Showing papers in "Electrochemistry in 2007"

Ionic Liquids for Electrochemical Devices

Abstract: A short review of ionic liquids (ILs) and their applications as electrolytes for electrochemical devices, such as electric double layer capacitors, fuel cells, lithium batteries, and solar cells, are presented here. The properties of ILs, such as non-volatility, non-flammability, wide liquid temperature ranges, and wide electrochemical windows, have the potential to be improved, including improvements in durability and safety, extending the operational temperature ranges and enabling improvements in power and energy densities of the devices.

Physical and Electrochemical Properties of Room Temperature Ionic Liquids Based on Quaternary Phosphonium Cations

Abstract: Room temperature ionic liquids based on tri-n-butylalkylphosphonium cations were physically and electrochemically characterized. Bis(trifluoromethylsulfonyl)imide-based phosphonium ionic liquids exhibited relatively low viscosities. Tri-n-butyloctylphosphonium cation gave low melting salts together with a wide variety of anions. Most phosphonium ionic liquids were thermally stable up to nearly 400°C. Linear sweep voltammetry showed that potential windows of bis(trifluoromethylsulfonyl)imide-based phosphonium ionic liquids were at least 5.7 V when a glassy carbon electrode was used. In cyclic voltammetric measurement, the redox response of Li in a phosphonium ionic liquid was observed.

Electrochemical Impedance and Complex Capacitance to Interpret Electrochemical Capacitor

Abstract: The electrochemical impedance Z and complex capacitance C for typical equivalent circuits were summarized systematically in order to support the frequency domain analysis of electric double layer capacitance (EDLC). In the present paper, the impedance and the complex capacitance were calculated in the case that the electrochemical response of porous electrode was presented by a transmission line model (TLM). The loci of Z and C on the complex were divided into the lumped constant and distributed constant ranges in low and high frequency ranges, respectively. The complex capacitance plot was superior to obtain the capacitance of porous electrode than the impedance plot.

Performance of Electrolyte Composed of Spiro-type Quaternary Ammonium Salt and Electric Double-layer Capacitor Using It

Abstract: An electrolyte composed of a tetra-alkyl ammonium cation having a spiro structure (spiro-(1,1′)-bipyrrolidinium: SBP) showed excellent performance in terms of electric conductivity, viscosity, and solubility in various solvents, especially propylenecarbonate (PC). The electric double-layer capacitor (EDLC) using spiro-(1,1′)-bipyrrolidinium tetrafluoroborate in PC (SBP-BF4/PC) has a performance better than that using tetraethylammonium tetrafluoroborate in PC (TEA-BF4/PC) and triethylmetylammonium tetrafluoroborate in PC (TEMA-BF4/PC), especially at low temperatures and high discharging rates.

Plastic Dye-sensitized Photovoltaic Cells and Modules Based on Low-temperature Preparation of Mesoscopic Titania Electrodes

Abstract: Recent trend in industries toward the development of thin opto-electronic devices requires flexibility and safety for versatile applications including ubiquitous electric generators. Dye-sensitized mesoporous semiconductor electrodes can be a powerful candidate for fabrication of low-cost flexible photovoltaic devices. This review introduces low-temperature coating methods for fixing mesoporous TiO2 films on conductive plastic electrodes. Characteristics of plastic dye-sensitized solar cells assembled by this technology are also summarized, including outdoor performance of large-area full-plastic modules in comparison with commercial silicon-based solar batteries.

Membrane Degradation in Polymer Electrolyte Fuel Cells under Low Humidification Conditions

Abstract: A durability test of a PEFC single cell using perfluorinated ionomer membrane as an electrolyte was carried out at 80°C under low humidification. The observed voltage drop under low humidification was mostly reversible up to 5200 h. However, hydrogen crossover increased with time and fluoride-ion was continuously detected in drain water, which indicated that membrane degradation proceeded steadily during the test under low humidification. Fluoride-ion release rate decreased with increasing current density, which suggested that hydrogen peroxide was formed at the anode catalyst layer upon direct combustion of hydrogen with crossover oxygen. Drain water analysis revealed that large amounts of sulfate ions and Fe(II or III) ions were released in addition to fluoride ions when the cell was temporarily operated under full humidification. From this fact, it was concluded that accumulation of impurities and hydrogen peroxide under low humidification is a reason why low humidification enhances membrane degradation.

Chemical Capacitors and Quaternary Ammonium Salts

Abstract: A capacitor is an electrical device that can store energy in an electric field between a pair of closely spaced conductors. Its application as an energy storage device —an alternative to rechargeable batteries— has been receiving considerable attention. There are chemical capacitors using liquid electrolytes such as aluminum electrolytic capacitors and double-layer capacitors, and they utilize quaternary ammonium salts in their nonaqueous electrolytes. After a brief explanation of the working principles of these chemical capacitors and their requirements for electrolytes, the practical applications of quaternary ammonium salts for aluminum electrolytic capacitors and double-layer capacitors are reviewed from the historical and technological viewpoints based on research conducted in our laboratory.

Electronic Behavior of WO2/Carbon Clusters Composite Materials

Abstract: Calcinations of a W12O41·[C5H5N(CH2)12CH3]7.8 complex under an argon atmosphere gave nano-sized WO2/carbon clusters composite materials. ESR spectral examinations suggest the possibility of an electron transfer from the carbon clusters to the WO2 particles.

Electrochemical Corrosion of Carbon Materials in an Aqueous Acid Solution

Abstract: In this fundamental study on corrosion of the carbon catalyst support in a PEMFC cathode, corrosion rates of electrodes made of carbon powder were investigated by measuring CO2 generation. The corrosion rate was affected by not only the retention potential but also by the presence of dissolved oxygen in the electrolyte. The corrosion rate showed two stages; initial corrosion was observed for several hours, and was followed by steady-state corrosion. Even in this latter stage, change to a lower potential induced a peak in the corrosion rate that exceeded the previous steady value just after the potential was restored. Corrosion was also accelerated by platinum catalyst.

Electrochemical Formation of Porous Silicon with Medium-Sized Pores

Abstract: Medium-sized pores of which diameter was over 100 nm were successfully prepared by anodization in 3 wt% HF aqueous solution containing KMnO4 and surfactant. The presence of an appropriate oxidizing agent and a surfactant was indispensable for the formation of medium-sized pores, but the concentration dependence was not significant. Similar method was applied to the p+-type silicon and led to the pore widening.

Electrochemical Quartz Crystal Microbalance Study on Dissolution of Platinum in Acid Solutions

Abstract: The influence of chloride ion on the oxidation and dissolution of platinum has been studied by using Electrochemical Quartz Crystal Microbalance (EQCM) in combination with Inductively Coupled Plasma (ICP) analysis to clarify the deterioration mechanism of platinum catalyst in the Polymer Electrolyte Fuel Cell (PEFC). Platinum was electroplated on an Au-QCM electrode in 2% H2PtCl6 solution and used as the working electrode. A mass gain of the platinum electrode was observed during anodic polarization in 0.5 M H2SO4 solution ([Cl−] free solution) due probably to the formation of Pt oxide. The oxide film was not stable in the whole range of potential used in this study. On the other hand, a mass loss due to the dissolution of Pt as chloride complex ions was observed in more positive potentials than 0.85 V vs. SHE in chloride-containing solutions. As the chloride concentration increased, platinum showed a higher dissolution rate with the formation of chloride complexes, PtCl42− and PtCl62−. The amount of dissolved PtCl62− tended to increase with the increase of anodic potential limit. The EQCM results revealed that the dissolution of Pt was accelerated when potential was cycled between cathodic and anodic limits. The ICP analysis of the solution after anodic polarization supported the results obtained by EQCM.

Highly Conductive Electrolytic Solution for Electric Double-layer Capacitor Using Dimethylcarbonate and Spiro-type Quaternary Ammonium Salt

Abstract: Use of a novel electrolyte, a spiro-type quaternary ammonium salt (SBP-BF4), in an electric double-layer capacitor (EDLC) has allowed the use of a linear-structure carbonate–which, although it has a low viscosity, had never been used as a solvent because it does not dissolve a conventional electrolyte–as a sub-solvent for an electrolytic solution in combination with a main solvent, propylenecarbonate (PC). Of such linear-structure carbonates, use of dimethylcarbonate (DMC) in an electrolytic solution attains not only low viscosity but also high conductivity of an electrolytic solution. Furthermore, an EDLC using the electrolytic solution exhibits very excellent capacitance, internal resistance, temperature properties, and discharging rate.

Bimodal Porous Carbon as a Negative Electrode Material for Lithium-Ion Capacitors

Abstract: Bimodal porous carbons consisting of interconnected macropores and spherical mesopores, were prepared by colloidal crystal templating method. The template was prepared by evaporation process of suspensions containing monodisperse poly[styrene-(co-2-hydroxyethyl methacrylate)] (PSHEMA) latex and colloidal silica in water. The carbonization of PSHEMA at 1000°C under inert atmosphere provided very thin carbon layer on the colloidal silica in the template, and the macropore corresponding to the PSHEMA particle size were formed simultaneously. After this procedure, bimodal porous carbons were obtained by removing the silica particles with hydrofluoric acid. Three kinds of bimodal porous carbons were prepared using PSHEMA latex of 450 nm and colloidal silica with three different average diameters of 10∼20 nm, 40∼50 nm, and 70∼100 nm, respectively. Bimodal porous structure was observed with a field emission-scanning electron microscope. Nitrogen adsorption/desorption measurements revealed that the prepared samples involved macropore and mesopore with small amount of micropore. The bimodal porous carbons were electrochemically evaluated as a negative electrode of lithium-ion capacitor in ethylene carbonate and diethyl carbonate containing 1 mol dm−3 LiClO4. The bimodal porous carbon prepared using silica of 10∼20 nm showed a large capacitance of 360 F g−1 at a high current density of 7.4 A g−1.

Oxygen Reduction Reaction of Ta-C-N Prepared by Reactive Sputtering with Heat Treatment

Abstract: Ta-C-N electrocatalysts were prepared by reactive R.F. sputtering with heat treatment in the temperature range from 70 to 800°C. The Ta-C-N was found to have high electrochemical stability in 0.1 mol dm−3 H2SO4. The catalytic activity for the oxygen reduction reaction of the sputtered Ta-C-N increased with the increasing heat treatment temperature. In particular, the current of the ORR on the Ta-C-N with the heat treatment temperature of 800°C was observed at about 0.73 V vs. RHE. It was found that the crystallinity of the Ta-C-N would affect the catalytic activity for the ORR.

High-performance Surface Plasmon Resonance Immunosensors for TNT Detection

Abstract: Detection of 2,4,6-trinitrotoluene (TNT) is an important environmental, security and health concern for the global community. TNT is a prime constituent of most of the landmines and bombs and is highly toxic and mutagenic. Various military and terrorist activities (e.g., manufacturing, waste discharge, testing and training) have resulted in extensive contamination of soil and ground water by TNT and its derivatives. Consequently, the development and application of new sensing techniques for detection and quantification of TNT has grown steadily over the years. Despite wide variety of analytical techniques, surface plasmon resonance (SPR) based immunosensors received great attention as a promising mean for TNT detection due to their advantages including high sensitivity, selectivity, good versatility and high throughput. This review explores the recent trend and advancements in immunochemical techniques for environmental monitoring and field detection of TNT. The advantages of the surface plasmon resonance as an optical signal transduction and indirect competitive immunoassay as the sensing principle are discussed with special emphasis on our investigations on TNT detection. A brief description on explosives, land-mines and the current detection techniques (bulk and trace detection) is also provided.

Accelerated Electron-Transfer Reaction of the O2/O2− Redox Couple on Multi-Walled Carbon Nanotubes—Modified Edge-Plane Pyrolytic Graphite Electrode in Room-Temperature Ionic Liquids

Abstract: An electrocatalysis of the multi-walled carbon nanotubes (MWCNTs)/Nafion-modified edge-plane pyrolytic graphite (EPPG) electrode for the one-electron reduction-oxidation reaction of the O2/O2− (superoxide ion) redox couple in three 1-n-alkyl-3-methylimidazolium tetrafluoroborate room temperature ionic liquids (RTILs), 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4), 1-n-propyl-3-methylimidazolium tetrafluoroborate (PMIBF4) and 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4) has been found We have observed that in all the RTILs both the cathodic and anodic peak currents and the peak potential separations (ΔEp) between the anodic and cathodic peak potentials at the MWCNTs/Nafion-modified electrode are increased and decreased, respectively, compared with those at the bare electrode The values of the standard rate constant (k0) were estimated from the ΔEp values, ie, the k0 values at the MWCNTs/Nafion-modified electrode are (83±01)×10−3, (104±01)×10−3 and (42±01)×10−3 cm s−1 in EMIBF4, PMIBF4 and BMIBF4, respectively, while the corresponding values at the bare electrode are (43±01)×10−3, (29±01)×10−3 and (23±01)×10−3 cm s−1, respectively The observed enhancement of the O2/O2− redox reaction is discussed briefly based on the catalytic effects and physicochemical properties of carbon nanotubes

Molecular Orbital Study on the Oxidation Mechanism of Hydrazine and Hydroxylamine as Reducing Agents for Electroless Deposition Process

Abstract: The oxidation mechanism of hydrazine and hydroxylamine were investigated using molecular orbital (MO) calculation. Two pathways for their reactions were verified. One is initiated by hydrogen elimination from the reducing agent, followed by coordination of hydroxyl group to center nitrogen atom accompanied with electron emission. Another is initiated by coordination of hydroxyl group to hydrogen atom to form H2O, which is eliminated later, followed by electron emission. The calculated results indicated that the oxidation reactions of hydrazine preferentially proceeded via the second pathway. It was also indicated that only the first electron emission steps of the hydrazine oxidation took place on Cu surface, while the following reactions proceeded at near the solid/liquid interface. The oxidation of hydroxylamine also proceeded via the elimination reaction of H2O. After the oxidation, calculated heats of reactions suggested that OH radical, generated from N2O as a product, decomposed hydroxylamine.

Proton Exchange Fuel Cell Materials and R&D Needs for Future Market Success

Abstract: I very much appreciate the privilege to write a preface for this special issue of Electrochemistry on the development of materials and evaluation methods for PEFCs (proton exchange fuel cells). During my past years at General Motors’ Fuel Cell Activities program, I was able to witness the extraordinary world-wide advances in PEFC R&D, resulting in H2/air PEFC stacks which now closely meet the demanding performance requirements for vehicle applications: power densities near 1 W/cm2 at~0.6 V, and much reduced MEA (membrane electrode assembly) platinum loadings to~0.4 mgPt/cmMEA. These accomplishments were largely driven by the implementation of thin membranes (~25 μm), highly conductive bipolar plate materials/coatings, and empirical improvements in electrode design. While this clearly demonstrates the viability of automotive PEFC applications, their future success in the market place requires further advances in materials performance and durability. Performance: Current PEFC power densities and Pt loadings equate to a Pt-specific power density of~0.4 gPt/kW, compared to automotive targets of<0.2 gPt/kW dictated by both platinum cost and supply constraints. Therefore, platinum loadings must be further decreased to~0.15 mgPt/cmMEA; owing to the fast hydrogen oxidation kinetics on Pt catalysts with pure H2, the challenge in reducing MEA Pt loadings lies in the development of cathode catalysts with improved Pt mass activity for the oxygen reduction reaction (ORR). Pt-based catalyst concepts with improved ORR activity are Pt-alloys as well as shell/core approaches, where a thin Pt-shell is supported by a non-noble metal core. Since the ORR activity of catalysts not only depends on catalyst composition but also on catalyst morphology (manifested by the well known particle-size effect and by recent reports on the outstanding activity of the (111)-face of Pt3Ni surfaces), effective catalyst development will require a closer alignment with detailed atomically resolved structural characterization and molecular modeling approaches. The latter might also yield novel routes towards non-Pt ORR catalysts, which currently suffer from both low turnover frequencies and low active-site densities. Another important area of materials development relates to simplifying PEFC systems, primarily by reducing humidification requirements for the reactant streams which is necessitated by the poor proton-conductivity of currently used membranes at low relative humidity (RH). Thus, the pursuit of novel proton-conducting membranes with high conductivity (~0.1 S/cm) at<50%RH in the temperature range of 60 to 120°C should be a major materials development focus. As in the case of catalyst development, close coupling between molecular modeling and materials synthesis is desired. Durability: Probably the most critical needs for materials and diagnostics development are related to PEFC durability under automotive conditions. Cyclic variations in reactant RH result in mechanical membrane failures due to volume expansion/contraction cycles, and improved membranes together with a fundamental understanding of their mechanical properties are desired for robust PEFC systems. Cell voltage cycles caused by dynamic operation lead to accelerated Pt dissolution/sintering on the cathode; this is significantly reduced for Pt-alloy catalysts, but the mechanism is poorly understood. Nevertheless, while implementation of Pt-alloys alleviates Pt dissolution/sintering, the longterm stability of these materials as well of core/shell-type catalysts towards non-noble metal dissolution is unclear. Additional materials degradation is produced by PEFC stack start-up/shut-down transients, leading to the corrosion of conventionally used carbon-supports and concomitant MEA performance degradation. This is partially alleviated by the use of graphitized carbon-supports, but improved novel supports would enable more robust PEFC systems. A related effect inducing excessive carbon-support corrosion is referred to as localized H2-starvation, and is due to transient maldistribution of H2 in the anode flow-field. To understand and counteract this effect, predictive 3D twophase transport models, describing the distribution of reactants and liquid water are required in conjunction with experimental liquid water imaging methods. Finally, very little is known about the long-term integrity of the electrode structure, particularly the microscopic distribution of the proton-conducting ionomer with respect to the electron-conducting carbon phase. Since ionomer dissolution, migration, its chemical degradation, and/or agglomeration within the porous carbon network may lead to MEA performance degradation, advanced diagnostics and spatially resolved imaging methods are required to assess MEA degradation contributions from electrode morphology changes over time. The topic of this Special Issue of Electrochemistry is centered on the above outlined R&D challenges to meet the demanding performance and durability targets for automotive PEFC systems. Therefore, this issue will be instrumental in aiding the R&D community in their work toward bringing PEFC systems to commercialization. Proton Exchange Fuel Cell Materials and R&D Needs for Future Market Success

Improvement in corrosion properties of carbon-coated Fe-based metals for PEFC bipolar plate

Abstract: Low cost metals are quite attractive as bipolar plate of polymer electrolyte fuel cell (PEFC). However, low cost metals easily corrode due to the absence of good passivation films. In order to give high electrical conductivity and corrosion resistance to low cost metals such as carbon steel (S25C), carbon-coat was carried out by plasma-assisted chemical vapor deposition. Carbon-coated samples were characterized by XRD, Raman spectroscopy, and interfacial contact resistance. Based on the results of polarization measurement, the combination of carbon-coat and electroless nickel plating was found to be effective for decreasing of anodic dissolution of base metal.

Solubility of Platinum in Acidic Media (I)—in Sulfuric Acid

Abstract: Platinum depositions have been observed in the electrolyte membrane of polymer electrolyte fuel cells (PEFCs) as in other types of fuel cells; PEFCs, MCFCs. The platinum solubility in the membrane must be a key issue to improve and guarantee the durability of PEFCs. In this basic study, the platinum solubility has been determined in an acidic medium as a function of temperature, pH, and potential which was controlled by the oxygen partial pressure. The solubility was 3.0×10−6 mol dm−3 at 23°C in 1 mol dm−3 H2SO4 under air, and increased with temperature and the decrease in pH. The platinum solubility slightly increased with the oxygen partial pressure in oxygen–nitrogen mixtures. However, the solubility in nitrogen was much smaller than the mixtures. Based on these results, the dissolution of platinum in an acidic medium would follow the acidic dissolution mechanism.

Capacitance Enhancement of Aqueous EDLC Systems by Electrochemical Treatment

Abstract: The electrochemical treatment of activated carbon electrodes has been investigated as a novel method enhancing the capacitance of electric double layer capacitor (EDLC) electrodes. The treatment at 0.7 V floating in a week at 70°C can be applied to EDLC with aqueous electrolytes. In 30 wt% H2SO4/H2O electrolyte, the treatment causes a capacitance increase in a low voltage region: 0–0.4 V. On the other hand, in 3.5 M NaBr/H2O electrolyte, it results in a remarkable capacitance increase of 2.4 times in a high voltage region: 0.4–1.0 V. These results suggest that the treatment mechanisms in the above electrolyte systems should be different. We report herein the origins and mechanisms of these capacitance increases.

An Electrochemical Investigation of Additive Effect in Trench-Filling of ULSI Interconnects by Electroless Copper Deposition

Abstract: The filling of trenches in ULSI interconnect structure by electroless copper deposition was investigated for the effect of bath additives. The additive effect was found to depend strongly on the reducing agent used in the bath. Void-free trench-filling was achieved by using polyethylene glycol (PEG) as an inhibiting additive in the bath containing glyoxylic acid as the reducing agent, while the combined addition of 8-hydroxy-7-iodo-5-quinoline sulfonic acid (HIQSA) and PEG was necessary for achieving void-free filling in the bath containing formaldehyde as the reducing agent. The effect of PEG on trench filling in the former bath was studied in detail based on electrochemical measurements. It is suggested that the rinse water remaining in trenches before electroless deposition causes a decrease in PEG concentration at the trench bottom during copper filling. The addition of PEG was found to shift the deposition potential in the negative direction. A new potential measuring apparatus was devised and used in model experiments, which revealed that the deposition potential depends on the local concentration of PEG at the trench bottom, where it is expected to be low. The observed preferential growth of copper deposit at the trench bottom is thus attributed to the effects of the variation of PEG concentration within the trenches on the deposition rate and potential.

Nb2O5 Deposition on Aluminum from NbCl5-used Sol and Anodizing of Nb2O5-coated Al

Abstract: Formation of Nb2O5 films on aluminum was attempted by sol-gel coating, using NbCl5 as a precursor. The structure and the dielectric properties of anodic oxide films formed on aluminum by sol-gel coating and the subsequent anodizing were examined. Anodic oxide film formed by this process was composed of an inner Al2O3 layer and an outer Nb2O5 layer. The capacitance of the anodic oxide films formed on Nb2O5-coated specimens was at most 34% higher than that of anodic oxide films on aluminum without coating.

Beneficial Effects of a Li Salt on Electrode Behavior in an Ionic Liquid for Electric Double Layer Capacitors

Abstract: Origins of cathodic stability of an ionic liquid electrolyte, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) containing lithium tetrafluoroborate (LiBF4), are investigated mainly by applying electrochemical impedance spectroscopy for a glassy-carbon electrode interface. Its Helmholtz layer capacitance in the EMIBF4 electrolyte containing Li+ becomes much higher than that in neat EMIBF4 with increasing cathodic polarization. On the basis of such analyses as well as our hypotheses, a reasonable mechanism for the stabilizing effect induced by the Li salt addition is discussed.

Charge and Discharge Performance of Over-Discharged Lithium-ion Secondary Battery—Lessons Learned from the Operation of the Interplanetary Spacecraft HAYABUSA

Abstract: The Japanese spacecraft, HAYABUSA, was launched on May 9, 2003 and spent more than 2.5 years approaching the asteroid ITOKAWA. This spacecraft used 13.2 Ah lithium-ion secondary cells. After HAYABUSA touched down on ITOKAWA in December 2005, it could not communicate for seven weeks due to a malfunction of the attitude control. During this period, four of 11 lithium-ion secondary cells were over-discharged, and solar power was unavailable due to the spacecraft’s tumbling motion. However, the battery power was still indispensable for sealing the container with the asteroid sample. The seven remaining healthy cells were slowly recharged using minimum current. During this time, ground simulation tests using similarly-built and intentionally short-circuited cells were carried out to evaluate the battery’s operational safety. After its safety was confirmed, the lithium-ion secondary battery was used to transfer, latch, and successfully seal the sample container into the reentry capsule. The necessary power for these actions was supplied by the battery.

Microscopic Visualization of State and Behavior of Liquid Water in a Gas Diffusion Layer of PEFC

Abstract: Water flooding in a cathode gas diffusion layer (GDL) is one of the critical problems of polymer electrolyte fuel cell (PEFC) because liquid water condensed in the porous GDL blocks oxygen transport to the active reaction sites. In this study, we experimentally investigate state of liquid water in a porous electrode of PEFC cathode using an optical diagnostic system and show that it is affected by operating parameters and GDL thickness. Furthermore, microscopic behavior of condensed water inside a cathode GDL of operated PEFC is also predicted by using the GDL with a groove.

Carbon Nanotube and Carbon Black Supported Platinum Nanocomposites as Oxygen Reduction Electrocatalysts for Polymer Electrolyte Fuel Cells

Abstract: Pt supported on a binary support of multi-walled carbon nanotube (CNT) and carbon black (CB) with different ratio of CNT and CB (Pt/CNT-CB) has been prepared by reducing H2PtCl6 with ethylene glycol. Pt/CNT-CB (CNT/CB=50/50, 80/20), Pt/CB and Pt/CNT have been used as electrocatalysts for cathode in polymer electrolyte fuel cells (PEFC). In the low current density region, the cathode with a CNT/CB ratio of 50/50 shows the best performance as compared to CNT based catalyst. Whereas in the high current density region, the cathode with a CNT/CB ratio of 80/20 exhibits the best performance.

Property, Electronic and Crystal Structures, Thermodynamic Stability, and Cathode Performance of Lix(Mn, Co, Ni, M)O2 (M=Al, Ti, Fe) as a Cathode Active Material for Li Secondary Battery

Abstract: 1 緒 言 LiCoO2は合成法が容易であり,良好な電極特性を示すこ とから,Li二次電池正極材料として最も幅広く用いられてい る1,2).しかし,希少金属のCoを用いるためコスト面,環境 面での問題を抱えているため,その代替材料の研究が進めら れている.近年,LiCoO2の代替材料として,LiNixCo1-2xMnxO2 が多く研究されている.LiMn1/3Co1/3Ni1/3O2を初めとした これらの材料は,150 mAh/g(2.5~4.2 V vs. Li/Li+),200 mAh/g(2.5~4.6 V vs. Li/Li+)の可逆容量を示し,良好な サイクル特性を持ち,熱安定性にも優れることからLiCoO2 の代替材料の候補として注目されている7).しかし,Li+と Ni2+のイオン半径が近いためカチオンミキシングが生じてい ることが報告されている8).カチオンミキシングは放電容量 の劣化などを引き起こすことが知られているが,その充放電 時の挙動,定量に関してはほとんど報告されていない. 著者らはこれまでに,熱量測定により,Li二次電池正極活 物質における熱力学的安定性について報告してきた.Liリッ チな Li4Mn5O12,欠陥スピネル構造を持つ Li2Mn4O9及び LiMn2O4のMnサイトの一部を置換,またはLi組成を変化さ せた試料LixMn2-yMyO4(M=Li, Mg, Zn, Ni, Co),LixMn1yMyO2(M=Mn, Al, Cu),LixMn1/3Co1/3Ni1/3O2についてサイ クル特性と熱力学安定性について検討してきた9-16).これら の中で,熱力学的安定性,構造安定性,サイクル特性の間に 相関関係があることを報告してきた. 本報では,まず固相法,溶液法によりLix(Mn, Co, Ni, M) O2(M=Al, Ti, Fe)を合成し,試料のキャラクタリゼーシ ョン,電池特性について検討した.さらに,これらの試料と 化学的にLi組成を制御したLixMn1/3Co1/3-0.1Ni1/3Al0.1O2につい て,カロリーメトリーにより溶解熱を測定し,単純酸化物か ら目的の酸化物を得る反応の1 g原子あたりの反応のエンタ ルピー変化∆HR及び標準生成エンタルピー∆ fHを求め,そ の熱力学安定性について検討した.また,粉末中性子・X線 回折による結晶構造解析,マキシマムエントロピー法 (MEM)により原子核・電子密度分布の検討を行い,その構 造安定性についても合わせて検討した.これらの結果から, 本系における物性,熱力学的安定性,構造安定性とサイクル 特性の関係について検討した. ― Article―