Ziliang Chen

Pseudocapacitance-tuned high-rate and long-term cyclability of NiCo2S4 hexagonal nanosheets prepared by vapor transformation for lithium storage

The high conductivity of bimetallic thiospinel NiCo2S4 endows energy storage devices with very fascinating performance. However, the unsatisfactory rate capability and long-term cyclability of this material series significantly limit their large-scale practical applications such as in electric vehicles and hybrid electric vehicles. Herein, we successfully synthesized NiCo2S4 hexagonal nanosheets with a large lateral dimension of ∼1.35 μm and a thickness of ∼30 nm through a vapor transformation method. The dynamic transformation process of the NiCo2S4 polycrystalline nanosheets from NiCo-hydroxide has been revealed in detail. Originating from their two-dimensional thin-sheet structure with a high aspect ratio, the induced extrinsic capacitive contribution as high as 91% makes them an ideal candidate for high-capacity and high-rate lithium-ion anodes. The NiCo2S4 nanosheets deliver a reversible capacity of 607 mA h g−1 upon 800 cycles at a current density of 2 A g−1. This outstanding long cycle performance sheds light on the structural design of electrode materials for high-rate lithium-ion batteries.

Temperature dependence of polarization switching properties of Bi3.15Nd0.85Ti3O12 ferroelectric thin film

The temperature dependences of the polarization switching properties of Bi3.15Nd0.85Ti3O12 (BNT) ferroelectric thin film in the range from 25 to 150°C have been investigated. With increasing temperature, the switchable polarization and switching time decrease. Meanwhile, the depolarization field due to the interfacial layer between the electrode and the BNT film increases with increasing temperature, which induces more domain back-switching. In addition, the local switching properties of BNT film have been studied using piezoresponse force microscopy, and it is found that the prepared BNT film has good local switching behaviors.

Embedding ZnSe nanodots in nitrogen-doped hollow carbon architectures for superior lithium storage

Transition metal chalcogenides represent a class of the most promising alternative electrode materials for high-performance lithium-ion batteries (LIBs) owing to their high theoretical capacities. However, they suffer from large volume expansion, particle agglomeration, and low conductivity during charge/discharge processes, leading to unsatisfactory energy storage performance. In order to address these issues, we rationally designed three-dimensional (3D) hybrid composites consisting of ZnSe nanodots uniformly confined within a N-doped porous carbon network (ZnSe ND@N-PC) obtained via a convenient pyrolysis process. When used as anodes for LIBs, the composites exhibited outstanding electrochemical performance, with a high reversible capacity (1,134 mA·h·g−1 at a current density of 600 mA·g−1 after 500 cycles) and excellent rate capability (696 and 474 mA·h·g−1 at current densities of 6.4 and 12.8 A·g−1, respectively). The significantly improved lithium storage performance can be attributed to the 3D architecture of the hybrid composites, which not only mitigated the internal mechanical stress induced by the volume change and formed a 3D conductive network during cycling, but also provided a large reactive area and reduced the lithium diffusion distance. The strategy reported here may open a new avenue for the design of other multifunctional composites towards high-performance energy storage devices.

Hierarchically porous-structured ZnxCo1−xS@C–CNT nanocomposites with high-rate cycling performance for lithium-ion batteries

Transition metal sulfides are of great interest as anodes for lithium-ion batteries (LIBs) due to their high theoretical capacity and low cost. However, the poor cycling stability and rate performance are the critical problems that hinder their practical applications. In this work, a unique hybrid nanocomposite constructed from starfish-like ZnxCo1−xS rooted in porous carbon and strongly coupled carbon nanotubes (ZnxCo1−xS@C–CNTs) is demonstrated to address this concern. The designed nanocomposite integrates the high theoretical capacity of ZnxCo1−xS and the excellent conductivity as well as the excellent mechanical stability of CNTs. When evaluated as anode materials for LIBs, ZnxCo1−xS@C–CNTs exhibited a high reversible capacity of 635 mA h g−1 at a current density of 1.2 A g−1 after 1000 cycles and excellent high-rate capability (890 mA h g−1 and 750 mA h g−1 at current densities of 3.2 and 6.4 A g−1, respectively). The excellent electrochemical performance of ZnxCo1−xS@C–CNTs can be ascribed to its hierarchically porous structure design and the synergistic effect between ZnxCo1−xS@C and CNTs.

In Situ Formation of Cobalt Nitrides/Graphitic Carbon Composites as Efficient Bifunctional Electrocatalysts for Overall Water Splitting

Developing cost-effective and highly efficient bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is of great interest for overall water splitting but still remains a challenging issue. Herein, a self-template route is employed to fabricate a unique hybrid composite constructed by encapsulating cobalt nitride (Co5.47N) nanoparticles within three-dimensional (3D) N-doped porous carbon (Co5.47N NP@N-PC) polyhedra, which can be served as a highly active bifunctional electrocatalyst. To afford a current density of 10 mA cm-2, the as-fabricated Co5.47N NP@N-PC only requires overpotentials as low as 149 and 248 mV for HER and OER, respectively. Moreover, an electrolyzer with Co5.47N NP@N-PC electrodes as both the cathode and anode catalyst in alkaline solutions can drive a current density of 10 mA cm-2 at a cell voltage of only 1.62 V, superior to that of the Pt/IrO2 couple. The excellent electrocatalytic activity of Co5.47N NP@N-PC can be mainly ascribed to the high inherent conductivity and rich nitrogen vacancies of the Co5.47N lattice, the electronic modulation of the N-doped carbon toward Co5.47N, and the hierarchically porous structure design.

Ultrafine Co Nanoparticles Encapsulated in Carbon-Nanotubes-Grafted Graphene Sheets as Advanced Electrocatalysts for the Hydrogen Evolution Reaction

The rational design of an efficient and inexpensive electrocatalyst based on earth-abundant 3d transition metals (TMs) for the hydrogen evolution reaction still remains a significant challenge in the renewable energy area. Herein, a novel and effective approach is developed for synthesizing ultrafine Co nanoparticles encapsulated in nitrogen-doped carbon nanotubes (N-CNTs) grafted onto both sides of reduced graphene oxide (rGO) (Co@N-CNTs@rGO) by direct annealing of GO-wrapped core-shell bimetallic zeolite imidazolate frameworks. Benefiting from the uniform distribution of Co nanoparticles, the in-situ-formed highly graphitic N-CNTs@rGO, the large surface area, and the abundant porosity, the as-fabricated Co@N-CNTs@rGO composites exhibit excellent electrocatalytic hydrogen evolution reaction (HER) activity. As demonstrated in electrochemical measurements, the composites can achieve 10 mA cm-2 at low overpotential with only 108 and 87 mV in 1 m KOH and 0.5 m H2 SO4 , respectively, much better than most of the reported Co-based electrocatalysts over a wide pH range. More importantly, the synthetic strategy is versatile and can be extended to prepare other binary or even ternary TMs@N-CNTs@rGO (e.g., Co-Fe@N-CNTs@rGO and Co-Ni-Cu@N-CNTs@rGO). The strategy developed here may open a new avenue toward the development of nonprecious high-performance HER catalysts.

Tunable electronic coupling of cobalt sulfide/carbon composites for optimizing oxygen evolution reaction activity

Hybrid nanocomposites consisting of non-precious transition-metal oxides/chalcogenides and a carbon matrix are of great interest for future applications in sustainable energy storage and conversion systems owing to their unique chemical and physical properties as well as the synergism. However, the construction of nanocomposites with tunable coupling effects to realize synergetic effects and optimal performance remains challenging. Herein, we develop a facile strategy for the synthesis of cobalt sulfide (Co9S8) nanoparticles encapsulated in the carbon matrix from a Prussian blue analogue (PBA) through a simultaneous sulfidation and carbonization process. With an increase in the degree of carbonization, the coordinated organic ligands (CN) liberated from the PBA can be evolved as N-doped amorphous carbon (N-AC), N-doped graphitic carbon (N-GC) and N-doped carbon nanotubes (N-CNTs), respectively, generating the tunable electronic coupling between the Co9S8 and the carbon matrix. As an example, we show that the oxygen evolution reaction (OER) activity of such nanocomposites can be made comparable to the state-of-the-art catalytic properties of precious catalysts by optimizing the electronic coupling between the components so that electron injection from the N-doped carbon to the catalytically active site is greatly facilitated. Furthermore, the in situ formation of cobalt hydroxides on the surface of Co9S8 is also confirmed during the OER process, which might induce the interfacial effect, i.e. electron interplay, thus altering the OER catalytic activity. The current work provides new insights into the rational design of advanced hybrid nanocomposites for energy and environmental applications.