Cuijuan Xuan

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.

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.

Hierarchically Porous Electrocatalyst with Vertically Aligned Defect-Rich CoMoS Nanosheets for the Hydrogen Evolution Reaction in an Alkaline Medium

Effective electrocatalysts for the hydrogen evolution reaction (HER) in alkaline electrolytes can be developed via a simple solvothermal process. In this work, first, the prepared CoMoS nanomaterials through solvothermal treatment have a porous, defect-rich, and vertically aligned nanostructure, which is beneficial for the HER in an alkaline medium. Second, electron transfer from cobalt to MoS2 that reduces the unoccupied d orbitals of molybdenum can also enhance the HER kinetics in an alkaline medium. This has been demonstrated via a comparison of the catalytic performances of CoMoS, CoS, and MoS2. Third, the solvothermal treatment time evidently impacts the electrocatalytic activity. As a result, after 24 h of solvothermal treatment, the prepared CoMoS nanomaterials exhibit the lowest onset potential (42 mV) and overpotential (98 mV) for delivering a current density of 10 mA cm-2 in a 1 M KOH solution. Thus, this study provides a simple method to prepare efficient electrocatalysts for the HER in an alkaline medium.

Porous Structured Ni–Fe–P Nanocubes Derived from a Prussian Blue Analogue as an Electrocatalyst for Efficient Overall Water Splitting

Exploring nonprecious metal electrocatalysts to replace the noble metal-based catalysts for full water electrocatalysis is still an ongoing challenge. In this work, porous structured ternary nickel-iron-phosphide (Ni-Fe-P) nanocubes were synthesized through one-step phosphidation of a Ni-Fe-based Prussian blue analogue. The Ni-Fe-P nanocubes exhibit a rough and loose porous structure on their surface under suitable phosphating temperature, which is favorable for the mass transfer and oxygen diffusion during the electrocatalysis process. As a result, Ni-Fe-P obtained at 350 °C with poorer crystallinity offers more unsaturated atoms as active sites to expedite the absorption of reactants. Additionally, the introduction of nickel improved the electronic structure and then reduced the charge-transfer resistance, which would result in a faster electron transport and an enhancement of the intrinsic electrocatalytic activities. Benefiting from the unique porous nanocubes and the chemical composition, the Ni-Fe-P nanocubes exhibit excellent hydrogen evolution reaction and oxygen evolution reaction activities in alkaline medium, with low overpotentials of 182 and 271 mV for delivering a current density of 10 mA cm-2, respectively. Moreover, the Ni-Fe-P nanocubes show outstanding stability for sustained water splitting in the two-electrode alkaline electrolyzer. This work not only provides a facile approach for designing bifunctional electrocatalysts but also further extends the application of metal-organic frameworks in overall water splitting.

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.

Various Structured Molybdenum-based Nanomaterials as Advanced Anode Materials for Lithium ion Batteries

A facile and scalable solvothermal high-temperature treatment strategy was developed to construct few-layered ultrasmall MoS2 with less than three layers. These are embedded in carbon spheres (MoS2-C) and can be used as advanced anode material for lithium ion batteries (LIBs). In the resulting architecture, the intimate contact between MoS2 surface and carbon spheres can effectively avert aggregation and volume expansion of MoS2 during the lithiation-delithiation process. Moreover, it improves the structural integrity of the electrode remarkably, while the conductive carbon spheres provide quick transport of both electrons and ions within the electrode. Benefiting from this unique structure, the resulting hybrid manifests outstanding electrochemical performance, including an excellent rate capability (1085, 885, and 510 mAh g-1 at 0.5, 2, and 5 A g-1), and a superior cycling stability at high rates (maintaining 100% of the initial capacity following 500 cycles at 0.5 A g-1). Using identical methods, molybdenum carbide and phosphide supported on carbon spheres (Mo2C-C, and MoP-C) were prepared for LIBs. As a result, MoS2-C exhibits outstanding lithium storage capacities due to its specific layered structure. This study investigates large-scale production capabilities of few-layered structure ultrasmall MoS2 for energy storage, and thoroughly compares lithium storage performance of molybdenum compounds.

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.

Effects of crystal phase and composition on structurally ordered Pt–Co–Ni/C ternary intermetallic electrocatalysts for the formic acid oxidation reaction

To enhance the electrocatalytic performance of the formic acid oxidation reaction (FAOR), structurally ordered face-centered tetragonal (fct) Pt–Co–Ni/C intermetallic nanoparticles were synthesized via an impregnation reduction method, followed by post heat-treatment. It was found that an ordered intermetallic PtCo phase prevails rather than PtNi as the principal part for the ternary Pt–Co–Ni alloy after being annealed at high temperature, namely, Ni atoms merely serve as the substitute for Co in the lattice of Pt–Co–Ni intermetallics possessing the same atomic stack as PtCo intermetallics. In addition, there is a limitation for Ni to replace Co for the intermetallic PtCo phase, otherwise, most likely excessive Ni would replace the Pt atoms and damage the atomically ordered structure. Benefiting from the ordered structural features and rational introduction of the third transition metal to modify the distance between Pt and Pt atoms, the Pt–Co–Ni/C ordered intermetallic nanoparticles exhibit an enhancement in catalytic activity for the FAOR compared with Pt/C, the PtNi/C alloy and ordered intermetallic PtCo/C nanoparticles. Furthermore, the presence of Ni in the ordered intermetallic Pt–Co–Ni/C catalyst leads to a noticeable improvement in durability compared with the ordered intermetallic PtCo/C catalyst. The present work reveals opportunities for the rational design of ternary electrocatalysts with enhanced catalytic performance for fuel cell applications.

Heteroatom (P, B, or S) incorporated NiFe-based nanocubes as efficient electrocatalysts for the oxygen evolution reaction

Exploring low-cost and highly efficient electrocatalysts toward the oxygen evolution reaction (OER) is of significant importance, although facing great challenges for sustainable energy systems. In this work, amorphous NiFe-based porous nanocubes (Ni–Fe–O–P, Ni–Fe–O–B, and Ni–Fe–O–S) are successfully synthesized via simple and cost-effective one-step calcination of Ni–Fe based metal–organic frameworks (MOFs) and heteroatom containing molecules. The resulting three materials maintain a well-defined porous nanocube morphology with heteroatoms uniformly distributed in the structure. The unique porous structure can effectively provide more active sites and shorten the mass transport distance. Additionally, the introduction of P, B or S can tune the electronic structure, which is favorable for accelerating the charge transfer, and may lead to the formation of the higher average oxidative valence of Ni species during the OER process. Benefiting from the above desirable properties, all three materials exhibit excellent OER electrocatalytic activities and outstanding long-term stability in a home-made zinc air battery. This work not only provides a general approach for the synthesis of highly efficient electrocatalysts based on earth-abundant elements but also highlights the potential prospects of MOFs in energy conversion and storage devices.

Coordination effect of network NiO nanosheet and a carbon layer on the cathode side in constructing a high-performance lithium–sulfur battery

Effectively preventing polysulfides from dissolving during the discharge/charge process to improve the performance of a lithium–sulfur battery is one of the essential requirements for commercialization. An NiO sheet with a network structure was developed via a simple two-step method and used as a mediator for a lithium–sulfur battery coupled with a carbon layer on the cathode side. Specifically, by introducing a thin carbon layer on the cathode side of the separator the dissolution of polysulfides was effectively hindered, and moreover the rate capability and long-term stability were effectively enhanced by the further addition of network NiO nanosheets owing to their mesoporous channels and the NiO–S interaction. Besides, in comparison with the cell performance at different S loadings, the cathode with 80 wt% of S in the total electrode exhibited a high specific capacity and excellent rate performance up to 5C. In addition, although the redox potential range of NiO did not form part of the recharging potential window, NiO exhibited an obvious interaction with polysulfides and limited their tendency to dissolve. The present study demonstrates that the joint action of network NiO and the carbon layer presents great potential to be used in low-cost and high-energy lithium–sulfur batteries and provides important guidance for the design of a multifunctional sulfur host for the battery cathodes.