Comparing Triflate and Hexafluorophosphate Anions of Ionic Liquids in Polymer Electrolytes for Supercapacitor Applications

Two different ionic liquid-based biopolymer electrolyte systems were prepared using a solution casting technique. Corn starch and lithium hexafluorophosphate (LiPF₆) were employed as polymer and salt, respectively. Additionally, two different counteranions of ionic liquids, viz. 1-butyl-3-methylimidazolium hexafluorophosphate (BmImPF₆) and 1-butyl-3-methylimidazolium trifluoromethanesulfonate (also known as 1-butyl-3-methylimidazolium triflate) (BmImTf) were used and studied in this present work. The maximum ionic conductivities of (1.47 ± 0.02) × 10⁻⁴ and (3.21 ± 0.01) × 10⁻⁴ S·cm⁻¹ were achieved with adulteration of 50 wt% of BmImPF₆ and 80 wt% of BmImTf, respectively at ambient temperature. Activated carbon-based electrodes were prepared and used in supercapacitor fabrication. Supercapacitors were then assembled using the most conducting polymer electrolyte from each system. The electrochemical properties of the supercapacitors were then analyzed. The supercapacitor containing the triflate-based biopolymer electrolyte depicted a higher specific capacitance with a wider electrochemical stability window compared to that of the hexafluorophosphate system.     

Composite Fe3O4-MXene-Carbon Nanotube Electrodes for Supercapacitors Prepared Using the New Colloidal Method

MXenes, such as Ti₃C₂T_x, are promising materials for electrodes of supercapacitors (SCs). Colloidal techniques have potential for the fabrication of advanced Ti₃C₂T_x composites with high areal capacitance (C_S). This paper reports the fabrication of Ti₃C₂T_X-Fe₃O₄-multiwalled carbon nanotube (CNT) electrodes, which show C_S of 5.52 F cm⁻² in the negative potential range in 0.5 M Na₂SO₄ electrolyte. Good capacitive performance is achieved at a mass loading of 35 mg cm⁻² due to the use of Celestine blue (CB) as a co-dispersant for individual materials. The mechanisms of CB adsorption on Ti₃C₂T_X, Fe₃O₄, and CNTs and their electrostatic co-dispersion are discussed. The comparison of the capacitive behavior of Ti₃C₂T_X-Fe₃O₄-CNT electrodes with Ti₃C₂T_X-CNT and Fe₃O₄-CNT electrodes for the same active mass, electrode thickness and CNT content reveals a synergistic effect of the individual capacitive materials, which is observed due to the use of CB. The high C_S of Ti₃C₂T_X-Fe₃O₄-CNT composites makes them promising materials for application in negative electrodes of asymmetric SC devices.     

基于活化多孔碳负载聚苯胺正极和活化多孔碳负极的有机非对称超级电容器

采用KOH活化法制得高比表面积的活化多孔碳（aHPC）,借助原位化学氧化法制得疏松多孔的活化多孔碳负载聚苯胺纳米复合材料（aHPC@PANI）,并分别以aHPC及aHPC@PANI为负极与正极,以四乙基氟硼酸-乙腈为电解液,构建有机非对称超级电容器。电化学测试结果显示：在1 A/g电流密度下,aHPC@PANI正极与aHPC负极分别呈现256.7 F/g（-0.6~0.8 V）及152.4 F/g（-2~-0.6 V）的比容量;所组装的有机非对称电容器呈现宽电位窗口（2.8 V）,高的能量密度（在0.75 kW/kg功率密度下为56.2 W·h/kg）及优异的循环稳定性（循环5 000次后其比电容保持率高达92.4%）。     

Inkjet Printing of Polypyrrole Electroconductive Layers Based on Direct Inks Freezing and Their Use in Textile Solid-State Supercapacitors

In this work, we propose a novel method for the preparation of polypyrrole (PPy) layers on textile fabrics using a reactive inkjet printing technique with direct freezing of inks under varying temperature up to −16 °C. It was found that the surface resistance of PPy layers on polypropylene (PP) fabric, used as a standard support, linearly decreased from 6335 Ω/sq. to 792 Ω/sq. with the decrease of polymerization temperature from 23 °C to 0 °C. The lowest surface resistance (584 Ω/sq.) of PPy layer was obtained at −12 °C. The spectroscopic studies showed that the degree of the PPy oxidation as well as its conformation is practically independent of the polymerization temperature. Thus, observed tendences in electrical conductivity were assigned to change in PPy layer morphology, as it is significantly influenced by the reaction temperature: the lower the polymerization temperature the smoother the surface of PPy layer. The as-coated PPy layers on PP textile substrates were further assembled as the electrodes in symmetric all-solid-state supercapacitor devices to access their electrochemical performance. The electrochemical results demonstrate that the symmetric supercapacitor device made with the PPy prepared at −12 °C, showed the highest specific capacitance of 72.3 F/g at a current density of 0.6 A/g, and delivers an energy density of 6.12 Wh/kg with a corresponding power density of 139 W/kg.     

二氧化锰氧化制备聚苯胺及其超级电容性能

以二氧化锰为氧化剂在酸水体系中化学氧化制备了聚苯胺（PANI）,考察了聚合条件对产率的影响。采用红外光谱、扫描电镜等手段对PANI的结构与形貌进行了表征,采用电化学工作站对其电化学电容性能进行了测试。结果表明：PANI产率随着体系中氧化剂用量的增加、苯胺用量的减少、反应温度的降低和反应时间的延长而增加,制备的聚苯胺主要是翡翠亚胺型聚苯胺,并以颗粒形式存在,大小在100 nm左右,局部有团聚现象,颗粒间堆积蓬松;该聚苯胺作为超级电容器活性电极材料,具有较好的电化学电容性能,最高比电容达到178 F/g。     

有机电解液对双电层电容器性能影响

采用石油焦基高比表面积活性炭作为电极材料,分别以1 mol/L Et4NBF4/PC(四乙基铵四氟硼酸盐/碳酸丙烯酯), Et4NBF4/AN(四乙基铵四氟硼酸盐/乙腈), Bu4NBF4/PC(四丁基铵四氟硼酸盐/碳酸丙烯酯)和Et4NBF4/AN(四丁基铵四氟硼酸盐/乙腈)作为电解液,组装成有机体系双电层电容器。采用恒流充放电、循环伏安及交流阻抗等电化学手段对各电解液体系下的电化学行为进行了对比。实验结果表明：对于高微孔比率的电极材料,由于Et4N+(四乙基铵离子)的溶剂化离子半径小于Et4N+ (四丁基铵离子)的溶剂化离子半径,因此,Et4NBF4体系下的电荷存储密度和有效表面利用率更高,电容性能优于Bu4NBF4。此外,虽然PC体系的比容量略高于AN体系,但由于PC的电导率低于AN,致使其功率特性不如AN 体系下的好。AN体系相比于PC体系具有更小的电荷传递阻抗和扩散阻抗,电容的频率响应性能要优于PC 体系,更适宜在大功率场合下应用。     

Flexible Supercapacitors Based on Graphene/Boron Nitride Nanosheets Electrodes and PVA/PEIGel Electrolytes

All-solid-state supercapacitors have gained increasing attention as wearable energy storage devices, partially due to their flexible, safe, and lightweight natures. However, their electrochemical performances are largely hampered by the low flexibility and durability of current polyvinyl alcohol (PVA) based electrolytes. Herein, a novel polyvinyl alcohol-polyethyleneimine (PVA-PEI) based, conductive and elastic hydrogel was devised as an all-in-one electrolyte platform for wearable supercapacitor (WSC). For proof-of-concept, we assembled all-solid-state supercapacitors based on boron nitride nanosheets (BNNS) intercalated graphene electrodes and PVA-PEI based gel electrolyte. Furthermore, by varying the electrolyte ions, we observed synergistic effects between the hydrogel and the electrode materials when KOH was used as electrolyte ions, as the Graphene/BNNS@PVA-PEI-KOH WSCs exhibited a significantly improved areal capacitance of 0.35 F/cm² and a smaller ESR of 6.02 ohm/cm². Moreover, due to the high flexibility and durability of the PVA-PEI hydrogel electrolyte, the developed WSCs behave excellent flexibility and cycling stability under different bending states and after 5000 cycles. Therefore, the conductive, yet elastic, PVA-PEI hydrogel represents an attractive electrolyte platform for WSC, and the Graphene/BNNS@PVA-PEI-KOH WSCs shows broad potentials in powering wearable electronic devices.     

High-Temperature Degradation Tests on Electric Double-Layer Capacitors: The Effect of Residual Voltage on Degradation

The demand for electric double-layer capacitors, which have high capacity and are maintenance-free, for use in a variety of devices has increased. Nevertheless, it is important to know the degradation behavior of these capacitors at high temperatures because they are expected to be used in severe environments. Therefore, degradation tests at 25 °C and 80 °C were carried out in the current study to analyze the degradation behavior. Steam-activated carbon, Ketjen black, and PTFE were used as the electrodes, conductive material, and binder, respectively, and KOH was used as the electrolyte. The impedance and capacitance were calculated from the voltage and current in the device using the alternating current (AC) impedance method. The results showed that the impedance increased and the capacitance decreased over 14 days at 80 °C, which is the inverse of what we observed at 25 °C. Rapid degradation was also confirmed from the 80 °C degradation test. The residual voltage after measuring the current and voltage was a prominent factor influencing this rapid degradation.     

Flexible and conductive MXene films and nanocomposites with high capacitance

MXenes, a new family of 2D materials, combine hydrophilic surfaces with metallic conductivity. Delamination of MXene produces single-layer nanosheets with thickness of about a nanometer and lateral size of the order of micrometers. The high aspect ratio of delaminated MXene renders it promising nanofiller in multifunctional polymer nanocomposites. Herein, Ti₃C₂T_x MXene was mixed with either a charged polydiallyldimethylammonium chloride (PDDA) or an electrically neutral polyvinyl alcohol (PVA) to produce Ti₃C₂T_x/polymer composites. The as-fabricated composites are flexible and have electrical conductivities as high as 2.2 × 10⁴ S/m in the case of the Ti₃C₂T_x/PVA composite film and 2.4 × 10⁵ S/m for pure Ti₃C₂T_x films. The tensile strength of the Ti₃C₂T_x/PVA composites was significantly enhanced compared with pure Ti₃C₂T_x or PVA films. The intercalation and confinement of the polymer between the MXene flakes not only increased flexibility but also enhanced cationic intercalation, offering an impressive volumetric capacitance of ∼530 F/cm³ for MXene/PVA-KOH composite film at 2 mV/s. To our knowledge, this study is a first, but crucial, step in exploring the potential of using MXenes in polymer-based multifunctional nanocomposites for a host of applications, such as structural components, energy storage devices, wearable electronics, electrochemical actuators, and radiofrequency shielding, to name a few.The history of exfoliated, or delaminated, nanosheets (2D materials) dates back to the 1950s (1); however, few of the produced nanosheets are conductive. In recent years, 2D materials have been receiving increased attention, with graphene as the star material owing to its excellent electric, mechanical, and other properties (2–5). In 2011, our group reported on a new family of 2D early transition metal carbides, which combined metallic conductivity and hydrophilic surfaces (6). This novel 2D family was labeled MXenes to denote that they are produced by etching out the A layers from the layered M_n+1AX_n phases (6–8) and their similarity to graphene (7).In the M_n+1AX_n, or MAX, phases, “M” is an early transition metal, “A” is a group A (mainly groups 13–16) element, “X” is carbon and/or nitrogen, and n = 1, 2, or 3 (9). So far, the MXene family includes Ti₃C₂, Ti₂C, (Ti_0.5,Nb_0.5)₂C, (V_0.5,Cr_0.5)₃C₂, Ti₃CN, Ta₄C₃ (10), Nb₂C, V₂C (8), and Nb₄C₃ (11). Because there are over 70 known MAX phases (9), many more MXenes can be expected. It is important to note here that MXene surfaces are terminated by O, OH, and/or F groups from the etching process. Henceforth, these terminated MXenes will be referred to as M_n+1X_nT_x, where T represents terminating groups (O, OH, and/or F) and x is the number of terminating groups.If they are not delaminated, MXenes are multilayered structures resembling those of exfoliated graphite, which have shown promising performance as electrodes in both lithium ion batteries and supercapacitors, as well as adsorbents for heavy metal ions (8, 12–16). Delamination of the multilayered materials into single- or few-layer nanosheets dramatically increases the accessible surface. Large quantities of dispersed 2D MXene flakes—delaminated Ti₃C₂T_x—have been produced by sonication at room temperature (7, 12, 14). The Ti₃C₂T_x flakes are one (single layer) to several (few layers) nanometers thick, with lateral sizes ranging from hundreds of nanometers to several micrometers.With the exception of graphene (17–20), to date, few 2D materials have been made into highly flexible free-standing films with good electrical conductivities. Other 2D materials have been made into free-standing hybrids by the addition of conductive carbon nanotubes or graphene (21). We have previously shown that Ti₃C₂T_x flakes can be easily assembled into additive-free, flexible Ti₃C₂T_x films that show excellent electrochemical performance (14).Ab initio simulations predict elastic moduli along the basal plane to be over 500 GPa for various MXenes (22), suggesting that they could be useful reinforcements for polymer composites. Their excellent intrinsic conductivity, in combination with their reactive and hydrophilic surfaces, also renders them attractive as fillers in a number of polymers. Furthermore, their atomic-scale thicknesses should, in principle, allow for the fabrication of nanocomposites with improved mechanical properties, which are conductive—a combination not offered by other hydrophilic additives with functionalized surfaces, such as clays (23–26), layered double hydroxides (27, 28), or graphene oxide (29–31), all of which are insulating. However, there have been no reports in the literature on MXene-based composites so far. Only a single study of MAX phase-polymer composites has been published (32). However, in that study, the MAX platelets had lateral sizes ranging from 100 to 200 nm and thicknesses of at least 4 nm.Herein, we report, for the first time (to our knowledge), on the fabrication of conductive, flexible free-standing MXene films and polymer composite films that possess excellent flexibility, impressive electrical conductivity, and hydrophilic surfaces. In this study, Ti₃AlC₂ was chosen as the MAX precursor because its exfoliation and delamination have already been well developed (7, 12, 14, 33). Two polymers were chosen: poly(diallyldimethylammonium chloride) (PDDA) and polyvinyl alcohol (PVA). The former was chosen because it is a cationic polymer and the Ti₃C₂T_x flakes are negatively charged. The PVA was chosen for several reasons, which include its solubility in water, the large concentration of hydroxyl groups along its backbone, and its extensive utilization in gel electrolytes and composites (23, 26, 34, 35). Both Ti₃C₂T_x/PDDA and Ti₃C₂T_x/PVA composite films were fabricated and characterized. A sketch explaining the route for fabricating MXene-based films and their resulting properties is shown in Fig. 1.Open in a separate windowFig. 1.(A) TEM and (B) SEM images of MXene flakes after delamination and before film manufacturing. (C) A schematic illustration of MXene-based functional films with adjustable properties.     

Engineering three-dimensional hybrid supercapacitors and microsupercapacitors for high-performance integrated energy storage

Supercapacitors now play an important role in the progress of hybrid and electric vehicles, consumer electronics, and military and space applications. There is a growing demand in developing hybrid supercapacitor systems to overcome the energy density limitations of the current generation of carbon-based supercapacitors. Here, we demonstrate 3D high-performance hybrid supercapacitors and microsupercapacitors based on graphene and MnO₂ by rationally designing the electrode microstructure and combining active materials with electrolytes that operate at high voltages. This results in hybrid electrodes with ultrahigh volumetric capacitance of over 1,100 F/cm³. This corresponds to a specific capacitance of the constituent MnO₂ of 1,145 F/g, which is close to the theoretical value of 1,380 F/g. The energy density of the full device varies between 22 and 42 Wh/l depending on the device configuration, which is superior to those of commercially available double-layer supercapacitors, pseudocapacitors, lithium-ion capacitors, and hybrid supercapacitors tested under the same conditions and is comparable to that of lead acid batteries. These hybrid supercapacitors use aqueous electrolytes and are assembled in air without the need for expensive “dry rooms” required for building today’s supercapacitors. Furthermore, we demonstrate a simple technique for the fabrication of supercapacitor arrays for high-voltage applications. These arrays can be integrated with solar cells for efficient energy harvesting and storage systems.As a result of the rapidly growing energy needs of modern life, the development of high-performance energy storage devices has gained significant attention. Supercapacitors are promising energy storage devices with properties intermediate between those of batteries and traditional capacitors, but they are being improved more rapidly than either (1). Over the past couple of decades, supercapacitors have become key components of everyday products by replacing batteries and capacitors in an increasing number of applications. Their high power density and excellent low-temperature performance have made them the technology of choice for backup power, cold starting, flash cameras, regenerative braking, and hybrid electric vehicles (2, 3). The future growth of this technology depends on further improvements in energy density, power density, calendar and cycle life, and production cost.According to their charge storage mechanism, supercapacitors are classified as either electric double-layer capacitors (EDLCs) or pseudocapacitors (2). In EDLCs, charge is stored through rapid adsorption–desorption of electrolyte ions on high-surface-area carbon materials, whereas pseudocapacitors store charge via fast and reversible Faradaic reactions near the surface of metal oxides or conducting polymers. The majority of supercapacitors currently available in the market are symmetric EDLCs featuring activated carbon electrodes and organic electrolytes that provide cell voltages as high as 2.7 V. Although these EDLCs exhibit high power density and excellent cycle life, they suffer from low energy density because of the limited capacitance of carbon-based electrodes. The specific pseudocapacitance of Faradaic electrodes (typically 300–1,000 F/g) exceeds that of carbon-based EDLCs; however, their performance tends to degrade quickly upon cycling.Studies during the past few years have demonstrated an attractive alternative to conventional EDLCs and pseudocapacitors by using hybrid systems. Using both Faradaic and non-Faradaic processes to store charge, hybrid capacitors can achieve energy and power densities greater than EDLCs without sacrificing the cycling stability and affordability that have so far limited the success of pseudocapacitors (4). Several combinations of materials, such as RuO₂ (5), Co₃O₄ (6), NiO (7), V₂O₅ (8), Ni(OH)₂ (9), and MnO₂ (10), have been studied for preparing hybrid supercapacitors. Among these, MnO₂-based systems are particularly attractive as MnO₂ is an earth-abundant and environmentally friendly material with a high theoretical specific capacitance of 1,380 F/g (11). However, the poor ionic (10⁻¹³ S/cm) and electronic (10⁻⁵–10⁻⁶ S/cm) conductivity of pristine MnO₂ often limits its electrochemical performance. Recent reports show that some high-performance results can be achieved only from ultrathin MnO₂ films that are a few tens of nanometers in thickness (12). Nevertheless, the thickness and the area-normalized capacitance of these electrodes are not adequate for most applications. A promising approach to realize practical applications of MnO₂ is to incorporate nanostructured MnO₂ on highly conductive support materials with high surface areas such as nickel nanocones (13), Mn nanotubes (14), activated carbon (15), carbon fabric (16), conducting polymers (17), carbon nanotubes (18, 19), and graphene (20–24). Although promising specific capacitances of 148–410 F/g have been achieved, such values were obtained only under slow charge–discharge rates and they were found to decrease rapidly as the discharge rate was increased. Moreover, many of these materials have low packing density with large pore volume, meaning that a huge amount of electrolyte is needed to build the device, which adds to the mass of the device without adding any capacitance (25). Accordingly, the energy density and power density of these systems are very limited on the device level. To solve these critical problems, we have developed promising hybrid electrodes based on 3D graphene doped with MnO₂ nanoflowers. By rationally designing the structure of the graphene substrate to achieve high conductivity, suitable porosity, and high specific surface area, one may expect to not only achieve a high gravimetric capacitance, but also to improve the volumetric capacitance (23). Furthermore, the high surface area of nanostructured MnO₂ provides more active sites for the Faradaic reactions and shortens the ion diffusion pathways that are crucial for realizing its full pseudocapacitance. We show that hybrid supercapacitors based on these materials can achieve energy densities of up to 42 Wh/l compared with 7 Wh/l for state-of-the-art commercially available carbon-based supercapacitors. Interestingly, these graphene–MnO₂ hybrid supercapacitors use aqueous electrolytes and are assembled in air without the need for the expensive dry rooms required for building today’s supercapacitors.Whereas great efforts have been made for the fabrication of macroscale hybrid supercapacitors, there are only a few studies on the design and integration of hybrid materials into microsupercapacitors (26). This is likely due to complicated microfabrication techniques that often involve building 3D microelectrodes with micrometer separations. Here, we present a simple, yet versatile technique for the fabrication of 3D hybrid microsupercapacitors based on graphene and MnO₂. These microdevices enable an ultrahigh capacitance per footprint approaching 400 mF/cm², which is among the highest values achieved for any microsupercapacitor (26). They can also provide an energy density of up to 22 Wh/l, more than two times that of lithium thin-film batteries. These developments are promising for microelectronic devices such as biomedical sensors and radiofrequency identification tags where high capacity per footprint is crucial.