We evaluate the potential of FeOCI/CDCA in practical applications by assembling an ASC device. High-performance supercapacitor Figure 1(a) depicts the schematic diagram of the structure of the ASC device assembled from FeOCI/CDCA anode and nickel foam-loaded MnO₂ cathode. Among them, the MnO₂ cathode shows an area specific capacitance of 318mF/cm² (about 127 F/g) at 2mA/cm² and good rate capability, and its detailed electrochemical performance is shown in Figure 2. Because the stable potential window range of FeOCICDCA is -1~0V (v.Ag/AgCI), and the stable potential window range of MnO₂ electrode is 0~0.8V (V.Ag/AgCI), the ASC can theoretically be applied as a device with a working voltage of 1.8V in 1mol/LNa₂SO₄ electrolyte. As shown in Figure 1(b), the stable electrochemical voltage of ASC can be extended from 0.8V to 18V. In addition, the irregular curves obtained by CV tests at different scan rates in the 18V potential window [Figure 1(c)] indicate that its capacitance mainly comes from redox reactions.

Figure 1 Electrochemical performance of MnO₂//FeOCI/CDCA ASC

(a) Schematic diagram of the structure of the ASC device assembled in a two-electrode system; (b) CV curves in different potential windows at a scan rate of 100 mV/s: (c) CV curves at scan rates of 5~100mV/s; (d) GCD curves at current densities of 2~50mA/cm²; (e) Rate thermal properties; (f) Ragone plot (compared with the energy/power density data of ASC devices in some literature); (g) Cyclic qualitative and Coulombic efficiency at a current density of 50mA/cm², the inset shows the Nyquist plot before and after the cycle

Figure 2 Electrochemical performance of MnO₂ electrode

(a) CV curves at scan rates of 5~100mV/s, the inset shows the enlarged image of the CV curve at 5mV/s; (b) GCD curves at a bubble density of 2~50mAcm² GCD curve; (c) rate performance; (d) Nyquisi plot, the inset shows the magnified view in the high frequency region

The GCD curves of ASC at different current densities were tested. As shown in Figure 1(d), at an operating voltage of 1.8V, its charge and discharge curves maintained good symmetry, proving its excellent reversibility and good Coulombic efficiency. In addition, ASC can provide area specific capacitances of 641mF/cm² and 372mF/cm² (128F/g and 74F/g) at current densities of 2mAcm² and 50mAcm², respectively, indicating that about 58% of the capacitance is still retained at high current density [Figure 1(e)], reflecting its excellent rate performance. According to the Ragone plot [Figure 1(f)], the ASC shows a maximum areal energy density of 289 μWh/cm² at a power density of 1.8 mW/cm², and a surface energy density of 168 μWh/cm² at the maximum power density (45 mW/cm²). This result is better than many recently reported ASCs with only pseudocapacitive energy storage mechanism [Figure 1(f) and Table 1]. In addition, after 10,000 cycles, the ASC specific capacitance retention rate is 88.7% [Figure 1(g)], reflecting its good cycling stability, further supporting its potential as a high-performance supercapacitor. In addition, according to previous studies, the possible reasons for the decrease in capacitance with the number of cycles are mainly the following three points: ① During the Faradaic reaction, a small amount of structural changes/disintegration occurs around the active sites due to the repeated insertion and consumption of ions in the electrode material; ② The dissolution/separation of active materials (such as the dissolution of a small part of iron) leads to a reduction in electrochemical active sites; ③ The consumption of electrolytes caused by very few irreversible side reactions and interfacial reactions. In addition, the device has a high Coulombic efficiency (>97%) during the cycle process, which is very consistent with the speculation of GCD analysis. The inset of Figure 1 (g) shows the Nyquist plot of ASC before and after the cycle test. Obviously, the solution resistance (R_s) does not change much before and after the test (about 3Ω), and the corresponding charge transfer resistance R_c is 1.0Ω and 2.4Ω, respectively. The slightly increased charge transfer resistance after the cycle test may be related to the destruction and dissolution of part of the electroactive material that has undergone a large number of violent redox reactions. This slight increase further indicates that ASC has good cycle stability. In addition, lower R_c and R_s values are expected to enhance its power density. These excellent electrochemical properties of MnO2//FeOCI/CDCA ASC indicate that FeOCI/CDCA is a cost-effective anode material suitable for high-performance supercapacitor.

Table 1 Comparison of area specific capacitance area energy density and cycling stability of MnO₂//FeOCI/CDCA ASC and recently reported ASC with Faraday energy storage mechanism

This chapter demonstrates that CDCA-supported self-stacked layered FeOCI is suitable as a new type of high-performance supercapacitor anode, which provides an ultra-high area specific capacitance of 1618mF/cm² (647 F/g) at 2mA/cm² and a capacitance loss of no more than 10% (about 9% after 10,000 cycles). The layered structure of FeOCl has excellent mechanical stability, which is due to the synergistic effect of CDCA, providing abundant channels and active sites for the migration and intercalation reaction of Na⁺. These structural advantages play a key role in obtaining high electrochemical activity. In addition, compared with the reported ASCs with pseudocapacitive energy storage mechanism, our assembled MnO₂//FeOCI/CDCA ASCs exhibit highly competitive energy/power density (energy density of 289μW/hcm² at power density of 1.8mW/cm²) and cycling stability. More importantly, the FeOCI/CDCA anode with excellent electrochemical performance is not only low-cost and easy to synthesize, but also suitable for assembling various high-performance energy storage devices (such as Li/Na ion batteries) not limited to high-performance supercapacitor.