High-voltage Aqueous Supercapacitors Research Articles

In-situ synthesis of N, S co-doped electrode material by an environmentally benign method towards designing of high voltage (3.0 V) binder-free sustainable aqueous symmetric supercapacitor

In aqueous electrolyte based symmetric supercapacitor, the decomposition of water to evolve hydrogen and oxygen occurs in relatively lower potential window, thereby, limits its cell voltage and needs to be modified so as to achieve high energy density. This manuscript reports a novel greener approach for an in-situ synthesis of N, S co-doped reduced graphene oxide (N, S co-doped rGO-10/20) nanohybrids as porous electrode material for designing a binder-free and up-scaled mass-loaded (5.0 mg cm−2) high voltage (3.0 V) aqueous symmetric supercapacitor in water-in-salt (WIS), 17 m NaClO4 electrolyte. A fairly rich co-doping/integration of heteroatoms (N (7.2 %) and S (1.5 %)) into the rGO matrix takes place through the formation of C-N, C-S and C-SOx-C bonds. The N, S co-doped rGO-20 nanohybrids in three-electrode system shows the pseudocapacitive behavior with the electrochemical stability of potential window (ESPW) of 3.0 V. In two electrodes set-up, the optimized aqueous symmetric supercapacitor cell (SSC) in WIS shows the superb cell voltage at 3.0 V (highest reported to-date), a fairly high areal energy density@areal power density (0.256 (mWh cm−2)@5.239 (mW cm−2)) at 0.7 A/g with excellent capacitance retention (93.7 %) and Coulombic efficiency (99.3 %) after 20 k cycles. Its energy storage capability is further validated by illumination of 125 white LEDs and 91 green LEDs upon lighting a single SSC for 50 s each and forming the tandem device with high cyclic stability (102 %). Under open circuit, SSC exhibits self-discharge to 2.4 V without any further appreciable discharge till 24 h.

Reconstructing Hydrogen Bond Network Enables High Voltage Aqueous Zinc-Ion Supercapacitors.

High-voltage aqueous rechargeable energy storage devices with safety and high specific energy are hopeful candidates for the future energy storage system. However, the electrochemical stability window of aqueous electrolytes is a great challenge. Herein, inspired by density functional theory (DFT), polyethylene glycol (PEG) can interact strongly with water molecules, effectively reconstructing the hydrogen bond network. In addition, N, N-dimethylformamide (DMF) can coordinate with Zn2+, assisting in the rapid desolvation of Zn2+ and stable plating/stripping process. Remarkably, by introducing PEG400 and DMF as co-solvents into the electrolyte, a wide electrochemical window of 4.27 V can be achieved. The shift in spectra indicate the transformation in the number and strength of hydrogen bonds, verifying the reconstruction of hydrogen bond network, which can largely inhibit the activity of water molecule, according well with the molecular dynamics simulations (MD) and online electrochemical mass spectroscopy (OEMS). Based on this electrolyte, symmetric Zn cells survived up to 5000 h at 1 mA cm-2, and high voltage aqueous zinc ion supercapacitors assembled with Zn anode and activated carbon cathode achieved 800 cycles at 0.1A g-1. This work provides a feasible approach for constructing high-voltage alkali metal ion supercapacitors through reconstruction strategy of hydrogen bond network.

Reconstructing Hydrogen Bond Network Enables High Voltage Aqueous Zinc‐Ion Supercapacitors

AbstractHigh‐voltage aqueous rechargeable energy storage devices with safety and high specific energy are hopeful candidates for the future energy storage system. However, the electrochemical stability window of aqueous electrolytes is a great challenge. Herein, inspired by density functional theory (DFT), polyethylene glycol (PEG) can interact strongly with water molecules, effectively reconstructing the hydrogen bond network. In addition, N, N‐dimethylformamide (DMF) can coordinate with Zn2+, assisting in the rapid desolvation of Zn2+ and stable plating/stripping process. Remarkably, by introducing PEG400 and DMF as co‐solvents into the electrolyte, a wide electrochemical window of 4.27 V can be achieved. The shift in spectra indicate the transformation in the number and strength of hydrogen bonds, verifying the reconstruction of hydrogen bond network, which can largely inhibit the activity of water molecule, according well with the molecular dynamics simulations (MD) and online electrochemical mass spectroscopy (OEMS). Based on this electrolyte, symmetric Zn cells survived up to 5000 h at 1 mA cm−2, and high voltage aqueous zinc ion supercapacitors assembled with Zn anode and activated carbon cathode achieved 800 cycles at 0.1 A g−1. This work provides a feasible approach for constructing high‐voltage alkali metal ion supercapacitors through reconstruction strategy of hydrogen bond network.

Effect of electrolyte and carbon material on the electrochemical performance of high-voltage aqueous symmetric supercapacitors

The energy storage capability of the aqueous supercapacitors is mainly attributed to the relatively low operating voltage of the device, as the thermodynamic decomposition voltage of water is 1.23 V. Therefore, the extension of the working voltage of the aqueous capacitor beyond the electrolyte decomposition limit is an important subject for the development of environmentally friendly energy storage devices. In this study, a commercial activated carbon (AC) and synthesized phosphorus-doped reduced graphene oxide (P-rGO) were used to gain insight into the influence of both textural properties and the surface chemistry on the electrochemical performance of high-voltage aqueous supercapacitors. Materials on the opposite end of the spectrum (highly porous, undoped AC and heteroatom-rich phosphorus-doped reduced graphene oxide with low porosity) were compared in a symmetric cell, operating in a wide voltage window of 2.0 V in 2 M NaClO4 electrolyte. Additionally, AC-based cell was tested in 1 M Na2SO4 solution to assess the differences in its performance in different sodium-based electrolytes. The obtained results demonstrate that both a porous structure and high contribution of heteroatoms, which improve the hydrophilicity of the electrode, are required to achieve high specific energy density values. However, with increasing current and higher power densities, a developed porous structure is required to maintain good energy storage characteristics. Achieving high operating voltage in the aqueous symmetric full-carbon supercapacitors is a promising energy storage solution. The assembled devices show a good specific energy density of up to 13 Wh kg−1 at a power density of 30 W kg−1.Graphical abstract

High-performance with a high voltage aqueous supercapacitor cell from a simple hybrid electrode of manganese oxide-phenanthrenequinone-graphite sheet

The present-day energy demand is growing research interest in synthesizing new hybrid electrode materials for supercapacitor applications. The main focus is on the development of new electrode materials which could be easily available, synthesizable, low-cost, environmentally friendly, with worthy electrochemical performance. In this scenario, a simple one-step synthesis of MnO2 is carried out via the solvothermal method. This MnO2 is mixed with a molecule of phenanthrenequinone (PQ). MnO2, PQ, and MnO2-PQ composite are characterized by FT-IR, XRD, TGA, and BET techniques and confirmed the formation of MnO2-PQ composite. The electrode is made by drop-casting the electro-active materials such as MnO2, PQ, and MnO2-PQ on a graphite sheet wherein a graphite sheet acts as an electrode and a current collector. These electrodes are studied in a three-electrode and also in the symmetric cell configuration in 1 M Na2SO4 electrolyte via CV, GCD, and EIS techniques. The device is constructed in Swagelok system exhibits a high energy density of 140 W h kg−1 at a power density of 6000 W kg−1 in the voltage range of 0 to 2 V. A coin cell is built using two MnO2-PQ-GS hybrid electrodes in an aqueous Na2SO4 electrolyte. The cycle life of the coin cell showed a retention capacitance of 74 % after 5000 cycles at a high voltage of 2 V, even at a high current density of 5 A g−1. EIS results carried out before and after 5000 cycles show that the cell works even at 5000 GCD cycles.

Biomass blend derived porous carbon for aqueous supercapacitors with commercial-level mass loadings and enhanced energy density in redox-active electrolyte

Herein, oxygen-enriched porous carbon (PC-x) was prepared via co-pyrolysis, and activation of a ternary mixture of biomass (viz. sugarcane bagasse, water hyacinth, and yellow oleander). The PC-x possessed large surface area (1439–2297 m2 g−1), high pore volume, rational micro/mesopore distribution, and surface oxygen functionalities (C/O = 2.5–3.1). The optimized PC-x electrodes at commercial electrode mass loading (∼10 mg cm−2) unveiled a gravimetric capacitance of 251.6F g−1, high rate capability, and cycle stability in 6 M KOH electrolyte. The high voltage (2 V) aqueous symmetric supercapacitor fabricated in 1 M Li2SO4 electrolyte exhibited an energy density of 22.75 W h kg−1 (at 200 W kg−1) with 96.8% capacitance retention over 10,000 cycles at 5 A g−1. The energy density of the supercapacitor was augmented to 37.24 W h kg−1 (at 200 W kg−1) using 0.1 M Na2MoO4 as the redox-additive in the Li2SO4. The approach of using biomass blends offers flexibility in terms of choosing multiple waste biomass as precursors for large-scale and sustainable production of carbon materials for energy storage applications.

Series module of quinone-based organic supercapacitor (>\u20096\xa0V) with practical cell structure

Inexpensive, high-performing, and environmentally friendly energy storage devices are required for smart grids that efficiently utilize renewable energy. Energy storage devices consisting of organic active materials are promising because organic materials, especially quinones, are ubiquitous and usually do not require harsh conditions for synthesis, releasing less CO2 during mass production. Although fundamental research-scale aqueous quinone-based organic supercapacitors have shown excellent energy storage performance, no practical research has been conducted. In this study, we aimed to develop a practical-scale aqueous-quinone-based organic supercapacitor. By connecting 12 cells of size 10 cm × 10 cm × 0.5 cm each in series, we fabricated a high-voltage (> 6 V) aqueous organic supercapacitor that can charge a smartphone at a 1 C rate. This is the first step in commercializing aqueous organic supercapacitors that could solve environmental problems, such as high CO2 emissions, air pollution by toxic metals, and limited electricity generation by renewable resources.

Perspectives on Working Voltage of Aqueous Supercapacitors.

Aqueous supercapacitors have the superiorities of high safety, environmental friendliness, inexpensive, etc. High energy density supercapacitors are not conducive to manufacturing due to the limitation of water thermodynamic decomposition potential, resulting in a narrow working voltage window. To address such challenges, a great endeavor has started to investigate high voltage aqueous supercapacitors as well as making some progress. This review summarizes key strategies regarding the realization of wide working voltage of aqueous supercapacitors and analyzes the involved mechanism, including the optimization of electrodes, electrolytes, diaphragms, and supercapacitor structures. From the perspective of extending the theoretical voltage window, electrode functionalization, heteroatom doping, neutral electrolyte, water-in-salt electrolyte, introducing redox mediators into electrolyte, and designing asymmetric structure are effective strategies for achieving this goal. Further, the actual voltage window can be maximized by optimizing the electrode mass ratio, adjusting potential of zero voltage, and electrode functionalization. The challenge and future of expanding working voltage of aqueous supercapacitors are further discussed. Importantly, this review provides inspiration for the development of supercapacitors with high energy density.

Ultrathin Polypyrrole Layers Boosting MoO3 as Both Cathode and Anode Materials for a 2.0 V High-Voltage Aqueous Supercapacitor.

An aqueous supercapacitor is an emerging energy storage unit on account of its low cost, fast energy delivery rate, and long service life. The energy density of an aqueous supercapacitor can be enlarged via extending the voltage window of electrode materials, while the aqueous electrolyte remains thermodynamically constant at 1.23 V. Herein, an aqueous supercapacitor with a 2.0 V high-voltage window is realized by core-shell MoO3-x/polypyrrole (MP) nanocomposites as both cathode and anode materials. The ultrathin PPy layer on the MoO3 core not only improves the conductivity and cycle stability of the nanocomposites but also acts as a reductant, leading to the formation of oxygen vacancies in the MoO3 core. When used as a cathode material, the potential range of the as-obtained MP nanocomposite is up to 1.0 V. As an anode material, the stable potential range could reach -1.0 V. Due to the large potential range of the cathode and anode, the as-obtained 2.0 V aqueous supercapacitor shows a remarkably high delivery energy of 58.5 Wh kg-1. The synthesis of MP nanocomposites is simple and the electrode performance is significantly enhanced; thus, it is a suitable candidate for high-energy-density aqueous supercapacitors.

Correction: Nucleotide(s)-mediated simultaneous N, P co-doped reduced graphene oxide (N, P-rGO) porous nanohybrids as high-performance electrode materials for designing sustainable binder-free high-voltage (2.8 V) aqueous symmetric supercapacitors and electrochemical sensors

Correction for ‘Nucleotide(s)-mediated simultaneous N, P co-doped reduced graphene oxide (N, P-rGO) porous nanohybrids as high-performance electrode materials for designing sustainable binder-free high-voltage (2.8 V) aqueous symmetric supercapacitors and electrochemical sensors’ by Ikrar Ahmad et al., Sustainable Energy Fuels, 2022, https://doi.org/10.1039/d2se00666a.

Nucleotide(s)-mediated simultaneous N, P co-doped reduced graphene oxide (N, P-rGO) porous nanohybrids as high-performance electrode materials for designing sustainable binder-free high-voltage (2.8 V) aqueous symmetric supercapacitors and electrochemical sensors

The 5′-AMP-mediated simultaneous reduction and co-doping of N and P into GO yielded high-performance electrode material (N, P-rGO) with defects and active centres for designing of binder-free 2.8 V aqueous SSC and sensing of dopamine and serotonin.

High-voltage aqueous symmetric supercapacitors based on 3D bicontinuous, highly wrinkled, N-doped porous graphene-like ultrathin carbon sheets

Biomass-derived 3D bicontinuous, highly-wrinkled, N-doped porous graphene-like ultrathin carbon sheets for high-performance aqueous symmetric supercapacitor.

Molecular crowding agents engineered to make bioinspired electrolytes for high-voltage aqueous supercapacitors

The development of low-cost and eco-friendly aqueous electrolytes with a wide voltage window is the key to achieving safe high energy density supercapacitors (SCs). In this work, a molecular crowding electrolyte is prepared by simulating the crowded environment in living cells. Ion transport in the molecular crowding electrolyte can be effectively improved via reducing the molecular weight of the crowding agent, polyethylene glycol (PEG). The results show that PEG with a molecular weight of 200 (PEG200) can significantly improve ionic conductivity while maintaining a wide voltage window. These advantages enable commercial activated carbon-based SCs to work at 2.5 V with high energy density, outstanding rate performance and good stability for more than 10,000 cycles. On this basis, three series of molecular crowding electrolytes using sodium perchlorate, lithium perchlorate, and sodium trifluoromethanesulfonate as salts are developed, demonstrating the versatility of PEG200 for wide-voltage aqueous electrolytes.

High voltage binder free hybrid supercapacitor based on reduced graphene oxide/graphene oxide electrodes and “water in salt” electrolyte

In the present work, a new low-cost and high-voltage aqueous symmetric supercapacitor cell using a blend of graphene oxide and reduced graphene oxide as binder free active material and “water-in-salt” electrolyte was developed. A reliable operating cell voltage of 2.3 V was achieved for this aqueous hybrid device. The symmetrical two electrode cell is able to exhibit a maximum specific energy of 43.8 W h/kg (325.7 µW h cm−2) at power density of 115.6 W kg−1 (858.6 µW cm−2) at a current density of 0.1 A/g with high cycle stability. It retains 84% of its initial specific energy even after 10,000 charge/discharge cycles. In addition, it exhibits outstanding charge storage capacity, reaching a specific capacity of 59.7 F g−1 (283 mF cm−2) in 17 m NaClO4 “water in salt” electrolyte at 0.1 A g−1. The superior energy storage capability is mainly attributed to the use of very high specific surface area of reduced graphene oxide and the participation of the electrochemical activity of graphene oxide surface functional groups in the charge storage process. The presence of graphene oxide eliminates the need to use non-conductive electro inactive adhesives and due to enhanced hydrophilicity of the electrode synergistically improves diffusion of the hydrated cations and anions to the surface of hydrophobic RGO.

Simple CVD growth of P-doped graphitic hallow carbon spheres for high-voltage (2.0\xa0V) aqueous symmetric supercapacitor

Doping of heteroatom into well-structured mesoporous carbon architecture can significantly augment the capacitive performance. In this work, we report P-doped graphitic hollow carbon spheres (P-GHCS) grown over Fe-KIT-6 through the in situ approach using the catalytic CVD technique. The obtained P-GHCS possesses a relatively high surface area with uniform mesoporous structure, good graphitization with tunable P-doping contents. The highly favorable structure and desirable heteroatom doping were taken into account to evaluate the P-GHCS as a modified electrode material towards high-performance supercapacitor. The optimized P-GHCS-800 sample exhibits superior specific capacitance (Csp) 321 F g−1 at 0.2 A g−1 with outstanding cycling stability with 2.9% loss of its initial capacitance after 2000 cycles in 6 M KOH electrolyte background in the three-electrode computerized system. More importantly, the fabricated P-GHCS-800 symmetric supercapacitor device can withstand at a wide potential width of 2.0 V, together with remarkable cyclic stability (89.09%) after 2000 cycles at a current density of 1 A g−1 in aqueous 1 M Na2SO4 as electrolyte providing a relatively high energy density of 10.83 Wh kg−1 with a power density of 222.78 W kg−1. Additionally, we demonstrated the single symmetric supercapacitor cell which provided sufficient energy to turn on a red LED of 20 mW and emit light over a certain period of time opens up possible realistic applications.

Promoting operating voltage to 2.3 V by a superconcentrated aqueous electrolyte in carbon-based supercapacitor

Aqueous supercapacitors (SCs) have attracted more and more attention for their safety, fast charge/discharge capability and ultra-long life. However, the application of aqueous SCs is limited by the low working voltage due to the narrow electrochemical stability window (ESW) of water. Herein, we report a new “water in salt” (WIS) electrolyte by dissolving potassium bis (fluorosulfonyl) amide (KFSI) in water with an ultra-high mass molar concentration of 37 mol/kg. The highly concentrated electrolyte can achieve a wide ESW of 2.8 V. The WIS electrolyte enables a safe carbon-based symmetrical supercapacitor to operate stably at 2.3 V with an ultra-long cycle life and excellent rate performance. The energy density reaches 20.5 Wh/kg at 2300 W/kg, and the capacity retention is 83.5% after 50,000 cycles at a current density of 5 A/g. This new electrolyte will be a promising candidate for future high-voltage aqueous supercapacitors.

From hierarchically porous carbon to Mn3O4/Carbon composites for high voltage aqueous supercapacitors

The energy density of aqueous supercapacitors can be enhanced by large capacitance and high operating voltage, which is mainly limited by the narrow stability window of the water. Herein, carbonaceous materials with nitrogen-doping and hierarchical porosity are successfully designed and prepared via a facile sol−gel route accompanied by the cooperative self-assembly of triblock copolymer and melamine-resorcinol-formaldehyde polymer, followed by carbonization and CO2 activation. Based on the as-prepared carbon material, symmetric supercapacitors achieve a high cell voltage of 1.8 V in 1 M Li2SO4, and exhibit a high energy density of 27.8 Wh kg−1 at power density of 0.23 kW kg−1. Taking advantage of the reversible redox reaction of Mn3+/Mn4+ accompanied with the insertion/extraction of Li+ of birnessite-type MnO2, the working potential of positive electrode is further expanded to 0–1.2 V (vs SCE). Then asymmetric supercapacitors with high cell voltage of 2.2 V have been successfully fabricated, exhibiting a higher energy density of 44.5 Wh kg−1 at power density of 0.27 kW kg−1 and an acceptable energy density of 21.8 Wh kg−1 even at a high power density of 5.6 kW kg−1.

Preparation of Graphene/Mn3O4 by Flash Irradiating for High Voltage Aqueous Supercapacitors

Graphene/Mn3O4 composites were synthesized by a facile flash irradiation method at room temperature. The flash irradiating triggers the deoxygenation reaction of graphene oxide and the decomposition reaction of manganese nitrate. Additionally, the graphene/Mn3O4 positive electrodes showed a wide operating potential of 0–1.2 V (vs SCE) in aqueous electrolyte for supercapacitors. The extended potential windows further expand the stable cell voltage (2.2 V) for aqueous asymmetric supercapacitors with a high energy density of 18.5 Wh kg−1 at power density of 0.28 kW kg−1.

Long term durability of solid-state supercapacitor based on reduced graphene oxide aerogel and carbon nanotubes composite electrodes

Extensive research efforts are dedicated to the development of high voltage aqueous supercapacitors to solve their low energy density concern. Whilst the operating voltage window is governed by decomposition of water (i.e. 1.23 V), this can be overcome by levelling potentials of oxygen (OER) and hydrogen (HER) evolution reactions. Here we report the design of a symmetrical supercapacitor (SC) in which the electrodes are fabricated from an ink containing reduced graphene oxide (rGO) aerogel, carbon nanotubes and binder by screen printing method on a conductive carbon cloth. For rGO aerogels, porous graphene oxide is prepared by freeze drying and reduced at 180 °C in vacuum. The solid-state supercapacitor is manufactured with two electrodes and a cation exchange electrolyte membrane and can operate stably at 1.6 V in a neutral environment. The initial supercapacitor power density increases from 61.7 to 602 W kg−1 when the current is raised from 0.1 to 1 A g−1. During the long-term durability test, the SC energy density decreases from 4.2 to 2.7 Wh kg−1 while the power density reduces from 61.7 to 53.1 W kg−1. The post-mortem analysis performed by X-ray photoelectron spectroscopy on electrodes demonstrats only small chemical modifications of positive electrode, while negative electrode is insignificantly altered, indicating promising perspectives for the usability of rGO-aerogel-based electrodes in practical supercapacitor applications. The self-discharge of the supercapacitor assessed after 10 000 charge/discharge cycles and 140 h floating at 1.6 V is mainly dependent on the activation contributions, likely due to Faradaic processes, which are more significant than that of diffusional ion species and voltage decay due to ohmic leakage mechanism. The solid-state rGO-based supercapacitor is found to be rather stable even under very stringent durability test, revealing low self-discharge rate that can promote possible commercial applications.

"Water-in-salt" electrolyte enhanced high voltage aqueous supercapacitor with carbon electrodes derived from biomass waste-ground grain hulls.

To design high specific surface area and optimize the pore size distribution of materials, we employ a combination of carbonization and KOH activation to prepare activated carbon derived from ground grain hulls. The resulting carbon material at lower temperature (800, BSAC-A-800) exhibits a porous structure with a high specific surface area of 1037.6 m2 g−1 and a pore volume of 0.57 m3 g−1. Due to the synergistic structural characteristics, BSAC-A-800 reveals preferable capacitive performance, showing a specific capacitance as high as 313.3 F g−1 at 0.5 A g−1, good rate performance (above 73%), and particularly stable cycling performance (99.1% capacitance retention after 10 000 cycles at a current density of 10 A g−1). More importantly, the assembled symmetric supercapacitor using a water-in-salt electrolyte (17 m NaClO4) with high discharge specific capacitance (59 F g−1 at 0.5 A g−1), high energy density (47.2 W h kg−1) and high voltage (2.4 V) represents significant progress towards performance comparable to that of commercial salt-in-water electrolyte supercapacitors (with discharge specific capacitance of 50 F g−1, energy densities of ∼28.1 W h kg−1 and voltages of 2.0 V).