3D hierarchical porous nitrogen-doped carbon/Ni@NiO nanocomposites self-templated by cross-linked polyacrylamide gel for high performance supercapacitor electrode

Ni/NiO@NC as a highly efficient and durable HER electrocatalyst derived from nickel(II) complexes: importance of polydentate amino-acid ligands

Research on Ni/NiO electrocatalysts has advanced significantly, but the main obstacles to their use and commercialization remain their relatively ordinary activity and stability. In this paper, a chelating structure based on the coordination of multidentate ligands and Ni(II) is proposed to limit the growth of Ni and Ni oxide grains. These features reduce the particle size of Ni/NiO, increase particle dispersion, and maintain the high activity and stability of the catalyst. Aspartic acid, as a polydentate ligand, could coordinate with Ni2+ to form structurally stable chelate rings. The latter can limit grain growth, but also coat the active core with thin carbon layers after calcination to further achieve the confinement and protection of nanoparticles. The hydrogen evolution overpotential of prepared nitrogen-doped graphitized carbon shells (Ni/NiO@NC) nanoparticles was 100 mV (vs. RHE) when the current density was 10 mA cm-2 in 1 M KOH. The hydrogen evolution overpotential increased by only 4 mV after 6000 continuous cyclic-voltammetry scans. Moreover, when coated on different conductive substrates, the overpotential of this catalyst dropped to 34.6 mV (vs. RHE) at a current density of 10 mV cm-2. The lowest overpotential of the composite was only 194.9 mV at a current density of 100 mA cm-2, which is comparable with that of noble metal-based electrocatalysts. This work provides a plausible method for designing high-performance electrocatalysts of small size.

Controllable reduction of NiCoO2@NiCo core-shell nanospheres on CNTs for high-performance electrochemical energy storage

The performances of energy storage devices are strongly dependent on the electrode materials. Owing to the high theoretical capacity, NiCoO₂ is a promising transition metal oxide for supercapacitors. Despite many efforts have been devoted, it still lacks of effective methods to overcome its shortcomings such as low conductivity and poor stability, in order to achieve its theoretical capacity. Herein, utilizing the thermal reducibility of trisodium citrate and its hydrolyzate, a series of NiCoO₂@NiCo/CNT ternary composites in which NiCoO₂@NiCo core-shell nanospheres deposited on CNT surface with adjustable metal contents are synthesized. Benefiting from the enhanced synergistic effect of both metallic core and CNTs, the optimized composite exhibits an extremely high specific capacitance (2660 F g^-1 at 1 A g^-1, the effective specific capacitance of the loaded metal oxide is 4199 F g^-1, close to the theoretical value), an excellent rate performance and stability, when the metal content is about 37%. After depolarized calculation, the energy storage mechanism of the composite is reasonably analyzed. By controlling the contents of hexamethylenetetramine, trisodium citrate and CNTs in the reactant, the roles of them are distinguished. This study reveals an efficient novel strategy for transition metal oxides to maximize the electrochemical performances.

Core-Shell Carbon Nanofibers@Ni(OH)2/NiO Composites for High-Performance Asymmetric Supercapacitors

The application of transition metal oxides/hydroxides in energy storage has long been studied by researchers. In this paper, the core-shell CNFs@Ni(OH)₂/NiO composite electrodes were prepared by calcining carbon nanofibers (CNFs) coated with Ni(OH)₂ under an N₂ atmosphere, in which NiO was generated by the thermal decomposition of Ni(OH)₂. After low-temperature carbonization at 200 °C, 250 °C and 300 °C for 1 h, Ni(OH)₂ or/and NiO existed on the surface of CNFs to form the core-shell composite CNFs@Ni(OH)₂/NiO-X (X = 200, 250, 300), in which CNFs@Ni(OH)₂/NiO-250 had the optimal electrochemical properties due to the coexistence of Ni(OH)₂ and NiO. Its specific capacitance could reach 695 F g^-1 at 1 A g^-1, and it still had 74% capacitance retention and 88% coulomb efficiency after 2000 cycles at 5 A g^-1. Additionally, the asymmetric supercapacitor (ASC) assembled from CNFs@Ni(OH)₂/NiO-250 had excellent energy storage performance with a maximum power density of 4000 W kg^-1 and a maximum functional capacity density of 16.56 Wh kg^-1.

Construction of hierarchical Co9S8@NiO synergistic microstructure for high-performance asymmetric supercapacitor

Metal-organic frameworks (MOFs) become a research hot-spot owing to their unique properties originating from the ultra-high porosity and large specific surface area with highly accessible active sites. However, the electrochemical performance of a single component is unsatisfied when MOFs are applied as electrode material in a supercapacitor. In this work, the hierarchical hollow framework involving interconnected Co₉S₈ structure and NiO nanosheets (Co₉S₈@NiO) are successfully prepared by MOFs derived methods and proposed to electrode materials. As a result, the prepared Co₉S₈@NiO electrode materials exhibit a superior specific capacitance of 1627 F g^-1 at a current density of 1 A g^-1. Moreover, an assembled hybrid supercapacitor shows a high energy density of 51.65 Wh Kg^-1 at a power density of 749.8 W Kg^-1 as well as excellent long-term cycling stability with 81.79% capacity retention after 10,000 cycles. Meanwhile, we concluded that the marvelous electrochemical performance is closely associated with the unique structure of NiO, in particular, the nanosheet surface provides a superior specific surface area and rich accessible redox reaction sites, thus enlarged the contact between the surface and interface of the electrode material. Finally, two supercapacitor devices connected in series can light up four light-emitting diodes (LEDs) for about 30 min. Hence, the presented strategy represents a general route for supercapacitor electrode material with promising electrochemical performance, which can combine the MOFs template and other hierarchical nanosheets together.

Preparation of NiO decorated CNT/ZnO core-shell hybrid nanocomposites with the aid of ultrasonication for enhancing the performance of hybrid supercapacitors

Supercapacitor (SC) electrodes fabricated with the combination of carbon nanotubes (CNTs) and metal oxides are showing remarkable advancements in the electrochemical properties. Herein, NiO decorated CNT/ZnO core-shell hybrid nanocomposites (CNT/ZnO/NiO HNCs) are facilely synthesized by a two-step solution-based technique for the utilization in hybrid supercapacitors. Benefitting from the synergistic advantages of three materials, the CNT/ZnO/NiO HNCs based electrode has evinced superior areal capacity of ~67 µAh cm-2 at a current density of 3 mA cm-2 with an exceptional cycling stability of 112% even after 3000 cycles of continuous operation. Highly conductive CNTs and electrochemically active ZnO contribute to the performance enhancement. Moreover, the decoration of NiO on the surface of CNT/ZnO core-shell increases the electro active sites and stimulates the faster redox reactions which play a vital role in augmenting the electrochemical properties. Making the use of high areal capacity and ultra-long stability, a hybrid supercapacitor (HSC) was assembled with CNT/ZnO/NiO HNCs coated nickel foam (CNT/ZnO/NiO HNCs/NF) as positive electrode and CNTs coated NF as negative electrode. The fabricated HSC delivered an areal capacitance of 287 mF cm-2 with high areal energy density (67 µWh cm-2) and power density (16.25 mW cm-2). The combination of battery type CNT/ZnO/NiO HNCs/NF and EDLC type CNT/NF helped in holding the capacity for a long period of time. Thus, the systematic assembly of CNTs and ZnO along with the NiO decoration enlarges the application window with its high rate electrochemical properties.

Nitrogen- and Oxygen-Containing Three-Dimensional Hierarchical Porous Graphitic Carbon for Advanced Supercapacitor

Three-dimensional hierarchical porous graphitic carbon (HPGC) were synthesized via one-step carbonization-activation and a catalytic strategy. The method can not only improve the graphitization degree of carbon materials, but also offer plentiful interfaces for charge accumulation and short paths for ion/electron transport. Polypyrrole, potassium hydroxide, and nickel acetate were used as the carbon precursors, activating agent, and catalyst, respectively. The retraction and dissolution of Ni caused the change of pore size in the material and led to the interconnected micro/nano holes. Nickel acetate played a significant role in enhancing the electrical conductivity, introducing pseudocapacitance, and promoting ion diffusion. In the supercapacitor, HPGC electrode exhibited a remarkable specific capacitance of 336.3 F g⁻¹ under 0.5 A g⁻¹ current density and showed high rate capability, even with large current densities applied (up to 50 A g⁻¹). Moreover, HPGC showed optimal cycling stability with 97.4% capacitance retention followed by 3000 charge-discharge cycles. The excellent electrochemical performances coupled with a facile large-scale synthesis procedure make HPGC a promising alternative for supercapacitors.