Hierarchical Construction Of Nanoarrays For Electrochemical Applications

Nanoarray materials have received worldwide concern and increasing research interest due to their many structure advantages (the assembled surface, exposed active crystal face, adjustable size, and good adhesion to the substrate, etc.), which have widely used in the fields of emission materials, interface materials, electrochemical electrodes, heterogeneous catalysts, and sensors, etc. Among of those nanoarray materials, transition metal oxides or hydroxides nanoarray with hierarchical structure have important prospects in electrochemical energy storage and catalysis because of their low price, high stability, and favorable structural characteristics.In this paper, a series of hierarchical nanoarrays were prepared by liquid-phase chemical deposition method, and used in the field of electrochemical energy storage, electrochemical catalysis, and heterogeneous catalysis. Based on the secondary hydrothermal evolution approach, multi-step deposition approach, and sacrificial-template transformation approach, the morphology, structure, composition, and metal ion doping of nanoarray could be adjusted, which will be helpful to improve the performances of nanoarrays. These researches provide valuable guidance for the fabrication of other hierarchical nanoarrays, which will be promising for a large spectrum of device applications. The main contents of this paper are as following:1. For the development of supercapacitor electrode with high specific capacitance, ultrathin CO3O4 nanosheet arrays on Ni foam were prepared by a secondary hydrothermal reaction and a subsequent calcination treatment. By using different sources of alkalinity (urea and hexamethylenetetramine), the obtained nanosheet showed different morphologies and sizes. The morphology evolutionary process and formation mechanism of the ultrathin nanosheets were also studied by changing the reaction time. As a supercapacitor electrode, a superior specific capacitance (1782 F·g-1) was obtained at the current density of 5 mA·cm-2, which is much larger than that of the thicker nanostructures (-351 F·g-1). The ultrathin nanosheet arrays electrode exhibited good rate capabilities, maintaining 51% of the initial capacity at current density of 30 mA·cm-2, and excellent long-term stability, remaining 90% of capacitance after 2000 cycles. After studing the relationship between the thickness and performance of the electrode, we found that constructing ultrathin structure increased the specific surface area of the nanosheet arrays, which resulted in the improvement of the contact area between electrode and electrolyte and facilitated the diffusion of electrolyte ion to the material, leading to more efficient utilization of the active material. Moreover, an asymmetric supercapacitor device based on CO3O4 NS-H/AC showed a high specific capacitance and specific energy (108 F·g-1 and 134 Wh·kg-1 within the potential range of 0-1.6 V), indicating this kind of electrodes is very promising for future supercapacitor or other energy storage applications.2. Construction of hierarchical nanoarrays can increase the specific surface area and mass-loading of the electrodes material, thereby improving the electrochemical performances of the materials. Around this goal, hierarchical Co3O4 nanosheet@nanowire arrays on 3D nickel foam substrate were prepared through a facile hydrothermal and annealing treatment. As a supercapacitor electrode, the hierarchical CO3O4 nanoarray showed more excellent electrochemical performances than the pure nanowire arrays and nanosheet arrays. The specific capacitance of hierarchical CO3O4 nanoarray could reach to 715 F·g-1 at 5 mA·cm-2, and maintained 69% rate characteristics at 30 mA·cm-2. Besides, the hierarchical CO3O4 nanoarray owned high cycle stability (100% after 1000 cycles), which made the hierarchical Co3O4 nanoarray an advanced supercapacitor material.3. High areal capacitance of electrodes is highly desirable for practical supercapacitor applications, which requires a combination of high mass-loading and high utilization of electrochemically active material. Hierarchical Co3O4@NiO nanowire@nanorod arrays with ultrathin NiO nanorods (～5 nm) directly grown on the CO3O4 nanowire arrays were prepared via a two-step hydrothermal reaction followed by a calcination process. And by simply adjusting the amount of the added nickel nitrate, the mass-loading and porosity of the active material could controlled. Due to the high mass-loading (19.5 mg·cm-2) and high utilization of active material, the hierarchical complex metal oxide nanoarrays showed high areal capacitance (39 F·cm-2) as a supercapacitor electrode, much higher than that of pure CO3O4 nanowire arrays (6.7 F·cm-2). Moreover, a remarkable rate capability (21.4 F·cm-2 at the current density of 30 mA·cm-2) and excellent cycling stability (100% after 1000 cycles) were observed. Compared with the pure CO3O4 nanowire arrays, the greatly enhanced capacitive performance is mainly attributed to the unique hierarchical porous architecture and the synergistic effect of the individual components. The electrode design concept can be readily generalized to other materials, which is helpful for a large spectrum of supercapacitor.4. Constructing hierarchical nanoarrays structure is also important in electrocatalytic field, since it can adress the issue of the loss of the active ingredients and the difficulty of bubbles overflow. Here, ultrathin NiCoFe layered double hydroxide (LDH) nanoplates, which directly grew on a cobalt based nanowire array, forming a hierarchical nanoarray structure, were constructed via a sacrificial template method by reacting cobalt-based nanowire with extra iron nitrate and urea. By changing the amount of added iron, the numbers and porosity of LDH nanosheets could be well controlled. In alkaline media, the ordered ultrathin hierarchical LDH nanoarray electrode showed dramatically increased catalytic activity compared to that of LDH nanoparticles and pure nanowire arrays due to the small size, large surface area, and high porosity of the NiCoFe LDH nanoarray. Only a small water oxidation overpotential of 0.23 V was needed for a current density of 30 mA·cm-2 with a Tafel slope of 53 mV·dec-1. The intrinsically high activity of the NiCoFe LDH and unique hierarchical architecture is believed to be responsible for the high electrocatalytic performance. This study affords a new strategy to achieve optimal performance in hierarchical nanoarray catalysts, and the electrode design concept can be readily generalized to other LDH-based materials.5. Immobilization of metal-organic framework (MOF) materials in three-dimensional (3D) and oriented arrangements would bring about many structural advantages, thus promoting the performance improvements of MOF-based nanodevices. A coordination replication strategy was developed to construct copper-based MOF into 3D hierarchical nanoarray architectures using Cu(OH)2 nanorod arrays as the sacrificial template. The Cu(OH)2 nanorod arrays can serve as the Cu source which coordinates with organic ligand to form MOF crystals, as well as the 3D substrate to support the growth of Cu-based MOF. This strategy was also found to be applicable to a variety of organic ligands, demonstrating the general efficacy. Due to the unique structural advantages (hierarchical porosity, large surface area and intimate contact with the 3D substrate), these MOF nanoarrays exhibited excellent catalytic activities and cycling stabilities in heterogeneous catalysis as structured catalysts. This strategy opens up new possibilities for constructing immobilized MOF-based 3D films, which will be of great promise for a large spectrum of device applications.