Yeyun Wang

Mo2C Nanoparticles Dispersed on Hierarchical Carbon Microflowers for Efficient Electrocatalytic Hydrogen Evolution

The development of nonprecious metal based electrocatalysts for hydrogen evolution reaction (HER) has received increasing attention over recent years. Previous studies have established Mo2C as a promising candidate. Nevertheless, its preparation requires high reaction temperature, which more than often causes particle sintering and results in low surface areas. In this study, we show supporting Mo2C nanoparticles on the three-dimensional scaffold as a possible solution to this challenge and develop a facile two-step preparation method for ∼3 nm Mo2C nanoparticles uniformly dispersed on carbon microflowers (Mo2C/NCF) via the self-polymerization of dopamine. The resulting hybrid material possesses large surface areas and a fully open and accessible structure with hierarchical order at different levels. MoO42- was found to play an important role in inducing the formation of this morphology presumably via its strong chelating interaction with the catechol groups of dopamine. Our electrochemical evaluation demonstrates that Mo2C/NCF exhibits excellent HER electrocatalytic performance with low onset overpotentials, small Tafel slopes, and excellent cycling stability in both acidic and alkaline solutions.

Iron-based sodium-ion full batteries

Rechargeable sodium-ion batteries have been an active area of research over the past several years. While a great deal of attention is now focused on the development and evaluation of single electrode materials, much less is paid to their combined performance in full batteries. Most full batteries currently available suffer from rapid capacity fading under extended cycling. In this study, we prepare ultra-small, poorly crystalline FeOx nanoparticles supported on carbon nanotubes as the anode material for sodium-ion batteries. It exhibits excellent half-cell performances; and, when combined with a Prussian blue cathode material, it leads to iron-based full batteries. Our prototypes have a working voltage of ∼2 V, specific energy density of ∼136 W h kg−1 and, most impressively, outstanding cycling stability at both low and high current rates with negligible capacity loss. Owing to their low material and fabrication cost, long cycle life and high efficiency, we believe that these iron-based sodium-ion batteries would be highly appealing toward the stationary energy storage.

A hierarchical α-MoC1−x hybrid nanostructure for lithium-ion storage

Transition metal carbides are promising electrode materials for electrochemical energy storage, yet to unveil their full potential requires judicious structural engineering at the nanoscale. In this study, we report a chrysanthemum-inspired nanoscale design to prepare a three-dimensional hierarchical molybdenum carbide hybrid. It consisted of an ensemble of numerous nanoflakes protruding out from the center, each formed by ultra-small (∼2 nm) α-MoC1−x nanoparticles uniformly supported on a N-doped carbonaceous support. Such a hybrid material has enlarged surface areas, shortened ionic diffusion length, great mechanical robustness, and buffer room for electrode volume change. Owing to the three-dimensional hierarchical arrangement, this hybrid material exhibits impressive performance toward active lithium-ion storage. It delivers a large reversible capacity of >1000 mA h g−1, great rate capacity with significant capacity at 10 A g−1, and excellent cycling stability with >95% capacity retention after 100 cycles at 500 mA g−1. Most impressively, we demonstrate that the structural integrity of the hybrid microflower is largely preserved even after prolonged cycling.

Engineering SnS2 nanosheet assemblies for enhanced electrochemical lithium and sodium ion storage

The reversible electrochemical storage of Li+ and Na+ ions is the operating basis of secondary lithium-ion and sodium-ion batteries. In recent years, there has been rapid growth in the search for appropriate electrode materials. Nevertheless, the development of host materials for active and durable electrochemical storage of both Li+ and Na+ ions remain challenging. In this study, we report a facile solvothermal method to prepare hierarchical assemblies of thin SnS2 nanosheets in N-methyl-2-pyrrolidone. The as-prepared product has an expanded layered structure due to the presence of organic intercalates. Mild annealing restores the normal 2H-SnS2 phase with the hierarchical architecture preserved. When annealed SnS2 was evaluated as the anode material of lithium-ion batteries, it exhibited large capacity in excess of 1200 mA h g−1 and decent short-term cycling stability. It was further coated with a thin carbon layer as the physical and electrical reinforcement, which led to a much improved cycle life at both low and high current rates. Moreover, carbon coated SnS2 also demonstrated a large capacity (∼600 mA h g−1) and decent cycling stability as the anode material of sodium-ion batteries.

Photoelectroreduction of Building-Block Chemicals

Conventional photoelectrochemical cells utilize solar energy to drive the chemical conversion of water or CO2 into useful chemical fuels. Such processes are confronted with general challenges, including the low intrinsic activities and inconvenient storage and transportation of their gaseous products. A photoelectrochemical approach is proposed to drive the reductive production of industrial building-block chemicals and demonstrate that succinic acid and glyoxylic acid can be readily synthesized on Si nanowire array photocathodes free of any cocatalyst and at room temperature. These photocathodes exhibit a positive onset potential, large saturation photocurrent density, high reaction selectivity, and excellent operation durability. They capitalize on the large photovoltage generated from the semiconductor/electrolyte junction to partially offset the required external bias, and thereby make this photoelectrosynthetic approach significantly more sustainable compared to traditional electrosynthesis.