B-Site Cation-Ordered Double-Perovskite Oxide as an Outstanding

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B-site cation-ordered double perovskite oxide as an outstanding electrode material for supercapacitive energy storage based on the anion intercalation mechanism Zhenye Xu, Yu Liu, Wei Zhou, Moses O Tade, and Zongping Shao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19391 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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ACS Applied Materials & Interfaces

B-site cation-ordered double perovskite oxide as an outstanding electrode material for supercapacitive energy storage based on the anion intercalation mechanism

Zhenye Xu ,† Yu Liu, ‡ Wei Zhou, † Moses O. Tade, ‡ Zongping Shao*,†,‡

†

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry & Chemical Engineering, Nanjing Tech University, Nanjing 210009, P. R. China

‡

Department of Chemical Engineering, Curtin University, Perth, Western Australia 6845, Australia

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KEYWORDS: Anion intercalation; supercapacitor; perovskite oxide; cation ordering; oxygen vacancy

ABSTRACT: Perovskite oxides are highly promising electrodes for oxygen-ion-intercalation-type supercapacitors owing to their high oxygen vacancy concentration, oxygen diffusion rate and tap density. Based on the anion intercalation mechanism, the capacitance is contributed by surface redox reactions and oxygen ion intercalation in the bulk materials. A high concentration of oxygen vacancies is needed because it is the main charge carrier. In this study, we propose B-site cation-ordered Ba2Bi0.1Sc0.2Co1.7O6-δ as an electrode material with an extremely high oxygen vacancy concentration and oxygen diffusion rate. A maximum capacitance of 1050 F g-1 was achieved, and a high capacitance of 780 F g-1 was maintained even after 3000 charge-discharge cycles at a current density of 1 A g-1 with an aqueous alkaline solution (6 M KOH) electrolyte, indicating excellent cycling stability. In addition, the specific volumetric capacitance of Ba2Bi0.1Sc0.2Co1.7O6-δ reaches up to 2549.4 F cm-3, based on the dense construction and high tap density (3.2 g cm-3). In addition, an asymmetric supercapacitor was constructed using activated carbon (AC) as a negative electrode, and it displayed the highest specific energy density of 70 Wh kg-1 at the power density of 787 W kg-1in this study.

1. INTRODUCTION Electrochemical energy conversion and storage technologies have attracted great attention during the past two decades owing to their important roles in modern personal electronics, electrified transportation and high-quality use of clean energy such as hydrogen energy, wind energy or solar power.1-3 To date, several important types of electrochemical devices, such as fuel cells,4,

5

lithium-ion batteries (LIBs),6,7 and supercapacitors,8,9 have been extensively exploited for the generation or storage of clean energy for applications. Among them, supercapacitors have been given equal importance because of their significant advantages in power density and cycling stability. Carbon-based electrodes are the most commonly investigated in supercapacitors, usually show excellent cycling stability based on electric double layer theory with continuous charge-discharge


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processes up to millions of cycles without failure;10 however, they always show poor energy density due to their intrinsically low electrical double-layer capacitance.11 A general trend in the research of supercapacitors is to introduce Faradaic pseudocapacitance.12-15 By introducing electrode materials with the capability of surface redox reactions, a significant increase in the energy density (by a factor of ten or higher) with minimal effect on the power density and cycling performance may be realized. To further improve the energy density, intercalation-type electrodes for supercapacitors were also proposed.16,17 In this type of supercapacitor, the electrode reactions are similar to those in LIBs with intercalation-type electrodes, i.e., the bulk material of the electrode also takes part in the energy storage process. However, typically no phase transition during the electrode reaction occurs, and the intercalation reaction is so fast that it resembles a capacitive behaviour. A significant advantage of such intercalation-type electrodes over ion absorption or pseudocapacitive electrodes for energy storage in supercapacitors is that a high surface area is not necessary to obtain a high capacitance. Thus, the electrode material can be fabricated with a very high tap density to achieve an ultrahigh volumetric energy, which is sometimes an important characteristic of power sources for personal electronics.18,19 The phenomenon of the oxygen anion intercalation process was first observed by Kudo et al. in perovskite oxides in an aqueous alkaline solution.20 Subsequently, capacitive energy storage based on the oxygen anion intercalation mechanism was reported by Meffold et al. in 2014.21 In their study, a perovskite-type LaMnO3-δ composite oxide exhibited a capacitance of approximately 609.8 F g-1, calculated from the related cyclic voltage curve. Perovskites make up a large family of composite oxides with the nominal composition ABO3, in which the A site is typically lanthanum or alkaline earth metal element in 12-coordination with oxygen ions, and the B site is a transition metal element in 6-coordination with oxygen anions.22 Both the A site and B site of the perovskite lattice can be doped while maintaining the perovskite structure, resulting in the formation of millions of compounds with highly versatile properties.23 Very recently, we demonstrated a perovskite-type SrCo0.9Nb0.1O3-δ (SCN) as an anion-intercalation-type electrode material for supercapacitors with excellent electrochemical performance,18 suggesting that perovskite-type materials are highly promising anion-intercalation-type electrodes for supercapacitors. Several considerations should be taken into account during the development of perovskite oxides for supercapacitors, including a high accessible oxygen vacancy concentration, high structural ACS Paragon Plus Environment


stability, and high oxygen ion diffusivity. Since the energy storage process is based on the incorporation of oxygen anions into the bulk oxygen, and oxygen vacancies are the carriers for oxygen ions, the theoretical capacitance of perovskite oxides is determined by the number of accessible oxygen vacancies within the perovskite lattice. Since the incorporation or de-incorporation of oxygen ions into/from the perovskite oxide lattice is accompanied by a change in the oxidation state of the B-site cation, the valence state change of the B-site cation with a low energy barrier is preferred. In addition, a high structural stability during operation is very important since the perovskite lattice is the foundation of oxygen anion incorporation, while the collapse of the perovskite structure will halt oxygen anion intercalation due to the loss of intercalation sites.19 To maximize the oxygen vacancy concentration, the A site of the perovskite lattice is usually substituted with alkaline earth cations (Ba2+, Sr2+). Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) is a benchmark perovskite oxide with a superior oxygen vacancy concentration, and it has found promising applications as a cathode material for SOFCs and as an oxygen-separation ceramic membrane and electrocatalyst for the oxygen evolution reaction in alkaline electrolyte at room temperature.24-26 However, we have demonstrated previously that the slow dissolution of Ba2+ into alkaline solution ultimately causes the collapse of the perovskite lattice, resulting in poor cycling stability of BSCF as an anion-intercalation-type electrode for supercapacitors.19 Normally, the A-site or the B-site cations are disordered and distributed homogeneously inside the perovskite lattice. However, under some situations, the ordering of the A-site cation with the formation of an A-site cation-ordered double perovskite27-29 or the ordering of B-site cation with the formation of a B-site cation-ordered double perovskite could occur.30-32 Cation ordering could result enhanced oxygen diffusivity in the perovskite oxide when used in solid oxide fuel cells.33 Therefore, it may also improve the oxygen diffusion rate when used as a supercapacitor at room temperature. In addition, cation ordering in the double perovskite is beneficial for the creation of oxygen vacancies.34 A high oxygen vacancy concentration will provide more space for the intercalation of oxygen ions, which will lead to a high specific capacitance when used as an electrode material in oxygen-ion-intercalation-type supercapacitors. For example, it was reported that the oxygen non-stoichiometry of Ba2Bi0.1Sc0.2Co1.7O6-δ reached as high as 0.62 (normalized as a simple perovskite) at room temperature, as determined based on iodometric titration; as a comparison, it was 0.42 for BSCF perovskite.35 The above results suggest that cation ordering may be a useful strategy ACS Paragon Plus Environment

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to develop high-performance perovskite-type oxides as oxygen-ion-intercalation-type electrode materials for supercapacitors. Herein, as a proof-of-concept, we prepared the B-site cation-ordered double perovskite oxide with the

nominal

composition

of

Ba2Bi0.1Sc0.2Co1.7O6-δ

(BBSC)

as

an

electrode

in

oxygen-ion-intercalation-type supercapacitors. For comparison, the oxygen vacancy-disordered BSCF electrode was studied, and the origin for the superiority of BBSC as an electrode in supercapacitors was exploited. The BBSC electrode showed higher specific capacitance and better stability than BSCF.

2. EXPERIMENTAL SECTION 2.1. Material synthesis BBSC oxide powder was synthesized using an EDTA-citrate complexing (EDTA-CA) method,36 with Ba(NO3)2 (≥99.5%), Bi(NO3)3·5H2O (≥99.0%), Co(NO3)3·6H2O (≥ 98.5%) and Sc2O3 (≥99.99%) in analytical grade of purity, as the cation sources. All the raw materials were purchased from Sino-chemistry, Ltd. and applied directly without any further purification. In a typical synthesis, stoichiometric amounts of Ba(NO3)2, Bi(NO3)3·5H2O, Co(NO3)3·6H2O, Sc2O3 were prepared into a homogeneous solution by dropping them in deionized water under continuous magnetic stirring and constant heating at 80 °C. The Bi(NO3)3·5H2O and Sc2O3 solid were dissolved in HNO3 (65.0%-68.0%) under stirring and heating. The required amounts of CA and EDTA were added into the solution at a molar ratio of metal ion to EDTA to CA of 1:1:2. NH3·H2O was added to regulate the pH of the mixture to ∼7 to ensure complete complexation and to avoid potential precipitation. Under constant heating and continuous stirring at 80 °C, a transparent gel was achieved, which was then heated at 250 °C for 5 h to form the precursor. Finally, the precursor was calcined at 1050 °C for 5 h in air with 5 °C/min of the heating rate. The synthesis of the BSCF oxide was similar to BBSC, but a calcination temperature of 1000 °C was selected for BSCF.

2.2. Material characterization The crystalline structures were analysed via X-ray diffraction (D8 Advance diffractometer, Bruker, Germany) using Cu Kα radiation in a 2-theta range of 10-90° with step size of 0.02 ° at room temperature. Rietveld refinement was performed using the GSAS program and the EXPGUI interface. The particulate morphology and size were conducted via field-emission ACS Paragon Plus Environment


scanning electron microscopy (FE-SEM, Hitachi S-4800). The N2 adsorption-desorption analysis of the samples was characterized at -196 °C using a BELSORP-MAX instrument. The electronic structures were analysed by X-ray photoelectron spectroscopy (XPS) conducted with a Thermo ESCALAB 250 system with Al Kα X-ray (1486.6 eV). The tap density was measured using a density tester at room temperature. For the determination of cation leaching from the perovskite in alkaline solution, the electrolyte solution after testing with the perovskite electrode was collected and neutralized to PH∼7 with hydrochloric acid, and the concentrations of Ba2+, Sc3+, Bi3+ and Cox+ in the solution were examined by ICP-MS using inductively coupled plasma-atomic emission spectroscopy (Optima 7000 DV, Perkin-Elmer, USA).

2.3. Electrochemical performance test For the electrochemical measurements, the working electrodes were made by mixing active material (80 wt%), Super P (15 wt%) and pol(tetrafluoroethylene) (PTFE, 5 wt%). The mass of the working electrode is about 3mg cm-2. First, they were mixed in a beaker with about 10 ml of an ethanol solution and dispersed by ultrasonic agitation for 2 h. Then, the turbid liquid was dehydrated flat in bottom the breaker for 12 h in a vacuum oven at 60 °C. Next, the mixture was fabricated into a round film by a 12-mm-diameter punch and dried in a vacuum oven at 100 °C for 12 h. It was then pressed between two layers of nickel foam (14-mm-diameter) by a tableting machine at a pressure of 0.1 MPa. The nickel foam was purchased from MTI with the thickness of 1.5mm and the porosity of 110 ppi. The nickel foam was cleaned by ultrasonication in ethanol, acetone, and deionized water for 10 minutes. The thickness of the electrode after pressing is about 0.3mm. The working electrode was soaked in the electrolyte solution for 12 h before testing. The three-electrode system consisted of a Hg/HgO electrode filled with 1 M KOH as the reference electrode and a Pt plate as the counter electrode. The electrolyte was a 6 M KOH aqueous solution for the electrochemical performance tests. The electrochemical performance of BBSC was tested via cyclic voltammograms, recorded in a Princeton Applied Research PARSTAT 2273 electrochemical workstation (Advanced Measurement Technology Inc., USA). The galvanostatic charge-discharge (GCD) tests were performed using a NEWARE BTS cell tester (Shenzhen, China). The AC electrodes was prepared by the same steps of BBSC electrodes, using the active carbon to replace BBSC. The asymmetric BBSC-AC capacitor cell consisting of the BBSC electrodes (anode) and the AC electrodes (cathode) with a distance of 0.5 cm was fabricated ACS Paragon Plus Environment

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to test the electrochemical performance in a 6 M KOH electrolyte. The specific capacitance was calculated from the CV curve according to the formula (1): =

() ∙∙∆

(1)

Where I (A g-1) is the response current density, (mV s-1) is the potential scan rate, m (g) is the mass of active material, ∆V (V) is the potential window. The specific capacitance, which is used to report capacitance values, was calculated from the GCD curve according to the formula (2): =

(2)

∆

Where is normalized by m and (A g-1) is the current density, and V (V) is the potential. ∆V (V) is the potential window. The energy density () and power density () can be calculated from the following equations: = =

∆

(3)

(4)

Where (F g-1) is the specific capacitance, ∆V (V) is the potential drop, and ∆t (s) is the discharge time. The diffusion rates of oxygen in the samples were measured using a chronoamperometric oxygen diffusion method as follows. A three-electrode cell (a Pt wire counter electrode, a Hg/HgO reference electrode, and a working electrode of BBSC on a glass carbon electrode) was used during the test. The glassy carbon electrodes were made using the following steps. First, 10 mg of BBSC, 10 mg of Super P conductive, and 0.1 mL of Nafion solution (5 wt%) were dispersed in 1 mL of ethanol under sonication for 2 h to obtain a homogeneous ink. Then, 5 µL of the ink was pipetted onto the glassy carbon electrode (4 mm diameter, 0.126 cm2) and dried at ambient atmosphere, leading to an approximate BBSC loading of 0.36 mg cm-2. The test was performed with the following steps. First, the pretreatment was performed by bubbling with O2 for half an hour to remove bubbles on the working electrode. Then, the cyclic voltammetry measurement was tested to obtain the value of E1/2. Following this, chronoamperometric method was performed with the E1/2 potential with an excess of 50 mV, and the working electrode was rotated at 1,600 rpm to eliminate any mass transfer effects.

The diffusion rate was calculated from the equation λ=√. Where λ is a shape factor, a is the radius



of the particle and D is the diffusion rate. The value of λ is 2 and the value of is 150nm according to the related literature.37-39

3. RESULTS AND DISCUSSION The B-site cation-ordered double perovskite Ba2Bi0.1Sc0.2Co1.7O6-δ was reported to have a high oxygen reduction activity for SOFCs by Zhou et al. because of its mixed ionic and electronic conducting characteristics.35 Goodenough et al. analysed the structure of the Ba2Bi0.2Sc0.2Co1.6O6-δ by neutron powder diffraction. They confirmed a double perovskite with the ordered structure Ba2BB´O6-δ with B = Co2+ and B´= Bi0.2Sc0.2(Co3+)0.6.40 Recently, Ba2BixSc0.2Co1.8-xO6-δ was applied in the oxygen evolution reaction (OER), exhibiting a higher OER activity than BSCF and PBC for the ordered dual-site cobalt environments.41 The room temperature XRD patterns is shown in Figure 1a, a phase-pure cubic double perovskite structure was formed for the as-prepared BBSC. Based on Rietveld refinement (Figure S1a), a face-centred-cubic structure with a space group Fm3m (#225) and lattice parameters of a=8.17 Å was fitted, which agrees well with our previously reported work.33 The BSCF sample displayed a cubic lattice with space group of Pm3m and the lattice parameters of a=3.977 Å (Figure S1b). To verify the oxygen state and oxygen vacancy in the BBSC perovskite oxide, which plays an important role in the oxygen intercalation process, XPS was performed. Figure 1b exhibited the O 1s XPS spectrum of BBSC, and it showed four characteristic peaks, such as lattice oxygen species（≈529.0 eV for O2-),42-45 highly oxidative oxygen species (≈530.5 eV for O2-/O-),46 hydroxyl groups or surface-adsorbed oxygen (≈531.2 eV for –OH or O2),47 and surface-adsorbed water (≈532.0 eV for H2O).48 The relative amounts of the oxygen species were calculated from the fitted sub-peaks, and the results are displayed in Table S1. The content of O22-/Oand –OH/O2 of BBSC associated with oxygen vacancy density is larger than that of SCN and BSCF,49 which should theoretically make a positive contribution to the specific capacitance when it used as an electrode of a supercapacitor.


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Figure 1. a) XRD pattern of the as-synthesized BBSC and BSCF. b) XPS spectrum of O 1s in BBSC and BSCF.

A typical nitrogen adsorption-desorption isotherm of the as-prepared BBSC is shown in Figure 2, and it can be classified as the typical I. The almost overlapped adsorption and desorption curves suggest the BBSC sample was pore free in nature. The derived pore size distribution profile shown in the inset clearly demonstrates that the sample contained negligible mesopores. The calculated specific

surface

area

based

on

the

adsorption-desorption

isotherm

curves

and

the

Brunner-Emmet-Teller (BET) method was only 0.76 m2 g-1. Assuming the spherical particle shape,

the average particle size of the synthesized BBSC was approximately 2.19 μm according to υ ρ , where υ is the volume, ρ is the density, S is the surface area, A is the specific surface area. This result has been proved by TEM in Figure S3. With a large particle size and small surface, one would expect a low capacitance when only the surface redox reactions take part in the energy storage process. However, the specific surface areas do not have an obvious effect on the specific capacitance of the BBSC electrode due to the oxygen intercalation process. As demonstrated from Figure S2, the as-synthesized BSCF sample had a low specific surface area of 0.88 m2 g-1. Shown in Figure S3 is the SEM image of the as-synthesized BBSC powder, which exhibited a seriously sintered nature of the particles with an irregular particulate shape. The aggregates showed sizes in the range of 2-4 µm. The rough surface of the grains suggests that the large aggregates are dense.



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Figure 2. N2 adsorption/desorption curves of BBSC.

It was reported that some perovskite oxides were capable for oxygen anion intercalation at room temperature in highly concentrated alkaline solutions, such as LaMnO3+δ, SCN, SrCoO3-δ, and BSCF.19,22,26 To investigate the capability of BBSC to store energy via oxygen ion intercalation, cyclic voltammetry (CV) was conducted in an alkaline electrolyte solution. Figure 3 shows typical CV curves at 20 mV s-1 in two different concentrations of a KOH aqueous solution (1 M and 6 M) and in a neutral KNO3 aqueous solution (1 M) at room temperature within the potential window of -

0.0-0.6 V. Since there was a negligible amount of OH in the neutral KNO3 aqueous electrolyte, any -

capacitance would arise from the electrostatic adsorption of K+ and NO3 over the electrode surface. As expected, no obvious redox reaction peaks were observed from the CV curves of the BBSC electrode in KNO3 aqueous solution, and the positive and negative scanning curves were narrowly separated, suggesting the small capacitance of BBSC through EDLC. The calculated capacitance from CV curve was only 2.5 F g-1, which may be explained by the ultralow specific surface area (0.76 m2 g-1) of the BBSC, which was synthesized through severe sintering during high-temperature calcination for the phase formation. However, once the electrolyte was changed to the KOH aqueous solutions, obvious redox peaks appeared. In a 1 M KOH solution, a pair of redox peaks with the oxidation peak at 0.5 V and the reduction peak at 0.4 V was demonstrated. By increasing the concentration of the KOH solution to 6 M, both the oxidation peak and the reduction peak were shifted to lower potentials, suggesting improved reaction kinetics. Interestingly, the oxidation peaks were separated into two peaks with the peak potential at 0.34 and 0.44 V. The oxidation peak(s) should be related with the oxidation of Co2+/Co3+ to Co3+/Co4+, while the reduction peak should be associated with the reduction of Co3+/Co4+ to Co2+/Co3+. From the fitting of the CV curve area, we ACS Paragon Plus Environment

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could conclude that the specific capacitance of the BBSC electrode in 6 M KOH electrolyte was much larger than that in 1 M KOH electrolyte. This suggests BBSC could obtain high capacitance in -

the anion intercalation system with the presence of OH .

Figure 3. CV curves of BBSC measured at a scan rate of 20 mV s-1 in KOH electrolyte (a) and 1 M KNO3 electrolyte (b).

It is well known that many defect-containing perovskite oxides possess many oxygen vacancies. The value of oxygen non-stoichiometry (δ) in BBSC was reported to reach as high as 1.24. In other words, 20.7% of the oxygen sites in BBSC may be vacant. Assuming all the oxygen vacancies in BBSC are accessible for oxygen anion intercalation, a theoretical capacity as high as 1105.9 F g-1 is expected, suggesting the great potential of BBSC as an anion-intercalation-type electrode for supercapacitors. In addition to a high concentration of accessible oxygen vacancies, it should also possess a sufficient oxygen anion intercalation rate. As we know, many perovskite oxides show high oxygen ion conductivity at elevated temperatures, making them attractive materials as oxygen-permeation membranes or cathodes of SOFCs. With the decrease in operation temperature, however, the oxygen ion conductivity is dramatically reduced due to the high activation energy associated with the oxygen ion conductivity. Interestingly, recent investigations have demonstrated that under voltage bias, the oxygen ion diffusion rate inside some perovskite oxides such as LaCoO3-δ (LC) and SCN is still favourably high at room temperature. For example, based on the chronoamperometric method, the oxygen ion diffusion rate in LC and SCN was found to reach 3.0!10-14 cm2 s-1 and 1.1!10-12 cm2 s-1,18,37 respectively. Similarly, the oxygen anion diffusion rates in fresh BBSC were also measured from the chronoamperometric curve. As shown in Figure 4, a high oxygen ion diffusion rate of 1.39!10-12cm2 s-1 was detected in 6 M KOH. It was approximately 2.40 !10-13 cm2 s-1 in 1 M KOH, while no obvious oxygen anion diffusion was found in 1 M KNO3.



The apparent oxygen diffusion rate through the as-prepared BBSC was slightly larger than the benchmark single-perovskite phase of SCN in 6 M KOH and larger than LC in 1 M KOH, suggesting that cation ordering had an effect in accelerating the oxygen diffusion and demonstrating its applicability for as an oxygen-ion-intercalation-type electrode for supercapacitors. With the increase -

in the concentration of OH , the oxygen anion diffusion rate also increased, corresponding to their electrical performance.

Figure 4. a）Chronoamperometric oxygen diffusion rate measurements, isolated current vs. t-1/2 for BBSC in different solutions. b) The value of the oxygen diffusion rate in different solutions.

Shown in Figure 5a are the CVs of BBSC at different scanning rates from 5 to 50 mV s-1 within the potential window of 0.0-0.6 V. The slight shift of oxidation peak to a higher potential and the reduction peak to a lower potential with an increase in the scanning rate was an indication of increased polarization. With the increase in the scanning rate, the general shape of the CV curve was maintained; the curves showed similar shapes and the peak current increased with the increasing scanning rate. All above phenomena indicated that the BBSC electrode possessed rapid redox reactions and good reversibility. The electrochemical performance was further supported by the galvanostatic charge-discharge (GCD) curves at different current densities (Figure 5b). It should be mentioned that the data presented here was after 200 GCD cycles to allow the activation of the electrode material. Typically, the activation process can lead to a substantial increase in the capacitance of the electrode material.19 The shape of the GCD curves demonstrated typical pseudocapacitance behaviour, suggesting the presence of Faradaic redox reactions. Specific capacitances of 796.7, 715.0, 620, and 550.0 F g-1 were obtained at current densities of 1, 2, 5, and 10 A g-1 from the GCD curves, respectively. The response current of the Ni foam from the CV curve


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is relatively weak and the specific capacitance calculated from the GCD curve is only 2 F g-1 (Figure S7), which indicates that the capacitance of Ni foam is negligible. Considering the large tap density of BBSC (3.2 g cm-3), the volumetric capacitances of BBSC reached as high as 2549.4 F cm-3 (1 A g-1).

Figure 5. a) CV curves at different scan rates. b) GCD curves at various current densities. c) Capacitance calculated from the GCD curves at various current densities.

A reduction in particle size or an increase in specific surface area could increase the contact area between the active material and the electrolyte, which may increase the specific capacitance of the material. Therefore, we have increased the specific surface area of BBSC by hand grinding for 2 hours (BBSC-H) with no obvious change to the phase structure. The BBSC has already been ground for 10 min before this. The XRD pattern of the ground BBSC is shown in Figure S4. The specific surface area of the ground BBSC increased to 1.3 m2 g-1 (Figure S5). The electrical performance was then conducted from the galvanostatic charge-discharge test (Figure S6). The specific capacitances of 792.0, 739.0, 614.7, and 568.7 F g-1 were reached for the ground BBSC electrode at current



densities of 1, 2, 5, and 10 A g-1. No obvious increase in the specific capacitance was observed for the ground BBSC compared to the pristine BSCF, although the specific surface area of the former is double that of the pristine BBSC, suggesting the capacitance of BBSC results mainly from the bulk -

diffusion of OH intercalation. The peak current and the scan rate obey the power law relationship: і = a & , where a, b are variables. It indicates that the bulk diffusion controls the current when the b value equals 0.5, while the surface reaction controls the current when the b value equals to 1. As shown in Figure 6, b values for both anodic and cathodic peaks exhibited 0.87 and 0.85 (close to 1) at low scan rates (0.5-10 mV s-1), respectively, suggesting a capacitive process and high oxygen diffusion rate of BBSC. From 20 to 200 mV s-1, the b values were 0.65 and 0.62 (close to 0.5) for the anodic and cathodic peaks, indicating an increase in the diffusion resistance, and oxygen diffusion controls the charge storage at much higher scan rates.

Figure 6. The ' value determination of the normalized anodic and cathodic peak currents.

Cycling stability is largely important for perovskite-type materials for anion-intercalation-type electrodes in supercapacitors. Previously, we demonstrated that BSCF, a benchmark perovskite oxide with a high oxygen vacancy concentration experienced a steady decrease with increasing cycling time after the activation process (for the initial 70 cycles). The peak capacitance of BSCF reached 610 F g-1 at around 100 cycles, only 370 F g-1 was retained after 3000 cycles.19The deterioration was related to the leaching of Ba2+ and Sr2+ from BSCF into the electrolyte solution, which caused the collapse of the perovskite lattice. The dependence of the BBSC electrode discharge capacitance with respect to cycle number at a constant galvanostatic charge-discharge current density of 1 A g-1 is


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shown in Figure 7; for comparison, the cycling stability of BSCF from the literature was also presented. A sharp increase in capacitance was observed in the first 200 cycles, corresponding to an activated process. The capacitance reached a maximum value of 1050 F g-1 at the 750th GCD process, and then decreased slowly during subsequent cycles. Nevertheless, a high capacitance of 780 F g-1 was still obtained at the 3000th cycle, indicating excellent cycling stability. Such performance stability was much better than that of BSCF, as reported previously.19 Table S2 shows the cation concentrations in the mother electrolyte liquid after the GCD cycling for 3000 times. It was found that both Ba and Bi were the primary elements leaching from the double perovskite oxide, which is similar to the observation of barium as the primary leaching element from BSCF in our previous study. It was reported that the double perovskite was able to tolerate a large A-site cation deficiency.50 In addition, the simultaneous leaching of Ba from the A site and Bi from the B site of BBSC can minimize the cation deficiency in BBSC, thus, protecting the collapse of the double perovskite lattice structure, which may contribute to the improved cycling stability of BBSC compared to BSCF.

Figure 7. Cycling performance at a current density of 1 A g-1. (6 M KOH solution, 0-0.5 V, 2.4 mg of active material)

To further evaluate the performance of BBSC electrode in practical usage, an asymmetric supercapacitor (ASC) was assembled by utilizing activated carbon (AC) as the negative electrode and BBSC as the positive electrode. The carbon material demonstrates a typical electric double-layer capacitance within -1.0-0 V in a KOH aqueous electrolyte.10 Therefore, the potential window of the fabricated ASC can be extended to 1.5 V. Figure 8a displayed the CV curves of the ASC at various scan rates between 0 V and 1.5 V. Unlike the nearly rectangular shape of the CV curves of the traditional electric double-layer capacitors, these curves exhibited shaped redox peaks, suggesting that a Faradaic redox reaction was the main mechanism resulting in the capacitance of BBSC. Figure



8b shows the GCD curves at various current densities. The specific capacitance of the ASC reached 188.0 F g-1 at 0.5 A g-1. Encouragingly, 92% of the initial capacitance was still retained when the current density increased from 0.5 to 10 A g-1. The energy densities (E) and the power densities (P) were calculated based on the GCD curves and manifested on the Ragone diagram (Figure 8c). A very high energy density of 58.8 Wh kg-1 was achieved at a power density of 362.8 W kg-1, and an energy density of 54.0 Wh kg-1 was retained even at a high power density of 8461.9 W kg-1 (Table S3). The BBSC//AC ASC also obtained a good stability (Figure 8d). The specific energy density increased fast in the first 200 cycles due to the activation process. A maximum specific energy of 70 Wh kg-1 was achieved at nearly the 500th cycle, and 49 Wh kg-1 was retained after 2000 cycles, indicating a long operational life and robust stability.

Figure 8. a) CV curves of BBSC//AC. b) GCD curves at different current densities of BBSC//AC. c) Ragone plots of BBSC//AC. d) Cycling stability of BBSC//AC at a current of 1 A g-1.

4. CONCLUSIONS In summary, B-site cation-ordered double perovskite BBSC with high cobalt content and


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face-centred-cubic structure was fabricated by a simple sol-gel method. The BBSC demonstrated a maximum capacitance of 1050 F g-1 and excellent cycling stability as electrodes for supercapacitors based on an anion-intercalation charge storage mechanism. In addition, with a high tap density value, it exhibited a remarkable volumetric energy density of 2549.4 F cm-3. As a further demonstration, a BBSC//AC asymmetric supercapacitor was assembled, and the ASC delivered a high energy density of 70 Wh kg-1 at the power density of 787 W kg-1 and excellent cycling stability. This suggests that BBSC will play an important role as an electrode material in next-generation supercapacitors.

ASSOCIATED CONTENT Supporting information Characterization date of BBSC and BSCF; GCD curves at different current densities of BBSC with hand grinding for 2 hours; CV and GCD curve of Ni foam; Calibration curves of Ba, Bi, Sc, Co; Isolated current vs t-1/2 for BBSC in 1M and 6M KOH solution; O 1s XPS peak deconvolution results of BBSC, SCN and BSCF; ICP results of the electrolyte after 3000 cycles; The results of the energy densities (E) and the power densities (P).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (ZP. SHAO). Tel.: +86 25 83172256; Fax: +86 25 83172242.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Nature Science Foundation of China (No. 21576135), Jiangsu Natural Science Foundation for Distinguished Young Scholars (No. BK20170043), the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Program for Changjiang Scholars, the Program for Jiangsu Specially-Appointed ACS Paragon Plus Environment


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Professors, and the Youth Fund in Jiangsu Province (No. BK20150945).

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SYNOPSIS TOC: The BBSC demonstrated high specific capacitance and excellent cycling stability as electrodes for supercapacitors based on an anion-intercalation charge storage mechanism.


B-Site Cation-Ordered Double-Perovskite Oxide as an Outstanding

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