Errol L. G. Samuel

Atomic cobalt on nitrogen-doped graphene for hydrogen generation

Reduction of water to hydrogen through electrocatalysis holds great promise for clean energy, but its large-scale application relies on the development of inexpensive and efficient catalysts to replace precious platinum catalysts. Here we report an electrocatalyst for hydrogen generation based on very small amounts of cobalt dispersed as individual atoms on nitrogen-doped graphene. This catalyst is robust and highly active in aqueous media with very low overpotentials (30 mV). A variety of analytical techniques and electrochemical measurements suggest that the catalytically active sites are associated with the metal centres coordinated to nitrogen. This unusual atomic constitution of supported metals is suggestive of a new approach to preparing extremely efficient single-atom catalysts.

Coal as an abundant source of graphene quantum dots

Coal is the most abundant and readily combustible energy resource being used worldwide. However, its structural characteristic creates a perception that coal is only useful for producing energy via burning. Here we report a facile approach to synthesize tunable graphene quantum dots from various types of coal, and establish that the unique coal structure has an advantage over pure sp2-carbon allotropes for producing quantum dots. The crystalline carbon within the coal structure is easier to oxidatively displace than when pure sp2-carbon structures are used, resulting in nanometre-sized graphene quantum dots with amorphous carbon addends on the edges. The synthesized graphene quantum dots, produced in up to 20% isolated yield from coal, are soluble and fluorescent in aqueous solution, providing promise for applications in areas such as bioimaging, biomedicine, photovoltaics and optoelectronics, in addition to being inexpensive additives for structural composites.

Laser-induced porous graphene films from commercial polymers

Synthesis and patterning of carbon nanomaterials cost effectively is a challenge in electronic and energy storage devices. Here report a one-step, scalable approach for producing and patterning porous graphene films with 3-dimensional networks from commercial polymer films using a CO2 infrared laser. The sp3-carbon atoms are photothermally converted to sp2-carbon atoms by pulsed laser irradiation. The resulting laser-induced graphene (LIG) exhibits high electrical conductivity. The LIG can be readily patterned to interdigitated electrodes for in-plane microsupercapacitors with specific capacitances of >4 mF·cm−2 and power densities of ~9 mW·cm−2. Theoretical calculations partially suggest that enhanced capacitance may result from LIG’s unusual ultra-polycrystalline lattice of pentagon-heptagon structures. Combined with the advantage of one-step processing of LIG in air from commercial polymer sheets, which would allow the employment of a roll-to-roll manufacturing process, this technique provides a rapid route to polymer-written electronic and energy storage devices.

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.

Large-Area Bernal-Stacked Bi-, Tri-, and Tetralayer Graphene

Few-layer graphene, with Bernal stacking order, is of particular interest to the graphene community because of its unique tunable electronic structure. A synthetic method to produce such large area graphene films with precise thickness from 2 to 4 layers would be ideal for chemists and physicists to explore the promising electronic applications of these materials. Here, large-area uniform Bernal-stacked bi-, tri-, and tetralayer graphene films were successfully synthesized on a Cu surface in selective growth windows, with a finely tuned total pressure and CH(4)/H(2) gas ratio. On the basis of the analyses obtained, the growth mechanism is not an independent homoexpitaxial layer-by-layer growth, but most likely a simultaneous-seeding and self-limiting process.

Nanocomposite of Polyaniline Nanorods Grown on Graphene Nanoribbons for Highly Capacitive Pseudocapacitors

A facile and cost-effective approach to the fabrication of a nanocomposite material of polyaniline (PANI) and graphene nanoribbons (GNRs) has been developed. The morphology of the composite was characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron microscopy, and X-ray diffraction analysis. The resulting composite has a high specific capacitance of 340 F/g and stable cycling performance with 90% capacitance retention over 4200 cycles. The high performance of the composite results from the synergistic combination of electrically conductive GNRs and highly capacitive PANI. The method developed here is practical for large-scale development of pseudocapacitor electrodes for energy storage.

Flexible and stackable laser-induced graphene supercapacitors.

In this paper, we demonstrate that by simple laser induction, commercial polyimide films can be readily transformed into porous graphene for the fabrication of flexible, solid-state supercapacitors. Two different solid-state electrolyte supercapacitors are described, namely vertically stacked graphene supercapacitors and in-plane graphene microsupercapacitors, each with enhanced electrochemical performance, cyclability, and flexibility. Devices with a solid-state polymeric electrolyte exhibit areal capacitance of >9 mF/cm2 at a current density of 0.02 mA/cm2, more than twice that of conventional aqueous electrolytes. Moreover, laser induction on both sides of polyimide sheets enables the fabrication of vertically stacked supercapacitors to multiply its electrochemical performance while preserving device flexibility.

Cobalt nanoparticles embedded in nitrogen-doped carbon for the hydrogen evolution reaction.

There is great interest in renewable and sustainable energy research to develop low-cost, highly efficient, and stable electrocatalysts as alternatives to replace Pt-based catalysts for the hydrogen evolution reaction (HER). Though nanoparticles encapsulated in carbon shells have been widely used to improve the electrode performances in energy storage devices (e.g., lithium ion batteries), they have attracted less attention in energy-related electrocatalysis. Here we report the synthesis of nitrogen-enriched core-shell structured cobalt-carbon nanoparticles dispersed on graphene sheets and we investigate their HER performances in both acidic and basic media. These catalysts exhibit excellent durability and HER activities with onset overpotentials as low as ∼70 mV in both acidic (0.5 M H2SO4) and alkaline (0.1 M NaOH) electrolytes, and the overpotentials needed to deliver 10 mA cm(-2) are determined to be 265 mV in acid and 337 mV in base, further demonstrating their potential to replace Pt-based catalysts. Control experiments reveal that the active sites for HER might come from the synergistic effects between the cobalt nanoparticles and nitrogen-doped carbon.

Functionalized Low Defect Graphene Nanoribbons and Polyurethane Composite Film for Improved Gas Barrier and Mechanical Performances

A thermoplastic polyurethane (TPU) composite film containing hexadecyl-functionalized low-defect graphene nanoribbons (HD-GNRs) was produced by solution casting. The HD-GNRs were well distributed within the polyurethane matrix, leading to phase separation of the TPU. Nitrogen gas effective diffusivity of TPU was decreased by 3 orders of magnitude with only 0.5 wt % HD-GNRs. The incorporation of HD-GNRs also improved the mechanical properties of the composite films, as predicted by the phase separation and indicated by tensile tests and dynamic mechanical analyses. The improved properties of the composite film could lead to potential applications in food packaging and lightweight mobile gas storage containers.

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.