The XRD measurements for Co(OH) 2 /GNS before electrochemical test and after 2000 charge-discharge cycles, in the mixed 1 M KOH and 0.04 M K 3 Fe(CN) 6 solution, at the current density of 80 Ag−1, show that XRD patterns of Co(OH) 2 /GNS before electrochemical test and after 2000 cycles are almost the same and no new diffraction peaks are detected after 2000 cycles (Supporting material 1). Also, the morphologies for original Co(OH) 2 /GNS and Co(OH) 2 /GNS after 2000 cycles are nearly the same (supporting material 2). XRD and SEM measurements confirm that the novel electrode-electrolyte system is stable.

In order to compare the redox reactions for Co(OH) 2 /GNS before and after adding K 3 Fe(CN) 6 into KOH electrolyte, the cyclic voltammograms (CV) measurements have been performed. Figure 1(a)–(c) display for three systems: (1) Co(OH) 2 /GNS electrode in 1 M KOH aqueous solution (Co(OH) 2 /GNS-KOH); (2) platinum plate electrode in mixed 1 M KOH and 0.04 M K 3 Fe(CN) 6 aqueous solution (Pt-KOH/K 3 Fe(CN) 6 ); (3) Co(OH) 2 /GNS electrode in mixed 1 M KOH and 0.04 M K 3 Fe(CN) 6 aqueous solution (Co(OH) 2 /GNS-KOH/K 3 Fe(CN) 6 ), at different scan rates (2, 5, 10, 25 and 50 mVs−1) in the potential range of −0.2–0.5 V, respectively. Here, system 1 and 2 is chosen for comparison. For Co(OH) 2 active material as electrode, the oxidation and reduction reactions observe the following faradaic reactions:

For system 1 with Co(OH) 2 /GNS as electrode in 1 M KOH aqueous solution, as shown in Fig. 1(a), a reversible charge-transfer process is observed: the charging process corresponds to the oxidation of Co(OH) 2 to CoOOH, while the discharging process corresponds to the reduction of CoOOH to Co(OH) 2 . The oxidation and reduction peaks appear at 0.033 V and 0.022 V, respectively and the peak potential separation (ΔE P ) is 0.011 V at a scan rate of 2 mVs−1. The oxidation peak upshifts and the reduction peak downshifts slightly with increasing scan rate. In addition, the peak current of anodic oxidation almost equals to that of the cathodic reduction for each curve. Hence the Co(OH) 2 /GNS electrode exhibits a high electrochemical activity and good charge/discharge reversibility. This capacitance behavior can be explained by that GNS provides a high electrical conductivity and high specific surface area, allowing rapid and effective electron and ion transport, while Co(OH) 2 nanosheets grown on GNS realizes a fast charge transfer, providing a high electrochemical activity and redox reversibility. For system 2 with the platinum plate as electrode in mixed 1 M KOH and 0.04 M K 3 Fe(CN) 6 aqueous solution, as shown in Fig. 1(b), a pair of reversible redox peaks are observed, where the oxidation peak is related to the charging process of K 4 Fe(CN) 6 to K 3 Fe(CN) 6 and reduced peak is from the reverse process, corresponding to the following reactions:

The characteristic CV shape of K 3 Fe(CN) 6 is not significantly influenced as the scan rate is increased. The redox reaction on platinum electrode is a fast and reversible electrochemical process, indicating that K 3 Fe(CN) 6 has a high electrochemical activity. For system 3 with the Co(OH) 2 /GNS as electrode in mixed 1 M KOH and 0.04 M K 3 Fe(CN) 6 aqueous solution, as shown in Fig. 1(c), two symmetric anodic/cathodic pairs superimposed on a broad redox background are recognized, indicating that reversible redox reactions of Co(OH) 2 solid electrode and K 3 Fe(CN) 6 liquid electrolyte occur simultaneously and independently, according to the equations (1) and (2).

Figure 1 Cyclic voltammograms at different scan rates (2, 5, 10, 25 and 50 mVs−1) for system: Co(OH) 2 /GNS electrode in 1 M KOH solution (a), platinum electrode in mixed 1 M KOH and 0.04 M K 3 Fe(CN) 6 solution (b) and Co(OH) 2 /GNS electrode in mixed 1 M KOH and 0.04 M K 3 Fe(CN) 6 solution (c). Full size image

The K 3 Fe(CN) 6 -dependent capacitive properties can be determined from the charge-discharge potential vs. time curve. Figure 2(a) and 2(b) show the charge-discharge potential vs. time curve for system 1 or 3 at high current density of 16 or 32 Ag−1 in the potential range of −0.1–0.45 V. From Fig. 2(a), the potential plateaus at about 0.05 V in charging process and 0.00 V in discharging processes correspond to the oxidation of Co(OH) 2 and the reduction of CoOOH, respectively. From Fig. 2(b), besides the potential plateaus at about 0.025 V in charging process and −0.025 V in discharging processes, corresponding to the redox reaction of Co(OH) 2 , there are other potential plateaus at about 0.25 V in charging process and 0.18 V in discharging process, corresponding to the redox reaction of K 3 Fe(CN) 6 . The potential plateaus result from a lot of exchanges occurred between electrons, corresponding to the anodic and cathodic peaks in the CV curves. The specific capacitance and coulombic efficiency for the two systems can be determined based on the equation (3) and (4), respectively. From the galvanostatic charge-discharge curve at 16 Ag−1 (32 Ag−1), the specific capacitance for system 1 is 567.3 Fg−1 (517.8 Fg−1), while that for system 3 is 2434.9 Fg−1 (1733.0 Fg−1). The coulombic efficiency for system 1 is 93.2% (90.6%) and 163.8% (121.6%) for system 3. The high specific capacitance for system 3 is due to that both Co(OH) 2 and K 3 Fe(CN) 6 directly do contribute the pseudocapacitance to the system 3, while only Co(OH) 2 gives a contribution of pseudocapacitance to system 1. As for the high coulombic efficiency for system 3, it can be explained by that for the initial additive of Fe(CN) 6 3−, according to the equation (2), in discharging process, only part of Fe(CN) 6 3− gains an electron and is reduced into Fe(CN) 6 4−, while in charging process, Fe(CN) 6 4− loses an electron and is completely oxidized into Fe(CN) 6 3−. Since the released charges include both discharging and no discharging Fe(CN) 6 3−, which can be much more than reserved charges in charging process. This is the reason why the coulombic efficiency is larger than 100%. For example, the electric charge with a quantity of 77.2 C for system 3 can be completely released from 0.04 M K 3 Fe(CN) 6 in 20 mL solution, if all K 3 Fe(CN) 6 is reduced into K 4 Fe(CN) 6 . In fact, in a single charge-discharge process for K 3 Fe(CN) 6 electrolyte, the electric charge with a quantity of 0.024 C is charged accompanied by the change of K 4 Fe(CN) 6 into K 3 Fe(CN) 6 and 0.027 C is discharged accompanied by the change of K 3 Fe(CN) 6 into K 4 Fe(CN) 6 for system 2 (both positive and negative electrodes consist of platinum plate). After each cycling, the K 3 Fe(CN) 6 concentration in KOH electrolyte changes slightly (only 0.04‰ of K 3 Fe(CN) 6 after each cycling is consumed) compared with that at the initial state, demonstrating that KOH/K 3 Fe(CN) 6 electrolyte is similar to a constant buffer solution. Hence the high coulombic efficiency (more than 100%) can be maintained for a long time. In particular, if the pseudocapacitance mainly comes from Co(OH) 2 , rather than K 3 Fe(CN) 6 , the K 3 Fe(CN) 6 will be consumed slowly, resulting in a long electrochemical stability for system 3. However, the coulombic efficiency decreases with increasing cycle number, which can be attributed to the consumption of Fe(CN) 6 3− in each discharging process. The high coulombic efficiency is essential for a battery-type supercapacitor device with a high energy density. In the practical application, the high coulombic efficiency can be recovered via only charging the electrolyte containing K 4 Fe(CN) 6 . As the electrolyte is applied at a high constant potential, Fe(CN) 6 4− will be oxidized completely. As a result, all K 4 Fe(CN) 6 can be transformed into K 3 Fe(CN) 6 and the high coulombic efficiency can be recovered (Supporting materials 3).

Figure 2 Galvanostatic charge-discharge curves at different current densities for Co(OH) 2 /GNS electrode in 1 M KOH solution (a) and Co(OH) 2 /GNS electrode in mixed 1 M KOH and 0.04 M K 3 Fe(CN) 6 solution (b). Full size image

The electrochemical stability is a very important factor for determining the capacitive properties of pseudocapacitors with introduction of additive into electrolyte. The cycling stability for either system 1 or 3 is examined by continuous charge-discharge experiments for 2000 cycles at a high current density of 80 Ag−1, as shown in Fig. 3(a). Both systems exhibit a high cycling stability, with retention of 94.4% and 91.1% initial capacitance after 2000 cycles. This indicates that either system 1 or 3 is stable and the addition of K 3 Fe(CN) 6 into KOH electrolyte does not obviously affect the stability of the electrode material, which is consistent with the results from XRD and SEM. The cycling stability of system 3 is also performed at 80 Ag−1 for 20000 continuous charge-discharge cycles, as shown in Fig. 3(b), from which the system maintains a high specific capacitance of 610.9 Fg−1 after 20000 cycles and retention of 75.0% the initial specific capacitance (814.5 Fg−1), indicating that the novel Co(OH) 2 /GNS-KOH/K 3 Fe(CN) 6 system exhibits a high electrochemical activity and excellent cycling stability.

Figure 3 Cycling performance of Co(OH) 2 /GNS electrode in 1 M KOH and Co(OH) 2 /GNS electrode in mixed 1 M KOH and 0.04 M K 3 Fe(CN) 6 solution for 2000 cycles (a) and Co(OH) 2 /GNS electrode in mixed 1 M KOH and 0.04 M K 3 Fe(CN) 6 solution for 20000 cycles (b), measured by using the galvanostatic charge-discharge technique at a current density of 80 Ag−1. Full size image

The comparison in electrical conductivity between system 1 and 3, determined by the electrochemical impedance spectroscopy (EIS) measurements, is shown in Fig. 4, from which system 3 possesses a smaller interfacial charge-transfer resistance (R ct , calculated from the span of the single semi-circle along the x-axis from the high to low frequency region) and lower internal resistance (R i , the intersecting point with the x-axis in the range of high frequency), compared to system 1. Such a pattern of the plots can be fitted by that from an equivalent circuit in the inset of Fig. 4, in which R ct = 0.1 mΩ and R i = 300.9 mΩ for system 3, while R ct = 10.0 mΩ and R i = 500.2 mΩ for system 126,27. The lower resistance for system 3 can afford a facile ionic and electronic transfer to ensure a high capacitive performance. Clearly, the electrochemical performances for system 3 are enhanced remarkably after introducing the redox mediator of K 3 Fe(CN) 6 into KOH electrolyte.

Figure 4 Nyquist plots of Co(OH) 2 /GNS electrode in 1 M KOH (a) and Co(OH) 2 /GNS electrode in mixed 1 M KOH and 0.04 M K 3 Fe(CN) 6 solution (b), in which the inset shows the equivalent circuit. Full size image

Finally, the effect of the K 3 Fe(CN) 6 concentration in KOH electrolyte on the pesudocapacitive properties for system 3 is explored and the CV curves for system 3 with a concentration K 3 Fe(CN) 6 of 0.00, 0.01, 0.02, 0.04, 0.06 and 0.08 M, respectively, at a scan rate of 25 mVs−1, are shown in Fig. 5(a). The peak current from Co(OH) 2 decreases slightly first, then increases and finally almost keeps constant and that from K 3 Fe(CN) 6 increases obviously with increasing the K 3 Fe(CN) 6 concentration (Supporting materials 4), indicating the capacitance contributed by Co(OH) 2 decreases slightly first, then increases and finally almost keeps constant. In contrast, the capacitance contributed by K 3 Fe(CN) 6 increases obviously. The peak currents for the oxidation of Co(OH) 2 and reduction of CoOOH are depressed, resulting from the adsorption of Fe(CN) 6 3−/Fe(CN) 6 4− on the Co(OH) 2 /GNS electrode surface, which suppresses the OH− diffusion. This suppression leads to a shortage of OH−, decreasing the reaction rate of Co(OH) 2 /CoOOH. On the other hand, upon further increasing the K 3 Fe(CN) 6 concentration, more Fe(CN) 6 3−/Fe(CN) 6 4− ions play the role of “electron shuttle” in the charge/discharge processes of Co(OH) 2 /CoOOH, promoting a high activity of Co(OH) 2 /CoOOH24. This promotion results in an increase in peak current. As above two effects are balanced, the peak current keeps almost constant. In addition, anodic and cathodic peak currents corresponding to redox reaction of Co(OH) 2 /CoOOH downshift, which is also caused by electrostatic adsorption, the more Fe(CN) 6 3−/Fe(CN) 6 4− ions are adsorbed on the Co(OH) 2 electrode surface, the larger the resistance to OH− diffusion is. In Fig. 5(b), as the K 3 Fe(CN) 6 concentration in system 3 is 0.01, 0.02, 0.04, 0.06 and 0.08 M, respectively, the specific capacitance, evaluated at a high current density of 16 Ag−1 from charge-discharge curves, will be 852.4, 1093.8, 2434.9, 4983.3 and 7514.2 Fg−1, respectively, corresponding to the coulombic efficiency of 103.9%, 125.8%, 163.8%, 285.5% and 541.4%, respectively (charge and discharge time are listed in Table 1). The system can be charged quickly and discharged slowly, which means that the promise can be offered to realize a battery-type supercapacitor. In addition, if the high-rate discharge ability (HRD) of the electrode is defined as the ratio of discharge capacitance at 32 Ag−1 to that at 16 Ag−1, the HRD for system 3 containing K 3 Fe(CN) 6 with a concentration of 0.01, 0.02, 0.04, 0.06 and 0.08 M, respectively, will be 86.0%, 80.9%, 71.2%, 52.8% and 30.4%, respectively. The typical data about the specific capacitance and HRD for system 3 containing various of K 3 Fe(CN) 6 concentrations are shown in Fig. 5(c). When the K 3 Fe(CN) 6 concentration is lower, the contribution of the redox reaction from K 3 Fe(CN) 6 is lower, resulting in a low specific capacitance. However, high concentration and charge-discharge current density will cause high concentration polarization, leading to a low rate property and poor electrochemical stability. From above, the K 3 Fe(CN) 6 concentration has a significant influence on the specific capacitance, coulombic efficiency and rate property. High K 3 Fe(CN) 6 concentration facilitates a high specific capacitance and coulombic efficiency, but worsens a rate property and electrochemical stability. Therefore, only the K 3 Fe(CN) 6 concentration is proper, the energy density, power density, coulombic efficiency and long-term cycle life can be compromised. For one system, a high specific capacitance indicates a high energy density, while a high power density associates with a large current charge-discharge characteristic. Through well controlling the K 3 Fe(CN) 6 concentration, a novel system with high energy density, power density, coulombic efficiency and long-term cycle life can be realized.

Table 1 Charge and discharge time for Co(OH) 2 /GNS electrode in 1 M KOH containing different concentrations of K 3 Fe(CN) 6 mixed solution, calculated from galvanostatic charge-discharge curves at a current density of 16 Ag−1 Full size table