Shujin Hou

ZnS nanoparticles decorated on nitrogen-doped porous carbon polyhedra: a promising anode material for lithium-ion and sodium-ion batteries

Rational fabrication and structure design of anode materials with high specific capacity and excellent cycling stability are of significant importance for the development of high-performance lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). In this paper, a zeolitic imidazolate framework-8 (ZIF-8) with a unique polyhedral morphology and large size (about 2 μm) was successfully synthesized through a facile co-precipitation method. After successive carbonization and sulfidation, ZnS nanoparticles decorated on nitrogen-doped porous carbon polyhedra (ZnS/NPC) were obtained. When applied as the anode material for LIBs, the ZnS/NPC hybrid displays the highest reversible specific capacity for ZnS-based electrodes reported so far (1067.4 mA h g−1 at 0.1 A g−1 after 200 cycles), excellent rate capability (364.6 mA h g−1 at 4 A g−1), and robust long-term cycling performance (856.8 mA h g−1 at 1 A g−1 after 1000 cycles). As for SIBs, the resultant ZnS/NPC also exhibits a desirable capacity of 370.6 mA h g−1 after 100 cycles at 0.1 A g−1 and 289.2 mA h g−1 after 1000 cycles at 1 A g−1. Such superior lithium and sodium storage performances should be attributed to the distinctive structure advantages inherited from ZIF-8, where the Zn ions were in situ converted to ZnS with high reactivity upon electrochemical cycling and the organic linkers were pyrolyzed to nitrogen-doped porous carbon polyhedra to enhance the conductivity of the hybrid and keep the structure stability during cycling.

Carbon-incorporated Janus-type Ni2P/Ni hollow spheres for high performance hybrid supercapacitors

Transition metal phosphides, especially Ni2P, are of great interest as promising battery-type electrode materials for hybrid supercapacitors, but their poor electrical conductivity and porosity limit their application. Here, for the first time, the synthesis of carbon-incorporated Janus-type Ni2P/Ni hollow spheres (Ni2P/Ni/C) was reported via simultaneous carbonization and phosphorization of Ni-based metal–organic frameworks (Ni-MOFs). Their unique structural merits include the incorporated carbon content, Janus-type Ni2P/Ni nanocrystals, and high-porosity hollow structure, thus endowing them with a high specific surface area, good electrical conductivity and low density. As a result, the optimized Ni2P/Ni/C exhibits a remarkable specific capacitance of 1449 F g−1 at 1 A g−1 in 2 M KOH aqueous electrolyte in a three-electrode system. A hybrid supercapacitor device was fabricated by using Ni2P/Ni/C as the positive electrode and active carbon as the negative electrode, and it achieves a very high energy density of 32.02 W h kg−1 at a power density of 700 W kg−1 and a remarkable cycling stability (about 99% capacitance retention after 5000 cycles). The Ni2P/Ni/C should be one of the most promising electrode materials for hybrid supercapacitor application.

Three-Dimensional Networked Metal–Organic Frameworks with Conductive Polypyrrole Tubes for Flexible Supercapacitors

Metal-organic frameworks (MOFs) with high porosity and a regular porous structure have emerged as a promising electrode material for supercapacitors, but their poor electrical conductivity limits their utilization efficiency and capacitive performance. To increase the overall electrical conductivity as well as the efficiency of MOF particles, three-dimensional networked MOFs are developed via using preprepared conductive polypyrrole (PPy) tubes as the support for in situ growth of MOF particles. As a result, the highly conductive PPy tubes that run through the MOF particles not only increase the electron transfer between MOF particles and maintain the high effective porosity of the MOFs but also endow the MOFs with flexibility. Promoted by such elaborately designed MOF-PPy networks, the specific capacitance of MOF particles has been increased from 99.2 F g-1 for pristine zeolitic imidazolate framework (ZIF)-67 to 597.6 F g-1 for ZIF-PPy networks, indicating the importance of the design of the ZIF-PPy continuous microstructure. Furthermore, a flexible supercapacitor device based on ZIF-PPy networks shows an outstanding areal capacitance of 225.8 mF cm-2, which is far above other MOFs-based supercapacitors reported up to date, confirming the significance of in situ synthetic chemistry as well as the importance of hybrid materials on the nanoscale.

Nitrogen-doped carbon spheres: A new high-energy-density and long-life pseudo-capacitive electrode material for electrochemical flow capacitor

One of the most challenging issues in developing electrochemical flow capacitor (EFC) technology is the design and synthesis of active electrode materials with high energy density and long cycle life. However, in practical cases, the energy density and cycle ability obtained currently cannot meet the practical need. In this work, we propose a new active material, nitrogen-doped carbon spheres (NCSs), as flowable electrodes for EFC application. The NCSs were prepared via one-pot hydrothermal synthesis in the presence of resorcinol/formaldehyde as carbon precursors and melamine as nitrogen precursor, followed by carbonization in nitrogen flow at various temperatures. The results of EFC experiments demonstrate that NCSs obtained at 800°C exhibit a high energy density of 13.5Whkg-1 and an excellent cycle ability, indicating the superiority of NCSs for EFC application.

Design of pomegranate-like clusters with NiS2 nanoparticles anchored on nitrogen-doped porous carbon for improved sodium ion storage performance

Nickel sulfide, a promising anode for sodium-ion batteries (SIBs), has drawn a lot of attention due to its natural abundance, low cost, rich types and high theoretical specific capacity (Ni3S2: 446, NiS: 591 and NiS2: 879 mA h g−1). However, the huge volume change induced severe electrode pulverization results in the low specific capacity and poor cycling stability of nickel sulfide electrodes. Herein, in this paper, we developed a metal–organic framework (MOF) strategy to prepare pomegranate-like clusters with small NiS2 nanoparticles anchored on nitrogen doped porous graphitic carbon networks (NiS2/NC) via successive carbonization and sulfidation. When evaluated as an anode for SIBs, the as-prepared NiS2/NC hybrid exhibited a high reversible capacity of 505.7 mA h g−1 after 100 cycles at 0.1 A g−1, excellent rate capability (294.4 mA h g−1 at 3 A g−1) and robust cycling stability with a capacity of 356.2 mA h g−1 after 300 cycles at 0.5 A g−1, which outperforms most of the nickel sulfide based electrodes reported so far. The excellent cycling performance and rate capability for SIBs can be attributed to the unique structure inherited from nickel based MOFs, in situ fabrication strategy, high capacity of NiS2, and conductive and buffering features of the nitrogen-doped graphitic carbon networks, demonstrating the great potential of the as-prepared NiS2/NC hybrid for high-performance SIBs.

Improved sodium-ion storage performance of Ti3C2Tx MXenes by sulfur doping

The sodium storage performance of recently reported Ti3C2Tx MXenes is seriously restricted by their low specific capacity due to their insufficient interlayer spacing. Herein, for the first time, a sulfur (S) doped multilayered Ti3C2Tx MXene was prepared by a simple sulfidation treatment of Ti3C2Tx using thiourea as the S source, which shows an increased interlayer spacing and enhanced electrical conductivity. When used as an anode for sodium-ion batteries (SIBs), the S-doped Ti3C2Tx exhibits a high reversible capacity of 183.2 mA h g−1 after 100 cycles at 0.1 A g−1, excellent rate capability (121.3 mA h g−1 at 2 A g−1 and 113.9 mA h g−1 at 4 A g−1) and robust long-term cycling stability with a reversible capacity of 138.2 mA h g−1 after 2000 cycles at 0.5 A g−1. Notably, the superior sodium storage performance should be attributed to the multilayered morphology, expanded interlayer spacing and enhanced electrical conductivity as well as the high contribution of surface-induced capacitive behavior after S doping, and it outperforms those of reported Ti3C2Tx based electrodes, highlighting the feasibility of the S doping strategy. Most importantly, this work offers a novel approach for smart design and rational fabrication of heteroatom-doped MXenes for energy storage and conversion applications.