Zejun Ding,Tianying Chen,Yiming Zhou,Peng Zhu,Feiyun Li,Yanjun Tang,2,*

1.Pulp and Papermaking Center,Zhejiang Sci-Tech University,Hangzhou,Zhejiang Province,310018,China

2.Key Laboratory of Intelligent Textile and Flexible Interconnection of Zhejiang Province,Zhejiang Sci-Tech University,Hangzhou,Zhejiang Province,310018,China

Abstract:Nanocellulose has served as an eye-catching nanomaterial for constructing advanced functional devices with renewability,light weight,flexibility,and environmental friendliness.In this study,Co3O4/graphene/cellulose nanofiber(CNF)flexible composite films,in which the CNF acted as a spacer for the graphene,were prepared via a facile and scalable vacuum filtration method.The effects of the CNF on the microstructure,hydrophilicity,thermal stability,tensile strength,surface resistance,and electrochemical performance of the Co3O4/graphene/CNF composite films were systematically investigated.The results showed that the synergistic interaction of the CNF and graphene substantially improved the overall properties of the Co3O4/graphene/CNF composite films,particularly their hydrophilicity and tensile strength.Meanwhile,Co3O4/graphene/CNF composite films with a CNF content of 4%appeared to have the optimal electrochemical performance,with an area specific capacitance of 56 mF/cm2 and prominent capacitance retention of 95.6%at a current density of 1 A/g after 1000 cycles.This work demonstrated that the prepared Co3O4/graphene/CNF flexible composite films have great application potential in the field of flexible energy storage devices.

Keywords: cellulose nanofiber; graphene; Co3O4; supercapacitor;composite films

1 Introduction

With the depletion of fossil fuels and severe environmental pollution,there is an urgent need for efficient,clean,and sustainable sources of energy,as well as new technologies associated with energy conversion and storage[1].Supercapacitors can be considered the missing link between traditional capacitors and batteries.They have advantages that include a fast charge/discharge rate,long cycle life,and high power density[2-3]. As the heart of a supercapacitor,the electrodes play a key role in energy storage. Therefore, the development of highperformance electrodes has attracted considerable attention.

To achieve high-performance electrode materials,major progress in theoretical and practical research has recently been made,which has proven that the intrinsic specific capacitance and effective surface area of the electrode material play decisive roles in energy storage applications[1,4-6].Hence,the further development of new materials with high capacitance and improvements in the performances of existing electrode materials have become the two most popular ways to overcome these challenges,with the latter being more attractive and easily achieved[1].Existing electrode materials can mainly be divided into three types[7]:(1)carbon materials such as activated carbon,graphene,and carbon nanotubes;(2)conductive polymers such as polyaniline;and(3)metal oxides such as Co3O4,MnO2,and RuO2.Carbon materials are considered to be the most promising electrode materials because of their abundance,non-toxicity,high specific surface area,good electronic conductivity,and high chemical stability[8].As a carbon material,graphene has received great attention because of its excellent properties,and it possesses the highest specific capacitance(550 F/g)among carbon materials[9-15].However,the van der Waals forces andπ-πbonding give graphene a tendency to agglomerate,significantly reducing its specific surface area and affecting its electrochemical energy storage performance[11,16].Hence,introducing spacers is an effective and common method to improve the specific surface area of graphene[2,17].Yu and Dai[18]fabricated carbon nanotube(CNT)/reduced graphene oxide(RGO)hybrid films with CNTs intercalated between adjacent RGO sheets for the first time.The resultant hybrid films exhibited rectangular cyclic voltammogram curves at a high scan rate of 1 V/s and a specific capacitance of 120 F/g.Huang et al[19]constructed compact RGO/poly(vinyl pyrrolidone)(PVP)composite films by casting a stable aqueous mixture of RGO and PVP,followed by air-drying.The intercalation of hydrophilic PVP between the RGO sheets not only efficiently inhibited the stacking of the RGO sheets but also improved the wettability of the electrodes by the electrolyte.Wu et al[20]fabricated hybrid films of RGO and polyaniline nanofibers by the vacuum filtration of mixed solutions of both components.The composite films exhibited excellent mechanical flexibility and good conductivity.The intercalated polyaniline nanofibers not only inhibited the restacking of the RGO sheets,but also provided some pseudocapacitance.Indeed,the introduction of spacers has proven to be an effective way to improve the utilization efficiency of graphene,allowing graphene-based composite films with excellent properties to be obtained.Therefore,it is important to find an environmentally friendly, biodegradable,renewable,and effective spacer that can not only inhibit the restacking of the graphene but also improve the properties of graphene-based composites for energy storage applications.

In our previous work,nanocellulose was found to possess various excellent properties such as good dispersibility,a film-forming ability,hydrophilicity,abundance, renewability, and environmental friendliness[21-24].Therefore,nanocellulose has the potential to act as a good spacer between graphene sheets.In the present work,cellulose nanofiber(CNF)was introduced to a Co3O4/graphene composite for the fabrication of Co3O4/graphene/CNF composite films via a facile and scalable vacuum filtration method.The effects of the CNF content on the various properties of the resulting Co3O4/graphene/CNF composite films

were systematically investigated.Generally,the results supported the conclusion that CNF has the ability to improve the capacitance of graphene,and the asprepared Co3O4/graphene/CNF composite films showed potential for flexible energy storage device applications.

2 Experimental

2.1 Materials

Commercial microcrystalline cellulose(MCC)powder was provided by the Shanghai Tonnor Material Science Co.,Ltd.,China.Co(NO3)2·6H2O(AR,99%)and urea(AR,99%)were purchased from the Shanghai Macklin Biochemical Co.,Ltd.,China.Graphene was supplied by the Ningbo Asia Pulp&Paper Co.,Ltd.,China.Distilled water was used for all the experiments.

2.2 Preparation of Co3O4

Hydrothermal treatment has been employed to synthesize Co3O4[25-26],i.e.,1.0 g of Co(NO3)2·6H2O and 0.7 g of urea were dissolved in 40 mL of a mixed ethanol/water solution with a 1:1 volume ratio and stirred for 10 min.The as-obtained solution was transferred to a Teflon-lined stainless steel reactor and heated at 150℃for 3 h.The resulting precipitates were dried and calcined for 3 h at 300℃to obtain the Co3O4samples.

2.3 Preparation of CNF

CNF was prepared from MCC via the previously reported high-pressure homogenization method[27].MCC(0.5 g)was dispersed in 100 mL of distilled water with stirring at 80℃for 2 h using an electric mixer(RW20,IKA GmbH.,Germany).The suspension was then homogenized 10 times at 80 MPa in a highpressure homogenizer(AH-BASIC,ATS Engineering Limited,China)to obtain the CNF suspension.The micromorphology (recorded using transmission electron microscopy,TEM)of the prepared CNF is shown in Fig.1.

Fig.1 TEM image of prepared CNF

2.4 Preparation of Co3O4/graphene/CNF composite films

First,Co3O4and graphene were mixed in deionized water at a solid mass ratio(1:4)to obt ai n the Co3O4/graphene suspension.Second,the Co3O4/graphene and CNF suspensions at different solid mass ratios(100:0,98:2,96:4,94:6,92:8,and 90:10)were mixed and stirred for 30 min.Finally,the obtained composite suspensions were filtered using a vacuum filtration device(circulating water vacuum pump,SHZ-D(Ⅲ),Gongyi Yuhua Instrument Co.,Ltd.,China;filter plant,1000 mL,Tianjin Jinteng Experimental Equipment Co.,Ltd.,China),followed by oven drying at 50℃for 5 h(DHG-9070A,Shanghai Yiheng Scientific Instrument Co.,Ltd.,China).The resulting Co3O4/graphene/CNF composite films were denoted as CNF-0,CNF-2,CNF-4,CNF-6,CNF-8,and CNF-10 according to the solid mass ratios.Digital photographs of the Co3O4/graphene/CNF composite films are shown in Fig.2.

Fig.2 Digital photographs of Co3O4/graphene/CNF composite films

2.5 Characterization

The synthetic Co3O4was characterized using X-ray photoelectron spectroscopy(XPS,K-α,Thermo Fisher Scientific Inc.,USA).The chemical structures of the Co3O4/graphene/CNF composite films were characterized with a Fourier transform infrared spectrometer(FT-IR,Nicolet IS50,Thermo Fisher Scientific Inc.,USA)using the attenuated total reflection(ATR)method.

The microstructures of the prepared CNF and Co3O4/graphene/CNF composite films were observed using TEM(JEM-1400Flas,JEOL Ltd.,Japan)and fieldemission scanning electron microscopy (FE-SEM,GeminiSEM500,Carl Zeiss AG,Germany).The thermal stability of each of the Co3O4/graphene/CNF composite films was analyzed using a thermogravimetric analyzer(TG209F3,NETZSCHGerätebau GmbH.,Germany) under a nitrogen atmosphere at a heating rate of 10℃/min.The water contact angles(WCA)of the Co3O4/graphene/CNF composite films were measured using a video contact angle tester(JB-82B,Chengde Dingsheng Testing Machine Co.,Ltd.,China).The tensile strengths of the Co3O4/graphene/CNF composite films were determined using an electronic universal material testing machine(EJA SERIES,Thwing-Albert Co.,Ltd.,USA).The surface electrical resistances of the Co3O4/graphene/CNF composite films were measured using a fourprobe tester(ST-2258C,Suzhou Jingge Electronic Co.,Ltd.,China).

An electrochemical workstation (CHI760E,Shanghai Chenhua Instrument Factory,China)was used to investigate the electrochemical performance of each of the Co3O4/graphene/CNF composite films.A three-electrode configuration was applied using a platinum sheet electrode and saturated calomel electrode as the counter electrode and reference electrode,respectively,where the Co3O4/graphene/CNF composite film directly served as the work electrode without the assistance of Ni foam as the current collector.Cyclic voltammetry(CV)and galvanostatic charge-discharge (GCD) measurements were performed in a 1 mol/L Na2SO4electrolyte at room temperature.The area specific capacitance(C,F/cm2)of each of the Co3O4/graphene/CNF composite films was calculated from the CV curve using the following equation[28]:

whereI(A),A(cm2),v(V/s),andΔV(V)are the current,area of the Co3O4/graphene/CNF composite film,scan rate,and potential range,respectively.

3 Results and Discussion

3.1 Characterization of Co3O4 and Co3O4/graphene/CNF composite films

To validate its synthesis,a component analysis of the Co3O4was performed using XPS.As shown in Fig.3,the XPS spectra of the Co 2p binding energy region generally displayed two peaks at 781.3 and 796.5 eV,which were mainly related to 2p3/2and 2p1/2of the Co2+ions in Co3O4,respectively.Moreover,the XPS spectra exhibited two other satellite peaks(i.e.,the 2p3/2(786.8 eV)and 2p1/2(805.0 eV)),which demonstrated the characteristic peaks of the Co2+/Co3+mixture present in Co3O4[25,29].Therefore,the above results provided evidence that Co3O4particles were successfully synthesized.

Fig.3 XPS spectra of Co 2p binding energy region

The chemical structures of the composite film samples with different amounts of CNF were characterized using FT-IR spectroscopy.As illustrated in Fig.4,the composite films without CNF exhibited four main peaks at 2910,1086,657,and 556 cm-1.The peaks centered at 2910 and 1086 cm-1are generally attributed to the C—H stretching vibration and C—O stretching vibration of graphene,respectively[30].The two remarkably distinctive peaks at 657 and 556 cm-1were primarily a result of the Co—O stretching vibration[29].The Co3O4/graphene/CNF composite films with 4%CNF exhibited three more peaks than the control sample,which were mainly ascribed to the characteristic groups of CNF,including the hydroxyl vibration peaks at 3332 and 1634 cm-1,and the C—H bending vibration peak at 1310 cm-1[27].The chemical structure analyses inferred that homogeneous Co3O4/graphene/CNF composite films were prepared.

Fig.4 FT-IR spectra of Co3O4/graphene/CNF composite films

3.2 Micromorphologies of Co3O4/graphene/CNF composite films

SEM images of the Co3O4/graphene/CNF composite films are shown in Fig.5.It can be observed that the planiform graphene sheets were stacked in parallel,presumably as a result of the van der Waals force andπ-πbonding,thus leading to a compact stacking structure[8,13].Meanwhile,the Co3O4particles appear to be agglomerated and interspersed on the surface of the graphene,which can largely be attributed to their poor dispersibility in water[31].Furthermore,with an increase in the CNF content,the compact stacking structure of the graphene was gradually destroyed,and the agglomerated Co3O4particles became smaller.Moreover,abundant wrinkles and apparent microgaps can be observed on the surfaces of the Co3O4/graphene/CNF composite films,indicating that the CNF could serve as an ideal dispersant and spacer to inhibit the stacking of graphene[2,32].In addition,a tangled network structure and nanopores formed by the CNF could also be observed in the Co3O4/graphene/CNF composite films with 8%CNF and 10%CNF,which would be beneficial for the penetration of a liquid[33-35].

Fig.5 SEM images of Co3O4/graphene/CNF composite films

3.3 Thermal stabilities of Co3O4/graphene/CNF composite films

The thermal stabilities of the Co3O4/graphene/CNF composite films with various amounts of CNF were evaluated by TG analyses.As shown in Fig.6,the TG curves of the Co3O4/graphene/CNF composite films with different amounts of CNF exhibited a trend similar to that of pyrolysis.However,compared with the Co3O4/graphene/CNF composite films without CNF,the Co3O4/graphene/CNF composite films with 4%CNF lost 3%more mass,which was mainly ascribed to the pyrolysis of the CNF at 250℃-350℃[36-37].In addition,both curves displayed the same pyrolysis stages.The first stage,within the range of 250℃-320℃was mainly due to the decomposition of residual CoO(OH)to Co3O4[38-40].The second stage,within the range of 600℃-800℃confirmed the partial decomposition of Co3O4to CoO[40].Consequently,based on the TG analysis,a small amount of CNF exerted a limited effect on the thermal stabilities of the Co3O4/graphene/CNF composite films.

Fig.6 TG curves of Co3O4/graphene/CNF composite films

3.4 Hydrophilicity values of Co3O4/graphene/CNF composite films

For comparison,the hydrophilicity values of the Co3O4/graphene/CNF composite films were evaluated using WCA testing.It could clearly be observed that the incorporation of CNF into the Co3O4/graphene composite resulted in an arresting effect on the hydrophilicity.As shown in Fig.7,the WCA of the Co3O4/graphene composite film in the absence of CNF was 75.3°.In addition,the Co3O4/graphene/CNF composite films with 2%,4%,6%,8%,and 10%CNF exhibited WCAs of 68.7°,61.9°,54.7°,46.4°,and 31.4°,respectively.Compared to the Co3O4/graphene composite films without CNF,the Co3O4/graphene/CNF composite films with 10%CNF showed a 58.3%decrease in the WCA,which indicates a remarkable improvement in the hydrophilicity of the Co3O4/graphene/CNF composite films.This conspicuous improvement was attributed to the contribution of hydrophilic groups in the CNF,such as hydroxyl groups[41].In addition,combined with the special microstructure of the Co3O4/graphene/CNF composite films, another possible explanation for the improvement in hydrophilicity is that the CNF with a tangled network structure could form micro-gaps and nanopores,which had the ability to facilitate the penetration of a liquid[33,42].

Fig.7 WCAs of Co3O4/graphene/CNF composite films

3.5 Tensile strengths of Co3O4/graphene/CNF composite films

The crucial role of the CNF in the tensile strengths of the Co3O4/graphene/CNF composite films was investigated,and the results are shown in Fig.8.In general,with an increase in the incorporation of CNF,the tensile strength increased significantly.Specifically,the tensile strength of the Co3O4/graphene/CNF composite films without CNF was found to be 4.0 MPa,which increased to 13.5,27.4,36.4,42.4,and 45.8 MPa in the presence of 2%,4%,6%,8%,and 10%CNF,respectively.Compared to the Co3O4/graphene/CNF composite films without CNF,the Co3O4/graphene/CNF composite films with 10%CNF showed an order of magnitude enhancement in the tensile strength,with the strong interaction between the graphene and CNF playing a pivotal role.The hydrophilic behavior of the CNF was attributed to the hydroxyl groups located on its surface.Meanwhile,the exposure of the hydrophobic C—H bonds caused hydrophobic faces, which enabled hydrophobic interactions with the hydrophobic graphene. In addition,the spatial gaps between graphene sheets were filled with flexible CNF.A large number of interfaces between the graphene and CNF were hydrogen bonded,which significantly enhanced the load transfer between the graphene sheets.The synergistic interaction between the graphene and CNF substantially increased the load resistance of the Co3O4/graphene/CNF composite films[43].In contrast,in the Co3O4/graphene/CNF composite films without CNF,there were only a limited number of hydrogen bonding sites in the graphene,and the interactions were essentially weak van der Waals forces.As a result,the Co3O4/graphene/CNF composite films without CNF were weak and easily fractured[16,30].

Fig.8 Tensile strengths of Co3O4/graphene/CNF composite films

3.6 Surface resistances of Co3O4/graphene/CNF composite films

The surface resistances of the Co3O4/graphene/CNF composite films were measured and studied,and the results are shown in Fig.9.In the Co3O4/graphene/CNF composite films without CNF,the sheet resistance was 0.063 kΩ/sq.However,the sheet resistances of the Co3O4/graphene/CNF composite films increased to 0.239,0.441,0.727,1.040,and 1.218 kΩ/sq in the presence of 2%,4%,6%,8%,and 10%CNF,respectively.In CNF-0,many continuous conductive channels were formed by compactly stacked graphene,which promoted the transfer of electrons in the Co3O4/graphene/CNF composite films[44].With the increasing CNF content in the Co3O4/graphene/CNF composite films,the interaction between the CNF and graphene led to the insulating CNF filling the gaps between graphene sheets[43],which broke the continuity of the conductive channels and inhibited the transfer of electrons.Thus,the surface resistance of the Co3O4/graphene/CNF composite films gradually increased with the CNF content.

Fig.9 Surface resistances of Co3O4/graphene/CNF composite films

3.7 Electrochemical performances of Co3O4/graphene/CNF composite films

The electrochemical properties of the Co3O4/graphene/CNF composite films were tested using a threeelectrode configuration.Fig.10 presents the CV curves of the Co3O4/graphene/CNF composite films at a scan rate of 10 mV/s.All the curves exhibited a quasirectangular shape,indicating an efficient and rapid charge transfer.Meanwhile,it was also obvious that as the amount of CNF increased from 0 to 4%,the area of the CV curves of the Co3O4/graphene/CNF composite films increased correspondingly.However,when the amount of CNF increased from 4%to 10%,the area of the CV curves visibly decreased.This unique change proved that the addition of CNF facilitated the area specific capacitance of the Co3O4/graphene/CNF composite films.This was mainly attributed to three factors:(1)the hierarchical pores formed by the CNF were beneficial for the transfer of ions;(2)the hydrophilicity of the Co3O4/graphene/CNF composite films promoted by the CNF was also advantageous for electrolyte penetration;and(3)the larger effective surface of the graphene improved by the CNF was stimulative for the absorption of ions,facilitating charge storage.In addition,Co3O4/graphene/CNF composite films with 4%CNF were selected as optimal candidates because of their high area specific capacitance of 56 mF/cm2.

Fig.10 CV curves of Co3O4/graphene/CNF composite films

Additional electrochemical experiments were performed to assess CNF-4.As shown in Fig.11,the CV curves of the Co3O4/graphene/CNF composite films with 4%CNF at scan rates between 1 and 100 mV/s have a quasi-rectangular shape,demonstrating an ideal electrical double-layer capacitance behavior.However,when the scan rate was increased to 1000 mV/s,the CV curve displayed a shuttle shape,which was mainly attributed to the deficient utilization efficiency of graphene at a high scan rate[6].Correspondingly,the GCD curves of the Co3O4/graphene/CNF composite films with 4%CNF at current densities of 0.01-1 A/g exhibited quasi-isosceles triangle shapes(Fig.12),which also proved the electrical double-layer capacitance behavior of the Co3O4/graphene/CNF composite films with 4%CNF[2].

Fig.11 CV curves of Co3O4/graphene/CNF composite films with 4%CNF at different scan rates

Fig.12 GCD curves of Co3O4/graphene/CNF composite films with 4%CNF at different current densities

Finally,the cycle lives of the Co3O4/graphene/CNF composite films with 4%CNF were measured using 100 CV cycles(10 mV/s)and 1000 GCD cycles(1 A/g),as shown in Figs.13 and 14,respectively.The shape of the CV curves after 100 cycles showed almost no change compared to the shape in the first cycle.Meanwhile,the results of the GCD cycles revealed that the area specific capacitance remained at 95.6%at a current density of 1 A/g after 1000 cycles,which exhibited a satisfactory life stability[45].

Fig.13 CV curves of Co3O4/graphene/CNF composite films with 4%CNF after different cycle numbers at scan rate of 10 mV/s

Fig.14 Cyclic stability of Co3O4/graphene/CNF composite films with 4%CNF for 1000 cycles at current density of 1 A/g

4 Conclusions

A facile and scalable vacuum filtration method was utilized for the preparation of Co3O4/graphene/cellulose nanofiber(CNF)composite films.The effects of the CNF content on the microstructure and properties were systematically investigated. In particular,the results indicated that 4%CNF appeared to be the optimal concentration for application in supercapacitors,at which the Co3O4/graphene/CNF composite films presented the highest area specific capacitance of 56 mF/cm2and a high capacitance retention of 95.6%at a current density of 1 A/g after 1000 cycles(galvanostatic charge-discharge(GCD)measurements). The improved electrochemical performance was mainly attributed to three factors:(1)the hierarchical pores formed by the CNF were beneficial for the transfer of ions;(2)the hydrophilicity of the Co3O4/graphene/CNF composite films promoted by the CNF was also advantageous for electrolyte penetration;and(3)the larger effective surface of the graphene improved by the CNF was stimulative for the absorption of ions,facilitating charge storage.The Co3O4/graphene/CNF composite films showed application potential for environmentally friendly,lightweight,renewable,and flexible energy-storage devices.Finally,we hope that our work can provide some references for the construction of CNF-based composite films.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (grant no.22078306),Key Research and Development Program of Zhejiang Province(grant no.2020C02021),521 Talent Cultivation Program of Zhejiang Sci-Tech University(grant no.11110132521310),and Qujiang Science and Technology Project(grant no.QJ2020023).