Youngchu Kang,Hailan Jin,Xianhui An,Xueren Qian

Key Laboratory of Bio-based Material Science&Technology(Northeast Forestry University)Ministry of Education,Harbin,Heilongjiang Province,150040,China

Abstract:Polypyrrole(PPy)@cellulose fiber-based composites have been widely investigated as electrode materials for use in flexible supercapacitors.However,they cannot readily provide high specific capacitance and cyclic stability owing to their inherent drawbacks,such as high resistance,Weber impedance,and volume expansion or collapse during charging/discharging.In this study,iron oxyhydroxide (FeOOH)is incorporated in the abovementioned composite to decrease the equivalent series resistance,charge transfer resistance,and Weber impedance,thereby enhancing electron transfer and ion diffusion,which results in superior electrochemical performance.The PPy-wrapped FeOOH@cellulose fiber-based composite electrode with the molar ratio of FeSO4 to NaBH4 of 1∶1 exhibits a high specific capacitance of 513.8 F/g at a current density of 0.2 A/g,as well as an excellent capacitance retention of 89.4%after 1000 cycles.

Keywords:cellulose fibers;iron oxyhydroxide;polypyrrole;electrode material;electrochemical performance

1 Introduction

Supercapacitors(SCs)with fast charge/discharge capabilities,high power density,and long cycling time have garnered significant interest from researchers[1−3].However,SCs have a low energy density,which hinders their practical application.Inaddition,owing to the active development of flexible wearable devices,the relevant energy storage devices have become increasingly important,thereby resulting in higher requirements for SCs,i.e.,they should exhibit excellent electrochemical performance and good mechanical properties,such as stretching,folding,and twisting[4−9].Electrode materials are vital to the development of SCs,and researchers have been attempting to identify electrode materials that can providehigher specific capacitances.

Researchers have performed extensive studies pertaining to electrode materials,which primarily include carbon materials[10−11],metal oxides[12−13],metal sulfides[14],metal carbides[15],conductive polymers(polyaniline[16−18],polypyrrole (PPy)[19−23],polythiophene[24−25], and poly- (3, 4-ethylenedioxythiophene)[26−27].Among these electrode materials,PPy is a promising high-quality electrode material owing to its high specific capacitance,high electrical conductivity,facile synthesis,environmental friendliness,and low cost.Nevertheless,some disadvantages,such as poor cyclic stability and low processability,limit its further application.Hence,a few strategies have been developed to restrain volumetric change during charging/discharging to enhance cyclic stability.One is to modify the morphology of PPy,such as nanorods and nanowires.Another strategy is to combine PPy with other materials,such as carbon materials and metal oxides.We attempted to grow PPy on the surface of metal oxyhydroxides@cellulose fiber-based composites to investigate their electrochemical performance.Cellulose fiber is a suitable substrate or skeleton for the growth of PPy and provides good cyclic stability.Additionally,cellulose fibers can be customized to improve their processability.Cellulose is the most abundant natural polymer on Earth and offers a distinct advantage in the preparation of composite materials.Furthermore,cellulose contains a significant number of hydroxyl functional groups;this enables it to exhibit hydrophilicity,which facilitates the storage and transfer of electrochemical energy.Iron is one of the most prevalent elements in the Earth's crust.Significant effort has been expended to investigate the application of iron-based composites in SCs,such as Fe2O3,Fe3O4,FeOx,and iron oxyhydroxide(FeOOH)[28−33].Among them,FeOOH offers a high specific surface area,environmental friendliness,richness,non-toxicity,and simple synthesis.In addition,its surface comprises a significant number of hydroxyl groups(—OH),which are conducive to its loading on the surface of cellulose fibers.To our knowledge,this strategy will effectively improve some inherent disadvantages(i.e.,higher equivalent series resistance,charge transfer resistance,and Weber impedance)of electrode materials such that a higher specific capacitance and cycle stability can be achieved.Cellulose fibers,which are flexible and freestanding,are promising substrates.

In this study,we prepared PPy-wrapped FeOOH@cellulose fiber-based composites via the liquid-phase reduction of sodium borohydride(NaBH4)in an open system at room temperature,followed byin situoxidative polymerization.This strategy introduces FeOOH onto the surface of cellulose fibers,thereby improving the electrochemical behavior and providing a higher specific capacitanceand cyclic stability of SCs.

2 Experimental

2.1 Materials

Cellulose fibers (CFs, Canada market-bleached softwood kraft pulp)were obtained from Mudanjiang Hengfeng Paper Co.,Ltd.,and beaten to 37°SR with a Valley beater(ZQS2-23)prior to use.Ferrous sulfate(FeSO4·7H2O),NaBH4,and ferric chloride(FeCl3·6H2O)were purchased from Sinopharm Chemical Reagent Co.,Ltd.,Shanghai Macklin Biochemical Co.,Ltd.,and Tianjin Hengxing Chemical Reagent Co.,Ltd., respectively. Pyrrole was purchased from Sinopharm Chemical Reagent Co.,Ltd.,and purified via distillation prior to use.All other chemicals were of analytical grade.

2.2 Preparation of FeOOH@CF-based composites

The FeOOH@CF-based composite(marked as IC)was prepared via the liquid-phase reduction of NaBH4in anopen system at room temperature.The cellulose fibers(0.5 g,dry weight)were dispersed in distilled water under vigorous stirring for 30 min,and then FeSO4·7H2O(1 mmol)was added dropwise.Subsequently,NaBH4(0.5−2.0 mmol)was added to the suspension to initiate the reaction,which was allowed to occur for 6 h in an open system at room temperature.The resulting composites with different molar ratios of FeSO4to NaBH4(2∶1,1∶1,and 1∶2)weredenoted IC-21,IC-11,and IC-12,respectively,and used to prepare PPywrapped FeOOH@CF-based composites(marked as PICs).

2.3 Preparation of PICs

The PICs were prepared via thein situpolymerization of pyrrole on the surface of ICs.First,pyrrole(0.25 mL)was dispersed in the abovementioned suspension of ICs,and then FeCl3·6H2O(0.973 g)was added to initiate polymerization(which was performed for 6 h)in an ice-bath system.Subsequently,the mixture was washed with distilled water several times to remove unreacted substances and other impurities.Finally,PPy-wrapped FeOOH@CF-based composites were obtained and named PIC-21,PIC-11,and PIC-12.For comparison,PPy/CF-based composites were prepared and marked as PC.

2.4 Characterizations

The morphologies of the cellulose fibers,IC,PC,and PIC were characterized via scanning electron microscopy(SEM,SEU 8010,Hitachi High-Tech Science Corporation)at an operating voltage of 5 kV,and the elemental distribution of PIC-11 was analyzed via energy dispersive spectrometer(EDS,Hitachi High-Tech Science Corporation). X-ray photoelectron spectroscopy(XPS)was performed using a Thermo ESCALAB 250XI(Thermo Tisher Scientific,USA)with Al Kαradiation,and Fourier transform infrared spectroscopy(FT-IR)was conducted using an FT-IR spectrometer(Nicolet 6700,Thermo Fisher Scientific Inc.)within therangeof 400−4000 cm-1.

2.5 Electrochemical measurements

Electrochemical measurements of the electrode materials were conducted on an electrochemical workstation(CHI-660E)using a three-electrode system(Pt counter electrode and Ag/AgCl reference electrode).Cyclic voltammetry(CV)and galvanostatic charge/discharge (GCD)tests were performed at different scan rates(5,10,20,30,40,and 50 mV/s)and current densities(0.2,0.5,1,2,and 3 A/g)in a 0.6 mol/L H2SO4electrolyte under a voltage range of-0.2−0.6 V (vs.Ag/AgCl).Electrochemical impedance spectroscopy(EIS)was conducted from 0.01 Hz to 100 kHz with a 5 mV amplitude.The specific capacitance(Cm)and Coulombic efficiency(Ce)of the composite electrode were calculated as follows[34]:

whereI(A/g)is the discharge current;mis the active mass(g);t1andt2are the discharge time(s)and charge time(s),respectively;ΔV(V)represents the potential window.

3 Results and discussion

Fig.1 shows the fabrication process and images of the flexible PIC electrode. The specific procedures performed were as follows:First,the FeOOH was depositedin situon the surface of cellulose fibers via NaBH4reduction at room temperature in open system conditions, based on previous studies[35−36].Subsequently,PPy was synthesizedin situon the surface of the composite in an ice-water bath using FeCl3·6H2O as an oxidant.Consequently,a PIC electrode was obtained. Because the composite electrode is prepared based on papermaking technology,a large-scale production can be realized easily(Fig.1(c),diameter of 9 cm).As shown in Fig.1(d)−Fig.1(f),the cellulose fibers as a flexible substrate exhibited excellent flexibility and customizability.Additionally,the electrode materials were binder-free,which can reduce the internal resistance and result in better electrochemical performance.

Fig.1 Illustration of fabrication processand imagesof flexible PICelectrode

The morphologies of the cellulose fibers,PC,IC,and PIC are shown in Fig.2.As shown by the SEMimage in Fig.2(a),the surface of the cellulose fibers was smooth.Meanwhile,the SEM image shown in Fig.2(b)reveals the presence of PPy on the surface of the cellulose fibers.However,the morphology appeared loose and exhibited a short rod shape,which may have contributed to the poor cyclic stability and specific capacitance.The SEM image presented in Fig.2(c)shows that FeOOH was deposited on the surface of the cellulose fibers.FeOOH contains a significant number of—OH groups,which is beneficial to its loading on the surface of cellulose fibers via intermolecular hydrogen bonds.Compared with the PPy in PC(Fig.2(b)),the PPy in PIC-11(Fig.2(d))exhibited a spherical shape and dense structure,which might restrain the collapse and expansion of the electrode materials during cycling,thereby improving the cyclic stability and specific capacitance.

XPS was used to analyze the elemental valence states and chemical compositions of the composite flexible electrodes.As shown in Fig.3(a)−Fig.3(c),the XPS spectrum of the cellulose fibers(Fig.3(a))indicates only elements C and O,whereas the XPS spectra of IC(Fig.3(b))and PIC-11(Fig.3(c))indicate additional Fe and N peaks,respectively.The presence of the Fe peak proves that FeOOH was successfully loaded on the surface of the cellulose fibers.The presence of the N peak suggests the deposition of PPy.The high-resolution XPS spectrum of Fe 2p of IC is shown in Fig.3(d).The Fe 2p exhibited two main peaks at 711.0 eV(satellite peak at 719.0 eV)and 724.2 eV (satellite peak at 732.4 eV),which are characteristic peaks of Fe3+in FeOOH[37−39].As shown in Fig.3(e),the N 1s spectrum can be fitted to three peaks corresponding to＝N—(398.8 eV),—NH—(399.7 eV), and —NH+— (401.5 eV), which demonstrates the successful coating of PPy.However,no iron was indicated in the XPS spectrum of the PIC-11 composite electrode.We assumed that PPy shielded FeOOH on the surface of the cellulose fibers.FT-IR was performed to determine the chemical structure of the composite materials(Fig.3(f)).The peaks of the cellulose fibers at 3339 and 2900 cm-1were due to O—H and C—H in the pyranoid ring,respectively[40−41].In addition,the peaks of the cellulose fibers at 1105,1057,and 1025 cm-1were attributed to C—O,C—C,and C—O—C, respectively. The IC exhibited characteristic peaks of the cellulose fibers.PIC-11 shows the characteristic absorption bands of PPy,which indicates that PPy was successfully coated on the surface of the cellulose fiber composites.The peaks of PIC-11 at 1548,1295,and 1098,962 cm-1can be assigned to the C＝C,C—N,N—H,and C—C bonds of the PPy ring,respectively[42−45].To further investigate the distribution of elements in the composite,we conducted EDS mapping tests,which suggested the uniform dispersion of C,O,Fe,and N elements in the PIC-11 electrode material,as shown in Fig.4,and hence the homogeneous distribution of FeOOH and PPy on thecellulose fibers.

Fig.3 XPSspectraof(a)cellulosefibers,(b)IC,(c)PIC-11,(d)Fe2p(IC),and(e)N 1s(PIC-11);(f)FT-IRspectraof cellulose fibers,IC,and PIC-11

Fig.4 Elemental mapping images of C,O,Fe,and N of PIC-11 via EDS

The electrochemical performances of the PC and PIC electrodes were measured using a three-electrode system in H2SO4electrolyte.As shown in Fig.5(a),cyclic voltage tests were performed at a scan rate of 5 mV/s to observe the electrochemical behavior of the different electrode materials.Compared with the CV curves of PC,PIC-21,and PIC-12 electrodes,the CV curve of PIC-11 electrode exhibited the largest integral area, suggesting its excellent electrochemical performance.Meanwhile,the PIC electrode with a distinct redox peak suggested that the PC electrode was improved via the modification of FeOOH,whichfacilitated the realization of better electrochemical performance.In addition,Fig.5(b)shows the GCD curves at a current density of 0.2 A/g.The PIC-11 electrode delivered a much longer discharge time than PC,PIC-21,and PIC-12 electrodes.The specific capacitances of PC,PIC-21,PIC-11,and PIC-12 electrodes were 308.8,364.0,513.8,and 435.6 F/g,respectively(Fig.5(c)).

The GCD profiles of the PC and PIC-11 electrodes at current densities of 0.2,0.5,1,2,and 3 A/g are shown in Fig.6(a)and Fig.6(b),respectively.The charge and discharge times of the PC and PICelectrodes decreased as the current density increased,which is a typical phenomenon in electrochemistry.This is attributed primarily to the short reaction time under a high current density,which prevented all the active materials from entering the electrode material;in fact,this aspect of the specific capacitance may only occur on the surface of the active material[46].The similar symmetric and linear curves indicate superior Coulombic efficiency,which was also demonstrated in the electrochemical cyclic process(Fig.7(a)and Fig.8(a)).The specific capacitances of PC and PIC-11 electrodes at various current densities were calculated based on Equation(1)derived from Fig.5(a)and Fig.5(b).The specific capacitances of PC electrode were 308.8,228.3,151.5,76.8,and 55.9 F/g at current densities of 0.2,0.5,1,2,and 3 A/g,respectively.The specific capacitances of PIC-11 electrode were higher than those of PC electrode,i.e.,513.8,418.7,357.5,287.2,and 244.1 F/g at current densities of 0.2,0.5,1,2,and 3 A/g(Fig.6(c)),respectively.In addition,approximately 47.5%specific capacitance of PIC-11 electrode remained when the current density increased from 0.2 to 3 A/g,which was higher than that of PC electrode(~18.1%),demonstrating better rate capability.The results indicated that the incorporation of FeOOH improved electrochemical performance.

Fig.5 (a)Comparative CV curvesof PC,PIC-21,PIC-11,and PIC-12 electrodesat 5 mV/sscan rate;(b)comparative GCD curvesof PC,PIC-21,PIC-11,and PIC-12 electrodes at 0.2 A/g current density;(c)bar graphs of specific capacitance for PC,PIC-21,PIC-11,and PIC-12 electrodes

To further evaluate the electrochemical performance of the electrode materials,a cyclic stability test was performed,and the results are shown in Fig.7(a).The capacitance retention of PIC-11 electrode reached 89.4%after 1000 cycles at a current density of 3 A/g,which was higher than that of PC electrode(72.2%,Fig.8(a)),suggesting that the introduction of FeOOH compensated for the volumetric expansion and collapse of the electrode material during charging/discharging.Based on the results,we performed EIS measurements in the frequency range from 0.01 Hz to 100 kHz to determine the root causes more comprehensively.As shown in Fig.7(d),the EIS spectrum of PIC-11 electrode primarily included three components:the equivalent series resistance(R1),transfer resistance(R2),and Weber impedance(W).The equivalent series resistance is the intersection of the semicircle andXaxis in the high-frequency area;it refers to the inherent resistance of the electrode material, electrolyte resistance,and contact resistance between the electrode material and current collector.The transfer resistance refers to the semicircle diameter in the high-frequency region,whereas the 45°straight section in the lowfrequency region represents the Weber impedance,which represents the ion diffusion process from the electrolyte to the electrode material.As shown in Fig.7(b),Fig.7(c),and Fig.8(b),the equivalent series resistance of the PIC-11 electrode is lower(~4.3Ω)than that of PC electrode(~5.9Ω),indicating that the incorporation of FeOOH can reduce the inherent resistance of the electrode material.In addition,the EIS curve of PIC-11 electrode exhibited a semicircle diameter,indicating a lower transfer resistance.The PIC-11 electrode shows a shorter Werburg-type line than PC electrode,signifying a better ion diffusion process.

Fig.7 (a)Capacitanceretention and Coulombic efficiency of PIC-11 electrodeover 1000 cyclesat 3A/g current density;(b)Nyquist plotsof PIC-11 electrodebeforeand after 1000 cycles;(c)Nyquist plotsof PIC-11 electrode;(d)EIScircuit of PIC-11 electrode

Fig.8 (a)Capacitance retention and Coulombic efficiency of PCelectrode over 1000 cycles;(b)Nyquist plots of PCelectrode before and after 1000 cycles

Based on the results above and compared with other electrode materials[16−27,47−51], PIC is an excellent electrode material for the following reasons:(1)cellulose is the most abundant natural polymer on Earth and exhibits distinct advantages of being flexible,porous,and customizable;(2)PPy as an active electrode material offers some unique advantages,such as high specific capacitance, high electrical conductivity, facile synthesis, environmental friendliness,and low cost;(3)compared with the PC electrodes,the incorporation of FeOOH decreased the equivalent series resistance,charge transfer resistance,and Weber impedance of the electrode materials,thereby enhancing electron transfer and ion diffusion,as well as providing a higher specific capacitance and cyclic stability. Therefore, PIC-11 with better electrochemical performance can be interpreted as having a lower resistance,which promotes ion diffusion,electron transfer,and the redox reaction.

4 Conclusions

In this study, polypyrrole (PPy)-wrapped iron oxyhydroxide@cellulose fiber-based composite(PIC)electrode materials were successfully fabricated via the liquid-phase reduction of NaBH4in an open system at room temperature andin situpolymerization under an ice bath.The incorporation of iron oxyhydroxide ameliorated the intrinsic defects of PPy@cellulose fiber-based composites.The prepared PIC-11 (the molar ratio of FeSO4to NaBH4was 1∶1)yielded a higher specific capacitance of 513.8 F/g at a current of 0.2 A/g and a capacitance retention cyclic stability of 89.4% after 1000 cycles.Owing to its excellent flexibility and electrochemical performance,PIC-11 can be utilized as an electrode material in the field of energy storage.

Acknowledgments

This study was supported by the National Natural Science Foundation of China(grant no.31770620).