1 Preparation of cellulose supported metal Co/Co(OH)2 functional aggregates
1.1 Innovative Research on Supercapacitors with Co Layer Deposition on Cellulose Paper for High Performance Electrodes
The deposition of the metal Co layer on the surface of cellulose paper was achieved by DC magnetron sputtering technology (VTC-600-2HD, Shenyang Kejing Automation Equipment Co., Ltd.), in which cellulose paper and Co were placed between the anode and cathode with a spacing of 60 mm. For the magnetron sputtering process, the vacuum degree of the reaction chamber was first evacuated to 3×10-3Pa and then argon was slowly introduced as the sputtering gas at a flow rate of 11 sccm. Under the conditions of 100W power and 20 r/min rotation speed, uniform sputtering of metal Co was achieved on the surface of the cellulose paper substrate. The water cooling system ensured that the maximum temperature of the reaction did not exceed 40℃, so the thermal stress of the cellulose substrate was at a very low level, which could prevent the deformation and diffusion of the deposited metal Co particles. The deposition thickness of the Co particle layer was set to 40µm. This process provides an important electrode material foundation for innovative research on supercapacitors and helps develop high-performance and sustainable energy storage devices.
1.2 In situ growth of honeycomb Co(OH)2 on the surface of metal Co by electrooxidation
The metal Co on the surface of cellulose paper was converted into Co(OH)2 by electrooxidation. The electrooxidation process was carried out using a three-electrode system in an electrochemical workstation (CS350, Wuhan Kosite Instrument Co., Ltd.). A 6mol/L KOH aqueous solution was used as the electrolyte solution. The prepared cellulose-supported metal Co, Pt (2cm×2 cm) and HgHgO electrodes were used as working electrodes, counter electrodes and reference electrodes, respectively. Co was then oxidized by cyclic polarization. The potential window and scan rate were set to 0~0.5V vs.Hg/gO and 10 mV/s, respectively. The oxidation time plays a key role in the surface area and electrochemical properties of the oxidation product [i.e., cellulose-supported metal Co/Co(OH)2]. By studying the effects of cycle time on the above factors, it was found that 100 cycles could achieve better electrochemical activity. This study provides new ideas for electrode materials for innovative research on supercapacitors and helps to improve the performance of energy storage devices.
2 Design strategies of functional aggregates and structural analysis of innovative research on supercapacitors
The design of functional aggregates has the following principles [as shown in Figure 1 (a) 1: ① Using a piece of ordinary paper as a cellulose substrate shows the versatility of the preparation method. Hydrophilic cellulose fibers (Figure 2) are similar to geological layers and can quickly absorb a large amount of electrolyte. Its cross-linked porous structure can be used as an electrolyte storage reservoir to promote ion transport [Figure 1 (b)]. In addition, cellulose has the characteristics of light weight and can give the composite material good flexibility (Figure 3). ② The metal Co component obtained by sputtering has a mountain-shaped structure with a thickness of about 40.3 µm [Figure 1 (c)~(e)], which brings rigidity and mechanical strength to the composite material. In addition, the presence of high-purity metal Co gives the composite material a high conductivity (about 5.1×102/cm). Therefore, the metal Co layer plays a key role as an electron high-speed channel for charge transport. ③ The Co(OH)2 grown in situ on the Co surface by electro-oxidation has a honeycomb nanostructure, which is composed of interwoven nanosheets with a thickness of 12~20 nm and a width of 300~500 nm [Figure 1(f), (g)]. Therefore, the Co(OH)2 nanostructure has a perfect interface with the inner layer Co and exhibits a large specific surface area of 318m2/g (Figure 4), which increases the electrochemical active sites for redox reactions.Based on the above strategies, we believe that this nature-inspired structural design is suitable for high-performance energy storage, especially in the innovative research on supercapacitors.
Figure 1 Nature-inspired design concept and microstructure of cellulose-supported metal Co/Co(OH)2 composites: (a) Schematic diagram of nature-inspired nanostructure design; (b) scanning electron microscope (SEM) images of cellulose paper and (c) cellulose-supported metal Co; (d, e) cross-sectional SEM images of cellulose-supported metal Co and (f) cellulose-supported metal Co/Co(OH)2; (g) high-magnification SEM image of honeycomb-shaped Co(OH)z; (h, i) transmission electron microscope (TEM) and (j, k) high-resolution transmission electron microscope (HRTEM) images of cellulose-supported metal Co/Co(OH)2. The insets of (c) and (g) show the corresponding magnified images
Figure 2 Water contact angle test of cellulose paper, cellulose-supported metal Co and cellulose-supported metal Co/Co(OH)2
Figure 3 Flexible characteristics of cellulose paper, cellulose-supported metal Co and cellulose-supported metal Co/Co(OH): (a) Cellulose paper and cellulose-supported metal Co have good flexibility and foldability; (b) Cellulose-supported metal Co/Co(OH)2 can be bent arbitrarily
Figure 4 (a) N2 adsorption-desorption isotherm and (b) pore size distribution of cellulose-supported metal Co/Co(OH)2
The morphology of the cellulose-supported Co/Co(OH)2 heterostructure was further studied by transmission electron microscopy (TEM). We found that the mountain-shaped metal Co is composed of many subcircular nanoparticles with a size of about 5nm [Figure 1(i)], while the Co(OH)2 nanosheets are directly attached to the Co surface [Figure 1(h)], which once again proves the good adhesion between their interfaces. Its crystal structure was studied by XRD. From Figure 5 It can be seen that due to the coverage of metal Co, the characteristic peaks of the cellulose I crystal structure disappeared, and three peaks corresponding to the (101), (002) and (100) crystal planes (JCPDS No. 05-0727) of Co were detected (located at 47.3°, 44.5° and 41.5°). In addition, some weak peaks related to a-Co(OH) and B-Co(OH) were also detected. This result shows that Co was partially converted to α-Co(OH)2 due to electro-oxidation. In addition, the HRTEM images corresponding to the marked areas in Figures 1(h) and (i) [as shown in Figures 1(j) and (k)] also show the presence of Co, and the crystal planes corresponding to α-Co(OH)2 and β-Co(OH)2 in the Co(OH)2 nanostructure are also observed, which is consistent with the XRD results.
Figure 5 XRD spectra of (a) cellulose substrate and (b) cellulose-supported metal Co, cellulose-supported metal Co/Co(OH)2
X-ray photoelectron spectroscopy (XPS) can explore the chemical valence state of each element. The Co 2p XPS spectrum of cellulose-supported metal Co/Co(OH)2 [Figure 6(a)] shows that the peaks centered at 795.9 eV and 780.6 eV are attributed to Co2p1/2 and Co2p3/2, respectively, indicating the presence of Co2+, indicating the successful synthesis of Co(OH)2. In addition, the weaker peaks at 804.0 eV and 789.9 eV correspond to the companion peaks of Co 2p1/2 and Co 2p3/2, indicating that a small amount of Co3+ is formed. In addition, the O 1s spectrum [Figure 6(b)] shows three fitting peaks at structural water (532.6 eV), Co(OH)2 (531.1 eV) and CoO (529.614 eV), respectively. The presence of CoO is attributed to the dehydroxylation of Co(OH)2 during drying.
Figure 6 (a) Co 2p (inset shows the fitting result) and (b) O 1s core level XPS spectra of cellulose-supported metal Co/Co(OH)2
Based on the above characterization results, we proposed the following possible formation mechanism of soil-mountain-vegetation structure, which may have an enlightening effect on the innovative research on supercapacitors.
⑴ Magnetron sputtering deposition of metal Co on the surface of cellulose paper
For magnetron sputtering (Figure 7), due to the potential difference between the negatively biased target (cathode) and the anode, the inert gas atoms (Ar) are ionized and accelerated; the interaction between the ions and the surface of the target (metal Co) causes the sputtering of Co atoms, which then condense on the surface of the cellulose matrix and eventually form a mountain-shaped structure. Magnetron sputtering has the following main advantages: ① High deposition rate; ② Easy to sputter any metal/alloy; ③ High purity of the prepared film ④) Strong film adhesion; ⑤ Excellent step coverage and small grain size; ⑥ Ability to coat heat-sensitive substrates: ⑦ Easy to automate; ⑧ Good uniformity on large-area substrates.
Figure 7 Schematic diagram of magnetron sputtering of metal Co on the surface of cellulose paper
⑵ Conversion of metal Co to Co(OH)2 by electro-oxidation
Metallic Co was converted to Co(OH)2 using an electro-oxidation method in a three-electrode system. The possible formation of honeycomb Co(OH)2 is shown in Figure 8. The expected chemical reactions are Co⟶ Co2++2e– and Co2++2OH+⟶ Co(OH)2. The oxidation reaction is located near the defects on the Co surface (such as surface deformation and depression). The newly generated Co2+ reacts with OH–, resulting in the nucleation of a large amount of Co(OH)2. These newly generated nanocrystals are unstable due to their high surface energy, so they tend to form more stable geometric forms to reduce the system energy. Brucite crystals [M(OH)2 or MOOH, M=Mg, Ca, Ni, Co, etc.] have a CdI2 type layered structure, and the interaction between layers is weak, while the binding force within the layer is strong. Therefore, brucite crystals are more likely to form a two-dimensional (2D) nanosheet structure. As the reaction proceeds, these nanosheets continue to aggregate to form a 3D honeycomb structure. This study provides new ideas for innovative research on supercapacitors.
Figure 8 Schematic diagram of the possible formation mechanism of honeycomb Co(OH)2
