Cheng-Hsien Yang

Graphene nanosheets, carbon nanotubes, graphite, and activated carbon as anode materials for sodium-ion batteries

The electrochemical sodium-ion storage properties of graphene nanosheets (GNSs), carbon nanotubes (CNTs), mesocarbon microbeads (MCMBs), and activated carbon (AC) are investigated. An irreversible oxidation occurs for the AC electrode during desodiation, limiting its use in sodium-ion batteries. The MCMB electrode shows a negligible capacity (∼2 mA h g−1), since the graphitic structure has a low surface area and is thus not capable of storing a sufficient amount of Na+. In contrast, the CNT and GNS electrodes exhibit reversible capacities of 82 and 220 mA h g−1, respectively, at a charge–discharge rate of 30 mA g−1. The high electro-adsorption/desorption area, large number of Na+ entrance/exit sites, and a large d-spacing of GNSs contribute to their superior Na+ storage capacity. At a high rate of 5 A g−1, the GNS electrode still delivers a capacity of as high as 105 mA h g−1, indicating great high-power ability. The charge storage mechanism of the electrode is examined using an ex situ X-ray diffraction technique.

Ionic liquid electrolytes with various sodium solutes for rechargeable Na/NaFePO4 batteries operated at elevated temperatures.

NaFePO4 with an olivine structure is synthesized via chemical delithiation of LiFePO4 followed by electrochemical sodiation of FePO4. Butylmethylpyrrolidinium-bis(trifluoromethanesulfonyl)imide (BMP-TFSI) ionic liquid (IL) with various sodium solutes, namely NaBF4, NaClO4, NaPF6, and NaN(CN)2, is used as an electrolyte for rechargeable Na/NaFePO4 cells. The IL electrolytes show high thermal stability (>350 °C) and nonflammability, and are thus ideal for high-safety applications. The highest conductivity and the lowest viscosity of the electrolyte are obtained with NaBF4. At an elevated temperature (above 50 °C), the IL electrolyte is more suitable than a conventional organic electrolyte for the sodium cell. At 75 °C, the measured capacity of NaFePO4 in a NaBF4-incorporated IL electrolyte is as high as 152 mAh g(-1) (at 0.05 C), which is near the theoretical value (154 mAh g(-1)). Moreover, 60% of this capacity can be retained when the charge-discharge rate is increased to 1 C.

Correlations between electrochemical Na+ storage properties and physiochemical characteristics of holey graphene nanosheets

Pores and surface functional groups are endowed on graphene nanosheets (GNSs) to improve their electrochemical Na+ storage properties. An optimal capacity of 220 mA g−1 is obtained (at a charge–discharge rate of 0.03 A g−1) in ethylene carbonate/diethyl carbonate mixed electrolyte containing 1 M NaClO4. Ex situ X-ray photoelectron spectroscopy and synchrotron X-ray diffraction techniques are employed to study the electrode charge storage mechanism. The results indicate that reversible surface redox reactions at the electrode surface are the predominant processes affecting charge storage in a potential range of 0.5–2.0 V (vs. Na/Na+), whereas Na+ intercalation/deintercalation between carbon layers occurs at lower potentials. When the charge–discharge rate was increased by >300 fold (to 10 A g−1), a capacity as high as 85 mA h g−1 is obtained, reflecting the excellent rate capability of the electrode. The physiochemical characteristics that affect the Na+ storage performance are explored. A highly promising GNS anode for sodium-ion batteries is proposed.

Holey Graphene Nanosheets with Surface Functional Groups as High‐Performance Supercapacitors in Ionic‐Liquid Electrolyte

Pores and surface functional groups are created on graphene nanosheets (GNSs) to improve supercapacitor properties in a butylmethylpyrrolidinium-dicyanamide (BMP-DCA) ionic liquid (IL) electrolyte. The GNS electrode exhibits an optimal capacitance of 330 F g(-1) and a satisfactory rate capability within a wide potential range of 3.3 V at 25 °C. Pseudocapacitive effects are confirmed using X-ray photoelectron spectroscopy. Under the same conditions, carbon nanotube and activated carbon electrodes show capacitances of 80 and 81 F g(-1) , respectively. Increasing the operation temperature increases the conductivity and decreases the viscosity of the IL electrolyte, further improving cell performance. At 60 °C, a symmetric-electrode GNS supercapacitor with the IL electrolyte is able to deliver maximum energy and power densities of 140 Wh kg(-1) and 52.5 kW kg(-1) (based on the active material on both electrodes), respectively, which are much higher than the 20 Wh kg(-1) and 17.8 kW kg(-1) obtained for a control cell with a conventional organic electrolyte.

High-selectivity electrochemical non-enzymatic sensors based on graphene/Pd nanocomposites functionalized with designated ionic liquids

Nano-sized Pd particles are uniformly dispersed on graphene nanosheets (GNSs) using a supercritical-fluid-assisted deposition technique to increase the electrochemical sensing properties. The incorporation of different kinds of ionic liquid (IL) can increase the electrode sensing current toward different analytes. Butylmethylpyrrolidinium-bis(trifluoromethanesulfonyl)imide (BMP-TFSI) IL is beneficial for glucose detection, whereas the electrode with butylmethylpyrrolidinium-dicyanamide (BMP-DCA) IL shows high sensitivity toward ascorbic acid (AA). The selective detection of glucose or AA from their mixture is for the first time demonstrated using a non-enzymatic electrode with the aid of an IL. Angle-resolved X-ray photoelectron spectroscopy analyses indicate that GNSs can create an aligned cation/anion orientation in the adsorbed IL film, with the anions preferentially occupying the topmost surface. As a result, the electrode sensitivity and selectivity are mainly determined by the IL constituent anions.

Nanostructured tin electrodeposited in ionic liquid for use as an anode for Li-ion batteries

Nanostructured tin (Sn) is fabricated via electrodeposition in an ionic liquid (IL). The advantages of electrodeposition in IL (compared to that in conventional aqueous solution) include increased deposition current efficiency and suppressed corrosion of the Cu current collector. The former is associated with the elimination of hydrogen evolution during the cathodic deposition and the latter is attributed to the chemical benignity of ILs. It is found that the Sn morphologies can be effectively manipulated by adjusting the deposition potential in the IL plating solution. The higher the deposition overpotential, the better is the electrochemical performance of the Sn electrode for Li-ion batteries. The electrode deposited at −2.4 V, consisting of Sn nanospheres, shows a reversible discharge (delithiation) capacity of 980 mA h g−1 (at 0.1 A g−1). When the charge–discharge rate is increased to 15 A g−1, the measured capacity is as high as 588 mA h g−1. These properties are clearly superior to those of the Sn electrode deposited in aqueous solution.

Facile electrochemical preparation of hierarchical porous structures to enhance manganese oxide charge-storage properties in ionic liquid electrolytes

A hierarchical porous MnO2 electrode is developed to enhance ionic liquid electrolyte transport and increase electroactive sites for charge-storage reactions. Superior cell energy and power densities of 90 W h kg−1 and 43 kW kg−1 were obtained, which are one-order higher than those for a conventional MnO2 cell with an aqueous electrolyte.

Eco-Efficient Synthesis of Highly Porous CoCO3 Anodes from Supercritical CO2 for Li+ and Na+ Storage

An eco-efficient synthetic route for the preparation of high-performance carbonate anodes for Li+ and Na+ batteries is developed. With supercritical CO2 (scCO2 ) as the precursor, which has gas-like diffusivity, extremely low viscosity, and near-zero surface tension, CoCO3 particles are uniformly formed and tightly connected on graphene nanosheets (GNSs). This synthesis can be conducted at 50 °C, which is considerably lower than the temperature required for conventional preparation methods, minimizing energy consumption. The obtained CoCO3 particles (ca. 20 nm in diameter), which have a unique interpenetrating porous structure, can increase the number of electroactive sites, promote electrolyte accessibility, shorten ion diffusion length, and readily accommodate the strain generated upon charging/discharging. With a reversible capacity of 1105 mAh g-1 , the proposed CoCO3 /GNS anode shows an excellent rate capability, as it can deliver 745 mAh g-1 in 7.5 min. More than 98 % of the initial capacity is retained after 200 cycles. These properties are clearly superior to those of previously reported CoCO3 -based electrodes for Li+ storage, indicating the merit of our scCO2 -based synthesis, which is facile, green, and can be easily scaled up for mass production.

Cost‐Effective Hierarchical Catalysts for Promoting Hydrogen Release from Complex Hydrides

Fe nanoparticles (∼10 nm), used to grow carbon nanotubes (CNTs), have an outstanding ability to catalyze the dehydrogenation of LiAlH4 . The CNTs help connect Fe and LiAlH4 and create microchannels among the composite, thus promoting the release of hydrogen. Inspired by these results, a supercritical-CO2 -fluid-assisted deposition technique is employed to decorate the Fe/CNTs with highly dispersed nanosized Ni (∼2 nm in diameter) for better performance. With the incorporation of 10 wt % of this hierarchical catalyst (Ni/Fe/CNTs), the initial dehydrogenation temperature of LiAlH4 is decreased from ∼135 to ∼40 °C. At 100 °C, this catalyzed LiAlH4 takes only ∼0.1 h to release 4.5 wt % hydrogen, which is more than 100 times faster than the time needed with pristine LiAlH4 . The dehydrogenation mechanism of the complex hydride is examined using in situ synchrotron X-ray diffraction.

Electrolyte Engineering: Optimizing High-Rate Double-Layer Capacitances of Micropore- and Mesopore-Rich Activated Carbon.

Various types of electrolyte cations as well as binary cations are used to optimize the capacitive performance of activated carbon (AC) with different pore structures. The high-rate capability of micropore-rich AC, governed by the mobility of desolvated cations, can outperform that of mesopore-rich AC, which essentially depends on the electrolyte conductivity.