NiCo₂S₄@graphene as a Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions

Here, the hybrid of NiCo₂S₄ nanoparticles grown on graphene in situ is first described as an effective bifunctional nonprecious electrocatalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the alkaline medium. NiCo₂S₄@N/S-rGO was synthesized by a one-pot solvothermal strategy using Co­(OAc)₂, Ni­(OAc)₂, thiourea, and graphene oxide as precursors and ethylene glycol as the dispersing agent; simultaneously, traces of nitrogen and sulfur were double-doped into the reduced graphene oxide (rGO) in the forms of pyrrolic-N, pyridinic-N, and thiophenic-S, which are often desirable for metal-free ORR catalysts. In comparison with commercial Pt/C catalyst, NiCo₂S₄@N/S-rGO shows less reduction activity, much better durability, and superior methanol tolerance toward ORR in 0.1 M KOH; it reveals higher activity toward OER in both KOH electrolyte and phosphate buffer at pH 7.0. NiCo₂S₄@graphene demonstrated excellent overall bicatalytic performance, and importantly, it suggests a novel kind of promising nonprecious bifunctional catalyst in the related renewable energy devices

Chemical Foaming Coupled Self-Etching: A Multiscale Processing Strategy for Ultrahigh-Surface-Area Carbon Aerogels

Due to the unique structure, carbon aerogels have always shown great potential for multifunctional applications. At present, it is highly desirable but remains challenging to tailor the microstructures with respect to porosity and specific surface area to further expand its significance. A facile chemical foaming coupled self-etching strategy is developed for multiscale processing of carbon aerogels. The strategy is directly realized via the pyrolysis of a multifunctional precursor (pentaerythritol melamine phosphate) without any special drying process and multiple steps. In the micrometer scale, the macroporous scaffold structures with interconnected and strutted carbon nanosheets are built up by chemical foaming from decomposition of melamine, whereas the meso/microporous nanosheets are formed via self-etching by phosphorus-containing species. The delicately hierarchical structures and record-breaking specific surface area of 2668.4 m² g^–1 render the obtained carbon aerogels great potentials for absorption (324.1–593.6 g g^–1 of absorption capacities for varied organic solvents) and energy storage (338 F g^–1 of specific capacitance). The construction of such novel carbon nanoarchitecture will also shed light on the design and synthesis of multifunctional materials

Efficient Oxygen Reduction Electrocatalyst Based on Edge-Nitrogen-Rich Graphene Nanoplatelets: Toward a Large-Scale Synthesis

The large-scale synthesis of nitrogen doped graphene (N-graphene) with high oxygen reduction reaction (ORR) performance has received a lot of attention recently. In this work, we have developed a facile and economical procedure for mass production of edge-nitrogen-rich graphene nanoplatelets (ENR-GNPs) by a combined process of ball milling of graphite powder (GP) in the presence of melamine and subsequent heat treatment. It is found that the ball milling process can not only crack and exfoliate pristine GP into edge-expanded nanoplatelets but also mechanically activate GP to generate appropriate locations for N-doping. Analysis results indicate that the doped N atoms mainly locate on the edge of the graphitic matrix, which contains ca. 3.1 at.% nitrogen content and can be well-dispersed in aqueous to form multilayer nanoplatelets. The as-prepared ENR-GNPs electrocatalyst exhibits highly electrocatalytic activity for ORR due to the synergetic effects of edge-N-doping and nanosized platelets. Besides, the stability and methanol tolerance of ENR-GNPs are superior to that of the commercial Pt/C catalyst, which makes the nanoplatelets a promising candidate for fuel cell cathode catalysts. The present approach opens up the possibility for simple and mass production of N-graphene based electrocatalysts in practice

Interconnected Phosphorus and Nitrogen Codoped Porous Exfoliated Carbon Nanosheets for High-Rate Supercapacitors

Carbon-based supercapacitors have high power density and long cycle life; however, they are known to suffer from problems related to low energy density and high inner resistance. Here, we report a novel hierarchically porous functional carbon that is made up of interconnected exfoliated carbon nanosheets with thickness of a few nanometers. Notably, these porous carbon nanosheets are doped with abundant nitrogen (N) dopants in the basal plane and phosphorus (P) functional groups at the edge of the graphene lattice. The specific surface chemistry and pore structure of the synthesized sample, combined with its large specific surface area, make it a high-performance active material for supercapacitor electrode. The obtained supercapacitor made with the optimized sample showed a high specific capacitance (265 F g^–1 at 0.5 A g^–1) as well as long-term stability (94% capacitance retention after 5000 cycles). Particularly, the enhanced electrochemical characteristics were maintained even at high electrode mass loading (time constant (τ₀) is 1.10 s for an electrode mass loading of 12.38 mg cm^–2 compared to 1.61 s for a mass loading of 4.17 mg cm^–2 for commercial activated carbon), which is important for a high packing factor of the capacitor

Identifying the Active Site in Nitrogen-Doped Graphene for the VO²⁺/VO₂⁺ Redox Reaction

Nitrogen-doped graphene sheets (NGS), synthesized by annealing graphite oxide (GO) with urea at 700–1050 °C, were studied as positive electrodes in a vanadium redox flow battery. The NGS, in particular annealed at 900 °C, exhibited excellent catalytic performance in terms of electron transfer (ET) resistance (4.74 ± 0.51 and 7.27 ± 0.42 Ω for the anodic process and cathodic process, respectively) and reversibility (Δ<i><i>E</i></i> = 100 mV, <i>I</i>_pa/<i>I</i>_pc = 1.38 at a scan rate of 50 mV s^–1). Detailed research confirms that not the nitrogen doping level but the nitrogen type in the graphene sheets determines the catalytic activity. Among four types of nitrogen species doped into the graphene lattice including pyridinic-N, pyrrolic-N, quaternary nitrogen, and oxidic-N, quaternary nitrogen is verified as a catalytic active center for the [VO]²⁺/[VO₂]⁺ couple reaction. A mechanism is proposed to explain the electrocatalytic performance of NGS for the [VO]²⁺/[VO₂]⁺ couple reaction. The possible formation of a N–V transitional bonding state, which facilitates the ET between the outer electrode and reactant ions, is a key step for its high catalytic activity

Controllable Synthesis of Cobalt Monoxide Nanoparticles and the Size-Dependent Activity for Oxygen Reduction Reaction

In this work, carbon-supported cobalt monoxides with an average size of 3.5, 4.9, and 6.5 nm are synthesized via a facile colloidal method avoiding any surfactants of long chains. Along with controlling the CoO particle size, we investigate the dependence of ORR activity on particle size of the CoO/C composite. It is discovered that the turnover frequency of the ORR per CoO site is largely independent of the particle size in the range of 3–7 nm, and the enhanced ORR activity for the smaller CoO particles is attributed to the enlarged interface between CoO and carbon