Keying Shi

Nitrogen-doped graphene with high nitrogen level via a one-step hydrothermal reaction of graphene oxide with urea for superior capacitive energy storage

Nitrogen-doped graphene nanosheets (NGS) with the nitrogen level as high as 10.13 atom% were synthesized via a simple hydrothermal reaction of graphene oxide (GO) and urea. N-doping and reduction of GO were achieved simultaneously under the hydrothermal reaction. In the fabrication, the nitrogen-enriched urea plays a pivotal role in forming the NGS with a high nitrogen level. During the hydrothermal process, the N-dopant of urea could release NH3 in a sustained manner, accompanied by the released NH3 reacting with the oxygen functional groups of the GO and then the nitrogen atoms doped into graphene skeleton, leading to the formation of NGS. The nitrogen level and species could be conveniently controlled by easily tuning the experimental parameters, including the mass ratio between urea and GO and the hydrothermal temperature. Remarkably, in 6 M KOH electrolyte, the synthesized NGS with both high nitrogen (10.13 atom%) and large surface area (593 m2 g−1) exhibits excellent capacitive behaviors (326 F g−1, 0.2 A g−1), superior cycling stability (maintaining initial capacity even) and coulombic efficiency (99.58%) after 2000 cycles. The energy density of 25.02 Wh kg−1 could be achieved at power density of 7980 W kg−1 by a two-electrode symmetric capacitor test. A series of experiments results demonstrated that not only the N-content but also the N-type are very significant for the capacitive behaviors. In more detail, the pyridinic-N and pyrrolic-N play mainly roles for improving pseudo-capacitance by the redox reaction, while quaternary-N could enhance the conductivity of the materials which is favorable to the transport of electrons during the charge/discharge process. Hence, the approach in this work could provide a new way for preparing NGS materials which could be used as advanced electrodes in high performance supercapacitors.

Small‐Sized and Contacting Pt–WC Nanostructures on Graphene as Highly Efficient Anode Catalysts for Direct Methanol Fuel Cells

The synergistic effect between Pt and WC is beneficial for methanol electro-oxidation, and makes Pt-WC catalyst a promising anode candidate for the direct methanol fuel cell. This paper reports on the design and synthesis of small-sized and contacting Pt-WC nanostructures on graphene that bring the synergistic effect into full play. Firstly, DFT calculations show the existence of a strong covalent interaction between WC and graphene, which suggests great potential for anchoring WC on graphene with formation of small-sized, well-dispersed WC particles. The calculations also reveal that, when Pt attaches to the pre-existing WC/graphene hybrid, Pt particles preferentially grow on WC rather than graphene. Our experiments confirmed that highly disperse WC nanoparticles (ca. 5 nm) can indeed be anchored on graphene. Also, Pt particles 2-3 nm in size are well dispersed on WC/graphene hybrid and preferentially grow on WC grains, forming contacting Pt-WC nanostructures. These results are consistent with the theoretical findings. X-ray absorption fine structure spectroscopy further confirms the intimate contact between Pt and WC, and demonstrates that the presence of WC can facilitate the crystallinity of Pt particles. This new Pt-WC/graphene catalyst exhibits a high catalytic efficiency toward methanol oxidation, with a mass activity 1.98 and 4.52 times those of commercial PtRu/C and Pt/C catalysts, respectively.

Facile synthesis of novel 3D nanoflower-like CuxO/multilayer graphene composites for room temperature NOx gas sensor application

3D nanoflower-like CuxO/multilayer graphene composites (CuMGCs) have been successfully synthesized as a new type of room temperature NOx gas sensor. Firstly, the expanded graphite (EG) was activated by KOH and many moderate functional groups were generated; secondly, Cu(CH3COO)2 and CTAB underwent full infusion into the interlayers of activated EG (aEG) by means of a vacuum-assisted technique and then reacted with the functional groups of aEG accompanied by the exfoliation of aEG via reflux. Eventually, the 3D nanoflower consisting of 5-9 nm CuxO nanoparticles homogeneously grow in situ on aEG. The KOH activation of EG plays a key role in the uniform formation of CuMGCs. When being used as gas sensors for detection of NOx, the CuMGCs achieved a higher response at room temperature than that of the corresponding CuxO. In detail, the CuMGCs show a higher NOx gas sensing performance with low detection limit of 97 ppb, high gas response of 95.1% and short response time of 9.6 s to 97.0 ppm NOx at room temperature. Meanwhile, the CuMGC sensor presents a favorable linearity, good selectivity and stability. The enhancement of the sensing response is mainly attributed to the improved conductivity of the CuMGCs. A series of Mott-Schottky and EIS measurements demonstrated that the CuMGCs have much higher donor densities than CuxO and can easily capture and migrate electrons from the conduction band, resulting in the enhancement of electrical conductivity.

One-pot synthesis of a nitrogen and phosphorus-dual-doped carbon nanotube array as a highly effective electrocatalyst for the oxygen reduction reaction

A nitrogen and phosphorus-dual-doped carbon nanotube (N, P-CNT) array has been successfully synthesized by a novel one-pot method, using an aminophosphonic acid resin as the N, P and C sources. The N, P-CNTs are open with large inner channels, allowing oxygen molecules to access a large number of catalytically active sites on the inner walls. The N, P-CNTs are not only comparable to Pt/C in electrocatalytic activity for the oxygen reduction reaction (ORR) in 0.1 M KOH, but are also highly stable and tolerant to methanol and CO poisoning. An onset potential of 0.95 V close to that of Pt/C and a well-defined limiting current plateau for the ORR are observed. Moreover, there is almost no visible current density decrease on N, P-CNTs after 5000 cycles.

Highly dispersed Ni-decorated porous hollow carbon nanofibers: fabrication, characterization, and NOx gas sensors at room temperature

Nanostructure arrays of porous hollow carbon nanofibers (HCFs) with highly dispersed functionalized Ni nanoparticles (Ni-NPs) are directly synthesized by the anodic aluminium oxide (AAO) template. HCFs with Ni-NPs are obtained after thermal decomposition of a mixture of glucose and nickel acetate inside the cylindrical nanochannels of the AAO template followed by removal of the AAO template. In the synthesis, Ni(II) ions could act as catalyst for the graphitization of amorphous carbon, and simultaneously be reduced to functionalized metal Ni-NPs. SEM, TEM, XRD, nitrogen adsorption–desorption and Raman spectroscopy reveal that the HCFs-Ni-NPs, with porous nanostructure and an average diameter of ∼60 nm, are successfully obtained, and Ni-NPs are uniformly distributed and highly dispersed on HCFs. The HCFs-Ni-NPs have the good adsorption property of carbon materials, and the functional Ni-NPs show performance for NOx gas sensor. It is demonstrated that the HCF-Ni-NP films detect NOx gas molecules with fast response time and good sensitivity in air at room temperature owing to the structural property of HCFs and highly dispersed functional Ni-NPs. This method is promising due to its inexpensive starting materials, no pollution, direct fabrication, highly dispersed functional nanoparticles and good gas-sensing properties.

Single-step pyrolytic preparation of Mo2C/graphitic carbon nanocomposite as catalyst carrier for the direct liquid-feed fuel cells

The work presented a practical and low-cost strategy for the preparation of Mo2C/graphitic carbon nanocomposite (Mo2C/GC). This synthesis has been conducted by a single-step heat treatment of resin–MoO42−–[Fe(CN)6]4− precursor in N2 atmosphere. The final material consists of well-dispersed Mo2C nanoparticles (typically 10–15 nm) on graphitic carbon. The formation process of Mo2C/GC has been investigated. The resultant material exhibits good supporting effect on Pt and Pd catalysts toward electro-oxidation of methanol, ethanol and formyl acid. This method is very simple and enables a systematic design of new, predictable catalysts.

Ni2P Entwined by Graphite Layers as a Low-Pt Electrocatalyst in Acidic Media for Oxygen Reduction

A simple and feasible strategy was reported to construct Ni2P nanostructures entwined by graphite layers (Ni2P/GC). In this process, a commercial amino phosphonic acid chelating resin was adopted as both the phosphorus and carbon resources. Then, Ni2+ was introduced into the resin framework via ionic exchange and chelation to form a resin-Ni2+ precursor. After carbonization, the highly dispersed Ni2P particles, coupled with thin graphite layers, were simultaneously synthesized in situ. A ternary 7.5% Pt-Ni2P/GC catalyst was further obtained by loading 7.5 wt % Pt on Ni2P/GC. For the oxygen reduction reaction in acidic media, the 7.5% Pt-Ni2P/GC catalyst exhibited even more positive onset (1.03 V) and half-wave (0.93 V) potentials, as well as a rather higher mass activity of 565.3 mA mgPt-1 and a better long-term stability than those of the commercial 20% Pt/C (JM) electrocatalyst. The improved reaction kinetics is mainly attributed to the synergistic effect between Pt and Ni2P/GC. This work not only provides a method for the synthesis of phosphides but also gives insight into the synergy between Pt and Ni2P, which is helpful for the development of more low-Pt catalysts in acidic media.