Tio2 Nps Assembled Into a Carbon Nanofiber Composite Electrode By

polymers

Article TiO2 NPs Assembled into a Carbon Nanoﬁber Composite Electrode by a One-Step Electrospinning Process for Supercapacitor Applications

Bishweshwar Pant 1, Mira Park 2,* and Soo-Jin Park 1,*

1 Department of Chemistry, Inha University, 100 Inharo, Incheon 22212, Korea; [email protected] 2 Department of Bioenvironmental Chemistry, College of Agriculture & Life Science, Chonbuk National University, Jeonju 561-756, Korea * Correspondence: [email protected] (M.P.); [email protected] (S.-J.P.)

Received: 18 April 2019; Accepted: 14 May 2019; Published: 17 May 2019

Abstract: In this study, we have synthesized titanium dioxide nanoparticles (TiO2 NPs) into carbon nanofiber (NFs) composites by a simple electrospinning method followed by subsequent thermal treatment. The resulting composite was characterized by state-of-the-art techniques and exploited as the electrode material for supercapacitor applications. The electrochemical behavior of the as-synthesized TiO2 NPs assembled into carbon nanofibers (TiO2-carbon NFs) was investigated and compared with pristine TiO2 NFs. The cyclic voltammetry and charge–discharge analysis of the composite revealed an enhancement in the performance of the composite compared to the bare TiO2 NFs. The as-obtained TiO2-carbon NF composite exhibited a specific capacitance of 106.57 F/g at a current density of 1 A/g and capacitance retention of about 84% after 2000 cycles. The results obtained from this study demonstrate that the prepared nanocomposite could be used as electrode material in a supercapacitor. Furthermore, this work provides an easy scale-up strategy to prepare highly efficient TiO2-carbon composite nanofibers.

Keywords: TiO2 nanoﬁber; TiO2-carbon; composite; electrospinning; supercapacitor

1. Introduction The ever-increasing requirements for clean and sustainable energy have drawn intensive attention to the development of high-performance energy storage systems. Among the various electrochemical energy storage systems, the supercapacitors (SCs) are presumed as one of the most promising candidates for energy storage devices due to their superior performance in terms of power density and specific energy density, higher charge–discharge rate, and long cyclic life as compared to batteries and conventional capacitors [1,2]. Since the performance of the electrode materials governs the performance of the supercapacitor, the recent trend in research on supercapacitors has focused on the fabrication of highly efficient electrode materials with high energy density, which can maintain a high power density and cycling stability [1,3]. The electrodes of most of the supercapacitors are made up of carbon. However, the active electrode surface area and pore size distribution restrict the maximum capacitance [2]. One of the strategies to enhance the performance of electrode materials is to integrate metal oxides into the different carbon-based nanostructures in order to achieve the combined effect of the pseudocapacitance materials with a double-layer capacitor [1,4]. In the past few years, various transition metal oxides such as MnO2, ZnO, NiO, Fe2O3, Co3O4, MgO, and TiO2 (titanium dioxide) have been used along with carbon as a potential electrode material for supercapacitors [1,5]. Recently, TiO2-based nanostructures have been applied in electronic

Polymers 2019, 11, 899; doi:10.3390/polym11050899 www.mdpi.com/journal/polymers Polymers 2019, 11, 899 2 of 13 and optoelectronic devices due to their low cost, nontoxicity, eco-friendly nature, and abundant availability [6–8]. Due to its good electrochemical activity and high specific energy density, TiO2 can be considered as a promising candidate for a supercapacitor electrode [9,10]. Despite these advantages, the poor conductivity of TiO2 is the main obstacle that restricts its electrochemical performance [11]. Therefore, it is necessary to combine TiO2 with other materials that have good conductivity to form composites. The preparation of a TiO2-carbon composite can be an effective strategy to overcome the limitations of TiO2 to be used in a supercapacitor. In this regard, several TiO2-carbon composite nanostructures based mainly on carbon nanotubes (CNTs) and graphene have been prepared [7,12–17]. The composite showed better electrochemical performances; however, the synthesis procedure is complicated, and multiple steps are required. Furthermore, the use of toxic chemicals during the synthesis process also limits their widespread application. For example, the functionalization of CNT is generally carried out by the acid oxidation method using nitric acid (HNO3) and sulfuric acid (H2SO4)[18,19]. Similarly, the reduction of graphene oxide (GO) to reduced graphene oxide (RGO) is accomplished using chemical reducing agents, which may be toxic [20]. Several methods such as microwave, hydrothermal treatment, dip coating, chemical wet-impregnation, and electrospinning have been recognized as strategies for producing a TiO2-carbon composite [21–24]. Electrospinning has been considered as an effective method for producing good morphology of organic/inorganic nanofibers. However, it is important to find a cheap and eco-friendly procedure in order to synthesize the TiO2-carbon-based nanostructures for a supercapacitor electrode. Recently, carbon nanofibers via an electrospinning process have gained extensive research interest due to their excellent conductivity, thermal and chemical stability, and easy fabrication process [25,26]. Mostly, carbon nanofibers are prepared from polyacrylonitrile (PAN) polymer. Besides PAN, various precursors, such as polyimide [27], cellulose [28–30], polyvinylidene fluoride (PVDF) [31], polyvinyl pyrrolidone (PVP) [32], and polyvinyl acetate (PVA) [33], have also been utilized to prepare carbon fibers via electrospinning followed by a suitable thermal treatment. The carbon fibers are highly applied as the electrode material in a supercapacitor due to their high operating voltage, attractive life span, and high capacity retention [34,35]. However, the specific capacitance and energy density need to be further enhanced in order to meet the growing energy demand. The introduction of metal oxides in the carbon nanofibers is beneficial for boosting the electrochemical properties of the resulting composite by the combined effect of the faradaic capacitance of the metal oxides and the double layer capacitance of the carbon nanofibers [1]. The composite of carbon nanofibers with TiO2 can be prepared easily by adding the modification additives in the precursor solution or TiO2 nanoparticles (NPs) formed earlier in the electrospinning process [1,32]. For example, Wang and coworkers [36] have synthesized TiO2 NP-decorated carbon nanofibers (NFs) from a P25/PAN solution by an electrospinning technique followed first by a heat treatment in air and then in a nitrogen environment. Similarly, Tang et al. [24] synthesized a TiO2-carbon NFs composite via an electrospinning technique by using a titanium oxycompound dispersed PAN/PVP solution. The as-fabricated nanocomposite showed good electrochemical performance [24]. The conversion of carbon fiber from PAN requires stabilization and carbonization processes under oxygen and inert atmosphere, respectively [24,25,37]. In this report, we have synthesized TiO2 NPs assembled into carbon nanofibers by an electrospinning process followed by a thermal treatment directly under an inert atmosphere and then investigated the performance of the as-synthesized nanocomposite in a supercapacitor. Importantly, this procedure simply requires the use of a polymer solution containing a TiO2 precursor. Upon thermal treatment, the prepared nanofibers simultaneously produce TiO2 NPs and carbon resulting in the formation of a TiO2 NP-embedded carbon nanofiber composite. We believe that the synthesis procedure is especially attractive since it offers a promising method for the assembly of TiO2 in carbon fibers and significantly boosts the electrochemical performance of the composite. Polymers 2019, 11, 899 3 of 13

Polymers 2019, 11, x FOR PEER REVIEW 3 of 13 2. Experimental 2. Experimental 2.1. Materials 2.1.Polyvinylpyrrolidone Materials (PVP), acetic acid, and titanium tetraisopropoxide (TTIP, 97%) were purchasedPolyvinylpyrrolidone from Sigma-Aldrich (PVP), (Seoul, acetic Korea). acid, Ethanol and titanium was purchased tetraisopropoxide from Samchun (TTIP, Pure 97%) Chemicals, were Co.purchased Ltd., Seoul, from Korea. Sigma-Aldrich All the chemicals (Seoul, were Korea) analytic. Ethanol grade was and werepurchase usedd as from received. Samchun Pure Chemicals, Co. Ltd., Seoul, Korea. All the chemicals were analytic grade and were used as received. 2.2. Synthesis of TiO2 NFs and TiO2-carbon NFs

2.2. Synthesis of TiO2 NFs and TiO2-carbon NFs The synthetic protocol is given in Figure1. TiO 2 nanofibers were synthesized in the laboratory followingThe our synthetic previous protocol report is [38 given]. Briefly, in Figure in the 1. beginning,TiO2 nanofibers 1.5 g ofwere TTIP synthesized was taken in with the 3laboratory g of acetic acidfollowing in a vial our and previous stirred report for 10 [38]. min. Briefly, Next, 0.5in the g of beginning, PVP and 1.5 4 g g of of ethanol TTIP was were taken added with to3 g the of acetic above mixtureacid in and a vial stirred and forstirred 3 h. for The 10 solution min. Next, was 0.5 electrospun g of PVP atand 20 4 kV g of with ethanol a 15 cm were distance added from to the the above tip to themixture collector. and Two stirred nanofiber for 3 h. mats The weresolution prepared was electrospun under identical at 20 kV conditions with a 15 and cm vacuum distance dried from atthe 60 tip◦C forto 12 the h. collector. The nanofiber Two nanofiber mats were mats subjected were prepared to thermal under treatment identical under conditions different and conditions vacuum dried (air and at argon).60 °C Afterfor 12 calcination h. The nanofiber in air atmats 600 were◦C for subjecte 2 h, thed TiOto thermal2 nanofibers treatment were obtained.under different For the conditions synthesis of(air the and TiO 2argon).-carbon After nanofiber calcination composite, in air anotherat 600 °C mat for was2 h, subjectedthe TiO2 nanofibers to carbonization were obtained. under the For argon the atmospheresynthesis of at the 900 TiO◦C2 for-carbon 2 h. nanofiber composite, another mat was subjected to carbonization under the argon atmosphere at 900 °C for 2 h.

FigureFigure 1. 1.Synthesis Synthesis protocol protocol of of pristine pristine titanium titanium dioxide dioxidenanofibers nanofibers (TiO(TiO22 NFs) and TiO 2-carbon-carbon NFs. NFs. 2.3. Characterization 2.3.The Characterization morphology of the prepared samples was checked with field-emission scanning electron microscopyThe morphology (FE-SEM, Hitachi of the prepared S-7400, Tokyo, samples Japan) was checked and transmission with field-emission electron microscopyscanning electron (TEM, JEOLmicroscopy Ltd., Tokyo, (FE-SEM, Japan). Hitachi The latticeS-7400, structures Tokyo, Japan) were and studied transmission by X-ray electron diffractometer microscopy with (TEM, Cu Kα radiationJEOL Ltd., (λ =Tokyo,0.15496 Japan). nm, RigakuThe lattice Co., structures Tokyo, Japan), were studied scanned by from X-ray 10 diffractometer to 80◦. The samples with Cu were Kα characterizedradiation (λ with= 0.15496 Raman nm, spectra Rigaku by Co., collecting Tokyo, on Japan), a Fourier scanned transform from infrared-Raman10 to 80°. The samples spectrometer were (RFS-100S,characterized Bruker, with Hamburg, Raman spectra Germany). by collecting The Fourier on a transformFourier transform infrared infrared-Raman (FTIR) spectra werespectrometer recorded using(RFS-100S, an ABB Bruker, Bomen MB100Hamburg, Spectrometer Germany). (Bomen, The Fourier QC, Canada). transform The infrared thermal (FTIR) behavior spectra was studied were byrecorded thermogravimetric using an ABB analysis Bomen (TGA, MB100 Perkin-Elmer, Spectrometer Akron, (Bomen, OH, QC, USA). Canada). For the The TGA thermal test, the behavior samples werewas kept studied in a by platinum thermogravimetric pan located analysis inside the (TGA, furnace Perkin-Elmer, and heated Akron, from 25 OH, to 800USA).◦C For under the airTGA flow test, at a heatingthe samples rate ofwere 10 ◦keptC/min. in a platinum pan located inside the furnace and heated from 25 to 800 °C under air flow at a heating rate of 10 °C/min. 2.4. Electrochemical Studies 2.4.The Electrochemical working electrodes Studies were prepared by coating the nickel foam with a homogenous slurry of as-preparedThe working samples, electrodes carbon black,were prepared and polyvinylidene by coating the fluoride nickel (PVDF)foam with at aa weighthomogenous ratio ofslurry 8:1:1 of in N-methyl-2-pyrrolidoneas-prepared samples, carbon (NMP). black, The and substrate polyvinylidene was then fluoride dried at (PVDF) 60 ◦C for at 12a weight h. The ratio morphology of 8:1:1 in of theN as-prepared-methyl-2-pyrrolidone slurry is given (NMP). in FigureThe substrate2, which was shows then that dried the at nanofibers 60 °C for 12 were h. The well morphology distributed of in the as-prepared slurry is given in Figure 2, which shows that the nanofibers were well distributed in

Polymers 2019, 11, 899 4 of 13 Polymers 2019, 11, x FOR PEER REVIEW 4 of 13 thethe slurry. slurry. The The electrochemical electrochemical performanceperformance was in investigatedvestigated with with a aconventional conventional three-electrode three-electrode electrochemicalelectrochemical analyzer analyzer at at room room temperature temperature in a 2 M KOH KOH solution solution as as an an electrolyte. electrolyte. Nickel Nickel foam, foam, AgAg/AgCl,/AgCl, and and Pt Pt wire wire were were used used as as thethe workingworking reference and and counter counter electrodes, electrodes, respectively. respectively. Cyclic Cyclic voltammetryvoltammetry (CV), (CV), galvanostatic galvanostatic charge–dischargecharge–discharge (GCD), electrochemical electrochemical impedance impedance spectroscopy spectroscopy (EIS),(EIS), and and the the stability stability by by charge–discharge charge–discharge cyclescycles werewere performed using using Iviumstat. Iviumstat.

FigureFigure 2. 2.Scanning Scanning electron electron microscopy microscopy (SEM)(SEM) image of of the the as-prepared as-prepared slurry slurry of of titanium titanium dioxide dioxide

nanofibersnanofibers (TiO (TiO2 NFs)2 NFs) ( A(A)) and and TiO TiO22-carbon-carbon NFs ( B)) with carbon carbon black black and and polyvinylidene polyvinylidene fluoride fluoride (PVDF)(PVDF) in inN -methyl-2-pyrrolidoneN-methyl-2-pyrrolidone (NMP). (NMP). 3. Results and Discussion 3. Results and Discussion FigureFigure3 shows3 shows the the XRD XRD patterns patterns of of the the nanofibersnanofibers synthesizedsynthesized under different di fferent calcination calcination conditions.conditions. The The nanofibers nanofibers werewere foundfound to be crystalline in in both both cases. cases. As As in in the the figure, figure, the the TiO TiO2 2 nanofibersnanofibers obtained obtained by by calcination calcination atat 600600 ◦°CC in an air atmosphere possessed possessed both both anatase anatase and and rutile rutile phasesphases [32 [32].]. On On the the other other hand, hand, the the TiOTiO22-carbon-carbon composite fibers fibers obtained obtained by by carbonization carbonization at at900 900 °C◦ C underunder an an argon argon atmosphere atmosphere consisted consisted mainly mainly ofof thethe rutilerutile phase [32]. [32]. It It is is noteworthy noteworthy to tomention mention that that thethe higher higher calcination calcination temperature temperature motivated motivated thethe transformationtransformation of of the the crystal crystal phase phase from from anatase anatase to to rutilerutile [39 [39].]. Furthermore, Furthermore, a a broad broad peak peak locatedlocated betweenbetween the 2 2 theta theta values values of of 22° 22 and◦ and 26° 26 was◦ was observed, observed, whichwhich is attributedis attributed to theto (002)the (002) plane plane of the of amorphous the amorphous carbon carbon [1]. Thus, [1]. theThus, XRD the suggests XRD suggests a successful a conversionsuccessful of conversion PVP to carbon of PVP under to carbon the inert under atmosphere. the inert Theatmosphere. crystal size The of crystal the as-synthesized size of the as- TiO2 andsynthesized TiO2-carbon TiO composite2 and TiO fibers2-carbon were composite calculated fibers by employingwere calculated the Debye–Scherrer by employing equationthe Debye– with respectScherrer to their equation highly with intensified respect to peaks their highly as below: intensified peaks as below: Dkλβcosθ, (1) D = kλ(β cos θ), (1) where “D” is the size of the crystal, “k” is a Scherer’s constant, “λ” is the wavelength of the X-ray, “β” whereis the “D” full iswidth the sizeat half of maximum the crystal, (FWHM) “k” is a of Scherer’s the mainconstant, peak, and “ “λθ”” isis thethe wavelengthdiffraction angle. of the From X-ray, “β”the is calculation, the full width the obtained at half maximum crystal sizes (FWHM) for TiO2 NFs of the and main TiO2 peak,-carbon and NFs “ θwere” is 25.11 the di andffraction 27.48 nm, angle. respectively. From the calculation, the obtained crystal sizes for TiO2 NFs and TiO2-carbon NFs were 25.11 and 27.48 nm, respectively.

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Figure 3. XRD spectra of as-synthesized TiO 2-carbon-carbon NFs NFs as as compared compared to pristine TiO 2 NFs.

FiguresFigure4 A–C4A, 4B, depict and the 4C morphologies depict the morpholo of the as-spungies of fiberthe as-spun membrane, fiber TiO membrane,2 NFs, and TiO TiO2 NFs,2-carbon and TiONFs,2-carbon respectively. NFs, Therespectively. as-spun nanofibers The as-spun showed nanofibers continuous showed and bead-free continuous morphology and bead-free with an morphologyaverage fiber with diameter an average of 290 fiber130 diameter nm (Figure of 2904A). ± 130 Well-preserved nm (Figure 4A). nanofibrous Well-preserved morphology nanofibrous was ± morphologyobtained after was the obtained thermal after treatment the thermal in both treatment cases. TiO in2 nanofibersboth cases. showedTiO2 nanofibers smooth showed and continuous smooth andmorphology. continuous It ismorphology. noticeable that It is during noticeable the calcination that during process, the calcination the PVP was process, selectively the PVP removed was selectivelyand continuous removed fibers and of TiOcontinuous2 were obtained fibers of (FigureTiO2 were4B). obtained The calcination (Figure under 4B). The the calcination inert atmosphere under theresulted inert inatmosphere the formation resulted of TiO in2 -incorporatedthe formation carbon of TiO nanofiber2-incorporated structure. carbon During nanofiber the calcination structure. Duringunder the the inert calcination atmosphere, under PVP wasthe graphitized inert atmosphere, and titanium PVP tetraisopropoxide was graphitized was decomposedand titanium to tetraisopropoxidea stable TiO2 form, was leading decomposed to the production to a stable of TiO TiO2 2form,-carbon leading composite to the fibers production (Figure of4C). TiO TiO2-carbon2 NFs compositeshowed an fibers average (Figure fiber 4C). diameter TiO2 NFs of 250 showed170 an nm, average while thefiber TiO diameter2-carbon of NFs 250 showed± 170 nm, an while average the ± TiOdiameter2-carbon of 160NFs 115showed nm. Thean average difference diameter in diameter of 160 distribution ± 115 nm. is attributedThe difference to the atmosphericin diameter ± distributioncondition during is attributed the thermal to the treatment. atmospheric The internal condition structure during of the as-preparedthermal treatment. samples The was internal studied structureby transmission of the as-prepared electron microscopy samples (TEM). was studied As in theby figure,transmission the pure electron TiO2 NFs microscopy revealed a(TEM). smooth As and in theuniform figure, fibrous the pure morphology TiO2 NFs (Figure revealed5A), whereasa smooth the and composite uniform nanofibers fibrous morphology were comprised (Figure of carbon 5A), whereasnanofibers the embedded composite with nanofibers TiO2 nanocrystals were compri (Figuresed5B). of The carbon existence nanofibers of carbon embedded nanofibers with and TiOTiO22 nanocrystalsin the composite (Figure was also5B). confirmedThe existence by theof mappingcarbon nanofibers images (Figure and TiO5C).2 in the composite was also confirmed by the mapping images (Figure 5C).

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Figure 4. Field-emission scanning electron microscopy (FE-SEM) images of an as-spun nanofiber Figure 4. Field-emission scanning electron microscopy (FE-SEM) images of an as-spun nanoﬁber Figuremembrane 4. Field-emission (A), pristine TiO scanning2 NFs ( Belectron), and TiO microscopy2-carbon NFs (FE-SEM) (C). The images insets showof an their as-spun corresponding nanofiber membrane (A), pristine TiO NFs (B), and TiO -carbon NFs (C). The insets show their corresponding membranediameter distributions. (A), pristine TiO22 NFs (B), and TiO22-carbon NFs (C). The insets show their corresponding diameterdiameter distributions.distributions.

Figure 5. Transmission electron microscopy (TEM) images of pristine TiO NFs (A) and TiO -CNFs (B). Figure 5. Transmission electron microscopy (TEM) images of pristine TiO2 2 NFs (A) and TiO2 2-CNFs C 2 2 ( Figure(B)). shows (C )5. shows theTransmission elemental the elemental electron mapping mapping microscopy of the of TiO the 2TiO(TEM)-carbon2-carbon images NF samples.NF of samples. pristine TiO NFs (A) and TiO -CNFs (B). (C) shows the elemental mapping of the TiO2-carbon NF samples.

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FigureFigure6 shows 6 shows the the Raman Raman spectra spectra of pristine of pristine TiO 2TiONFs2 NFs and TiOand2 -CNFs.TiO2-CNFs. As in As Figure in Figure6A, the 6A, pristine the 1 TiOpristine2 NFs areTiO rich2 NFs in are the rich anatase in the phase anatase showing phase majorshowing peaks major at 142,peaks 388, at 142, 516, 388, and 516, 638 cmand− 638[36 cm,40−,411 ]. After[36,40,41]. calcination After undercalcination argon under atmosphere argon atmosphere at 900 ◦C at (Figure 900 °C6 B),(Figure themajor 6B), the anatase major anatase peak at peak 142 wasat highly142 was suppressed highly suppressed and other and peaks other disappeared. peaks disappeared. Instead, Instead, three three new new peaks peaks appeared appeared at aboutat about 245, 1 −1 420,245, and 420, 600 and cm 600− ,cm which, which were were ascribed ascribed to theto the rutile rutile phase phase of of TiO TiO22 [[36,40].36,40]. These These changes changes indicate indicate a transformationa transformation of of the the anatase anatase to to rutile rutile phasephase uponupon a higher degree degree of of thermal thermal treatment. treatment. Besides Besides the TiO2 peaks, two broad peaks corresponding to the D-band and G-band of carbon were observed the TiO2 peaks, two broad peaks corresponding to the D-band and G-band of carbon were observed around 1350 and 1590 cm1 −1, respectively, which further proves that the PVP had been converted to around 1350 and 1590 cm− , respectively, which further proves that the PVP had been converted to the carbonthe carbon structure structure [25]. The[25]. resultsThe results obtained obtained from from Raman Raman are are consistent consistent with with the the XRD XRD data. data.

FigureFigure 6. 6.Raman Raman spectra spectra ofof TiOTiO22 NFs ( (AA)) and and TiO TiO2-carbon2-carbon NFs NFs (B ().B ).

FigureFigure7A 7A represents represents the the thermogravimetric thermogravimetric analysis analysis (TGA)(TGA) proﬁlesprofiles of of the the samples. samples. The The pristine pristine TiOTiO2 NFs2 NFs show show high highthermal thermal stabilitystability (~99.42% (~99.42% at at 800 800 °C).◦C). In Inthe the case case of the of thecomposite composite sample, sample, a a gradualgradual decrease in in the the weight weight was was observed observed until until 550 550 °C ◦andC and after after that thattemperature, temperature, there therewas no was noweight weight loss. loss. Since Since TiO TiO2 did2 didnot notdecompose decompose at the at given the giventemper temperature,ature, the weight the weightloss (23.27%) loss (23.27%)could couldbe attributed be attributed to the to loss the lossof the of carbon the carbon content content in the in composite the composite sample. sample. From Fromthe obtained the obtained data, the data, thecarbon carbon and and TiO TiO2 2contentscontents in in the the composite composite sample sample were were estimated estimated to tobe be ~23.27% ~23.27% and and ~76.73%, ~76.73%, respectively.respectively. The The interaction interaction between between the the TiOTiO22 and carbon was was studied studied by by Fourier Fourier transform transform infrared infrared spectroscopy (FTIR) and the results are given in Figure 7B. The pristine TiO2 NFs showed absorption spectroscopy (FTIR) and the results are given in Figure7B. The pristine TiO 2 NFs showed absorption peaks around 500–700 and 3400 1cm−1 for Ti–O vibration and O–H bending of water molecules peaks around 500–700 and 3400 cm− for Ti–O vibration and O–H bending of water molecules absorbed, absorbed, respectively [42]. In the case of TiO2-carbon composite nanofibers, the peaks at about 1200 respectively [42]. In the case of TiO2-carbon composite nanoﬁbers, the peaks at about 1200 and and 15501 cm−1 are attributed to the C–C stretching vibration and asymmetric and symmetric 1550 cm− are attributed to the C–C stretching vibration and asymmetric and symmetric stretching −1 stretching band of COO–, respectively [1]. The presence of Ti–O–C bonds around 1500–750 cm band of COO–, respectively [1]. The presence of Ti–O–C bonds around 500–750 cm− represents the represents the interaction of TiO2 with the carbon, suggesting the successful formation of the interaction of TiO with the carbon, suggesting the successful formation of the composite [6,23]. composite [6,23].2

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FigureFigureFigure 7. 7.7.Thermogravimetric ThermogravimetricThermogravimetric analysis analysisanalysis (TGA)(TGA) ((AA))) and andand Fourier Fourier transformtransform infraredinfrared infrared spectroscopyspectroscopy spectroscopy (FTIR)(FTIR) (FTIR) spectra (B) of the TiO -carbon NFs as compared to the TiO NFs. spectraspectra ((BB)) ofof thethe TiOTiO2 22-carbon-carbon NFsNFs asas comparedcompared toto thethe TiOTiO22 NFs.NFs.

The electrochemical performance of the as-synthesized TiO -carbon NFs compared with the TheThe electrochemicalelectrochemical performanceperformance ofof thethe as-synthesizedas-synthesized TiOTiO22-carbon-carbon NFsNFs comparedcompared withwith thethe pristine TiO NFs was evaluated by CV, GCD, and EIS tests in a three-electrode system electrochemical pristinepristine TiOTiO2 22 NFsNFs waswas evaluatedevaluated byby CV,CV, GCD,GCD, andand EISEIS teststests inin aa three-electrodethree-electrode systemsystem cellelectrochemicalelectrochemical by using 2 M cellcell KOH byby usingusing as an 22 electrolyte. MM KOHKOH asas anan Figure electrelectr8olyte.A,Bolyte. show FiguresFigures the 8A8A CV andand curves 8B8B showshow of pristine thethe CVCV curves TiOcurves2 and ofof TiOpristinepristine2-carbon TiOTiO NF22 andand samples TiOTiO22-carbon-carbon at diff erent NFNF samplessamples scan rates atat differentdifferent from 10 scanscan to 200 ratesrates mV fromfrom/s with 1010 atoto potential 200200 mV/smV/s window withwith aa potentialpotential from 0 to 0.6windowwindow V, respectively. fromfrom 00 toto As 0.60.6 in V,V, the respectively.respectively. figure, the CV AsAs curve inin thethe of figure,figure, TiO2-carbon thethe CVCV NFs curvecurve shows ofof TiOTiO a larger22-carbon-carbon area NFsNFs compared showsshows aa to thatlargerlarger of pristine areaarea comparedcompared TiO2 NFs, toto thatthat suggesting ofof pristinepristine a better TiOTiO22 NFs,NFs, capacitive suggestingsuggesting behavior. aa betterbetter The capacitivecapacitive quasi-rectangular behavior.behavior. The shapeThe quasi-quasi- of all curvesrectangularrectangular clearly shapeshape indicates ofof pseudocapacitiveallall curvescurves clearlyclearly characteristics. indicatesindicates pseudocapacitivepseudocapacitive The slight diff erencecharacteristics.characteristics. in the redox TheThe peaks slightslight of thedifferencedifference composite inin thethe compared redoxredox peakspeaks to the ofof pristine thethe compositecomposite TiO2 NFs comparedcompared may be toto due thethe to pristinepristine the structural TiOTiO22 NFsNFs di mayffmayerence bebe duedue in the toto thethe two samples.structuralstructural It differencedifference is noteworthy inin thethe to twotwo mention samples.samples. that ItIt theisis noteworthynoteworthy pristine TiO toto 2 mentionmentionNFs were thatthat composed thethe pristinepristine of TiOTiO both22 NFsNFs rutile werewere and anatasecomposedcomposed phases, ofof bothboth whereas rutilerutile the andand TiO anataseanatase2-carbon phases,phases, NFs whereaswhereas were composed thethe TiOTiO22-carbon-carbon mainly NFs ofNFs the werewere rutile composedcomposed phase. In mainlymainly addition, ofof thethethe TiO rutilerutile2-carbon phase.phase. sample InIn addition,addition, could the havethe TiOTiO the22-carbon-carbon combined samplesample effect couldcould of the havehave faradaic thethe combinedcombined capacitance effecteffect of the ofof thethe TiO faradaic2faradaicand the double-layercapacitancecapacitance capacitanceofof thethe TiOTiO22 andand of the thethe carbon. double-layerdouble-layer capacitancecapacitance ofof thethe carbon.carbon.

FigureFigureFigure 8. 8.8.Cyclic CyclicCyclic voltammetry voltammetryvoltammetry (CV) (CV)(CV) curves curvescurves of pristine ofof pristinepristine TiO2 TiOTiONFs22 (NFsANFs) and ((AA) TiO) andand2-carbon TiOTiO22-carbon-carbon NFs (B NFs)NFs at di ((BﬀB)erent) atat scandifferentdifferent rates. scanscan rates.rates.

TheThe specificspecific capacitancecapacitance ofof thethe compositecomposite matematerialrial waswas quantifiedquantified byby GCDGCD studiesstudies overover thethe potentialpotential windowwindow betweenbetween 00 andand 0.450.45 VV atat variousvarious currentcurrent densitiesdensities (1,(1, 2,2, andand 33 A/g)A/g) asas shownshown inin FigureFigure 9.9. ItIt cancan bebe seenseen thatthat whenwhen thethe currentcurrent densidensityty waswas increased,increased, thethe dischargedischarge timetime ofof thethe materialmaterial decreaseddecreased significantly.significantly. ThisThis couldcould bebe duedue toto thethe slsluggishuggish kineticskinetics ofof thethe redoxredox reactionreaction toto fastfast potentialpotential changechange [5].[5]. ManyMany factorsfactors suchsuch asas specificspecific surfacesurface area,area, porepore size,size, andand conductivityconductivity cancan affectaffect

Polymers 2019, 11, 899 9 of 13

The specific capacitance of the composite material was quantified by GCD studies over the potential window between 0 and 0.45 V at various current densities (1, 2, and 3 A/g) as shown in FigurePolymers9. 2019 It can, 11 be, x FOR seen PEER that REVIEW when the current density was increased, the discharge time of the material9 of 13 decreased significantly. This could be due to the sluggish kinetics of the redox reaction to fast potential changethe specific [5]. Many capacitance factors suchof the as electrode specific surfacematerials area, [43]. pore The size, specific and capacitance conductivity of can the a electrodesffect the specific was capacitancecalculated ofby thethe electrodeequation materialsbelow: [43]. The specific capacitance of the electrodes was calculated by the equation below: 𝐼. ∆𝑡 𝐶 , (2) 𝑚.I ∆ t∆𝑉 C = · , (2) m ∆V where “C” is the specific capacitance (F/g), “I” is the· charge–discharge current (A), “t” is the discharge wheretime (s), “C” “ ism” the is the specific mass capacitance of the active (F material/g), “I” is(g), the and charge–discharge “V” is the potential current window (A), “(V).t” is The the potential discharge timewindow (s), “m in” the is the charge–discharge mass of the active curves material match (g), with and the “V cyclic” is the voltammetry potential window curves and (V). suggest The potential that windowthe specific in the capacitance charge–discharge of the curvesmaterials match is due with mostly the cyclic to voltammetrythe faradaic curvesreaction and [1]. suggest The specific that the specificcapacitances capacitance of TiO of2 NFs the materialswere 45.31, is due16.62, mostly and to5.92 the F/g faradaic at the reactioncurrent densities [1]. The specificof 1, 2, capacitancesand 3 A/g, ofrespectively TiO2 NFs were (Figure 45.31, 10A). 16.62, As expected, and 5.92 the F/g specific at the currentcapacitance densities values of were 1, 2, found and 3to A be/g, increased respectively in (Figurethe case 10 ofA). TiO As2-carbon expected, NFs. the The specific specific capacitance capacitance values of the TiO were2-carbon found NF to becomposite increased was in calculated the case of TiOto 2be-carbon 106.57, NFs. 34.44, The and specific12.59 F/g capacitance at the current of densities the TiO2 -carbonof 1, 2, and NF 3 composite A/g, respectively was calculated (Figure 10A). to be 106.57,It is obvious 34.44, andthat 12.59the capacitance F/g at the of current the electro densitiesde decreased of 1, 2, and with 3 Aincreasing/g, respectively current (Figuredensities, 10 A).which It is obviouswas due that to thethe capacitancekinetic resistance of the electrodeand insufficient decreased charge with transfer increasing across current the electrode–electrolyte densities, which was dueinterface to the at kinetic higher resistance current densities and insu [5,43].fficient A charge drop in transfer the discharge across thecurve electrode–electrolyte in the case of pristine interface TiO2 atNFs higher was current noticed, densities which could [5,43 ].be A due drop to inthe the equivale dischargent series curve resistance in the case (ESR). of pristine As compared TiO2 NFs to the was noticed,pristine which TiO2 NFs, could the be composite due to the showed equivalent better series performanc resistancee. The (ESR). electron-transfer As compared kinetics to the pristineof the electrode materials were studied by EIS analysis (Figure 10B). The smaller semicircular arc in the TiO2 NFs, the composite showed better performance. The electron-transfer kinetics of the electrode materialshigh-frequency were studied range byin EISthe analysisNyquist (Figure plots of 10 theB). TheTiO smaller2-carbon semicircular NF composite arc inas thecompared high-frequency to the pristine TiO2 NFs (inset in Figure 10B) shows a better charge transfer property than that of the pristine range in the Nyquist plots of the TiO2-carbon NF composite as compared to the pristine TiO2 NFs TiO2 NFs [1,44]. The inset in Figure 10A displays cyclic stability of the TiO2-carbon NF composite. As (inset in Figure 10B) shows a better charge transfer property than that of the pristine TiO2 NFs [1,44]. in the figure, it revealed about 84% capacitance retention after 2000 cycles (Figure 10A; inset). A The inset in Figure 10A displays cyclic stability of the TiO2-carbon NF composite. As in the figure, comparison of the electrochemical performance of the as-synthesized TiO2-carbon composite it revealed about 84% capacitance retention after 2000 cycles (Figure 10A; inset). A comparison of nanofibers with some previously investigated electrodes is given in Table 1, which indicates a the electrochemical performance of the as-synthesized TiO2-carbon composite nanofibers with some satisfactory performance of the as-prepared TiO2-carbon NF composite for supercapacitor previously investigated electrodes is given in Table1, which indicates a satisfactory performance of the applications. as-prepared TiO2-carbon NF composite for supercapacitor applications.

Figure 9. Galvanostatic charge–discharge (GCD) curves of pristine TiO2 NFs (A) and TiO2-carbon NFs Figure 9. Galvanostatic charge–discharge (GCD) curves of pristine TiO2 NFs (A) and TiO2-carbon NFs (B) at diﬀerent current densities. (B) at different current densities.

PolymersPolymers2019 2019, 11, 11, 899, x FOR PEER REVIEW 1010 of of 13 13

Figure 10. The speciﬁc capacitance (A) and electrochemical impedance spectroscopy (EIS) test (B) of Figure 10. The specific capacitance (A) and electrochemical impedance spectroscopy (EIS) test (B) of diﬀerent electrodes. Inset A and B represent the stability of the TiO2-carbon NF composite and semicircle different electrodes. Inset A and B represent the stability of the TiO2-carbon NF composite and area in the EIS spectra, respectively. semicircle area in the EIS spectra, respectively.

Table 1. Comparison of the electrochemical performance of as-synthesized TiO2/carbon composite nanofibersTable 1. Comparison for supercapacitor of the applicationselectrochemical with performance some investigated of as-synthesized electrodes. TiO2/carbon composite nanofibers for supercapacitor applications with some investigated electrodes. S. Electrode Fabrication Electrolyte Specific S. Electrode Electrolyte Specific Stability Ref. N. Material FabricationMethod Method Used Capacitance Stability Ref. N. Material Used Capacitance TiO2-activated 0.1 N 92 F/g ~89% 1 TiO2-activated Microwave 0.1 N 92 F/g ~89% (5000 [22] 1 carbon Microwave Na2SO4 (at 5 mV/s) (5000 cycles) [22] carbon Na2SO4 (at 5 mV/s) cycles) 1 M 64.364.3 F/g F/g 85% 2 rGO/TiO2/rGO Hydrothermal 1 M 2 85% (4000 [21] 2 rGO/TiO2/rGO Hydrothermal Na2SO4 (at 0.1 mA(at/cm 0.1 ) (4000 cycles) [21] Na2SO4 cycles) Dip coating and mA/cm2) 1 M 250.8 F/g 84.4% 3 BC-G-TiO2 Diphigh coating temperature and high 1 M 250.8 F/g 84.4% (100 [45] 3 BC-G-TiO2 H2SO4 (at 2 A/g) (100 cycles) [45] temperaturetreatment treatment H2SO4 (at 2 A/g) cycles) Chemical 1 M 110110 F/g F/g 4 TiO -CNT Chemical wet- 1 M -[46] 4 TiO2-CNT2 wet-impregnation H SO (at 0.05mA(at/cm 2) - [46] impregnation 2 H42SO4 0.05mA/cm2) Fe-TiO2/C 1 M 137 F/g 5 Fe-TiO2/C Electrospinning 1 M 137 F/g -[47] 5 nanofibers Electrospinning KOH (at 5 mV/s) - [47] nanofibers KOH (at 5 mV/s) 1 M 210.5 F/g 97% 6 MPTNF/rGO Electrospinning 97% [13] H SO1 M (at 1210.5 A/g) F/g (1000 cycles) 6 MPTNF/rGO Electrospinning 2 4 (1000 [13] H2SO4 (at 1 A/g) 6 M 151.5 F/g 97.8%cycles) 7 TiO2@CNFs Electrospinning [24] KOH (at 1A/g) (400097.8% cycles) 6 M 151.5 F/g 7 TiOTiO2@CNFs-carbon Electrospinning 2 M 106.57 F/g 84%(4000 This[24] 8 2 Electrospinning KOH (at 1A/g) nanofibers KOH (at 1A/g) (2000cycles) cycles) study 84% TiO2-carbon 2 M 106.57 F/g This 8 Electrospinning (2000 4. Conclusionsnanofibers KOH (at 1A/g) study cycles)

TiO2 NPs embedded into carbon nanofibers were successfully prepared by an electrospinning technique4. Conclusions and post thermal treatment under argon atmosphere. Upon heating at 900 ◦C, the anatase to rutileTiO phase2 NPs transformationembedded into carbon was achieved. nanofibers The were loading successfully of TiO prepared2 into the by amorphousan electrospinning carbon showedtechnique anextraordinary and post thermal enhancement treatment inunder terms argon of electrochemical atmosphere. Upon properties. heating The at 900 cyclic °C, voltammetry,the anatase galvanostaticto rutile phase charge–discharge, transformation was and achieved. EIS test The results loading exhibited of TiO a2 combinedinto the amorphous synergistic carbon effect showed of TiO 2 andan carbonextraordinary fibers. enhancement Furthermore, in theterms higher of electrochemical conductivity of properties. the rutile The phase cyclic and voltammetry, lower charge limitationgalvanostatic also charge–discharge, favored the electrochemical and EIS test property results exhibited enhancement. a combined Overall, synergistic the results effect obtainedof TiO2 fromand thecarbon electrochemical fibers. Furthermore, study and the the easyhigher synthesis conductivity protocol of showthe rutile potential phase application and lower in charge several applications,limitation also including favored supercapacitors. the electrochemical property enhancement. Overall, the results obtained from

Polymers 2019, 11, 899 11 of 13

Author Contributions: Conceptualization, S.-J.P.; methodology, B.P.; software, B.P.; validation, M.P. and S.-J.P.; formal analysis, B.P.; investigation, S.-J.P.; resources, M.P. and S.-J.P.; data curation, B.P.; writing—original draft preparation, B.P.; writing—review and editing, S.-J.P.; visualization, B.P.; supervision, S.-J.P.; project administration, S.-J.P.; funding acquisition, M.P. and S.-J.P. Funding: This research was supported by the Korea Evaluation institute of Industrial Technology (KEIT) through the Carbon Cluster Construction project (10083586, Development of petroleum based graphite fibers with ultra-high thermal conductivity) funded by the Ministry of Trade, Industry and Energy (MOTIE, Korea). This study was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2019R1A2C1004467). Conflicts of Interest: The authors declare no conflict of interest.

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