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

Recent advances and challenges in biomass-derived porous carbon nanomaterials for supercapacitors

Although aqueous asymmetric supercapacitors are promising technologies because of their high-energy density and enhanced safety, their voltage window is still limited by the narrow stability window of water. Redox reactions at suitable electrodes near the water splitting potential can increase the working potential. Here, we demonstrate a kinetic approach for expanding the voltage window of aqueous asymmetric supercapacitors using in situ activated Mn3O4 and VO2 electrodes. The underlying mechanism indicates a specific potential of ∼1 V vs Ag/AgCl for the oxidation of Mn4+-to-Mn7+ at the positive electrode and ∼ –0.8 V vs Ag/AgCl for the reduction of V3+-to-V2+ at the negative electrode, which limits oxygen and hydrogen evolution reactions, respectively. The as-fabricated aqueous asymmetric supercapacitor exhibited a working voltage of 2.2 V with a high-energy density of 42.7 Wh/kg and a power density of ∼1.1 kW/kg. This mechanism improves the voltage window and energy and power densities.

Redox-Driven Route for Widening Voltage Window in Asymmetric Supercapacitor

The article reviews the recent progress of electrochemical techniques on synthesizing nano-/microstructures as supercapacitor electrodes. With a history of more than a century, electrochemical techniques have evolved from metal plating since their inception to versatile synthesis tools for electrochemically active materials of diverse morphologies, compositions, and functions. The review begins with tutorials on the operating mechanisms of five commonly used electrochemical techniques, including cyclic voltammetry, potentiostatic deposition, galvanostatic deposition, pulse deposition, and electrophoretic deposition, followed by thorough surveys of the nano-/microstructured materials synthesized electrochemically. Specifically, representative synthesis mechanisms and the state-of-the-art electrochemical performances of exfoliated graphene, conducting polymers, metal oxides, metal sulfides, and their composites are surveyed. The article concludes with summaries of the unique merits, potential challenges, and associated opportunities of electrochemical synthesis techniques for electrode materials in supercapacitors.

/pdf/a-review-on-nano-microstructured-materials-constructed-by-18i76e7erh.pdf

A Review on Nano-/Microstructured Materials Constructed by Electrochemical Technologies for Supercapacitors

Rational construction of bowl-like MnO2 nanosheets with excellent electrochemical performance for supercapacitor electrodes

In this paper, we report an effective, simple, and cost-effective strategy of electrochemical deposition to prepare hausmannite Mn3O4 thin films for the applications of supercapacitors. Various precursor concentrations and deposition durations were manipulated to tailor the surface morphologies of Mn3O4 nanostructures and to optimize their electrochemical performances. The Mn3O4 samples prepared at 0.05 M Mn(NO3)2 solution for 30 min delivered a large gravimetric specific capacitance of 210 F g−1 at a current density of 0.5 A g−1, and a good rate capability over other samples. This superior electrochemical performance may be attributed to the improved electrode conductivity with increased accessible area for electrolytes ions. Furthermore, a nanocomposite film based on Mn3O4/carbon foam was fabricated by utilizing the developed optimized conditions. The Mn3O4/carbon foam films exhibit an excellent specific capacitance with negligible degradation in retaining specific capacitance values up to 4000 cycles. These findings could further broaden the applications of hausmannite Mn3O4 in electrochemical energy storage electrodes.

/pdf/a-facile-and-template-free-one-pot-synthesis-of-mn3o4-12km8t40iv.pdf

A Facile and Template-Free One-Pot Synthesis of Mn3O4 Nanostructures as Electrochemical Supercapacitors.

To overcome the obstacle of low energy density, one of the most intensive approaches is the development of new materials for supercapacitor electrodes. Most explored materials today are carbon particle materials, which have high surface areas for charge storage. But in spite of these large specific surface areas, the charges physically stored on the carbon particles in porous electrode layers are unfortunately limited. Regarding advanced supercapacitor electrodes, metal oxides are considered the most promising material for the next generation of supercapacitors owing to their unique physical and chemical properties. In this chapter, the rational design and fabrication of metal oxide nanostructures for supercapacitor applications are addressed.

/pdf/nanostructured-metal-oxides-based-electrode-in-x8asx31751.pdf

Nanostructured Metal Oxides-Based Electrode in Supercapacitor Applications

The dissociation of H2 is an essential elementary step in many industrial chemical transformations, typically requiring precious metals. Here, we report a hierarchical nanoporous Cu catalyst doped with small amounts of Ti (npTiCu) that increases the rate of H2-D2 exchange by approximately one order of magnitude compared to the undoped nanoporous Cu (npCu) catalyst. The promotional effect of Ti was measured via steady-state H2-D2 exchange reaction experiments under atmospheric pressure flow conditions in the temperature range of 300-573 K. Pretreatment with flowing H2 is required for stable catalytic performance, and two temperatures, 523 and 673 K, were investigated. The experimentally determined H2-D2 exchange rate is 5-7 times greater for npTiCu vs the undoped Cu material under optimized pretreatment and reaction temperatures. The H2 pretreatment leads to full reduction of Cu oxide and partial reduction of surface Ti oxide species present in the as-prepared catalyst as demonstrated using in situ ambient pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. The apparent activation energies and pre-exponential factors measured for H2-D2 exchange are substantially different for Ti-doped vs undoped npCu catalysts. Density functional theory calculations suggest that isolated, metallic Ti atoms on the surface of the Cu host can act as the active surface sites for hydrogen recombination. The increase in the rate of exchange above that of pure Cu is caused primarily by a shift in the rate-determining step from dissociative adsorption on Cu to H/D atom recombination on Ti-doped Cu, with the corresponding decrease in activation entropy that it produces.

Facilitating Hydrogen Dissociation over Dilute Nanoporous Ti-Cu Catalysts.

Electrochemical CO2 reduction (ECR) promises the replacement of fossil fuels as the source of feedstock chemicals and seasonal storage of renewable energy. While much progress has been made in catalyst development and electrochemical reactor design, few studies have addressed the effect of catalyst integration on device performance. Using a microfluidic gas diffusion electrolyzer, we systematically studied the effect of thickness and the morphology of electron beam (EB) and magnetron-sputtered (MS) Cu catalyst coatings on ECR performance. We observed that EB-Cu outperforms MS-Cu in current density, selectivity, and energy efficiency, with 400 nm thick catalyst coatings performing the best. The superior performance of EB-Cu catalysts is assigned to their faceted surface morphology and sharper Cu/gas diffusion layer interface, which increases their hydrophobicity. Tests in a large-scale zero-gap electrolyzer yielded similar product selectivity distributions with an ethylene Faradaic efficiency of 39% at 200 mA/cm2, demonstrating the scalability for industrial ECR applications.

Scalable Gas Diffusion Electrode Fabrication for Electrochemical CO2 Reduction Using Physical Vapor Deposition Methods.

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Papers

A Facile and Template-Free One-Pot Synthesis of Mn3O4 Nanostructures as Electrochemical Supercapacitors.

Nanostructured Metal Oxides-Based Electrode in Supercapacitor Applications

Facilitating Hydrogen Dissociation over Dilute Nanoporous Ti-Cu Catalysts.

Scalable Gas Diffusion Electrode Fabrication for Electrochemical CO2 Reduction Using Physical Vapor Deposition Methods.