Mixed analogous heterostructure based on MXene and prussian blue analog derivative for high-performance flexible energy storage

Journal Pre-proofs Mixed Analogous Heterostructure Based on MXene and Prussian Blue Analog Derivative for High-Performance Flexible Energy Storage Meng Zhang, Jie Zhou, Jiali Yu, Ludi Shi, Muwei Ji, Huichao Liu, Dongzhi Li, Caizhen Zhu, Jian Xu PII: DOI: Reference:

S1385-8947(19)32582-3 https://doi.org/10.1016/j.cej.2019.123170 CEJ 123170

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

14 August 2019 25 September 2019 13 October 2019

Please cite this article as: M. Zhang, J. Zhou, J. Yu, L. Shi, M. Ji, H. Liu, D. Li, C. Zhu, J. Xu, Mixed Analogous Heterostructure Based on MXene and Prussian Blue Analog Derivative for High-Performance Flexible Energy Storage, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123170

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Published by Elsevier B.V.

Mixed Analogous Heterostructure Based on MXene and Prussian Blue Analog Derivative for High-Performance Flexible Energy Storage

Meng Zhang#, Jie Zhou#, Jiali Yu*, Ludi Shi, Muwei Ji, Huichao Liu, Dongzhi Li, Caizhen Zhu*, Jian Xu

Institute of Low-dimensional Materials Genome Initiative, College of Chemistry and Environmental Engineering of Shenzhen University, Shenzhen 518060, China #These authors contributed equally to this work. E-mail: [email protected] (Jiali Yu); E-mail: [email protected] (Caizhen Zhu) Declarations of interest: none Abstract In this work, we addresses the fabrication of a flexible film electrode based on 2D MXene wrapped 3D Ni-Fe oxide nanocube mixed analogous heterostructure. The resulted composite film electrode successfully inherits the merit of different building blocks: MXene layers works as binders and conductive additives that can connect cubic Ni-Fe oxide nanoparticals, facilitate the charge transfer and avoid a significant conductivity decrease in the resulting electrode. While cubic Ni-Fe oxide serves as an active spacer inside the adjacent MXene layers to increase the interlayer space, facilitate the electrolyte diffusion and enhance the electrochemical activity of the composite film. As a result, the optimized composite film manifests excellent specific 1

capacitance of 1038.43 mF cm-2 at current density 0.5 mA cm−2. Meanwhile by assembling into all-solid-state flexible supercapacitor, an excellent specific areal capacitance of 328.35 mF cm−2 at 0.2 mA cm−2 was achieved. Additionally, the excellent energy storage performance is well maintained with a capacitance retention of 90.9% during 10 000 charging-discharging long cycles. Furthermore, a high mechanical robustness with 88.9% capacitance remained after subjected to bending at 90° for 50 cycles, suggesting great potentials for the applications in future flexible and wearable devices. Keywords: MXene, prussian blue analog, Ni-Fe oxide, flexibility, supercapacitor 1. Introduction Recently, heterostructured architectures have sparked great research interests in the field of energy storage, electronics, optoelectronic, catalysis, etc.,[1-5] due to their ability of creating synergistic advanced materials that combine the collective advantages of individual building blocks and eliminate their disadvantages. Since 2004, the isolation of graphene give birth to a new era of integrating graphene into heterostructures which can further be directly assembled into flexible and conductive film electrodes.[6,7] These flexible electrodes require no binders, current collectors and conductive additives, showing promising prospective in future flexible electronic applications. However, the weak electrochemical redox activity of graphene limits their charge storage capabilicity.[8] Therefore, the challenge is to find an eligible alternative of graphene, which combines not only the characteristics of graphene, such as high conductivity, excellent flexibility, easy preparation and large lateral size, but 2

also high electrochemical activity. In 2011, a family of 2D transition metal carbides, nitrides, and carbonitrides, collectively known as MXenes, have caught extraordinary attention as a new paradigm in 2D material science, attributing to its striking signatures such as high metallic conductivity (up to 8000 S cm-1), [5] packing density (4 g cm-3), good mechanical strength and pseudocapacitive charge storage.[9,10] Especially, MXenes own high metallic conductivity even in the presence of surface functional groups, which makes it appropriate to be assembled into flexible freestanding electrodes. In fact, MXenes demonstrate outstanding energy storage performances that significantly outperform graphenes.[11,12] The volumetric capacitance of pure Ti3C2Tx electrodes in an aqueous electrolyte can reach up to 900 F cm−3,[13] exceeding the highest value reported so far for graphene-based electrodes(376 F cm−3).[14] However, like other 2D materials, the nano-sized 2D MXene sheets trend to re-stack in the flexible film electrode that only contains MXene, which decreases the energy storage performance of the electrodes by hindering electrolyte accessibility and ion transport.[15] To address this problem, one of the most promising strategies is to build heterostructured or analogous heterostructured film electrodes by stacking MXenes with different nanomaterials as spacers to weaken the re-stack of MXene as well as strengthen the energy storage ability by the synergistic effect of each building blocks.

As

a

result,

MXene/graphene,[16,17]

heterostructures/analogous MXene/CNT,[18]

heterostructures

MXene/MoS2

of

nanosheets,[19]

MXene/MnO2 nanowire,[20, 21] etc. binder free flexible electrodes have been widely 3

fabricated to enlarge the interlayer spacing and increase electrolyte accessibility, thus improving ion transport. However, most of the reported works are based on 2D-2D or 2D-1D geometries.[18,22-26] Until now, there is only a few papers addressed the fabrication of 2D-3D constructions based on MXenes. While the design, fabrication and in-depth investigations on the 2D MXene-3D nanoparticals based flexible film electrode are also favourable for widening the category of MXene based flexible electrodes. In this work, we demonstrate the fabrication of a flexible hybrid film electrode composed of 2D Ti3C2TX layers and 3D cubic Ni-Fe oxide derived from prussian blue analog (PBA) by a solution processing process. The cubic Ni-Fe oxide was wrapped by MXene layers and continuously stacked forming a macroscopic flexible film. The 2D-3D analogous heterostructure (2D-3D AHS) based flexible film successfully inherited the merit of heterostructures that combined the advantages of different building blocks: MXene layers worked as binders and conductive additives connecting cubic Ni-Fe oxide nanoparticals, facilitating charge transfer in the electrode and avoiding a significant conductivity decrease in the resulting electrode. While cubic Ni-Fe oxide served as a spacer between the MXene layers to enlarge their interlayer

space

and

improve

the

electrolyte

diffusion.

Furthermore,

the

electrochemical activity of the flexible electrode can also be effectively improved due to the pseodocapacitive performance provided by the complex valence change of Ni-Fe oxide during charging-discharging process. As a result, the optimized composite film manifests a remarkable areal capacitance of 1038.43 mF cm−2 at 0.2 4

mA cm−2, which is about 2 times higher than the pristine Ti3C2TX film. Based on the flexible composite film electrode, an all solid-state flexible supercapacitor is fabricated, which shows highly stable cycling stability with capacitance retention of 90.88% during 10 000 charging-discharging cycles and maintains stable energy storage performance during the mechanical bending for 50 cycles. 2. Material and methods 2.1. Synthesis of Ni-Fe oxide

In a typical synthesis process, 0.8 mmol (0.264 g) of potassium ferricyanide(Ⅲ) (K3Fe[CN]6) was dissolved into 60 mL deionized (DI) water

as solution A. Then 1.2

mmol (0.30 g) of nickel (II) acetate tetradrate and 1.5 mmol (0.441 g) trisodium citrate dehydrate were dissolved in 40 mL DI water as solution B. Then, solution A was dropped into solution B under magnetic stirring. After continuous stirring for 5 min, the bright yellow green mixed solution was aged at room temperature for 24 h. The yellow precipitates (Ni-Fe PBA) was collected by centrifugation, washed with DI water and ethanol 3 times, dried at 60 oC overnight. Then Ni-Fe PBA nanocube was sintered at 450 oC for 4 h under air atmosphere to synthesis Ni-Fe oxide for further use. 2.2 Preparation of the MXene nanosheets First, 0.8 g LiF was added into 10 mL 9 M HCl followed by continuous stirring for 5 min. Then 0.5 g Ti3AlC2 power was gradually added into the above solution and the reaction was conducted under magnetic stirring for 24 h at 40 oC with water bath. 5

The acidic mixture was washed through repeatedly centrifugation at 3500 rpm for 5 min repeatedly until pH>6. The dark-green supernatant of MXene was obtained and freeze-dried for further use. 2.3 Fabrication of Freestanding Flexible MXene/Ni-Fe oxide Composite Film First, a certain amount of Ni-Fe oxide nanocubes was dispersed in 20 mL DI water under ultrasonication for 30 min to obtain a homogeneous dispersion. Then the freeze-dried Ti3C2Tx was dispersed in DI water by hand shaking to get MXene solution (1 mg mL-1). Subsequently, 20 mL MXene solution was added into the Ni-Fe oxide dispersion under ultrasonication for 1 min and magnetic stirring for 10 min to get a homogeneous mixture of MXene/Ni-Fe oxide. Subsequently, the hybrid film was collected by vacuum filtration through polyethersulfone membrance (PES, 0.1 μm). Then the MXene/Ni-Fe oxide composite film was dried at vacuum drying oven at 40 oC for 6 h. A series of hybrid film with different Ni-Fe oxide weight ratios were produced by adjusting the adding amount of Ni-Fe oxide. 2.4 Fabrication of Flexible Supercapacitors Device The energy storage device was assembled by employing two pieces of the composite film, which were cut to same size and adhered to polyethylene terephthalate flexible substrate for electrochemical measurements. And, cellulose membrane was soaked PVA/LiCl as separator inside between the two film electrodes. The loading of the electrode was about 1.59 mg cm-2. 2.5 Materials Characterizations The microstructure and morphology of the samples were characterized by Field emission scanning electron microscopy (SEM, JSM-7800F) and transmission 6

electron microscope (TEM, JEM-2100F). The crystalline phases of the samples were investigated by power X-ray diffraction (XRD, MiniFlex600, Rigaku, Japan) using Cu-Kα radiation. X-ray photoelectron spectroscopy (XPS) measurements were carried out with Escalab-250Xi (Thermo Scientific, USA) electron spectrometer with Al-Kα radiation. The Brunner-Emmet-Teller (BET, ASAP2020M+C) surface area were measured by N2 adsorption-desorption isotherms at 77 K. All the electrochemical measurements were collected by an electrochemical workstation (Ivium-n-Stat, Netherlands).

3. Results and Discussion As is shown in Figure 1, 2D-3D AHS based flexible film was synthesized by simple procedures merely including nanoparticle fabrication, mixing and filtration. The delaminated Ti3C2TX MXene were prepared through a previously reported minimally intensive layer delamination (MILD) method,[5] as described detailedly in experimental section. Meanwhile, the highly uniform Ni-Fe PBA was prepared by a facile coprecipitation method.[27,28] Subsequently, Ni-Fe PBA nanocubes were transformed into mixed-metal-oxide nanocubes (Ni-Fe oxide) by a simple annealing process in air atmosphere, resulting in porous Ni-Fe mixed-metal-oxide nanocubes. The porous structure is generated from an outward gas flow accompanying with the continuous decomposition of PBA nanocubes during the thermally induced oxidative decomposition. The Ti3C2TX/Ni-Fe oxide composite film was then obtained by filtering the mixture of the 2D nanosheets and 3D nanocubes. The resulted film exhibits good flexibility that can be bent and folded, due to ultrathin and foldable

7

features of Ti3C2TX.

Figure 1. Schematic illustration of the fabrication process for the composite film electrode. The morphology and microstructure of Ti3C2TX nanosheets and Ni-Fe oxide nanocubes were investigated via scanning electron microscope (SEM) and transmission electron microscope (TEM). Ti3C2TX produced through the mild etching and delamination process demonstrates large lateral size of 3–6 μm, combined with ultrathin and foldable features (Figure 2a, b and Figure S1). This graphene-like morphology endows Ti3C2Tx to perfectly serve as suitable assembly platforms for assembling into flexible freestanding films. The morphology of PBA particles is demonstrated in Figure 2c, which displays uniform 3D nanocubes with an average size of 110 nm. After calcination, PBA was directly transformed into Ni-Fe oxide nanocubes with well-maintained cubic morphology of the precursor, except that the edges and surfaces of the produced mixed-metal-oxide nanocubes become rough (Figure 2d), resulting in a much improved specific surface area (Figure S5). It should 8

be noted that apart from the porous structure, the obtained Ni-Fe oxide products demonstrate decreased particle sizes, which are caused by the release of C, N and H species during the calcination.[29] To further confirm the structure of Ni-Fe oxide, high-resolution TEM images are given in Figure S2. Clear lattice fringes with interatomic distance of 0.21 nm and 0.29 nm can be observed, which are corresponded to the (200) and (220) planes of NiO and NiFe2O4, respectively.[28,29] Additionally, the corresponding element distributions in Figure S3e-o reflect the successful synthesis of Ni-Fe oxide. Figure 2e shows the TEM image of the Ti3C2TX/Ni-Fe oxide AHS. It can be seen that the cubic Ni-Fe oxide is fully wrapped by the 2D MXene layer and form a uniform 2D-3D analogous heterostructure. Since Ti3C2TX is a highly conductive 2D material with metallic conductivities, meanwhile porous 3D nanocube can increase the effective area and active site for electrochemical reaction, hence the intimate contact between Ti3C2TX and cubic Ni-Fe oxide will facilitates the charge transfer in composite film and accelerate the redox reaction during energy storage process. The cross-sectional SEM image of the flexible freestanding Ti3C2TX/Ni-Fe oxide composite film is given in Figure 2f. The film with a thickness of about 3.8 μm demonstrates a regular layered structure with cubic nanoparticals inserted in the MXene layers. The elemental mapping images of selected area in the composite film demonstrate evenly distributed Ti, C, O, Ni, Fe elements, which further proved the high-quality hybridization of MXene and Ni-Fe oxide.

9

Figure 2. a) SEM and b) TEM images of Ti3C2TX MXene. c) SEM image of Ni-Fe PBA nanocubes and d) Ni-Fe oxide. e) TEM image of Ti3C2TX/Ni-Fe oxide AHS. f) Cross-sectional SEM image of Ti3C2TX/Ni-Fe oxide composite film and elemental mapping images of g) titanium, h) carbon, i) oxygen j) nickel and k) iron in the marked area in Figure 2f. To further identified the crystallographic structure of the as prepared pure MXene film, PBA, Ni-Fe oxide and Ti3C2TX/Ni-Fe oxide composite film, X-ray diffraction (XRD) analysis was conducted and the results are shown in Figure 3a and Figure S3. The XRD pattern of pure PBA (Figure S3) possesses featured prominent peaks at 2θ = 17o, 24o, 35o, 39o, and 50o, which are corresponded to the (200), (220), (400), (420), and (440) planes of Ni-Fe PBA respectively (JCPDS card no.51-1987).[27] After annealing, the diffraction peaks of Ni-Fe oxide matched well with the stand patterns of JCPDS card no.71-1179 for NiO and JCPDS card 10

no.54-0964 for NiFe2O4.[31-33] No other detectable peaks from impurities can be found in the XRD pattern of Ni-Fe oxide, confirming that the Ni-Fe PBA precursor was completely converted to Ni-Fe oxide. The MXene nanosheets are confirmed by the prominent peak at 2θ = 6.2o, which is the characteristic of the (002) plane for Ti3C2TX MXene (Figure 3a). Both the diffraction peaks of Ti3C2Tx and Ni-Fe oxide appear simultaneously in the composite film, indicating the successful hybridization. Particularly, the (002) plane diffraction peak of the composite film shifts from 6.2o to 5.9o as compared with pure MXene due to the insertion of Ni-Fe oxide caused an increased interlayer spacing of MXene,[21,34] which is beneficial for the electrolyte diffusion between MXene layers, allowing the analogous heterostructure (Ni-Fe oxide) to provide improved overall electrochemical performances. The hybridization of MXene and Ni-Fe oxide was further supported by X-ray photoelectron spectroscopy (XPS). Survey spectrum for the Ti3C2TX/Ni-Fe oxide sample gives the peaks of C, Ti, O, F, Fe and Ni elements (Figure 3b). The High-resolution XPS spectra of Ti 2p can be fitted with three doublets, corresponding to Ti-C, Ti (II), and Ti-O bonds, respectively.[17,35] The high-resolution XPS spectrum of Ni 2p (Figure 3d) demonstrates Ni 2p3/2 and Ni 2p1/2 peaks along with their two satellite peaks (marked as “sat.”). The Ni 2p3/2 and Ni 2p1/2 peaks can further be divided into two spin-orbit doublets, indicating the coexistence of Ni2+ and Ni3+ in the composite.[36,37] The Fe 2p spectra are shown in Figure 3e, which gives two dominant peaks of Fe 2p3/2 and Fe 2p1/2. This two peak can further be divided into two spin-orbit doublets (713.8 and 720.2 eV), indicating the presents of Fe2+ and 11

Fe3+.[29,30,38] Meanwhile, the O 1s XPS are consisted four peaks with binding energies of 529.6, 530.5, 531.5 and 532.9 eV, related to the bonds of metal-O, Ti-OH, O-C, Ti-O, respectively. The complex valence states in the composite film is beneficial for multiple electrochemical reactions and pseudocapacitive energy storage, which will leads to improved capacitance.

Figure 3. a) The XRD patterns of different samples. b) Survey XPS spectrum of the composite film. High-resolution XPS spectra of c) Ti 2p, d) Ni 2p, e) Fe 2p and f) O 1s of the composite film. Due to cubic Ni-Fe oxide can provide complex valence change during the electrochemical reaction and the insertion of Ni-Fe oxide can increase the MXene interlayer space and improve the electrolyte diffusion, it is expected that the resulted Ti3C2TX/Ni-Fe oxide film will demonstrate effectively improved pseodocapacitive performance during charging-discharging process. This expectation was proved by exploring the energy storage performance of pure MXene film, Ti3C2TX/PBA with a mass ratio of 4 (Ti3C2TX/PBA = 4) and Ti3C2TX/Ni-Fe oxide film with a weight ratio 12

of 4 (Ti3C2TX/Ni-Fe oxide = 4). The results are presented in Figure S4. Apparently, the Ti3C2TX/Ni-Fe oxide = 4 composite film electrode presents the largest integral area in cyclic voltammetry (CV) curves and has a sharp boosted discharge time in GCD than those of the pure MXene film electrode and un-annealed PBA composite film electrode, indicating that the transition of PBA into Ni-Fe oxide is important for the improved energy storage performance. Thus, further explorations of the energy storage performance of Ti3C2TX/Ni-Fe oxide composite films with different Ni-Fe oxide mass ratios were investigated. Figure 4a presents the CV curves of pristine MXene and Ti3C2TX/Ni-Fe oxide films. At the same scan rate, the response current in the CVs are significantly enlarged for the composite films as compared with the pure MXene film (Figure 4a). As the loading amount of Ni-Fe oxide increases, the integrated area and discharge time of the composite film demonstrates an rising trend and the highest value is obtained by the Ti3C2TX/Ni-Fe oxide = 3 sample (Figure 4b and c), indicating its highest specific capacitance than others. At the current density of 0.5 mA cm-2, the composite papers with Ti3C2Tx/Ni-Fe oxide weight ratios of 3, 4, 6 and 13 exhibit specific areal capacitance of 1218.00, 1038.43, 916.00 and 666.57 mF cm-2, respectively (Figure 4c). However, since the flexibility of the composite film was totally depended on the connection and stacking of MXene layers, the high loading amount of Ni-Fe oxide would hinder the connection and stacking of MXenes inside the film, leading to the deterioration of the flexibility. Therefore, although the Ti3C2TX/Ni-Fe oxide=3 sample demonstrates the highest energy storage performance, it is prone to rupture due to its comparatively higher Ni-Fe oxide content (Figure S6), 13

which makes it inappropriate for flexible applications. Instead, the Ti3C2TX/Ni-Fe oxide = 4 electrode could achieve a high capacitance, while maintained the film flexibility well. Thus, the Ti3C2Tx/Ni-Fe oxide = 4 was selected as the optimized sample for further electrochemical performance explorations. Based on the excellent energy storage performance and flexibility of Ti3C2TX/Ni-Fe oxide = 4 electrode, a high-performance all-solid state flexible supercapacitor was constructed. Figure 4d shows the CV curves of the supercapacitor tested at different scan rates. By virtue of the increased scan rates, the current densities of the CVs demonstrate distinctive enlargement, signifying good I-V response of the flexible device. In addition, the triangular shapes of the GCD curves maintain well within the current density range from 0.2 to 1 mA cm−2, confirming the good reversibility and coulombic efficiency of the device (Figure S7). The specific capacitance of the supercapacitor was calculated based on GCD curves and the results are given in Figure 4e. It can be seen that the specific capacitance of the composite film supercapacitor reaches 328.4 mF cm-2 at 0.2 mA cm-2 and maintains more than 85.5% as the current density increases to 1 mA cm-2, indicating its good rate performance. The Ragone plots in Figure 4f compared the power densities and energy densities of our device with other previously reported flexible supercapacitors. A high volumetric energy density of 76.8 mWh cm−3 at a power density of 0.4 W cm-3 and 65.7 mWh cm-3 at a high power density of 2.1 W cm-3 are obtained by the supercapacitor based on Ti3C2TX/Ni-Fe oxide = 4 electrode, which are higher than 2.8 mWh cm-3 at a power density of 0.2 W cm-3 for MXene electrochemical microsupercapacitor,[39] 61.6 mWh cm-3 at a power density of 14

0.36W cm-3 for MXene/carbon nanotube yarn Supercapacitors,[40] 24.72 mWh cm-3 at a power density of 1.9 W cm-3 for Mo1.33C MXene and PEDOT:PSS solid-state supercapacitor,[41] and 2.55 mWh cm-3 at a power density of 0.046 W cm-3 for MXene/CNT hybrid fiber.[42] The values obtained in this work are also superior than Ti3C2TX MXene-based fiber supercapacitor (7.13 mWh cm-3, 0.142 W cm-3),[43] MXene/GO fiber supercapacitor (5.1 mWh cm-3, 0.020 W cm-3),[44] MXene-based planar micro-supercapacitor device (5.48–6.10 mWh cm-3, 0.1-1 W cm-3),[45] and freestanding Ti3C2TX Film micro-supercapacitor (12.4 mWh cm-, 0.22 W cm-3).[46]

Figure 4. a) CV curves for different samples tested at the scan rate of 10 mV s-1. b) GCD curves for different samples tested at the current density of 2 mA cm-2. c) Specific areal capacitance of different samples versus current density. d) CV curves of the supercapacitor based on Ti3C2TX/Ni-Fe oxide = 4 electrode at different scan rates. e) Specific areal capacitance of the supercapacitor based on Ti3C2TX/Ni-Fe oxide = 4 electrode at different current density. f) Ragone plots of the supercapacitor based on Ti3C2TX/Ni-Fe oxide = 4 sample compared with previously reported supercapacitors. 15

Further investigations of the charge storage mechanism of the supercapacitor based on Ti3C2TX/Ni-Fe oxide = 4 sample were explored and the results are given in Figure 5. It has been reported that the total response current (i) in CV curves under slow potential sweep rate is a sum of the fast faradaic reaction current (icap) and slow diffusion controlled current (idiff), as demonstrated in the following Equation (1).[47] i(v) = icap+ idiff = avb

(1)

Where a and b are adjustable parameters. The b value was generally used to evaluate the domination of the fast capacitive contribution or diffusion controlled contribution in the electrochemical process. Ideally, when the value of b is equaled to 0.5, it can be concluded that a total diffusion controlled process was happened during the energy storage process. While for the b-value equals to 1, a fully capacitive controlled process was triggered in the device.[47] b-value can be determined from the slope of the linear analysis by plotting logi vs. logv based on Equation (2) log𝑖 = log𝑎 + 𝑏log𝑣

(2)

The b-values of the supercapacitor at different voltages are given in Figure 5b. Since most of the values are above 0.75, it can be concluded that the kinetics of the Ti3C2TX/Ni-Fe oxide = 4 based supercapacitor is predominantly fast capacitive controlled. Quantitative distinguishment of the fast capacitive controlled contribution and diffusion controlled contribution is therefore investigated through equation (3) and (4).[19,45]

i(v)  k1v  k2 v1 / 2 16

(3)

i( v )  k1v 1 / 2  k 2 1/2 v

(4)

1/ 2 where v is the scan rate (mV s−1), k1v and k2v represent the currents from surface

capacitance contribution and the diffusion-controlled Faradaic processes, respectively. In this work, with the scan rate ranging from 2-10 mV s-1, the capacitive contribution increases from 67.21% to 82.23% (Figure 5c and d), which also indicates that the capacitive contribution dominates in electrochemical kinetics. The high capacitive contribution is resulted from the rapid electron transfer and ion diffusion in 2D-3D analogous heterostructure of the composite film. Moreover, the total amount of the stored charges (qT) and outer surface stored charges (qo, more accessible) were innvestigated through the Trasatti analysis method, and the results are given in Figure 5e and f. A high qo of 272.21 C cm−3 and qT of 485.44 C cm−3 were obtained by the supercapacitor, indicating its excellent energy storage capability.

Figure 5. a) CV curves of supercapacitor based on Ti3C2TX/Ni-Fe oxide = 4 at 17

different scan rate. b) Variation of b value as a function of voltage. c) CV curves at 6 mV s−1, where shaded area represents the contribution of capacitive current. d) Capacitive contributions at different scan rates. e, f) Calculations of qo and qT. In order to meet the demand for widespread practical applications of flexible and wearable electronics, a power source should be not only sufficiently flexible, but also deliver stable energy storage performance under deformations. In the repeated mechanical deformation process, the electrode with a high mass loading of Ni-Fe oxide is trend to break due to uneven internal and external stress. Consequently, the capacitance retention of Ti3C2TX/Ni-Fe oxide = 3 have a sharp reduction of the capacitance to about a half of the original value after it was bended several times at the bending angle of 90° (Figure 6a). Whereas the sample of Ti3C2TX/Ni-Fe oxide = 4 manifests a well capacitance retention just like pure MXene film and it kept a retention of 88.89 % after bending for 50 times. In addition, the Ti3C2TX/Ni-Fe oxide = 4 device demonstrates almost consistent CVs under different bending angles (Figure 6b) and excellent capacitance retention after 50 bending cycles under different angles (Figure 6c), indicating its high electrochemical stability and flexibility (Figure S8). As a demonstration of its feasibility in practical applications, since a single device only demonstrates an potential window of 0.8 V, thus 2-4 devices were connected in series with the voltage window steadily increased to 1.6, 1.8, 3.2 V, respectively (Figure 6d). The output voltage of the 4 serial connected devices reaches 3.2 V and it can easily lighten a red LED and maintain more than 2 minutes (Video S1). On the other hand, the device assemblies in parallel can be used to enlarge output capacitance, which are presented intuitively in the discharge time in Figure 6e. These results demonstrate that the supercapacitor are easily scalable in series or in parallel for complex practical 18

applications. To further examine the cyclic stability of the Ti3C2TX/Ni-Fe oxide = 4 supercapacitor, the device was charging-discharging at a current density of 1 mA cm-2 for 10 000 cycles. The result shows that our device displays an outstanding capacitance retention of 90.88% after 10 000 cycles, demonstrating an excellent electrochemical cycling stability.

Figure 6. a) The capacitance retentions of different samples after repeated bending to 90° for 50 cycles. b) CV curves of the flexible supercapacitor based on Ti3C2TX/Ni-Fe oxide = 4 at different bending angles. c) The capacitance retentions of Ti3C2TX/Ni-Fe oxide = 4 supercapacitor after repeated bending at different angles. d) CV curves for devices connected in series; the inset shows the optical image of four devices connected in series and lightened a red LED. e) GCD curves for devices connected in series or parallel. f) The capacitance retention of Ti3C2TX/Ni-Fe oxide = 4 sample after 10 000 charging-discharging cycles at 1 mA cm-2. 4. Conclusion In summary, a 2D-3D AHS based composite paper electrode with high 19

flexibility and excellent electrochemical performance was reported in this work. The optimized composite film demonstrated excellent energy storage performance with a specific capacitance of 1038.43 mF cm-2 and a well preserved flexibility. When assembled into all-solid-state flexible supercapacitor, the resultant device, an excellent specific areal capacitance of 328.35 mF cm−2 at 0.2 mA cm−2 was achieved. Additionally, it displays an outstanding cycling stability with a capacitance retention of 90.9% after 10 000 cycles, and a high mechanical robustness with 88.9% capacitance remained after subjected to 50 cycles bending test at 90°. Furthermore, the supercapacitors can be assembled into serious or parallel connected assemblies to with multiplied operation voltage windows or enlarged capacitance, demonstrating broad application potentials in flexible devices. Acknowledgements This work was supported by National Natural Science Foundation of China (51673117, 21805193, 51574166, 51602199), the Science and Technology Innovation Commission of Shenzhen (JSGG20160226201833790, JCYJ20170818093832350, JCYJ20170818112409808, JSGG20170824112840518, JCYJ20180507184711069, JCYJ20170818100112531, JCYJ20170818101016362, JCYJ20170817094628397), The

Key

R&D

Programme

of

Guangdong

Province

(2019B010929002,

2019B010941001). References [1] E. Pomerantseva, Y. Gogotsi, Two-dimensional heterostructures for energy 20

storage, Nature Energy 2 (2017) 17089. [2] H. Wang, F. C. Liu, W. Fu, Z. Y. Fang, W Zhou, Z. Liu, Two-dimensional heterostructures: fabrication, characterization, and application, Nanoscale. 6 (2014) 12250-12272. [3] F. Shahzad, M. Alhabeb, C. B Hatter, B. Anasori, S. M. Hong, C. M. Koo, Y. Gogotsi, Electromagnetic interference shielding with 2D transition metal carbides (MXenes), Science 353 (2016) 1137-1140. [4] K. S. Novoselov, A. Mishchenko, A. Carvalho, A. H. Castro Neto, 2D materials and van der Waals heterostructures, Science 353 (2016) aac9439. [5] M. Alhabeb, K. Maleski, B. Anasori, P. Lelyukh, L. Clark, S. Sin, Y. Gogotsi, Guidelines for synthesis and processing of 2D titanium carbide (Ti3C2Tx MXene), Chem. Mater. 29 (2017) 7633-7644. [6] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Electric feld effect in atomically thin carbon films, Science 306 (2004) 666–669. [7] D.Jariwala, T. J. Marks, M. C Hersam, Mixed-dimensional van der Waals heterostructures, Nat Mater. 16 (2017) 170-181 [8] B. Mendoza-Sanchez, Y. Gogotsi, Synthesis of two-dimensional materials for capacitive energy storage, Adv. Mater. 28 (2016) 6104-6135. [9] B. Anasori, M. R. Lukatskaya, Y. Gogotsi, 2D metal carbides and nitrides (MXenes) for energy storage, Nat. Rev. Mater. 2 (2017) 16098. [10] V. M. H. Ng, H. Huang, K. Zhou, P. S. Lee, W. X. Que, J. Z.C. Xu, L. B. Kong, 21

Recent progress in layered transition metal carbides and/or nitrides (MXenes) and their composites: synthesis and applications, J. Mater. Chem. A. 5 (2016) 8769-8769. [11] M. R. Lukatskaya, S. Kota, Z. F. Lin, M.-Q. Zhao, N. Shpigel, M. D Levi, J. Halim, P.-L. Taberna, M. W. Barsoum, P. Simon, Ultra-high-rate pseudocapacitive energy storage in two-dimensional transition metal carbides, Nature Energy 2 (2017) 17105. [12] M. R. Lukatskaya, O. Mashtalir, C. E. Ren, Y. Dall'Agnese, P. Rozier,; P. L. Taberna, M. Naguib, P. Simon, M. W. Barsoum, Y. Gogotsi, Cation intercalation and high volumetric capacitance of two-dimensional titanium carbide, Science 341 (2013) 1502-1505. [13] M.Ghidiu1, M. R. Lukatskaya, M. -Q. Zhao, Y. Gogotsi, M. W. Barsoum, Conductive two-dimensional titaniumcarbide ‘ clay ’ with high volumetric capacitance, Nature 216 (2014) 78-81. [14] Y. Tao, X. Y. Xie, W. Lv, D. M. Tang, D. B. Kong, Z. H. Huang, H. Nishihara, T. Ishii, B. H. Li, D. Golberg, F. Y. Kang, T. Kyotani, Q. H. Yang, Towards ultrahigh volumetric capacitance: graphene derived highlydense but porous carbons for supercapacitors, Sci. rep. 3 (2013) 2975. [15] D. B. Xiong, X. F. Li, Z. M. Bai, S.G.Lu, Recent advances in layered Ti3C2Tx MXene for electrochemical energy storage, Small. 14 (2018) 1703419. [16]Sh. K. Xu, G. D. Wei, J. Zh. Li, W. Han, Y. Gogotsi, Flexible MXene–graphene electrodes with high volumetric capacitance for integrated co-cathode energy conversion/storage devices, J. Mater. Chem. A. 5 (2017) 17442-17451. 22

[17] J. Yan, C. E. Ren, K. Maleski, C. B. Hatter, B. Anasori, P. Urbankowski, A. Sarycheva, Y. Gogotsi, Flexible MXene/Graphene films for ultrafast supercapacitors with outstanding volumetric capacitance, Adv. Funct. Mater. 27 (2017) 1701264. [18] M. Q. Zhao, C. E. Ren, Z. Ling, M. R. Lukatskaya, Ch. F. Zhang, K. L.Van Aken, M. W. Barsoum, Y. Gogotsi, Flexible MXene/carbon nanotube composite paper with high volumetric capacitance, Adv. Mater. 27 (2015) 339-345. [19] C. Chen, X. Q. Xie, B. Anasori, A. Sarycheva, T. Makaryan, M. Q. Zhao, P. Urbankowski, L. Miao, J. J. Jiang, Y. Gogotsi, MoS2-on-MXene heterostructures as highly reversible anode materials for lithium-ion batteries, Angew. Chemie. 57 (2018) 1846-1850. [20] W. H. Liu, Zh. Q. Wang, Y. L. Su, Q. W. Li,. Zh. G. Zhao, Z. F. X. Geng, Molecularly stacking manganese dioxide/titanium carbide sheets to produce highly flexible and conductive film electrodes with improved pseudocapacitive performances, Adv. Energy Mater. 7 (2017) 1602834. [21] J. Zhou, J. L. Yu, L. D. Shi, Z. Wang, H. C. Liu, B. Yang, C. H. Li. C. Z. Zhu, J. Xu, A Conductive and highly deformable all-pseudocapacitive composite paper as supercapacitor electrode with improved areal and volumetric capacitance, Small 14 (2018) 1803786. [22] Z. Y. Zhan, Q. C. Song, Z. H. Zhou, C. H. Lu, Ultrastrong and conductive MXene/cellulose nanofiber films enhanced by hierarchical nano-architecture and interfacial interaction for flexible electromagnetic interference shielding, J. Mater. Chem. C. 2019.(10.1039/C9TC03309B) 23

[23] K. L. Wang, B. C. Zheng, M. Mackinder, N. Baule, H. Qiao, H. Jin, T. Schuelke, Q. H. Fan, Graphene wrapped MXene via plasma exfoliation for all-solid-state flexible supercapacitors, Energy Storage Mater. 20 (2019) 299-306. [24] T. X. Shang, Z. F. Lin, C. S. Qi, X. C. Liu, P. Li, Y. Tao, Zh. T, Wu, D. W. Li, P. Simon, Q. H. Yang, 3D macroscopic architectures from self-assembled MXene hydrogels, Adv. Funct. Mater. (2019) 1903960. [25] P. Flouda, X.Feng, J. G. Boyd, E. L.Thomas, D. C. Lagoudas, J. L Lutkenhaus, Interfacial engineering of reduced graphene oxide for aramid nanofiber-enabled structural supercapacitors, Batteries & Supercaps 2 (2019) 464-472. [26] J. Azadmanjiri, V. K. Srivastava, P.Kumar, J. Wang, A. M. Yu, Graphene-supported 2D transition metal oxide heterostructures, J. Mater. Chem. A. 6 (2018) 13509-13537. [27]J. W. Nai, Y. Lu, L. Yu, X. Wang, .X. W. D. Lou, Formation of Ni-Fe mixed diselenide nanocages as a superior oxygen evolution electrocatalyst, Adv. Mater. 29 (2017) 1703870. [28] Y. X. Xu, Sh. Sh. Zheng, H. F. Tang, X. T. Guo, H. G. Xue, H. Pang, Prussian blue and its derivatives as electrode materials for electrochemical energy storage, Energy Storage Mater. 9 (2017) 11-30. [29] J. X. Shao, J. H. Feng, H. Zhou, A. H. Yuan, Graphene aerogel encapsulated Fe-Co oxide nanocubes derived from Prussian blue analogue as integrated anode with enhanced Li-ion storage properties, Appl. Surf. Sci. 471 (2019) 745-752. [30] J. L. Liu, D. D. Zhu, T. Ling, A. Vasileff, S.-Z. Qiao, S-NiFe2O4 ultra-small 24

nanoparticle built nanosheets for efficient water splitting in alkaline and neutral pH, Nano Energy 40 (2017) 264-273. [31]

G.

Q.

Zhang,

Y.

F.

layered-double-hydroxide-derived architectures

for

enhanced

Li,

Y.

F.

Zhou,

F.

NiO-NiFe2O4/Reduced electrocatalysis

of

alkaline

L.

Yang,

graphene water

NiFe oxide

splitting,

ChemElectroChem. 3 (2016), 1927-1936. [32] D. J. Du, W. B. Yue, X. L. Fan, K. Tang, X. J. Yang, Ultrathin NiO/NiFe2O4 nanoplates decorated graphene nanosheets with enhanced lithium storage properties, Electrochim. Acta. 194 (2016) 17-25. [33] J. Landon, E. Demeter, N. İnoğlu, C. Keturakis, I. E. Wachs,R. Vasić, A. I. Frenkel, J. R. Kitchin, Spectroscopic characterization of mixed Fe−Ni oxide electrocatalysts for the oxygen evolution reaction in alkaline electrolytes, ACS Cat. 2 (2012) 1793-1801. [34] J. Liu, H. B. Zhang, X. Xie, R. Yang, Zh. Sh. Liu, Y. F. Liu, Zh. Zh.

Yu,

Multifunctional, superelastic, and lightweight MXene/polyimide aerogels, Small 14 (2018) 1802479. [35] Y. H. Yoon, M. H. Lee, S. K. Kim, G. Bae, W. Song, S. Myung, J. Lim, S. S. Lee, T. Zyung, K.-S. An, A strategy for synthesis of carbon nitride induced chemically doped 2D MXene for high-performance supercapacitor electrodes, Adv. Energy Mater. 8 (2018) 1703173. [36] X. Zhang, Ch. Y. Wang, H. H. Li, X.-G. Wang, Y.-N. Chen,.Zh. J.

Xie, Zh.

Zhou, High performance Li–CO2 batteries with NiO–CNT cathodes, J. Mater. Chem. 25

A. 6 (2018) 2792-2796 [37] N. Wang, Y. B. Hu, X. Ke, L. Xiao, X. Zhou, Sh. S. Peng, G. Z. Hao, W. Jiang, Enhanced-absorption template method to prepare double-shell NiO hollow nanospheres with controllable particle size for the application in nanothermite, Chem. Eng. J. 2019 122330(10.1016/j.cej.2019.122330) [38] Q. Zhang, L. Fu, J. Y. Luan, X. B. Huang, Y. G. Tang, H. L. Xie, H. Y. Wang, Surface engineering induced core-shell Prussian blue@polyaniline nanocubes as a high-rate and long-life sodium-ion battery cathode, J. Power Sources. 395 (2018) 305-313. [39] Q. Jiang, Ch. Sh. Wu, Zh. J. Wang, A. C. Wang, J.-H. He, Zh. L. Wang, H. N. Alshareef, MXene electrochemical microsupercapacitor integrated with triboelectric nanogenerator as a wearable self-charging power unit, Nano Energy 45 (2018) 266-272. [40] Zh. Y. Wang, S. Qin, S. Seyedin, J. Zh. Zhang, J. T. Wang, A. Levitt, N. Li, C. Haines, R. Ovalle-Robles, W. W. Lei, Y. Gogotsi, R. H. Baughman, J. M. Razal, High-performance biscrolled MXene/carbon nanotube yarn supercapacitors, Small. 14 (2018) 1802225. [41] L. Q. Qin, Q. Zh. Tao, A. El Ghazaly, J. Fernandez-Rodriguez, Persson, P. O. A. J. Rosen, F. L. Zhang, High-performance ultrathin flexible solid-state supercapacitors based on solution processable Mo1.33C MXene and PEDOT:PSS, Adv. Funct. Mater. 28 (2018) 1703808. [42] Ch. Y. Yu, Y. J. Gong, R. Y. Chen, M. Y. Zhang, J. Y. Zhou, J. L. An, L. Lv, Sh. 26

J. Guo,G. Zh. Sun, A solid-state fibriform supercapacitor boosted by host-guest hybridization between the carbon nanotube scaffold and MXene nanosheets, Small. 14 (2018) 1801203. [43] J. Z. Zhang, Sh. Y. Seyedin, S. Qin, Zh.Y. Wang, S. Moradi, F. L. Yang, P. A. Lynch, W. R. Yang, J. Q. Liu, X. G. Wang, J. M. Razal, Highly conductive Ti3C2Tx MXene hybrid fibers for flexible and elastic fiber-shaped supercapacitors, Small. 15 (2019) 1804732. [44] S. Seyedin, E. Roswell, S. Yanza, J. M. Razal, Knittable energy storing fiber with high volumetric performance made from predominantly MXene nanosheets, J. Mater.Chem. A. 5 (2017) 24076–24082. [45] H. B. Hu, T. Hua, An easily manipulated protocol for patterning of MXenes on paper for planar micro-supercapacitors, J. Mater.Chem. A. 5 (2017) 19639–19648. [46] H.Ch. Huang, H. Su, H.T. Zhang, L. D. Xu, X. Chu, Ch. F. Hu, H. Liu, N. J. Chen, F. Y. Liu, W. Deng, B.N. Gu, H. P. Zhang, and W. Q. Yang, Extraordinary areal and volumetric performance of flexible solid-state micro-supercapacitors based on highly conductive freestanding Ti3C2Tx films, Adv. Electron. Mater. 4 (2018) 1800179. [47] J. L. Liu, J. Wang, Ch. H. Xu, H. Jiang, Ch. Zh. Li, L. L. Zhang, J. Y. Lin, Z. X. Shen, Advanced energy storage devices: basic principles, analytical methods, and rational materials design, Adv. Sci. 5 (2018) 1700322. Highlights:

1.

A 2D MXene wrapped Ni-Fe oxide nanocube based flexible electrode was 27

2. 3. 4.

synthesized. The composite film functioned like a mixed analogous heterostructure. The film successfully combined the advantages of different components. The film demonstrated satisfied electrochemical performance and flexibility.

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

28