Bilayer Electrolyte Research Articles

NiMo-ceria-zirconia catalytic reforming layer for solid oxide fuel cells running on a gasoline surrogate

This paper describes an application of a NiMo-ceria-zirconia (NiMo-CZ) catalyst as a micro-reforming layer for solid oxide fuel cells running on isooctane (i.e., a gasoline surrogate). The catalyst layer was applied on top of a conventional anode supported single cell with a configuration of Ni-yttria-stabilized zirconia (YSZ) anode, YSZ/Ce0.8Sm0.2O1.9 bi-layer electrolyte and La0.6Sr0.4Co0.2Fe0.8O3-δ cathode. Our findings show that the application of the novel catalyst layer was an effective way to reform the mixture of isooctane and air into H2 and CO, which facilitated the electrochemical oxidation of complex hydrocarbons at the anode. At 750°C, the single cell with the micro-reforming layer exhibited a low polarization resistance of 1.36Ω cm2 and a maximum power density of 405mWcm−2 in the direct feeding condition of an isooctane/air mixture. At the current density of 500mAcm−2, the cell voltage presented a fairly low degradation rate of 3.0mVh−1 during a 12h stability test. The excellent electrochemical performance suggests the high catalytic activity of the NiMo-CZ catalyst layer for reforming isooctane and suppressing degradation of the single cell in the direct feeding of isooctane/air.

An evaluation of electrochemical performance of a solid oxide electrolyzer cell as a function of co-sintered YSZ-SDC bilayer electrolyte thickness

Using porous Ni-YSZ (Yttria Stabilized Zirconia, ZrO2-Y2O3) as a fuel electrode, dense YSZ-SDC (Samaria Doped Ceria, Sm0.2Ce0.8O1.9) as an electrolyte, and porous 20wt%SDC-80wt% BSCF (Ba0.5Sr0.5Co0.8Fe0.2O3−δ) as an air electrode, with the three distinguishable layers well adhered to each other, a Ni-YSZ/YSZ-SDC/BSCF-SDC solid oxide electrolyzer cell (SOEC) was constructed in order to satisfy the aim of this research which is studying the effect of YSZ blocking layer on its electrochemical performance over the temperature range of 650–800°C. Coating of SDC with a very thin layer of YSZ to form a YSZ-SDC bilayer electrolyte was performed using the spin coating technique in order to prevent the open circuit voltage (OCV) loss and enhance the chemical stability of doped ceria electrolyte in reducing atmospheres. For further examination to find out which microstructure is crack-free in order to provide electrical current leakage prevention in the SDC layer and at the same time to have an optimum thickness, impedance measurement under 0.1V bias with different hydrogen humidity level as well as cell polarization study were conducted at 800°C. Detailed investigations on the microstructural, electrochemical, and power measurement data confirmed that abilayer electrolyte structure with 3.5µm YSZ thickness can provide chemical, mechanical, and structural stability of the YSZ-SDC bilayer electrolyte in the Ni-YSZ/YSZ-SDC/BSCF-SDC single cell.

Optimizing the Thickness of Esb-GDC Bilayer Electrolytes for Efficient Low Temperature Operating SOFCs

Bilayer electrolytes with high oxide ion conductive materials such as Er2O3-stabilized Bi2O3 (ESB) and Gd-doped CeO2- δ (GDC) were developed to lower the operating temperature of solid oxide fuel cells (SOFCs) without compromising their efficiency[1]. In a bilayer electrolyte design, optimizing the thickness of ESB to GDC (r ESB/GDC) and the total electrolyte thickness (h) can help to maximize the SOFC performance. However, such optimization of electrolyte thicknesses requires elaborate understanding on the voltage drop across SOFC components. In this work, efforts are made to study the voltage drop across various layers of SOFCs on a Ni-GDC based anode-supported cell. The anode-supports were deposited with ESB-GDC bilayer electrolytes with varying r ESB/GDC and h. In particular, the voltage drop across ESB (V ESB), GDC+anode (V GDC+anode) and the whole cell (V total) were determined at open circuit voltage and under load conditions. These measurements were performed by positioning the voltage probes between ESB/GDC and ESB/cathode. The behavior of V ESB, V GDC+anode, and V total with a varying r ESB/GDC and h were monitored and analyzed. Specific losses that could potentially limit the overall SOFC performance with reference to electrolyte thicknesses are addressed. [1] E.D. Wachsman, K.T. Lee, Science, 334 (2011) 935-939.

Development of Stabilized Bismuth Oxide Based Composite Cathodes for Low Temperature SOFCs

Commercialization of SOFCs will be advanced by the development of cell materials and components which operate well at lower temperatures. Lower operating temperatures allow the use of inexpensive ferritic stainless steels as interconnect, improve the chemical and mechanical stability of the cells, and increase stack lifetime. However, oxygen reduction reaction (ORR) kinetics at cathode of the cell is slower at lower temperatures, which significantly reduces the maximum power density of the cell. Electrochemical impedance spectroscopy (EIS) measurements on anode supported bilayer electrolyte SOFCs at 600oC with stabilized bismuth oxide based composite cathode indicated that the non-ohmic polarization accounted for half to two-third of the total cell area specific resistance (ASR). In this work, composite cathodes of various compositions, i.e. weight ratio of electron conducting phase (LSM or BRO) to stabilized bismuth oxide, were fabricated and tested on symmetrical cells using EIS. The starting powder was made by solid-state and wet-chemical methods followed by ball milling to achieve different particle size distributions. Reactivity of stabilized bismuth oxide and electron conducting phase at high temperatures was studied using XRD. EIS spectrum was studied to deconvolute the kinetics of ORR and the performance of the composite cathode as a function of weight ratio and particle size distribution of the two phases was looked at. Lastly, long term performance stability of selected cathode compositions was determined.

Numerical modeling of ceria-based SOFCs with bi-layer electrolyte free from internal short circuit: Comparison of two cell configurations

A numerical model for ceria-based solid oxide fuel cells (SOFCs) with bi-layer electrolyte is proposed to evaluate the internal short circuit by the comparison of two cell configurations: the electronic barrier electrolyte adjacent to cathode and anode, respectively. In this model, the activation polarization of the electrode reaction and the charge transport of the electrolyte with both n/p-type electronic and oxygen ion conductivity are considered. The activation polarization and the charge transport are described by the Butler-Volmer equation and the Nernst-Planck equation, respectively. Parametric simulations are performed to compare the two bi-layer electrolyte configurations in terms of the open circuit voltage, I–V relationship, leakage current density, power density at 0.7V, oxygen partial pressure distribution and electrochemical efficiency as functions of the temperature and thickness ratio of the electronic barrier electrolyte. From our modeling results, the cell configuration of which the barrier electrolyte is adjacent to cathode has significant p-type leakage current, leading to the lower open circuit voltages and electrochemical efficiency than the other one. The oxygen partial pressure distribution under the open circuit displays the “S” type in the barrier layer, which is related to the change of the n/p-type conductivity of the barrier layer. Besides, the activation polarization greatly influences the open circuit voltage and the oxygen partial pressure distribution between boundaries of electrolytes under open circuit. It is also found that the thickness ratio of the electronic barrier electrolyte can be optimized to maximize the electrochemical performance by balancing the open circuit voltage and ohmic polarization loss.

Performance Characteristics of Solid Oxide Fuel Cells with YSZ/CGO Electrolyte

This paper presents the results of performance evaluation of anode-supported solid oxide fuel cells (SOFC) with magnetron sputtered YSZ/CGO bilayer electrolyte, and composite LSCF-CGO cathode. Deposition of the YSZ/CGO electrolyte with the thickness of up to 14 microns was performed on the commercial anode substrates with dimensions of 5×5 cm2. The LSCF-CGO cathode of the fuel cells was formed by the screen-printing method. The microstructure of the YSZ/CGO bilayer electrolyte and LSCF-CGO cathode was studied by scanning electron microscopy. Comparison of the fuel cells performance with different thicknesses of the YSZ and CGO layers was carried out by measuring current-voltage and power characteristics, and also by testing the long-term stability of cell power at the temperature of 750 °C and voltage of 0.7 V.

A Preliminary Study on Bi-Layered Ceramic Electrolytes of Mixed Conductor and Proton Conductor

In this contribution, we present an experimental study of single-layered electrolyte with mixed ion and electron conducting ceramic material SDC (Sm0.2Ce0.8O2-x) and bi-layered electrolyte of the SDC layer with proton conducting ceramic material BCY (BaCe0.9Y0.1O3-x) to enhance the electrolyte performance. Anode supported cell structure was adopted and the button cells were fabricated through conventional tape casting-screen printing and high temperature sintering processes. It was found that the open circuit voltages of the bi-layered electrolyte cells were higher than that of the single-layered SDC electrolyte cells. The bi-layered cells showed a decent performance in the temperature range of 500-650°C. Interaction between the two electrolyte materials occurred at high sintering temperature, which led to an increase in electrolyte resistance as well as electrode polarization values.

Spatial Distribution of Oxygen Chemical Potential Profile across Zr0.84Y0.16O1.92 / Ce0.9Gd0.1O1.95 Bilayer Electrolyte under SOFC Operating Conditions

Partial electronic conductivity and total conductivity of Zr0.84Y0.16O1.92 and Ce0.9Gd0.1O1.95 have been measured by Hebb-Wagner polarization method and D.C. 4-probe conductivity measurements, respectively as a function of temperature and oxygen partial pressure. (600 ≤ temp/˚C ≤ 900, 10-18 ≤ PO 2/atm ≤ 0.21) Theoretical current-voltage characteristics and power density were calculated by integrating partial conductivities of charge carriers under various DC bias condition at fixed oxygen chemical potential gradient at both sides of the electrolyte (YSZ side: PO 2=10-22 GDC side: PO 2=0.21). The spatial distribution of oxygen partial pressure across the bilayer electrolyte with various thickness ratio was calculated based on Choudhury and Patterson’s model by considering zero electrode polarization. From the spatial distribution, oxygen partial pressures of interface were calculated with various operating condition. Interfacial oxygen chemical potential with various thickness ratio (YSZ/GDC) shows strong dependence upon cell operating potential.

Ceria/Bismuth Oxide Bilayer Electrolyte based Low-Temperature SOFCs with Stable Electrochemical Performance

SOFCs are electrochemical devices that directly convert the electrochemical energy of a fuel into electricity. One common approach to lower the operating temperature of the SOFC is to replace the widely used yttria – stabilized zirconia (YSZ) with higher conductivity materials such as doped ceria. Doped-ceria materials such as GDC, are mixed ionic electronic conductors (MIECs) that are reduced from Ce4+ to Ce3+ in a fuel atmosphere, resulting in electronic conduction in the electrolyte by small-polaron hopping mechanism. In order to block the electronic leakage current through the electrolyte that induces lower OCV and power dissipation, we are developing advanced SOFCs based on ceria/bismuth oxide bilayer electrolyte, which can operate at low temperatures (450-650 °C). Stabilized-Bi2O3 has been reported to be an ionic conductor under oxidizing conditions that can effectively suppress the leakage current through GDC. Recently developed stabilized bismuth oxide (SBO) electrolyte material, which showed stable conductivity at lower temperatures was used to block the electronic leakage current. In this work, the effect of SBO/GDC thickness ratio and overall thickness of the electrolyte on the OCV of the cell was studied. Thickness of GDC and SBO layers was varied. The results showed the addition of SBO layer was effective in blocking electronic conduction and increased the OCV compared to the baseline single layer GDC cell. Further, the relative and total thickness of the two layers had a significant impact on the OCV of the cell at different temperatures with best performance obtained with cells with lower GDC thickness and higher SBO thickness. Lastly, long-term stability tests showed exceptional low degradation rates for both OCV and power density of the bilayer SBO/GDC cells.

A Preliminary Study on Bi-Layered Ceramic Electrolytes of Mixed Conductor and Proton Conductor

In this contribution, we present an experimental study of single-layered electrolyte with mixed ion and electron conducting ceramic material SDC (Sm0.2Ce0.8O2-x) and bi-layered electrolyte of the SDC layer with proton conducting ceramic material BCY (BaCe0.9Y0.1O3-x) to enhance the electrolyte performance. Anode supported cell structure was adopted and the button cells were fabricated through conventional tape casting, screen printing and high temperature sintering processes. It was found that the open circuit voltages of the bi-layered electrolyte cells were higher than that of single-layered SDC electrolyte cells. The bi-layered cells showed a decent performance in the temperature range of 500-650°C. Interaction between the two electrolyte materials occurred at high sintering temperature, which led to an increase in electrolyte resistance as well as electrode polarization values.

Ceria/Bismuth Oxide Bilayer Electrolyte based Low-Temperature SOFCs with Stable Electrochemical Performance

In this work, we report on the fabrication and performance of anode supported cells based on gadolinium doped ceria (GDC) and stabilized bismuth oxide (xSB) bilayer electrolyte that can operate at low temperatures (450-650 °C). Thickness of GDC and xSB layers was varied between 23-70 μm and 0-25 μm, respectively. The results showed the addition of xSB layer was effective in blocking electronic conduction which increased the OCV compared to the baseline single layer GDC cell. Further, the relative and total thickness of the two layers showed a significant impact on the OCV of the cell at different temperatures with best performance obtained with cells with lower GDC thickness and higher xSB thickness. Long-term stability tests showed exceptionally low degradation rates for OCV of the bilayer GDC/xSB cells. At 650 oC, a GDC/xSB bilayer fuel cell showed a maximum power density and total area specific resistance (ASR) of 1.06 W/cm2 and 0.17 Wcm2, respectively.

Spatial Distribution of Oxygen Chemical Potential Profile across Zr0.84Y0.16O1.92 / Ce0.9Gd0.1O1.95 Bilayer Electrolyte under SOFC Operating Conditions

Theoretical current-voltage characteristics and power density were calculated by integrating partial conductivities of charge carriers under various DC bias condition at fixed oxygen chemical potential gradient at both sides of the electrolyte (YSZ side: PO 2=10-22 GDC side: PO 2=0.21). The spatial distribution of oxygen partial pressure across the bilayer electrolyte with various thickness ratios was calculated based on Choudhury and Patterson’s model by considering zero electrode polarization. From the spatial distribution, oxygen partial pressures of an interface were calculated with the various operating condition. Interfacial oxygen chemical potential with different thickness ratios (YSZ/GDC) shows strong dependence upon cell operating potential.

Development of Ceramic Materials and Application of Novel Physical Analysis Methods to Enhance Solid Oxide Fuel Cells and Solid Oxide Electrolysis Cells

Different physical (FIB-TOF-SIMS, SXAS, XRD, high-temperature XRD under electrochemical working conditions, FIB-SEM-EDX, AFM, gas chromatography-mass spectrometry, BET) and electrochemical (cyclic voltammetry, chronoamperometry and electrochemical impedance) methods have been applied for detailed analysis of solid oxide fuel cell materials. Solid oxide fuel and co-electrolysis cell single cells based on Ni-free anodes/cathodes as well as Ni-Ce0.9Gd0.1O2- d or Ni-Zr0.92Y0.08O2- d supporting anodes/cathodes, bi-layered Zr0.92Y0.08O2- d | Ce0.9Gd0.1O2- d or Zr0.94Sc0.06O2- d | Ce0.9Gd0.1O2- d electrolytes and micro-meso-porous La0.6Sr0.4CoO3- d, Gd0.6Sr0.4CoO3- d, La0.6Sr0.4Co0.8Fe0.2O3- d or Pr0.6Sr0.4CoO3- d cathodes/anodes, respectively. For preparation of electrodes and electrolytes various nano-microporous powders, different sintering temperatures and pore former additions in the raw cathode and anode pastes have been applied. Ni-free anodes/cathodes have been synthesized using the impregnation method and tested at temperatures from 650 to 800 °C. Influence of the electrodes porosity, chemical composition and also of the bi-layered electrolyte properties on the power density of the SOFC and SOEC single cells has been established and analyzed. The synchrotron radiation based X-ray absorption spectroscopy under electrochemical polarization and different fuel feeding (pressure, gas flow rate, etc.), chemical composition (H2S and other sulphur containing compounds, halides, etc.) and temperature cycling conditions has been applied for chemical analysis of surface composition of the SOFC anodes.

Scale‐up of Solid Oxide Fuel Cells with Magnetron Sputtered Electrolyte

AbstractThe possibility of fabricating large‐area solid oxide fuel cells (SOFC) with thin film electrolyte using a commercial physical vapor deposition technology is investigated. Yttria‐stabilized zirconia (YSZ)/gadolinium‐doped ceria (GDC) bilayer electrolyte is successfully deposited on a 10 × 5 cm2 commercial NiO/YSZ anode support by reactive magnetron sputtering. The microstructure of the fuel cells was studied by scanning electron microscopy. Current‐voltage characteristics of fuel cells at a temperature of 750°C and their power stability under electrical load were investigated. Single cells with La0.6Sr0.4Co0.2Fe0.8O3/ Gd0.1Ce0.9O1.95 (LSCF/GDC) cathode had an open cell voltage of 1.14 V and a maximum power density of 490 mW cm−2 at 750 °C using H2/N2 gas mixture as fuel and air as the oxidant. Three‐cell planar SOFC stack using 10 × 5 cm2 anode‐supported unit cells with power density of 450 mW cm−2 at a voltage of 0.7 V per cell has been assembled and tested.

Low-temperature co-sintering for fabrication of zirconia/ceria bi-layer electrolyte via tape casting using a Fe2O3 sintering aid

Abstracts Bilayer electrolytes have potential in solid oxide cells to improve ionic conduction whilst blocking electronic conduction. GDC/YSZ bilayer electrolyte processing has proven problematic due to thermochemical instability at high sintering temperatures. We first match the shrinkage profile of the two bulk materials using a Fe 2 O 3 sintering additive. Additions of 5 mol% of Fe 2 O 3 in the GDC layer and 2 mol% of Fe 2 O 3 in the YSZ layer prevents delamination during co-sintering. The addition of Fe 2 O 3 promotes densification, enabling achievement of a dense bilayer at a reduced sintering temperature of 1300 °C; ∼150 °C below conventional sintering temperatures. Elemental analysis showed the compositional distribution curves across the bilayer interface to be asymmetric when Fe 2 O 3 is employed. The Fe 2 O 3 increases the total conductivity of the bilayer electrolyte by an order of magnitude; this is explained by the effect of Fe 2 O 3 on reducing the resistive solid solution interlayer at YSZ/GDC interface from ∼15 to ∼5 μm.

Functionally Graded Bismuth Oxide/Zirconia Bilayer Electrolytes for High-Performance Intermediate-Temperature Solid Oxide Fuel Cells (IT-SOFCs).

A functionally graded Bi1.6Er0.4O3 (ESB)/Y0.16Zr0.84O1.92 (YSZ) bilayer electrolyte is successfully developed via a cost-effective screen printing process using nanoscale ESB powders on the tape-cast NiO-YSZ anode support. Because of the highly enhanced oxygen incorporation process at the cathode/electrolyte interface, a novel bilayer solid oxide fuel cell (SOFC) yields extremely high power density of ∼2.1 W cm-2 at 700 °C, which is a 2.4 times increase compared to that of the YSZ single electrolyte SOFC.

Experimental and thermodynamic study on the performance of water electrolysis by solid oxide electrolyzer cells with Nb-doped Co-based perovskite anode

In this work, Solid Oxide Electrolyzer Cell (SOEC) based on Ba0.9Co0.7Fe0.2Nb0.1O3-δ (BCFN) air electrode and YSZ-GDC bilayer electrolyte was systematically investigated and the efficiency of high-temperature water electrolysis by such a cell was analyzed. Firstly, chemical compatibility test between BCFN and YSZ showed that BaZrO3 formed after heat treatment at 1000°C for 5h, which adversely influenced the performance of BCFN dramatically. A fully dense GDC interlayer was thus developed by co-sintering GDC layer, with addition of 0.5at.% Fe2O3, with YSZ electrolyte at only 1300°C. The as-prepared fuel electrode-supported eletrolyzer cell consisting of Ni-YSZ fuel electrode, YSZ-GDC bilayer electrolyte and BCFN air electrode was evaluated for water electrolysis. Specifically, at 800°C using a feedstock of 60% H2O-40% H2, the cell showed total area specific resistance of 0.195Ωcm2 and the cell voltage was 1.13V with an electrolysis current of 1Acm−2. After short-term stability test for 120h with 1Acm−2 electrolysis current at 800°C, the cell showed no microstructural changes as observed by scanning electron microscopy. At last, a high-temperature water electrolysis system based on the cell studied was proposed and the system analysis shows that the overall electricity to hydrogen efficiency can reach 73% based on lower heating value of hydrogen, with a hydrogen generation rate of 4180Lh−1m−2.

Electrical and microstructural characterization of Gd-doped Ceria/Y-doped Zirconia bi-layer electrolyte prepared by tape casting

GDC-YSZ bi-layer SOFC electrolytes are an interesting manner to protect GDC electrolytes of the low oxygen partial pressure present at anode-electrolyte interface. This oxygen partial pressure condition and high temperatures during the operation of SOFC permit GDC electrolyte reduction injuring the performance of SOFC. In this work, GDC-YSZ bi-layer electrolytes were prepared using double tape casting technique. Single layers of YSZ and GDC electrolytes were also prepared by the same technique for comparison. All samples were sintered at 1600 °C/2 h in air and analyzed by SEM and impedance spectroscopy technique. A good adhesion was established between the layer GDC and YSZ and no cracks were detected despite the differential firing shrinkage. The electrical conductivity of GDC-YSZ bi-layer sample at 350 °C was 7.98x10-5 S.cm-1 and for GDC and YSZ single layer samples electrical conductivities were 1.89x10-4 S.cm-1 and 4.71x10-5 S.cm-1, respectively. These results show that GDC-YSZ bi-layer electrolyte is superior to YSZ single layer electrolyte.

PEALD YSZ-based bilayer electrolyte for thin film-solid oxide fuel cells

Yttria-stabilized zirconia (YSZ) thin film electrolyte deposited by plasma enhanced atomic layer deposition (PEALD) was investigated. PEALD YSZ-based bi-layered thin film electrolyte was employed for thin film solid oxide fuel cells on nanoporous anodic aluminum oxide substrates, whose electrochemical performance was compared to the cell with sputtered YSZ-based electrolyte. The cell with PEALD YSZ electrolyte showed higher open circuit voltage (OCV) of 1.0 V and peak power density of 182 mW cm−2 at 450 °C compared to the one with sputtered YSZ electrolyte(0.88 V(OCV), 70 mW cm−2(peak power density)). High OCV and high power density of the cell with PEALD YSZ-based electrolyte is due to the reduction in ohmic and activation losses as well as the gas and electrical current tightness.

Electrochemical Behavior of Li4Ti5O12 in Ionic Liquid/Water Bilayer Electrolyte

Introduction Lithium-ion batteries (LIBs) have been used in portable electronic devices, and recently expected to be used in large-scale energy facilities for electric vehicle or large energy storage systems. Flammable organic solvents were used in LIBs and it threaten the safety of LIBs. Therefore, nonflammable aqueous electrolytes have been investigated[1]. However, its narrow potential windows restrict the use of negative electrodes, which makes it difficult to obtain high voltage. In this study, we utilized stable liquid-liquid interface between water and hydrophobic ionic liquids in order to enhance the negative potential window. The electrochemical behavior of Li4Ti5O12(LTO) thin-film electrode, which cannot be used in aqueous solution, was investigated. Experimental LTO thin-film electrodes were prepared on a Pt substrate by a sol-gel method. LTO thin-film electrode was used as a working electrode. The bilayer electrolyte was formed on the working electrode: the upper layer was 1.0 mol dm −3 LiNO3 aqueous solution, and the lower layer was butyltrimethylammonium bis(trifluoromethylsulfonyl)amide (BTMATFSA) containing 1.0 mol kg −1 of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA). The counter electrode was platinum wire and the reference electrode was Ag/AgCl electrode (sat. KCl), and these electrodes were placed in the aqueous solution layer. Cyclic voltammetry was performed. Hereafter, all potentials are referred to as Ag/AgCl. Results Figure 1(a) shows the cyclic voltammograms of LTO thin-film electrode in bilayer electrolyte. At volume ratio of ionic liquid:aqueous solution = 3:4, clear redox peaks around ca. −1.33 V were observed. This reaction can be assigned to lithium-ion insertion/extraction into/from LTO. In contrast, Figure 1(b) shows that only large reduction current was observed and the redox peaks of LTO were not detected at volume ratio = 1:29. At this ratio, ionic liquid was miscible with aqueous solution. Thus, the reduction current below 0.9 V can be assigned to the hydrogen evolution reaction from water. These results reveal that bilayer electrolyte with clear interface prevent the hydrogen evolution reaction and enable lithium-ion to insert/extract into/from LTO. Reference [1] W. Li, J. R. Dahn, and D. S. Wainwright, Science, 264, 1115 (1994). Acknowledgment This work was partially supported by CREST, JST. Figure 1