Study on Capacitance of Zn-Based Electrode in Redox Electrolyte System

Electrode material is one of the most important factors affecting the performance of supercapacitors, and electrolyte solution is another. In this work, electrochemical properties of hydroxide zinc carbonate composite electrode (HZC) in KOH + K 3 [Fe(CN) 6 ] electrolyte were studied. It was proved that [Fe(CN) 6 ] 3− in electrolyte participated in electrochemical reactions and pro-moted electron transfer. The specific capacitance of HZC electrode was as high as 920.5 F∙g −1 at 1.0 A∙g −1 in 1 mol∙L −1 KOH and 0.04 mol∙L −1 K 3 [Fe(CN) 6 ] electrolyte, which is 172.9% higher than that in KOH. The combination of HZC electrode and low alkalinity aqueous electrolyte provided the supercapacitor system with good capacitance performance, safety, and environmen-tally friendly.


Introduction
Nowadays, the world is facing a series of challenges related to energy, including energy depletion, environmental pollution, and so on. Researchers are committed to studying green and high-performance energy storage devices to meet the needs of sustainable innovative development [1] [2]. Supercapacitor (SC) is one of the most promising energy storage devices. The electrolytes used in the supercapacitor system are mainly divided into three categories: aqueous phase, organic phase and ionic liquid [3]. Water phase system is the most widely used electrolyte in supercapacitors. However, limited by water decomposition potential, the operating voltage window of capacitors in aqueous solution is generally lower than 1 V [4]. Organic phase and ionic liquid can achieve higher operating voltage window (2.2 -2.9 V and 2.6 -4 V), which can effectively improve the energy density of capacitors, but there are still certain safety and environmental risks [5].
In recent years, redox active additives, or electrically active substances as supplement are introduced in traditional electrolyte systems to improve the performance of SC [6] [7]. In this way, the capacitance can be effectively improved to achieve the energy density close to that of battery [6]. Compared with electrode material synthesis and selection, this method is simpler, safer, low-cost, and non-toxic, which is easy to mass production [7]. Senthilkumar et al. [8] achieved about 2 times higher specific capacitance of carbon electrodes in sol-electrolyte systems containing hydroquinone and potassium iodide. They believed that the increased specific capacitance was due to the rapid Faraday reaction of redox components in the electrolyte solution at the interface between the electrode and electrolyte.
Hydroxide zinc carbonate composite electrode (HZC) has been synthesized in our previous studies, which showed novel electrochemical activity in high concentration KOH electrolyte [9]. However, higher concentration of KOH will undoubtedly increase the corrosion of electrodes and equipment, which is unfavorable to the long-term operation and performance of supercapacitors. In this work, K 3 [Fe(CN) 6 ] redox active components were introduced into low concentration KOH electrolyte to optimize the system and maintain capacitive performance. The influence of redox component on electrochemical performance of electrode and supercapaictor, and the interaction between active substance and electrolyte were analyzed.

Preparation of Working Electrode
Zn(NO 3 ) 2 ·6H 2 O 0.11 g, Al(NO 3 ) 2 ·6H 2 O 0.03 g, and CO(NH 2 ) 2 0.14 g were dissolved in 36 ml deionized water and stirred for 15 min. Then the solution was transferred to 50 ml hydrothermal reactor. After pretreatment, nickel foam was added in above solution and the temperature was maintained at 120˚C for 4 h.
Nickel foam was washed to neutral and dried overnight after the reaction.
Active carbon (AC, BP2000), acetylene black and PTFE mixed together with mass proportion of 8:1:1, using ethanol as dispersant. Paste was stirred and smeared evenly on 1 × 1 cm 2 nickel foam, then dried 6 hours at 80˚C.

Characterization
The morphological studies of HZC were performed using field emission scan- Electrochemical performance of supercapacitor assembled with HZC electrode and activated carbon electrode was tested on CHI660E electrochemical workstation (China). Cycle stability of SC was tested by LANHE (China). The voltage window is 0 -1.6 V. Figure 1 shows SEM analysis of HZC material on foam Ni. The morphology of HZC shows regular hexagon nanosheet with about 1 μm in diameter and 100 -200 nm for thickness (measured in FESEM image, 20,000×). All partials constitute a three-dimensional network electrode layer. This special structure provides three-dimensional nucleation site for in-situ growth of electrode active substances.

Cyclic Voltammetry Analysis
In three-electrode system, the electrochemical performance of the electrode in the K 3 [Fe(CN) 6 ] + KOH electrolyte was tested firstly. The results were analyzed and compared with the cyclic voltammetry (CV) curve of the electrode in 1 mol•L −1 KOH electrolyte, as shown in Figure 2.
In KOH electrolyte, a pair of main redox peaks appeared in CV curve at about 0.34 V and 0.56 V (vs HgO/Hg) at scanning rate of 0.01 V•s −1 , which could be attributed to the transition between Zn(II) and Zn(III) of HZC electrode. The HZC electrode showed typical pseudocapacitance characteristics [10] [11]. In  , and the deviation of redox peak may be related to properties of substrate, surface roughness of substrate, or superpotential of electrode [12]. On the other hand, the peak current and peak area of CV curves increased significantly [13], and the peak potential deviated to higher and lower potential respectively in K 3 [Fe(CN) 6 ] + KOH electrolyte system. This result indicates that more species are involved in redox process on the electrode surface, which effectively improves the capacitance. Secondly, these changes of strength and position of redox peaks, indicating possible interaction between two electric pairs during charging and discharging [3] [7].

Galvanostatic Charge and Discharge Test
Galvanostatic charge and discharge curves at 1 A•g −1 are shown in Figure 3. In KOH electrolyte, there is a charging/discharging platform, while there are two obvious charging/discharging platforms at about 0.4 and 0.5 V in K 3 [Fe(CN) 6 ] + KOH electrolytes, which further confirms existence of multi-step redox process in K 3 [Fe(CN) 6 ] + KOH system [10].
On the other hand, in K 3 [Fe(CN) 6 ] + KOH electrolyte system, discharging time of electrode is significantly longer than that in KOH system, especially containing 0.04 mol•L −1 K 3 [Fe(CN) 6 ] in electrolyte.
The specific capacitance of HZC composite electrode can be estimated from the discharge current using following equation [6]: where, C (F•g −1 ) is the specific capacitance of the electrode. I (A) is charge and discharge current. t (s) is discharge time and ΔV (V) for charging and discharging voltage window, m (g) as electroactive material quality.
As can be seen in    [14]. Electrochemical performance of electrode can be improved through contribution of pseudocapacitance at electrode-electrolyte interface. Su et al. [14] proposed a mechanism of "Electron Shuttle" when studying electrochemical performance of Co-Al double layered hydroxide electrodes. Researchers believe that electron transmission of Co(II) is key to determine the reaction rate of whole electrode process. It is considered that addition of K 3 [Fe(CN) 6 ] or K 4 [Fe(CN) 6 ] in KOH promotes the oxidation and electron loss of Co(II) effectively. According to literature reports and our results in this experiment, we believe that the mechanism of "Electron Shuttle" is also suitable for electrochemical reactions on the surface of HZC electrode. Journal of Materials Science and Chemical Engineering As shown in Figure 4, Zn(II) loses an electron and is oxidized to Zn(III) during charging process, while [Fe(CN) 6 Discharge : Zn III e Zn II + → According to mechanism analysis of charge and discharge process on the

Asymmetric Supercapacitor Performance
Electrochemical performance of AC electrode in K 3 [Fe(CN) 6 ] + KOH electrolyte is studied, and the activity is compared with that in KOH, as shown in Figure 5.
It can be seen that CV curve of AC presents a typical rectangular shape, which is similar to that in KOH, indicating that AC electrode still exhibits relatively ideal Electrochemical performance of the assembled asymmetric supercapacitor HZC//AC was tested and compared ( Figure 6). It can be seen that current density  Compared with the result in KOH electrolyte, the response current in K 3 [Fe(CN) 6 ] + KOH electrolyte showed significant changes at range of 0 -0.2 V, and peak current increases with K 3 [Fe(CN) 6 ] content in electrolyte. Based on these results, it is speculated that the excellent performance of SC comes from the pseudo-capacitance behavior of redox electrolyte components and additional double electric layer effect of redox electrolyte components on AC electrode [15].

Conclusion
Electrochemical performance of HZC electrode was measured in redox electro-  6 ] increases to 920.5 F•g −1 at 1.0 A•g −1 , which is much higher than that in KOH. Combination of Zn-based electrode and low alkalinity electrolyte achieved SC equipment good performance and friendly environment. Next works will focus on electrolyte composition and stability optimization.