Metal Nanowire-Based Hybrid Electrodes Exhibiting High Charge/Discharge Rates and Long-Lived ElectrocatalysisRakesh K. Pandey,§,†,‡ Yuto Kawabata,§,† Satoshi Teraji,† Tomohisa Norisuye,† Qui Tran-Cong-Miyata,†
Siowling Soh,*,‡ and Hideyuki Nakanishi*,†
†Department of Macromolecular Science and Engineering, Graduate School of Science and Technology, Kyoto Institute ofTechnology, Matsugasaki, Kyoto 606-8585, Japan‡Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585,Singapore
*S Supporting Information
ABSTRACT: Nanostructured electrodes are at the forefront of advanced materials research, and have been studied extensivelyin the context of their potential applications in energy storage and 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. Ingeneral, the specific capacitance of electrochemical capacitors can be increased by faradaic reactions, but their charge transferresistance impedes charge transport, decreasing the capacitance with increasing charge/discharge rate. The specific capacitance ofthe hybrid electrodes is enhanced due to the pseudocapacitance of the polypyrrole shells; moreover, the electrodes operate as anideal 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-basedsupercapacitors. The mechanisms behind these fast charge/discharge rates are elucidated by electrochemical impedancespectroscopy, and are ascribed to the reduced internal resistance associated with the fast charge transport ability of the goldnanowire 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 forelectrocatalysis during potential cycling. This study demonstrates that the gold-polypyrrole hybrid electrodes can store anddeliver charge at fast rates, and that the polypyrrole shells of the nanowires extend the catalytic lifetime of the gold cores.
■ INTRODUCTIONNanowires (NWs) have been of great interest for use aselectrodes for electrochemical capacitors (ECs)1−3 and photo-electrochemical cells,4,5 since their one-dimensional (1D)nanostructures create short diffusion pathways for ions andexcitons, leading to an increase in charge storage capacity andphotoelectric conversion efficiency. Metal oxides have beenextensively studied as electrode materials for these devices, andformed into complex NW structures, giving rise to intriguingfunctionality and properties (e.g., core−shell p−n junctionallowing efficient charge separation, and radially grown NWs on
fibrils for wearable energy devices).6,7 On the other hand, metalNWs, and hybrid electrodes using metal NWs and conductingpolymers, have not been investigated nearly as extensively, eventhough metals exhibit high electrical conductivity, preventingenergy dissipation,8,9 and have the ability to catalyze electro-chemical reactions.10,11 Here, we demonstrate that gold (Au)NWs hybridized with polypyrrole (Ppy) exhibit enhanced
Received: June 1, 2017Accepted: September 25, 2017Published: September 25, 2017
electrochemical energy storage and electrocatalysis. Ppy is apromising pseudocapacitive material with high electricalconductivity that is environmentally friendly.12 Our electrodesare based on arrays of AuNWs covered with a thin layer of Ppy,forming a core−shell (Au-Ppy) NW structure. Whencharacterized for use in ECs, these hybrid NW electrodesexhibit a dramatic increase in capacitance, due to thepseudocapacitance of Ppy, while their electrical resistance isremarkably low, allowing for ECs with simultaneously highenergy and power densities. When used for the electro-oxidation of ethanol, the outer Ppy layer imparts a longer-livedcatalytic activity to the Au cores, over repeated cycles ofpotential sweep. Achieving a balanced combination of highenergy and power densities in ECs, and extending the lifetimeof catalytic activity in noble metals, have been major challengesin developing high-performance ECs and electrocatalysts. Inthis work, we demonstrate the unique characteristics of hybridAu-PpyNW electrodes, which represent significant progresstoward meeting these challenges.
■ RESULTS AND DISCUSSION
We prepared arrays of AuNWs by electrodepositing Au throughthe cylindrical pores of anodic aluminum oxides, AAOs, on apreformed underlying Au film.8 After removing the AAOs,pyrrole was electropolymerized on the AuNWs to form theouter layer of Ppy. This procedure produced arrays of hybridNWs, with a core−shell nanostructure, with an Au core and aPpy shell. The length and diameter of the inner AuNWs weremodified by varying the time for the electrodeposition and thepore diameter of AAO, respectively. The thickness of the outerPpy was tuned by monitoring the time for the electro-polymerization.To demonstrate the level of control in tailoring the nanoscale
structure of the hybrid Au-PpyNW electrodes, we presentscanning electron microscopy (SEM) images, and energydispersive spectroscopy (EDS) element maps, for a typicalhybrid NW array fabricated for use in an EC (Figure 1). Theheight of the array (∼20 μm) was uniform over the entireelectrode, 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 aflexible, free-standing, and monolithic film, fabricated withoutthe use of binders and additives. A close-up SEM image revealsa thin (∼10 nm), uniform coating of Ppy over the AuNWs(Figure 1c), and EDS elemental mapping of a single hybridNW confirms the formation of the core−shell NW structure(Figure 1d-f).An EC was assembled, by placing an ion-permeable separator
(8 μm thick porous membrane, Whatman) between twoidentical electrodes, using a 50 μm thick Teflon spacer, andfilling the space between the electrodes with 2 M KClelectrolyte solution. To determine the effects of hybridizationthrough control, we also tested electrodes fabricated withpristine AuNWs (Supporting Information (SI) Figure S1), andplanar Au electrodes with an electropolymerized Ppy film ontheir surface prepared at the same charge consumption used infabricating the hybrid Au-PpyNW electrodes (the thickness ofthe Ppy film is approximately 1−2 μm, as determined by thecross-sectional SEM image and EDS elemental mapping shownin SI Figure S2).An ideal EC can be characterized by the relation, I = C × dV/
dt, where I, C, and dV/dt, are current (alternatively, currentdensity), capacitance (specific capacitance), and time rate ofchange in voltage, respectively. In reality, most ECs deviatefrom this ideal capacitive behavior, due to irreversible reactions,self-discharging, and the equivalent series resistance of theelectrodes.13 Among these undesirable contributions, theequivalent series resistance (internal resistance) is one of themost important factors dominating the actual performance ofECs, and it causes adverse effects, such as energy dissipation (ie.Joule heating) and the slowing of charge transport (related tochanges in the equivalent RC time constant). In order toimprove the performance of ECs, much effort has been devotedto reducing their internal resistance, through the modificationof electrode materials and structures.14−16 With this in mind,we characterized our electrodes by galvanostatic charge/discharge (CC) measurement. Figure 2a shows CC curvesobtained at a slow charge/discharge rate of I = | ± 390| mA/cm3. The Au-PpyNW electrode exhibits a symmetric triangularCC 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 whenthe Au/Ppy film begins charging and discharging (at 0 and 12.2s), respectively. Such sudden changes in voltage are caused by alarge internal resistance,13 and become more visible at a fastercharge/discharge rate of I = | ± 4690| mA/cm3 (Figure 2b).Compared with the Au/Ppy film electrode, the Au-PpyNWelectrode retains its ideal capacitive behavior, even at the fasterrate, demonstrating its reduced internal resistance. Figure 2cshows the rate dependence of C, determined from the CCcurves. At slow rates, the Au-PpyNW and Au/Ppy filmelectrodes show similar values for C (7.4 F/cm3 and 6.5 F/cm3 at 160 mA/cm3, respectively), and both of the electrodesexhibit their maximum charge storage capacity (the specificcapacitance at slow rates is most likely determined by theamount of Ppy; when the thickness of the Ppy for the Au/Ppyfilm is decreased from 1−2 μm to 10−20 nm, the specificcapacitance is decreased accordingly with the Ppy thickness, asshown in SI Figure S3). The value of C for the Au/Ppy filmelectrode, however, declines with increasing rate. This apparentdecrease in C is possibly due to a process that limits chargetransport, which manifests itself as a large internal resistance.
Figure 1. (a,b) SEM images of the arrays of the hybrid NWs. (c) Aclose-up of the NWs shows the thin layer of Ppy formed on theAuNWs. The average diameter of the hybrid NWs is 200 nm. (d−f)EDS elemental mapping of a single hybrid NW with correspondingSEM image.
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Analogous trends have also been reported for systems ofagglomerated, granular electrodes, such as activated carbonusing binders.17 Similarly, metal oxide NW-based ECs sufferfrom poor rate performance (reduction of C in response toincreasing charge/discharge rate) due to their low electricalconductivity, although they achieve substantial specificcapacitance at slow charge/discharge rates (e.g., 7 mF/cm2 at0.2 mA/cm2 for Fe3O4@SnO2 core−shell nanorods,18 and 27mF/cm2 at 0.2 mA/cm2 for ZnO@MnO2 core−shell NWs,19
and more data available in the literature20). Similar trends havebeen also observed in asymmetric supercapacitors when thecharge/discharge rates are increased.21,22 The Au-PpyNWelectrode is distinguished by its excellent rate performance,which is even superior to leading-edge electric double layer(EDL) capacitance-based graphene electrodes (Figure 2c).17
The Au-PpyNW electrodes exhibit a value for C (7.0 F/cm3 at4690 mA/cm3, or 37 mF/cm2 at 25 mA/cm2) that is 8.4 times
higher than that for the pristine AuNW electrodes at theidentical rate.To verify the properties of the hybrid Au-PpyNW electrodes,
we performed cyclic voltammetry, and recorded current undervarious scan rates, |dV/dt|. An ideal capacitive element issupposed to present a rectangular cyclic voltammogram (CV),and a linear variation between current and scan rate, accordingto the relation, I = C × dV/dt. First, we examined CVs for thepristine AuNW electrode, and they exhibited nearly rectangularshapes, even at extraordinary fast scan rates of up to 100 V/s(Figure 2d). This scan rate is more than 2 orders of magnitudefaster than most EDL-based, and metal oxide NW-based ECs,for which maximal rates are usually limited to 1 V/s, due tocharge transport and/or charge transfer resistance. Beyond thislimit, CVs deform into tilted ellipsoids, due to the influence ofinternal resistance. Rectangular CVs at such fast rates indicaterapid EDL formation, and fast charge transport within theAuNW array. Due to the superior transport properties of the
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 scanrate. (h) C determined from the CVs.
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AuNW array at the core of the hybrid Au-PpyNW electrode, itmaintains rectangular CVs up to 10 V/s (Figure 2e). Incontrast, the Au/Ppy film does not function as an EC, and it isincapable of storing charge, as indicated by its completelycollapsed CVs (Figure 2f). Furthermore, the charge current forthe 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 thecurrent for the Au/Ppy film electrode deviates from a linearrelation at much slower scan rates (Figure 2g). C is evaluatedfrom the CVs, and it verifies the enhancement of the specificcapacitance and excellent rate performance for the Au-PpyNWelectrode (Figure 2h), and coincides with the values for Cobtained from the CC measurement (Figure 2c). The Au-PpyNW exhibits a value for C of 26.4 mF/cm2 at 10 V/s. Thisspecific 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 thedevice performance for the Au-PpyNW exhibits a cleardeparture from metal NW-based electrodes prepared on asubstrate with a random mesh structure. For example, thespecific capacitance of an EC utilizing Ag−Au core−shellnanowire 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−Ppycore−shell nanowire mesh films declines by ∼45% (from 580to 320 μF/cm2) with a 6-fold increase in rate (from 5.8 to 35μA/cm2).28 The values for the specific capacitance, charge/discharge rate, and rate performance observed for the meshfilms are far below those for the Au-PpyNW, whose specificcapacitance decreases by only ∼7% (from 40 to 37 mF/cm2, orfrom 7.4 to 6.9 F/cm3) with a thirty-fold increase in rate (from0.84 mA/cm2 to 25 mA/cm2, or from 160 to 4690 mA/cm3), asshown in Figure 2c. The electrical conductivities of individualmetal-based NWs should be high, and differences in perform-ance should arise from other factors, such as ion transport andcharge collection from individual NWs. We attribute thereduced internal resistance of the Au-PpyNW to its uniquestructural features, which effectively eliminate possible rate-limiting processes. First, the vertical alignment of the NWdecreases resistance by creating a short path length for ion andelectron transport to the electrode’s surfaces. Second, themonolithic and seamless structure of the electrode is effective atcircumventing the electrode’s interfacial resistance and thusenhances charge collection. Charge collection has been aproblem with agglomerated electrodes composed of granular,or anisotropic materials, such as activated carbon, carbonnanotubes, graphene, or nanowires, which are often mixed with
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 imagesof 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. forthe different electrodes. (f) Retention of capacitance is demonstrated by cycling voltage from 0 and 0.85 V at 1 V/s.
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binders and additives. Finally, both the core AuNW array, andthin Ppy shell are highly conductive. To gain further insightinto internal resistance, we performed electrochemicalimpedance spectroscopy (EIS), at frequencies ranging from100 kHz to 10 mHz. The complex plane plots of the impedanceat low, intermediate, and high frequencies are shown in Figure3(a), its inset, and 3(b), respectively (the bode plots, and thefrequency dependence of the specific capacitance are shown inSI Figure S4). All plots show nearly vertical curves (∼90°) toreal axis at low frequency (Figure 3(a)), indicative of acapacitive behavior. The plots for the pristine AuNW electrodeintersect vertically with the real axis, indicating that all surfacesof the electrode contribute to charge storage, up to highfrequencies (Figure 3b).29,30 On the other hand, the plots forthe Au-PpyNW and Au/Ppy film electrode incline at ∼45°,with increasing frequency (Figure 3(b), and inset in Figure3(a)), showing Warburg-type lines, whose projected length onthe real axis characterizes the ion migration process, andcorresponds to one-third of the ionic resistance of the Ppy.31
The ionic resistance of Ppy is significantly reduced from 70.4 Ω(Au/Ppy electrode) to 0.2 Ω (Au-PpyNW electrode) byforming it into the thin, uniform layer on the AuNWs. Themost insightful feature of the plots for the Au-PpyNWelectrode is that the intersection with the real axis shifts closerto the original point (Zre = 0.33 Ω) than for the pristine AuNWelectrode (Zre = 0.44 Ω). Since the distance between the twoNW arrays is kept constant by the Teflon spacer, the differencein the impedance would be due to the electrical resistanceacross the interface between the NWs and the underlyingelectrode, rather than the ionic resistance of bulk electrolytelayer. Resistance across the interface between active materialsand the underlying electrode has often been an issue, and maylimit overall electron flow in ECs.32−34 To verify thishypothesis, 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 arepartially 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 away that wraps and thickens the bottlenecks, and fills in thesepartial disconnects, strengthening the electrical connection ofthe AuNWs to the underlying film, resulting in a reduction ofthe internal resistance of the electrode. As a consequence of thereduced internal resistance and increased capacitance that resultfrom the optimal Ppy deposition (∼10 nm-thick Ppy; for detail,see the SI), the power density for the Au-PpyNW is enhancedby more than 1 order of magnitude, and the energy density ismaintained at a very high level, compared to the Au/Ppyelectrode (Figure 3e). The Au-PpyNW electrode is durableagainst the structural breakdown of the Ppy layer,36 andwithstands thousands of charge/discharge cycles (Figure 3f).Additionally, the core AuNWs can be replaced with affordablemetals, such as nickel (Ni), which is often used as a currentcollector in ECs. NiNWs have been previously prepared using asimilar electrodeposition technique, and show excellent electro-chemical properties that are comparable to AuNWs.8 Pyrrolehas been electropolymerized to form Ppy on a wide variety ofcommon metals, such as nickel,37 copper and brass,38 andzinc−lead-silver alloys.39 We believe that similar hybrid NWstructures are feasible in combination with inexpensive metalsand Ppy, and analogous properties to the Au-PpyNW may beobtained.Nanoscopic Au, such as Au nanoparticles40 and nanoporous
Au,41 have attracted great interest as high efficiency electro-catalysts for alcohol oxidation, in the context of their potentialapplications in direct fuel cells. We have tested small- diameterAuNWs (35 nm in diameter and 6 μm in length) for use inethanol electro-oxidation, as shown in Figure 4a, where CVs
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 thesolution. Scan rate is 10 mV/s. Thickness of Ppy is ∼50 nm. (c) CVs in 0.5 M NaOH. Scan rate is 50 mV/s. Thickness of Ppy is ∼100 nm. (d)Retention of the anodic current observed for the electrodes shown in (c), at 0.1 and 0.5 V under potential cycling between −0.8 and 1.0 V in 0.5 MNaOH with 1.0 M ethanol. Scan rate is 50 mV/s. For (a)−(b), current was divided by the electroactive surface area of the electrodes, determinedfrom the CVs shown in SI Figure S7. The same method was used to transform current into current density for (c).
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establish the electrocatalytic activity of the AuNWs, and thatthe peak current in the anodic sweep monotonically increaseswith the concentration of ethanol in solution. Interestingly, theoxidation current does not change significantly with theaddition of the Ppy coating on the AuNWs (Figure 4b). Toverify that catalytically active sites on the AuNWs are preserved,we investigated the surface activity (electroactive surface), inthe absence of ethanol, by means of cyclic voltammetry (Figure4c). For the Au-PpyNW electrode, a large anodic currentappears at ∼0.06 V, indicating the formation of gold surfaceoxides, the precursors of which (chemisorbed OH− anions)play a crucial role in promoting catalysis in alkalinesolutions.42,43 Gold surface oxides are subsequently removedduring the cathodic sweep, as represented by the reductionpeak at 0.05 V. The CV of the Au-PpyNW electrode overlapswith that of the pristine AuNW electrode, where the onsetpotential and peak potential, corresponding to the gold surfaceoxide formation, and its removal, respectively, are the same forboth electrodes. These results emphasize that the surfaceactivity of Au cores in the Au-PpyNW electrode is not affectedby the Ppy coating, and is maintained at a nearly equivalentlevel to the pristine AuNW. The catalytic lifetime of the Au-PpyNW electrode for ethanol electro-oxidation was charac-terized by cyclic voltammetry. A potential was cycled between−0.8 and 1.0 V. The anodic current at 0.1 and 0.5 V wasrecorded and normalized with the initial current (Figure 4d).The Au-PpyNW electrode demonstrates a longer catalyticlifetime than the pristine AuNW electrode, as the Ppy coatingimparts a longer catalytic lifetime to the core AuNW catalyst. Ithas been reported that nanoporous Au undergoes a degradationof catalytic activity, associated with a change in the surfacestructure of the Au. The extent of the degradation becomesmore prominent with decreasing pore size, although catalyticactivity increases accordingly, possibly due to metastablesurface states at nanoscale.42 With this in mind, we haveexamined the structures of the electrodes using SEM, EDS,TEM, and HRTEM to gain insight into the longer catalyticlifetime observed for the Au-PpyNW. Figure 5a shows a SEMimage of the pristine AuNWs, before conducting the potentialcycling test demonstrated in Figure 4d. The SEM imageconfirms the well-defined AuNW structures, with an averagediameter of 35 nm. However, the AuNWs coalesce into largestructures after cycling potential over 50 times (Figure 5b), andthe changes in the surface structure of Au could be responsiblefor the degradation of the electrocatalyst (cf. Figure 4d). Insharp contrast, the AuNW structures in the Au-PpyNWelectrodes with 100 nm thick Ppy shell are maintained afterpotential cycling 50-times, and no noticeable structural changesin the core AuNWs and the encapsulating Ppy are observed, asshown in the SEM, TEM, and HRTEM images (Figure 5c−e),and in EDS elemental mapping (SI Figure S5,6). It should benoted that the Ppy shell has been adjusted to an optimalthickness of 100 nm, 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 thedegradation of the AuNW surface structure. Additionally, thePpy coating on the AuNWs may protect the AuNWs againstelectrode poisoning caused by chemisorbed intermediates.44
■ CONCLUSIONWe have designed hybrid NW electrodes composed of AuNWcores with Ppy shells, and have demonstrated their uniqueproperties, and their suitability for use in ECs, and in the
electrocatalysis of ethanol oxidation. Galvanostatic CCmeasurements and cyclic voltammetry reveal that an ECassembled with these hybrid electrodes functions as an idealcapacitive element, and is capable of charging and dischargingat fast rates, with simultaneously high energy and powerdensities. The rate performance achieved is remarkable, andsuperior to metal- and metal oxide-NW based electrodes. Thehigh charge/discharge rates possible with these hybridelectrodes is the result of a reduced internal resistance, asconfirmed by EIS measurements, and these high rates areessential for surge-power delivery applications. The presentwork suggests that the ion transport in the active layers, and thecharge collection from the active materials, are important rate-limiting processes that need to be addressed to maximize theperformance of metal NW-based hybrid electrodes. It isdemonstrated that the device performance is significantlyenhanced by designing overall electrode structures that addressthe rate-limiting processes. Additionally, the outer Ppy layer ofthe hybrid electrodes does not impede the electrocatalyticactivity of the core AuNWs, and serves to protect the activesites of the AuNW surface from degradation, thus prolongingthe catalytic lifetime of the AuNW electrocatalyst for ethanoloxidation.
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 ofthe Au-PpyNWs after the potential cycling test, shown in Figure 4d.(d) TEM and (e) HRTEM images of a single Au-PpyNW from theNW array shown in (c). The Au and Ppy interfaces remain intact afterpotential cycling. For (a-e), the NWs are cleaved from their array, andtransferred onto a silicon wafer or Cu grid for observation.
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■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.7b07794.
Methods for electrode preparation and characterization(PDF)
■ AUTHOR INFORMATIONCorresponding Authors*(S.S.) E-mail: [email protected].*(H.N.) E-mail: [email protected] Soh: 0000-0002-4294-6772Hideyuki Nakanishi: 0000-0001-8065-6373Author Contributions§R. K. P. and Y. K. contributed equally to this work. Themanuscript was written through contributions of all authors. Allauthors have given approval to the final version of themanuscript.NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTSThis work was supported by the Ministry of Education,Singapore, under grant R-279-000-408-112 and R-279-000-496-114 (to S.S.), and JSPS KAKENHI Grant NumbersJP15H05410 and JP16K13627 (to H.N.), the OgasawaraFoundation (to H.N.), and the Project for Enhancing Researchand Education in Polymer and Fiber Science at KIT (to S.S. &H.N.).
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