Ti3C2Tx MXene as Intriguing Material for Electrochemical Capacitor.

This study provides meaningful insight into the charge storage in Ti3C2Tx MXene (M-transition metal, X-carbon, T-Cl, F, O) for electrochemical capacitor (EC) application. The experiments show that this 2D material is especially adapted for the hydrogen electrosorption under negative polarization. It is found that hydrogen bonding to the Ti3C2Tx surface occurs through interactions of various strength. Different mechanisms are suggested to explain the nature of H stored at the electrode/electrolyte interface depending on pH and potential range. For the negative potentials, both capacitive and faradaic currents are involved, and the electrode can operate in a relatively wide range. On the other hand, the narrow range of positive potentials limits whole voltage of EC. Such charge disproportion has a major impact on the performance failure of symmetric MXene-based ECs. New design of MXene cells with a wide operating voltage is introduced. To equalize the charge storage of both electrodes, the positive Ti3C2Tx electrode is replaced by the porous carbon (BP2000) with a wide working potential and a good capacitive response. Thus, EC operating voltage is considerably expanded to 1.3, 1.4, 2 V in acidic, basic, neutral medium, respectively. During cycling tests at 1 A g-1, the asymmetric cell MXene/BP2000 maintains 80% of initial capacitance after 22 000 cycles.

Ambiguous Role of Cations in the Long-Term Performance of Electrochemical Capacitors with Aqueous Electrolytes.

A comprehensive comparison of electrochemical capacitors (ECs) with various aqueous alkali metal sulfate solutions (Li2SO4, Na2SO4, Rb2SO4, and Cs2SO4) is reported. The EC with a less conductive 1 mol L-1 Li2SO4 solution demonstrates the best long-term performance (214 h floating test) compared to the EC with a highly conductive 1 mol L-1 Cs2SO4 solution (200 h). Both the positive and negative EC electrodes are affected by extensive oxidation and hydrogen electrosorption, respectively, during the aging process, as proven by the SBET fade. Interestingly, carbonate formation is observed as a minor cause of aging. Two strategies for optimizing sulfate-based ECs are proposed. In the first approach, Li2SO4 solutions with the pH adjusted to 3, 7, and 11 are investigated. The sulfate solution alkalization inhibits subsequent redox reactions, and as a result, EC performance is successfully enhanced. The second approach exploits so-called bication electrolytic solutions based on a mixture of Li2SO4 and Na2SO4 at an equal concentration. This concept allows the operational time to be significantly prolonged, up to 648 h (+200% compared to 1 mol L-1 Li2SO4). Therefore, two successful pathways for improving sulfate-based ECs are demonstrated.

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.

Electrochemical Capacitor Performance of Nanotextured Carbon/Transition Metal Dichalcogenides Composites.

Transition metal dichalcogenides (TMDs) are emerging low-dimensional materials with potential applications for electrochemical capacitors (EC). Here, physicochemical and electrochemical characterizations of carbon composites with two sulfides ReS2 and FeS2 are reported. To enhance conductivity, multiwalled carbon nanotubes (NTs) serve as a support for ReS2 while 3D graphene-like network (3DG) is utilized for FeS2 deposition. Unique structure of carbon/TMDs composites allows a faradaic contribution of sulfides to be exploited. Capacitance values, charge/discharge efficiency, capacitance retention, charge propagation, cyclabilty, and voltage limits of EC with carbon/sulfide composites in aqueous neutral solutions (Li2 SO4 , Na2 SO4 ) are analyzed. Special attention is devoted to energetic efficiency of capacitive charge/discharge processes. Structure-to-capacitance correlation for the composites with various TMDs loading is thoroughly emphasized. The more defected structure of layered NTs/ReS2 composite is responsible for the lower capacitor voltage (0.8 V) owing to quicker electrolyte decomposition. Additionally, the catalytic effect of Re for hydrogen evolution reaction plays a crucial role in EC voltage restriction. Contrary, the operating voltage of capacitor based on 3DG/FeS2 is able to be extended until 1.5 V in sodium sulfate electrolytic solution.

Link between Alkali Metals in Salt Templates and in Electrolytes for Improved Carbon-Based Electrochemical Capacitors.

Various alkali metal (Li+, Na+, K+, Rb+, and Cs+) chlorides with Pluronic F127 were used as a soft-salt template for tuning the textural and structural properties of carbon. Highly conductive metal hydroxide solutions, where the cations are the same as those in the salt template, have been used as electrolytes. By increasing the size of the cation in the template, the textural properties of carbon, such as the specific surface area, micropore volume, and pore size, were remarkably enhanced. It directly translates to an increase in the specific capacitance of the electrode material. For a constant current charge/discharge at 0.1 A g-1, the electrode composed of LiCl-T and operating with 1 mol L-1 LiOH demonstrates the capacitance of 124 F g-1, whereas CsCl-T with the same electrolyte has a capacitance of 216 F g-1. Moreover, the materials show the highest capacitance retention (up to 75%) vs. the current regime applied when the cation used during synthesis matches the cation present in the electrolyte (i.e., LiCl-T with LiOH). Interestingly, capacitance normalized by specific surface area has been found to be the highest when LiOH solution is applied as an electrolyte. Thus, for this metric, the size of ions seems to be a crucial parameter. The importance of mesoporosity is highlighted as well by using materials with a similar fraction of micropores and with or without mesopores. Briefly, the presence of mesopore fraction proved to be essential for improved capacity retention (69% vs. 30%). Besides textural properties, the graphitization degree impacts the electrochemical performance as well. It increases among the samples, in accordance with cation-π binding energy, e.g., LiCl-T is the most "graphitic-like" material and CsCl-T is the most disordered. Thus, the more graphitic-like materials demonstrate higher rate capability and cycle stability.

Towards more Durable Electrochemical Capacitors by Elucidating the Ageing Mechanisms under Different Testing Procedures.

Electrical double-layer capacitors (EDLCs) commonly denoted supercapacitors are rechargeable energy storage devices with excellent power and energy delivery metrics intermediate to conventional capacitors and batteries. High-voltage aqueous electrolyte based EDLCs are particularly attractive due to their high-power capability, facile production, and environmental advantages. EDLCs should last for thousands of cycles and evaluation of future cell chemistries require long-term and costly galvanostatic cycling. Voltage holding tests have been proposed to shorten evaluation time by accelerating cell degradation processes. Whether voltage holding can replace cycling completely remains undemonstrated. In this work, a systematic investigation of the influence of testing procedure on cell performance is presented. The state-of-the-art post-mortem and operando experimental techniques are implemented to elucidate ageing mechanisms and kinetics inside EDLC cells under different testing procedures. Carbon corrosion occurring on the positively polarized electrode leads to the lower active surface area and higher oxygen content. On the contrary, an increase of surface area and micropore volume are observed on the negatively polarized electrode. Repeated galvanostatic cycles at U<1.6 V appears to facilitate the depletion of oxygen species on the positively polarized electrode in comparison with voltage holding, which indicates a more complex degradation mechanism during cycling. Caution is advised when comparing results from different test procedures.

Redox-active electrolyte for supercapacitor application.

This paper reports the electrochemical behaviour of supercapacitor carbon electrodes operating in different aqueous solutions modified by various redox-active species (hydroxybenzenes, bromine derivatives and iodide). Three dihydroxybenzenes with varying stereochemistry, i.e., -OH substitution, have been considered as electrolyte additives (0.38 mol L(-1)) in acidic, alkaline and neutral solutions. High capacitance values have been obtained, especially for the acidic and alkaline solutions containing 1,4-dihydroxybenzene (hydroquinone). Bromine derivatives of dihydroxybenzenes were also considered as the additive in alkaline solution for use as a supercapacitor electrolyte, and a significant increase in capacitance value was observed. The redox couple investigated next was an iodide/iodine system, where 2 mol L(-1) NaI aqueous electrolyte was utilized. In this case, the most promising faradaic contribution during capacitor operation was achieved. In particular, stable capacitance values from 300-400 F g(-1) have been confirmed by long-term galvanostatic cycling (over 100 000 cycles), cycling voltammetry and floating. The mechanism of pseudocapacitance phenomena was discussed and supported by electrochemical and physicochemical measurements, e.g., in situ Raman spectroscopy.

Carbons and electrolytes for advanced supercapacitors.

Electrical energy storage (EES) is one of the most critical areas of technological research around the world. Storing and efficiently using electricity generated by intermittent sources and the transition of our transportation fleet to electric drive depend fundamentally on the development of EES systems with high energy and power densities. Supercapacitors are promising devices for highly efficient energy storage and power management, yet they still suffer from moderate energy densities compared to batteries. To establish a detailed understanding of the science and technology of carbon/carbon supercapacitors, this review discusses the basic principles of the electrical double-layer (EDL), especially regarding the correlation between ion size/ion solvation and the pore size of porous carbon electrodes. We summarize the key aspects of various carbon materials synthesized for use in supercapacitors. With the objective of improving the energy density, the last two sections are dedicated to strategies to increase the capacitance by either introducing pseudocapacitive materials or by using novel electrolytes that allow to increasing the cell voltage. In particular, advances in ionic liquids, but also in the field of organic electrolytes, are discussed and electrode mass balancing is expanded because of its importance to create higher performance asymmetric electrochemical capacitors.

Electrochemistry serving people and nature: high-energy ecocapacitors based on redox-active electrolytes.

Positive Poles: A new type of electrochemical capacitor with two different aqueous solutions, separated by a Nafion membrane is described. High capacitance values as well as excellent energy/power characteristics are reported and discussed. The neutral character of the applied electrolytes makes this capacitor an environmentally friendly, easy to assemble, and cost-effective device for energy storage.

Triethylammonium bis(tetrafluoromethylsulfonyl)amide protic ionic liquid as an electrolyte for electrical double-layer capacitors.

This study describes the preparation, characterization and application of [Et(3)NH][TFSA], either neat or mixed with acetonitrile, as an electrolyte for supercapacitors. Thermal and transport properties were evaluated for the neat [Et(3)NH][TFSA], and the temperature dependence of viscosity and conductivity can be described by the VTF equation. The evolution of conductivity with the addition of acetonitrile rendered it possible to determine the optimal mixture at 25 °C, with a weight fraction of acetonitrile of 0.5. This mixture was also evaluated for transport properties, and showed a Newtonian behavior, as the neat PIL. An electrochemical study demonstrated, at first, a passivation on Al after the second cyclic voltammogram. Subsequently, the electrochemical window was estimated using a three-electrode cell to 4 V on a platinum electrode, and to 2.5 V on activated carbon. Finally, the neat PIL was found to exhibit good performances as promising electrolyte for supercapacitor applications.

Carbon materials for supercapacitor application.

The most commonly used electrode materials for electrochemical capacitors are activated carbons, because they are commercially available and cheap, and they can be produced with large specific surface area. However, only the electrochemically available surface area is useful for charging the electrical double layer (EDL). The EDL formation is especially efficient in carbon pores of size below 1 nm because of the lack of space charge and a good attraction of ions along the pore walls. The pore size should ideally match the size of the ions. However, for good dynamic charge propagation, some small mesopores are useful. An asymmetric configuration, where the positive and negative electrodes are constructed from different materials, e.g., activated carbon, transition metal oxide or conducting polymer, is of great interest because of an important extension of the operating voltage. In such a case, the energy as well as power is greatly increased. It appears that nanotubes are a perfect conducting additive and/or support for materials with pseudocapacitance properties, e.g. MnO(2), conducting polymers. Substitutional heteroatoms in the carbon network (nitrogen, oxygen) are a promising way to enhance the capacitance. Carbons obtained by one-step pyrolysis of organic precursors rich in heteroatoms (nitrogen and/or oxygen) are very interesting, because they are denser than activated carbons. The application of a novel type of electrolyte with a broad voltage window (ionic liquids) is considered, but the stability of this new generation of electrolyte during long term cycling of capacitors is not yet confirmed.

Synthesis and properties of trigeminal tricationic ionic liquids.

Novel trigeminal tricationic ionic liquids (TTILs) have been successfully synthesized in high yields by means of Menschutkin quaternization via an S(N)1 mechanism. This reaction presents a new convenient method for transforming glycerol into multifunctional compounds. The physical properties of a series of TTILs were characterized by using a variety of techniques. The prepared salts were tested for antimicrobial activity. Electrochemical characterization of TTILs was also performed, which allowed the estimation of the conductivity of these new compounds, to establish their electrochemical stability window and capacitance properties over a wide range of temperatures. A good correlation of the physical properties of TTILs with capacitance values was observed.

High yield of pure multiwalled carbon nanotubes from the catalytic decomposition of acetylene on in-situ formed cobalt nanoparticles.

For the first time, multiwalled carbon nanotubes (MWNTs) could be formed selectively in a high yield, free of any disordered carbon by-product, from the catalytic decomposition of acetylene at 600 degrees C on a CoxMg(1-x)O solid solution. Starting from 1 g of catalytic substrate, 4 g of pure MWNTs were obtained after its dissolution in boiling concentrated HCl, without any additional purification in strongly oxidizing medium, as is required for other methods of nanotube production. In situ reduction of CoO by dihydrogen liberated from acetylene decomposition allows highly divided metal particles to be continuously produced as synthesis proceeds. This is undoubtedly the reason for the good performance of the catalyst and for the ability to produce nanotubes in a narrow diameter range, namely from 10 to 15 nm. With the use of acetylene instead of methane, the synthesis proceeds at low temperature, which prevents the growth of carbon shells, in which the metal particles are generally embedded, decreasing their activity. Because of the very low specific surface area of the catalyst support, the amount of disordered carbon by-product formed is negligible.