Junjie Cai

Hierarchical Mesoporous Zinc–Nickel–Cobalt Ternary Oxide Nanowire Arrays on Nickel Foam as High-Performance Electrodes for Supercapacitors

Nickel foam supported hierarchical mesoporous Zn-Ni-Co ternary oxide (ZNCO) nanowire arrays are synthesized by a simple two-step approach including a hydrothermal method and subsequent calcination process and directly utilized for supercapacitive investigation for the first time. The nickel foam supported hierarchical mesoporous ZNCO nanowire arrays possess an ultrahigh specific capacitance value of 2481.8 F g(-1) at 1 A g(-1) and excellent rate capability of about 91.9% capacitance retention at 5 A g(-1). More importantly, an asymmetric supercapacitor with a high energy density (35.6 Wh kg(-1)) and remarkable cycle stability performance (94% capacitance retention over 3000 cycles) is assembled successfully by employing the ZNCO electrode as positive electrode and activated carbon as negative electrode. The remarkable electrochemical behaviors demonstrate that the nickel foam supported hierarchical mesoporous ZNCO nanowire array electrodes are highly desirable for application as advanced supercapacitor electrodes.

Heteroatoms dual doped porous graphene nanosheets as efficient bifunctional metal-free electrocatalysts for overall water-splitting

Nitrogen and fluorine dual-doped porous graphene nanosheets (NFPGNS) have been successfully synthesized as efficient bifunctional metal-free electrocatalysts for overall water splitting via a simple chemical-etching method. Pyridinic N doping rich configurations have been proven beneficial for the electrochemical process. The onset voltage of water splitting on the NFPGNS is lower than 1.60 V, only slightly higher than that found for Pt/C electrocatalysts. Particularly, an onset potential of 1.45 V vs. RHE on the NFPGNS for the OER is lower than some metal based electrocatalysts, involving Pt/C. DFT calculations reveal the origin of the electrocatalytic activity on the NFPGNS for the HER and OER; heteroatom-doped graphene materials modify the electron acceptor–donor properties of carbon via a synergistic coupling effect between heteroatoms. This leads to favorable electronic structures tuning the C sites around the heteroatoms, introducing a stronger adsorption strength and consequently, a lower value for the Gibbs free energy.

Hybrid Reduced Graphene Oxide Nanosheet Supported Mn–Ni–Co Ternary Oxides for Aqueous Asymmetric Supercapacitors

Hybrid reduced graphene oxide (RGO) nanosheet supported Mn-Ni-Co ternary oxides (MNCO) are prepared through a facile coprecipitation reaction with a subsequent calcination process as electrodes for supercapacitors. Electrochemical measurements prove that RGO can significantly improve the supercapacitive behaviors, compared with the pure MNCO electrode. A high specific capacity of 646.1 C g-1 at 1 A g-1 can be achieved and about 89.6% of the capacity can be remained at 30 A g-1 relative to that of the low-current capacity, indicating attractive rate capability of the RGO-MNCO electrode. Moreover, an asymmetric supercapacitor (ASC) device is fabricated with nitrogen-enriched RGO as the negative electrode and the synthesized RGO-MNCO as the positive electrode. Electrochemical performances investigated at different potential range reveal that the ASC device presents excellent capacitive behavior and reversibility. A maximum energy density of 35.6 Wh kg-1 at power density of 699.9 W kg-1 can be delivered. Furthermore, stable cycle capability with 100% Coulombic efficiency and 77.2% the capacitance retention is also achieved after 10000 cycles. The achieved outstanding electrochemical properties indicate that the obtained RGO-MNCO electrode materials are fairly ideal for progressive supercapacitors.

Activated Microporous Carbon Derived from Almond Shells for High Energy Density Asymmetric Supercapacitors

Via the activation treatment of carbonized almond shells with HNO3 and KOH, activated microporous carbon (AMC-3 and AMC-2) was successfully synthesized. These two AMC electrodes demonstrate remarkable electrochemical behaviors such as high rate capability, high specific capacitance, and excellent cycle stability when serving as electrodes for supercapacitors. More importantly, through the use of a Zn-Ni-Co ternary oxide (ZNCO) positive electrode and the AMC negative electrode, asymmetric supercapacitors (ASC) were assembled that deliver superior energy density (53.3 Wh kg(-1) at a power density of 1126.1 W kg(-1) for ASC-2 and 53.6 Wh kg(-1) at a power density of 1124.5 W kg(-1) for ASC-3) and excellent stability (82.7% and 83.4% specific capacitance retention for ZNCO//AMC ASC-2 and ZNCO//AMC ASC-3, respectively, after 5000 cycles). Through these two methods, low-cost, renewable, and environmentally friendly electrode materials can be provided for high energy density supercapacitors.

3D hierarchically porous zinc–nickel–cobalt oxide nanosheets grown on Ni foam as binder-free electrodes for electrochemical energy storage

Three-dimensional (3D) hierarchically porous transition metal oxides, particularly those involving different metal ions of mixed valence states and constructed from interconnected nano-building blocks directly grown on conductive current collectors, are promising electrode candidates for energy storage devices such as Li-ion batteries (LIBs) and supercapacitors (SCs). This study reports a facile and scalable chemical bath deposition process combined with simple calcination for fabricating 3D hierarchically porous Zn–Ni–Co oxide (ZNCO) nanosheet arrays directly grown on Ni foam with robust adhesion. The resulting nanostructures are then evaluated as a binder-free electrode for LIBs and SCs. Given its unique architecture and compositional advantages, the electrode exhibits a reversible capacity of 1131 mA h g−1 after 50 cycles at a current density of 0.2 A g−1, an excellent long-term cycling stability at a high current density of 1 A g−1 for 1000 cycles, and a desirable rate capability when tested as an anode for LIBs. When used for SCs, the electrode demonstrates a high specific capacitance (1728 F g−1 at 1 A g−1), an outstanding rate capability (72% capacitance retention from 1 A g−1 to 50 A g−1), and an excellent cycling stability (capacitance of 1655 F g−1 after 5000 cycles at a current density of 20 A g−1 with 108.6% retention). Overall, the unique 3D hierarchically porous ZNCO nanosheets hold a great promise for constructing high-performance energy storage devices.

Graphene-Encapsulated Nanosheet-Assembled Zinc–Nickel–Cobalt Oxide Microspheres for Enhanced Lithium Storage

The appropriate combination of hierarchical transition-metal oxide (TMO) micro-/nanostructures constructed from porous nanobuilding blocks with graphene sheets (GNS) in a core/shell geometry is highly desirable for high-performance lithium-ion batteries (LIBs). A facile and scalable process for the fabrication of 3D hierarchical porous zinc-nickel-cobalt oxide (ZNCO) microspheres constructed from porous ultrathin nanosheets encapsulated by GNS to form a core/shell geometry is reported for improved electrochemical performance of the TMOs as an anode in LIBs. By virtue of their intriguing structural features, the produced ZNCO/GNS core/shell hybrids exhibit an outstanding reversible capacity of 1015 mA h g(-1) at 0.1 C after 50 cycles. Even at a high rate of 1 C, a stable capacity as high as 420 mA h g(-1) could be maintained after 900 cycles, which suggested their great potential as efficient electrodes for high-performance LIBs.

Lithiophilic Cu‐CuO‐Ni Hybrid Structure: Advanced Current Collectors Toward Stable Lithium Metal Anodes

Metallic lithium (Li) is a promising anode material for next-generation rechargeable batteries. However, the dendrite growth of Li and repeated formation of solid electrolyte interface during Li plating and stripping result in low Coulombic efficiency, internal short circuits, and capacity decay, hampering its practical application. In the development of stable Li metal anode, the current collector is recognized as a critical component to regulate Li plating. In this work, a lithiophilic Cu-CuO-Ni hybrid structure is synthesized as a current collector for Li metal anodes. The low overpotential of CuO for Li nucleation and the uniform Li+ ion flux induced by the formation of Cu nanowire arrays enable effective suppression of the growth of Li dendrites. Moreover, the surface Cu layer can act as a protective layer to enhance structural durability of the hybrid structure in long-term running. As a result, the Cu-CuO-Ni hybrid structure achieves a Coulombic efficiency above 95% for more than 250 cycles at a current density of 1 mA cm-2 and 580 h (290 cycles) stable repeated Li plating and stripping in a symmetric cell.

Seed-assisted smart construction of high mass loading Ni-Co-Mn hydroxide nanoflakes for supercapacitor applications

Smartly designed nanoarchitectures with effective hybridization of transition metal oxides/hydroxides are promising to realize high performance electrodes for energy storage devices. To promote the applications of high-power supercapacitors, a seed-assisted method is firstly applied to prepare mesoporous Ni–Co–Mn hydroxide nanoflakes (NCMH) on nickel foam with practical mass loadings (higher than 5 mg cm−2). Further mechanism study reveals that the Ni(OH)2 nanorod arrays, which are firstly prepared by a hydrothermal process, serve as seeds for the successful deposition of NCMH nanoflakes. Through this convenient and cost effective method, this design results in a more orderly spatial distribution, lower intrinsic resistance and shorter electron transport pathways. The proof-of-concept application of NCMH as a binder-free supercapacitor electrode reveals an impressive specific capacity of 1043.1 μA h cm−2 at a high mass loading of 5.2 mg cm−2. The NCMH//activated carbon asymmetric device delivered a maximum energy density of 55.42 W h kg−1 at a power density of 750 W kg−1, exhibiting great potential as an energy storage device and shedding light on the structural design of nanomaterials.

Templated and Catalytic Fabrication of N-Doped Hierarchical Porous Carbon–Carbon Nanotube Hybrids as Host for Lithium–Sulfur Batteries

Nitrogen-doped hierarchical porous carbon and carbon nanotube hybrids (N-HPC-CNTs) are fabricated by simple pyrolysis of the N-rich raw material melamine-formaldehyde (MF) resin in the presence of nano-CaCO3 and a bimetallic combination of Fe-Co catalyst. During carbonization, nano-CaCO3 acts as a template for creating a hierarchical porous carbon, and the N atoms originated from MF resin are in situ doped into the carbon matrix simultaneously. Meanwhile, volatile gases generated by the thermal decomposition of MF resin could serve as carbon and nitrogen sources to grow nitrogen-doped CNTs on HPC. The growth mechanism is the same as that for conventional chemical vapor deposition (CVD) growth of CNTs on the metal catalysts, but the technological requirements are obviously not as harsh as those for the CVD method. Low-cost raw materials and simple equipment are sufficient for the growth. Moreover, the density and length of the CNTs are tunable, which can be simply adjusted via applying different amounts of Fe-Co catalysts. Such an N-doped hybrid structured carbon with mesopores can not only effectively prompt the physical and chemical adsorption of polysulfides but also ensures a fast electron transfer because of the incorporation of CNTs, which provides sufficient conducting pathways and effective connections between the CNTs and HPC. Furthermore, CNTs grown on HPC can act as physical barriers to block the large pores on HPC, thereby reducing the polysulfide loss. Benefiting from the advantages, the N-HPC-CNT hybrids are a desirable host prospect for Li-S batteries.

Nanoforest of hierarchical core/shell CuO@NiCo2O4 nanowire heterostructure arrays on nickel foam for high-performance supercapacitors

Nickel foam-supported CuO@NiCo2O4 nanoforests with a mesoporous hierarchical core/shell structure are prepared by combining a facile, scalable, and cost-effective thermal oxidation method with a simple hydrothermal method followed by a calcination procedure. The smart hybridization of CuO nanowires and NiCo2O4 nanosheets into a hierarchical core/shell array configuration results in remarkably enhanced electrochemical performances with high specific capacitance, excellent rate capability and good cycle performance compared with pure nickel foam-supported NiCo2O4 nanosheets. A high specific capacitance of 1298.8 F g−1 at a current density of 1 A g−1 has been exhibited and excellent rate capability of about 96.3% capacitance retention at 5 A g−1 can be obtained. The CuO@NiCo2O4-based supercapacitor exhibits a very long cycle life with only 2.1% capacitance loss after 2000 cycles and the coulombic efficiency remains about 100% during the cycling. In addition, the assembled CuO@NiCo2O4//AC ASC device delivers an energy density of 27.9 W h kg−1 at a power density of 749.6 W kg−1, and the energy density is as much as 17.7 W h kg−1 even at a high power density of 7496.5 W kg−1. These excellent electrochemical performances demonstrate that the nickel foam-supported hierarchical core/shell CuO@NiCo2O4 nanowire heterostructure array electrodes are highly desirable for application as advanced supercapacitor electrodes.