Double-layer Electrode Research Articles

Phase II SBIR Double Layer Electrode Technology for Advanced Space Power Technologies

Under Phase II of this SBIR, Quallion is continuing development of its double-layer electrode technology for lithium ion batteries. The double-layer electrode technology increases adhesion of active materials to improve mechanical strength and the increased surface area increases conductivity. Quallion will refine its electrode design based on lessons learned in Phase I by adjusting the composition of coating materials and the electrode coating process, and incorporating the double-layer electrode into commercial satellite cell designs. Quallion is building prototype cells and will perform testing to validate their performance and to generate data for further analysis. Quallion will present initial data which will include rate capability, capacity, and cycle life characterization on test cells.

Influence of Surface/Interface on the Performance of MIEC Cathode for SOFC

La0.6Sr0.4CoO3 has been studied as an SOFC cathode because of its high mixed ionic electronic conductivity. The mixed conductivity makes the zone of oxygen reduction reaction extended from triple-phase boundary to two-phase boundary. The rate limiting step of the cathode reaction is oxygen reduction reaction on two-phase boundary (1). Sase et al. reported that the oxygen reduction is activated at hetero-interface of La0.6Sr0.4CoO3 and La1.5Sr0.5CoO4 (2). The hetero- interface effect leads to development of higher performance electrode. However, the detail mechanism is still not clear.This study aims to elucidate the mechanism of the hetero-interface effect by the relationship between the electrode performance and the surface chemistry.Thin-film electrodes were fabricated on the electrolyte substrate of Ce0.9Gd0.1O1.95 (10GDC) by pulse laser deposition (PLD) method. The two-layer electrodes were made: the first layer was La0.6Sr0.4CoO3 (LSC40); the second layer, La1.5Sr0.5CoO4(LSC155), was prepared exactly onto the first layer. Additionally single layer electrodes of LSC155 and LSC40 were also formed on the same 10GDC electrolyte to evaluate the hetero- interface effect. Several samples were prepared with changing thickness of LSC155.A micro probe apparatus equipped with a potentio-galvano stat and a frequency response analyzer was employed for the electrochemical measurement. The measured electrode could be selected by manipulating probes of the micro prober. This setup enables to measure four electrodes simultaneously without changing the sample and its thermal history. The thin-film electrodes, the porous Pt on the side surface, and the porous Pt on the opposite side of the sample were used as working, reference, and counter electrodes, respectively. The measurement conditions were at 873, 973, and 1073 K, in air.The electrode reaction resistances of the hetero-interface electrodes were measured by an impedance measurement. It was expected that the resistance of the double layer electrodes should become larger with increase in thickness of LSC155 due to longer ionic diffusion pathway. However, the measured electrode resistances did not depend much on LSC155 thickness. Electrode resistances of most of the double-layered electrodes were smaller than those of the LSC155 single-phase electrode in spite of LSC155 thickness. This phenomenon implies the electrochemical properties of LSC155 film on LSC40 might be modified and different from those of LSC155 film on a substrateFocusing on the LSC40 single-phase electrode, the electrode resistance increased from 873 to 973 K. It may be the influence of rearrangement of cations, which can occur over 923 K: the deposition temperature of the electrodes. The resistances of the layered electrodes were smaller than that of LSC40 single-phase electrode. Element distribution was evaluated by EDX analysis. The enrichment of strontium was observed at the surface grains of the degraded LSC40. The concentrations of lanthanum and cobalt were lower at the surface grains. This means the surface composition changed from that of bulk. Therefore, the segregation of strontium on the surface of LSC40 may degrade the electrode performance.This research was supported by JST-PREST and JST-CREST project.

Influence of Surface/Interface on the Performance of MIEC Cathode for SOFC

La0.6Sr0.4CoO3 (LSC40) has been studied as an SOFC cathode. It is reported that the oxygen reduction reaction was enhanced at hetero-interface of LSC40 and La1.5Sr0.5CoO4 (LSC155). Relationship between the electrode performance and the surface chemistry was elucidated to find out the mechanism of the hetero-interface effect. Thin film electrodes of LSC40, LSC155, and double-layer electrode of LSC155-on-LSC40 were fabricated by pulsed laser deposition. The electrode performance was measured by impedance measurement. LSC40 electrode degraded during the measurement, and enrichment of strontium was observed at the surface of the degraded LSC40 by EDX analysis. On the other hand, the segregation of strontium was suppressed on the double-layer electrode, whose electrode performance was relatively higher than that of LSC40 single-phase electrode. It implies that the layered LSC155 may suppress the electrode degradation.

Improvement in Photo-Potential Characteristics of TiO2 Electrode Assembling to Marine Microbial Fuel Cell

It has been reported that the potential of stainless steel (S.S.) ennobles to 400 mV by the formation of biofilm in seawater. This phenomenon can lead to the development of a battery by coupling an active counter electrode. On the other hand, TiO2 is well known as an n-type semiconductor which exhibits the photo-catalytic effect under the UV irradiation. When a TiO2 coated metal is irradiated by the UV light, excited electrons are transferred to the base metal, this phenomenon leads to the active shift of electrode potential. This effect can be applied to microbial fuel cell as a non-sacrifice electrode. We have investigated the electrochemical characteristics of the marine microbial fuel cell (MMFC) composed of biofilm covered stainless steel cathode and TiO2 anode in seawater. However, it is needed to improve the photo-potential characteristics of electrodes for the practical application.The present work attempted to improve the battery performance by assembling the double layered TiO2 anode electrode.Results from experimental, by using the double layered anode for MFC, cell voltage increases 120 mV and maximum power density enhances 1.4 times compared with the value of single layered one coated by squeegee printing method. The double layered electrode has anatase / rutile hetero-junction, therefore it promotes the electron transfer between base metal and TiO2 coating interface. In addition, the double layered electrode has a high durability against the cyclic irradiation because TiO2 intermediate layer behaves as a corrosion protective barrier which suppresses the direct contact between electrolyte and base metal.

Double-Layer TiO2 Electrodes with Controlled Phase Composition and Morphology for Efficient Light Management in Dye-Sensitized Solar Cells

The light-scattering effect in the dye-sensitized solar cells (DSCs) was studied by controlling TiO2 phase composition and morphology by fabrication of double-layer cells with different arrangement modes. The starting material for preparation of TiO2 cells was synthesized by an aqueous sol–gel process. X-ray diffraction and field emission scanning electron microscopic analyses revealed that TiO2 nanoparticles had particle size ranging between 18 and 44 nm. The optical property and band gap energy of TiO2 nanoparticles were studied through UV–Vis absorption. The indirect optical band gap energy of anatase and rutile nanoparticles was found to be 3.47 and 3.41 eV, respectively. The double-layer DSC made of nanostructured TiO2 film with phase composition of 78 % anatase and 22 % rutile as the under-layer and mixtures of anatase-nanoparticles and anatase-microparticles as the over-layer (i.e., NM solar cell) was shown the highest power conversion efficiency (PCE) of 6.04 % and open circuit voltage of 795 mV. This was achieved due to the optimal balance between the light scattering effect and dye sensitization parameters. Optimum light scattering of photoanode led to improve the PCE of NM double-layer solar cell which was demonstrated by diffuse reflectance spectroscopy.

Preparation of Pt/C electrode with double catalyst layers by electrophoresis deposition method for PEMFC

Pt/C double catalyst layer (DCL) electrodes are prepared by pulsed electrophoresis deposition (PED) method from a Pt colloidal solution as a plating bath. The PED is optimized by varying the deposition time in a galvanostatic mode. The catalyst layers of the electrodes prepared by this method are structurally characterized by EDX and SEM studies. The catalytic activities of Pt/C DCL electrodes are evaluated by cyclic voltammetry technique. The loading amount of the Pt catalyst is controlled by varying the deposition time. With the same Pt catalyst loading, the DCL electrode has enhanced catalytic activity than single catalyst layer (SCL) electrode.

Mathematical Modeling of Hybrid Asymmetric Electrochemical Capacitors

Electrochemical capacitors may be ideal energy storage devices for applications ranging from renewable energy storage to transportation. Electrochemical capacitors are characterized by high power density and cycle life, and can be classified based on the properties of their electrodes. Symmetric capacitors have two electrodes that exhibit the same behavior, while asymmetric capacitors have two electrodes that exhibit different behavior. A hybrid asymmetric capacitor is a device that has two electrodes that behave differently because of different processes occurring in them. In this paper, we present a mathematical model of hybrid asymmetric capacitors made up of a redox couple electrode and a double-layer electrode. We show that, because of the different behavior of each electrode, the hybrid asymmetric capacitor possesses higher energy and power density than the symmetric capacitor. Our model is generic and can be extended to any device so long as the properties of the electrodes are known.

Electrochemical properties of CVD grown pristine graphene: monolayer- vs. quasi-graphene

We report the electrochemical properties of pristine monolayer, double layer and few-layer (termed quasi-) graphene grown via CVD and transferred using PMMA onto an insulating substrate (silicon dioxide wafers). Characterisation has been performed by Raman spectroscopy, optical spectroscopy, Atomic Force Microscopy and X-ray Photoelectron Spectroscopy, revealing 'true' pristine single-layer graphene (O/C of 0.05) at the former and pristine quasi-graphene at the latter (O/C of 0.07); the term "quasi-graphene" is coined due to the surface comprising on average 4-graphene-layers. The graphene electrodes are electrochemically characterised using both inner-sphere and outer-sphere redox probes with electrochemical performances of the graphene electrodes compared to other available graphitic electrodes, namely that of basal- and edge- plane pyrolytic graphite electrodes constructed from Highly Ordered Pyrolytic Graphite (HOPG), with information on heterogeneous rate constants (k(o)) obtained. The electrochemical rate constants are predominantly influenced by the electronic properties of the graphene surfaces. Monolayer graphene is found to exhibit slow heterogeneous electron transfer (HET) kinetics towards the redox probes studied, with HET rates ca. 2 and 8 times faster at quasi-graphene and HOPG respectively, relative to that of the monolayer graphene electrode. Critically contrasting the performance of monolayer graphene to quasi-graphene and HOPG electrodes reveals that increasing the number of graphene layers results in improved electrochemical properties, where in terms of the electrochemical reversibility of the probes studied: monolayer-graphene < quasi-graphene < HOPG, as governed by the respective HET electrochemical rate constants. Given that edge plane sites are the predominant origin of fast electron transfer kinetics at graphitic materials, the slow HET rates at pristine single-layer graphene electrodes are likely due to graphene's fundamental geometry, which comprises a small edge plane and large basal plane contribution. In the case of quasi-graphene and HOPG, they possess increasing global coverage of electrochemically reactive edge plane sites (respectively) and thus exhibit superior electrochemical performances over that of monolayer graphene. Last, the case of a double-layer graphene electrode is considered, which as a result of its fabrication possesses a large global coverage of edge plane like- sites/defects. In agreement with the former conclusions, the double-layered defect-graphene electrode is found to exhibit fast/favourable electrochemical properties, which is attributed to its large edge plane content (i.e. defect abundant graphene) and thus is further evidence that the electrochemical response is dependent on the density of edge plane sites at graphene based electrodes (influenced by the coverage of graphene-defects and the number of graphene layers).

ナノ多孔質炭素材料の吸着材および電気二重層キャパシタ電極としての応用に関する熱力学

ナノ多孔質炭素材料の吸着材および電気二重層キャパシタ電極としての応用に関する熱力学

Electrochemical Kinetics of Nanostructured Nb2O5 Electrodes

Pseudocapacitive charge storage is based on faradaic charge-transfer reactions occurring at the surface or near-surface of redox-active materials. This property is of great interest for electrochemical capacitors because of the substantially higher capacitance obtainable as compared to traditional double-layer electrode processes. While high levels of pseudocapacitance have been obtained with nanoscale materials, the development of practical electrode structures that exhibit pseudocapacitive properties has been challenging. The present paper shows that electrodes of Nb2O5 successfully retain the pseudocapacitive properties of the corresponding nanoscale materials. For charging times as fast as one minute, there is no indication of semi-infinite diffusion limitations and specific capacitances of 380 F g−1 and 0.46 F cm−2 are obtained in 40-μm thick electrodes at a mean discharge potential of 1.5 V vs Li+/Li. In-situ X-ray diffraction shows that the high specific capacitance and power capabilities of Nb2O5 electrodes can be attributed to fast Li+ intercalation within specific planes in the orthorhombic structure. This intercalation pseudocapacitance charge-storage mechanism is characterized as being an intrinsic property of Nb2O5 that facilitates the design of electrodes for capacitive storage devices. We demonstrate the efficacy of these electrodes in a hybrid electrochemical cell whose energy density and power density surpass that of commercial carbon-based devices.

Double layered nanoarchitecture based on anodic TiO2 nanotubes for dye-sensitized solar cells

Based on the TiO2 nanotube (TONT) with three different thicknesses (10, 18, 28 μm) prepared by electrochemical anodization, a double layered electrode consisting of TONT and TiO2 nanoparticles (TONPs) with a size of approximately 20 nm was developed. The anodic TONT electrode has been actively investigated due to its beneficial properties such as a favorable electron transport rate and light scattering ability. However, the small specific surface area caused by the large pores and vacant space between 1-D NTs impedes the fabrication of high efficient DSSCs. Hence, the TONP layer with a thickness of 10 μm is additionally deposited on the top surface of the TONT film to offer a sufficient surface area for large dye loading. Compared to a single TONT electrode, the double layered TONT/NP electrodes provide a high short-circuit current density (Jsc) and fill factor (FF) while maintaining a constant open-circuit voltage (Voc), finally disclosing a higher conversion efficiency (η), attributed to the high loading of dye molecules, light scattering ability, low resistance in the photoanode/electrolyte interface, and longer electron lifetimes. In particular, the optimal η was attained with the TONT/NP (18 μm) electrode having a Voc of 0.80 V, a Jsc of 7.80 mA/cm2, a FF of 62.7%, and a η of 3.93%. Incidentally, when using a longer TONT film (28 μm), η decreases steadily due to the fast charge recombination process. A more detailed discussion is included below.

In-plane switching of vertically aligned negative liquid crystals for high transmittance and wide viewing angle

We propose a method for in-plane switching of vertically aligned negative liquid crystals (LCs) for high transmittance and wide viewing angle. By applying an in-plane electric field using a double-layered electrode structure, LC molecules can be rotated by the vertical as well as the in-plane components of the applied field over the entire region so that high transmittance can be achieved. The threshold voltage difference can be obtained simply by varying the electrode structure, which can reduce the off-axis gamma shift in a multidomain vertical alignment LC cell.

Improvement of Transparent Metal Top Electrodes for Organic Solar Cells by Introducing a High Surface Energy Seed Layer

AbstractWe present highly transparent and conductive silver thin films in a thermally evaporated dielectric/metal/dielectric (DMD) multilayer architecture as top electrode for efficient small molecule organic solar cells. DMD electrodes are frequently used for optoelectronic devices and exhibit excellent optical and electrical properties. Here, we show that ultrathin seed layers such as calcium, aluminum, and gold of only 1 nm thickness strongly influence the morphology of the subsequently deposited silver layer used as electrode. The wetting of silver on the substrate is significantly improved with increasing surface energy of the seed material resulting in enhanced optical and electrical properties. Typically thermally evaporated silver on a dielectric material forms rough and granular layers which are not closed and not conductive below thicknesses of 10 nm. With gold acting as seed layer, the silver electrode forms a continuous, smooth, conductive layer down to a silver thickness of 3 nm. At 7 nm silver thickness such an electrode exhibits a sheet resistance of 19 Ω/□ and a peak transmittance of 83% at 580 nm wavelength, both superior compared to silver electrodes without seed layer and even to indium tin oxide (ITO). Top‐illuminated solar cells using gold/silver double layer electrodes achieve power conversion efficiencies of 4.7%, which is equal to 4.6% observed in bottom‐illuminated reference devices employing conventional ITO. The top electrodes investigated here exhibit promising properties for semitransparent solar cells or devices fabricated on opaque substrates.

On Practical Charge Injection at the Metal/Organic Semiconductor Interface

We have revealed practical charge injection at metal and organic semiconductor interface in organic field effect transistor configurations. We have developed a facile interface structure that consisted of double-layer electrodes in order to investigate the efficiency through contact metal dependence. The metal interlayer with few nanometers thickness between electrode and organic semiconductor drastically reduces the contact resistance at the interface. The improvement has clearly obtained when the interlayer is a metal with lower standard electrode potential of contact metals than large work function of the contact metals. The electrode potential also implies that the most dominant effect on the mechanism at the contact interface is induced by charge transfer. This mechanism represents a step forward towards understanding the fundamental physics of intrinsic charge injection in all organic devices.

High Mobility IGZO/ITO Double-layered Transparent Composite Electrode: A Thermal Stability Study

ABSTRACTThe fabrication of a thin film optoelectronic device involves the exposure of the transparent conductive oxide (TCO) to a high process temperature. Indium gallium zinc oxide (InGaZnO4 or IGZO) is a well known TCO with high optical transparency, moderate conductivity and high mobility. However, its electrical properties deteriorate after subsequent high temperature processes in air atmosphere. On the other hand indium tin oxide (ITO) has higher conductivity than IGZO and better thermal stability. Therefore, IGZO/ITO bilayers have been deposited on glass by radio frequency magnetron sputtering at room temperature and subsequently annealed at high temperatures in order to study their thermal stability. In the present work, a-IGZO layers with a thickness ranging from 10 nm to 100 nm were deposited over a 50 nm thick ITO layer. Results are compared with those from a single IGZO layered thin film without the ITO bottom layer. The structural, optical and electrical properties of the multilayers are studied with the use of scanning electron microscopy, UV–Vis spectroscopy and Hall measurement. An IGZO optimal thickness of 50 nm is found to improve the bilayer thermal stability at temperatures upto 400 °C keeping good opto-electrical properties. The sheet resistance for the optimized IGZO/ITO composite films is about 22 Ohm/sq, and the transmittance in the visible range is about 90%. The composite shows an excellent mobility above 40 cm2 /V-s and thus can be potentially applied as channel layer in thin film transistors (TFTs)

Transparent, flexible conducting graphene hybrid films with a subpercolating network of silver nanowires

Herein we report a double layer structured transparent, flexible hybrid electrode from a subpercolating network of silver nanowires (Ag NWs) with CVD graphene coating on a PET substrate. The resistance of the obtained hybrid graphene/Ag NW film, with T = 81.5% (at 550 nm) and Rs = 32.5 Ω sq−1, is comparable to that of the resistance of ITO. In comparison with the Ag NW film, the resulting hybrid film has a lower surface roughness and a high DC to optical conductivity ratios. In such an electrode structure, graphene and Ag NWs can serve as the short-range and long-range conductors respectively, which should be more efficient in collecting and transporting the carriers to the external circuits. In addition, the graphene/Ag NW film possesses a desirable mechanical property. It is expected that the graphene/Ag NW double layered electrode might serve as a flexible transparent electrode.

Engineering aspects of the hybrid supercapacitor with H-insertion electrode

A hybrid supercapacitor, which was assembled with an H-insertion negative carbon electrode and an electrochemical double-layer positive electrode, was studied and its structure was optimized. The electrochemical performance of the individual electrode with different negative to positive electrode mass ratios was investigated. It has been demonstrated that the performance of the hybrid supercapacitor is closely related to the electrode balance. The results indicated that for carbon electrodes used in the study, the capacity of the H-insertion electrode could be fully utilized when the mass ratio of the negative to positive electrode is about 1:6.

A CoOx/carbon double-layer thin film air electrode for nonaqueous Li-air batteries

A double-layer structural air cathode consisting of diamond like carbon (DLC) active layer and CoOx catalytic layer is designed and its catalytic effect for Li-air batteries both in carbonate and ether based electrolytes are tested. Ethylene carbonate (EC)/dimethyl carbonate (DMC) based Li-air cell using this double-layer electrode as air cathode exhibits significant improvement in discharge/charge electrochemical performance but suffers from electrolyte decomposition as proved by Fourier transform infrared (FTIR) and secondary ion mass spectrometry (SIMS) measurements. In 1,2-dimethoxyethane (DME) based electrolyte, CoOx/DLC double layer electrode shows high catalytic activity towards oxygen evolution reaction (OER). Furthermore, over 30 cycles is achieved with a capacity of more than 2000 mAh g−1 by using CoOx/DLC double layer electrode, which makes it a promising air electrode for Li-air batteries.

Thermal Treatment Effects on Charge Storage Performance of Graphene-Based Materials for Supercapacitors

Graphene materials were synthesized by reduction of exfoliated graphite oxide and then thermally treated in nitrogen to improve the surface area and their electrochemical performance as electrical double-layer capacitor electrodes. The structural and surface properties of the prepared reduced graphite oxide (RGO) were investigated using atomic force microscopy, scanning electron microscopy, Raman spectra, X-ray diffraction pattern analysis, and nitrogen adsorption/desorption studies. RGO forms a continuous network of crumpled sheets, which consist of large amounts of few-layer and single-layer graphenes. Electrochemical studies were conducted by cyclic voltammetry, impedance spectroscopy, and galvanostatic charge-discharge measurements. The modified RGO materials showed enhanced electrochemical performance, with maximum specific capacitance of 96 F/g, energy density of 12.8 Wh/kg, and power density of 160 kW/kg. These results demonstrate that thermal treatment of RGO at selected conditions is a convenient and efficient method for improving its specific capacitance, energy, and power density.

Adhesion Force Change by Electrowetting on a Polymer Microlens Array

In addition to contact angle changes, adhesion force changes by electrowetting on a polymer microlens are reported in this paper. A flexible poly(dimethylsiloxane) (PDMS) microlens array was fabricated with an embedded Cr/Au double-layered electrode and a dielectric layer of parylene and Teflon. On microlens structures with a Teflon layer, a deionized water droplet on the microlens array surface shows a high contact angle of 137° before electrowetting. The contact angle is decreased to 63° by electrowetting with an external bias of 250 V. The electrowetting process carried out on the microlens array also enhances the adhesion force between the water droplet and the microlens array. This change in the adhesion force after electrowetting is demonstrated by a water droplet that hangs upside down on the fabricated microlens array. From the computed work of adhesion, it is shown that a larger difference in the adhesion force by electrowetting can be achieved on the fabricated microlens array than on a flat Teflon surface.