Aqueous Electrochemical Capacitors Research Articles

Dilute aqueous hybrid electrolyte endows a high-voltage window for supercapacitors

Developing aqueous electrolyte with wide electrochemical stability window (ESW) is one of the key technologies to boosting the energy density of aqueous electrochemical double-layer capacitors (EDLCs). However, expanding the ESW using super-concentrated electrolytes comes at the expense of cost, ionic conductivity and the mass density of the device. Herein, we selected a cost-effective and environmentally friendly molecular crowding agent, polyethylene glycol dimethyl ether (PEGDME-250), as the electrolyte additive designed to achieve high ESW at low salt concentrations. First, PEGDME can reduces the activity of water and increases the ESW of the electrolyte by forming hydrogen bonds with free water molecule. In addition, PEGDME does not have the interaction between terminal groups compared to conventional polyethylene glycols (PEG), which helps to reduce the viscosity and improve the ionic conductivity of the electrolyte. By adjusting the composition of the electrolyte, the optimal electrolyte with a high ESW of 2.96 V was achieved with the formula of 3 m NaClO4-25 % H2O-75 % PEGDME. The EDLCs assembled with this diluted aqueous electrolyte and YP-50 F electrodes presents a high operational voltage window of 2.4 V and outstanding cycle stability (>10000 cycles), and maintains excellent rate performance in a temperature range of −10–35 ℃.

Fabrication of high voltage and energy dense supercapacitor with Manilkara zapota seeds derived porous activated carbon in acidic electrolyte

Achieving a high working potential window beyond the water electrolysis limit (> 1.23 V) has always been a big challenge in the fabrication of aqueous supercapacitors (SCs). Since energy density has quadratic dependence on voltage, broadening of cell voltage can remarkably increase the energy density of the capacitor. Unfortunately, most of the aqueous electrochemical capacitors operated in strongly acidic or alkaline electrolytes show poor energy densities owing to their narrow operating voltage limit. To obtain a high voltage range of aqueous symmetric carbon/carbon capacitors and achieve high energy density, the chemistry of electrode materials and the electrochemical kinetics of electrolytes play a critical role. Hence, in the present work, we explore a porous activated carbon produced from the seeds of Manilkara zapota (ACMZ-700) in combination with sulfuric acid electrolyte to obtain an encouraging electrochemical stable working voltage window. Thus, the resulting symmetric SC effectively exhibited an enlarged cell voltage of 1.5 V in an acidic medium. Importantly, due to the positive role of oxygenated surface functional groups of the ACMZ-700 active material, the fabricated symmetric cell delivered an attractive specific capacitance of 334 F g−1 @ 1 A/g. As a result of high capacitance and broader working voltage, the symmetric device delivered an excellent energy density of 26 Wh kg−1 at a power density of 748 W kg−1 and held excellent cycling stability over 10,000 charge/discharge cycles.

Unveiling the charge storage mechanisms of perovskite fluoride KMnF3 in neutral aqueous electrolyte

Perovskite fluorides (ABF3) have attracted much attention as an emerging and promising electrode material for electrochemical energy storage. However, to reveal the charge storage mechanisms of ABF3 in neutral media and further facilitate their energy storage utilizations remains very challenging. Herein, we have, for the first time, elucidated the charge storage mechanisms of KMnF3 in neutral aqueous electrolyte (1 M Na2SO4) via diverse ex/in situ physicochemical/electrochemical techniques, showing the dominated conversion reactions along with partial insertion/extraction reactions and adsorption during the charging/discharging processes. Specifically, the KMnF3 nanocrystals transform into amorphous MnOOH/(hydrated)MnO2 nanosheets via adsorption and conversion mechanisms by formation/oxidation of [Mn-OH] intermediates and Mn2+ accompanying with partial H+ extraction of MnOOH to MnO2 in the first charging period, while the partial Na+ and H+ insert into the MnO2 to form Na0.5MnO2 and MnOOH and further (hydrated)MnO2 and MnOOH convert into Mn3O4 and/or Mn2+ in the discharging period. The as-constructed aqueous electrochemical capacitors (ECs) by matching KMnF3 positive electrode with activated carbon (AC) negative electrode exhibit good energy/power density and spectacular cycling behavior. Overall, this work addresses an in-depth understanding of charge storage mechanisms of KMnF3 in neutral media and plays a milestone-like role in developing state-of-the-art ABF3 in aqueous storage systems.

Synthesis and characterization of electron beam irradiated glutinous rice husk-derived biochar and activated carbon for aqueous electrochemical capacitors

Glutinous rice husk, an abundant agricultural biowaste in Thailand, was pretreated with high energy electron beam irradiation (EBI) at doses of 500 kGy, 1000 kGy, and 1500 kGy prior to fabrication into biochar by carbonization at 500℃ under nitrogen atmosphere. The biochar was then treated with KOH and subsequently heated at 800℃, yielding activated carbon (GAC). The physical, chemical, and electrochemical properties of the as-received biochar (GB) and activated carbon (GAC) were investigated. Scanning electron microscopic images (SEM) suggested that biochar irradiated with 1500 kGy (GB-1500) has the highest porosity compared to the other samples. The electrochemical properties of GB and GAC in 3 M H2SO4 using a three-electrode system indicated that EBI affects the electrochemical performance of the material. The specific capacitance of GB-1500 (6.15 F·g-1 at 0.05 A·g-1) is higher than that of the as-received biochar, and the improved performance of the former is potentially due to the formation of structural defects upon irradiation. Finally, we observed that the specific capacitances of the GAC were much higher than those of their corresponding GB with the same irradiation doses, and the capacitances of the GAC decrease with increasing EBI dose.

Electrochemical Oxidation of Anthracene to Anthraquinone inside the Nanopores of Activated Carbon: Implications for Electrochemical Capacitors

Anthracene (ANT) is oxidized to anthraquinone (AQ) inside the nanopores of activated carbon (AC) without any metal catalysts. The resulting hybrid of AC and AQ shows high performance as an anode for aqueous electrochemical capacitors due to the reversible redox reaction of AQ inside the nanopores of AC. ANT is first adsorbed inside the nanopores of AC in the gas phase and finely dispersed at the nanolevel. Consequently, the adsorbed ANT has a large contact area with the conductive carbon pore surface. The adsorbed ANT is then electrochemically oxidized to AQ in aqueous H2SO4 electrolyte. The oxidation of ANT requires conductive surfaces for charge transfer, and ANT is efficiently oxidized at the large contact interface between the conductive carbon pore surface and ANT by using AC. The hybrid is capable of fast charging and discharging through rapid charge transfer at the large contact interface. In addition, since the hybridization of AQ inside the AC pores does not expand the volume of AC particles, the volumetric capacitance is enhanced by the hybridization. Furthermore, the nanopores of AC prevent AQ from desorbing into the electrolyte during charging and discharging. Consequently, the hybridization endows the hybrid with a high volumetric capacitance, a high rate capability, and a long cycle lifetime. We demonstrate that nanopores can provide the reaction environment not only for efficient oxidation of ANT as nanoreactors but also for a fast redox reaction of AQ.

Polysaccharide-based hydrogel electrolytes enriched with poly(norepinephrine) for sustainable aqueous electrochemical capacitors

One of the main drawbacks of conventional energy storage systems is the possibility of electrolyte leakage due to faulty encapsulation or accidental overcharge. In this work, novel environmentally benign all-biopolymer-based hydrogels have been designed to be used as electrolytes in aqueous electrochemical capacitors. A primary component of gels, i.e. microcrystalline cellulose dissolved in NaOH/urea mixture and cross-linked with epichlorohydrin, was used in combination with agarose, or subsequently introduced agarose and poly(norepinephrine) to form the three-dimensional interpenetrated polymer network of enhanced mechanical stability. The effect of introduced components has been studied independently and investigated in terms of morphology, conductivity, swelling degree, and mechanical/structural properties. The electrochemical diagnostic tests of electrochemical capacitors utilizing acidic (H2SO4, H4SiW12O40) and neutral (Na2SO4) electrolyte-saturated hydrogels of the best overall characteristics have revealed the performance approaching the one characteristic of the liquid electrolyte-based counterparts but with lower self-discharge. The systems were characterized by the capacitance of 97–127 F g−1 at 0.1 A g−1 with retention of 33–89% at 20 A g−1, and cycling life of 10 000 cycles with a maximum of 10% of capacitance loss. The effect of intentionally improved hydrophilic properties of activated carbon electrodes, on the interfacial compatibility between electrode/hydrogel electrolyte is discussed as well. The results are in line with the global trend of green and sustainable energy storage designed for specific applications, i.e. in-vivo systems and wearable electronics.

Redox enhanced membraneless electrochemical capacitor with CO2-derived hierarchical porous carbon electrodes

Aqueous redox electrochemical capacitors (redox ECs) generally suffer from low energy density and uncontrolled cross-diffusion of soluble redox components, causing serious self-discharge and capacity fade. We have designed a membraneless potassium bromide electrochemical capacitor (ML-PBEC) single cell generating high energy density with reduced self-discharge. From the perspective of CO2 utilization, highly porous carbon with a hierarchical structure was synthesized from CO2 gas and the change of the electrochemical performance by doping nitrogen on the CO2-derived carbon materials was investigated. Using the synthesized carbon as electrodes with a Br−/Br3− redox active aqueous electrolyte, the designed ML-PBEC single cell produced enhanced energy density due to suppressed cross-diffusion of soluble bromine molecules through electrostatic interaction between the protons of N-doped sites and polybromide anions even without membrane. The designed single cell provided a high energy density equivalent to the highest energy density of membrane contained aqueous electrochemical capacitors, 14.3 Wh kg−1 at 2 A g−1 over 10,000 cycles.

Metal-Free Homocoupling of Pyrene inside the Pores of Mesoporous Carbons via Electrochemical Oxidation: Application for Electrochemical Capacitors.

A pyrene dimer (PYD) is synthesized by electrochemical oxidation via homocoupling of pyrene (PY) inside the pores of MgO-templated mesoporous carbons without any metal catalysts or organic solvents. The resulting MgO-C/PYD hybrids can be used as high-performance aqueous electrochemical capacitor electrodes due to the reversible redox property of PYD and large contact area between the hybridized PYD and conductive carbon surfaces, which enable rapid charge transfer at the large contact interface. In our previous study, PY was considered to polymerize through electrochemical oxidation, and activated carbon with the pore sizes of ∼4 nm was used as a porous carbon substrate. In this study, the MgO-templated carbons have the average pore sizes of 5, 10, and 30 nm, and their large mesopore volumes can accommodate a large amount of PYD for enhancing the capacitance. To develop high-performance electrochemical capacitors, the dependence of the capacitance enhancement and the capacitance retention on the amount of PY and the pore sizes of MgO-templated carbons are studied. It is found that mesopores are necessary for fast charging/discharging, but the capacitance retention and capacitance enhancement decrease with increasing the mesopore sizes and the amount of PY due to the decreased utilization ratio of PY.

Operando Monitoring of Local pH Value Changes at the Carbon Electrode Surface in Neutral Sulfate-Based Aqueous Electrochemical Capacitors.

The operando monitoring of pH during the charging and discharging of an electrochemical capacitor in an aqueous neutral salt solution is presented. Proper knowledge of transient and limiting pH values allows for a better understanding of the phenomena that take place during capacitor operation. It also enables the proper assignment of the reaction potentials responsible for water decomposition. It is shown that the pH inside the capacitor is strongly potential-dependent and different for individual electrodes; therefore, the values of the evolution potentials of hydrogen and oxygen cannot be precisely calculated based only on the initial pH of the electrolyte. The operando measurements indicate that the pH at the positive electrode reaches 4, while at the negative electrode, it is 8.5, which in theory could shift the theoretical operating voltage well beyond 1.23 V. On the other hand, high voltage cannot be easily maintained since the electrolyte of both electrode vicinities is subjected to mixing. Operando gas monitoring measurements show that the evolution of electrolysis byproducts occurs even below the theoretical decomposition voltage. These reactions are important in maintaining a voltage-advantaged pH difference within the cell. At the same time, the electrochemical quartz crystal microbalance (EQCM) measurements indicated that the ions governing the pH (OH–) that initially accumulated in the vicinity of the positive electrode enter the carbon porosity, losing their pH-governing abilities. pH fluctuations in the cell are important and play a vital role in the description of its performance during the cyclability at a given voltage. This is especially noticeable in cell floating at 1.3 V, where the pH difference between electrodes is the highest (6 units). The increase of the electrode separation distance acts similarly to the introduction of a semipermeable membrane toward the increase of the capacitor cycle life. During floating at 1.6 V, where the pH difference is not as high anymore (4 units), the influence of separation in terms of electrode stability, although present, is less notable.

High frequency response of adenine-derived carbon in aqueous electrochemical capacitor

Electrochemical capacitors are attractive power sources, especially when they are able to operate at high frequency (high current regime). In order to meet this requirement their constituents should be made of high conductivity materials with a suitable porosity. In this study, enhanced power and simultaneously high capacitance (120 F g−1 at 1 Hz or 10 A g−1) electrode material obtained from carbonized adenine precursor is presented. A micro/mesoporous character of the carbon with optimal pore size ratio and high surface area was proven by the physicochemical characterization. The beneficial pore structure and morphology resembling highly conductive carbon black, together with a significant nitrogen content (5.5%) allow for high frequency response of aqueous capacitor to be obtained. The carbon/carbon symmetric capacitor (in 1 mol L−1 Li2SO4) has been tested to the voltage of 1.5 V. The cyclic voltammetry indicates a good electrochemical response even at high scan rate (50 mV s−1). The cyclability of the capacitor is comparable to the one operating with commercial carbon (YP50F). The adenine-based capacitor is especially favourable for stationary applications requiring high power.

Fast Activation of Graphene with a Highly Distorted Surface and Its Role in Improved Aqueous Electrochemical Capacitors

Fast Activation of Graphene with a Highly Distorted Surface and Its Role in Improved Aqueous Electrochemical Capacitors

Potassium formate-based electrolytes for high performance aqueous electrochemical capacitors

In this work we report the use of a water-in-salt (WiSE) electrolyte based on potassium formate (HCOOK). We showed that a solution of 14 M HCOOK in H2O displays promising transport properties and can be successfully utilized for the realization of electrochemical double layer capacitors (EDLCs) operating at 1.7 V, displaying high energy and power densities and extraordinary cycling stability (90% of capacitance retention after 1700 h of float at 1.7 V). Taking these results into account, and the fact that 14 M HCOOK in H2O is cheap, environmentally friendly and not toxic, this electrolyte can be certainly considered very promising for the realization of a novel generation of cheap and high performance aqueous EDLCs.

Particle Dynamics-Based Stochastic Modeling of Carbon Particle Charging in the Flow Capacitor Systems

Aqueous electrochemical flow capacitors (EFCs) have demonstrated high-power capabilities and safety at low cost, making them promising energy storage devices for grid applications. A primary performance metric of an EFC is the steady-state electrical current density it can accept or deliver. Performance prediction, design improvements, and up-scaling are areas in which modeling can be useful. In this paper, a novel stochastic superparticle (SP) modeling approach was developed and applied to study the charging of carbon electrodes in the EFC system, using computational superparticles representing real carbon particles. The model estimated the exact values of significant operating parameters of an EFC, such as the number of particles in the flow channel and the number of electrolytic ions per carbon particle. Optimized model parameters were applied to three geometrical designs of an EFC to estimate their performance. The modeling approach allowed study of the charge per carbon particle to form the electric double-layer structure. The linear relationship between the concentration of SPs and the ionic charge was observed when optimized at a constant voltage of 0.75 V. The simulation results are in excellent agreement with experimental data, providing a deep insight into the performance of an EFC and identifying limiting parameters for both engineers and material scientists to consider.

Prussian‐Blue Analogue‐Derived Hollow Structured Co3S4/CuS2/NiS2 Nanocubes as an Advanced Battery‐Type Electrode Material for High‐Performance Hybrid Supercapacitors

The facile and cost-effective fabrication of hybrid nanostructures comprised of hollow mixed metallic chalcogenides has attracted growing interest in the development of high-performance energy storage devices. Herein, multi-component (nickel-cobalt-copper-sulfides/selenides (NCCS/NCCSe)) hollow nanocubes (HNCs) are prepared via a single-step sulfurization/selenization process. The NCCS material shows interior HNCs, and the NCCSe material exhibits slightly formed porous cubes. Both the prepared materials demonstrate higher charge storage performance than the precursor NCC NCs owing to the improved surface morphology and addition of sulfur and selenium ions. Particularly, the NCCS HNCs electrode reveals superior specific capacity (capacitance) (70.32 mAh g-1 (666.20 F g-1 ) at 5mA cm-2 ) along with excellent cycling stability of 108.6% even after 10000 cycles. Interestingly, the electrode delivers a good rate capability of 83.5% at a high current density of 20mA cm-2 . The feasibility of the battery-type NCCS HNCs as a positive electrode is explored by constructing an aqueous electrochemical hybrid capacitor (AEHC). The AEHC exhibits maximum energy and power densities of 23.15Wh kg-1 and 7899.08 W kg-1 , respectively. Remarkably, it demonstrates superior long-life cycling stability even after 10000 cycles (120.6% retention). The suitability of AEHC for practical application is also tested by driving electronic devices.

Continuous PEDOT:PSS nanomesh film: Towards aqueous AC line filtering capacitor with ultrahigh energy density

Filtering capacitors with high energy densities are of critical importance in stabilizing the pulse signal of circuits that require the conversion of alternating current to direct current, which is however far from satisfactory at current stage due to lack of a reasonable design. Here, we report an ultrafast aqueous electrochemical capacitor based on highly conductive and continuously cross-linked poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) nanomesh films fabricated using a rational morphology engineering strategy. The interpenetrating polymer network promoted an efficient electron transfer and provided smooth channels for ion transport while exposing a significant number of accessible interfacial area to electrolyte. In addition to the large phase angle of − 84° at 120 Hz, this polymer-based electrochemical capacitor exhibited an extremely high areal specific capacitance of 1087 μF cm−2 and areal specific energy density of 544 μF V2 cm−2, far superior to those of most reported aqueous filtering capacitors. Furthermore, it can efficiently smoothen the ripples generated when arbitrary alternating current waveforms were converted into straight signals over a wide frequency range of 1–10,000 Hz. Moreover, it was conveniently integrated with a rotating disk triboelectric nanogenerator for ripple filtering and pulse energy smoothing. This work brings a view for aqueous filtering capacitors with high performance.

Activation Scan Rate Dependence on the Power Performance of an Artificial Interface for High Potential Aqueous Electrochemical Capacitors

Increasing the potential window of electrochemical capacitors (EDLCs) operating in aqueous to improve their energy density remains a major challenge. We propose here a new approach where by designing a passivation layer, termed as artificial interface (ai), to extend the electrolyte potential widow beyond its thermodynamic stability. This is achieved by coating the carbon electrode with a conformal, passivating hybrid ionogel layer, which is electrically insulating but allows ionic conductivity to promote the formation of the electrochemical double layer.Moreover, we observed a strong dependence of the first cycle - called activation process - on the power performance of the ai-modified electrodes, with a marked improvement even compared to pristine electrode.The set of results presented here helps in developing our basic understanding of the charge storage mechanism in porous carbon electrodes in the presence of an ai layer. It also offers opportunities for designing high energy density EDLCs operating in aqueous electrolyte. Figure 1

Enhancing capacitor lifetime by alternate constant polarization

This paper reports the successful attempts of symmetric aqueous electrochemical capacitor (EC) lifespan extension. A novel exploitation procedure, i.e., alternate floating, is herein proposed. Since only the positive electrode ages remarkably in EC systems with LiNO3 as the electrolyte, it is demonstrated that alternate polarization allows the properties of the negative electrode to be fully exploited. In our approach both electrodes play the role of the positive electrode and the negative one alternately; carbon electrodes were subjected to standard and alternate voltage-holding polarization. To observe the capacitor behavior under alternate floating at different conditions, various electrolyte concentrations of LiNO3 (0.2–5.0 mol L−1) were used. It was found that the utilization of alternate polarization during the aging test allows the lifespan of the capacitor to be noticeably prolonged. Moreover, the lifetime improvement strongly depends on the electrolyte concentration. A maximum improvement of 83% of capacitor operation time is achieved for 0.2 mol L−1 LiNO3, i.e., almost doubling the operation time. The aged electrodes were tested postmortem using SEM, TEM, XPS and N2 sorption at 77 K. The results demonstrated that the alternate floating protocol ages both electrodes almost equally.

Electrode Design for MnO2-Based Aqueous Electrochemical Capacitors: Influence of Porosity and Mass Loading

The purpose of this study is to highlight the influence of some fabrication parameters, such as mass loading and porosity, which are not really elucidated and standardized during the realization of electrodes for supercapacitors, especially when using metal oxides as electrode materials. Electrode calendering, as one stage during the fabrication of electrodes, was carried out step-by-step on manganese dioxide electrodes to study the decreasing porosity effect on the electrochemical performance of a MnO2 symmetric device. One other crucial parameter, the mass loading, which has to be understood and well used for realistic supercapacitors, was investigated concurrently. Gravimetric, areal and volumetric capacitances are highlighted, varying the porosity for low-, medium- and large-mass loading. Low-loading leads to the best specific capacitances but is not credible for realistic supercapacitors, except for microdevices. Down 50% porosities after calendering, capacitances are increased and become stable faster, suggesting a faster wettability of the dense electrodes by the electrolyte, especially for high-mass loading. EIS experiments performed on electrodes without and with calendering lead to a significant decrease of the device’s time response, especially at high loading. A high-mass loading device seems to work as a power battery, whereas electrode calendaring, which allows decreasing the time response, leads to an electrical behavior closer to that expected for a supercapacitor.

Supercapacitor with Carbon/MoS2 Composites

Transition metal dichalcogenides (TMDs) with a two-dimensional character are promising electrode materials for an electrochemical capacitor (EC) owing to their unique crystallographic structure, available specific surface area, and large variety of compounds. TMDs combine the capacitive and faradaic contribution in the electrochemical response. However, due to the fact that the TMDs have a strong catalytic effect of promoting hydrogen and oxygen evolution reaction (HER and OER), their usage in aqueous ECs is questioned. Our study shows a hydrothermal l-cysteine–assisted synthesis of two composites based on different carbon materials—multiwalled carbon nanotubes (NTs) and carbon black (Black Pearl-BP2000)—on which MoS2 nanolayers were deposited. The samples were subjected to physicochemical characterization such as X-ray diffraction and Raman spectroscopy which proved that the expected materials were obtained. Scanning electron microscopy coupled with electron dispersive spectroscopy (SEM/EDS) as well as transmission electron microscopy images confirmed vertical position of few-layered MoS2 structures deposited on the carbon supports. The synthetized samples were employed as electrode materials in symmetric ECs, and their electrochemical performance was evaluated and compared to their pure carbon supports. Among the composites, NTs/MoS2 demonstrated the best electrochemical metrics considering the conductivity and capacitance (150 Fg−1), whereas BP2000/MoS2 reached 110 Fg−1 at a current load of 0.2 Ag−1. The composites were also employed in a two-electrode cell equipped with an additional reference electrode to monitor the potential range of both electrodes during voltage extension. It has been shown that the active edge sites of MoS2 catalyze the hydrogen evolution, and this limits the EC operational voltage below 1 V. Additional tests with linear sweep voltammetry allowed to determine the operational working voltage for the cells with all materials. It has been proven that the MoS2/carbon composites possess limited operating voltage, that is, comparable to a pure MoS2 material.

Reversible Intercalation of Al‐Ions in Poly(3,4‐Ethylenedioxythiophene):Poly(4‐Styrenesulfonate) Electrode for Aqueous Electrochemical Capacitors with High Energy Density

Due to the high capacity of the three‐electron redox mechanism, Al‐ions‐based energy‐storage devices have the potential to provide a viable solution to meet the growing demand for powering electronic products. However, discovering suitable electrode materials for reversible insertion of Al ions remains a difficult task. Herein, it is reported that a classical conductive polymeric material poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) can perform the reversible Al‐ions intercalation for aqueous electrochemical capacitors. The as‐prepared PEDOT:PSS film on a carbon cloth composite electrode exhibits a large magnitude of faradaic currents and sharp redox peaks in cyclic voltammetry (CV) curves in aluminum sulfate electrolyte, and delivers a high capacitance of 269 F g−1 (78 mAh g−1). Diffusion‐controlled Al‐ions intercalation/deintercalation as the charge‐storage mechanism is demonstrated here, which is not observed in other ions‐based electrolytes (H+, Mg2+, Li+, Na+). An asymmetric electrochemical capacitor based on Al ions, composed of such an electrode and activated carbon electrode is assembled and displays a high energy density of 41.6 Wh kg−1 at a power density of 0.24 kW kg−1, demonstrating a promising aqueous electrochemical capacitor with an advanced energy density via polyvalent ions intercalation.