Double-Porous Polyaniline-Based Cryogels with Carbon Nanofibers for Supercapacitors

Abstract

Polyaniline-poly(N-vinylpyrrolidone) cryogels/aerogels with carbon nanofibers (PANI−CNF−PVP) were prepared by oxidative cryopolymerization of aniline in water:isopropanol medium in the presence of various amounts of dispersed carbon nanofibers (CNF; 0.05−2.5 mg mL −1 ). Scanning electron microscopy showed that all prepared PANI−CNF−PVP and PANI−PVP aerogels have double-porous morphology, which is represented by macropores (5−35 μm), constituting the main threedimensional network of the materials, and smaller pores (≈200 nm − 3 μm) in the macropore walls. The incorporation of CNF into PANI−CNF−PVP aerogels was visualized by scanning and transmission electron microscopy and additionally supported by X-ray photoelectron spectroscopy. Surface area of the aerogels was found to be ≈13−25 m 2 g −1 . Based on full decomposition temperatures determined by TGA, PANI−CNF−PVP aerogels had better thermal stability (792 °C; 2.5 mg mL −1 of CNF) compared to PANI−PVP (750 °C). Starting from the lowest used CNF content (0.05 mg mL −1 ), PANI−CNF−PVP aerogels demonstrated significantly higher gravimetric capacitance (≈3−6 times), compared to PANI−PVP aerogel, reaching 201 F g −1 (1 A g −1 ; 1 mg mL −1 of CNF), in the three-electrode setup. These data were supported by electrochemical impedance spectroscopy, showing lower charge transfer resistance for PANI−CNF−PVP aerogels, which decreased with an increasing CNF fraction. CNF-containing aerogels also showed enhanced cycling stability. A two-electrode symmetrical supercapacitor was assembled using PANI−CNF−PVP aerogel (1 mg mL −1 CNF) as the active electrode material. The device reached a gravimetric capacitance of 213 F g −1 (0.5 A g −1 ) with energy and power densities of 30 Wh kg −1 and 1000 W kg −1 , respectively, and showed 95% cycling stability after 1000 cycles. The performance of this supercapacitor is comparable or often exceeds that of previously reported electrode materials, based on conducting polymers and carbon derivatives. Therefore, the prepared PANI−CNF−PVP aerogels are the promising materials for energy storage.