Enhanced Electrochemical Behavior of Peanut-Shell Activated Carbon/Molybdenum Oxide/Molybdenum Carbide Ternary Composites.

Biomass-waste activated carbon/molybdenum oxide/molybdenum carbide ternary composites are prepared using a facile in-situ pyrolysis process in argon ambient with varying mass ratios of ammonium molybdate tetrahydrate to porous peanut shell activated carbon (PAC). The formation of MoO2 and Mo2C nanostructures embedded in the porous carbon framework is confirmed by extensive structural characterization and elemental mapping analysis. The best composite when used as electrodes in a symmetric supercapacitor (PAC/MoO2/Mo2C-1//PAC/MoO2/Mo2C-1) exhibited a good cell capacitance of 115 F g-1 with an associated high specific energy of 51.8 W h kg-1, as well as a specific power of 0.9 kW kg-1 at a cell voltage of 1.8 V at 1 A g-1. Increasing the specific current to 20 A g-1 still showcased a device capable of delivering up to 30 W h kg-1 specific energy and 18 kW kg-1 of specific power. Additionally, with a great cycling stability, a 99.8% coulombic efficiency and capacitance retention of ~83% were recorded for over 25,000 galvanostatic charge-discharge cycles at 10 A g-1. The voltage holding test after a 160 h floating time resulted in increase of the specific capacitance from 74.7 to 90 F g-1 at 10 A g-1 for this storage device. The remarkable electrochemical performance is based on the synergistic effect of metal oxide/metal carbide (MoO2/Mo2C) with the interconnected porous carbon. The PAC/MoO2/Mo2C ternary composites highlight promising Mo-based electrode materials suitable for high-performance energy storage. Explicitly, this work also demonstrates a simple and sustainable approach to enhance the electrochemical performance of porous carbon materials.

Ex-situ nitrogen-doped porous carbons as electrode materials for high performance supercapacitor.

Nitrogen (N) doping of porous carbon materials is an effective strategy for enhancing the electrochemical performance of electrode materials. Herein, we report on ex-situ (post) nitrogen-doped porous carbons prepared using a biomass waste, peanut shell (PS) as a carbon source and melamine as the nitrogen source. The synthesis method involved a two-step mechanism, initial chemical activation of the PS using KOH and post N-doping of the activated carbon. The effect of the activating agent/precursor ratio and the ex-situ N-doping on the structural, textural, electrochemical properties of the porous carbons was studied. The ex-situ N-doped porous carbon with an optimum amount of KOH to PS exhibited the best capacitance performance with a specific surface area (SSA) of 1442 m2 g-1 and an enriched nitrogen content (3.2 at %). The fabricated symmetric device exhibited a 251.2 F g-1 specific capacitance per electrode at a gravimetric current of 1 A g-1 in aqueous electrolyte (2.5 M KNO3) at a wide cell voltage of 2.0 V. A specific energy of 35 Wh kg-1 with a corresponding specific power of 1 kW kg-1 at 1 A g-1 was delivered with the device still retaining up to 22 Wh kg-1 and a 20 kW kg-1 specific power even at 20 A g-1. Moreover, long term device stability was exhibited with an 83.2% capacity retention over 20 000 charge/discharge cycles and also a good rate capability after 180 h of floating at 5 A g-1. This great performance of the symmetric supercapacitor can be correlated to the surface porosity and post nitrogen-doping effect which increased the electrochemically-active sites resulting in a remarkable charge storage capability.

Nitridation Temperature Effect on Carbon Vanadium Oxynitrides for a Symmetric Supercapacitor.

In this work, porous carbon-vanadium oxynitride (C-V2NO) nanostructures were obtained at different nitridation temperature of 700, 800 and 900 °C using a thermal decomposition process. The X-ray diffraction (XRD) pattern of all the nanomaterials showed a C-V2NO single-phase cubic structure. The C-V2NO obtained at 700 °C had a low surface area (91.6 m2 g-1), a moderate degree of graphitization, and a broader pore size distribution. The C-V2NO obtained at 800 °C displayed an interconnected network with higher surface area (121.6 m2 g-1) and a narrower pore size distribution. In contrast, at 900 °C, the C-V2NO displayed a disintegrated network and a decrease in the surface area (113 m2 g-1). All the synthesized C-V2NO yielded mesoporous oxynitride nanostructures which were evaluated in three-electrode configuration using 6 M KOH aqueous electrolyte as a function of temperature. The C-V2NO@800 °C electrode gave the highest electrochemical performance as compared to its counterparts due to its superior properties. These results indicate that the nitridation temperature not only influences the morphology, structure and surface area of the C-V2NO but also their electrochemical performance. Additionally, a symmetric device fabricated from the C-V2NO@800 °C displayed specific energy and power of 38 W h kg-1 and 764 W kg-1, respectively, at 1 A g-1 in a wide operating voltage of 1.8 V. In terms of stability, it achieved 84.7% as capacity retention up to 10,000 cycles which was confirmed through the floating/aging measurement for up to 100 h at 10 A g-1. This symmetric capacitor is promising for practical applications due to the rapid and easy preparation of the carbon-vanadium oxynitride materials.

Three dimensional vanadium pentoxide/graphene foam composite as positive electrode for high performance asymmetric electrochemical supercapacitor.

The electrochemical performance of hydrothermal synthesized three dimensional (3D) orthorhombic vanadium pentoxide (V2O5) nanosheets and vanadium pentoxide/graphene foam (V2O5/GF) composites at different mass loading of GF were successfully studied. The optimized V2O5/GF-150 mg composite provided a high specific capacity of 73 mA h g-1, which was much higher than that the pristine V2O5 (60 mA h g-1) nanosheets at a specific current of 1 A g-1. A hybrid capacitor was also fabricated by adopting a carbon-based negative electrode obtained from the pyrolysis of an iron-PANI polymer (C-Fe/PANI) mixture and the 3D V2O5/GF-150 mg composite as the positive electrode in 6 M KOH electrolyte. The hybrid device of V2O5/GF-150 mg//C-Fe/PANI demonstrated a high energy density of 39 W h kg-1 with a corresponding high power density of 947 W kg-1 at a specific current of 1 A g-1 in an operating voltage window of 0.0-1.6 V. The hybrid device also exhibited an excellent cycling stability with 74% capacity retention recorded for up to 10,000 constant charging-discharge cycles and an excellent ageing test at a specific current of 10 A g-1.