Kedong Xia

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.

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.

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.

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.

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.

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.