**1. Introduction**

A supercapacitor is a promising energy storage device that has been an essential component in most fields, ranging from portable electronics to hybrid electric vehicles and large industrial equipment [1]. A supercapacitor can also be used in various applications such as power electronics, energy storage at intermittent generators including windmills, and smart grid applications because it has a high power density, high rate of charge/discharge, long life, etc. [2].

The energy storage method of the supercapacitor can be divided into two types: electrical double-layer capacitors (EDLC) and pseudocapacitors [3]. EDLC, made of materials with a high surface area, has a high power density but a lower volumetric energy density than traditional batteries. In contrast, the pseudocapacitor has an energy density higher than EDLC because it stores electrons through a redox reaction on the surface of the electrode. In this regard, metal-based pseudocapacitors can take the intermediate role of conventional electrostatic capacitors and batteries. Electric double-layer capacitors (EDLC) and commercial devices can store between 3 and 6 Wh [3].

The pseudocapacitor has a very fast charge/discharge rate compared to lithium-ion batteries but has the disadvantage of a somewhat low energy density. In this regard, there have been a number of efforts to increase energy density through the selection of new materials or the improvement of the electrode structure, current-collecting method, etc. For example, metal oxides have the potential to improve energy density significantly due to reversible faradaic surface reactions [4].

There are three methods of charge storage for pseudocapacitor electrodes [3]: The first is the adsorption of electrolyte ions on the metal surface with monolayers. This adsorption can be revealed through partial charge transfer between the metal center of the electrode and the electrolyte anion, and underpotential deposition of the so-called electrosorption valence [5,6]. The second method, pseudocapacitance, includes redox reactions on the surface of the electrode. These redox reactions are chemical reactions on the surface that result in a charge transfer with strong bonds [7]. The third is the reversibly rapid insertion of ions into the bulk of the material. As the ions are inserted into the crystalline material, the intercalation pseudocapacitance is observed and eventually behaves like a capacitor [8,9]. All of these can be described as pseudocapacitive because of the change of the oxidation state of the transition metal.

Established electrode materials include activated carbon (AC), carbon nanotube (CNT), polymers, graphene, etc. [10–12]. Among them, graphene is widely studied because of its high specific surface area, aspect ratio, and conductivity [13]. Additionally, graphene has high charge mobility (>200,000 cm2/V·S), zero effective mass, and ballistic transport even at room temperature [14]. Graphene nanoplatelets are composed of several graphene layers and have similar properties to single graphene and are much easier to produce and handle [15,16]. When AC was used as an electrode at a current density of 0.5 A/g, specific capacitance values at 1 V (voltage window) were maintained at 242 F/g initially to 204 F/g after 7000 cycles, which was 84% of the initial value [17]. Yan et al. reported that the capacitance of PANI (polyaniline)-based graphene electrodes was 1046 F/g at a scan rate of 1 mV/s among the graphene electrodes. However, the graphene and PANI composite electrodes showed a linear decrease after the 1000th cycle, with the capacitance dropping to 50% of the initial value [18].

In this respect, perovskite has got attention to improve the cyclic stability of the graphene-based electrode. La0.8Sr0.2Mn0.5Co0.5O3-<sup>δ</sup> perovskites (LSMCO) are known to be good catalysts for total oxidation due to a large amount of oxygen in the perovskite structure and the redox behavior of the Mn- and Co-ions. Their catalytic activity is comparable with some noble metal catalysts. ABO3-type perovskite structure has been studied as an electrode for solid oxide fuel cells and an oxide ion-conducting electrolyte with stable structural stability at high temperatures [19–21]. A is the lanthanum group or the alkali earth element, and B is the transition metal [22]. The adjustment of the site of A and B can control the electrical properties of the material, which induces the role of anion vacancy as a charge storage site for pseudocapacitance [23]. For example, SrRuO3 exhibits metallic conductivity and is stable in alkaline electrolytes [24]. In addition, the microemulsion technique can enhance electrochemical performance by generating perovskite nanostructure [25].

Mefford et al. presented an anion-based mechanism through a supercapacitor using LaMnO3 perovskite, which is the first example of anion-based intercalation pseudocapacitance and the first oxygen insertion method used for fast energy storage [23]. The composite electrode was fabricated by growing Ag nanoparticles on a La0.7Sr0.3CoO3 (LSCO) substrate by Liu et al. The Ag/LSCO electrode maintained 81% capacitance after 3000 cycles at the current density of 50 mA/cm2. Liu and coworkers have released anion-intercalation type electrodes. When the cycling performance of SrCoO3 (SCO) was measured with an activated carbon electrode at 1 A/g current density, the specific capacitance of approximately 80% was maintained after the 2000th cycle [26]. Previous research using perovskite LaSrMnO3 shows the capacitive behavior (102 F/g) and charge storage performance for supercapacitors through the anion-intercalation mechanism [27–32].

Structurally stable perovskite is expected to help maintain the stability of the supercapacitor electrode. Our goal is to maintain the initial capacitance of the pseudo-supercapacitor and to improve the capacitance stability after 4990 cycles using a perovskite-mixed electrode. In our experiment, we compared the electrochemical characteristics, including cyclic voltammetry, galvanostatic

charge-discharge, etc. with an electrode G95 (graphene nanoplatelets 95 wt%) and electrode G70L25 (graphene nanoplatelets 70 wt%, LSMCO 25 wt%).

## **2. Materials and Methods**
