Constant Current Measurements Research Articles

Quantitative Field Emission Imaging for Studying the Doping-Dependent Emission Behavior of Silicon Field Emitter Arrays.

Field emitter arrays (FEAs) are a promising component for novel vacuum micro- and nanoelectronic devices, such as microwave power amplifiers or fast-switching X-ray sources. However, the interrelated mechanisms responsible for FEA degradation and failure are not fully understood. Therefore, we present a measurement method for quantitative observation of individual emission sites during integral operation using a low-cost, commercially available CMOS imaging sensor. The emission and degradation behavior of three differently doped FEAs is investigated in current-regulated operation. The measurements reveal that the limited current of the p-doped emitters leads to an activation of up to 55% of the individual tips in the array, while the activation of the n-type FEA stopped at around 30%. This enhanced activation results in a more continuous and uniform current distribution for the p-type FEA. An analysis of the individual emitter characteristics before and after a constant current measurement provides novel perspectives on degradation behavior. A burn-in process that trims the emitting tips to an integral current-specific ideal field enhancement factor is observed. In this process, blunt tips are sharpened while sharp tips are dulled, resulting in homogenization within the FEA. The methodology is described in detail, making it easily adaptable for other groups to apply in the further development of promising FEAs.

Multi-Modal and Self-Reconstruction Conditioning Circuit of Semiconductor Gas Sensors

Semiconductor gas sensors (chemiresistors) have been widely used in many fields. The emerging novel gas sensors usually have large initial resistance and ultra-high sensitivity, and their resistance value may change significantly during the response process. It is difficult for conventional front-end conditioning circuits to meet the requirements of large measurement range and high accuracy. Meanwhile, the circuit needs to be compatible with several forms of signals generated from different types of sensors, such as voltage, current, and resistance. This study designs a front-end conditioning circuit that is compatible with various types of gas sensors and can adjust the measurement range in real time. The circuit has different functional states, which can realize four functions: constant voltage measurement, constant current measurement, electrochemical measurement, and parameter calibration. The modal transformation of the circuit is controlled by a multiplexer, which can perform automatic transformation according to software instructions. Our work provides a flexible circuit framework composed of different functional units, with the characteristics of multi-modal and self-reconstruction. It is suitable for designing integrated or fixed-function circuits and is convenient for miniaturization and cost reduction.

Different Efforts but Similar Insights in Battery R&D: Electrochemical Impedance Spectroscopy vs Galvanostatic (Constant Current) Technique

Electrochemical impedance spectroscopy (EIS) using alternating currents is a widely established technique to investigate kinetic aspects of batteries and their components, though it requires an interruption of battery operation with extra measurement time and effort. In this work, EIS is compared with the conventional galvanostatic (constant current) technique, which is based on direct currents, being the standard operation mode of batteries. Data from constant current measurements not only are representing application conditions but also are automatically and continuously generated during routine charge/discharge processes, i.e., without extra measurement efforts, and do give kinetic insights via the characteristic overvoltage (= resistance-reasoned voltage rise/decrease), as well. In fact, distinguishing between even very similar values for ohmic (RΩ), charge transfer (Rct), and mass transport (Rmt) resistances can be done via analysis of overvoltage data from constant current measurements, as exemplarily demonstrated in symmetric Li||Li and LiNi0.6Mn0.2Co0.2O2 (NMC622)||Li cells with poly(ethylene oxide)-based solid polymer electrolyte, finally proving their validity. From a practical point of view, direct-current methods can be beneficial for R&D of kinetic aspects in batteries, as data is directly obtained and, thus, application-oriented.

Effects of Electrode Support Structure on Electrode Microstructure, Transport Properties, and Gas Diffusion within the Gas Diffusion Layer.

The effects of gas diffusion layer (GDL) and electrode microstructure, which influence the catalyst layer and catalyst–membrane interface on the performance of a membrane electrode assembly (MEA) for gas-phase electrolysis and the separation of CO2 were experimentally characterized. Several types of GDL materials, with and without a microporous layer (MPL), were characterized using scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) surface area analysis. The diffusion of reactants through the GDL materials was measured to determine the effects on the microstructure and chemical properties on mass transport. The effects on the GDL structure and chemistry were determined through evaluation of Pt–IrO2 MEAs with different GDL materials using constant-current measurements. Increasing the thickness of the MPL and hydrophobicity within the GDL assist with retaining water within the membrane and catalyst layers, which results in greater performance at high current densities.

(Invited) Leak Current Analysis of Stop Operation and Its Modeling for the Development of Bipolar Alkaline Water Electrolyzer Electrodes

Introduction Water electrolysis is expected a key device to introduce large-scale renewable electricity under management of power grid and electrification of non-electric sector. While alkaline water electrolysis (AWE) systems are well-developed large system, degradation under fluctuated operation with start and stop operation is significant issue to combine photovoltaic and/or wind turbine generation is significant issue. In this study, we have been investigated reverse current, which is leak current through manifold of bipolar alkaline water electrolyzers, and electrode potential behavior of stop operation, and proposed accelerated durability test (ADT) protocol for start and stop operation. Experimental and modeling Figure 1 shows configuration of 4-cells bipolar alkaline water electrolyzer that was consisted with end plates of EP(-) and EP(+), bipolar plates of BP1 to BP3, anodes, separators, and cathodes with principle of reverse current. The end and bipolar plates were made of nickel. Anodes and cathodes were commercially available oxygen and hydrogen electrocatalyst coated nickel mesh electrodes (De Nora Permelec Ltd) with 27.8 cm2 of projected area. Zirfon Perl UTP500 (Agfa) was used for separators. Manifolds were made of Teflon tubes and 15 mL/min of 7 M (= mol/dm3) was circulated for each anolyte and catholyte chambers during measurements. An anode and a cathode set on a bipolar plate. During operation anodes and cathodes are oxidized and reduces, respectively. After stop, the anode and the cathode on a bipolar plate connects both electronically and ionically, so oxidized anode and reduced cathode surface discharges to same potential. In this study, we measured electrode potential and reverse current after 1 h water electrolysis of 80oC at 0.6 A/cm2. The reverse current was measured ionic current through communicating tube with D. C. clamp meter (KEW2510, Kyoritsu).Reverse current behavior was analyzed with COMSOL Multiphysics version 5.5 based 2-dimensional model of stack and height direction using experimentally anode and cathode potentials as functions of discharge for anode and cathodes. Results and discussion Figure 2 shows cell performances in the stacks and a picture of the lab-scale zero-gap configuration electrolyzer. All cells in a stack showed almost the same performance. The cell voltage was 2 V at 400 mA/cm2 at 30oC and was 1.8 V at 500 mA/cm2 at 80oC. Therefore, we think all cells and parts work well.Figure 3 shows reverse currents and electrode potentials as a function of time after 1 h electrolysis under 600 mA/cm2 at 30 or 80 oC. Here, the dashed lines in Fig. 3-a) were simulated value and were almost same after 20 s. The measured reverse current increased around 20 s. At this moment, outlet manifolds filled with electrolyte to increase ionic conduction among the chambers, which was not considered in the simulation. After degassed of manifolds, the reverse current decreased with time with the largest reverse current for the BP2. The reverse current at 80oC was significantly larger than that at 30oC. This difference could be explained the dependence of ionic resistance of manifold on temperature. At same moment, anode and cathode potentials on the end plates were almost constant. The anode cathode potentials on the bipolar plates decreased and increased with time, respectively. Both anode and cathode showed significantly potential change region. The final potentials of all electrodes were around 0.9 V vs. RHE. The potential change regions of the electrodes on the BP2 were earlier than others. Here, the anode and cathode potentials as functions of discharge were almost same for the electrode on all bipolar plates. Therefore, the average discharge functions for anode and cathode were treated as characteristics of electrodes of this study. Using this function, reverse current as a function of time could be expressed accurately using the developed model. From this model, reverse current, and electrode potentials as a function of time would be expected with discharge function of each electrode and ionic resistance of manifold.Figure 4 shows that a start & stop operation simulated ADT protocol based on bipolar electrolyzer measurements. We propose the combination of constant current electrolysis, potential sweep, and chronoamperometry as the inset of illustration in Fig. 4, because constant current measurement is easier to get reproductivity in high current that need accurate iR correction for constant potential measurement and current control measurement never simulate stop operation. Acknowledgements This study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in Advancement of Hydrogen Technologies and Utilization Project (P14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Figure 1

Improving Catalytic Activity in the Electrochemical Separation of CO2 Using Membrane Electrode Assemblies

The direct electrochemically driven separation of CO2 from a humidified N2, O2, and CO2 gas mixture was conducted using an asymmetric membrane electrode assembly (MEA). The MEA was fabricated using a screen-printed ionomer bound Pt cathode, an anion exchange membrane (AEM), and ionomer bound IrO2 anode. Electrocatalyst materials were physically and chemically characterized prior to inclusion within the electrode. Electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) measurements using a rotating disk electrode (RDE) were used to quantify the catalytic activity and determine the effects of the catalyst-to-ionomer ratio. Catalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) surface analysis, and (dynamic light scattering) DLS to evaluate catalyst structure, active surface area, and determine the particle size and bulk particle size distribution (PSD). The electrocatalyst layer of the electrodes were fabricated by screen printing a uniformly dispersed mixture of catalyst, dissolved anionic ionomer, and a solvent system onto an electrode supporting gas diffusion layer (GDL). Pt-IrO2 MEAs were fabricated and current-voltage relationships were determined using constant-current measurements over a range of applied current densities and flow rates. Baseline reaction kinetics for CO2 separation were established with a standard set of Pt-IrO2 MEAs.

Neuromorphic Dynamics at the Nanoscale in Silicon Suboxide RRAM

Resistive random-access memories, also known as memristors, whose resistance can be modulated by the electrically driven formation and disruption of conductive filaments within an insulator, are promising candidates for neuromorphic applications due to their scalability, low-power operation and diverse functional behaviors. However, understanding the dynamics of individual filaments, and the surrounding material, is challenging, owing to the typically very large cross-sectional areas of test devices relative to the nanometer scale of individual filaments. In the present work, conductive atomic force microscopy is used to study the evolution of conductivity at the nanoscale in a fully CMOS-compatible silicon suboxide thin film. Distinct filamentary plasticity and background conductivity enhancement are reported, suggesting that device behavior might be best described by composite core (filament) and shell (background conductivity) dynamics. Furthermore, constant current measurements demonstrate an interplay between filament formation and rupture, resulting in current-controlled voltage spiking in nanoscale regions, with an estimated optimal energy consumption of 25 attojoules per spike. This is very promising for extremely low-power neuromorphic computation and suggests that the dynamic behavior observed in larger devices should persist and improve as dimensions are scaled down.

Silicon Field Emission Electron Source With Individually Controllable Single Emitters

An electron source with an array of individually controllable single tips allows the observation of each emitter current simultaneously as well as the measurement of the integral current. Furthermore, the initial activation process of the individual tips and current distribution of the source can be examined. The Si cathode consists of an array of 2×2 conically shaped emitter structures on a borosilicate glass substrate. The emitters were fabricated by laser micromachining and wet etching. Integral field emission (FE) measurements were performed in a diode configuration in a vacuum chamber at pressures of about 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> mbar with a self-aligned Si extraction grid, due to adjustment pins. The total emission current was regulated to a predefined value (4.0, 10, 20, 40, and 50 μA) by an external regulating circuit and recorded individually during the measurement. The onset voltage for a current of 1 nA varied between 320 and 430 V. The voltage conversion factor is in the range of 7.6×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> to 1.3×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Constant current measurements over a longer period of time (1800 values with a sample rate of 0.5 Hz) were performed and show a total current fluctuation less than 0.6% due to the regulation circuit. Despite the stable total current in regulated operation, it was observed that the individual emitters fluctuate up to 75% for low (~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-8</sup> A) and 5% for high (~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> A) currents.

High Capacity Lactate Biofuel Cell Using Enzyme Cascade without NAD

We describe a modification of the lactate biofuel cell to increase its capacity. A new multi-step enzyme reaction system without NAD was prepared using a bioanode modified with lactate oxidase, pyruvate decarboxylase, and pyrroloquinoline quinone-dependent aldehyde dehydrogenase. Its operating time was 6 times longer than that of the biofuel cell using the lactate oxidase-modified bioanode at constant current measurement of 1 mA cm−2 because of suppression of the diffusion overpotential due to the decrease in the fuel concentration.

Silicon field emitters fabricated by dicing-saw and wet-chemical-etching

Silicon field emitter arrays (FEAs) with different tip sizes and quantities were fabricated by saw dicing and anisotropic wet chemical etching by tetramethylammonium hydroxide. The tip is formed by the different etching rates of the crystal facets leading to a sharp pyramid based on {103} planes on the top and a hexadecagon based on {331} and {221} planes on the bottom. Electrical measurements at 10−5 mbar up to 10 μA show good reproducibility for FEAs with the same process parameters and higher uniformity and stability with an increasing number of tips. Constant current measurements at the same conditions and 10 μA show a mean electric field increase of about 0.06(3) V/(μm h) for p-type FEAs with a tip quantity of 3600. The shift increases with lower tip quantity and is higher for n-type FEAs compared to p-type. The degradation during the constant current measurement of n-type samples is found to be partly reversible by heating to 200 °C during emission. In contrast, heating of p-type FEAs induced further degradation instead of a regeneration effect.

Femtosecond Laser-Induced Surface Modification of the Electrolyte in Solid Oxide Electrolysis Cells

Electrolyte-supported solid oxide cells are often used for steam electrolysis. Advantages are high mechanical stability and a low degradation rate. The aim of this proof of concept study was to use a femtosecond laser to process the electrolyte of an electrolyte-supported solid oxide cell and evaluate the effect of this laser treatment on the electrochemical performance. The femtosecond laser treatment induces a macroscopic and a superimposed microscopic structure. It can be proven that the electrolyte remains gas tight and the electrochemical performance increases independently of the laser parameters. The initial area-specific resistance degradation during a constant current measurement of 200 h was reduced from 7.9% for a non-treated reference cell to 3.2% for one of the laser-treated cells. Based on electrochemical impedance measurements, it was found that the high frequency resistance of the laser-treated cells was reduced by up to 20% with respect to the reference cell. The impedance spectra were evaluated by calculating the distribution of relaxation times, and in advance, a novel approach was used to approximate the gas concentration resistance, which was related to the test setup and not to the cell. It was found that the low frequency polarization resistance was increased for the laser-treated cells. In total, the area-specific resistance of the laser-treated cells was reduced by up to 14%.

Setting up the Reference Performance Tests of an Ageing Campaign for Non-Intrusive Diagnosis on Li4Ti5012 and LiMn1-XFexPO4 Based Cells

Non-intrusive diagnosis and prognosis methods are increasingly adopted to track Li-ion batteries ageing without additional cost due to destructive processes for post-mortem analyses. To qualify and quantify internal degradation mechanisms of a cell, Incremental Capacity Analysis (ICA) and Differential Voltage Analysis (DVA) have proven themselves to provide precise outputs on the battery state of health. These differential methods require close-to-equilibrium Open Circuit Voltage (cte-OCV) measurements to decipher ageing paths depending on the utilization.This poster presents an ageing campaign preliminary study on some Graphite//LiMn1-xFexPO4 and Li4Ti5O12/Ni1-x-yMnxCoyO2 based cells. It focuses on the reference performance tests chosen to be the most complete on both cell and electrode levels.As the battery cte-OCV measurement is a lengthy process of at least 25h, the reference tests should be well-defined to acquire as many data as possible in a single shot. A comparison is carried between constant current measurements and incremental charges with relaxation (GITT) to approach thermodynamic equilibrium, as well as the conditions and sampling rates to leverage differential method curves without further smoothing.The electrode study is implemented as references for models to evaluate both thermodynamic and kinetic behaviours of the battery components. To find out the closest half-cell replicates, electrode versus lithium coin cells are compared with a 3 electrodes PAT-cell assembly depending on the temperature and measurements.

Peat-Derived Hard Carbons for High Capacity Sodium-Ion Batteries: Synthesis and Characterization

Hard carbons have emerged as one of the most promising candidates for high capacity anode materials in sodium-ion batteries (SIB). It has been demonstrated that the degree of graphitization, disorder, and porosity of obtained hard carbons have an enormous effect on capacity.1 Various biomass precursors (banana peels, peanut shells, leaves, etc.) have been used for obtaining hard carbons, which have shown promising results as SIB anodes.2–4 In this study, peat was used as the precursor for obtaining peat-derived hard carbon anode materials (PDCs). Peat has a distinctive advantage as a precursor for its ease of collection. It can be excavated in millions of tons, as opposed to collecting shells or peels of different fruit. Hard carbons were synthesized using three steps – pre-pyrolysis at 300-800 oC, base-acid treatment, and post pyrolysis at 1000-1500 oC (Fig. 1). Each step was deemed necessary to improve the capacity of obtained hard carbon material (Fig. 1a). The materials were labeled as follows: PDC – pre-pyrolysis temperature – post-pyrolysis temperature A (A appended if KOH-HCl treatment was used).5 The structure of hard carbon materials was investigated with XRD, SEM-EDX, TEM, Raman spectroscopy, and laser powder diffraction methods. The specific surface area was examined by gas sorption analysis (with N2 and Ar). XRF was used to determine the elemental composition of hard carbon materials before and after the KOH-HCl treatment. TGA was performed to evaluate the burn-off of peat’s organic content during pre-pyrolysis.Cycling performance of hard carbon electrodes was investigated using the galvanostatic charge-discharge method at current densities ranging from 25 to 2000 mA g-1 and cyclic voltammetry at a potential sweep rate of 0.1 mV s-1. NaClO4 in EC:DEC (1:1) and NaPF6 EC:PC (1:1) were used as electrolytes to compare two rather common electrolytes and their effect on capacity (Fig. 1b).The highest capacities were achieved for PDC-450-1400 A. The constant current measurements show very high capacities of 350 mAh g -1 with a plateau region (E < 0.2 V vs. Na/Na+) of 250 mAh g-1 at the current density of 25 mA g -1. At the current density of 50 mA g-1, a capacity of 328 mAh g-1 with a plateau region of 243 mAh g -1 was achieved. Long-term stability of PDC-450-1400 A was also demonstrated, after 130 charge-discharge cycles at the current density of 25 mA g-1, the capacity stabilized at 280 mAh g-1 with the initial capacity of 320 mAh g-1.Overall, the highest capacities were achieved with materials that had the lowest impurity contents and specific optimal physical properties – low surface areas of around 6 m2 g-1, broad 002 reflexes in XRD diffractograms, interlayer spacing of about 3.9 Å and optimal I D/I G values of approximately 1.50. These characteristics might indicate the existence of local graphitic domains and therefore explain the high capacities.5 Acknowledgments This work was supported by ‘Strengthening of sectoral R&D’ project LLTOM17351, by EU through the European Regional Development Fund project 2014-2020.3.01.15-0011 and Personal Research under Grant PRG676. References I. El Moctar et al., Functional Materials Letters, 11, 1830003 (2018).E. M. Lotfabad et al., ACS Nano, 8, 7115–7129 (2014).J. Ding et al., Energy Environ. Sci., 8, 941–955 (2015).H. Li et al., ACS Appl. Mater. Interfaces, 8, 2204–2210 (2016).A. Adamson et al., RSC Adv., 10, 20145–20154 (2020). Figure 1

(Invited) Reverse Current Behavior and ADT Protocol for Start & Stop Operation of Bipolar Alkaline Water Electrolyzer

Introduction Water electrolysis is expected a key energy conversion system to introduce large-scale renewable electricity under power grid management. Here, less expensive large system is needed. Alkaline water electrolysis (AWE) systems are well-developed large system to combine hydropower generation, but fluctuated operation with start and stop operation is not considered for conventional systems. Therefore, improvement of durability to combine photovoltaic and/or wind turbine generation is significant issue. In this study, we have been investigated reverse current behavior of bipolar alkaline water electrolyzers and proposes accelerated durability test (ADT) protocol. Theoretical and Experimental Figure 1 shows configuration of bipolar alkaline water electrolyzer, principle of reverse current. An equivalent circuit to analyze reverse current is also shown in the figure. During electrolysis, surface Ni of anode oxidizes to Ni(III) and that of cathode reduces to Ni(0). They electronically connect on a bipolar plate and ionically connect through manifold to distribute electrolyte. In the equivalent circuit, electrochemical potential difference between anode and cathode on a bipolar plate correspond to electromotive force of Uk(t). The R 1 and R 2 correspond ionic resistances of branch and communicating tubes, respectively. Relationship among these parameters can describe as eq. 1 in Fig. 1 based on the Kirchhoff's laws, so reverse current would be estimated. In this study, we measured electrode potential and reverse current after 1 h water electrolysis at 0.6 Acm-2 for 2 to 4 cells stack bipolar electrolyzer using commercially available industrial anode and cathode (De Nora Permelec Ltd). The reverse current was measured ionic current through communicating tube with D. C. clamp meter.As bipolar electrolyzer evaluation, we measured electrode potential and reverse current after 1 h water electrolysis at 0.6 Acm-2 for 2 to 4 cells stack bipolar electrolyzer using the commercially available anode and cathode. Effective electrode size was 27 cm2. The reverse current was measured ionic current through communicating tube with D. C. clamp meter (KEW2510, Kyoritsu).As start & stop operation simulated ADT protocol, we propose the combination of constant current electrolysis, potential sweep, and chronoamperometry as the inset of illustration in Fig. 4, because constant current measurement is easier to get reproductivity in high current that need accurate iR correction for constant potential measurement and current control measurement never simulate stop operation. Results and discussion Figure 2 shows the setup of lab-scale electrolyzer and cell performances in the stacks. All cells showed almost the same performance. Figure 3 shows reverse currents and electrode potentials as a function of time for 2 to 4 cells stack electrolyzer. The longer cell stack had lager reverse current at initial period of stop operation. As the potential difference between anode and cathode decreased, the reverse current decreased. The anode potential significantly decreased certain time, and that for longer cell stack was shorter. For the 4 cells stack, the time for the BP2 was shorter than that for the BP1 and BP3. Simultaneously, the I 2 was larger than the I 1 and I 3. These behaviors could be explained with eq. 1 in the Fig. 1 with the same electromotive force of the U(t=0)s for initial period and time function of U(t)s with same surface condition for all cells at the end of electrolysis. Remarkable point is that the cathode potential of BP2 was higher than the anode, which is impossible with discharge of BP2 itself. This opposite potential of the BP2 would be forced by electromotive force of the BP1 and BP3.Figure 4 shows the anode potential at 10 mAcm-2 (E @10mAcm-2) of oxygen evolution reaction (OER) current during the ADT protocol with various minimum potential: E min. The all anodes significantly degraded for around 3-days test protocol, although DN851 was developed as an active and durable anode for conventional OER electrode. The degradation increased with decreases of the E min. Above 0 V vs. RHE of the E min, The E @10mAcm-2 increased after ca. 800 cycles of introduction period and reached around 1.69 V vs. RHE after 1900 cycles, although it increased from initial period and the increase continued around 2000 cycles for -0.3 V vs. RHE of the E min. From these experimental results, the E min of between 0 to 0.3 V vs. RHE might be suitable for the ADT protocol because of rapid degradation with robustness to the E min. Acknowledgements This study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in Advancement of Hydrogen Technologies and Utilization Project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Figure 1

Adhesion of Bacteria to a Graphene Oxide Film.

Graphene oxide (GO)-based materials have recently received increasing attention in antibacterial fields due to their unique two-dimensional structure, high specific surface area, water solubility, easy functionality, low cost, etc. In this work, we investigated the interaction of GO films with several bacteria, including Escherichia coli, Staphylococcus aureus, Bacillus, and Shewanella-MR 1. Surprisingly, it was found that GO films had no influence on the growth of bacteria; bacteria could selectively attach on the GO films, and the adhesion efficiency was dependent on the genus of bacteria and the quality of GO films, as confirmed by quartz crystal microbalance (QCM) measurement. X-ray photoelectron spectroscopy, Ramam spectra analyses, and current-voltage curves further indicated that the bacteria could reduce GO to reductive GO (rGO) or graphite. Through constant current measurements, a possible mechanism that involved electron transfer from bacteria to the GO film was proposed for the adherence of bacteria to GO films and reduction of GO films by bacteria. The study suggested that GO films could act as biocompatible sites for adhesion of bacteria on their surfaces, providing a paradigm for exploring the interaction of GO-related materials with bacteria.

A Reversible Electrochemical Side Reaction of Li Electrodeposition / Dissolution Using N(SO2)X(SO2)Y Anion-Containing Molten Salt (X, Y = F, CF3)

LiTFSA (Lithium Bis(trifluoromethanesulfonyl)imide) and LiFSA (Lithium bis(fluorosulfonyl)imide) are well-known as a salt of an electrolyte of a Li-based secondary battery, and also used as a salt of an electrolyte of a secondary battery using metal Li as a negative electrode. In many cases, when using these electrolytes to electrodeposit metal Li, side reactions occur at about 1.8 V vs Li+/Li.1,2 Recently, we have discovered that this side reaction is reversible and appears prominently at temperatures as high as 150 ° C. The electrolyte was prepared by dissolving 1 M of Li [TFSA] in Pyr13 [TFSA] (N-Propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide). A Ni (Nickel) substrate was immersed in this electrolytic solution to form a working electrode. A metal Li was used as a counter electrode to form a two-electrode cell. A current was flowed to the reduction side at a current density of 0.2 and 1 mA cm-2, and a current was flowed to the oxidation side when it reached 0.1 V. The voltage-time profiles are shown in Figure (a)(b). At the time of reduction, it has a plateau at a voltage of 1.5 V, indicating that some electrochemical reaction has occurred. The oxidation has a plateau near 1.9 V, suggesting that this reaction is electrochemically reversible. The result of performing cyclic voltammetry measurement for the cells of the same configuration is shown in Figure (c). Reversible reduction and oxidation waves can be confirmed at the potential (ca. 1.5 V and 1.9 V) corresponding to the plateau potential in constant current measurement. Also, it was seen that the peak increases with each cycle. At the meeting site, we will also describe the results of using LiFTA (lithium (fluorosulfonyl)(trifluoromethylsulfonyl)amide), which is a molten Li single salt, and discuss the origin of side reactions. Acknowledgments This work was performed as a cross-appointment project between Osaka University and the National Institute of Advanced Industrial Science and Technology (AIST).

Lithium- and sodium-ion storage properties of modulated titanate morphologies in reduced graphene oxide nanocomposites

Electrochemical energy storage (EES) systems rely on novel materials to meet stringent performance demands in terms of both high energy and high power. Although several new materials have been identified for EES, one method that seeks to amplify material properties has been to modulate morphologies at the nanoscale. This improvement in electrochemical properties is in large part due to the enhancement in surface area and mitigation of solid-state diffusion at the nanoscale domain. By using sodium titanate (Na2Ti3O7) as a model compound, we employ efficient synthetic routes to prepare titanate nanosheets and nanotubes, and assess their charge-storage properties in both Li- and Na-ion non-aqueous electrolytes. Moreover, nanocomposites comprising reduced graphene oxide are constructed to further enhance rate capability. We perform a fundamental electroanalytical treatment of the charge-storage properties for titanate nanosheets and nanotubes and identify the underlying rate-determining mechanisms by kinetic analysis. In both Li- and Na-ion electrolytes, the nanotube–reduced graphene oxide nanocomposite exhibits pseudocapacitive behavior, indicated by broad voltammetric features and sloping charge-discharge voltage profiles under galvanostatic mode. This corresponding charge-storage process supports high capacities of 170 and 80 mAh g−1 in a Li- and Na-ion system, respectively, based on constant current measurements at a C-rate of 5C.

V2O5·nH2O nanosheets and multi-walled carbon nanotube composite as a negative electrode for sodium-ion batteries

Two dimensional (2D) transition metal oxides and chalcogenides demonstrate a promising performance in sodium-ion batteries (SIBs) application. In this study, we investigated the use of a composite of freeze dried V2O5·nH2O nanosheets and multi-walled carbon nanotube (MWCNT) as a negative electrode material for SIBs. Cyclic voltammetry (CV) results indicated that a reversible sodium-ion insertion/deinsertion into the composite electrode can be obtained in the potential window of 0.1–2.5 V vs. Na+/Na. The composite electrodes delivered sodium storage capacities of 140 and 45 mAh g−1 under applied current densities of 20 and 100 mA g−1, respectively. The pause test during constant current measurement showed a raise in the open circuit potential (OCP) of about 0.46 V, and a charge capacity loss of ∼10%. These values are comparable with those reported for hard carbon electrodes. For comparison, electrodes of freeze dried V2O5·nH2O nanosheets were prepared and tested for SIBs application. The results showed that the MWCNT plays a significant role in the electrochemical performance of the composite material.

Revealing the Impact of Anions in Lithium Oxygen Batteries

Despite their high theoretical capacity, lithium oxygen cells cannot be fully discharged to the limit of their theoretical energy density. The main reason for that is the insoluble and insulating character of the discharge product Li2O2, when it is formed as a thin film form on the cathode, creating the passivation layers, and causing a stop of the charge transport. Furthermore, the discharge/side products can block the cathode’s pores, therefore limiting capacity too. Capacity is also limited by oxygen transport especially at higher current rates. The solubility of oxygen in the electrolyte and the diffusion is important in that case, and can be increased with the right choice of salt. A big challenge in building lithium oxygen batteries is choosing proper electrolyte for the system, which should be stable against species such as LiO2 -, LiO2, O2-, O2 2-. New combinations are necessary to improve issues such as minimizing electrolyte breakdown and/or enhancing the stability of the battery. In this work the electrochemical performances are investigated using constant current measurements and cyclic voltammetry of the lithium nitrate and bis(trifluoromethane) sulfonamide lithium salt mixtures in tetragylme. Furthermore, post mortem analysis in the form of Raman Spectroscopy, Scanning Electron Microcopy, and Energy – dispersive X ray Spectroscopy is performed. The contribution of this work is an analysis of the effect of various lithium salts, by maintaining the activity of Li+ but changing the anion. This yields insights into the extent of ion pairing and its effects on the Li-O2 battery system. Higher capacity results are coming from the increased stability and solubility of ion pair species that are formed in the electrolyte between the initial oxygen reduction reaction product (superoxide), O2 - and solvated the Li+. Figure 1. Constant current first discharge performances of LiNO3 and LiTFSI salt mixture concentrations and Scanning Electron Micrographs (scale 10µm) of their air cathode. Figure 1

Distribution Analysis of Electrode Reaction of Lithium Manganese Oxide By Means of Time-Resolved XAFS Technique

Lithium ion secondary batteries (LIBs) are widely used as energy storage devices, and it is thus essential to evaluate the heterogeneous reaction distribution appeared in the electrode in order to improve the battery performance. The chemical state of the electrode active material is analyzed by the operando XAFS technique during the charging and discharging processes, and the 2D XAFS imaging technique provides the information about the in-plane reaction distribution. The previous study using the 2D XAFS imaging technique revealed that the heterogeneous distribution reaction appeared on the lithium iron phosphate (LFP) cathode due to the spatial difference in the electron conductivity. The distribution analyses for the other cathode materials with different crystal structure are important to understand the generation mechanism of the heterogeneous distribution. In the case of lithium manganese oxide (LMO) cathode, the heterogeneous distribution like LFP was not observed because the Li ion diffusion in the LMO particles could be enhanced by the spinel structure of LMO. In order to understand the difference of heterogeneity between LFP with the olivine structure and LMO, the vertically dispersive XAFS (VDXAFS) instrument was developed to achieve the space- and time-resolved XAFS measurements. In this study, the time course change of the chemical state map on the Mn species for the LMO cathode was revealed using the VDXAFS instrument with applying the constant voltage for the operating LIB. An LMO cathode sheet, two separator sheets, and a Li anode plate were assembled in a battery cell with Al-laminated films for the VDXAFS measurements. The cathode was composed of LMO (2 different particle sizes of 0.5-1.0 and 5-10 micrometers), acetylene black, and polyvinylidene difluoride with the mass ratio of 14:3:3. The VDXAFS measurements at the Mn K edge were performed at NW2A of PF-AR (KEK, Japan) and BL-5 of SR Center (Ritsumeikan University, Japan). The charging process at 4.5 V and the discharging process at 3.0 V were observed until the electrode current becomes less than 0.1 C. The time course change of the chemical state map for the charging process was obtained on the basis of the observed XANES spectra as shown in Figure. The blue pixel indicates that the average chemical state of Mn at that position corresponds to that before the charge, and the chemical state shifts to the higher valence state as depicted by the red pixels during the charging process. As clearly seen in Figure, the electrode reaction proceeded heterogeneously for the LMO cathode, although such heterogeneity was not observed in the previous study for constant current measurements at 0.5 C. It means that the heterogeneous distribution is dynamically appeared in the LMO cathode during the constant voltage charging process. At the beginning, the electrode current was relatively high, and about 25 reaction channels, at which the oxidation reaction proceeded earlier, were appeared in the measured linear area of 10 mm length. These reaction channels were fused during the progress of charge with decreasing the electrode current, and the chemical states of the Mn species become homogeneous when the electrode current reached less than 1 C. It is considered that the lower electric resistance at the reaction channel is a reason to generate such the heterogeneous electrode reaction on the LMO cathode in the case of large current. In this study, the heterogeneous distribution in the LMO cathode was first observed using the VDXAFS instrument during the constant voltage charging process. The present results are important to optimize the electrode reaction of LIBs. Figure 1