Reversible Electrode Reaction Research Articles

Discrepant roles of adsorbed OH* species on IrWOx for boosting alkaline hydrogen electrocatalysis

Improving the slow kinetics of alkaline hydrogen electrode reactions, involving hydrogen oxidation and evolution reactions (HOR/HER) is highly desirable for accelerating the commercialization of alkaline exchange membrane-based fuel cells (AEMFCs) and water electrolyzers (AEMWEs). However, fundamental understanding of the mechanism for HOR/HER catalysis under alkaline media is still debatable. Here we develop an amorphous tungsten oxide clusters modified iridium-tungsten nanocrystallines (IrWOx) which exhibited by far the highest exchange current density and mass activity, about three times higher than the commercial Pt/C toward alkaline HOR/HER. Density functional theory (DFT) calculations reveal the WOx clusters act as a pivotal role to boost reversible hydrogen electrode reactions in alkaline condition but via different mechanisms, which are, hydrogen binding energy (HBE) mechanism for HOR and bi-functional mechanism for HER. This work is expected to promote our fundamental understanding about the alkaline HOR/HER catalysis and provide a new avenue for rational design of highly efficient electrocatalysts toward HOR/HER under alkaline electrolytes.

Ionic Liquid Electrolytes Enabling the Development of Highly Efficient High-Voltage Potassium-Ion Full Cells with a Potassium Manganese Hexacyanoferrate Cathode

Potassium-ion (K-ion) batteries are a promising complementary technology to lithium-ion in which less scarce and less expensive elements can be used; potassium instead of lithium, aluminum instead of copper, and cobalt-free cathodes. They have the advantage over sodium-ion batteries in that graphite, the standard commercial anode for lithium-ion, can be used[1] and potassium has a lower reduction potential (K+/K) than that of sodium and even lithium. Currently, the research focus is on developing energy-dense, high-voltage cathode materials. Prussian blue analogues, with their open-framework structure, are well-suited to reversibly insert the relatively large potassium ion.[2] Potassium manganese hexacyanoferrate (KMF) is of particular interest due to its high theoretical capacity (155 mAh g-1) and high voltage (around 4 V vs. K+/K) making it one of the most promising K-ion candidate cathode materials.[3] However, significant challenges remain before KMF K-ion batteries are suitable for commercialization. Current collector corrosion at high voltages and material instability lead to low coulombic efficiency on cycling and severe capacity fade.[4]In this work we report a significant improvement in KMF electrode performance by utilizing a highly stable ionic liquid electrolyte. This electrolyte suppresses corrosion of the current collector and deleterious reactions on the cathode. Performance of the KMF material is significantly controlled by its composition and particle size, which can be controlled during synthesis. We employ high-throughput techniques to systematically study a wide synthesis parameter space in order to minimize hexacyanoferrate vacancy and water content whilst controlling for particle size. We found that changing particle size considerably impacted specific capacity and cyclability, and a compromise must be made. Our optimized KMF material had a specific capacity of 119 mA h g-1, a coulombic efficiency of 99.3%, and relatively good cyclability. Crucially, graphite also displayed excellent electrochemical performance in the same ionic liquid electrolyte (maintaining 99% of initial capacity after 400 cycles and a coulombic efficiency of more than 99.9%) and exhibited a highly reversible electrode reaction studied by in operando XRD. Optimized KMF | ionic liquid electrolyte | graphite full-cell chemistry reported here proves a viable strategy for highly efficient high-voltage potassium-ion batteries.[1] Komaba et al., Potassium intercalation into graphite to realize high-voltage/high-power potassium-ion batteries and potassium-ion capacitors, Electrochemistry Communications (2015), 60, 172-175[2] Hurlbutt et al., Prussian Blue Analogs as Battery Materials, Joule (2018), 2, 10, 1950-1960[3] Bie et al., A novel K-ion battery: hexacyanoferrate(II)/graphite cell, J. Mater. Chem. A (2017), 5, 9, 4325-4330[4] Hosaka et al. Highly concentrated electrolyte solutions for 4 V class potassium-ion batteries, Chem. Commun. (2018), 54, 8387-8390

Voltammetry at microelectrodes of reversible electrode reactions with complex stoichiometry: A general analytical theoretical framework

The complex theoretical problem of higher-order reversible electron transfers at electrodes and microelectrodes of different geometries is addressed by analytical mathematical methods. A novel and general approach is established, valid for uniformly and non-uniformly accessible electrodes, and it is particularized for the most common geometries (micro-discs, hemispheres, bands and cylinders) and stoichiometries (2:1, 1:2, 3:1 and 1:3). General and explicit solutions are derived for the surface concentrations, the half-wave potential and the current-potential response in any voltammetric technique under transient and stationary conditions, whatever the initial concentrations of the oxidized and reduced species.With the theoretical results obtained, the response in cyclic voltammetry (CV) is analyzed in detail for a variety of a:b stoichiometries of interest. The influence of the main working parameters (microelectrode shape and size, scan rate and bulk concentration of redox species) is investigated, pointing out similarities and differences between the stoichiometries considered and with respect to the 1:1 case. In relation to the similarities, in all cases the surface concentrations are only potential-dependent and the thickness of the linear diffusion layer takes the same value regardless of the stoichiometric coefficients. Also, the effects of the electrode size and the scan rate (v) are analogous for any a:b values, including the appearance of isopoint(s) between voltammograms of different scan rates, the potential and current of which have a remarkable diagnostic value. Regarding differences between stoichiometries, the peak current, the peak-to-peak separation and the shape of transient CV curves, as well as the slope of the stationary current-potential response, depend on the a:b values. Thus, simple elucidation procedures are proposed based on the peak-to-peak separation, on the current and potential of the isopoint(s), as well as on an extension of the Tomeš criterion (∣E3/4 − E1/4∣) to CV under transient and stationary conditions.

Staircase voltammetry of dissolved redox couple in a thin layer twin electrode cell

A model of reversible and quasi-reversible electrode reaction of dissolved redox couple is developed for the staircase voltammetry and the twin electrode thin layer cell. It is assumed that the neutral molecule is oxidized to a monovalent cation. The calculations were performed for the absence of supporting electrolyte and for its various concentrations. The influence of migration of cations and of IRΩ drop on the peak currents and peak potentials was investigated. Also, the kinetically controlled electrode reactions were simulated. These reactions can be distinguished from the reactions influenced by the resistance in the solution.

Rechargeable High‐Capacity Aluminum‐Nickel Batteries

AbstractIn this paper, we designed a novel rechargeable Al−Ni battery. Nickel foil was firstly used as a cathode material for aluminum batteries and realized a reversible electrode reaction in a weak Lewis acidic electrolyte. Al−Ni battery can provide a high discharge capacity of more than 0.4 mA h cm−2 and exhibit excellent stability. Furthermore, the electrode reaction mechanism and proposed reasonable reaction equations of Al−Ni battery was studied deeply. We also explored the failure process of Al−Ni batteries and obtained the composition of side reaction products. This Al−Ni battery could offer a high reversible capacity of 0.66 mA h at a current density of 0.5 mA cm−2. Even at 1 mA cm−2, it still delivers a capacity of 0.415 mA h (0.27 mA h cm−2) after 100 cycles.

Cage-confinement pyrolysis route to size-controlled molybdenum-based oxygen electrode catalysts: From isolated atoms to clusters and nanoparticles

Metal-nitrogen-carbon materials with properly tailored metal particle sizes offer an ideal model system for electrocatalysis due to their readily tunable dimension, geometric and electronic effects at the catalytically active sites. Herein, a cage-confinement pyrolysis route is proposed to realize size-controllable synthesis of molybdenum-based catalysts ranging from isolated single Mo atoms to sub-nanometer clusters (<1 nm) and nanoparticles (2–5 nm). The effective control in the degree of dispersion facilitates a rational investigation into the electrocatalytic behavior of metal species from the angstrom to the nanometer scales. Notably, the Mo single atom catalysts show superior activity for bifunctional oxygen reduction and evolution reactions (ORR/OER) with a smaller potential gap of 0.65 V, which are compared favorably with the analogous sub-nanometer clusters and nanometer-level particles. X-ray fine structure analyses coupled with theoretical calculations demonstrate that the active Mo sites in the Mo1N1C2 local coordination environment have promoted the reversible oxygen electrode reactions. As a demonstration of their real application potentials, the zinc-air batteries assembled by using the highly active Mo single atom catalysts as the air cathode deliver a specific capacity of 750 mA h gZn−1 with an energy density of 673 W h kgZn−1, comparable to a mixed Pt/C-RuO2 benchmark.

Three-Dimensional Graphene-Supported Ni3Fe/Co9S8 Composites: Rational Design and Active for Oxygen Reversible Electrocatalysis.

The development of low-cost and efficient electrocatalysts with a bicomponent active surface for reversible oxygen electrode reactions is highly desirable and challenging. Herein, we develop an effective calcination-hydrothermal approach to fabricate graphene aerogel-anchored Ni3Fe-Co9S8 bifunctional electrocatalyst (Ni3Fe-Co9S8/rGO). The mutually beneficial Ni3Fe-Co9S8 bifunctional active components efficiently balance the performance of oxygen reduction and oxygen evolution reactions (ORR/OER), in which Co9S8 promotes the ORR and Ni3Fe facilitates the OER. This balance behavior has an obvious advantage over that of monocomponent Ni3Fe/rGO and Co9S8/rGO catalysts. Meanwhile, the additional synergy between porous rGO aerogels and Ni3Fe-Co9S8 endows the composite with more exposed active sites, faster electrons/ions transport rate, and better structural stability. Benefiting from the reasonable material selection and structural design, the Ni3Fe-Co9S8/rGO exhibits not only outstanding ORR activity with the high onset- and half-wave potentials ( Eonset = 0.91 V and E1/2 = 0.80 V) but also satisfactory OER activity with a low overpotential at 10 mA cm-2 (0.39 V). Moreover, rechargeable Zn-air cells equipped with Ni3Fe-Co9S8/rGO exhibit excellent rechargeability and a fast dynamic response.

Theory of staircase cyclic voltammetry of two electrode reactions coupled by a chemical reaction

Two reversible electrode reactions that are connected by either reversible or totally irreversible chemical reactions are theoretically analysed by staircase cyclic voltammetry. The dependence of peak potentials on the thermodynamic and kinetic parameters is calculated. If the mechanism is permanently in equilibrium, the stability constant of the reversible chemical reaction can be determined. Furthermore, the critical kinetic parameter is determined and its application to the measurement of the forward rate constant of the chemical reaction is demonstrated. Also, the influence of the kinetics of electrode reactions is discussed. Keywords: ECE mechanism; Cyclic voltammetry; Theory

Exploring Indium‐Based Ternary Thiospinel as Conceivable High‐Potential Air‐Cathode for Rechargeable Zn–Air Batteries

AbstractReversible oxygen reactions in Zn–air batteries require cost‐effective and highly‐active bifunctional electrocatalysts to substitute traditional noble‐metal based catalysts. Herein, a new and promising electrocatalytic material, ternary CoIn2S4 thiospinel, is demonstrated for effectively catalyzing oxygen reduction and oxygen evolution reactions (ORR and OER) with S‐doped reduced graphene oxide (S‐rGO) as an electronic conductor. Compared with Co9S8/S‐rGO (without In doping), the newly developed CoIn2S4/S‐rGO reveals superior electrocatalytic properties for the ORR (half‐wave potential of 0.83 V) and OER (overpotential of 0.37 V at 10 mA cm−2), demonstrating that the introduction of In can promote the reversible oxygen electrode reactions of CoIn2S4. The superior experimentally‐observed electrocatalytic properties are corroborated via density function theory investigations. Meanwhile, the synergistic improvements in the bifunctional activities resulting from the combination of CoIn2S4 and S‐rGO are also confirmed. As a proof of concept, home‐made Zn–air cells are assembled with CoIn2S4/S‐rGO as an air‐cathode. The developed Zn–air cells exhibit a high peak power density (133 mW cm−2) with an energy density of 951 Wh kgZn−1 and robust cycling stability over 150 cycles for 50 h, exceeding of those commercial Pt/C+RuO2 which highlights the practical viability of CoIn2S4/S‐rGO for rechargeable Zn–air batteries.

Characterization of Lithium Vanadate Film Electrode Formed on Garnet-Type Oxide Solid Electrolyte by Aerosol Deposition

Aerosol deposition (AD) method has many advantages compared to the conventional film deposition process such as sputtering deposition and pulsed laser deposition [1]. This method uses room temperature impact consolidation (RTIC) between raw ceramic powders and substrate during aerosolized powders crash onto the substrate. The film formed by AD method has dense structure made of nanocrystalline particles and its structural and physical properties is similar to base powder material. This feature is attractive for the fabrication of oxide-based solid state batteries, because an electrode layer can be formed on solid electrolyte without any thermal treatment [2, 3]. In this study, we fabricated lithium vanadate LiV3O8 (LVO) film electrode by AD on both SUS316L plate and garnet-type Ta doped Li7La3Zr2O12 (LLZT) solid electrolyte as substrates. Ball-milled LVO powder with the size of 0.5-2 µm was used as raw material for LVO film fabrication. The powders were aerosolized with N2 carrier gas at flow rate of 20L/min and splayed through the nozzle onto SUS316L plate or LLZT pellet fixed on X-Y stage in vacuumed deposition chamber to form LVO film. Crystal phase and microstructure of LVO film was investigated by XRD, FE-SEM and EDX. In addition, two-electrode coin cell was constituted using LVO film on SUS316L plate as working electrode, Li metal foil as counter one and 1 mol/L LiPF6-EC:DMC(1:1v/v%) electrolyte to measure the electrochemical properties of film electrode at 25ºC. On the other hand, for LVO film on LLZT pellet, Li metal foil was attached on the other end face of LLZT pellet to consist all-solid-state cell. Electrochemical properties for LVO/LLZT/Li cell were measured at 50 and 100ºC. From XRD measurement results, diffraction peaks for LVO were clearly confirmed in films formed by AD and other impurity phases were not observed, while the diffraction peaks become broader than raw powders. From SEM observation, LVO films have highly dense structure composed of deformed and fractured LVO particles via impact consolidation, and their thicknesses were in the range of 2.5-10 μm depending on the deposition times. In the cell with liquid organic electrolyte, LVO film with the thickness of 2.5 μm showed initial discharge capacity around 300 mAh/g, which is close to the theoretical capacity of LVO with V5+/V4+ redox couple. Capacity retention of LVO film electrode in liquid electrolyte is approximately 80% after 30 cycles. LVO film with the thickness of 5 µm formed on LLZT pellet has also dense structure and pores were not observed at LVO/LLZT interface. From AC impedance measurment, charge transfer resistance at LVO/LLZT interface is estimated to be around 103 Ωcm2 at room temperature range, which is much higher than at Li/LLZT interface (= 50 Ωcm2). Reversible charge and discharge reaction in LVO/LLZT/Li all-solid-state cell was demonstrated successfully and the specific capacites were 100 and 290 mAh g-1 at 50 and 100ºC, respectively. Good reversibility of electrode reaction indicates strong adhesion between LVO film electrode formed via impact consolidation and LLZT. This work was partly supported by JSPS KAKENHI Grant numbers 16K06218 and 16KK0127, and Research Foundation for the Electro-technology of Chubu (R-28241). Reference s : [1] J. Akedo, Journal of the American Ceramic Society 89, 1834-1839, 2006. [2] T. Kato, S. Iwasaki, Y. Ishii, M. Motoyama, W.C. West, Y. Yamamoto, Y. Iriyama, Journal of Power Sources 303, 65-72, 2016. [3] R. Inada, S. Yasuda, M. Tojo, K. Tsuritani, T. Tojo, Y. Sakurai, Frontiers in Energy Research 4, 28, 2016. Figure 1

A comparative study on LiFePO4/C by in-situ coating with different carbon sources for high-performance lithium batteries

LiFePO4/C materials are synthesized via in-situ coating of LiFePO4 by using 1-butyl-3-methylimidazolium dicyanamide ([BMIm][N(CN)2]) and glucose as carbon sources, respectively. The structure characterization results indicate that the carbon sources have no effect on the LiFePO4 particles, but have remarkable influence on the coated carbon films. It is found that the [BMIm][N(CN)2] carbon source leads to ultrathin (1–2 nm), uniform and highly graphitized carbon films. Due to this unique structure, the electrode materials fabricated by using [BMIm][N(CN)2] exhibit a superior electrode reaction reversibility, a capacity retention of 160.6 mAhg−1 (1.47% decay rate) after 50 cycles and a specific discharge capacity of 143.6 mAhg−1 at 1C, which is much better than the performance of the electrode materials synthesized with glucose.

A thermally regenerative ammonia battery with carbon-silver electrodes for converting low-grade waste heat to electricity

Thermally regenerative ammonia batteries (TRABs) have shown great promise as a method to convert low-grade waste heat into electrical power, with power densities an order of magnitude higher than other approaches. However, previous TRABs based on copper electrodes suffered from unbalanced anode dissolution and cathode deposition rates during discharging cycles, limiting practical applications. To produce a TRAB with stable and reversible electrode reactions over many cycles, inert carbon electrodes were used with silver salts. In continuous flow tests, power production was stable over 100 discharging cycles, demonstrating excellent reversibility. Power densities were 23 W m−2-electrode area in batch tests, which was 64% higher than that produced in parallel tests using copper electrodes, and 30 W m−2 (net energy density of 490 Wh m−3-anolyte) in continuous flow tests. While this battery requires the use a precious metal, an initial economic analysis of the system showed that the cost of the materials relative to energy production was $220 per MWh, which is competitive with energy production from other non-fossil fuel sources. A substantial reduction in costs could be obtained by developing less expensive anion exchange membranes.

Simulation of square wave voltammetry of three electrode reactions coupled by two reversible chemical reactions

&lt;p style="margin: 0cm 0cm 8pt; line-height: 150%;"&gt;&lt;span lang="EN-GB"&gt;Three fast and reversible electrode reactions that are connected by two reversible chemical reactions that are permanently in the equilibrium are analysed theoretically for square wave voltammetry. The dependence of peak potentials on the dimensionless equilibrium constants of chemical reactions is calculated. The influence of the basic thermodynamic parameters on the square wave voltammetric responses is analysed.&lt;/span&gt;&lt;/p&gt;

Nitrogen doped graphene anchored cobalt oxides efficiently bi-functionally catalyze both oxygen reduction reaction and oxygen revolution reaction

A series of efficient bifunctional electrocatalyst composed of nitrogen-doped graphene-cobalt oxide nanoparticles nano-hybrids (Co-N/G) are fabricated by one-pot hydrothermal synthesis, which involves in-situ growth of cobalt oxides nanocrystals and nitrogen doping into graphene. 2, 4, 6-Triaminopyrimidine used as nitrogen precursor could anchor cobalt oxides nanoparticles on graphene oxide as active centers for oxygen electrode reactions. The TEM, Raman, XRD and XPS confirm the germination of nitrogen doped graphene and nano-sized cobalt oxides. The optimized Co-N/G catalyst consists of the highest contents of pyridinic nitrogen and CoO, which efficiently catalyze both ORR and OER. The strong coupling between cobalt oxides and N-graphene endows Co-N/G 600 with a low reversible overvoltage of 0.96 V between ORR and OER in alkaline medium, which is superior to novel catalysts such as Pt/C (1.24 V), IrO2 (1.43 V) and RuO2 (1.35 V), rendering it bi-catalyst efficient and economical for reversible oxygen electrode reactions.

Investigation of electrochemical behaviour of cefuroxime axetil using hanging mercury drop electrode and graphene oxide modified glassy carbon electrode

The electrochemical reduction and oxidation behaviour of Cefuroxime Axetil was investigated by square wave voltammetry in different supporting electrolytes. Cefuroxime Axetil has both reducible and oxidacible groups in its structure. The optimum conditions were obtained in 0.1M phosphate borate buffer pH7.0 for electroreduction at hanging mercury drop electrode. A well-defined peak was observed at −1.06V vs. Ag/AgCl/4.6M KCl. The optimum conditions were obtained in Britton-Robinson buffer pH2.0 for electrooxidation at modified glassy carbon electrode. A well-defined peak was observed at 1.30V vs. Ag/AgCl/4.6M KCl. Current type, reversibility of electrode reactions and the number of electrons transferred was investigated by using cyclic voltammetry, chronoamperometry and chronocoulometry. Besides calculation of diffusion coefficients and the rate constants of electron transfer, the reduction and oxidation mechanisms were also proposed.

Simulation of differential pulse polarography of irreversible EC mechanism

A reversible electrode reaction that is followed by totally irreversible chemical reaction is investigated theoretically. The influence of chemical reaction on two components of the net response of differential pulse polarogram is analyzed. It is demonstrated that this EC mechanism depends on two kinetic parameters and that their critical values can be used for the measurement of the rate constant of chemical reaction.

Influence of MgO on the Electrochemical Performance of Nickel Electrode in Alkaline Aqueous Electrolyte

β-nickel hydroxide (β-Ni(OH)2) was prepared using precipitation method. Magnesium oxide (MgO) was synthesized by solu-tion combustion method using magnesium nitrate as oxidizer and urea as a fuel. The effects of MgO additive on the structure and electro-chemical performance of β-Ni(OH)2 electrode are examined. The structure and property of the MgO added β-Ni(OH)2 were characterized by X-ray diffraction (XRD), thermal gravimetric-differential thermal analysis (TG-DTA), Scanning electron microscopy (SEM), and Energy Dispersive X-ray (EDX) analysis. The results of the TG-DTA studies indicate that the MgO added β-Ni(OH)2 contains adsorbed water mol-ecules and anions. Partial substitution of MgO for graphite to β-nickel hydroxide is found to exhibit improvement in the electrochemical activity. Anodic peak potential (Epa) and cathodic peak potential (Epc) values are found to decrease remarkably after the incorporation of MgO into the β-Ni(OH)2 electrode. Further, addition of MgO is found to enhance the reversibility of the electrode reaction. Compared with β-Ni(OH)2 electrode, MgO substituted β-Ni(OH)2 electrode is found to exhibit higher proton diffusion coefficient. These findings suggest that the MgO substituted β-Ni(OH)2 electrode possess improved electrochemical properties such as enhanced reversibility of electrode reaction and higher proton diffusion coefficient and thus can be recognized as a promising candidate for the battery electrode applications.

Preparation and electrochemical properties of Pb-0.3wt%Ag/Pb-WC composite inert anodes

We prepared Pb-0.3wt%Ag/Pb-WC (WC stands for tungsten carbide, the same below) composite inert anodes by double-pulse electrodeposition on the surface of Pb-0.3wt%Ag substrates, and investigated the electrochemical properties of the composite inert anodes, which were obtained under different forward pulse average current densities from 2 A/dm2 to 5 A/dm2 and WC concentrations from 0 g/L to 40 g/L in bath. The kinetic parameters of oxygen evolution, corrosion potential and corrosion current of the composite inert anodes were obtained in a synthetic zinc electrowinning electrolyte of 50 g/L Zn2+ and 150 g/L H2SO4 at 35 °C, by measuring the anodic polarization curves, Tafel polarization curves and cyclic voltammetry curves. The results show that Pb-0.3wt%Ag/Pb-WC composite inert anodes obtained under forward pulse average current density of 3 A/dm2 and WC concentration of 30 g/L in an original acid plating bath, possess higher electrocatalytic activity of oxygen evolution, lower overpotential of oxygen evolution, better reversibility of electrode reaction and corrosion resistance in [ZnSO4+H2SO4] solution. The overpotential of oxygen evolution of the composite inert anode is 0.926 V under 500 A/m2 in [ZnSO4+H2SO4] solution, and 245 mV lower than that of Pb-1%Ag alloy; the corrosion current of the composite inert anode is 0.95×10-4 A which is distinctly lower than that of Pb-1%Ag alloy, showing the excellent corrosion resistance.

The impedance spectroscopy response for a reversible electrode reaction based on diffusion limited chronoamperometry at an inlaid disk electrode

First, a simple relationship is derived between the diffusion-controlled current transient resulting from application of a large potential step (PSCA method) and the diffusion impedance (EIS method) calculated at the equilibrium potential of a uniformly accessible electrode when a one-step redox reaction takes place at the electrode surface. The derivation is valid under one-dimensional (1D) diffusion conditions in the electrolyte, assuming in addition the same diffusion coefficient for both redox species involved in the electrochemical reaction. The theoretical relationship also applies to 2D diffusion from/towards an inlaid (micro)disk electrode provided the electron transfer reaction displays reversible kinetics at the electrode surface. Accurate values for the diffusion impedance at the equilibrium potential of (micro)disk electrode are obtained starting from the explicit formulation of the diffusion-controlled current transient in the article by L.K. Bieniasz, Electrochim. Acta 199 (2016) 1–11. These values can be used as benchmark data for checking the accuracy of numerical methods, e.g. finite element methods, employed for solving the initial boundary value problem under consideration. Finally, approximation formulae are derived for the diffusion impedance either in closed form or using a representation model together with a complex nonlinear least squares procedure.

Investigation of Structural Changes in Layered NaxNi0.5Mn0.5O2 Cathode Materials (0.5≤ x ≤1.2) and Their Electrochemical Performance

Compared to lithium, sodium is a highly abundant and cheap element. Although it has obvious weakness on gravimetric energy density due to the higher weight (M(Li)= 6.941 g/mol vs. M(Na)= 22.989 g/mol) (Hollemann et al.) and redox potential (the standard redox potential of sodium is 0.3 V above that of lithium), this cost benefit still makes it attractive to employ sodium ion batteries for large scale applications (Ellis und Nazar 2012). Such an application is for instance stationary energy storage for fluctuating renewable energy sources (e.g. solar energy or wind energy) (Barpanda et al. 2014) (Palomares et al. 2012). To keep the costs low, it is also necessary not to employ expensive and toxic transition metals, such as cobalt. Layered transition metal oxides, having the formula AMO2 (A- alkali metal, M- transition metals, O-oxygen), have already gained a lot of success in lithium-ion technology. There material are commonly crystallize in a O3-type layered structure, such as LiCoO2 and LiNiO2, which means that Li+ ions are located between sheets of transition metal oxides and are octahedrally coordinated by oxygen anions. Sodium based materials, which structure in the similar order, may also be the proper cathode candidate for sodium ion batteries. Several layered sodium oxides with different compositions have been synthesized. A promising one is the compound Na1.0Ni0.5Mn0.5O2, offering a high theoretical capacity of around 240 mAh/g when being desodiated completely. Practically, it can deliver a reversible capacity of around 185 mAh/g in the voltage range of 2.2V – 4.5V against Na/Na+ (Komaba et al. 2012). Starting from the initial O3 layered structure, many phase transitions are reported to take place during desodiation in the charge process and sodiation process, leading to a complex voltage profile. However, the final structure of the compound strongly depends on the sodium content as well as on the transition metal ratio. Beside O3 layered type structures, P2 and P3 layered type structures are reported as well for varied molar ratios between Na, Mn and Ni. In this work, NaxNi0.5Mn0.5O2 cathode material was synthesized with varying sodium content, ranging from 0.5≤ x ≤1.2. Half cells were cycled to examine the influence of sodium content on structural changes and electrochemical performance. The electrodes and active materials were examined by x-ray diffraction and scanning electron microscopy. Since best electrochemical performance as well as highest phase purity was found for a sodium content of x=0.9, this material was chosen for further investigations. In detail, in situ x-ray diffraction was conducted to observe structural changes during charging / discharging, the influence of different cut off potentials on cycling stability were tested and corresponding electrode surfaces were examined via scanning electron microscopy and x-ray photoelectron spectroscopy (XPS). Literature Barpanda, Prabeer; Oyama, Gosuke; Nishimura, Shin-ichi; Chung, Sai-Cheong; Yamada, Atsuo (2014): A 3.8-V earth-abundant sodium battery electrode. In: Nat Comms 5. DOI: 10.1038/ncomms5358. Ellis, Brian L.; Nazar, Linda F. (2012): Sodium and sodium-ion energy storage batteries. In: Current Opinion in Solid State and Materials Science 16 (4), S. 168–177. DOI: 10.1016/j.cossms.2012.04.002. Hollemann, A. F.; Wiberg, E.; Wiberg, N.: Lehrbuch der anorganischen Chemie. In: de Gruyter, zuletzt geprüft am 11.11.2014. Komaba, Shinichi; Yabuuchi, Naoaki; Nakayama, Tetsuri; Ogata, Atsushi; Ishikawa, Toru; Nakai, Izumi (2012): Study on the Reversible Electrode Reaction of Na 1– x Ni 0.5 Mn 0.5 O 2 for a Rechargeable Sodium-Ion Battery. In: Inorg. Chem. 51 (11), S. 6211–6220. DOI: 10.1021/ic300357d. Palomares, Verónica; Serras, Paula; Villaluenga, Irune; Hueso, Karina B.; Carretero-González, Javier; Rojo, Teófilo (2012): Na-ion batteries, recent advances and present challenges to become low cost energy storage systems. In: Energy Environ. Sci. 5 (3), S. 5884. DOI: 10.1039/c2ee02781j.