The future of energy storage devices relies on energy storage systems with both high energy and power densities. Supercapacitors, a hybrid of capacitors and batteries, have the potential to deliver both high energy and power densities. Materials suitable for supercapacitors are those capable of storing charges via faradaic redox reactions or pseudocapacitive reactions in addition to electrostatic double-layer storage. High-surface-area early transition metal nitrides such as vanadium nitride (VN) have promising properties for use in energy storage devices, especially supercapacitors, due to their pseudocapacitive charge storage mechanisms. The nature of their pseudocapacitive function, however, remains poorly understood. Further development of these materials requires a detailed understanding of their pseudocapacitive charge storage mechanisms. This paper reports a detailed and comprehensive analysis of the pseudocapacitive charge-storage mechanisms of the VN material in aqueous alkaline electrolytes using in-situ small angle neutron scattering (SANS) and in-situ x-ray absorption spectroscopy (XAS) combined with physical and electrochemical characterization techniques. Contrary to conventional wisdom that large pores are more accessible to electrolyte ions and thus store more charge, our results showed that pseudocapacitive charge storage mechanism for VN in aqueous alkaline media is induced by the insertion/extraction of anions (i.e. OH- OD-) in and out of micropores. The anion insertion/extraction leads to the reduction/oxidation of the V metal, as the material was electrochemically charged and dis-charged within its operating potential window. These results suggest that high pseudocapacitances in excess of 1300 Fg(-1) could be achieved by the VN material in 1.2 V aqueous alkaline electrolytes. This work undoubtedly provides a clear and generic conceptual and fundamental characterization approach for energy storage devices and related systems.

Mg/O2 cells employing a MgCl2/AlCl3/DME (MACC/DME) electrolyte are cycled and compared to cells with modified Grignard electrolytes, showing that performance of magnesium/oxygen batteries depends strongly on electrolyte composition. Discharge capacity is far greater for MACC/DME-based cells, while rechargeability in these systems is severely limited. The Mg/O2-MACC/DME discharge product comprises a mixture of Mg(ClO4)2 and MgCl2, with the latter likely formed from slow decomposition of the former. The presence of Cl in these compounds suggests that the electrolyte participates in the cell reaction, or reacts readily with the initial electrochemical products. A rate study suggests that O2 diffusion in the electrolyte limits discharge capacities at higher currents. Formation of an insulating product film on the positive electrodes of Mg/O2-MACC/DME cells following deep discharge increases cell impedance substantially, and likely explains the poor rechargeability. An additional impedance rise consistent with film formation on the Mg negative electrode suggests the presence of detrimental O2 crossover. Minimizing O2 crossover and bypassing charge transfer through the discharge product would improve battery performance.

Mg/O-2 cells employing a MgCl2/AlCl3/DME (MACC/DME) electrolyte are cycled and compared to cells with modified Grignard electrolytes, showing that performance of magnesium/oxygen batteries depends strongly on electrolyte composition. Discharge capacity is far greater for MACC/DME-based cells, while rechargeability in these systems is severely limited. The Mg/O-2-MACC/DME discharge product comprises a mixture of Mg(ClO4)(2) and MgCl2, with the latter likely formed from slow decomposition of the former. The presence of Cl in these compounds suggests that the electrolyte participates in the cell reaction or reacts readily with the initial electrochemical products. A rate study suggests that O-2 diffusion in the electrolyte limits discharge capacities at higher currents. Formation of an insulating product film on the positive electrodes of Mg/O-2-MACC/DME cells following deep discharge increases cell impedance substantially and likely explains the poor rechargeability. An additional impedance rise consistent with film formation on the Mg negative electrode suggests the presence of detrimental O-2 crossover. Minimizing O-2 crossover and bypassing charge transfer through the discharge product would improve battery performance.

The development of a practical magnesium-anode battery requires electrolytes that allow for highly efficient magnesium exchange while also being compatible with cathode materials. Here, a one-dimensional continuum-scale model is developed to simulate cyclic plating/stripping voltammetry of a model magnesium-based electrolyte system employing magnesium borohydride/dimethoxyethane [Mg(BH4)2/DME] solutions on a gold substrate. The model is developed from non-electroneutral dilute-solution theory, using Nernst-Planck equations for the mass flux and Poisson's equation for the electrostatic potential. The electrochemical reaction is modeled with multistep Butler-Volmer kinetics, with a modified current/overpotential relationship that separately accounts for the portions of the current responsible for nucleating new deposits and propagating or dissolving existing ones. The diffusivities of the electrolyte species, standard heterogeneous rate constant, charge-transfer coefficient, formal potential, and nucleation overpotential are determined computationally by reproducing experimental voltammograms. The model is computationally inexpensive and therefore allows for broad parametric studies of electrolyte behavior that would otherwise be impractical.

Introduction Early transition-metal carbides and nitrides are potential electrode materials for supercapacitor applications due to their high accessible surface areas, high electric conductivities and low cost. They possess high capacitances, good capacitance retention during cycling and wide voltage windows. For example, the capacitance for VN has been reported to be as high as 1340 Fg-1 in aqueous electrolyte [1]. The origin of this high capacitance was attributed to a combination of electric double-layer formation and faradaic redox reactions occurring on the nitride or oxynitride (VNxOy) surface. However, the contribution of the each individual mechanism is ill-defined. Despite efforts to date, the nature of the pseudocapacitive properties of early transition-metal carbides and nitrides remains vague. Full exploitation of the properties of these materials will require an understanding of the pseudocapacitive charge-storage mechanism. Here we report a detailed investigation of the charge-storage mechanisms for early transition-metal carbides and nitrides in aqueous media. The pseudocapacitive charge-storage mechanism has been investigated using x-ray absorption spectroscopy and neutron scattering and a combination of electrochemical techniques including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS). Experimental High-surface-area Ti, V, Mo, and W carbides and nitrides were prepared from their oxide precursors TiO2 (Alfa Aesar), V2O5 (Alfa Aesar), (NH4)6Mo7O24.4H2O (81-83% as MoO3, Alfa Aesar) and WO3 (Alfa Aesar) respectively, by temperature-programmed reactions (TPR) [2]. Characterization of the structural properties was performed using nitrogen physisorption (BET surface area) and X-ray diffraction. The total capacitance and extent of pseudocapacitance was determined based on results from CV and EIS. For selected materials details regarding the adsorption of actives species and metal oxidation state changes during electrochemical cycling were determined using neutron scattering and x-ray absorption. Results and Discussion Table 1 lists the total capacitances and percent contribution of pseudocapacitance of each material in aqueous electrolytes. The pseudocapacitive charge storage was obtained by subtracting the double-layer capacitance from the total capacitance. The extent of pseudocapacitive charge storage contribution ranged from 61% for TiN to 88% for WC1-x in acid and base electrolytes, respectively. This result indicates that pseudocapacitance is the dominant charge-storage mechanism in carbides and nitrides. This is expected given that these materials are electroactive and can go through several oxidation state changes during electrochemical cycling. Figure 1 shows the oxidation state changes for Mo2N in acidic electrolyte during electrochemical cycling. There was removal of one electron per Mo as the potential was increased. This result suggests reduction of the metal during electrochemical cycling. Concomitant with changes in oxidation state, neutron scattering indicated that hydrogen was inserted into the material. The amount of hydrogen exceeded the amount attributable to adsorption on the surface. These and other results will be discussed during the presentation. References (1) Kumta, P.N. et al., Adv. Mater., 2006, 18, 1178. (2) Djire. A, et al., J. Power Sources, 2015,275, 159-166. Figure 1

Early transition metal nitrides are promising materials for use in electrochemical capacitors and as electrocatalysts due to their high electronic conductivities, high surface areas and electrochemical stabilities. In addition to double-layer (electrostatic) charge storage, these materials store charge via a pseudocapacitive mechanism involving charge-transfer reactions. This paper describes electrochemical and in-situ X-ray absorption spectroscopic results that provide important insights regarding the pseudocapacitive charge storage mechanism for molybdenum nitrides in aqueous acids. From analysis of the rest potentials at varying pH values, we concluded that electrons and protons were coupled during charge transfer with a ratio of ∼2 electrons per proton. Cycling the Mo2N electrodes between −0.73 and −0.19 V, the stability window for this material, resulted in the removal of approximately one electron per Mo from a band that was primarily Mo in character. The extended X-ray absorption fine structure spectra indicated minimal changes in the immediate local structure around Mo as the potential was varied. The results are consistent with reversible hydrogen insertion into and extraction from interstitial sites in the Mo2N lattice. Charge storage capacitances in excess of 1500 Fg−1 are predicted for materials with full occupation of these interstitial sites.

The effect of surface oxygen on the physical and electrochemical properties of high surface area Mo, V and W carbides and nitrides has been investigated. These materials hold promise for use in supercapacitors and other electrochemical conversion devices. The carbides and nitrides were synthesized using a temperature programmed reaction method and passivated to avoid bulk oxidation on exposure to air. The passivated materials were activated in 0.3 mol dm(-3) NaOH solution to remove the oxygen-rich passivation layer. This activation resulted in significant changes in the electrochemical stabilities and capacitances. The surface areas for the Mo and W-based materials were higher after activation, with the effect ranging from an 11% increase for Mo2N to a 208% increase for W2C. An increase in pore volume and mesopore density was also observed for most of the materials. Interestingly, the VC and W2C, which were electrochemically unstable in acidic electrolyte in their passivated form, were stable after activation. The capacitances of all of the materials were increased after activation with the effect ranging from 48% for Mo2N to a 79% increase for (alpha + beta)-Mo2C. This activation process could be used to improve the performance of carbide and nitride-based supercapacitor electrode materials. (C) 2014 Elsevier B.V. All rights reserved.

Non-aqueous Li/O 2 batteries could achieve much higher specific energies than lithium-ion (3-5x) 1 , making battery powered electric vehicles more competitive with traditional combustion vehicles. 2 Realizing this promise however requires significant improvement in cycle life and cycling efficiency. Understanding the charge mechanism is crucial to lowering the charging overpotential, which in turn would increase efficiency. Observing the process of deposition and dissolution of electrodeposited Li 2 O 2 in Li/O 2 cells can shed light on the charging mechanism. During charge, the amount of energy necessary to decompose Li 2 O 2 likely depends on the morphology of the Li 2 O 2 particles deposited 3-5 : higher surface energy morphologies should require less energy. The morphology formed on discharge depends heavily on cell potential with higher energy particles formed at higher overpotentials. 4,5 Observing how Li 2 O 2 is consumed during charge could further explain the relationship between cell overpotentials and Li 2 O 2 morphology, as well as probe any differences in the discharge and charge mechanisms. In this talk we will show how the morphology of electrochemically deposited Li 2 O 2 changes at intermediate states of charge in a Li/O 2 cell. Cells are discharged and charged at constant rate of 0.5 mAcm –2 as described in a previous report. 5 After discharge is complete, various intermediate states of charge are sampled by stopping the charge at cell potentials of 3, 3.375, 3.75, 4.125, and 4.5 V. Partially cycled electrodes are harvested from the cell and imaged with scanning electron microscopy. Figure 1 shows a typical discharge charge curve, with an image of a control electrode and the morphologies observed for a fully discharged cell and a cell charged to 4.5 V (coincidentally corresponding to about half of the discharge current). From this it is clear that Li 2 O 2 is not consumed uniformly throughout the electrode during charge. The implications of the non-uniform consumption of Li 2 O 2 for electron and O 2 transport in the electrode during charge will be discussed. Figure 1

By combining a multivalent cation with the high capacity of a gas-breathing positive electrode, the energy density of a non-aqueous Mg/O2 battery is projected to surpass that of other ‘beyond-Li-ion’ chemistries. Nevertheless, research into Mg/O2 systems has been limited. A rechargeable, non-aqueous Mg/O2 cell that operates at room temperature will be discussed and used to clarify fundamental electrochemical characteristics, including the nature of the product phase(s) and the reaction mechanism for discharge. The discharge product differs from alkali-metal-based chemistries in that it is a mixed phase, primarily comprising crystalline MgO, with a substantial minority of MgO2. The open-circuit cell voltage is 2.0 V, lower than the theoretically expected ~2.9 V. Both the low voltage and the two-phase discharge product are consistent with a multi-step discharge reaction in which a superoxide (O2 –) intermediate forms at ~2 V vs. Mg/Mg2+. Chemical disproportionation reactions subsequently yield MgO and MgO2, but do not contribute to the cell’s electrical energy output. During the charging step, MgO2 is preferentially decomposed. Bypassing this multi-step mechanism in favor of direct electrochemical MgO x formation would allow for higher discharge potential and consequently higher energy density.

Non-aqueous Li/O2 batteries could achieve much higher specific energies than lithium-ion (3-5x)1, making battery powered electric vehicles more competitive with traditional combustion vehicles.2 Realizing this promise however requires significant improvement in cycle life and cycling efficiency. Understanding the charge mechanism is crucial to lowering the charging overpotential, which in turn would increase efficiency. Observing the process of deposition and dissolution of electrodeposited Li2O2 in Li/O2 cells can shed light on the charging mechanism. During charge, the amount of energy necessary to decompose Li2O2 likely depends on the morphology of the Li2O2 particles deposited3-5: higher surface energy morphologies should require less energy. The morphology formed on discharge depends heavily on cell potential with higher energy particles formed at higher overpotentials.4,5 Observing how Li2O2 is consumed during charge could further explain the relationship between cell overpotentials and Li2O2 morphology, as well as probe any differences in the discharge and charge mechanisms. In this talk we will show how the morphology of electrochemically deposited Li2O2 changes at intermediate states of charge in a Li/O2 cell. Cells are discharged and charged at constant rate of 0.5 mAcm–2 as described in a previous report.5 After discharge is complete, various intermediate states of charge are sampled by stopping the charge at cell potentials of 3, 3.375, 3.75, 4.125, and 4.5 V. Partially cycled electrodes are harvested from the cell and imaged with scanning electron microscopy. Figure 1 shows a typical discharge charge curve, with an image of a control electrode and the morphologies observed for a fully discharged cell and a cell charged to 4.5 V (coincidentally corresponding to about half of the discharge current). From this it is clear that Li2O2 is not consumed uniformly throughout the electrode during charge. The implications of the non-uniform consumption of Li2O2 for electron and O2 transport in the electrode during charge will be discussed. Figure 1

Metal/O2 batteries have the potential to achieve extremely high energy densities, and metal/O2 batteries based on magnesium are predicted to have one of the highest.1 Shiga et al. have demonstrated rechargeable, non-aqueous Mg/O2 cells that operate above room temperature.2 , 3 The need for elevated temperatures can be negated by using a Mg2+ electrolyte that can deposit and dissolve Mg at room temperature. We have produced Mg/O2 test cells using a modified-Grignard-reagent electrolyte, which has oxidative stability higher than 4V vs. Mg/Mg2+ and is viable at room temperature.4 We will discuss our progress in exploring discharge and recharge characteristics of non-aqueous Mg/O2 cells. Although Mg/O2 batteries have been demostrated using non-aqueous electrolytes in the past, the nature of the discharge product is not well understood. Figure 1 shows the discharge product on a porous-carbon positive electrode after first discharge. We have elucidated the Mg/O2 discharge-product composition using several characterization techniques, including scanning electron microscopy, energy dispersive spectroscopy, Auger electron spectroscopy, X-ray diffraction and Raman spectroscopy. The discharge product predominantly comprises MgO. In addition to MgO, there is evidence of a substantial minority of MgO2, as well as trace Cl. The energy efficiency for the first cycle is 42%, lower than other metal/O2 chemistries,5 , 6 but comparable to prior elevated-temperature Mg/O2 cells.2 The open-circuit potential (OCP) prior to first discharge of the non-aqueous Mg/O2 cell is 2.0V, which is lower than would be expected for the direct electrochemical formation of MgO or MgO2 (2.95V and 2.91V vs. Mg/Mg2+, respectively). This observation is consistent with discharge-product formation via a superoxide-ion intermediate, which forms from molecular O2 at 2.03 V vs. Mg/Mg2+. Superoxide then drives an O2-releasing precipitation of MgO2, which subsequently disproportionates to O2 and MgO – chemical reactions that do not affect the cell voltage, and rationalize the observed discharge-product composition. (1) Zu, C.-X.; Li, H. Energy Environ. Sci. 2011, 4, 2614. (2) Shiga, T.; Hase, Y.; Kato, Y.; Inoue, M.; Takechi, K. Chem. Commun. (Camb) 2013, 49, 9152. (3) Shiga, T.; Hase, Y.; Yagi, Y.; Takahashi, N.; Takechi, K. J. Phys. Chem. Lett. 2014, 5, 1648. (4) Nelson, E. G.; Brody, S. I.; Kampf, J. F.; Bartlett, B. M. J. Mater. Chem. A 2014. (5) Hartmann, P.; Bender, C. L.; Vračar, M.; Dürr, A. K.; Garsuch, A.; Janek, J.; Adelhelm, P. Nat. Mater. 2013, 12, 228. (6) Ren, X.; Wu, Y. J. Am. Chem. Soc. 2013, 135, 2923. Figure 1

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTIdentifying the Discharge Product and Reaction Pathway for a Secondary Mg/O2 BatteryGulin Vardar†, Emily G. Nelson‡, Jeffrey G. Smith§, Junichi Naruse∥, Hidehiko Hiramatsu⊥, Bart M. Bartlett‡, Alice E. S. Sleightholme#, Donald J. Siegel*†§∇, and Charles W. Monroe*#⊗View Author Information# †Materials Science and Engineering Department, ‡Chemistry Department, §Mechanical Engineering Department, and #Chemical Engineering Department, University of Michigan, Ann Arbor, Michigan 48109, United States∥ North America Research & Development, DENSO International America, Inc., 24777 Denso Drive, Southfield, Michigan 48086, United States⊥ Research Laboratories, DENSO CORPORATION, 500-1, Minamiyama, Komenoki-cho, Nisshin 470-0111, Japan∇ Department of Energy Conversion and Storage, Technical University of Denmark, Fysikvej, Building 309, 2800 Kgs Lyngby, Denmark⊗ Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, United Kingdom*D.J.S. E-mail: [email protected]*C.W.M. E-mail: [email protected]Cite this: Chem. Mater. 2015, 27, 22, 7564–7568Publication Date (Web):November 6, 2015Publication History Received14 September 2015Revised6 November 2015Published online16 November 2015Published inissue 24 November 2015https://pubs.acs.org/doi/10.1021/acs.chemmater.5b03608https://doi.org/10.1021/acs.chemmater.5b03608rapid-communicationACS PublicationsCopyright © 2015 American Chemical SocietyRequest reuse permissionsArticle Views2771Altmetric-Citations56LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Electrochemical cells,Electrodes,Electrolytes,Magnesium oxide,Oxides Get e-Alerts

The discharge rate is critical to the performance of lithium/oxygen batteries: it impacts both cell capacity and discharge-phase morphology, and in so doing may also affect the efficiency of the oxygen-evolution reaction during recharging. First-discharge data from tens of Li/O2 cells discharged across four rates are analyzed statistically to inform these connections. In the practically significant superficial current-density range of 0.1 to 1 mA cm(-2), capacity is found to fall as a power law, with a Peukert's-law exponent of 1.6 ± 0.1. X-ray diffractometry confirms the dominant presence of crystalline Li2O2 in the discharged electrodes. A completely air-free sample-transfer technique was developed to implement scanning electron microscopy (SEM) of the discharge product. SEM imaging of electrodes with near-average capacities provides statistically significant measures of the shape and size variation of electrodeposited Li2O2 particles with respect to discharge current. At lower rates, typical "toroidal" particles are observed that are well approximated as cylindrical structures, whose average radii remain relatively constant as discharge rate increases, whereas their average heights decrease. At the highest rate studied, air-free SEM shows that particles take needle-like shapes rather than forming the nanosheets or compact films described elsewhere. Average particle volumes decrease with current while particle surface-to-volume ratios increase dramatically, supporting the notion that Li2O2 grows by a locally mass-transfer-limited nucleation and growth mechanism.

Introduction Early transition-metal carbides and nitrides are potential electrode materials for supercapacitor applications due to their high accessible surface areas, high electric conductivities and low cost. They possess high capacitances, good capacitance retention during cycling and wide voltage windows. For example, the capacitance for VN has been reported to be as high as 1340 Fg -1 in aqueous electrolyte [1]. The origin of this high capacitance was attributed to a combination of electric double-layer formation and faradaic redox reactions occurring on the nitride or oxynitride (VNxOy) surface. However, the contribution of the each individual mechanism is ill-defined. Despite efforts to date, the nature of the pseudocapacitive properties of early transition-metal carbides and nitrides remains vague. Full exploitation of the properties of these materials will require an understanding of the pseudocapacitive charge-storage mechanism. Here we report a detailed investigation of the charge-storage mechanisms for early transition-metal carbides and nitrides in aqueous media. The pseudocapacitive charge-storage mechanism has been investigated using x-ray absorption spectroscopy and neutron scattering and a combination of electrochemical techniques including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS). Experimental High-surface-area Ti, V, Mo, and W carbides and nitrides were prepared from their oxide precursors TiO 2 (Alfa Aesar), V 2 O 5 (Alfa Aesar), (NH 4 ) 6 Mo 7 O 24 .4H 2 O (81-83% as MoO 3 , Alfa Aesar) and WO 3 (Alfa Aesar) respectively, by temperature-programmed reactions (TPR) [2]. Characterization of the structural properties was performed using nitrogen physisorption (BET surface area) and X-ray diffraction. The total capacitance and extent of pseudocapacitance was determined based on results from CV and EIS. For selected materials details regarding the adsorption of actives species and metal oxidation state changes during electrochemical cycling were determined using neutron scattering and x-ray absorption. Results and Discussion Table 1 lists the total capacitances and percent contribution of pseudocapacitance of each material in aqueous electrolytes. The pseudocapacitive charge storage was obtained by subtracting the double-layer capacitance from the total capacitance. The extent of pseudocapacitive charge storage contribution ranged from 61% for TiN to 88% for WC 1-x in acid and base electrolytes, respectively. This result indicates that pseudocapacitance is the dominant charge-storage mechanism in carbides and nitrides. This is expected given that these materials are electroactive and can go through several oxidation state changes during electrochemical cycling. Figure 1 shows the oxidation state changes for Mo 2 N in acidic electrolyte during electrochemical cycling. There was removal of one electron per Mo as the potential was increased. This result suggests reduction of the metal during electrochemical cycling. Concomitant with changes in oxidation state, neutron scattering indicated that hydrogen was inserted into the material. The amount of hydrogen exceeded the amount attributable to adsorption on the surface. These and other results will be discussed during the presentation. References (1) Kumta, P.N. et al., Adv. Mater ., 2006 , 18, 1178. (2) Djire. A, et al., J. Power Sources, 2015 ,275, 159-166. Figure 1

Introduction Early transition-metal carbides and nitrides are promising candidates for use in supercapacitor electrodes due to their high electronic conductivities, high surface areas (can exceed 200 m2g-1), good electrochemical stabilities and high capacitance [1, 2]. For example, the capacitance for VN has been reported to be as high as 1340 Fg-1 in aqueous KOH [3]. This high capacitance has attributed to a combination of electric double-layer formation and faradaic reactions occurring on the nitride or oxynitride (VNxOy) surface [3]. Despite efforts to date, the nature of the faradaic redox reactions or pseudocapacitive properties of early transition-metal carbides and nitrides remains ill-defined. This presents a challenge to the full exploitation of these materials. Here we report a detailed investigation of the charge-storage mechanisms in early transition-metal carbides and nitrides in aqueous media. The contributions of both double-layer and pseudocapacitive mechanisms have been deconvoluted using a combination of electrochemical techniques including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and electrochemical quartz crystal microbalance (EQCM), x-ray absorption spectroscopy and neutron scattering. Experimental High-surface-area Ti, V, Nb, Mo, and W carbides and nitrides were prepared from their oxide precursors TiO2 (Alfa Aesar), V2O5 (Alfa Aesar), N2O5 (Alfa Aesar), (NH4)6Mo7O24.4H2O (81-83% as MoO3, Alfa Aesar) and WO3 (Alfa Aesar) respectively, by temperature-programmed reaction (TPR) synthesis with 15% CH4 / H2 (Cryogenic Gases) or NH3 (Cryogenic Gases), respectively, then passivated using a flowing mixture of 1% O2/He (Cryogenic Gases). Characterization of the structural properties was performed using nitrogen physisorption (BET surface area) and X-ray diffraction. The CV was used to establish the stability windows and capacitances for these materials. The capacitance was deconvoluted into double-layer capacitance and pseudocapacitance by means of CV and EIS. EQCM was used to characterize the nature of the adsorbed/desorbed species during charge/discharge. For selected materials details regarding key species and hydrogen adsorption were determined from the x-ray absorption and neutron scattering results. Results and Discussion Figure 1 shows the response of VN in the frequency range of 10 kHz to 10 mHz in acidic medium for selected potentials within the stable potential window. We observed plateaus at -0.7 and -0.44V bias potentials. This region is believed to be a signature of double-layer and surface adsorption, respectively. This behavior is expected given that both processes depend strongly on accessible surface area. As shown in Figure 2, the double-layer capacitance determined for VN in acidic medium was approximately 96 μFcm-2. The charge-storage mechanism was found to be a combination of surface redox reaction, adsorption and double-layer charging. Similar examinations have been applied to the other carbides and nitrides listed above in aqueous media and the results will be discussed. References (1) Cladridge, J. B.; York, A. P. E.; Brungs, A. J.; Green Malcolm L. H.; Chem. Mater. 2000, 12, 132. (2) Wixom, M.R.; Tarnowski, D. J.; Parker, J. M.; Lee , J.Q.; Chen, P. –L.; Song, I.; Thompson, L. T.; Mat. Res. Soc. Symp. Proc. 1998, 496, 643. (3) Choi, D.; Kumta, P. N.; Electrochem. Solid-State Lett. 2005, 8, 8, A418.

Properties of supporting electrolytes and solvents were examined for use with vanadium acetylacetonate – a member of the class of metal(β-diketonate) active species – in non-aqueous redox flow batteries. Twenty supporting-electrolyte/solvent combinations were screened for ionic conductivity and supporting-electrolyte solubility. Hexane, tetrahydrofuran, and dimethylcarbonate solvents did not meet minimal conductivity and solubility criteria for any of the electrolytes used, which included tetraethylammonium tetrafluoroborate, tetrabutylammonium tetrafluoroborate, tetrabutylammonium hexafluorophosphate, and (1-butyl, 3-methyl)imidazolium bis(trifluoromethanesulfonyl)imide. Ionic conductivities and solubilities for solutions of these electrolytes passed screening criteria in acetonitrile and dimethylformamide solvents, in which maximum supporting-electrolyte and active-species solubilities were determined. Active-species electrochemistry was found to be reversible in several solvent/support systems; for some systems the voltammetric signatures of unwanted side reactions were suppressed. Correlations between supporting-solution properties and performance metrics suggest that an optimal solvent for a vanadium acetylacetonate RFB should have a low solvent molar volume for active-species solubility, and a high Hansen polarity for conductivity.

Li-O2 batteries have the potential to achieve a much higher (3-5x)1 specific energy than current state of the art Li-ion batteries, which could allow battery-powered electric vehicles to compete with traditional gas-powered cars.2 The greatest barriers to Li-O2 technology are limited cycle life and low battery efficiency. A better understanding of the mechanism of discharge product formation and dissolution would help to overcome these barriers. Controlled observation of discharge-product (Li2O2) morphology may provide insight into reaction mechanisms. Several reports of the morphology of electrochemically formed Li2O2 show the size and shape of Li2O2 particles and relate these characteristics to battery performance3,4,5. Typically, the discharge products are discussed as either small particles4 or nanosheets5 on the order of 10 nm, or larger toroidal shapes on the order of 100 nm. The small particles have a lower overpotential during recharge than the larger particles. If Li2O2 is formed by an elementary electron exchange, then the formation of large particles suggests a discharge mechanism where electrons travel over hundreds of nanometers, which suggests a complex charge transfer mechanism since bulk Li2O2is an electrical insulator. In this talk we will report a study of the rate dependence of Li-O2 discharge-product morphology during first discharge. At each of four rates ten independent Li-O2 cells were discharged to provide a statistically significant assessment of discharge capacity. Figure 1 shows the variation of discharge capacity with discharge rate with the inset showing the discharge curves at 0.2 mA/cm2. Cathodes with near-average capacity were selected for further characterization. X-ray diffraction showed that Li2O2 was the primary crystalline discharge product at all rates. Scanning electron microscopy (SEM) showed that the Li2O2 deposits comprised roughly cylindrical particles at lower rates, and needle-like particles at the highest rate. The cylinders had constant radii at different rates, which were comparable to the lengths of the needles. The heights of cylindrical products appeared to rise with decreasing rate. Figure 2 shows the volume of the Li2O2 particles as a function of discharge rate. The inset SEM images show the cylinders at end-of-discharge within 1 µm x 1 µm areas. The characteristic shapes (and aspect ratios) of particles yielded by image processing of > 25 particles from each electrode are also shown schematically. A nucleation and growth mechanism for Li2O2will be proposed and the impacts of this mechanism on battery performance will be discussed. 1. Lu, Y. C., B. M. Gallant, et al. (2013). Energy & Environmental Science 6(3): 750-768. 2. Christensen, J., P. Albertus, et al. (2012). J. Electrochem. Soc. 159(2): R1-R30. 3. Adams, B. D., C. Radtke, et al. (2013). Energy & Environmental Science 6(6): 1772-1778. 4. Gallant, B. M., D. G. Kwabi, et al. (2013). Energy & Environmental Science 6(8): 2518-2528. 5. Xu, J. J., Z. L. Wang, et al. (2013). Nature Communications 4.

Li-O 2 batteries have the potential to achieve a much higher (3-5x) 1 specific energy than current state of the art Li-ion batteries, which could allow battery-powered electric vehicles to compete with traditional gas-powered cars. 2 The greatest barriers to Li-O 2 technology are limited cycle life and low battery efficiency. A better understanding of the mechanism of discharge product formation and dissolution would help to overcome these barriers. Controlled observation of discharge-product (Li 2 O 2 ) morphology may provide insight into reaction mechanisms. Several reports of the morphology of electrochemically formed Li 2 O 2 show the size and shape of Li 2 O 2 particles and relate these characteristics to battery performance 3,4,5 . Typically, the discharge products are discussed as either small particles 4 or nanosheets 5 on the order of 10 nm, or larger toroidal shapes on the order of 100 nm. The small particles have a lower overpotential during recharge than the larger particles. If Li 2 O 2 is formed by an elementary electron exchange, then the formation of large particles suggests a discharge mechanism where electrons travel over hundreds of nanometers, which suggests a complex charge transfer mechanism since bulk Li 2 O 2 is an electrical insulator. In this talk we will report a study of the rate dependence of Li-O 2 discharge-product morphology during first discharge. At each of four rates ten independent Li-O 2 cells were discharged to provide a statistically significant assessment of discharge capacity. Figure 1 shows the variation of discharge capacity with discharge rate with the inset showing the discharge curves at 0.2 mA/cm 2 . Cathodes with near-average capacity were selected for further characterization. X-ray diffraction showed that Li 2 O 2 was the primary crystalline discharge product at all rates. Scanning electron microscopy (SEM) showed that the Li 2 O 2 deposits comprised roughly cylindrical particles at lower rates, and needle-like particles at the highest rate. The cylinders had constant radii at different rates, which were comparable to the lengths of the needles. The heights of cylindrical products appeared to rise with decreasing rate. Figure 2 shows the volume of the Li 2 O 2 particles as a function of discharge rate. The inset SEM images show the cylinders at end-of-discharge within 1 µm x 1 µm areas. The characteristic shapes (and aspect ratios) of particles yielded by image processing of > 25 particles from each electrode are also shown schematically. A nucleation and growth mechanism for Li 2 O 2 will be proposed and the impacts of this mechanism on battery performance will be discussed. 1. Lu, Y. C., B. M. Gallant, et al. (2013). Energy & Environmental Science 6 (3): 750-768. 2. Christensen, J., P. Albertus, et al. (2012). J. Electrochem. Soc. 159 (2): R1-R30. 3. Adams, B. D., C. Radtke, et al. (2013). Energy & Environmental Science 6 (6): 1772-1778. 4. Gallant, B. M., D. G. Kwabi, et al. (2013). Energy & Environmental Science 6 (8): 2518-2528. 5. Xu, J. J., Z. L. Wang, et al. (2013). Nature Communications 4 .

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-n-butyl-3-methylimidazolium (BMIM)-Tf2N, N-methyl-N-propylpiperidinium (PP13)-Tf2N, and N,N-diethyl-N-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/Mg(2+) 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 Mg(2+) dissociation.