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Nanowire modified carbon fibers for enhanced electrical energy storageMohammad Arif Ishtiaque Shuvo, 1 Tzu-Liang (Bill) Tseng, 2 Md. Ashiqur Rahaman Khan, 1

Hasanul Karim, 1 Philip Morton,1 Diego Delfin, 1 and Yirong Lin 1, a)

1 Department of Mechanical Engineering, The University of Texas at El Paso, El Paso, Texas 79968, USA2 Department of Industrial, Manufacturing & Systems Engineering, The University of Texas at El Paso, El Paso, Texas 79968, USA

(Received 5 July 2013; accepted 24 August 2013; published online 11 September 2013)The study of electrochemical super-capacitors has become one of the most attractive topics in bothacademia and industry as energy storage devices because of their high power density, long lifecycles, and high charge/discharge efciency. Recently, there has been increasing interest in thedevelopment of multifunctional structural energy storage devices such as structural super-capacitors for applications in aerospace, automobiles, and portable electronics. Thesemultifunctional structural super-capacitors provide structures combining energy storage and loadbearing functionalities, leading to material systems with reduced volume and/or weight. Due totheir superior materials properties, carbon ber composites have been widely used in structuralapplications for aerospace and automotive industries. Besides, carbon ber has good electricalconductivity which will provide lower equivalent series resistance; therefore, it can be an excellentcandidate for structural energy storage applications. Hence, this paper is focused on performing apilot study for using nanowire/carbon ber hybrids as building materials for structural energystorage materials; aiming at enhancing the charge/discharge rate and energy density. This hybridmaterial combines the high specic surface area of carbon ber and pseudo-capacitive effect of metal oxide nanowires, which were grown hydrothermally in an aligned fashion on carbon bers.The aligned nanowire array could provide a higher specic surface area that leads to highelectrode-electrolyte contact area thus fast ion diffusion rates. Scanning Electron Microscopy andX-Ray Diffraction measurements are used for the initial characterization of this nanowire/carbonber hybrid material system. Electrochemical testing is performed using a potentio-galvanostat.The results show that gold sputtered nanowire carbon ber hybrid provides 65.9% higher energydensity than bare carbon ber cloth as super-capacitor. VC 2013 AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4820942 ]

I. INTRODUCTION

The evolving interest on portable and exible electronicdevices in recent times has intrigued researchers for develop-ing lightweight, bendable, and en vironment friendly ene rgystorage devices, such as batteries 1 ,2 and super-capacitors 3 – 5

The electrical double layer capacitor (EDLC) or super-capacitor has become an excellent candidate among thesedevices with fast charging/discharging rate, high power density, s ustainable cycling life and excellent cyclingstability. 3 – 7 It has higher capacitance than g eneral capacitor and can deliver more power than batteries. 8 ,9 However, con-ventional super-capacitors are typically separated parts fromload bearing structures, besides, they are heavy and bulkythus leading to system complexity and extra add-onweight. 10 Therefore, there is an increasing need to embedthese energy storage devices such as super-capacitor into theload bearing structures to form multifunctional materials.

Prototypes of some thin exi ble super-capacitors havebeen proposed i n literature. 11 ,12 Carbonaceo us materialsuch as gr aphe ne13 – 17 and carbon na notu bes, 18 – 20 hybridcomposites, 21 – 23 conducting polymers 24 – 27 and transition

metal oxides 28 – 30 inclusion with carbonaceous material havebeen used for developing exible electrodes. Chen et al.fabricated hybrid In 2O3 nanowires/carbon nanotubes lmson polyethylene terephthalate (PET) as exible electrodesfor super-capacitor with 1 M LiClO 4 as electrolyte. 31

Transparent graphene/PET lm was used as exible electro-des for super-capacitor with 2 M KCl as electrolyte byYu et al. 15 Single wall carbon nanotube suspension wascoated on both sides of a printing paper pretreated by polyvi-nylidine uoride (PVDF) for exible paper based electrodesby Hu et al. 32 For this approach, they used coffee bag to en-capsulate the electrodes and electrolyte for super-capacitor fabrication. But all these developed super-capacitors could

not carry mechanical loads. Recently, Lin et al. developed aZnO/Carb on ber hybrid that has shown improved structuralproperties, 33 however, its energy storage property has notbeen studied yet. Therefore, this paper investigates electro-chemical performance of ZnO/Carbon ber hybrid for super-capacitor electrode, aiming at developing multifunctionalcomposites with embedded energy storage functionalities.

Herein, we report a simple two-step hydrothermalmethod for growing uniformly distributed ZnO nanowires(ZnO NWs) on carbon ber cloth, compared with bare car-bon ber for super-capacitor electrode. This low temperaturegrowth of ZnO NWs on carbon ber cloth is biocompatible,

a) Author to whom correspondence should be addressed. Electronic mail:ylin3@utep.edu

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JOURNAL OF APPLIED PHYSICS 114 , 104306 (2013)

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environmentally friendly and could provide mor e specicsurface area for highly efcient super-capacitor. 34 – 38 For improved performance, Au was coated on nanowire/carbonber electrode and bare carbon ber electrode. Coated nano-wire/carbon ber electrode showed about 65.9% better per-formance than bare carbon ber cloth in super-capacitor application with 5.5 M KOH aqueous electrolyte.

II. EXPERIMENTAL DETAILS

A. Carbon fiber/zinc oxide nanowire hybrids synthesis

The ZnO nanowire growth is a two-step hydrothermalgrowth method including seeding and nanowire growth.Briey, ZnO nanoparticle seeds were rst synthesized in anorganic solution, and then the seeding process was per-formed to coat ZnO nanoparticles onto carbon ber cloth.The seeded carbon cloth was transferred to ZnO nanowirehydrothermal growth solution at elevated temperature andatmospheric pressure. After the nanowire growth, the nano-wire/carbon ber was rinsed extensively and dried in air for super-capacitor assembly.

1. ZnO seed particles

Carbon cloth (made of carbon ber, diameter 7 l m) of 50mm 30 mm was glued (Devcon, 5 min epoxy) on theedges. Then carbon cloth was rst cleaned in bath sonication(Branson, 2510). Acetone, Deionized water (DI) and ethanolwere used as cleaning solvent. Carbon cloth was placed in abeaker with cleaning solvent and bath sonication was per-formed for 10 min using each solvent. Then carbon clothwas dried at ambient temperature in open air. The clean car-bon cloth was taken on a glass petri dish to soak in the nano-particle solution for 10 min and subsequently annealed on ahot plate at 150 C for 10 min to enhance adhesion betweenthe substrates and nanoparticles. This soaking and annealingwas repeated two more times to ensure th e mass coating of ZnO nanoparticle all over the carbon bers. 33 ,39

2. ZnO nanowires

After seeding, hydrothermal growth of ZnO nanowireswas performed using a low temperature hydrothermalmethod detailed elsewhere. 4 Briey, an aqueous solution of 25 mM zinc nitrate hexahydrate (Zn(NO 3)2•6H 2O, 99.9%purity, Sigma-Aldrich), 25 mM hexamethylenetetramine

(HMTA) (C 6H12 N4 , 99.9% purity, Sigma-Aldrich), and5–7 mM Polyethylenemine (PEI, Branched) were preparedat room temperature. Carbon ber cloth was immersed ingrowth solution when the temperature was at 85 C. Thetime duration of this growth was 2 h for all the carbon ber samples. After the reaction had been completed, carbon ber samples were taken out of the solution and rinsed with DIwater and dried at 100 C on a hot plate for 2 h.

B. Super-capacitor fabrication

A two electrode testing set up was prepared as shown inFigure 1 because it provides most accurate measure for an

electrode’s performance for electrochemical capacitors.40

Both bare carbon ber (CF) and ZnO NWs on carbon ber samples were sputtered with gold (Au) for 120 s to preparefour kinds of electrodes, i.e., (a) CF, (b) carbon ber coatedwith Au (CF-Au), (c) ZnO nanowires on carbon ber (ZnO-CF), and (d) ZnO nanowires on carbon ber coatedwith Au (ZnO-CF-Au). A hand punch with 19 mm diameter was used for preparing circular electrodes and copper tapewas used as current collector. An aqueous solution of 5.5 MKOH was prepared for electrolyte and Celgard 3501 wasused as separator.

III. RESULTS AND DISCUSSION

The morphology of zinc oxide nanowires grown oncarbon cloth was analyzed with scanning electron micro-scope as shown in Figure 2. As indicated by the gure, ZnOnanowires were grown uniformly throughout the carbonber. The nanowires had a length between 1 and 3.14 l mand diameter between 40 and 110 nm. No residue or ower was found.

The X-Ray Diffraction (XRD) scanning result of theZnO nanowires on carbon cloth is shown in Figure 3. The as-terisk (*) marked peak represents carbon of rhombohedracrystal structure. The scan result matched with the standardcard JCPDS (01-075-0444) and has lattice parameters of a ¼ 2.29597, b ¼ 2.29597, and c ¼ 10.15404. The rest of thepeaks represent zinc oxide which matched with the standardcard JCPDS (01-070-8070). The lattice parameters for hexagonal zinc oxide are a ¼ 3.24890, b ¼ 3.24890, andc ¼ 5.20490. No other peaks were found which indicates thesample has only zinc oxide and carbon.

FIG. 1. Two-electrode test cell conguration.

FIG. 2. SEM images, (a, b, and d) ZnO/carbon ber, (c) bare carbon ber.

104306-2 Shuvo et al. J. Appl. Phys. 114 , 104306 (2013)

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Four types of electrodes (CF, carbon ber coated withAu [CF-Au], ZnO NWs on carbon ber [CF-ZnO], and ZnONWs on carbon ber coated with Au [CF-ZnO-Au]) wereprepared for the two-electrode testing setup of the super-capacitor. The electrochemical properties of the super-capacitors in 5.5 M KOH aqueous electrolyte were evaluatedby cyclic voltammetry (CV) and galvanostatic charge

discharge method using a potentio-galvanostat from EZ-stat.Figures 4(a) – 4(d) show cyclic voltammetry for the four dif-ferent samples at different scan rates of 50 mV/s, 100 mV/s,250 mV/s, and 500 mV/s with a scan range between 0 and500 mV. Area under each CV curve suggests that all fabri-cated super-capacitors have good electrochemical stabilityand capacitance. Top left corner and bottom right corner of

each CV curve indicate the presence of little resi stive proper-ties due to equivalent series resistance (ESR). 43 Moderateleakage resistance of each sample was perceived as low lea k-age resistance would skew all CV curves vertically. 42

Pseudo-capacitive behavior of all samples had been observedwhich can be ensured by the mirror like image at the anodicand cathodic regimes. 43

Six cycles of galvanostatic charging-discharging curvesfor all four samples at 56 l A are shown in Figures 4(e) – 4(h)within the potential window of 0–0.5 V. Capacitance for thefour ber super-capacitors or nanowire hybrid ber super-capacitors originates either at the electrolyte/ber interfaceor electrolyte/nanowire interface, which is indicated by thetypical triangular nature of the curves. The charging-discharging curves for all four samples showed very sym-metric nature indicating good electrochemical capacitive

FIG. 3. XRD result for ZnO NWs coated carbon cloth. Asterisk (*) peak isfor carbon. The rest of the peaks belong to ZnO.

FIG. 4. Typical cyclic voltammogramsof (a) CF, (b) CF-Au, (c) CF-ZnO, (d)CF-ZnO-Au electrode super-capacitor and galvanostatic charge dischargecurve at 56 l A within a voltage win-dow of 0–0.5V for (e) CF, (f) CF-Au,(g) CF-ZnO, (h) CF-ZnO-Au electrodesuper-capacitor.

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property and high reversibility, consistent with earlier CVobservation. Specic capacitance for the super-capacitorscan be calculated by the following equation: 41

Cs ¼4 I m

D t D V

; (1)

where I is the discharging current, t is the discharging time,V is the discharging voltage, and m is the mass of active ma-terial in the electrodes.

Specic capacitance for the samples was calculated fromthe discharge slope of the galvanostatic charge dischargecurve. 40 Carbon ber coated with Au (CF-Au) has a speciccapacitance value of 2.35 F/g which is about 7% higher thanthat of bare CF (2.2 F/g). ZnO NWs on carbon ber couldprovide more surface area for the pseudo-capacitance of theelectrodes, therefore, provided 16% higher specic capacitance(2.55 F/g) than CF. Highest specic capacitance value wasobtained from ZnO NWs on carbon ber coated with goldwhich showed about 65.9% higher value (3.65 F/g) than CF.

We also measured specic capacitance in exible/twisted con-dition for all the samples but no apparent change was observedwhich conrms highly exible nature of the electrodes. Toimprove the value of the specic capacitance of the electro-chemical capacitor further optimization might be done, i.e.,matching the pore sizes and ion size of the electrolyte, control-ling the density of ZnO NWs.

IV. CONCLUSION

Multifunctional structural energy storage devices are oneof the key research interests of recent time to obtain load bear-ing and energy storage capability in one structure. It has beendemonstrated nanowire carbon ber hybrid could provide bet-ter mechanical strength than bare carbon ber for composites.In this pilot research project, we have incorporated the samenanowire hybrid carbon ber for electrode materials to exam-ine its electrochemical properties for potential structural energyapplications. A low temperature hydrothermal method hasbeen utilized to synthesize ZnO nanowires on carbon ber cloth and sputtered for high performance super-capacitor elec-trode. Scanning Electron Microscopy (SEM) images demon-strated good coverage of uniformly distributed nanowires allover carbon ber cloth. XRD analysis result conrmed exis-tence of carbon and ZnO only. The growth of nanowires has apositive synergistic effect which helps pathways for better ion

diffusion incrementing electrode-electrolyte contact area.Based on the obtained result and previous report, it is obviousthat ZnO NWs on carbon ber has a promising future for struc-tural energy storage with appropriate solid electrolyte. Thissimple method can be utilized for making high performancestructural super-capacitor for next generation aerospace, auto-mobile and electronic application.

ACKNOWLEDGMENTS

The authors would like to thank Celgard for providingthe Celgard 3501 separator.

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