Shengxue Yu

Synthesis of Capsule-like Porous Hollow Nanonickel Cobalt Sulfides via Cation Exchange Based on the Kirkendall Effect for High-Performance Supercapacitors.

To construct a suitable three-dimensional structure for ionic transport on the surface of the active materials for a supercapacitor, porous hollow nickel cobalt sulfides are successfully synthesized via a facile and efficient cation-exchange reaction in a hydrothermal process involving the Kirkendall effect with γ-MnS nanorods as a sacrificial template. The formation mechanism of the hollow nickel cobalt sulfides is carefully illustrated via the tuning reaction time and reaction temperature during the cation-exchange process. Due to the ingenious porous hollow structure that offers a high surface area for electrochemical reaction and suitable paths for ionic transport, porous hollow nickel cobalt sulfide electrodes exhibit high electrochemical performance. The Ni(1.77)Co(1.23)S4 electrode delivers a high specific capacity of 224.5 mAh g(-1) at a current density of 0.25 A g(-1) and a high capacity retention of 87.0% at 10 A g(-1). An all-solid-state asymmetric supercapacitor, assembled with a Ni(1.77)Co(1.23)S4 electrode as the positive electrode and a homemade activated carbon electrode as the negative electrode (denoted as NCS//HMC), exhibits a high energy density of 42.7 Wh kg(-1) at a power density of 190.8 W kg(-1) and even 29.4 Wh kg(-1) at 3.6 kW kg(-1). The fully charged as-prepared asymmetric supercapacitor can light up a light emitting diode (LED) indicator for more than 1 h, indicating promising practical applications of the hollow nickel cobalt sulfides and the NCS//HMC asymmetric supercapacitor.

All-solid-state high performance asymmetric supercapacitors based on novel MnS nanocrystal and activated carbon materials.

All-solid-state high-performance asymmetric supercapacitors (ASCs) are fabricated using γ-MnS as positive electrode and porous eggplant derived activated carbon (EDAC) as negative electrode with saturated potassium hydroxide agar gel as the solid electrolyte. The laminar wurtzite nanostructure of γ-MnS facilitates the insertion of hydroxyl ions into the interlayer space, and the manganese sulfide nanowire offers electronic transportation channels. The size-uniform porous nanostructure of EDAC provides a continuous electron pathway as well as facilitates short ionic transportation pathways. Due to these special nanostructures of both the MnS and the EDAC, they exhibited a specific capacitance of 573.9 and 396 F g−1 at 0.5 A g−1, respectively. The optimized MnS//EDAC asymmetric supercapacitor shows a superior performance with specific capacitance of 110.4 F g−1 and 89.87% capacitance retention after 5000 cycles, a high energy density of 37.6 Wh kg−1 at a power density of 181.2 W kg−1 and remains 24.9 Wh kg−1 even at 5976 W kg−1. Impressively, such two assembled all-solid-state cells in series can light up a red LED indicator for 15 minutes after fully charged. These impressive results make these pollution-free materials promising for practical applications in solid aqueous electrolyte-based ASCs.

Synthesis of graphene oxide anchored porous manganese sulfide nanocrystals via the nanoscale Kirkendall effect for supercapacitors

Graphene oxide (GO) anchored porous manganese sulfide nanocrystals (MnS/GO-NH3) were obtained via a facile hydrothermal method based on the Kirkendall effect. The honeycomb-like manganese sulfide nanocrystals (40–80 nm) and the three-dimensional sandwich structure endow the MnS/GO-NH3 with high supercapacitive performance when it was used as a supercapacitor material. The MnS/GO-NH3 electrode exhibits high specific capacitance (390.8 F g−1 at 0.25 A g−1), high rate capacity (78.7% retention at 10 A g−1) and stable cycle life (81.0% retention after 2000 cycles), which are superior to those of GO anchored MnS floccules (MnS/GO) and manganese hydroxide (Mn(OH)2/GO). As a novel material for supercapacitors, the charge–discharge mechanism of the MnS/GO-NH3 composite is proposed via detailed investigation. Asymmetric supercapacitors, assembled with MnS/GO-NH3 as the positive material and activated carbon as the negative electrode, reveal a high specific capacitance (73.63 F g−1), a high energy density of 14.9 W h kg−1 at 66.5 W kg−1 and even 12.8 W h kg−1 at a high power density of 4683.5 W kg−1.

Synthesis of peanut-like hierarchical manganese carbonate microcrystals via magnetically driven self-assembly for high performance asymmetric supercapacitors

To construct a suitable structure for both electronic conduction and ionic transport towards supercapacitors, peanut-like hierarchical manganese carbonate (MnCO3) microcrystals assembled with floss-like nanowires are synthesized via a hydrothermal process and primarily used as an active material for supercapacitors. The formation mechanism is illustrated by means of a dissolution–recrystallization process and magnetically driven self-assembly. The electrode with peanut-like hierarchical MnCO3 microcrystals exhibits a high specific capacitance of 293.7 F g−1 and a superior cycle stability of 71.5% retention after 6000 cycles, which are higher than those of the reported Mn-based active materials in alkaline electrolytes. The asymmetric supercapacitor, assembled with the peanut-like MnCO3 electrode as the positive electrode and a home-made porous carbon electrode as the negative electrode, exhibits an energy density of 14.7 W h kg−1 at a power density of 90.2 W kg−1 and an energy density of up to 11.0 W h kg−1 at 3.3 kW kg−1. An as-assembled all-solid-state supercapacitor series can light up a LED indicator for 10 min, indicating a promising practical application of peanut-like MnCO3 microcrystals.

Comparative study on three commercial carbons for supercapacitor applications

Three commercial carbon materials for supercapacitor were investigated by physicochemical characterization, electrochemical measurements and surface treatment to explore the effects of specific surface area, electrolyte and surface functional groups on the specific capacitance, charge storage mode and high rate performance of carbon materials. Results indicate that the specific surface area of carbon material plays dominate role in the specific capacitance. The electrolytes have remarkable effects on the specific capacitance and high rate performance. Investigation of HNO3 treated Vulcan XC-72 carbon material reveals that the treatment can increase the specific surface area and surface functional groups, which observably improve the specific capacitance of the XC-72 carbon material. The surface functional groups contribute to the pseudo-capacitance of the carbon material.

Investigation of oxygen reduction reaction and methanol tolerance on the carbon supported Pt-Pd catalysts

Carbon supported Pt-Pd (Pt-Pd/C) catalysts with different Pt/Pd ratios were prepared by microwave assisted ethylene glycol method and characterized by X-ray diffraction (XRD), transmission electron spectroscopy (TEM) and electrochemical measurements. The results reveal that the metal nanoparticles of Pt-Pd/C catalysts are uniformly dispersed on the carbon support and exist as alloys with face centered cubic (FCC) structures. The effects of both preparation process and Pt/Pd ratios on the structure and catalytic activity of the Pt-Pd/C catalysts were investigated. With the increase of Pd content, the catalytic activity for ORR is deteriorated due to the increased metal particle size of Pt-Pd/C catalysts, while improving the methanol tolerance. Among the Pt-Pd/C catalysts with different Pt/Pd ratios, the Pt3Pd/C catalyst, which have the highest Pt/Pd ratios, exhibits higher specific mass activities toward oxygen reduction reaction (ORR) than that of the commercial Pt/C catalyst.

In Situ and Ex Situ Studies on the Degradation of Pd/C Catalyst for Proton Exchange Membrane Fuel Cells

To investigate the degradation mechanism of the as-prepared Pd/C catalyst, in situ and ex situ accelerated stress tests were carried out via potential cycling. Durability tests of the single cells with Pd/C catalysts were performed through an interval constant current density mode. Electrochemical impedance spectroscopy (EIS) was applied to measure the impedance of the single cell during degradation tests. Results indicate that the degradation of Pd/C catalyst may be attributed to the phase transition of absorbed α-phase PdH to β-phase PdH, the dissolution of Pd metal, and the size increase of Pd nanoparticles. Moreover, the degradation of single cell may be predominantly ascribed to the degradation of catalyst, the deterioration of contact between electronic/ionic conductors, as well as the flooding of gas diffusion channels.