen pressure also changes the product distribution from superoxide to peroxide, which we attributed to the pronounced precipitation of the sparingly soluble superoxide. A further essential issue which is addressed in this work it the influence of water on the non-aqueous electrochemistry, as it is a nearly ubiquitous "contaminant" and difficult to remove quantitatively. In principle, two different trends are observable. In the case of the rather irreversible reduction of oxygen in presence of Li⁺, water leads to a shift of the peroxide formation to larger overpotentials. However, for the largely reversible K–O₂ and Cs–O₂ systems, addition of water leads to the pronounced formation of the peroxide, probably due to an irreversible follow-up reaction of the peroxide. <br /> The importance of electromobility and storage systems for renewable energies has attracted not only electrochemists, who are interested in non-aqueous electrochemistry, but researchers from various fields of science. Scientists nowadays make great use of simulation techniques to predict the performance of electrical storage systems rather than testing every possible electrolyte combination. However, modelling of electrochemical cells requires knowledge of different parameters, one of which are the transport properties of oxygen. As state-of-the art methods of determining the transport properties were either too slow, too inaccurate or too expensive, we developed a new, non-electrochemical measurement cell which enables simultaneous measurement of the gas diffusivities and solubilities without external knowledge. Interestingly, the solubility of oxygen in DMSO increases for increasing temperatures. Moreover, a significant salting-in effect has been observed in the presence of lithium bis(trifluoromethane)sulfonimide, while the usual salting-out effect has been identified in the presence of the perchlorate salts of different cations. From the temperature-dependent diffusivities we were able to evaluate the activation barrier for the diffusion of oxygen in DMSO-based electrolyte, which will certainly be of great help for modelling electrochemical cells at different temperatures (think of cold winter and hot summer days). <br /> While battery-related research usually focuses on the reversibility of the reaction and the energy density of a potential energy storage system, we aimed at elucidating more fundamental properties in this work and there is nothing more fundamental for an electrochemist than the interface between electrode and electrolyte. Therefore, the final section of this work deals with the investigation of the electrode–electrolyte interface via surface-enhanced infrared spectroscopy. The adsorption of cyanide and carbon monoxide from propylene carbonate have been studied and it was found that the observed shift of the vibrational bands are similar to the aqueous system, indicating similar adsorption geometries. Careful analysis of the adsorption of acetonitrile from acetonitrile-based electrolytes revealed that the solvent is adsorbed via the methyl-group for potential negative of the point of zero charge and that the solvent is electrochemically decomposed to cyanide. Moreover, there is a strong interaction between the cation of the conducting salt and acetonitrile, which shows up as a significant shift of the vibration bands of the methyl- and cyanide-groups. <br /> In the present work we could give some deeper insights in the course of reactions in non-aqueous electrolytes as well as in the importance of the cation of the conducting-salt, the electrode material, the partial pressure of oxygen and the presence of water. Using a newly developed measurement cell for simultaneous determination of the gas diffusivities and solubilities new data concerning diffusivities and solubilities of oxygen in DMSO-based electrolytes could be collected. Moreover, further insights in the adsorption processes at the electrode electrolyte interface could be given by investigations using the surface-enhanced infrared spectroscopy. The presented results describe fundamental aspects of the electrochemistry in non-aqueous electrolytes and thus contribute to a better understanding of the underlying mechanisms, which will possibly help to improve the development of new batteries in practice.