Browsing by Author "Fan, Xiujun"
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Item Atomic H-Induced Mo2C Hybrid as an Active and Stable Bifunctional Electrocatalyst(American Chemical Society, 2017) Fan, Xiujun; Liu, Yuanyue; Peng, Zhiwei; Zhang, Zhenhua; Zhou, Haiqing; Zhang, Xianming; Yakobson, Boris I.; Goddard, William A. III; Guo, Xia; Hauge, Robert H.; Tour, James M.; NanoCarbon CenterMo2C nanocrystals (NCs) anchored on vertically aligned graphene nanoribbons (VA-GNR) as hybrid nanoelectrocatalysts (Mo2C–GNR) are synthesized through the direct carbonization of metallic Mo with atomic H treatment. The growth mechanism of Mo2C NCs with atomic H treatment is discussed. The Mo2C–GNR hybrid exhibits highly active and durable electrocatalytic performance for the hydrogen-evolution reaction (HER) and oxygen-reduction reaction (ORR). For HER, in an acidic solution the Mo2C–GNR has an onset potential of 39 mV and a Tafel slope of 65 mV dec–1, and in a basic solution Mo2C–GNR has an onset potential of 53 mV, and Tafel slope of 54 mV dec–1. It is stable in both acidic and basic media. Mo2C–GNR is a high-activity ORR catalyst with a high peak current density of 2.01 mA cm–2, an onset potential of 0.93 V that is more positive vs reversible hydrogen electrode (RHE), a high electron transfer number n (∼3.90), and long-term stability.Item Hydrogen Diffusion and Stabilization in Single-Crystal VO2 Micro/Nanobeams by Direct Atomic Hydrogenation(American Chemical Society, 2014) Lin, Jian; Ji, Heng; Swift, Michael W.; Hardy, Will J.; Peng, Zhiwei; Fan, Xiujun; Nevidomskyy, Andriy H.; Tour, James M.; Natelson, Douglas; Smalley Institute for Nanoscale Science and TechnologyWe report measurements of the diffusion of atomic hydrogen in single crystalline VO2 micro/nanobeams by direct exposure to atomic hydrogen, without catalyst. The atomic hydrogen is generated by a hot filament, and the doping process takes place at moderate temperature (373 K). Undoped VO2 has a metal-to-insulator phase transition at ∼340 K between a high-temperature, rutile, metallic phase and a low-temperature, monoclinic, insulating phase with a resistance exhibiting a semiconductor-like temperature dependence. Atomic hydrogenation results in stabilization of the metallic phase of VO2 micro/nanobeams down to 2 K, the lowest point we could reach in our measurement setup. Optical characterization shows that hydrogen atoms prefer to diffuse along the c axis of rutile (a axis of monoclinic) VO2, along the oxygen “channels”. Based on observing the movement of the hydrogen diffusion front in single crystalline VO2 beams, we estimate the diffusion constant for hydrogen along the c axis of the rutile phase to be 6.7 × 10–10 cm2/s at approximately 373 K, exceeding the value in isostructural TiO2 by ∼38×. Moreover, we find that the diffusion constant along the c axis of the rutile phase exceeds that along the equivalent a axis of the monoclinic phase by at least 3 orders of magnitude. This remarkable change in kinetics must originate from the distortion of the “channels” when the unit cell doubles along this direction upon cooling into the monoclinic structure. Ab initio calculation results are in good agreement with the experimental trends in the relative kinetics of the two phases. This raises the possibility of a switchable membrane for hydrogen transport.Item Three-Dimensional Nanoporous Fe2O3/Fe3C-Graphene Heterogeneous Thin Films for Lithium-Ion Batteries(American Chemical Society, 2014) Yang, Yang; Fan, Xiujun; Casillas, Gilberto; Peng, Zhiwei; Ruan, Gedeng; Wang, Gunuk; Yacaman, Miguel Jose; Tour, James M.; Smalley Institute for Nanoscale Science and TechnologyThree-dimensional self-organized nanoporous thin films integrated into a heterogeneous Fe2O3/Fe3C-graphene structure were fabricated using chemical vapor deposition. Few-layer graphene coated on the nanoporous thin film was used as a conductive passivation layer, and Fe3C was introduced to improve capacity retention and stability of the nanoporous layer. A possible interfacial lithium storage effect was anticipated to provide additional charge storage in the electrode. These nanoporous layers, when used as an anode in lithium-ion batteries, deliver greatly enhanced cyclability and rate capacity compared with pristine Fe2O3: a specific capacity of 356 μAh cm–2 μm–1 (3560 mAh cm–3 or ∼1118 mAh g–1) obtained at a discharge current density of 50 μA cm–2 (∼0.17 C) with 88% retention after 100 cycles and 165 μAh cm–2 μm–1(1650 mAh cm–3 or ∼518 mAh g–1) obtained at a discharge current density of 1000 μA cm–2(∼6.6 C) for 1000 cycles were achieved. Meanwhile an energy density of 294 μWh cm–2 μm–1(2.94 Wh cm–3 or ∼924 Wh kg–1) and power density of 584 μW cm–2 μm–1 (5.84 W cm–3 or ∼1834 W kg–1) were also obtained, which may make these thin film anodes promising as a power supply for micro- or even nanosized portable electronic devices.