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  . 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 . 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 . 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 .
In recent years, redox active additives, or electrically active substances as supplement are introduced in traditional electrolyte systems to improve the performance of SC  . In this way, the capacitance can be effectively improved to achieve the energy density close to that of battery . Compared with electrode material synthesis and selection, this method is simpler, safer, low-cost, and non-toxic, which is easy to mass production . Senthilkumar et al.  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 . 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, K3[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.
2.1. Preparation of Working Electrode
Zn(NO3)2·6H2O 0.11 g, Al(NO3)2·6H2O 0.03 g, and CO(NH2)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 cm2 nickel foam, then dried 6 hours at 80˚C.
The morphological studies of HZC were performed using field emission scanning electronic microscope (FESEM, JEOLJEM-6360LV, Japan) with an accelerating voltage of 20 - 30 kV. The sample was processed with gold spraying before testing.
Classic three electrode system was used to electrochemical analysis with Pt electrode as counter electrode, HgO/Hg electrode as reference electrode, HZC electrode as working electrode, and 1 mol∙L−1 KOH containing a certain amount of K3[Fe(CN)6] as electrolyte.
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.
3. Results and Discussion
3.1. Cyclic Voltammetry Analysis
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.
In three-electrode system, the electrochemical performance of the electrode in the K3[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  . In K3[Fe(CN)6] + KOH electrolyte system, a new oxidation peak appeared in CV curves at about 0.42 V (vs HgO/Hg), and the strength of oxidation peaks increased with K3[Fe(CN)6] content in electrolyte. It can be speculated that the
Figure 1. SEM of HZC electrode.
Figure 2. CV curves comparison of electrodes at 0.01 V∙s−1 in KOH electrolyte with K3[Fe(CN)6].
new redox peak of CV curve comes from transformation between [Fe(CN)6]3− and [Fe(CN)6]4−  . Theoretical analysis shows that standard potential of [Fe(CN)6]3−/[Fe(CN)6]4− is 0.2577 V vs. HgO/Hg (0.3557 V vs. SHE), and the deviation of redox peak may be related to properties of substrate, surface roughness of substrate, or superpotential of electrode . On the other hand, the peak current and peak area of CV curves increased significantly , and the peak potential deviated to higher and lower potential respectively in K3[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  .
3.2. 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 K3[Fe(CN)6] + KOH electrolytes, which further confirms existence of multi-step redox process in K3[Fe(CN)6] + KOH system .
On the other hand, in K3[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 K3[Fe(CN)6] in electrolyte.
The specific capacitance of HZC composite electrode can be estimated from the discharge current using following equation  :
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 Table 1, the specific capacitance of electrodes in K3[Fe(CN)6] + KOH electrolyte system is generally higher than that in KOH. Under the
Figure 3. Galvanostatic charge and discharge curves of HZC electrode in KOH electrolytes with K3[Fe(CN)6] at 1 A∙g−1.
Table 1. Comparisons of specific capacitance (F∙g−1) of HZC electrode in redox electrolytes.
same current density, the electrode capacitance shows an overall increasing tendency with increase of K3[Fe(CN)6] concentration in electrolyte solution. For example, when the current density is 1 A∙g−1, the specific capacitance of HZC electrode is 337.3 F∙g−1 in the 1 mol∙L−1 KOH, while they are 672.8, 678.6, 691.2 and 920.5 F∙g−1, respectively when K3[Fe(CN)6] from 0.01 to 0.04 mol∙L−1 in electrolyte, which is 99.5%, 101.2%, 104.9% and 172.9% higher. It indicates that presence of [Fe(CN)6]3− ions significantly improves electrochemical performance of HZC electrode.
Lots of research results show that redox components or media in electrolyte solution can directly participate in electron transfer in redox reaction on electrode surface . Electrochemical performance of electrode can be improved through contribution of pseudocapacitance at electrode-electrolyte interface. Su et al.  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 K3[Fe(CN)6] or K4[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.
As shown in Figure 4, Zn(II) loses an electron and is oxidized to Zn(III) during charging process, while [Fe(CN)6]3− obtains electron and converts to [Fe(CN)6]4−. Similarly, during discharge, [Fe(CN)6]4− gives one electron to [Fe(CN)6]3−, and electron is transferred to Zn(III), which gets the electron and converts to Zn(II).
Electrode surface reaction equation can be expressed as follows:
According to mechanism analysis of charge and discharge process on the electrode surface, redox component in electrolyte can effectively promote electron transfer and reduce charge transfer resistance. Consumption of electrons in Equation (3) also promotes oxidation reaction of Zn(II) (Equation (2)), making more Zn(II) on electrode surface become active center and participate in electrode reaction, thereby improving the utilization rate of Zn(II) and increasing the capacitance performance of electrode. Redox components, on the other hand, store charge and energy synchronously through their conversion between multi-valent species, providing additional capacitance to the electrode.
3.3. Asymmetric Supercapacitor Performance
Electrochemical performance of AC electrode in K3[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 double layer capacitance characteristics in K3[Fe(CN)6] + KOH. No significant redox peak of [Fe(CN)6]3− is observed in CV curve. It is worth noting that closed area of CV curve for AC in K3[Fe(CN)6] + KOH is nearly 1 time larger than that in KOH, which indicates that AC electrode presents higher specific capacitance in mixed electrolyte solution. In addition, compared with CV curve in KOH, CV curve of AC electrode in K3[Fe(CN)6] + KOH is significantly shifted downward, showing decreased charge current and increased discharge current. It can be speculated that [Fe(CN)6]3− exerts strong gravitation on K+ in mixed electrolyte,
Figure 4. Schematic of charge and discharge process on HZC electrode in K3[Fe(CN)6] + KOH electrolyte.
causing some K+ ions form ion layer around [Fe(CN)6]3− ions, which reduces migration ability of K+ in solution. During charging process, K+ in solution migrates to the surface of AC electrode. Under external electric field and [Fe(CN)6]3− ions electric field competition, it is possible that not all K+ can escape from gravity of [Fe(CN)6]3− ions electric field. So, part of K+ ions migrate to AC electrode and participate in formation of double electric layer. At the same time, some K+ may exist in loose layers outside the double electric layer of AC electrode, so as to form small charging current. During discharge process, external electric field disappears. K+ ions in double electric layer and loose layer outside the double electric layer migrate under electrostatic gravity and [Fe(CN)6]3− ion electric field, which effectively improves discharge capacity.
Figure 5 shows CV curves of AC and HZC electrodes in three electrode system, where K3[Fe(CN)6] + KOH as electrolyte. According to the charge conservation principle, it can be obtained that the mass ratio of HZC and AC materials is about 1.5:1.
Electrochemical performance of the assembled asymmetric supercapacitor HZC//AC was tested and compared (Figure 6). It can be seen that current density
Figure 5. (a) CV curves of AC electrode in K3[Fe(CN)6] + KOH solution, (b) CV curves of AC and HZC electrodes performed in three-electrode system.
Figure 6. CV curves of HZC//AC asymmetric supercapacitors with different amount of K3[Fe(CN)6] in electrolyte.
in CV curves increases with K3[Fe(CN)6] content in electrolyte at 10 mV∙s−1. Compared with the result in KOH electrolyte, the response current in K3[Fe(CN)6] + KOH electrolyte showed significant changes at range of 0 - 0.2 V, and peak current increases with K3[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 . K3[Fe(CN)6] in electrolyte improves electrochemical performance of two electrodes at the same time, thereby improving the capacitance of asymmetric chemical capacitors.
Electrochemical performance of HZC electrode was measured in redox electrolyte system composed of KOH and K3[Fe(CN)6]. [Fe(CN)6]3− improved capacitance performance of electrodes through participation and synergistic action. Redox component K3[Fe(CN)6] in electrolyte can promote electron transfer effectively. The results show that specific capacitance of HZC electrode in KOH + K3[Fe(CN)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.
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