Can Faradaic Processes in Residual Iron Catalyst Help Overcome

Subscriber access provided by ISTANBUL UNIVERSITESI

Article

Can Faradaic Processes in Residual Iron Catalyst help Overcome Intrinsic EDLC limits of Carbon Nanotubes? Robert K Emmett, Mehmet Karakaya, Ramakrishna Podila, Margarita Arcila-Velez, Jingyi Zhu, Apparao M. Rao, and Mark E. Roberts J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5097184 • Publication Date (Web): 29 Oct 2014 Downloaded from http://pubs.acs.org on November 5, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Can Faradaic Processes in Residual Iron Catalyst help Overcome Intrinsic EDLC limits of Carbon Nanotubes? Robert K. Emmett†,#, Mehmet Karakaya‡,⊥,#, Ramakrishna Podila‡,⊥, Margarita R. Arcila-Velez†, Jingyi Zhu‡,⊥, Apparao M. Rao‡,⊥,*, Mark E. Roberts†,⊥,* #

Both authors contributed equally to this work.

†

Dept. of Chemical & Biomolecular Engineering, Clemson University, Clemson, SC 29634

‡

Dept. of Physics & Astronomy, Clemson University, Clemson, SC 29634

⊥

Clemson Nanomaterials Center, Clemson University, Clemson, SC 29634

KEYWORDS: Carbon Nanotubes, Iron Catalyst, Faradaic Pseudocapacitance, Supercapacitors ABSTRACT. The promise of multi-walled carbon nanotubes (MWNTs) for supercapacitor electrodes remains unfulfilled due to their poor energy density, which is limited by their redox inactivity. Here, we show a simple, alternative path to achieve Faradaic charge storage by harnessing intrinsic heterogeneity (e.g. Fe catalyst) of as-synthesized MWNTs, obviating the challenges of combining disparate materials in hybrid composite electrodes. In acidic solutions, MWNTs are ruptured by voltammetric cycling beyond the electrolysis limit, thereby exposing residual catalyst nanoparticles. The addition of Faradaic charge storage associated with the Fe2+/Fe3+ transition, results in a 4-fold increase in peak capacitance of MWNT electrodes (290

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 19

F/g) compared to purified MWNT electrodes (70 F/g), along with a 60% increase in charge capacity.

With ever increasing power and energy demands for applications ranging from portable electronics to hybrid electric vehicles, there is a tremendous need to improve the performance of electrical energy storage materials and systems.1-3 While nanomaterial-based electrochemical double layer capacitors (EDLCs) are promising energy storage systems with power densities comparable to electrolytic capacitors, they exhibit much lower energy densities compared to conventional batteries.4-6 Porous carbon nanomaterials are currently being used to commercialize EDLC systems; however, their theoretical capacitance is limited to 150-200 F/g.7-9 As our efforts continue to surpass such intrinsic limits posed by carbon materials, an in-depth understanding of electrochemical interfaces at the nanoscale will be the key to realize power and energy densities necessary for future applications. In this regard, carbon nanotubes (CNTs) have been used as EDLC electrodes due to their high surface area, high electrical conductivity, chemical and thermal inertness, entangled percolating network, and controllable porosity.10-11 Indeed, the unique properties of CNTs have allowed the fabrication of EDLCs with specific capacitances as high as 100 F/g in multi-walled CNTs (MWNTs) and up to 180 F/g in single-walled CNTs (SWCNTs).12 Although their performance is moderately higher than that of activated carbon, there is still a need to further improve the specific capacitance while retaining their ability to rapidly charge and discharge over hundreds of thousands of cycles for practical applications. As mentioned earlier, the intrinsic double layer capacitance of CNTs is theoretically limited to 150-200 F/g since charges


2

Page 3 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


can only be stored physically along the sidewalls (similar to the basal plane in pyrolytic graphite) and open ends (similar to the edge plane of graphite) rather than through Faradaic charge transfer.12 Accordingly, inherent defects or impurities within the CNT framework present additional opportunities to further increase their electrochemical activity. Indeed, open-end defects in CNTs are known to increase the electron transfer-rate constant (k) about 7 orders of magnitude, while residual impurities (e.g. Fe catalyst) exhibit the most prominent electrocatalytic activity.13-14 While CNTs are actively pursued as materials for EDLC electrodes, there is very little research aimed at harnessing the electrochemical properties of metal catalyst particles confined within CNTs. The metal catalyst particles in CNTs are often covered by many impermeable layers of graphene and thereby remain electrochemically inactive;15-16 however, when activated, they can participate in electrochemical redox reactions that could be beneficial to surpass the intrinsic double layer capacitance limits of inert carbon materials.17 In this letter, we describe a Faradaic process by which the residual iron nanoparticles residing within multiwall carbon nanotubes (MWNT) are electrochemically activated leading to carbon nanomaterials with enhanced capacitance. The residual iron nanoparticles (~5-6 atomic %) within the MWNTs are activated by applying potentials exceeding 0.9 V (vs. Ag/AgCl) under acidic conditions (H2SO4, HNO3, and HClO4). During this process, the nanotubes predominantly rupture around the endcaps (where Fe catalyst particles are confined), causing rapid oxidation of the catalyst along with the formation a stable and reversible Fe2+ / Fe3+ redox couple. Fecontaining MWNTs activated through this process show greater than 50% increase in the capacitance and charge capacity compared to Fe-deficient (purified) MWNTs. This approach provides a novel method to increase the capacitance of carbon nanomaterials by leveraging Faradaic processes in confined catalyst materials.


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 19

Free-standing buckypaper electrodes provide an ideal platform to electrochemically activate and characterize carbon nanotubes with varying physical and chemical properties.18-20 BP electrodes (Fig. 1a) prepared using MWNT-CT (dia.=15 nm, length=1-12 µm) and MWNT-NTL (dia.=70-80 nm, length=10 µm) exhibited entangled networks of nanotubes, as shown in Fig. 1b and Fig 1c, respectively (see supporting information for MWNT information). Regardless of the MWNT type, these electrodes were very mechanically robust, flexible, and could be soaked in aqueous acid, base, and salt electrolyte solutions (up to 3M) without noticeable physical, chemical, or mechanical damage. Typically, it is well known that MWNTs grown by the ferrocene-xylene CVD method contain ~5-10% iron catalyst and ~5-10% amorphous carbon impurities.21 In this study, TGA (Fig. 1d) and EDXS (Table 1) data indicate that MWNT-NTL contained ~ 5.7% Fe by weight compared to 0.3% for MWNT-CT. The remaining mass of MWNT-NTL electrodes following the TGA consisted of a dense red powder, which is likely an oxidized form of the residual Fe nanoparticles (e.g., Fe2O3).


4

Page 5 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Physical properties of MWNT electrodes. a) Optical image of a MWNT buckypaper (40 mg, 33 mm diameter). Scanning electron microscopy (SEM) images of electrodes prepared with (b) MWNT-CT and (c) MWNT-NTL. d) Weight % vs. temperature data obtained from thermal gravimetric analysis (TGA) of MWNT-CT (black) and MWNT-NTL (red) at a heating rate of 40 °C/min. The inset shows an expansion of the weight % for the electrodes slightly above the MWNT decomposition temperature range.

Table 1. Elemental composition of MWNT-NTL and MWNT-CT electrodes as determined by Energy Dispersive X-ray Analysis (EDX). Element [Wt%] C Fe O Ni Si S

MWNT-NTL

MWNT-CT

90.4 ± 0.1 5.7 ± 0.1 3.1 ± 0.2 0.5 0.3

93.5 ± 0.2 0.3 ± 0.1 3.3 ± 0.1 1.4 ± 0.2 1.1 0.3

In order to access the redox activity of the embedded Fe nanoparticles, any enclosing graphite layers must first be removed to expose the Fe particles to the electrolyte solution. Previously, we observed that the Fe nanoparticles are often trapped inside the graphitic planes of MWNTs in body-centered cubic α-Fe, face-centered cubic γ-Fe, and Fe3C that cannot be completely removed by acid purification.13,22 The encapsulating carbon layers prevent electrolyte from accessing and activating Fe/Fe3C nanoparticles, and thereby result in low EDLC current profiles for MWNTs-NTL electrodes.


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 19

As shown in Figure 2a, we demonstrate an activation process that exposes the embedded Fe/Fe3C nanoparticles by rupturing the graphitic layers when MWNT-NTL electrodes are subjected to electrochemical potentials above 0.9 V in 0.5 M sulfuric acid (pH 0.3). During the first CV cycle, the applied potential is increased at a rate of 100 mV/s from Vi=-0.5 V up to Vf=1.5 V, and then decreased back to Vi=-0.5 V. As the potential is increased up to ~0.8 V (i), a negligible anodic current is measured as the Fe nanoparticles are shielded from the electrolyte. Above 1 V (ii), electrolysis of water occurs resulting in the production of oxygen gas and a significant, irreversible anodic current. On the reverse potential scan (Vf->Vi), a slight cathodic current is observed near 0.5 V (iii) due to the reduction of any exposed Fe nanoparticles that may exist in the amorphous carbon region. As the potential continues to decrease, a strong anodic current spike is observed near 0.2-0.3 V (iv), which we attribute to the rapid oxidation of the exposed iron as a result of the rupturing of the MWNT endcaps. Subsequent voltammetric cycling shows a pronounced, reversible Fe2+/Fe3+ redox couple arising from the activated Fe nanoparticles (Fig 2b).


6

Page 7 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Electrochemical treatment of MWNT-NTL electrodes. a) Initial cyclic voltammetry (CV, 100 mV/s) scan of a MWNT-NTL electrode in 0.5 M H2SO4 (pH 0.3) (scan direction is shown by red arrows). b) CV cycling of MWNT-NTL electrodes in 0.5 M H2SO4, with sequential current profiles during electrode treatment (inset shows the voltage at which the anodic spike occurs in different pH solutions). c) Voltammetric cycling (100 mV/s) of MWNTNTL electrodes starting with Vi = -0.3 V to gradually increasing positive potentials, Vf = 0.6 V (black), 0.7 V (green), and 0.8 V (red). d) Voltammetric cycling (100 mV/s) of MWNT-NTL electrodes starting with Vi = -0.3 V to increasingly positive potentials, Vf = 1.0 (black), 1.1 V (purple), 1.2 V (blue), 1.3 V (green), 1.4 V (orange), and 1.5 V (red). e) CV cycling (50 mV/s) of MWNT-NTL electrodes in 5 mM H2SO4 (pH 2), with sequential current profiles during electrode treatment; 1 (black), 2 (purple), 3 (blue), 4 (green), 5 (orange). f) Normalized peak current of the


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 19

irreversible oxidation peak (e, peak 1) with increasing cycle number. Peak current values are normalized by the peak current of the 10th CV profile.

Importantly, we found that the anodic current spike (iv) is not observed unless Vf applied to the MWNT-NTL electrodes exceeds 0.9V, the thermodynamic potential of oxygen evolution in the acidic solution (Fig. 2b). Nonetheless, the small, reversible redox couple present in Fig. 2c is attributed to the exceedingly low amount of exposed Fe in the MWNT-NTL electrodes prior to electrochemical activation described above. When the positive potential vertex, Vf, was further increased above 0.9 V, the magnitude of the anodic current spike and redox couple correspondingly increased (Fig. 2d). In a similar manner, we found that the magnitude and position of the anodic current spike was dependent upon the pH of the electrolyte during the activation process (Vf = 1.5 V). While the Fe nanoparticles within the MWNT-NTL electrodes can be completely activated within the initial CV scan (-0.5 to 1.5 V) in low pH electrolytes, the activation process is prolonged at higher pH values. When the pH is increased from 0.3 to 1.2 (decrease proton concentration), the anodic current spike decreases and downshifts to a lower potential. Unlike CV profile shown in Fig 2b (pH 0.3), a new irreversible oxidation peak (0.2V, peak 1) is observed during the 2nd CV scan in the vicinity of the anodic current spike potential (Fig. 2e). Once the Fe nanoparticles are exposed, the lower proton concentration limits the extent of oxidation during each CV cycle (40 sec), resulting in the additional irreversible peak. As shown in Fig. 2f, this peak quickly decreases with subsequent voltammetric cycling and reaches a low steady-state value within 4 cycles. In higher pH solutions, peak 1 persists over more


8

Page 9 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


voltammetric cycles (>7 cycles), indicating that the activation process is a combination of physical breaking and chemical activation. Based on these observations, the electrochemical activation process can be rationalized as follows. When Vf increases above 0.9 V, electrolysis of water produces oxygen gas that induces physical stress within the MWNTs (in the acidic electrolyte) resulting in the etching of graphiticlayers (Fig 3), as evidenced by our TEM images. On the reverse scan, the electrolyte gains access to the Fe/Fe3C nanoparticles as the MWNT rupture, initiating their rapid oxidation. During this process, some Fe2+ ions are released into the electrolyte along with an equivalent flow of electrons measured as the anodic current spike (see blue panel in Fig. 3e). After complete activation of Fe nanoparticles and their oxidation into FeO, a stable Fe3+/Fe2+ redox couple is observed (Figs. 3d and 3f). Although Fe nanoparticles are completely activated during the first scan at low pH, the activation process is prolonged (cf Fig 2e) in higher pH electrolytes, consistent with the above mechanism. For solutions with pH > 6, however, no significant MWNT activation was observed. It should be noted that similar trends were observed in other acidic electrolytes (e.g. HClO4, HNO3, HCl, etc).


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 19

Figure 3. Schematic representation of the electrochemical activation process of MWNT-NTL electrodes, inferred from TEM images (shown in insets) and electrochemical analysis. The initial structure of MWNT electrodes containing Fe/Fe3C catalyst particles (a) is physically stressed (b) when the electrochemical potential is increased beyond 0.9 V, leading to etching (c) and rapid oxidation (d) upon electrolyte infiltration. The electrochemistry associated with these processes are illustrated by the cyclic voltammetry scan (e), which shows the current during electrolysis (pink) and an anodic current spike associated with Fe ion dissolution during the etching (blue). The cyclic voltammetry profile after treatment (f) shows the contribution of the Fe2+/Fe3+ redox couples (green) to the electrochemical behavior of the MWNT electrodes.

The activation process described above transforms MWNTs from EDLC to Faradaic materials with increased capacitance and energy density, obviating the need for any external redox additives. Accordingly, MWNT-NTL electrodes exhibit a peak capacitance of 286 F/g


10

Page 11 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(avg 79 F/g) after electrochemical activation compared to 68 F/g (avg 48 F/g) for MWNT-CT at a scan rate of 3 mV/s. Under similar electrochemical activation conditions as described for MWNT-NTL, MWNT-CT electrodes exhibit primarily EDLC behavior due to their exceedingly low Fe content. As shown in Figs. 4a and 4b, MWNT-NTL electrodes containing 5.7 wt% Fe exhibited an increase in capacitance relative to MWNT-CT electrodes due to the activation of the redox elements. The redox peaks in Fig. 4a align well with the electrochemical behavior of Fe(CN)63- to Fe(CN)64- on platinum electrodes, confirming existence of the Fe3+ / Fe2+ redox couple. Each electrode type exhibited some degree of scan-rate dependence on the measured capacitance as shown in Fig. 4c. A purely EDLC device is characterized by constant capacitance with varying scan rate in the absence of ion-diffusion limitations. Due to the use of a relatively thick (~ 1-2 mm) MWNT mat electrode with randomly oriented and bundled nanotubes, even the low Fe-content MWNT electrodes displayed an increase in capacitance with decreasing scan rate (for the range of scan rates examined). As expected, the average capacitance over the voltage range tested (0 to 1V) is fairly similar to the peak capacitance for these materials. When electrodes contain the Fe3+ / Fe2+ redox couple (MWNT-NTL) from residual iron catalyst, however, the peak capacitance exhibits a strong dependence on scan rate, as is expected for diffusion limited redox processes and slow reaction kinetics. The activation of the residual Fe catalysts provides a significant increase in capacitance through Faradaic processes; however, it should be noted that this redox couple possesses limited stability in acidic environments. As shown in Fig. 4d, extended voltammetric cycling reveals that the Fe3+ / Fe2+ redox couple gradually decreases over hundreds of CV cycles (ν = 30 mV/s). The inset indicates that the shape of the CV profile maintains constant, but decreases in magnitude


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 19

with extended cycling as the Fe iron particles are slowly etched away in H2SO4. Nonetheless, this approach to heterogeneous nanomaterial synthesis will provide a new route to developing materials for energy storage systems with increased charge capacity. Material stability limitations and performance in various electrolyte systems are the subject of ongoing research.

Figure 4. Electrochemical analysis of MWNT electrodes treated with voltammetric cycling in 0.5M H2SO4 (pH 0.3). Gravimetric capacitance (current normalized by mass and scan rate) versus potential for (a) MWNT-NTL and (b) MWNT-CT measured in 1.5M H2SO4. c) Average and peak capacitance values versus scan rate for MWNT-NTL (blue) and MWNT-CT (red) electrodes. d) % capacitance versus cycle number from repeated voltammetric cycles recorded at a scan rate of 30 mV/s for MWNT-NTL (solid) and MWNT-CT (dashed) (inset shows CV


12

Page 13 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


profiles for cycles 1, 10, 100, 200 of MWNT-NTL). The initial capacitance for MWNT-NTL and MWNT-CT were 75 F/g and 46 F/g, respectively. e) Nyquist plots from the electrochemical impedance analysis (EIS) of MWNT-NTL (blue) and MWNT-CT (red) electrodes (inset: expansion at low resistance). f) Galvanostatic discharge measurements for MWNT-NTL and MWNT-CT electrodes at current densities of 0.53 A/g (red) and 1.0 A/g (black, dashed).

Electrode resistance characteristics were determined from electrochemical impedance spectroscopy and are presented as a Nyquist plot in Fig. 4e. As expected, MWNT-CT electrodes show a low charge transfer (0.62 Ω) due to the exceedingly small amount of residual iron and small diameter (OD = 4 - 2 nm), whereas MWNT-NTL exhibit a much higher charge transfer resistance (13.5 Ω), as evidenced by the larger diameter hemisphere in the Nyquist plot, due primarily to the redox processes associated with the Fe2+/Fe3+ transition and also their larger diameter (OD=70-80 nm). The increase in charge capacity associated with the Faradaic process can be quantified using charge-discharge measurements (Fig. 4f). The addition of Faradaic capacitance results in an increase in charge capacity from 10 mAh/g (MWNT-CT) to 16 mAh/g (MWNT-NTL). Electrodes comprised of MWNT-NTLs show battery-like behavior with a relatively constant electrode potential (around ~0.4-0.5 V) during constant-current discharge. These electrodes also exhibit some EDLC behavior, which is manifested in the two distinct slopes in voltage above and below the Fe3+/Fe2+ redox potential. On the other hand, MWNT-CT electrodes show purely EDLC behavior, as evidenced by the nearly linear decrease in current during discharge.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 19

In summary, this work demonstrates that Faradaic processes associated with residual iron catalyst can be used to enhance the capacitance of MWNTs beyond their EDLC limitations. When subjected to electrochemical potentials above the electrolysis limit in acidic solutions, MWNT electrodes are physically stressed, and then subsequently rupture to expose the encapsulated Fe nanoparticles. The interaction between Fe nanoparticles and electrolyte leads to rapid oxidation and the manifestation of a stable Fe2+/Fe3+ redox couple, thereby improving the electrochemical performance of MWNT-NTL relative to MWNT-CT electrodes. These results provide motivation for further investigating the electrochemical activity of carbon nanomaterials, such as single wall carbon nanotubes (SWNT), with other catalysts or impurities as a simple design approach to increasing the performance of electrode materials.

AUTHOR INFORMATION Corresponding Author *Dr. Mark E. Roberts, [email protected] *Dr. Apparao M. Rao, [email protected] Funding Sources NSF Scalable Nanomanufacuring grant CMMI 1246800 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


14

Page 15 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MER and AMR acknowledge NSF Scalable Nanomanufacuring grant CMMI 1246800 and MER acknowledges 3M for support through their NTFG Award.

ASSOCIATED CONTENT Supporting Information. (i) Materials, (ii) Fabrication of MWNT buckypaper, (iii) Experimental Details for Scanning Electron Microscopy, Thermal Gravimetric Analysis, Energy Dispersive X-ray Spectrometry, (iv) Electrochemical analysis including Cyclic Voltammetry, Electrical Impedance Spectroscopy and Charge-Discharge measurements. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES 1. Dunn, B.; Kamath, H.; Tarascon, J.M. Electrical Energy Storage for the Grid: A Battery of Choices. Science. 2011, 334, 928-935. 2. Thackeray, M.M.; Wolverton, C.; Isaacs, E.D. Electrical Energy Storage for Transportation—Approaching the Limits of, and Going Beyond, Lithium-Ion Batteries. Energy & Environ. Sci. 2012, 5, 7854-7863. 3. Dai, L.; Chang, D.W.; Baek, J.B.; Lu, W. Carbon Nanomaterials for Advanced Energy Conversion and Storage. Small. 2012, 8, 1130-1166. 4. Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nature Mater. 2008, 7, 845-854.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 19

5. Sun, Y; Wu, Q; Shi, G. Graphene Based New Energy Materials. Energy & Environ. Sci. 2011, 4, 1113-1132. 6. Khaligh A.; Li, Z. Battery, Ultracapacitor, Fuel-Cell, and Hybrid Energy Storage Systems for Electric, Hybrid Electric, Fuel Cell, and Plug-In Hybrid Electric Vehicles: State of the Art. IEEE Trans. Veh. Technol. 2010, 59, 2806-2814. 7. Zhai, Y.; Dou, Y; Zhao, D; Fulvio, P.F.; Mayes, R.T.; Dai, S. Carbon Materials for Chemical Capacitive Energy Storage. Adv. Mater. 2011, 23, 4828-4850. 8. Merlet, C.; Rotenberg, B.; Madden, P.A.; Taberna, P.L.; Simon, P.; Gogotsi, Y.; Salanne, M. On the Molecular Origin of Supercapacitance in Nanoporous Carbon Electrodes. Nature Mater. 2012, 11, 306-310. 9. Burke, A. R&D Considerations for the Performance and Application of Electrochemical Capacitors. Electrochim. Acta. 2007, 53, 1083-1091. 10. Lu, W.; Dai, L. Carbon Nanotube Supercapacitors. In Carbon Nanotubes. Marulanda, J.M., Ed.; ISBN: 978-953-307-054-4, InTech, DOI: 10.5772/39444. 11. Kim, D.W.; Kim, K.S.; Park, S.J. Synthesis and Electrochemical Performance of Polypyrrole- Coated Iron Oxide/Carbon Nanotube Composites. Carbon Lett. 2012, 13, 157160. 12. Pumera, M. The Electrochemistry of Carbon Nanotubes: Fundamentals and Applications. Chem. Eur. J. 2009, 15, 2970-2978. 13. Banks, C.E.; Crossley, A.; Slater, C.; Wilkins, S.J.; Compton, R.G. Carbon Nanotubes Contain Metal Impurities Which are Responsible for the “Electrocatalysis” Seen at Some Nanotube-Modified Electrodes. Angew. Chem. 2006, 45, 2533-2537.


16

Page 17 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14. Yuan, W.; Zhou, Y.; Li, Y.; Li, C.; Peng, H.; Zhang, J.; Liu, Z.; Dai, L.; Shi, G. The Edgeand Basal-Plane-Specific Electrochemistry of a Single-Layer Graphene Sheet. Sci. Rep. 2013, 3, 1-7. 15. Kim, H.; Kaufman, M.J.; Sigmund, W.M.; Jacques, D.; Andrews, R. Observation and Formation Mechanism of Stable Face-Centered-Cubic Nanorods in Carbon Nanotubes. J. Mater. Res. 2003, 18, 1104-1108. 16. Azam, M.A.; Manaf, N.S.A.; Talib, E.; Bistamam, M.S.A. Aligned Carbon Nanotube from Catalytic Chemical Vapor Deposition Technique for Energy Storage Device: A Review. Ionics. 2013, 19, 1455-1476. 17. Lota, G.; Fic, K.; Frackowiak, E. Carbon Nanotubes and Their Composites in Electrochemical Applications. Energy Environ. Sci. 2011, 4, 1592-1605. 18. Cheng, Y.; Liu, J. Carbon Nanomaterials for Flexible Energy Storage. Mater. Res. Lett. 2013, 1, 175-192. 19. Bahr, J.L.; Yang, J.; Kosynkin, D.V.; Bronikowski, M.J.; Smalley, R.E.; Tour, J.M. Functionalization of Carbon Nanotubes by Electrochemical Reduction of Aryl Diazonium Salts: A Bucky Paper Electrode. J. Am. Chem. Soc. 2001, 123, 6536-6542. 20. Meng, C.; Liu, C.; Fan, S. Flexible Carbon Nanotube/Polyaniline Paper-Like Films and Their Enhanced Electrochemical Properties. Electrochem. Comm. 2009, 11, 186-189. 21. Sinnott, S.B.; Andrews, R.; Qian, D.; Rao, A.M.; Mao, Z.; Dickey, E.C.; Derbyshire, F. Model of Carbon Nanotube Growth Through Chemical Vapor Deposition. Chem. Phys. Lett. 1999, 315, 25-30.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 19

22. Khasanov, A.; He, J.; Gaillard, J.; Yang, K.; Rao, A.M.; Cameron, C.M.; Schmeltzer, J.M.M.; Stevens, J.G.; Nath, A. The Role of γ-Iron Nanoparticulates in the Growth of Carbon Nanotubes. Appl. Phys. Lett. 2008, 93, 013103-1-3.


18

Page 19 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC GRAPHIC (2” x 2”)


19

Can Faradaic Processes in Residual Iron Catalyst Help Overcome

Recommend Documents