Surface Electrolyte Interface Research Articles

Effect of charge-discharge with higher capacitance performance of Ni-substituted CoFe2O4 magnetic nanoparticles for energy storage

In this work, we synthesized NixCo1-xFe2O4 (0 ≤ x ≤ 0.8) magnetic nanoparticles (MNPs) by using ultrasonication-assisted reverse chemical co-precipitation method. The X-ray diffraction (XRD) patterns confirm the single-phase with mixed spinel structure of the NixCo1-xFe2O4 (0 ≤ x ≤ 0.8) MNPs with the crystallite size range between 12 nm - 17 nm. The molecular dynamics and oleic acid coated Ni-substituted CoFe2O4 magnetic nanoparticles were confirmed by FTIR measurements. The surface composition of NixCo1-xFe2O4 (0 ≤ x ≤ 0.8) MNPs (Ni, Co, Fe and O) and their oxidation states were estimated using X-ray photoelectron spectroscopy. The NixCo1-xFe2O4 (0 ≤ x ≤ 0.8) MNPs are spherical in shape and polycrystalline in nature, confirmed by FESEM and HRTEM measurements. The Ni-substituted CoFe2O4 MNPs with higher zeta potential (ζ) from −24 mV to -41 mV with an increase in Ni concentration, signifies the stable colloidal stability due to the electrostatic repulsion between MNPs. The DC magnetization (M-vs-H) measurements reveal the ferrimagnetic behavior of NixCo1-xFe2O4 (0 ≤ x ≤ 0.8) MNPs, which suppress the saturation magnetization (MS) from 48.6 emu/g to 5.3 emu/g with the enhanced coercivity (HC) from 335 Oe to 1700 Oe with the substitution of Ni2+ ions is in corroboration with electron paramagnetic resonance (EPR) measurements. The enhanced EPR resonance field (Hres = 2335 Oe - 3261 Oe) with the substitution of Ni2+ ions is attributed to the generation of oxygen vacancies and defects, which are predominant in Ni0.8Co0.2Fe2O4, resulting in significantly enhanced electrochemical performance. Therefore, the three-electrode system was utilized to investigate the electrochemical properties of NixCo1-xFe2O4 (0 ≤ x ≤ 0.8) MNPs. The Ni-substituted CoFe2O4 MNPs have demonstrated significantly enhanced capacity retention of 77 % even after 5000 cycles in 1 M H2SO4 electrolyte solution. NixCo1-xFe2O4 (0 ≤ x ≤ 0.8) MNPs exhibit an excellent specific capacity (Cs) of 794.5 F/g at a current density of 1 A/g, signifies the rate of redox reaction at surface electrolyte interface (SEI) is greatly influenced by the surface to volume ratio. Thus, with the enhancement of Ni2+ ions, redox reaction kinetics might become more efficient resulting in the enhanced charge storage capacity.

Post-mortem analysis of the Li-ion battery with charge/discharge deterioration in high- and low-temperature environments

Li-ion batteries have widespread applications. However, their deterioration mechanisms at different temperature conditions remain unclear. In this study, we investigate the effect of high- and low-temperature environments on the charge–discharge performance of an 18650 Li-ion battery having a Li(Ni,Co,Al)O2-family cathode and a graphite anode. After 50 cycles under three temperature conditions, the surfaces of the cathode and anode removed from the disassembled batteries are analyzed using scanning electron microscopy. The crystal and electronic structures of the cathode and anode materials are investigated using X-ray diffraction and X-ray absorption spectroscopy. The anode after 50 cycles at 5 °C is also investigated using X-ray photoemission spectroscopy. Results of the charge–discharge experiments indicate that, under identical charge–discharge conditions, the deterioration in the battery performance at a low-temperature environment of 5 °C is greater than that at environments of 25 and 80 °C. The post-mortem analyses of the cathode and anode samples of the battery cycled at 5 °C reveal the primary reasons for the decrease in battery capacity. The generation of deposits, including metallic Li, Li2CO3, and LiF, on the anode surface is clarified. During this process, the Li ions available for reinsertion into the cathode decrease. Furthermore, the Ni3+ component is partly reduced to Ni2+ near the surface of the cathode active material, and the reactive oxygen species are generated simultaneously. The reactive oxygen species can promote Li deficiency and surface-electrolyte interface formation. The above understanding of the deterioration mechanism can contribute to developing safer battery usage methods.

Determining the electrochemical transport parameters of sodium-ions in hard carbon composite electrodes

Sodium-ion batteries offer advantages over conventional Li-ion batteries, including cost and safety. However, much less is known about their operation and performance properties, particularly at the anode. The electron and ion transport in the active materials and composite electrode significantly impact battery performance. Understanding the changes in transport properties as a function of state-of-charge and state-of-health is essential for effective electrode design and performance assessment. In this work, the resistivity and diffusivity of sodium transport in hard carbon composite electrodes are studied at different states-of-health, using Galvanostatic Intermittent Titration Technique (GITT), Electrochemical Impedance Spectroscopy (EIS), and Electrochemical Potential Spectroscopy (EPS) in a stable 3-electrode test cell configuration. The reference electrode eliminated some voltage errors arising from the overpotentials on the counter electrode. The resistance contributions from the surface electrolyte interface, electrolyte transport in the electrode pores, and the charge transfer resistance are extrapolated from the impedance measurements and the diffusion coefficient from the GITT and EPS. The different techniques indicate similar trends in the diffusion coefficient during sodiation, desodiation, and ageing, although different orders of magnitude were observed between the EPS and GITT data. The accuracy of the parameters calculated using the different electrochemical techniques is discussed in detail.

Structure and reaction mechanism of binary Ni-Al oxides as materials for lithium-ion battery anodes.

A nanometer sized solid solution of NiO and Al2O3 was synthesized by calcination of Ni-Al layered double hydroxides (LDHs). The crystal structure of the obtained compound was determined by XRD and XAFS analyses: Ni2+ and Al3+ ions are located at the metal ion site of the rock salt structure and a certain amount of cation vacancies are also introduced for charge compensation. The electrochemical properties of the Ni-Al binary metal oxide as an anode material for lithium ion batteries were examined by the constant current charge-discharge test. Ni-Al oxide showed higher charging capacity in comparison with pristine NiO. In particular, the capacity in the lower voltage region (below 1.5 V), the limited capacity in this region is the weak point of the conversion anode, was improved to 540 mA h g-1 that is about twice that of pristine NiO. This improvement in the capacity in the lower voltage region is concluded to be due to the redox activity of Al3+ ions during the charge-discharge on the basis of the results of electrochemical measurements and ex situ XAFS measurements at the Ni and Al edge. The reaction mechanism of this compound is investigated using ex situ XRD and XAFS methods. For the charge (reduction) in the higher voltage region (OCV-1.0 V), lithium ion intercalation into the cation vacancy sites and/or lithium ion adsorption on the surface of particles are proceeding. For the charge in the lower voltage region (1.0-0.03 V), conversion reaction occurs by the reduction of Ni2+ and Al3+ ions to metal particles with surface electrolyte interface (SEI) layer formation. For the discharge in the lower voltage region (0.03-1.5 V), only Al metal particles are oxidized to Al3+ ions and some intermediate complexes are formed. For the discharge in the higher voltage region (1.5-3.0 V), the lattice of the Ni-Al binary oxide solid solution is reconstructed with the oxidation of Ni to Ni2+.

Interfaces and Electrolyte Additives for Long-Life Sodium-Ion and Lithium-Ion Batteries

Sodium-ion batteries (NIBs) are attracting considerable attention as post lithium-ion batteries (LIBs) as they are a likely ‘drop-in’ technology which can be produced using existing manufacturing lines. One of the processes in manufacturing which is least understood is the formation and conditioning step; both battery technologies require a formation of the surface electrolyte interface (SEI) and cathode electrolyte interface (CEI) for long life cells. Here we discuss the formation, conditioning and electrolyte additives for the creation of the stable interfaces for high cycle life.In particular we highlight the similarities of the formation step in the manufacturing process for a NIB layered oxide-hard carbon chemistry and LIB NMC-graphite, and the subsequent differences in the SEI observed1. Methods for faster formation protocols for NIB and LIB where targeted voltage windows in active formation methods produce more stable interfaces are introduced2,3 and we show the effect upon the composition of the SEI layer using these different protocols.In all cases full cells were made with in a coin-cell figuration, these cells were subjected to different formation protocols and electrolytes additives such as FEC, VC and a new inorganic solid state additive. After formation the impedance of the cells were measured, and then cycled for 500 cycles at a higher rate to accelerate the ageing process. The resistance and the capacity of the cells were monitored during cycling. We show that even in a coin cell configuration significant differences in the cycle life are observed with different formation protocols and electrolyte additives. In addition we show the difference in the stability of the SEI in carbonate-based electrolytes between NIBs and LIBs. The effect upon the stability of the SEI and the performance of a NIB with a sodium metal oxide cathode and a hard carbon anode is discussed in detail and the role of the formation protocols and the inorganic additive. We also suggest the role of the additives for NIB and suggest the mechanism for which the stability is enhanced. Roberts, S. & Kendrick, E. The re-emergence of sodium ion batteries: testing, processing, and manufacturability. Nanotechnol. Sci. Appl. Volume 11, 23–33 (2018).Pathan, T. S., Rashid, M., Walker, M., Widanage, W. D. & Kendrick, E. Active formation of Li-ion batteries and its effect on cycle life. J. Phys. Energy 1, 044003 (2019).Kendrick, E. Optimisation Of Formation And Cell Ageing Protocols For Lithium Ion EV Batteries. Batter. Supercaps (2020). doi:10.1002/batt.202000048

Electrolyte Effects on Single Crystal Cyclic Voltammograms

Insights into the governing mechanisms relevant for electrocatalytic energy conversion is of fundamental interest for future energy schemes aimed at combating fossil dependence and anthropogenic CO2 emissions.1 Electrocatalytic reactions occur at an electrode’s surface-electrolyte interface (see figure 1a). In this interface both geometric and electronic effects determine the selectivity and efficiency of the electrocatalytic reaction.2,3 Simultaneously, the structure and properties of the electrochemical interface are affected not only by the potential applied to the electrocatalyst, but also the pH of the solute4 as well as the cations/anions present in the electrolyte.3,5 Given recent advances on the generalized computational hydrogen electrode (GCHE)6 it is now possible to predict the most probable surface structure using ab initio molecular dynamics (AIMD) and density functional theory (DFT). These structures can be represented as 2D phase diagrams of the interface at any given potential. Herein, we present the basic methodology for creating such diagrams and how to correlate these with experimental cyclic voltammograms (CVs).7 CVs of Pt(111) under different electrolyte conditions (see figure 1b) have been investigated using a rotating disk electrode (RDE) setup and compared with DFT calculations. Our work addresses the impact cations, anions and pH have on the surface of Pt(111) electrodes and what one may extrapolate when contemplating real fuel cell devices.We herein show that combining experimental model studies with theoretical calculations through the GCHE framework opens up for a new and powerful tool for interpreting CVs of well-ordered metallic M(hkl) interfaces8 for a wide range of electrocatalytic reactions.References S. Chu, and A. Majumdar, Nature, 488, 294–303 (2012);K. D. Jensen, J. Tymoczko, J. Rossmeisl, A. S. Bandarenka, I. Chorkendorff, M. Escudero-Escribano, and I. E. L. Stephens, Angew. Chemie Int. Ed., 57, 2800–2805 (2018);D. Strmcnik, M. Escudero-Escribano, K. Kodama, V. R. Stamenkovic, A. Cuesta, and N. M. Marković, Nat. Chem., 2, 880–885 (2010);A. Bagger, L. Arnarson, M. H. Hansen, E. Spohr, and J. Rossmeisl, JACS, 141, 1506–1514 (2019);R. Rizo, E. Herrero, and J. M. Feliu, PCCP, 15, 15416 (2013);J. Rossmeisl, K. Chan, R. Ahmed, V. Tripkovic, and M. E. Björketun, PCCP, 15, 10321 (2013);J. Rossmeisl, K. D. Jensen, A. S. Petersen, L. Arnarson, A. Bagger, and M. Escudero-Escribano, Submitted-under review, (2020);A. Bagger, R. M. Arán‐Ais, J. H. Stenlid, E. Campos dos Santos, L. Arnarson, K. D. Jensen, M. Escudero‐Escribano, B. R. Cuanya, and J. Rossmeisl, ChemPhysChem, 20, 3096-3105 (2019); Figure 1

Electrochemical Study on Porous Silicon Anodes in Sulfide-Based All-Solid-State Lithium-Ion Batteries

All-solid-state lithium-ion battery (LIB) with high safety and reliability is the dominant power source for electric and hybrid vehicles. To meet the energy demands of near-future automobile technology, the anode materials should possess high energy density and long cycle life. Though Si with a high theoretical capacity (4200 mAh g-1) is the most attractive choice, there are few applications for the all-solid-state LIBs. This is because the large volumetric fluctuation (>300%) in lithiation/delithiation causes the drastic capacity loss and low coulombic efficiency. Very recently, we have found that sulfide-based all-solid-state LIBs with porous Si anodes exhibited high capacity retention [1,2]. In the present study, the electrochemical impedance was measured to provide further insight into the stable cyclability. Porous Si particles were prepared through air-oxidation demagnesiation of Mg2Si. The details have been described in our previous paper [1]. Commercially available non-porous Si particles were also used as a reference active material. Anode composite was comprised of Si particles, Li3PS4 solid electrolyte (SE), and acetylene black. We adopted two mixing methods, ball milling and mortar milling, to vary the dispersion degree of Si particles. In an electric insulation tube, the anode composite and SE powder were pressed to make two-layered pellet. As the counter electrode, Li-In foil was attached on the SE side. Finally, the three-layered pellet was compressed using stainless-steel disks. AC impedance of the half-cells thus prepared was measured in the range from 2×10-3 to 1×106 Hz. The ball-milled porous Si anode cells exhibited the capacity retention over 80% up to 150 cycles. In contrast, the lower retention, 40%, was observed for the mortar-milled porous Si anode cells. The ball-milled and mortar-milled non-porous Si anode cells possessed the poorer cycle stability with the capacity retention of 10% or less. As shown in the Cole-Cole plots, in the ball-milled porous Si anode cell, there was few changes in the arc size between 1st and 20th cycles. On the other hand, the arc of the mortar-milled porous Si anode cell became gradually large with the increase of cycle number. In addition, huge arcs were observed in the non-porous Si anode cells, regardless of the mixing method. The experimental results described above can be explained as follows. By utilizing the ball milling, Si particles are highly dispersed in the SE matrix. In this case, a lot of pores on the porous Si surface are exposed to the electrolyte. As the result, surface electrolyte interface (SEI) is easily formed at 1st cycle. However, SEI become hardly thick with the increase of charge/discharge cycle. This is because SE possesses high stability and low fluidity. In addition, the volumetric expansion is buffered by the shrinkage of Si pores. Therefore, the slight stress arising from the Si particles is relieved by the elastic deformation of SE. These findings strongly suggest that almost constant interfacial resistance causes the cycle stability of the ball-milled porous Si anode cells.

Improving the electrochemical cycling performance of anode materials via facile in situ surface deposition of a solid electrolyte layer

In this paper, we report a facile method for the in situ deposition of a thin solid electrolyte layer on the surface of anode materials, which can significantly improve the rate capability and reduce the time for reaching capacity maximum. The thin solid electrolyte layer (∼8 nm) is formed by adding a small amount of LiNO3 into the anode materials; the added LiNO3 decomposes irreversibly into Li3N and LiNxOy on the surface of the anode materials during the first cycle, facilitating fast diffusion of lithium ions and limiting the formation of the surface electrolyte interface films. This is verified by adding LiNO3 into graphite half-cells which shows a fast activation process at high current density: in particular, the charge capacity reaches its maximum after only ∼30 and ∼40 cycles at 170 and 340 mA g−1, respectively, compared to ∼50 and ∼80 cycles for the graphite half-cell. Furthermore, the LiNO3 additive results in high capacity retention at high C-rates, with a value of 82.4% at 680 mA g−1, compared with 22.4% for the graphite half-cell. The facile and low-cost method reported here is expected to be readily applicable to other electrode materials for high-performance lithium-ion batteries.

Operando Mapping of Li Concentration Profiles and Phase Transformations in Graphite Electrodes by Magnetic Resonance Imaging and Nuclear Magnetic Resonance Spectroscopy

An innovative combined magnetic resonance imaging and nuclear magnetic resonance methodology, which enables the visualization and spatially resolved quantification of the lithiation/delithiation process in porous Li-ion battery electrodes in real time, is reported. We demonstrate that polarization of the thick graphite electrode correlates with the appearance of energy barriers during the graphite phase transformations and the resulting significant reduction of the lithium chemical diffusion, which must be addressed in any attempt at the fast charging of Li-ion batteries. It is also shown that a portion of the transported Li+ is temporarily stored in the electrode surface film prior to its intercalation into graphite, indicating reversible lithium storage within the surface electrolyte interface. The inclusion of a short current reversal, which could be considered as part of a shaped charging process, facilitates the complete lithiation of thick graphite electrodes, an attractive strategy for increasing u...

Study on Li Metal Deposition, SEI Formation on Anodes and Cathode Potential Change during the Pre-Lithiation Process in a Cell Prepared with Laminated Porous Anodes and Cathodes

The pre-doping of Li+ ions (pre-lithiation) in graphite anodes, which is needed to improve the initial charging/discharging efficiency of lithium ion batteries, was examined from the viewpoints of lithium metal deposition, surface electrolyte interface (SEI) formation and electrode potential changes in the cathode during the pre-lithiation. Using a cell composed of pre-laminated, through-holed anodes and cathodes, the anodes were pre-lithiated with the perpendicular pre-doping method, as explained in this work, which can effectively enhance the pre-lithiation process in laminated cells. In the pre-lithiation system, Li deposition was not observed on the anode surfaces during pre-lithiation, and the thickness of the SEI layers formed on the anodes did not increase. Moreover, the SEI layer has the same composition as that formed by the electrochemical lithiation (charging) process, even when the pre-lithiation of the anodes is accelerated by through-holes formed on the anode and cathode electrodes. In addition, the electrode potential of cathodes inserted between pre-lithiated anodes does not change during the pre-lithiation process, and the capacity of the cathodes does not degrade upon pre-lithiation. The perpendicular pre-doping method examined in this study is found to be applicable to the production process of lithium ion batteries with high charging/discharging efficiency.

β-Sn Nanorods with Active (001) Tip Induced LiF-Rich SEI Layer for Stable Anode Material in Lithium Ion Battery

β-Sn nanorod (NR) with (200) facets and (001) tip is a potential anode material in lithium ion battery (LIB) due to its good cycle stability that can retain ∼600 and 550 mA h g–1 after 100 cycles using 0.2 C for high and low aspect ratios, respectively. The high stability compared to that of spherical nanoparticles can be attributed to the combination of the nanorod morphology that buffers large volumetric change, and the LiF-rich F-containing surface electrolyte interface (SEI) layer that allows for a stable SEI layer and a good ionic and electronic conductivity. The lower SEI resistivity and high Li+ diffusivity of low aspect ratio β-Sn NRs resulted in a better cyclability compared to high aspect ratio β-Sn NRs. These can be attributed to the higher specific surface area of highly reactive (001) surface, i.e., the tip of the rod, that produces a LiF-rich F-containing SEI layer of the lower aspect ratio compared to that of the higher aspect ratio β-Sn NRs.

The role of graphene as an overlayer on nanostructured hematite photoanodes for improved solar water oxidation

Photocatalysis is a clean, sustainable method of producing hydrogen fuel. Hematite (α-Fe2O3) is a promising photocatalytic material due to its favorable band gap, earth abundance and low cost. However, overcoming hematite's poor surface and electrical properties is crucial for reaching its theoretical potential. To this end, we show that single layer graphene transferred to the surface of nanostructured, titanium doped hematite results in a photocurrent density 1.6 times greater than bare hematite. Understanding the mechanism behind this enhancement is important for optimizing graphene coatings on photocatalysts. Electrical Impedance Spectroscopy reveals that graphene affects the surface-electrolyte interface without modifying the bulk of the hematite. Kinetic studies were performed using Intensity Modulated Photocurrent Spectroscopy, revealing that hematite with the graphene overlayer is able to transfer charge more efficiently due to a decrease in the surface recombination rate. Therefore, the main contribution of graphene on hematite is reduced surface recombination, resulting in a higher yield of charge carriers on the catalytic surface to participate in the oxygen evolution reaction. Single layer graphene overlayers have the potential to act as a general surface-modification technique to improve the photocurrent density of other metal oxide photocatalysts.

In Situ Porous Structure Characterization of Electrodes for Energy Storage and Conversion by EQCM-D: a Review

A great advantage of non-gravimetric vs. conventional gravimetric mode of quartz-crystal microbalance (QCM) has been demonstrated for quantification of the hydrodynamic interactions of rigid porous electrode coatings with contacting electrolyte solutions. The complex frequency change of these electrodes measured by multiharmonic EQCM-D (Electrochemical QCM with Dissipation Monitoring) contains implicit information about the initial porous electrode structure at open-circuit potential and its minute changes caused by Li-ions intercalation into the electrode host. Fitting suitable hydrodynamic models to the experimental complex frequency changes allows determination of the initial geometric parameters of the porous electrode in contact with solution as well as their potential-dependent changes. Validation of structural parameters derived by the hydrodynamic models for a large number of porous structure geometries using penetration depth as a unique mesoscopic independent variable should be done using complementary techniques with different resolution power. A meaningful application of EQCM-D in Li-ion batteries R&D is only possible under strict control over numerous contributions to the total complex frequency change from the processes accompanying intercalation of ions into the electrode (viscoelastic effects of binders, formation of surface-electrolyte interface, side reactions with gas evolution, etc.)

Modeling and Simulation of Thermal Runaway in Cylindrical 18650 Lithium-Ion Batteries

The thermal runaway of a single Li-ion battery (LIB) in larger battery package is the worst case scenario which must be avoided under all circumstances. In this work the electrochemical-thermal model for a LIB based on porous electrode theory is extended with contribution coming from exothermic side reactions to model the abuse mechanisms in Li-ion batteries with respect to the temperature which could lead to a thermal runaway. The electrochemical-thermal model is extended with several new contributions which occur in several temperature regimes. In detail the heat source in the corresponding heat equation is extended with the heat generated by various exothermic reactions, for example at the surface-electrolyte interface at temperatures in the interval T=[T1, T2], reactions between anode resp. cathode and the electrolyte between temperatures in the interval T=[T3, T4] and the destruction of the electrolyte above the temperature T>T5, where T1<T2 ≤T3<T4 ≤T5. These extensions will be modeled with a solid fuel or a constant fuel model as simplification coming from combustion theory. For this extended model simulations are performed for specified current profiles and exterior temperature profiles to simulate an oven trial and cell cycling. In the simulations a cylindrical 18650 LIB is investigated with a LiCoO2 cathode. The purpose of the simulations is to compute the temperature-field during the time evolution of a thermal runaway, therefore the cross section in horizontal and vertical direction are considered as well as the one-dimensional temperature profiles along these directions. Finally the time evolution of the overall mean cell temperature T is used to classify the different stages of a thermal runaway in a T-dT/dt-plot. The results will be compared with experimental ARC-measurements in pure oven experiments and cycling experiments.

1H,1H,5H-Perfluoropentyl-1,1,2,2-tetrafluoroethylether as a co-solvent for high voltage LiNi1/3Co1/3Mn1/3O2/graphite cells

1H,1H,5H-Perfluoropentyl-1,1,2,2-tetrafluoroethylether (F-EAE) mixed with ethylene carbonate (EC), diethyl carbonate (DEC), and lithium hexafluorophosphate (LiPF6) is evaluated as a co-solvent high-potential electrolyte of LiNi1/3Co1/3Mn1/3O2/graphite batteries. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) indicate that the EC/DEC-based electrolyte with F-EAE possesses a high oxidation potential (>5.2 V vs. Li/Li+) and excellent film-forming characteristics. With 40 wt% F-EAE in the electrolyte, the capacity retention of the LiNi1/3Co1/3Mn1/3O2/graphite pouch cells that are cycled between 3.0 and 4.5 V is significantly improved from 28.8% to 86.8% after 100 cycles. In addition, electrochemical impedance spectroscopy (EIS) of three-electrode pouch cells, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) are used to characterize the effects of F-EAE on the enhanced capacity retention. It is demonstrated that F-EAE facilitates the formation of a stable surface electrolyte interface (SEI) layer with low impedance on the anode and effectively suppresses an increase in the charge-transfer resistance on the cathode. These results suggest that F-EAE can serve as an alternative electrolyte solvent for 4.5 V high voltage rechargeable lithium-ion batteries.

Cathodic Corrosion of Cu Substrates as a Route to Nanostructured Cu/M (M=Ag, Au, Pd) Surfaces

AbstractThe electrochemical formation of nanostructured materials is generally achieved by reduction of a metal salt onto a substrate that does not influence the composition of the deposit. In this work, we report that Ag, Au and Pd electrodeposited onto Cu under conditions where galvanic replacement is not viable and hydrogen gas is evolved results in the formation of nanostructured surfaces that unexpectedly incorporate a high concentration of Cu in the final material. Under cathodic polarisation conditions, the electrodissolution/corrosion of Cu occurs, which provides a source of ionic copper that is reduced at the surface–electrolyte interface. The nanostructured Cu/M (M=Ag, Au and Pd) surfaces are investigated for their catalytic activity for the reduction of 4‐nitrophenol by NaBH4, where Cu/Ag was found to be extremely active. This work indicates that a substrate electrode can be utilised in an interesting manner to make bimetallic nanostructures with enhanced catalytic activity.

Large area graphene ion sensitive field effect transistors with tantalum pentoxide sensing layers for pH measurement at the Nernstian limit

We have fabricated and characterized large area graphene ion sensitive field effect transistors (ISFETs) with tantalum pentoxide sensing layers and demonstrated pH sensitivities approaching the Nernstian limit. Low temperature atomic layer deposition was used to deposit tantalum pentoxide atop large area graphene ISFETs. The charge neutrality point of graphene, inferred from quantum capacitance or channel conductance, was used to monitor surface potential in the presence of an electrolyte with varying pH. Bare graphene ISFETs exhibit negligible response, while graphene ISFETs with tantalum pentoxide sensing layers show increased sensitivity reaching up to 55 mV/pH over pH 3 through pH 8. Applying the Bergveld model, which accounts for site binding and a Guoy-Chapman-Stern picture of the surface-electrolyte interface, the increased pH sensitivity can be attributed to an increased buffer capacity reaching up to 1014 sites/cm2. ISFET response was found to be stable to better than 0.05 pH units over the course of two weeks.

Electrochemical Performances of Carbon Electrode Materials Depending on Electrode Density for Hybrid Capacitors

Supercapacitors are of practical interest owing to their high power electrical energy storage and exceptionally long cycle life. Also, it has been used and developed such as memory back up for electric home appliance and information transport devices, renewable energy and power assist for transportation and machinery, voltage sag and renewable energy sources for stable electric power. Significantly, electric double layer capacitor (EDLC) consist of stacked activated carbon shows superior high power characteristics whereas it has low energy density as a drawback. Therefore, hybrid capacitor is one of possible candidates and is developing for high energy density.The hybrid capacitor is made up with activated carbon electrode for electric double layer capacity and anode for intercalation and de-intercalation of lithium ions. Although it shows inevitable increase of energy density [1-3], the total output power is lower than that the EDLC due to chemical reaction on the anode. To increase the power density, the decrease of intercalation length for low diffusion resistance is necessary. The hybridization of activated materials with carbon nanotubes, carbon nanofiber, and graphene is intriguing for the application of highly conductive cathode electrode.Here, we introduce the preparation of carbon electrodes to enhance the output power on the anode without high-dimensional and high cost method. In general, high density electrode has high power characteristics because of low contact resistance. However, in this study, the decrease of electrode density can be increase dramatically the output power in the high current density. This result may give rise to the decrease of diffusion resistance due to the low electrode density and the stable surface electrolyte interface at the reacting surfaces.

Transmission Electron Forward Scattered Diffraction and Low Voltage SEM/STEM Characterization of Binder-Free TiO2 Electrodes

Graphite, the current commercial anode material in batteries suffers from the formation of a surface/electrolyte interface (SEI) layer that adversely affects performance. However, titania (TiO2) does not form an SEI layer and is both abundant and inexpensive, making it a prime anode material replacement [1]. While much work has been done to determine the effects of morphology and phase of TiO2 on lithium intercalation, the effect of the method fabrication of the electrode on its electrochemical performance is still unknown. Currently, electrodes are fabricated using active material, conductive additives, and a non-electroactive polymer binder mixed mechanically with a solvent. In this work, we introduced a chemo-mechanical fabrication method that produces binder-free electrodes. Without binder, more loading of active material is possible. Further, agglomeration of both TiO2 and conductive additives that occurs with the binder-based method is limited by using dispersants. Finally, the electrode is sintered, forming a 3-dimensional mesoporous structure that provided a 25% increase in capacity over the binder-based method, as well as better capacity retention over multiple cycles.

Studying the Reversibility of Multielectron Charge Transfer in Fe(VI) Cathodes Utilizing X-ray Absorption Spectroscopy

The superiron salts BaFeO4 and K2FeO4 when utilized as battery cathodes both undergo a three electron charge transfer; however, they exhibit significantly different physical and electrochemical properties. K2FeO4 exhibits higher solid-state stability and higher intrinsic 3e– capacity (406 mAh/g) than BaFeO4 (313 mAh/g); however, the rate of cathodic charge transfer is considerably higher for BaFeO4. To understand these differences, primary coin cells of alkaline batteries containing either μm-BaFeO4, μm-K2FeO4, or nm-K2FeO4 (nm = nanometer, or μm = micrometer size particles) were constructed and discharged to various depths under a constant load. Discharged cathode composite were studied by ex-situ X-ray absorption measurements. The oxidation state of discharge product of the Fe local symmetry was followed by the magnitude of K-edge and pre-edge Fe 1s to 3d peak. To track structural changes, the extended X-ray absorption fine structure (EXAFS) χ functions of the partially discharged cathodes were subject to linear combination fitting. The expanded BaFeO4 lattice, or the much larger surface-electrolyte interface in the nm-K2FeO4 materials, significantly increased their capacities compared to μm-K2FeO4. In the case of nm-K2FeO4, electron density is more distributed by water intercalation about the Fe hydrous environment, which relieves the “stress” of full Fe6+ to Fe3+ reduction. The stronger Ba–FeO4 anion–cation interaction and increased lattice size apparently slows the rate of lattice rearrangement into the discharge product.