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

We developed two-step solution-phase reactions to form hybrid materials of Mn3O4 nanoparticles on reduced graphene oxide (RGO) sheets for lithium ion battery applications. Mn3O4 nanoparticles grown selectively on RGO sheets over free particle growth in solution allowed for the electrically insulating Mn3O4 nanoparticles wired up to a current collector through the underlying conducting graphene network. The Mn3O4 nanoparticles formed on RGO show a high specific capacity up to ~900mAh/g near its theoretical capacity with good rate capability and cycling stability, owing to the intimate interactions between the graphene substrates and the Mn3O4 nanoparticles grown atop. The Mn3O4/RGO hybrid could be a promising candidate material for high-capacity, low-cost, and environmentally friendly anode for lithium ion batteries. Our growth-on-graphene approach should offer a new technique for design and synthesis of battery electrodes based on highly insulating materials.

Mn3O4-Graphene Hybrid as a High Capacity Anode Material for Lithium Ion Batteries

Exploring noble-metal-free electrocatalysts with high efficiency for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) holds promise for advancing the production of H2 fuel through water splitting. Herein, one-pot synthesis was introduced for MoS2–Ni3S2 heteronanorods supported by Ni foam (MoS2–Ni3S2 HNRs/NF), in which the Ni3S2 nanorods were hierarchically integrated with MoS2 nanosheets. The hierarchical MoS2–Ni3S2 heteronanorods allow not only the good exposure of highly active heterointerfaces but also the facilitated charge transport along Ni3S2 nanorods anchored on conducting nickel foam, accomplishing the promoted kinetics and activity for HER, OER, and overall water splitting. The optimal MoS2–Ni3S2 HNRs/NF presents low overpotentials (η10) of 98 and 249 mV to reach a current density of 10 mA cm–2 in 1.0 M KOH for HER and OER, respectively. Assembled as an electrolyzer for overall water splitting, such heteronanorods show a quite low cell voltage of 1.50 V at 10 mA...

MoS2–Ni3S2 Heteronanorods as Efficient and Stable Bifunctional Electrocatalysts for Overall Water Splitting

Mixed transition-metal oxides (MTMOs), including stannates, ferrites, cobaltates, and nickelates, have attracted increased attention in the application of high performance lithium-ion batteries. Compared with traditional metal oxides, MTMOs exhibit enormous potential as electrode materials in lithium-ion batteries originating from higher reversible capacity, better structural stability, and high electronic conductivity. Recent advancements in the rational design of novel MTMO micro/nanostructures for lithium-ion battery anodes are summarized and their energy storage mechanism is compared to transition-metal oxide anodes. In particular, the significant effects of the MTMO morphology, micro/nanostructure, and crystallinity on battery performance are highlighted. Furthermore, the future trends and prospects, as well as potential problems, are presented to further develop advanced MTMO anodes for more promising and large-scale commercial applications of lithium-ion batteries.

Recent Developments and Understanding of Novel Mixed Transition-Metal Oxides as Anodes in Lithium Ion Batteries

Mixed metal sulfides (MMSs) have attracted increased attention as promising electrode materials for electrochemical energy storage and conversion systems including lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), hybrid supercapacitors (HSCs), metal–air batteries (MABs), and water splitting. Compared with monometal sulfides, MMSs exhibit greatly enhanced electrochemical performance, which is largely originated from their higher electronic conductivity and richer redox reactions. In this review, recent progresses in the rational design and synthesis of diverse MMS-based micro/nanostructures with controlled morphologies, sizes, and compositions for LIBs, SIBs, HSCs, MABs, and water splitting are summarized. In particular, nanostructuring, synthesis of nanocomposites with carbonaceous materials and fabrication of 3D MMS-based electrodes are demonstrated to be three effective approaches for improving the electrochemical performance of MMS-based electrode materials. Furthermore, some potential challenges as well as prospects are discussed to further advance the development of MMS-based electrode materials for next-generation electrochemical energy storage and conversion systems.

Mixed Metal Sulfides for Electrochemical Energy Storage and Conversion

High performance NiMoO4 nanowires supported on carbon cloth as advanced electrodes for symmetric supercapacitors

Enhanced electrochemical performance of CoMoO4 nanorods/reduced graphene oxide as anode material for lithium-ion batteries

Porous NiCo2O4-reduced graphene oxide (rGO) composite with superior capacitance retention for supercapacitors

Well-ordered manganese oxide (MnO2) nanosheet arrays (NSAs) were grown directly on carbon cloth via a simple in situ redox replacement reaction between potassium permanganate (KMnO4) and carbon cloth without any other oxidant or reductant addition. The morphology of MnO2 NSAs was examined by scanning and transmission electron microscopy and the phase structure of nanosheets (NSs) was analyzed by X-ray diffraction spectroscopy. Based on a series of time-dependent experiments, a possible growth process for this structure was proposed. The MnO2 NSAs supported on carbon cloth were directly used as integrated electrodes for electrochemical capacitors. The ordered MnO2 NSAs yielded high-capacitance performance with a high specific capacitance of 2.16 F cm−2 at a charge and discharge current density of 5 mA cm−2 and 1.01 F cm−2 at 20 mA cm−2 with a cycling ability (61.4% of the initial specific capacitance remains after 3000 cycles). The MnO2 nanosheet arrays with large surface area and high degree of ordering, combined with the flexible carbon cloth substrate can offer great promise for large-scale supercapacitor applications.

Facile synthesis of well-ordered manganese oxide nanosheet arrays on carbon cloth for high-performance supercapacitors

NiO–CoO nanoneedles are grown on carbon fibers by a solvothermal strategy to form nanobrushes. The density of nanobrushes can be easily controlled by altering the solvents. The synthesis mechanism of NiO–CoO/carbon fiber nanobrushes is investigated by the time-dependent experiments in detail. As anodes for lithium ion batteries, the NiO–CoO/carbon fiber nanobrushes synthesized in ethanol show excellent properties with a discharge capacity of 801 mA h g–1 after 200 cycles at a current density of 200 mA g–1. The improvement can be ascribed to the carbon fibers as the highway for electrons and the interspace between NiO–CoO nanoneedles to accommodate the volume change and maintain the structural stability.

Yazi Luo

Papers

High performance NiMoO4 nanowires supported on carbon cloth as advanced electrodes for symmetric supercapacitors

Enhanced electrochemical performance of CoMoO4 nanorods/reduced graphene oxide as anode material for lithium-ion batteries

Porous NiCo2O4-reduced graphene oxide (rGO) composite with superior capacitance retention for supercapacitors

Facile synthesis of well-ordered manganese oxide nanosheet arrays on carbon cloth for high-performance supercapacitors

Solvent-Controlled Synthesis of NiO–CoO/Carbon Fiber Nanobrushes with Different Densities and Their Excellent Properties for Lithium Ion Storage