Guoping Xiong

Hyperbolically Patterned 3D Graphene Metamaterial with Negative Poisson's Ratio and Superelasticity.

A hyperbolically patterned 3D graphene metamaterial (GM) with negative Poissons ratio and superelasticity is highlighted. It is synthesized by a modified hydrothermal approach and subsequent oriented freeze-casting strategy. GM presents a tunable Poissons ratio by adjusting the structural porosity, macroscopic aspect ratio (L/D), and freeze-casting conditions. Such a GM suggests promising applications as soft actuators, sensors, robust shock absorbers, and environmental remediation.

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).

Au nanoparticles on graphitic petal arrays for surface-enhanced Raman spectroscopy

We report a unique substrate for surface-enhanced Raman scattering (SERS) based on Au nanoparticle-decorated, thin graphitic petals. The petals were grown on Si substrates by microwave plasma chemical vapor deposition without catalyst, followed by Au nanoparticle decoration on the oxygen plasma-treated petals by electrodeposition. The substrates possess high surface area and sharp nanoscale features that enable high SERS sensitivity to detect 1×10−7 M rhodamine 6G in methanol solution. The obtained SERS enhancement is comparable to the best values reported in the literature and is determined to result from high surface area and increased density of Au nanoparticles on the petal surfaces.

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.

Flyweight 3D Graphene Scaffolds with Microinterface Barrier-Derived Tunable Thermal Insulation and Flame Retardancy

In this article, flyweight three-dimensional (3D) graphene scaffolds (GSs) have been demonstrated with a microinterface barrier-derived thermal insulation and flame retardancy characteristics. Such 3D GSs were fabricated by a modified hydrothermal method and a unidirectional freeze-casting process with hierarchical porous microstructures. Because of high porosity (99.9%), significant phonon scattering, and strong π-π interaction at the interface barriers of multilayer graphene cellular walls, the GSs demonstrate a sequence of multifunctional properties simultaneously, such as lightweight density, thermal insulating characteristics, and outstanding mechanical robustness. At 100 °C, oxidized GSs exhibit a thermal conductivity of 0.0126 ± 0.0010 W/(m K) in vacuum. The thermal conductivity of oxidized GSs remains relatively unaffected despite large-scale deformation-induced densification of the microstructures, as compared to the behavior of reduced GSs (rGSs) whose thermal conductivity increases dramatically under compression. The contrasting behavior of oxidized GSs and rGSs appears to derive from large differences in the intersheet contact resistance and varying intrinsic thermal conductivity between reduced and oxidized graphene sheets. The oxidized GSs also exhibit excellent flame retardant behavior and mechanical robustness, with only 2% strength decay after flame treatment. In a broader context, this work demonstrates a useful strategy to design porous nanomaterials with a tunable heat conduction behavior through interface engineering at the nanoscale.

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.

Influence of Temperature on Supercapacitor Components

Thermophysical properties of supercapacitor components determine the thermal behavior of supercapacitors at different application temperatures. A fundamental understanding of the influence of temperature on these properties is necessary to design supercapacitors with high performance for practical applications. Major supercapacitor elements include electrolytes, electrodes (active electrode materials, current collectors, and binders) and separators. As discussed in Chap. 2, supercapacitor electrolytes can be broadly classified into two types: liquid electrolytes and solid-state/polymer gel electrolytes (Xiong et al. in Electroanalysis 26:30–51, 2014 [24]). Conventional liquid electrolytes include: (i) aqueous electrolytes, (ii) organic electrolytes and (iii) ionic liquid electrolytes. The commonly used solid-state polymer gel electrolytes are water-containing (proton-conducting/alkaline), organic solvent-containing, and ionic liquid-containing polymer electrolytes. Active electrode materials for supercapacitors are broadly classified into three categories (Xiong et al. in Electroanalysis 26:30–51, 2014 [24]): (1) carbon materials, (2) conducting polymers, and (3) transition metal oxides. The importance of these electrolytes, electrode materials and separators has been addressed in prior reviews (Xiong et al. in Electroanalysis 26:30–51, 2014 [24], Simon and Gogotsi in Nat Mater 7:845–854, 2008 [39], Ye et al. in J Mater Chem A 1:2719–2743, 2013 [84], Zhang in J Power Sources 164:351–364, 2007 [193], Huang in J Solid State Electr 15:649–662, 2011 [194]). This chapter discusses the effects of temperature on the thermophysical properties of these components.

Thermal Management in Electrochemical Energy Storage Systems

Thermal management of electrochemical energy storage systems is essential for their high performance over suitably wide temperature ranges. An introduction of thermal management in major electrochemical energy storage systems is provided in this chapter. The general performance metrics and critical thermal characteristics of supercapacitors, lithium ion batteries, and fuel cells are discussed as a means of setting the stage for more detailed analysis in later chapters.

Thermal Modeling of Supercapacitors

The previous chapter reviewed the experimentally observed variations in electrochemical performance with temperature. The performance of supercapacitors depends strongly on operating temperature; therefore it is necessary to model temperature variations inside a supercapacitor. The major advantage of theoretical models is that they provide an opportunity to avoid time-consuming and expensive experiments by predicting performance in a wide range of applications and then building guidelines based on those predictions (Ike et al. in J Power Sources 273:264–277, 2015 [13]). Models can be used to study the thermal behavior of supercapacitors and thereby to develop new thermal management strategies. In this chapter, fundamentals of thermal modeling and various modeling approaches for temperature evolution are discussed from a theoretical standpoint.