Rechargeable Magnesium/Oxygen Batteries: Reaction Mechanisms and Their Dependence on Electrolyte Composition.

Electrochemical energy storage devices that are robust, energy-dense, and cheap will accelerate the commercialization of electric vehicles. Magnesium/Oxygen (Mg/O2) batteries are a promising system with the potential for very high energy densities. The goal of this dissertation is to explore candidate magnesium electrolytes for use in Mg/O2 batteries, and to assess the reaction mechanisms and performance of Mg/O2 cells that employ these electrolytes. We first consider electrolytes based on ionic liquids (ILs), which are attractive because they are nonflammable and nonvolatile. The absence of voltammetric signatures of Mg plating from ILs with Tf2N– and BF4– suggests that strong Mg/anion Coulombic attraction inhibits electrodeposition. Cosolvent additions to Mg(Tf2N)2/PP13-Tf2N were explored but did not result in enhanced plating/stripping activity. The results highlight the need for IL solvents or cosolvent systems that promote Mg2+ dissociation. We next describe a room-temperature, non-aqueous, reversible Mg/O2 cell using a modified Grignard electrolyte. Electrochemical, microscopic, and spectroscopic analyses reveal characteristics that distinguish Mg/O2 chemistry from its alkali-metal/O2 analogues. The open-circuit voltage is 2.0 V, lower than the ~2.9 V expected for direct electrochemical formation of MgOx. The low voltage and two-phase product are consistent with a multi-step discharge reaction in which a superoxide-ion (O2–) intermediate forms at ~2 V vs. Mg/Mg2+. Bypassing the multi-step mechanism in favor of direct electrochemical MgOx formation would raise the discharge potential and the energy density. The performance of the preceding modified-Grignard-based cell is then compared with one based on an all-inorganic magnesium aluminum chloride complex (MACC) electrolyte. Mg/O2 cells using the MACC electrolyte exhibit higher discharge capacities than those based on the modified Grignard electrolyte. However, rechargeability is limited. The discharge product is found to be an inhomogeneous mixture of MgCl2 and Mg(ClO4)2. Electrochemical impedance spectroscopy measurements are used to identify that film formation on the Mg anode is main source of impedance in Mg/O2 cells. This research shows that the performance of Mg-based batteries is highly sensitive to the electrolyte. For future work, it would be useful to develop electrolytes that limit SEI film formation on the Mg anode and could result in a MgO2 discharge product to enhance rechargeability.PhDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/120792/1/gvardar_1.pd

A robust and active hybrid catalyst for facile oxygen reduction in solid oxide fuel cells

The sluggish oxygen reduction reaction (ORR) greatly reduces the energy efficiency of solid oxide fuel cells (SOFCs). Here we report our findings in dramatically enhancing the ORR kinetics and durability of the state-of-the-art La[subscript 0.6]Sr[subscript 0.4]Co[subscript 0.2]Fe[subscript 0.8]O[subscript 3](LSCF) cathode using a hybrid catalyst coating composed of a conformal PrNi[subscript 0.5]Mn[subscript 0.5]O[subscript 3](PNM) thin film with exsoluted PrOxnanoparticles. At 750°C, the hybrid catalyst-coated LSCF cathode shows a polarization resistance of ∼0.022 Ω cm[superscript 2], about 1/6 of that for a bare LSCF cathode (∼0.134 Ω cm[superscript 2]). Further, anode-supported cells with the hybrid catalyst-coated LSCF cathode demonstrate remarkable peak power densities (∼1.21 W cm[superscript -2]) while maintaining excellent durability (0.7 V for ∼500 h). Near Ambient X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-Ray Absorption Fine Structure (NEXAFS) analyses, together with density functional theory (DFT) calculations, indicate that the oxygen-vacancy-rich surfaces of the PrOxnanoparticles greatly accelerate the rate of electron transfer in the ORR whereas the thin PNM film facilitates rapid oxide-ion transport while drastically enhancing the surface stability of the LSCF electrode

Long-Run and Short-Run Dynamics among the Sectoral Stock Indices: Evidence from Turkey

Abstract This paper investigates the short-run and long-run dynamics among the major sectoral stock indices of the Istanbul Stock Exchange over the period 1997-2011. Long-run relationship among these indices is analyzed by using both conventional Engle and Granger (1987) an

The impact of interest rate and exchange rate volatility on banks' stock returns and volatility: Evidence from Turkey

This paper investigates the effects of interest rate and foreign exchange rate changes on Turkish banks' stock returns using the OLS and GARCH estimation models. The results suggest that interest rate and exchange rate changes have a negative and significant impact on the conditional bank stock return. Also, bank stock return sensitivities are found to be stronger for market return than interest rates and exchange rates, implying that market return plays an important role in determining the dynamics of conditional return of bank stocks. The results further indicate that interest rate and exchange rate volatility are the major determinants of the conditional bank stock return volatility. (C) 2011 Elsevier B.V. All rights reserved

Consolidation and commercial bank net interest margins: Evidence from the old and new European Union members and candidate countries

This paper examines the effects of financial reforms on the determinants of commercial bank net interest margin in the banking systems of the new EU member countries and candidate countries by dividing the sample period (1995-2006) into two sub-periods: consolidation period (1995-2000) and post-consolidation period (2001-2006). The paper also compares the new and old EU members to check whether differences with respect to the determinants of net interest margins between these two groups of countries exist within the same time period. The results indicate that size and managerial efficiency are negatively and significantly related to net interest margins in the two sub-periods. Regulators should promote merger and acquisition and market entry in order to increase the scale and efficiency of banks operating in the sector. Exploitation of the scale economies seems to be important in decreasing the interest rate spread in the sampled banking sectors. The results further indicate that all macroeconomic variables are statistically insignificant in the second sub-period, suggesting that differences in macroeconomic fundamentals have decreased among the sampled countries due to the increased convergence process in recent years. As for the comparison of the new and old EU members, the results suggest that the financial and economic convergence between the new and old members has not been completed. Macroeconomic differences within the group and between the groups still exist. (C) 2010 Elsevier B.V. All rights reserved

Consolidation and commercial bank net interest margins: Evidence from the old and new European Union members and candidate countries

This paper examines the effects of financial reforms on the determinants of commercial bank net interest margin in the banking systems of the new EU member countries and candidate countries by dividing the sample period (1995-2006) into two sub-periods: consolidation period (1995-2000) and post-consolidation period (2001-2006). The paper also compares the new and old EU members to check whether differences with respect to the determinants of net interest margins between these two groups of countries exist within the same time period. The results indicate that size and managerial efficiency are negatively and significantly related to net interest margins in the two sub-periods. Regulators should promote merger and acquisition and market entry in order to increase the scale and efficiency of banks operating in the sector. Exploitation of the scale economies seems to be important in decreasing the interest rate spread in the sampled banking sectors. The results further indicate that all macroeconomic variables are statistically insignificant in the second sub-period, suggesting that differences in macroeconomic fundamentals have decreased among the sampled countries due to the increased convergence process in recent years. As for the comparison of the new and old EU members, the results suggest that the financial and economic convergence between the new and old members has not been completed. Macroeconomic differences within the group and between the groups still exist.Consolidation Net interest margin EU banking Transition economies

Threshold catalytic onset of carbon formation on CeO 2 during CO 2 electrolysis: mechanism and inhibition

Carbon deposition from CO and other carbon-containing fuels is a major cause of the performance degradation of catalysts and electrocatalysts in many energy conversion devices, including low-temperature solid oxide cells (LT-SOCs). In this work, we present direct observation of carbon deposition on thin-film CeO2 electrodes at LT-SOC operating temperatures (450 °C) in a CO/CO2 atmosphere by in operando X-ray photoelectron spectroscopy. In contrast to the general view that CeO2 is a carbon tolerant material, significant carbon formation was observed on CeO2 during CO2 electrolysis, with no other catalyst present. Moreover, carbon deposition on CeO2 demonstrated an intriguing threshold onset formation against surface Ce3+ concentration. With the aid of Monte Carlo simulations, we propose the neighboring Ce3+-Ce3+ pairs to be a critical catalytic structure that facilitates carbon deposition from CO. Finally, we propose mitigation of carbon deposition on CeO2 by doping CeO2 with non-redox-active cations, and proved this concept using 50% Gd- and 50% Zr-doped CeO2 as an example system. These findings provide an in-depth understanding of the mechanism of carbon deposition on CeO2 during electrochemical reactions and can guide the design of carbon-resistant CeO2-based electrocatalysts. ©201

Electrochemistry of Magnesium Electrolytes in Ionic Liquids for Secondary Batteries

The electrochemistry of Mg salts in room-temperature ionic liquids (ILs) was studied using plating/stripping voltammetry to assess the viability of IL solvents for applications in secondary Mg batteries. Borohydride (BH₄^–), trifluoromethanesulfonate (TfO^–), and bis­(trifluoromethanesulfonyl)­imide (Tf₂N^–) salts of Mg were investigated. Three ILs were considered: l-<i>n</i>-butyl-3-methylimidazolium (BMIM)-Tf₂N, <i>N</i>-methyl-<i>N</i>-propylpiperidinium (PP13)-Tf₂N, and <i>N</i>,<i>N</i>-diethyl-<i>N</i>-methyl­(2-methoxyethyl)­ammonium (DEME⁺) tetrafluoroborate (BF₄^–). Salts and ILs were combined to produce binary solutions in which the anions were structurally similar or identical, if possible. Contrary to some prior reports, no salt/IL combination appeared to facilitate reversible Mg plating. In solutions containing BMIM⁺, oxidative activity near 0.8 V vs Mg/Mg²⁺ is likely associated with the BMIM cation, rather than Mg stripping. The absence of voltammetric signatures of Mg plating from ILs with Tf₂N^– and BF₄^– suggests that strong Mg/anion Coulombic attraction inhibits electrodeposition. Cosolvent additions to Mg­(Tf₂N)₂/PP13-Tf₂N were explored but did not result in enhanced plating/stripping activity. The results highlight the need for IL solvents or cosolvent systems that promote Mg²⁺ dissociation