The high performance of a pseudocapacitor electrode relies largely on a scrupulous design of nanoarchitectures and smart hybridization of bespoke active materials. We present a powerful two-step solution-based method for the fabrication of transition metal oxide core/shell nanostructure arrays on various conductive substrates. Demonstrated examples include Co(3)O(4) or ZnO nanowire core and NiO nanoflake shells with a hierarchical and porous morphology. The "oriented attachment" and "self-assembly" crystal growth mechanisms are proposed to explain the formation of the NiO nanoflake shell. Supercapacitor electrodes based on the Co(3)O(4)/NiO nanowire arrays on 3D macroporous nickel foam are thoroughly characterized. The electrodes exhibit a high specific capacitance of 853 F/g at 2 A/g after 6000 cycles and an excellent cycling stability, owing to the unique porous core/shell nanowire array architecture, and a rational combination of two electrochemically active materials. Our growth approach offers a new technique for the design and synthesis of transition metal oxide or hydroxide hierarchical nanoarrays that are promising for electrochemical energy storage, catalysis, and gas sensing applications.
Electrochemical splitting of water to produce hydrogen and oxygen is an important process for many energy storage and conversion devices. Developing efficient, durable, low-cost, and earth-abundant electrocatalysts for the oxygen evolution reaction (OER) is of great urgency. To achieve the rapid synthesis of transition-metal nitride nanostructures and improve their electrocatalytic performance, a new strategy has been developed to convert cobalt oxide precursors into cobalt nitride nanowires through N2 radio frequency plasma treatment. This method requires significantly shorter reaction times (about 1 min) at room temperature compared to conventional high-temperature NH3 annealing which requires a few hours. The plasma treatment significantly enhances the OER activity, as evidenced by a low overpotential of 290 mV to reach a current density of 10 mA cm(-2) , a small Tafel slope, and long-term durability in an alkaline electrolyte.
yields a synergism for the HER. [ 25 ] In accordance with the "volcano plot," the activity for the evolution of hydrogen is a function of the M H (metal hydride) bond strength and exhibits a peak value for metal Pt, which has an optimal M H bond strength. [ 26 ] Therefore, designing a material on the molecular scale which combines an M H-weak metal (Ni) with an M H-strong metal (Mo) is a feasible method to acquire ideal catalysts. Sasaki's group synthesized NiMoN x nanosheets on carbon support [ 21 ] and mixed close-packed Co 0.6 Mo 0.14 N 2 particles [ 20 ] via annealing corresponding precursors under ammonia gas. Both of the materials show the high HER electrocatalytic activity with low overpotential and small Tafel slope. In addition to the pristine activity of the catalysts, a variety of other parameters can limit their performance, such as roughness, conductivity, stability of the catalyst, the attachment of catalysts on electrodes. [ 27 ] In general, hazardous and unfriendly ammonia is applied as nitrogen source for synthesis of metal nitrides from metal oxide precursors at relatively high temperatures (600-800 °C). [ 21, In addition, this method often results in incomplete nitrifi cation, leading to inferior electronic, mechanical and thermal properties of the as-obtained composites. In this work, we employed a novel method to synthesize 3D porous nickel molybdenum nitride on carbon cloth (NiMoN) by treating electrodeposited NiMo alloy fi lms with N 2 plasma at a relatively low reaction temperature (450 °C) and shorter duration (15 min). The obtained bimetallic nitrides exhibit a 3D porous hierarchical structure with high roughness factor (1050), and outstanding catalytic performance for HER. It is believed that this method can be employed for the synthesis of many other bimetallic nitride nanostructures for applications in battery and supercapacitors.The overall synthesis procedure for the porous NiMoN fi lms on carbon cloth is illustrated schematically in Figure 1 a. First, a dense and shiny grey NiMo alloy fi lm is deposited on commercial carbon cloth (Figure 1 c,f) via optimized pulse-electrodeposition (PED) method. Then, after being treated under N 2 RF plasma at 450 °C for 15 min, a porous and black NiMoN fi lm is obtained (Figure 1 d,g). According to the previous report, [ 31 ] the metal Mo cannot be electroplated from an aqueous solution directly without the assistant of metal Ni. With increasing the molybdenum content, the deposited alloys tend to the amorphous state and an amorphous pattern appears when the content of molybdenum is over 20 at%. [ 32 ] A further increase in the molybdenum content causes crack in the deposited fi lm. Hence, a molybdenum concentration of 20 at% in the alloy is found optimal by tuning the deposition parameters (Supporting The fossil-fuel crisis and the increasing environment issues have triggered the urgent demand for renewable and clean energy sources. Hydrogen is considered as a promising alternative energy carrier to fossil fuels because of its zero ca...
This article provides an overview of solution-based methods for the controllable synthesis of metal oxides and their applications for electrochemical energy storage. Typical solution synthesis strategies are summarized and the detailed chemical reactions are elaborated for several common nanostructured transition metal oxides and their composites. The merits and demerits of these synthesis methods and some important considerations are discussed in association with their electrochemical performance. We also propose the basic guideline for designing advanced nanostructure electrode materials, and the future research trend in the development of high power and energy density electrochemical energy storage devices.
Lithium‐ion capacitors (LICs) are hybrid energy storage devices that have the potential to bridge the gap between conventional high‐energy lithium‐ion batteries and high‐power capacitors by combining their complementary features. The challenge for LICs has been to improve the energy storage at high charge−discharge rates by circumventing the discrepancy in kinetics between the intercalation anode and capacitive cathode. In this article, the rational design of new nanostructured LIC electrodes that both exhibit a dominating capacitive mechanism (both double layer and pseudocapacitive) with a diminished intercalation process, is reported. Specifically, the electrodes are a 3D interconnected TiC nanoparticle chain anode, synthesized by carbothermal conversion of graphene/TiO2 hybrid aerogels, and a pyridine‐derived hierarchical porous nitrogen‐doped carbon (PHPNC) cathode. Electrochemical properties of both electrodes are thoroughly characterized which demonstrate their outstanding high‐rate capabilities. The fully assembled PHPNC//TiC LIC device delivers an energy density of 101.5 Wh kg−1 and a power density of 67.5 kW kg−1 (achieved at 23.4 Wh kg−1), and a reasonably good cycle stability (≈82% retention after 5000 cycles) within the voltage range of 0.0−4.5 V.
A new and generic strategy to construct interwoven carbon nanotube (CNT) branches on various metal oxide nanostructure arrays (exemplified by V2 O3 nanoflakes, Co3 O4 nanowires, Co3 O4 -CoTiO3 composite nanotubes, and ZnO microrods), in order to enhance their electrochemical performance, is demonstrated for the first time. In the second part, the V2 O3 /CNTs core/branch composite arrays as the host for Na(+) storage are investigated in detail. This V2 O3 /CNTs hybrid electrode achieves a reversible charge storage capacity of 612 mAh g(-1) at 0.1 A g(-1) and outstanding high-rate cycling stability (a capacity retention of 100% after 6000 cycles at 2 A g(-1) , and 70% after 10 000 cycles at 10 A g(-1) ). Kinetics analysis reveals that the Na(+) storage is a pseudocapacitive dominating process and the CNTs improve the levels of pseudocapacitive energy by providing a conductive network.
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