Cellulose Nanofibers/Reduced Graphene Oxide/Polypyrrole Aerogel Electrodes for High-Capacitance Flexible All-Solid-State Supercapacitors

Abstract

Flexible supercapacitors with considerable energy storage performance from green/sustainable materials have attracted significant attention in many fields, such as portable and wearable electronics. In this work, flexible cellulose nanofibers/reduced graphene oxide/polypyrrole (CNFs/rGO/PPy) aerogel electrodes with well-defined three-dimensional porous structures are prepared using citric acid-Fe3+ (CA-Fe3+) complexes as oxidant precursors to command the deposition of PPy. The in situ gradual release of Fe3+ leads to the formation of thin and uniform polypyrrole in the composites. A flexible all-solid-state supercapacitor is then prepared by the CNFs/rGO/PPy aerogel film electrode and poly­(vinyl alcohol) (PVA)/H2SO4 gel electrolyte and separator. Due to the porous structure, high electrical conductivity, and remarkable wettability of the electrodes, the assembled supercapacitors show excellent electrochemical properties with maximum areal capacitance of 720 mF cm–2 (405 F g–1 for single electrode) at 0.25 mA cm–2 and good cycle stability (95% retention after 2000 cycles). The device with maximum energy density of 60.4 μW h cm–2 also exhibits nearly constant capacitance under different bending conditions, suggesting their great potential for applications in flexible electronics.

Wood‐Derived Lightweight and Elastic Carbon Aerogel for Pressure Sensing and Energy Storage

“…The corresponding gravimetric and areal capacitances are 62.4 F g −1 at 0.5 A g −1 and 231 mF cm −2 at 0.5 mA cm −2 (based on the whole mass of the two carbon electrodes), respectively (Figure 6e), superior to those of many other carbon‐based all‐solid‐state SSC reported in literature (Table S6, Supporting Information) 56–58,61,63,65–67. The rate capacity of the assembled SSC (58.5%, from 0.5 to 4.0 A g −1 ) is also comparable to those of many other carbon‐derived SSC 57,67–71. More impressively, the SSC reveals a high energy density of 8.6 Wh kg −1 at a power density of 250 W kg −1 , and maintains 5.0 Wh kg −1 at a power density of 2000 W kg −1 (Figure 6f), which are not only better than those of many other carbon‐derived SSC,56–58,61,63 but also comparable to those of some conducting polymer or metal oxides‐based supercapacitors66,67 (Table S6, Supporting Information).…”

Core–Shell Structured Cellulose Nanofibers/Graphene@Polypyrrole Microfibers for All‐Solid‐State Wearable Supercapacitors with Enhanced Electrochemical Performance

“…[ 101,102 ] The XRD patterns of CNFs/RGO‐1.62 and CNFs/RGO@PPy‐2 microfibers are similar to that of cellulose I crystalline structure, only the peaks become relatively weak and broad which demonstrates that CNFs are well preserved in the composite microfibers. [ 103 ] According to the Prague formula (), the inter layer spacing of GO, RGO, CNFs/RGO‐1.62 and CNFs/RGO@PPy‐2 are 8.48, 3.74, 3.93, and 3.93 Å, respectively. The results demonstrate that: i) the layered GO microfibers preserved very ordered lamellar structures from the oriented GO liquid crystals; ii) RGO microfibers have compact re‐packing of nanosheets and smaller layer spacing compared with GO after the removing of OCGs; iii) CNFs can act as “spacers” to effectively reduce the π–π stacking of RGO nanosheets; iv) the interlayer spacing of CNFs/RGO‐1.62 microfibers has little change after PPy interfacial polymerization.…”

“…[ 106 ] The presence of ‐N + confirms that PPy has been successfully doped and in an oxidized state. [ 103 ] The –N + polaron is crucial for PPy to exhibit conductivity which can be greatly facilitated by the direct electrostatic interaction between the positive charges and electrons. [ 76 ] Similarly, N 1s XPS spectrum of CNFs/RGO@PPy‐2 also shows that three component peaks originating from –N , N–H, and –N + ; while the BEs shift to a higher region at 400.4, 401.96, and 403.2 eV, respectively.…”

“…[ 122 ] The slight distorted rectangular‐like shapes are ascribed to the redox reactions on the interface of electrode/electrolyte, which indicates a typical characteristic of pseudo capacitance. [ 103 ] Furthermore, the CV curves exhibit near mirror‐image symmetry with rapid current responses, demonstrating the desirable fast charge‐discharge property and ideal capacitive behaviors for power devices. [ 123–125 ] The GCD curves of CNFs/RGO@PPy‐2 FSCs in Figure 5b resemble a symmetric triangle without any apparent voltage drop in the potential window of 0–0.8 V at different current densities from 0.1 to 1.0 mA cm −2 , demonstrating good charge transmission across the microfibers and relatively high coulombic efficiency.…”

“…The simulated equivalent circuit is shown in inset (Figure 5d), where R S is the intrinsic ohmic resistance related to the ionic resistance of electrolyte and the intrinsic resistance of electrode materials [ 130 ] ; R CT is interfacial charge transfer resistors in the surface of electrolyte/electrode, [ 22 ] Warburg impedance Z W reflects ion diffusion/transport at the electrode/electrolyte interface and constant phase element CPE is associated with the double‐layer capacitance considering the actual morphology of microfibers. [ 103 ] RGO, CNFs/RGO‐1.62 and CNFs/RGO@PPy FSCs have relatively close R S of 0.065, 0.069, and 0.141 kΩ on the real axis at high frequency, respectively. It is indicated that no significant increase of R S occurs after the addition of poorly conductive CNF and PPy into electrode materials.…”