Fast self-assembled microfibrillated cellulose@MXene film with high-performance energy storage and superior mechanical strength

Zhirong Zhang,Zhongping Yao,Zhaohua Jiang

School of Chemistry and Chemical Engineering,State Key Laboratory of Urban Water Resource and Environment,Harbin Institute of Technology,Harbin 150001,China

ABSTRACT The trade-off between the electrochemical performance and mechanical strength is still a challenge for Ti3C2Tx free-standing electrode.Herein,a facile approach was proposed to fabricate a Microfibrillated cellulose@Ti3C2Tx(MFC@Ti3C2Tx)self-assembled microgel film by means of hydrogen bonding linkage.Benefiting from the rich hydroxyl groups on the MFC,the Ti3C2Tx nanosheets coated on the MFC in a time scale of minutes(within 1 min)instead of hours.The ultralong 1D frame of MFC effectively mitigated the re-aggregation of Ti3C2Tx nanosheet.The fluffy MFC@Ti3C2Tx film structure and the constructed 1D/2D conducting Ti3C2Tx pathways in horizontal and vertical directions endowed the fast ion transport of the electrolytes and the improved accessibility to the Ti3C2Tx surface.As a result,the freestanding MFC@Ti3C2Tx microgel film delivered a high specific capacitance of 451F/g.And the rate performance was increased to 71% from the 64% of that of pristine Ti3C2Tx film.Furthermore,the tensile strength of MFC@Ti3C2Tx film was also promoted to 46.3 MPa,3 folds of that of the pristine Ti3C2Tx film,due to the high strength of MFC and the hydrogen bonding effect.

Keywords:MXene Microfibrillated cellulose Supercapacitor Self-assemble Microgel

Recently,2D layered materials always gain tremendous attraction in the fields of electromagnetic interference shielding[1],catalysis[2],photothermal conversion[3]and energy storage field[4,5]for their unique characteristics,such as their planar geometry with atomic or molecular thickness[6,7],easy to process[8,9]and more exposed atom[10].Among the existing 2D nanomaterials,the thriving family,MXene(Mn+1XnTx),as one of the most promising electrode materials,is standing out for their unique combination of metallic conductivity,nanosheet processability,hydrophilicity and widely tunable properties[11–13],where,M is an early transition metal such as Ti,X is C or N,and Txindicates the different functional groups(--O,--OH and --F)on the surface of metal layers.However,the self-aggregation of Ti3C2Txnanosheets due to the strong interlayer van der Waals interaction or hydrogen bonding deteriorates ionic dynamical diffusion,bringing about low accessibility of the electrolyte ions[14,15].As a result,their performance in many fields is hindered severely by the aggregate phenomenon.

Hitherto,two approaches have been proposed for alleviating the self-stacking as well as enhancing ion transport kinetics,including construction of porous 3D structures[16,17]and introduction of interlayer pillars[18].Nevertheless,the mechanical performance is rarely reported in above works,which is also crucial for practical supercapacitor application as a free-standing film.Actually,numerous efforts have been devoted to reinforce the mechanical strength of Ti3C2Txfilm,typically,introducing the nano-scale polymers(such as polyvinyl alcohol,bacteria cellulose and cellulose nanofiber)[19–21].Unfortunately,the improvement of mechanical properties is at the expense of electrochemical performance.Generally,the capacitance of these nanocomposite films decreased quickly with the addition of nano-scale polymers due to the reduced interlayer conductivity.Thereby,there is much room in developing MXene-based materials to realize the trade-off between mechanical properties and electrochemical performance.

Herein,a novel strategy was proposed to fabricate antiaggregation MFC@Ti3C2Txfilm electrode towards promoted electrochemical performance as well as mechanical strength.Cellulose with micron diameter(MFC)was selected as reinforcing materials due to their high aspect ratio,abundant of--OH groups and high Young’s modulus(＞150 GPa)[22].In designed architecture,the Ti3C2Txnanosheets would be coating 1D MFC to produce MFC@Ti3C2Txmicrogel quickly owing to the rich --OH groups on MFC,hereby the 1D/2D conducting Ti3C2Txpathways in horizontal and vertical directions was constructed.As-prepared MFC@Ti3C2Txmicrogel not only suppresses the aggregation of Ti3C2Txnanosheets efficiently,but the mechanical properties of MFC@Ti3C2Txfilm have enhanced due to the framework of 1D MFC.As a result,the rate performance of MFC@Ti3C2Txfilm is increased from 64%to 71% profit by the unique anti-aggregation structure for fast ion transport.What is more,the self-assembled MFC@Ti3C2Txfilm could deliver an enhanced specific capacitance of 451 F/g at 1 A/g and superior tensile strength of 46.3 MPa,the balance of the energy storage and mechanical strength is realized in such anti-aggregation MFC@Ti3C2Txfree-standing film.

Fig.1.(a)Schematic illustrating the fabrication process of MFC@Ti3C2Tx microgel,the inset is digital photo of Ti3C2Tx colloid solution(left),MFC solution(middle)and diluted MFC@Ti3C2Tx microgel dispersions(right).(b)Zeta potentials of Ti3C2Tx and MFC@Ti3C2Tx dispersions.(c)HR-TEM image of Ti3C2Tx nanosheets,(d)TEM image of MFC@Ti3C2Tx dispersions.(e)STEM image of MFC@Ti3C2Tx and the corresponding EDX elemental mapping of(f)C,(g)F and(h)Ti.

The Ti3C2Txcolloid solution was first prepared by etching Ti3AlC2in HCl and LiF solution.After the introduction of MFC with abundant --OH groups(Fig.S2 in Supporting information)[23],the MFC@Ti3C2Txmicrogel was synthesized successfully within 1 min(Video S1 in Supporting information)due to the strong hydrogen bonding linkage between the terminations(e.g.,--F,--O and--OH)on the Ti3C2Txnanosheets and--OH groups on the MFC.Consequently,Because of the micron-sized average diameter of MFC(around 2 μm),the formed large-scale MFC@Ti3C2Txmicrogel greatly decreased the vacuum filtration time(filter membrane with 0.22 μm pore size),and the acquisition of MFC@Ti3C2Txfilm only took several minutes.Based on the huge size differences of Ti3C2Txnanosheets to MFC,Ti3C2Txnanosheets were self-assembled parallel to the radial direction of MFC.After coating,each 1D MFC@Ti3C2Txwould interconnect each other to generate larger microgel,the schematic diagram of synthesis process of MFC@Ti3C2Txviaself-assembly process is illustrated in Fig.1a.The digital photos of the obtained MFC@Ti3C2Txfilm by vacuumassist filtration in Fig.S3(Supporting information)indicates much looser and coarser than that of the Ti3C2Txfilm,which means that the alleviation of stacking,accompanied by the improved dispersity of Ti3C2Txnanosheets,such anti-aggregation structure could be seen in Fig.S4d.As a result,more electrochemical active sites would be exposed for such anti-aggregation architecture of MFC@Ti3C2Txmicrogel.As for the MFC@Ti3C2Txfree-standing films,distinct ultralong MFC could be observed in Fig.S4c(Supporting information),which is the guarantee for the robust mechanical strength.Fig.1b shows the zeta potentials of Ti3C2Txand MFC@Ti3C2Txdispersions are-23 mV and-3 mV,respectively.The positive shift of the zeta potential demonstrates the negative potential on Ti3C2Txsurface is partially offset by the strong hydrogen bonding linkage,further confirming the successful self-assembly process of the MFC@Ti3C2Txmicrogel.

Fig.S5(Supporting information)shows the SEM images of Ti3C2Txnanosheet,MFC and MFC@Ti3C2Txmicrogel.Ultrathin 2D Ti3C2Txnanosheet is similar to the graphene,large-scaled cellulose is composed of much ultralong nanocellulose.Basis on the characteristic of MFC,MFC@Ti3C2Txmicrogel featuring 1D structure,the diameter and the length of are～2.5 μm,～120 μm,separately,with the high aspect ratio of 50.Fig.1c is the TEM image of Ti3C2Txnanosheets,stacked 3～4 layers,which the thickness is about 3 nm(Fig.S6 in Supporting information).In addition,the interlayer spacing of Ti3C2Txis 1.43 nm,corresponding to the(002)plane of Ti3C2Tx.Fig.1d is the TEM image of MFC@Ti3C2Tx,as expected,distinct analogous core-shell architecture was formed.The thickness of MFC@Ti3C2Txis about 2.5 μm,in good agreement with that from SEM characterizations(Figs.S5c and d in Supporting information).Figs.1e–h are the EDX elemental mapping results,which also show a clear interface between MFC and Ti3C2Tx,further indicating explicitly the formation of MFC@Ti3C2Txarchitecture.

Fig.2.(a)C 1s region and(b)O 1s region component peaks fitting of XPS spectra of Ti3C2Tx and MFC@Ti3C2Tx film.(c)XRD patterns of Ti3AlC2,Ti3C2Tx and MFC@Ti3C2Tx.(d)Corresponding enlarged XRD patterns of Ti3C2Tx and MFC@Ti3C2Tx.(e)Schematic illustrating the expansion of interlayer spacing in MFC@Ti3C2Tx.

Fig.3.(a)Conductivity of Ti3C2Tx and MFC@Ti3C2Tx films.(b)GCD curves at 1 A/g and(c)CV profiles at 5 mV/s for Ti3C2Tx and MFC@Ti3C2Tx electrodes.(d)Rate performance calculated from GCD and CV measurements.(e)EIS data of Ti3C2Tx and MFC@Ti3C2Tx electrodes.The inset shows the magnified high-frequency region.(f)Schematic illustration of ion transport channel of different films.(g)Cycling performance of Ti3C2Tx and MFC@Ti3C2Tx electrodes at 10 A/g.(h)Tensile stress–strain curves of Ti3C2Tx and MFC@Ti3C2Tx films.(i)Comparison of the capacitance versus mechanical strength of the MFC@Ti3C2Tx film with some other notable reported composites in the literature.

The XPS spectra of Ti3C2Txand MFC@Ti3C2Txfilms in the C 1s region and O 1s region is shown in Figs.2a and b.It could be seen that the contents of C-O/CHxincrease significantly due to the introduction of organic MFC.In addition,the contents of T-C bonding have negligible change,which proves that connecting with MFC would not break the internal Ti-C bonding of Ti3C2Tx.This result demonstrates that the self-assembling strategy is not only facile but also nondestructive,and it could be further demonstrated in Ti 2p region(Fig.S7 in Supporting information).The XRD data of Ti3C2Txand MFC@Ti3C2Txis represented in Fig.2c.The(104)main peak(2θ=39°)of precursor MAX is disappeared in composition of the Ti3C2Tx,suggesting that the Al element is successfully removed during the etching process.Additionally,the(002)peak is broadened and shift to 5.9°from 9.5°,manifesting that the c-lattice parameter and the interlayer spacing of Ti3C2Txis 30 ? and 15 ?,respectively.The value of interlayer spacing(15 ?)is closed to the results of TEM analysis(1.43 nm).After bonding with MFC,the(002)peak shift from 5.9°to 5.5°(Fig.2d),revealing that the interlayer spacing is expanded from 15 ? to 15.9 ?.Considering

that the large diameter of the MFC,it is impossible to intercalation the narrow interlayer of Ti3C2Txnanosheets(only 1.5 nm).Thus,the increasing interlayer spacing may be due to the pulling effect of hydrogen bonding.According to the results of TEM of MFC@Ti3C2Txmicrogel and Fig.1a,single coated MFC@Ti3C2Txwould connect each other,thereby the intermediate Ti3C2Txnanosheets would be pulled and expanded due to the stronger hydrogen linkage between MFC and Ti3C2Tx,as described in Fig.2e.Such expanded interlayer spacing in favor of the intercalation of H+ion,and thus more faradic reaction could be occurred as follows:Different from the decreased conductivity arise from introduction of nano-scale polymers,the micro-scale MFC would not affect the charge transport of Ti3C2Txnanosheets.From Fig.3a,strikingly,the conductivity MFC@Ti3C2Txfilm is enhanced compared to the encounter Ti3C2Txfilm.Ultralong 1D/2D conducting Ti3C2Txpathway is responsible for the promoted charge transport,however,as increasing the contents of MFC,the conductivity of composites film became decreased due to the excess insulator MFC,some exposed 1D MFC would block the electron transport in the planar dimension,thus the conductivity investigated by fourpoint probe technique would be decreased,as shown in Fig.S8(Supporting information).The electrochemical performance of Ti3C2Txand MFC@Ti3C2Txelectrode is carried out in threeelectrode configuration.Fig.3b shows the GCD curves of different films at 1 A/g,it could be observed that the discharge time of MFC@Ti3C2Txelectrode is longer than that of the pristine Ti3C2Txelectrode,and the specific capacitance of Ti3C2Txelectrode and MFC@Ti3C2Txelectrode are 375 F/g and 451 F/g,respectively.The value of 451 F/g at 1 A/g in MFC@Ti3C2Txelectrode is higher than most ever-reported anti-aggregated Ti3C2Tx-based electrode for supercapacitors(Table S1 in Supporting information).The enhanced capacitance in MFC@Ti3C2Txelectrode is not only due to the increase of interlayer spacing,more the point,attributing to the shorten of ion diffusion channel[24,25].Fig.3c exhibits the CV curves of different films at 5 mV/s.The shape of the deviated from rectangle indicated the pseudocapacitance of the Ti3C2Txmaterial,and the larger CV area in MFC@Ti3C2Txreveals the promoted charge storage.Based on specific capacitance from the GCD and CV curves at different current density and scan rate(Figs.S9 and S10,Table S2 in Supporting information),the rate performance is shown in Fig.3d.It is not hard to find that the rate performance both increased by 7%,more importantly,the capacitance retained 71% after current density up to 5 A/g.Such enhanced rate performance is ascribing to the more accessible surface of Ti3C2Txnanosheet for electrolyte.Similar results could be found in EIS curves(Fig.3e).After adding MFC,the high frequency region still maintained a negligible semicircle,proving excellent conductivity of MFC@Ti3C2Txelectrode.Comparing the slope of the straight line in the low frequency region,it could be found that the MFC@Ti3C2Txelectrode possessed higher slope,indicating that the diffusion impedance has reduced in MFC@Ti3C2Txelectrode.Consequently,introduction of MFC could ease the stacking phenomenon between the layers as well as shortened the ion transport path greatly.Based on above result,compared with the compact Ti3C2Txfilm,looser MFC@Ti3C2Txmicrogel film provide the faster ion transport,as displayed in Fig.3f.The cycling performance of pristine Ti3C2Txand MFC@Ti3C2Txelectrodes at 10 A/g is shown in Fig.3g.The MFC@Ti3C2Txelectrode could keep initial capacitance after 5000 cycles,which better than that of pristine Ti3C2Txelectrode,confirming its good cycle stability.Concluding the factor of improved electrochemical performance of MFC@Ti3C2Txelectrode,the coated anti-aggregation structure is mainly account for larger interlayer spacing(hydrogen pulling effect),fast ion(looser architecture)and charge(1D/2D conducting pathway)transport.Considering the practical application in aqueous electrolyte,the mechanical properties of Ti3C2Tx-based film must be taken into account.Fig.3h and Fig.S11(Supporting information)are the tensile stress–strain curves of Ti3C2Txand MFC@Ti3C2Txfilms.The stress strength of MFC@Ti3C2Txfilm is up to 46.3 MPa owning to the synergistic bonding effect,leading to 3-fold improvement in competition of pristine Ti3C2Txfilm(16.1 MPa).Fig.S12(Supporting information)is the photograph of a piece of 7 mm wide strip made from MFC@Ti3C2Txfilm,which can hold a 105 g weight,～20,000 times its own weight.The plot of the gravimetric specific capacitance versus mechanical strength in Fig.3i shows that the MFC@Ti3C2Txelectrodes display the superior integration of the capacitance and mechanical strength among some of the previously reported notable supercapacitor electrodes,such as Ti3C2Tx/PVA[19],Ti3C2Tx/BC[20],Ti3C2Tx/CNF[21],RGO/ANF[26],GO/PANI[27],GO/cellulose[28]and RGO/MnO2[29],This indicates that in MFC@Ti3C2Txcomposites,an optimized trade-off between mechanical properties and energy storage is realized,where the mechanical strength is greatly improved compared to pristine Ti3C2Txfilm while the electrochemical properties of Ti3C2Txalso improved.

In summary,the green and accessible 1D MFC was introduced into 2D Ti3C2Txnanosheets to fast fabricate anti-aggregation MFC@Ti3C2Txmicrogel.This facile and time-saving strategy to assembling 2D Ti3C2Txinto a 1D/2D conducting channel is definitely nondestructive.Surprisingly,benefiting from the alleviation of self-stacking and shortened ion transport pathway of Ti3C2Txnanosheets,71%rate performance was realized in aqueous electrolyte.Besides,the self-assembly MFC@Ti3C2Txmicrogel film exhibited an improved capacitance of 451F/g as well as the outstanding mechanical stress of 46.3 MPa,which is a guarantee for the practical potential.In addition,the cycling performance of MFC@Ti3C2Txelectrode could withstand 5000 cycles without any capacitance decay.This fast self-assembly approach provides a new sight to construct anti-aggregation structure between 1D fibers and 2D nanosheets.

Declaration of competing interest

The authors declare that they have no financial and personal relationships with other people or organizations that can inappropriately influence our work,there is no professional or other personal interest of any nature or kind in any product,service and/or company that could be construed as influencing the position presented in,or the review of,the manuscript entitled.

Acknowledgments

This work was supported by National Natural Science Foundation of China(No.51571076)and Open project of State Key Laboratory of Urban Water Resource and Environment of Harbin Institute of Technology(No.HCK202115).

Appendix A.Supplementary data

Supplementarymaterialrelatedtothisarticlecanbefound,inthe online version,at doi:https://doi.org/10.1016/j.cclet.2021.03.025.