Study on electrochemical performance of activated carbon in aqueous Li2SO4, Na2SO4 and K2SO4 electrolytes

Constant chemical potential-quantum mechanical-molecular dynamics simulations of the graphene-electrolyte double layer

We present the coupling of two frameworks-the pseudo-open boundary simulation method known as constant potential molecular dynamics simulations (CμMD), combined with quantum mechanics/molecular dynamics (QMMD) calculations-to describe the properties of graphene electrodes in contact with electrolytes. The resulting CμQMMD model was then applied to three ionic solutions (LiCl, NaCl, and KCl in water) at bulk solution concentrations ranging from 0.5 M to 6 M in contact with a charged graphene electrode. The new approach we are describing here provides a simulation protocol to control the concentration of electrolyte solutions while including the effects of a fully polarizable electrode surface. Thanks to this coupling, we are able to accurately model both the electrode and solution side of the double layer and provide a thorough analysis of the properties of electrolytes at charged interfaces, such as the screening ability of the electrolyte and the electrostatic potential profile. We also report the calculation of the integral electrochemical double layer capacitance in the whole range of concentrations analyzed for each ionic species, while the quantum mechanical simulations provide access to the differential and integral quantum capacitance. We highlight how subtle features, such as the adsorption of potassium graphene or the tendency of the ions to form clusters contribute to the ability of graphene to store charge, and suggest implications for desalination.

Partial Oxidation to Extend the Lifetime of Nanoporous Carbon in an Ultracapacitor with Li2SO4 Electrolyte

A TiC-derived carbon (CDC) and its partially oxidized derivative (ox-red-CDC), oxidized by a modified Hummers method, were studied as promising electrode materials for electrochemical energy storage. To evaluate the electrochemical properties of the carbon materials, cyclic voltammetry, galvanostatic cycling, and electrochemical impedance spectroscopy measurements were performed in 1 M Li2SO4 using 2- and 3-electrode cells. A partially oxidized surface was shown to improve the capacitance and electrochemical stability of a nanoporous CDC at positive potential values. The respective anodic capacitance of 80 F cm−3 reveals a 15% improvement over the non-oxidized CDC. At negative potential values, the capacitance of two carbon materials is almost equal, 97 vs. 93 F cm−3, for the non-oxidized and partially oxidized CDC materials, respectively. An asymmetric 2-electrode ultracapacitor containing ox-red-CDC as the anode and pristine CDC as the cathode demonstrated an excellent cycle life. The temporary repolarization of the 2-electrode cell after thousands of charge–discharge cycles increased the capacitance and improved the cycling characteristics, likely due to regeneration and cleaning of the electrode surface.

Exploration of Charge Storage Behavior of Binder-Free EDL Capacitors in Aqueous Electrolytes

Charge storage in electrochemical double-layer capacitors (EDLCs) is via the adsorption of electrolyte counterions in their positive and negative electrodes under an applied potential. This study investigates the EDLC-type charge storage in carbon nanotubes (CNT) electrodes in aqueous acidic (NaHSO₄), basic (NaOH), and neutral (Na₂SO₄) electrolytes of similar cations but different anions as well as similar anions but different cations (Na₂SO₄ and Li₂SO₄) in a two-electrode Swagelok-type cell configuration. The physicochemical properties of ions, such as mobility/diffusion and solvation, are correlated with the charge storage parameters. The neutral electrolytes offer superior charge storage over the acidic and basic counterparts. Among the studied ions, SO₄ ^2- and Li⁺ showed the most significant capacitance owing to their larger solvated ion size. The charge stored by the anions and cations follows the order SO₄ ^2- > HSO₄ ^- > OH^- and Li⁺ > Na⁺, respectively. Consequently, the CNT//Li₂SO₄//CNT cell displayed outstanding charge storage indicators (operating voltage ∼0-2 V, specific capacitance ∼122 F·g^-1, specific energy ∼67 W h·kg^-1, and specific power ∼541 W·kg^-1 at 0.5 A·g^-1) than the other cells, which could light a red light-emitting diode (2.1 V) for several minutes. Besides, the CNT//Li₂SO₄//CNT device showed exceptional rate performance with a capacitance retention of ∼95% at various current densities (0.5-2.5 A·g^-1) after 6500 cycles. The insights from this work could be used to design safer electrochemical capacitors of high energy density and power density.

Effects of Valence States of Working Cations on the Electrochemical Performance of Sodium Vanadate

Supercapacitors have received much attention as large-scale energy storage devices for high power density and ultralong cycling life. In this work, sodium vanadate Na_0.76V₆O₁₅/poly(3,4-ethylenedioxythiophene) (PEDOT) nanocables with deficient bridge oxygen at the interface (denoted Vo^••-PNVO) have been tailored for supercapacitors through the in situ polymerization of 3,4-ethylenedioxythiophene and studied using three different electrolytes. Experiments and theoretical calculations reveal that all Na⁺, Zn²⁺, and Al³⁺ ions appear as hydrates in aqueous solutions but insert into the crystal structure as Na⁺ ions and Zn²⁺-H₂O and Al³⁺-H₂O hydrates, respectively. In comparison with the Zn²⁺-H₂O and Al³⁺-H₂O hydrates, Na⁺ ions with a smaller radius diffuse more quickly in Vo^••-PNVO. Thus, Vo^••-PNVO delivers better charge storage capability and stability when an electrolyte with Na⁺ ions is used. The results strongly suggest that an electrostatic interaction is significant in determining transport properties and storage capacities, rather than hydrate radii or valence states.

High performance asymmetric supercapacitors based on Ti3C2T x MXene and electrodeposited spinel NiCo2S4 nanostructures

Spinel metal sulfides have been investigated for a wide range of applications mostly in electrochemical energy storage owing to their better electronic conductivity and high reversible redox activity. Herein, we report a facile fabrication approach for the binder-free supercapacitor electrodes based on spinel NiCo₂S₄ (NCS) on various substrates such as Cu-foil (CF), Ni-foam (NF), and vertical graphene nanosheets grown on carbon tape (VG) via a single step-controlled electrodeposition technique. The obtained electrodeposited NiCo₂S₄ grown on Cu-foil (denoted as CF-NCS) in symmetric assembly shows a high specific capacitance of 167.28 F g^-1 compared to NCS grown on Ni-foam and VG substrates, whereas, symmetric NiCo₂S₄ grown on a VG substrate device exhibits better cycling performance (81% for 3000 cycles) compared to CF-NCS and NF-NCS. Furthermore, an asymmetric supercapacitor was assembled in combination with MXene (Ti₃C₂T _x ) as a negative electrode (denoted as TCX). As a result, the CF-NCS//TCX device exhibits a high areal capacitance of 48.6 mF cm^-2 at 2 mA cm^-2 of current density. We also report good specific capacitance of 54.57 F g^-1 at 2 A g^-1; in addition, the CF-NCS//TCX assembly delivers maximum areal and gravimetric energy density of 14.86 mWh cm^-2 and 14.86 Wh kg^-1 respectively. In contrast, the VG-NCS//TCX device showed improved cycling stability with 85% of capacitance retention over 5000 cycles owing to its highly porous structure and multiple conductive networks in the VG substrate and provides structural stability to NCS with fast ion diffusion. This experiment favors 2D MXene as a capacitive electrode that provides a replacement for carbon-based electrodes in asymmetric assembly with superior electrochemical performance. Hence, the hierarchical NCS structure grown on the various substrates in combination with MXene serve as a promising material for energy storage application.

An evaluation of the capacitive behavior of supercapacitors as a function of the radius of cations using simulations with a constant potential method

We report on the atomistic molecular dynamics, applying the constant potential method to determine the structural and electrostatic interactions at the electrode-electrolyte interface of electrochemical supercapacitors as a function of the cation radius (Cs⁺, Rb⁺, K⁺, Na⁺, Li⁺). We find that the electrical double layer is susceptible to the size, hydration layer volume, and cations' mobility and analyzed them. Besides, the transient potential shows an increase in magnitude and length as a function of the monocation size, i.e., Cs⁺ > Rb⁺ > K⁺ > Na⁺ > Li⁺. On the other hand, the charge distribution along the electrode surface is less uniform for large monocations. Nonetheless, the difference is not observed as a function of the radius of the cation for the integral capacitance. Our results are comparable to studies that employed the fixed charge method for treating such systems.

Combined effect of nitrogen-doped functional groups and porosity of porous carbons on electrochemical performance of supercapacitors

In this work, nitrogen-doped porous carbons obtained from chitosan, gelatine, and green algae were investigated in their role as supercapacitor electrodes. The effects of three factors on electrochemical performance have been studied-of the specific surface area, functional groups, and a porous structure. Varying nitrogen contents (from 5.46 to 10.08 wt.%) and specific surface areas (from 532 to 1095 m2 g-1) were obtained by modifying the carbon precursor and the carbonization temperature. Doping nitrogen into carbon at a level of 5.74-7.09 wt.% appears to be the optimum for obtaining high electrochemical capacitance. The obtained carbons exhibited high capacitance (231 F g-1 at 0.1 A g-1) and cycle durability in a 0.2 mol L-1 K2SO4 electrolyte. Capacitance retention was equal to 91% at 5 A g-1 after 10,000 chronopotentiometry cycles. An analysis of electrochemical behaviour reveals the influence that nitrogen functional groups have on pseudocapacitance. While quaternary-N and pyrrolic-N nitrogen groups have an enhancing effect, due to the presence of a positive charge and thus improved electron transfer at high current loads, the most important functional group affecting energy storage performance is graphite-N/quaternary-N. The study points out that the search for the most favourable organic precursors is as important as the process of converting precursors to carbon-based electrode materials.

Electrolyte Technologies for High Performance Sodium-Ion Capacitors

Bridging the energy gap between batteries and capacitors, while in principle delivering a supercapacitor-like high power density and long lifespan, sodium-ion capacitors (SIC) have been considered promising energy storage devices that could be commercialized in the near future due to the natural abundance of sodium sources and the performance superiority of SIC devices. Therefore, in the past decade, substantial research efforts have been devoted to their structure and property improvements. With regard to the electrochemical performance of an ion capacitor, except for the electrode, the composition and structure of the electrolytes are also of great importance. Thus, in this mini review, we present a brief summary of the electrolytes developed recently for high performance SIC, including aqueous, organic, and ionic liquid based electrolytes. The influence factors such as ionic conductivities, electrolyte concentrations, electrochemical stable windows, as well as the cost and safety issues are discussed. Furthermore, the future perspectives and challenges in the science and engineering of new electrolytes are also considered. We hope that this review may be helpful to the energy storage community regarding the electrolytes of advanced SIC systems.

A QM/MD Coupling Method to Model the Ion-Induced Polarization of Graphene

We report a new Quantum Mechanical/Molecular Dynamics (QM/MD) simulation loop to model the coupling between the electron and atom dynamics in solid/liquid interfacial systems. The method can describe simultaneously both the quantum mechanical surface polarizability emerging from the proximity to the electrolyte and the electrolyte structure and dynamics. In the current setup, Density Functional Tight Binding calculations for the electronic structure calculations of the surface are coupled with classical molecular dynamics to simulate the electrolyte solution. The reduced computational cost of the QM part makes the coupling with a classical simulation engine computationally feasible and allows simulation of large systems for hundreds of nanoseconds. We tested the method by simulating both a noncharged graphene flake and a noncharged and charged infinite graphene sheet immersed in an NaCl electrolyte solution. We found that, when no bias is applied, ions preferentially remained in solution, and only cations are mildly attracted to the surface of the graphene. This preferential adsorption of cations vs anions seems to persist also when the surface is moderately charged and rules out any substantial ions/surface charge transfer.

Biomass-based Hierarchical Porous Carbon for Supercapacitors: Effect of Aqueous and Organic Electrolytes on the Electrochemical Performance

Biomass-based hierarchical porous carbon (SCPC) exhibits excellent electrochemical performance in electric double layer capacitors, prepared by carbonization and activation of straw cellulose. To investigate the potential applications of SCPC in supercapacitors, the effect of aqueous and organic electrolytes on the electrochemical performance of SCPC was studied in detail. In H₂ SO₄ , the SCPC electrode exhibits higher specific capacitance (358 F g^-1 ) and outstanding cycling stability with 95.6 % capacitance retention over 10 000 cycles. The SCPC electrode shows superior rate capability with 90.7 % capacitance retention in KOH, and higher energy density of 17.9 Wh kg^-1 in Na₂ SO₄ . The SCPC electrode exhibits ideal capacitance characteristics, superior rate capability with capacitance retention of 95.8 %, and high energy density of 36.0 Wh kg^-1 in tetraethylammonium tetrafluoroborate/propylene carbonate (Et₄ NBF₄ /PC). The significant difference of capacitive performance of SCPC electrode in various electrolytes is mainly attributed to the difference in the electrolyte ion size, ionic conductivity, matching between the electrolyte ions and pore structure, and matching between anions and cations adsorbed on the positive and negative electrodes. This work not only establishes the relationship between the structure of SCPC and its electrochemical performance in different electrolytes, but also provides a reference for the high value-added utilization of SCPC.

Reduced Faradaic Contributions and Fast Charging of Nanoporous Carbon Electrodes in a Concentrated Sodium Nitrate Aqueous Electrolyte for Supercapacitors

The Faradaic processes related to electrochemical water reduction at the nanoporous carbon electrode under negative polarization are reduced when the concentration of aqueous sodium nitrate (NaNO3) is increased or the temperature is decreased. This effect enhances the relative contribution of ion electrosorption to the total charge storage process. Hydrogen chemisorption is reduced in aqueous 8.0 m NaNO3 due to the low degree of hydration of the Na+ cation; consequently, less free water is available for redox contributions, driving the system to exhibit electrical double-layer capacitive characteristics. Hydrogen adsorption/desorption is facilitated in 1.0 m NaNO3 due to the high molar ratio. The excess of water shifts the local pH in carbon nanopores to neutral values, giving rise to a high overpotential for dihydrogen evolution in the latter. The dilution effect on local pH shift in 1.0 m NaNO3 can be reduced by decreasing the temperature. A symmetric activated carbon cell assembled with 8.0 m NaNO3 exhibits a high capacitance and coulombic efficiency, a larger contribution of ion electrosorption to the overall charge storage process, and a stable capacitance performance at 1.6 V.

Recent Advances in Layer-by-Layer Assembled Conducting Polymer Based Composites for Supercapacitors

Development of well-designed electrodes is the key to achieve high performance supercapacitors. Therefore, as one of the effective methods, a layer-by-layer (LBL) approach is often fruitfully employed for the fabrication of electrode material. Benefiting from a tunable parameter of the LBL approach, this approach has paved a way to design a highly ordered nanostructured electrode material with excellent performance. Conducting polymers (CPs) are the frontrunners in supercapacitors and notably, the LBL assembly of CPs is attracting extensive attention. Therefore, this critical review covers a comprehensive discussion on the research progress of CP-based composites with special importance on the LBL approach predominately for supercapacitors. Following a brief discussion on supercapacitors and CPs, the most up-to-date techniques used in LBL are highlighted.

Specific Ion Effects in Polyampholyte Hydrogels Dialyzed in Aqueous Electrolytic Solutions

Polyampholyte hydrogels (PAHs) constitute a class of physical gels with cross-linking originating from inter- and intrachain ionic cross-linking between countercharged functional groups. In our previous report, we have shown that PAH has the potential to be a gel electrolyte in electrochemical energy storage devices. In this work, we further our understanding of charge-balanced PAH as a host material for gel electrolytes by studying the effect of dialysis on the mechanical properties and ionic conductivities of PAHs, whereas these properties are compared with those of poly(vinyl alcohol) (PVA)-based gel electrolytes. Here, various electrolyte solutions were investigated as dialyzing agents. The ionic species in the electrolytes form ion pairs with countercharged functional groups in PAH, whereas such interactions govern the ionic conductivity and mechanical strength of PAH in various electrolytes. For anions, the trend in ionic interactions follows the Hofmeister series in an exact manner, whereas some anomaly is observed among cations. We anticipate that our study provides a design criterion for fabricating gel electrolytes. In a broader context, this work can shed light on understanding the behavior of PAHs in various operational environments, such as under physiological conditions and in antifouling coatings for biomedical and maritime applications, respectively.

Performance of Partially Exfoliated Nitrogen-Doped Carbon Nanotubes Wrapped with Hierarchical Porous Carbon in Electrolytes

The preparation of highly conductive, high-surface-area, heteroatom-doped, porous carbon nanocomposite materials with enhanced electrochemical performance for sustainable energy-storage technologies, such as supercapacitors, is challenging. Herein, a route for the large-scale synthesis of nitrogen-doped porous carbon wrapped partially exfoliated carbon nanotubes (N-PPECNTs) with an interconnected hierarchical porous structure, as an advanced electrode material that can realize several potential applications for energy storage, is presented. Polypyrrole conductive polymer acts as both nitrogen and carbon sources that contribute to the pseudocapacitance. Partially exfoliated carbon nanotubes (PECNTs) provide a high specific surface area for ion and charge transportation and act as a conductive matrix. The derived porous N-PPECNT displays a nitrogen content of 6.95 at %, with a specific surface area of 2050 m²  g^-1 , and pore volume of 1.13 cm³  g^-1 . N-PPECNTs, as an electrode material for supercapacitors, exhibit an excellent specific capacitance of 781 F g^-1 at 2 A g^-1 , with a high cycling stability of 95.3 % over 10 000 cycles. Furthermore, the symmetric supercapacitor exhibits remarkable energy densities as high as 172.8, 62.7, and 53.55 Wh kg^-1 in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][TFSI]), organic, and aqueous electrolytes, respectively. Also, biocompatible hydrogel and polymer gel electrolyte based, stable, flexible supercapacitors with excellent electrochemical performance could be demonstrated.

Sustainable Carbon/Carbon Supercapacitors Operating Down to -40 °C in Aqueous Electrolyte Made with Cholinium Salt

Cholinium chloride at a concentration of 5 mol kg^-1 in water is proposed as a low-cost and environmentally friendly aqueous electrolyte, enabling extension of the operating range of carbon/carbon supercapacitors (SCs) down to -40 °C. This solution has a pH close to neutrality (pH 6.1) and high conductivity of 88 mS cm^-1 at 24 °C. The supercapacitors demonstrate a high capacitance of 126 F g^-1 (per mass of one electrode) and long life span at voltages up to 1.5 V. At -40 °C, the carbon/carbon SCs display excellent electrochemical characteristics with only slightly reduced capacitance of 106 F g^-1 and negligible ohmic losses. As compared to previous works, where antifreezing additives were introduced in traditional neutral electrolytes, the low solubility of the salt and related poor conductivity of the solution is no longer an issue, which makes cholinium salt aqueous solutions very promising for SCs operating at sub-ambient temperature conditions.

Performance enhancement of asymmetric supercapacitors with bud-like Cu-doped Mn3O4 hollow and porous structures on nickel foam as positive electrodes

Cu-doped Mn₃O₄ hollow nanostructures supported on Ni foams as high-performance electrode materials for supercapacitors were successfully synthesized through a facile hydrothermal method and subsequent calcination. The morphology, structure, and electrochemical performance of the as-prepared Mn₃O₄ nanostructures can be tuned just by varying the Cu doping content. Benefiting from the unique bud-like hollow structure, the 1.5 at% Cu-doped Mn₃O₄ sample has a high specific capacitance of 257.6 F g⁻¹ at 1 A g⁻¹ and remarkable stability (about 90.6% retention of its initial capacitance after 6000 electrochemical cycles). Besides, an asymmetric supercapacitor (ASC) cell based on the 1.5 at% Cu-doped Mn₃O₄ exhibits a high specific capacitance of 305.6 F g⁻¹ at 1 A g⁻¹ and an energy density of 108.6 W h kg⁻¹ at a power density of 799.9 W kg⁻¹. More importantly, the ASC shows good long-term stability with 86.9% capacity retention after charging/discharging for 6000 cycles at a high current density of 5 A g⁻¹.

The effect of Cu doping on the electrochemical performance of bud-like Mn₃O₄ nanostructures for supercapacitor application was comparatively investigated.

Kinetic-Dominated Charging Mechanism within Representative Aqueous Electrolyte-based Electric Double-Layer Capacitors

The chemical nature of electrolytes has been demonstrated to play a pivotal role in the charge storage of electric double-layer capacitors (EDLCs), whereas primary mechanisms are still partially resolved but controversial. In this work, a systematic exploration into EDL structures and kinetics of representative aqueous electrolytes is performed with numerical simulation and experimental research. Unusually, a novel charging mechanism exclusively predominated by kinetics is recognized, going beyond traditional views of manipulating capacitances preferentially via interfacial structural variations. Specifically, strikingly distinctive EDL structures stimulated by diverse ion sizes, valences, and mixtures manifest a virtually identical EDL capacitance, where the dielectric nature of solvents attenuates ionic effects on electrolyte redistributions, in stark contradiction with solvent-free counterpart and traditional Helmholtz theory. Meanwhile, corresponding kinetics evolve conspicuously with ionic species, intimately correlated with ion-solvent interactions. The achieved mechanisms are subsequently illuminated by electrochemical measurements, highlighting the crucial interplay between ions and solvents in regulating EDLC performances.

Synthesis and electrochemical evaluation of the PANI/δ-MnO2 electrode for high performing asymmetric supercapacitors

Structure–property correlation and electrochemistry of in situ polymerized PANI on MnO₂ and an asymmetric supercapacitor using them are presented.

Template-free synthesis of nitrogen doped carbon materials from an organic ionic dye (murexide) for supercapacitor application

The present study describes a template-free single step carbonization route to prepare hierarchically structured nitrogen-doped carbon materials (NCMs) by using an organic ionic dye (OID), ammonium purpurate (murexide).

Pseudocapacitive behaviors of mesoporous nickel–cobalt oxide nanoplate electrodes in different electrolyte systems

A freestanding mesoporous NiCo₂O₄ nanoplate electrode exhibits superior pseudocapacitive performance and long-life stability.

A review of electrolyte materials and compositions for electrochemical supercapacitors

Electrolytes have been identified as some of the most influential components in the performance of electrochemical supercapacitors (ESs), which include: electrical double-layer capacitors, pseudocapacitors and hybrid supercapacitors. This paper reviews recent progress in the research and development of ES electrolytes. The electrolytes are classified into several categories, including: aqueous, organic, ionic liquids, solid-state or quasi-solid-state, as well as redox-active electrolytes. Effects of electrolyte properties on ES performance are discussed in detail. The principles and methods of designing and optimizing electrolytes for ES performance and application are highlighted through a comprehensive analysis of the literature. Interaction among the electrolytes, electro-active materials and inactive components (current collectors, binders, and separators) is discussed. The challenges in producing high-performing electrolytes are analyzed. Several possible research directions to overcome these challenges are proposed for future efforts, with the main aim of improving ESs' energy density without sacrificing existing advantages (e.g., a high power density and a long cycle-life) (507 references).