Pingge He

Plasma-grown graphene petals templating Ni–Co–Mn hydroxide nanoneedles for high-rate and long-cycle-life pseudocapacitive electrodes

Ni–Co–Mn triple hydroxide (NCMTH) nanoneedles were coated on plasma-grown graphitic petals (GPs) by a facile one-step hydrothermal method for high-rate and long-cycle-life pseudocapacitive electrodes. Structural and compositional characteristics of NCMTHs indicate that the multi-component metal elements distribute homogeneously within the NCMTHs. Comparison of the electrochemical performance of the three-dimensional NCMTH electrodes to Ni–Co double hydroxides reveals that a synergistic effect of the hierarchical structure of GPs and NCMTHs enables their high rate capability and long cycle life. The NCMTH electrode maintains over 95% of its capacitance at a high charge/discharge rate of 100 mA cm−2 relative to its low-current (1 mA cm−2) capacitance; and it exhibits very high specific capacitance of approximately 1400 F g−1 (based on the mass of NCMTH), high specific energy density (≈30 W h kg−1) and power density (≈39 kW kg−1) at a high current density of 100 mA cm−2, and excellent long-term cyclic stability (full capacitance retention over 3000 cycles). To assess functional behavior, two-terminal asymmetric supercapacitor devices with NCMTHs on graphitic petals as positive electrodes were assembled and tested to reveal ultrafast charge/discharge rates up to 5000 mV s−1 (approx. two orders of magnitude faster than conventional asymmetric devices based on metal hydroxides) with high rate capabilities, and excellent long-term cyclic stability (full capacitance retention over 10 000 cycles).

Large-scale synthesis and activation of polygonal carbon nanofibers with thin ribbon-like structures for supercapacitor electrodes

Polygonal carbon nanofibers (PCNFs) were prepared on a large scale by chemical vapor deposition using Ni3Sn2 intermetallic compound as a catalyst. The PCNFs feature polygonal cross sections with side lengths ranging from 200 nm to 400 nm, as primarily determined by the orthorhombic structure of the Ni3Sn2 compound. The PCNFs were subsequently activated by KOH with different concentrations, denoted as a-PCNFs, for supercapacitor electrode applications. The PCNFs were significantly etched during the activation process under a high KOH concentration, forming a unique thin ribbon-like nanostructure with large specific surface area and high content of oxygen-containing functional groups. The electrochemical measurements reveal that a-PCNFs, activated by KOH at a KOH : C weight ratio of 4 : 1 under 800 °C, exhibit favorable electrochemical properties with a specific capacitance of 186 F g−1 at a current density of 3 A g−1 in 1 M Na2SO4, good rate capability, low internal resistance, and reasonably stable cycle life. These promising electrochemical properties indicate significant potential for use as scalable supercapacitor electrodes.

Synthesis of Porous Ni–Co–Mn Oxide Nanoneedles and the Temperature Dependence of Their Pseudocapacitive Behavior

Porous Ni-Co-Mn oxide nanoneedles have been synthesized on Ni foam by a facile one-step hydrothermal method for use as supercapacitor electrodes. Structural and compositional characterizations indicate that Ni, Co and Mn elements are homogeneously distributed within the multi-component metal oxides. Such multi-component metal oxides with a homogenous structure exhibit high specific capacitance of 2023 F g-1 at 1 mA cm-2, high coulombic efficiencies (greater than 99%), and good long-term cycle life (approximately 7% loss in specific capacitance over 3000 charge/discharge cycles) at room temperature. Moreover, the influence of temperature on the electrochemical performance of the electrodes has been characterized at temperatures ranging from 4 to 80°C in aqueous electrolytes. The thermal behavior of the electrodes reveals that elevated operating temperature promotes higher capacitance and lower internal resistance by decreasing the ionic conductivity of the electrolyte and increasing redox reaction rates at the interface of the electrodes and electrolytes. The capacitance of the electrodes increases by 84% at a nominal temerature of 80°C and decreases by 18% at 4°C, compared to that at room temperature (RT). The overall set of results demonstrates that the new Ni-Co-Mn oxide nanoneedle electrodes are promising for high-performance pseudocapacitive electrodes with a wide usable temperature range.

Bioinspired leaves-on-branchlet hybrid carbon nanostructure for supercapacitors

Designing electrodes in a highly ordered structure simultaneously with appropriate orientation, outstanding mechanical robustness, and high electrical conductivity to achieve excellent electrochemical performance remains a daunting challenge. Inspired by the phenomenon in nature that leaves significantly increase exposed tree surface area to absorb carbon dioxide (like ions) from the environments (like electrolyte) for photosynthesis, we report a design of micro-conduits in a bioinspired leaves-on-branchlet structure consisting of carbon nanotube arrays serving as branchlets and graphene petals as leaves for such electrodes. The hierarchical all-carbon micro-conduit electrodes with hollow channels exhibit high areal capacitance of 2.35 F cm−2 (~500 F g−1 based on active material mass), high rate capability and outstanding cyclic stability (capacitance retention of ~95% over 10,000 cycles). Furthermore, Nernst–Planck–Poisson calculations elucidate the underlying mechanism of charge transfer and storage governed by sharp graphene petal edges, and thus provides insights into their outstanding electrochemical performance.One way to improve the performance of supercapacitors is to use hybrid carbon nanomaterials. Here the authors show a bioinspired electrode design with graphene petals and carbon nanotube arrays serving as leaves and branchlets, respectively. The structure affords excellent electrochemical characteristics.

Controllable synthesis of Ni–Co–Mn multi-component metal oxides with various morphologies for high-performance flexible supercapacitors

Ni–Co–Mn multi-component metal oxides with various morphologies for high-performance supercapacitor electrodes were controllably synthesized on carbon cloth through a facile hydrothermal method followed by a subsequent annealing process. The crystalline structure, morphology and electrochemical property of the metal oxides can be manipulated by simply adjusting the Ni/Co/Mn ratio in the solution. The metal oxide prepared in the solution containing a Ni/Co/Mn ratio of 1/1/2 presents a structure consisting of nanosheets and nanoneedles, and exhibits the highest specific capacitance of 1434.2 F g−1 at a current density of 2 mA cm−2, desirable rate capability and excellent cyclic stability with a capacitance retention of 94% over 3000 cycles. To demonstrate practical application, flexible asymmetric supercapacitors based on such metal oxides were prepared and show high capacitance, low internal resistance, excellent cyclic stability and outstanding flexibility, indicating great potential as high-performance energy storage devices.

Vertically-oriented graphene nanosheet as nano-bridge for pseudocapacitive electrode with ultrahigh electrochemical stability

A hierarchical structure consisting of Ni–Co hydroxide nanosheets (NCHN) electrodeposited on vertically-oriented graphene nanosheets (GN) on carbon cloth (CC) was fabricated for high-performance pseudocapacitive electrodes. NCHN was uniformly distributed on GN, forming a sheet-on-sheet hierarchical structure. Such NCHN/GN/CC hybrid electrodes exhibit high capacitance and ultrahigh electrochemical-stability that structure and electrochemical properties of hybrid electrodes are not affected by the cyclic low-rate scanning (at 5 mV s−1 even over 1000 cycles). GN vertically grown on CC is used as nano-bridge between NCHN active materials and CC current collector, which effectively facilitates ion/charge transfer between the electrolyte and electrode, consequently leading to the ultrahigh electrochemical-stability of hybrid electrodes. To assess functional behavior, two-terminal flexible asymmetric supercapacitor devices with NCHN/GN/CC as positive electrode and GN/CC as negative electrode were assembled and electrochemically treated to demonstrate the ultrahigh electrochemical stability.

Correction to: Structural evolution of vertically oriented graphene nanosheet templating Ni–Co hydroxide as pseudocapacitive electrode

In the original article author name Qiangqiang Zhang was misspelled. It is correct here.