Huagui Nie

Sulfur-Doped Graphene as an Efficient Metal-free Cathode Catalyst for Oxygen Reduction

Tailoring the electronic arrangement of graphene by doping is a practical strategy for producing significantly improved materials for the oxygen-reduction reaction (ORR) in fuel cells (FCs). Recent studies have proven that the carbon materials doped with the elements, which have the larger (N) or smaller (P, B) electronegative atoms than carbon such as N-doped carbon nanotubes (CNTs), P-doped graphite layers and B-doped CNTs, have also shown pronounced catalytic activity. Herein, we find that the graphenes doped with the elements, which have the similar electronegativity with carbon such as sulfur and selenium, can also exhibit better catalytic activity than the commercial Pt/C in alkaline media, indicating that these doped graphenes hold great potential for a substitute for Pt-based catalysts in FCs. The experimental results are believed to be significant because they not only give further insight into the ORR mechanism of these metal-free doped carbon materials, but also open a way to fabricate other new low-cost NPMCs with high electrocatalytic activity by a simple, economical, and scalable approach for real FC applications.

A Lightweight TiO2/Graphene Interlayer, Applied as a Highly Effective Polysulfide Absorbent for Fast, Long‐Life Lithium–Sulfur Batteries

DOI: 10.1002/adma.201405637 using microporous carbon paper and achieved signifi cant improvements not only in the use of the active material but also in the capacity retention. [ 12 ] More recently, various free-standing carbon interlayers including carbonized paper, a carbonized eggshell membrane, and an acetylene black mesh have been developed for the interception of migrating PS ions. [ 13–15 ] Investigating different categories of carbon interlayers has become a major avenue of current research into the insertion of interlayers. Meanwhile, non-carbon interlayers also appeared on stage. A typical example is that the Li + selective permselective membrane based on a coating layer of Nafi on blocks the diffusion of PS anions across the membrane to the Li anode, which greatly suppresses the shuttling of PS. [ 16 ] Since the diffusion of PS was localized on the cathode side, the cycling stability of the Li–S battery can be dramatically improved. Although these carbon and non-carbon interlayers have shown that it is possible to suppress the diffusion of PS and to improve the cycle-ability, some crucial issues remain to be resolved: i) As for these carbon and non-carbon interlayers, the complexity of the processes required for the synthesis of unique interlayers hinders their large-scale application, furthermore, the unsatisfactory thickness/weight of the applied interlayer may lead to a sharp decrease in the overall energy density, which may offset the gains in cell performance; ii) when the carbon interlayer acts only as a physical barrier, its nonpolar nature leads to weak interaction with polar PS anions, greatly reducing their ability to bind and confi ne these species during cycling. Moreover, the Li + ion transfer may be impeded by the physical barrier; iii) with regard to the non-carbon interlayer, a lower initial discharge capacity compared to the counterpart cathode was arisen likely due to increasing the resistance to some extent. To address shuttling of PS issues, the adjustment of the interlayer components may be a desirable strategy; in theory, an ideal interlayer should be able to selectively control the shuttling of PS anions via strong chemical interactions between them, while not disturbing the Li + ion transfer. Developing a lightweight and chemically selective interlayer carbon is therefore seems to be urgent. Previous reports have demonstrated the successful coupling of mesoporous TiO 2 additives to a C–S composite to improve the cycle life and the capacity retention. [ 6,17 ] It was shown recently that Li–S batteries could achieve 1000 cycles when the sulfur was coated with TiO 2 to create yolk–shell structures. [ 18 ] These results indicated that the mesoporous TiO 2 used in the coating layers promoted the interaction between TiO 2 and S, which was believed to be an electrostatic attraction (S–Ti–O) [ 6,19 ] that improved the surface adsorption of PS on the TiO 2 . Inspired by these results, and after comprehensively considering the three The development of advanced electrode materials with high energy/power density for energy storage is critical for a sustainable society. [ 1,2 ] Among existing materials, lithium–sulfur (Li–S) batteries show great potential for next-generation electrical energy storage applications. Sulfur cathodes, as well as being cost-effective and environmental friendly, provide a high theoretical capacity of 1675 mA h g −1 , a value that is an order of magnitude greater than typical values for conventional lithiated cathodes. [ 3 ] Despite the great promise of Li–S batteries, two main technical challenges must be addressed before they can fi nd practical use. First, the intrinsically poor electronic conductivity of sulfur leads to low use of the active material. Second, the high solubility of the polysulfi de’s (PS) reaction intermediaries (Li 2 S x , 4 < x < 8), and the action of their notorious “shuttle” mechanism in organic electrolytes, produce a rapid decline in the capacity, and a short cycle life. [ 4 ] To facilitate the development of the Li–S system, it is therefore crucial to improve the conductivity of the sulfur cathode and maintain/ reuse the soluble PS within the cathode structure. [ 5 ]

INVESTIGATION OF HOMOLOGOUS SERIES AS PRECURSORY HYDROCARBONS FOR ALIGNED CARBON NANOTUBE FORMATION BY THE SPRAY PYROLYSIS METHOD

The precursory carbon source is one of the key parameters which govern the formation of carbon nanotubes (CNTs). In this work, by selecting four homologous series, namely n-pentane, n-hexane, n-heptane and n-octane, as investigated targets, we comparatively study the relationship between thermodynamic properties of the precursory carbon source and formation of aligned CNTs. We find that all of these alkanes are favored for the growth of aligned CNTs in a suitable growth environment. But only n-heptane can yield the aligned CNTs with relatively high quality, high yield and narrow diameter distribution. Furthermore, after considering the link between thermodynamic properties of the precursory carbon source and the morphology characteristic of the nanotube samples, we find that the Gibbs free energy and formation enthalpy of precursory carbon sources play critical roles in the nanotube formation. In additions some possible explanations are proposed to better understand these phenomena. These rules will be ve...

Sulfur–nitrogen co-doped three-dimensional carbon foams with hierarchical pore structures as efficient metal-free electrocatalysts for oxygen reduction reactions

Despite the good progress in developing doped carbon catalysts for oxygen-reduction reaction (ORR), the current metal-free carbon catalysts are still far from satisfactory for large-scale applications of fuel cell. Developing new metal free doped carbon materials with abundance active sites as well as excellent electron transfer and reactant transport rate towards ORR may be a potential solution. Herein, we develop a novel three-dimensional (3D) sulfur-nitrogen co-doped carbon foams (S-N-CF) with hierarchical pore structures, using a convenient, economical, and scalable method. The experimental results have demonstrated that the obtained 3D S-N-CF exhibited better catalytic activity, longer-term stability and higher methanol tolerance than a commercial Pt/C catalyst. Such excellent performances may be attributed to the synergistic effect, which includes high catalytic sites for ORR provided by high S-N heteroatom loading, excellent reactant transport caused by hierarchical pore structures and high electron transfer rate provided by 3D continuous networks. Our results not only develop a new type of catalysts with excellent electrocatalytic performance by a commercially valid route, but also provide useful information for further clarification of the relationship between the microstructures of metal-free carbon materials and catalyst properties for ORR. More importantly, the idea to design hierarchical pore structures could be applied to other catalytic materials and serve as a general strategy for improving the activity of various ORR catalysts.

Nonenzymatic electrochemical detection of glucose using well-distributed nickel nanoparticles on straight multi-walled carbon nanotubes

A nonenzymatic electrochemical sensor device was fabricated for glucose detection based on nickel nanoparticles (NiNPs)/straight multi-walled carbon nanotubes (SMWNTs) nanohybrids, which were synthesized through in situ precipitation procedure. SMWNTs can be easily dispersed in solution after mild sonication pretreatment, which facilitates the precursor of NiNPs binding to their surface and results in the homogeneous distribution of NiNPs on the surface of SMWNTs. The morphology and component of the nanohybrids were characterized by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD), respectively. Cyclic voltammetry (CV) and amperometry were used to evaluate the catalytic activity of the NiNPs/SMWNTs nanohybrids modified electrode towards glucose. It was found that the nanohybrids modified electrode showed remarkably enhanced electrocatalytic activity towards the oxidation of glucose in alkaline solution compared to that of the bare glass carbon electrode (GCE), the NiNPs and the SMWNTs modified electrode, attributing to the synergistic effect of SMWNTs and Ni(2+)/Ni(3+) redox couple. Under the optimal detection conditions, the as-prepared sensors exhibited linear behavior in the concentration range from 1 μM to 1 mM for the quantification of glucose with a limit of detection of 500 nM (3σ). Moreover, the NiNPs/SMWNTs modified electrode was also relatively insensitive to commonly interfering species such as ascorbic acid (AA), uric acid (UA), dopamine (DA), galactose (GA), and xylose (XY). The robust selectivities, sensitivities, and stabilities determined experimentally indicated the great potential of NiNPs/SMWNTs nanohybrids for construction of a variety of electrochemical sensors.

Metal-free selenium doped carbon nanotube/graphene networks as a synergistically improved cathode catalyst for oxygen reduction reaction

The ongoing search for new non-precious-metal catalysts (NPMCs) with excellent electrocatalytic performance to replace Pt-based catalysts has been viewed as an important strategy to promote the development of fuel cells. Recent studies have proven that carbon materials doped with atoms which have a relatively small atomic size (e.g. N, B, P or S), have also shown pronounced catalytic activity. Herein, we demonstrate the successful fabrication of CNT/graphene doped with Se atoms, which has a relatively large atomic size, by a simple, economical, and scalable approach. The electrocatalytic performance of the resulting Se-doped CNT-graphene catalyst exhibits excellent catalytic activity, long-term stability, and a high methanol tolerance compared to commercial Pt/C catalysts. Our results confirmed that combining CNTs with graphene is an effective strategy to synergistically improve ORR activity. More importantly, it is also suggested that the development of graphite materials doped with Se or other heteroatoms of large size will open up a new route to obtain ideal NPMCs with realistic value for fuel cell applications.

Porous carbon nanotubes etched by water steam for high-rate large-capacity lithium–sulfur batteries

The current investigation of lithium–sulfur (Li–S) batteries faces three practical problems: (1) the poor conductivity of sulfur; (2) the notorious shuttle mechanism; and (3) the volume variation of the sulfur cathode. In principle, carbon nanotubes (CNTs) have a strong potential for improving sulfur usage because of their high electrical conductivity. Furthermore, opening holes in CNTs or creating pores on the walls is also a useful approach to not only enhance the diffusion of Li ions, but also enable more sulfur to fill the interior of the CNTs, which would be beneficial in retaining the soluble poly-sulfide intermediates and accommodate volume variations. Herein, we designed a mild one-step oxidation approach to create porous CNTs (PCNTs) through a chemical reaction between CNTs and rare oxygen sourced from a nebulized water stream at high temperatures. The higher specific surface area and pore volume values confirmed that PCNTs had significant porosity, compared with raw CNTs. When the PCNTs–S composites were tested as cathode materials in Li–S batteries, the cathode with 78 wt% S content exhibited an initial reversible capacity of 1382 mA h g−1 at 0.2 C. Furthermore, a reversible capacity of 150 mA h g−1 can be preserved, even at a very high current rate of 15 C. More importantly, it is also confirmed that a cathode with 89 wt% S content unexpectedly delivered a reversible capacity as high as 1165 mA h g−1sulfur/830 mA h g−1electrode at the initial cycle, and 792 mA h g−1sulfur/564 mA h g−1electrode after 200 cycles at a current rate of 0.2 C. To the best of our knowledge, such a high rate performance (15 C) and S loading (89 wt%) in cathodes of advanced Li–S batteries have been infrequently reported in previous research.

Facile construction of manganese oxide doped carbon nanotube catalysts with high activity for oxygen reduction reaction and investigations into the origin of their activity enhancement.

MnOx-doped carbon nanotube (MnOx-CNTs) catalysts for the oxygen reduction reaction (ORR) were fabricated using a simple electrochemical deposition method. MnOx-CNTs (0.85 wt % MnOx) could exhibit an improved electrocatalytic activity, long-term stability and excellent resistance to crossover-effect compared to Pt/C catalysts. High-resolution transmission electron microscopy (HRTEM) and X-ray diffraction analysis confirm that the MnOx in the MnOx-CNTs exists in an amorphous state. Moreover, compared to the catalytic performances of MnOx on other substrates, the MnOx-CNTs exhibit a high ORR activity. X-ray photoelectron spectroscopy results suggest that the electron transfer, from the CNTs to the Mn ions occurs and the high positive charge is generated on the MnOx-CNT surface. This is believed to be origin of the catalytic activity observed in the ORR using MnOx-CNTs.

A facile and general approach for the direct fabrication of 3D, vertically aligned carbon nanotube array/transition metal oxide composites as non-Pt catalysts for oxygen reduction reactions.

A simple and effective strategy involving nebulized ethanol assisted infiltration for the general synthesis of 3D structure-based vertically aligned carbon nanotube arrays (VACNTs) uniformly and deeply decorated with various transition-metal oxide (MOs) has been developed. Furthermore, it is demonstrated that the 3D structure-based VACNTs with decorated MnO2 can exhibit superior electrocatalytic oxygen reduction reaction activity, long-term stability, and an excellent resistance to crossover effects compared to the commercial Pt/C catalyst.

Polysulfide-Scission Reagents for the Suppression of the Shuttle Effect in Lithium–Sulfur Batteries

Lithium-sulfur batteries have become an appealing candidate for next-generation energy-storage technologies because of their low cost and high energy density. However, one of their major practical problems is the high solubility of long-chain lithium polysulfides and their infamous shuttle effect, which causes low Coulombic efficiency and sulfur loss. Here, we introduced a concept involving the dithiothreitol (DTT) assisted scission of polysulfides into lithium-sulfur system. Our designed porous carbon nanotube/S cathode coupling with a lightweight graphene/DTT interlayer (PCNTs-S@Gra/DTT) exhibited ultrahigh cycle-ability even at 5 C over 1100 cycles, with a capacity degradation rate of 0.036% per cycle. Additionally, the PCNTs-S@Gra/DTT electrode with a 3.51 mg cm-2 sulfur mass loading delivered a high initial areal capacity of 5.29 mAh cm-2 (1509 mAh g-1) at current density of 0.58 mA cm-2, and the reversible areal capacity of the cell was maintained at 3.45 mAh cm-2 (984 mAh g-1) over 200 cycles at a higher current density of 1.17 mA cm-2. Employing this molecule scission principle offers a promising avenue to achieve high-performance lithium-sulfur batteries.