Source: https://pubs.rsc.org/en/content/articlehtml/2018/ra/c8ra03963a
Timestamp: 2019-04-25 06:17:25+00:00

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Carbon nanomaterials show great promise for a wide range of applications due to their excellent physicochemical and electrical properties. Since their discovery, the state-of-the-art has expanded the scope of their application from scientific curiosity to impactful solutions. Due to their tunability, carbon nanomaterials can be processed into a wide range of formulations and significant scope exists to couple carbon structures to electronic and electrochemical applications. In this paper, the electrochemical performance of various types of CNT films, which differ by the number of walls, diameter, chirality and surface chemistry is presented. Especially, chirality-sorted (6,5)- and (7,6)-based CNT films are shown to possess a high charge storage capacity (up to 621.91 mC cm−2), areal capacitance (262 mF cm−2), significantly increased effective surface area and advantageous charge/discharge characteristics without addition of any external species, and outperform many other high capacity materials reported in the literature. The results suggest that the control over the CNT structure can lead to the manufacture of macroscopic CNT devices precisely tailored for a wide range of applications, with the focus on energy storage devices and supercapacitors. The sorted CNT macroassemblies show great potential for energy storage technologies to come from R&D laboratories into real life.
Carbon nanomaterials such as carbon nanotubes (CNTs) or graphene have shown great promise for a wide range of applications due to their excellent electrical,1–3 mechanical,4–6 thermal7–9 and optical properties.10–13 Since their discovery,14,15 the state-of-the-art has matured enough to prove the concept of many nanocarbon-based devices2,16–19 and to provide practical implementation. For instance, carbon nanomaterials have been employed successfully as atomic force microscopy probes,20 in the manufacture of light-weight bicycle frames21 and as light absorbing materials.22 Carbon nanomaterials can exist in numerous forms, differing in structure, size, chemistry, physicochemical and mechanical properties23 and it is possible to further enhance the desired properties of these materials through chemical and physical functionalization.24–26 Critically, significant leverage exists still for improving the efficacy of carbon nanomaterials in specific applications through tailoring the chemistry, and structure of carbon material formulation at the micro and nanoscales.
In this report, we present on free-standing CNT films made of various types of CNTs, which differ by the number of walls, tube diameter, chirality and surface chemistry and show that these materials possess advantageous electrochemical properties, including exceptionally high capacitance, large effective surface area and beneficial charge/discharge characteristics without addition of any external species or supporting matrices. In particular, single-wall chirality-defined (6,5) and (7,6) CNTs revealed excellent charge-storage capacity (CSC) performance with measured CSC one order of magnitude higher than that of a platinum reference and an effective surface area enlargement factor of more than 10. It was confirmed that by careful synthesis of CNTs it is possible to reduce the wall defect frequency and obtain highly conductive films with significantly reduced impedance relative to commercially available CNTs and charge/discharge characteristics suitable for supercapacitor applications.
The aforementioned materials were used for the preparation of free-standing CNT films of predetermined structure according to a method developed previously.49 In brief, described CNT formulations were sonicated in iso-propanol in the presence of a binding agent (ethyl cellulose) until a uniform dispersion was obtained. Then, the material was deposited onto a Kapton foil by spray coating, detached from the substrate and separated from the binder by flash annealing. Specimens of 28.5 mm2 area were used for the study.
Scanning Electron Microscopy and Transmission Electron Microscopy (SEM – FEI Nova NanoSEM at 10 kV accelerating voltage, TEM – Tecnai Osiris FEGTEM at 200 kV accelerating voltage) were used to visualize the microstructure of the materials.
Raman spectroscopy (Renishaw Ramascope 1000 with 633 nm emission wavelength) was used to analyze the surface chemistry. A ratio of intensity of disorder peak (D) to the intensity of peak of vibrations of graphitic lattice (G) gave information about the pristinity of the samples.
XPS investigations were carried out with PREVAC EA15 hemispherical electron energy analyzer equipped with 2D-MCP detector. The samples were irradiated with X-ray source (PREVAC dual-anode XR-40B, Al-Kα line, energy 1486.60 eV). The system base pressure was 2 × 10−8 Pa. For the survey spectra the scanning step was set to 0.9 eV with pass energy 200 eV while for particular energy regions to 0.05 eV with pass energy 100 eV. All of the measurements were performed with analyzer's axis perpendicular to samples' plane. The binding energy (BE) scale of the analyzer was calibrated to Au 4f7/2 (84.0 eV). Recorded data were fitted utilizing CASA XPS® embedded algorithms and relative sensitivity factors. Shirley function was used for the background subtraction. The estimated uncertainty for components' energy position determination was 0.1 eV.
Thermogravimetric analysis (TGA – Mettler Toledo TGA/DSC system) was used to measure changes in chemical and physical properties as a function of temperature. The samples were heated from room temperature to 1000 °C at 10 °C min−1 in the flow of air (20 ml min−1). The gas adsorption parameters were measured using nitrogen adsorption at 77 K (Tristar3000). 100 mg samples were used for analysis. BET model was employed to calculate specific surface area.
The voltammetric experiments were carried out using PARSTAT 2273 Advanced Electrochemical System (Princeton Applied Research) in a three-electrode electrochemical Teflon cell equipped with O-ring, CNT working electrode, Ag/AgCl (3 M KCl) reference electrode and platinum coil counter electrode. CV scans were recorded in 0.1 M KCl solution, in the potential range from −1.0 V to 1.2 V at a scan rate of 0.1 V s−1. CV curves were used to determine charge storage capacity (CSC), calculated as the electric charge integrated under corresponding CV curve during one CV cycle.50 To ensure the good infiltration of electrolyte into CNT films, the samples were immersed in the solution of electrolyte for one hour before the electrochemical measurements. The relative contribution from faradaic and diffusion controlled processes was investigated by conducting CV experiments at different voltage scan rates, from 10 mV s−1 up to 200 mV s−1. The measurements were performed in triplicates and the results are expressed as a mean ± standard deviation.
where ip is the reduction/oxidation peak current (A), n is the number of electrons participating in the redox reaction, A is the area of the electrode (cm2), D is the diffusion coefficient of Fe(CN)64−in KCl solution (6.3 × 10−6 cm2 s−1),53 C is the concentration of the Fe(CN)64− in the bulk solution (mol cm−3) and ν is the scan rate (V s−1).
The enlargement factor was calculated basing on the difference in the electroactive surface area (ESA) between CNT film and bare Pt electrode.
where D is the diffusion coefficient (cm2 s−1), ω is the frequency of the transition point (Hz), and R is the electrode thickness (cm).
where C is the film capacitance (F g−1), ΔV is the potential difference between charging and discharging processes (V), and t is the discharge time (s).
In the galvanostatic (chronopotentiometric) experiment, CNT films were subjected to the current densities of 10 mA cm−2. When the potential reached 1.0 V (vs. Ag/AgCl), the current was stopped. The process of discharging was monitored until the time when the potential of CNT electrode reached the value of its open circuit potential.
The electrochemical properties of CNT films, such as CSC, impedance and effective surface area, were compared with a reference platinum foil electrode (0.1 mm thickness, 99.9% purity, produced by Mennica-Warsaw, Poland).
The microstructure of the prepared CNT films was analyzed by SEM (Fig. 1). Macroscopic CNT assemblies made from NC7000 demonstrated a relatively uniform structure (Fig. 1a). SEM micrographs of CNT carpets indicated that the synthesized CNTs possessed considerable polydispersity with larger tube diameters (Fig. 1b).
Fig. 1 Microstructures of (a) NC7000, (b) carpet, (c) (6,5), (d) (7,6) based CNT films as observed by SEM.
The presence of some carbonaceous residue of a non-CNT form was noted, particularly on chirality-sorted samples (Fig. 1c and d). Further investigation by TEM confirmed that NC7000 contained multi-wall CNTs (Fig. 2a). CNT carpets were confirmed again to possess multiple walls and considerable polydispersity in CNT diameter, which was observed to approach 33% (Fig. 2b, standard deviation/average outer diameter). CNT carpet also exhibited reduced graphitization degree relative to other CNT formulations (Fig. 2b). Chirality-sorted (6,5) and (7,6) CNTs were indeed observed to possess single-walls (Fig. 2c and d). The average outer diameter was found to be 10 ± 1 nm, 45 ± 14 nm, 0.80 ± 0.05 nm and 0.85 ± 0.06 nm, respectively.
Fig. 2 Structure and diameter of individual CNTs in (a) NC7000, (b) carpet, (c) (6,5), (d) (7,6) based CNT films observed by TEM.
Raman spectroscopy was employed to probe the surface chemistry of all investigated CNT formulations (Fig. 3). Resulting spectra revealed that commercial grade NC7000 CNTs demonstrated significantly elevated sp3 contamination relative to other investigated CNT formulations (ID/IG as high as 1.58). The corrugated structure of individual CNTs (Fig. 2a) may be indicative of the abundance of defects and presence of external functional groups. As a consequence of the observed low graphitization levels observed in CNT carpet films, these materials also showed an elevated ID/IG ratio equal to 0.42. Interestingly, chirality-defined (6,5) and (7,6) CNT films were associated with the highest observed purity, with ID/IG ratios of 0.045 and 0.033, respectively. Splitting of G-mode into two components G− and G+, out of which the former is of Lorentzian line shape confirms the semiconducting character of the (6,5) and (7,6) CNT films.
Fig. 3 (a) Raman spectra and (b) ratios of intensities of defect-induced band (D) to the band of graphitic lattice (G) of NC7000, carpet, (6,5), (7,6) based CNT films as measured by Raman spectroscopy.
More detailed analysis by XPS confirmed the surface composition of the CNT films (Table 1, Fig. S2†). As expected, analysis of the C1s region reveals that the most intense signal comes from C C component at 284.8 eV. The following components according to available literature and databases can be ascribed to C–C (at 285.5 eV indicative of sp3 functionalization), C–O (at 286.7 eV, also standing for C–OH component), C O (287.8 eV) and broad COOH component (288.9 eV). The last one (291 eV) shall be identified as π–π* shake-up feature. Chirality enriched films mostly composed of (6,5) and (7,6) CNTs had much higher sp2/sp3 ratio, what is in accordance with high degree of structural perfection confirmed by Raman spectroscopy.
Thermogravimetric analysis (Fig. 4) revealed that films formed from single-wall CNTs as expected were associated with a reduced thermal stability relative to multi-wall CNT films.57 Furthermore, NC7000- and carpet-based multi-wall CNT films decomposed at temperatures of ∼600 °C whereas chirality-predominant (6,5)- and (7,6)-based single-wall CNT films decomposed at reduced temperatures, ∼500 °C. Furthermore, NC7000 CNTs had the highest content of residual catalyst (13%) relative to other CNT film formulations (5–7%) (Fig. 4).
Fig. 4 Thermograms of NC7000-, carpet-, (6,5)-, (7,6)-based CNT films as measured by TGA. The arrows show the temperature of maximum rate of decomposition, whereas the values on the right give the amount of residue left after combustion of the samples.
Nitrogen adsorption experiments showed that the CNT films have a well developed surface area (Fig. 5). Multi-wall based NC7000 and CPT CNT films had on average three times lower surface area than (6,5) and (7,6) CNT films built from small diameter single-wall CNTs. The measured values are within the theoretical surface area window calculated for CNTs of various number of walls and diameter.58 Because of the fact that the surface is much more developed for (6,5) and (7,6) enriched CNT films more adventitious contaminants can adsorb on the surface. As a consequence, the content of adulterants for these samples as estimated by XPS is probably overestimated.
Fig. 5 BET surface area calculated by means of nitrogen adsorption.
The electrochemical behavior of CNT films was studied by cyclic voltammetry (CV) as well as electrochemical impedance spectroscopy (EIS), and compared with the electrochemical behavior of a platinum electrode. CVs recorded for different types of CNT working electrodes (Fig. 6) show the discrepancies in electrochemical behavior of CNT films composed of NC7000, carpets and chirality-sorted CNTs.
Fig. 6 Cyclic voltammograms collected in 0.1 M KCl at a scan rate of 0.1 V s−1 of all experimental CNT film formulations (a) NC7000, (b) carpet, (c) (6,5) CNTs, (d) (7,6) CNTs overlaid onto the Pt foil reference. Shading represents the area that was used to calculate charge storage capacities (CSC).
The featureless CV recorded for unsorted CNT carpet films was as a result of the wide distribution of the structure of CNTs, which is in accordance with electron microscopy studies. Because of the fact that this film is composed of CNTs having structural variations, such as length, diameter and chirality, the CV shows the average of many closely spaced peaks representing electron transfer into each CNT.59 Resulting CVs of (7,6)-based CNT film also did not reveal evident reversible redox processes occurring on the electrode surface. This was however, not observed in the CVs of sorted CNT (6,5) film electrodes, for which the anodic (0 V) and cathodic (−0.6 V) peaks, characteristic for this type of structure, were observed. The presence of two weak oxidation peaks (−0.10 V and 0.25 V) and one reduction peak (−0.40 V) in the CV of NC7000-based CNT film confirmed the abundance of functional groups on the surface, what is in accordance with results from Raman spectroscopy.
CVs recorded for all CNT films exhibit double layer capacitance behavior, which is represented by large areas under corresponding CV curves.59,62,63 Consequently, these CNT films were associated with significantly high values of CSC (Fig. 7), defined as the charge passing through the electrode and calculated as the time integral of the corresponding CV curve. The highest CSC (621.91 ± 21.61 mC cm−2) was observed with (7,6) CNT film formulations, which were composed of high quality and low diameter CNTs; this measured charge storage capacity was 25% higher than for (6,5) CNTs and 12 times higher than for pristine Pt.
Fig. 7 Charge storage capacities of all experimental CNT film formulations and Pt calculated by the integration of corresponding CV curves recorded in 0.1 M KCl in the potential range from −1.0 V to 1.2 V (vs. Ag/AgCl) at a scan rate of 0.1 V s−1.
This is an exceptionally high CSC, significantly higher than for many other CNT-based high charge storage capacity materials reported in the literature, including iridium oxide/CNT (101.2 mC cm−2),64 CNT yarns (98.6 mC cm−2)65 and PEDOT/MWCNT (202.9 mC cm−2).66 The high value of CSC is caused by the regular structure and low defect number of employed single-wall CNTs, which indicated the improved electrochemical and electroanalytical parameters relative to multi-wall CNTs.67 The enhanced double layer capacitance behavior was additionally confirmed by collecting CV curves at different scan rates (Fig. S3a–d†) and plotting the current vs. square root of scan rate plot (Fig. S4†). The linear character of this dependency in case of CPT (R2 > 0.99) and NC7000 (R2 > 0.98) confirm that electrochemical processes for these materials are controlled by the diffusion. The decreased value of R2 for chirality-sorted (6,5) CNT (R2 > 0.97) and especially for (7,6) CNT (R2 > 0.91) show that the process of charge transfer is not fully diffusion-controlled, but is most probably accompanied by faradaic reaction.
Electrochemical impedance spectroscopy indicated that all experimental CNT film formulations exhibited significantly lower impedance relative to control Pt electrode, especially in the low frequency region (below 1 kHz) (Fig. 8). In this region these were chirality-sorted CNTs that outperformed both unsorted CNTs and bare Pt with respect to low impedance. With frequencies higher than 1 kHz, the impedance spectra of Pt were observed to decrease to a value comparable with that of investigated CNT film formulations, as well as the impedance of NC7000 and CPT decreased to the values lower than for (6,5) and (7,6) CNTs. This discrepancy can be explained by the fact that the unsorted CNTs exhibit strong diffusion-controlled capacitive behavior, just as it was shown by the analysis of CVs collected at different scan rates. Due to their mixed electron transport mechanism, the impedance profile of chirality-sorted CNTs must differ from the behavior of ideal capacitor,70 manifested by almost flat line in the whole frequency range.
Fig. 8 Bode plots showing (a) impedance modules and (b) phase profiles as a function of frequency, as well as (c) Nyquist plots for different types of CNT films (NC7000, carpet, (6,5) and (7,6) CNTs) and Pt. The inset indicates the linear part in the low frequency region characteristic for the ideal capacitor.
The energy storage performance of the all experimental CNT film formulations was studied in both potentiostatic and galvanostatic modes. In the potentiostatic (chronoamperometric) mode (Fig. S5†), CNT films were subjected to a constant potential of 1.3 V (vs. Ag/AgCl) for 30 s (charging) and to an open circuit potential for the time needed to reach a fully discharged state (300 s). The cumulative charge passing through the CNT films during this charging/discharging process, presented in Fig. 9a, showed that the most effective capacitance behavior was exhibited by the chiral (7,6) CNT film. In only 30 s of potential application, the cumulative charge reached 495 mC cm−2.
Fig. 9 Cumulative charge accumulated and released during (a) chronoamperometric charging/discharging process and (b) chronopotentiometric curve of charging/discharging process of all experimental CNT film formulations.
With the accumulated charge of 275 mC cm−2 (212 mF cm−2), also chiral (6,5) CNT film could be treated as a high capacity material. The unsorted NC7000 and CPT films are characterized with the charge capacities of 101 mC cm−2 (78 mF cm−2) and 74 mC cm−2 (57 mF cm−2), respectively, still remarkable when compared with literature.
In summary, CNT films of various number of walls, diameter, chirality and surface chemistry were formulated and evaluated with respect to their electrochemical performance. Comparison with other electrode materials reported in the literature indicated that chirality-defined pristine single-wall CNT films (7,6 in particular) had excellent charge storage capacity (up to 621.91 mC cm−2), areal capacitance (262 mF cm−2), very high effective surface area and preferential charge/discharge characteristics. High degree of crystallinity of both (6,5) and (7,6) enriched s-SWCNT films and significantly reduced presence of extraneous impurities makes such ensembles (as compared with polydisperse mixtures of MWCNTs of moderate quality and significant content of metallic CNTs) very attractive for charge storage applications. They could already offer competitive advantage over traditionally employed materials, especially the materials applied as supercapacitors. We believe that with further progress in the sorting of CNTs (and manufacture of macroscopic ensembles from such materials), we will not only be able to better understand the nature of carbon nanomaterials, but it will enable the design of CNT devices with properties tailored for a specific application. The results of this study demonstrate that sorted CNT macroassemblies show great potential for energy storage technologies to be discharged from R&D laboratories to the real life.
K. K. and D. J. would like to thank National Science Center, Poland (under the Polonez program, grant agreement UMO-2015/19/P/ST5/03799) and the European Union's Horizon 2020 research and innovation programme (Marie Sklodowska-Curie grant agreements 665778 and 713690). D. J. would also like to acknowledge the Rector of the Silesian University of Technology in Gliwice for funding the research in the framework of Pro-Quality grant (04/020/RGJ18/0057). This publication has emanated from research supported in part by a research grant from Science Foundation Ireland (SFI) and is co-funded under the European Regional Development Fund under Grant Number 13/RC/2073. M. J. Biggs is also an SFI, Starting Investigator SIRG COFUND fellow (11/SIRG/B2135). Authors acknowledge ESPEFUM laboratory (at Institute of Physics – CSE, Silesian University of Technology) for access to XPS experimental setup.
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