Effects of Nanowire Length on Charge Transport in Vertically Aligned

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Effects of Nanowire Length on Charge Transport in Vertically Aligned Gold Nanowire Array Electrodes Hideyuki Nakanishi, Ikuo Kikuta, Satoshi Teraji, Tomohisa Norisuye, and Qui Tran-Cong-Miyata Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03089 • Publication Date (Web): 28 Nov 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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Effects of Nanowire Length on Charge Transport in Vertically Aligned Gold Nanowire Array Electrodes Hideyuki Nakanishi,* Ikuo Kikuta, Satoshi Teraji, Tomohisa Norisuye, and Qui Tran-CongMiyata Department of Macromolecular Science and Engineering, Graduate School of Science and Technology, Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japan. KEYWORDS: energy storage, supercapacitor, battery, current collector, electric double-layer capacitance

ABSTRACT: In this study, we demonstrate that vertically aligned gold nanowire array electrodes provide rapid ion and electron transport to the electrode-electrolyte interface. The charge-transport properties of the nanowire electrodes were investigated through cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy under a constant-volume device configuration. The total charge stored in the corresponding devices increases monotonically with the length of the nanowires owing to the concomitant increase in the electroactive real surface area of the electrode. A remarkable feature of the electrodes is that the internal resistance associated with charge transport decreases with increasing nanowire length. The electric double-layer capacitance per unit electroactive surface

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area remains constant up to high charge/discharge rates. Our results demonstrate that charge migration occurs rapidly on the surfaces of the nanowires regardless of their length and the charge/discharge rate used. Thus, vertically aligned nanowire array electrodes show promise as current collectors for next-generation electrochemical energy-storage devices.

1. INTRODUCTION Electrochemical energy-storage devices such as supercapacitors and batteries have attracted increasing research interest in recent years owing to their importance in the development of portable electronics and large-scale power sources for next-generation grid energy-storage systems. A large number of active materials, including metal oxides, conductive polymers, and 1

2

composite hybrids, have been investigated to achieve higher charge capacities than those 3-6

currently available. However, most active materials have low ionic and electronic conductivity. Unfortunately, charge transport in the active layer is often a rate-limiting process; accordingly, the charge capacities of devices decrease with increasing charge/discharge rates. Thus, to develop electrodes with improved rate capabilities, the creation of pathways therein that efficiently transport charge carriers and collect electrons from the active material is desired. In this regard, the surface structure of the current collector that contacts the active material plays a central role in charge transport, and is a dominant factor in the rational design of charge-transport pathways and, consequently, in determining device performance. The energy and power densities of devices are largely dependent on the choice of the current collector, even when the same active materials are used. In particular, electrodes 7-9

fabricated with interpenetrating three-dimensional (3D) ion and electron transport pathways have


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been proven to exhibit charge-transport properties superior to those of their planar counterparts.

10-

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These electrodes are prepared through the deposition of the active layer onto suitable current

collectors with stereoscopic 3D surface structures (henceforth, “3D current collectors”) such as copper pillar arrays, nickel foams, 13

materials,

24-25

14-18

and nanoporous gold.

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stainless steel meshes,

19-20

polymer scaffolds,

21-23

carbonaceous

Alternatively, gold-nanowire current collectors can provide

superior performance in combination with active materials such as carbon nanotubes and manganese dioxides.

29-30

Nevertheless, the charge capacity per unit mass of the active material

often decreases with an increase in the loading of the active material.

31-32

This adverse effect is

most likely related to the increased charge-transport path length on thickening of the active layer. To circumvent this limitation, 3D current collectors should be engineered in such a way as to increase their volumes and surface areas, i.e., the height and roughness factor (R.F.), so that they can be coated with large amounts of thinner active layers. However, increasing the volume and surface area of a current collector inevitably influences the overall charge-transport properties of the corresponding device, often resulting in unfavorable effects on device performance. In addition, as regards strategies for improving the charge capacities of devices, parameters that influence the performances of 3D current collectors have been studied much less extensively than the development of new types of active material. In this study, we demontrate that ion and electron transport in vertically aligned gold nanowire (AuNW) array electrodes is extremely fast, allowing the charge carriers to migrate to the nanowire surfaces and form electric double layers (EDLs), even at the high charge/discharge rates of 500 V/s and ≈ 5000 mA/cm , respectively. A remarkable feature of the electrodes is their 3

ability to maintain fast charge transport even though the nanowire lengths are extended by up to 19 µm within a 54-µm-thick device. The internal resistances of nanowire-based electrochemical


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devices were assessed using direct current (DC) and alternating current methods, indicating that the equivalent series resistance of a device decreases with increasing nanowire length.

2. EXPERIMENTAL SECTION 2.1. Preparation of gold films. Commercial anodic aluminum oxides (Synkera Technologies) were used as templates to prepare vertically aligned AuNW array electrodes. Gold was sputtered onto one side of the anodic aluminum oxide to form a thin, electrically conductive gold layer. Following this procedure, a 2-µm-thick gold layer was electrodeposited onto the sputtered gold to create a robust, flexible gold film. The gold electrodeposition was performed using a potentiostat/galvanostat (VersaSTAT4, Princeton Applied Research) with a threeelectrode setup. A Ag/AgCl electrode containing saturated potassium chloride and a platinum mesh were used as the reference and counter electrodes, respectively. The electrolytic solution contained 0.74 wt% potassium dicyanoaurate (I) and 7.0 wt% trisodium citrate. A constant voltage of -1.0 V was applied during gold electrodeposition. The film thickness (2 µm) was controlled by monitoring the electrodeposition time. 2.2. Preparation of AuNWs. The AuNWs were formed utilizing the same electrodeposition technique as that used above, allowing the cylindrical pores of the anodic aluminum oxide template to be filled with the electrolyte solution. The AuNWs were electrodeposited onto the surface of the underlying gold film through the pores of the template. During the electrodeposition, the length of the AuNWs was tuned by controlling the electrodeposition time. Finally, the anodic aluminum oxide template was dissolved in a


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concentrated sodium hydroxide solution (≈10 M), allowing the fabrication of free-standing AuNW array electrodes. 2.3. Scanning electron microscopy measurements. Cross-sectional scanning electron microscopy (SEM) images of the vertically aligned AuNW array electrodes were obtained using a field-emission scanning electron microscope (JSM-7600F, JEOL) under an accelerating voltage of 15 kV. Before the measurements, the electrodes were cut into pieces to allow examination of their cross sections. 2.4. Evaluation of electroactive surface area. The AuNW electrodeposition area was maintained at 0.48 cm using a Teflon O-ring. The R.F. of the thus-prepared AuNW array 2

electrodes was determined by dividing the electroactive real surface areas (ESRAs) of the electrodes by their projected area (0.48 cm ). 2

2.5. Device assembly. The electrochemical devices used to assess charge-transport properties were assembled by placing an 8-µm-thick separator between two identical AuNW array electrodes. The devices were filled with 2 M Li SO solution. The thicknesses (volumes) of 2

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the devices were kept constant at 54 µm (2.6 × 10 cm ) using a 50-µm-thick Teflon spacer. -3

3

2.6. Characterization. The properties and performances of the thus-assembled devices were assessed by cyclic voltammetry, galvanostatic charge/discharge analysis, and electrochemical impedance spectroscopy (EIS) using the same potentiostat/galvanostat (VersaSTAT4, Princeton Applied Research) as that used above. The capacitances (C) of the devices were determined from galvanostatic charge/discharge measurements using the relationship C = i/(dV/dt), where i and dV/dt are the charge/discharge current (in amperes) and voltage variation over time (in volts per second), respectively. In the present study, a single


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device consists of two identical electrodes, and an EDL forms on each electrode, accordingly. Thus, the double-layer capacitance (C ) was derived using the equation C EDLs

EDLs

= 2 × C/ERSA,

where the ERSA for a single electrode was used for the calculation. EIS was performed under open-circuit conditions by applying a sinusoidal voltage of 10 mV amplitude at frequencies ranging from 100 kHz to 10 mHz.

3. RESULTS AND DISCUSSION The AuNW arrays were electrodeposited on gold films through the cylindrical pores of an anodic aluminum oxide template with the nanowire length being dictated by careful monitoring of the electrodeposition time. In the final step of the array preparation, the template was removed using an alkaline solution. Thus, free-standing, vertically aligned AuNW array electrodes were prepared (see the Experimental section for details and Figure S1 for the photos of the electrode; the method can also be used with affordable metals such as nickel ). Figure 1a shows a cross32

sectional SEM image of an 8-µm-long AuNW array electrode. The average nanowire diameter is 180 nm, as demonstrated by the inset in Figure 1a. We connected the AuNWs directly to the gold films to improve charge transport under the assumption that the electrodeposited nanowire junctions would be electrically conductive and would effectively enhance electron flow across the interfaces of the AuNWs and the underlying gold film. Furthermore, the vertically aligned AuNWs provide the device with extremely short ion and electron transport pathways to the electrode-electrolyte interface. All the electrode structures were rationally designed and engineered to reduce the ionic and electronic resistances associated with charge transport. As a result, we systematically increased the average AuNW length (L) from 8 to 19 µm, thus


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increasing the electrode surface area (Figure 1a-c). The surface areas of electrodes are often characterized by R.F., which is defined as the ratio of the actual surface area of the electrode to its projected area. To determine the R.F., we performed cyclic voltammetry analysis using a three-electrode system, thereby measuring the ERSA of the AuNW array electrodes. Figure 1d shows the cyclic voltammograms (CVs) that we used to derive the ERSA values. During the voltage sweep in the positive direction, a large anodic current began to flow at around 1.15 V. This anodic current is ascribed to the formation of gold surface oxides. These gold oxides are subsequently removed by sweeping the voltage in the negative direction, as evidenced by the cathodic current peaks presented near 0.85 V, which indicate gold oxide reduction. It has been reported that the charge required for gold oxide reduction is 390 µC/cm for polycrystalline 2

gold. Based on this value, we evaluated the ERSA from the charge consumption during oxide 33

reduction. Thus, the R.F. values of the AuNW arrays were determined as shown in the inset of Figure 1d. R.F. is linearly related to the length of the AuNWs, reaching its highest value of 181 for the 19-µm-long AuNWs.


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Figure 1. (a-c) Cross-sectional SEM images of the vertically aligned AuNW array electrodes with nanowire lengths (L) of (a) 8 µm, (b) 16 µm, and (c) 19 µm. The inset in (a) shows the magnified image of a single nanowire. (d) CVs used for the evaluation of the ERSA of the nanowire array electrodes. The voltage sweep rate was 100 mV/s. Sulfuric acid (0.5 M) was used. A Ag/AgCl electrode with saturated KCl solution and a Pt mesh were used as the reference and counter electrodes, respectively. The inset in (d) shows the electrode roughness factor (R.F.) determined from reduction of the gold surface oxides. The values of R.F. are 91, 140, and 181 (with corresponding ERSAs of 43, 66, and 86 cm ) for L = 8, 16, and 19 µm, respectively. 2


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For all the experiments, we set two identical AuNW array electrodes across a 50 µm gap to maintain the device volume constant regardless of AuNW length (Figure 2a). For the thusassembled constant-volume device, the charge transport of the AuNW array electrodes was examined with respect to nanowire length. Figure 2b-d shows the CVs obtained for the 8-µm-, 16-µm-, and 19−µm-long AuNWs. All the CVs show nearly rectangular shapes, which is an indication of the ideal capacitive property of the electrodes. The observed capacitive property is due to the EDL formation associated with ion and electron transport to the AuNW-electrolyte interface. A remarkable feature is that the rectangular CVs are sustained up to the extremely fast scan rate of 500 V/s. Although active materials are not deposited on the AuNWs, for reference, this scan rate is approximately two orders of magnitude faster than those for typical supercapacitors,

34-35

and is comparable or even superior to the ultrahigh-power supercapacitors

based on onion-like carbon . This indicates that ion and electron migration to the electrode 36

surface occurs even at such high scan rates, demonstrating the fast charge-transport capability of the AuNW array electrodes. In contrast, rectangular CVs generally collapse, and thus, their inner areas decrease with increasing scan rate because charge migration to the nanowire surface is delayed under rapid voltage change. To further examine the charge-transport properties of the electrodes, the charge current (i) was plotted against the scan rate (dV/dt), as shown in Figure 2e. The plots indicate nearly linear relationships between i and dV/dt regardless of AuNW length. The charge current (i.e., slope of the current vs. scan rate) increases linearly on elongating the AuNWs from 8 to 19 µm. These observations indicate that the device is governed up to 500 V/s by the purely capacitive component (C) characterized by the relationship i = C × dV/dt. Extension of the AuNWs does not interrupt the overall charge transport in the constant-volume devices examined. An increase in the ERSAs of the AuNWs (i.e., the electrode R.F.) favorably


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increases the amount of charge carriers stored in the device without sacrificing the rate capability, as evidenced by the increased charge current for L = 19 µm.


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Figure 2. (a) Schematic illustration of device configuration. (b-d) CVs obtained for devices with nanowire lengths (L) of (b) 8 µm, (c) 16 µm, and (d) 19 µm. (e) Charge current obtained at 1.2 V vs. scan rate.


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To obtain further insight into their ion transport behaviors, the AuNW array electrodes were tested by means of galvanostatic charge/discharge (CC) measurement in which ion migration to the electrode surfaces was examined under constant electron flow. Figure 3a shows the CC curve obtained for the 8-µm-long AuNWs. Voltage varies linearly with charge/discharge time (i.e., dV/dt is constant). The linear CC plot indicates the occurrence of ion migration to the AuNW surfaces and concomitant EDL formation in response to the constant electron flow. Similar CC plots are observed for the AuNWs with L = 16 and 19 µm (Figure 3b,c). Although no noticeable differences in the shape of the CC curves are observed with the current density used (I = 980 mA/cm ), the time required for charging and discharging (for five cycles) increases with 3

AuNW length (2.7, 4.1, and 4.9 s for L = 8, 16, and 19 µm, respectively). This indicates an increase in the total number of ions that are delivered to the AuNW surface. To examine the efficiency of the ion transport, we evaluated the EDL capacitance (C ) EDLs

per unit ERSA of the AuNWs. Figure 3d shows plots of C

EDLs

The C

EDLs

vs. charge/discharge current density.

curves are almost superimposable. The ion transport in the AuNW arrays is most likely

governed by a transport mechanism that is analogous across all samples (i.e., L = 8, 16, and 19 µm). Although the C

EDLs

values are slightly higher for the low current densities (possibly due to

the contribution of a Faradaic process), they remain relatively constant at 23-28 µF/cm up to the 2

high charge/discharge current density of ≈5000 mA/cm . These values of C 3

EDLs

are appropriate for

the widely acknowledged double-layer capacitance of gold. Owing to the invariant values for 37

C , the specific (stack and areal) capacitance of the devices are monotonically improved by EDLs

increasing L (ERSA) (Figure S2). The results suggest that ion migration occurs to virtually all the AuNW surfaces and is not limited by the extension of the AuNWs. Additionally, the


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capacitance shows no noticeable degradation during the voltage sweep for 10000 cycles (Figure S3).


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Figure 3. (a-c) Galvanostatic charge/discharge curves obtained for a current density of 980 mA/cm . The lengths of the nanowires were (a) 8 µm, (b) 16 µm, and (c) 19 µm. (d) Electric 3

double-layer capacitance (C ) vs. current density. EDLs


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Internal resistance is a measure of the charge-transport properties of the whole device. It causes undesirable effects such as poor charge transport and heat loss, and thus deteriorates device performance parameters such as energy and power density. Hence, device structures must be considered in such a way as to decrease their internal resistances. In general, the internal resistance of devices comprises multiple contributions involving electron conduction across electrode interfaces (in the case of particulate electrodes) and ion diffusion in the electrode and electrolyte. These contributions are often represented by the equivalent series resistance (R ). esr

Figure 4a shows the CC curves obtained at the high charge/discharge rate of 2730 mA/cm . The 3

voltage suddenly drops on switching the current polarity from charging to discharging. This voltage drop (V ) is caused by R , and these two parameters are directly related by the drop

esr

expression V = R × 2i, where i is the charge/discharge current. Figure 4b shows the plots of V drop

esr

drop

vs. i. V varies linearly with i, according to the expression above, and their slopes become less drop

steep with increasing AuNW length. It should be noted that the device volume was kept constant for all experiments, with the total thickness of the bulk electrolyte and AuNW array layers preserved at 54 µm. Under these particular conditions, two major resistive components (resistance in the electrode and in the bulk electrolyte layers) systematically changes with the ratio of these layers, which is dictated by the AuNW length (Figure 4c). The change in the AuNW length alters the relative (resistive) contributions of the nanowire arrays and bulk electrolyte layers to their combined resistance of the devices. With this in mind, R values were esr

evaluated from the slopes of the fitted straight lines shown in Figure 4b and are compared with the length of the AuNWs (Figure 4d). Unexpectedly, R decreases with increasing AuNW length. esr

Thus, elongation of the AuNWs favorably influences the ion and electron transport in the


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constant-volume devices. The reduction of the R leads to an improvement of the energy and esr

power densities of the devices (Figure S4).


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Figure 4. (a) Galvanostatic charge/discharge curves obtained for a current density of 2730 mA/cm . V denotes the voltage drop that arises on switching from charging to discharging. (b) 3

drop

Variation of V

drop

with current (alternatively, current density). (c) Schematic illustration of the

devices comprising short and long nanowire array electrodes. (d) Equivalent series resistance (R ) vs. nanowire length (L). esr


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We also investigated the devices using EIS and examined the effect of AuNW length on R . Figure 5a shows the frequency dependence of the impedance phase angle for AuNWs esr

ranging from L = 8 to 19 µm. The devices exhibit capacitive properties with phase angles close to -90˚ in the low-frequency region, but they become resistive at high frequency because of the effect of internal resistance. The charge-transport properties of devices are often examined with reference to the characteristic frequency (f ) at which the magnitudes of the capacitive and 0

resistive impedance match at a phase angle of -45˚. f is approximately 900, 500, and 300 Hz (the 0

corresponding time constants are 1.1, 2.0, and 3.3 ms) for the 8-µm-, 16-µm-, and 19-µm-long AuNWs, respectively. The observed time constants are quite low compared with those for other porous electrode systems (e.g., ≈1 s for typical electric double-layer capacitors ), indicating the 38

lowered internal resistance of the AuNW-based electrodes. Figure 5b shows the complex plane plots of the impedance. With increasing frequency, the plots approach the real axis along the imaginary axis, and they intersect with the real axis nearly vertically. In this way, the impedance fit a near-vertical line, that indicates the devices can be characterized by a series-RC circuit model. In this model, 38

the impedance at the intersection of the plots and the real axis

corresponds to the R value of the device. The intersection shifts in the direction of the original esr

point on increasing the AuNW length. This confirms the reduction of R on elongation of the esr

AuNWs, showing qualitative agreement with the results obtained by the DC method (i.e. Figure 4a). In general, with an increase in the thickness of porous electrodes, the value of RC time constant is distributed inside pores, and varies along the direction from the top (near bulk electrolyte layer) to the bottom of electrodes. The distribution of the RC time constants can be attributed to the ionic resistance in the pores. In such cases, the electrode surfaces deep inside the


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pores cannot form EDLs in the high-frequency region because of the slow ion transport. This behavior is widely known and is reproduced by the transmission line model (TLM), where the plots of impedance on the complex plane are inclined at an angle of 45º in the high-frequency region.

39-41

However, the impedance shown in Figure 5b does not exhibit such an incline, and the

plots nearly fit a series-RC circuit model instead of the TLM. With this frequency dependence of the impedance, C

EDLs

remained constant at high charge/discharge rates (Figure 3d), and R

esr

decreased as the AuNW length increased (Figure 4d). Nevertheless, these trends in the capacitance and internal resistance might change due to the aforementioned porous electrode behavior, if the AuNW length is increased using devices thicker than those used in the present study. We believe that the upper limit of the AuNW length that will still enhance device performance is affected by the diameter of the AuNWs, and the pore features. These factors are related to the ionic resistance in the pores of the AuNW arrays.


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Figure 5. (a) Bode plots (phase angle vs. frequency) for the devices assembled with 8-µm-, 16µm-, and 19-µm-long nanowires. (b) Nyquist plots (complex plane plots) of the impedances of the same devices. The inset shows magnification of the high-frequency region.


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4. CONCLUSIONS The effects of AuNW length on charge transport were examined in constant-volume devices fabricated with vertically aligned AuNW array electrodes. The ERSAs, which are related to the R.F. values of the AuNW electrodes, increase linearly with AuNW length. The double-layer capacitance per unit ERSA exhibits a constant value of 23-28 µF/cm , demonstrating that charge 2

migration to all AuNW surfaces occurs, irrespective of AuNW length or charge/discharge rate. Besides the improved ERSA, another remarkable effect of the extended AuNWs is the lowering of the internal resistances of the devices and improvement in charge transport. We believe that the present study demontrates the importance of designing overall device structures, including device volume and electrode thickness, to enhance ion and electron transport. Thus, the vertically aligned AuNWs allow fast ion and electron transport, and they would be suitable for use as current collectors for electrochemical energy-storage devices.

ASSOCIATED CONTENT

Supporting Information.

The following files are available free of charge.

Photos of the electrode, specific (stack and areal) capacitance, capacitance retention, and Ragone plot (PDF)

AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected] (H.N.). Author Contributions I.K. and S.T. conducted the experiments and data analysis and created the figures. Q.T.-C.-M. and T.N. revised the manuscript. H.N. conceived the experiments and wrote the paper. The manuscript was written through contributions from all authors. All authors have approved the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT H.N. acknowledges funding support from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) via JSPS KAKENHI (grant nos. JP16K13627 and JP18H01828), the Ogasawara Foundation for the Promotion of Science & Engineering, and the Iwatani Naoji Foundation. REFERENCES (1) Jiang, J.; Li, Y.; Liu, J.; Huang, X.; Yuan, C.; Lou, X. W. Recent advances in metal oxidebased electrode architecture design for electrochemical energy storage. Adv. Mater. 2012, 24, 5166-5180. (2) Snook, G. A.; Kao, P.; Best, A. S. Conducting-polymer-based supercapacitor devices and electrodes. J. Power Sources 2011, 196, 1-12.


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Effects of Nanowire Length on Charge Transport in Vertically Aligned

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