Transparent flexible conductive thin films based on cellulose nanofibers by layer-by-layer assembly method and its fabricated electrochromic flexible supercapacitors

The cellulose nanofibers (CNFs) film exhibit high visible light transmittance, high mechanical strength, and excellent flexibility. Therefore, CNFs film may be an excellent substrate material for flexible transparent electronic devices. In this paper, we endeavor to prepare the CNFs-based hybrid multilayer thin film CNFs/[Cu²⁺-GO]₅/[PANI-PEDOT: PSS]₁₀ by layer-by-layer assembly method using divalent copper ions (Cu²⁺) as the crosslinking agent. Then the conductive thin film of CRGPP-10 was got by removing copper ions with solution and using reduction reaction of graphene oxide (GO) with hydroiodic acid solution for reduction of oxide graphene (RGO) from CNFs/[Cu²⁺-GO]₅/[PANI-PEDOT:PSS]₁₀ film. After that, the supercapacitor fabricated S-RGPP was fabricated using double CRGPP-10 films as electrode when H₂SO₄-PVA gel was used as the electrolyte. The crystalline characteristic and configuration of CRGPP thin films, along with the absorbency characteristics of CRGPP thin films, were analyzed. Meanwhile, the electrochemical characters of the supercapacitor S-RGPP were tested, such as cyclic vollammetry (CV) curves, galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). The results indicated that the method of the flexible electrochromic film with layer-by-layer assembly method is practicable. The conducting film CRGPP-10 was uniform with controlled transparency. The supercapacitor S-RGPP exhibits an excellent areal capacitance of 8.15 mF/cm² at a current density of 0.0043 mA/cm². Owing to the addition of RGO reagent, the cycle stability of supercapacitor S-PGPP improved, which is fabricated by CPRPP-10 electrodes. The device exhibited good electrochemical performance, such as the characters of the low resistance, with the capacitive behavior of the electrical double-layer capacitor (EDL) capacitor and pseudo capacitor at the same time. Meanwhile, it showed so better flexible that there was no difference between normal and bent state. The device shows some electrochromism. This study provides a novel method using CNFs as substrate to prepare hybrid electrodes for future flexible supercapacitors.