The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

Metal oxide nanostructures are promising electrode materials for lithium-ion batteries and supercapacitors because of their high specific capacity/capacitance, typically 2-3 times higher than that of the carbon/graphite-based materials. However, their cycling stability and rate performance still can not meet the requirements of practical applications. It is therefore urgent to improve their overall device performance, which depends on not only the development of advanced electrode materials but also in a large part "how to design superior electrode architectures". In the article, we will review recent advances in strategies for advanced metal oxide-based hybrid nanostructure design, with the focus on the binder-free film/array electrodes. These binder-free electrodes, with the integration of unique merits of each component, can provide larger electrochemically active surface area, faster electron transport and superior ion diffusion, thus leading to substantially improved cycling and rate performance. Several recently emerged concepts of using ordered nanostructure arrays, synergetic core-shell structures, nanostructured current collectors, and flexible paper/textile electrodes will be highlighted, pointing out advantages and challenges where appropriate. Some future electrode design trends and directions are also discussed.

Recent Advances in Metal Oxide-based Electrode Architecture Design for Electrochemical Energy Storage

Liming Dai,*,†,‡ Yuhua Xue,†,‡ Liangti Qu,* Hyun-Jung Choi, and Jong-Beom Baek* †Center of Advanced Science and Engineering for Carbon (Case4Carbon), Department of Macromolecular Science and Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Department of Chemistry, School of Science, Beijing Institute of Technology, Beijing 100081, People’s Republic of China School of Energy and Chemical Engineering/Center for Dimension-Controllable Covalent Organic Frameworks, Ulsan National Institute of Science and Technology (UNIST), 100 Banyeon, Ulsan, 689-798, South Korea

Metal-free catalysts for oxygen reduction reaction.

It is estimated that the world will need to double its energy supply by 2050. Nanotechnology has opened up new frontiers in materials science and engineering to meet this challenge by creating new materials, particularly carbon nanomaterials, for efficient energy conversion and storage. Comparing to conventional energy materials, carbon nanomaterials possess unique size-/surface-dependent (e.g., morphological, electrical, optical, and mechanical) properties useful for enhancing the energy-conversion and storage performances. During the past 25 years or so, therefore, considerable efforts have been made to utilize the unique properties of carbon nanomaterials, including fullerenes, carbon nanotubes, and graphene, as energy materials, and tremendous progress has been achieved in developing high-performance energy conversion (e.g., solar cells and fuel cells) and storage (e.g., supercapacitors and batteries) devices. This article reviews progress in the research and development of carbon nanomaterials during the past twenty years or so for advanced energy conversion and storage, along with some discussions on challenges and perspectives in this exciting field.

Carbon Nanomaterials for Advanced Energy Conversion and Storage

With the ever-increasing power and energy needs in applications ranging from next-generation plug-in hybrid electric vehicles (PHEVs) and modern consumer electronics to microand nanoelectromechanical systems, recent research and development has focused on new electrode materials for advanced energy storage devices. [ 1–5 ] Of the various power source devices, supercapacitors, also known as electrochemical capacitors (ECs), have attracted great interest due to a number of desirable properties, including fast charging and discharging, long cycle life, and the ability to deliver up to ten times more power than conventional batteries. [ 6–10 ] In addition, ECs play an important role in complementing fuel cells in future all-electric vehicles based on clean and renewable energy media. [ 11 ] There are three major types of electrode materials reported for ECs: carbonaceous materials, [ 12 ] metal oxides/hydroxides, [ 13 ] and conducting polymers. [ 14 ] Carbon-based materials store charge electrostatically from the reversible adsorption of ions onto their surfaces, leading to high power delivery at the cost of low energy density. By contrast, metal oxides/hydroxides and conducting polymers store charge in a faradic or redox-type process similar to batteries, which enables high energy density but is in general kinetically unfavorable. To bridge the performance gap between these materials, attempts at novel electrode design have been extensively made. Despite a huge number of publications, nearly all of them can be clarifi ed into one general concept, that is, the use of pseudocapacitive material–conductive matrix hybrid nanostructures. [ 15 , 16 ] In this regard, materials presenting high pseudocapacitence (metal oxides) are incorporated directly into highly conductive nanostructured carbons (carbon nanotubes, [ 17–20 ]

Co3O4 Nanowire@MnO2 Ultrathin Nanosheet Core/Shell Arrays: A New Class of High‐Performance Pseudocapacitive Materials

Recently, the role of nanostructured materials in addressing the challenges in energy and natural resources has attracted wide attention. In particular, oriented nanostructures demonstrate promising properties for energy harvesting, conversion, and storage. In this Review, we highlight the synthesis and application of oriented nanostructures in a few key areas of energy technologies, namely photovoltaics, batteries, supercapacitors, and thermoelectrics. Although the applications differ from field to field, a common fundamental challenge is to improve the generation and transport of electrons and ions. We highlight the role of high surface area to maximize the surface activity and discuss the importance of optimum dimension and architecture, controlled pore channels, and alignment of the nanocrystalline phase to optimize the transport of electrons and ions. Finally, we discuss the challenges in attaining integrated architectures to achieve the desired performance. Brief background information is provided for the relevant technologies, but the emphasis is focused mainly on the nanoscale effects of mostly inorganic-based materials and devices.

Oriented nanostructures for energy conversion and storage.

https://depts.washington.edu/solgel/documents/pub_docs/journal_docs/2010/10-EC-Pan.pdf

Guozhong H. Cao

Papers

Oriented nanostructures for energy conversion and storage.

Nano-Structured Li3V2(PO4)3 /Carbon Composite for High Rate Lithium Ion Batteries