Porous nickel disulfide/reduced graphene oxide nanohybrids with improved electrocatalytic performance for hydrogen evolution

Single porous nickel disulfide (NiS₂) nanoballs and nanohybrids of NiS₂ with reduced graphene oxide (NiS₂/rGO) were successfully prepared by a simple hydrothermal process in the absence or presence of graphene oxide. NiS₂/rGO nanocomposites exhibit remarkable electrocatalytic performance for hydrogen evolution from water splitting due to the plentiful active sites in the porous NiS₂, the improved conductivity and the positive synergetic effect between NiS₂ and rGO. The nanocomposites displayed superior activity for the hydrogen evolution reaction (10 mA cm^− 2 vs. − 200 mV, Tafel slope of 52 mV dec^− 1) and an excellent electrocatalytic stability.     

Heteroatom-doped carbon nanorods with improved electrocatalytic activity toward oxygen reduction in an acidic medium

High-performance heteroatom-doped carbon catalysts with large surface areas were prepared by pyrolyzing nanorod precursors that had been synthesized by polymerizing a mixture of aniline (An) and β-naphthalene sulfonic acid (NSA). The catalysts were characterized by scanning and transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, N₂ adsorption/desorption isotherms, and elemental analysis. We intensively investigated how the catalysts’ structure and catalytic performance were affected by (i) the ratio of NSA to An and (ii) the addition of Fe. The catalysts retained their nanorod morphology after pyrolysis. The optimal NSA/An ratio was 3/2 and the optimal Fe content was 3 wt%. The catalysts showed excellent activity toward oxygen reduction in an acidic medium, with the onset potential, half-wave potential, and limiting current density values reaching 0.86, 0.73 V (vs. reversible hydrogen electrode), and 5.28 mA cm⁻², respectively. We suggest that the catalysts’ high performance may be due to the co-doping effects of nitrogen, sulfur, and iron, as well as the large surface area created by the nanorod structures.     

Reaction mechanisms of graphene oxide chemical reduction by sulfur-containing compounds

We used density functional theory to study the reaction mechanisms of chemical reduction of graphene oxide (GO) by the sulfur-containing compounds HSO₃⁻ and H₂SO₃. We studied the reaction energy profiles for the following reactions: dehydroxylation of GO with one and two hydroxyl groups, de-epoxidation of GO with one or two epoxy groups and decarboxylation and decarbonylation of GO with carboxyl and carbonyl groups. We found that hydroxyl and epoxide groups could be easily reduced because of the lower energy barriers, whereas decarboxylation and decarbonylation reactions are not kinetically and thermodynamically easy because of the higher energy barriers. These reaction mechanisms at the atomistic level are not only supported by Chen’s experimental results [J. Phys. Chem. C 2010, 114, 19885], but are also beneficial for the development of new agents that could efficiently reduce GO.     

The improved electrocatalytic activity of palladium/graphene nanosheets towards ethanol oxidation by tin oxide

Tin oxide (SnO₂)/graphene nanosheets (GNS) composite was prepared by a simple chemical-solution method as the catalyst support for direct ethanol fuel cells. Then the SnO₂-GNS composites supporting Pd (Pd/SnO₂-GNS) catalysts were synthesized by a microwave-assisted reduction process. The Pd/SnO₂-GNS catalysts were characterized by using X-ray diffraction, transmission electron microscopy and energy-dispersive spectroscopy techniques. The electrocatalytic performances of Pd/SnO₂-GNS catalysts for ethanol oxidation were studied by cyclic voltammetric and chronoamperometric measurements. It was found that compared with Pd/GNS, the Pd/SnO₂-GNS catalyst showed superior electrocatalytic activity for ethanol oxidation when the mass ratio of SnCl₂·2H₂O precursor salt to graphite oxide was about 1:2.     

Multivalent manganese oxides with high electrocatalytic activity for oxygen reduction reaction

A noble-metal-free catalyst based on both Mn₃O₄ and MnO was prepared by using the dielectric barrier discharge technique at moderate temperature. The prepared catalyst shows a higher electrocatalytic activity towards the oxygen reduction reaction than the catalyst prepared by using the traditional calcination process. The enhanced activity could be due to the coexistence of manganese ions with different valences, the higher oxygen adsorption capacity, and the suppressed aggregation of the catalyst nanoparticles at moderate temperature. The present work would open a new way to prepare low-cost and noble-metal-free catalysts at moderate temperature for more efficient electrocatalysis.

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One-step solid state preparation of reduced graphene oxide

We have developed an easy and scalable chemical reduction method assisted by microwave irradiation for the synthesis of reduced graphene oxide (RGO) nanosheets in solid state. The as-synthesized RGO is characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, thermogravimetry, X-ray diffraction, X-ray photoelectron spectroscopy and atomic force microscopy. It is revealed that the bulk of the oxygen-containing functional groups are removed from graphene oxide with this one-step reduction method and monolayer RGO sheets are got from its N,N-dimethyl formamide solution. It is found that the ammonium bicarbonate plays a key role in the preparation of RGO. Considering the analysis results, a mechanism for the formation of RGO is proposed. Besides being eco-friendly, when compared to previous chemical techniques, this process has several advantages like low cost, simplicity and short processing times, which may find practical applications in the preparation of graphene-based composites.     

氮掺杂石墨烯/多孔碳复合材料的制备及其氧还原催化性能

采用简单、无模板的方法制备了氮掺杂多孔石墨烯/碳复合材料(NPGC)。采用SEM、XRD、Raman、XPS等分析手段对NPGC的形貌、组成以及结构进行了表征，利用旋转圆盘电极技术测试了其电催化氧还原反应(ORR)活性。结果表明，葡萄糖在水热后生成的碳与石墨烯成功复合，并在950℃炭化、活化后形成了相互渗透、结构良好的三维片状多孔网络结构；其氮含量高达9.47%。NPGC作为一种高效的非金属ORR电催化剂，在碱性溶液中具有较高的起始电位[0.87 V(vs RHE)]和较大的极限电流密度(4.7 mA?cm^?2)，以及其ORR平均转移电子数为3.8。与商业Pt/C催化剂相比，NPGC具有较强的耐甲醇性和长期耐久性，且制备成本较低，具有广阔的应用前景。     

The reduction of graphene oxide

Graphene has attracted great interest for its excellent mechanical, electrical, thermal and optical properties. It can be produced by micro-mechanical exfoliation of highly ordered pyrolytic graphite, epitaxial growth, chemical vapor deposition, and the reduction of graphene oxide (GO). The first three methods can produce graphene with a relatively perfect structure and excellent properties, while in comparison, GO has two important characteristics: (1) it can be produced using inexpensive graphite as raw material by cost-effective chemical methods with a high yield, and (2) it is highly hydrophilic and can form stable aqueous colloids to facilitate the assembly of macroscopic structures by simple and cheap solution processes, both of which are important to the large-scale uses of graphene. A key topic in the research and applications of GO is the reduction, which partly restores the structure and properties of graphene. Different reduction processes result in different properties of reduced GO (rGO), which in turn affect the final performance of materials or devices composed of rGO. In this contribution, we review the state-of-art status of the reduction of GO on both techniques and mechanisms. The development in this field will speed the applications of graphene.     

The physical–chemical properties and electrocatalytic performance of iridium oxide in oxygen evolution

An IrO₂ anode catalyst was prepared by using the Adams method for the application of a solid polymer electrolyte (SPE) water electrolyzer. The effect of calcination temperature on the physical–chemical properties and the electrochemical performance of IrO₂ were examined to obtain a low loading and a high catalytic activity of oxygen evolution at the electrode. The physical–chemical properties were studied via thermogravimetry–differential scanning calorimetry (TG–DSC), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The electrochemical activity was investigated by using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronopotentiometry in 0.1 mol L⁻¹ H₂SO₄ at room temperature. The optimum condition was found to be at the calcination temperature of 500 °C, where the total polarization reached a minimum at high current densities (>200 mA cm⁻²). The optimized catalyst was also applied to a membrane electrode assembly (MEA) and stationary current–potential relationships were investigated. With an optimized catalytic IrO₂ loading of 1.5 mg cm⁻² and a 40% Pt/C loading of 0.5 mg cm⁻², the terminal applied potential difference was 1.72 V at 2 A cm⁻² and 80 °C in a SPE water electrolysis cell.     

Simple preparation of nanoporous few-layer nitrogen-doped graphene for use as an efficient electrocatalyst for oxygen reduction and oxygen evolution reactions

We prepared nitrogen-doped graphene (NG) by simple pyrolysis of graphene oxide and polyaniline, which was selected as the N source. The resulting NG contains 2.4 at.% N, of which as high as 1.2 at.% is quaternary N. Electrochemical characterizations reveal that the NG has excellent catalytic activity toward oxygen reduction reaction (ORR) in an alkaline electrolyte, including a desirable four-electron pathway for the formation of water, large kinetic-limiting current density, long-term stability and good tolerance to methanol crossover. In addition, we demonstrate that the NG also has high catalytic activity toward oxygen evolution reaction (OER), rendering its potential application as a bi-functional catalyst for both ORR and OER.     

In situ chemical reduction and functionalization of graphene oxide for electrically conductive phenol formaldehyde composites

We report an efficient one-step approach to reduce and functionalize graphene oxide (GO) during the in situ polymerization of phenol and formaldehyde. The hydrophilic and electrically insulating GO is converted to hydrophobic and electrically conductive graphene with phenol as the main reducing agent. Simultaneously, functionalization of GO is realized by the nucleophilic substitution reaction of the epoxide groups of GO with the hydroxyl groups of phenol in an alkali condition. Different from the insulating GO and phenol formaldehyde resin (PF) components, PF composites are electrically conductive due to the incidental reduction of GO during the in situ polymerization. The electrical conductivity of PF composite with 0.85 vol.% of GO is 0.20 S/m, nearly nine orders of magnitude higher than that of neat PF. Moreover, the efficient reduction and functionalization of GO endows the PF composites with high thermal stability and flexural properties. A striking increase in decomposition temperature is achieved with 2.3 vol.% of GO. The flexural strength and modulus of the PF composite with 1.7 vol.% GO are increased by 316.8% and 56.7%, respectively.     

Fe/N/C catalysts systhesized using graphene aerogel for electrocatalytic oxygen reduction reaction in an acidic condition

Graphene aerogel was modified with polyaniline and Fe precursors to produce Fe/N/C catalysts for electrocatalytic oxygen reduction reaction in the acidic condition. The graphene aerogel was produced by a simple hydrothermal treatment of graphene oxide dispersion with a high surface area. Aniline was polymerized with the graphene aerogel powder, and the pyrolysis of the resulting material with FeCl₃ produced Fe/N/C catalyst. The loading amount on the electrode and the catalyst ink concentration was carefully selected to avoid the mass transfer limitation inside the catalyst layer. The pyrolysis temperature affected the states of nitrogen sites on the catalyst; the sample prepared at 900 °C presented the highest mass activity. The sulfur was also doped with various amounts of FeSO₄ with enhanced mass activity of up to 2.1 mA/mg at 0.8 V in 0.5 M H₂SO₄ solution. Its durability was also tested by repeating cyclic voltammetry in a range of 0.6–1.1 V 5000 cycles. This graphene-aerogel-based carbon catalysts showed improved activity and durability for the oxygen reduction reaction in the acidic condition.     

Platinum crystallite size considerations for electrocatalytic oxygen reduction—I

An examination of the activity contributions for atoms at corners, edges and planes of platinum electro-catalyst crystallites was carried out for the reduction of molecular oxygen in 1M H₂SO₄. Varying the mean crystallite diameters between 30 and 400 Å caused at least a 200-fold and 30-fold change in the surface densities of corner and edge sites, respectively. Deliberate sintering of the platinum crystallites caused crystallite growth, with a reduction of surface and lattice defects. The catalysts showed a uniform activity for oxygen reduction of 0·018 ± 0·003 mA/real cm² Pt at 900 mV (nhe) with a Tafel slope of 65 ± 5 mV at 50 and 70°C. Platinum atoms at corners, edges, kink sites or dislocations are not more active than atoms on the crystallite faces for this reaction.     

Solvent-free mechanochemical reduction of graphene oxide

We report a versatile and eco-friendly approach for the reduction of graphene oxide into high-quality graphene nanoplatelets by simple solid-state mechanochemical ball-milling in the presence of hydrogen. After the ball-milling process, the resultant graphene nanoplatelets show the efficient restoration of the graphitic structure completely free from any heteroatom doping (e.g., nitrogen, sulfur) and enhanced electrical conductivities up to 120 and 3400 S/m before and after an appropriate heat treatment (e.g., 900 °C for 2 h under nitrogen).     

In-situ graphene oxide reduction during UV-photopolymerization of graphene oxide/acrylic resins mixtures

The preparation of electrically conductive acrylic resins containing reduced graphene oxide (rGO) by photopolymerization is presented. The synthesis consists of a single-step procedure starting from a homogeneous water dispersion of GO, which undergoes reduction induced by the UV radiation during the photopolymerization of an acrylic resin. The role played by the amount of radical photoinitiator added to the resin has been evaluated in relation to the in-situ reduction of GO, that was monitored by X-ray photoelectron spectroscopy. Results show that the UV-induced photopolymerization of acrylic resins with added GO gives rise to conductive acrylic composites thanks to the simultaneous reduction of GO to rGO and crosslinking of the resin. On this basis UV-induced photopolymerization is proposed as a sustainable strategy for the production of conductive graphene/polymer composites.