Jiaoxing Xu

Sulfur and Nitrogen Co-Doped, Few-Layered Graphene Oxide as a Highly Efficient Electrocatalyst for the Oxygen-Reduction Reaction

S and N co-doped, few-layered graphene oxide is synthesized by using pyrimidine and thiophene as precursors for the application of the oxygen reduction reaction (ORR). The dual-doped catalyst with pyrrolic/graphitic N-dominant structures exhibits competitive catalytic activity (10.0 mA cm(-2) kinetic-limiting current density at -0.25 V) that is superior to that for mono N-doped carbon nanomaterials. This is because of a synergetic effect of N and S co-doping. Furthermore, the dual-doped catalyst also shows an efficient four-electron-dominant ORR process, which has excellent methanol tolerance and improved durability in comparison to commercial Pt/C catalysts.

Sulfur- and nitrogen-doped, ferrocene-derived mesoporous carbons with efficient electrochemical reduction of oxygen.

Development of inexpensive and sustainable cathode catalysts that can efficiently catalyze the oxygen reduction reaction (ORR) is of significance in practical application of fuel cells. Herein we report the synthesis of sulfur and nitrogen dual-doped, ordered mesoporous carbon (SN-OMCs), which shows outstanding ORR electrocatalytic properties. The material was synthesized from a surface-templating process of ferrocene within the channel walls of SBA-15 mesoporous silica by carbonization, followed by in situ heteroatomic doping with sulfur- and nitrogen-containing vapors. After etching away the metal and silica template, the resulting material features distinctive bimodal mesoporous carbon frameworks with high nitrogen Brunauer-Emmett-Teller specific surface area (of up to ∼1100 m(2)/g) and uniform distribution of sulfur and nitrogen dopants. When employed as a noble-metal-free electrocatalyst for the ORR, such SN-OMC shows a remarkable electrocatalytic activity; improved durability and better resistance toward methanol crossover in oxygen reduction can be observed. More importantly, it performs a low onset voltage and an efficient nearly complete four-electron ORR process very similar to the observations in commercial 20 wt % Pt/C catalyst. In addition, we also found that the textural mesostructure of the catalyst has superseded the chemically bonded dopants to be the key factor in controlling the ORR performance.

A layered porous ZrO2/RGO composite as sulfur host for lithium-sulfur batteries

A new layered porous nanostructure with ZrO2 nanoparticles attached on the reduced graphene oxide (ZrO2/RGO) was synthesized by a facile solvothermal process. The resulting ZrO2/RGO composite with well-designed mesoporous structure and excellent conductivity not only served as scaffold to house sulfur but also as polysulfide reservoir for lithium–sulfur batteries. This nanostructured S@ZrO2/RGO electrode exhibits enhanced cycling stability, high specific capacity, and superior coulombic efficiency.

Toward understanding the active site for oxygen reduction reaction on phosphorus-encapsulated single-walled carbon nanotubes

Positively-charged single-walled carbon nanotubes (SWNTs) induced by physically encapsulated phosphorus demonstrate a little-promoted oxygen reduction reaction (ORR) electrocatalytic activity as compared with the empty SWNTs, including ORR current once increased at low potential and bare ORR-overpotential improvement. It implied that the substitutional doped P in hexagonal carbon framework should be the active center responsible for the excellent ORR activity concerning the P-doped carbon nanostructures reported recently.

Porous cobalt–nitrogen-doped hollow graphene spheres as a superior electrocatalyst for enhanced oxygen reduction in both alkaline and acidic solutions

Porous cobalt–nitrogen-doped hollow graphene spheres were prepared by a template synthesis method. As a catalyst for the oxygen reduction reaction, they exhibit an excellent electrocatalytic activity, superior methanol tolerance and strong durability, not only in alkaline solution, but also in acidic solution. The unprecedented electrocatalytic performance of the catalyst is attributed to the well-defined morphology, high specific surface area (321 m2 g−1), large pore volume (1.8 cm3 g−1) and homogeneous distribution of cobalt–nitrogen active sites.

MnO2 Nanofilms on Nitrogen-Doped Hollow Graphene Spheres as a High-Performance Electrocatalyst for Oxygen Reduction Reaction

Platinum is commonly chosen as an electrocatalyst used for oxygen reduction reaction (ORR). In this study, we report an active catalyst composed of MnO2 nanofilms grown directly on nitrogen-doped hollow graphene spheres, which exhibits high activity toward ORR with positive onset potential (0.94 V vs RHE), large current density (5.2 mA cm-2), and perfect stability. Significantly, when it was used as catalyst for air electrode, a zinc-air battery exhibited a high power density (82 mW cm-2) and specific capacities (744 mA h g-1) comparable to that with Pt/C (20 wt %) as air cathode. The enhanced activity is ascribed to the synergistic interaction between MnO2 and the doped hollow carbon nanomaterials. This easy and cheap method paves a way of synthesizing high-performance electrocatalysts for ORR.

Strong-coupled Co-g-C3N4/SWCNTs composites as high-performance electrocatalysts for oxygen reduction reaction

The hybrid materials of cobalt doped graphitic carbon nitride (g-C3N4) attached on single-walled carbon nanotubes (SWCNTs) were synthesized by a simple pyrolysis process. Electrochemical measurements revealed that the composites exhibited excellent electrocatalytic activity for oxygen reduction reaction (ORR), with a more positive onset potential (−0.03 V), half-wave potential (−0.15 V), high efficiency four-electron process (n = 3.97) and much higher stability than that of commercial Pt/C catalysts in alkaline media. The ORR activity mainly originates from the strong coupling of Co-g-C3N4 derived active sites on the SWCNTs.

Space-confinement-induced synthesis of hierarchically nanoporous carbon nanowires for the enhanced electrochemical reduction of oxygen

Hierarchically nanoporous N-doped carbon nanowires (N-CWs) were prepared by a novel space-confinement-induced assembly strategy, for which nitrogen-enriched pyrimidine and anodic aluminium oxide (AAO) template bearing metal oxides are employed as precursor and nanoscale channels, respectively, and the Fe/Co metal oxide not only blocks the AAO surface from the original surface-templating carbonization, but also introduces nanoporosity with acid etching. Thus-obtained carbon nanowires composed of N-doped graphene-like carbon nanosheets not only contain a high N content (up to ∼12%), but also possess a hierarchically meso/microporous structure (∼1.3 cm3 g−1) with high specific surface area (∼455 m2 g−1). This protocol allows for the simultaneous optimization of graphitization, porous structure and surface functionalization. As a result, the prepared N-CWs demonstrate an attractive electrocatalytic capability towards oxygen reduction reaction (ORR). Specifically, in addition to the improved kinetic current density and overpotential, the N-CWs prepared at 700 °C show the optimized ORR performance with an electron-transfer number of ∼4.0, which very close to that of a commercial Pt/C catalyst.

Oxygen reduction electrocatalysts based on spatially confined cobalt monoxide nanocrystals on holey N-doped carbon nanowires: the enlarged interfacial area for performance improvement

Due to the high cost of Pt-based materials used in the electrocatalysis of the oxygen reduction reaction (ORR), an alternative composed of non-precious metals is highly desirable. Herein a hybrid with cobalt monoxide nanocrystals spatially confined in holey N-doped carbon nanowires (CoO/NCWs) was synthesized via metal oxide assisted surface pitting of chemical vapor deposited carbon nitrogen nanowires and colloidal assembly. The catalyst consists of a Co2+ enriched surface and delivers a remarkably higher ORR electrocatalytic activity and stability than its surface smooth N-doped carbon nanotube supported counterpart, with a kinetically limited current density (30.3 mA cm−2 at 0.7 V) nearly three times that of the latter. It also outperformed the commercial Pt/C catalyst. As characterized by cyclic voltammetry and XPS, the enlarged interfacial area by spatially confined hybridization is believed to be responsible for the improved ORR performance, which might create more active catalytic sites for the ORR. We propose that in-depth consideration of interfacial construction is essential when designing carbon supported metal oxide catalysts for the ORR in alkaline media.