Construction And Supercapacitor Performances Of 3D Nickel/Cobalt Based Electrode Materials

Because of short charge time, large power density, long lifespan and high safety, supercapacitors have been applied in the daily consumption electronic products, new energy automobile, national defense and military. At present, commercial supercapacitors mostly make use of activated carbon material. The activated carbon materials deliver an electric double layer capacitance of often lower than 300 F/g, seriously restricting applications of supercapacitors in large energy devices. Thereby, development of new electrode materials has attracted academia and industry common interest. Nickel/cobalt electrode materials with the advantages of natural abundance, low price and green non-toxicity have turned out to possess high specific capacitance, good rate performance and cycle life, which have been regarded as a class of the most potential electroactive materials for supercapacitors. However, thier electrochemical performances mainly depend on their components and space structures. The article designed and constructed series of 3D different nanostructured nickel/cobalt hydroxides, oxides and sulfides by innovation of the synthesis theory and method, and achieved the largely boosted supercapacitive performances.The study adopted hard-template, soft-template and free-template methods to fabricate different 3D nanostructured nickel/cobalt hydroxide, respectively. First, using SiO₂ as hard template, Co?NO3?2 and Ni?NO3?2 as the precursor salts and ethanol as reaction medium, one-step hydrothermal method was employed for synthesis of hydrangea-like nickel/cobalt hydroxide hollow microspheres. The investigations found that during the synthesis, ethanol and nitrate underwent a redox reaction for releasing OH- ions. The released OH- source not only can combine with Ni2+/Co2+ to form ultrathin Ni/Co-LDH nanoflakes, but also can dissolve SiO₂ template. Interestingly, perfect match between generation rate of Ni/Co-LDH nanoflakes and removal rate of the template creates an elaborate three-dimensional structure with well-defined hollow interior and hydrangea-like exterior. The formed hollow microspheres are assembled by of a host of nanosheets with 6.0 nm thickness, showing perfect dispersion and particle uniformity. At the current density of 2 A/g, the electrode delivers a high specific capacitance of 2106.8 F/g, higher than that of nickel/cobalt solid microspheres?1480.5 F/g?. The capacitance still keeps at 1508.7 F/g when the current densities rise to 17 A/g, and the capacitance loss is 2.5% after 1500 charge-discharge cycles at 10 A/g. Next, one-step hydrothermal reaction was employed for synthesis of flower-like nickel/cobalt hydroxide hollow microspheres by using tert-butanol?TBA? as soft template. The investigations demonstrated that TBA and H2 O can form specific W/O and O/W microemulsion systems, and nickel/cobalt hydroxide with various morphologies can be obtained by adjusting the volume ratio of TBA/H2 O. Particularly, flower-like nickel/cobalt hydroxide hollow microspheres are created when TBA/H2 O ratio is 8:2. The formed microspheres show an interior core with 354.2 nm diameter and flower-like exterior essembled from abundant nanosheets with about 11.2 nm thickness. The electrode delivers large specific capacitance of 2107.9 F/g at 1 A/g, 1502.6 F/g at 12 A/g and 2.97% capacitance loss after 1500 cycles. Finally, flowercluster-like nickel/cobalt hydroxide was prepared by free-template method through microwave assisted heating mode. The flowercluster-like architecture is composed of abundant ultrathin nanosheets, showing unique 3D network-like structure. As the electrode, it displays the maxiumal specific capacitance of 2100.5 F/g and the capacitance retention of 97.8 % after 1500 cycles at 10 A/g.Compared with the hydroxides, nickel/cobalt oxides are more stable in the air or in the alkaline solution, thus possessing superior cyclic stability for supercapacitors. Considering ultrathin feature of flowercluster-like nickel/cobalt hydroxide above, the study adopted it as the precursor and calcinated it to form unique coral-like NiCo₂O₄. The resulting NiCo₂O₄ is of 3D nanocoral-like network architecture. Interestingly, the size of coral can be easily controlled by adjusting the temperature, and the size of coral increases with the increasement of calcination tempeatures. The NiCo₂O₄ prepared at 400? gives a minimum building block size?10.2 nm? and maximum specific surface area?108.8 m2/g?. These features will greatly improve electron transport and mass transfer during faradaic redox reaction. At the current density of 1 A/g, the NiCo₂O₄ electrode shows the largest specific capacitance of 870.7 F/g. When the current density increases to 20 A/g, the capacitance still can remain at 700.8 F/g, and the capacitance found without any decay after 3000 cycles at 10 A/g. To further lift up the performance of NiCo₂O₄, it was directly grown on the conductive network of graphene to form double-network nanostructured NiCo₂O₄/graphene. The results show that the conductivity of NiCo₂O₄/graphene?22.6 S/m? is around 266 times higher than pure NiCo₂O₄?0.085 S/m?. The NiCo₂O₄/graphene as the electrode displays a high capacitance of 1710.7 F/g, good rate performance of 1467.7 F/g at 20 A/g and outstanding cycle life of 4.4% capacitance loss after 3000 cycles at 10 A/g.Eectronic flow of sulphur atom endows nickel/cobalt sulfides with superior electron conduction comparable to their hydroxides or oxides. Tremella-like NiCo₂S₄ was successfully prepared by S2- ion hydrothermal exchange reaction with flowercluster-like nickel/cobalt hydroxide prepared above. The prepared NiCo₂S₄ displays 3D network framework assembled from lots of incomplete nanosheets. At the current density of 1 A/g, it exhibits a specific capacitiance of 1410.7 F/g. In subsequence, the electroactive NiCo₂S₄ was directly grown on the surface of Ni foam via a simple hydrothermal method combined with the sulfuration treatment to form 3D hierarchical NiCo₂S₄ microflowers@NiCo₂S₄ nanosheet arrays. The formation process also has been investigated in detail. In the configuration, high-conductivity NiCo₂S₄ nanosheet arrays paves a “bridge” between NiCo₂S₄ microflowers and current collector, speeding up the electron transport; the large exterior surface of NiCo₂S₄ microflowers provides more superficial electroactive sites to take part in electrode reaction, resulting in high utilization rate of the materials. The unique hierarchical configuration achieves a stunning interface contact between NiCo₂S₄ microflowers and NiCo₂S₄ nanosheets, potentially resulting in a strong synergy effect. Thus, the NiCo₂S₄ microflowers@NiCo₂S₄ nanosheet arrays manifest outstanding electrochemical performances in the three-electrode test. Further, an asymmetric supercapacitor was assembled with the NiCo₂S₄ microflowers @NiCo₂S₄ nanosheet arrays as the positive and the activated carbon from biomass waste pomegranate peel as the negative. The device demonstrates a high energy density of 58.9 Wh/kg at the power density of 800 W/kg, even when the power density reaches to 6400 W/kg, the energy density of 30.9 Wh/kg still can be achieved, showing outstanding capacitive performance.How to combine nickel/cobalt hydroxide with the oxides or sulfides to realize their complementary advantages has become a research hotspot in recent years. Finally, three different types of 3D hierarchical multi-component electrode materials were developed. They include rose-like Ni/Co-LDH@NiCo₂O₄ nanosheet arrays, urchin-like Ni/Co-LDH@NiCo₂S₄ nanowire arrays and peony-like Ni/Co-LDH@NiCo₂S₄ nanoleaf arrays@graphene. Among them, Ni/Co-LDH@NiCo₂S₄ @ graphene gives the maximal loading mass of 7.6 mg/cm2, the largest apparent heterogeneous electron transfer constant of 0.324 cm/s and the smallest integrated impedance of 0.775 ?, indicating a superior electronic conductivity. As the electrode, Ni/Co-LDH@NiCo₂S₄@graphene shows outstanding specific capacitance of 2001.2 F/g at the current density of 1 A/g, high rate performance of 1645.6 F/g at 8 A/g and long cycle life of 3.0% capacitance decay after 3000 cycles. To further improve the capacitive performance of Ni/Co-LDH@NiCo₂S₄@graphene, a mixed electrolyte of K3Fe?CN?6/KOH aqueous solution was developed. As a result, Ni/Co-LDH@NiCo₂S₄@graphene electrode displays an extremely high specific capacitance of 6282.6 F/g and coulombic efficiency of 484.3% at the current density of 6 A/g. Meanwhile, the assembled asymmetric supercapacitor by Ni/Co-LDH@NiCo₂S₄@graphene as the positive and the activated carbon electrode as the negative also achieves a high energy density of 102.8 Wh/kg at the power density of 800 W/kg which is superior to 62.5 Wh/kg of the device in KOH aqueous solution.