Development of novel and ultrahigh-performance asymmetric supercapacitor based on redox electrode-electrolyte system

Accepted Manuscript Title: Development of novel and ultrahigh-performance asymmetric supercapacitor based on redox electrode-electrolyte system Authors: Cuimei Zhao, Ting Deng, Xiangxin Xue, Limin Chang, Weitao Zheng, Shumin Wang PII: DOI: Reference:

S0013-4686(17)30348-1 http://dx.doi.org/doi:10.1016/j.electacta.2017.02.083 EA 28948

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

26-11-2016 14-2-2017 15-2-2017

Please cite this article as: Cuimei Zhao, Ting Deng, Xiangxin Xue, Limin Chang, Weitao Zheng, Shumin Wang, Development of novel and ultrahigh-performance asymmetric supercapacitor based on redox electrode-electrolyte system, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2017.02.083 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Development

of

novel

and

asymmetric

supercapacitor

ultrahigh-performance based

on

redox

electrode-electrolyte system Cuimei Zhaoa, Ting Dengb, Xiangxin Xuea, Limin Changa*, Weitao Zhengb*, Shumin Wangc a

Key Laboratory of Preparation and Applications of Environmental Friendly Materials (Jilin Normal

University), Ministry of Education, Changchun, China b

Department of Materials Science, Key Laboratory of Mobile Materials, MOE, and State Key

Laboratory of Superhard Materials, Jilin University, Changchun, China c

Light Industry College, Liaoning University, Shenyang, China.

Abstract The conventional enhancement in capacitive performance that only relies on electrode materials is limited. Here, based on the system-level design principle, we explore the possibility of enhancing capacitance through both electrode and electrolyte. A novel and ultrahigh-performance asymmetric supercapacitor has been fabricated using two redox electrode systems. The positive electrode system consists of graphene supported Co(OH)2 nanosheet (Co(OH)2/GNS) electrode and mixed KOH and K3Fe(CN)6 aqueous electrolyte. And the negative electrode system comprises carbon fiber paper supported activated carbon (AC/CFP) electrode in mixed KOH and p-phenylenediamine (PPD) aqueous electrolyte. The novel asymmetric supercapacitor exhibits a significantly improved capacitive performance (specific capacitance of 204.5 Fg-1, operational voltage of 2.0 V) in comparison with that of the conventional asymmetric supercapacitor (66.8 Fg-1, 1.5 V) fabricated without redox electrolyte. The improvement is attributed high reversibility and conductivity of electrode materials and redox electrolyte, as well as the synergistic effect between the two electrode systems, resulting in a ultrahigh energy density (114.5 Whkg-1 at a power density of 1000 Wkg-1), excellent power density (4000 Wkg-1 at an energy density of 31.6 Whkg-1) and long-term cycling stability (after 20000 cycles, initial 1

capacitance remains well). These encouraging results afford a facile and efficient way to fabricate ultrahigh-performance supercapacitors for the increasing demands on the energy storage devices.

Keywords: Asymmetric supercapacitor, Cobalt hydroxide, Graphene nanosheet, Activated carbon, Redox electrode system *Corresponding author: Key Laboratory of Preparation and Applications of Environmental Friendly Materials, Ministry of Education, College of Chemistry, Jilin Normal University, Changchun 130103, PR China E-mail address: [email protected] (Limin Chang). Department of Materials Science, Key Laboratory of Mobile Materials, MOE, and State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China. E-mail address: [email protected] (Weitao Zheng).

1.

Introduction

With the increasing power, energy and stability demands in next-generation energy devices, research efforts have mostly focused on developing compatible energy storage devices [1-3]. Supercapacitors (SCs), also known as electrochemical capacitors (ECs), have recently become promising candidates in the energy storage areas, owing to their higher power densities, longer cycle life and safer operation performance than batteries [4-7]. However, the disadvantages of limited energy density (usually less than 10 Whkg-1 for commercial ECs) and high fabrication cost (expensive to obtain high-performance electrode materials, such as RuO2) have been identified as the major challenge for the capacitive storage science. The energy density (E) is usually limited to the specific capacitance (C) and operating voltage (V) according to the equation E=1/2CV2. Therefore, developing ECs with high specific capacitance and wide operating voltage are two strategies to improve the energy density. However, two problems exist, firstly, although using organic electrolytes or ionic liquids can extend the operating voltage of ECs, the nonaqueous electrolytes suffer from low conductivity, high cost, flammability and 2

environmental problems [8, 9]; secondly, the researches for improving the capacitive performance of ECs mainly focus on the electrode materials recently, although developing nanostructured electrode materials can enhance specific capacitance, the enhancing is limited. An effective approach to increase energy density is to design aqueous asymmetric supercapacitors (ASCs) combining a battery-like faradic electrode as energy source and a capacitive electrode as power source [9-11]. The current aqueous symmetric supercapacitors are mainly electric double layer capacitors (EDLCs), based on high surface area carbon materials in the two electrodes, showing a relatively low energy density (less than 10 Whkg-1) [6]. The restriction for symmetric supercapacitors is a limited operating voltage of about 1.23 V at which water decomposes [12]. To obtain high energy density, various types of aqueous ASCs, such as Li(Na, K)Mn2O4//activated carbon (AC) [13-15],

MxOy//AC (M = Co, Ni, Mn, Mo, V, etc.) [8, 16-18], Mx(OH)y//AC (M = Co, Ni, Al etc.) [9, 19, 20], have been developed to achieve a high cell voltage of 1.4–2.0 V by taking advantage of different potential windows of the two electrodes to widen the device operating voltage. As shown in Table 1, advanced ASCs with pseudocapacitive materials as positive electrodes and carbon materials as negative electrode have been developed, providing a pronounced improvement in energy density (~50 Whkg-1) by giving a specific capacitance about 2~10 times as large as that of carbon materials as well as an operating voltage higher than ∼1 V of EDLCs for symmetric supercapacitors with aqueous electrolytes [21-35]. Although the construction of ASCs is an effective approach to extend the operating voltage window of aqueous electrolytes by electrode materials working in well-separated potential windows and thus improve the energy density, a problem exists that the specific capacitance for ASCs is not high enough to improve the energy density effectively compared with batteries. Since the ASCs 3

research mainly focuses on the discovery of new electrode materials and new synthesis methodology, the specific capacitance cannot be improved significantly via only relying on the active electrode materials. Recently, there have been a few reports that redox additives are introduced into the conventional electrolyte for symmetric carbon EDLCs to substantially enhance the capacitance via redox reactions of the additives between the electrode and electrolyte. For example, Chen et al. [36] have introduced K3Fe(CN)6 as a redox additives into conventional Na2SO4 electrolyte for the graphene-paper electrode, and a much higher areal specific capacitance (475 mFcm-2) than that (93 mFcm-2) with Na2SO4 electrolyte has been achieved. Similarly, Senthilkumar et al. [37] have reported that with KI introduced to conventional H2SO4 electrolyte, the AC|H2SO4-KI|AC supercapacitor system delivered a high specific capacitance and energy density of 912 Fg-1 at 2 mAcm-2 and 19.04 Whkg-1 at 224.43 Wkg-1, which is nearly twice the specific capacitance and energy density of AC|H2SO4|AC (472 Fg-1, 9.5 Whkg-1). Based on these reports, it can be concluded that the redox additives can increase the original capacitance of the supercapacitor by contributing additional pseudocapacitance. As mentioned above, although more attention has been paid to investigating either electrode or electrolyte for enhancing the capacitance, no any efforts have been done to improve both solid electrode and liquid electrolyte simultaneously for ASCs. In previous work, we have studied the performance of graphene supported Co(OH)2 (Co(OH)2/GNS) electrode in mixed KOH and K3Fe(CN)6 aqueous solution [38]. In this work, we prepared Co(OH)2/GNS-KOH/K3Fe(CN)6 positive electrode system and AC/carbon fiber paper-KOH/p-phenylenediamine (AC/CFP-KOH/PPD) negative electrode system and fabricated an ASC with the novel positive and negative electrode system. We aimed at developing a novel ASC device based on redox electrode-electrolyte system with ultrahigh 4

electrochemical performance, realizing that solid electrode and liquid electrolyte provide capacitance simultaneously and stably. 2.

Experimental

2.1. Preparation of Co(OH)2/GNS-KOH/K3Fe(CN)6 electrode system Our previous work has confirmed the possibility of enhancing pseudocapacitance through both Co(OH)2/GNS electrode and KOH/K3Fe(CN)6 electrolyte [38]. For the electrode material, vertically oriented GNS were synthesized by plasma-enhanced chemical vapour deposition on Ni foam, and that was used as substrates for cathodic electrodeposition of Co(OH) 2 nanosheets in Co(NO3)2 aqueous solution, the detailed description about the preparation of the Co(OH)2/GNS electrode can be found in ref. [39]. The electrolytes were 1 M KOH aqueous solution mixed with 0.04 M K3Fe(CN)6. 2.2. Preparation of AC/CFP-KOH/PPD electrode system The influence of AC on the performance of capacitor is very important, therefore high-quality commercial AC (TF-02, Xinjiang Tianfu Electric Co. Ltd) with a surface area of 2000 m2g-1 and average particle size of 8 μm was chosen. The AC electrode was prepared by a simple and low-cost spray method. With high speed stirring and ultrasonic processing, 85 wt% AC, 5 wt% Nafion, and 10 wt% conductive graphite were mixed in ethanol to form homogeneous slurry. Then the homogeneous slurry was sprayed on CFP current collector. After being dried at 120 ˚C for 5 h, an AC/CFP electrode was obtained. The electrolytes were 1 M KOH aqueous solution mixed with 0.025 M PPD. 2.3. Supercapacitor device fabrication The novel ASC was fabricated using Co(OH)2/GNS-KOH/K3Fe(CN)6 positive electrode system and AC/CFP-KOH/PPD negative electrode system with a selective permeability diaphragm (Dupont Fluoroproducts) in the middle. In addition, we prepared a conventional ASC using Co(OH)2/GNS 5

positive electrode and AC/CFP negative electrode in KOH electrolyte for comparison. The total mass of the active materials on the positive and negative electrodes is 0.8 and 2.4 mg, respectively. 2.4. Characterisation The mass of the active material of Co(OH)2 or AC was measured from the weight difference before and after deposition by means of a micro-balance (Sartorius BT125D) with an accuracy of 0.01 mg. Electrochemical measurements were carried out on a computer-controlled electrochemical working station (CHI760D or AUTOLABPGSTAT302N). The corresponding electrochemical performance such as the specific capacitance (Cm, Fg-1), energy density (E, Whkg-1) and power density (P, Wkg-1), can be calculated from the following equations [30, 35, 40-45]:

Cm =

2im ∫ V dt V2 |Vf Vi

(1) 1

E = Cm ∆V 2 /3.6 2

(2)

P=

3600E ∆t

(3) where im (A) is the charge-discharge current density, ∫ V dt is the integral area under charge-discharge curve, ∆V (V) is the potential range, ∆t (s) is the discharge time, and m (g) is the total mass of the active material in the electrode. 3.

Results and Discussion As shown in Fig. 1, a sandwich-shaped ASC was obtained using Co(OH)2/GNS-KOH/K3Fe(CN)6

positive electrode system and AC/CFP-KOH/PPD negative electrode system, and in the middle a selective permeability diaphragm was chosen only for K+ transfer by pore effect, electrostatic effect 6

and diffusion effect. From the SEM images of Co(OH)2/GNS or AC/CFP electrode in Fig. 1, Co(OH)2 grows interlaced sheets with channels and pores, and AC/CFP electrode displays the porous morphology with network structure for AC on CFP. The different electrochemical properties of Co(OH)2/GNS or AC/CFP electrode in different electrolytes were tested using Cyclic voltammograms (CV) and Galvanostatic charge-discharge (GCD) measurements in a three-electrode cell with a SCE reference electrode (Fig. 2 and 3). Fig. 2(a) shows the CV curves for Co(OH)2/GNS electrode in 1M KOH or 1M KOH+0.04M K3Fe(CN)6 aqueous solution at a scan rate of 25 mVs-1, the CV curve of Co(OH)2/GNS electrode in KOH electrolyte displays one pair of well-defined redox peaks, ascribed to the reversible conversion from Co 2+/Co3+. In contrast, the Co(OH)2/GNS electrode in KOH+K3Fe(CN)6 electrolyte shows two pairs of strong redox peaks, corresponding to the reversible faradic processes from Co 2+/Co3+ and Fe(CN)64−/Fe(CN)63−, indicating that reversible redox reactions of Co(OH) 2 solid electrode and K3Fe(CN)6 liquid electrolyte occur simultaneously and independently. Fig. 2(b) shows the GCD curves for Co(OH)2/GNS electrode in 1M KOH or 1M KOH+0.04M K3Fe(CN)6 aqueous solution at a current density of 8 Ag-1, the specific capacitances were calculated based on the GCD curves are 442.7 and 2545.1 Fg-1, respectively. From the CV curves at 25 mVs-1 in Fig. 3(a), the AC electrode in 1M KOH reveals a nearly rectangle shape without redox peaks, implying an ideal electric double layer capacitance. The CV curve of the AC electrode in 1M KOH+0.025M PPD displays one pair of remarkable redox peaks, indicating typical behavior of the combination of the electric double layer capacitance of AC electrode materials and pseudocapacitance from the reversible redox reaction of PPD electrolyte. The GCD curves of AC electrode at 8 Ag-1 in 1M KOH or 1M KOH+0.025M PPD are shown in Fig. 3(b), the specific capacitances were 96.5 and 382.7 Fg-1, respectively. 7

Fig. 4 (a) shows the typical CV curves for the novel ASC measured at various scan rates of 2, 5, 10, 25 mVs-1, and Fig. 4 (b) is the conventional ASC for comparison. It is observed that obvious redox peaks appear in the CV curves of the novel ASC, which is attributed to the faradic pseudocapacitance from the conversion of Co2+/Co3+, Fe(CN)64−/Fe(CN)63− and p-phenylenediamine/p-phenylenediimine [38, 40], suggesting that reversible redox reactions of solid electrode and liquid electrolyte occur

simultaneously and independently, therefore high capacitance can be expected. And the operating voltage window for the novel ASC is 2.0 V, much higher than the conventional ASC (1.5V). The

high specific capacitance and voltage window are important for high-energy supercapcitors. In addition, the characteristic CV shape is not significantly influenced as the scan rate is increased,

and remains very well even at a high scan rate of 25 mVs-1, indicating that ideal rate capability and excellent reversibility, desirable for high-power supercapcitors. To further evaluate the electrochemical performance, GCD measurement was conducted at various current densities 1, 2 and 4 Ag−1 for novel ASC (Fig. 5 (a)) or conventional ASC (Fig. 5 (b)). From Fig. 5 (a), the potential plateaus in charge-discharge process result from a lot of electron exchange, corresponding to the reversible redox conversions from Co2+/Co3+, Fe(CN)64−/Fe(CN)63− and p-phenylenediamine/p-phenylenediimine, in agreement with the results from CV test. In addition, the initial voltage loss (i.e. IR drop) for novel ASC observed on the discharge curve is rather small, demonstrating low internal resistance and fast I–V response in the supercapacitor [22]. The specific

capacitances, evaluated at 1, 2 and 4 Ag−1 from GCD curves for the novel ASC, will be 204.5, 106.3 and 56.4 Fg−1 with a wide voltage window of 2.0 V, superior to the conventional ASC (66.8, 65.1 and 61.9 Fg−1 with a voltage window of 1.5 V.), corresponding to the coulombic efficiency of 127.8%, 98.3% and 96. 5%. The high coulombic efficiency is essential for a battery-type 8

supercapacitor device with a high energy density. The capacitive performance for the novel ASC obviously outperforms most of the supercapacitors reported previously for conventional asymmetric supercapacitors (table 1) and novel symmetric supercapacitors (table 2). Power density and energy density are the two key parameters to characterize the performance of the electrochemical supercapacitors. Fig. 6 shows the Ragone plot of the novel ASC and conventional ASC based on the GCD at different current densities. It is worth noting that the maximum energy density obtained for our novel ASC is 114.5 Whkg-1 at a power density of 1000 Wkg-1. Even at a high power density of 4000 Wkg-1, the novel ASC still achieves a high energy density of 31.6 Whkg-1. The maximum energy density and power density of the novel ASC is much higher than that of conventional ASC (20.7 Whkg-1, 3000 Wkg-1), indicating the superior properties of novel ASC in terms of energy and power. Furthermore, the maximum energy density and power density of the novel ASC also much outperform the supercapacitors in the reported literature for conventional asymmetric supercapacitors (table 1) and novel symmetric supercapacitors (table 2). The cycling stability for the novel ASC is examined by continuous charge-discharge experiments for 20000 cycles at a high current density of 10 Ag-1, as shown in Fig. 7. The capacitance of the supercapacitor shows a slight increase and decrease, and remains quite constant with cycle number, after 20000 charge-discharge cycles, ~100% capacitance remains, indicating excellent cycling stability of the novel ASC. The initial increase of the capacitance is due to the material activation process. The decrease of the capacitance for the following cycles is probably due to the dissolution and detachment of the active material during the long-time cycling [12]. Electrochemical impedance spectroscopy (EIS) measurement (frequency range of 100 mHz to 1000 kHz) was employed to investigate the impedance behaviour at the electrode/electrolyte interface, in 9

order to further evaluate the electrochemical behaviour of the novel ASC. The Nyquist plot composed of approximate semicircles in the high frequency region and a slope in the low frequency region for the novel ASC is shown in Fig. 8. The semicircle is related to faradic reaction with its diameter representing the interfacial charge-transfer resistance (RCT). Such a small semicircle during long-term cycling exhibits low RCT and high charge-transfer conductivity, indicating the ease of pseudocapacitance redox reactions to occur. The slope is related to the diffusion resistance of the electrolyte in the electrode pores. Such a great slope (more than 45°) indicates small diffusion resistance, promoting good pseudocapacitative behaviour. During 20000 cycles, the EIS shows small changes (semicircles diameter keeps very small and slope maintains more than 45°), demonstrating excellent electrochemical stability of the ASC. Our novel asymmetric supercapcitor displays excellent electrochemical performance with ultrahigh energy density, excellent power density, and long-term cycling stability. The superior properties of the novel ASC can be attributed to the following factors: (1) Because of the high individual capacitive performance of Co(OH)2/GNS-KOH/K3Fe(CN)6 and AC/CFP-KOH/PPD electrode systems, as well as the synergistic effect between the two electrode systems, the operating voltage window is extended to 2.0 V. (2) The introduction of K3Fe(CN)6 or PPD in the electrolyte, do contribute the extra pseudocapacitance, leading to the obvious increase of specific capacitance. (3) With active electrode materials Co(OH)2 or AC directly grown on GNS or CFP, respectively, carbon-material substrate not only serves as a highly conductive path, facilitating fast electron transport, but also enhances the mechanical strength of the composite, resulting in the excellent cycle stability. Conclusions A facile and cost-effective approach was developed to fabricate ultrahigh-performance ASC through 10

introducing redox active substances into the conventional KOH electrolyte to give extra pseudocapacitance

and

extend

voltage

window.

The

novel

ASC

device

based

on

Co(OH)2/GNS-KOH/K3Fe(CN)6 positive electrode system and AC/CFP-KOH/PPD negative electrode system can achieve an ultrahigh specific capacitance of 204.5 Fg-1 at a current density of 1 Ag-1 with an extended operational voltage of 2.0 V. A maximum energy density of 114.5 Whkg-1 at a power density of 1000 Wkg-1, and a maximum power density 4000 Wkg-1 at an energy density of 31.6 Whkg-1 have been obtained for the novel ASC, which were much higher than that of conventional ASC (20.7 Whkg-1, 3000 Wkg-1). In addition, the novel ASC device displays a long-term cycling stability (the specifc capacitance remains quite constant after 20000 charge-discharge cycles). This novel asymmetric supercapacitor holds great promise for application in ultrahigh-performance energy storage devices. Acknowledgments The support from NSF of China (grant no. 51372095 and 51602140), the projects of Jilin Province Department of Education (No.2016-221 and 2016-222) and the project of Liaoning Province Department of Education (No. LYB201619) is highly appreciated.

11

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17

18

Figure captions Fig. 1. A schematic diagram illustrating the architecture of the novel ASC based on Co(OH)2/GNS-KOH/K3Fe(CN)6 positive electrode system and AC/CFP-KOH/PPD negative electrode system. Inset: physical prototype of the novel ASC. Fig. 2 CV curves at 25 mVs-1 (a) and GCD curves at 8Ag-1 (b) for the systems of Co(OH)2/GNS electrode in 1M KOH and 1M KOH+0.04M K3Fe(CN)6 solution. Fig. 3 CV curves at 25 mVs-1 (a) and GCD curves at 8Ag-1 (b) for the systems of AC/CFP electrode in 1M KOH and 1M KOH+0.025M PPD solution. Fig. 4. CV curves at different scan rates for novel ASC (a) and conventional ASC (b). Fig. 5. GCD curves at different current densities for novel ASC (a) and conventional ASC (b). Fig. 6. Ragone plots for the novel ASC and conventional ASC. Fig. 7. Cycling performance of the novel ASC measured at a high current density of 10 Ag-1. Inset: GCD curve for the ASC device measured at a high current density of 10 Ag-1. Fig. 8. EIS for the novel ASC during the 20000 charge-discharge cycles.

19

Fig. 1

Fig. 2

Fig. 3 20

Fig. 4

Fig. 5

Fig. 6

21

Fig. 7

Fig. 8

22

Table Table 1. Summary of research describing energy storage parameters of the conventional ASC devices reported in literature. NR means not reported. Max

ES(Whkg-1)@

CS

(Fg-1)

V (V)

PS(kWkg-1)

(%)/cycle number

3M NaOH

NR

1.8

[email protected]

91.5/20000

[21]

MnO2/carbon nanofiber//AC

0.5M Na2SO4

56.8

2.0

[email protected]

94/5000

[22]

Co(OH)2/Co3O4//AC

2M KOH

~62

1.5

22.3@3

104/2000

[23]

Co/Al LDHs//rGO

6MKOH

97.5

1.6

[email protected]

93/2000

[24]

CoMn2O4/GNR//GNR

0.5M Na2SO4

~100

1.9

84.69@22

96/1500

[25]

NiCo2O4@MnO2//AC

1M NaOH

112

1.5

35@3

71/5000

[26]

NiCo2O4 NSs@HMRAs//AC

1 M KOH

70.04

1.5

[email protected]

106/2500

[27]

Ni–Co oxide NWAs//AC

1 M KOH

NR

1.8

[email protected]

73.1/3000

[28]

1 M KOH

87.9

1.6

[email protected]

82/3000

[29]

NiO// rGO

1 M KOH

50

1.7

39.9@4

95/3000

[30]

NiMoO4.xH2O//AC

2 M KOH

96.7

1.6

[email protected]

80.6/1000

[31]

Co3O4/Co3(VO4)2//AC

2 M KOH

105

1.6

[email protected]

94.7/5000

[32]

Co3O4@MnO2//graphene

1 M LiOH

49.8

1.8

17.7@158

81.1/10000

[33]

MnO2//CNT-CNF

0.5 M Na2SO4

93.99

2.0

52.22@1

92/2000

[34]

Co(OH)2/NMEG//PPy/rG-O

1 M KOH

74

1.6

[email protected]

60/6000

[35]

204.5

2.0

114.5@4

100/20000

SC configuration

Electrolyte

CoO@Polypyrrole//AC

[email protected](OH)2//C MK-3

Co(OH)2/GNS//AC/CFP

KOH+K3Fe(CN) 6//KOH+PPD

CS

23

retention

Ref.

present work

Table 2. Summary of research describing energy storage parameters of the novel symmetric supercapacitor devices reported in literature. NR means not reported. ES(Whkg-1)@

CS

(V)

PS(kWkg-1)

(%)/cycle number

78.01

1.0

[email protected]

90.68/10000

[41]

H2SO4+HQ

220

1.0

NR

NR

[42]

H2SO4+KI

726

1.0

[email protected]

97/500

[43]

H2SO4+HQ

205

2.0

[email protected]

96/1000

[44]

204.5

2.0

114.5@4

100/20000

SC configuration

Electrolyte

CS (Fg-1)

AC//AC

KOH+MPD

AC//AC MWCNTs/PANI// MWCNTs/PANI PANI/F-MWCNTs// PANI/F-MWCNTs

Co(OH)2/GNS//

KOH+K3Fe(CN)6

AC/CFP

//KOH+PPD

24

Max

V

retention

Ref.

present work