A comparison of the aging of electrochemical double layer capacitors with acetonitrile and propylene carbonate-based electrolytes at elevated voltages

Unveiling a Novel Decomposition Pathway in Propylene Carbonate-Based Supercapacitors: Insights from a Jelly Roll Configuration Study

This research elucidates novel insights into the electrochemical properties and degradation phenomena of propylene carbonate (PC)-based supercapacitors at a large-scale 18650 cylindrical jelly-roll cell level. Central to our findings is the identification of 2-ethyl-4-methyl-1,3-dioxolane (EMD) as a hitherto undocumented decomposition by-product, highlighting the nuanced complexity of PC electrolyte stability. We further demonstrate that elevated operational voltages precipitate accelerated electrolyte degradation, underscoring the criticality of defining the operational voltage window for maximizing device longevity. Employing advanced analytical techniques, including gas chromatography-mass spectrometry (GC-MS), this study meticulously analyzes electrolyte decomposition mechanisms. The outcomes offer pivotal insights into the operational constraints and chemical resilience of PC-based supercapacitors, contributing significantly to the optimization of supercapacitor design and application. By delineating a specific decomposition pathway, this investigation enriches the understanding of electrochemical dynamics in supercapacitor systems, providing a foundation for future research and technological advancement in energy storage devices.

Hierarchical Carbon Composites for High-Energy/Power-Density and High-Reliability Supercapacitors with Low Aging Rate

A facile method for preparing hierarchical carbon composites that contain activated carbon (AC), carbon nanospheres (CNSs), and carbon nanotubes (CNTs) for use as the electrode material in supercapacitors (SCs) was developed. The CNS/CNT network enabled the formation of three-dimensional conducting pathways within the highly porous AC matrix, effectively reducing the internal resistance of an SC electrode. The specific capacitance, cyclability, voltage window, temperature profile during charging/discharging, leakage current, gas evolution, and self-discharge of the fabricated SCs were systematically investigated and the optimal CNS/CNT ratio was determined. A 2.5 V floating aging test at 70 °C was performed on SCs made with various hierarchical carbon electrodes. Electrochemical impedance spectroscopy, postmortem electron microscopy, Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy analyses were conducted to examine the electrode aging behavior. A hierarchical carbon architecture with an appropriate AC/CNS/CNT constituent ratio could significantly improve charge-discharge performance, increase cell reliability, and decrease the aging-related degradation rate.

Emerging trends in anion storage materials for the capacitive and hybrid energy storage and beyond

Electrochemical capacitors charge and discharge more rapidly than batteries over longer cycles, but their practical applications remain limited due to their significantly lower energy densities. Pseudocapacitors and hybrid capacitors have been developed to extend Ragone plots to higher energy density values, but they are also limited by the insufficient breadth of options for electrode materials, which require materials that store alkali metal cations such as Li⁺ and Na⁺. Herein, we report a comprehensive and systematic review of emerging anion storage materials for performance- and functionality-oriented applications in electrochemical and battery-capacitor hybrid devices. The operating principles and types of dual-ion and whole-anion storage in electrochemical and hybrid capacitors are addressed along with the classification, thermodynamic and kinetic aspects, and associated interfaces of anion storage materials in various aqueous and non-aqueous electrolytes. The charge storage mechanism, structure-property correlation, and electrochemical features of anion storage materials are comprehensively discussed. The recent progress in emerging anion storage materials is also discussed, focusing on high-performance applications, such as dual-ion- and whole-anion-storing electrochemical capacitors in a symmetric or hybrid manner, and functional applications including micro- and flexible capacitors, desalination, and salinity cells. Finally, we present our perspective on the current impediments and future directions in this field.

Application of Ionic Liquids in Electrochemistry-Recent Advances

In this review, the roles of room temperature ionic liquids (RTILs) and RTIL based solvent systems as proposed alternatives for conventional organic electrolyte solutions are described. Ionic liquids are introduced as well as the relevant properties for their use in electrochemistry (reduction of ohmic losses), such as diffusive molecular motion and ionic conductivity. We have restricted ourselves to provide a survey on the latest, most representative developments and progress made in the use of ionic liquids as electrolytes, in particular achieved by the cyclic voltammetry technique. Thus, the present review comprises literature from 2015 onward covering the different aspects of RTILs, from the knowledge of these media to the use of their properties for electrochemical processes. Out of the scope of this review are heat transfer applications, medical or biological applications, and multiphasic reactions.

Phenylacetonitrile (C6H5CH2CN) Ionic Liquid Blends as Alternative Electrolytes for Safe and High-Performance Supercapacitors

The increasing need in the development of storage devices is calling for the formulation of alternative electrolytes, electrochemically stable and safe over a wide range of conditions. To achieve this goal, electrolyte chemistry must be explored to propose alternative solvents and salts to the current acetonitrile (ACN) and tetraethylammonium tetrafluoroborate (Et4NBF4) benchmarks, respectively. Herein, phenylacetonitrile (Ph-ACN) has been proposed as a novel alternative solvent to ACN in supercapacitors. To establish the main advantages and drawbacks of such a substitution, Ph-ACN + Et4NBF4 blends were formulated and characterized prior to being compared with the benchmark electrolyte and another alternative electrolyte based on adiponitrile (ADN). While promising results were obtained, the low Et4NBF4 solubility in Ph-ACN seems to be the main limiting factor. To solve such an issue, an ionic liquid (IL), namely 1-ethyl-3-methylimidazolium bis [(trifluoromethyl)sulfonyl] imide (EmimTFSI), was proposed to replace Et4NBF4. Unsurprisingly, the Ph-ACN + EmimTFSI blend was found to be fully miscible over the whole range of composition giving thus the flexibility to optimize the electrolyte formulation over a large range of IL concentrations up to 4.0 M. The electrolyte containing 2.7 M of EmimTFSI in Ph-ACN was identified as the optimized blend thanks to its interesting transport properties. Furthermore, this blend possesses also the prerequisites of a safe electrolyte, with an operating liquid range from at least -60 °C to +130 °C, and operating window of 3.0 V and more importantly, a flash point of 125 °C. Finally, excellent electrochemical performances were observed by using this electrolyte in a symmetric supercapacitor configuration, showing another advantage of mixing an ionic liquid with Ph-ACN. We also supported key structural descriptors by density functional theory (DFT) and COnductor-like Screening Model for Real Solvents (COSMO-RS) calculations, which can be associated to physical and electrochemical properties of the resultant electrolytes.

In Operando Calorimetric Measurements for Activated Carbon Electrodes in Ionic Liquid Electrolytes under Large Potential Windows

This study aims to investigate the effect of the potential window on heat generation in carbon-based electrical double layer capacitors (EDLCs) with ionic-liquid (IL)-based electrolytes using in operando calorimetry. The EDLCs consisted of two identical activated-carbon electrodes with either neat 1-butyl-1-methylpyrrolidinium bis(trifluoromethane-sulfonyl)imide ([Pyr₁₄ ][TFSI]) electrolyte or 1.0 m [Pyr₁₄ ][TFSI] in propylene carbonate (PC) as electrolyte. The instantaneous heat generation rate at each electrode was measured under galvanostatic cycling for different potential windows ranging from 1 to 4 V. First, the heat generation rates at the positive and negative electrodes differed significantly in neat IL owing to the differences in the ion sizes and diffusion coefficients. However, these differences were minimized when the IL was diluted in PC. Second, for EDLC in neat [Pyr₁₄ ][TFSI] at high potential window (4 V), a pronounced endothermic peak was observed at the beginning of the charging step at the positive electrode owing to TFSI^- intercalation in the activated carbon. On the other hand, for EDLC in 1.0 m [Pyr₁₄ ][TFSI] in PC at potential window above 3 V, an endothermic peak was observed only at the negative electrode owing to the decomposition of PC. Third, for both neat and diluted [Pyr₁₄ ][TFSI] electrolytes, the irreversible heat generation rate increased with increasing potential window and exceeded Joule heating. This was attributed to the effect of potential-dependent charge redistribution resistance. A further increase in the irreversible heat generation rate was observed for the largest potential windows owing to the degradation of the PC solvent. Finally, for both types of electrolyte, the reversible heat generation rate increased with increasing potential window because of the increase in the amount of ion adsorbed/desorbed at the electrode/electrolyte interface.

Natural Polymers as Green Binders for High-Loading Supercapacitor Electrodes

The state-of-the-art aqueous binder for supercapacitors is carboxymethyl cellulose (CMC). However, it limits the mass loading of the coatings owing to shrinkage upon drying. In this work, natural polymers, that is, guar gum (GG), wheat starch (WS), and potato starch (PS), were studied as alternatives. The flexibility and adhesion of the resulting coatings and electrochemical performance was tested. The combination of 75:25 (w/w) ratio PS/GG showed a promising performance. Electrodes were characterized by SEM, thermal, adhesion, and bending tests. Their electrochemical properties were determined by cyclic voltammetry, electrochemical impedance spectroscopy, and cycling experiments. The PS/GG mixture conformed well to criteria for industrial production, enabling mass loadings higher than CMC (7.0 mg cm^-2 ) while granting the same specific capacitance (26 F g^-1 ) and power performance (20 F g^-1 at 10 A g^-1 ). Including the mass of the current collector, this represents a +45 % increase in specific energy at the electrode level.

Lifetime and Performance Assessment of Commercial Electric Double-Layer Capacitors Based on Cover Layer Formation

The lifetime and performance of energy storage systems are essential characteristics for a major success of clean energy innovations, especially regarding the automotive sector. In this area, electric double-layer capacitors represent the cutting edge of nonfaradaic, high power, and long lifetime energy storages. Usually, degradation is neglected while operating or it is assessed by referring to simple rules of thumb. To better address the aging effects, commercial specimens have been investigated in-depth. The works can be split into two parts: first, extensive accelerated aging for more than 3 years to statistically analyze the degradation trend and significantly improve the current rules. Second, cell opening and surface characterization of the electrodes to gain a profound understanding of the ongoing processes and to correlate aging mechanisms to the statistics of the first part. It is found that a prominent cover layer forms during degradation on the positive electrode scaling with lost capacitance. The used methods for in-depth characterization include microscopy, X-ray diffraction, and thermogravimetric analysis with subsequent mass spectrometry. The newly formed layer consists of poly(tetrafluoroethylene) (also known as Teflon) that is still passable by charge carriers, albeit with a longer time constant. Additionally, the negative electrode shows corrosion and loss of contact. The composition of the cover layer has not been known yet. Thus, materials and especially electrolyte development can benefit from the results.

High energy density and high working voltage of a quasi-solid-state supercapacitor with a redox-active ionic liquid added gel polymer electrolyte

A redox-active gel polymer electrolyte with a high working voltage was synthesized and used for assembling a quasi-solid-state supercapacitor possessing high energy density.

Wire-Shaped Supercapacitors with Organic Electrolytes Fabricated via Layer-by-Layer Assembly

A wire-shaped supercapacitor (WSS) has structural advantages of high flexibility and ease of incorporation into conventional textile substrates. In this work, we report a thin reproducible WSS fabricated via layer-by-layer (LbL) assembly of multiwalled carbon nanotubes (MWCNTs), combined with an organic electrolyte of propylene carbonate (PC)-acetonitrile (ACN)-lithium perchlorate (LiClO₄)-poly(methyl methacrylate) (PMMA) that extends the voltage window to 1.6 V. The MWCNTs were uniformly deposited on a curved surface of a thin Au wire using an LbL assembly technique, resulting in linearly increased areal capacitance of the fabricated WSS. Vanadium oxide was coated on the LbL-assembled MWCNT electrode to induce pseudocapacitance, hence enhancing the overall capacitance of the fabricated WSS. Both the cyclic stability of the WSS and the viscosity of the electrolyte could be optimized by controlling the mixing ratio of PC to ACN. As a result, the fabricated WSS exhibits an areal capacitance of 5.23 mF cm^-2 at 0.2 mA cm^-2, an energy density of 1.86 μ W h cm^-2, and a power density of 8.5 mW cm^-2, in addition to a high cyclic stability with a 94% capacitance retention after 10 000 galvanostatic charge-discharge cycles. This work demonstrates a great potential of the fabricated scalable WSS in the application to high-performance textile electronics as an integrated energy storage device.

Thermally Reduced Graphene Oxide Electrochemically Activated by Bis-Spiro Quaternary Alkyl Ammonium for Capacitors

Thermally reduced graphene oxide (RGO) electrochemically activated by a quaternary alkyl ammonium-based organic electrolytes/activated carbon (AC) electrode asymmetric capacitor is proposed. The electrochemical activation process includes adsorption of anions into the pores of AC in the positive electrode and the interlayer intercalation of cations into RGO in the negative electrode under high potential (4.0 V). The EA process of RGO by quaternary alkyl ammonium was investigated by X-ray diffraction and electrochemical measurements, and the effects of cation size and structure were extensively evaluated. Intercalation by quaternary alkyl ammonium demonstrates a small degree of expansion of the whole crystal lattice (d002) and a large degree of expansion of the partial crystal lattice (d002) of RGO. RGO electrochemically activated by bis-spiro quaternary alkyl ammonium in propylene carbonate/AC asymmetric capacitor exhibits good activated efficiency, high specific capacity, and stable cyclability.

Organoaqueous calcium chloride electrolytes for capacitive charge storage in carbon nanotubes at sub-zero-temperatures

The road de-icing salt, calcium chloride, can decrease the freezing point of mixed water and formamide to below −60 °C, forming an excellent electrolyte that promises electricity storage in supercapacitors in ultra-cold winters and places.

Fast diffusion supercapacitors via an ultra-high pore volume of crumpled 3D structure reduced graphene oxide activation

In order to obtain a high performance supercapacitor, our a-NSrGO for crumpled 3D structure graphene maintains a high specific surface area and total pore volume with a wide pore size distribution, leading to the fast diffusion rate of electrolyte ions.

Investigation of Imbalanced Activated Carbon Electrode Supercapacitors

Imbalanced supercapacitor was constructed by using various ratio of activated carbon (AC) of positive to negative electrode. The electrochemical behavior of imbalanced supercapacitor was investigated using 1.0 M spiro-(1,1′)-bipyrrolidinium tetrafluoroborate electrolyte in propylene carbonate. The results showed that there are some factors that influenced the imbalanced supercapacitor with different AC ratio of positive to negative electrode, the utilization of AC, electrode potential distribution, and life cycle. The imbalanced supercapacitor with an AC weight ratio of 80 : 120 of positive to negative electrode has an average potential distribution in each electrode, and it revealed the best electrochemical performance: specific capacitor was 39.6 F·g−1, while the charge-discharge efficiency was 97.2% after 2000 life cycle tests.

Effects of ion insertion on cycling performance of miniaturized electrochemical capacitor of carbon nanotubes array

Capacity degradation and ion insertion of a miniaturized electrochemical capacitor are studied using ionic liquid [EMI] [TFSI] as the electrolyte. This capacitor is featured with two comb-like electrodes of vertical carbon nanotubes, ∼70 μm in height and 20 μm in interelectrode gap. We quantify the levels of ion insertion damage with Raman spectroscopy after the electrode experiences 120 consecutive voltammetric cycles to various potential limits. Distinct structural damage emerges due to [EMI] when the negative potential reaches -1.7 V, and those due to [TFSI] arise when the positive potential reaches 1.7 V vs. RHE. Judging from the peak broadenings, [EMI] is more detrimental than [TFSI]. When the voltage window ΔU is set as less than or equal to 2.8 V, both electrode potentials are within the two intercalation limits, little or no decay is observed in 10(4) charge/discharge cycles. When ΔU is 3.4 V, the positive potential exceeds the upper limit, but the negative potential stays within the lower limit, the cell capacitance decreases moderately. When ΔU increases to 3.8 V, both electrodes suffer from damages because of exceeding the intercalation limits. And the cell capacitance decreases substantially, even leading to a premature failure.