Flexible solid-state energy storage devices play an important role in the development of stretchable, wearable and portable electronic devices. As a typical flexible solid-state supercapacitor, liquid electrolytes usually require a strong package to prevent liquid leakage, so they are usually not as safe as solid electrolytes. In this chapter, a flexible solid-state supercapacitor symmetric supercapacitor (SSC) device based on RPC-2.5 (labeled as RPC-SSC) was fabricated by assembling two RPC-2.5 electrodes and using H₃PO₄/PVA gel as the electrolyte. The RPC-SSC device has excellent flexibility, and CV tests confirm that bending stress has little effect on the capacitance performance [Figure 1(a)]. In addition, the CV curves of RPC-SSC maintain a quasi-rectangular shape at different scan rates from 5 to 100 mV/s (Figure 2). The GCD curve of RPC-SSC shown in Figure 1(b) is symmetrical, and its discharge/charge voltage is approximately linear with time. These characteristics indicate that the device has ideal capacitive performance. In addition, the RPCSSC exhibits the highest surface capacitance of 0.51 F/cm at a discharge current of 0.1 mA/cm: (Fig. 1(b) inset 1), and the corresponding volumetric capacitance is calculated to be about 8.5 F/cm’ (the thickness of the device is as low as 600 mm), which is higher than the data of many other flexible solid-state supercapacitor (Table 1).

Figure 1 (a) CV curves of RPC-SSC at a scan rate of 5 mV/s in normal and bent states; (b) GCD curves of RPC-SSC at different currents of 0.1 mA/cm², 0.2 mA/cm², 0.5 mA/cm², 1.0 mA/cm² and 2.0 mA/cm², with the corresponding surface capacitance shown in the inset; (c) Comparison of the power density/energy disk density of RPC-SSC with the data of recently reported quasi/all-solid-state supercapacitor devices (Rogono plot)

Figure 2 CV curves of RPC-SSC at different scan rates

Table 1 Comparison of area specific capacitance and volume specific capacitance of RPC-SSC with other recently reported solid-state supercapacitors

The volume energy density (E) of RPC-SSC is calculated using the following formula:

                                                       (1)

Where E, C and ∆V are volume energy density, volume specific capacitance and voltage window, respectively.

The power density (P) of RPC-SSC can be calculated based on the ratio of energy density to discharge time:

                                                                  （2）

Where E, t and P are volume energy density, discharge time and volume power density, respectively.

The energy density of this RPC-SSC is compared with the data of other recently reported quasi/all-solid-state supercapacitor devices, as shown in Figure 6-11(c). The calculated maximum volume energy density of RPC-SSC is 1.18 mWh/cm³, which is higher than other quasi/all-solid-state supercapacitor devices, such as carbon nanotube-based SSC (0.008 mWh/cm³, H₂SO₄PVA), single-walled carbon nanotube-based SSC (0.01 mWh/cm³, H₃PO₄/PVA), TiN-based SSC (0.05 mWh/cm³, KOH/PVA), carbon/MnO₂ linear supercapacitor (0.22 mWh/cm³), TiO₂@MnO₂//TiO₂@C-based asymmetric supercapacitor (ASC0.30 mWh/cm³, LiCI/PVA), rGO-GO-rG0 supercapacitor (0.43 mWh/cm³), VO_x//VN-based ASC (0.61 mWh/cm³, LiCI/PVA) and PPy paper-based SSC (1.0 mWh/cm³, H₃PO₄/PVA). The excellent energy density indicates that RPC-2.5 is expected to be a potential candidate material for manufacturing high-performance flexible solid-state supercapacitor devices.

Conclusion

This chapter demonstrates an easy-to-operate and scalable method to fabricate an RPC composite paper electrode. By optimizing the preparation parameters, the cellulose fiber framework-supported RGO/PPy interpenetrating structure (RPC-2.5) obtains an extremely low layer resistance of only 1.7Ω/sg. This structure gives RPC-2.5 a high surface capacitance of 1.20F/cm² at a discharge current of 2mA/cm², good cycling stability (capacitance retention rate of 89.5% after 5000 cycles), excellent conductivity stability, and good mechanical flexibility. In addition, the all-solid-state flexible SSC device fabricated using RPC-2.5 and H₃PO₄/PVA gel electrolyte has a high surface capacitance of 0.51F/cm² at 0.1mA/cm², and a volumetric energy density of up to 1.18 mWh/cm³. These results indicate that flexible, highly conductive, and self-supporting RPC-2.5 can be used as a candidate electrode material for the manufacture of a variety of high-performance flexible energy storage devices.