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The reversibility of current Li-O2 batteries suffers from high charging overpotentials. To address this problem, the use of redox mediators has been proposed, which are supposed to improve the sluggish reaction kinetics of the oxygen evolution reaction via a solution mediated oxidation of lithium peroxide. In this study, we present a new thin layer cell for battery related differential electrochemical mass spectrometry (DEMS) experiments, which exhibits a high electrode surface area to electrolyte volume ratio which is closer to the situation in batteries other approaches/cells with their usually large electrolyte excess. The confined volume also allows a better distinction between the mediating activity of a redox system and a near continuous electrochemical reaction of this species. One further benefit of the new thin layer cell is that experiments can easily be performed under different O2-partial pressures. This new set-up allows the highly sensitive detection of volatile species formed during the OER. Therefore, small changes in the number of electrons transferred per oxygen molecule are observable. These changes help to identify side reactions and possible decomposition of the reaction products. During our experiments, we investigated the impact of TTF, TMPD, Fc and TEMPO on the oxidation of Li2O2. Within our experiments, we are able to precisely determine the potential at which the catalytic activity of the redox mediation starts. A comparison between the potential at which we observe the activity of the redox mediator to the half wave potential of the redox system could be explained with an outer sphere electron transfer for the oxidation of Li2O2 by a redox mediator. This observation is confirmed by a theoretical treatment of the redox mediation mechanism. Moreover, insights into the number of transferred electrons per oxygen molecule during the activity of the different redox mediators reveal the presence of side reactions. This finding is also underlined by an unexpected shift of the CO2 evolution onset for the redox mediator containing electrolytes. Our experiments also reveal that a Li-O2 cell, which contains a redox mediator, undergoes less fluctuation in its reversibility compared to a cell without a redox mediator.
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Correction for 'Oxygen reduction and oxygen evolution in DMSO based electrolytes: the role of the electrocatalyst' by C. J. Bondue et al., Phys. Chem. Chem. Phys., 2015, 17, 25593-25606.
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In the present paper the role of the electrode material in oxygen reduction in DMSO based electrolytes is elucidated using DEMS. We have found, employing platinum, gold, ruthenium rhodium, selenium decorated rhodium and boron doped diamond (BDD) as electrode materials, that the actual mechanism of oxygen reduction largely depends on the electrode material. At platinum, rhodium and selenium decorated rhodium the final reduction product, peroxide, is formed electrochemically. At gold and at low overpotentials oxygen is reduced to superoxide and peroxide is only formed by disproportionation of the latter. No oxygen reduction takes place at the diamond surface of the BDD-electrode, hence, showing unambiguously that oxygen reduction is an inner sphere reaction. Also, the rate of oxygen evolution varies with the electrode material, although the onset potential of oxygen evolution is not influenced. The amount of peroxide formed is limited to 1-2 monolayers. Contrary to intuition oxygen reduction and oxygen evolution from peroxide, therefore, are heterogeneous, electrocatalytic reactions. The finding of such an electrocatalytic effect is of great importance for the development and optimization of lithium-air batteries. Aside from the electrode material there are also effects of water as well as of the cation used in the electrolyte. This suggests an influence of the double layer at the interface between the electrode and the electrolyte on oxygen reduction in addition to the well-known higher stability of Na2O2 and K2O2. Electrospray ionization (ESI) results show that any effect of water in the Li(+) containing electrolyte is not due to an altered solvation of the cation.
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PURPOSE: Three methods have been recently proposed for the delineation of the primary tumor clinical target volume (CTV-P) in Head and Neck Cancers: the anatomic method popularized in the French literature by Lapeyre et al. the geometric methods proposed by the DAHANCA group, and more recently the international guidelines promoted by Grégoire et al. integrating the latter two. The aim of this study was to perform a volumetric and dosimetric comparison of the French and the International consensus methods in laryngeal SCC. PATIENTS AND METHODS: Two radiation oncologists independently delineated the high dose and low dose primary tumor CTV in four patients with T2 or T3 N0-M0 laryngeal SCC following either the so-called French guidelines or the International guidelines. For the 4 cases, the GTV was delineated by a single radiation oncologist. Nodal CTVs were delineated by one radiation oncologist for the 4 cases using International guidelines. Dose optimization was then performed with VMAT (MONACO version 5.11) using 6â¯MeV photons. Differences in target volumes and dose distributions in OARs and PTVs were then evaluated with various metrics such as the DICE Similarity Coefficient and the homogeneity index. RESULTS: Major differences were observed in the CTV delineation between the 2 delineation methods for the low dose volumes and to a lower extend for the high dose volumes. These differences translated into variations in dose distribution favoring the International guidelines for decreasing dose to various OARs. Such differences toned down when dose distribution on the primary tumors PTVs and nodal PTVs were combined. CONCLUSION: This study demonstrated large differences in CTV delineation between the 2 delineation guidelines. Such differences translated into differences in dose distribution.