MXene coupled with molybdenum dioxide nanoparticles as 2D-0D pseudocapacitive electrode for high performance flexible asymmetric micro-supercapacitors

Journal Pre-proof MXene coupled with molybdenum dioxide nanoparticles as 2D-0D pseudocapacitive electrode for high performance flexible asymmetric micro-supercapacitors Liangzhu Zhang, Guoliang Yang, Zhiqiang Chen, Dan Liu, Jiemin Wang, Yijun Qian, Cheng Chen, Yuchen Liu, Lifeng Wang, Joselito Razal, Weiwei Lei PII:

S2352-8478(19)30239-4

DOI:

https://doi.org/10.1016/j.jmat.2019.12.013

Reference:

JMAT 261

To appear in:

Journal of Materiomics

Received Date: 7 November 2019 Revised Date:

9 December 2019

Accepted Date: 30 December 2019

Please cite this article as: Zhang L, Yang G, Chen Z, Liu D, Wang J, Qian Y, Chen C, Liu Y, Wang L, Razal J, Lei W, MXene coupled with molybdenum dioxide nanoparticles as 2D-0D pseudocapacitive electrode for high performance flexible asymmetric micro-supercapacitors, Journal of Materiomics (2020), doi: https://doi.org/10.1016/j.jmat.2019.12.013. 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 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.

MXene Coupled with Molybdenum Dioxide Nanoparticles as 2D0D Pseudocapacitive Electrode for High Performance Flexible Asymmetric Micro-supercapacitors Liangzhu Zhang,a⊥ Guoliang Yang,a⊥ Zhiqiang Chen,a Dan Liu,*a Jiemin Wang,a Yijun Qian,a Cheng Chen,a Yuchen Liu,a Lifeng Wang,a Joselito Razal,a Weiwei Lei*a a.

Institute for Frontier Materials, Deakin University, Waurn Ponds Campus, Locked Bag 20000,

Victoria 3220, Australia. Liangzhu Zhang and Guoliang Yang contributed equally to this work. E-mails: [email protected]; [email protected]

Keywords: 2D-0D structure, Flexible Material; MXene, Molybdenum dioxide, Asymmetric micro-supercapacitors, Energy storage

Abstract Recently, two-dimensional (2D) transition metal carbides and carbonitrides (MXene), has shown great potential in micro-supercapacitors (MSCs). However, the maximum voltage output of symmetric MXene MSCs is limited to 0.6 V due to the oxidation effects at high anodic potentials. Herein, we developed asymmetric micro-supercapacitors (AMSCs) based on titanium carbide MXene (Ti3C2Tx) and MXene-MoO2 electrodes with an enlarged voltage window of 1.2 V, which is twice wider than that of symmetric MXene MSCs. The 2D-0D MXene-MoO2 microelectrode is fabricated by homogenous dispersing zero-dimensional (0D) MoO2 nanoparticles into MXene layers to impede layers stacking and MoO2 nanoparticles aggregation. Notably, the AMSCs delivered good electrochemical performances of areal 1

capacitance of ~19 mF cm-2 and volumetric capacitance of 63 F cm-3 at a scan rate of 2 mV s1

, and high energy density of 9.7 mW·h·cm-3 at a power density of 0.198 W cm-3. The

AMSCs also presented exceptionally mechanical flexibility under different bending states and excellent cyclic stability, with 88% capacitance retention after 10000 cycles at a discharge current density of 0.5 mA cm-2. For practical application, the serially connected AMSCs is fully affordable to power electronics, which is beneficial for soft and wearable power devices.

1. Introduction Progress in micro-scale energy storage units is highly demanded for flexible electronics that serve as an ideal platform for wearable biomedical and environmental monitors, wireless and smart sensors [1, 2]. To date, thin film battery is mainly used as micro-scale energy unit, but is limited by its low power density, short cycle span and leakage of electrolyte [3]. On the other hand, on chip micro-electrochemical capacitor, named as micro-supercapacitors, have the advantages of superior power density, long cycle life, and free of liquid electrolyte. They can also be integrated with triboelectric or wireless energy harvesters to harvest variable current for maintains free systems [4]. There are various electrode materials for MSCs, such as carbon-based materials, sulphides and conductive polymers [5-12]. However, most electrode materials suffer from poor electrical conductivity and low packing density, which leads to poor areal and volumetric capacitance [13]. Recently, MXenes have shown great promise on energy storage, such as supercapacitors and batteries, due to their excellent conductivity ~6500 S cm-1, large packing density ~4 g cm-3, surface hydrophilicity and superior ion intercalation behaviour [14-20]. Unlike graphene that can be exfoliated from natural graphite, MXenes are produced by selectively etching the “A” layers from synthetic MAX phases. This synthesis process introduces hydrophilic surface functional groups on exfoliated nanosheets, such as -OH,-O 2

and -F, which enable good dispersibility in water. Therefore, free-standing film and film on substrate (paper, metal foils and glass) with MXenes are easily fabricated by solutionprocessed method, such as vacuum filtration, spray/spin coating and casting. These methods are compatible with the MSCs fabrication technology. For example, the symmetric MSCs based on few layer Ti3C2Tx delivered large areal and volumetric capacitances of 27 mF cm-2 and 335 mF cm-3, respectively, which outperform most of the carbon-based microsupercapacitors (<10 mF cm-2) [13]. However, the small voltage window of 0.6 V remains a challenge due to the oxidation of Ti3C2Tx nanosheets in aqueous electrolyte, which limits the energy density and further practical application [21, 22]. In order to increase the output voltage of MXene based MSCs, combining electrode materials with negative and positive working voltage windows in an asymmetric configuration becomes an alternative choice [2326]. MXenes operates on a negative potential window. Therefore, an optimized positive electrode material with large specific capacitance and good solution dispersibility, is required. In past decades, metal oxides have been used as electrodes for supercapacitors due to their high redox activity and cost [27]. MoO2, among many metal oxides, is a promising candidate for positive electrodes due to its high-conductivity, high specific capacitance and environmental friendliness [28, 29]. For example, the RGO@MoO2/C demonstrated a high specific capacitance of 1224.5 F g-1 at 1 A g-1 [30]. However, the MoO2 nanoparticles suffer from severe aggregation, which impede cycling stability. Therefore, the hybridization of nanostructured 0D MoO2 with few layer 2D MXene could prevent aggregation as well as the restacking of MXene layers and result in synergistic effects in electrode properties and device performance [31-33]. Here, we designed a 2D-0D Ti3C2Tx -MoO2 hybrid film and pure Ti3C2Tx film for a planar all-solid-state asymmetric micro-supercapacitors. Few layered Ti3C2Tx nanosheets with merits of high electrochemical performance and thickness of 2 nm were selected as the

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negative electrode material. The ultra-fine MoO2 nanoparticles with diameter of 5-10 nm were prepared as the positive electrode material. Then, MXene and MXene-MoO2 dispersions were vacuum filtrated on cellulose paper and tailored into microelectrodes by laser cutting. The obtained microelectrodes were further transferred to polyethylene terephthalate (PET) substrates for in-plane AMSCs assembly, which was denoted as MXene//MXene-MoO2-AMSCs. The obtained device demonstrates excellent energy density of 9.7 mWh cm-3, outstanding areal and volumetric capacitance of 19 mF cm-2 and 63 F cm-3 at a scan rate of 2 mV s-1, respectively. The devices shows good long-term stability with 88% capacitance retention after 10000 cycles during a voltage window of 1.2 V. More importantly, the AMSCs shows excellent flexibility with negligible capacitance loss even under highly bent state ~180°. For practical applications, the two tandem AMSCs are able to power LED for 10 s and timer display for 180 s, illustrating the potential as integrated power devices for flexible electronics

2. Experimental section 2.1 Synthesis of Ti3C2Tx MXene All chemicals were used without further purification. Ti3C2Tx MXene solution with a concentration of 3 mg/mL was prepared by hydrochloric acid and lithium fluoride etching methods [34]. Typically, the solution for etching was prepared by mixing 1.6 g LiF (Sigma Aldrich, 99%) and 20 ml 9 M HCl (Sigma Aldrich, analytical grade,35%). 1 g Ti3AlC2 MAX powder (Sigma Aldrich, 99%) was slowly mixed with the etching solution under mild stirring for 24 hours. After that, the etched solution was washed by centrifugation for several times with DI water until the pH around 6 and disposed the upper solution. The Ti3C2Tx MXene sediment was redissolved in DI water and sonicated for 1 h under Ar-protection in the ice bath. Finally, the dark green Ti3C2Tx solution was obtained after 1 h centrifugation at 3500 rpm. MoO2 solution were prepared by hydrothermal method with a concentration of 3.4 4

mg/ml [35]. In detail, 0.3 g bulky MoO3 powder (Sigma Aldrich, 99%) was dissolved in solution containing 10 ml DI water and 10 ml absolute ethylene glycol (Sigma Aldrich). The mixture was mixed 30 minutes and poured into a 100 ml Teflon-lined stainless steel autoclave. The autoclave was reacted at 120 °C for 6 h. A dark solution was obtained and washed with ethanol three times. Finally, the MoO2 colloid solution with a concentration of 3.4 mg/ml was collected by centrifuging at 7000 rpm for 30 min. 2.2 Fabrication of MXene//MXene-MoO2-AMSCs First, we prepared MXene and MXene-MoO2 composite films by vacuum filtrated the corresponding MXene solution (10 mg) and MXene-MoO2 solution (MXene 10mg and MoO2 1.1 mg). The prepared films were dried in oven for 1h at 60 °C before further treatment. Subsequently, the microelectrodes with different shape were fabricated by laser cutting. Finally, the patterned microelectrodes were assembled on the PET using dual adhesive tape. A polymer gel electrolyte of PVA-LiCl was carefully drop-casted onto the surface of MXene//MXene-MoO2-AMSCs. 2.3 Materials Characterization Electrodes chemical structure and morphology were characterized by XRD (PAN analytical X’pert Pro.), FTIR (Bruker Vertex-70), Raman spectra (Renishaw), SEM (Zeiss Supra 55VP), AFM (Asylum Research), and Transmission Electron Microscope (TEM) (JEOL 2100). 2.4 Electrochemical Measurement The electrochemical performance of the fabricated devices were measured by CV tests from 5 to 100 mV s-1, GCD test at different current density, electrochemical impendence spectroscopy recorded in the frequency range of 0.1 Hz ∼ 100 kHz with a 5 mV AC amplitude under an electrochemical workstation (reference 600+, Garmy Co., LTD). 5

3. Results and discussion

Figure 1. (a) Schematic diagram of the fabrication process of MXene//MXene-MoO2-AMSCs. MXene//MXene-MoO2-AMSCs with different sizes and shapes of (b) Spiral, (c and d) parallel interdigital fingers, (e) nine parallel interdigital AMSCs integrated on one paper. AMSCs transferred onto various substrates, (f) glass and (g) cloth. Scale bar is 0.5 mm for Figure b to g.

Figure 1a schematically presents the fabrication procedure for all-solid-state in-plane MXene//MXene-MoO2-AMSCs. First, the MXene film and MXene-MoO2 film were prepared by vacuum filtration of MXene solution 20 ml (10 mg) and MXene-MoO2 solution (10 mg MXene and 1.1 mg MoO2) on a cellulose paper, respectively. After vacuum filtration, the obtained films were dried in oven for 1h at 60 oC. MXene and MXene-MoO2 films were precisely cut into arbitrary shapes microelectrodes by laser cutting. Notably, it is scalable for fabrication of in-plane AMSCs in various configurations with desirable geometries and sizes, e.g., spiral, parallel strips, interdigital fingers (Figure 1b-e). Further, the AMSCs were easy 6

transferred to suitable substrates, e.g., glass and cloth (Figure 1f-1g). Finally, the all-solidstate planar MXene//MXene-MoO2-AMSCs were coated with the PVA/LiCl gel electrolyte.

Figure 2. Schematic view of the crystal structure of (a) Ti3C2Tx nanosheets. (b) MoO2. Schematic view of film structure of (c) MXene and (d) MXene-MoO2. (e) Low magnification of Ti3C2Tx nanosheets, inset is HRTEM and corresponding Fast Fourier transformation (FFT) images. (f) AFM image of Ti3C2Tx nanosheets on a mica substrate; inset graph presents the height of Ti3C2Tx nanosheet. (g) TEM image of MoO2 nanoparticles. (h) HRTEM of MoO2 nanoparticle. (i) SEM top view of MXene film. (j) SEM top view of MXene-MoO2 film. (k-p) Elemental mapping analysis of MXeneMoO2 film on a conductive tape. 7

Ti3C2Tx nanosheets and MoO2 nanoparticles were synthesized for micro-supercapacitors’ electrode. Figure 2a-2d shows schematic view of the layer structure of Ti3C2Tx nanosheets, crystal structure of MoO2, films structure of MXene and MXene-MoO2. The morphology of Ti3C2Tx nanosheets in Figure 2e shows a flat and 2D structure without cracks and impurity and the HRTEM and FFT images demonstrate the Ti3C2Tx nanosheets with a hexagonal crystal structure. As indicated from AFM measurements in Figure 2f, the Ti3C2Tx nanosheets present a thickness ~2 nm with a lateral size of 1 µm, demonstrating two layer thickness. The prepared Ti3C2Tx nanosheets are well dispersed in water and ideal for further vacuum filtration to form freestanding film on different substrates (Figure S1). The XRD profile of Ti3C2Tx nanosheets and bulk MAX phase are shown in Figure S2. In the Figure S2, most peaks of Ti3C2Tx are weakened, suggesting the successfully etched Al layer. FTIR spectra of MXene film in Figure S3 confirmed the existence of –O-H and –F terminated groups. On the other hand, ultra-fine MoO2 nanoparticles were obtained by hydrothermal treating MoO3 in ethylene glycol as positive electrode materials. The morphology was confirmed by the TEM image in Figure 2g. The MoO2 nanoparticles show a uniform size distribution with diameter about 5-10 nm. The fringes with a distance of 0.24 nm are assigned to lattice plane of (211) in HRTEM (Figure 2h). As shown in Figure S4, the diffraction peaks are in good agreement with the standard card of JCPDS 32-0671 of MoO2. MXene film and MXene-MoO2 film were fabricated by vacuum filtrating diluted Ti3C2Tx solution and Ti3C2Tx-MoO2 mixed solution on cellulose paper, respectively. The schematic view of pure MXene film with layer structure and MXene-MoO2 film with 2D-0D structure were demonstrated in Figure 2c, 2d. The morphology of MXene and MXene-MoO2 films were showed in Figure 2i and 2j, respectively. It can be found that Ti3C2Tx nanosheets were assembled into a uniform film with considerable wrinkles. Compared with pure MXene film, MXene-MoO2 film is rougher and less wrinkles, which may result from the MoO2 dispersing

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on MXene layer as steric. Cross-section image of Figure S5 shows MXene film and MXene-

MoO2 film are well-aligned on cellulose paper with a thickness around 3 µm and 3.1 µm, respectively. Moreover, elemental mapping images of MXene-MoO2 film in Figure 2k-p show the evenly distributed C, Mo, Ti, O, and F elements in MXene-MoO2 film, confirming that abundant MoO2 nanoparticles are uniformly dispersed in the functionalized MXene surface.

Figure 3. Electrochemical characterization of planar MXene//MXene-MoO2-AMSCs. (a) Mechanism illustration of MXene//MXene-MoO2-AMSCs. (b) CV curves of MXene and MXene-MoO2 films under a scan rate of 40 mV/s by a three-electrode test in 1 M LiCl solution, using Ag/AgCl as

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reference electrode and Pt foil as counter electrode. (c) CV curves of MXene//MXene-MoO2-AMSCs under voltages from 0.8 to 1.2 V at a scan rate of 2 mV/s. (d) GCD curves performed at a current density of 0.5 mA cm-2 from 0.8 to 1.2 V. (e) CV curves tested at various scan rates from 2 to 20 mV s-1. (f) GCD curves performed at different current densities from 0.1 to 1 mA cm-2. (g) Areal capacitance (h) Complex plane plots of MXene//MXene-MoO2-AMSCs. (i) Cycling stability of MXene//MXene-MoO2-AMSCs at a current density of 0.5 mA cm-2. Inset is the GCD curves of before and after 10000 cycle.

The all-solid-state MXene//MXene-MoO2-AMSCs are totally binder and separator free with the adopted in-plane configuration, which is beneficial for ion transport (Figure.3a). As shown in Figure 3a, during charging, MoO2 nanoparticles in the MXene layer harvest a redox reaction (Li++MoO2-LiMoO2) to function as energy source while pure MXene layer could store charges through Cl- ions adsorption and desorption to offer fast power. For further study of the electrochemical performances of MXene//MXene-MoO2-AMSCs, we first investigated the voltage output of AMSCs by testing the electrode materials operating windows in a typical three-electrode test. From the CV curves in Figure 3b, the operating potential windows were found to be -0.7 to 0 V and 0 to 0.5 V vs. Ag/AgCl for MXene and MXeneMoO2 films, respectively. It is calculated that the optimal area ratio of MXene and MXeneMoO2 is 1:1 based on areal capacitance. Thus, the MXene//MXene-MoO2-AMSCs can deliver a maximum working voltage up to 1.2 V. As expected, the CV and GCD curves show a uniform electrochemical response between 0 and 1.2 V (Figure 3c and 3d). More importantly, the applied voltage extended to 1.2 V without sudden current increase, indicating no oxygen evolution reaction for MXene negative electrode. The quasi-rectangular profile of CV confirms that two microelectrodes areal capacitance is well matched. The electrochemical performance of AMSCs was further tested by CV at a scan rate in the range of 2-20 mV s-1 (Figure 3e). The curves present a rectangle shape with a large enclosed area.

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Pure MoO2 nanoparticles is a pseudocapacitive material. However, unlike conducting polymers, the obtained supercapacitor presented a rectangular CV profiles without obvious redox peaks may because of the large surface area which lead to electric double layer domination [30, 36]. As a result, the AMSCs based on hybrid MXene-MoO2 and MXene microelectrodes produced rectangular shapes for CV curves [34]. GCD performed at various current densities ranging from 0.1 and 1 mA cm-2 are showed in Figure 3f. The symmetric linear shapes further indicated a good coulombic efficiency. Notably, the all-solid-state MXene//MXene-MoO2-AMSCs delivered excellent specific areal capacitance of 19 mF cm-2 and volumetric capacitance of 63.3 F cm-3 at 2 mV s-1, respectively (Figure 3g and Figure S6). A decline in capacitance at higher scan rates was noted, which is also observed for other MXene based supercapacitors [37]. Nyquist plot in Figure 3h presents that the MXene//MXene-MoO2-AMSCs have a relatively low equivalent series resistance (ESR) of ~180 Ω, contributing to the fast electron transfer. More importantly, Figure 3i shows the asfabricated MXene//MXene-MoO2-AMSCs have a good reversibility with 88% capacitance retention after 10000 cycles test under a voltage window of 0-1.2 V at a current density of 0.5 mA cm-2. The observed deterioration capacitance is due to the decreasing of electrochemical active sites from the agglomeration of MoO2 nanoparticles during revisable cycling test. MoO2 nanoparticles is partial soluble in the PVA-H3PO4 gel electrolytes due to a long time testing, contributing the instability of the AMSCs.[38].

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Figure 4 (a) CV curves tested at various bending angles at 20 mV s-1. (b) Capacitance retention as a function of bending angle. Inset is the AMSCs device without bending with bending at 150o. CV curves of up to four MXene//MXene-MoO2-AMSCs connected (c) in series, (e) in parallel. GCD curves of four MXene//MXene-MoO2-AMSCs connected (d) in series, (f) in parallel. (g and h) a red LED and liquid crystal display (LCD) were powered by two serially connected MXene//MXeneMoO2-AMSCs.

Flexibility, integration ability and output voltage are crucial characteristics for flexible electronics. Figure 4a shows the CV profiles tested under different angle. The CV curves are almost overlapped under different bending angles. The capacitance nearly keep the same as the initial state after bending test in Figure 4b. Therefore these AMSCs have the potential as energy storage units for smart flexible electronics. Moreover, AMSCs are easy to be connected in series or parallel. Four AMSCs in series can readily increase the voltage output from 1.2 V to 4.8 V, as evidenced by CV and GCD curves (Figure 4c and 4d), respectively. The tandem AMSCs showed a gradually improvement of output current and discharged time (Figure 4e and 4f), indicative of uniformity and repeatability of the AMSC units. In addition, two serially connected micro-device can easily power a red LED for 10 seconds and timer display for 180 seconds (Figure 4g and 4h, SI Movie 1 and SI Movie 2), suggesting the high-

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voltage output and charge-storage capability of MXene//MXene-MoO2-AMSCs. These results indicate that our MXene//MXene-MoO2-AMSCs with high-voltage output are promising for wearable and portable electronics.

Energy density (Wh cm

-3

)

10-1 10-2

MXene//MXene-MoO2-AMSCs 4V/500 µΑh µΑ

10-3 10-4

Li thin-film battery

5.5V/100 mF SCs

2.7 V/44 mF AC-MSCs

10-5

3V/300 µF Electrolytic capacitor

-6

10 10-3

10-2 10-1 100 101 102 Power density (W cm-3)

Figure 5. Ragone plot presents volumetric energy density and power density of in-plane MXene//MXene-MoO2-AMSCs compared with commercially available energy storage devices.

As a micro-scale energy storage units, high volumetric energy density and power density are highly demanded for high performance wearable and flexible electronics. The Ragone plot in Figure 5 shows the comparison of volumetric energy density and power density of MXene//MXene-MoO2-AMSCs to commercially available energy storage devices. Notably, the as-prepared MXene//MXene-MoO2-AMSCs delivered a maximum volumetric energy density of 9.72 mWh-3 at a power density of 0.8 W cm-3, which is much higher than commercial supercapacitors (5.5 V/100 mF, 2.7 V/24 mF and 3.5 V/25 mF), Al electrolytic capacitors and comparable to Li thin-film battery (≤10 mWh cm-3) [39]. Further, such exceptional volumetric energy density of MXene//MXene-MoO2-AMSCs is also better than those of the recent reported high-power MSCs (Table S1), e.g., photochemically reduced

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graphene (1.5 mWh cm-3),[40] EGMX (1.4 mWh cm-3),[41] LIG-FeOOH//LIG-MnO2 (2.4 mWh cm-3),[42] GP/PANI-G/GP (3.1 mWh cm-3) and K2Co3(P2O7)2//GS (0.96 mWh cm-3) [43, 44].

4. Conclusion In summary, we have fabricated high-energy all-solid-state planar AMSCs by applying highly conductive MXene and pseudocapacitive 2D-0D MXene-MoO2 as electrode materials. The AMSCs exhibit an enhanced voltage output 1.2 V, which is twice higher than the symmetric MXene MSCs. The fabricated devices can deliver remarkable volumetric capacitance of 63 F cm-3 and energy density of 19 mWh cm-3, which are superior to most reported MSCs. What’s more, the micro-device shows excellent flexibility, outstanding integration ability, long cycle life, ability of powering a red LED and LCD, and compatibility for various substrate. Therefore, this work provides a promising route for scalable fabrication of MXene-based asymmetric micro-supercapacitors with a combination of high voltage output, high volumetric energy density and excellent flexibility.

Conflicts of interest 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.

Supporting Information Supporting Information is available from the ELSEVIER or from the author.

Funding

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This work was financially supported by the Australian Research Council Discovery Program (DP190103290) and Australian Research Council Discovery Early Career Researcher Award scheme (DE150101617) Reference [1] N. Liu, Y. Gao, Recent Progress in Micro‐Supercapacitors with In‐Plane Interdigital Electrode Architecture, Small, 13 (2017) 1701989. [2] N.A. Kyeremateng, T. Brousse, D. Pech, Microsupercapacitors as miniaturized energy-storage components for on-chip electronics, Nat Nanotechnol, 12 (2017) 7. [3] Z.-S. Wu, X. Feng, H.-M. Cheng, Recent advances in graphene-based planar microsupercapacitors for on-chip energy storage, National Science Review, 1 (2014) 277-292. [4] S. Park, H. Lee, Y.-J. Kim, P.S. Lee, Fully laser-patterned stretchable microsupercapacitors integrated with soft electronic circuit components, NPG Asia Materials, 10 (2018) 959. [5] C. Yin, L. He, Y. Wang, Z. Liu, G. Zhang, K. Zhao, C. Tang, M. Yan, Y. Han, L. Mai, Pyrolyzed carbon with embedded NiO/Ni nanospheres for applications in microelectrodes, RSC Advances, 6 (2016) 43436-43441. [6] Z. Su, C. Yang, B. Xie, Z. Lin, Z. Zhang, J. Liu, B. Li, F. Kang, C.P. Wong, Scalable fabrication of MnO 2 nanostructure deposited on free-standing Ni nanocone arrays for ultrathin, flexible, highperformance micro-supercapacitor, Energy & Environmental Science, 7 (2014) 2652-2659. [7] C. Xia, Y. Zhou, D.B. Velusamy, A.A. Farah, P. Li, Q. Jiang, I.N. Odeh, Z. Wang, X. Zhang, H.N. Alshareef, Anomalous Li Storage Capability in Atomically Thin Two-Dimensional Sheets of Nonlayered MoO2, Nano letters, 18 (2018) 1506-1515. [8] A. Ferris, S. Garbarino, D. Guay, D. Pech, 3D RuO2 microsupercapacitors with remarkable areal energy, Advanced Materials, 27 (2015) 6625-6629. [9] J. Wu, J. Peng, Z. Yu, Y. Zhou, Y. Guo, Z. Li, Y. Lin, K. Ruan, C. Wu, Y. Xie, Acid-Assisted Exfoliation toward Metallic Sub-nanopore TaS2 Monolayer with High Volumetric Capacitance, Journal of the American Chemical Society, 140 (2017) 493-498.

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1.2D-0D MXene-MoO2 hybrid structure was designed for MXene-based flexible asymmetric microsupercapacitors. 2.Vaccum filtration and laser technology can effortlessly prepare various shape of in-plane AMSCs. 3.The micro-device delivered exceptional energy density (9.7 mWh cm-3) as well as excellent cyclability.

Liangzhu Zhang is PhD students of Institute for Frontier Materials, Deakin University, Australia. He obtained his Master’s Degree in 2016 in Materials Science and Engineering from Shanghai Institute of Ceramics, Chinese Academy of Sciences, P.R. China. His current research interests focus on the two-dimensional based materials for micro-energy storage. Dr Weiwei Lei is a Senior Research Fellow at the Institute for Frontier Materials, Deakin University, Australia. He received his Ph.D. degree from Jilin University in 2009. From 2010 to 2011, he worked as a research fellow at Max Planck Institute of Colloids and Interfaces in Germany. Afterwards, he was awarded an Alfred Deakin Postdoctoral Research Fellowship (2011) and ARC Discovery Early Career Researcher (2014) at Deakin University. His research includes the synthesis of two- and threedimensional nanomaterials and their applications in sustainable energy and water applications.

The authors declare no competing financial interest.