8,665 research outputs found
Increases Overpotential of Electrocatalytic Water Oxidation in Lithium Hydroxide Electrolytes
No full textChemical and structural changes preceding electrocatalysis obfuscate the nature of the active state of electrocatalysts for the oxygen evolution reaction (OER), which calls for model systems to gain systematic insight. We investigated the effect of bulk oxidation on the overpotential of ink-casted LiMn2 O4 electrodes by a rotating ring-disk electrode (RRDE) setup and X-ray absorption spectroscopy (XAS) at the K shell core level of manganese ions (Mn-K edge). The cyclic voltammogram of the RRDE disk shows pronounced redox peaks in lithium hydroxide electrolytes with pH between 12 and 13.5, which we assign to bulk manganese redox based on XAS. The onset of the OER is pH-dependent on the scale of the reversible hydrogen electrode (RHE) with a Nernst slope of -40(4) mV/pH at -5 μA monitored at the RRDE ring. To connect this trend to catalyst changes, we develop a simple model for delithiation of LiMn2 O4 in LiOH electrolytes, which gives the same Nernst slope of delithiation as our experimental data, i. e., 116(25) mV/pH. From this data, we construct an ERHE -pH diagram that illustrates robustness of LiMn2 O4 against oxidation above pH 13.5 as also verified by XAS. We conclude that manganese oxidation is the origin of the increase of the OER overpotential at pH lower than 14 and also of the pH dependence on the RHE scale. Our work highlights that vulnerability to transition metal redox may lead to increased overpotentials, which is important for the design of stable electrocatalysts.collaborative research center (CRC)HZB http://dx.doi.org/10.13039/10001311
A Modular Double Electrode Flow Cell with Exchangeable Generator and Detector Electrodes
No full textGenerator-collector experiments offer insights into the mechanisms of electrochemical reactions by correlating the product and generator currents. Most commonly, these experiments are performed using commercially-available rotating ring-disk electrodes (RRDE). We developed a modular double electrode flow cell (DEFC) with exchangeable generator and detector electrodes where the electrode width equals the channel width. As a test case, we considered the ferri-/ferrocyanide redox couple in experiments, analytical calculations and multiphysics simulations. Wall effects reduce the current density by less than 10 % in our geometry for the investigated conditions and the analytical solution for the limiting current at the generator electrode applies to widths up to 5 mm. The collection efficiency for all investigated electrode widths is close to the expected 35.4 % above a flow rate of 1.0 (mL/min)1/3 but only independent of the flow rate for electrodes with width 5 mm and larger. Kinetic constants of 1.3–1.9 ⋅ 10−3 cm/s are obtained from Koutecký-Levich analysis and 21.0–5.0 ⋅ 10−3 cm/s from Nicholson analysis for the DEFC, which falls within the range reported previously. We conclude that our DEFC with exchangeable electrodes is an attractive alternative to commercial RRDEs which offers the flexibility to optimize both the generator and collector materials for the desired reaction.Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)Germany's Excellence StrategyGerman Helmholtz AssociationHelmholtz Energy Materials Foundry (HEMF)German Helmholtz AssociationOpen-Access-Publikationsfonds 202
A criticial view on eg occupancy as a descriptor for oxygen evolution catalytic activity in LiMn2O4 nanoparticles
No full textWe investigate the effect of the surface electronic structure and composition of LiMn^{2+}^+^{2+}. This is surprising since the e emptying fails to correlate with OER activity here indicates that octahedral cation valence is not a fundamental measure of activity, either because the active surface state is not affected by tetrahedral Mn or because other details of the band structure or metal-oxygen bonding character, more strongly regulate the rate-limiting steps for OER
Manganese dissolution in alkaline medium with and without concurrent oxygen evolution in LiMn2O4
No full textEnseñanza de la escritura de Max Aub: comprensión y memoria
No full textEste texto analiza a obra testimonial de Max Aub sobre su experiencia en los campos de concentración en Francia desde una perspectiva de discursos comparados. Para destacar las estrategias de la escritura del autor recuperables por otros proyectos discursivos que persigan la sensibilización y la denuncia a través del cruce entre la comunicación y la éticaThis text analyses the testimonial work of Max Aub about his experience in the French concentration camps in France from comparative discourses approach. It emphasizes the writing strategies used by the author useful for other awareness and denounce discourses through the dialogue among communication and ethic
Influence of particle size on the apparent electrocatalytic activity of LiMn2O4 for oxygen evolution
No full textWe investigate LiMn2O4 as a model catalyst for the oxygen evolution reaction (OER), because it shares the cubane structure with the active site of photosystem II. Specifically, we study the influence of different particle sizes of LiMn2O4 on the OER in a sodium hydroxide electrolyte. The product currents of manganese corrosion and oxygen evolution were obtained by rotating ring disk electrodes (RRDE). Physical characterization by various methods supports identical surface chemistry and microstructure of the pristine powders. We obtained similar oxygen current densities of 40(14) μA cmECSA−2 and 26(5) μA cmECSA−2 for micro- and nano-sized particles at 1.68 V vs. RHE. However, the total current densities differed drastically and while the micro-powder had a high disk current density of 205(2) μA cmECSA−2, its faradaic efficiency was only 25%. In contrast, the faradaic efficiency of the nanopowder was at least 75%. We hypothesize that a Mn redox process may occur in the bulk in parallel and possibly in combination with oxygen evolution on the surface based on the observed difference between the total and product current densities. Knowledge of the product currents is crucial for distinguishing the mechanisms of corrosion and catalysis and for designing better catalysts with high faradaic efficiency
A criticial view on eg occupancy as a descriptor for oxygen evolution catalytic activity in LiMn2O4 nanoparticles
No full textWe investigate the effect of the surface electronic structure and composition of LiMn^{2+}^+^{2+}. This is surprising since the e emptying fails to correlate with OER activity here indicates that octahedral cation valence is not a fundamental measure of activity, either because the active surface state is not affected by tetrahedral Mn or because other details of the band structure or metal-oxygen bonding character, more strongly regulate the rate-limiting steps for OER
Max Brooks literary reading flier
No full text2012 Bismarck State College Visiting Writers Series and ArtsQuest present: Max Brooks. April 25, 7:30 p.m.; Belle Mehus Auditorium. Max Brooks is the author of World War Z: An Oral History of the Zombie War and the graphic novel The Zombie Survival Guide: Recorded Attacks
Max Frisch's novel: Stiller. A study
No full text The attempt is made in the following study to present an interpretation of the novel "Stiller" by the Swiss author, Max Frisch, by tracing through the novel the dominant themes of the graven-image or 'Bildnis' and that of the problem of freedom with reference to the novel's main character. ThesisMaster of Arts (MA
A Transfer Report on the Development of a Framework to Evaluate Search Interfaces for their Support of Different User Types and Search Tactics
No full textAs the understanding of search systems, user needs and seeking strategies is developing, the design of search user interfaces is evolving to support more complicated and exploratory forms of search. With the design of new search features that enable these richer modes of exploration, comes the need to better understand the support they provide. In this report a new evaluation framework is presented that analyses search features for how they a) contribute to an overall interface, b) allow users to carry out different search tactics, and c) support different types of users and their needs. The novel contributions of the framework improve on some of the limitations of typical user studies, and allow search systems to be systematically analysed in much more detail and in much less time. The presented evaluation framework is then validated in three ways. First the validity of the models used as the building blocks of the framework are investigated through related work. Second the method of integrating these building-block models is validated and strengthened by consensus of expert opinion. Third, the overall approach is validated by comparing its analyses to the results of previously carried out user studies. The validation process has shown both the value of the framework and identified areas of future work that should be addressed for the framework to be completed. This report concludes with the set of contributions that the framework makes, and why the remaining work will be challenging, but critical to the final design
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