Boosting the Pseudocapacitive and High Mass‐Loaded Lithium

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Boosting the Pseudocapacitive and High Mass-Loaded Lithium/Sodium Storage through Bonding Polyoxometalate Nanoparticles on MXene Nanosheets

Huixia Chao, Haiquan Qin, Mengdi Zhang, Yunchun Huang, Linfang Cao, Hailing Guo, Kai Wang, Xiaoling Teng, Jingkang Cheng, Yukun Lu, Han Hu,* and Mingbo Wu*

Achieving rate-capable and high mass-loaded lithium/sodium storage plays a pivotal role in promoting real world applications of many emerging electrode materials. Herein, such an electrode material is reported made of Mo and Fe-based polyoxometalates firmly bonded on MXene nanosheets through a mild in situ growth procedure. The polyoxometalate nanopar-ticles can efficiently prevent the restacking of the MXene nanosheets, enabling an electrolyte-permeable architecture at high mass loadings while the metallic conductivity of MXenes allows rapid electron transfer contrib-uting to the full exploration of the rich redox capability of polyoxometalates even at high rates. The optimal structure delivers a high capacity of 297 and 191 mA h g–1 at 1.0 A g–1 for lithium and sodium storage, respectively, even after a thousand of cycles. The kinetic analysis suggests high capacitive contributions of 81.6% and 67.4% for lithium and sodium uptake/release at 1.0 mV s–1, respectively. Moreover, a decent capacity remains even after 13.5-fold increase of loading mass. The lithium/sodium-ion hybrid super-capacitors based on this composite deliver remarkable energy density and power capability. The results demonstrated in this work may offer an alternative selection of electrodes based on rationally designed polyoxo-metalates and MXenes.

DOI: 10.1002/adfm.202007636

H. Chao, H. Qin, Dr. M. Zhang, Y. Huang, L. Cao, Prof. H. Guo, X. Teng, J. Cheng, Prof. Y. Lu, Prof. H. Hu, Prof. M. WuState Key Laboratory of Heavy Oil Processing, Institute of New EnergyCollege of Chemical EngineeringChina University of Petroleum (East China)Qingdao 266580, ChinaE-mail: [email protected]; [email protected]. ChaoGuangxi Colleges and Universities Key Laboratory of Beibu Gulf Oil and Natural Gas Resource Effective UtilizationCollege of Petroleum and Chemical EngineeringBeibu Gulf UniversityQinzhou 535011, ChinaProf. K. WangCollege of Electrical EngineeringQingdao UniversityQingdao 266071, China

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202007636.

1. Introduction

Due to the impendent shortage of fossil fuels and soaring concern about envi-ronmental pollution, tremendous efforts have been devoted to exploring renewable and sustainable energy in the past dec-ades.[1] However, the intermittent nature of these resources calls for innovative energy storage technologies with high energy density and large power capability. Metal-ion batteries, such as lithium/sodium-ion batteries and supercapaci-tors delivering high energy and power density, respectively, are currently the dominant energy storage systems in the market.[2] Nevertheless, neither metal-ion batteries nor supercapacitors can satisfy the increasing demand of the increasingly electrified society, which largely delays the broad deployment of new energy.[3,4] As a conceptually new energy storage device, hybrid supercapacitors that smartly com-bine the advantages of metal-ion batteries and supercapacitors represent a prom-ising power source for many current and

emerged applications. Typically, a hybrid supercapacitor uses an anode of lithium-ion batteries or sodium-ion batteries as the energy source and a supercapacitor cathode as the power reservoir.[5,6] Because of the different operation mechanisms involved in anode and cathode separately, the key to promote the practical performance of hybrid supercapacitors is to ration-ally reconcile these two distinct electrodes.

The battery-type anodes in hybrid supercapacitors use the sluggish ion diffusion process for energy storage while the capacitive cathodes store charges through fast surface adsorp-tion/desorption.[7] To fully explore the potential of hybrid super-capacitors, it is imperative to address the imbalanced kinetics between these two electrodes. In this regard, battery-type elec-trode materials with multiple-electron involved redox reactions capable of rapid lithium/sodium ion transfer are urgently needed.[8] A viable solution for this purpose is to form a stable connection between redox-active sites for lithium/sodium storage and electrically conductive substrates facilitating charge transfer.[9] Recently, polyoxometalates (POMs), consisting of

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several transition metal oxyanions, have been rising as prom-ising electrode materials for reversible lithium/sodium storage because of their rich redox properties.[10–14] Besides, POMs offer tunable structures and diverse compositions, contributing to a vast spectrum of potential candidates for energy storage appli-cations.[15] Despite these advantages, the poor electrical conduc-tivity of POMs largely restricts their practical performance.[16] As a result, the electrically conductive substrates are urgently needed to rationally hybridize with POMs for improved elec-trochemical properties.[17,18] Among all the available substrates, 2D materials are the preferable choice of substrates because of their large surface facilitating the formation of robust interfaces with various nanostructured materials.[19,20] The vigorous syner-gistic effects can be essentially harvested, especially with strong chemical bonding formed between them.[21] As a result, tremen-dous research efforts have been devoted to strongly connecting redox-active materials with electrically conductive 2D materials, for example, the recently emerged MXenes.[22–28] As a group of atomically thick metal carbides/nitrides, MXenes feature metallic conductivity and solution processability, offering excel-lent possibilities for energy storage applications.[29–31] Moreover, MXene nanosheets are stable in the acid conditions of synthe-sizing POMs that affords a large possibility of in situ forma-tion of a strong connection between them for improved charge storage.[32] Through rational regulation of the hybrid structures consisting of POMs and MXenes, there is a good chance to har-vest decent electrochemical performance at high mass loadings.

Herein, we report the in situ growth of densely and uni-formly dispersed Mo and Fe-based POM (MF POM) nanopar-ticles on MXenes, namely Ti3C2X nanosheets through rational utilization of their interaction for rate-capable lithium/sodium storage. MXene nanosheets enable effective electron transfer due to their metallic conductivity while their strong tendency of restacking is prohibited by the POM nanoparticles. The syner-gistic effects between these two components contribute to out-standing lithium/sodium storage performance in terms of high specific capacities, large-rate capability, and long-term stability even at high mass loadings. Then, the lithium-ion capacitors (LICs) and sodium-ion capacitors (SICs) based on this electrode material are fabricated, which deliver a high energy density, large power capability, and excellent cycle stability.

2. Results and Discussion

2.1. Synthesis and Characterization of MF POMs/MXenes

Typically, POMs could be prepared by aging a mild acid solu-tion of the precursors at the ambient condition.[13,33] On the other hand, the MXene nanosheets are generally obtained through etching the MAX phase using a solution containing hydrofluoric acid at room temperature followed by exfoliation to produce an aqueous dispersion of MXene nanosheets with high stability.[34] The syntheses of these two distinct materials share some similarities, for example, both of which are car-ried out in an aqueous solution at the ambient condition. As a result, there would be a strong possibility to rationally com-bine the synthesis processes of POMs and MXenes for in situ construction of their hybrid materials with strong bonding.

Figure 1a illustrates the in situ synthesis procedure of the hybrid structure made of Mo and Fe-based POM nanoparticles and Ti3C2X MXene nanosheets. The Ti3C2X nanosheets are prepared through etching the MAX phase Ti3AlC2 in a mixed solution of hydrochloric acid (HCl) and lithium fluoride (LiF) followed by a subsequent ethanol-mediated delamination.[35,36] The thus-obtained MXene nanosheets possess a highly func-tionalized surface that provides abundant anchoring sites for the subsequent growth of other nanostructures. Then, the dis-persion containing MXene nanosheets is mixed with a mild acid solution containing Fe3+ and [Mo7O24]6– ions for synthe-sizing MF POMs. After aging at the ambient condition for more than 24 h, the MF POM nanoparticles decorated MXene nanosheets are then collected after freeze-drying, donated as MF POMs/MXenes. Since all these steps are carried out at the ambient condition without using sophisticated apparatus, the hybrid structures suggested here may hold promise potential to be massively produced.

The two individual components were first analyzed sepa-rately, providing the structural evolution of the hybrid struc-ture. Scanning electron microscopy (SEM) observation reveals that the as-prepared multilayered MXene powder (Figure S1a, Supporting Information) could be exfoliated into atomic-thick MXene nanosheets (Figure S1b, Supporting Information) which are dispersible in water as shown in Figure S1c (Supporting Information). Transmission electron microscopy (TEM) obser-vation (Figure S1d, Supporting Information) intuitively reflects the sheet-shape morphology of MXenes, and the high-resolu-tion TEM image (Figure S1e, Supporting Information) exhibits the lattice fringe with a spacing of 0.32 nm, corresponding to the (002) plane of Ti3C2X MXenes. Moreover, the high-resolution XPS spectra of Ti 2p peak of MXenes (Figure S2, Supporting Information) reveals the existence of Ti2+ (455.5 eV), Ti3+ (457.2 eV) and a small quantity of TiO2 (459.2 eV).[37] The MF POMs were harvested by aging the acidic solution with Fe3+ and [Mo7O24]6– ions. The as-obtained MF POM particles show a rhombic prism-like morphology of several micrometers (Figure S3a, Supporting Information) where Fe, Mo, and O ele-ments are distributed uniformly throughout the whole particles (Figure S3b-d, Supporting Information). To identify the exact composition of the POMs, inductively coupled plasma optical emission spectroscopy (ICP-OES) and X-ray photoelectron spectroscopy (XPS) were conducted. The Mo/Fe molar ratio is around 2.6:1, where Mo exists as Mo (VI) while Fe is in the form of Fe (III)/Fe (II) in a molar ratio of 4.6:1 (Figure S4a,b, Supporting Information).[38]

The micromorphology of MF POMs/MXenes is depicted in Figure 1b–h. As revealed by the SEM image (Figure 1b), the composites present a sandwich-like structure with a large quantity of nanoparticles between the layers. It can be more obviously seen from the TEM image (Figure 1c) that MF POM nanoparticles are densely distributed on the surface of MXene nanosheets with the abundant anchoring sites.[39] The HAADF-STEM image (Figure 1d) and the corresponding EDS elemental mapping images (Figure 1e–h) further demonstrate the uniform dispersion of MF POM nanoparticles with a size of ≈10–20 nm within the composites. The dramatic decrease in particle sizes of as-obtained MF POMs may be caused by the distinct growth environment in the presence of MXene nanosheets. Specifically,



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the Fe3+ and [Mo7O24]6– ions could supply from all directions to the crystal nucleus for the growth of POMs without MXenes while the access to the crystal nucleus is largely denied from quite some directions in the presence of MXene nanosheets, as illustrated in Figure S5 (Supporting Information).

The XRD pattern and Raman spectrum of MF POMs/MXenes are compared with those of the individual components shown in Figure 2a,b. Clearly, the profiles of the composite show the overlap of different components but with decreased intensity, indicative of the successful nucleation of MF POMs on MXene nanosheets. XPS spectra (Figure 2c,d) of the composite reveal that both the Mo 3d and Fe 2p peaks shift toward lower binding energy after hybridizing with MXenes, which is mainly attributed to the strong chemical interaction between the functional groups on the MXene surface and metal ions in POMs.[40,41]

2.2. Lithium Storage Performance of MF POMs/MXenes

The lithium storage performance of the hybrid structure was evaluated in a lithium half-cell configuration with a lithium foil as the counter and reference electrodes. Meanwhile, the elec-trochemical property of the MF POMs and MXenes was also assessed under identical conditions as reference. The MF POMs give a decent initial specific capacity of around 1200 mA h g–1 but poor capacity retention. After 100 discharge/charge cycles, only 17% of the initial capacity remains (Figure S6a, Supporting Information). The specific capacity of MF POMs also decays rapidly with the increase of the current density as shown in Figure S6b (Supporting Information). In addition, the Nyquist plot of MF POMs shows a high charge transfer resist-ance (Figure S6c, Supporting Information). Although the ini-tial capacity reduces a little bit after hybridizing MF POMs

Figure 1. a) Schematic illustration of the synthesis procedure of the MF POMs/MXenes composite. b) SEM images of MF POMs/MXenes. c) TEM image and d) HRTEM image of MF POMs/MXenes. The HAADF TEM element mapping images of e) Ti, f) Fe, g) Mo, and h) the combination of C, Ti, Mo and Fe.



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with MXenes (Figure 3a), the cyclic stability of the MF POMs is essentially enhanced after hybridizing with MXenes via chemical bonding as illustrated in Figure 3b and Figure S6d (Supporting Information). Meanwhile, the composite made of physically mixed MF POMs and MXenes only offers mildly enhanced electrochemical performance (Figure S6e, Sup-porting Information) compared with the MF POMs, high-lighting the prominent effect of chemical bonding. The hybrid material with an equal amount of POMs and MXenes exhibits the optimal performance (Figure S6d, Supporting Information). As a result, the subsequent electrochemical performance was mainly evaluated based on this hybrid material. The CV curves of the MF POMs/MXenes electrode were scanned between 0.01 and 3.0 V versus Li+/Li, and the scan rate was set as 0.2 mV s−1 (Figure S7a, Supporting Information). In the first cycle, an irreversible reduction peak at around 1.0 V versus Li+/Li indi-cates the formation of solid electrolyte interphase (SEI) which disappears in the following cycles. The peak at 0.01-0.5 V versus Li+/Li in the reduction process is mainly caused by the conver-sion reaction of Li+ ions with the MF POMs.[42] An extraordi-nary rate capacity is afforded by this composite which is mainly due to the synergistic effects between MF POMs and MXenes. As shown in Figure 3c, the capacitance retention of 51% is observed with a 60-fold increase in the current density. By gradually reducing the current density from 6.0 to 0.1 A g–1, the specific capacity of MF POMs/MXenes can be largely restored, indicative of the excellent reversibility. To further unveil the kinetics behavior, the CV tests were carried out at the scan rates from 0.2 to 2.0 mV s–1 (Figure S7b, Supporting Information),

and the dependence of the current density (i) on the scan rate (ν) was determined using the power law:[43]

i a bν= (1)

where the value of a and b is tunable. Specifically, the b value of 0.5 suggests the diffusion-controlled process, while the value of 1.0 indicates that the capacitive behavior is dominated. As shown in Figure 3d, the b values are much larger than 0.5 for the anodic and cathodic processes, revealing the charge transfer is a surface-controlled process.[44] For further quantify the capacitive contribution, the current density at a fixed potential i(V) is separated as the surface-dominated contribution (k1ν) and insertion effect (k2ν1/2) and expressed as:[7,45,46]

k k1 21/2i V ν ν( ) = + (2)

Figure 3e exhibits the CV curve of the MF POMs/MXenes at a scan rate of 0.6 mV s–1 which offers a capacitive contri-bution of 76.2%. Even at a scan rate as low as 0.2 mV s–1, the capacitive contribution can still be as high as 66.1% (Figure 3f). These results suggest that the MF POMs/MXenes mainly use a pseudocapacitive effect for charge storage, a highly demanded behavior for power-capable energy storage devices. Further-more, we tested the electrochemical impedance spectroscopy (EIS) to evaluate the charge transfer. In the high-frequency and medium frequency ranges, the Nyquist plot of the compos-ites in Figure S7c (Supporting Information) gives depressed semicircles even after a hundred of cycles, suggesting a low

Figure 2. a) XRD patterns and b) Raman spectra of MF POMs, MXenes, and MF POMs/MXenes. High resolution XPS profiles of c) Mo 3d and d) Fe 2p of MF POMs/MXenes and MF POMs.



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charge transfer resistance. Then, galvanostatic intermittent titration technique (GITT) was employed to evaluate the diffu-sion coefficient of Li+ (DLi

+) in the MF POMs/MXenes.[47,48] The calculated coefficient values were plotted as a function of the potential during charge and discharge, as shown in Figure S7d–f (Supporting Information). The DLi

+ value remains high and stable, indicative of a very fast diffusion rate of Li+ in the frame-work of MF POMs/MXenes. These results are in very good agreement with the aforementioned measurements.

The long-term stability of the hybrid anodes was evaluated at a current density of 1.0 and 4.0 A g–1. As shown in Figure 3g, the capacity retention at both current densities remains high even after 1000 cycles, which is superior to most of the previously

reported POM-based electrodes (Table S1, Supporting Informa-tion).[10,13,18,49–60] Specifically, the hybrid with rationally com-bined POMs and MXenes offers a decent specific capacity, the longest cyclic stability, and a profound rate performance among these hybrid structures. Figure 3h shows the evolu-tion of the mass capacity and areal capacity with the loading mass of the active materials. With a 13.5-fold increase of the loading mass, the MF POMs/MXenes electrode displays a high mass capacity retention of 83% and an area specific capacity up to 2.17 mA h cm–2. Moreover, a high capacitive contribution is still maintained even at a high mass loading (Figure S7g-i, Supporting Information). For example, the capacitance contri-bution is around 71% at a scan rate of 0.6 mV s–1 even at a mass

Figure 3. Lithium storage performance of MF POMs/MXenes. a) CV tests of MF POMs/MXenes, MF POMs, and MXenes at a scan rate 2.0 mV s–1. b) Cycling performance of MF POMs/MXenes at 0.1 A g–1. c) Rate capability of MF POMs/MXenes evaluated between the current density of 0.1 and 6.0 A g–1. d) Evolution of peak current with the scan rate ranging from 0.2 to 2.0 mV s–1. e) Evaluation of the capacitive current at a scan rate of 0.6 mV s–1 and f) capacitive contribution at different scan rates. g) Cycling performance recorded at 1.0 and 4.0 A g–1. h) Evolution of the specific capacity with the loading mass at a current density of 1.0 A g–1.



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loading as high as 6 mg cm–2. Such an intriguing property is of paramount importance for the real application, of which the outstanding performance of nanoscale materials could be essentially harvested at the electrode level.

Considering the outstanding pseudocapacitive contribution and long-term stability, the MF POMs/MXenes was employed as the anode to construct LICs by pairing with a nitrogen-doped activated carbon cathode in Figure S8 (Supporting Information). The detailed characterization and electrochem-ical investigation of the cathode are illustrated in Figure S9 (Supporting Information). To fully explore the stable poten-tial window of the electrolyte, the mass ratio of AC cathode and MF POMs/MXenes was set to be 2.8:1.[61] The perfor-mance evaluation of MF POMs/MXenes//AC LIC is shown in Figure 4. The CV curves of the LIC, anode, and cathode are exhibited in Figure 4a, all of which show typical capaci-tive behavior. The galvanostatic charge/discharge (GCD) curves of the LIC offer triangular shapes at different current densities, as displayed in Figure 4b. These symmetric curves suggest good reversibility. To understand the charge storage

mechanism, the GCD curve of MF POMs/MXenes//AC cell was recorded at 1.0 A g–1 where the corresponding potential profiles of cathode and anode referring to Li-metal (Figure 4c) were simultaneously detected.[62] The anode and cathode work in the potential range of 0.1–1.9 V and 2.1–4.3 V versus Li+/Li, respectively. The operation window of the anode and cathode remains quite stable in a wide range of current densities, indicative of the well-matched electrochemical performance for these two electrodes (Figure 4d).[63] Figure 4e shows that the MF POMs/MXenes//AC LIC exhibits low charge transfer resistances based on the Nyquist plot, which supports the fast charge transfer kinetics. To get a clear view of the practical performance of the LIC, the function of energy density as the power density is illustrated in Figure 4f. As shown, this cell delivers a high energy density of 164.7 Wh kg–1 even at the power density of 0.1 kW kg–1 while a large power output of 6 kW kg–1 is realized at an energy density of 66.2 Wh kg–1. Furthermore, the MF POMs/MXenes//AC LIC exhibits good cycling performance with a capacity retention after 800 cycles at 1.0 A g−1 (Figure 4g).

Figure 4. Electrochemical performance of the LIC containing the MF POMs/MXenes anode. a) CV curves of anode and cathode as well as the full device at a scan rate of 5.0 mV s–1. b) GCD curves recorded between 0.2 and 4.2 V. c) GCD curves of the individual electrodes along with the voltage profile of the full device at a current density of 1.0 A g–1 (based on the total mass of cathode and anode). d) GCD curves of the anode and cathode during operating the LIC at different current densities. e) Nyquist plot, f) Ragone plot, and g) cycling performance of the LIC.



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2.3. Sodium Storage Performance of MF POMs/MXenes

Then, the sodium storage behavior of MF POMs/MXenes com-posite was also evaluated using a half-cell configuration. As shown in Figure S10a–c (Supporting Information), without the electrically conductive MXenes substrate, the MF POMs elec-trode almost fully loses its capacity by increasing the current density from 1.0 to 2.0 A g–1 within a few cycles. In addition, 66% of the initial capacity is lost only after 40 cycles where a high charge transfer resistance is observed. The poor per-formance denies the direct use of the MF POMs for sodium storage. After hybridizing with MXenes, an enlarged CV curve is observed in Figure 5a which may derive from the synergistic

effects of different components. Similar to their lithium storage capability, the optimal performance for sodium storage is also realized on the composite with an equal mass of MF POMs and MXenes (Figure S10d, Supporting Information). More-over, broad anodic and cathodic peaks centered at 0.48/0.28, 0.80/0.79, and 2.15/1.66 V versus Na+/Na are observed corre-sponding to the Mo4+/Mo5+, Fe3+/Fe2+, and Mo5+/Mo6+ redox couples, respectively.[56,64,65] The typical SEI can be formed in the first cycle and a dynamic equilibrium can be obtained in the subsequent test (Figure S11a, Supporting Information). In addi-tion to the boosted specific capacity, the stability is essentially enhanced in the presence of the MXene nanosheets (Figure 5b). The excellent rate capability of the composite anode is exhibited

Figure 5. Electrochemical sodium storage of the MF POMs/MXenes anode. a) CV curves of MF POMs/MXenes, MF POMs, and MXenes recorded a scan rate 2.0 mV s–1. b) Cycling performance at a low current density of 0.1 A g–1. c) Rate performance evaluated at current densities ranging from 0.1 and 4.0 A g–1. d) Evolution of peak current with the scan rate ranging from 0.2 to 2.0 mV s–1. e) Separation of capacitive current from the total current at a scan rate of 0.6 mV s–1. f) Evolution of capacitive contribution with the scan rate. g) Cycling performance at a current density of 1.0 and 3.0 A g–1. h) Evolution of specific capacity of the MF POMs/MXenes for sodium storage with loading mass.



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in Figure 5c. The discharge capacity gradually decreases from 301 to 115 mA h g–1 with the increase of the current density from 0.1 to 4.0 A g–1 which would then be mostly recovered back to the initial value by decreasing the current density to 0.1 A g–1. To acquire an insight into the kinetic process, the CV curves were recorded at scan rates from 0.2 to 2.0 mV s–1 (Figure S11b, Supporting Information), and the power law was employed to reveal the dependence of the current density and scan rate. As given in Figure 5d, the calculated b value for anodic and cathodic peaks is between 0.78 and 0.85, indicating a capacitive-dominated process. The capacitive contribution was quantified to be 62.5% at 0.6 mV s–1 (Figure 5e). When slightly increasing the scan rate to 2.0 mV s–1, the capacitive contribution can be increased to 80.9% (Figure 5f). The recorded EIS offers a small semicircle at the high-frequency range, revealing the fast charge transfer (Figure S11c, Supporting Information). The large diffu-sion coefficient of Na+ (DNa

+) in the MF POMs/MXenes elec-trodes evaluated by GITT suggests a very fast diffusion rate of Na+ in the framework of MF POMs/MXenes (Figure S11d-f, Supporting Information). The cyclic stability of the MF POMs/MXenes anode was evaluated by repeatedly charging and

discharging at different current densities for over 1000 cycles. After a mild decrease at the very early stage, the capacity at 1.0 and 3.0 A g–1 remains stable, which gives capacity retention of 83.5% and 80.6%, respectively, as illustrated in Figure 5g. Similar to the lithium storage, a decent mass and area capacity can still be delivered at very high mass loading (Figure 5h). Spe-cifically, the specific capacity of 271 mA h g–1 and 0.13 mA h cm–2 at a loading mass of 0.35 mg cm–2 become to 198 mA h g–1 and 1.37 mA h cm–2 at a loading mass of 6.0 mg cm–2 and a current density of 1.0 A g–1, suggesting the well-maintained electrolyte-permeable electrode architectures at high loading masses. Moreover, the high rate-performance can also be well-maintained even at high loading masses as illustrated in Figure S11g-i (Supporting Information).

Based on the dominated pseudocapacitive sodium storage capability of the MF POMs/MXenes composite, the SIC was assembled using a nitrogen-doped AC as the cathode and MF POMs/MXenes as the anode at a mass ratio of around 1.9:1. Figure S12 (Supporting Information) provides the electrochem-ical signature of the AC cathode for SICs. The CV curves of the SIC, anode, and cathode are shown in Figure 6a, which

Figure 6. Electrochemical performance of the SIC with the MF POMs/MXenes anode. a) CV curves of anode, cathode, and the SIC at a scan rate of 2.0 mV s–1. b) GCD curves of the SIC between 0.4 and 4.2 V at different current densities. c) GCD curves of the SIC at current density of 0.5 A g–1 with potential variation of anode and cathode. d) Potential variation of the anode and cathode recorded at different current densities. e) Nyquist plot, f) Ragone plot, and g) cyclic performance of the SIC.



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affords a decent capacitive capability. Moreover, the highly symmetrical GCD curves were obtained at current densities ranging from 0.4 to 4.2 A g–1 as shown in Figure 6b. To obtain the detailed operation process, the GCD curve of the SIC, as well as the potential profiles of anode and cathode, was detected simultaneously. As shown in Figure 6c, the anode and cathode operate in the potential range of 0.2–1.9 and 2.3–4.4 V versus Na+/Na, respectively. The well-matched relationship could be maintained at different current densities because the potential profiles of both cathode and anode remain undistorted as illus-trated in Figure 6d.[66] This observation provides further evi-dence of the excellent pseudocapacitive behavior of MF POMs/MXenes for sodium storage and the kinetics gap between the two electrodes is essentially alleviated.[61,67] Figure 6e shows that the MF POMs/MXenes//AC SIC also exhibits low charge transfer resistance, which supports the fast charge transfer kinetics. Then, the Ragone plot of the SIC is drawn to illus-trate its practical performance. As exhibited in Figure 6f, a high energy density of 117.3 W h kg–1 is obtained at a power density of around 0.19 kW kg–1 while a decent energy density of 58.7 W h kg–1 still is delivered even at a large power output of 3.8 kW kg–1. Furthermore, the MF POMs/MXenes//AC SIC exhibits relatively good cycling performance with good capacity retention after 800 cycles at 1.0 A g−1 (Figure 6g), and the Cou-lombic efficiency is nearly 100% during the cycling test.

3. Conclusion

In summary, we developed a mild in situ technology to chemi-cally bond Mo and Fe-based POMs nanoparticles on MXenes nanosheets for pseudocapacitive and high mass-loaded lithium/sodium storage. Such a composite can essentially har-vest the rich redox-activity of POMs and metallic conductivity of MXenes. Because of these merits, the MF POMs/MXenes affords outstanding pseudocapacitive performance for lithium/sodium storage with long-term stability. Moreover, the decent electrochemical capability can be offered even at very high mass-loadings. When fabricated into LIC and SIC by pairing the composite with a nitrogen-doped AC cathode, the hybrid devices with large energy densities, high power capabilities, and excellent stability are demonstrated. Because of the simple procedure and mild synthesis condition, the strategy suggested here may contribute to a broad spectrum of composites made of different types of POMs and MXenes, considering the wide choices of them, for enhanced energy storage applications.

4. Experimental SectionMaterials Preparation: MF POMs were synthesized via a simple

wet-chemical route. Specifically, 5.6 g of ((NH4)6Mo7O24.4H2O) and 1.1 g of FeCl3.6H2O were dissolved in 400 mL water to form a turbid liquid under stirring. Then, the pH value of the solution was adjusted to 1–2 by adding glacial acetic acid dropwise to form a homogeneous solution. After aging for 3 d, the MF POMs would precipitate out from the solution as yellow sediments. The Ti3C2X nanosheets were prepared as follow. First, 2.0 g of Ti3AlC2 powder (11 technology co., LTD, China) was etched in 40 mL HCl (9 m) solution containing 2.0 g of LiF for 24 h at 35 °C. Then, the supernate was removed after centrifuging at 3500 rpm for 10 min. After redispersion of the sediment in deionized

water, another centrifugation and supernate removal were conducted. The dispersion, centrifugation, and supernate removal processes were repeated until the pH of the supernate reached 6. Then, the sediment was dispersed in ethanol for full exfoliation under sonication. Finally, the exfoliated nanosheets were collected by centrifugation and redispersed in deionized water for subsequent use. To prepare the MF POMs/MXenes, 1.0 mL of the precursor solution of MF POMs (100 mg mL–1 based on the mass of (NH4)6Mo7O24 and FeCl3) were prepared and then mixed with 50 mL of MXenes dispersion (2 mg mL–1). After aging for more than 24 h, the product was filtrated and collected by freeze-drying. The mass ratio of MF POMs to MXenes in the composite is set as 1:1. Meanwhile, composites with MF POMs to MXenes mass ratio of 2:1 and 1:2 were also synthesized by modulating the content of the MF POMs precursor solution. Moreover, the as-synthesized MF POMs and MXenes were physically mixed to produce the composite as reference. The nitrogen-doped activated carbon (AC) (YF-8P KURARAY Chemical Co. Ltd. in Japan) was prepared by heating the AC at 800 °C for 5 h in the mixed gases of NH3 and N2 (1:3 v/v).

Materials Characterization: XRD patterns of different samples were recorded on an X′ Pert PRO MPD diffractometer equipped with Cu Ka radiation (λ = l.5406 Å). The element contents were collected using Agilent 710 ICP-OES. Scanning electron microscopy (SEM) observation was performed on JEOL SM-7900F and transmission electron microscopy (TEM) images were obtained on the FEI Talos F200 STEM. Energy dispersive spectroscopy (EDS) was collected on an EDS system attached on the SEM. Raman spectra were detected on a HORIBA HR800 (512 nm laser). The nitrogen sorption isotherms were measured by a Micromeritics ASAP 2020 analyzer. The surface elemental compositions were detected using an X-ray photoelectron spectroscopy (XPS, Escalab 250XI, Thermo Fisher Scientific) with Al Kα radiation. The thermal gravimetric analysis (TGA) curves were obtained on a thermal gravimetric analyzer (PerkinElmer TGA 4000) with a heating rate of 10 °C min–1 in flowing nitrogen. The Atomic Force Microscope (AFM) images were obtained on DI MULTIMODE system with MultiMode SPM microscope and AS-12 scanner.

Electrochemical Measurements: Before assembling the working electrodes, the POMs were dehydrated by heating at 220 °C 6 h in a vacuum oven. The working electrode was prepared by mixing the active material, carbon black, and polyvinylidene difluoride with a mass ratio of 7:2:1. The slurry containing the aforementioned components was then coated on the current collector and then vacuum dried at 80 oC for 12 h to produce the electrode. The loading mass of active species was 0.35–6.0 mg cm–2. CR2032 coin-cells were assembled with Li/Na metals as the counter and reference electrodes which employed the 1 m LiPF6 in dimethyl carbonate (DMC), ethylene carbonate (EC), and ethyl methyl carbonate (EMC) (1:1:1 vol%) with 2.0 wt% fluoroethylene carbonate (FEC) and 1 M NaClO4 in EC and DMC (1:1 vol%) as the electrolytes for lithium and sodium storage, respectively. To assemble the LICs and NICs, the working electrode and nitrogen-doped AC were employed as the anode and cathode, respectively. The mass ratio of cathode/anode was determined by balancing the charge of the cathode and anode where the value was set as 2.8 and 2.0 for LICs and NICs, respectively. To simultaneously record the potential variation of the cathode and anode during the device operation, the there-electrode split cell was employed where the lithium/sodium metal wire was introduced into the device. The GCD profiles and cyclical stabilities were recorded using the NEWARE BTS-5V Measurement System while CV curves and electrochemical impedance spectroscopy (EIS) were obtained on an IVIUMn STAT electrochemical workstation. The Li+/Na+ diffusivities of the MF POMs/MXenes electrode were measured by GITT via which the lithium/sodium storage were evaluated at a pulse current of 200/100 mA g–1 for 20 min at 20 min rest intervals.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.



2007636 (10 of 11) © 2021 Wiley-VCH GmbH

AcknowledgementsThe authors acknowledge the financial support from National Natural Science Foundation of China (21975287, 22005341, 21878336), the startup support grant from China University of Petroleum (East China), Technological Leading Scholar of 10000 Talent Project (No. W03020508), Taishan Scholar Project (No. ts201712020), and Shandong Provincial Natural Science Foundation (ZR2018ZC1458, ZR2018MB035, ZR2020QB128).

Conflict of InterestThe authors declare no conflict of interest.

Data Availability StatementResearch data are not shared.

Keywordshybrid supercapacitors, MXenes, polyoxometalates, pseudocapacitive contribution

Received: September 7, 2020Revised: January 20, 2021

Published online:

[1] Y. Lu, J. Nai, X. Lou, Angew. Chem., Int. Ed. 2018, 57, 2899.[2] C. Zhang, M. Liang, S.-H. Park, Z. Lin, A. Seral-Ascaso, L. Wang,

A. Pakdel, C. Coileáin, J. B. Boland, O. Ronan, N. McEvoy, B. Lu, Y. Wang, Y. Xia, J. N. Coleman, V. Nicolosi, Energy Environ. Sci. 2020, 13, 2124.

[3] J. Lang, B. Yang, H. Li, X. Yan, Sci. Sin.: Chim. 2018, 48, 1478.[4] P. Zhang, B. Guan, L. Yu, X. Lou, Angew. Chem., Int. Ed. 2017, 56,

7141.[5] J. Lang, X. Zhang, B. Liu, R. Wang, J. Chen, X. Yan, J. Energy Chem.

2018, 27, 43.[6] J. Ding, W. Hu, E. Paek, D. Mitlin, Chem. Rev. 2018, 118, 6457.[7] Q. Wang, Z. Wen, J. Li, Adv. Funct. Mater. 2006, 16, 2141.[8] M. Yang, B. Choi, S. Jung, Y. Han, Y. Huh, S. Lee, Adv. Funct. Mater.

2014, 24, 7301.[9] J. Chen, M. D. Symes, S. Fan, M. Zheng, H. Miras, Q. Dong,

L. Cronin, Adv. Mater. 2015, 27, 4649.[10] D. Ma, L. Liang, W. Chen, H. Liu, Y. Song, Adv. Funct. Mater. 2013,

23, 6100.[11] X. Yang, M. Li, N. Sheng, J. Li, G. Liu, J. Sha, J. Jiang, Cryst. Growth

Des. 2018, 18, 5564.[12] H. Wang, H. Shun, N. Yoshio, I. Stephan, Y. Toshihiko, Y. Hirofumi,

A. Kunio, J. Am. Chem. Soc. 2012, 134, 4918.[13] S. Hartung, N. Bucher, H. Chen, R. Al-Oweini, S. Sreejith, P. Borah,

Y. Zhao, U. Kortz, U. Stimming, H. Hoster, M. Srinivasan, J. Power Sources 2015, 288, 270.

[14] P. Barpanda, G. Liu, C. Ling, M. Tamaru, M. Avdeev, S. Chung, Y. Yamada, A. Yamada, Chem. Mater. 2013, 25, 3480.

[15] H. Miras, J. Yan, D. Long, L. Cronin, Chem. Soc. Rev. 2012, 41, 7403.

[16] J. Ye, J. Chen, R. Yuan, D. Deng, M. Zheng, L. Cronin, Q. Dong, J. Am. Chem. Soc. 2018, 140, 3134.

[17] X. Yang, P. Zhu, X. Ma, W. Li, Z. Tan, J. Sha, CrystEngComm 2020, 22, 1340.

[18] W. Chen, L. Huang, J. Hu, T. Li, F. Jia, Y. Song, Phys. Chem. Chem. Phys. 2014, 16, 19668.

[19] P. Zhang, Q. Zhu, R. Soomro, S. He, N. Sun, N. Qiao, B. Xu, Adv. Funct. Mater. 2020, 30, 2000922.

[20] H. Shi, M. Yue, C. Zhang, Y. Dong, P. Lu, S. Zheng, H. Huang, J. Chen, P. Wen, Z. Xu, Q. Zheng, X. Li, Y. Yu, Z. Wu, ACS Nano 2020, 14, 8678.

[21] C. Wang, S. Chen, H. Xie, S. Wei, C. Wu, L. Song, Adv. Energy Mater. 2018, 9, 1802977.

[22] K. Ma, H. Jiang, Y. Hu, C. Li, Adv. Funct. Mater. 2018, 28, 1804306.[23] M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon,

L. Hultman, Y. Gogotsi, M. W. Barsoum, Adv. Mater. 2011, 23, 4248.[24] Y. Dong, H. Shi, Z. Wu, Adv. Funct. Mater. 2020, 30, 2000706.[25] J. Luo, W. Zhang, H. Yuan, C. Jin, L. Zhang, H. Huang, C. Liang,

Y. Xia, J. Zhang, Y. Gan, X. Tao, ACS Nano 2017, 11, 2459.[26] X. Wang, S. Kajiyama, H. Iinuma, E. Hosono, S. Moriguchi,

M. Okubo, A. Yamada, Nat. Commun. 2015, 6, 6544.[27] M. Naguib, O. Mashtalir, J. Carle, V. Presser, J. Lu, L. Hultman,

Y. Gogotsi, M. W. Barsoum, ACS Nano 2012, 6, 1322.[28] Y. Ando, M. Okubo, A. Yamada, M. Otani, Adv. Funct. Mater. 2020,

30, 2000820.[29] R. L. Maria, M. Olha, E. R. Chang, D. A. Yohan, R. Patrick,

L. T. Pierre, N. Michael, S. Patrice, W. B. Michel, Y. Gogotsi, Science 2013, 341, 1502.

[30] M. Naguib, J. Come, B. Dyatkin, V. Presser, P.-L. Taberna, P. Simon, M. W. Barsoum, Y. Gogotsi, Electrochem. Commun. 2012, 16, 61.

[31] Q. Tang, Z. Zhou, P. Shen, J. Am. Chem. Soc. 2012, 134, 16909.[32] F. Bu, M. M. Zagho, Y. S. Ibrahim, B. Ma, A. A. Elzatahry, D. Zhao,

Nano Today 2020, 30, 100803.[33] A. Müller, S. Sarkar, S. Q. N. Shah, H. Bögge, M. Schmidtmann,

S. Sarkar, P. Kögerler, B. Hauptfleisch, A. X. Trautwein, V. Schünemann, Angew. Chem., Int. Ed. 1999, 38, 3238.

[34] M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L. Hultman, Y. Gogotsi, M. W. Barsoum, Adv. Mater. 2011, 23, 4248.

[35] M. Ghidiu, M. R. Lukatskaya, M. Q. Zhao, Y. Gogotsi, M. W. Barsoum, Nature 2014, 516, 78.

[36] T. Zhang, L. Pan, H. Tang, F. Du, Y. Guo, T. Qiu, J. Yang, J. Alloys Compd. 2017, 695, 818.

[37] M. Ghidiu, J. Halim, S. Kota, D. Bish, Y. Gogotsi, M. Barsoum, Chem. Mater. 2016, 28, 3507.

[38] M. Biesingera, B. Paynec, A. Grosvenord, L. Laua, A. Gersonb, R. Smart, Appl. Surf. Sci. 2011, 257, 2717.

[39] H. Yu, Y. Wang, Y. Jing, J. Ma, C. Du, Q. Yan, Small 2019, 15, 1901503.

[40] Y. Hao, S. Wang, Y. Shao, Y. Wu, S. Miao, Adv. Energy Mater. 2019, 10, 1902836.

[41] D. Zhou, L. Yin, N. Li, F. Li, H. Cheng, ACS Nano 2012, 6, 3214.

[42] J. Ge, B. Wang, J. Wang, Q. Zhang, B. Lu, Adv. Energy Mater. 2020, 10, 1903277.

[43] J. Chen, B. Yang, H. Hou, H. Li, L. Liu, L. Zhang, X. Yan, Adv. Energy Mater. 2019, 9, 1803894.

[44] S. Niu, Z. Wang, M. Yu, M. Yu, L. Xiu, S. Wang, X. Wu, J. Qiu, ACS Nano 2018, 12, 3928.

[45] L. Shen, H. Lv, S. Chen, P. Kopold, P. A. Aken, X. Wu, J. Maier, Y. Yu, Adv. Mater. 2017, 29, 1700142.

[46] P. Xiong, X. Zhang, F. Zhang, D. Yi, J. Zhang, B. Sun, H. Tian, D. Shanmukaraj, T. Rojo, M. Armand, R. Ma, T. Sasaki, G. Wang, ACS Nano 2018, 12, 12337.

[47] Z. Li, Y. Dong, J. Feng, T. Xu, H. Ren, C. Gao, Y. Li, M. Cheng, W. Wu, M. Wu, ACS Nano 2019, 13, 9227.

[48] X. Hu, Y. Liu, J. Chen, L. Yi, H. Zhan, Z. Wen, Adv. Energy Mater. 2019, 9, 1901533.

[49] Y. Nie, S. Liang, W. Yu, H. Yuan, J. Yan, Chem. Asian J. 2018, 13, 1199.



2007636 (11 of 11) © 2021 Wiley-VCH GmbH

[50] P. Huang, X. Wang, D. He, H. Wu, C. Qin, M. Du, C. Lai, Z. Su, Dalton Trans. 2017, 46, 13345.

[51] H. Wu, M. Huang, C. Qin, X. Wang, H. Hu, P. Huang, Z. Su, Cryst-EngComm 2019, 21, 1862.

[52] Y. Ding, J. Peng, S. Khan, Y. Yuan, Chem. - Eur. J. 2017, 23, 10338.

[53] X. Meng, H. Wang, Y. Zou, L. Wang, Z. Zhou, Dalton Trans. 2019, 48, 10422.

[54] H. Wu, H. Hu, C. Qin, P. Huang, X. Wang, Z. Su, Chem. Commun. 2020, 56, 2403.

[55] Y. Wang, M. Zhang, S. Li, S. Zhang, W. Xie, J. Qin, Z. Su, Y. Lan, Chem. Commun. 2017, 53, 5204.

[56] X. Jia, J. Wang, H. Hu, Y. Song, Chem. - Eur. J. 2020, 26, 5257.

[57] J. Hu, H. Diao, W. Luo, Y. Song, Chem. - Eur. J. 2017, 23, 8729.

[58] X. Zhao, G. Niu, H. Yang, J. Ma, M. Sun, M. Xu, W. Xiong, T. Yang, L. Chen, C. Wang, CrystEngComm 2020, 22, 3588.

[59] Y. Yue, Y. Li, Z. Bi, G. Veith, C. Bridges, B. Guo, J. Chen, D. Mullins, S. Surwade, S. Mahurin, H. Liu, M. Paranthaman, S. Dai, J. Mater. Chem. A 2015, 3, 22989.

[60] J. Hu, Y. Ji, W. Chen, C. Streb, Y. Song, Energy Environ. Sci. 2016, 9, 1095.

[61] R. Naderi, A. Shellikeri, M. Hagen, W. Cao, J. Zheng, J. Electrochem. Soc. 2019, 166, A2610.

[62] Y. Wang, Z. Hong, M. Wei, Y. Xia, Adv. Funct. Mater. 2012, 22, 5185.

[63] D. Dubal, K. Jayaramulu, J. Sunil, Š. Kment, P. Gomez-Romero, C. Narayana, R. Zbořil, R. Fischer, Adv. Funct. Mater. 2019, 29, 1900532.

[64] R. Tripathi, S. M. Wood, M. S. Islam, L. F. Nazar, Energy Environ. Sci. 2013, 6, 2257.

[65] X. Jia, Doctoral Thesis, Shangdong University, 2007.[66] J. Dong, Y. He, Y. Jiang, S. Tan, Q. Wei, F. Xiong, Z. Chu, Q. An,

L. Mai, Nano Energy 2020, 73, 104838.[67] Y. Gogotsi, R. Penner, ACS Nano 2018, 12, 2081.


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