Metal oxide applications in organic-based photovoltaics

Abstract

Organic-based photovoltaics (PV) have attracted increasing attention in recent years and efficiencies exceeding 8% have recently been confirmed. These low cost, lightweight and mechanically flexible devices offer unique advantages and opportunities currently unavailable with crystalline silicon technology. Progress in the field of organic PV has been achieved in part due to the incorporation of transition metal oxides. These offer a wide range of optical and electronic properties, making them applicable in organic-based PV in many capacities. Transparent electrodes can be made from doped metal oxides. The high intrinsic charge carrier mobility of many undoped metal oxides makes them attractive as active materials and charge collectors. Metal oxides can increase the charge selectivity of the electrodes due to the energetic positioning of their valence and conduction bands. Thin films of these materials can manipulate the light distribution inside of organic devices, allowing for improved light harvesting. Metal oxides are stable and can be processed at low temperatures. Consequently, they have been demonstrated as suitable intermediate layer materials in tandem cells. Finally, oxygen-deficient metal oxides can improve the stability of the oxygen- sensitive organic semiconductors. The present work reviews the various applications of metal oxide layers in organic PV devices and summarises the challenges associated with organic/oxide interfaces.     

Research on key materials for potassium ion batteries

钾具有资源丰富、价格低廉以及较低的电化学电势的特点,利用其开发的钾离子电池具有低成本、长寿命、能量密度高等特点,可满足储能领域需要。然而,钾离子半径大和质量大,给电池电极材料与电解质材料开发提出新的挑战。近年来,在电动汽车与储能应用等重大需求的牵引下,人们对钾离子电池的高容量电极材料和电解液进行了大量的研究工作。其中普鲁士蓝及其类似物、过渡金属氧化物和聚阴离子材料等正极材料展现了应用前景;负极材料主要包括碳基、钛基和合金类材料;电解质材料有酯类电解质和醚类电解质,这些研究成果为钾离子电池的基础与应用研究提供了重要的指导意义。     

Electrode and electrolyte materials for electrochemical capacitors

Among different electric energy storage technologies electrochemical capacitors are used for energy storage applications when high power delivery or uptake is needed. Their energy and power densities, durability and efficiency are influenced by electrode and electrolyte materials however due to a high cost/performance ratio; their widespread use in energy storage systems has not been attained yet.Thanks to their properties such as high surface area, controllable pore size, low electrical resistance, good polarizability and inertness; activated carbons derived from polymeric precursors are the most used electrode materials in electrochemical capacitors at present. Other electrode materials such as shaped nano-carbons or metal oxides are also investigated as electrode materials in electrochemical capacitors, but only as useful research tools.Most commercially used electrochemical capacitors employ organic electrolytes when offering concomitant high energy and high power densities. The use of aqueous based electrolytes in electrochemical capacitor applications is mainly limited to research purposes as a result of their narrow operating voltage. Recent studies on room temperature ionic liquids to be employed as electrolyte for electrochemical capacitor applications are focused on fine tuning their physical and transport properties in order to bring the energy density of the device closer to that of batteries without compromising the power densities.In this paper a performance analysis, recent progress and the direction of future developments of various types of materials used in the fabrication of electrodes for electrochemical capacitors are presented. The influence of different types of electrolytes on the performance of electrochemical capacitors such as their output voltage and energy/power densities is also discussed.     

Progress of electrochemical capacitor electrode materials: A review

The electrode is the key part of the electrochemical capacitors (ECs), so the electrode materials are the most important factors to determine the properties of ECs. In this paper, the storage principles and characteristics of electrode materials, including carbon-based materials, transition metal oxides and conductive polymers for ECs are depicted briefly. Among them, more work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and organic electrolytes. But the composites of pseudocapacitive and carbonaceous materials are promising electrode materials for ECs because of their good electrical conductivity, low cost and high mass density.     

Opportunities and challenges of electrochromic phenomena in transition metal oxides

During the past two decades, electrochromic phenomena in transition metal oxides have been very widely studied. This unique phenomenon of electrically induced reversible coloration and charge storage in this films has diverse applications that range from information display to neural networking in optical computation. Electrochromic devices using metal oxides can be classified into two broad categories (1) all-solid-state systems, and (2) semi-solid systems involving wet electrolytes. The principles underlying the operation of both systems appear quite similar, but there remain both scientific and technological issues that are poorly understood. In this paper we briefly discuss some of the opportunities and challenges for the application of electrochromic phenomena in transition metal oxides.     

Synthesis of porous flower-like Ni-Co-Mo-S nanostructures on Ni foam for battery-supercapacitor hybrid devices

The synergistic effects of multiple components and unique nanostructures were contributed to prepare the high-performance battery-type electrode materials. In this work, Mo element was introduced to form the ternary transition metal oxides/hydroxides of Ni-Co to improve conductivity, and then charge transfer was accelerated to enhance the capacity storage. After sulfidation, the electrical conductivity was further improved, and a porous flower-like nanostructure was formed. Except for that, the composites of transition metal oxides/hydroxides and sulfides were formed via sulfidation. With the help of the synergistic effects of multiple components and a porous flower-like nanostructure, more Faradic redox reactions occurred. Therefore, the as-prepared porous flower-like Ni-Co-Mo-S nanostructures on Ni foam exhibited an excellent areal capacitance of 7.22 C·cm⁻² at 5 mA·cm⁻² and long-cycle stability (96.9% retention after 5000 cycles). Furthermore, a coin-type battery-supercapacitor hybrid (BSH) device was assembled, which achieved 54.54 Wh·kg⁻¹ at 540 W·kg⁻¹ and displayed 74.8% capacitance retention after 3500 cycles. All mentioned above demonstrated that ternary transition metal oxides/hydroxides precursors via sulfidation can form special structures and the composites of transition metal oxides/hydroxides and sulfides to prepare high-performance battery-type electrodes for energy storage.     

A review of performance optimization of MOF‐derived metal oxide as electrode materials for supercapacitors

Metal‐organic frameworks (MOFs), as new class of porous materials, are constructed by inorganic metal centers and bridging organic links. Recently, MOFs have been proved to be effective templates for preparing metal oxides with large surface areas and controlled shape by directly annealing in air. There are lots of reports about metal‐organic framework‐derived metal oxides as electrode materials for supercapacitors. Metal‐organic framework‐derived metal oxides can offer higher capacitances compared with that prepared by other synthetic methods, likely attribute to high surface areas and optimal pore sizes. However, at present, the specific capacitances of MOF‐derived metal oxides received are far lower than theoretical values, and the cycle numbers could not meet practical demands. Accordingly, much effort has been made to improve the performance by further modifying MOFs. Thus, this paper focused on the advances in performance optimization of MOF‐derived metal oxide as electrode materials for supercapacitors as follows:

1. Dual metal MOF‐derived binary metal oxides. Metal oxides with 2 metal cations possess better electrical conductivity and richer redox active sites than single metal oxides.
2. Metal‐organic framework‐derived carbon/metal oxide composites (MO@C) or graphene/MOF‐derived graphene/metal oxide composites. Doping carbon not only facilitate transportation of electrodes but also contribution to extra double‐layer capacitance.
3. Hybrid MOF‐derived metal oxide composites (MO@MO). Metal oxide composites can produce some synergistic effects that the individuals cannot provide.
4. Metal‐organic framework‐derived metal oxides with a hollow structure. The Hollow structure could shorten ion diffusion distance and adapt to volume expansion generated during the ion intercalated/extracted process.

     

Electrochemical behaviour of metallic oxides

Some of the physico-chemical properties of oxides used as cathodes in batteries or as electrocatalysts are briefly reviewed. Possible relationships between these properties and the electrochemical reactivity of the oxides are considered, and attention is drawn to the important influence of the surface state of electrodes on this reactivity. From consideration of the available information on three metal oxides of technological importance and of spinel type mixed oxides it is apparent that reduction processes take place in the solid state, and that for the metal oxides at least, the presence of OH groups and possibly H₂O molecules is necessary for their activity. For all these materials there appears to be a correlation between electrochemical reactivity and the presence of redox couples in octahedral sites, and it is suggested that further investigation based on this new concept may lead to the development of power sources with increased specific outputs.     

Production and characterization of ITO-Pt semiconductor powder containing nanoscale noble metal particles catalytically active in dye-sensitized solar cells

Doped indium tin oxide (ITO) samples of a surface area around 60 m²/g have been synthesized. As doping component platinum and gold atoms were utilized, respectively. The powders were produced by pyrolyzing mixed metal oxide precursors in a first zone in a flame consisting of hydrogen and air, metering a noble metal compound and reducing gas into a second zone of the flame and separating off the solid obtained in a third zone. Well-defined solid phases In₂Pt and In₂Au in ITO were obtained, respectively. The production of a mixture with discrete SnO₂ entities could be avoided. The mixed oxide powder doped with platinum can be used as a catalyst. The structure and composition of materials obtained at different levels of platinum dosing into a flame reactor has been determined. The size of the noble metal-containing entities ranged at about 3.5–5.5 nm. The catalytic properties of these oxides containing noble metal have been utilized in the fabrication of dye-sensitized solar cells (DSCs). Long term stability of DSCs containing liquid electrolytes requires a glass sealing of the two electrodes at a temperature of over 600 °C. Such high temperatures are detrimental to the catalytic activity of the conventional platinum layers in the counter electrode. Improved catalytic activity could be achieved by platinum entities bound to indium tin oxide. This catalytic material can effectively be used in the DSCs with glass sealing, the sealing temperature being over 600 °C. Spray pyrolysis in a flame reactor is highly suitable to produce noble metal-containing oxides in one single production step. Platinum-containing ITO particles from flame pyrolysis production exhibit superior catalytic activity in dye-sensitized solar cells compared to material from conventional thermal decomposition.     

Fabrication of CuO film with network-like architectures through solution-immersion and their application in lithium ion batteries

A novel CuO negative electrode with network-like architectures was fabricated on copper substrate by a simple solution-immersion step and subsequent heat treatment, which avoids the use of binder and conducting agent that necessary to the conventional electrode-preparation process. The as-prepared CuO electrodes exhibit not only high reversible capacity but also long cycling life, high rate capability in Li ion batteries. The result of this approach creates a new and attractive negative electrode with good electrochemical performance, which is simple, mild conditions, low cost, and easy control. It also opens a pathway for the application of other nanostructured materials of transition metal oxides in lithium ion batteries.     

Tin‐based materials as negative electrodes for Li‐ion batteries: Combinatorial approaches and mechanical methods

Graphite has been used as the negative electrode in lithium‐ion batteries for more than a decade. To attain higher energy density batteries, silicon and tin, which can alloy reversibly with lithium, have been considered as a replacement for graphite. However, the volume expansion of these metal elements upon lithiation can result in poor capacity retention. Alloying the active metal element with an inactive material can limit the overall volume expansion and improve cycle life. This paper presents a summary of tin‐based materials as negative electrodes. After reviewing attempts to improve and understand the electrochemical behaviour of metallic tin and its oxides, the focus turns to alloys of tin with a transition metal (TM) and, optionally, carbon. To do so, a combinatorial sputtering technique was used to simultaneously prepare many different compositions of Sn‐TM‐based materials. The structural and electrochemical results of these samples are presented and they show that cobalt is the preferred TM to give optimal performance. Finally, a comparison of a Sn–Co–C negative electrode material prepared by a rapid quenching method (sputtering) with a material prepared by an economical milling method (mechanical attrition) is presented and discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

A critical review on progress of the electrode materials of vanadium redox flow battery

Nowadays, renewable energy sources are taken great attention by the researchers and the investors around the world due to increasing energy demand of today's knowledge societies. Since these sources are non-continuous, the effective storage and re-use of the energy produced from renewable energy sources have great importance. Although classical energy storage systems such as lead acid batteries and Li-ion batteries can be used for this goal, the new generation energy storage system is needed for large-scale energy storage applications. In this point, vanadium redox flow batteries (VRFBs) are shinning like a star for this area. VRFBs consist of electrode, electrolyte, and membrane component. The battery electrodes as positive and negative electrodes play a key role on the performance and cyclic life of the system. In this work, electrode materials used as positive electrode, negative electrode, and both of electrodes in the latest literature were complained and presented. From graphene-coated and heteroatom-doped carbon-based electrodes to metal oxides decorated carbon-based electrodes, a large scale on the modification of carbon-based electrodes is available on the electrode materials of the VRFBs. By the discovering of novel electrode components for the battery system, the using of the VRFBs probably increase in a short time for many industrial and residential applications.     

Investigation of the gas-diffusion-electrode used as lithium/air cathode in non-aqueous electrolyte and the importance of carbon material porosity

The gas-diffusion-electrode used in a Li-air cell has been studied in a unique homemade electrochemical cell. Three major obstacles for the development of a feasible Li-air system were discussed with a focus on the development of a functional gas-diffusion-electrode in non-aqueous electrolytes and the way of avoiding the passivation of gas-diffusion-electrodes caused by the deposition of the reduction products. It is the first time that the importance of establishing the 3-phase electrochemical interface in non-aqueous electrolyte is demonstrated by creating air-diffusion paths and an air saturated portion for an air cathode. A model mechanism of electrode passivation by the reaction products was also proposed. Lithium oxides formed during O₂ reduction tend to block small pores, preventing them from further utilization in the electrochemical reaction. On the other hand, lithium oxides would accumulate inside the large pores during the reduction until the density of oxides becomes high enough to choke-off the mass transfer. Carbon materials with a high surface area associated with larger pores should be selected to make the gas-diffusion-electrode for Li-air battery. For the first time, a near linear relationship between the capacity of GDE in a non-aqueous electrolyte and the average pore diameter was demonstrated, which could be used to estimate the capacity of the GDE quantitatively.     

层状富镍过渡金属氧化物正极材料衰减机理研究进展

层状富镍过渡金属氧化物正极材料因具有比容量高、价格低廉以及对环境友好等特性而受到广泛关注,并已在产品中应用。但其本身固有的一些缺陷,如在循环过程中结构稳定性较差、高温循环衰减过快、导电系数较低及储存性能不佳等,极大地限制了其在各个领域的广泛应用。本文对近年来高镍层状过渡金属氧化物正极材料在循环过程中容量衰减有关机理进行概括总结,并针对不同衰减机理给出简要改进方法。     

Electrode designs and concepts for bipolar alkaline fuel cells

Fuel cells as direct energy converters and their economic applications in a future hydrogen economy are discussed briefly. Emphasis is put on the merits of a new way to construct and mass-produce alkaline fuel cells, eliminating the traditionally used expensive metal screens and porous nickel plaques and replacing them with plastic-bonded conductive materials for structures and porous carbons for electrodes.Carbon electrodes have been used successfully in many types of fuel cell systems. Multi-layered thin carbon electrodes of high power output and long life can be produced by pressing, rolling and spraying methods.Acetylene black, furnace blacks and graphites are used as carbon materials, mixed with PTFE and filler materials and evaluated in electrodes which are tested in laboratory cells, before they are built into stacks.Compositions of the different mixtures for the fabrication of the electrodes are given and the advantage of the bipolar cell design from the construction and operating points of view is discussed.     

Investigation of new types of lithium-ion battery materials

This paper reports part of the activities in progress in our laboratory in the investigation of electrode and electrolyte materials which may be of interest for the development of lithium-ion batteries with improved characteristics and performances. This investigation has been directed to both anode and cathode materials, with particular attention to convertible oxides and defect spinel-framework Li-insertion compounds in the anode area and layered mixed lithium–nickel–cobalt oxide and high voltage, metal type oxides in the cathode area. As for the electrolyte materials, we have concentrated the efforts on composite polymer electrolytes and gel-type membranes. In this work we report the physical, chemical and electrochemical properties of the defect spinel-framework Li-insertion anodes and of the high voltage, mixed metal type oxide cathodes, by describing their electrochemical properties in cells using either “standard” liquid electrolytes and “advanced” gel-type, polymer electrolytes.The results illustrated here demonstrate that the spinel-framework anodes of the Li[Li_1/3Ti_5/3]O₄ type can be combined with high voltage cathodes of the Li₂M_xMn_(4−x)O₈ family for the fabrication of new types of lithium-ion battery systems cycling around 3.5 V. The development of this interesting concept is however still limited by the availability of highly stable electrolytes.     

Ambient operation of Li/Air batteries

In this work, Li/air batteries based on nonaqueous electrolytes were investigated in ambient conditions (with an oxygen partial pressure of 0.21 atm and relative humidity of ∼20%). A heat-sealable polymer membrane was used as both an oxygen-diffusion membrane and as a moisture barrier for Li/air batteries. The membrane also can minimize the evaporation of the electrolyte from the batteries. Li/air batteries with this membrane can operate in ambient conditions for more than one month with a specific energy of 362 Wh kg⁻¹, based on the total weight of the battery including its packaging. Among various carbon sources used in this work, Li/air batteries using Ketjenblack (KB) carbon-based air electrodes exhibited the highest specific energy. However, KB-based air electrodes expanded significantly and absorbed much more electrolyte than electrodes made from other carbon sources. The weight distribution of a typical Li/air battery using the KB-based air electrode was dominated by the electrolyte (∼70%). Lithium metal anodes and KB-carbon account for only 5.12% and 5.78% of the battery weight, respectively. We also found that only ∼20% of the mesopore volume of the air electrode was occupied by reaction products after discharge. To further improve the specific energy of the Li/air batteries, the microstructure of the carbon electrode needs to be further improved to absorb much less electrolyte while still holding significant amounts of reaction products.     

活性炭材料用于烟气脱硫的研究进展

活性炭材料因具有丰富的孔结构和较大的比表面积，而被用于大气污染治理。对活性炭材料用于烟气脱硫的研究现状(目前主要集中在含氧、含氮官能团的引入以及表面负载金属及其化舍物的研究)进行了综述。最后展望了活性炭材料用于烟气脱硫的未来。     

纳米石墨化碳在锂离子电池中的应用进展

纳米石墨化碳因其优异的导电、导热及力学性能近年来备受重视,并在锂离子电池体系中得到广泛运用。纳米石墨化碳具有的优异电学性能及纳米尺度结构特征使其在解决锂离子电池中高导电性、导热性、充放电过程中的柔性及结构稳定性等方面发挥了重要作用。本文综述了近年来纳米石墨化碳在锂离子电池应用中的最新进展和研究热点,包括纳米石墨化碳在锂离子电池中直接充当高容量负极材料,纳米石墨化碳作为高性能骨架材料为电极提供导电及力学网络,与硅、金属氧化物等高容量电极材料复合形成同轴、核壳等结构的高容量电极材料甚至柔性电极等。如何进一步认识纳米石墨化碳储锂机制,发展其精确可控制备科学和工程技术,进而在三维尺度上构建高效的锂离子电池电极材料结构仍是未来的重点研究方向。     

CO-tolerant electrodes developed with PhosphoMolybdic Acid for Polymer Electrolyte Fuel Cell (PEFCs) application

Several approaches were used to improve the CO-tolerant electrodes for polymer electrolyte fuel cells (PEFCs) when using processed H₂ as a fuel. The employment of transition metals oxides (WO_x, MoO_x) promotes CO oxidation and, for this reason, heteropolyacids (like PWA, PMoA, SiWA, etc.) containing these oxides were selected in this work, for the development of CO-tolerant electrodes.