As shown in Figure 1(a), the electrochemical performance of the cellulose-supported metal Co/Co(OH)₂ composite electrode without any binder and conductive agent was studied using a conventional three-electrode system. The cyclic voltammetry (CV) curve of the electrode material in the range of 5~100 mV/s was tested in the potential window of 0~0.5V (vs.Hg/HgO) [Figure 1(b)], indicating that the material has good supercapacitor performance. It can be clearly observed that there is a pair of redox peaks that appear as intercalation reactions, which are caused by the 3D adsorption of electroactive substances in the solid bulk electrode. In addition, it is observed that the potential difference between the anode and cathode peaks is much higher than the theoretical value (0.058V). Therefore, two possible reactions may occur during the potential scan, as shown below:

                       (1)

                          (2)

In addition to reaction (1), the second redox reaction has also been discussed in previously reported work and is related to many structural factors (such as specific surface area). In addition, the interlayer spacing between adjacent Co/Co(OH)₂ monolayers is also likely to affect the redox reaction. The oxidation to CoOOH is generally considered to be the main reaction process. The XPS spectrum of the composite after electrochemical testing also showed a significant increase in the proportion of Co³⁺ (Figure 2). In addition, with the increase in scan rate, the redox peak shifted slightly, but the shape of the CV curve remained unchanged. This result shows that the unique soil-mountain-vegetation structure and honeycomb-shaped Co/Co(OH)₂ are conducive to rapid redox reactions and excellent supercapacitor performance.

In order to study the electrochemical capacitance characteristics of the composite electrode, the electrochemical capacitance of the composite electrode was 2~50 mA/cm² (3.0~74.6 A/g). As shown in Figure 1(c), the shape of the GCD curve reflects the characteristics of pseudocapacitance, which is consistent with the results of the CV test. In addition, there is no obvious drop, which again shows that the soil-mountain-vegetation structure has a fast I-V response capability and low intrinsic resistance, further improving the supercapacitor performance. Figure 1(d) shows the rate performance of the composite electrode. The electrode reaches a maximum specific capacitance of 642 mF/cm²/958 F/g at 2mA/cm² (3.0 Ag), which is much higher than other forms of Co(OH)₂ electrode materials, such as nanowalls (205 F/g), nanoflowers (405 F/g), nanowire arrays (422.36 F/g), staggered nanosheets (693.8 F/g) and nanocones (729 F/g), as shown in Table 1. In addition, when the current density increases from 2mA/cm² to 50 mA/cm², the capacitance retention rate is 57.6% (specific capacitance is 552 F/g at 50 mA/cm²), indicating that the composite electrode has excellent rate performance. In addition, the capacitance loss after 10,000 cycles is negligible (about 2.1% loss), reflecting its excellent cycling stability [Figure 1(e)]. Compared with other Co(OH)₂-based electrodes (Table 1), the composite material prepared by us has good cycling stability, which may be due to the porous and fluffy honeycomb structure that slows down the mechanical deformation and volume expansion of the material [Figure 1(g)]. In addition, the SEM image of the electrode after 10,000 cycles verifies that the honeycomb structure is well retained [inset of Figure 1(e)], indicating its good shape stability.

Figure 1 Electrochemical performance of cellulose-supported metal Co/Co(OH)₂ composite electrode (a) Schematic diagram of the three-electrode system in the electrochemical workstation; (b) CV at a scan rate of 5~100 mV/s Curve; (c) GCD curve at current density of 2~50 mA/cm²; (d) rate characteristics; (e) cycle stability at current density of 20 mA/cm², the inset shows the microscopic morphology of Co/Co(OH)₂ after 10,000 cycles; (f) Nyquist plot [equivalent circuit composed of solution resistance R_s, charge transfer resistance R_c and two constant phase angle elements (CPE)]; (g) schematic diagram of electrochemical energy storage mechanism of honeycomb structure

2 Co 2p core level XPS spectrum of cellulose-supported metal Co/Co(OH)₂ composites after electrochemical testing, the inset shows the corresponding fitting results

Table 1 Fiber-supported metal Comparison of mass specific capacitance and cycle stability of Co/Co(OH)₂ composite materials with other morphologies of Co/Co(OH)₂-based electrode materials

Based on the characterization results, we believe that the excellent electrochemical behavior of the cellulose-supported metal Co/Co(OH)₂ composite electrode is mainly attributed to the synergistic gain characteristics between the soil-mountain-vegetation components, that is, the cellulose paper (soil) with strong hydrophilicity and porosity can adsorb and store electrolytes, thereby promoting ion migration. ② The highly conductive mountain-shaped metal Co acts as an electron highway for charge transport and forms a perfect interface bond between the electrodes, while Co and in-situ grown Co/Co(OH)₂ (vegetation) minimize the interfacial resistance and also promote the charge transfer reaction. In particular, no semicircle is formed in the high-frequency region of the Nyquist plot of cellulose-supported metal Co/Co(OH)₂, which further proves its ultra-low charge transfer resistance [Figure 1(f)]. ③Co(OH)₂ shows a honeycomb-like and vertically oriented nanostructure, with a large specific surface area of ​​318m²/g and good hydrophilicity, which may significantly increase the contact area between the electrolyte and the electrode, shorten the ion diffusion path, increase the ion diffusion rate which is closely related to the rate performance, and further improve the supercapacitor performance [Figure 1(g)]. In addition to these advantages inspired by natural structures, according to the research of Deng et al., Co(OH)₂ and CoOOH both have the same layered structure and large interlayer spacing, which is conducive to the embedding of H⁺. In addition, from the perspective of supercapacitors, the structural similarity avoids structural changes during phase transition, which helps to improve capacitance and extend cycle life. From the perspective of batteries, H⁺ embedding/de-embedding for energy storage/release is similar to Li⁺ embedding/de-embedding of lithium batteries. Therefore, this battery-like energy storage mechanism of Co(OH)₂ also plays an important role in achieving high electrochemical performance.