The electrochemical performance of FeOCICDCA as a supercapacitor anode was evaluated in a three-electrode test system [Figure 1(a)], including cyclic voltammetry (CV) and constant current charge-discharge (GCD) curves, cycle stability and electrochemical impedance spectroscopy (EIS), and compared with CDCA and FeOCI electrodes. Figure 1(c) compares the CV curves of CDCA, FeOCl and FeOCI/CDCA electrodes at a scan rate of 5mV/s. The effect of nickel foam on capacitance is negligible (Figure 2). The closed area in the CV curve corresponds to the charge storage capacity. FeOCI with pseudocapacitive characteristics has a much larger CV closed area than CDCA with electric double layer capacitor (EDLC) characteristics, and the combination of the two materials leads to a larger area of the CV curve closed area of the FeOCI/CDCA composite material, reflecting its stronger charge storage capacity. The CV curves of FeOCI/CDCA at different scan rates are shown in Figure 1(d), where a pair of redox peaks are identified, representing the redox intercalation reaction of the 3D adsorption of active substances in the electrode material. In addition, the XPS spectrum of the FeOCI/CDCA electrode after electrochemical testing shows that Fe(III) is transformed into Fe(II) (Figure 3). Therefore, the possible charge storage process of FeOCl in Na₂SO₄ electrolyte is described as:

                                         (1)

Figure 1 Kinetic analysis of electrochemical behavior of FeOCI/CDCA electrode

(a) Schematic diagram of electrochemical test equipment using three-electrode system and (b) electrochemical reaction mechanism; (c) CV curve at a scan rate of 5mV/s; (d) CV curve of FeOCI/CDCA at a scan rate of 5~100 mV/s; (e) According to the voltammogram in Figure (d), the b value is determined using the relationship between peak current and scan rate; (f) The relationship between i(V)/v^1/2 and v^1/2 is obtained using the anodic current at a potential of -0.15V. The inset shows the contribution of surface capacitance and insertion capacitance at different scan rates

Figure 2 CV curves of CDCA and bubble nickel at a scan rate of 5mV/s

Figure 3 XPS spectra of Fe 2p in FeOCI/CDCA before and after electrochemical testing

To further verify the proposed Na⁺ intercalation mechanism, we also measured the CV curves of FeOCI/CDCA supercapacitor anode in 1 mol/LMgSO₄ electrolyte. Because the diameter of Mg²⁺ hydrated ions (0.856 nm) is larger than the interlayer distance of FeOCl (0.783 nm), the capacitance measured in MgSO₄ electrolyte is significantly reduced, which will help to prove its intercalation mechanism. As expected, the area contained in the CV curve measured in MgSO₄ solution is much smaller than that contained in the CV curve measured in Na₂SO₄ solution [Figure 4(a)]. In addition, we also tested the GCD curve of FeOCI/CDCA in MgSO₄ solution at a current density of 5mA/cm². The corresponding capacitance value was 759 mA/cm² [Figure 4(b)], which is much lower than the capacitance value measured in Na₂SO₄ solution (1552 mA/cm²). These experimental results further prove its intercalation mechanism and the reduction of intercalation reaction sites in MgSO₄ solution.

Figure 4 Comparison of (a) CV and (b) GCD of FeOCI/CDCA electrode tested in MgSO₄ and Na2S04 electrolytes

The charge storage kinetics of electrode materials for Na⁺ are evaluated. The peak current behavior is analyzed by assuming that the current (i) conforms to the power law relationship of the scan rate (v):

                                                                                       (2)

Where a and b are adjustable parameters. The value of 6 can be determined by the slope of the log(v)-log(i) graph. When the b value is 0.5, it means that the charge storage is completely controlled by the diffusion process; when the value is 1.0, it indicates that it is a surface-controlled capacitive process. Figure 1(e) shows the log(v)-log(i) plots of CDCA, FeOCl, and FeOCI/CDCA supercapacitor anode. It is calculated that for the anode current, the b value of CDCA is 0.935, which indicates that most of the current is capacitive at the peak potential. The results show that capacitive behavior dominates the electrochemical reaction of CDCA with fast kinetics. In contrast, the b value of FeOCl is 0.579 (close to 0.5), indicating that the kinetic behavior of FeOCI is mainly controlled by the diffusion process. In addition, the b value of the FeOCI/CDCA composite electrode is 0.622, which proves the positive contribution of CDCA to the charge storage kinetics of FeOCI/CDCA. CDCA has a charge storage kinetics dominated by capacitive behavior, while the kinetic behavior of FeOCl is mainly controlled by the diffusion process.

To study the capacitive contribution, we calculated the surface capacitance and intercalation capacitance of FeOCI/CDCA at different scan rates. The current response i at a fixed potential V represents the sum of the two contributions of surface and intercalation capacitance, as described below.

                                                                         (3)

                                                                   (4)

where v represents the scan rate, and k₁v and k₂v^1/2 represent the current contributions of the surface capacitance (surface pseudocapacitance and double layer) and the intercalation process (intercalation pseudocapacitance), respectively. By determining k₁ and k₂, we can distinguish the proportion of the current generated by the surface capacitance and the intercalation process at a specific potential. At a scan rate of 5mV/s [Figure 1(f)], there is about 9% of the total current is capacitive. Similarly, the contribution ratio between the two different processes at other scan rates was calculated. The results show that the capacitance contribution gradually increases with the increase of the scan rate. The reason may be that higher scan rates will limit the ion diffusion process, thereby preventing the ions from being embedded in the stacked layered structure. Therefore, at high scan rates, the contribution of the electrode surface capacitance becomes more obvious. In addition, at scan rates of 5~100mV/s, the proportion of intercalation capacitance is higher (68%~91%), indicating that the kinetic behavior of the FeOCI/CDCA supercapacitor anode is mainly controlled by the diffusion process. The results are consistent with the b-value analysis. In order to further study the ion migration process, the apparent diffusion coefficients of FeOCl and FeOCI/CDCA electrodes were calculated according to the Randles-Sevick equation:

               (5)

Wherein, I_p is the peak current, n is the number of electrons transferred per molecule during the electrochemical reaction, A is the effective reaction area of the electrode, C is the concentration of sodium ions in the electrolyte, D is the ion diffusion coefficient (including the diffusion of sodium ions and hydroxide ions), and v is the scan rate. From Figure 2, it can be calculated that the ion diffusion coefficient of FeOCI/CDCA is 8.21×10^-7cm²/s, which is almost 11.9 times that of FeOCl (6.92×10^-8cm²/s). These results show that FeOCI/CDCA exhibits a faster ion diffusion rate, thereby improving the charge storage capacity.

Figure 5 Surface diffusion coefficients of FeOCI/CDCA and FeOCl electrodes calculated by the Randes-Sevick equation

GCD is a reliable means to determine the capacitance of supercapacitors. As shown in Figure 6(a), the nonlinear shape reflects the pseudocapacitive characteristics of FeOCI/CDCA, which is consistent with the results of CV analysis. The good symmetry between the charge and discharge curves indicates good reversibility and Coulombic efficiency (i.e., the ratio of charge and discharge capacitance). According to the GCD curve, FeOCI/CDCA can obtain the highest area specific capacitance of about 1618mF/cm² (647 F/g) at a discharge current of 2mA/cm² (0.8A/g) [Figure 6(b)], which is 1.3 times and 14.7 times that of FeOCI and CDCA electrodes (about 696 mF/cm² and 103 mF/cm²), respectively. The GCD curves of CDCA and FeOCl electrodes at different current densities are shown in Figure 7. When the current density increases significantly from 2mAcm² to 50mA/cm², FeOCl/CDCA still shows a high capacitance retention rate of 54% (875 mF/cm²/350F/g), indicating that it has excellent signal performance. The capacitance value of FeOCI/CDCA electrode is better than some reported supercapacitor anode materials, such as y-Fe₂O₃/graphene (76 F/g), mesoporous MoO₂ (146 F/g), polypyrrole@V₂O₅ (308 F/g), Ti-Fe₂O₃@PEDOT (311.6 F/g), highly functionalized activated carbon (525 F/g) and Fe nanosheet/graphene (717 F/g), as shown in Table 1.

Figure 6 Electrochemical performance of FeOCI/CDCA electrode

(a) GCD curve of FeOCI/CDCA at current density of 2~50 m/cm²: (b) Rate performance: (c) FeOCI/CDCA at Cyclic stability and Coulombic efficiency at 50 mA/cm², the inset shows the SEM image after 10,000 cycles; (d) Nyquist plot, the inset shows the magnified image of the high-frequency region and the equivalent circuit diagram used for data fitting; (e) Schematic diagram of the functions of CDCA and FeOCI components in electrochemical reactions

Figure 7 GCD curves of (a) CDCA and (b) FeOCl electrodes at current densities of 2~50 mA/cm²

Table 1 Comparison of physical and electrochemical properties of various aqueous electrolyte anode materials reported in the literature in recent years

In addition, it is known that the crystal structure of FeOCl is similar to that of FeOOH. By comparing their capacitances, the area specific capacitance of FeOCI/CDCA electrode (1.62 F/cm², 0.8 A/g) is comparable to or even higher than that of FeOOH-based electrodes, such as FeOOH nanoparticles (1.71 F/cm², 1 A/g), nanopillared β-FeOOH (1.16 F/cm², 0.5 A/g), and FeOOH nanorods/carbon nanotube networks (0.79 F/cm², 0.5 A/g). Due to the high mass loading (2.5 mg/cm²), the corresponding mass specific capacitance of FeOCI/CDCA is 647 F/g, which is lower than that of FeOOH nanoparticles (1066 F/g, 1.6 mg/cm²), but higher than that of nanopillared β-FeOOH (116 F/g, 10 mg/cm²) and FeOOH nanorods/carbon nanotube networks (396 F/g, 2 mg/cm²). In addition, it is known that FeOOH has poor thermal stability. When the isostructural y-FeOOH is treated with an organic base at 120-140°C, it will be quantitatively converted to y-Fe₂O₃. In addition, according to the research results of Tang et al., a-FeOOH will produce a-Fe₂O₃ nanorods after calcination at 250°C for 2h. In contrast, FeOCl has better thermal stability because FeOCI is obtained by calcining a mixture of a-Fe₂O₃ and FeCl₃ at 380°C. Therefore, compared with FeOOH, FeOCI has unique advantages when acting as a supercapacitor anode.

In addition to the above properties, FeOCI/CDCA also exhibits excellent cycle stability. After 10,000 charge and discharge cycles [Figure 6(c)], the capacitance loss does not exceed 10%, which is lower than FeOCl (loss 25%) (Figure 8). The excellent cycling performance of FeOCI/CDCA is mainly attributed to the contribution of CDCA to the stability of FeOCI, which may include the following two points: ① 3D interconnected CDCA can act as a buffer layer to relieve the strain of FeOCI during ion intercalation and deintercalation; ② The good mixing between CDCA and FeOCI can effectively prevent the aggregation of FeOCl. As shown in the inset of Figure 6(c), the SEM image of FeOCI/CDCA after 10,000 cycles shows that its layered structure is well preserved with good shape stability and dispersibility. Similar results can be found for other composite materials composed of carbon materials and metal oxides/hydroxides. In addition, during the cycling process, the Coulombic efficiency can be stabilized between 96% and 100%.

Figure 8 Cyclic stability and Coulombic efficiency of FeOCl electrode at 50 mA/cm² current density

EIS was measured to quantify the resistance of the electrode/electrolyte interface and to analyze the reasons for the performance differences among CDCA, FeOCl, and FeOCI/CDCA [Figure 6(d)]. The equivalent circuit diagram used to fit the EIS data is shown in Figure 6(d). By comparing the Nyquist plots, we can see that combining FeOCI with CDCA can reduce the diameter of the semicircle (representing the charge transfer resistance R_c at the electrode material/electrolyte interface) from 1Ω (FeOCI) to 0.4Ω (FeOCI/CDCA) because the highly conductive CDCA builds a dense conductive network and provides abundant mobile charge carriers and conductive channels. This result is consistent with the FeOCI conductivity measurement results (the conductivity increases significantly from <10⁵S/cm of FeOCl to 0.26 S/cm of FeOCI/CDCA). In addition, the projection length of the 45° Warburg region on the real axis (Z´) reflects the diffusion process of ions. Obviously, the projection length of FeOCI/CDCA is shorter than that of CDCA and FeOCl, indicating that the ion diffusion path is shorter. Therefore, the EIS results show that FeOCI/CDCA has better ion/electron transport ability. In the low-frequency region, compared with FeOCI, FeOCI/CDCA presents a line that is more vertical along the imaginary axis (Z”), indicating a more ideal capacitance characteristic because it introduces a non-Faraday charge storage process, which also indicates that FeOCI/CDCA has stronger capacitance performance as a supercapacitor anode material.

Based on the above analysis, the superior electrochemical performance of FeOCI/CDCA may be attributed to the synergistic gain effect of CDCA and FeOCl, which can be summarized as follows: 1. The large specific surface area of FeOCl composed of stacked microsheets can provide sufficient space and expose more active sites to promote electrochemical reactions; 2. The theoretical capacitance value of FeOCI can reach about 899 F/g, and the FeOCl stacked structure supported by CDCA has excellent mechanical stability, which helps to prolong its service life; 3. The porous CDCA with a large specific surface area can act as a reservoir for electrolytes, shortening the ion diffusion path between ions and layered FeOCl [Figure 6(e)]; 4. Highly conductive CDCA It can provide abundant mobile charge carriers and conductive pathways during the reaction process, thereby enhancing its performance as a supercapacitor anode.