Effect of Redox Electrolyte on the Specific Capacitance of SrRuO3

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C: Energy Conversion and Storage; Energy and Charge Transport

Effect of Redox Electrolyte on the Specific Capacitance of SrRuO-Reduced Graphene Oxide Nano-Composite 3

Ahmed Galal, Hagar K. Hassan, Nada F Atta, and Timo Jacob J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02068 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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The Journal of Physical Chemistry

Effect of Redox Electrolyte on the Specific Capacitance of SrRuO3-Reduced Graphene Oxide Nano-Composite Ahmed Galal, *a Hagar K. Hassan,a Nada F. Atta,a Timo Jacob*bcd a Department of Chemistry, Faculty of Science, Cairo University, 12613 Giza, Egypt E-mail: [email protected] b Institute of Electrochemistry, Ulm University, 89081 Ulm, Germany E-mail: [email protected] c Helmholtz-Institute-Ulm (HIU) [d] Helmholtzstr. 11, 89081 Ulm, Germany d Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany Abstract In this work, we studied the effect of adding a redox electrolyte on the specific capacitance of SrRuO3-reduced graphene oxide nano-composite (SRGO) electrode. Two redox systems were considered, quinone/hydroquinone (Q/HQ) and triiodide/iodide (I3-/I-) in acidic (H3PO4) and neutral (NaNO3) solutions. The employment of RGO reduces the amount of Ru loading to the surface down to 20% to achieve specific capacitance (Cs) of 62.4 F.g-1 in 1.0 M NaNO3 and 101 F.g-1 in 1.0 M H3PO4. The Cs of the electrode increased 21.6 folds in KI-containing NaNO3 solutions compared to NaNO3 electrolyte. The presence of the redox system in the electrolyte enhanced the cycling stability of the electrode to 173% compared to its initial value after 1000 (galvanic charging/discharging) GCD cycles compared to 93% in its absence. The presence of redox-intermediates including iodine (I2), participate in the surface reaction and contribute to the observed enhancement in both Cs and cycling GCD. A maximum Cs value of 1037 F.g-1 (733 mF. cm-2) at 5 A.g-1 was obtained by using SRGO/0.08 M KI-1.0 M NaNO3.

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Introduction The development of the energy storage devices attracts considerable attention nowadays. More interest is devoted to renewable energy sources rather than fossil fuels 1. Supercapacitors are devices that differ from batteries in their high power densities, long cycling stabilities, fast charging/discharging time 2–5. Supercapacitors have a wide range of applications such as mobile electronic devices, hybrid electric vehicles, space or military devices and large industrial equipment 6. There are three types of supercapacitors: electric double layer capacitors (EDLCs) (their capacitances arise from the storage of charges at the interface between electrode and electrolyte), pseudocapacitors (due to the faradic reaction that occurs at the double layer) and hybrid supercapacitors (a combination between EDLCs and pseudocapacitor) 7. Although, EDLCs have the advantage of high cycling stability, their main drawback is their low energy density and specific capacitance compared to pseudocapacitors. In contrast to EDLCs, pseudocapacitors exhibit much higher theoretical capacitance due to the presence of the reversible faradaic redox reactions 6. However, pseudocapacitive materials lack from poor cycling stability due to their poor conductivity

8–10

. So, the combination between EDLCs and

pseudocapacitors significantly improves their specific capacitance, cycling stability and energy density 11. Among the promising materials used in pseudocapacitors, transition metal oxides attracted a significant interest. For instance, RuO2 has high specific capacitance ranging from 1300 to 2200 F·g−1

12

, high reversible faradaic reaction, high thermal stability, and high rate capability

13

.

However, the high cost of Ru, the rapid decrease of power density at high charge/discharge and the poor cycling stability of RuO2-based supercapacitors limit their commercial use

14

. The

formation of Ru-based perovskite decreases the mass percentage of Ru to 43% in SrRuO 3 compared to 60% in RuO2.nH2O. Further combination of Ru-based perovskite with graphene sheets enhances the specific capacitance, cycling stability, energy and power density of the resulting SrRuO3-based supercapacitor 15. The development of supercapacitors is usually targeting the electrode surface. While the significant enhancement in the supercapacitor should be from the integration of the optimization of its components such as electrode surface, electrolyte, separator, etc. Recently, some researches targeted the modification of electrolyte solution itself upon the addition of redox system to the 2 ACS Paragon Plus Environment

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electrolyte solution 6,16–22. This enhances significantly the pseudocapacitance character and leads to an enhancement in both specific capacitance and energy density 6. One of these reports used phenylene-diamine as a redox mediator in KOH electrolyte to improve carbon-based supercapacitors

16

. In this report, the addition of phenylene-diamine leads to an increase in specific

capacitance to the value of 605.2 F.g-1 compared to only 144.0 F.g-1 in KOH electrolyte without phenylene-diamine. Senthilkumar et al.

17

reported that the addition of KI or KBr to H2SO4 or

Na2SO4 greatly enhances the specific capacitance of activated carbon-based supercapacitor. The addition of K3[Fe(CN)6] as a redox system to KOH electrolyte was also used as an electrolytic improvement to Co(OH)2/rGO electrode by Zhao et al. 6. They found that the addition of K3[Fe(CN)6] to KOH solution enhances the pseudocapacitance of Co(OH) 2/rGO and promotes the electron gain and loss from Co 2+ ions. Q/HQ redox system in H2SO4 electrolyte as an electrolytic modification to rGO/PAni electrode surface was also reported by Chen et al.23. Roldan et al.24 reported that an improved specific capacitance was obtained for activated carbonbased supercapacitor upon adding Q/HQ redox system into the same electrolyte (H 2SO4). On the other hand, Park et al.25 reported that the addition of a redox system, deca-methyl-ferrocene, in an organic electrolyte results in an approximately 27-fold increase in the energy density of carbon nanotube-based supercapacitors, increases the cell voltage from 1.1 to 2.1 and enhances the specific capacitance from 8.3 F. g-1 to 61.3 F. g-1. Herein, we introduce supercapacitors based on SrRuO3/RGO nano-composite in a redox systemcontaining electrolyte. In our previous work, the inclusion of SrRuO3 into RGO led to a decrease in Ru loading while maintaining a relatively high Cs 15. The effect of introducing redox couples, namely Q/HQ or I3-/I-, to H3PO4 and NaNO3 should result in further improvement in the pseudocapacitive characteristics of the electrode. We aimed in this work to evaluate the adverse effects of the redox systems on the capacitance, energy and power densities of the Ru-based supercapacitor. 2 Experimental 2.1 Preparation of SrRuO3- RGO nanocomposite using citrate method SrRuO3 has been prepared via microwave-assisted citrate method according to the method mentioned in

26

. Briefly, 0.33 mmol of both Sr(NO3)3 and RuCl3.xH2O were dissolved in a small

volume of distilled water and ultrasonicated till the complete solubility. Then 0.66 mmol of citric acid was added so that the molar ratio between citric acid and total metal nitrates becomes 1:1. The 3 ACS Paragon Plus Environment


mixture was sonicated again until homogeneity followed by adjusting the pH to 8.0 using NH 4OH solution. The mixture was subjected to microwave irradiation using a power of 700-watt for 30 minutes (the power was switched on and off for 20 s and 10 s, respectively). The precursor was initially dehydrated and became viscos with time and finally the ignition process occurred. The asresulted fluffy powder was grind and transferred into a ceramic crucible then calcined at 600 oC for three hours in an oven. The reduced graphene oxide (RGO) was prepared by modified Hummer

method following the same procedure as reported previously 7. SrRuO3-RGO (SRGO) nano-composite was prepared by mixing 5 mg of SRO with 5.0 mg RGO in 1.0 mL DMF to form a total concentration of 10 mg/mL. The suspension was then ultrasonicated for an hour until a homogeneous suspension prepared and to facilitate the formation of SRGO nano-composite. 2.2 Electrode fabrication and electrochemical measurements 10 µL of 1% Nafion solution was added on the surface of a well-polished (mirror-like) Ni electrode (99.5% Ni rod with a surface area of 0.283 cm2 from Johnson Matthey Inc.) and dried at 75 oC. A 20 µL of SRGO was immobilized on the surface of Ni/Nafion (Ni/N) and left to dry in an oven to obtain Ni/N/SRGO. The mass of active material was calculated as 0.2 mg. The electrochemical measurements were performed in a conventional three-electrode system using Ag/AgCl (4.0 M NaCl) as a reference electrode and Pt wire as a counter electrode. Cyclic voltammetry (CV), galvanostatic charging/discharging (GCD) and electrochemical impedance spectroscopy (EIS) experiments were performed using Voltalab PGZ301 potentiostat. Cyclic voltammetry curves were recorded at scan rates ranging from 2 to 200 mV.s-1. Galvanostatic charging/discharging curves were recorded using chrono-potentiomety at various current densities. HQ was used in the concentrations of 0.01, 0.02, 0.04 and 0.06 M in 1.0 M H 3PO4. KI concentrations were 0.02, 0.04, 0.06 and 0.08 in 1.0 M NaNO3. The specific capacitance was calculated from CV curves using equation (1.1) and from galvanostatic charging discharging curves using equation (1.2). Where, Cs is the specific capacitance in Farad.g-1, Ī is the integration of area enveloped inside the CVs, m is the mass of electroactive material in grams, 𝜟V is the potential window in Volts (Vf-Vi), I is the current in Ampere and td is the discharge time in seconds.


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̅ Cs

(1.1)

Cs

(1.2)

3. Results and discussion The combination between pseudocapacitance of SRO with the EDLC of RGO characteristics exhibited a significant improvement in Cs value as well as energy and power densities compared to the individual material as presented in our previous work

15

. SRGO composite represents a

surface modification for the Ru-based perovskite to enhance its energy storage properties while lowering the Ru loading. Complete surface characterization from XRD-, FESEM-, HRTEM- and SAED-data can be found elsewhere

15

. This work is concerned with the use of simple redox

systems in both neutral and acidic electrolyte to further improve the performance of the electrode capacitance. 3.1 Effect of adding HQ to H3PO4 electrolyte The incorporation of redox system to the electrolyte solution not only affects the shape of the CV but also alters the electrochemical storage mechanism. Figure (1A) shows the CVs of SRGO in both 1.0 M H3PO4 and 0.02 M HQ/1.0 M H3PO4 at scan rate 50 mV.s-1. A distinct difference in the shape of the CVs is observed; two redox peaks are identified in the case of 0.02 M HQ/1.0 M H3PO4 at 595 and 269 mV corresponding to Q/HQ electrochemical redox reaction (Scheme 1). A semi-rectangular shape is observed in 1.0 M H3PO4 with pseudocapacitive behavior. Consequently, galvanic charging/discharging (GCD) curves display two different behaviors for the two electrolytic systems measured at 10 A.g-1. In 1.0 M H3PO4 solution the GCD curve displays a triangular shape while in 0.02 M HQ/1.0 M H3PO4 solution a deviation from the triangular shape was observed (Figure 1B). In this respect, two different mechanisms are suggested for the storage process: capacitive and faradaic mechanisms, as indicated from the change in the charging/discharging slopes and the presence of a semi-plateau in the discharge cycles when comparing the two curves in the studied electrolytes processes. The first slope in the charging cycle is due to the double layer charging, which is a relatively faster process. The second slope is arising from the faradaic process that took place following the double layer charging which is a rather slower process and manifested by a decrease in the slope as shown in Figure 1B. The loss of the rectangular shape in the presence of HQ and the change in the 5 ACS Paragon Plus Environment


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charging/discharging slopes in GCD curves suggest that the charge/discharge process is governed by two different mechanisms.

Scheme 1. Q/HQ redox system The aforementioned results indicated that the pseudocapacitive mechanism involves the nature of the electrode material while the faradaic (or battery-like) mechanism is mainly dependent on the components of the electrolytic solution. The work published by Brousse et al. discussed in detail the controversial description of mechanisms involving pseudocapacitive and battery-like behavior

27

. In their description

27

, the type of material employed such as MnO2 and Ni(OH)2

exhibit distinct electrochemical behaviors, the first is comparable to that of carbon based supercapacitors and the second with “battery-like” performance. The applications of redox electrolytes proved to enhance the capacitive behavior of the electrodes

17,28,29

. Conway described in his

classic book that EDLCs contain 1-5% faradaic component arising from the oxygenated functional groups presumably formed on the surface of carbon-based materials 2. While 10% of the batteries capacities arise from electrical double layer charging 2. From the above discussion, a common description is established that pseudocapacitors are considered when involved in threeelectrode measurements. This definition will be true either when the electrode material contains a redox component or if a redox electrolyte is used. From Figures 1A and 1B, the presence of 0.02 M HQ increased the specific capacitance from 148 to 272 F.g-1, at scan rate 50 mV.s-1, and from 64.2 to 127.5 F.g-1 at current density 10 A.g -1. The effect of scan rate provides information regarding the energy storage mechanism as well as the nature of the electrochemical process. Figure 1C shows CVs of SRGO in 0.02 M HQ/1.0 M H3PO4 electrolyte at various scan rates from 2 to 200 mV.s-1. It is observed that as the scan rate increases the anodic and cathodic peak-potential separation increases. The relation between the anodic and cathodic peak currents vs. the square root of the scan rate is shown in Figure 1D. The observed linear relation indicates a diffusion controlled process which is associated with the 6 ACS Paragon Plus Environment

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faradaic reaction of the redox couple Q/HQ. The inset of figure 1D depicts the relation between anodic and cathodic peak-currents with the scan rate displaying a symmetry that is characteristic of the reversible charging/discharging processes.

A)

B)

C)

D)

Figure (1): A) CVs of SRGO in the absence and the presence of 0.02 M HQ-containing 1.0 M H3PO4 at scan rate of 50 mV.s-1, B) the corresponding GCD curve measured at current density of 10 A.g-1, C) the CVs of SRGO in 0.02 M HQ/1.0 M H3PO4 at various scan rates: 2, 10, 20, 50, 100, 150 and 200 mV.s-1, and D) the relation between charging and discharging peak currents, and square root of scan rate (inset is the relation between peak current and scan rate). It is advisable at this stage to optimize the concentration of HQ that results in an “optimum” capacitance value. Figure 2A shows the CVs of SRGO in 1.0 M H3PO4; 0.01, 0.02 and 0.04 M



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HQ/1.0 M H3PO4 at scan rate of 50 mV.s-1. It is observed that the Cs value increases as the concentration of HQ increases up to 0.04 M then starts to decrease again. The relation between the calculated Cs and the scan rate for all electrolyte concentrations studied is shown in Figure 2B. Similar trend is observed at various scan rates and the highest Cs was obtained when using 0.04 M HQ/1.0 M H3PO4 as electrolyte. A maximum Cs value of 590 F.g-1 at scan rate 2 mV.s-1 was calculated for 0.04 M HQ/1.0 M H3PO4 electrolyte compared to 208 F.g-1 in absence of HQ. GCD curves of SRGO in 0.01, 0.02, 0.04 and 0.06 M HQ/1.0 M H3PO4 at current density of 10 A.g-1 are shown in Figure 2C. The discharge time increases with HQ-concentration. It is worth noting that the charging time increases while the discharge time decreases when using 0.06 M compared to 0.04 M HQ. The relation between Cs and charging/discharging current densities is presented in Figure 2D. The data confirm that 0.04 M HQ would be an “optimum” concentration that results in relatively higher CS values. Table (1) summarizes the electrochemical parameters calculated for SRGO in absence and presence of different concentrations of HQ/Q redox system. The diffusion coefficients (Dc is the diffusion coefficient during the charging step; Dd is the diffusion coefficient during the discharging step) are calculated from the Randles Sevcik equation: Ip = (2.687 × 105) n3/2 ν1/2 D1/2 A Co

(3.1)

Ip is the peak current (A), n is the number of electrons exchanged in electrochemical process (n = 2), ν is the scan rate (V.s-1), Co is the concentration of HQ (mole.cm-3), A is the geometrical electrode area = 0.283 cm2, and D is the diffusion coefficient (cm2.s-1).

[HQ] (M)

Cs* (F.g-1)

Dc (cm2.s-1) ×10-8

Dd (cm2.s-1) ×10-8

Cs** (F.g-1)

IRdrop** (V)

E** (Wh.Kg-1)

P** (kW.Kg-1)

Cs¶ (%)

ηi (%)

η1000 (%)

0.0 0.01 0.02 0.04 0.06

208 369 567 590 427

---11 6.5 1.5 1.2

---6.8 4.9 1.2 0.86

84.0 99.8 206 233 163§

0.0800 0.116 0.0660 0.150 0.200

14.6 16.3 36.7 35.6 22.6

1.50 1.47 2.12 1.34 2.09

78.0 100 84.1 90.0 54.4

40.0 82.6 84.6 78.0 37.6

92.8 100 94.1 40.5 91.7

Table (1): The electrochemical properties of SRGO in the absence and in the presence of various concentrations of HQ/1.0 M H3PO4 electrolyte solution (*: measured at 2 mV.s-1; **: measured at 2.1 mA.cm-2 ; §: measured at 3.5 mA.cm-2; ¶: retention after 1000 cycles).


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The effect of the potential limit on the Cs value was evaluated by GCD tests at 15 A.g-1. The lower potential limits varied between -0.2 and -0.6 V; the upper potential limit is 1.0 V in 0.04 M HQ/1.0 M H3PO4 (Figure 3A). The observed change in the slope of the discharging section of the curve (for 0.04 M HQ/1.0 M H3PO4) at about -0.4 mV vs Ag/AgCl (4 M NaCl) is attributed to the onset of hydrogen evolution reaction (HER). The results indicate that this supercapacitor electrode performs effectively up to 1.4 V without electrolytic complications. The relation between the voltage and the calculated Cs values (shown in the inset of the Figure 3A) revealed that SRGO in 0.04 M HQ/1.0 M H3PO4 provides the optimum Cs value with voltage of 1.2 V without any electrolytic decomposition.

A)

B)

C)

D)

Figure (2): A) CVs of SRGO in various concentrations of HQ/1.0 M H3PO4 at scan rate of 50 mV.s-1, B) the relation between scan rate and the calculated Cs values of SRGO in various HQ concentrations, C) GCD of SRGO in various HQ concentrations at current density of 10 A.g-1, and D) the relation between the current density and the calculated Cs of SRGO in various HQ concentrations.



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Cycling stability is a parameter of merit in the performance of the capacitor, in this respect repeated GCD tests for 1000 cycles were performed (Figure 3B). SRGO showed Cs retention values of 100, 84, 90 and 26% after 1000 cycles of GCD tests at current density 25 A.g -1 in 0.01, 0.02, 0.04 and 0.06 M HQ, respectively. These results indicate high stability of SRGO in 0.04 M HQ/1.0 M H3PO4. Using HQ concentration of 0.06 M in the electrolyte, results in a decrease in the Cs value. This could be attributed to the blocking of surface of SRGO by the bulky HQ and its oxidation product. Table (1) shows the values of Cs calculated from CV and GCD measurements, IRdrop calculated from GCD curves, the specific capacitance retention after 1000 cycles, energy density (E), and power density (P) of SRGO in 0.01, 0.02, 0.04 and 0.06 M HQ/1.0 M H3PO4. The energy and power densities were calculated as mentioned elsewhere 7. Coulombic efficiency (η) is defined as the ratio of the charge time and discharge time when the charge and discharge currents are equal. It can be calculated using the following equation: η=

(3.2)

Where, td is the discharge time and tc is the charge time. It can be used to evaluate the cycling stability by comparing the Coulombic efficiency of the first cycle to that of the last cycle. Here, ηi (of the first cycle) is compared to η1000 (of the 1000th cycle) in 0.01, 0.02, 0.04 and 0.06 M HQ/1.0 M H3PO4. The results are listed in Table (1).

A)

B)

Figure (3): A) The effect of lower potential limits of SRGO in 0.04 M HQ/1.0 M H 3PO4 at current density of 15 A.g-1, black line for lower potential limit -200 mV, red line for -300 mV, green line for -400mV, yellow line for -500 mV and blue line for -600 mV. (The inset is the relation between the calculated Cs value and the corresponding voltage), and B) the cycling stability of SRGO in various HQ concentrations. 10 ACS Paragon Plus Environment

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A series of EIS experiments were performed in various concentrations of HQ to provide more information about the impedance parameters in each electrolyte concentration. Nyquist plots of SRGO in 0.01, 0.02, 0.04 and 0.06 M HQ/1.0 M H3PO4 showed a semi-circle in the highfrequency region and vertical lines in the low-frequency region as presented in Figure 4A. Except for the concentration of 0.04 M HQ, the slope of the vertical line in the low frequency region decreases as the concentration of HQ increases. The introduction of the redox electrolyte results in a mixed process that includes faradaic, non-ideal capacitive and diffusional components. Ionic diffusion is frequency-dependent and implies the use of a Warburg-diffusion element in the equivalent circuit used for data fitting. This finding is in agreement with the CV measurements from which we assumed that the electrochemical process is diffusion-controlled in the studied electrolytes. Taking into account the possible “blocking” of the electrode surface when using HQ concentration of 0.06 M or higher, 0.04 M HQ is more suitable to use to realize less diffusional resistance and optimum pseudocapacitance. The Nyquist plot of SRGO in 0.04 M HQ/1.0 M H3PO4 displays a relatively higher slope in the vertical line region indicating lower diffusional resistance and higher electrical double-layer capacitance. The solution resistance is calculated from the intersection of the semi-circle with x-axis while the diameter of the semi-circle and has been used to calculate the charge transfer resistance. In order to obtain more insights on the values of EIS elements, EIS data has been fitted with an equivalent circuit. The equivalent circuit used to fit the data is shown in Figure 4B. It consists of a solution resistance (Rs) in series with a constant phase element representing the EDLC (Ydl with an exponent n), a second constant phase element representing the pseudocapacitance (Yp, with an exponent m) in parallel with the charge transfer resistance (Rct) and a Warburg diffusion element (W). The values of EIS elements deduced from the fitting data are shown in Table (2). From the data of Table (2), relatively lower charge transfer and diffusion resistances are observed when using 0.04 M HQ. The values of Ydl increase with the concentration of HQ up to a concentration of 0.04 M and then decrease when using 0.06 M HQ. The higher Rct and lower Ydl values obtained when using 0.06 M HQ is reflected on the obtained relatively low Cs value compared to 0.04 M HQ. This finding is in good agreement with the data obtained from CV and GCD measurements.

Therefore, we

concluded that using 0.04 M HQ is an “optimum” concentration that should enhance the Cs value of SRGO in H3PO4 electrolyte. 11 ACS Paragon Plus Environment


A)

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B)

Figure (4): A) The Nyquist plot of SRGO in various concentrations of HQ/1.0 M H3PO4 (measured from 100 mHz to 100 KHz, symbols and solid lines represent the experimental and the fitting data, respectively), and B) the equivalent circuit used for the fitting process of the EIS data. [HQ] Rs Ydl Rct Yp Cp* W σ** (M) (Ω) (μS. sn), n (Ω) (mS. sm), m (mF) (mS. s1/2) (Ω. s-1/2) 0.01 16.4 15.8, 0.79 9.9 8.5, 0.85 5.4 29.0 34.5 0.02 33.6 27.5, 0.86 21.5 15.8, 0.75 11 40.7 24.5 0.04 32.1 36.9, 0.81 7.7 12.7, 0.92 10.4 75.2 13.3 0.06 16.2 29.5, 0.88 59.6 19.6, 0.39 --42.8 23.5 (*calculated from Yp; **: Warburg coefficient) Table (2): The values of the EIS components calculated from the fitting of EIS measurements of SRGO in various HQ concentrations (data from Figure 4a). 3.2 Effect of adding KI to NaNO3 electrolyte Potassium iodide was used as a redox additive to the acidic electrolyte (H2SO4)

17

or basic

electrolyte (KOH) 21 to enhance the Cs of EDLC of commercial activated carbon. Iodide (I-) ions can produce various redox pairs through faradaic reactions following equations (3.2-3.5) 17. All these faradaic reactions will possibly result in an increase in the current enveloped within a certain potential range as well as the discharge time at a certain current density. To the best of our knowledge, the effect of adding iodide to neutral electrolyte on the performance of perovskite-based pseudo-capacitors has not been studied. 3I¯ ⇌ I3¯ + 2e-

(3.2)

2I¯ ⇌ I2 +2e-

(3.3) 12 ACS Paragon Plus Environment

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2I3¯ ⇌ 3I2 +2e-

(3.4)

I2 +6H2O ⇌ 2IO3 + 12 H+ +10e-

(3.5)

Figure 5A shows CVs of SRGO in both 1.0 M NaNO3 and 0.04 M KI/1.0 M NaNO3 at scan rate 50 mV.s-1. It is shown that addition of KI to NaNO3 solution leads to an increase in the enveloped current through the faradaic reactions associated with the above-mentioned reactions. The addition of 0.04 M KI results in obtaining a non-rectangular shaped CV for SRGO electrode. Two well-defined redox peaks can be identified at 450 and 243 mV. It is important to mention that EDLC component is still contained within the faradaic current envelopes. The contribution of faradaic redox reactions when using iodide enhances the Cs value to 204 F.g-1 compared to 99.1 F.g-1 in 1.0 M NaNO3. GCD curves of SRGO in 0.04 M KI/1.0 M NaNO3 compared to 1.0 M NaNO3 at relatively high current density of 15 A.g-1 are shown in Figure 5B. The addition of KI changes the charge storage mechanism from an ideal capacitive behavior (triangular GCD curve) to a non-ideal capacitive one (with the appearance of a faradaic plateau). GCD curve of 0.04 M KI/1.0 M NaNO3 showed the following processes: fast EDL followed by a slow faradaic reaction in the charging direction and a slow faradaic reverse reaction followed by fast EDL discharge step as shown in Figure 5B. The presence of slow faradaic reaction leads to increasing the discharge time and consequently the Cs value. The calculated Cs of SRGO at 15 A.g-1 is 105 F.g-1 in 0.04 M KI/1.0 M NaNO3 compared to 40.5 F.g-1 in the absence of KI.

A)

B)

Figure (5): A) CVs of SRGO in the absence and the presence of 0.04 M KI-containing 1.0 M NaNO3 at scan rate of 50 mV.s-1, and B) the corresponding GCD curve measured at current density of 15 A.g-1 showing the two processes involved in the energy storage mechanism.



It was worthy to investigate the effect of KI concentration on the values of Cs. The KI concentrations studied are 0.02, 0.04, 0.06 and 0.08 M. Figure 6A shows CVs of SRGO in various KI concentrations at scan rate 20 mV.s-1. It is shown that by increasing the concentration of KI the enveloped current increases in higher Cs values due to the increase in I--ions concentrations participating in the redox reactions. The broadening of the redox peaks increases with increasing KI concentrations due to the large possibilities of different redox reactions of I ions. It was observed the appearance of a yellow color in the electrolyte during the electrochemical test indicating the possibility of I2 formation. The high tendency of adsorption of I2 molecules has a large contribution to the high Cs values at the relatively low scan rate. The effect of scan rate (2 to 200 mV.s-1) on Cs values shown in Figure 6B revealed that 0.06 M KI resulted in maximum Cs value at scan rate 2 mV.s-1 of 907.7 F.g-1 followed by 0.04 M KI that showed 860 F.g-1 then 0.08 M KI (800 F.g-1) at the same scan rate. The observed sudden decrease in Cs values at high scan rates suggests I2 adsorption-dependence of Cs values on the electrode. Relatively high concentrations of KI (ca. 0.08 M) result in adsorption over the electrode surface that reflects on the relatively lower Cs values. GCD tests can provide more information about the nature and mechanism of the storage process in such systems. Figure 6C shows GCD curves of SRGO in various KI concentrations at current density of 20 A.g-1. As the concentration of KI increases both charging and discharge times increases except for 0.08 M. In the later case, the charging time starts to decrease compared to the lower concentration of 0.06 M (the discharge time is nearly the same for both concentrations). The appearance of a potential plateau in GCD curves especially at the relatively low current densities indicates the adsorption-dependence of the storage process of SRGO in KINaNO3 electrolyte Figure 6D shows the effect of the current density on Cs values. It is shown that using 0.08 M KI can provide a Cs value of 1037 F.g-1 at a current density of 5 A.g-1 compared to 48 F.g-1 in case of 1.0 M NaNO3. While at relatively high current densities such as 15 A.g-1 (fast charging/discharging) comparable Cs values are obtained when using 0.06 and 0.08 M KI. In the case of HQ-H3PO4 system, Cs values did not show an adsorption-dependency. The high number of electrons involved in I-/I3 -/I2 redox reactions compared to HQ/Q two-electrons process and the high adsorption tendency of I2 molecules as well as the small size of I- ions and their product compared to hydroquinone and its product, Q, result in higher Cs values for KINaNO3 system compared to HQ-H3PO4. 14 ACS Paragon Plus Environment

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The effect of varying the lower and upper potential limits on the Cs value of SRGO in 0.04 M KI/1.0 M NaNO3 has been investigated by GCD at current density 15 A.g -1 (Figure 6E). The results of figure 6E show that, the SRGO can be used effectively as a supercapacitor in 0.04 M KI/1.0 M NaNO3 electrolyte and provides up to 1.8 V. The stability of SRGO in KI-NaNO3 redox system was investigated by GCD at 25 A.g-1 for 1000 cycles as shown in Figure 6F. It is observed that the Cs values increase upon cycling in all the studied KI concentrations. A specific capacitance retention of 151% has been obtained in 0.08 M KI/1.0 M NaNO3 after 1000 cycles of repeated GCD test. It is observed during the stability experiment that the color of solution turned brown with cycling that indicates increasing the concentration of the liberated I2 molecules. It is suggested that the amount of adsorbed I2 increases leading to high capacitance value and this explains the increase in the Cs values upon cycling. Table (3) shows the values of Cs calculated from CV and GCD; IRdrop calculated from GCD curves, the specific capacitance percent retention after 1000 cycles of SRGO, energy density (E), power density (P), initial Coulombic efficiency (ηi) and Coulombic efficiency after 1000 cycles (η1000) in 0.02, 0.04, 0.06 and 0.08 M KI/1.0 M NaNO3.

[KI] (M)

Cs* (F.g-1)

Cs** (F.g-1)

IRdrop** (V)

E** (Wh.Kg-1)

P** (kW.Kg-1)

Cs¶ (%)

ηi (%)

η1000 (%)

0.0 0.02 0.04 0.06 0.08

214.9 489.7 859.7 906.8 798.8

48 176.2 718 896 1037.5

0.040 0.185 0.085 0.140 0.130

9 25.2 124 139.8 165

3.36 2.15 2.74 2.38 2.39

93.0 167 173 117 151

100 90.3 88.4 82.7 89.8

85.7 48.4 88.4 89.6 41

Table (3): The electrochemical properties of SRGO in the absence and the presence of various concentration of KI/1.0 M NaNO3 electrolyte solution. (*: measured at 2 mV.s-1 ; **: measured at 3.5 mA.cm-2 ; ¶: retention after 1000 cycles). A series of EIS measurements were performed in 0.02, 0.04, 0.06 and 0.08 M KI/1.0 M NaNO3 from 100 mHz to 100 KHz. Figure 7A shows the Nyquist plot of SRGO in the above-mentioned electrolytes. Contrary to the HQ/H3PO4 electrolyte, KI/NaNO3 system did not show a frequencydependent diffusion process. The Nyquist plot of SRGO in 0.02, 0.04, 0.06 and 0.08 M KI/1.0 M NaNO3 showed a semi-circle with relatively small diameter in the high-frequency region and a vertical line with nearly 90o-angle in the low-frequency region. The equivalent circuit used to fit the EIS data is shown in Figure 7B. Two constant phase elements represent EDLC and pseudo15 ACS Paragon Plus Environment


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capacitance (Ydl and Yp) in parallel with charge transfer resistance (Rct) and film resistance (Rf) respectively. It is noticed from the value of EIS elements tabulated in Table (4) that the solution resistance as well as the charge transfer resistance decrease upon using 0.08 M KI. From the EIS measurements, it is noticed that the double layer capacitance increases as the concentration of KI increases. The film resistance is the same in all concentration and equals 7.73 KΩ.

A)

B)

C)

D)

E)

F)


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Figure (6): A) CVs of SRGO in various concentrations of KI/1.0 M NaNO3 at scan rate of 20 mV.s-1, B) the relation between scan rate and the calculated Cs values of SRGO in various KI concentrations, C) GCD of SRGO in various HQ concentrations at current density of 20 A.g-1, D) the relation between the current density and the calculated Cs of SRGO in various KI concentrations, E) the effect of the upper and lower potential limits of 0.04 M KI/1.0 M NaNO3 at current density of 15 A.g-1, black line for lower potential limit -200 mV, red line for -300 mV, green line for -400mV, yellow line for -500 mV, blue line for -600 mV to 1000 mV; pink line for -600 to 1200 mV. (The inset is the relation between the calculated Cs values and the corresponding total voltages), and F) the cycling stability of SRGO in various KI concentrations. Moreover, the time necessary to discharge the capacitor to 36.8% which is known as the time constant (τ) provides information about the rate of the charging/discharging process. It can be calculated by finding the characteristic frequency, f* and using the following equation 30: τ=

(3.6)

The characteristic frequency is the frequency at which the resistive and capacitive impedance are equal at a phase angle of 45°. The larger f* and the smaller the τ value refer to relatively faster charge/discharge process. From the values of f* and τ in Table (4) that are calculated for all electrolytic solutions studied, we can conclude that as the concentration of KI increases, the f* increases and consequently τ decreases indicating faster charge/discharge process.

A)

B)

Figure (7): A) The Nyquist plot of SRGO in various concentrations of KI/1.0 M NaNO3 (measured from 100 mHz to 100 KHz, symbols and solid lines represent the experimental and the fitting data, respectively), and B) the equivalent circuit used for the fitting process of the EIS data.



[KI] Rs Cdl (M) (Ω) (mF) 0.02 12.1 0.13 0.04 19.8 0.156 0.06 20.5 0.256 0.08 12.7 0.456 (¶: calculated from Yp)

Rct (Ω) 6.3 3.0 2.99 2.77

Yp, m (mS.sm) 6.2, 0.808 6.4, 0.880 4.3, 0.812 4.3, 0.880

Cp¶ (mF) 2.9 3.7 1.6 2.3

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Rf (KΩ) 7.73 7.73 7.73 7.73

f*

τ

1.11 1.11 1.58 2.5

0.14 0.14 0.10 0.06

Table (4): The values of the EIS components calculated from the fitting of EIS measurements of SRGO in various KI concentrations as well as the calculated characteristic frequencies and time constants (data from Figure 7a). SEM images and elemental mapping have been used to track any morphological changes for SRGO before and after testing in both 0.04 M HQ/1.0 M H3PO4, and 0.04 M KI/1.0 M NaNO3. Figure 8 shows SEM images of freshly prepared SRGO before electrochemical test and after repeated 150 GCD cycles at current density of 25 A.g-1. It is noticed that, the particles of SrRuO3 are very small and hardly noticed due to the high folding of graphene sheets. The particle morphology was studied and presented in a previous study15. However some small aggregates have been observed on the freshly prepared SRGO surface. There are no significant surface changes between the freshly prepared surface and those after electrochemical tests in both solutions. The surfaces still maintain their integrities even after electrochemical test at relatively high current density indicating the high stability of SRGO in both electrolytes. However, after electrochemical test the particles of SrRuO3 are less aggregated especially after testing in KI/NaNO3 electrolyte. Moreover, some adsorbed I2 have been detected on the surface of SRGO after electrochemical test in KI/NaNO3 which confirms the possible adsorption of iodine as discussed earlier.


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(A)

SRO

CK

OK

Ru L

Sr L

(B)

SRO

CK

OK

Ru L

OK

Ru L

Sr L (C)

SRO

CK



Sr L

IL

Figure (8): SEM images and the corresponding elemental mapping of SRGO: A) before test, B) after repeated 150 GCD cycles in 0.04 M HQ/1.0 M H3PO4 at current density 25 A.g -1, and C) after repeated 150 GCD cycles in 0.04 M KI/1.0 M NaNO3 at current density 25 A.g-1. 4. Conclusion A further modification of SRGO composite through the addition of simple redox systems to, either the acidic or the neutral electrolyte, HQ/1.0 M H3PO4 or KI/1.0 M NaNO3 has been proposed in this study.

This electrolyte modification represents another method for the

supercapacitor modification that differs from the solid electrode surface modifications to achieve higher specific capacitance. We presented an extensive electrochemical study of SRGO supercapacitor in both redox systems-containing electrolytes. The presence of HQ in H3PO4 increased the Cs value to about 2.6 times compared to 1.0 M H3PO4. While a higher enhancement has been achieved upon the addition of KI to NaNO 3 electrolyte and the Cs value increased to 21.6 times compared to that measured in 1.0 M NaNO3. Moreover, the addition of the redox systems to the electrolyte solution not only enhanced the Cs value but also provided higher operating voltage of 1.4 V and 1.5 V for, HQ/1.0 M H3PO4 or KI/1.0 M NaNO3, respectively. Additionally, the energy storage mechanism has been changed from EDLC or pseudocapacitance to a hybrid energy storage mechanism (a combination between pseudocapacitance and batteries) upon the addition of the redox systems to the electrolyte solutions. It is also investigated that the enhancement in the Cs value of SRGO in HQ-containing 1.0 M H3PO4 arises from the combination of the pseudocapacitance of SRGO with the diffusioncontrolled faradaic reaction of Q/HQ transformation. While in the case of KI/1.0 M NaNO3 electrolyte, the adsorption of the liberated I 2 molecules on SRGO surface is the key step in the energy storage mechanism and is responsible for the great enhancement of the Cs value of SRGO in KI-containing NaNO3 electrolyte. The high adsorption tendency of I2 molecules as


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well as the small size of I - ions and their product compared to HQ and its product, Q, are responsible to the large enhancement in Cs using KI/NaNO3 system compared to HQ/H3PO4. Our proposed electrode, Ni/N/SRGO, in KI-containing NaNO3 electrolyte provided an excellent Cs value and stability compared to the other mentioned work as represented in Figure (9). Based on our results, Ni/N/SRGO in KI-containing NaNO3 may be a good candidate to be used for high capacitance and high voltage supercapacitor electrode.

*The value of current density mentioned in Ref [17] was converted from mA.cm2 to A. g-1 for comparison. Figure (9): A comparison between the Cs values of the current work with the other work concerning addition of redox systems to the electrolyte solution showing the higher Cs value of SRGO in KI-containing NaNO3 compared to the others. Acknowledgement The authors would like to acknowledge the partial financial support from Cairo University through the Vice President Office for Research Funds. References (1)

Cao, Y.; Lin, B.; Sun, Y.; Yang, H.; Zhang, X. Synthesis, Structure and Electrochemical Properties of Lanthanum Manganese Nanofibers Doped with Sr and Cu. J. Alloys Compd. 2015, 638, 204–213. 21 ACS Paragon Plus Environment


(2)

Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, 2nd ed.; Conway, B. E., Ed.; Plenum: New York, 1999.

(3)

Burke, A. Ultracapacitors: Why, How, and Where Is the Technology. J. Power Sources 2000, 91 (1), 37–50.

(4)

Hall, P. J.; Mirzaeian, M.; Fletcher, S. I.; Sillars, F. B.; Rennie, A. J. R.; Shitta-Bey, G. O.; Wilson, G.; Cruden, A.; Carter, R. Energy Storage in Electrochemical Capacitors: Designing Functional Materials to Improve Performance. Energy Environ. Sci. 2010, 3 (9), 1238–1251.

(5)

Winter, M.; Brodd, R. J. What Are Batteries , Fuel Cells , and Supercapacitors? Chem. Rev. 2004, 104 (10), 4245–4270.

(6)

Zhao, C.; Zheng, W.; Wang, X.; Zhang, H.; Cui, X.; Wang, H. Ultrahigh Capacitive Performance from Both Co(OH)2/graphene Electrode and K3 Fe(CN)6 Electrolyte. Sci. Rep. 2013, 3, 3–8.

(7)

Hassan, H. K.; Atta, N. F.; Hamed, M. M.; Galal, A.; Jacob, T. Ruthenium NanoparticlesModified Reduced Graphene Prepared by a Green Method for High-Performance Supercapacitor Application in Neutral Electrolyte. RSC Adv. 2017, 7 (19), 11286–11296.

(8)

Wang, H.; Casalongue, H. S.; Liang, Y.; Dai, H. Ni (OH)2 Nanoplates Grown on Graphene as Advanced Electrochemical Pseudocapacitor Materials. J. Am. Chem. Soc. 2010, 132 (21), 7472–7477.

(9)

Augustyn, V.; Come, J.; Lowe, M. A.; Kim, J. W.; Taberna, P.-L.; Tolbert, S. H.; Abruña, H. D.; Simon, P.; Dunn, B. High-Rate Electrochemical Energy Storage through Li+ Intercalation Pseudocapacitance. Nat. Mater. 2013, 12 (6), 518.

(10) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7 (11), 845–854. (11) Li, Z.; Wang, J.; Liu, S.; Liu, X.; Yang, S. Synthesis of Hydrothermally Reduced graphene/MnO2 Composites and Their Electrochemical Properties as Supercapacitors. J. Power Sources. 2011, 196 (19), 8160–8165. (12) Wang, P.; Liu, H.; Xu, Y.; Chen, Y.; Yang, J.; Tan, Q. Supported Ultrafine Ruthenium Oxides with Specific Capacitance up to 1099Fg−1 for a Supercapacitor. Electrochim. Acta. 2016, 194, 211–218. (13) He, X.; Xie, K.; Li, R.; Wu, M. Microwave-Assisted Synthesis of Ru/mesoporous Carbon Composites for Supercapacitors. Mater. Lett. 2014, 115, 96–99. (14) Lin, N.; Tian, J.; Shan, Z.; Chen, K.; Liao, W. Hydrothermal Synthesis of Hydrous Ruthenium Oxide/graphene Sheets for High-Performance Supercapacitors. Electrochim. Acta. 2013, 99, 219–224. (15) Galal, A.; Hassan; H. K.; Jacob, T; Atta, N. F. Enhancing the specific capacitance of SrRuO3 and reduced graphene oxide in NaNO3, H3PO4 and KOH electrolytes. Electrochim. Acta. 2018, 260, 738–747. (16) Wu, J.; Yu, H.; Fan, L.; Luo, G.; Lin, J.; Huang, M. A Simple and High-Effective Electrolyte Mediated with P-Phenylenediamine for Supercapacitor. J. Mater. Chem. 2012, 22 ACS Paragon Plus Environment

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22 (36), 19025–19030. (17) Senthilkumar, S. T.; Selvan, R. K.; Lee, Y. S.; Melo, J. S. Electric Double Layer Capacitor and Its Improved Specific Capacitance Using Redox Additive Electrolyte. J. Mater. Chem. A. 2013, 1 (4), 1086–1095. (18) Su, L.-H.; Zhang, X.-G.; Mi, C.-H.; Gao, B.; Liu, Y. Improvement of the Capacitive Performances for Co–Al Layered Double Hydroxide by Adding Hexacyanoferrate into the Electrolyte. Phys. Chem. Chem. Phys. 2009, 11 (13), 2195–2202. (19) Lota, G.; Milczarek, G. The Effect of Lignosulfonates as Electrolyte Additives on the Electrochemical Performance of Supercapacitors. Electrochem. Commun. 2011, 13 (5), 470–473. (20) Yu, H.; Wu, J.; Fan, L.; Lin, Y.; Xu, K.; Tang, Z.; Cheng, C.; Tang, S.; Lin, J.; Huang, M. A Novel Redox-Mediated Gel Polymer Electrolyte for High-Performance Supercapacitor. J. Power Sources. 2012, 198, 402–407. (21) Yu, H.; Wu, J.; Fan, L.; Xu, K.; Zhong, X.; Lin, Y.; Lin, J. Improvement of the Performance for Quasi-Solid-State Supercapacitor by Using PVA–KOH–KI Polymer Gel Electrolyte. Electrochim. Acta. 2011, 56 (20), 6881–6886. (22) Yu, H.; Fan, L.; Wu, J.; Lin, Y.; Huang, M.; Lin, J.; Lan, Z. Redox-Active Alkaline Electrolyte for Carbon-Based Supercapacitor with Pseudocapacitive Performance and Excellent Cyclability. RSC Adv. 2012, 2 (17), 6736–6740. (23) Chen, W.; Rakhi, R. B.; Alshareef, H. N. Capacitance Enhancement of Polyaniline Coated Curved-Graphene Supercapacitors in a Redox-Active Electrolyte. Nanoscale. 2013, 5 (10), 4134–4138. (24) Roldán, S.; Granda, M.; Menéndez, R.; Santamaría, R.; Blanco, C. Mechanisms of Energy Storage in Carbon-Based Supercapacitors Modified with a Quinoid Redox-Active Electrolyte. J. Phys. Chem. C. 2011, 115 (35), 17606–17611. (25) Park, J.; Kim, B.; Yoo, Y. E.; Chung, H.; Kim, W. Energy-Density Enhancement of Carbon-Nanotube-Based Supercapacitors with Redox Couple in Organic Electrolyte. ACS Appl. Mater. Interfaces. 2014, 6 (22), 19499–19503. (26) Atta, N. F.; Galal, A.; Ali, S. M. The Catalytic Activity of Ruthenates ARuO 3 (A= Ca, Sr or Ba) for the Hydrogen Evolution Reaction in Acidic Medium. Int. J. Electrochem. Sci. 2012, 7, 725–746. (27) Brousse, T.; Belanger, D.; Long, J. W. To Be or Not To Be Pseudocapacitive? J. Electrochem. Soc. 2015, 162 (5), A5185–A5189. (28) Béguin, F.; Presser, V.; Balducci, A.; Frackowiak, E. Carbons and Electrolytes for Advanced Supercapacitors. Adv. Mater. 2014, 26 (14), 2219–2251. (29) Fic, K.; Frackowiak, E.; Béguin, F. Unusual Energy Enhancement in Carbon-Based Electrochemical Capacitors. J. Mater. Chem. 2012, 22 (46), 24213–24223. (30) Balakrishnan, A., Subramanian, K. R. V. Nanostructured Ceramic Oxides for Supercapacitor Applications; CRC Press: Boca Roten, FL. USA, 2014.



List of Tables Table (1): The electrochemical properties of SRGO in the absence and in the presence of various concentration of HQ/1.0 M H3PO4 electrolyte solution. Table (2): The values of the EIS components calculated from the fitting of EIS measurements of SRGO in various HQ concentrations (data from Figure 4a). Table (3): The electrochemical properties of SRGO in the absence and the presence of various concentration of KI/1.0 M NaNO3 electrolyte solution. Table (4): The values of the EIS components calculated from the fitting of EIS measurements of SRGO in various KI concentrations as well as the calculated characteristic frequencies and time constants (data from Figure 7a).

List of Figures Figure (1): A) CVs of SRGO in the absence and the presence of 0.02 M HQ-containing 1.0 M H3PO4 at scan rate of 50 mV.s-1, B) the corresponding GCD curve measured at current density of 10 A.g-1, C) the CVs of SRGO in 0.02 M HQ/1.0 M H3PO4 at various scan rates: 2, 10, 20, 50, 100, 150 and 200 mV.s-1, and D) the relation between charging and discharging peak currents and square root of scan rate (inset is the relation between peak current and scan rate). Figure (2): A) CVs of SRGO in various concentrations of HQ/1.0 M H3PO4 at scan rate of 50 mV.s-1, B) the relation between scan rate and the calculated Cs values of SRGO in various HQ concentrations, C) GCD of SRGO in various HQ concentrations at current density of 10 A.g-1, and D) the relation between the current density and the calculated Cs of SRGO in various HQ concentrations. Figure (3): A) The effect of lower potential limits of SRGO in 0.04 M HQ/1.0 M H3PO4 at current density of 15 A.g-1, black line for lower potential limit -200 mV, red line for -300 mV, green line for -400mV, yellow line for -500 mV and blue line for -600 mV. (The inset is the relation between the calculated Cs value and the corresponding voltage), and B) the cycling stability of SRGO in various HQ concentrations. Figure (4): A) The Nyquist plot of SRGO in various concentration of HQ/1.0 M H3PO4 and (measured from 100 mHz to 100 KHz, symbols and solid lines represent the experimental and the fitting data, respectively), and B) the equivalent circuit used for the fitting process of the EIS data.


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Figure (5): A) CVs of SRGO in the absence and the presence of 0.04 M KI-containing 1.0 M NaNO3 at scan rate of 50 mV.s-1, and B) the corresponding GCD curve measured at current density of 15 A.g-1 showing the two processes involved in the energy storage mechanism Figure (6): A) CVs of SRGO in various concentrations of KI/1.0 M NaNO3 at scan rate of 20 mV.s-1, B) the relation between scan rate and the calculated Cs values of SRGO in various KI concentrations, C) GCD of SRGO in various HQ concentrations at current density of 20 A.g-1, D) the relation between the current density and the calculated Cs of SRGO in various KI concentrations, E) the effect of the upper and lower potential limits of 0.04 M KI/1.0 M NaNO3 at current density of 15 A.g-1, (the inset is the relation between the calculated Cs values and the corresponding total voltages), and F) the cycling stability of SRGO in various KI concentrations. Figure (7): A) The Nyquist plot of SRGO in various concentrations of KI/1.0 M NaNO3 (measured from 100 mHz to 100 KHz, symbols and solid lines represent the experimental and the fitting data, respectively), and B) the equivalent circuit used for the fitting process of the EIS data. Figure (8): SEM images and the corresponding elemental mapping of SRGO: A) before test, B) after repeated 150 GCD cycles in 0.04 M HQ/1.0 M H3PO4 at current density 25 A.g -1, and C) after repeated 150 GCD cycles in 0.04 M KI/1.0 M NaNO3 at current density 25 A.g-1. Figure (9): A comparison between the Cs values of the current work with the other work concerning addition of redox systems to the electrolyte solution showing the higher Cs value of SRGO in KI-containing NaNO3 compared to the others.



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Effect of Redox Electrolyte on the Specific Capacitance of SrRuO3

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