Xiujun Fan

Boron- and Nitrogen-Doped Graphene Quantum Dots/Graphene Hybrid Nanoplatelets as Efficient Electrocatalysts for Oxygen Reduction

The scarcity and high cost of platinum-based electrocatalysts for the oxygen reduction reaction (ORR) has limited the commercial and scalable use of fuel cells. Heteroatom-doped nanocarbon materials have been demonstrated to be efficient alternative catalysts for ORR. Here, graphene quantum dots, synthesized from inexpensive and earth-abundant anthracite coal, were self-assembled on graphene by hydrothermal treatment to form hybrid nanoplatelets that were then codoped with nitrogen and boron by high-temperature annealing. This hybrid material combined the advantages of both components, such as abundant edges and doping sites, high electrical conductivity, and high surface area, which makes the resulting materials excellent oxygen reduction electrocatalysts with activity even higher than that of commercial Pt/C in alkaline media.

M3C (M: Fe, Co, Ni) Nanocrystals Encased in Graphene Nanoribbons: An Active and Stable Bifunctional Electrocatalyst for Oxygen Reduction and Hydrogen Evolution Reactions

Transition metal carbide nanocrystalline M3C (M: Fe, Co, Ni) encapsulated in graphitic shells supported with vertically aligned graphene nanoribbons (VA-GNRs) are synthesized through a hot filament chemical vapor deposition (HF-CVD) method. The process is based on the direct reaction between iron group metals (Fe, Co, Ni) and carbon source, which are facilely get high purity carbide nanocrystals (NCs) and avoid any other impurity at relatively low temperature. The M3C-GNRs exhibit superior enhanced electrocatalystic activity for oxygen reduction reaction (ORR), including low Tafel slope (39, 41, and 45 mV dec(-1) for Fe3C-GNRs, Co3C-GNRs, and Ni3C-GNRs, respectively), positive onset potential (∼0.8 V), high electron transfer number (∼4), and long-term stability (no obvious drop after 20 000 s test). The M3C-GNRs catalyst also exhibits remarkable hydrogen evolution reaction (HER) activity with a large cathodic current density of 166.6, 79.6, and 116.4 mA cm(-2) at an overpotential of 200 mV, low onset overpotential of 32, 41, and 35 mV, small Tafel slope of 46, 57, and 54 mV dec(-1) for Fe3C-GNRs, Co3C-GNRs, and Ni3C-GNRs, respectively, as well as an excellent stability in acidic media.

WC Nanocrystals Grown on Vertically Aligned Carbon Nanotubes: An Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction.

Single nanocrystalline tungsten carbide (WC) was first synthesized on the tips of vertically aligned carbon nanotubes (VA-CNTs) with a hot filament chemical vapor deposition (HF-CVD) method through the directly reaction of tungsten metal with carbon source. The VA-CNTs with preservation of vertical structure integrity and alignment play an important role to support the nanocrystalline WC growth. With the high crystallinity, small size, and uniform distribution of WC particles on the carbon support, the formed WC-CNTs material exhibited an excellent catalytic activity for hydrogen evolution reaction (HER), giving a η10 (the overpotential for driving a current of 10 mA cm(-2)) of 145 mV, onset potential of 15 mV, exchange current density@ 300 mV of 117.6 mV and Tafel slope values of 72 mV dec(-1) in acid solution, and η10 of 137 mV, onset potential of 16 mV, exchange current density@ 300 mV of 33.1 mV and Tafel slope values of 106 mV dec(-1) in alkaline media, respectively. Electrochemical stability test further confirms the long-term operation of the catalyst in both acidic and alkaline media.

Three-Dimensional Nanoporous Fe2O3/Fe3C-Graphene Heterogeneous Thin Films for Lithium-Ion Batteries

Three-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.

Hydrothermally formed three-dimensional nanoporous Ni(OH)2 thin-film supercapacitors.

A three-dimensional nanoporous Ni(OH)2 thin-film was hydrothermally converted from an anodically formed porous layer of nickel fluoride/oxide. The nanoporous Ni(OH)2 thin-films can be used as additive-free electrodes for energy storage. The nanoporous layer delivers a high capacitance of 1765 F g(-1) under three electrode testing. After assembly with porous activated carbon in asymmetric supercapacitor configurations, the devices deliver superior supercapacitive performances with capacitance of 192 F g(-1), energy density of 68 Wh kg(-1), and power density of 44 kW kg(-1). The wide working potential window (up to 1.6 V in 6 M aq KOH) and stable cyclability (∼90% capacitance retention over 10,000 cycles) make the thin-film ideal for practical supercapacitor devices.

High thermal conductivity of suspended few-layer hexagonal boron nitride sheets

AbstractThe thermal conduction of suspended few-layer hexagonal boron nitride (h-BN) sheets was experimentally investigated using a noncontact micro-Raman spectroscopy method. The first-order temperature coefficients for monolayer (1L), bilayer (2L) and nine-layer (9L) h-BN sheets were measured to be −(3.41 ± 0.12) × 10−2, −(3.15 ± 0.14) × 10−2 and −(3.78 ± 0.16) × 10−2 cm−1·K−1, respectively. The room-temperature thermal conductivity of few-layer h-BN sheets was found to be in the range from 227 to 280 W·m−1·K−1, which is comparable to that of bulk h-BN, indicating their potential use as important components to solve heat dissipation problems in thermal management configurations.

Preparation of carbon-coated iron oxide nanoparticles dispersed on graphene sheets and applications as advanced anode materials for lithium-ion batteries

We report a novel chemical vapor deposition (CVD) based strategy to synthesize carbon-coated Fe2O3 nanoparticles dispersed on graphene sheets (Fe2O3@C@G). Graphene sheets with high surface area and aspect ratio are chosen as space restrictor to prevent the sintering and aggregation of nanoparticles during high temperature treatments (800 °C). In the resulting nanocomposite, each individual Fe2O3 nanoparticle (5 to 20 nm in diameter) is uniformly coated with a continuous and thin (two to five layers) graphitic carbon shell. Further, the core-shell nanoparticles are evenly distributed on graphene sheets. When used as anode materials for lithium ion batteries, the conductive-additive-free Fe2O3@C@G electrode shows outstanding Li+ storage properties with large reversible specific capacity (864 mAh/g after 100 cycles), excellent cyclic stability (120% retention after 100 cycles at 100 mA/g), high Coulombic efficiency (∼99%), and good rate capability.

Bandgap Engineering of Coal-Derived Graphene Quantum Dots

Bandgaps of photoluminescent graphene quantum dots (GQDs) synthesized from anthracite have been engineered by controlling the size of GQDs in two ways: either chemical oxidative treatment and separation by cross-flow ultrafiltration, or by a facile one-step chemical synthesis using successively higher temperatures to render smaller GQDs. Using these methods, GQDs were synthesized with tailored sizes and bandgaps. The GQDs emit light from blue-green (2.9 eV) to orange-red (2.05 eV), depending on size, functionalities and defects. These findings provide a deeper insight into the nature of coal-derived GQDs and demonstrate a scalable method for production of GQDs with the desired bandgaps.

Lithium Batteries with Nearly Maximum Metal Storage

The drive for significant advancement in battery capacity and energy density inspired a revisit to the use of Li metal anodes. We report the use of a seamless graphene-carbon nanotube (GCNT) electrode to reversibly store Li metal with complete dendrite formation suppression. The GCNT-Li capacity of 3351 mAh g-1GCNT-Li approaches that of bare Li metal (3861 mAh g-1Li), indicating the low contributing mass of GCNT, while yielding a practical areal capacity up to 4 mAh cm-2 and cycle stability. A full battery based on GCNT-Li/sulfurized carbon (SC) is demonstrated with high energy density (752 Wh kg-1 total electrodes, where total electrodes = GCNT-Li + SC + binder), high areal capacity (2 mAh cm-2), and cyclability (80% retention at >500 cycles) and is free of Li polysulfides and dendrites that would cause severe capacity fade.

Atomic H-Induced Mo2C Hybrid as an Active and Stable Bifunctional Electrocatalyst

Mo2C 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.