(MXene) with Superior Electrochemical Performance for Supercapacitors

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Ag Nanoparticles Decorated 2D Titanium Carbide (MXene) with Superior Electrochemical Performance for Supercapacitors Lu Li, Na Zhang, Mingyi Zhang, Lili Wu, Xitian Zhang, and Zhiguo Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00047 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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ACS Sustainable Chemistry & Engineering

Ag Nanoparticles Decorated 2D Titanium Carbide (MXene) with Superior Electrochemical Performance for Supercapacitors Lu Lia, b, Na Zhangc, Mingyi Zhangb, Lili Wud, Xitian Zhang∗, b, Zhiguo Zhang∗, a a

Condensed Matter Science and Technology Institute, Department of Physics, Harbin

Institute of Technology, Harbin 150001, People’s Republic of China. b

Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of

Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, People’s Republic of China. c

d

Department of Chemistry and Chemical Biology, Cornell University, Ithaca, USA. Center for Engineering Training and Basic Experimentation, Heilongjiang

University of Science and Technology, Harbin 150022, People’s Republic of China.

Keywords: Ti3C2Tx nanosheets, Ag nanoparticles, Negative electrode, Asymmetric supercapacitors

∗ ∗

Corresponding author: E–mail: [email protected] (X. T. Zhang) Corresponding author: E–mail: [email protected] (Z. G. Zhang) 1

ACS Paragon Plus Environment

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Abstract: 2D Ti3C2Tx decorated with Ag Nanoparticles (NPs) has been rationally designed and successfully fabricated by filtering the mixture of Ti3C2Tx nanosheets and Ag NPs aqueous dispersion solution. The Ti3C2Tx/Ag NPs hybrid film electrode exhibits high specific surface area (107 m2 g-1), high areal capacitance (332.2 mF cm-2 at 2 mV s-1), good rate performance (63.2 % of its initial value at 2 mV s-1 as the scan rate increases to 100 mV s-1), and long-term cycling stability (87% capacitance retention over 10000 cycles). Furthermore, even when the mass loading is as high as 15.0 mg cm-2, a high areal capacitance of 1173 mF cm-2 can be obtained. In addition, an asymmetric supercapacitor (ASC) has been assembled based on Ti3C2Tx/Ag NPs negative electrode and MnO2/ESCNF positive electrode using 1 M Na2SO4 as electrolyte. The integrated device not only delivers excellent capacitive performance (246.2 mF cm-2 at 2 mA cm-2, 69.4 % capacitance retention at 20 mA cm-2 and 82 % capacitance retention over 10 000 cycles), but also exhibits the maximum energy density of 121.4 µWh cm-2 and maximum power density of 17395 µW cm-2. A red light-emitting diode is lighted by the ASC, suggesting the ability of its practical application.

2


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Introduction Supercapacitors have been attracting considerable attention because of their high power densities and long cycle lives.1-2 In particular, with the recently burgeoning growth of portable and wearable electronics, flexible supercapacitors are arousing tremendous attention for their great potential applicability as energy storage devices.3-4 Electrode material is an important component of supercapacitors. In recent years, 2D electrode materials have been extensively studied due to their high specific surface area, excellent electronic and mechanical properties.5 Particularly, MXenes, a new large family of 2D materials with metallic conductivity and hydrophilic surfaces,6 are first reported by Gogotsi et al. in 2011.7 The 2D MXenes can be synthesized via selective etching of the “A” layer by hydrofluoric acid (HF) or other fluorine source treatments from MAX phases.8 MXenes have already shown great potential

for

applications

in

energy

storage

systems,9-10

especially

for

supercapacitors.11-12 Many efforts have been devoted to explore the electrochemical properties of MXenes as electrode materials for supercapacitors. For example, Lukatskaya et al. reported that a range of mono- and multivalent cations (such as Li+, Na+, K+, NH4+ and Mg2+) can intercalate MXenes (chemically or electrochemically), occupying electrochemically active cites on the MXene surfaces, and can participate in energy storage.13 Li et al. obtained a significant gravimetric capacitance enhancement of 517 F g−1 after K+ intercalating and terminal groups (OH-/F-) removing from MXene sheets.14 However, like graphene, aggregation and face-to-face self-restacking of MXene nanosheets (NSs) are usually inevitable during 3



drying and electrode fabrication processes owing to the strong van der Waals interaction between adjacent NSs. The restacking gives rise to the increase of “dead” area, results in a fatal loss in available surface area, prevents electrolyte penetration into layers, and limits the electrochemical performance of electrode materials and practical applications.15-18 Introducing interlayer spacers between the sheets is an effective strategy to prevent the restacking, simultaneously, increase specific surface area.16-17,19-23 The electrochemical performance can also be improved via an increase in the electrode/electrolyte interface areas and a decrease in the ion diffusion length within active materials. Recently, using metal nanoparticles (NPs) as spacers, the graphene-metal nanocomposites showed promising properties as electrode materials for electrochemical capacitors.24 In particular, silver (Ag) nanoparticles have the highest conductivity among the noble metals, and exhibit large surface area-to-volume ratio at the nanoscale level and hence are excellent nanomaterials for energy storage devices where a large surface area is required.25 For instance, Khamlich et al. prepared the composites of Ag NPs on 3D graphene networks by microwave-assisted, which exhibited excellent performance in supercapacitors.24 He et al. reported the fabrication of Ag/graphite composite and obtained six times enhancement in specific capacitance.26 This strategy also applies to the design of MXene materials. Zou et al. demonstrated the accordion-like MXene/Ag composites for Li-ion batteries, which showed superior rate capability and excellent long-term cyclability.27 To the best of our knowledge, there is no report on Ag NPs decorated 2D Ti3C2Tx NSs as electrode for supercapacitors, and the Ti3C2Tx/Ag NPs hybrid film is 4


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expected to further enhance the capacitive performance. In this paper, a simple and scale-up method for the synthesis of flexible Ti3C2Tx/Ag NPs hybrid film electrode through filtering the mixture of Ti3C2Tx NSs and Ag NPs aqueous dispersion solution was reported. The Ti3C2Tx/Ag NPs hybrid film electrode exhibits a high specific surface area, large areal capacitance (332.2 mF cm-2 at 2 mV s-1), good rate performance (63.2 % of its initial value at 2 mV s-1 as the scan rate increases to 100 mV s-1), and long-term cycling stability (87% capacitance retention over 10000 cycles). Furthermore, even when the mass loading is as high as 15.0 mg cm-2, the areal capacitance of 1173 mF cm-2 can be obtained in our work. On the other hand, we further fabricate an asymmetric supercapacitor (ASC) based on Ti3C2Tx/Ag NPs negative electrode and MnO2/ESCNF positive electrode using 1 M Na2SO4 as electrolyte. The integrated device not only delivers excellent capacitive performance (246.2 mF cm-2 at 2 mA cm-2, 69.4 % capacitance retention at 20 mA cm-2 and 82 % capacitance retention over10 000 cycles), but also exhibits a higher energy density and power density (the maximum energy density of 121.4 µWh cm-2 and maximum power density of 17395 µW cm-2). A red light-emitting diode was lighted by the ASC, suggesting the ability of its practical application.

Experimental Preparation of Ti3C2Tx/Ag NPs hybrid film The colloidal solution of delaminated 2D Ti3C2Tx NSs was successfully prepared by etching Al from Ti3AlC2 powder as previously reported

28

and the detail is

available in supplementary. Ag NPs were prepared using a synthetic method by 5



following the reference.29 Briefly, 10 mL AgNO3 (1.0 mM) and sodium dodecyl sulfate (10.0 mM) mixture solution was added dropwise to NaBH4 solution (2.0 mM, 30 mL) under magnetic stirring in an ice bath. The clear yellow colloidal Ag dispersion is obtained. To make the hybrid film, the obtained aqueous dispersions of Ti3C2Tx NSs (1.2 mg mL-1) and Ag NPs (0.027 mg mL-1) were mixed, ultrasonicated to achieve homogeneity, and filtered through a polytetrafluoroethylene separator membrane (50 mm diameter, 0.22 µm pore size) using vacuum filtration. Hybrid films with different Ti3C2Tx NSs and Ag NPs solution volume ratios of 2:1, 1:1, and 1:2 were prepared. For comparison purposes, pure Ti3C2Tx NSs film was also produced from the colloids of Ti3C2Tx NSs following the same procedure. The detailed experimental parameters are listed in Table 1. Preparation of MnO2/ESCNF hybrid film The carbon nanofibers paper was fabricated by electrospinning as described previously.28 Nanostructured MnO2 was eletrodeposited on the electrospinning carbon nanofiber film at a constant current of 0.5 mA for 90 min in a mixed solution of 0.1 M of Mn(CH3COO)2 and Na2SO4. The loading amount of MnO2 was found to be about 2.3 mg cm-2. Characterization The morphology and chemical composition of Ti3C2Tx/Ag NPs hybrid films were characterized by field-emission scanning electron microscopy (FE-SEM; SU70, Hitachi, Japan) equipped with energy dispersive X-ray (EDX) spectrometer and transmission electron microscopy (TEM; FEI, Tecnai TF20). The X-ray powder 6


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diffraction (XRD) spectra were collected on a Rigaku D/max2600 X-ray diffractometer. Surface area was calculated using the Brunauer-Emmett-Teller (BET, Micromeritics ASAP 2010) equation. Electrochemical measurements. Electrochemical measurements, using VMP3 electrochemical workstation (BioLogic, France), were conducted with a standard three-electrode electrochemical cell containing 1 M Na2SO4 electrolyte. The as-obtained Ti3C2Tx/Ag NPs hybrid film was cut into 1.0 × 0.5 cm2 and placed on nickel foam. A carbon rod and Ag/AgCl was used as the counter and reference electrode, respectively. The electrochemical impedance spectroscopy (EIS) was measured at a frequency range from 0.01 Hz to 100 kHz with an AC perturbation of 10 mV at an open circuit potential. In the case of the two-electrode system, a Ti3C2Tx/Ag//MnO2/ESCNF asymmetric supercapacitor using a 2016-type coin cell was assembled and investigated. The Ti3C2Tx/Ag, MnO2/ESCNF and cellulose paper were regarded as the negative electrode, positive electrode and separator, respectively. 1 M Na2SO4 aqueous solution was used as electrolyte. The areal capacitance could be calculated from CV curves and GCD curves by the following equations. For CV studies: Ca =

∫ IdV 2 ⋅ A ⋅ν ⋅ ∆V

For GCD studies:

7


(1)


Ca =

i ⋅ ∆t ∆V

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(2)

Here, Ca is the areal capacitance (mF cm-2), I is the response current (A), V is the potential vs. reference electrode (V), A is electrode area (cm2), ν is the scan rate (mV s-1), ∆V is the potential window (V), i is the current density (mA cm-2), ∆t is the discharging time (s). The areal capacitance (C), energy density (E) and power density (P) of the ASC could be calculated by the following equations. C=

i ⋅ ∆t ∆V

1 C ⋅ (∆ V)2 2 E P= ∆t

E=

(3) (4) (5)

Here, C is the areal capacitance (mF cm-2), i is the current density (mA cm-2), ∆t is the discharging time (s), ∆V is the cell voltage (V).

Results and discussion Figure 1a schematically illustrates the procedure to prepare Ti3C2Tx/Ag NPs hybrid film electrodes. The aqueous dispersions of Ti3C2Tx NSs and Ag NPs were mixed, ultrasonicated and filtered by vacuum-assisted to form a hybrid film. Figure 1b shows the digital photos of the Ag NPs, Ti3C2Tx NSs and the mixture dispersions, respectively. A strong Tyndall scattering effect can be observed when a side incident light beam is directed on the colloidal dispersion. In general, for Ag NP alone, there is high dispersion difficulty because of their strong adhesion forces between particles with high surface energy. However, in this work, the Ag NPs were obtained by reducing AgNO3 with NaBH4 in water suspension with sodium dodecyl sulfate as 8


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surfactant to obtain the negative potential-functionalized Ag NPs, which can form a homogeneous colloidal solution. For Ti3C2Tx NSs, the surface was covered by functional groups such as -O, -OH and/or -F, which endow Ti3C2Tx NSs highly dispensability in aqueous solution, and the surface of Ti3C2Tx NSs was negatively charged. After mixing the two colloids, the electrostatic repulsion between Ag NPs and Ti3C2Tx NSs helps to keep the stability of the colloidal mixture and without apparent agglomeration. The as-fabricated Ti3C2Tx/Ag NPs hybrid films are easily detachable from the filter membrane to form a free-standing paper. Additionally, this film is easily wrapped around glass tube without noticeable damage to the structure, as shown in Figure 1c, indicating its flexibility. The Ag NPs have no capacitive contributions but act as conducive spacer in the structure of Ti3C2Tx/Ag NPs hybrid film. Besides, mixing with high amount of Ag NPs decreases the flexibility of the Ti3C2Tx/Ag NPs hybrid paper, and thus, the Ag content is limited to less than 5 wt%. The weight ratio of Ag NPs to Ti3C2Tx is varied to 1.125%, 2.25%, and 4.5% by adjusting the volume ratios of Ti3C2Tx NSs and Ag NPs solution in 2:1, 1:1, and 1:2. Typical XRD patterns of pure Ti3C2Tx film and Ti3C2Tx/Ag NPs hybrid films with different Ag NPs contents are presented in Figure 2. As shown in this figure, for the pure Ti3C2Tx film, the XRD pattern exhibits highly ordered in the basal face and apparent (000l) peaks. Besides, the (0002) peak is the strongest in intensity. After mixing with Ag NPs, the (0002) peak positions of Ti3C2Tx have no change, implying that no Ag intercalation occurs. However, no peaks can be assigned to Ag in the patterns of Ti3C2Tx/Ag NPs (2:1) and Ti3C2Tx/Ag NPs (1:1) samples, which may be 9



attributed to the little amount of Ag NPs. With the increase of Ag NPs content, the sample Ti3C2Tx/Ag NPs (1:2), a small XRD peak located at 38.1o appears, which can be indexed to the (111) plane of the cubic Ag crystal (JCPDS, No. 04-0783), showing the Ag NPs in the composites are well crystallized. In addition, the (0002) peak intensity of Ti3C2Tx become stronger and sharper with increasing Ag NPs content, suggesting more ordered along c direction. The general morphology and microstructure of the Ag NPs and Ti3C2Tx/Ag NPs (1:2) hybrid films were characterized by FESEM and TEM. Figure 3a shows the TEM image of Ag NPs. The Ag NPs have good mono-dispersity and the average size of the Ag NPs is about 8.5±2.8 nm (Figure 3b) as calculated by counting 200 Ag NPs. Before mixing with Ag NPs, the surfaces of Ti3C2Tx NSs are relatively smooth, as SEM image shown in Figure 3c. When the particles are combined with the Ti3C2Tx NSs, as TEM images shown in Figure 3e, f, Ag NPs can uniformly disperse on Ti3C2Tx NSs without aggregation, which benefits from the electrostatic repulsion between Ag NPs and Ti3C2Tx NSs. And the clear lattice fringes with fringes spacing of 0.24 nm can be indexed as the d-spacing of the (111) plane of Ag, consistent with the results of XRD analysis. Figure 3d shows the top-view SEM image of Ti3C2Tx/Ag NPs (1:2) hybrid film, indicating that the surface of the Ti3C2Tx/Ag NPs (1:2) hybrid film becomes rough due to the Ag NPs attached. A scanning transmission electron microscopy (STEM) image and corresponding element mapping images of the Ti3C2Tx/Ag NPs composite structure are shown in Figure 3g, exhibiting a highly homogeneous distribution of Ti and Ag. The Ti3C2Tx/Ag NPs composite structure 10


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could have an inhibition effect on restacking and provide excess and faster diffusion paths for electrolyte ions. Thus, we deduce that Ti3C2Tx/Ag NPs hybrid films would exhibit an improved electrochemical performance when used as supercapacitor electrodes. The exposed surface area and pores distribution of the pure Ti3C2Tx film and Ti3C2Tx/Ag

NPs

(1:2)

hybrid

film

were

further

investigated.

The

adsorption-desorption isotherm curve of Ti3C2Tx/Ag NPs (1:2) hybrid film shows type IV curve with H3 hysteresis loops, as shown in Figure 4a, indicating the presence of uniform mesoporous with high specific surface area and large total pore volume. The specific surface area of Ti3C2Tx/Ag NPs (1:2) hybrid film is calculated to be 107 m2 g-1 by Brunauer-Emmett-Teller (BET) analysis, which is about seven times larger than pure Ti3C2 film (16.2 m2 g-1). The increased specific surface area indicates that wedging Ag NPs can effectively hinder the restacking of Ti3C2Tx NSs, which in turn could improve its electrochemical performance. Figure 4b shows the pore-size distribution of the pure Ti3C2Tx film and the Ti3C2Tx/Ag NPs (1:2) hybrid film. The pore distribution for pure Ti3C2Tx film is mainly located at 4 nm, which can be attributed to the gap between few layer Ti3C2Tx NSs, and the pore volume is just 0.034 cm3 g-1. But for the Ti3C2Tx/Ag NPs (1:2) hybrid film, in addition to the distribution at 4 nm, the larger pore size distribution at 5 nm could originate from the channel pores, which derives from Ag NPs supporting between Ti3C2Tx NSs. The pore volume is as high as 0.259 cm3 g-1, which is 7.6 times larger than pure Ti3C2 Tx film. From this comparison, the high specific surface area (107 m2 g-1) of Ti3C2Tx/Ag NPs 11



(1:2) hybrid film can be ascribed to the large pore volume. The mesoporous structure and large surface area of the Ti3C2Tx/Ag NPs (1:2) hybrid film render this composite with fast electron and ion transport, which is conductive to improve electrochemical performance. The electrochemical behavior of pure Ti3C2Tx film and Ti3C2Tx/Ag NPs hybrid films was studied in 1.0 M aqueous Na2SO4 electrolyte at potential interval from -0.95 to -0.25 V in three-electrode system. Although the Na2SO4 electrolyte has a larger resistance than the 1 M H2SO4 or 1 M KOH electrolyte, it can put up with a broader potential window and does not react with Ag NPs during the electrochemical process. To evaluate the effect of Ag NPs on the electrochemical properties of the Ti3C2Tx, we synthesized all the electrode materials with the same mass loading of Ti3C2Tx per unit area. Figure 5a shows the CV curves for different electrode materials at a scan rate of 5 mV s-1. Obviously, the enclosed area of the CV curve for Ni foam is small enough as compared with that for Ti3C2Tx and Ti3C2Tx/Ag NPs so that the effective capacitive contribution from Ni foam to the overall capacitance is negligible. The CV curves of Ti3C2Tx film and Ti3C2Tx/Ag NPs hybrid film electrodes show a rectangular shape and have no distinct Faradaic peaks, that can be ascribed to a continuous change in the titanium oxidation state during electrochemical process. Furthermore, it can be discerned that an ameliorative capacitive behavior for hybrid electrode than pure Ti3C2Tx electrode under the same measurement conditions, which manifested an increase in the integrated area, indicating remarkable enhancement in capacitance by wedging Ag NPs. The Ti3C2Tx/Ag NPs (1:2) hybrid electrode shows the largest CV 12


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area, suggesting that Ti3C2Tx/Ag NPs (1:2) possesses the best electrochemical capacitive performance. Notably, the nearly rectangular CV curves for Ti3C2Tx/Ag NPs (1:2) hybrid electrode at a high scan rate of 100 mV s-1 (Figure 5b) and the nearly triangular GCD curves (Figure 5c) indicate remarkable rate capability and quick charge-propagation capability. The relationships of areal capacitances as a function of scan rates are presented in Figure 5d. As expected, the Ti3C2Tx/Ag NPs (1:2) exhibits the highest areal capacitance of 332.2 mF cm-2 at 2 mV s-1, and retains as high as 63.2 % of its initial value at 2 mV s-1 as the scan rate increases to 100 mV s-1, slightly higher than those of pure Ti3C2Tx, Ti3C2Tx/Ag NPs (2:1) and Ti3C2Tx/Ag NPs (1:1). The high areal capacitance is highly comparable with, or in some cases much higher than those of previously reported Ti3C2-based materials, such as binder free Ti3C2 film by the modified electrophoretic deposition (186 mF cm-2, 1 M KOH),30 accordion-like Ti3C2 (211 mF cm-2, 1 M KOH),31 Ti3C2 aerogel (174.2 mF cm-2, 1 M KOH),28 transparent Ti3C2Tx film (3.4 mF cm-2, 3 M H2SO4),32 Ti3C2Tx/RGO-3 (268.8 mF cm-2, 2 M KOH),33 PPy/l-Ti3C2 (203 mF cm-2, 0.5 M KOH),34 and so forth. The outstanding electrochemical performance of the hybrid Ti3C2Tx/Ag NPs electrode can be attributed to (i) the improved accessibility of the ions in electrolyte due to pillaring of the Ag NPs; (ii) the high specific surface area; and (iii) high conductivity of the hybrid film. Cycling stability was tested by CV at a scan rate of 50 mV s-1 for 15 000 cycles as shown in Figure 5e. The result shows that the capacitance retention progressively increases in the first 400 cycles, due to activation process, and retain 78.2 % over the 15 000 cycles. The capacitance retention is much lower than that of 13



Ti3C2-based materials, for example, our group reported Ti3C2Tx thin film exhibiting a 100% capacitance retention after 10 000 cycles.35 To further explore this, EIS measurements before and after 15 000 cycles were performed, as the Nyquist plots exhibited in Figure 5f. As shown in the inset in Figure 5f, the electrode after 15 000 cycles shows the equivalent series resistance (Rs) value (≈ 3.0 Ω) obtained from the X-intercept of the Nyquist plot, higher than that of the initial state of the electrode (≈ 2.5 Ω), suggesting the increase of resistance after 15 000 cycles. The diameter of the semicircle arc is related to the charge-transfer resistance (Rct) of the electrode and electrolyte interface, and the Rct of electrode after 15 000 cycles is much larger than the initial state, indicating the charge transport is slower after 15 000 cycles. Additionally, the electrode after 15 000 cycles has a large characteristic relaxation time constant τ0 (1/f0, the maximum C″ at frequency f0) of 25 s, which is about 6 time larger than the initial state (4.2 s), as shown in Figure S1, implying the ion diffusion is blocked after 15 000 cycles. Based on the above analysis, the fading of areal capacitance during charge-discharge process could be ascribed to the increase of resistance and the decrease of diffusion, which may derive from the following reasons: (1) Ti3C2Tx NSs have some degree of damage. From the XRD patterns in Figure S2a, the (0002) peak of Ti3C2Tx after 15 000 cycles is weaker and broader than before, indicating the defect of Ti3C2Tx may bring about the increase of resistance. And the damage can also be observed from SEM images in Figure S2c, comparing with the sample before cycles in Figure S2b, the edge of Ti3C2Tx NSs become rougher and dirtier after 15 000 cycles, while the overall structure is still intact. (2) The 14


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agglomeration of Ag NPs. After 15 000 cycles, in Figure S2c, the size of the Ag NPs increases and ranges from 25 to 45 nm. The agglomeration of Ag NPs results in the non-uniform distribution of Ag on Ti3C2Tx NSs, leading to the partial restacking of Ti3C2Tx NSs and loss of partial ion-accessible tunnels for charge storage, resulting in the decrease of diffusion and fading of areal capacitance. (3) In addition, the capacitance has undergone an accelerated decay process, which retain 93.7% of the initial capacitance over the 5 000 cycles (corresponding to an approximate decay rate of 0.00126% per cycle), retain 87% over the 10 000 cycles (corresponding to an approximate decay rate of 0.00134% per cycle for the second 5 000 cycles), even retain 78.2 % over the 15 000 cycles (corresponding to an approximate decay rate of 0.00176% per cycle for the last 5 000 cycles). This phenomenon can be attributed to the increased resistance, which causes more energy to dissipate on the internal resistance, resulting in heat loss and reduced capacitance. In spite of this, the cycling stability of Ti3C2Tx/Ag NPs electrode (87% over the 10 000 cycles) is still better than some metal oxides and conducting polymers electrodes, such as FeOOH/PPy@CF (84% over the 5 000 cycles),36 Mn3O4/NGP (82.1% over the 10 000 cycles),37 ZnCo2O4@Ni(OH)2 (70% over the 2 200 cycles),38 and rGO/PANI (76.5% over the 2 000 cycles).39 In order to assess the impact of mass loading on capacitance performance, electrochemical properties of Ti3C2Tx/Ag NPs (1:2) electrodes with mass loadings of 3.3 mg cm-2, 6.7 mg cm-2 and 15.0 mg cm-2 were measured. Figure 6a shows the GCD curves of the Ti3C2Tx/Ag NPs (1:2) electrodes at a current density of 5 mA cm-2, the 15



symmetrical

charge-discharge

characteristics

demonstrate

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an

excellent

electrochemical reversibility of the electrodes. As the mass loading increases, the discharge time increases at the same current density, which implies the remarkable enhancement in capacitance. Figure 6b shows the relationship between the areal capacitance and the discharge current density. The areal capacitance increases with mass loading increases. At current density of 5 mA cm-2, areal capacitance for samples of 3.3, 6.7 and 15.0 mg cm-2 is 265, 575 and 1173 mF cm-2, respectively. However, it is worth noting that the specific capacitance (78.2 F g-1) of Ti3C2Tx/Ag NPs (1:2) with mass loadings of 15.0 mg cm-2 is not high due to the use of the neutral electrolyte or alkaline electrolyte. In fact, a high specific capacitance of the Ti3C2Tx materials in the neutral electrolyte or alkaline electrolyte are difficult to be obtained. The corresponding data are shown in Table S2. However, the specific capacitance of Ti3C2Tx materials in 1 M H2SO4 electrolyte is much higher than that in neutral electrolyte or alkaline electrolyte due to the high ionic conductivity of H2SO4 electrolyte and protons being the smallest cations. We conclude that the specific capacitance of Ti3C2Tx is greatly affected by the used electrolyte based on our experimental results 35 and data reported by others. 13, 40 The rate capability is a critical factor of supercapacitors used for power applications. Even at a high current density of 50 mA cm-2, the areal capacitance is 198, 387 and 672 mF cm-2, corresponding to the capacitance retention of 74.6%, 67.2% and 57.0%. To explore the reasons for the rate capability decreasing with the increase in mass loading, EIS spectrum (Figure 6c) was conducted to understand the internal 16


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resistance and ion-diffusion process of the Ti3C2Tx/Ag NPs (1:2) electrodes. Nyquist plots show that Rs is 2.57, 2.69 and 2.80 Ω for 3.3 mg cm-2, 6.7 mg cm-2 and 15.0 mg cm-2, respectively. The low Rs reveals that all the Ti3C2Tx/Ag NPs (1:2) electrodes show low intrinsic ohmic resistance. Besides, the Rs is almost the same for the three electrodes, indicating that the mass loading has little effect on Rs. However, the ionic diffusion with a response time increasing in the order of 4.2 s, 10.6 s and 25.0 s (Figure 6d). This result proves that the ion migration rate is the primary affecting factors of the reduced rate capability. The diffusion and migration of electrolytic ions into the interior of the active materials is easy at low mass. As the high-mass loading leads to a slower electrolytic ion diffusion to access all the active materials, the block diffusion effect results in more active surface areas of the electrodes becoming inaccessible for charge storage under large current densities, bringing about the reduced rate capability. Assembling ASC is an effective design to increase the energy density and the cell voltage in aqueous electrolyte. To fabricate a complete ASC device, a suitable positive electrode is crucial. MnO2, with high theoretical specific capacitance (1370 F g-1) and low cost, is a preferred candidate in positive electrodes.41 The MnO2/ESCNF film positive electrode was characterized as shown in Figure S3. Combined with high capacitance and high rate performance, Ti3C2Tx/Ag NPs (1:2) electrode with mass loading of 6.7 mg cm-2 was selected as negative electrode. Figure S4a shows the CV curves of the Ti3C2Tx/Ag NPs and MnO2/ESCNF at a scan rate of 10 mV s-1, the two quasi-rectangular shaped curves occupy a potential window from -0.95-0.8 V (vs. 17



Ag/AgCl), suggesting the operating cell voltage can be greatly enhanced. Moreover, the GCD curves have two approximate linear lines in Figure 7a, the positive potential for MnO2/ESCNF can reach 0.8 V and the negative potential for Ti3C2/Ag can reach -0.95 V, further implying the operational voltage of the ASC can be raised to 1.75 V. Figure 7b shows CV curves of the Ti3C2Tx/Ag NPs//MnO2/ESCNF device at different cell voltages at a scan rate of 20 mV s-1, and an extended voltage window of 1.9 V is obtained, indicating the possibility to provide high energy densities. The CV curves of the ASC are quasi-rectangular in shape, as shown in Figure 7c, even at a high scan rate of 100 mV s-1, indicating a high rate capability. And as shown in Figure 7d, the GCD curves are perfectly linear at different current densities. The areal capacitance calculated from GCD curves are shown in Figure S4b, and it decreases from 246.2 to 170.8 mF cm-2 as the current density increases by 10 times, the corresponding capacitance retention is 69.4%. Such high rate performance may be ascribed to superior electrical conductivity of Ti3C2Tx/Ag NPs and MnO2/ESCNF. Energy density and power density are two important parameters for the ASC, and Figure 7e shows the Ragone plot of our ASC calculated from galvanostatic discharge curves. The ASC exhibits an energy density of 121.4 µWh cm-2 at a current density of 2 mA cm-2 with the power density of 1885 µW cm-2. And the energy density still retains 71.8 µWh cm-2 at the current density of 20 mA cm-2, when the power density increases to 17395 µW cm-2. As compared to other ASCs reported previously, such as MnO2//Bi2O3 (43 µWh cm-2, 12.9 mW cm-2),42 MnO2/CNT//PI/CNT (36.4 µWh cm-2, 0.78 mW cm-2),43 MnO2/CNT/AC SC (141 µWh cm-2, 4466 µW cm-2),44 N-CNFs/RGO/BC SC (110 18


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µWh cm-2, 25 000 µW cm-2),45 RGO/MnO2//RGO (35.1 µWh cm-2, 3800 µW cm-2),46 MnO2/microporous carbon SC (150 µWh cm-2, 4500 µW cm-2),47 Ti3C2 aerogel//ESCNF film (120 µWh cm-2, 26123µW cm-2),28 as shown in Figure 7e, our Ti3C2Tx/Ag NPs//MnO2/ESCNF ASC exhibits a high energy density and power density. The cycling stability of the ASC was further conducted at 10 mA cm-2 for 10000 cycles and it is clearly seen that the supercapacitor retains 82 % of the initial capacitance, demonstrating its outstanding long-term cycling durability, as shown in Figure 7f. And a commercial red light-emitting diode is lighted by a ASC device, as shown in the inset of Figure 7f. Moreover, to evaluate the feasibility of the as-fabricated ASC for flexible energy storage device, we tested its electrochemical performances under different bending conditions. The corresponding CV curves shown in Figure S5 were collected at a scan rate of 20 mV s-1. However, they exhibit an excellent electrochemical performance without capacitance change when the ASC is blended into different angles, indicating that the ASC has a good flexibility, which ensures its potential application in flexible electronic device. These results imply that theTi3C2Tx/Ag NPs composite electrode is expected to be a highly promising candidate for application as the negative electrode in high-performance energy storage systems.

Conclusions In summary, we have developed a simple and scale-up method for the synthesis of flexible Ti3C2Tx/Ag NPs hybrid film electrode through filtering the mixture of Ti3C2Tx NSs and Ag NPs aqueous dispersion solution. The Ti3C2Tx/Ag NPs hybrid 19



film electrode exhibits an excellent electrochemical performance. Furthermore, even when the mass loading is as high as 15.0 mg cm-2, the areal capacitance of 1173 mF cm-2 can be obtained in our work. In addition, the fabricated ASC based on Ti3C2Tx/Ag NPs negative electrode and MnO2/ESCNF positive electrode exhibits a higher energy density and power density (the maximum energy density of 121.4 µWh cm-2 and maximum power density of 17395 µW cm-2). A red light-emitting diode is lighted by the ASC, suggesting the ability of its practical application.

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Acknowledgments This work was partially supported by the National Natural Science Foundation of China (No. 51472066 and 51772069).

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Supporting information Experimental details, EDX results for samples, characterization methods and results, and electrochemical results.

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Figure 1. (a) Schematic illustration for formation mechanism of Ti3C2Tx/Ag NPs hybrid film electrodes, (b) digital images of Ag NPs, Ti3C2Tx NSs and the mixture dispersions, all displaying a clear Tyndall scattering effect with laser light, (c) digital images showing flexibility of Ti3C2Tx/Ag NPs hybrid film.

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Figure 2. XRD patterns of Ti3C2Tx film and Ti3C2Tx/Ag NPs hybrid film with different amounts of Ag NPs.

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Figure 3. (a) TEM image of Ag NPs, (b) Ag NPs size distribution, (c) SEM image of pure Ti3C2Tx film, (d) SEM image of Ti3C2Tx/Ag NPs (1:2) hybrid film, (e) TEM image of Ti3C2Tx/Ag NPs (1:2), (f) high-resolution TEM image of Ti3C2Tx/Ag NPs (1:2) (dotted circle showing Ag NP), (g) TEM image and corresponding element mapping using Ti and Ag.

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Figure 4. (a) Nitrogen adsorption and desorption isotherms of Ti3C2Tx/Ag (1:2) hybrid film and Ti3C2Tx film, (b) the pore size distribution of Ti3C2Tx/Ag NPs (1:2) hybrid film and Ti3C2Tx film.

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Figure 5. (a) CV curves of Ni current collector, Ti3C2Tx film electrode and Ti3C2Tx/Ag NPs with different amounts of Ag NPs electrodes at 2 mV s-1, (b) CV curves of Ti3C2Tx/Ag (1:2) electrode at various scan rates, (c) GCD curves of Ti3C2Tx/Ag NPs (1:2) at different current density, (d) areal capacitance and specific capacitance as a function of scan rates, (e) cycling stability of Ti3C2Tx/Ag NPs (1:2) electrode at a scan rate of 50 mV s-1, the inset shows the CV curves with the selected cycles, (f) Nyquist plots of Ti3C2Tx/Ag NPs (1:2) electrode before and after 15000 cycles.

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Figure 6. (a) GCD curves of Ti3C2Tx/Ag NPs (1:2) electrodes with different mass loading at the current density of 5 mA cm-2, (b) areal capacitance as a function of current density for electrodes with different mass loading, (c) Nyquist plots of Ti3C2Tx/Ag NPs (1:2) electrodes with different mass loading, (d) progression of the imaginary (C’’) parts of the areal capacitance of Ti3C2Tx/Ag NPs (1:2) electrodes with different mass loading as a function of frequency.

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Figure 7. (a) comparison of GCD curves collected for Ti3C2Tx/Ag NPs and MnO2/ESCNF electrodes at a current density of 5 mA cm-2, (b) CV curves of Ti3C2Tx/Ag // MnO2/ESCNF at different voltages at a scan rate of 20 mV s-1, (c) CV curves at different scan rates, (d) GCD curves at different current densities in the voltage range of 0-1.9 V, (e) Ragone plots of the Ti3C2Tx/Ag // MnO2/ESCNF ASC. The values reported for other ASCs are added for comparison. (f) cycling stability at a current density of 10 mA cm-2, and the inset shows the photographs of a red LED powered by the Ti3C2Tx/Ag // MnO2/ESCNF ASC.

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Table 1. Experimental parameters of the prepared samples. Precursors Sample Ti3C2Tx Ti3C2Tx/Ag NPs (2:1)

Ti3C2Tx NSs (mL) 30 30

Ag NPs (mL) 0 15

Ti3C2Tx/Ag NPs (1:1)

30

30

Ti3C2Tx/Ag NPs (1:2)

30

60

a

The value is determined by EDX (Table S1).

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Ag/ Ti atom ratioa 0 0.004：1 0.010：1 0.027：1

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TOC

This work opens up new design space for high-performance MXene electrode materials for sustainable supercapacitors.

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(MXene) with Superior Electrochemical Performance for Supercapacitors

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