The crystallographic structures of the prepared materials were estimated by X-ray diffraction measurement. XRD pattern of the product is presented in Figure 1. The product shows diffraction peaks at 12.8˚, 19.3˚, 34.4˚, etc. corresponding to typical lattice planes of Zn₄CO₃(OH)₆·H₂O (JCPDS: 011-0287). The diffraction peaks of these planes are especially strong while other peaks have relatively weak intensities, indicating the growth of the crystal along a definite direction. It can be seen from Figure 1 that Zn₄CO₃(OH)₆·H₂O has been formed even after 1 h hydrothermal process at 120˚C. With the extension of hydrothermal reaction time, the peak strength of characteristic diffraction peaks of products slightly increases, indicating that the material can be synthesized easily with low energy consumption. And more importantly, the crystal structure and composition of products are stable which is critical to cycle stability.

The FTIR spectrum of the product is given in Figure 2, confirming the presence of CO 3 2 − as well as water molecules intercalated in the interlayer. The absorption band around 3452 cm⁻¹ corresponds to the O-H stretching vibration of water molecules in the interlayer, H-bonded hydroxyl group and possible Metal-OH stretching vibration. The band at 1350 - 1550 cm⁻¹ assigned to O-C-O

groups asymmetric vibration of single-base carbonate group ligand. Other absorption bands at 400 - 700 cm⁻¹ are due to attributes to metal-oxygen (M-O) stretching and bending modes, including zinc-O-O, etc. All observations above are in good agreement with the previous reports [8] [9].

Hydrothermal technique over chemical routes is beneficial to control the morphology of products by properly choosing the temperature, time of reaction, solvent used for reaction, with or without the use of any major structure-directing agents and templates. The morphology of the prepared HZC growing on the surface of Ni foam is examined via field-emission scanning electron microscopy (FESEM). Figure 3(a) and Figure 3(b) show the morphological features of the HZC prepared in the present work at different magnifications. As presented in Figure 3(a), all of the HZC particles display nanosheets that are relatively loosely, irregularly grown on the substrate surface of foam Ni. The morphology of product particles is nearly hexagonal nanosheet with about 1 - 2 μm of diameter and 50 - 100 nm thickness. The interconnected nanosheets form a mesoporous film on foam Ni surface.

The electrochemical richness of HZC composite electrode has been extensively studied using cyclic voltammetry, galvanostatic charge-discharge studies and electrochemical impedance techniques. The CV traces (Figure 4) of HZC shows a pair of redox peaks in the range of 0 - 0.6 V (vs Hg/HgO) of HZC composite electrode in three electrode system, which indicates that HZC composite electrode exhibits typical energy storage mode of Faraday pseudocapacitance [3]. It is generally believed that the pseudocapacitance energy storage mechanism of Zn(II)-based oxide or hydroxide electrode in alkaline electrolyte solution is derived from the redox reaction of Zn(II), which is completed with the assistance of ion insertion and removal in electrolyte solution. Theoretical analysis shows that E^θ(Zn²⁺/Zn) = −0.762 V, so it can be inferred that E^θ(Zn(OH)₂/Zn) = −1.240 V. In the experiment, the redox peak of was about 0.45 - 0.5 V (vs Hg/HgO). Based on the literature, it was preliminarily speculated that the redox process of the active component of HZC electrode under the study conditions was between Zn (II) and Zn (III).

Galvanostatic charge-discharge curves (Figure 5) of HZC were recorded at different current densities in the potential range of 0 - 0.6 V. From the constant-current charge and discharge curves, it can be seen that the curves are nearly symmetrical, with obvious charging and discharging platforms around 0.45 V. The charging curves with relatively slow voltage change at the range of 0.45 - 0.6 V and the discharging curves at the range of 0.3 - 0.45 V both reflect the typical pseudocapacitive properties of HZC composite electrode, and correspond to the redox peaks in the CV curve of the electrode in Figure 4.

The specific capacitance of HZC composite electrode can be estimated from the discharge current using the following equation [3]:

C = I × t / ( m × Δ V ) (1)

where, C (F∙g⁻¹) is the specific capacitance of the electrode. I (A) is charge and

discharge current. T (s) is discharge time and ΔV (V) for charging and discharging voltage window, m (g) as electroactive material quality. The estimated specific capacitance of HZC composite electrode is 1329.2 F∙g⁻¹ at a current density of 1 A∙g⁻¹ and 882.8 F∙g⁻¹ at 10 A∙g⁻¹, respectively. HZC in situ grown on foam Ni exhibits 40 fold enhancement in the capacity to store charges, when compared to HZC powders coated on foam Ni under the same test condition (32.7 F∙g⁻¹ at 1 A∙g⁻¹). By comparing the research progress and electrochemical performance results of Zn-based electrode materials in recent years, it can be seen that a one-step hydrothermal process using inorganic precursors in this experiment is facile, and the synthesized HZC electrode materials show superior electrochemical activities [10] [11].

More advantageous crystal structure and the nanosheet morphology of HZC on foam Ni are the possible reasons for the excellent charge storage characteristics. Thin nanosheet arrays of uniform lamellar provide both large active surface area and good electrical conduction for fast redox kinetics. The layed structure of HZC can increase the rate of active centers participate in electrochemical reaction. This result shows that the ordered electrode material has a significant influence on the electrochemical performance of the HZC electrode. It is calculated that the energy density can reach 38.9 Wh∙kg⁻¹ when the power density is 2000 W.