Hongsheng Fan

Layer-controlled Pt-Ni porous nanobowls with enhanced electrocatalytic performance

Hollow and porous Pt-based nanomaterials are promising catalysts with applications in many sustainable energy technologies such as fuel cells. Economical and green synthetic routes are highly desirable. Here, we report a facile approach to prepare double- and single-layered Pt-Ni nanobowls (DLNBs and SLNBs) with porous shells. Microstructural analysis revealed that the shells were constructed of alloyed Pt-Ni nanocrystals and small amounts of Ni compounds. X-ray photoelectron spectra showed that their Pt 4f binding energies shifted in the negative direction compared to those of the commercial Pt/C catalyst. Furthermore, the DLNBs contained greater contents of oxidized Ni species than the SLNBs. The layer-controlled growth processes were confirmed by microscopy, and a formation mechanism was proposed based on the assistance of citrate and poly(vinylpyrrolidone) (PVP). For the methanol oxidation reaction, the DLNBs and SLNBs exhibited 2.9 and 2.5 times higher mass activities than that of the commercial Pt/C catalyst, respectively. The activity enhancements were attributed to electronic effects and a bifunctional mechanism. Chronoamperometry and prolonged cyclic voltammetry indicated that the Pt-Ni bowl-like structures had better electrochemical properties and structural stability than the commercial Pt/C catalyst, thus making the Pt-Ni nanobowls excellent electrocatalysts for use in direct methanol fuel cells.

From channeled to hollow CoO octahedra: controlled growth, structural evolution and energetic applications

Single-crystalline hollow CoO nanooctahedra were successfully synthesized by the combination of self-gas-leaching and Ostwald ripening processes through a template-free route. Elaborate microscopy evidence discloses that hollow CoO nanocrystals (NCs) evolve from the channeled CoO NCs, which is an essential intermediate state to construct the hollow structure. Electrochemical measurements indicate that hollow CoO NCs exhibit three times higher specific capacitance than the solid CoO and show excellent cycling stability with the specific capacitance retaining more than 97% after 4500 cycles at a current density of 2 A g−1. Meanwhile, as a catalyst for oxygen evolution reaction and hydrogen production from hydrolysis of sodium borohydride, the hollow and channeled CoO NCs exhibit much better performances than their solid counterparts. The hollow CoO NCs afford a current density of 10 mA cm−2 at a small overpotential of merely 0.375 V and a small Tafel slope of 55 mV decade−1, and they also possess more steady catalytic activity than the channeled CoO.

Porous Pt–NiOx nanostructures with ultrasmall building blocks and enhanced electrocatalytic activity for the ethanol oxidation reaction

Oxidized species on surfaces would significantly improve the electrocatalytic activity of Pt-based materials. Constructing three-dimensional porous structures would endow the catalysts with good stability. Here, we report a simple strategy to synthesize porous Pt–NiOx nanostructures composed of ultrasmall (about 3.0 nm) building blocks in an ethanol–water solvent. Structure and component analysis revealed that the as-prepared material consisted of interconnected Pt nanocrystals and amorphous NiOx species. The formation mechanism investigation revealed that the preformed amorphous compounds were vital for the construction of porous structure. In the ethanol oxidation reaction, Pt–NiOx/C exhibited current densities of 0.50 mA cmPt−2 at 0.45 V (vs. SCE), which were 16.7 times higher than that of a commercial Pt/C catalyst. Potentiostatic tests showed that Pt–NiOx/C had much higher current and better tolerance towards CO poisoning than the Pt/C catalyst under 0.45 V (vs. SCE). In addition, the NiOx species on the surface also outperformed an alloyed Ni component in the test. These results indicate that the Pt–NiOx porous nanomaterial is promising for use in direct ethanol fuel cells.

Morphology-Controlled Synthesis of Hematite Nanocrystals and Their Optical, Magnetic and Electrochemical Performance

A series of α-Fe2O3 nanocrystals (NCs) with fascinating morphologies, such as hollow nanoolives, nanotubes, nanospindles, and nanoplates, were prepared through a simple template-free hydrothermal synthesis process. The results showed that the morphologies could be easily controlled by SO42− and H2PO4−. Physical property analysis showed that the α-Fe2O3 NCs exhibited shape- and size-dependent ferromagnetic and optical behaviors. The absorption band peak of the α-Fe2O3 NCs could be tuned from 320 to 610 nm. Furthermore, when applied as electrode material for supercapacitor, the hollow olive-structure exhibited the highest capacitance (285.9 F·g−1) and an excellent long-term cycling stability (93% after 3000 cycles), indicating that it could serve as a candidate electrode material for a supercapacitor.

Pd–Zn nanocrystals for highly efficient formic acid oxidation

Highly uniform and compositionally graded Pd–Zn nanocrystals (NCs) with particle diameters of approximately 5.0–6.5 nm are obtained via a facile one-pot synthesis process in dimethylformamide (DMF) and water solution. The Pd–Zn NCs are formed by the reduction of transition metal ions coexisting in ascorbic acid (AA) and noble metals. The NCs are characterized by X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy. Investigations indicated that the NCs have compositionally graded PdxZn1−x NCs with x ranging from 0.68 to 1.00. Electrochemical measurements reveal that the PdxZn1−x NCs exhibit superior electrocatalytic performance of both activity and stability for the formic acid oxidation (FAO) reaction compared with commercial Pd/C. The optimized Pd0.77Zn0.23 NCs exhibit a FAO catalytic oxidation current density of 1945 mA mgPd−1, which is 8.2 times higher than that of commercial Pd/C (237 mA mgPd−1). CO stripping measurement indicates that the tolerance to CO poisoning has been significantly improved by alloying Zn. The excellent performance can be attributed to the synergistic effect between Pd and Zn in Pd–Zn NCs.

Hollow Co2P nanoflowers organized by nanorods for ultralong cycle-life supercapacitors

Hollow Co2P nanoflowers (Co2P HNFs) were successfully prepared via a one-step, template-free method. Microstructure analysis reveals that Co2P HNFs are assembled from nanorods and possess abundant mesopores and an amorphous carbon shell. Density functional theory calculations and electrochemical measurements demonstrate the high electrical conductivity of Co2P. Benefiting from the unique nanostructures, when employed as an electrode material for supercapacitors, Co2P HNFs exhibit a high specific capacitance, an outstanding rate capability, and an ultralong cycling stability. Furthermore, the constructed Co2P HNF//AC ASC exhibits a high energy density of 30.5 W h kg-1 at a power density of 850 W kg-1, along with a superior cycling performance (108.0% specific capacitance retained after 10 000 cycles at 5 A g-1). These impressive results make Co2P HNFs a promising candidate for supercapacitor applications.