**3. Results and discussion**

The plasma functionalization of the fiber was confirmed by Raman data **Figure 3a** and XPS data **Figure 3b–d**. From the Raman spectra in **Figure 3a**, we observe an increment in the ratio of intensities between the D and G peak, from 0.776 to 1.195, signifying the destruction of the carbon sp2 bonds during plasma functionalization. In **Figure 3b**, there is a documented increase in the atomic weight percent of oxygen from 9.1% in the pristine state to 28.17% for the oxygen plasma functionalized thread. **Figure 3c** and **d** is deconvoluted high-resolution C1s and O1s peaks from the XPS data, showing the various oxygen functional groups found on the surface of the fiber which is in close agreement with data reported in the literature [39, 56].

PANI-CNT composite fibers were created from four ratios of aniline to APS (1:1, 2:1, 5:1 and 10:1). The OPFCNT fibers were placed with the chemicals as they polymerized for an hour. From our electrochemical half-cell tests, we observed that a 2:1 aniline to APS ratio gave the best specific capacitance, as seen in **Figure 4a**. Further testing of OPFCNT fibers with varying durations (10 minutes to 6 hours) of polymerization revealed that the composite fibers that

**Figure 3.**

*(a) Raman spectra of pristine and plasma functionalized fiber; (b) XPS survey scans of pristine CNT fiber and plasma functionalized fiber; (c) high-resolution C1s scan of the pristine and plasma functionalized CNT; (d) high-resolution O1s scan of the pristine and plasma functionalized CNT.*

underwent polymerization for an hour had the best electrochemistry data, as seen in the inset of **Figure 4a** and in **Figure 4b**. We observed that the polymerization of PANI increased with a higher concentration of APS as well as duration of polymerization. A 1:1 ratio therefore produced more PANI than a 2:1 ratio in the same time frame. PANI in the right amounts improves capacitance of the fibers, however when it becomes deposited in agglomerate morphologies, it leads to the inefficient usage of PANI and reduced capacitance [35–37, 46] . Thus, in the same manner, if polymerization is allowed to take place for longer time these agglomerate morphologies will form and subtract from the synergistic effects of the PANI-CNT composite.

#### **Figure 4.**

*Half-cell test data for PANI-CNT composite. (a) Specific capacitance vs. scan rates for fibers created at different ratios of aniline to APS for an hour, Inset: specific capacitance vs. scan rate for 2:1 aniline to APS ratio at different times; (b) specific capacitance vs. different times for 2:1 aniline to APS ratio polymerization at 1 A/g.*

**45**

for polymerization.

**Figure 5.**

more like PANI.

*Fiber Supercapacitors Based on Carbon Nanotube-PANI Composites*

The structures of the PANI-CNT fibers were observed by SEM. The morphologies and amount of PANI formed were found to correlate strongly to the duration of the polymerization. At 10 minutes, a thin film of PANI forms across the surface of the fiber and as the duration of polymerization increases, PANI nanorods begin to develop in dendritic structures on the fiber. **Figure 5** shows SEM images of the fiber as it progresses from its pristine state to 6 hours of oxidation polymerization. For ease of referencing, we have labeled the fibers by the number of minutes they were polymerized (minutes-PANI-CNT). **Figure 6** compares pristine CNT, 10-PANI-CNT and 360-PANI-CNT at higher magnifications to reveal the PANI structures being formed. **Figure 6a** shows the pristine fiber which has no PANI on it. In **Figure 6b** we find the onset of the formation of PANI as thin films in the fiber. The agglomerate morphologies of PANI are observed in **Figure 6c**. This shows the increment of PANI morphologies on the surface of the fibers with increasing time

*SEM images showing the route of polymerization of fibers up to 6 hours (magnification 1000, scale: 25 μm).*

From the Raman data presented in **Figure 7**, we observe the gradual increment

Devices were created with PANI-CNT fibers, pristine CNT fibers, and OPFCNT fibers. Asymmetrical supercapacitors were also fabricated combining a PANI-CNT fiber and an OPFCNT fiber. The energy density of the PANI-CNT fiber supercapacitor was 3.77 Wh/kg at 0.5 A/g and a power density of about 188 W/kg when using PVA-H2SO4. These parameters were dramatically increased to 6.16 Wh/kg and 630 W/kg when using EMIMBF4 corresponding to an almost 64% increment in energy density and 235% increment in power density. **Figure 8** presents a Ragone plot to give a more holistic view of the data as well as a comparison to other previously reported in the literature fiber supercapacitor devices. For ease of comparison, this plot was presented with the areal capacitance, as most fiber supercapacitor data is published with respect to the surface area of the electrodes [57]. The best

in PANI formation on the composite fibers as the duration of polymerization increases. The spectra for pristine CNT and pure PANI are also incorporated, so the gradual transformation from one extreme to the other can be seen. We observe as the duration of polymerization increases the spectra becomes less like CNT and

*DOI: http://dx.doi.org/10.5772/intechopen.80487*

*Fiber Supercapacitors Based on Carbon Nanotube-PANI Composites DOI: http://dx.doi.org/10.5772/intechopen.80487*

*Science, Technology and Advanced Application of Supercapacitors*

underwent polymerization for an hour had the best electrochemistry data, as seen in the inset of **Figure 4a** and in **Figure 4b**. We observed that the polymerization of PANI increased with a higher concentration of APS as well as duration of polymerization. A 1:1 ratio therefore produced more PANI than a 2:1 ratio in the same time frame. PANI in the right amounts improves capacitance of the fibers, however when it becomes deposited in agglomerate morphologies, it leads to the inefficient usage of PANI and reduced capacitance [35–37, 46] . Thus, in the same manner, if polymerization is allowed to take place for longer time these agglomerate morphologies will form and subtract from the synergistic effects of

*Half-cell test data for PANI-CNT composite. (a) Specific capacitance vs. scan rates for fibers created at different ratios of aniline to APS for an hour, Inset: specific capacitance vs. scan rate for 2:1 aniline to APS ratio at different times; (b) specific capacitance vs. different times for 2:1 aniline to APS ratio polymerization* 

*(d) high-resolution O1s scan of the pristine and plasma functionalized CNT.*

*(a) Raman spectra of pristine and plasma functionalized fiber; (b) XPS survey scans of pristine CNT fiber and plasma functionalized fiber; (c) high-resolution C1s scan of the pristine and plasma functionalized CNT;* 

**44**

**Figure 4.**

*at 1 A/g.*

the PANI-CNT composite.

**Figure 3.**

**Figure 5.** *SEM images showing the route of polymerization of fibers up to 6 hours (magnification 1000, scale: 25 μm).*

The structures of the PANI-CNT fibers were observed by SEM. The morphologies and amount of PANI formed were found to correlate strongly to the duration of the polymerization. At 10 minutes, a thin film of PANI forms across the surface of the fiber and as the duration of polymerization increases, PANI nanorods begin to develop in dendritic structures on the fiber. **Figure 5** shows SEM images of the fiber as it progresses from its pristine state to 6 hours of oxidation polymerization.

For ease of referencing, we have labeled the fibers by the number of minutes they were polymerized (minutes-PANI-CNT). **Figure 6** compares pristine CNT, 10-PANI-CNT and 360-PANI-CNT at higher magnifications to reveal the PANI structures being formed. **Figure 6a** shows the pristine fiber which has no PANI on it. In **Figure 6b** we find the onset of the formation of PANI as thin films in the fiber. The agglomerate morphologies of PANI are observed in **Figure 6c**. This shows the increment of PANI morphologies on the surface of the fibers with increasing time for polymerization.

From the Raman data presented in **Figure 7**, we observe the gradual increment in PANI formation on the composite fibers as the duration of polymerization increases. The spectra for pristine CNT and pure PANI are also incorporated, so the gradual transformation from one extreme to the other can be seen. We observe as the duration of polymerization increases the spectra becomes less like CNT and more like PANI.

Devices were created with PANI-CNT fibers, pristine CNT fibers, and OPFCNT fibers. Asymmetrical supercapacitors were also fabricated combining a PANI-CNT fiber and an OPFCNT fiber. The energy density of the PANI-CNT fiber supercapacitor was 3.77 Wh/kg at 0.5 A/g and a power density of about 188 W/kg when using PVA-H2SO4. These parameters were dramatically increased to 6.16 Wh/kg and 630 W/kg when using EMIMBF4 corresponding to an almost 64% increment in energy density and 235% increment in power density. **Figure 8** presents a Ragone plot to give a more holistic view of the data as well as a comparison to other previously reported in the literature fiber supercapacitor devices. For ease of comparison, this plot was presented with the areal capacitance, as most fiber supercapacitor data is published with respect to the surface area of the electrodes [57]. The best

**Figure 6.** *SEM Images at 25000 magnification (scale: 1 μm). a) Pristine CNT; b)10-PANI-CNT; c) 360-PANI-CNT.*

devices (superior energy density and power density) from this Ragone plot were observed in our asymmetric devices. The latter was attributed to the combined redox reactions between the PANI and oxygen functional groups on the surface of the fibers, as well as to the synergistic effect of the pseudocapacitance (PANI-CNT) and EDLC (OPFCNT). Oxygen functional groups have been reported in other works to have improved capacitance of carbon-based materials [58–61] and this also plays a role in the enhanced electrochemical properties of the asymmetrical device.

**Figure 9** shows cyclic voltammetry graphs of all the devices at 200 and at 5 mV/s. It can be clearly seen from these graphs that the devices had the characteristic curves

**47**

**Figure 9.**

*Cyclic voltammetry curves of devices at 5 and 200 mV/s.*

*Fiber Supercapacitors Based on Carbon Nanotube-PANI Composites*

of supercapacitors. At lower scan rates (5 mV/s), where redox reactions are more

The stability of a supercapacitor is an important parameter since its practical application can be evaluated from this data. **Figure 10** shows the cycling stability of the PANI-CNT (EMIMBF4) device over 1000 cycles. The device retains 88% of its capacitance even after 1000 charge-discharge cycles. This shows good stability and

visible, we see larger voltammetry curves for PANI doped threads.

*Ragone plot comparing devices with others in literature [61–63].*

long lifetime of devices.

**Figure 8.**

*DOI: http://dx.doi.org/10.5772/intechopen.80487*

**Figure 7.** *Raman spectra of CNT, OPFCNT and PANI-CNT composites polymerized at different times.*

*Fiber Supercapacitors Based on Carbon Nanotube-PANI Composites DOI: http://dx.doi.org/10.5772/intechopen.80487*

*Science, Technology and Advanced Application of Supercapacitors*

devices (superior energy density and power density) from this Ragone plot were observed in our asymmetric devices. The latter was attributed to the combined redox reactions between the PANI and oxygen functional groups on the surface of the fibers, as well as to the synergistic effect of the pseudocapacitance (PANI-CNT) and EDLC (OPFCNT). Oxygen functional groups have been reported in other works to have improved capacitance of carbon-based materials [58–61] and this also plays a role in the enhanced electrochemical properties of the asymmetrical device. **Figure 9** shows cyclic voltammetry graphs of all the devices at 200 and at 5 mV/s. It can be clearly seen from these graphs that the devices had the characteristic curves

*Raman spectra of CNT, OPFCNT and PANI-CNT composites polymerized at different times.*

*SEM Images at 25000 magnification (scale: 1 μm). a) Pristine CNT; b)10-PANI-CNT; c) 360-PANI-CNT.*

**46**

**Figure 7.**

**Figure 6.**

**Figure 8.** *Ragone plot comparing devices with others in literature [61–63].*

of supercapacitors. At lower scan rates (5 mV/s), where redox reactions are more visible, we see larger voltammetry curves for PANI doped threads.

The stability of a supercapacitor is an important parameter since its practical application can be evaluated from this data. **Figure 10** shows the cycling stability of the PANI-CNT (EMIMBF4) device over 1000 cycles. The device retains 88% of its capacitance even after 1000 charge-discharge cycles. This shows good stability and long lifetime of devices.

**Figure 9.** *Cyclic voltammetry curves of devices at 5 and 200 mV/s.*

**Figure 10.**

*Cyclic stability for PANI-CNT EMIMBF4 device Inset: galvanostatic charge/discharge curve after 500 cycles.*

#### **4. Conclusion**

In this chapter, we have discussed the increased attention being given to fiber supercapacitors and their relevance to wearable electronics. We also revealed the role of carbon nanostructured fiber as energy storage devices and the challenges they face. We have successfully synthesized CNT fibers by CVD and dry spinning, applied a post-processing technique to these fibers (oxygen plasma functionalization) and by means of oxidation polymerization doped these fibers with PANI. These fibers were characterized electrochemically, by Raman spectroscopy and with SEM. These fibers were then used as electrodes to create simple fiber devices. The obtained devices produced energy densities of up to 6.16 Wh/ kg and 630 W/kg when using EMIMBF4 as electrolytes corresponding to almost a 64% increment in energy density and 335% increment in power density from devices fabricated with PVA-H2SO4 (3.77 Wh/kg, 188 W/kg). These devices also maintained excellent capacitance retention (88%) over 1000 charge-discharge cycles. When a comparison was however made with other devices with respect to areal energy density and power density it was observed that the asymmetrical device comprising of an OPFCNT and PANI-CNT showed the best data. This was attributed to the combined redox reactions of both the OPFCNT and PANI-CNT electrodes with the electrolyte.

#### **Acknowledgements**

This work was funded by NASA NNX13AF46A and the National Institute for Occupational Safety and Health through the Pilot Research Project Training Program of the University of Cincinnati Education and Research Center Grant # T42OH008432. One of the authors (P. K. A.) would like to thank the Department of Chemical and Environmental Engineering at UC for a partial financial support.

**49**

**Author details**

Paa Kwasi Adusei1

Cincinnati, OH, USA

Cincinnati, OH, USA

Kevin Johnson2

provided the original work is properly cited.

, Yu-Yun Hsieh1

, Noe T. Alvarez<sup>3</sup>

*Fiber Supercapacitors Based on Carbon Nanotube-PANI Composites*

The authors declare there is no conflict of interest.

*DOI: http://dx.doi.org/10.5772/intechopen.80487*

**Conflict of interest**

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

, Sathya Narayan Kanakaraj1

and Vesselin Shanov1,2\*

1 Department of Mechanical and Materials Engineering, University of Cincinnati,

2 Department of Chemical and Environmental Engineering, University of

3 Department of Chemistry, University of Cincinnati, OH, USA

\*Address all correspondence to: vesselin.shanov@uc.edu

, Yanbo Fang1

,

*Fiber Supercapacitors Based on Carbon Nanotube-PANI Composites DOI: http://dx.doi.org/10.5772/intechopen.80487*
