Zexing Wu

Nitrogen and sulfur co-doping of 3D hollow-structured carbon spheres as an efficient and stable metal free catalyst for the oxygen reduction reaction

Three-dimensional, hollow-structured carbon sphere nanocomposites (N,S-hcs) doped with nitrogen and sulfur were prepared using a soft template approach followed by a high-temperature treatment. The synthesized N,S-hcs nanomaterials exhibited favourable catalytic activity for the oxygen reduction reaction (ORR) compared to carbon spheres doped solely with nitrogen (N-hcs), polypyrrole (PPY) solid nanoparticles and irregular fragments of polyaniline (PAN). These results demonstrated the co-doping of N/S and the relatively large surface area of the mesoporous carbon structure that enhanced the catalytic activity of the resulting material. Notably, the prepared N,S-hcs electrocatalysts provided four electron oxygen reduction selectivity, long-term durability and high resistance to methanol poisoning, all of which represented improvements over the conventional Pt/C electrocatalyst. The progress represented by this reported work is of great importance in the development of outstanding non-metal based electrocatalysts for the fuel cell industry.

Synergistic enhancement of nitrogen and sulfur co-doped graphene with carbon nanosphere insertion for the electrocatalytic oxygen reduction reaction

A carbon black incorporated nitrogen and sulfur co-doped graphene (NSGCB) nanocomposite has been synthesized through one-pot annealing of a precursor mixture containing graphene oxide, thiourea, and acidized carbon black (CB). The NSGCB shows excellent performance for the oxygen reduction reaction (ORR) with the onset and half-wave potentials at 0.96 V and 0.81 V (vs. RHE), respectively, which are significantly higher compared to those of the catalysts derived from only graphene (0.90 V and 0.76 V) or carbon nanospheres (0.82 V and 0.74 V). The enhanced catalytic activity of the NSGCB electrode could be attributed to the synergistic effect of N/S co-doping and the enlarged interlayer space resulted from the insertion of carbon nanospheres into the graphene sheets. The four-electron selectivity and the limiting current density of the NSGCB nanocomposite are comparable to those of the commercial Pt/C catalyst. Furthermore, the NSGCB nanocomposite is superior to Pt/C in terms of long-term durability and tolerance to methanol poisoning.

Nitrogen and sulfur co-doping of partially exfoliated MWCNTs as 3-D structured electrocatalysts for the oxygen reduction reaction

Preventing the stacking of graphene sheets is of vital importance for highly efficient and stable fuel cell electrocatalysts. In the present work, we report a 3-D structured carbon nanotube intercalated graphene nanoribbon with N/S co-doping. The nanocomposite is obtained by using high temperature heat-treated thiourea with partially unzipped multi-walled carbon nanotubes. The unique structure preserves both the properties of carbon nanotubes and graphene, exhibiting excellent catalytic performance for the ORR with similar onset and half-wave potentials to those of Pt/C electrocatalysts. Moreover, the stereo structured composite exhibits distinct advantages in long-term stability and methanol poisoning tolerance in comparison to Pt/C.

Hollow-Structured Carbon-Supported Nickel Cobaltite Nanoparticles as an Efficient Bifunctional Electrocatalyst for the Oxygen Reduction and Evolution Reactions

The exploration of efficient electrocatalysts for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is essential for fuel cells and metal‐air batteries. In this study, we developed 3 D hollow‐structured NiCo2O4/C nanoparticles with interconnected pores as bifunctional electrocatalysts, which are transformed from solid NiCo2 alloy nanoparticles through the Kirkendall effect. The unique hollow structure of NiCo2O4 nanoparticles increases the number of active sites and improves contact with the electrolyte to result in excellent ORR and OER performances. In addition, the hollow‐structured NiCo2O4/C nanoparticles exhibit superior long‐term stability for both the ORR and OER compared to commercial Pt/C. The template‐ and surfactant‐free synthetic strategy could be used for the low‐cost and large‐scale synthesis of hollow‐structured materials, which would facilitate the screening of high‐efficiency catalysts for energy conversion.

3D hollow structured Co2FeO4/MWCNT as an efficient non-precious metal electrocatalyst for oxygen reduction reaction

A template- and surfactant-free strategy is developed to prepare a hollow structured Co2FeO4/MWCNT electrocatalyst, which has been successfully used as a highly efficient non-precious metal electrocatalyst for the oxygen reduction reaction (ORR) in alkaline media. The hollow structured Co2FeO4 particles are transformed from solid Co2Fe nanoparticles via the Kirkendall effect, which increases their active sites and improves the contact between the electrolyte and catalyst surfaces and then enhances the electrocatalytic activity. Furthermore, the hollow structured Co2FeO4/MWCNT exhibits excellent long term stability and high methanol tolerance compared to commercial Pt/C. The hollow structured Co2FeO4/MWCNT electrocatalysts synthesized herein are promising electrode materials for fuel cell applications and the facile synthesis method could be used in low-cost and large-scale materials production, facilitating the screening of high efficiency catalysts.

Controllable synthesis of molybdenum-based electrocatalysts for a hydrogen evolution reaction

In order to explore low-cost, high efficiency, precious metal-free materials for electrochemical water splitting, three types of molybdenum-based compounds (MoO2, MoC and Mo2C) were synthesized by tuning the ratio of glucose and ammonium molybdate via a two-step procedure. TEM images reveal a uniform dispersion of the three molybdenum-based nanoparticles on the carbon support, and in particular, MoC and Mo2C exhibit ultra-small particle sizes which are lower than 3 nm. When used as catalysts for the HER in both acid and basic media, Mo2C exhibits the best catalytic activity with a small overpotential of 135 mV in acid media and 96 mV in alkaline media at a current density of 10 mA cm−2, which is about 105 mV and 30 mV higher than that with Pt/C, respectively. The enhanced catalytic activity of Mo2C could originate from the excellent crystal structure, the high electronic conductivity of the carbon support with a high degree of graphitization and the ultra-small particle size, which provides a large surface area and active sites.

A general approach for the direct fabrication of metal oxide-based electrocatalysts for efficient bifunctional oxygen electrodes

A simple one-pot synthetic strategy for the general preparation of nitrogen doped carbon supported metal/metal oxides (Co@CoO/NDC, Ni@NiO/NDC and MnO/NDC) derived from the complexing function of (ethylenediamine)tetraacetic acid (EDTA) is developed. EDTA serves not only as a resource to tune the morphology in terms of the complexation constant for M–EDTA, but also as a nitrogen and oxygen source for nitrogen doping and metal oxide formation, respectively. When the materials are used as electrocatalysts for the oxygen electrode reaction, Co@CoO/NDC-700 and MnO/NDC-700 show superior electrocatalytic activity towards the oxygen reduction reaction (ORR), while Co@CoO/NDC-700 and Ni@NiO/NDC-700 exhibit excellent oxygen evolution reaction (OER) activities. Taken together, the resultant Co@CoO/NDC-700 exhibits the best catalytic activity with favorable reaction kinetics and durability as a bi-functional catalyst for the ORR and OER, which is much better than the other two catalysts, Pt/C and Ir/C. Moreover, as an air electrode for a homemade zinc–air battery, Co@CoO/NDC-700 shows superior cell performance with a highest power density of 192.1 mW cm−2, the lowest charge–discharge overpotential and high charge–discharge durability over 100 h.

Structurally ordered Pt–Zn/C series nanoparticles as efficient anode catalysts for formic acid electrooxidation

Controlling the size, composition, and structure of bimetallic nanoparticles is of particular interest in the field of electrocatalysts for fuel cells. In the present work, structurally ordered nanoparticles with intermetallic phases of Pt3Zn and PtZn have been successfully synthesized via an impregnation reduction method, followed by post heat-treatment. The Pt3Zn and PtZn ordered intermetallic nanoparticles are well dispersed on a carbon support with ultrasmall mean particle sizes of ∼5 nm and ∼3 nm in diameter, respectively, which are credited to the evaporation of the zinc element at high temperature. Meanwhile, these catalysts are less susceptible to CO poisoning relative to Pt/C and exhibited enhanced catalytic activity and stability toward formic acid electrooxidation. The mass activities of the as-prepared catalysts were approximately 2 to 3 times that of commercial Pt at 0.5 V (vs. RHE). This facile synthetic strategy is scalable for mass production of catalytic materials.

Nitrogen-Doped Hierarchical Porous Carbons Derived from Sodium Alginate as Efficient Oxygen Reduction Reaction Electrocatalysts

The exploration of earth‐abundant and low‐cost eco‐friendly materials with an excellent electrocatalytic performance is crucial for sustainable energy development. In this work, 3 D N‐doped hierarchical porous carbon (NC) materials with an interconnected mesoporous/macroporous structure have been synthesized by the simple single‐step pyrolysis of naturally available sodium alginate in the presence of urea. The systematic investigation of the pyrolysis temperature on the performance in the oxygen reduction reaction in 0.1 m KOH solution indicates that the catalyst obtained at 900 °C (NC‐900) exhibits the best catalytic performance because of the high degree of graphitization and the unique hierarchical porous structure. Furthermore, NC‐900 exhibits an excellent durability and a remarkable resistance to methanol poisoning compared to Pt/C in alkaline solution. This work highlights the significance and potential of biomass‐derived hierarchical porous carbon materials for applications in energy conversion devices.

MoS2–MoP heterostructured nanosheets on polymer-derived carbon as an electrocatalyst for hydrogen evolution reaction

Electrocatalytic production of hydrogen from water is a promising and sustainable strategy. In this study, a simple and general strategy is demonstrated for the synthesis of a polymer-derived, heteroatom-doped carbon supporting MoS2–MoP nanosheets (MoS2–MoP/C), which acts as an efficient hydrogen evolution reaction (HER) catalyst. The unique composition of the MoS2–MoP/C enables catalysis of HER, demonstrating a Tafel slope of 58 mV dec−1 and overpotentials of 102 mV and 130 mV, and is therefore able to deliver current densities of 10 mA cm−2 and 20 mA cm−2, respectively. Moreover, the MoS2–MoP/C nanosheets demonstrate superior long-term durability in acid electrolytes and the developed strategy is easily adapted to large-scale production of efficient electrocatalysts. This excellent HER performance is likely attributed to electronic interactions between P and S, a relatively-high specific surface area of the electrocatalyst (162 m2 g−1), the characteristic nanosheet morphology, and high electrical conductivity, which promotes rapid charge transport and collection.