Metal Nanowire-Based Hybrid Electrodes Exhibiting High Charge

Subscriber access provided by UNIV OF UTAH

Article

Metal Nanowire-Based Hybrid Electrodes Exhibiting High Charge/Discharge Rates and Long-Lived Electrocatalysis Rakesh K Pandey, Yuto Kawabata, Satoshi Teraji, Tomohisa Norisuye, Qui Tran-Cong-Miyata, Siowling Soh, and Hideyuki Nakanishi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07794 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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.

ACS Applied Materials & Interfaces 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 22

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

ACS Applied Materials & Interfaces

Metal

Nanowire-Based

Hybrid

Electrodes

Exhibiting High Charge/Discharge Rates and LongLived Electrocatalysis Rakesh, K. Pandey,†,‡,§ Yuto, Kawabata,†,‡ Satoshi, Teraji,‡ Tomohisa Norisuye,‡ Qui TranCong-Miyata,‡ Siowling Soh,*,§ and Hideyuki Nakanishi*,‡ ‡

Department of Macromolecular Science and Engineering, Graduate School of Science and

Technology, Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japan. §

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4

Engineering Drive 4, Singapore 117585, Singapore. KEYWORDS electrochemical energy storage, capacitance, rate performance, ethanol electrooxidation, conducting polymer, catalytic activity

ABSTRACT

Nanostructured electrodes are at the forefront of advanced materials research, and have been studied extensively in the context of their potential applications in energy storage and

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 22

conversion. Here, we report on the properties of core-shell (gold-polypyrrole) hybrid nanowires and their suitability as electrodes in electrochemical capacitors and as electrocatalysts. In general, the specific capacitance of electrochemical capacitors can be increased by faradaic reactions, but their charge transfer resistance impedes charge transport, decreasing the capacitance with increasing charge/discharge rate. The specific capacitance of the hybrid electrodes is enhanced due to the pseudocapacitance of the polypyrrole shells; moreover, the electrodes operate as an ideal capacitive element and maintain their specific capacitance even at fast charge/discharge rates of 4690 mA/cm3 and 10 V/s. These rates far exceed those of other types of pseudocapacitors, and are even superior to electric double layer-based supercapacitors. The mechanisms behind these fast charge/discharge rates are elucidated by electrochemical impedance spectroscopy, and are ascribed to the reduced internal resistance associated with the fast charge transport ability of the gold nanowire cores, low ionic resistance of the polypyrrole shells, and enhanced electron transport across the nanowire’s junctions. Furthermore, the hybrid electrodes show great catalytic activity for ethanol electro-oxidation, comparable to bare gold nanowires, and the surface activity of gold cores is not affected by the polypyrrole coating. The electrodes exhibit improved stability for electrocatalysis during potential cycling. This study demonstrates that the gold-polypyrrole hybrid electrodes can store and deliver charge at fast rates, and that the polypyrrole shells of the nanowires extend the catalytic lifetime of the gold cores.

INTRODUCTION


2

Page 3 of 22

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


Nanowires (NWs) have been of great interest for use as electrodes for electrochemical capacitors (ECs)1-3 and photoelectrochemical cells4-5, since their one-dimensional (1D) nanostructures create short diffusion pathways for ions and excitons, leading to an increase in charge storage capacity and photoelectric conversion efficiency. Metal oxides have been extensively studied as electrode materials for these devices, and formed into complex NW structures, giving rise to intriguing functionality and properties (e.g., core-shell p-n junction allowing efficient charge separation, and radially grown NWs on fibrils for wearable energy devices).6-7 On the other hand, metal NWs, and hybrid electrodes using metal NWs and conducting polymers, have not been investigated nearly as extensively, even though metals exhibit high electrical conductivity, preventing energy dissipation,8-9 and have the ability to catalyze electrochemical reactions.10-11 Here, we demonstrate that gold (Au) NWs hybridized with polypyrrole (Ppy) exhibit enhanced electrochemical energy storage and electrocatalysis. Ppy is a promising pseudocapacitive material with high electrical conductivity that is environmentally friendly.12 Our electrodes are based on arrays of AuNWs covered with a thin layer of Ppy, forming a core-shell (Au-Ppy) NW structure. When characterized for use in ECs, these hybrid NW electrodes exhibit a dramatic increase in capacitance, due to the pseudocapacitance of Ppy, while their electrical resistance is remarkably low, allowing for ECs with simultaneously high energy and power densities. When used for the electro-oxidation of ethanol, the outer Ppy layer imparts a longer-lived catalytic activity to the Au cores, over repeated cycles of potential sweep. Achieving a balanced combination of high energy and power densities in ECs, and extending the lifetime of catalytic activity in noble metals, have been major challenges in developing high-performance ECs and electrocatalysts. In this work, we demonstrate the unique characteristics of hybrid Au-PpyNW electrodes, which represent significant progress toward meeting these challenges.


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 22

RESULTS AND DISCUSSION We prepared arrays of AuNWs by electrodepositing Au through the cylindrical pores of anodic aluminum oxides, AAOs, on a preformed underlying Au film.8 After removing the AAOs, pyrrole was electropolymerized on the AuNWs to form the outer layer of Ppy. This procedure produced arrays of hybrid NWs, with a core-shell nanostructure, with an Au core and a Ppy shell. The length and diameter of the inner AuNWs were modified by varying the time for the electrodeposition and the pore diameter of AAO, respectively. The thickness of the outer Ppy was tuned by monitoring the time for the electropolymerization. To demonstrate the level of control in tailoring the nanoscale structure of the hybrid Au-PpyNW electrodes, we present scanning electron microscopy (SEM) images, and energy dispersive spectroscopy (EDS) element maps, for a typical hybrid NW array fabricated for use in an EC (Figure 1). The height of the array (~ 20 µm) was uniform over the entire electrode, and the AuNW cores were grown directly from, and aligned perpendicularly to the underlying film (Figure 1a,b). Unlike granular electrodes, the hybrid NW electrodes are a flexible, free-standing, and monolithic film, fabricated without the use of binders and additives. A close-up SEM image reveals a thin (~ 10 nm), uniform coating of Ppy over the AuNWs (Figure 1c), and EDS elemental mapping of a single hybrid NW confirms the formation of the core-shell NW structure (Figure 1d-f).


4

Page 5 of 22

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. (a,b) SEM images of the arrays of the hybrid NWs. (c) A close-up of the NWs shows the thin layer of Ppy formed on the AuNWs. The average diameter of the hybrid NWs is 200 nm. (d-f) EDS elemental mapping of a single hybrid NW with corresponding SEM image.

An EC was assembled, by placing an ion-permeable separator (8 µm thick porous membrane, Whatman) between two identical electrodes, using a 50 µm thick teflon spacer, and filling the space between the electrodes with 2 M KCl electrolyte solution. To determine the effects of hybridization through control, we also tested electrodes fabricated with pristine AuNWs (Figure S1), and planar Au electrodes with an electropolymerized Ppy film on their surface prepared at the same charge consumption used in fabricating the hybrid Au-PpyNW electrodes (the thickness of the Ppy film is approximately 1-2 µm, as determined by the cross-sectional SEM image and EDS elemental mapping shown in Figure S2).


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 22

An ideal EC can be characterized by the relation, I = C × dV/dt, where I, C, and dV/dt, are current (alternatively, current density), capacitance (specific capacitance), and time rate of change in voltage, respectively. In reality, most ECs deviate from this ideal capacitive behavior, due to irreversible reactions, self-discharging, and the equivalent series resistance of the electrodes.13 Among these undesirable contributions, the equivalent series resistance (internal resistance) is one of the most important factors dominating the actual performance of ECs, and it causes adverse effects, such as energy dissipation (ie. Joule heating) and the slowing of charge transport (related to changes in the equivalent RC time constant). In order to improve the performance of ECs, much effort has been devoted to reducing their internal resistance, through the modification of electrode materials and structures.14-16 With this in mind, we characterized our electrodes by galvanostatic charge/discharge (CC) measurement. Figure 2a shows CC curves obtained at a slow charge/discharge rate of I = |± 390| mA/cm3. The Au-PpyNW electrode exhibits a symmetric triangular CC curve, with a linear voltage variation (|dV/dt| = const.), indicating that the EC operates as an ideal capacitive element. On the other hand, the voltage suddenly rises and drops when the Au/Ppy film begins charging and discharging (at 0 s and 12.2 s), respectively. Such sudden changes in voltage are caused by a large internal resistance,13 and become more visible at a faster charge/discharge rate of I = |± 4690| mA/cm3 (Figure 2b). Compared with the Au/Ppy film electrode, the Au-PpyNW electrode retains its ideal capacitive behavior, even at the faster rate, demonstrating its reduced internal resistance. Figure 2c shows the rate dependence of C, determined from the CC curves. At slow rates, the Au-PpyNW and Au/Ppy film electrodes show similar values for C (7.4 F/cm3 and 6.5 F/cm3 at 160 mA/cm3, respectively), and both of the electrodes exhibit their maximum charge storage capacity (the specific capacitance at slow rates is most likely determined by the amount of Ppy; when the


6

Page 7 of 22

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


thickness of the Ppy for the Au/Ppy film is decreased from 1 ~ 2 µm to 10 ~ 20 nm, the specific capacitance is decreased accordingly with the Ppy thickness, as shown in Figure S3). The value of C for the Au/Ppy film electrode, however, declines with increasing rate. This apparent decrease in C is possibly due to a process that limits charge transport, which manifests itself as a large internal resistance. Analogous trends have also been reported for systems of agglomerated, granular electrodes, such as activated carbon using binders.17 Similarly, metal oxide NW-based ECs suffer from poor rate performance (reduction of C in response to increasing charge/discharge rate) due to their low electrical conductivity, although they achieve substantial


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 22

Figure 2. (a,b) CC curves obtained by charging/discharging at current densities of (a) 390 mA/cm3, and (b) 4690 mA/cm3. (c) Specific capacitance, C, evaluated from the CC curves. (d-f) CVs for (d) pristine AuNW, (e) hybrid Au-PpyNW, and (f) Au/Ppy film. (g) Charge current versus scan rate. (h) C determined from the CVs.

specific capacitance at slow charge/discharge rates (e.g., 7 mF/cm2 at 0.2 mA/cm2 for Fe3O4@SnO2 core-shell nanorods,18 and 27 mF/cm2 at 0.2 mA/cm2 for ZnO@MnO2 core-shell NWs19, and more data available in the literature20). Similar trends have been also observed in asymmetric supercapacitors when the charge/discharge rates are increased.21-22 The Au-PpyNW electrode is distinguished by its excellent rate performance, which is even superior to leadingedge electric double layer (EDL) capacitance-based graphene electrodes (Figure 2c).17 The AuPpyNW electrodes exhibit a value for C (7.0 F/cm3 at 4690 mA/cm3, or 37 mF/cm2 at 25 mA/cm2) that is 8.4 times higher than that for the pristine AuNW electrodes at the identical rate. To verify the properties of the hybrid Au-PpyNW electrodes, we performed cyclic voltammetry, and recorded current under various scan rates, |dV/dt|. An ideal capacitive element is supposed to present a rectangular cyclic voltammogram (CV), and a linear variation between current and scan rate, according to the relation, I = C × dV/dt. First, we examined CVs for the pristine AuNW electrode, and they exhibited nearly rectangular shapes, even at extraordinary fast scan rates of up to 100 V/s (Figure 2d). This scan rate is more than 2 orders of magnitude faster than most EDL-based, and metal oxide NW-based ECs, for which maximal rates are usually limited to 1 V/s, due to charge transport and/or charge transfer resistance. Beyond this limit, CVs deform into tilted ellipsoids, due to the influence of internal resistance. Rectangular CVs at such fast rates


8

Page 9 of 22

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


indicate rapid EDL formation, and fast charge transport within the AuNW array. Due to the superior transport properties of the AuNW array at the core of the hybrid Au-PpyNW electrode, it maintains rectangular CVs up to 10 V/s (Figure 2e). In contrast, the Au/Ppy film does not function as an EC, and it is incapable of storing charge, as indicated by its completely collapsed CVs (Figure 2f). Furthermore, the charge current for the Au-PpyNW electrode linearly increases with the scan rate, showing no noticeable resistive contributions up to 10 V/s (100 V/s for the pristine AuNW electrode), whereas the current for the Au/Ppy film electrode deviates from a linear relation at much slower scan rates (Figure 2g). C is evaluated from the CVs, and it verifies the enhancement of the specific capacitance and excellent rate performance for the Au-PpyNW electrode (Figure 2h), and coincides with the values for C obtained from the CC measurement (Figure 2c). The Au-PpyNW exhibits a value for C of 26.4 mF/cm2 at 10 V/s. This specific capacitance and rate exceed those for typical EDL-based microcapacitors (0.4 – 2 mF/cm2 at 1 – 100 mV/s),23-25 and an EC utilizing electrodes of Ppy-decorated nanoporous Au (1.8 mF/cm2 at 100 mV/s)26. One of the important findings in the present work is that the device performance for the AuPpyNW exhibits a clear departure from metal NW-based electrodes prepared on a substrate with a random mesh structure. For example, the specific capacitance of an EC utilizing Ag-Au coreshell nanowire mesh films decreases by ~ 35 % (from 209.9 to 136.5 µF/cm2) after increasing the rate 20 times (from 10 to 200 µA/cm2).27 Similarly, the specific capacitance for Ag-Au-Ppy coreshell nanowire mesh films declines by ~ 45 % (from 580 to 320 µF/cm2) with a six-fold increase in rate (from 5.8 to 35 µA/cm2).28 The values for the specific capacitance, charge/discharge rate, and rate performance observed for the mesh films are far below those for the Au-PpyNW, whose specific capacitance decreases by only ~ 7 % (from 40 to 37 mF/cm2, or from 7.4 to 6.9 F/cm3)


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 22

with a thirty-fold increase in rate (from 0.84 mA/cm2 to 25 mA/cm2, or from 160 to 4690 mA/cm3), as shown in Figure 2c. The electrical conductivities of individual metal-based NWs should be high, and differences in performance should arise from other factors, such as ion transport and charge collection from individual NWs. We attribute the reduced internal resistance of the Au-PpyNW to its unique structural features, which effectively eliminate possible rate-limiting processes. First, the vertical alignment of the NW decreases resistance by creating a short path length for ion and electron transport to the electrode’s surfaces. Second, the monolithic and seamless structure of the electrode is effective at circumventing the electrode’s interfacial resistance and thus enhances charge collection. Charge collection has been a problem with agglomerated electrodes composed of granular, or anisotropic materials, such as activated carbon, carbon nanotubes, graphene, or nanowires, which are often mixed with binders and additives. Finally, both the core AuNW array, and thin Ppy shell are highly conductive. To gain further insight into internal resistance, we performed electrochemical impedance spectroscopy (EIS), at frequencies ranging from 100 kHz to 10 mHz. The complex plane plots of the impedance at low, intermediate, and high frequencies are shown in Figure 3(a), its inset, and 3(b), respectively (the bode plots, and the frequency dependence of the specific capacitance are shown in Figure S4). All plots show nearly vertical curves (~ 90°) to real axis at low frequency (Figure 3(a)), indicative of a capacitive behavior. The plots for the pristine AuNW electrode intersect vertically with the real axis, indicating that all surfaces of the electrode contribute to charge storage, up to high frequencies (Figure 3b).29-30 On the other hand, the plots for the Au-PpyNW and Au/Ppy film electrode incline at ~ 45°, with increasing frequency (Figure 3(b), and inset in Figure 3(a)), showing Warburg-type lines, whose projected length on the real axis characterizes the ion migration process, and corresponds to one third of the ionic resistance of the Ppy.31 The


10

Page 11 of 22

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


ionic resistance of Ppy is significantly reduced from 70.4 Ω (Au/Ppy electrode) to 0.2 Ω (AuPpyNW electrode) by forming it into the thin, uniform layer on the AuNWs. The most insightful feature of the plots for the Au-PpyNW electrode is that the intersection

Figure 3. (a,b) Complex plane plots of the impedance in the (a) low, (inset in (a)) intermediate, and (b) high frequency regions. (c,d) SEM images of the Au-PpyNWs showing (c) their junctions with the underlying Au film ((d) is a close-up image). (e) Energy density, E, vs. power density, P. for the different electrodes. (f) Retention of capacitance is demonstrated by cycling voltage from 0 V and 0.85 V at 1 V/s.

with the real axis shifts closer to the original point (Zre = 0.33 Ω) than for the pristine AuNW electrode (Zre = 0.44 Ω). Since the distance between the two NW arrays is kept constant by the


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 22

teflon spacer, the difference in the impedance would be due to the electrical resistance across the interface between the NWs and the underlying electrode, rather than the ionic resistance of bulk electrolyte layer. Resistance across the interface between active materials and the underlying electrode has often been an issue, and may limit overall electron flow in ECs.32-34 To verify this hypothesis, we examined the structure of the junctions between the Au-PpyNWs and the underlying electrodes using SEM (Figure 3c,d). The core AuNWs become narrow and are partially disconnected at their junction with the electrode. These junction defects (bottlenecks and partial disconnects) can arise from pore defects in the AAO.35 The Ppy forms in a way that wraps and thickens the bottlenecks, and fills in these partial disconnects, strengthening the electrical connection of the AuNWs to the underlying film, resulting in a reduction of the internal resistance of the electrode. As a consequence of the reduced internal resistance and increased capacitance that result from the optimal Ppy deposition (~ 10nm-thick Ppy; for detail, see the supporting information), the power density for the Au-PpyNW is enhanced by more than 1 order of magnitude, and the energy density is maintained at a very high level, compared to the Au/Ppy electrode (Figure 3e). The Au-PpyNW electrode is durable against the structural breakdown of the Ppy layer,36 and withstands thousands of charge/discharge cycles (Figure 3f). Additionally, the core AuNWs can be replaced with affordable metals, such as nickel (Ni), which is often used as a current collector in ECs. NiNWs have been previously prepared using a similar electrodeposition technique, and show excellent electrochemical properties that are comparable to AuNWs.8 Pyrrole has been electropolymerized to form Ppy on a wide variety of common metals, such as nickel,37 copper and brass,38 and zinc-lead-silver alloys.39 We believe that similar hybrid NW structures are feasible in combination with inexpensive metals and Ppy, and analogous properties to the Au-PpyNW may be obtained.


12

Page 13 of 22

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


Nanoscopic Au, such as Au nanoparticles40 and nanoporous Au,41 have attracted great interest as high efficiency electrocatalysts for alcohol oxidation, in the context of their potential applications in direct fuel cells. We have tested small- diameter AuNWs (35nm in diameter and 6 µm in length) for use in ethanol electro-oxidation, as shown in Figure 4a, where CVs establish the electrocatalytic activity of the AuNWs, and that the peak current in the anodic sweep monotonically increases with the concentration of ethanol in solution. Interestingly, the oxidation current does not change significantly with the addition of the Ppy coating on the AuNWs (Figure 4b). To verify that catalytically active sites on the AuNWs are preserved, we investigated the surface activity (electroactive surface), in the absence of ethanol, by means of cyclic voltammetry (Figure 4c). For the Au-PpyNW electrode, a large anodic current appears at ~ 0.06

Figure 4. (a,b) CVs of the (a) pristine AuNW and (b) Au-PpyNW in 0.5 M NaOH. The annotation shows the concentration of ethanol in the solution. Scan rate is 10 mV/s. Thickness of Ppy is ~ 50nm. (c) CVs in 0.5 M NaOH. Scan rate is 50 mV/s. Thickness of Ppy is ~ 100nm. (d) Retention of the anodic current observed for the electrodes shown in (c), at 0.1 V and 0.5 V under potential cycling between -0.8 V and 1.0 V in 0.5 M NaOH with 1.0 M ethanol. Scan rate


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 22

is 50 mV/s. For (a)-(b), current was divided by the electroactive surface area of the electrodes, determined from the CVs shown in Figure S7. The same method was used to transform current into current density for (c).

V, indicating the formation of gold surface oxides, the precursors of which (chemisorbed OHanions) play a crucial role in promoting catalysis in alkaline solutions.42-43 Gold surface oxides are subsequently removed during the cathodic sweep, as represented by the reduction peak at 0.05 V. The CV of the Au-PpyNW electrode overlaps with that of the pristine AuNW electrode, where the onset potential and peak potential, corresponding to the gold surface oxide formation, and its removal, respectively, are the same for both electrodes. These results emphasize that the surface activity of Au cores in the Au-PpyNW electrode is not affected by the Ppy coating, and is maintained at a nearly equivalent level to the pristine AuNW. The catalytic lifetime of the AuPpyNW electrode for ethanol electro-oxidation was characterized by cyclic voltammetry. A potential was cycled between -0.8 V and 1.0 V. The anodic current at 0.1 V and 0.5 V was recorded and normalized with the initial current (Figure 4d). The Au-PpyNW electrode demonstrates a longer catalytic lifetime than the pristine AuNW electrode, as the Ppy coating imparts a longer catalytic lifetime to the core AuNW catalyst. It has been reported that nanoporous Au undergoes a degradation of catalytic activity, associated with a change in the surface structure of the Au. The extent of the degradation becomes more prominent with decreasing pore size, although catalytic activity increases accordingly, possibly due to metastable surface states at nanoscale.42 With this in mind, we have examined the structures of the electrodes using SEM, EDS, TEM, and HRTEM to gain insight into the longer catalytic lifetime observed for the Au-PpyNW. Figure 5a shows a SEM image of the pristine AuNWs, before


14

Page 15 of 22

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


conducting the potential cycling test demonstrated in Figure 4d. The SEM image confirms the well-defined AuNW structures, with an average diameter of 35nm. However, the AuNWs coalesce into large structures after cycling potential over 50 times (Figure 5b), and the changes in the surface structure of Au could be responsible for the degradation of the electrocatalyst (cf. Figure 4d). In sharp contrast, the AuNW structures in the Au-PpyNW electrodes with 100nm thick Ppy shell are maintained after potential cycling 50-times, and no noticeable structural changes in the core AuNWs and the encapsulating Ppy are observed, as shown in the SEM, TEM, and HRTEM images (Figure 5c-e), and in EDS elemental mapping (Figure S5,6). It should be noted that the Ppy shell has been adjusted to an optimal thickness of 100nm, beyond which the individual NWs overlap, and pathways for mass transport within their arrays are lost. Based on these observations, we expect that the Ppy inhibits the degradation of the AuNW surface structure. Additionally, the Ppy coating on the AuNWs may protect the AuNWs against electrode poisoning caused by chemisorbed intermediates.44


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 22

Figure 5. (a,b) SEM images of the pristine AuNWs (a) before and (b) after the potential cycling test shown in Figure 4d. (c) A SEM image of the Au-PpyNWs after the potential cycling test, shown in Figure 4d. (d) TEM and (e) HRTEM images of a single Au-PpyNW from the NW array shown in (c). The Au and Ppy interfaces remain intact after potential cycling. For (a-e), the NWs are cleaved from their array, and transferred onto a silicon wafer or Cu grid for observation.

CONCLUSION We have designed hybrid NW electrodes composed of AuNW cores with Ppy shells, and have demonstrated their unique properties, and their suitability for use in ECs, and in the


16

Page 17 of 22

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


electrocatalysis of ethanol oxidation. Galvanostatic CC measurements and cyclic voltammetry reveal that an EC assembled with these hybrid electrodes functions as an ideal capacitive element, and is capable of charging and discharging at fast rates, with simultaneously high energy and power densities. The rate performance achieved is remarkable, and superior to metaland metal oxide-NW based electrodes. The high charge/discharge rates possible with these hybrid electrodes is the result of a reduced internal resistance, as confirmed by EIS measurements, and these high rates are essential for surge-power delivery applications. The present work suggests that the ion transport in the active layers, and the charge collection from the active materials, are important rate-limiting processes that need to be addressed to maximize the performance of metal NW-based hybrid electrodes. It is demonstrated that the device performance is significantly enhanced by designing overall electrode structures that address the rate-limiting processes. Additionally, the outer Ppy layer of the hybrid electrodes does not impede the electrocatalytic activity of the core AuNWs, and serves to protect the active sites of the AuNW surface from degradation, thus prolonging the catalytic lifetime of the AuNW electrocatalyst for ethanol oxidation.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Methods for electrode preparation and characterization (PDF) AUTHOR INFORMATION Corresponding Author


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 22

*E-mail: [email protected] *E-mail: [email protected] Author Contributions † R. K. P. and Y. K. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the Ministry of Education, Singapore, under grant R-279-000-408112 and R-279-000-496-114 (to S.S.), and JSPS KAKENHI Grant Numbers JP15H05410 and JP16K13627 (to H.N.), the Ogasawara Foundation (to H.N.), and the Project for Enhancing Research and Education in Polymer and Fiber Science at KIT (to S.S. & H.N.).

REFERENCES 1. Gaboriau, D.; Boniface, M.; Valero, A.; Aldakov, D.; Brousse, T.; Gentile, P.; Sadki, S., Atomic Layer Deposition Alumina-Passivated Silicon Nanowires: Probing the Transition from Electrochemical Double-Layer Capacitor to Electrolytic Capacitor. ACS Appl. Mater. Interfaces 2017, 9, 13761-13769. 2. Yan, W.; Thai, M. L.; Dutta, R.; Li, X.; Xing, W.; Penner, R. M., A Lithographically Patterned Capacitor with Horizontal Nanowires of Length 2.5 mm. ACS Appl. Mater. Interfaces 2014, 6, 5018-5025. 3. Liu, X.; Liu, J.; Sun, X., NiCo2O4@NiO Hybrid Arrays with Improved Electrochemical Performance for Pseudocapacitors. J. Mater. Chem. A 2015, 3, 13900-13905. 4. Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D., Nanowire DyeSensitized Solar Cells. Nat. Mater. 2005, 4, 455-459.


18

Page 19 of 22

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


5. Wang, G.; Wang, H.; Ling, Y.; Tang, Y.; Yang, X.; Fitzmorris, R. C.; Wang, C.; Zhang, J. Z.; Li, Y., Hydrogen-Treated TiO2 Nanowire Arrays for Photoelectrochemical Water Splitting. Nano Lett. 2011, 11, 3026-3033. 6. Moniz, S. J. A.; Zhu, J.; Tang, J., 1D Co-Pi Modified BiVO4/ZnO Junction Cascade for Efficient Photoelectrochemical Water Cleavage. Adv. Energy Mater. 2014, 4, 1301590. 7. Bae, J.; Song, M. K.; Park, Y. J.; Kim, J. M.; Liu, M. L.; Wang, Z. L., Fiber Supercapacitors Made of Nanowire-Fiber Hybrid Structures for Wearable/Flexible Energy Storage. Angew. Chem. Int. Ed. 2011, 50, 1683-1687. 8. Nakanishi, H.; Kikuta, I.; Kawabata, Y.; Norisuye, T.; Tran-Cong-Miyata, Q.; Segawa, H., Fast Ion and Electron Transport in a Supercapacitor Based on Monolithic Nanowire-Array Electrodes Prepared from a Defect-Free Anodic Aluminium Oxide Mold. Adv. Mater. Interfaces 2015, 2, 1500354. 9. Nakanishi, H.; Grzybowski, B. A., Supercapacitors Based on Metal Electrodes Prepared from Nanoparticle Mixtures at Room Temperature. J. Phys. Chem. Lett. 2010, 1, 1428-1431. 10. Zhu, C.; Guo, S.; Dong, S., PdM (M = Pt, Au) Bimetallic Alloy Nanowires with Enhanced Electrocatalytic Activity for Electro-Oxidation of Small Molecules. Adv. Mater. 2012, 24, 2326-2331. 11. Hong, W.; Wang, J.; Wang, E., Facile Synthesis of Highly Active PdAu Nanowire Networks as Self-Supported Electrocatalyst for Ethanol Electrooxidation. ACS Appl. Mater. Interfaces 2014, 6, 9481-9487. 12. Wang, G.; Zhang, L.; Zhang, J., A Review of Electrode Materials for Electrochemical Supercapacitors. Chem. Soc. Rev. 2012, 41, 797-828. 13. Yu, A.; Chabot, V.; Zhang, J., Electrochemical Supercapacitors for Energy Storage and Delivery: Fundamentals and Applications. CRC Press: Boca Raton, pp.62-79, 2013. 14. Hu, L. B.; Chen, W.; Xie, X.; Liu, N. A.; Yang, Y.; Wu, H.; Yao, Y.; Pasta, M.; Alshareef, H. N.; Cui, Y., Symmetrical MnO2-Carbon Nanotube-Textile Nanostructures for Wearable Pseudocapacitors with High Mass Loading. ACS Nano 2011, 5, 8904-8913. 15. Wu, M. S.; Fu, Y. H., Electrophoretic Self-Assembly of Expanded Mesocarbon Microbeads with Attached Nickel Nanoparticles as a High-Rate Electrode for Supercapacitors. Nanoscale 2014, 6, 4195-4203. 16. Yan, J.; Fan, Z.; Wei, T.; Qian, W.; Zhang, M.; Wei, F., Fast and Reversible Surface Redox Reaction of Graphene-MnO2 Composites as Supercapacitor Electrodes. Carbon 2010, 48, 3825-3833. 17. El-Kady, M. F.; Strong, V.; Dubin, S.; Kaner, R. B., Laser Scribing of High-Performance and Flexible Graphene-Based Electrochemical Capacitors. Science 2012, 335, 1326-1330. 18. Li, R.; Ren, X.; Zhang, F.; Du, C.; Liu, J., Synthesis of Fe3O4@SnO2 Core-Shell Nanorod Film and Its Application as a Thin-Film Supercapacitor Electrode. Chem. Commun. 2012, 48, 5010-5012. 19. Sun, X.; Li, Q.; Lü, Y.; Mao, Y., Three-Dimensional ZnO@MnO2 Core@Shell Nanostructures for Electrochemical Energy Storage. Chem. Commun. 2013, 49, 4456-4458. 20. Yang, P.; Xiao, X.; Li, Y.; Ding, Y.; Qiang, P.; Tan, X.; Mai, W.; Lin, Z.; Wu, W.; Li, T.; Jin, H.; Liu, P.; Zhou, J.; Wong, C. P.; Wang, Z. L., Hydrogenated ZnO Core-Shell Nanocables for Flexible Supercapacitors and Self-Powered Systems. ACS Nano 2013, 7, 2617-2626. 21. Wang, F.; Zhan, X.; Cheng, Z.; Wang, Z.; Wang, Q.; Xu, K.; Safdar, M.; He, J., Tungsten Oxide@Polypyrrole Core-Shell Nanowire Arrays as Novel Negative Electrodes for Asymmetric Supercapacitors. Small 2015, 11, 749-755.


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

Page 20 of 22

22. Zhou, C.; Zhang, Y.; Li, Y.; Liu, J., Construction of High-Capacitance 3D CoO@Polypyrrole Nanowire Array Electrode for Aqueous Asymmetric Supercapacitor. Nano Lett. 2013, 13, 2078-2085. 23. Pech, D.; Brunet, M.; Durou, H.; Huang, P. H.; Mochalin, V.; Gogotsi, Y.; Taberna, P. L.; Simon, P., Ultrahigh-Power Micrometre-Sized Supercapacitors Based on Onion-Like Carbon. Nat. Nanotechnol. 2010, 5, 651-654. 24. In, H. J.; Kumar, S.; Shao-Horn, Y.; Barbastathis, G., Origami Fabrication of Nanostructured, Three-Dimensional Devices: Electrochemical Capacitors with Carbon Electrodes. Appl. Phys. Lett. 2006, 88, 083104. 25. Kaempgen, M.; Chan, C. K.; Ma, J.; Cui, Y.; Gruner, G., Printable Thin Film Supercapacitors Using Single-Walled Carbon Nanotubes. Nano Lett. 2009, 9, 1872-1876. 26. Meng, F. H.; Ding, Y., Sub-Micrometer-Thick All-Solid-State Supercapacitors with High Power and Energy Densities. Adv. Mater. 2011, 23, 4098-4102. 27. Lee, H.; Hong, S.; Lee, J.; Suh, Y. D.; Kwon, J.; Moon, H.; Kim, H.; Yeo, J.; Ko, S. H., Highly Stretchable and Transparent Supercapacitor by Ag-Au Core-Shell Nanowire Network with High Electrochemical Stability. ACS Appl. Mater. Interfaces 2016, 8, 15449-15458. 28. Moon, H.; Lee, H.; Kwon, J.; Suh, Y. D.; Kim, D. K.; Ha, I.; Yeo, J.; Hong, S.; Ko, S. H., Ag/Au/Polypyrrole Core-Shell Nanowire Network for Transparent, Stretchable and Flexible Supercapacitor in Wearable Energy Devices. Sci. Rep. 2017, 7. 29. Itagaki, M.; Suzuki, S.; Shitanda, I.; Watanabe, K.; Nakazawa, H., Impedance Analysis on Electric Double Layer Capacitor with Transmission Line Model. J. Power Sources 2007, 164, 415-424. 30. Taberna, P. L.; Simon, P.; Fauvarque, J. F., Electrochemical Characteristics and Impedance Spectroscopy Studies of Carbon-Carbon Supercapacitors. J. Electrochem. Soc. 2003, 150, A292-A300. 31. Hughes, M.; Shaffer, M. S. P.; Renouf, A. C.; Singh, C.; Chen, G. Z.; Fray, J.; Windle, A. H., Electrochemical Capacitance of Nanocomposite Films Formed by Coating Aligned Arrays of Carbon Nanotubes with Polypyrrole. Adv. Mater. 2002, 14, 382-385. 32. An, K. H.; Kim, W. S.; Park, Y. S.; Choi, Y. C.; Lee, S. M.; Chung, D. C.; Bae, D. J.; Lim, S. C.; Lee, Y. H., Supercapacitors Using Single-Walled Carbon Nanotube Electrodes. Adv. Mater. 2001, 13, 497-500. 33. Talapatra, S.; Kar, S.; Pal, S. K.; Vajtai, R.; Ci, L.; Victor, P.; Shaijumon, M. M.; Kaur, S.; Nalamasu, O.; Ajayan, P. M., Direct Growth of Aligned Carbon Nanotubes on Bulk Metals. Nat. Nanotechnol. 2006, 1, 112-116. 34. Portet, C.; Taberna, P. L.; Simon, P.; Laberty-Robert, C., Modification of Al Current Collector Surface by Sol-Gel Deposit for Carbon-Carbon Supercapacitor Applications. Electrochim. Acta 2004, 49, 905-912. 35. Ono, S.; Ichinose, H.; Masuko, N., Defects in Porous Anodic Films Formed on High Purity Aluminum. J. Electrochem. Soc. 1991, 138, 3705-3710. 36. Zhao, Y.; Liu, B.; Pan, L.; Yu, G., 3D Nanostructured Conductive Polymer Hydrogels for High-Performance Electrochemical Devices. Energy Environ. Sci. 2013, 6, 2856-2870. 37. Jacques, A.; Barthélémy, B.; Delhalle, J.; Mekhalif, Z., 1-Pyrrolyl-10Decylammoniumphosphonate Monolayer: A Molecular Nanolink between Electropolymerized Pyrrole Films and Nickel or Titanium Surfaces. Electrochim. Acta 2015, 170, 218-228.


20

Page 21 of 22

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


38. Bazzaoui, M.; Martins, J. I.; Bazzaoui, E. A.; Reis, T. C.; Martins, L., Pyrrole Electropolymerization on Copper and Brass in a Single-Step Process from Aqueous Solution. J Appl Electrochem 2004, 34, 815-822. 39. Bazzaoui, M.; Bazzaoui, E. A.; Martins, L.; Martins, J. I., Electropolymerization of Pyrrole on Zinc-Lead-Silver Alloys Electrodes in Acidic and Neutral Organic Media. Synth. Met. 2002, 130, 73-83. 40. Choi, Y.; Gu, M.; Park, J.; Song, H. K.; Kim, B. S., Graphene Multilayer Supported Gold Nanoparticles for Efficient Electrocatalysts toward Methanol Oxidation. Adv. Energy Mater. 2012, 2, 1510-1518. 41. Xia, H.; Ran, Y.; Li, H.; Tao, X.; Wang, D., Freestanding Monolayered Nanoporous Gold Films with High Electrocatalytic Activity Via Interfacial Self-Assembly and Overgrowth. J. Mater. Chem. A 2013, 1, 4678-4684. 42. Zhang, J.; Liu, P.; Ma, H.; Ding, Y., Nanostructured Porous Gold for Methanol ElectroOxidation. J. Phys. Chem. C 2007, 111, 10382-10388. 43. Burke, L. D.; Nugent, P. F., The Electrochemistry of Gold: II The Electrocatalytic Behaviour of the Metal in Aqueous Media. Gold Bull. 1998, 31, 39-50. 44. Nagaraju, D. H.; Lakshminarayanan, V., Electrochemically Grown Mesoporous Gold Film as High Surface Area Material for Electro-Oxidation of Alcohol in Alkaline Medium. J. Phys. Chem. C 2009, 113, 14922-14926.


21


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 22 of 22

TOC


22

Metal Nanowire-Based Hybrid Electrodes Exhibiting High Charge

Recommend Documents