No abstract

In the present study, we investigated the reduction and the evolution of oxygen from lithium containing DMSO based electrolytes at gold. The number of electrons that are transferred per O 2 (z-value) during oxygen reduction depends on the structure of the electrode: Despite the presence of Li + , O 2 is reduced electrochemically to superoxide at smooth gold electrodes and at low overpotentials. At porous electrodes a z-value close to 2 e − /O 2 indicates Li 2 O 2 -formation even at low overpotentials. This is ascribed to a reaction of superoxide, which is catalyzed by gold-particles at open circuit. This behavior is also responsible for the non-proportionality between reduced and evolved amounts of oxygen. Furthermore, we observed a linear relationship between evolved amounts of CO 2 and reduced amounts of oxygen, indicative for electrolyte decomposition during oxygen reduction. Combined electrochemical quartz crystal microbalance (eQCMB) and Differential electrochemical mass spectroscopy (DEMS) measurements reveal that mass changes that occur in the anodic sweep are due to the evolution of CO 2 , whereas oxygen evolution takes place without any mass changes. The observed m.p.e-values (mass changes per transferred electron) are affected by convection due to the formation of soluble reduction products which observed in rotating ring disc electrode measurements. It is common knowledge that, in order to electrify automotive traffic, portable storage systems for electrical energy are required. For the time being lithium-ion-batteries are the most promising candidates for a real life technical application. However, due to the requirement of heavy metal oxides as cathode material their theoretical specific energy density is too low to replace current fuels.1 Due to the low weight and very negative standard potential of lithium, a lithiumoxygen battery has, in theory, a specific energy density of 13.8 kJ/g (considering the weight of the discharged state and the formation of Li 2 O 2 rather than Li 2 O) and a theoretical electrochemical efficiency of 90.2% as can easily be calculated from thermodynamics 2,3 (10% of the energy is lost due to the heat flow caused by the entropy).It is in general accepted that reduction of oxygen in Li + -containing organic solvents yields Li 2 O 2 as a reaction product. [4][5][6] The process of Li 2 O 2 formation in DMSO seems to be unique as it involves the formation of a superoxide intermediate. [7][8][9][10][11] This has not only been shown by CV and rotating ring disc electrode (RRDE) measurements but also by combining electrochemistry with spin-trap experiments and EPRspectroscopy 12 as well as by DEMS experiments. 7 The remarkable stability of superoxide, despite the presence of Li + -cations, was ascribed to the large donor number of DMSO 8,13 of nearly 125 kJ/mol 14 as compared to acetonitrile with a donor number of 58.9 kJ/mol. 14 In the latter solvent superoxide intermediates have not been reported so far. However, whether Li 2 O 2 forms via lithium induced disproportionation of...

SummarySeveral transitions in the friction coefficient with increasing load are found on Au(111) in sulfuric acid electrolyte containing Cu ions when a monolayer (or submonolayer) of Cu is adsorbed. At the corresponding normal loads, a transition to double or multiple slips in stick–slip friction is observed. The stick length in this case corresponds to multiples of the lattice distance of the adsorbed sulfate, which is adsorbed in a √3 × √7 superstructure on the copper monolayer. Stick–slip behaviour for the copper monolayer as well as for 2/3 coverage can be observed at F N ≥ 15 nN. At this normal load, a change from a small to a large friction coefficient occurs. This leads to the interpretation that the tip penetrates the electrochemical double layer at this point. At the potential (or point) of zero charge (pzc), stick–slip resolution persists at all normal forces investigated.

Secondary lithium-air batteries are promising candidates for a future energy storage system as they provide in theory high capacity; however, in practice they show bad cyclebility. The key step in discharging these batteries is the reduction of oxygen present in air. So far the oxygen reduction reaction (ORR) has been investigated thoroughly in aqueous media, but little research has been done in non-aqueous solvents. Organic carbonates are unsuitable for the use in lithium-air batteries because they decompose upon charging [1]. Recently Scrosati [2] showed by XRD that tetraethylene glycol dimethyl ether (tetraglyme) is an appropriate electrolyte for Li-air battery whereas Bruce [3] using XRD and FTIR techniques demonstrated that, tetraglyme is not the best electrolyte because it decomposes in the first few cycles. On the other hand, Li2O2 has been reported as the major discharge product using dimethyl sulfoxide (DMSO) as an electrolyte [4]. Since spectroscopic techniques are superior to diffraction methods in detecting charge/discharge products, mass spectrometry in combination with electrochemistry (DEMS) has been used in this study to determine the electrolyte stability, water content, the amount of O2 reduced and evolved in a variety of aprotic electrolytes such as ethers (tetraglyme) and sulfoxide (DMSO). O2 solubility in aqueous and different non-aqueous electrolyte and its dependence on the flow rate of the electrolyte has been studied. Crucial is a knowledge of the O2 solubility in these electrolytes. We will present a new method for its determination using differential electrochemical mass spectrometry (DEMS) based on the convection and diffusion behaviour characteristices of the dual thin layer cell used for DEMS. [5] References: [1] F. Mizuno, et al., Electrochemitry, 2010, 78, 403. [2] H Jung et al., Nat. Chem., 2012, 4, 579. [3] S. Freunberger, et al., Angew. Chem. Int. Ed., 2011, 50, 8609. [4] Z. Peng, et al., Science, 2012, 337, 563. [5] Fuhrmann, J.; Linke, A.; Langmach, H.; Baltruschat, H., Numerical calculation of the limiting current for a cylindrical thin layer flow cell Electrochimica Acta 2009, 55, 430-438.

not Available.