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Incorporating redox additives in sodium hydroxide electrolyte for energy storage device with the nickel cobalt molybdenum oxide active material
Journal of Energy Storage 25 (2019) 100823
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Incorporating redox additives in sodium hydroxide electrolyte for energy storage device with the nickel cobalt molybdenum oxide active material Kuan-Hsien Lina,1, Lu-Yin Lina,b, a b
⁎,1
T
, Wei-Lun Honga
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1 Sec. 3, Zhongxiao E. Rd., Taipei, 10608, Taiwan Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors, Taiwan
A R T I C LE I N FO
A B S T R A C T
Keywords: Nickel cobalt molybdenum oxide Battery supercapacitor hybrid Electrolyte Energy storage Redox additive
To enhance the redox reactions and improve the energy storage ability, redox additives have been widely incorporated in the electrolyte to assemble the battery supercapacitor hybrid (BSH). However, due to the possible generation of numerous redox reactions for the metal oxide with abundant redox states, it is limited to apply the redox additives in the multiple metal oxide system for producing more redox reactions and promoting the energy storage. In this study, two redox additives of p-phenylenediamine (PPD) and K3[Fe(CN)6] are incorporated in KOH electrolyte for fabricating BSH with the nickel cobalt molybdenum oxide active material. The addition of PPD in the electrolyte causes the reduction of the specific capacitance (CF) for the BSH, while the greatly enhanced CF value is obtained for the BSH applying K3[Fe(CN)6] as the redox additive in the electrolyte. With the optimized K3[Fe(CN)6] concentration in the electrolyte, the BSH shows the CF value of 3.13 F/cm2 at 10 mV/s along with the maximum energy density of 48.0 Wh/kg at 756.0 W/kg. The Coulombic efficiency higher than 95% and capacitance retention of 75% are obtained for the optimized BSH. This work firstly achieves the opposite results for the BSH with different sorts of redox additives in electrolyte. It is suggested that the ion size and the compatibility between the ions in the electrolyte and the pores in the active material are important to decide the capability for energy storage.
1. Introduction The energy issue has been intensively addressed using numerous ways including generating energies via green routes, converting pollutions to useful energies, and storing energies efficiently [1–5]. Nickel cobalt oxides have been intensively used as the active material of battery supercapacitor hybrid devices (BSH) [6–9]. Primary studies for enhancing the energy storage ability of BSH is to refine the properties of the active material, such as designing efficient morphologies [10–13] or combining several materials [14–16] to produce synergic effects. However, the electrolyte for the BSH also play indispensable roles on the energy storage ability. The electrolyte is responsible for providing ions for carrying out the redox reactions and building the electric field to induce ions adsorption [17–19]. Also, the state of the electrolyte such as liquid, gel or solid states can determine the viscosity, the ionic conductivity and the hence the possible cycling stability of the BSH [20–22]. Adding extra redox additives in the electrolyte has been intensively verified to enhance the capacitance of the BSH by generating
more Faradaic redox reactions [23–25]. The redox additive was previously added in the electrolyte for the metal compound systems to enhance the electrochemical performance [26–28]. Among the numerous redox additives used for BSH, the watersoluble redox couple Fe(CN)63−/Fe(CN)64− has been widely selected as the redox additive in the electrolyte, due to the low toxicity, the high reversibility, and the high stability [29,30]. Su et al. added hexacyanoferrate(II) and (III) into 1 M KOH to reduce the charge-transfer resistance and increase the exchange current density, and hence to improve the capacitive properties of the cobalt aluminum layered double hydroxide electrode [31]. Ma et al. prepared gel polymer polyvinyl alcohol (PVA)-KOH-K3[Fe(CN)6] for solid-state supercapacitor with the activated carbon electrode to attain flexible, high ionic conductivity and wide potential properties [32]. Chen et al. developed a graphene paper electrode in the redox-electrolyte of K3[Fe (CN)6] to achieve both the electric double-layer capacitance and pseudo-capacitance [33]. On the other hand, the p-phenylenediamine (PPD) is consider to be one of the effective organic redox mediators
⁎
Corresponding author at: Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1 Sec. 3, Zhongxiao E. Rd., Taipei, 10608, Taiwan. E-mail address: [email protected] (L.-Y. Lin). 1 The authors contributed equally. https://doi.org/10.1016/j.est.2019.100823 Received 10 June 2019; Received in revised form 3 July 2019; Accepted 3 July 2019 Available online 19 August 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
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cobalt oxide (NCO) on the Ni foam. After carrying out hydrothermal process at 120 °C for 6 h and rinsing the samples using DIW and ethanol, the calcination process at 350 °C in air for 2 h was applied to obtained the NCO/Ni foam electrode. The NCM nanostructure was in turn synthesized on the NCO/Ni foam via the second step of the hydrothermal process and the calcination process. The precursor solution for the hydrothermal process contains 1 mmole Ni(NO3)2·6H2O and 1 mmole Na2MoO4·2H2O (≥99%, Acros) in 40 ml DIW. After carrying out the hydrothermal process at 130 °C for 6 h and rinsing the samples using DIW and ethanol, the calcination process at 350 °C in air for 2 h was applied to obtain the NMC electrode.
with quick reversible Faradaic reactions [34–36]. Wu et al. introduced the redox intermedium PPD to the KOH electrolyte for assembling the supercapacitor to attain five-fold higher capacitance comparing to the system without the PPD addition [37]. Lin et al. fabricated porous biocarbon electrode for supercapacitor with the redox additive of PPD in the KOH electrolyte. A specific capacitance of 216 F/g was obtained for the supercapacitor [38]. Ma et al. utilized redox-mediated PVAKOH-PPD as the gel electrolyte and activated carbon as the electrode to fabricate supercapacitors, which attained higher ionic conductivity and energy density owing to the quick Faradaic redox reactions [39]. In our previous work, hydroquinone and PPD were added in electrolyte of the activated carbon supercapacitors. The specific capacitance of 116.23 F/ g at 2 A/g and the maximum energy density of 1.85 Wh/kg at the power density of 150 W/kg were obtained for the supercapacitor [40]. The redox additives are usually coupled with the carbon-based system, which stores energy primarily by the electric double-layer capacitance [41–43]. The single metal oxide, hydroxide and sulfide systems can also have better energy storage ability with the addition of redox additives, which can compensate for less Faradaic reactions produced by the simple single metal compounds [44–46]. However, applying the redox additives on the system using the multiple metal oxide as active material in electrolyte is limited, probably owing to the abundant redox states of the multiple metal oxide which is already capable of generating numerous Faradaic redox reactions for energy storage. Higher amounts of the Faradaic redox reactions are probably produced by adding extra redox additives in electrolyte for multiple metal compounds systems. In this study, the redox additives of K3[Fe(CN)6] and PPD were incorporated in 2 M KOH and used as the electrolyte for assembling the BSH with nickel cobalt molybdenum oxide (NCM) electrode. Due to multiple redox states for the active material with three metals, applying redox additives cannot surely enhance energy storage capability of the BSH. The amounts of K3[Fe(CN)6] added in the KOH electrolyte was optimized and the highest specific capacitance of 3.13 F/cm2 was obtained at 10 mV/s for the BSH with the optimized amount of K3[Fe (CN)6] in KOH. The maximum energy density of 48.0 Wh/kg at 756 W/ kg was achieved. The Coulombic efficiency of larger than 95% and the CF retention of 75% were achieved in 3000 charging/discharging cycles.
2.2. Battery supercapacitor hybrid fabrications The BSH were assembled with a NCM positive electrode coupled with the negative electrode with activated carbon (AC). The doctorblade technique was applied to make the AC electrode according to our previous study [47]. The active materials on positive and negative electrodes should be balanced according to Eq. (1).
m+ I × t− = − m− I+ × t +
(1)
where I and t are respectively current and time for discharging in the Galvanostatic charge/discharge (GC/D) plot; m is active material mass. An electrolyte used for the BSH is based on 2 M sodium hydroxide (KOH, analytical reagent grade, Fisher). Two redox additives were added in electrolyte to enhance the redox reactions. The first kind of the redox-electrolyte contains 6, 12, 18, 24, and 30 mM K3[Fe(CN)6] in 2 M KOH. The other kind of the redox-electrolyte contains in 50 mM PPD in 2 M KOH. 2.3. Physical and electrochemical characterizations The morphology of NCM was observed using the field-emission scanning electron microscopy (FE-SEM, Nova NanoSEM 230, FEI, Oregon, USA). The compositions of NCM was examined using the energy-dispersive X-ray spectroscopy (EDX) equipped in the FE-SEM. The electrochemical performance of the BSH was evaluated by using the cyclic voltammetry (CV), GC/D technique, and the electrochemical impedance spectroscopy (EIS). The CV and GC/D plots were evaluated via the potentiostat/galvanostat instrument (PGSTAT 204, Autolab, Eco–Chemie, the Netherlands). The EIS was measured using the same instrument but with the equipment of an FRA2 module, and the frequencies from 0.01 Hz to 0.1 mHz were applied to measure EIS at opencircuit potential (OCP).
2. Experimental 2.1. Fabrication of NCM electrode The NCM was fabricated according to our previous study [47]. In brief, Ni foam (110PPI, thickness = 1.05 mm, Innovation Materials Co., Ltd., Taiwan) was used as the substrate for growing the NCM nanostructure. After cleaning the Ni foam using 6 M HCl (37%, Sigma–Aldrich), deionized water (DIW) and ethanol, the cleaned Ni foam was dried in vacuum oven overnight before being used as the substrate. The NCM were grown on Ni foam via the two-step hydrothermal process coupled with the calcination process. In the first step of the hydrothermal process, the precursor solution was used to grow the nickel
3. Results and discussion The NCM electrode was used as ternary metallic oxide active material in the positive electrode for BSH. The morphology and the composition of NCM were examined using SEM image and EDX spectrum, respectively. The SEM image of NCM was shown in Fig. 1(a). The bulkassembled nanosheet array with the sheet staggered with each other was observed for the NCM. The gap between the sheets is around
Fig. 1. (a) The SEM image and (b) EDX spectrum for the NiCo2O4@NiMoO4 electrode. 2
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Fig. 2. (a) The CV curves at 10 mV/s, (b) the GC/D curves at 50 mA/cm2 and (c) the Nyquist plot for the BSH composed of different electrolytes.
of the same cation, K+ ions, from K3[Fe(CN)6] can strengthen the cation diffusion. Therefore, the capacitance was enhanced with the addition of K3[Fe(CN)6] for the NCM system. Since the redox peaks located at different potentials (Fig. 2(a)) for the NCM electrodes in different electrolytes, it is inferred that the redox reactions from the redox couples play more importance roles on deciding the redox reactions in the charge/discharge process. Also, there is no redox couple in the KOH electrolyte (none). The peaks at the potentials at around 0.65 and 0.90 V for the CV curve (none) could be attributed to the nickel cobalt oxide active material. On the other hand, the shifted peaks in the CV curves for the PPD and K3[Fe(CN)6] systems could be attributed to the redox couples. In turn, GC/D plot was also measured for the BSH with different electrolytes, as shown in Fig. 2(b). The discharge time of the curve is in proportional to the energy storage ability for the BSH. The same result was obtained that the curve for the BSH with K3[Fe (CN)6] shows the longest discharge time and the device with PPD presents the smallest discharge time. The Nyquist plot for the BSH with different electrolyte was shown in Fig. 2(c) to examine the chargetransfer resistance, ionic conductivity and the capacitive property. Three primary points could be observed in the Nyquist plot. At high frequency region, a semicircle indicates the resistance for chargetransfer, the x-intercept can refer to the series resistance (RS) which is in inverse proportional to the ionic conductivity, and the slope of the nearly straight line can be a pointer for the capacitive property of the BSH. The Nyquist curve for the BSH with K3[Fe(CN)6] shows the smallest semicircle at the high frequency region while the largest semicircle was found for the BSH with PPD. The x-intercept in the Nyquist plot is similar for the three BSH with different electrolytes, suggesting the ionic conductivity was not greatly influenced with the addition of redox additive for the NCM system. Also, the slope of the curve at low frequency region is higher for the BSH with K3[Fe(CN)6] and with pure KOH, while the smaller slope was obtained for the BSH with PPD, inferring the better capacitive properties for the former cases. These phenomena suggest that the charge-transfer resistance and the capacitive property play dominated roles on the specific capacitance and the charge storage capability of the BSH. With the best-performed BSH with the redox additive of K3[Fe (CN)6], the concentration of K3[Fe(CN)6] in the electrolyte was further optimized. The electrolytes containing 6, 12, 18, 24 and 30 mM K3[Fe (CN)6] in 2 M KOH were used for assembling the BSH. As found in the CV curves in Fig. 3(a), the BSH with higher K3[Fe(CN)6] concentrations in the electrolyte show larger CV integrated area, indicating that the better energy storage ability could be obtained with more additive ions in the electrolyte. However, the increases on the CV integrated area is limited for the BSH with the K3[Fe(CN)6] concentration higher than 18 mM. This result suggests that the ions in the electrolyte with 18 mM K3[Fe(CN)6] may almost fully fill the pores of NCM. Therefore, more ions in the electrolyte for the BSH with 24 and 30 mM K3[Fe(CN)6] could have limited enhancements on the energy storage capacity. Same results could be observed in GC/D curves, as shown in Fig. 3(b). The longer discharge time was obtained for the BSH with higher K3[Fe
20 nm, which is inferred to be favorable to provide the large surface area to conduct redox reactions and the suitable pore size for promoting the electrolyte diffusion. On the other hand, Fig. 1(b) shows the EDX spectrum of NCM. The signals of Ni, Co, Mo and O were obviously observed in this spectrum, inferring the successful synthesis of nickel cobalt molybdenum oxide. With the NCM positive electrode, the BSHs were assembled using the electrolytes of 2 M KOH with the redox additives of PPD and K3[Fe (CN)6]. The BSH was also fabricated using the pure 2 M KOH to compare the performance with those with the redox additives. The BSH with the pure KOH was indicated as none in all the plots in Fig. 2. To evaluate the electrocapacitive properties of the BSH, the CV plot was measured as presented in Fig. 2(a). The CV curve for the BSH with pure KOH only shows one oxidation peak and one reduction peak. These redox peaks are attributed to the redox reactions from the redox states of nickel ions and cobalt ions [48]. The CV curves show different shapes for the BSH with and without the redox additives in the electrolyte. When the PPD was incorporated, the BSH shows a smaller CV integrated area, which is embedded in the curve of BSH without redox additive. Even though the addition of redox additives is aimed to produce more the redox reactions and hence to enhance specific capacitance, the addition of PPD in the electrolyte otherwise leads to the reduction of the current at certain ranges of potentials. The redox reaction for PPD is referred to our previous work as shown in Eq. (2) [40].
(2) The size of PPD is inferred to be too large to diffuse efficiently with the nickel and cobalt ions. The redox reactions of nickel and cobalt ions could be hindered with the participation of PPD. It is found that the currents reduced greatly for the BSH with PPD at the potentials of the redox peaks obtained for the BSH without redox additive. That is, when PPD was used in the electrolyte to assemble the BSH, the currents would decrease largely at the main potentials for the redox reaction contributed from the NCM. Therefore, the addition of PPD in the electrolyte could not enhance the specific capacitance of BSH but oppositely reduce the electrochemical performance of BSH due to the incompatible properties between PPD and nickel cobalt ions. On the other hand, the BSH with K3[Fe(CN)6] in the electrolyte shows much larger CV integrated area. The shape of this CV curve is also different from that for the BSH with pure KOH. One obvious oxidation/reduction peaks and several smaller redox peaks were found in the CV curve. The obvious redox peaks are resulted from the redox reaction of NCM, and the smaller redox peaks may be attributed to the redox additive of K3[Fe(CN)6] in the electrolyte according to Eq. (3). Fe(CN)63− + e− ↔ Fe(CN)64−
(3)
The size of Fe(CN)63− is relatively smaller than the PPD, and release 3
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Fig. 3. (a) The CV curves, (b) the GC/D curves, (c) the Nyquist plot and (d) the equivalent circuit for fitting plot (c) for the BSH with 2 M KOH and different K3[Fe (CN)6] concentrations in electrolyte.
for the BSH with pure KOH as the electrolyte. The 1.3 V was got as the optimized potential window since the much serious water oxidation was found at the potential larger than 1.3 V. Based on the potential window of 1.3 V, the scan rates of 5, 10, 20, 30, 40 and 50 mV/s were applied to measure the CV curves, as shown in Fig. 4(b). The distortion of the CV curve shape is small, suggesting the excellent high-rate charge/discharge ability. In addition, the plot for the relation between the specific capacitance and the scan rate was shown in Fig. 4(c). The specific capacitance (CF) was calculated using Eq. (4) as follows from the CV curves.
(CN)6] concentrations, suggesting better energy storage capability for BSH with more redox additive ions in the electrolyte with K3[Fe(CN)6]. The Nyquist plot was shown in Fig. 3(c) for examining the chargetransfer resistance, ionic conductivity and capacitive properties of the BSH. Fig. 3(d) shows the equivalent circuit for fitting the Nyquist plot (Fig. 3(c)) to obtain the resistance data. The x-intercept is similar for the BSH with different concentrations of K3[Fe(CN)6], suggesting the concentration increases cannot largely enhance the ionic conductivity of the electrolyte. The charge-transfer resistance of around 1 Ω was obtained for all the BSH with different concentrations of K3[Fe(CN)6], indicating the concentration variation have limited influence on the transfer of charges. The only difference for the Nyquist plot of the BSH with different concentrations of K3[Fe(CN)6] in the electrolyte lies on the capacitive properties. The slopes of the straight line in the low frequency region are larger for the BSH with the K3[Fe(CN)6] concentration of 18, 24, and 30 mM. The remaining slopes of the straight lines in the low frequency region for the BSH with smaller K3[Fe(CN)6] concentrations are smaller. The better capacitive properties for the BSH with the K3[Fe(CN)6] concentration higher than 18 mM is consistent with their largest CV integrated areas. Therefore, BSH with the electrolyte containing 2 M KOH and 18 mM K3[Fe(CN)6] is considered to be the optimized energy storage device in this work. Further electrochemical analysis was carried out on the BSH with pure KOH as well as with the redox additives of PPD and optimized concentration of K3[Fe (CN)6] in KOH to understand the energy storage ability difference between them. Furthermore, the high-rate capacity and potential window were examined for BSH with different electrolytes. Figs. 4–6 respectively shows the electrochemical analysis for the BSH with 2 M KOH, 2 M KOH/PPD, and 2 M KOH/K3[Fe(CN)6] in the electrolyte. Firstly, the CV curves measured at different potential windows was shown in Fig. 4(a)
CF =
∫ IdV ν∙ΔV∙A
(4)
where ∫ IdV is the integrated area of the CV curve, ν is the scan rate, ΔV is the potential window, and A is the geometric area of the active material. The specific capacitance decay at the high scan rate region is found to be smaller than that in the low scan rate region, and the decay is considered to be small in all the scan rate region. The GC/D curves with different potential windows was shown in Fig. 4(d). At the largest potential window of 1.3 V, the GC/D curve still maintains high symmetry. This result indicates the suitable potential window of 1.3 V for this case. Fig. 4(e) presents the GC/D curve measured at the current densities of 10, 20, 30, 40 and 50 mA/cm2. The high symmetry was observed for all the GC/D plot at different current densities, indicating the good high-rate charge/discharge capacity. The plot for relating the specific capacitance to the current density was shown in Fig. 4(f). The specific capacitance (CF) was calculated using Eq. (5) as follows from the GC/D curves.
CF = 4
I∙t ΔV
(5)
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Fig. 4. The CV curves (a) at varied potential windows measured at 40 mV/s and (b) at 1.3 V and varied scan rates; (c) the plot for the CF value and scan rate relations; the GC/D curves (d) with varied potential windows measured at 10 mA/cm2 and (e) at 1.3 V and varied current densities; (f) the plot for the CF value and current density relations for the BSH composed of 2 M KOH electrolyte.
KOH. With 1.3 V as the potential window of, the scan rates of 5, 10, 20, 30, 40 and 50 mV/s were used to measure the CV curves (Fig. 5(b)). The highly similar shape for the CV curves indicates the excellent high-rate charge/discharge ability for this case. The plot for the relation between the specific capacitance and the scan rate in Fig. 5(c) suggests the larger specific capacitance decay with increasing scan rates. More over 50% decay on the specific capacitance was observed when five-fold of the scan rate was applied for the measurement. Fig. 5(d) shows GC/D plot at varied potential windows. All the GC/D curves remains high symmetry at varied potential windows. Fig. 5(e) shows the GC/D plot
where I is the current density for measuring the GC/D curve, t is the discharge time, and ΔV is the potential window. Nearly no decay on the specific capacitance was found for GC/D curves at different current densities, implying a high maintenance of the specific capacitance even using the five-fold current density to measure the GC/D curve. The CV curves at varied potential windows was shown in Fig. 5(a) for the BSH with KOH and PPD in the electrolyte. The current increases owing to the oxidation of water at the large potential region was observed almost for all the curves, so the optimized potential window was set at 1.3 V to be consistent with the optimized potential window for BSH with pure
Fig. 5. The CV curves (a) with varied potential windows measured at 40 mV/s and (b) at 1.3 V and varied scan rates; (c) the plot for the CF value and scan rate relations; the GC/D curves (d) with varied potential windows measured at 10 mA/cm2 and (e) at 1.3 V and varied current densities; (f) the plot for the CF value and current density relations for the BSH composed of 2 M KOH and PPD electrolyte. 5
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Fig. 6. The CV curves (a) with varied potential windows measured at 40 mV/s and (b) at 1.2 V and varied scan rates; (c) the plot for the CF value and scan rate relations; the GC/D curves (d) with varied potential windows measured at 10 mA/cm2 and (e) at 1.2 V and varied current densities; (f) the plot for the CF value and current density relations for the BSH composed of 2 M KOH and K3[Fe(CN)6] electrolyte.
obtained at varied current densities, and all GC/D curves present high symmetry, suggesting the high reversibility in the charge/discharge process for this case. Fig. 5(f) presents the specific capacitance and the current density relations. The large decay on the specific capacitance was again obtained, implying the worse high-rate charge/discharge ability. Additionally, the CV plot at various potential windows was presented in Fig. 6(a) for the BSH with KOH and K3[Fe(CN)6] in the electrolyte. The optimized potential window of 1.2 V was obtained. At the optimized potential window of 1.2 V, various scan rates were used for testing CV curves (Fig. 6(b)). At all the scan rates, the CV curves present almost the same shapes. The plot of the relation between the specific capacitance and the scan rate was in Fig. 6(c). Fig. 6(d) shows the GC/D plot with various potential windows. At the largest potential window of 1.2 V the GC/D curve still maintains high symmetry. Fig. 6(e) shows the GC/D curves at different current densities. The high symmetry was obtained for all the curves. Fig. 6(f) shows the relation between the specific capacitance and the current density. With five-fold increases on the current density, the specific capacitance can still remain 70% retention, suggesting the excellent high-rate charge/discharge ability. Other than the specific capacitance, the power and energy densities are more important for energy storage devices. Fig. 7 shows the Ragone plot for the BSH with the electrolyte containing 2 M KOH and 18 mM K3[Fe(CN)6]. With the power density increases, only slight decay was observed on the energy density. Also, the maximum energy density (48.0 Wh/kg) was attained at the power density of 756.0 W/kg. The energy density can still maintain 33.8 Wh/kg at 3757.7 W/kg. The results indicate the outstanding energy storage ability for the BSH with K3[Fe(CN)6] incorporated in electrolyte. Last but not least, the excellent cycling stability is indispensable. The cycling stability of the device was measure in 3000 times repeatedly charge/discharge process. Fig. 8(a) shows the GC/D plot at the first and last four cycles. Even if the charge/ discharge time becomes shorter at the last four curves comparing to that for the first four curves, the high symmetry was observed at the all the GC/D curves at initial and final stages of the stability measurement. In addition, the plot for Coulombic efficiency and capacitance retention relating to the cycle time were shown in Fig. 8(b). The Coulombic
Fig. 7. The Ragone plot for the optimized BSH with KOH and K3[Fe(CN)6] in the electrolyte.
efficiency larger than 95% and the CF retention of 75% were achieved in 3000 times charge/discharge process. The high capacitance retention and Coulombic efficiency indicate the excellent charge/discharge cycling stability and high reversibility for the BSH with the NCM as the active material and the K3[Fe(CN)6] as the redox addictive in 2 M KOH as the electrolyte. 4. Conclusions The redox additives of PPD and K3[Fe(CN)6] were incorporated in the KOH electrolyte to enhance the energy storage ability of the BSH with the NCM energy storage material. Largely enhanced CF value was obtained when K3[Fe(CN)6] was added in the electrolyte, owing to the generation of extra redox reactions as well as the reduced charge transfer resistance and the better capacitive property. However, due to the large size of PPD, the addition of PPD in the electrolyte oppositely reduced the redox reactions of the nickel ions and cobalt ions provided by the active material and reduced the charge storage capability of BSH. The concentration of K3[Fe(CN)6] was optimized and the BSH 6
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Fig. 8. (a) The GC/D curves at first and last four cycles and (b) capacitance retention and Coulombic efficiency relating to the cycle times in the 3000 charging/ discharging cycles for BSH composed of optimized electrolyte with KOH and K3[Fe(CN)6].
reached the maximum CF value when the K3[Fe(CN)6] concentration of higher than 18 mM was incorporated in the electrolyte. The potential window of 1.2 V, the CF of 3.13 F/cm2 at 10 mV/s, the maximum energy density of 48.0 Wh/kg at 756 W/kg, as well as 75% as the capacitance retention and higher than 95% as Coulombic efficiency in 3000 times charging/discharging process were obtained for BSH with the optimized concentration of K3[Fe(CN)6] in the KOH electrolyte.
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Acknowledgements This study is financially supported by the “Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. This study is partially supported by the Ministry of Science and Technology of Taiwan (MOST) under grant number of 107-2636-E-027-003-. References [1] S.-C. Lin, C.-S. Hsu, S.-Y. Chiu, T.-Y. Liao, H.M. Chen, J. Am. Chem. Soc. 139 (2017) 2224–2233. [2] S.-F. Hung, Y.-T. Chan, C.-C. Chang, M.-K. Tsai, Y.-F. Liao, N. Hiraoka, C.-S. Hsu, H.M. Chen, J. Am. Chem. Soc. 140 (2018) 17263–17270. [3] C.-W. Tung, Y.-Y. Hsu, Y.-P. Shen, Y. Zheng, T.-S. Chan, H.-S. Sheu, Y.-C. Cheng, H.M. Chen, Nat. Commun. 6 (2015) 8106. [4] Y. Zhu, H.-C. Chen, C.-S. Hsu, T.-S. Lin, C.-J. Chang, S.-C. Chang, L.-D. Tsai, H.M. Chen, ACS Energy Lett. 4 (2019) 987–994. [5] J. Gu, C.-S. Hsu, L. Bai, H.M. Chen, X. Hu, Science 364 (2019) 1091–1094. [6] W. Zuo, R. Li, C. Zhou, Y. Li, J. Xia, J. Liu, Adv. Sci. 4 (2017) 1600539. [7] F. Wang, X. Wu, X. Yuan, Z. Liu, Y. Zhang, L. Fu, Y. Zhu, Q. Zhou, Y. Wu, W. Huang, Chem. Soc. Rev. 46 (2017) 6816–6854. [8] L.G. Beka, X. Li, W. Liu, Sci. Rep. 7 (2017) 2105. [9] G.Z. Chen, Int. Mater. Rev. 62 (2017) 173–202. [10] W.-L. Hong, L.-Y. Lin, L.-Y. Lin, Electrochim. Acta 255 (2017) 309–322. [11] L.-Y. Lin, L.-Y. Lin, Electrochim. Acta 250 (2017) 335–347. [12] L.-Y. Lin, H.-Y. Lin, W.-L. Hong, L.-Y. Lin, Thin Solid Films 667 (2018) 69–75. [13] J.-W. Cheng, L.-Y. Lin, W.-L. Hong, L.-Y. Lin, H.-Q. Chen, H.-X. Lai, Electrochim. Acta 283 (2018) 1245–1252. [14] C.-C. Tu, P.-W. Peng, L.-Y. Lin, Appl. Surf. Sci. 444 (2018) 789–799. [15] H.-X. Lai, L.-Y. Lin, J.-Y. Lin, Y.-K. Hsu, Electrochim. Acta 273 (2018) 115–126. [16] T.-W. Chang, L.-Y. Lin, P.-W. Peng, Y.X. Zhang, Y.-Y. Huang, Electrochim. Acta 259 (2018) 348–354.
7
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Journal of Energy Storage journal homepage: www.elsevier.com/locate/est
Incorporating redox additives in sodium hydroxide electrolyte for energy storage device with the nickel cobalt molybdenum oxide active material Kuan-Hsien Lina,1, Lu-Yin Lina,b, a b
⁎,1
T
, Wei-Lun Honga
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1 Sec. 3, Zhongxiao E. Rd., Taipei, 10608, Taiwan Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors, Taiwan
A R T I C LE I N FO
A B S T R A C T
Keywords: Nickel cobalt molybdenum oxide Battery supercapacitor hybrid Electrolyte Energy storage Redox additive
To enhance the redox reactions and improve the energy storage ability, redox additives have been widely incorporated in the electrolyte to assemble the battery supercapacitor hybrid (BSH). However, due to the possible generation of numerous redox reactions for the metal oxide with abundant redox states, it is limited to apply the redox additives in the multiple metal oxide system for producing more redox reactions and promoting the energy storage. In this study, two redox additives of p-phenylenediamine (PPD) and K3[Fe(CN)6] are incorporated in KOH electrolyte for fabricating BSH with the nickel cobalt molybdenum oxide active material. The addition of PPD in the electrolyte causes the reduction of the specific capacitance (CF) for the BSH, while the greatly enhanced CF value is obtained for the BSH applying K3[Fe(CN)6] as the redox additive in the electrolyte. With the optimized K3[Fe(CN)6] concentration in the electrolyte, the BSH shows the CF value of 3.13 F/cm2 at 10 mV/s along with the maximum energy density of 48.0 Wh/kg at 756.0 W/kg. The Coulombic efficiency higher than 95% and capacitance retention of 75% are obtained for the optimized BSH. This work firstly achieves the opposite results for the BSH with different sorts of redox additives in electrolyte. It is suggested that the ion size and the compatibility between the ions in the electrolyte and the pores in the active material are important to decide the capability for energy storage.
1. Introduction The energy issue has been intensively addressed using numerous ways including generating energies via green routes, converting pollutions to useful energies, and storing energies efficiently [1–5]. Nickel cobalt oxides have been intensively used as the active material of battery supercapacitor hybrid devices (BSH) [6–9]. Primary studies for enhancing the energy storage ability of BSH is to refine the properties of the active material, such as designing efficient morphologies [10–13] or combining several materials [14–16] to produce synergic effects. However, the electrolyte for the BSH also play indispensable roles on the energy storage ability. The electrolyte is responsible for providing ions for carrying out the redox reactions and building the electric field to induce ions adsorption [17–19]. Also, the state of the electrolyte such as liquid, gel or solid states can determine the viscosity, the ionic conductivity and the hence the possible cycling stability of the BSH [20–22]. Adding extra redox additives in the electrolyte has been intensively verified to enhance the capacitance of the BSH by generating
more Faradaic redox reactions [23–25]. The redox additive was previously added in the electrolyte for the metal compound systems to enhance the electrochemical performance [26–28]. Among the numerous redox additives used for BSH, the watersoluble redox couple Fe(CN)63−/Fe(CN)64− has been widely selected as the redox additive in the electrolyte, due to the low toxicity, the high reversibility, and the high stability [29,30]. Su et al. added hexacyanoferrate(II) and (III) into 1 M KOH to reduce the charge-transfer resistance and increase the exchange current density, and hence to improve the capacitive properties of the cobalt aluminum layered double hydroxide electrode [31]. Ma et al. prepared gel polymer polyvinyl alcohol (PVA)-KOH-K3[Fe(CN)6] for solid-state supercapacitor with the activated carbon electrode to attain flexible, high ionic conductivity and wide potential properties [32]. Chen et al. developed a graphene paper electrode in the redox-electrolyte of K3[Fe (CN)6] to achieve both the electric double-layer capacitance and pseudo-capacitance [33]. On the other hand, the p-phenylenediamine (PPD) is consider to be one of the effective organic redox mediators
⁎
Corresponding author at: Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1 Sec. 3, Zhongxiao E. Rd., Taipei, 10608, Taiwan. E-mail address: [email protected] (L.-Y. Lin). 1 The authors contributed equally. https://doi.org/10.1016/j.est.2019.100823 Received 10 June 2019; Received in revised form 3 July 2019; Accepted 3 July 2019 Available online 19 August 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
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cobalt oxide (NCO) on the Ni foam. After carrying out hydrothermal process at 120 °C for 6 h and rinsing the samples using DIW and ethanol, the calcination process at 350 °C in air for 2 h was applied to obtained the NCO/Ni foam electrode. The NCM nanostructure was in turn synthesized on the NCO/Ni foam via the second step of the hydrothermal process and the calcination process. The precursor solution for the hydrothermal process contains 1 mmole Ni(NO3)2·6H2O and 1 mmole Na2MoO4·2H2O (≥99%, Acros) in 40 ml DIW. After carrying out the hydrothermal process at 130 °C for 6 h and rinsing the samples using DIW and ethanol, the calcination process at 350 °C in air for 2 h was applied to obtain the NMC electrode.
with quick reversible Faradaic reactions [34–36]. Wu et al. introduced the redox intermedium PPD to the KOH electrolyte for assembling the supercapacitor to attain five-fold higher capacitance comparing to the system without the PPD addition [37]. Lin et al. fabricated porous biocarbon electrode for supercapacitor with the redox additive of PPD in the KOH electrolyte. A specific capacitance of 216 F/g was obtained for the supercapacitor [38]. Ma et al. utilized redox-mediated PVAKOH-PPD as the gel electrolyte and activated carbon as the electrode to fabricate supercapacitors, which attained higher ionic conductivity and energy density owing to the quick Faradaic redox reactions [39]. In our previous work, hydroquinone and PPD were added in electrolyte of the activated carbon supercapacitors. The specific capacitance of 116.23 F/ g at 2 A/g and the maximum energy density of 1.85 Wh/kg at the power density of 150 W/kg were obtained for the supercapacitor [40]. The redox additives are usually coupled with the carbon-based system, which stores energy primarily by the electric double-layer capacitance [41–43]. The single metal oxide, hydroxide and sulfide systems can also have better energy storage ability with the addition of redox additives, which can compensate for less Faradaic reactions produced by the simple single metal compounds [44–46]. However, applying the redox additives on the system using the multiple metal oxide as active material in electrolyte is limited, probably owing to the abundant redox states of the multiple metal oxide which is already capable of generating numerous Faradaic redox reactions for energy storage. Higher amounts of the Faradaic redox reactions are probably produced by adding extra redox additives in electrolyte for multiple metal compounds systems. In this study, the redox additives of K3[Fe(CN)6] and PPD were incorporated in 2 M KOH and used as the electrolyte for assembling the BSH with nickel cobalt molybdenum oxide (NCM) electrode. Due to multiple redox states for the active material with three metals, applying redox additives cannot surely enhance energy storage capability of the BSH. The amounts of K3[Fe(CN)6] added in the KOH electrolyte was optimized and the highest specific capacitance of 3.13 F/cm2 was obtained at 10 mV/s for the BSH with the optimized amount of K3[Fe (CN)6] in KOH. The maximum energy density of 48.0 Wh/kg at 756 W/ kg was achieved. The Coulombic efficiency of larger than 95% and the CF retention of 75% were achieved in 3000 charging/discharging cycles.
2.2. Battery supercapacitor hybrid fabrications The BSH were assembled with a NCM positive electrode coupled with the negative electrode with activated carbon (AC). The doctorblade technique was applied to make the AC electrode according to our previous study [47]. The active materials on positive and negative electrodes should be balanced according to Eq. (1).
m+ I × t− = − m− I+ × t +
(1)
where I and t are respectively current and time for discharging in the Galvanostatic charge/discharge (GC/D) plot; m is active material mass. An electrolyte used for the BSH is based on 2 M sodium hydroxide (KOH, analytical reagent grade, Fisher). Two redox additives were added in electrolyte to enhance the redox reactions. The first kind of the redox-electrolyte contains 6, 12, 18, 24, and 30 mM K3[Fe(CN)6] in 2 M KOH. The other kind of the redox-electrolyte contains in 50 mM PPD in 2 M KOH. 2.3. Physical and electrochemical characterizations The morphology of NCM was observed using the field-emission scanning electron microscopy (FE-SEM, Nova NanoSEM 230, FEI, Oregon, USA). The compositions of NCM was examined using the energy-dispersive X-ray spectroscopy (EDX) equipped in the FE-SEM. The electrochemical performance of the BSH was evaluated by using the cyclic voltammetry (CV), GC/D technique, and the electrochemical impedance spectroscopy (EIS). The CV and GC/D plots were evaluated via the potentiostat/galvanostat instrument (PGSTAT 204, Autolab, Eco–Chemie, the Netherlands). The EIS was measured using the same instrument but with the equipment of an FRA2 module, and the frequencies from 0.01 Hz to 0.1 mHz were applied to measure EIS at opencircuit potential (OCP).
2. Experimental 2.1. Fabrication of NCM electrode The NCM was fabricated according to our previous study [47]. In brief, Ni foam (110PPI, thickness = 1.05 mm, Innovation Materials Co., Ltd., Taiwan) was used as the substrate for growing the NCM nanostructure. After cleaning the Ni foam using 6 M HCl (37%, Sigma–Aldrich), deionized water (DIW) and ethanol, the cleaned Ni foam was dried in vacuum oven overnight before being used as the substrate. The NCM were grown on Ni foam via the two-step hydrothermal process coupled with the calcination process. In the first step of the hydrothermal process, the precursor solution was used to grow the nickel
3. Results and discussion The NCM electrode was used as ternary metallic oxide active material in the positive electrode for BSH. The morphology and the composition of NCM were examined using SEM image and EDX spectrum, respectively. The SEM image of NCM was shown in Fig. 1(a). The bulkassembled nanosheet array with the sheet staggered with each other was observed for the NCM. The gap between the sheets is around
Fig. 1. (a) The SEM image and (b) EDX spectrum for the NiCo2O4@NiMoO4 electrode. 2
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Fig. 2. (a) The CV curves at 10 mV/s, (b) the GC/D curves at 50 mA/cm2 and (c) the Nyquist plot for the BSH composed of different electrolytes.
of the same cation, K+ ions, from K3[Fe(CN)6] can strengthen the cation diffusion. Therefore, the capacitance was enhanced with the addition of K3[Fe(CN)6] for the NCM system. Since the redox peaks located at different potentials (Fig. 2(a)) for the NCM electrodes in different electrolytes, it is inferred that the redox reactions from the redox couples play more importance roles on deciding the redox reactions in the charge/discharge process. Also, there is no redox couple in the KOH electrolyte (none). The peaks at the potentials at around 0.65 and 0.90 V for the CV curve (none) could be attributed to the nickel cobalt oxide active material. On the other hand, the shifted peaks in the CV curves for the PPD and K3[Fe(CN)6] systems could be attributed to the redox couples. In turn, GC/D plot was also measured for the BSH with different electrolytes, as shown in Fig. 2(b). The discharge time of the curve is in proportional to the energy storage ability for the BSH. The same result was obtained that the curve for the BSH with K3[Fe (CN)6] shows the longest discharge time and the device with PPD presents the smallest discharge time. The Nyquist plot for the BSH with different electrolyte was shown in Fig. 2(c) to examine the chargetransfer resistance, ionic conductivity and the capacitive property. Three primary points could be observed in the Nyquist plot. At high frequency region, a semicircle indicates the resistance for chargetransfer, the x-intercept can refer to the series resistance (RS) which is in inverse proportional to the ionic conductivity, and the slope of the nearly straight line can be a pointer for the capacitive property of the BSH. The Nyquist curve for the BSH with K3[Fe(CN)6] shows the smallest semicircle at the high frequency region while the largest semicircle was found for the BSH with PPD. The x-intercept in the Nyquist plot is similar for the three BSH with different electrolytes, suggesting the ionic conductivity was not greatly influenced with the addition of redox additive for the NCM system. Also, the slope of the curve at low frequency region is higher for the BSH with K3[Fe(CN)6] and with pure KOH, while the smaller slope was obtained for the BSH with PPD, inferring the better capacitive properties for the former cases. These phenomena suggest that the charge-transfer resistance and the capacitive property play dominated roles on the specific capacitance and the charge storage capability of the BSH. With the best-performed BSH with the redox additive of K3[Fe (CN)6], the concentration of K3[Fe(CN)6] in the electrolyte was further optimized. The electrolytes containing 6, 12, 18, 24 and 30 mM K3[Fe (CN)6] in 2 M KOH were used for assembling the BSH. As found in the CV curves in Fig. 3(a), the BSH with higher K3[Fe(CN)6] concentrations in the electrolyte show larger CV integrated area, indicating that the better energy storage ability could be obtained with more additive ions in the electrolyte. However, the increases on the CV integrated area is limited for the BSH with the K3[Fe(CN)6] concentration higher than 18 mM. This result suggests that the ions in the electrolyte with 18 mM K3[Fe(CN)6] may almost fully fill the pores of NCM. Therefore, more ions in the electrolyte for the BSH with 24 and 30 mM K3[Fe(CN)6] could have limited enhancements on the energy storage capacity. Same results could be observed in GC/D curves, as shown in Fig. 3(b). The longer discharge time was obtained for the BSH with higher K3[Fe
20 nm, which is inferred to be favorable to provide the large surface area to conduct redox reactions and the suitable pore size for promoting the electrolyte diffusion. On the other hand, Fig. 1(b) shows the EDX spectrum of NCM. The signals of Ni, Co, Mo and O were obviously observed in this spectrum, inferring the successful synthesis of nickel cobalt molybdenum oxide. With the NCM positive electrode, the BSHs were assembled using the electrolytes of 2 M KOH with the redox additives of PPD and K3[Fe (CN)6]. The BSH was also fabricated using the pure 2 M KOH to compare the performance with those with the redox additives. The BSH with the pure KOH was indicated as none in all the plots in Fig. 2. To evaluate the electrocapacitive properties of the BSH, the CV plot was measured as presented in Fig. 2(a). The CV curve for the BSH with pure KOH only shows one oxidation peak and one reduction peak. These redox peaks are attributed to the redox reactions from the redox states of nickel ions and cobalt ions [48]. The CV curves show different shapes for the BSH with and without the redox additives in the electrolyte. When the PPD was incorporated, the BSH shows a smaller CV integrated area, which is embedded in the curve of BSH without redox additive. Even though the addition of redox additives is aimed to produce more the redox reactions and hence to enhance specific capacitance, the addition of PPD in the electrolyte otherwise leads to the reduction of the current at certain ranges of potentials. The redox reaction for PPD is referred to our previous work as shown in Eq. (2) [40].
(2) The size of PPD is inferred to be too large to diffuse efficiently with the nickel and cobalt ions. The redox reactions of nickel and cobalt ions could be hindered with the participation of PPD. It is found that the currents reduced greatly for the BSH with PPD at the potentials of the redox peaks obtained for the BSH without redox additive. That is, when PPD was used in the electrolyte to assemble the BSH, the currents would decrease largely at the main potentials for the redox reaction contributed from the NCM. Therefore, the addition of PPD in the electrolyte could not enhance the specific capacitance of BSH but oppositely reduce the electrochemical performance of BSH due to the incompatible properties between PPD and nickel cobalt ions. On the other hand, the BSH with K3[Fe(CN)6] in the electrolyte shows much larger CV integrated area. The shape of this CV curve is also different from that for the BSH with pure KOH. One obvious oxidation/reduction peaks and several smaller redox peaks were found in the CV curve. The obvious redox peaks are resulted from the redox reaction of NCM, and the smaller redox peaks may be attributed to the redox additive of K3[Fe(CN)6] in the electrolyte according to Eq. (3). Fe(CN)63− + e− ↔ Fe(CN)64−
(3)
The size of Fe(CN)63− is relatively smaller than the PPD, and release 3
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Fig. 3. (a) The CV curves, (b) the GC/D curves, (c) the Nyquist plot and (d) the equivalent circuit for fitting plot (c) for the BSH with 2 M KOH and different K3[Fe (CN)6] concentrations in electrolyte.
for the BSH with pure KOH as the electrolyte. The 1.3 V was got as the optimized potential window since the much serious water oxidation was found at the potential larger than 1.3 V. Based on the potential window of 1.3 V, the scan rates of 5, 10, 20, 30, 40 and 50 mV/s were applied to measure the CV curves, as shown in Fig. 4(b). The distortion of the CV curve shape is small, suggesting the excellent high-rate charge/discharge ability. In addition, the plot for the relation between the specific capacitance and the scan rate was shown in Fig. 4(c). The specific capacitance (CF) was calculated using Eq. (4) as follows from the CV curves.
(CN)6] concentrations, suggesting better energy storage capability for BSH with more redox additive ions in the electrolyte with K3[Fe(CN)6]. The Nyquist plot was shown in Fig. 3(c) for examining the chargetransfer resistance, ionic conductivity and capacitive properties of the BSH. Fig. 3(d) shows the equivalent circuit for fitting the Nyquist plot (Fig. 3(c)) to obtain the resistance data. The x-intercept is similar for the BSH with different concentrations of K3[Fe(CN)6], suggesting the concentration increases cannot largely enhance the ionic conductivity of the electrolyte. The charge-transfer resistance of around 1 Ω was obtained for all the BSH with different concentrations of K3[Fe(CN)6], indicating the concentration variation have limited influence on the transfer of charges. The only difference for the Nyquist plot of the BSH with different concentrations of K3[Fe(CN)6] in the electrolyte lies on the capacitive properties. The slopes of the straight line in the low frequency region are larger for the BSH with the K3[Fe(CN)6] concentration of 18, 24, and 30 mM. The remaining slopes of the straight lines in the low frequency region for the BSH with smaller K3[Fe(CN)6] concentrations are smaller. The better capacitive properties for the BSH with the K3[Fe(CN)6] concentration higher than 18 mM is consistent with their largest CV integrated areas. Therefore, BSH with the electrolyte containing 2 M KOH and 18 mM K3[Fe(CN)6] is considered to be the optimized energy storage device in this work. Further electrochemical analysis was carried out on the BSH with pure KOH as well as with the redox additives of PPD and optimized concentration of K3[Fe (CN)6] in KOH to understand the energy storage ability difference between them. Furthermore, the high-rate capacity and potential window were examined for BSH with different electrolytes. Figs. 4–6 respectively shows the electrochemical analysis for the BSH with 2 M KOH, 2 M KOH/PPD, and 2 M KOH/K3[Fe(CN)6] in the electrolyte. Firstly, the CV curves measured at different potential windows was shown in Fig. 4(a)
CF =
∫ IdV ν∙ΔV∙A
(4)
where ∫ IdV is the integrated area of the CV curve, ν is the scan rate, ΔV is the potential window, and A is the geometric area of the active material. The specific capacitance decay at the high scan rate region is found to be smaller than that in the low scan rate region, and the decay is considered to be small in all the scan rate region. The GC/D curves with different potential windows was shown in Fig. 4(d). At the largest potential window of 1.3 V, the GC/D curve still maintains high symmetry. This result indicates the suitable potential window of 1.3 V for this case. Fig. 4(e) presents the GC/D curve measured at the current densities of 10, 20, 30, 40 and 50 mA/cm2. The high symmetry was observed for all the GC/D plot at different current densities, indicating the good high-rate charge/discharge capacity. The plot for relating the specific capacitance to the current density was shown in Fig. 4(f). The specific capacitance (CF) was calculated using Eq. (5) as follows from the GC/D curves.
CF = 4
I∙t ΔV
(5)
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Fig. 4. The CV curves (a) at varied potential windows measured at 40 mV/s and (b) at 1.3 V and varied scan rates; (c) the plot for the CF value and scan rate relations; the GC/D curves (d) with varied potential windows measured at 10 mA/cm2 and (e) at 1.3 V and varied current densities; (f) the plot for the CF value and current density relations for the BSH composed of 2 M KOH electrolyte.
KOH. With 1.3 V as the potential window of, the scan rates of 5, 10, 20, 30, 40 and 50 mV/s were used to measure the CV curves (Fig. 5(b)). The highly similar shape for the CV curves indicates the excellent high-rate charge/discharge ability for this case. The plot for the relation between the specific capacitance and the scan rate in Fig. 5(c) suggests the larger specific capacitance decay with increasing scan rates. More over 50% decay on the specific capacitance was observed when five-fold of the scan rate was applied for the measurement. Fig. 5(d) shows GC/D plot at varied potential windows. All the GC/D curves remains high symmetry at varied potential windows. Fig. 5(e) shows the GC/D plot
where I is the current density for measuring the GC/D curve, t is the discharge time, and ΔV is the potential window. Nearly no decay on the specific capacitance was found for GC/D curves at different current densities, implying a high maintenance of the specific capacitance even using the five-fold current density to measure the GC/D curve. The CV curves at varied potential windows was shown in Fig. 5(a) for the BSH with KOH and PPD in the electrolyte. The current increases owing to the oxidation of water at the large potential region was observed almost for all the curves, so the optimized potential window was set at 1.3 V to be consistent with the optimized potential window for BSH with pure
Fig. 5. The CV curves (a) with varied potential windows measured at 40 mV/s and (b) at 1.3 V and varied scan rates; (c) the plot for the CF value and scan rate relations; the GC/D curves (d) with varied potential windows measured at 10 mA/cm2 and (e) at 1.3 V and varied current densities; (f) the plot for the CF value and current density relations for the BSH composed of 2 M KOH and PPD electrolyte. 5
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Fig. 6. The CV curves (a) with varied potential windows measured at 40 mV/s and (b) at 1.2 V and varied scan rates; (c) the plot for the CF value and scan rate relations; the GC/D curves (d) with varied potential windows measured at 10 mA/cm2 and (e) at 1.2 V and varied current densities; (f) the plot for the CF value and current density relations for the BSH composed of 2 M KOH and K3[Fe(CN)6] electrolyte.
obtained at varied current densities, and all GC/D curves present high symmetry, suggesting the high reversibility in the charge/discharge process for this case. Fig. 5(f) presents the specific capacitance and the current density relations. The large decay on the specific capacitance was again obtained, implying the worse high-rate charge/discharge ability. Additionally, the CV plot at various potential windows was presented in Fig. 6(a) for the BSH with KOH and K3[Fe(CN)6] in the electrolyte. The optimized potential window of 1.2 V was obtained. At the optimized potential window of 1.2 V, various scan rates were used for testing CV curves (Fig. 6(b)). At all the scan rates, the CV curves present almost the same shapes. The plot of the relation between the specific capacitance and the scan rate was in Fig. 6(c). Fig. 6(d) shows the GC/D plot with various potential windows. At the largest potential window of 1.2 V the GC/D curve still maintains high symmetry. Fig. 6(e) shows the GC/D curves at different current densities. The high symmetry was obtained for all the curves. Fig. 6(f) shows the relation between the specific capacitance and the current density. With five-fold increases on the current density, the specific capacitance can still remain 70% retention, suggesting the excellent high-rate charge/discharge ability. Other than the specific capacitance, the power and energy densities are more important for energy storage devices. Fig. 7 shows the Ragone plot for the BSH with the electrolyte containing 2 M KOH and 18 mM K3[Fe(CN)6]. With the power density increases, only slight decay was observed on the energy density. Also, the maximum energy density (48.0 Wh/kg) was attained at the power density of 756.0 W/kg. The energy density can still maintain 33.8 Wh/kg at 3757.7 W/kg. The results indicate the outstanding energy storage ability for the BSH with K3[Fe(CN)6] incorporated in electrolyte. Last but not least, the excellent cycling stability is indispensable. The cycling stability of the device was measure in 3000 times repeatedly charge/discharge process. Fig. 8(a) shows the GC/D plot at the first and last four cycles. Even if the charge/ discharge time becomes shorter at the last four curves comparing to that for the first four curves, the high symmetry was observed at the all the GC/D curves at initial and final stages of the stability measurement. In addition, the plot for Coulombic efficiency and capacitance retention relating to the cycle time were shown in Fig. 8(b). The Coulombic
Fig. 7. The Ragone plot for the optimized BSH with KOH and K3[Fe(CN)6] in the electrolyte.
efficiency larger than 95% and the CF retention of 75% were achieved in 3000 times charge/discharge process. The high capacitance retention and Coulombic efficiency indicate the excellent charge/discharge cycling stability and high reversibility for the BSH with the NCM as the active material and the K3[Fe(CN)6] as the redox addictive in 2 M KOH as the electrolyte. 4. Conclusions The redox additives of PPD and K3[Fe(CN)6] were incorporated in the KOH electrolyte to enhance the energy storage ability of the BSH with the NCM energy storage material. Largely enhanced CF value was obtained when K3[Fe(CN)6] was added in the electrolyte, owing to the generation of extra redox reactions as well as the reduced charge transfer resistance and the better capacitive property. However, due to the large size of PPD, the addition of PPD in the electrolyte oppositely reduced the redox reactions of the nickel ions and cobalt ions provided by the active material and reduced the charge storage capability of BSH. The concentration of K3[Fe(CN)6] was optimized and the BSH 6
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Fig. 8. (a) The GC/D curves at first and last four cycles and (b) capacitance retention and Coulombic efficiency relating to the cycle times in the 3000 charging/ discharging cycles for BSH composed of optimized electrolyte with KOH and K3[Fe(CN)6].
reached the maximum CF value when the K3[Fe(CN)6] concentration of higher than 18 mM was incorporated in the electrolyte. The potential window of 1.2 V, the CF of 3.13 F/cm2 at 10 mV/s, the maximum energy density of 48.0 Wh/kg at 756 W/kg, as well as 75% as the capacitance retention and higher than 95% as Coulombic efficiency in 3000 times charging/discharging process were obtained for BSH with the optimized concentration of K3[Fe(CN)6] in the KOH electrolyte.
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Acknowledgements This study is financially supported by the “Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. This study is partially supported by the Ministry of Science and Technology of Taiwan (MOST) under grant number of 107-2636-E-027-003-. References [1] S.-C. Lin, C.-S. Hsu, S.-Y. Chiu, T.-Y. Liao, H.M. Chen, J. Am. Chem. Soc. 139 (2017) 2224–2233. [2] S.-F. Hung, Y.-T. Chan, C.-C. Chang, M.-K. Tsai, Y.-F. Liao, N. Hiraoka, C.-S. Hsu, H.M. Chen, J. Am. Chem. Soc. 140 (2018) 17263–17270. [3] C.-W. Tung, Y.-Y. Hsu, Y.-P. Shen, Y. Zheng, T.-S. Chan, H.-S. Sheu, Y.-C. Cheng, H.M. Chen, Nat. Commun. 6 (2015) 8106. [4] Y. Zhu, H.-C. Chen, C.-S. Hsu, T.-S. Lin, C.-J. Chang, S.-C. Chang, L.-D. Tsai, H.M. Chen, ACS Energy Lett. 4 (2019) 987–994. [5] J. Gu, C.-S. Hsu, L. Bai, H.M. Chen, X. Hu, Science 364 (2019) 1091–1094. [6] W. Zuo, R. Li, C. Zhou, Y. Li, J. Xia, J. Liu, Adv. Sci. 4 (2017) 1600539. [7] F. Wang, X. Wu, X. Yuan, Z. Liu, Y. Zhang, L. Fu, Y. Zhu, Q. Zhou, Y. Wu, W. Huang, Chem. Soc. Rev. 46 (2017) 6816–6854. [8] L.G. Beka, X. Li, W. Liu, Sci. Rep. 7 (2017) 2105. [9] G.Z. Chen, Int. Mater. Rev. 62 (2017) 173–202. [10] W.-L. Hong, L.-Y. Lin, L.-Y. Lin, Electrochim. Acta 255 (2017) 309–322. [11] L.-Y. Lin, L.-Y. Lin, Electrochim. Acta 250 (2017) 335–347. [12] L.-Y. Lin, H.-Y. Lin, W.-L. Hong, L.-Y. Lin, Thin Solid Films 667 (2018) 69–75. [13] J.-W. Cheng, L.-Y. Lin, W.-L. Hong, L.-Y. Lin, H.-Q. Chen, H.-X. Lai, Electrochim. Acta 283 (2018) 1245–1252. [14] C.-C. Tu, P.-W. Peng, L.-Y. Lin, Appl. Surf. Sci. 444 (2018) 789–799. [15] H.-X. Lai, L.-Y. Lin, J.-Y. Lin, Y.-K. Hsu, Electrochim. Acta 273 (2018) 115–126. [16] T.-W. Chang, L.-Y. Lin, P.-W. Peng, Y.X. Zhang, Y.-Y. Huang, Electrochim. Acta 259 (2018) 348–354.
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