Interfaces in ionic devices

The technology of Ionics is based on the availability of materials with fast ion transport. Individual materials are, however, meaningless from a practical point of view; all applications require combinations of materials with appropriate ionic and electronic properties. This situation is similar to Electronics which requires combinations of semi-conducting or metallic conducting materials with differences in the chemical potentials of the electrons. The technology of Ionics requires interfaces between ionic and electronic conductors which generate strong electrical fields or allow to modify the field by the application of external voltages. Ions and electrons equilibrate both at these “ionic junctions”. While semi-conductor junctions have commonly a width in the μm-range, the space charge region is several orders of magnitude smaller in the case of ionic junctions, i.e. in the nm or even sub-nm-range. The interfaces have to be chemically stable for the lifetime of the device which is difficult to achieve in view of the commonly large number of components present in both phases and the existence of mobile species with sometimes large variations in the activity of the electroactive component. Furthermore, the kinetics of transfer of ions across the interface has to be fast to allow high current densities which are required in many cases. In addition, two such interfaces are required to convert the electronic current into an ionic one and again back into an electronic current at the opposite side of the electrolyte. The development of ionic devices depends to the strongest extent on the engineering of appropriate interfaces. Examples of the role and engineering of interfaces will be presented for applicationes such as chemical sensors, electrochromic devices, fuel cells, batteries and photogalvanic solar cells.     

Fundamental electrochemical materials aspects of solid state fuel cells

The development of fuel cells and water electrolysis cells requires careful materials engineering in order to overcome the present limitations of degradation and high cost. For this reason, the fundamental parameters of operation of the galvanic cells are discussed. It has to be taken into account that the voltage drops over a very narrow region in the nanometer range which requires a high degree of materials stability. The electrical current depends on the redox processes at the interfaces and the electronic properties of the employed materials. The possibilities to control these parameters by dopants are discussed. Some general rules are concluded. Furthermore, long-term effects of cell polarization on the charge transfer resistance are reported. The described aspects of fuel cells may also provide a general guidance for the selection of appropriate materials for other solid state ionic devices. Paper presented at the 97th Xianshan Science Conference on New Solid State Fuel Cells, Xianshan, Beijing, China, June 14–17, 1998.     

Ionics—a key technology for our energy and environmental needs on the rise

Ionics is a key technology for storing, converting and using energy efficiently as well as protecting the environment. Major progress has been achieved in recent years in the understanding and development of individual materials components needed for ionic devices. It should be emphasized that only combinations of materials are eventually important and at least four interfaces exist with electronic and ionic junctions. The electrical fields exist over distances in the atomic range. Examples are given of recent successful developments of practically useful solids for lithium and oxide ion conduction in combination with appropriate electrodes. In addition, recent approaches to the design of ionic devices are described, notably the SEA concept for generating voltages in fuel cells and the coloration of single phase electrochromic materials. In order to overcome the tremendous problems in developing wide spread commercial applications, it is necessary to intensify our efforts in fundamental materials research drastically.     

The vision of ionics

Materials with fast ionic motion will have a major impact on the solution of some of our largest problems in areas of environmental protection and energy-storage, -conversion and-savings. A large variety of upcoming technologies is described which may not or not as well become realized without ionic materials. These include sensors, high performance batteries, electrochromic windows and displays, fuel and water electrolysis cells, chemotronics, semiconductor ionics, electrocatalysis, thermoelectric converters and photogalvanic solar cells.     

离子导体载流子输运与存储的界面调控

离子导电材料既可以是载流子主要为离子的纯离子导体,也可以是载流子同时包括离子和电子的混合离子导体.这两类材料是电化学能量转换与储存、化学传感以及选择性透过膜等器件的关键材料.在这些器件中,均存在电极与电解质、颗粒与颗粒、不同晶粒之间的二维或三维的异质结界面.因此,离子与电子在异质结中的输运性质对器件性能有重要的影响.但...     

Development and investigation of perovskite (ABO3)-type oxides for power generation

Critical problems of the present approach of electrolytes for Solid Oxide Fuel Cells (SOFCs) for commercialization are discussed. Major progress is expected from the development of materials based on the “SEA (Single Element Arrangement)” concept. The galvanic cell consists in this case basically of a single chemically homogeneous compound, which functions as electrodes at high and low activity and as electrolyte at intermediate activities of the electroactive component. In view of the large structural flexibility of the chemical nature of the constitutents, we explored perovskite (ABO₃)-type compounds to be used as SEAs for SOFCs. Results of studies on Pr-substituted LSGM and Fe-substituted SrSnO₃ perovskite-type oxides are presented. For instance, SrSn_1-xFe_xO_3-δ with x=0.1 exhibits p and n-type electronic conduction at the cathode and anode sides of the SOFC, respectively, while oxide ion conduction prevails at intermediate oxygen partial pressures. The SEA concept is also applicable for other devices in the field of Ionics.     

Electrode kinetic phenomena of solid state ionic devices

With the development of functional materials with high ionic and combined ionicelectronic conductivity, the engineering of interfaces became prerequisite. Technological applications demand combinations of materials with different electrical properties to form junctions. The occurring kinetic electrode phenomena play a significant role to the overall performance and may be rate determining. We show here the significance of the equilibration between platinum and the solid electrolyte with respect to the response time in potentiometric oxygen sensors. Fundamental aspects of voltage generation are discussed as well, and the response time is correlated to the equilibration along the contact zone between platinum and the electrolyte. A common problem of potentiometric devices for gas sensing applications is the cross sensitivity to species other than those under detection. The proposed kinetic method based on the theta concept is investigated for selective detection in the presence of a multiple number of complex gases by employing a single electrochemical cell. Information from current-voltage plots is converted to generate complex plane plot for the Fourier coefficients. The various polarizations under the applied electric perturbation are modelled and compared to experimental data for the determination of the rate determining processes.     

Advances on fluorine ion conductors,basic and applied research

Fluorides which are insulators, often possess a high anionic conductivity due to weak chemical bonding of the F^- ion. The examination of the known ionic conductors allows us to point out some general criteria characteristic of high mobility of F^-. The mechanisms of conduction determined on the basis of neutron diffraction are described. The possibility of utilization of such materials for thermodynamic measurements, specific electrodes, solid state batteries, electrochromic devices and gas sensors are examined.     

Experimental studies on high-performance supercapacitor based on nanogel polymer electrolyte with treated activated charcoal

Electrochemical capacitors, based on the double-layer capacitance of high specific surface area carbon materials, are attracting major fundamental and technological interest as highly reversible, electrical-charge storage and delivery devices, capable of being operated at high power densities. In the present paper, studies have been carried out on nanocomposite gel polymer electrolyte comprising poly(vinylidene fluoride-co-hexafluoropropylene)-propylene carbonate-magnesium perchlorate-nanofumed silica with a view to use them as electrolyte in electrochemical double-layer capacitors (EDLCs) based on chemically treated activated charcoal as electrodes. The optimized composition of nanogel polymer electrolyte exhibits high room-temperature ionic conductivity of 5.4?×?10^?3 S cm^?1 with good mechanical and dimensional stability which is suitable for their application as electrolyte in EDLCs. Detailed chemical and microstructural characterization of chemically treated and untreated activated charcoal was conducted using scanning electron microscopy and Brunauer–Emmett–Teller (BET). BET studies reveal that the effective surface area of treated activated charcoal powder (1,515 m² g^?1) increases by more than double-fold compared with untreated one (721 m² g^?1). Performance characteristics of EDLCs have been tested using cyclic voltammetry, impedance spectroscopy, prolonged cyclic test, and charge–discharge techniques. Analysis shows that the treated activated charcoal electrodes have almost five times more capacitance values as compared with the untreated one. The maximum capacitance of 324 mF cm^?2, equivalent to single electrode specific capacitance of 216 F?g^?1 was achieved. It corresponds to an energy density of 20 Wh kg^?1 and a power density of 2.2 kW kg^?1.     

固体电解质Rb4Cu16I7Cl13制备及离子导电性研究

液态固体电解质材料的离子电导率低,安全性问题在一定程度上限制了其发展与应用,而固体电解质材料在室温下具有很好的稳定性和高的离子电导率值,具有较好的应用前景.本文采用机械化学球磨法制备固体电解质Rb₄Cu₁₆I₇Cl₁₃粉末,探索制备工艺和球磨参数,对其晶体结构进行解析、观察粉体微观结构、通过交流阻抗谱及等效电路分析得到了离子电导率与活化能、并详细探讨其离子传导性能与晶体结构的关系以及化学成分稳定性进行研究.实验结果表明,在480 rpm转速下球磨6 h时可得到纯的固体电解质Rb₄Cu₁₆I₇Cl₁₃物相.粉体晶粒尺寸分布均匀,均在20 nm-400 nm之间,室温下固体电解质Rb₄Cu₁₆I₇Cl₁₃离子电导率可达到0.213 S/cm且活化能为0.087(9)eV.在真空干燥条件下存放5天和12天后观察了微观形貌和化学稳定性...     

Computational screening of doping schemes for LiTi2(PO4)3 as cathode coating materials

In all-solid-state lithium batteries,the impedance at the cathode/electrolyte interface shows close relationship with the cycle performance.Cathode coatings are helpful to reduce the impedance and increase the stability at the interface effectively.LiTi2(PO4)3(LTP),a fast ion conductor with high ionic conductivity approaching 10^-3S·cm^-1,is adopted as the coating materials in this study.The crystal and electronic structures,as well as the Li^+ion migration properties are evaluated for LTP and its doped derivatives based on density functional theory(DFT)and bond valence(BV)method.Substituting part of Ti sites with element Mn,Fe,or Mg in LTP can improve the electronic conductivity of LTP while does not decrease its high ionic conductivity.In this way,the coating materials with both high ionic conductivities and electronic conductivities can be prepared for all-solid-state lithium batteries to improve the ion and electron transport properties at the interface.     

Sol-gel ionics

The sol-gel synthesis of metal oxides offers new possibilities in the field of solid state ionics. The chemical design of molecular precursors allows a better control of the polymerization process so that taylor-made materials could be obtained. New hybrid organic-inorganic compounds exhibit high ionic conductivities and could be used as electrolytes. Oxide gels are actually particle hydrates. Water adsorption and dissociation at the oxide-water interface are responsible for their properties as ion exchangers or fast proton conductors. Colloidal particles are very small so that the large solid-liquid interface provides improved electrochemical properties which can be used for making reversible cathodes or electrochromic devices. The sol-gel deposition of thin coatings of large area could open the possibility to make multilayer micro-ionic devices.     

Elementary processes at semiconductor/electrolyte interfaces: perspectives and limits of electron spectroscopy

Semiconductor device properties based on electrolyte contacts or modified by electrochemical reactions are dominated by the electronic structure of the interface. Electron spectroscopy as e.g. photoemission is the most appropriate surface science techniques to investigate elementary processes at semiconductor/electrolyte interfaces. For such investigations a specific experimental set-up (SoLiAS) has been built-up which allows performing model experiments as well as surface analysis after emersion under different experimental conditions. The experimental approach is presented by a number of experiments performed during the last years with GaAs as substrate material. Model experiments by adsorption and coadsorption of electrolyte species give information on fundamental aspects of semiconductor/electrolyte interactions. Emersion experiments give information on a final composition and the related electronic structure of electrodes after electrochemical reactions. The use of frozen electrolytes will help to bridge the gap between these two approaches. With the combination of the experimental procedures one may expect a detailed analysis of electrolyte (modified) interfaces covering chemical composition, electronic structure of surfaces/interfaces as well as surface/interface potentials.     

基于中性水凝胶/取向碳纳米管阵列高电压柔性固态超级电容器

随着科技发展和时代进步,发展质轻便携、安全环保的高性能储能器件变得日趋重要,对柔性固态超级电容器的研究也应运而生.柔性电极材料及电解质的选用是设计柔性固态超级电容器的关键因素,近年来一直是研究的热点.考虑到环境污染及实际需求问题,本文采用中性凝胶电解质对具有高比表面积、良好导电性及取向性的碳纳米管阵列进行包埋处理,所形成的柔性复合薄膜作为电极材料,设计制备三明治结构的柔性超级电容器件.通过改变凝胶电解质中所加入的无机盐电解质种类,调控器件的电化学储能性质.最终在聚乙烯醇PVA-NaCl作为凝胶电解质时,整个器件比容量最高达104.5 mF·cm~(–3),远高于有机离子凝胶与碳管阵列形成的复合器件以及无规分布的碳纳米管与水凝胶形成的复合器件,同时获得了0.034 mW·h·cm~(–3)的最大能量密度,并且具有良好的倍率性能、循环稳定性及抑制自放电的效果,并在高电压1.6 V下依然保持良好的化学稳定性.这种中性凝胶/碳管阵列复合超级电容器件不仅满足了绿色安全、柔性便携的要求,未来在医学可植入器件等领域也具有很好的应用前景.     

硫钒铜矿化合物电子结构和电致变色特性的第一原理研究

基于密度泛函的第一原理赝势平面波方法,计算晶体结构、电子结构和光学性质,研究硫钒铜矿化合物Cu₃VS₄、Cu₃NbS₄和Cu₃TaS₄的电子输运及电致变色特性,探讨作为透明半导体材料应用于太阳能电池和电致变色器件的可能性.电子结构的计算表明这类化合物是间接带隙半导体,其电子能带的导带底和价带顶分别位于布里渊区的X点和R点.价带顶的电子本征态主要来自于Cu原子的d电子轨道,而导带底电子态主要来源于VB族元素原子的d电子轨道.能带结构、电荷布居分析、电子局域化函数和光吸收及反射谱的计算表明这些硫钒铜矿化合物属于极性共价半导体,具有较高的电荷迁移率和优良的电致变色特性,可应用于高效电致变色器件.     

In Situ Transmission Electron Microscopy Observation of Melted Germanium Encapsulated in Multilayer Graphene

Germanene is a two-dimensional germanium (Ge) analogous of graphene, and its unique topological properties are expected to make it a material for next-generation electronics. However, no germanene electronic devices have yet been reported. One of the reasons for this is that germanene is easily oxidized in air due to its lack of chemical stability. Therefore, growing germanene at solid interfaces where it is not oxidized is one of the key steps for realizing electronic devices based on germanene. In this study, the behavior of Ge at the solid interface at high temperatures is observed by transmission electron microscopy (TEM). To achieve such in situ heating TEM observation, this work fabricates a graphene/Ge/graphene encapsulated structure. In situ heating TEM experiments reveal that Ge like droplets move and coalesce with other Ge droplets, indicating that Ge remains as a liquid phase between graphene layers at temperatures higher than the Ge melting point. It is also observed that Ge droplets incorporate the surrounding amorphous Ge as Ge nuclei, thereby increasing its size (domain growth). These results indicate that Ge crystals can be grown at the interface of van der Waals materials, which will be important for future germanene growth at solid interfaces.     

Electrochemical properties of disordered cathode materials

Disordered and amorphous compounds sometimes show better properties as cathodes than crystalline compounds of the same material in lithium electrochemical devices. In the present work, we have considered molybdenum compounds such as MoO₃ and MoS₂ having different degrees of structural disorder which are being extensively investigated as intercalation hosts of lithium for applications in rechargeable batteries or electrochromic devices. In this connection, their potential-composition curves have been measured as thermodynamic characteristic which is very important from both fundamental and practical point of view. Considering both ionic and electronic contributions to the charge/discharge behavior, the electrochemical features are discussed from the point of view of energy diagrams. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Highly uniform electrochromic tungsten oxide thin films deposited by e-beam evaporation for energy saving systems

In the last few decades, there has been a surge of interest in using tungsten oxide thin films as an active layer of electrochromic device. These devices have several practical applications such as smart window of buildings and automobile glazing for energy saving. The main objective of this work was to construct highly homogeneous and uniform e-beam evaporated amorphous WO_3-x based films into electrochromic devices, which were fully characterized for switching speed, coloration efficiencies and cycling voltammetry responses. Fabricated devices contain indium doped transparent oxide coated glass as the transparent conductive electrode, ～200?nm thickness of WO_3-x as the cathodically coloring material and a lithium perchlorate based conducting gel electrolyte. X-ray diffraction patterns indicate that all as-deposited films are amorphous. Experimental results showed that both solid and liquid electrolyte electrochromic devices are initially very transparent that exhibit perfect optical modulation and coloration efficiency (up to 68.7?cm²/C and 52.6?cm²/C at 630?nm, respectively) due to easier intercalation of the Li⁺ within their structure. One of the more significant findings to emerge from this study is that e-beam coated electrochromic devices based on tungsten oxide thin films showed superior performance among to other coating methods. Therefore, excellent reversibility of color change behavior is attractive for pertinent use in electrochromic energy storage devices.