An ionic aqueous pseudocapacitor system: electroactive ions in both a salt electrode and redox electrolyte

Abstract

We designed a new type of ionic pseudocapacitor system with excellent contributions from ionic-state redox mediators, including a redox couple [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ in the electrolyte and redox cations in highly electroactive colloid electrodes. The highest pseudocapacitance value of 12 658 mF cm⁻² was obtained, which is a 7-fold increase in the specific capacitance of the CoCl₂ electrode for K₃Fe(CN)₆ in a KOH alkaline electrolyte at a current density of 20 mA cm⁻² and at a potential interval of 0.55 V. The currently designed supercapacitor systems show a versatile strategy to design high-performance supercapacitors with CoCl₂, CuCl₂, NiCl₂, and FeCl₃ electrodes.

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Supercapacitors can deliver energy at higher rates than batteries and maintain their specific power for extended periods.¹ Inorganic pseudocapacitors consisting of a transition metal oxide/hydroxide electrode and an aqueous electrolyte function as a type of an energy storage device; their specific capacitance originates from the redox reaction of active cations with fast charge–discharge rates.^2,3 The synthetic transition metal oxide/hydroxide-based solid materials were broadly selected as electrode materials of pseudocapacitors due to the fact that most oxides and hydroxides have multiple oxidation states.^2,4 The theoretical specific capacitance of a solid material is calculated on the basis of fully utilizing the involved active cations in Faradaic redox reactions.^5,6 However, the condensed status of transitional metal cations failed to fully release them as electroactive cations. To solve this problem, we proposed a new concept of an ionic pseudocapacitor system by designing water-soluble inorganic salts electrodes with the direct use of commercial inorganic salts as electrode materials in an alkaline electrolyte. In this designed system, highly electroactive colloids can be formed by electric-field-assisted chemical coprecipitation. Inorganic salt pseudocapacitors can show ultrahigh capacitance because the active cations can be utilized to their maximum extent possible in this existing status.

Systems materials engineering emphasizes the study of the interactions between individual components of a system.⁷ A supercapacitor is a multiple component system that requires positive and negative electrodes to be precisely coupled with an aqueous electrolyte. This stimulates us to design an electrolyte system for further increasing the specific capacitance of pseudocapacitors except for the electrode system. One high-efficiency strategy to promote rapid Faradaic reactions is the utilization of the redox electrolyte.^8–11 These increases in capacitance were attributed to the rapid Faradaic reactions at the electrode/electrolyte interface from the introduced mediators (redox pairs) in electrolytes, which augmented the additional pseudocapacitive contribution to the system. A water-soluble redox couple Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ as the aqueous cathode and the redox electrolyte has been reported, which can store electric charge in an alkaline aqueous electrolyte.^11–14 Therefore, we can utilize the Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox couple in the electrolyte to further increase the electrochemical performance of our designed ionic pseudocapacitor system.

Herein, we proposed a new concept of an ionic pseudocapacitor system, which excellently utilizes two types of ionic-state redox mediators: (1) redox couple [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ in the electrolyte, and (2) redox cations in highly electroactive colloid electrode. The electrochemical reactions at the electrode can be written as follows:

redox electrolyte [Fe(CN)₆]³⁻ + e⁻ ↔ [Fe(CN)₆]⁴⁻ (1)

redox electrode M^z+ ↔ M^(z+n)+ + ne⁻ (2)

where M represents Co²⁺, Cu²⁺, Fe³⁺ or Ni²⁺ cations, and 1 ≤ n ≤ z. In this work, CoCl₂·6H₂O, CuCl₂·2H₂O, FeCl₃·6H₂O, or NiCl₂·6H₂O, respectively, were used as starting electrode materials, and K₃Fe(CN)₆/KOH as the redox electrolyte. We demonstrated that pseudocapacitance can be achieved in our designed redox electrolyte and redox electrode systems, which suggests that it is an attractive way for charge storage in a liquid electrolyte and an inorganic salt electrode system.

The schematic of the new concept of an ionic pseudocapacitor system is shown in Scheme 1, which displays an example of the inorganic salt electrode in alkaline K₃Fe(CN)₆ aqueous electrolyte. First, inorganic salts were transformed to electroactive hydroxide/oxide colloids by electric-field-assisted chemical coprecipitation in a KOH solution (Scheme 1a). The electroactive cations in the inorganic salt electrode were transformed from the free-ion state to electroactive media. Therefore, pseudocapacitance can be obtained from electroactive hydroxide/oxide colloids at the electrode. With the introduction of Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox couple to the KOH aqueous electrolyte, an additional capacitance contribution to inorganic salt pseudocapacitors can be obtained (Scheme 1b). In our designed system, pseudocapacitance originated from two types of charge storage reactions in the redox electrode and redox electrolyte, both of which have electroactive ions.

Scheme 1 Schematic of the new concept of an ionic pseudocapacitor system. An example of an inorganic salt electrode in K₃Fe(CN)₆ electrolyte is shown. (a) The cations in inorganic salts were transformed to electroactive hydroxide colloids by electric-field-assisted chemical coprecipitation in a KOH aqueous solution. (b) Enhanced capacitance originates from the inorganic salt electrode and redox electrolyte. In the presence of Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ couple in the redox electrolyte, an additional capacitance contribution can be obtained.

Working electrodes of supercapacitors were prepared by mixing transition metal chloride salts (CoCl₂·6H₂O, CuCl₂·2H₂O NiCl₂·6H₂O, or FeCl₃·6H₂O), carbon black and poly(vinylidene fluoride) (PVDF) in a weight ratio of 70 : 20 : 10. The detail experimental method can be found in ESI.† Metal chloride salts can be directly used in the electrochemical measurement, neglecting complex synthesis procedure. All electrochemical experiments were carried out using a classical three-electrode cell configuration in 2 M KOH and an electrolyte mixture of K₃Fe(CN)₆ and KOH aqueous solution.

Fig. 1a and b show the cyclic voltammograms (CV) and galvanostatic charge–discharge curves of CoCl₂ salt electrodes in an electrolyte mixture of 2 M KOH and K₃Fe(CN)₆ of different concentrations. Without the use of K₃Fe(CN)₆ in the KOH electrolyte, distinct redox peaks can be found, which originate from the Faradaic reaction of electroactive Co(OH)₂ or Co₃O₄ colloids.¹⁵ After the CoCl₂ salt electrode measured in KOH solution, electric-field-assisted chemical coprecipitation occurred immediately accompanying the occurrence of the Faradaic redox reaction. Then, Co(OH)₂ or Co₃O₄ colloids were immediately formed by the coprecipitation of CoCl₂ salts in the KOH solution. After adding K₃Fe(CN)₆ into KOH electrolyte, the intensities of redox peaks are significantly increased with the change of peak potentials (Fig. 1a), indicating the occurrence of an additional redox reaction in this supercapacitors system. The redox peaks from the redox electrolyte and redox electrode overlapped, and only one pair of redox peak was found in CV curves. The characteristic CV shape was not significantly influenced with the increase in the concentration of K₃Fe(CN)₆. In addition, the intensity and area of redox peaks increased with the concentration of K₃Fe(CN)₆. The result confirmed that the specific capacitance increased with the increase in the concentration of K₃Fe(CN)₆. The enhanced specific capacitance of the CoCl₂ electrode originated from the redox reaction of Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ (eqn (1)). Besides the redox reaction of Co²⁺/Co³⁺, the Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ couple gave an additional capacitance contribution to the inorganic salt pseudocapacitors.

Fig. 1 Electrochemical performance of CoCl₂ salt electrodes in electrolyte mixture of 2 M KOH and different concentrations of K₃Fe(CN)₆. (a) CV curves (current density versus potential) at a scan rate of 3 mV s⁻¹ and a potential range from −0.2 to 0.45 V. A pair of redox peaks is present in the CV curves, indicating pseudocapacitive characteristic. (b) Charge–discharge curves (time versus potential) measured at a current density of 20 mA cm⁻² and at a potential interval of 0.55 V. (c) Specific capacitance vs. current density in different mixture electrolytes.

The non-linear galvanostatic charge–discharge curves also prove the presence of redox reactions in the pseudocapacitor system (Fig. 1b). The discharge curves were significantly enhanced with adding K₃Fe(CN)₆, indicating the presence of new redox couples in the present pseudocapacitor system. The additional Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox couple in the electrolyte significantly enhanced the discharge process of the pseudocapacitor system.

Because the pseudocapacitive reaction (1) occurred at the electrode/electrolyte interface, to evaluate the actual capacitance contribution from both electrode and electrolyte, specific areal capacitances were used in this work. The specific capacitance of the system was calculated from galvanostatic charge–discharge curves according to the equation S_c = IΔt/AΔV, where I (A) is the current used for charge–discharge, Δt (s) is the time elapsed for the discharge cycle, A (cm²) is the area of the electrode, and ΔV is the potential interval of the discharge. The specific capacitances calculated from galvanostatic charge–discharge curves are shown in Fig. 1c. The specific capacitance is enhanced with the addition of a higher concentration of K₃Fe(CN)₆. The specific capacitances are 1676, 2567, 4174, 6578 and 12 658 mF cm⁻², respectively, when adding 0, 0.05, 0.1, 0.2 and 0.3 M K₃Fe(CN)₆ to the KOH electrolyte, at a current density of 20 mA cm⁻² and at a potential interval of 0.55 V. We realized a 7-fold increase in the specific capacitance of the CoCl₂ electrode in 0.3 M K₃Fe(CN)₆ and 2 M KOH alkaline electrolyte compared with only the 2 M KOH electrolyte (Table 1).

Table 1 Comparison of the capacitance of different electrodes in the 2 M KOH electrolyte and mixed 2 M KOH and 0.3 M K₃Fe(CN)₆ solution

Electrode	Potential window (V)	Capacitance (mF cm^−2)		Increment in capacitance
		KOH	KOH/K3Fe(CN)6
CoCl2	0.55	1676	12 658	755%
CuCl2	0.45	5031	25 253	502%
NiCl2	0.45	3280	12 569	383%
FeCl3	0.40	2965	12 680	428%

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The electrochemical performance of the CoCl₂ electrode in 0.3 M K₃Fe(CN)₆ and 2 M KOH electrolyte is shown in Fig. 2. With the increase in the scan rate, the oxidation peak upshifts, and the reduction peak downshifts slightly. Fig. 2b shows the charge–discharge curves of CoCl₂ electrodes at various current densities. The non-linear discharge curves are the reflection of Faradaic redox reactions of pseudocapacitors. Specific capacitances of 8635, 6443, 5500, 4920, 3273 and 1091 mF cm⁻² were obtained from the discharge curves at current densities of 30, 40, 50, 60, 100, and 200 mA cm⁻², respectively. Specific capacitance decreased with the increase in the current density, where the electrons and ions cannot efficiently transfer into the electrodes due to diffusion limitation at high current density.¹⁶

Fig. 2 Electrochemical performance of the CoCl₂ electrode in 0.3 M K₃Fe(CN)₆ and 2 M KOH electrolyte. (a) CV curves at various scan rates and a potential range from −0.2 to 0.45 V. (b) Charge–discharge curves measured at various current densities and at a potential interval of 0.55 V.

To prove the formation of Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox couple in the electrolyte, we study the aqueous electrolyte before and after electrochemical measurement. The physical characterization of the redox electrolyte was performed in an effort to understand the mechanism of its electrochemical activity. The electrolyte was examined using Fourier transform infrared spectroscopy (FT-IR, as shown in Fig. 3 and S1†). The 2 M KOH, 0.3 M K₃Fe(CN)₆ and 0.3 M K₄Fe(CN)₆ aqueous electrolytes were first examined. Then, the mixture of KOH and K₃Fe(CN)₆ electrolyte was examined before and after charge–discharge cycling. Fig. S1† shows the global view of infrared spectra with wavenumber range from 500 to 4000 cm⁻¹, showing strong bands of water at 3354, 1635 and 650 cm⁻¹. Fig. 3 shows the enlarged view of infrared spectroscopy with a wavenumber range from 2200 to 1900 cm⁻¹. For FTIR measurements, ferro-/ferricyanide is excellent for characterization, since the C≡N stretching modes of the two members of the redox couple have relatively large absorptivities, producing bands that are well separated in wavenumber and are easy to detect.^17,18 The infrared stretching frequency of Fe^III–CN was found to be 2114 cm⁻¹, while the stretching frequency of Fe^II–CN was 2039 cm⁻¹. After the initial electrochemical reaction, [Fe(CN)₆]³⁻ was reduced to [Fe(CN)₆]⁴⁻; thus, the redox couple of [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ was formed. The redox reactions (1) can occur at the surface of electrode, which contributes additional capacitance to inorganic salt pseudocapacitors.

Fig. 3 Fourier transform infrared spectra of K₃Fe(CN)₆ and KOH aqueous electrolyte. Infrared spectra of the mixing electrolyte including 0.3 M K₃Fe(CN)₆ and 2 M KOH after (a) and before (b) the electrochemical test, and (c) 0.3 M K₄Fe(CN)₆, (d) 0.3 M K₃Fe(CN)₆, and (e) 2 M KOH aqueous solution. The infrared stretching frequency of Fe^III–CN was found to be 2114 cm⁻¹, while the stretching frequency of Fe^II–CN was 2039 cm⁻¹.

To prove the generality of our ionic pseudocapacitor system, we studied the electrochemical performances of CuCl₂, NiCl₂ and FeCl₃ electrodes in an alkaline redox electrolyte of K₃Fe(CN)₆. As shown in Fig. 4, without adding K₃Fe(CN)₆ to the KOH electrolyte, redox peaks are found in all CV curves, which indicate that the inorganic salts were transformed to electroactive hydroxide/oxide colloids. After adding K₃Fe(CN)₆ to the KOH electrolyte, a pair of reversible redox peaks is also observed in CV curves, where the oxidation peak is related to the charging process of K₄Fe(CN)₆ to K₃Fe(CN)₆ and M^z+ to M^(z+n)+ (M = Cu, Ni, Fe), and the reduced peak is from the reverse process.

Fig. 4 Electrochemical performance of CuCl₂ (a), NiCl₂ (b) and FeCl₃ (c) electrodes in different concentrations of K₃Fe(CN)₆ and 2 M KOH. CV curves (current density versus potential) obtained at a scan rate of 3 mV s⁻¹ and a potential range from −0.1 to 0.45 V.

The galvanostatic charge–discharge curves of CuCl₂, NiCl₂ and FeCl₃ electrodes are shown in Fig. 5a–c. The non-linear charge–discharge curves confirmed that the capacitance originated from the pseudocapacitance of reactive cations in the electrode and the Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox couple in the electrolyte. The specific capacitances are significantly increased with the increase in the concentration of K₃Fe(CN)₆. Specific capacitances of 25 253, 12 569 and 12 680 mF cm⁻² were obtained from the discharge curves of CuCl₂, NiCl₂ and FeCl₃ electrodes in 0.3 M K₃Fe(CN)₆ and 2 M KOH electrolyte at a current density of 20 mA cm⁻² (Table 1). In only KOH electrolyte, specific capacitances of 5031, 3280 and 2965 mF cm⁻² were obtained from the discharge curves of CuCl₂, NiCl₂ and FeCl₃ electrodes at a current density of 20 mA cm⁻² (Table 1). We can obtain 5-fold, 4-fold and 4-fold increase in capacitance for CuCl₂, NiCl₂ and FeCl₃ electrodes in the redox electrolyte, respectively.

Fig. 5 Electrochemical performances of CuCl₂ (a), NiCl₂ (b) and FeCl₃ (c) electrodes in different concentrations of K₃Fe(CN)₆ and 2 M KOH. (a and b) Charge–discharge curves (time versus potential) measured at a current density of 20 mA cm⁻² and at a potential interval of 0.45 V. (c) Charge–discharge curves measured at a current density of 20 mA cm⁻² and at a potential interval of 0.4 V.

To further evaluate the electrochemical performance of these salt electrodes, we performed cyclic voltammetry and galvanostatic charge–discharge tests at different scan rates and discharge–charge current densities. As shown in Fig. 6a, the oxidation peaks shift to the positive direction, and the reduction peaks downshift in CV curves with the increase in the scan rate. With different current densities, non-linear charge–discharge curves can be found in Fig. 6b. Area capacitances of CuCl₂, NiCl₂ and FeCl₃ electrodes vs. concentration of K₃Fe(CN)₆ and current density are shown in Tables S1–S3.† The capacitances increase with the increase in the concentration of K₃Fe(CN)₆, while the capacitance values decrease with the current density. It should be noted the Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox couple significantly enhanced the discharge process. All discharge times are greater than that the charge time. This result is important for the practical application of a pseudocapacitor, where we can obtain more energy in short charge time. The enhanced capacitance in our ionic pseudocapacitor system originated from the inorganic salt electrode and redox electrolyte. The new ionic pseudocapacitor system can be a versatile strategy to design high-performance supercapacitors.^19–21

Fig. 6 Electrochemical performances of the CuCl₂ electrode in 0.3 M K₃Fe(CN)₆ and 2 M KOH electrolyte. (a) CV curves (current density versus potential) at different scan rates and a potential range from −0.1 to 0.45 V. (b) Charge–discharge curves (time versus potential) measured at various current densities and at a potential interval of 0.45 V.

In conclusion, we demonstrated a new type of an ionic pseudocapacitor system with excellent contributions of ionic-state redox mediators, including redox couple[Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ in the electrolyte and redox cations in highly electroactive colloid electrode. In our designed system, pseudocapacitance originated from two types of charge storage reactions in redox electrode and redox electrolyte, both of which have highly electroactive ions. We realized a 7-fold, 5-fold, 4-fold and 4-fold increase in specific capacitance of CoCl₂, CuCl₂, NiCl₂ and FeCl₃ electrodes in 0.3 M K₃Fe(CN)₆ and 2 M KOH alkaline electrolyte compared with only 2 M KOH electrolyte. The K₃Fe(CN)₆ redox electrolyte highly enhanced the discharge process so that a coulombic efficiency >100% of inorganic salt electrodes was obtained. The improvement is attributed to the synergistic effect between electrolyte ions and electroactive cations in the electrode. The currently designed novel supercapacitor systems show a versatile strategy to design high-performance supercapacitors, which has been validated in CoCl₂, CuCl₂, NiCl₂ and FeCl₃ electrodes.

Financial support from the National Natural Science Foundation of China (grant no. 50872016, 20973033 and 51125009), the National Natural Science Foundation for Creative Research Group (grant no. 21221061) and the Hundred Talents Program of the Chinese Academy of Sciences is acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1 and Table S1–S3. See DOI: 10.1039/c4ra03037k

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