Polyaniline Nanorods Grown on Hollow Carbon Fibers as High‐Performance Supercapacitor Electrodes

Abstract

Hollow carbon fibers (aCFs) with a high specific surface area are applied as conductive scaffolds for polyaniline (PANi) growth. aCFs oxidized by HNO3 treatment have more surface negative charges, which interact with protonated aniline molecules in acidic media. The subsequent chemical oxidative polymerization assisted by (NH4)2S2O8 yields aCF–PANi composites with PANi nanorod arrays strongly coupled on both the external and internal surfaces of the aCFs. Serving as binder‐free supercapacitor electrode, the aCF–PANi composite with 31 wt % PANi exhibits a specific capacitance of 364 F g−1, and retains 70 % of this value at 20 A g−1. More interestingly, it exhibits excellent cycling stability with 92.5 % capacitance retention, and outstanding reversibility with 98.6 % coulombic efficiency, even after 5000 continuous cycles; this performance is far superior to that of both aCF and PANi electrodes. The strongly coupled PANi arrays on the aCF surface greatly reduce the interface resistance, thus facilitating both ion and electron transport for enhanced capacitive performance.

Promoting Oriented Growth of a Polyaniline Array on Carbon Cloth through in Situ Chemical Polymerization under a High Voltage Electric Field for a Flexible Supercapacitor with High Areal Capacity and Stability

“…Moreover, the long tails are all approximately vertical to the real axis in the low frequency region except for the electrode prepared at the aniline concentration of 1.5 M. Generally, the slope line appearing in the low frequency region is related to the electrolyte diffusion/transport, and the larger slope means a faster redox reaction, closer to a pure capacitive behavior. ,,, Generally, the slope in the low frequency represents electrolyte diffusion/transport, and the larger slope means a faster redox reaction, closer to a pure capacitive behavior. ,,, The equivalent series resistances (ESRs) obtained by the intercept between the x axis and the Nyquist plot are 1.37, 1.42, 1.57, 1.52, 1.55, and 1.77 ohm for the aniline concentrations of 0.2, 0.5, 0.8, 1.0, 1.3, and 1.5 M, respectively. These demonstrate that the PANI array/CCs have relatively low resistances and fast ion responses ing a high frequency range that makes the electrolyte ion pass through the surface and inner layer structure easily; as a result, a large capacitance can remain even under a fast charge–discharge condition. ,,, Figure b further gives the Bode plots of the impedance phase angles for all the electrodes. The values of the phase angles are 85°, 86°, 83°, 85°, 86°, 87°, and 84° for the CC and PANI array/CCs prepared at 0.2, 0.5, 0.8, 1.0, 1.3, and 1.5 M, respectively, implying the nearly ideal capacitive behaviors.…”

“…The areal capacitance of the electrodes prepared at the aniline concentrations of 0.2, 0.5, 0.8, 1.0, 1.3, and 1.5 M are 49.3, 174.3, 237.0, 356.9, 604.7, and 505.9 mF cm –2 , while the corresponding gravimetric capacitances are 205.4, 311.3, 329.2, 446.1, 419.9, and 266.3 F g –1 , respectively. The maximum gravimetric capacitance of 446.1 F g –1 is better than 364 F g –1 for neat PANI/aCF fabricated though chemical polymerization and 408 F g –1 for the SPAN/FC prepared by electrochemical polymerization. Moreover, the rate capabilities for all the PANI array/CCs are further tested by varying the current density from 1 to 20 mA cm –2 .…”

“…More clearly, the rate retentions of the areal capacitances are 81%, 75%, 79%, 88%, 84%, and 78% for the aniline concentrations of 0.2, 0.5, 0.8, 1.0, 1.3, and 1.5 M, respectively, indicating all the electrodes possess relatively high rate capabilities. These rate capabilities are better than 70% 11 for PANI-NWs/GF, 70% 26 for neat PANI/aCF, 41% 30 for PANI/CKF, 60% 31 for PANI/GECF, and 74%, 28 70%, 32 and 51% 33 for PANI/GO and similar to 84% 22 for PANI/GNM prepared though chemical polymerization, while higher than 70% 15 for PANI/SS and close to 88.4% 27 for PANI/GO fabricated by electrochemical polymerization. This can be mainly attributed to the orderly compact morphology, short ion transport path, and low internal resistance.…”

“…All the areal and gravimetric capacitances first rise with the increase of the mass of the PANI nanofibers and then decline once the mass exceeds a certain value, corresponding to the different morphologies of PANI from the nanocone to nanowire to nanofiber arrays. The decrease in the gravimetric capacitance occurs when the aniline concentration exceeds 1.0 M, suggesting a relatively lower utilization of PANI due to a relatively long and thick nanofiber and collapse structure. ,,− However, the maximum areal capacitance of 1459.2 mF cm –2 appears at the aniline concentration of 1.3 M, while the maximum gravimetric capacitance of 1269.5 F g –1 appears for 1.0 M. Furthermore, a gradual reduction of the areal capacitance of all the PANI array/CCs occurs at a higher scan rate as displayed in Figure c. When the scan rate changes from 5 to 100 mV s –1 , the retentions in the areal capacitance are 82%, 76%, 70%, 82%, 76%, and 69% for 0.2, 0.5, 0.8, 1.0, 1.3, and 1.5 M aniline solutions, respectively.…”

“…These demonstrate that the PANI array/CCs have relatively low resistances and fast ion responses ing a high frequency range that makes the electrolyte ion pass through the surface and inner layer structure easily; as a result, a large capacitance can remain even under a fast charge−discharge condition. 17,26,32,38 Figure 4b further gives the Bode plots of the impedance phase angles for all the electrodes. The values of the phase angles are 85°, 86°, 83°, 85°, 86°, 87°, and 84°for the CC and PANI array/CCs prepared at 0.2, 0.5, 0.8, 1.0, 1.3, and 1.5 M, respectively, implying the nearly ideal capacitive behaviors.…”