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     Combining the characteristics of aqueous and organic electrolytes into a hybrid version increases VGN performance in supercapacitors, while KOH activation improves nanostructure and charge storage capacity.

    Charge storage schematic of vertical graphene nanosheets in TEABF4/H2SO4.

     

    Providing a larger surface area

    Supercapacitors can store more energy than and are preferable to batteries because they are able to charge faster, mainly due to the vertical graphene nanosheets (VGNs) that are larger and positioned closer together. VGNs are 3-D networks of carbon nanomaterial that grow in rows of vertical sheets, providing a large surface area for greater charge storage capacity. Also called carbon nanowalls or graphene nanoflakes, VGNs offer promise in high-power energy storage systems, fuel cells, bio sensors and magnetic devices, amongst others.

    Using VGNs as the material for supercapacitor electrodes offers advantages due to their intriguing properties such as an interconnected porous nanoarchitecture, excellent conductivity, high electrochemical stability, and its array of nanoelectrodes. Advantages of VGNs can be enhanced depending on how the material is grown, treated and prepared to work with electrolytes.

    “Performance of a supercapacitor not only depends on the geometry of electrode material, but also depends on the type of electrolyte and its interaction with the electrode,” said Subrata Ghosh of the Indira Gandhi Centre for Atomic Research at Homi Bhabha National Institute. “To improve the energy density of a device, [electric] potential window enhancement will be one key factor.”

    In a paper published this week in the Journal of Applied Physics, from AIP Publishing, Ghosh and a team of researchers discovered ways to improve the material’s supercapacitance properties.

    According to modeling, VGNs should be able to provide high charge storage capabilities, and the scientific community is trying to unlock the keys to reaching the levels of efficiency that are theoretically available. Needed improvements to be viable include, for instance, greater capacitance per unit of material, greater retention, less internal resistance, and greater electrochemical voltage ranges (operating potential windows).

    “Our motivation was to improve VGN performance,” Ghosh said. “We have taken two strategies. One is inventing a novel electrolyte, and another is improving the VGN structure by chemical activation. The combination of both enhances the charge storage performance remarkably.”

    The researcher team treated VGNs with potassium hydroxide (KOH) to activate the electrodes and then allowed the treated electrodes to interact with a hybrid electrolyte, testing the formation of the electric double layer at the electrode/electrolyte interface. They also examined the morphology, surface wettability, columbic efficiency and areal capacitance of VGN.

    The novel electrolyte they created is a hybrid that combines the advantages of aqueous and organic electrolytes for a novel hybrid organo-aqueous version that works to increase supercapacitor performance of VGNs. Using an organic salt, Tetraethylammonium tetrafluoroborate (TEABF4), in an acidic aqueous solution of sulfuric acid (H2SO4), they created an electrolyte that extended the device’s operating window.

    Improvement of VGN architecture was associated with the process of KOH activation, which grafted the oxygen functional group onto the electrode, improved electrode wettability, reduced internal resistance and provided a fivefold improvement in capacitance of the VGNs. The activation approach in the paper can be applied to other supercapacitor devices that are based on nanoarchitecture, Ghosh said.

    “Aqueous and organic electrolytes are extensively used, but they have their own advantages and disadvantages,” he said. “Hence the concept of hybrid electrolyte arises.”

    Reference

    The article, “Enhanced supercapacitance of activated vertical graphene nanosheets in hybrid electrolyte,” is authored by Subrata Ghosh, Gopinath Sahoo, S.R. Polaki, Nanda Gopala Krishna, M. Kamruddin and Tom Mathews. The article appeared in the Journal of Applied Physics Dec. 5, 2017 (DOI: 10.1063/1.5002748) and can be accessed at http://aip.scitation.org/doi/full/10.1063/1.5002748.

    Abstract

    Supercapacitors are becoming the workhorse for emerging energy storage applications due to their higher power density and superior cycle life compared to conventional batteries. The performance of supercapacitors depends on the electrode material, type of electrolyte, and interaction between them. Owing to the beneficial interconnected porous structure with multiple conducting channels, vertical graphene nanosheets (VGN) have proved to be leading supercapacitor electrode materials. Herein, we demonstrate a novel approach based on the combination of surface activation and a new organo-aqueous hybrid electrolyte, tetraethylammonium tetrafluoroborate in H2SO4, to achieve significant enhancement in supercapacitor performance of VGN. As-synthesized VGN exhibits an excellent supercapacitance of 0.64 mF/cm2 in H2SO4.
    However, identification of a novel electrolyte for performance enhancement is the subject of current research. The present manuscript demonstrates the potential of the hybrid electrolyte in enhancing the areal capacitance (1.99 mF/cm2) with excellent retention (only 5.4% loss after 5000 cycles) and Coulombic efficiency (93.1%). In addition, a five-fold enhancement in the capacitance of VGNs (0.64 to 3.31 mF/cm2) with a reduced internal resistance is achieved by the combination of KOH activation and the hybrid electrolyte.
    Since the last two decades, vertical graphene nanosheets (VGN), sometimes called carbon nanowalls or graphene nanoflakes, have captured significant attention as envisioned materials for supercapacitors (SCs), fuel cells, field emission, magnetic device applications and so on due to their intriguing properties.1–4 VGN comprise a three-dimensional interconnected network of vertically standing graphene sheets composed of few layers. Their remarkable properties include the high surface area, plenty of edges, excellent thermal and electrical conductivity, good thermal stability, chemical inertness, and easy functionalization.5–12
    Owing to their high power density and superior cycle life, SCs are promising energy storage devices in comparison to conventional batteries.13 However, the low charge storage capacity of SCs compared to batteries limits their use in commercial applications. VGN fulfill the criteria of an ideal SC electrode because of their distinct morphology and abovementioned remarkable properties. The charge storage mechanism of VGN is based on the electric double layer (EDL) formation at the electrode/electrolyte interface. They have a non-agglomerated, self-supported porous structure and serve as electrolyte ion-reservoirs where each vertical sheet acts as a nanoelectrode.14 The advantage of VGN over other electrode materials is that they do not need any binder, conductive additives, and even additional current collector.5,11,14–24
    Generally, the areal capacitance of VGN is achieved in the order of micro-Farad to milli-Farad per cm2.1–25 However, theoretical modelling demonstrates that VGN have high charge storage capability compared to commercially available SC devices (MaxwellBoostcap® Ultracapacitor, BCAP3000-P270-T04).15 In addition, the areal capacitance of VGN is very less compared to the theoretical value and that of other commercially used carbon materials, which made us to focus on the adoptable strategies to reach that goal. The SC performance of VGN depends on the vertical height, sheet density, surface charging, and electrolyte.25 The charge storage performance has been considerably enhanced by increasing defect density26 and nitrogen and boron doping into the VGN structure.27,28 Since the edges are privileged to several times higher charge storage capacity than basal planes,29 growths of VGN on nanocups and hence three times enhancement in capacitance have been also reported.30
    Improvement in the supercapacitance of carbon based electrode materials including VGN has also be done by either decorating pseudocapacitance materials11,31 or nanoparticles,32 using novel electrolytes,20,21,33 or by designing asymmetric SCs.34,35 Pseudocapacitors are promising candidates in providing higher charge storage capacity compared to the carbon based materials.35–38Metal oxides or hydroxides and conducting polymers fall in this category.39–41The charge storage mechanism of pseudocapacitors relies on fast redox kinetics. However, stability, electrical conductivity, agglomeration, and the need of binder, additives, and current collector are the critical issues. Hence, a lot of research has been dedicated toward hybrid structures.42,43 In the case of hybrid nanostructures, VGN not only contribute the EDL capacitance but also serve as a skeleton for the pseudocapacitive materials and provide conducting paths for fast electron transport. They also prevent the agglomeration and restacking.11,22To achieve the theoretical value and to compete with the imposed energy demand in real-life applications, the present research is focused on innovative and viable strategies to improve the SC performance of VGN exclusively.
    Although the electrode material is crucial, the choice of electrolyte and its concentration also play significant roles in determining the performance of SCs.13,36,44 The potential window of a device strongly depends on the electrolyte and its nature of interaction with the electrode material. Since VGN are proven to be promising SC electrode materials, our focus in this research is mainly to find out a potential electrolyte. Recently, thorough investigation of aqueous electrolytes (Na2SO4, H2SO4, and KOH) has demonstrated their suitability to use with VGN.14 The observed superior capacitance performance of VGN in H2SO4 is ascribed to their lower charge transfer resistance, higher Coulombic efficiency, and better electrochemical stability.14 Even then, the ever rising demand for high energy storage capacities propels the research activities to focus on novel electrolytes for further enhancement of capacitance.44
    At present, a lot of research is being focused on organic electrolytes and ionic liquids because of their extended operating potential windows. However, the use of an aqueous alkaline electrolyte of 6 M KOH offers higher capacitance compared to the organic counterpart, 1 M tetraethylammonium tetrafluoroborate (TEABF4) in acetonitrile (AN), but organic electrolytes are found to have better wettability.45Moreover, water based electrolytes are environmental friendly, safe, and cost effective. The limitation of the narrow operational potential window of the aqueous electrolyte needs to be overcome for developing a high performance SC device. Recently, considerable attention is devoted to the use of novel electrolytes in innovative ways. Besides the use of aqueous, organic, or ionic electrolytes, a great deal of research is concentrated on hybrid electrolytes for improving the SC performance. For example, acetate aqueous electrolytes,46 iso-propanol added aqueous electrolytes,47 aqueous solution of acidic ionic liquid,48ionic liquid-added polymer electrolytes,49 and hydroquinone doped hybrid gel electrolytes50 are opted for enhanced stability for an extended potential window. It is reported that with an addition of 10−4 M KOH to 1 M Na2SO4 electrolyte solution, an extended potential window of 1.07 V was achieved for the VGN electrode.16 Despite the progress in the specific capacitance of VGNs in the aqueous electrolyte, research towards extending the potential window of VGNs is still under active research.
    Additionally, the coverage of the entire electrode surface with an electrolyte also has a key role in determining the SC performance. It is mainly controlled by the wetting nature of the electrode surface. Porous carbon nanomaterials including VGNs are, in general, hydrophobic in nature.7 The incomplete wetting of the electrode can be overcome by its functionalization, which is another feasible strategy towards improving SC performance.51 In this context, hydrophilic electrode materials are more beneficial. Functionalization by a polar group is used to improve the wetting nature of the electrode material. The in-situ oxygen plasma exposure to the VGN surface is found to enhance the wettability, which further leads to improved SC performance.17 However, chemical functionalization is very easy, cost-effective, and quick compared to plasma assisted functionalization. Non-covalent functionalization of the graphene surface allows attachment of oxygen containing functional groups on the surface, leading to high pseudocapacitance.52 Oxygen functionalization of carbon materials by HNO3 treatment is found to enhance their specific capacitance.51 However, the functionalization process should not deteriorate the morphology and crystalline structure. It is reported that KOH is widely used in the activation process to fabricate activated graphene oxides and activated carbon to improve SC performance.53–55 Also, the chemical activation process by KOH is reported to generate nanoscale pores in carbon nanostructures.55Recently, better wetting nature is found for VGN in the 1 M KOH medium, leading to a higher capacitance value.14 Hence, our interest is to activate the VGN and to investigate its SC performance.
    Our endeavor here is to combine a novel electrolyte and chemical activation of the electrode surface towards achieving enhanced SC performance. First, we formulated a hybrid organo-aqueous electrolyte made up of an organic salt (TEABF4) mixed in acidic aqueous solution. It extended the potential window up to 0.8 V, compared to 0.5 V in H2SO4. Furthermore, VGN electrodes were activated by KOH for use in combination with the hybrid electrolyte. This combination manifested in superior charge storage performance. These findings seem to be promising for the usage of VGN based SCs for future energy storage applications.

     

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