Recent advances in perovskite oxides as electrode materials for supercapacitors

Abstract

Owing to the high power density and ultralong cycle life, supercapacitors represent an alternative to electrochemical batteries in energy storage applications. However, the relatively low energy density is the main challenge for supercapacitors in the current drive to push the entire technology forward to meet the benchmark requirements for commercialization. To effectively solve this issue, it is crucial to develop electrode materials with excellent electrochemical performance since the electrode used is closely related to the specific capacitance and energy density of supercapacitors. With the unique structure, compositional flexibility, and inherent oxygen vacancy, perovskite oxides have attracted wide attention as promising electrode materials for supercapacitors. In this review, we summarize the recent advances in perovskite oxides as electrode materials for supercapacitors. Firstly, the structures and compositions of perovskite oxides are critically reviewed. Following this, the progress in various perovskite oxides, including single perovskite and derivative perovskite oxides, is depicted, focusing on their electrochemical performance. Furthermore, several optimization strategies (i.e., modulating the stoichiometry of the anion or cation, A-site doping, B-site doping, and constructing composites) to improve their electrochemical performance are also discussed. Finally, the significant challenges facing the advancement of perovskite oxide electrodes for supercapacitor applications and future outlook are proposed.

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Xuping Sun

Xuping Sun received his PhD degree from the Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences, in 2006. During 2006–2009, he carried out postdoctoral research at Konstanz University, University of Toronto, and Purdue University. In 2010, he started his independent research career as a full Professor at CIAC and then moved to Sichuan University in 2015. In 2018, he joined the University of Electronic Science and Technology of China, where he found the Research Center of Nanocatalysis & Sensing. He was recognized as a highly cited researcher (2018–2020) in both areas of chemistry and materials science by Clarivate Analytics. He has published over 490 papers with total citations over 42 000 and an h-index of 108. His research mainly focuses on the rational design of functional nanostructures toward applications in electrochemistry for energy conversion and storage, sensing, and environment.

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1. Introduction

The energy paradigm has changed from an era of fossil fuel resources to sustainable energy sources and new technologies associated with energy conversion and storage to address the rapid depletion of fossil fuels. Currently, energy storage and conversion technologies mainly include batteries, fuel cells, and supercapacitors (SCs).^1–3 Among them, SCs have aroused considerable interest due to their high power density (>10 K W kg⁻¹), long cycle life (>10⁵ cycles), and bridging function of the power/energy gap between traditional dielectric capacitors with high power output and batteries/fuel cells with high energy storage (Fig. 1).^4–7

Fig. 1 Comparison of specific power vs. specific energy of capacitors and batteries. Reproduced from ref. 6 with permission from Materials Research Society, copyright 2011.

Based on the charge storage mechanism, SCs can be classified into two categories:⁸ electrochemical double layer capacitors (EDLCs) and pseudocapacitors (PCs). EDLCs store charge through charge accumulation at the interface between the electrolyte and the electrode's surface. This process is a physical process without any electrochemical reaction (non-faradaic), guaranteeing fast energy uptake and delivery and avoiding the electrode materials' swelling. As such, EDLCs have high power density (>15 K W kg⁻¹) and long cycle life (>10⁵ cycles).^9,10 Because of their high specific surface area (1000–3000 m² g⁻¹) and desirable conductivity, carbon-based materials (such as carbon nanotubes, activated carbon, and graphene) are the most attractive to date for EDLCs.¹¹ However, experimentally, owing to the finite conductivity and unavailability of all of the active sites, the practical specific capacitance of pure carbon-based EDLCs achieved has usually been limited to ∼100–350 F g⁻¹.^12–14 In comparison to EDLCs, PCs store charge via reversible redox reaction (faradaic reaction) between the electrode materials and the electrolyte. Since the electrochemical reactions occur both on the surface and in the bulk near the solid electrode surface, PCs often show far higher capacitance values and energy density (by a factor of 10 or higher) than EDLCs.¹⁵ Metal oxides and conductive polymers typically work as electrode materials in PCs due to their fast reversible redox reactions, cost-effectiveness as well as easy processability.^16,17 Although PCs have a high energy density, they are not satisfactory in terms of power density and stability due to their poor electrical conductivity and framework swelling during cycling.¹⁸ Therefore, achieving high energy density while maintaining the high power density of SCs has become a challenging task.

As per the calculation formula (E = 1/2CV²), the energy capability of SCs is in proportion to the specific capacitance (C) and the square of voltage windows (V). Hence, the energy density can be improved by enhancing either the potential windows or the specific capacitance value. For the measurement of electrochemical response, electrochemical cells can be designed in either a half cell or a full cell configuration. The half cell is composed of three electrodes (working electrode, counter electrode, and reference electrode), while the full cell is composed of two electrodes (positive and negative electrodes).¹⁹ Generally, the cell potential can be increased by assembling asymmetric electrodes (two different electrode materials for the anode and the cathode) or by using organic electrolytes.^19,20 Typically, in an aqueous-based symmetric system, the working voltage is limited to less than 1.2 V (due to the thermodynamic breakdown potential of water molecules).²¹ Based on the same active material, when it is assembled into an asymmetric supercapacitor (ASC), the voltage window will be expanded to 2.5 V, while when using organic electrolytes, the potential window will be expanded to 3 V.^22,23 On the other hand, finding sustainable electrode materials is an effective strategy to improve the specific capacitance. Among the many candidate materials, metal oxides have attracted significant interest because they can provide higher energy density than conventional carbon materials and have better cycle lives than polymer materials.²⁴ With a wide potential window (about 1.2 V), excellent electronic conductivity (3 × 10² S cm⁻¹), and high theoretical specific capacitance (1358 F g⁻¹), RuO₂ is the most active electrode material.²⁵ Nonetheless, high cost and limited abundance restrict its widespread applications. In this context, non-noble metal oxides such as MnO₂, NiO, and Fe₃O₄ have been widely explored.^26–28 Nevertheless, some inherent shortcomings, such as low electrical conductivity and poor cycling stability, limit their practical applications.

As star materials, perovskite oxides have rapidly drawn intensive attention owing to their low cost, robust skeleton structure, high tap density, and inherent nature of containing oxygen vacancies and distinctive chemical tailoring.^29,30 Over the past few decades, perovskite oxides have traditionally been used as functional materials in energy-related fields. In 2014, a breakthrough work on the oxygen anion intercalation mechanism for nanostructured lanthanum-based perovskite oxides was reported by Meffold et al.³¹ The mechanism is as follows: in the first step, the diffusion of oxygen is in the form of OH⁻ from the electrolyte. Subsequently, the oxygen vacancies are filled by OH⁻ groups and they diffuse along the octahedral edges through the crystal concomitant with the oxidation of two Mn²⁺ centers to two Mn³⁺ and yielding water as a product. In the next step of the reaction, excess oxygen is intercalated at the surface by diffusion of manganese to the surface and oxidation of two Mn³⁺ centers to two Mn⁴⁺ (Fig. 2).^31,32 As mentioned above, the bulk of perovskite oxide is involved in the storage of charge, which means that perovskite oxide does not require a high surface area to achieve high energy storage. There are metal oxides (such as TiO₂-B, T-Nb₂O₅, and α-MoO₃) with similar intercalation behavior to perovskite oxides but are based on positive ions (such as Li⁺ or Na⁺) as the charge carrier.^33–35 In principle, O²⁻ can carry two negative charges per unit, which means that the intercalation pseudocapacitance of O²⁻ can store twice charges in one charge/discharge cycle than Li⁺ intercalation. Therefore, perovskite oxides have been regarded as tremendously promising candidate materials for SCs.

Fig. 2 Mechanism of oxygen intercalation into perovskite oxides. Reproduced from ref. 32 with permission from Elsevier B.V., copyright 2020.

To date, some high-quality reviews on perovskite oxides for energy conversion and storage applications have been published.^36–39 However, there is still a lack of application and development of perovskite oxides in SCs. In this review, we first review the structure and composition of perovskite oxides. Secondly, perovskite oxides as electrode materials for SCs are introduced. Thirdly, the optimization strategies and applications of perovskite oxides in SCs are discussed. Finally, the application opportunities and challenges of perovskite oxides in SCs are presented.

2. Structure and composition of perovskite oxides

Perovskite oxides with the general formula of ABO₃, originating from CaTiO₃, are a family of oxides.⁴⁰ An ideal structure of perovskite oxides is shown in Fig. 3a. The A cation is located at the cube's body center, the B cation is at each of the eight corners, and O²⁻ is at each of the centers of the 12 edges. In this structure, twice the B–O bond distance is equal to a (a is the cubic unit cell parameter), and the A–O bond distance is equal to a/2^1/2.

Fig. 3 Crystal structure of (a) the ideal ABO₃-type perovskite oxides, (b) Ruddlesden–Popper perovskite oxides ((AO)(ABO₃)_n), (c) A-site ordered double perovskite oxides (AA′B₂O₆) and (d) B-site ordered double perovskite oxides (A₂BB′O₆).

In reality, it was found that the cubic structure was still retained in ABO₃ compounds, even though the bond length relationship between A–O and B–O does not strictly meet the above rule. To measure the ideal deviation, Goldschmidt introduced a tolerance factor (t), defined by the equation⁴¹

Here R_A, R_B, and R_O are the radii of A-site ions, B-site ions, and oxygen ions, respectively. Based on Shannon's ionic radii data,³⁵t = 1 corresponds to an ideal cubic structure, and different distortions are observed as t decreases.⁴² The perovskite oxide structure is stable when t is in the range from 0.75 to 1.00. Notably, in addition to the above ion radius rule, the other requirement that needs to be met for the perovskite structure is the principle of electrical neutrality: the sum of the charges of A- and B-site cations must be equal to the total of oxygen anions. Adding these constraints together, the perovskite oxide structure displaying incomparable flexibility can accommodate variable occupancy of different structural and electronic lattice defects.^43,44 For instance, partial substitution of A and B-site cations by doping with different cations with varying ionic radius and valence has been extensively adopted to tailor perovskite oxide properties in both physical and chemical terms, such as lattice structure, electronic structure, lattice defects, and diffusion behavior.^45,46

In addition to the ABO₃-type perovskite, other oxides are also associated with perovskites, such as Ruddlesden–Popper (RP) perovskites ((AO)(ABO₃)_n, Fig. 3b),⁴⁷ A-site ordered double perovskite oxides (AA′B₂O₆, Fig. 3c),⁴⁸ and B-site ordered double perovskite oxides (A₂BB′O₆, Fig. 3d).⁴⁹ From the composition, about 90% of the metal elements in the periodic table can be the cations in ABO₃-type perovskite oxides.⁵⁰ The common elements are shown in Fig. 4. Generally, A-site metal's significant role is to elevate the thermodynamic stability, and the B-site metal regulates the electrochemical reactions. For the electrode material applications in SCs, alkaline-earth (such as Sr and Ba) and rare-earth metals (such as La and Sm) are the principal A-site cations, and the B-site cation is usually a transition metal (such as Mn, Co, and Ni).

Fig. 4 Common elements in ABO₃-type perovskite oxides.

3. Perovskite oxides as electrode materials for SCs

3.1 The single perovskite oxides

ABO₃ holds great promise as an active material for SCs due to its high tap density and oxygen vacancy concentration. In this section, perovskite oxides are divided into four categories, lanthanum-based, strontium-based, cerium-based, and other perovskite oxides, to review their application.

3.1.1 Lanthanum-based perovskite oxides. In recent years, lanthanum-based perovskite oxides have attracted great interest in SCs, due to their good thermal stability, facile synthesis, oxygen storage, low cost, and improved electrical conductivity.⁵¹

LaMnO₃ is the first lanthanum-based perovskite oxide to be applied in SCs. Because of the defective cation-deficient lattice and the presence of manganese in two oxidation states (Mn 3p/Mn 4p), LaMnO₃ shows a relatively stable and constant oxygen excess.⁵² In 2014, a specific capacitance of about 609.8 F g⁻¹ for LaMnO₃ was reported by Mefford et al.³¹ LaNiO₃ is reported to exhibit a similar charge storage mechanism to LaMnO₃ but with lower electrical resistivity (about 10⁻⁴ Ω).^53–58 Shao et al.⁵⁹ developed a template-free solvothermal approach to fabricate a hollow spherical structure of LaNiO₃, showing a high specific capacitance (422 F g⁻¹ at 1 A g⁻¹) and good cycle stability (83.3% capacitance retention after 5000 cycles).

Compared with LaNiO₃, LaFeO₃ is more stable because Fe³⁺ has a stable electronic configuration of 3d⁵.⁶⁰ Zhang et al.⁶¹ reported mesoporous LaFeO₃ nanoparticles as the electrode material for SCs (Fig. 5a). A two-electrode symmetric SC cell (SSC) based on this material displayed a high energy density of 34 W h kg⁻¹ at a power density of 900 W kg⁻¹ with 92.2% capacitance retention after 5000 cycles (Fig. 5b and c). In Harikrishnan's work,⁶² LaCoO₃ nanoparticles were prepared by the co-precipitation method. Because cobalt atoms have many oxidation states (2+, 3+, and 4+), LaCoO₃ exhibited good electrochemical redox properties with a specific capacitance of 299.64 F g⁻¹ at 10 A g⁻¹. Recently, Hussain's group used the sol–gel method to synthesise a hierarchical mesoporous nanostructure of LaCrO₃. The obtained sample revealed superior specific capacitance (1268 F g⁻¹ at 2 A g⁻¹) and good stability (91.5% capacitance retention after 5000 cycles).⁶³

Fig. 5 (a) Schematic illustrations showing the synthesis of perovskite LaFeO₃. (b) Cycling stability versus cycle number graph, inset: the first and last five GCD cycles. (c) Ragone plots (energy density vs. power density) of this work and other devices. Reproduced from ref. 61 with permission from Elsevier B.V., copyright 2020.

3.1.2 Strontium-based perovskite oxides. Strontium-based perovskite oxides have attracted substantial research attention as SC electrode materials because of their natural abundance, good ionic and electronic conductivity, and tolerance to reduction–oxidation cycling.⁶⁴

The earliest application of strontium-based perovskite oxides in the field of SCs can be traced back to 1999.⁶⁵ Garche et al.⁶⁶ reported SrRuO₃ with a capacitance of 270 F g⁻¹. Other strontium-based perovskite oxides (such as SrCoO_2.5, SrMnO₃, and SrTiO₃) also show potential for SCs. Xiao et al.⁶⁷ used a conventional solid-state reaction route to synthesize the orthorhombic structure of SrCoO_2.5, and the resultant SrCoO_2.5 displayed a specific capacitance of 168.5 F g⁻¹. George et al.⁶⁸ fabricated SrMnO₃ nanofibers using sol–gel assisted electrospinning, which was followed by calcination at different temperatures (600 °C, 700 °C, and 800 °C). It was found that the nanofibers synthesized at 700 °C with small grains comprising a porous structure performed (321.7 F g⁻¹ at 0.5 A g⁻¹) better than other samples. The high porosity and surface area are likely to improve the SC performance with more ion diffusion and adsorption in the charge–discharge process. Additionally, with high porosity, sufficient electrochemically active sites are exposed, which can hold a large quantity of the electrolyte, providing more electrode–electrolyte accessibility.⁶⁹ Similarly, Sharma et al.⁷⁰ demonstrated the charge storage properties of mesoporous SrTiO₃ (STO) with a cubic crystal structure. In their study, the Barrett-Joyner-Halenda (BJH) average pore diameter and pore volume of STO were tested as 15.18 nm and 0.249 cm³ g⁻¹, respectively. Such pores provide a large number of active sites and low charge transfer resistance.⁷¹ Consequently, STO displayed a high capacitance of 592 F g⁻¹ at 5 mV s⁻¹. Interestingly, based on this material, a SSC was constructed, which exhibited maximum energy (27.8 W h kg⁻¹) and power density (1921 W kg⁻¹) with excellent cycling stability (99% capacitance retention after 5000 cycles).

3.1.3 Cerium-based perovskite oxides. Cerium-based perovskite oxides are attractive candidates due to their high dielectric constant, low cost, high bandgap, and variable valence states between Ce³⁺ and Ce⁴⁺.^72,73 Their high SC performance in an alkaline solution at room temperature has also been confirmed recently. In this context, Hu and coworkers utilized electrospinning and consequent calcination processes to synthesize CeMnO₃ nanofibers (NFs).⁷⁴ It is generally well accepted that A-site cations of perovskite oxides cannot contribute to the electronic structure near the Fermi level.³⁷ However, because cerium has high redox ability between Ce³⁺ and Ce⁴⁺, the faradaic redox reaction (Ce⁴⁺/Ce³⁺ and Mn³⁺/Mn⁴) occurred on the CeMnO₃ electrode surface. As a result, the specific capacitance of CeMnO₃ NFs was 159.59 F g⁻¹ at a current density of 1 A g⁻¹. Very recently, other cerium-based perovskite oxides (CeCoO₃, CeNiO₃, and CeCuO₃) showed a specific capacitance of 128, 189, and 117 F g⁻¹, respectively.⁷⁵ Similarly, the capacitance of CeBO₃ (B = Co, Ni, Cu) perovskite oxides is ascribed to the redox reactions of Ce⁴⁺/Ce³⁺ and B^m+/B^(m+1)+.

3.1.4 Other single perovskite oxides. In addition to the several types discussed above, other single perovskite oxides (such as BiFeO₃, NiMnO₃, NiTiO₃, and CoTiO₃) have also received attention recently.^76–81 Yin et al.⁷⁷ prepared BiFeO₃ nanoplates by a hydrothermal method. The obtained BiFeO₃ displayed a specific capacitance (254.6 F g⁻¹) that was almost 3.1 times higher than that of the electrochemically deposited counterpart.⁷⁸ Kim et al.⁷⁹ utilized the same approach to synthesize three-dimensional NiMnO₃. The prepared NiMnO₃ delivered a specific capacitance of 99.03 F g⁻¹. The cycling-stability test showed a decrease in capacitance retention after 7000 cycles (77% capacitance retention after 7000 cycles), which can be attributed to the loss of the transition metal oxide during the redox reaction. Beyond that, the metal titanate MTiO₃ (M = Ni, Co) has also been studied extensively as a SC material. In Kitchamsetti's work, mesoporous nickel titanate (NTO) rods were synthesized via the sol–gel method.⁸⁰ The obtained sample showed a good specific capacitance (542.26 F g⁻¹). Similarly, Kitchamsetti et al.⁸¹ reported the highly porous CoTiO₃ (CTO) micro rods with high specific capacitance (608.4 F g⁻¹), specific power (4835.7 W kg⁻¹), and specific energy (9.77 W h kg⁻¹).

3.2 Optimization strategy of perovskite oxides

Because the oxygen vacancies can provide charge-storage sites, increasing the number of oxygen vacancies is an effective strategy to enhance capacitance. For perovskite oxides, the number of oxygen vacancies is highly dependent on their structure and composition. Therefore, well-designed perovskite oxides are critical for achieving high electrode performance. Moreover, composite materials generally have better properties than the individual components. In the following sections, we discuss two aspects to optimize the SC of perovskite oxides: creating oxygen vacancies and compositing with other materials.

3.2.1 Creating oxygen vacancies in perovskite oxides. The flexibility of the structure and composition of perovskite oxides presents a variety of defect structures. In this section, we discuss three ways of improving perovskite's oxygen vacancy concentration (modulating the stoichiometry of the anion or cation, A-site doping, and B-site doping) to affect the performance of SCs.
3.2.1.1 Modulating the stoichiometry of the anion or cation. The non-stoichiometry phenomenon in perovskite oxides is widespread, arising from either anion deficiency (ABO_3−δ) or anion excess (ABO_3+δ) or cation deficiency in the A/B-site.⁸² Modulating the stoichiometry of the anion or cation to enhance electrochemical performance is crucial because it generates oxygen vacancies.³⁷

Due to stability issues, there have been few studies on oxygen-excess perovskite ABO_3+δ with interstitial oxygen atoms.⁸³ However, oxygen-deficient perovskite oxides have been studied extensively. Typically, ABO_3−δ is prepared by post-processing methods, such as, thermal treatment of perovskite oxides at elevated temperatures under a low-oxygen partial pressure or inert or reducing atmosphere (such as nitrogen, argon, hydrogen, or their mixture, or a vacuum).^84–88 One typical example is that of Che et al.,⁸⁹ who fabricated two kinds of nonstoichiometric LaNiO_3−δ through the sol–gel method and further calcination at different temperatures (800 and 850 °C), which were named LNO and LNO-HT, respectively. In their experiment, the oxygen non-stoichiometry δ of LNO and LNO-HT was 0.37 and 0.27, respectively. They found that the specific capacitance of LNO increased by 75 F g⁻¹ compared with LNO-TH. Interestingly, LNO showed cycling stability (94.5% capacitance retention after 15 000 cycles) superior to that of hollow spherical LaNiO₃.⁵⁶ LaMnO₃ suffers from similar limitations as other oxides, such as low conductivity and short cycle stability.⁹⁰ Elsiddig et al.⁹¹ synthesized a series of LaMn_1±xO₃ by controlling Mn/La molar ratios (0.90, 0.95, 1, 1.05, and 1.1). Compared with the stoichiometric LaMnO₃ sample, significant differences were observed regarding Mn⁴⁺ ions and oxygen vacancies, which improve their conductivity and provide additional sites to accept more ions from the electrolyte and hence improve the electrochemical performance (Fig. 6a and b). In particular, LaMn_1.1O_3−δ showed the lowest equivalent series resistance (R_s = 3.5 Ω) and the best specific capacity (727.6 C g⁻¹ at 1 A g⁻¹).

Fig. 6 (a) Cyclic performance of the LM_1±xO₃ samples at a scan rate of 0.01 V s⁻¹. (b) Charge–discharge curves in a potential window of −1 to 0.56 V at 1 A g⁻¹. (C) EIS of the LM_1±xO₃ samples with the inset of enlarged R_s at high frequencies. (d) Charge storage mechanism in LM_1±xO₃ perovskite oxide materials. Reproduced from ref. 91 with permission from Elsevier B.V., copyright 2017.

3.2.1.2 A-site doping. In general, the SC performance of perovskite oxides is closely related to the B-site transition metal element. The A-site element (such as lanthanide and alkaline earth element) is inert for the redox reaction directly. However, the A-site cation may affect the electronic structure and coordination. Partial substitution of the A-site by a low-valence cation (such as Sr²⁺ and Ca²⁺) generates addition oxygen vacancies and shifts a large proportion of the B-site transition-metal ions to unstable oxidation states (B^m+/B^(m+1)+ redox couple).⁹² As a result, electronic conductivity is increased, and electrochemical performance is enhanced.

Since the alkaline earth metals and rare earth metals share similar atomic radius, it is energetically favorable for the former to substitute the latter.³⁸ In this direction, Mo et al.⁹³ prepared Ca-doped perovskite lanthanum manganates (La_0.5Ca_0.5MnO₃) by a sol–gel method. The specific capacitance of La_0.5Ca_0.5MnO₃ was 2.4 times that of pure LaMnO₃ (72 F g⁻¹). Similarly, Wang et al.⁹⁴ doped LaMnO₃ with Sr and reported that its capacitance was slightly increased from 187 to 198 F g⁻¹ at 0.5 A g⁻¹, and the cycle life was enhanced from 40% to 80% after 1000 cycles. To gain more insight into the influence of the substitution degree on electrochemical performance, Tian's group synthesized La_1−xSr_xMnO₃ (x = 0, 0.15, 0.3, 0.5) by the sol–gel method.⁹⁵ It was observed that the aggregation degree of nanoparticles, specific capacitance, and charge transfer resistance are affected by the value of x. In particular, when x = 0.15 (La_0.85Sr_0.15MnO₃), the lowest R_ct (1.6 Ω) and the best specific capacitance (102 F g⁻¹) were observed. Furthermore, the ASCs employing La_0.85Sr_0.15MnO₃ as the anode material and AC as the cathode material reached an energy density of 3.6 W h kg⁻¹ at a low power density of 120 W kg⁻¹. Recently, Alexander and coworkers synthesized a series of perovskite oxides with the composition La_1−xSr_xBO_3−δ (x = 0–1; B = Fe, Mn, Co) to study anion-based PCs systematically.⁹⁶ They found that more significant oxygen vacancy content upon systematic incorporation of Sr²⁺ linearly increases the surface-normalized capacity with a slope controlled by the B-site element. Moreover, La_0.2Sr_0.8MnO_2.7 exhibited the best specific capacitance (492 A g⁻¹) relative to the Fe and Co oxides. The formation energy of oxygen vacancies due to aliovalent substitution depends on the alkaline-earth metals.⁹⁷ The influence of Ba and Ca doping on the SC of SrMnO₃ nanofibers was studied by Luo et al.⁶⁸ Doping 20 mol% Ba to the SrMnO₃ matrix significantly enhanced the specific capacitance from 321.7 F g⁻¹ to up to 446.8 F g⁻¹. Moreover, the ASC fabricated using Sr_0.8Ba_0.2MnO₃ showed an energy density of 37.3 W h kg⁻¹ at a power density of 400 W kg⁻¹ and a superior capacitance retention of 87% after 5000 cycles.

3.2.1.3 B-site doping. The charge storage mechanism embraces electrochemical reactions. Incorporating oxygen ions into the vacancies requires a simultaneous change in the B cation's oxidation state. B-site doping is another effective measure to enhance electrical performance because it can produce oxygen vacancies and improve the crystal structure's stability.⁹⁸

Cobalt-based perovskite oxides are superior to Mn-based ones due to their larger oxygen vacancy concentration and higher oxygen-ion mobility.⁹⁹ However, some cobalt-based perovskite oxides (such as cubic phase SrCoO_3−δ) are not stable at room temperature. To widely implement cobalt-based perovskite oxides application in SCs, Sharma and coworkers doped Mo to substitute Co in SrCoO_3−δ partially (Fig. 7a).¹⁰⁰ It was found that the oxygen vacancies of SrCo_0.9Mo_0.1O₃ (SCM) were 2.1 times greater than that of SrCoO_3−δ. Additionally, SCM's specific capacitance (1223.34 F g⁻¹ at 1 A g⁻¹) was increased by 1.36 times that of SrCoO_3−δ (Fig. 7b). Notably, the ASC constructed by using lacey reduced graphene oxide nanoribbon (LRGONR) as a negative electrode showed long-term stability (165.1% activated specific capacitance retention after 10 000 cycles) (Fig. 7c). Most importantly, it also achieved a specific energy density of 74.8 W h kg⁻¹ at a power density of 734.5 W kg⁻¹ (Fig. 7d). Similarly, Nb-doped SrCoO_3−δ with a gravimetric capacitance of ca. 773.6 F g⁻¹ (2034.6 F cm⁻³) and great cycling stability (95.7% capacitance retention after 3000 cycles) was reported by Shao et al.¹⁰¹ Furthermore, an ASC was assembled using SrCo_0.9Nb_0.1O_3−δ (SCN) and AC as the cathode and anode. The device possessed an energy density of 37.6 W h kg⁻¹ at a power density of 433.9 W kg⁻¹, and it still maintained an energy density of 32.9 W h kg⁻¹ at a higher power density of 9864.2 W kg⁻¹. Also, it displayed long-duration cycling performance (about 1.7% loss after 5000 cycles). The B-site doping can also affect the potential window of perovskite oxides. Garche's team studied the influence of B site element doping on the stability window of SrRuO₃. Interestingly, 20 mol% Mg doping was found to increase the specific capacitance of SrRuO₃ without changing its stability window.⁶⁶ In contrast, Fe or Co substitution could lead to a decreased stability window.

Fig. 7 (a) Schematic illustration of the synthesis of SCM. (b) Electrochemical performance of the SCM electrode evaluated through GCD at different current densities. (c) Cycling stability of the ASC at 10 A g⁻¹. The inset shows the last five GCD cycles. (d) Ragone plot of a hybrid SCM cell compared to literature reports. Reproduced from ref. 100 with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, copyright 2018.

3.2.2 Perovskite oxide composites. Based on previous work, the defective structure of perovskite oxides and excellent oxygen ion mobility can boost the conversion of B^x+ to a high valence state, thus obtaining superior electrochemical properties. Despite this, the drawbacks of perovskite oxides, including the low surface area and high transport resistance of aggregated nanoparticles, hinder the further improvement of electrochemical performance. An effective strategy to address these problems is to combine ABO₃ with other materials (such as noble metals, metal oxides, and carbon materials), forming composites with enriched chemistry.
3.2.2.1 Perovskite oxides/noble metals. Noble metals, including gold (Au) and platinum (Pt) group metals with good electric conductivity, can facilitate the transport of electrons that arise from the oxidation/reduction of PCs to the current collectors.¹⁰² Nevertheless, owing to the scarcity and high cost of noble metals, their integration with other sustainable and cheap materials is considered one of the most attractive strategies to minimize their consumption. Perovskite oxides possess characteristics of natural abundance and low cost. Unfortunately, the electrical conductivity of perovskite oxides is still not satisfactory. Therefore, combining perovskite oxides with noble metals is expected to improve their SC performance by creating synergistic effects.

Ag has the highest conductivity among the noble metals. Besides, it also has the advantage of reasonable cost and has an acceptable activity.¹⁰³ Lang et al.¹⁰⁴ reported that Ag nanoparticle decorated La_0.85Sr_0.15MnO₃ was designed and used as an electrode for SCs. Since Ag has much higher electrical conductivity than that of carbon materials, it could build electron transfer channels. Additionally, the redox reaction between Ag and Ag₂O in an alkaline electrolyte solution could contribute a small amount of pseudocapacitance. As a result, the Ag@LSM15 composite delivered a high specific capacitance (186 F g⁻¹ at 1 A g⁻¹) and a long cycle life (100% capacitance retention after 1000 cycles). Moreover, based on this material, the ASC was constructed, showing a maximum energy density of 20.6 W h kg⁻¹ at a power density of 1700 W kg⁻¹. In another study, Ag nanoparticles were grown directly on a porous perovskite La_0.7Sr_0.3CoO_3−δ (LSC) substrate (Ag/LSC) (Fig. 8a).¹⁰⁵ The effect of different mass loading of Ag (10.61, 30.60, and 51.31 mg) on performance was studied. The porous structure of LSC was preserved, and the surface became rough when the content of Ag was 30 mg (30 Ag/LSC) or less. This favorable structure can provide more active surface sites and facilitate rapid mass transport. Besides, the Ag/LSC electrode with 30 mg Ag loading also displayed lower R_s (1.28 Ω cm²) and R_ct (0.61 Ω cm²), giving rise to the best performance (14.8 F cm⁻²) (Fig. 8b). Besides, an ASC based on 30 Ag/LSC as the negative electrode and carbon cloth (CC) as the positive electrode was constructed, displaying an energy density of 21.9 mW h cm⁻³ and a power density of 90.1 mW cm⁻³ at 5 mA cm⁻² (Fig. 8c). Visibly, two ASCs in series were used to light an LED blue for 15 min after charged for 0.5 min at 50 mA cm⁻² (Fig. 8c).

Fig. 8 (a) Schematic illustration of the preparation of Ag nanoparticles grown on a LSC substrate. (b) The areal capacitance of 30 Ag/LSC. (c) Electrochemical measurements of the asymmetric supercapacitor: Ragone plot at different current densities (inset: an LED bulb lighted by two asymmetric cells in series). Reproduced from ref. 105 with permission from Elsevier B.V., copyright 2017.

3.2.2.2 Perovskite oxides/metal oxides. Metal oxides have received more and more attention because their multiple numbers of oxidation states allow for up to an order of magnitude more generous energy storage compared to carbon-based EDLCs.¹² However, most metal oxides suffer from low conductivity, poor rate capability, and poor durability during charge/discharge processes. In contrast, perovskite oxides possess a stable structure and can produce much higher oxygen ion/electron conductivity and better surface oxygen exchange kinetics.¹⁰⁶ Hence, the composites of perovskite oxides and metal oxides are attractive alternatives because of their low cost in addition to the possibility to achieve high specific capacitance/capacity values and superb stability.

Among the metal oxides, MnO₂ stands out due to its high theoretical specific capacitance (about 1370 F g⁻¹), low cost, and natural abundance.^107,108 Lv et al.¹⁰⁹ synthesized (La_0.75Sr_0.25)_0.95MnO_3−δ ((LSM)/MnO₂) composite as an electrode for SCs via a hydrothermal method. The resultant sample exhibited a larger specific capacitance (437.2 F g⁻¹ at 2 mV s⁻¹). Compared with MnO₂, CeO₂ has low theoretical capacitance, but it has unique chemical properties. That is, it can be easily oxidized and reduced during the oxidation–reduction process.^110,111 Nagamuthu and co-workers reported LaMnO₃ mixed CeO₂ (CeO₂/LaMnO₃) nanocomposites with a higher specific capacitance (262 F g⁻¹ at 1 A g⁻¹) in 1 M Na₂SO₄ solution.¹¹² Interestingly, they found that the CeO₂/LaMnO₃ nanocomposite was more suited for ASC negative electrodes. More specifically, an ASC device was assembled using the CeO₂/LaMnO₃ nanocomposites as the negative electrode and AC as the positive electrode, revealing an energy density of 17.2 W h kg⁻¹ at a power density of 1015 W kg⁻¹. It is well established that the energy density (E) is proportional to the potential window (V). Thus, another way to elevate the energy density is to enlarge the potential window. A La_0.8Sr_0.15MnO₃@NiCo₂O₄ (LSM15@NC) core–shell nanoflower structure directly grown on Ni foam (Fig. 9a) was constructed by Lang et al.¹¹³ LSM15@NC showed a wide window and the coexistence of PC as well as EDLC behavior (Fig. 9b). Benefitting from the unique structure acting as a buffer for the volume change during the long-term charge–discharge tests, the specific capacitance of LSM15@NC was twice as much as that of the first cycle after 10 000 cycles (Fig. 9c). Moreover, the ASC using AC as the negative electrode and the LSM15@NC composite as the positive electrode delivered an energy density of 63.5 W h kg⁻¹ at a power density of 900 W kg⁻¹ (Fig. 9d).

Fig. 9 (a) TEM images of LSM15@NC scratched down from the Ni foam. (b) Comparative CV curves at a scan rate of 25 mV s⁻¹ within different potential windows of −0.8 to 0.5 V and −0.2 to 0.5 V. (c) Cycling performance (the insets show the morphology of LSM15@NC after 10 000 cycles). (d) Ragone plot. Reproduced from ref. 113 with permission from Elsevier B.V., copyright 2018.

3.2.2.3 Perovskite oxides/carbon materials. It is well known that carbonaceous materials (such as AC, graphene, graphene oxide, and reduced graphene oxide) with large specific surface area, good electronic conductivity, and high chemical stability have been widely applied in SCs.¹¹⁴ The low intrinsic conductivity of perovskite oxides is the main drawback that limits their further application in SCs. The addition of carbonaceous materials to form perovskite oxides/carbon composites is an effective strategy to remedy this deficiency.

Due to the extra-large theoretical specific surface area (2630 m² g⁻¹) and superior electrical conductivity (6000 S cm⁻¹), graphene is usually added into perovskite oxides to enhance SC performance.^114,115 Graphene can effectively restrain the agglomeration of perovskite oxides. Meanwhile, it can provide a high-speed electron transport channel.¹¹⁶ In return, the loading perovskite oxide nanoparticles are also available to maintain the structural nature of monolayer graphene for their function of increasing the distance between the graphene sheets.¹¹⁶Fig. 10a shows BiFeO₃ (BFO) nanowires immobilized on nanometer-thin RGO.¹¹⁷ Based on this material, the specific capacitance (368.28 F g⁻¹) and charge transfer resistance are found to be superior to those of BFO and RGO (Fig. 10b and c). Notably, the electrolyte also has a more significant impact on the performance of BFO–RGO (Fig. 10d). To investigate the capacitive behavior of graphene–perovskite oxide compound materials in aqueous electrolytes of various acidity or basicity, Ataa et al.¹¹⁸ tested reduced graphene performance sheets decorated SrRuO₃ (SRGO) in different electrolytes (1.0 M NaNO₃, 1.0 M H₃PO₄, and 1.0 M KOH). The SRGO exhibited the largest capacitance (160 F g⁻¹) in 1.0 M KOH.

Fig. 10 FESEM micrographs of (a) BFO. (b) Cyclic voltammetry curves of BFO, RGO, and BFO–RGO electrodes at a scan rate of 10 mV s⁻¹ in a 3 M KOH electrolyte. (c) Electrochemical impedance spectra of RGO, BFO, and BFO–RGO electrodes in a 3 M KOH electrolyte. (d) Change of the specific capacitance of BFO and BFO–RGO electrodes with changing current density from 5 to 10 A g⁻¹. Reproduced from ref. 117 with permission from American Chemical Society, copyright 2018.

As stated above, the electrochemical performance is enhanced compared with pure perovskite oxides after carbon introduction. The redox reaction of oxygenated groups on carbon nanostructures' surface contribute to pseudocapacitance.¹¹⁹ In this regard, introducing surface functional groups or heteroatoms on the surface of carbon materials appears to be an effective way of improving the composite electrode's capacitance.^120,121 Elsiddig et al.¹²² introduced a heteroatom to reduced graphene oxide (rGO) by substituting the hydroxyl groups with the nitrogen atoms. Then, the as-prepared nitrogen-doped graphene (N-rGO) sheets can be directly integrated with LaMnO₃ (LMO) through electrostatic interactions to form a three-dimensional network (LMO/N-rGO). Compared with pristine graphene and LMO, the obtained nanocomposites displayed the best specific capacitance (687 F g⁻¹ at 5 mV s⁻¹) and stability (79% capacitance retention after 2000 cycles at 10 A g⁻¹). In another study, Shafi et al.¹²³ synthesized a composite material containing LaMnO₃, RGO, and polyaniline (PANI) by in situ chemical polymerization. Since the RGO support and PANI coating again provided good structural stability and good electrical conductivity, the synthesized ternary composite exhibited a specific capacitance of 802 F g⁻¹ at 1 A g⁻¹. More importantly, the ASC with RGO as the negative electrode and ternary nanocomposite as the positive electrode exhibited a maximum energy density of 50 W h kg⁻¹ at a power density of 2.25 kW kg⁻¹, with admirable long-term stability (117% capacitance retention after 100 000 cycles).

4. Derivative perovskite oxides

Some derivative perovskite oxides were reported to possess fast oxygen ion diffusion channels, in which the oxygen diffusion exhibited shallow activation energy.^124,125 Additionally, the oxygen-ion conductivity of some derivative perovskite oxides is even higher than that of many benchmark ABO₃.^45,126 Moreover, many derivative perovskite oxides also can present high concentrations of variable oxygen vacancies. In this context, derivative perovskite oxides have some potential for application in SCs, as shown in Table 1.

Table 1 Applications and performances of different perovskite oxides in supercapacitors

Perovskite oxides	Methods	Structure	Electrolyte	Potential window	Capacitance	Ref.
LaMnO2.97	Reverse-phase hydrolysis approach	Nanoparticles	1 M KOH	−1.2 to 0.0 V	609.8 F g^−1 at 2 mV s^−1	31
LaMn1.1O3	Sol–gel method	Mesoporous	1 M KOH	−1.0 to 0.56 V	727.6 C g^−1 at 1 A g^−1	90
LaNiO3	Template-free solvothermal method	Hollow spheres	6 M KOH	0.0–0.45 V	422 F g^−1 at 1 A g^−1	59
LaNiO2.63	Sol–gel method	Nanoparticles	1 M KOH	0.2–0.6 V	478.7 F g^−1 at 0.1 mV s^−1	89
LaCoO3	Co-precipitation method	Nanoparticles	3 M KOH	0.0–0.5 V	299.64 F g^−1 at 10 A g^−1	62
LaCrO3	Sol–gel method	Hierarchical mesoporous	1 M LiCl	0.0–1.0 V	1268 F g^−1 at 2 A g^−1	63
BiFeO3	Hydrothermal method	Nanoplates	1 M NaOH	0.0–0.5 V	254.6 F g^−1 at 1 mV s^−1	77
LaFeO3	The MOF gel method	Mesoporous	1 M Na2SO4	−1.0 to 0.0 V	241.3 F g^−1 at 1 A g^−1	61
SrRuO3	Pyrolysis route	Porous	6 M KOH	−0.9 to 0.3 V	270 F g^−1 at 20 mV s^−1	66
SrTiO3	Sol–gel method	Mesoporous	3 M KOH	−0.2 to 0.6 V	592 F g^−1 at 5 mV s^−1	70
CeMnO3	Electrospinning	Nanofibers	6 M KOH	−0.1 to 0.6 V	155 F g^−1 at 1 A g^−1	74
CeCoO3	Electrospinning	Nanofibers	6 M KOH	−0.1 to 0.6 V	128 F g^−1 at 0.5 A g^−1	75
CeNiO3	Electrospinning	Nanofibers	6 M KOH	−0.1 to 0.65 V	189 F g^−1 at 0.5 A g^−1	75
CeCuO3	Electrospinning	Nanofibers	6 M KOH	−0.1 to 0.7 V	117 F g^−1 at 0.5 A g^−1	75
NiTiO3	Sol–gel method	Rod-like	2 M KOH	0.0–0.6 V	542.26 F g^−1 at 1 A g^−1	80
NiMnO3	Hydrothermal method	Three-dimensional hierarchical	6 M KOH	0.0–0.9 V	99.03 F g^−1 at 100 mV s^−1	79
CoTiO3	Sol–gel method	Mesoporous microrods	2 M KOH	0.0–0.6 V	608.4 F g^−1 at 5 mV s^−1	81
La0.85Sr0.15MnO3	Sol–gel method	Sphere	1 M KOH	−0.96 to 0.65 V	198 F g^−1 at 0.5 A g^−1	94
La0.5Ca0.5MnO3	Sol–gel method	Nanoparticles	1 M KOH	−1.0 to 0.6 V	170 F g^−1 at 1 A g^−1	93
Sr0.8Ba0.2MnO3	Sol–gel and electrospinning	Nanofibers	1 M Na2SO4	0.0–0.8 V	446.8 F g^−1 at 0.5 A g^−1	68
SrCo0.9Mo0.1O3	Sol–gel method	Nanoparticles	6 M KOH	−0.1 to 0.5 V	1223.34 F g^−1 at 1 A g^−1	100
SrCo0.9Nb0.1O3	Solid-state	Nanoparticles	1 M KOH	0.0–0.5 V	773.6 F g^−1 at 0.5 A g^−1	101
La2ZnMnO6	Hydrothermal route	Nanoflakes	2 M KOH	0.0–0.6 V	718.6 F g^−1 at 1 mV s^−1	128
La2CuMnO6	Hydrothermal route	Nanoparticles	2 M KOH	0.0–0.45 V	205.5 F g^−1 at 0.5 A g^−1	129
Gd2NiMnO6	Wet chemical route	Nanoparticles	4 M KOH	0.2–0.6 V	400.46 F g^−1 at 1 A g^−1	130
La2CoMnO6	Template impregnation method	Hollow spheres	1 M Na2SO4	−0.1 to 1.0 V	376 F g^−1 at 1 A g^−1	131
Y2NiMnO6	Hydrothermal route	Nanowires	0.5 M KOH	0.2–0.5 V	77.76 F g^−1 at 30 mA g^−1	132
PrBaMn2O6−δ	Sol–gel method	Nanoparticles	6 M KOH	0.0–0.5 V	1034.8 F g^−1 at 1 A g^−1	133
PrBaCo2O5+δ	Sol–gel method	Nanoparticles	1 M KOH	−0.1 to 0.55 V	428.2 C g^−1 at 1 mV s^−1	136
Ba2Bi0.1Sc0.2Co1.7O6−δ	Sol–gel method	Nanoparticles	6 M KOH	0.0–0.6 V	796.7 F g^−1 at 1 A g^−1	137
Sr2CoMoO6−δ	Sol–gel method	Spherical particles	6 M KOH	−0.1 to 0.4 V	747 F g^−1 at 1 A g^−1	138
La2NiO4+δ	Citrate method	Porous	3 M KOH	0.0–0.6 V	657.4 F g^−1 at 2 mV s^−1	141

---

4.1 Double perovskite oxides

In 1951, double perovskite oxides were first discovered by Steward and Rooksby, expressed by the general formula of AA'BB'O₆, a derivative of ABO₃, extending the pristine framework to a super-lattice.¹²⁷ To date, double perovskite oxides have been widely investigated, because they have high oxygen surface exchange kinetics and fast oxygen ion diffusion rates, and good mixed ionic and electronic conductivity at elevated temperature. Compared with element-doped perovskite oxides, AA′BB′O₆ holds a more orderly structure arrangement, thereby preventing lattice distortion and improving cycling stability.

In general, cations are disordered and distributed homogeneously inside the AA′BB′O₆ lattice. Nevertheless, in some situations, cation ordering could occur (such as AA′B₂O₆ and A₂BB′O₆). It is worth mentioning that cation ordering could display better physical and chemical properties. Kumar's team reported the electrochemical performance of R₂MMnO₆ (R = La, Gd; M = Zn, Cu, Ni) perovskite oxides including La₂ZnMnO₆, La₂CuMnO₆, and Gd₂NiMnO₆, which showed a specific capacitance of 718.6 F g⁻¹, 205.5 F g⁻¹, 400.46 F g⁻¹, respectively, at different current densities.^128–130 To improve the SC properties, increasing the specific surface area and improving the nanoscale charge transport are useful measures. Meng and coworkers fabricated a hollow spherical porous structure of La₂CoMnO₆ by template impregnation (HS-LCMO).¹³¹ For comparison, La₂CoMnO₆ was also prepared by the sol–gel method (SG-LCMO). The specific surface area of HS-LCMO and SG-LCMO was estimated to be 22.14 and 10.36 m² g⁻¹, respectively. Owing to its large specific surface area, HS-LCMO displayed a specific capacitance (376 F g⁻¹) which was 4 times that of SG-LCMO (94 F g⁻¹). Similarly, Y₂NiMnO₆ nanowires (NWs) were prepared by a hydrothermal route. The resultant Y₂NiMnO₆ NWs exhibited a specific capacitance (77.76 F g⁻¹) which was higher than that of the counterparts prepared by the sol–gel method.¹³² The crystal structure of perovskite oxides is closely related to the calcination temperature. Shao's team demonstrated that the reduction of PrBaMn₂O_6−δ (r-PBM) in a hydrogen atmosphere at 850 °C led to a phase transition from a mixed hexagonal and cubic phase to a cubic phase PBM (Fig. 11a and b).¹³³ Based on density functional theory (DFT) calculations, it is somewhat easier to form an oxygen vacancy in cubic PBM than in the hexagonal phase. Correspondingly, the formation of a layered double perovskite oxide structure after hydrogen reduction (800 °C for 45 min) significantly increased the oxygen vacancy concentration and oxygen anion diffusion rate, thus contributing to the capacitance (Fig. 11c). Notably, the r-PBM with a low specific surface area (7.83 m² g⁻¹) exhibited a gravimetric capacitance of 1034.8 F g⁻¹ (Fig. 11d).

Fig. 11 DFT+U calculated density of states (DOS) for (a) cubic and (b) hexagonal PBM. (c) A schematic diagram of oxygen intercalation into r-PBM during the energy storage process. (d) Galvanostatic charge–discharge profiles of r-PBM. Reproduced from ref. 133 with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, copyright 2018.

To further explore the effect of cation ordering on the performance of SCs, more effort has been made.^134,135 Wang et al.¹³⁶ synthesized A-site cation-ordered double perovskite oxide PrBaCo₂O_5+δ (PBCO) by the sol–gel method. They found that oxygen atoms located in the Pr³⁺ planes can be partially or even entirely removed, producing numerous oxygen vacancies in the perovskite oxide structure. Accordingly, the synthesized PBCO showed a good specific capacity (428.2 C g⁻¹). Notably, PBCO displayed cycling stability (93% capacitance retention after 2000 cycles at 10 A g⁻¹) superior to that of some single perovskite oxides (such as BiFeO₃ and LaMnO₃).^77,91 Beyond that, Xu et al.¹³⁷ reported that B-site cation-ordered double perovskite oxide Ba₂Bi_0.1Sc_0.2Co_1.7O_6−δ (BBSC) was a good anion-intercalation-type electrode for SCs. For comparison, the oxygen vacancy-disordered Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) electrode was also studied. It was found that the oxygen vacancy density of BBSC is larger than that of BSCF, and the simultaneous leaching of Ba from the A site and Bi from the B site of BBSC could minimize the cation deficiency in BBSC. As a result, BBSC delivered a specific capacitance (1050 F g⁻¹) higher than that of BSCF. Hence, the ASC fabricated using BBSC displayed an energy density of 70 W h kg⁻¹ at a power density of 787 W h kg⁻¹. Sr₂CoMoO_6−δ (DP-SCM) as an intercalation PC was reported by Tomar et al.¹³⁸ In their study, B site-ordering was based on the difference between oxidation states of Co and Mo (greater than 2) with a large ionic radii difference. It was found that the synergy of Co/Mo has good redox ability, further facilitating high oxygen mobility, resulting in DP-SCM achieving a specific capacitance of 747 F g⁻¹ at 1 A g⁻¹ with a rate capability of 56% up to 10 A g⁻¹.

4.2 Ruddlesden–Popper oxides

RP perovskite oxides were first studied in 1958 by Ruddlesden and Popper.¹³⁹ Alternating perovskite (ABO₃)⁻ and rock-salt (AO)⁺ layers in the homologous series of titanates (Sr_n+1TinO_3n+1) were discovered.¹⁴⁰ The unique layered structure allows two sorts of oxygen species (oxygen vacancy, interstitial oxygen). The oxygen vacancy exists in the perovskite layer, while the interstitial oxygen is found in the rock-salt layer. Compared with ABO₃-type, RP perovskite oxides show superior mixed ionic and electronic conductivity. Sang et al.¹⁴¹ prepared RP-type perovskite oxide La₂NiO_4+δvia a citrate method and used it as the active material for SCs. It was found that there are two ways of oxygen extrusion and only one way of oxygen insertion in La₂NiO_4+δ. Interestingly, La₂NiO_4+δ showed a high specific capacitance of 657.4 F g⁻¹ at a scan rate of 2 mV s⁻¹, which was better than the capacitance of LaMnO₃ (609.8 F g⁻¹).³¹ Recently, to optimize the surface properties of RP particles, Wei and coworkers synthesized La₂NiO_4+δ coated with Ag (La₂NiO_4+δ@Ag).¹⁴² Because of the unique layered structure of La₂NiO_4+δ and the quick reversible redox process of Ag/AgO₂, the La₂NiO_4+δ@Ag electrode showed better anion intercalation performance compared to La₂NiO_4+δ, such as large capacity (466.4 C g⁻¹ at 1 mV s⁻¹) and good cycling stability (average efficiency approximated 93% in 10 000 cycles). Furthermore, a hybrid SC device assembled using La₂NiO_4+δ@Ag and AC as the cathode and the anode showed a high energy density of 44.7 W h kg⁻¹ at a power density of 800 W h kg⁻¹.

5. Conclusions and outlooks

With the unique structure, compositional flexibility, and inherent oxygen vacancy, perovskite oxides have recently been widely employed as electrode materials for SCs. Notably, as an active material of intercalation-type capacitors, perovskite oxides are characterized by high oxygen vacancy concentrations and do not require high surface area to achieve a high energy storage capacity. This article mainly summarizes the recent advances in perovskite oxides (i.e., single, double, and RP perovskite oxides) for SCs. Further improvement of perovskite oxides' electrochemical properties by increasing oxygen vacancy concentration and constructing composites is discussed. Despite many efforts in this field, several aspects still need to be considered in the design of perovskite oxide electrode materials in the future.

(1) During the charge storage process, the valence state change of the B-site cation with a low energy barrier in perovskite oxides is preferred.¹³⁷ To date, Mn, Ni, Fe, and Co have been studied as B site elements. Some early transition metals, such as Ti, Zr, V, and Nb, share similar ionization energies and ionization energy differences with Mn,¹⁴³ which is expected to be the B site element of perovskite oxide electrodes.

(2) B-site substitution by a metal element has been proven as a feasible approach to create oxygen vacancies in perovskite oxides, thereby boosting their performance as SC electrode materials.^100,101 In contrast, non-metal doping at the B-site was shown to improve the phase stability, electrical conductivity, and oxygen vacancies of perovskite oxides.^144–146 Therefore, more research work on non-metal doping at the B-site is expected in this field.

(3) The electronic conductivity of RP perovskite oxides is anticipated to increase as their n value increases.¹⁴⁷ However, most of the reports are on RP perovskite oxides with n = 1. RP perovskite oxides with higher n values as advanced SC electrode materials are therefore expected.

(4) The morphology of perovskite oxides plays a crucial role in boosting electrochemical performance. However, perovskite oxides generally are synthesized at high temperatures above 600 °C, leading to limited low surface areas (<10 m² g⁻¹) and less exposure to active sites.¹⁴⁸ Therefore, controllable synthesis of nanostructures (such as hollow, core–shell, and nanoarray) with large specific surface area and high redox activity is vital for achieving high energy/power densities of perovskite oxide SC electrodes.

(5) Apart from the architecture and electronic configurations, electrolytes also optimize perovskite oxides' energy-storage performance. Aqueous electrolytes like KOH with a narrow working voltage range of ∼1.23 V and Na₂SO₄ (∼2 V) are by far the most investigated media. The utilization of organic (2.5–3.5 V) or ionic liquid electrolytes (2–6 V) offers a wide voltage window, further improving the energy density. Notably, the introduction of additional redox-active compounds such as CuCl₂, KMnO₄, KI, KBr, Na₂MoO₄, and K₃Fe(CN)₆, i.e., the formation of a redox electrolyte, can further improve the electrode materials' electrochemical performance. For instance, an additional 0.1 M K₄[Fe(CN)₆] in 3 M KOH causes a nearly two-fold increase in the BFO–RGO electrode's specific capacitance.¹¹⁷ Therefore, the use of a redox electrolyte may be promising to boost the electrochemical performance of SCs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 22072015).

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Footnote

† Both authors contributed equally to this work.

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