Effect of redox additive in aqueous electrolyte on the high specific capacitance of cation incorporated MnCo2O4@Ni(OH)2 electrode materials for flexible symmetric supercapacitor
Graphical abstract
Flexible symmetric supercapacitor device with chrysanthemum flower like MnCo2O4@Ni(OH)2 as electrode materials delivers high energy density on using a redox mediated K3[Fe(CN)6] aqueous alkali gel electrolyte.
Introduction
Green energy conversion/storage has attracted a significant attention worldwide due to the continuous depletion of fossil fuel and global warming. Among the various types of energy storages devices, the future of supercapacitors (SCs) is promising in the areas of portable electronics, heavy industries, defence, automobile, aerospace, etc. It can deliver power at a higher rate than batteries and maintains constant current supply during discharging. However, the low specific energy confines their uses for long run applications like battery [1,2]. In order to mitigate this lacuna, improvement of energy density of the SC electrodes through the modification of electrode materials is the major focus of research. The capacitive performance of electrochemical double layer capacitors (EDLCs) is proportional to the surface area of the materials and dielectric separator [3], [4], [5], [6]. However, the specific capacitance of the EDLCs electrode materials suffers from the low specific capacitance due to the limited porosity of carbonaceous materials [7,8]. In contrast, the Faradic electrode materials store charges via faradic redox reaction in aqueous electrolyte [[9], [10], [11]]. In addition to the EDLCs and Faradic capacitor, the electrolyte also controls the total capacitance of a SC device [12]. Addition of redox additive into the aqueous electrolyte is an effective approach to promote the efficiency of SCs via electron transfer at the electrode/electrolyte interface through the reversible oxidation/reduction reaction. Utilization of redox additives such as CuSO4 [13], KI [14], VOSO4 [15,16], hydroquinone [17,18], K3[Fe(CN)6] [[19], [20], [21], [22]], phenylenediamine [23,24], etc. in aqueous electrolyte were found to improve the overall specific capacitance of SCs as summarized in Table S1 of the supporting information. Gao et al. showed significant improvement in specific capacitance in KI-H2SO4 mixed electrolyte (616 F g−1) as compared to the 1 M H2SO4 electrolyte (184 F g−1) [25]. Roldan et al. reported that the addition of redox-active quinine/hydroquinone (Q/HQ) into 1 M H2SO4 improved the specific capacitance up to 5017 F g−1 [26]. The redox active Q/HQ improved the faradic contribution in SCs. The specific capacitance improved by 10 orders of magnitude with the addition of CuSO4 as redox additive into the 1 M H2SO4 as described by Mai et al. for the functionalized carbon electrode. The retention in specific capacitance was ~99.4% after 5,000 galvanostatic charge-discharge (GCD) cycles recorded at 60 A g−1 current density [1]. Zhao et al. explored the electrochemical performance of Co(OH)2/rGO electrode using K3[Fe(CN)6]-1 M KOH mixed electrolyte. The use of 1 M KOH/0.08 M K3[Fe(CN)6] electrolyte contributed to the high specific capacitance of 7514 F g−1 at 16 A g−1 current density [27]. Recently, all-solid-state SCs have been investigated extensively to meet the demand of flexible and wearable energy storage device [17,[28], [29], [30]]. The electrolyte of all-solid-state SCs is not as mobile as liquid electrolyte, so it is possible to separate from the other parts of the device.
Considering the recent research trends on redox-active additives into the aqueous electrolyte for electrochemical energy storage, the present work focused on the redox additive-assisted alkaline aqueous electrolyte as a new paradigm for the improvement of capacitance for energy storage system. Herein, the focus is to improve the specific capacitance of the redox-active electrode by adding K3[Fe(CN)6] into the aqueous alkaline electrolyte-based on the eco-friendly and safe operation. K3[Fe(CN)6] showed standard redox potential in between 0 and 0.4 V which contributed additional specific capacitance from the specific reaction Fe(CN)63−/Fe(CN)64− system in presence of KOH electrolyte. A facile and cost-effective cation-intercalated MnCo2O4@Ni(OH)2 hybrid electrode was synthesized by a soft chemical route. Flowerlike morphology with nanostructured hexagonal Ni(OH)2 decorated spinel MnCo2O4 facilitated ion migration and provided an additional support for the intercalation/de-intercalation of the ion during the oxidation/reduction processes. The specific capacitance recorded for the K+ intercalated MnCo2O4@Ni(OH)2 (KMNC) hybrid electrode in KOH/K3[Fe(CN)6] mixed electrolyte (KFCN) was 5413.12 F g−1 at 5 A g−1 current density. Similarly, ~2378.75 and 1800.62 F g−1 were recorded in NaOH/K3[Fe(CN)6] (NFCN) and LiOH/K3[Fe(CN)6] (LFCN) electrolyte for Na+ intercalated MnCo2O4@Ni(OH)2 (NMNC) and for Li+ intercalated MnCo2O4@Ni(OH)2 electrode (LMNC), respectively. To evaluate the practical application of flexible SCs, all-solid-state symmetric SCs were fabricated using KMNC electrode in KFCN mixed electrolyte. The fabricated solid-state SCs exhibited ~1656.6 F g−1 specific capacitance at 1 A g−1 current density, excellent flexibility and maintained the electrochemical properties even after several bending cycles. The device displayed remarkable reproducibility and durability. The KMNC hybrid electrode showed the retention in cyclic stability up to ~77.32 and 75.84% after 5000 GCD cycles in three-electrode system and two-electrode symmetric device.
Section snippets
Materials
Lithium hydroxide (LiOH), sodium hydroxide (NaOH), potassium hydroxide (KOH), N, N-dimethyl formamide (DMF), manganese nitrate (Mn(NO3)2. xH2O), cobalt nitrate (Co(NO3)2. xH2O), and nickel nitrate (Ni(NO3)2. xH2O) were procured from Merck Specialities Pvt. Ltd. (Mumbai, India). Poly vinyl alcohol (PVA) and hydrogen peroxide (H2O2) were purchased from Sigma Aldrich, India. Polyvinylidene fluoride (PVDF) and carbon black were purchased from Akzo Nobel Amides Co. Ltd. (Kyungpuk, South Korea).
Physical characterization
The powder X-ray diffraction (PXRD) (Fig. 1) of KMNC, NMNC and LMNC showed the diffraction peaks at 2θ ≈ 30.65, 36.2, 43.97, 54.6, 58.25, and 64.2° owing to the (220), (311), (400), (422), (511), and (400) planes of the face-centred cubic spinal of MnCo2O4, respectively (JCPDS No. 1-1130). Appearance of sharp peaks in the PXRD pattern indicated highly crystalline nature of MnCo2O4 spinel. Other diffraction peaks were well defined at 2θ ≈ 19.5, 22.6, 33.7, 39, and 59.6° due to the (001), (002),
Conclusions
In summary, a novel supercapacitor or supercapattery electrode material composed of spinel and hexagonal multi-component MnCo2O4@Ni(OH)2 nano-flower was reported by a facile and cost-effective soft-chemical route. The existence of intercalated cations was confirmed by the FE-SEM, HR-TEM, and XPS analysis. Crystal growth of the KMNC, NMNC, and LMNC electrode material occurred according to the cation intercalation as evidenced from the CV analysis. Such type of chrysanthemum flower like structure
Authors statement
Mr. Prakas Samanta: Literature review, conducting experiment and structural and morphological characterization, data analysis, electrochemical studies, writing the manuscript.
Mr. Souvik Ghosh: Electrochemical characterization and data analysis.
Dr. N C Murmu: Management and coordination responsibility for the research activity planning and execution.
Dr. Tapas Kuila: Oversight and leadership responsibility for the research activity planning and execution, including mentorship external to the core
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are thankful to the Director of CSIR-CMERI, Durgapur. Authors are also thankful to GAP219012 project for financial support.
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