An Aqueous Al-ion Supercapacitor with V2O5 Mesoporous Carbon

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An Aqueous Al-ion Supercapacitor with V2O5 Mesoporous Carbon Electrodes Meng Tian, Ruihan Li, Chaofeng Liu, Donghui Long, and Guozhong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02030 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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ACS Applied Materials & Interfaces

An Aqueous Al-ion Supercapacitor with V2O5 Mesoporous Carbon Electrodes

Meng Tian,a,b Ruihan Li,a Chaofeng Liu,b Donghui Long,a,c

a



Guozhong Cao,b,

State Key Laboratory of Chemical Engineering, East China University of Science and

Technology, Shanghai 200237, China b

Department of Materials Science and Engineering, University of Washington, Seattle, WA

98195, USA c

Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China

University of Science and Technology, Shanghai 200237, China



Corresponding author:

Donghui Long ([email protected]); Guozhong Cao ([email protected])

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Abstract A high-performance low-cost aqueous Al-ion supercapacitor was fabricated based on nanostructured V2O5 impregnated mesoporous carbon microspheres (MCM/V2O5) electrodes and Al2(SO4)3 electrolyte for the efficient energy storage. MCM/V2O5 composites exhibit high dispersion of nanostructured V2O5 in mesoporous carbon matrix, beneficial to fast reversible redox reactions with a short diffusion path. The corresponding capacitor illustrates the distinguishable redox behavior, most likely due to the Al3+ intercalation/deintercalation leading to the reduction/oxidation of V5+/V4+. It delivers a high energy density of 18.0 Wh kg-1 at 147 W kg-1 and a long cycling lifespan with over 88% capacitance retention over 10,000 cycles. The competitive performance can be ascribed to the integration of the electric double layer capacitance provided from MCM with pseudocapacitance contributed by nanostructured V2O5. This work offers the possibilities of high-performance aqueous capacitors based on trivalent Al-ion as guest species, providing new directions for future development of supercapacitors.

Keywords: Al-ion capacitors; Vanadium pentoxide; Pseudocapacitance; Mesoporous carbon; Symmetric capacitors.


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1 Introduction Electrochemical capacitors, also known as supercapacitors, are of great interest for energy storage with numerous applications in back-up power sources, electric vehicles and intermittent wind and solar energy systems due to their high power density, long cycling time and great reversibility.1–3 Supercapacitors can be classified into electric double-layered capacitors (EDLCs) and pseudocapacitors (PCs) based on different charge storage mechanisms. The former is mostly based on carbon materials which achieves energy storage by fast ion adsorption on the electrode surface, highly dependent on available surface area of materials, thus demonstrating high power density and long lifespan but low energy density.4,5 The latter always shows higher capacitance originating from pseudocapacitive materials (e.g. metal oxides, conductive polymers) triggering fast redox reactions at electrode/electrolyte interface, while exhibits limited cycling performance due to the structural expansion and shrink of electrode materials in the reaction process.6–8 Accordingly, the composites of carbon and redox-active materials always are fabricated to achieve the maximization of capacitors performance combining the advantages of EDLCs and PCs simultaneously.9,10 Vanadium pentoxide (V2O5) is a promising electrochemical redox-active material with abundant resources, layered structure and high theoretical capacity (440 mAh g-1 as battery cathode with three inserted monovalent cations),11 but the applications are limited by the slow ion diffusion (10-13-10-12 cm2 s-1 for Li+) and low electric conductivity (10-3-10-2 S cm-1).12 Many efforts have been contributed to design and synthesize carbon materials as impressive conductive network to enhance electron transport in V2O5-based electrodes with the promise to achieve high-performance capacitors.13–15 For instance, nanostructured rod-like V2O5 covered reduced graphene oxide (rGO) composites were fabricated by a solvothermal method, which achieves a high capacitance of 537 F g-1 at 1 A g-1 and 84% capacitance retention after 1,000 cycles compared to 30% retention of pure V2O5 due to the enhanced conductivity.16 3 ACS Paragon Plus Environment


Carbon nanotubes (CNT) and V2O5 nanowires were filtered to a flexible electrode film, with an energy density of 46.3 Wh kg-1 and the capacitance decay of 25% after 1,000 cycles.17 Nitrogen enriched mesoporous carbon spheres (n-MPC) were also excellent conductive matrix for V2O5. The composites prepared by a hydrothermal method display the flaky structured V2O5 is anchored on the surface of n-MPC, and demonstrate 91 F g-1 of cell capacitance and 16% decay up to 2,000 cycles.18 However, the uncontrolled growth of V2O5 on conductive materials impair the structure stability and obstruct the intimate contact between conductive materials and electrolyte ions, leading to the fast performance degradation, which cannot satisfy the requirements for the long-life requirement for supercapacitors. Therefore, multifunctional carbon matrix needs to be designed, with uniform porous structure for controlled nanostructured V2O5 growth without agglomeration, large volume for high mass loading of V2O5 and at the same time conductive network for rapid electrons transfer under long cycling process. Electrolyte also plays a key role in capacitors. Many literatures reported the use of aqueous electrolytes with advantages of high ionic conductivity, cost effectivity and safety compared to organic systems.19–22 Aqueous alkali-ion containing electrolytes (e.g. Li+,23 Na+ 24 and K+ 25) have drawn significant attentions due to the integration of electric-double-layer capacitance and pseudocapacitance. Less concerns focus on pseudocapacitive behaviors of multivalent metal ions just like Mg2+ and Al3+, ascribed to the strong ionic interactions than that of monovalent ions, which leads to a main constraint for ions diffusion.26,27 However, it should not be denied that aluminum is the third abundant element in the earth’s crust (82,000 ppm) with lower price than lithium and sodium. Aluminum-based redox couple engages a threeelectron transportation during the electrochemical charge/discharge reactions, offering higher storage capacity relative to the Na-ion or Mg-ion capacitors from single or two electrons.28–30 Thus, higher requirements are put forward to fabricate an efficient Al-ion capacitor and 4 ACS Paragon Plus Environment

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overcome the strong ionic interaction without sacrificing electrochemical performance. Herein, we present the design and fabrication of nanostructured V2O5 embedded in mesoporous carbon microspheres (MCM) electrode materials via a facile spray-drying method for MCM preparation and followed by a wet-impregnation to obtain MCM/V2O5 composites. MCM possesses uniform pore structure for V2O5 growth without agglomeration, high surface area with enormous active sites and large pore volume for high-content V2O5 impregnation. The corresponding MCM/V2O5 composites can achieve effective capacitance enhancement by integrating electric double layer capacitance contributed by MCM with pseudocapacitance provided by V2O5. The symmetric supercapacitor based on MCM/V2O5 electrodes and 1M Al2(SO4)3 electrolyte delivers high energy density of 18.0 Wh kg-1 at 147 W kg-1 and maintains 88% capacitance retention after 10,000 cycles. CV tests, ex-situ XPS and SEM elemental mappings were measured to provide the proof for the Al3+ intercalation/deintercalation behavior. Wet-impregnated MCM/V2O5 provides a promising candidate electrode material for low-cost long-cycle-life supercapacitors.

2 Experimental 2.1 Preparation of the MCM Chemicals were bought from Titanchem Co. with no more treatment unless stated otherwise. MCM was prepared by a spray drying method reported previously.31,32 Typically, 10 mL of aqueous solution containing 2.58 g of resorcinol and 3.8 g of formaldehyde (37 wt.%) was dropped into 70 mL aqueous solution with 28 g of 30 wt.% silica sol solution (Ludox SM30, Grace, USA). Following that, the solution was stirred at 45 ℃ for 1 h and then pumped into a spray dryer to be dispersed to the droplets and then dried quickly. The collected polymer microspheres were carbonized in a nitrogen flow at 800 ℃ for 3 h and the silica template was etched by 10% NaOH solution. MCM was obtained after water washing and drying at 80 ℃ 5 ACS Paragon Plus Environment


for 12 h. 2.2 Preparation of the MCM/V2O5 composites The MCM/V2O5 composites were synthesized by a facile wet impregnation method using as-prepared MCM as the matrix, which is always used for heterogeneous catalysts preparation.33,34 In a typical procedure, 0.2 g of MCM was dispersed in 5 mL of aqueous solution including 0.4 g of NH4VO3 and 0.43 g of oxalic acid with ultrasound for 1 h. Subsequently, the suspension was heated at 60 ℃ under vigorous stirring for 3 h and then dried at 50 ℃. The sample was finally obtained after heat treatment under N2 at 400 ℃ for 3 h, named as MCM/V2O5-20%. MCM/V2O5-40% was also prepared by doubling the concentration of V2O5 precursors. 2.3 Materials characterization The as-prepared materials morphologies were characterized by SEM (FE-SEM, Nova NanoSEM450, USA) equipped with an EDS analysis system (SEM, FEI Q-300, USA). TEM observations were captured by a transmission electron microscopy (TEM, JEOL 2100F, Japan) to show the detailed structure of materials. V2O5 contents in composites were confirmed by the thermogravimetric analysis (TG) using a TA Instrument Q600 Analyser in the air atmosphere. N2 sorption isotherms of materials were recorded by a Quadrasorb SI analyzer (Quantachrome, USA) and the corresponding porosity parameters were calculated. BrunauerEmmett-Teller (BET) method was adopted to analyze the specific surface area. The pore size distribution (PSD) was derived from the desorption branch with the Barrett-Joyner-Halenda (BJH) model for mesoporous structure. The total pore volume was calculated at a single point of P/P0 = 0.985. The X-ray diffraction (XRD) patterns were performed on a Rigaku D/max 2550 diffractometer (Japan) with Cu (Kα) radiation (λ = 1.5406 Å). Raman spectra were displayed with a Renishaw system 1000 using a 514 nm Ar-ion laser. The surface chemistry of samples was analyzed using an Axis Ultra DLD X-ray photoelectron spectrometer (XPS). 6 ACS Paragon Plus Environment

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2.4 Electrochemical measurements The working electrodes fabricated by mixing 85% of as-prepared materials, 10% of carbon black and 5% of polytetrafluoroethlene (PTFE, 60 wt.% in H2O) was punched into circle electrodes (8.4 mm in diameter). The mass loadings on electrodes were calculated to be 3.8, 5.1 and 6.1 mg cm-2 for MCM, MCM/V2O5-20% and MCM/V2O5-40%, respectively. The electrochemical performance of capacitors was characterized based on a modified symmetric two-electrode setup, with two electrode films separated by a cellulose separator and 1M Al2(SO4)3 solution as electrolyte. Capacitors in 1M Na2SO4 and 1M MgSO4 solution were also fabricated with the same configuration but different electrolytes. The Arbin BT2000 system (USA) was used to test the electrochemical performance of capacitors. MCM electrodes were tested at the voltage range of 0-1.0 V limited by water electrolysis. MCM/V2O5-20% and MCM/V2O5-40% electrodes were tested at 0-1.6 V because the introduction of V2O5 into MCM improves the materials activity at the positive potential to broaden the voltage window with no gas evolution. Cyclic voltammetry (CV) was tested at the different scan rates from 2 to 100 mV s-1. Galvanostatic charge-discharge (GCD) was performed at various current densities from 0.5 to 20 A g-1. The specific capacitance was calculated based on the discharging profiles of GCD using the following equation: Csp =

2 ∆E

∙ ∫idt m

(1)

where i is the discharging current, t is the discharging time, ∆E is the voltage window and m means the mass of one-electrode active materials. For MCM/V2O5, both MCM and V2O5 are regarded as active materials in electrodes to calculate the mass-referred values of electrochemical performance of capacitors. The cycle performance was performed with GCD method at 1 A g-1. The energy density and the corresponding power density were also figured out: 1

E = 2CU2 7 ACS Paragon Plus Environment

(2)


P = E/t

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(3)

where C, U and t represent the specific capacitance of cell, the discharging voltage change and the discharging time, respectively.

3. Results and discussion 3.1 Materials characterization

Figure 1. (a) Synthesis of MCM/V2O5 composites with a spray drying process for MCM preparation and followed by a wet-impregnation process to obtain composites. (b) SEM image of MCM. (c) SEM image and (d) HR-TEM image of MCM/V2O5-40%. Inset in Figure 1(d) is the enlarged lattice fringes. The fringe spacing marked by arrows relates to the planar distance of (202) in V2O5. (e-h) elemental mapping images of MCM/V2O5-40%. Elements of C, V and O are uniformly distributed in structure. 8 ACS Paragon Plus Environment

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The fabrication process of MCM/V2O5 composites is illustrated in Figure 1a. MCM was prepared firstly by a spray drying method with resorcinol (R) and formaldehyde (F) as carbon precursors and silica sol as meso-structure template, carbonized in N2 and then etched in alkali. The obtained MCM in Figure 1b shows a smooth surface and great sphericity. SEM images in Figure 1c and Figure S1a illustrate MCM/V2O5 composites still remain excellent sphericity and monodispersity without aggregation. HR-SEM images in Figure S1b and Figure S2 show the similar porous surface of MCM and MCM/V2O5 composites, indicating impregnation of V2O5 using NH4VO3 and oxalic acid as precursors do not change the surface of carbon materials. The typical lattice fringes are captured by HRTEM images (Figure 1d and Figure S1c), with an interplanar spacing of 0.206 nm, assigned to (202) plane of V2O5 (PDF41-1426). Elemental mapping images (Figure 1e-h and Figure S1d-g) further demonstrate the homogeneous distribution of vanadium and oxygen in MCM, indicating MCM acts as a porous conductive matrix for accommodating the uniform V2O5 nanoparticles and at the same time restricting the agglomeration, suggesting the advantages of the wet-impregnation process on facile preparation of metal oxide and carbon composites for capacitors. TG results in Figure S3 confirm the content of V2O5 in composites, near to the desired value.



Figure 2. (a) N2 sorption isotherms and (b) corresponding pore size distributions of samples, with typical mesoporous structure. (c) XRD patterns and (d) Raman spectra of samples. (e) survey XPS spectra and (f) V 2p XPS spectra of samples. The pore structure of MCM/V2O5 are shown in Figure 2a and b. MCM exhibits the typical Ⅳ isotherms with a hysteresis loop at high P/P0, suggesting dominant mesoporous structure, with the average pore size of 9.48 nm. MCM also displays a high surface area of 1,282 m2 g-1 and large pore volume of 2.90 m3 g-1. Adsorption branch in low P/P0 indicates the existence of micropores in MCM, with the microporous surface area of 454 m2 g-1. Table S1 summarizes the detailed porosity parameters. With V2O5 impregnated into MCM, the specific surface area 10 ACS Paragon Plus Environment

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of materials decreases, with the value of 847 m2 g-1 and 689 m2 g-1 for MCM/V2O5-20% and MCM/V2O5-40%, respectively, while the isotherm type of MCM/V2O5 composites keep the same as those of MCM. Introduction of nanostructured V2O5 into MCM sacrifices some adsorption sites but the composites still remain mesoporous structure and large pore volume. Figure 2c depicts XRD patterns of MCM/V2O5 composites, all of which display a broad diffraction peak located at the 2θ of around 24°, revealing amorphous carbon structure. It is worth to notice that no typical peaks of V2O5 appear in XRD patterns, which is ascribed to the difficulty of collecting signals of nanostructured low-crystallinity V2O5 in porous carbon spheres by XRD analysis.35 Low-crystallinity V2O5 could render less electrostatic interaction that commonly hinders insertion of high valence cations, and its less well-packed structure leads to a low density and a large space between adjacent ions/atoms. These two advantages contribute to the high-efficiency Al3+ intercalation/deintercalation behavior. Raman spectra in Figure 2d show two typical peaks at about 1341 cm-1 and 1592 cm-1 corresponding to D-band and G-band of carbon materials. The weakened peak intensity is observed for MCM/V2O5 composites compared to that of MCM, but all samples show the similar intensity ratios of Dband to G-band (ID/IG), indicating the existence of V2O5 do not introduce more defects into carbon lattice. XPS spectra are recorded to confirm the valence state of V, shown in Figure 2e-f. In the survey spectra (Figure 2e), MCM/V2O5 composites show typical C, V and O peaks without impurities. MCM/V2O5-40% displays more intensive V 2p and O 1s peak than that of MCM/V2O5-20% due to the higher content of vanadium pentoxide. Figure 2f compares highresolution V 2p XPS spectra of samples. For MCM/V2O5-40%, V 2p3/2 peak is decomposed into two synthetic peaks (dominant peak at 517.5 eV and slight peak at 516.6 eV), associating with V5+ and V4+ of 82.5% and 17.5%, respectively.36 MCM/V2O5-20% shows similar phenomenon but with weaker intensity of V4+ ion.



3.2 Electrochemical performance

Figure 3. Symmetric two-electrode capacitors: (a) CV curves of samples at 5 mV s-1. (b) CV curves of MCM/V2O5-20% at 5 mV s-1 in different electrolytes. (c) V 2p and (d) Al 2p XPS spectra of discharged and charged positive electrode of MCM/V2O5-40%. The transformation between V5+ and V4+ and the intensity variation of Al 2p peak indicate redox reactions during charging/discharging process.

The electrochemical performance of capacitors is evaluated using a symmetrical twoelectrode system. Figure 3a shows CV curves at the scan rate of 5 mV s-1 in 1M Al2(SO4)3 electrolyte. MCM exhibits a typical rectangular shape, suggesting an ideal double layer capacitive behavior. MCM/V2O5-20% and MCM/V2O5-40% show the typical redox peaks, attributed to the pseudocapacitive contributions from V2O5, with no polarization peaks. The 12 ACS Paragon Plus Environment

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comparison of CV curve areas demonstrates the as-obtained MCM/V2O5 composites possess a significantly higher specific capacitance than MCM, and MCM/V2O5-40% is the highest. More detailed CV curves at different scan rates are depicted in Figure S4a-c. As the scan rate increases to 50 mV s-1, MCM still retains near-box shape with a great charging/discharging reversibility. MCM/V2O5-20% shows a similar shape as that at 5 mV s-1, while MCM/V2O540% displays an evident distortion due to the decrease of material conductivity with high V2O5 loading, leading to limited ion transfer and redox reactions, especially at high scan rate. Figure S4d displays the b values of MCM/V2O5 electrodes, which is commonly used to evaluate if the charge storage is surface reaction controlled (b=1) or bulk diffusion controlled process (b=0.5). The b value decreases with the increased loading of V2O5 in MCM/V2O5 composites, indicating the introduction of more V2O5 leads to a more diffusion-controlled process. Capacitors in 1M Na2SO4 and 1M MgSO4 electrolytes were also tested. Figure. 3b compares CV curves at 5 mV s-1 of MCM/V2O5-20% in Na-ion, Mg-ion and Al-ion capacitors, and more details are shown in Figure. S5. Both Na-ion and Mg-ion capacitors exhibit near-box shape with a slight peak indicating weak redox reactions. Compared to them, Al-ion capacitor shows more distinguish redox peaks and larger CV area, demonstrating more effective reactions thus achieving higher capacitance enhancement. Theoretically, alkali metal ions in aqueous electrolyte are in the form of hydrate ions with six water molecules showing an octahedron configuration.37 Hydrate Alion possesses shorter metal-oxygen bond distance (RAl-O : 1.89 Å) and smaller metal ion radius (RAl3+ : 0.55 Å), beneficial to faster ion diffusion and higher-efficiency intercalation behavior, compared to hydrate Na-ion (RNa-O : 2.43 Å, RNa+ : 1.09 Å) and hydrate Mg-ion (RMg-O : 2.10 Å, RMg2+ : 0.76 Å).37,38 And solvation shell on hydrate ions can shield the positive charge on ions especially high-charge-density Al3+ so that it releases the restrictions of strong electrostatic interactions between Al3+ and intercalated structure of V2O5, therefore enhance the insertion kinetics.27 Furthermore, it should be emphasized that proton intercalation could exist in 13 ACS Paragon Plus Environment


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capacitors. If so, proton intercalation should provide the same capacitance contributions to the capacitors in Na2SO4, MgSO4 and Al2(SO4)3 electrolytes because of the same proton conditions. Therefore, the different redox behaviors and electrochemical capacitance of capacitors in threetype electrolytes are ascribed to different metal cations intercalation instead of proton intercalation. To fully understand the intercalation/deintercalation behavior, ex-situ XPS spectra of discharged and charged positive electrode are recorded in Figure 3c-d. For V 2p spectra in Figure 3c, discharged electrode shows a larger area of V4+ compared to the pristine electrode in Figure 2f, probably attributed to the Al-ion intercalation, which leads to a reduction of V5+ to V4+, with 34.3% of V4+. The spectrum of charged electrode corresponds to Al-ion deintercalation and an oxidation from V4+ to V5+, with 17.3% of V4+, near to 17.5% in the pristine

electrode.

The

reduction/oxidation

of

vanadium

correlates

to

the

intercalation/deintercalation of aluminium. In Figure 3d, Al 2p peak is evident at 75.1 eV of discharged electrode. When charged, the intensity of Al 2p peak decrease possibly due to Al ion deintercalation from materials.39 Weak Al signal still exists, which is ascribed to little adsorbed species from electrolytes in porous structure of MCM/V2O5 composites. These results provide

informative

proofs

for

Al-ion

intercalation

and

deintercalation

during

charging/discharging process. SEM elemental mapping images of discharged electrode (Figure S6) clearly reveal C, O, V and Al are uniformly distributed in materials.


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Figure 4. (a) GCD profiles at 0.5 A g-1 and (b) the rate performance of samples. (c) Cycling performance and (d) Ragone plot of samples. All samples show long cycling performance with up to 88% capacitance retention after 10,000 cycles. Highest energy density and highest power density are achieved by MCM/V2O5-40% and MCM/V2O5-20%, respectively.

Figure 4a shows the GCD profiles of samples at the current density of 0.5 A g-1. MCM exhibits symmetric linear charging and discharging curves, indicating a good reversible charging/discharging behavior at the electrolyte/electrode interfaces. In particular, MCM/V2O5-20% and MCM/V2O5-40% depict symmetric non-linear curves with two plateaus at about 0.4 V and 1.2 V, consistent with the position of redox peaks in CV curves, demonstrating a pseudocapacitive behavior contributed by reversible redox reactions from nanostructured V2O5. And triple longer charging/discharging time than that of MCM indicates higher capacitance after V2O5 impregnation. More detailed GCD profiles are summarized in Figure S7 and S8. Rate performance is recorded in Figure 4b to evaluate the power applications of capacitors. MCM/V2O5-20% and MCM/V2O5-40% maintain 65% and 48% of initial specific 15 ACS Paragon Plus Environment


capacitance when the current density increases from 0.5 to 20 A g-1, respectively. It is lower than that of MCM (80%) because the introduction of semi-conductive V2O5 into carbon matrix sacrifices part of surface area and conductivity of materials, thus leading to little faster capacitance decay at high current densities. However, the capacitance enhancement is still achieved at low current densities when impregnating V2O5 into MCM matrix. MCM/V2O5-20% and MCM/V2O5-40% show the specific capacitance of 207 F g-1 and 290 F g-1 at 0.5 A g-1, respectively, with around 60% and 120% increase compared to that of MCM (129 F g-1). Therefore, impregnating more V2O5 into MCM is beneficial to the specific capacitance enhancement at the expense of the rate performance. The corresponding volumetric capacitance is recorded in Figure S9a. MCM/V2O5-20% and MCM/V2O5-40% show enhanced volumetric capacitance compared to MCM. The cycling stability is also tested, which can be seen in Figure 4c. MCM displays an excellent cycling performance, with a 95% capacitance retention in 1M Al2(SO4)3 electrolyte after 10,000 cycles at 1 A g-1. For MCM/V2O5, cycling performance could be restricted by lower conductivity of V2O5. But high capacitance retentions after 10,000 cycles are still maintained at 90% and 88% for MCM/V2O5-20% and MCM/V2O5-40%, respectively. All samples show high coulombic efficiency with a great reversibility during consecutive charging/discharging process (Figure S9b). In Figure 4d, Ragone plot compares the energy density and corresponding power density of samples in Al-ion capacitors. MCM delivers an energy density of 4.0 Wh kg-1 at 118 W kg-1. Increasing V2O5 content is beneficial to energy density improvement, with 13.2 Wh kg-1 at 147 W kg-1 for MCM/V2O5-20% and 18.0 Wh kg1

at 147 W kg-1 for MCM/V2O5-40%, nearly four-fold as that of MCM, higher than that in

Na2SO4 and MgSO4 electrolytes (shown in Figure. S10). And the electrochemical performance of MCM/V2O5 electrodes is comparative to the previously reported symmetric supercapacitors in Table S2. Futhermore, MCM/V2O5-20% can retain 7.0 Wh kg-1 at 5,840 W kg-1, showing 16 ACS Paragon Plus Environment

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the great potential for high-power-density applications.

4. Conclusions An aqueous rechargeable Al-ion symmetric capacitor was fabricated using MCM/V2O5 electrodes and Al2(SO4)3 electrolyte. MCM was synthesized based on a scalable spray-drying method and followed by a facile wet impregnation method to obtain MCM/V2O5 composites. The as-prepared materials possess uniform mesoporous structure for fast ion diffusion, great conductivity for long cycling stability and high redox activity for capacitance improvement in aqueous Al-ion capacitor. The results of controlling different V2O5 content in MCM matrix demonstrate the increase of V2O5 contributes to capacitance enhancement, although the rate performance and cycling stability of systems are slightly restrained due to the decreased surface area and lower conductivity after V2O5 introduction. The corresponding capacitor illustrates the distinguishable redox behavior, most likely due to the Al3+ intercalation/deintercalation leading to the reduction/oxidation of V5+/V4+. It exhibits high energy density up to 18.0 Wh kg-1 at 147 W kg-1 and great cycling stability up to 88% capacitance retention after 10,000 cycles. Therefore, electrochemical capacitors based on MCM/V2O5 electrodes in Al-ion electrolyte provide the possibilities of high safety, cost efficiency, together with a threeelectron intercalated redox behavior, being regarded as a promising candidate to satisfy the continuous increasing energy storage demands.

Supporting Information SEM and TEM images, TG curves, porosity parameters and electrochemical performances.

Author Information Corresponding Author 17 ACS Paragon Plus Environment


*Email: [email protected] *Email: [email protected] ORCID: Donghui Long: 0000-0002-3179-4822 Guozhong Cao: 0000-0001-6539-0490 Notes The authors declare no competing financial interest.

Acknowledgments This work was partly supported by MOST (2014CB239702) and National Science Foundation of China (No. 21576090), and Fundamental Research Funds for the Central Universities (222201718002).

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An Aqueous Al-ion Supercapacitor with V2O5 Mesoporous Carbon

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