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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., ACS Applied Materials & Interfaces, 15 (19), ISSN:1944-8244, ISSN:1944-8252

This is a revised version of the manuscript, submitted for further review. Abstract: Step Potential Electrochemical Spectroscopy (SPECS) analysis of lithium intercalation into graphite was performed. Four different solutions of lithium bis(fluorosulfonyl)imide (LiFSI) salt in EC:DMC solvent (1:1, v:v) with various concentrations (0.1, 1, 5 and 10 molL-1) were selected as electrolytes. SPECS allowed for calculating the capacity resulting from lithium intercalation between the graphite layers and the capacity that comes from the adsorption of ions on the outer electrode surface, resistance, as well as the diffusion coefficient.Additionally,to compare theresults obtained,the galvanostatic intermittent titration technique (GITT) was performedunderthe same conditions. However, it turned out that initially, similar diffusion parameters are not identical in their meaning. Hence, SPECS can offer fresh insights into the lithium intercalation process in highly concentrated electrolytic solutions, enhancing existing approaches.

The point of zero charge (PZC) is crucial for investigating molecular level charging mechanisms in energy storage systems, as demonstrated in electrochemical capacitors. Three electrochemical techniques were studied: cyclic voltammetry (CV), staircase potentio electrochemical impedance spectroscopy (SPEIS) and step potential electrochemical spectroscopy (SPECS) for two activated carbons (ACs) with 0.1 mol L− 1 aqueous solutions of LiNO3, Li2SO4, and KI. The porous AC charging process in aqueous electrolytes is a complex phenomenon; the ion mixing zone covered a wide potential region. Inadequate PZC determination could lead to obscure data evaluation, which could further provide a misguided mechanism description at the molecular level. In the aqueous solutions studied, the adsorption of specific ions and active participation of all ionic species in the electrical double-layer formation were considered. The SPECS technique was determined to be the most beneficial for the PZC determination in electrochemical quartz crystal microbalance applications.

The paper puts forward the concept of a double-redox electrochemical capacitor operating in an aqueous electrolyte. The redox activity of sulphur from insoluble Bi2S3 nanocrystals embedded in the negative electrode material (up to 10 wt%) operating in 1 mol L−1 Li2SO4 electrolyte is demonstrated. It is also shown that the performance is significantly boosted using MPA (3-mercaptopropionic acid) as a ligand attached to the surface of the nanocrystals, which allows for more efficient use of Bi2S3 redox active species. This redox activity is combined with the reactions of iodides, which occur at the opposite electrode with 1 mol L−1 NaI. This enables the formation of a discharge voltage plateau that effectively boosts the capacitance (275 F g−1), and thus specific energy of the device owing to the relatively high cell voltage of 1.5 V. This performance is possible due to the advantageous electrode mass ratio (m− : m+ = 2 : 1), which helps to balance the charge. The rate capability test of the device demonstrates its capacitance retention of 73% at 10 A g−1 of the discharge current. The different states of the redox species ensure their operation at separate electrodes in an immiscible manner without a shuttling effect. The specific interactions of the redox active species with carbon electrodes are supported by operando Raman spectroscopy.

Selection of the materials with the most promising energy storage capabilities in terms of long-term performance (cyclability and voltage hold tests) and short-term storage (self-discharge, leakage currents)

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

The presentation of WP5 shown during the final meeting of NOEL project

Electrochemistry is strongly related to redox reactions. Charge transfer processes are used for the current generation in all electrochemical cells. Nowadays, redox reactions are still of evitable importance for energy storage/conversion technology. For instance, the charge and discharge of batteries exploit redox reactions. Moreover, these processes can also be used to improve the operating parameters of other energy storage devices like electrochemical capacitors. Although, in principle, the energy in electrochemical capacitors is stored in an electrostatic manner (by electrical double-layer formation), the redox reactions introduce an additional charge and improve the energy of these systems. This chapter presents the principles of electrochemical capacitors’ operation and provides comprehensive insights into this technology with special attention focused on hybrid systems, exploiting the redox activity of the electrolytic solution.

WP5 progress meeting 19.05.2022

This paper reports on the ion fluxes at the interfaces of various porous carbon electrodes/aqueous solutions of alkali metal cations (Na+, K+ and Rb+) and iodide anions, monitored by an electrochemical quartz crystal microbalance (EQCM). Different electrode material compositions as well as various electrolyte concentrations have also been considered. By tracking the ions during electrochemical polarization, we aimed to identify the reasons for the fading capacitor performance that occurs during long-term operation. The mass change profile suggests that hydroxide anions are responsible for counterbalancing the charge stored on the negative electrode. Furthermore, we found that iodide-based species are physically adsorbed on the carbon surface immediately after the electrodes come into contact with the electrolyte, regardless of the textural properties of the activated carbon used or electrode composition. Apart from the qualitative description, the mass change profiles allow the sequence of pore occupation to be determined. Additionally, the solvation numbers for alkali metals (Na+, K+ and Rb+) and hydroxide-based species have been determined. It is claimed that the solvation number is strongly affected by the electrolyte composition. Apparently, the concentration of water molecules available in the electrolyte cannot be neglected. The outcome of this research has fundamental and application context.

Meeting 2022-03-23 WP5

This paper reports on the electrochemical performance of activated carbon electrodes in aqueous solutions of bromide‐based species and discusses the results in the broader context of redox‐active electrolytes in electrochemical capacitor applications. Operando techniques such as Raman spectroscopy and electrochemical quartz crystal microbalance have been implemented to describe the interactions at the electrode/electrolyte interface. Ex situ experiments, including XPS profiles at different electrode thicknesses, support the discussion by providing insights into surficial and bulk‐related processes. Finally, the electrochemical activity of bromides and their advantages and drawbacks are discussed.

The model activated carbon spheres (ACSs) were optimized to be used as negative electrode in asymmetric electrochemical capacitors (ECs). A microporous commercial activated carbon was used as positive electrode. Carbon spheres (CSs) were synthesized by precipitation polymerization at room temperature followed by pyrolysis and activation. By enhancing the carbon:KOH ratio from 1:2 to 1:4, an increase in the specific surface area from 380 m2 g−1 to 2835 m2 g−1 (1:4) was achieved along with an increase in pore volume/size. Consequently, the electrochemical performance in the aqueous electrolyte was improved. After activation, low-temperature-pyrolyzed polymer spheres (400 °C) result in random-like particles with macroporous structure, while intermediate- and high-temperature-pyrolyzed materials (550 °C and 700 °C) lead to partially and totally conserved spheres. The interaction between the electrode material and electrolyte is important and is related to the texture, morphology, and surface chemistry of ACS. In 1 mol L−1 H2SO4, capacitance and rate handling are mostly affected by electrode morphology and wettability with the electrolyte. In 1 mol L−1 Li2SO4 electrolyte solution, texture and surface chemistry of the electrode material are crucial to obtain high-performance EC. Regardless of morphology, poor wetting properties of synthesized materials were found in Li2SO4, compared to the H2SO4 solution., Journal of Power Sources, 542, ISSN:0378-7753, ISSN:1873-2755

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., Partners acknowledge M-ERA.NET network, MCIN/AEI/10.13039/501100011033 (Ref. PCI2019–103637), CIBER-BBN, ICTS ‘‘NANBIOSIS’’, ICTS ELECMI node "Laboratorio de Microscopias Avanzadas", National Science Centre, Poland (2018/30/Z/ST4/00901), and Ministrstvo za izobraževanje, znanost in šport for financial support and the grant of Ministry of Science and Higher Education in Poland, no. 0911/SBAD/2101. A.V., B.T., E.T. and R.D. additionally acknowledge financial support from the Slovenian Research Agency (ARRS) research core funding P2–0393.

This is the deliverable D5.1

This paper describes a hybrid capacitor based on identical activated carbon (AC) electrodes and the 1-ethyl-3-methylimidazolium selenocyanate, [EMIm][SeCN], liquid salt as redox-active electrolyte. [EMIm][SeCN] is a fluorine-free ionic liquid (IL) which combines good charge and mass transport properties (σ = 25 mS cm−1 and η = 17 mPa s−1), as well as redox activity originating from the pseudohalide anion. Owing to the impact of the SeCN−/(SeCN)2 redox couple, the AC/AC capacitor based on this electrolyte displays remarkable discharge (from 1.5 V) capacitance of 49 and 56 F g−1 (expressed per total active mass of electrodes) using aluminum and stainless steel (SSt) current collectors, respectively. Although higher capacitance values were displayed for the hybrid capacitor with SSt components, more advantageous E0 localization for cell hybridization was observed when using Al current collectors.

International audience; 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.

This review focuses on describing the current state-of-the-art in the synthesis of 3D carbon architectures for the electrochemical capacitor application. The selection is based on both template and non-template strategies. Particular attention has been paid to carbon materials because of their structural interconnection, which not only creates the desired hierarchical porous channels but also ensures high conductivity and mechanical stability. A comprehensive overview of electrode materials is presented here with a detailed discussion of composite solutions, including their advantages and disadvantages. Numerous examples from the literature are presented for individual solutions. The future challenges posed for this type of material are finally summarized.

International audience; Soft-and salt-templating methods have been combined to obtain highly microporous carbon materials with mesopores in the narrow pore size range. Phenolic resin was used as a carbon source, and rubidium and caesium chloride were used as salt-templates, giving a well-developed microporosity with a high specific surface area, whereas a sacrificial triblock polymer Pluronic F-127 (soft-template) induced the mesopores of essential importance for fast access of the electrode surface area for the electrolytic solution. The combination of a high specific surface area (up to 2556 m 2 g À1) with a suitable pore size (0.77 e0.88 nm) resulted in an excellent performance of the electrochemical capacitor. High specific energies of 16.7 Wh•kg À1 at 300 W kg À1 of specific power have been achieved for a CsCl-T-based high-voltage (1.5 V) device with a 0.5 mol,L À1 Li 2 SO 4 electrolytic solution. The improved rate handling was allowed to maintain 10 Wh,kg À1 of specific energy at 4 kW kg À1 of specific power. In contrast to the other carbons with well-developed porosities, the material density obtained allowed our device to reach competitive, remarkably higher volumetric characteristics.

This paper reports on the “water-in-salt” electrolyte concept, applying CsF as an ionic compound in a symmetric electrochemical capacitor. CsF has been selected as a highly soluble inorganic salt that allows the amount of free water in the electrolyte to be significantly reduced. Several activated carbons with various microtextural parameters have been investigated to identify the most suitable electrode material. A wide range of electrolyte concentrations have been investigated in terms of conductivity, pH and viscosity. The electrochemical characteristics have been determined for three electrolyte concentrations and indicate critical changes in the electrolyte physicochemical properties. It seems that ion-solvent interactions, residual water content, electrolyte viscosity and charging/discharging conditions remarkably affect the electrochemical performance of capacitors with elevated concentrations of electrolyte. In addition to the capacitance increase, a positive impact on the self-discharge and charge/discharge efficiency has been observed for higher CsF concentrations.

This paper reports on the electrochemical performance of symmetric carbon/carbon electrochemical capacitors operating in aqueous electrolytic solutions with various benzoquinones as a source of pseudocapacitance. The benzoquinones have been introduced for the enhancement of capacitance values and the energy density in the final device. Three isomers, namely hydroquinone, catechol and resorcinol as electrolyte modifiers displayed different electrochemical activity in acidic electrolyte, affecting final electrochemical performance of the capacitors subjected to investigations. Furthermore, the paper demonstrates the electrochemical impedance spectroscopy as a powerful technique for electrochemical capacitor characterization and discusses the potential risk of the overestimation of the data recorded.

This paper reports on the operando study of degradation mechanisms in carbon-based electrochemical capacitors with Li2SO4 and LiNO3 electrolytes. Several electrochemical techniques, coupled with mass and Raman spectrometry have been implemented to recognize and describe the processes occurring at the carbon electrode/aqueous electrolyte interface. It has been shown that for the systems operating with mild, water-based electrolytes, the performance fade might have a different origin; dependently on the salt used for electrolyte preparation, carbon surface might be either oxidized or anion might be reduced. A detrimental effect of oxygen evolution at elevated voltages (up to 1.8 V) has also been observed. An in-depth study involving the implementation of the operando techniques also allowed the most optimal voltage for both systems to be determined, taking into account their long-term performance as well.

This manuscript reports on the novel insight into the development of high voltage carbon/carbon electrochemical capacitors operating in aqueous solutions of alkali metals and ammonium thiocyanates (KSCN, NaSCN, LiSCN, and NH4SCN). The effect of salt concentration, electrode porosity and current collectors on the capacitance value, system stability, and power performance has been investigated. Therefore, thiocyanate-based electrolytes were recognized as cheap and highly conductive electrolytic solutions (up to 401 mS cm−1 for NH4SCN at RT) allowing a cell voltage of 1.6 V in a symmetric carbon/carbon system to be achieved. At the same time, they display an attractive redox activity, enhancing the energy of the device with a good performance during cycling.

In this work, we report on the performance of carbon-basedelectrochemical capacitorsoperating in an aqueous solution of potassium selenocyanate (KSeCN) as redox active electrolyte, applied forcapacitanceand energy enhancement. The source of faradaic contribution is pseudohalide anion, SeCN−, demonstrating reversibleredox reactionon the positive electrode. Since the standard potential of SeCN–/(SeCN)2/(SeCN)3–is close to the I–/I2/I3–redox couple, thecapacitor demonstrates similar hybrid operation. With cost-effective stainless steel (SSt) current collectors, thesystem displayshigh capacitance 43 F·g−1(0.5 A·g−1) and performs at 1.4 V (1 A·g−1) for 9 000 cycles with 90% capacitance retention.

Char and Carbon Materials Derived from Biomass: Production, Characterization and Applications provides an overview of biomass char production methods (pyrolysis, hydrothermal carbonization, etc.), along with the characterization techniques typically used (Scanning Electronic Microscopy, X-Ray Fluorescence, Nitrogen adsorption, etc.) In addition, the book includes a discussion of the various properties of biomass chars and their suitable recovery processes, concluding with a demonstration of applications. As biomass can be converted to energy, biofuels and bioproducts via thermochemical conversion processes, such as combustion, pyrolysis and gasification, this book is ideal for professionals in energy production and storage fields, as well as professionals in waste treatment, gas treatment, and more.

Char and Carbon Materials Derived from Biomass: Production, Characterization and Applicationsprovides an overview of biomass char production methods (pyrolysis, hydrothermal carbonization, etc.), along with the characterization techniques typically used (Scanning Electronic Microscopy, X-Ray Fluorescence, Nitrogen adsorption, etc.) In addition, the book includes a discussion of the various properties of biomass chars and their suitable recovery processes, concluding with a demonstration of applications. As biomass can be converted to energy, biofuels and bioproducts via thermochemical conversion processes, such as combustion, pyrolysis and gasification, this book is ideal for professionals in energy production and storage fields, as well as professionals in waste treatment, gas treatment, and more.

International audience; Rapid development of the technologies based on electric energy in the last decades have stimulated intensive research on efficient power sources. Electrochemical energy conversion and storage systems are based on Faradaic reactions (charge transfer) and electrostatic attraction of ions at the electrode/electrolyte interface. The latter might be an interesting solution for applications requiring moderate energy density, high power rates, and long cycle life. Electrochemical capacitors (ECs) store the charge in a physical manner, hence, their energy density is moderate. At the same time, the lack of electrochemical reactions ensures very high power and long cycle life compared to batteries. Activated carbons with their versatile properties (like specific surface area, well-developed and suitable porosity, heteroatoms in the graphene matrix) are the most popular materials in EC application. This chapter provides a comprehensive overview of the carbon-based materials recently developed, with special attention devoted to those obtained by biomass carbonization and activation. Electrochemical properties demonstrated by such carbons are discussed in respect to their physicochemical characteristics

This paper provides a comprehensive overview of the recent state of the art in research on a neutral, water-based electrolyte, i.e., 1 mol⋅L−1Li2SO4solution, for sustainable electrochemical capacitors. The fundamental physicochemical properties of aqueous electrolytic solutions (conductivity, viscosity, etc.) are discussed together with the electrochemical performance of carbon-based electrochemical capacitors. In addition to a critical review of the recent findings in the field, the paper discusses new data obtained with theoperandoapproach; the discussion is supported by recent findings from the electrochemical quartz crystal microbalance, Raman spectroscopy and scanning electrochemical microscopy techniques. In this respect, interesting results concerning specific and selective ion adsorption have been obtained. In addition, apost-mortemanalysis of carbon electrodes (N2adsorption) subjected to an ageing protocol is presented. Furthermore, the influence of an anti-ageing additive (tocopherol) on the electrode material and electrolyte has been investigated. Finally, the mechanisms governing capacitive- and redox-based interfacial interactions are proposed.

This paper reports on the ageing mechanisms in symmetric, carbon/carbon electrochemical capacitors operating with a 1 mol/LLiNO3aqueous electrolyte. The carbon electrodes were subjected to a constant polarization protocol (i.e.,floating) at various voltages and were analysedpost-mortemby N2sorption at 77 K, Raman spectroscopy, Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), and elemental analysis. This approach allowed real capacitor performance deterioration to be simulated. Herein, the relationship between the electrochemical performance of high-voltage aqueous capacitors and the porosity of activated carbon cloth electrodes was investigated. Interestingly, two different ageing mechanisms, which mainly depended on the applied voltage, were identified. The oxidation of the positive electrode was the main reason for the capacitance fading. However, the oxidation of carbon was not the only reason, and it was postulated that solid-state deposits originating from CO and CO2evolution block the electrode pores and induce capacitance fading. A proof-of-concept experiment presented in this paper confirmed a partial electrode surface recovery after ageing.

Hybrid materials play an essential role in the development of the energy storage technologies since a multi-constituent system merges the properties of the individual components. Apart from new features and enhanced performance, such an approach quite often allows the drawbacks of single components to be diminished or reduced entirely. The goal of this paper was to prepare and characterize polymer-metal hydroxide (polypyrrole-nickel hydroxide, PPy-Ni(OH)2) nanowire arrays demonstrating good electrochemical performance. Nanowires were fabricated by potential pulse electrodeposition of pyrrole and nickel hydroxide into nanoporous anodic alumina oxide (AAO) template. The structural features of as-obtained PPy-Ni(OH)2 hybrid nanowires were characterized using FE-SEM and TEM analysis. Their chemical composition was confirmed by energy-dispersive x-ray spectroscopy (EDS). The presence of nickel hydroxide in the synthesized PPy-Ni(OH)2 nanowire array was investigated by X-ray photoelectron spectroscopy (XPS). Both FE-SEM and TEM analyses confirmed that the obtained nanowires were composed of a polymer matrix with nanoparticles dispersed within. EDS and XPS techniques confirmed the presence of PPy-Ni(OH)2 in the nanowire array obtained. Optimal working potential range (i.e., available potential window), charge propagation, and cyclic stability of the electrodes were determined with cyclic voltammetry (CV) at various scan rates. Interestingly, the electrochemical stability window for the aqueous electrolyte at PPy-Ni(OH)2 nanowire array electrode was remarkably wider (ca. 2 times) in comparison with the non-modified PPy electrode. The capacitance values, calculated from cyclic voltammetry performed at 20 mV s&minus, 1, were 25 F cm&minus, 2 for PPy and 75 F cm&minus, 2 for PPy-Ni(OH)2 array electrodes. The cyclic stability of the PPy nanowire array electrode up to 100 cycles showed a capacitance fade of about 13%.

It is often said that the highly developed surface area of electrode materials ensures high capacitance, power and energy in electrochemical capacitors. However, the real ‘efficiency’ of the surface exploited in charge storage remains unknown. Recently, it turned out that the Step Potential Electrochemical Spectroscopy (SPECS) technique can be a useful tool for determining it. The SPECS technique allows for the comprehensive analysis of the total current output and evaluation of the capacitive, redox and other components. Together with the physicochemical data from nitrogen and CO2 adsorption techniques, we aim at the determination of the real electrode/electrolyte interface surface. Based on the knowledge gained to date, we assume that the efficiency of the interface ‘exploitation’ depends on many factors: pore size distribution, wettability of the electrode material by electrolyte, as well as the size of the ions. Thus, inappropriate selection of one of these parameters can aggravate the device performance, even if the excellent carbon material is applied as electrode component. The paper will provide the results from SPECS correlated with the physicochemical properties of carbon materials. It appears that the electrochemically accessible surface does not exceed 30% of the values estimated by N2 or CO2adsorption. The results will be supported by the recent findings from electrochemical dilatometry experiments.

Electrochemical capacitors store the energy by the formation of the electrical double-layer at the electrode/electrolyte interface. These devices are characterized by high power density, short charge/discharge time and excellent cycling stability. The main goal of supercapacitor development nowadays is to increase their energy density and mentioned cycling stability. The former parameter could be improved by increasing the capacitance or operating voltage of the system, while the second by preserving current collectors from electrochemical corrosion phenomena. The increase of capacitance could be realized by the so-called pseudocapacitance effect, which can originate from the quick faradaic reactions at the electrode/electrolyte interface. These reactions can be induced by the appropriate modification of electrode in order to enforce the pseudocapacitive effect [1-4]. Electrochemical capacitors are composed of two electrodes separated by a separator and soaked in the electrolyte solution. Electrode material (mainly activated carbon) is coated on the surface of the current collectors made either of metals or stainless steel. In such a case, current collectors are exposed to corrosive electrolytes and can be subjected to corrosion processes [5-7]. There are several methods that are utilized to protect metals and steel from the harmful effects of electrochemical corrosion. One of them is the utilization of ionic liquids (ILs), which are added to an aggressive aqueous environment [8]. They act as corrosion inhibitors. This form of protection is being discussed in the scientific literature from about 20 years. A majority of articles describes the ability of this class of compounds to provide anti-corrosive protection for metal and steel. For these materials, an inhibitive influence on corrosion processes has been documented for dozens of ILs, vastly diversified in their chemical structure. A list of almost 90 investigated substances can be found in the literature [8]. Ionic liquids (ILs) are organic compounds which exhibit ionic structure and melting points below 100 degrees of Celsius. Due to their numerous advantageous properties, from some decades ILs stay in the scope of intense scientific interest, what results in the extensive research on possible ways to utilize them industrially. It covers various fields of IL application, for example as a solvent, an electrolyte, a catalyst, as a pharmaceutical or as corrosion inhibitors. In this work, the influence of ionic liquids additives on the performance of electrochemical capacitors is presented. Strictly, ionic liquids have been added to aqueous 1 M Na2SO4 electrolyte to inhibit corrosion processes on the surface of 316L stainless steel current collectors, which also influence the performance of electrochemical capacitors. Ionic liquids compounds were based on a common 2,5-dihydroxybenzenesulfonic anion. Electrochemical measurements were performed for a 2,5-dihydroxybenzenesulfonic acid, introduced to the electrolyte solution in 4 different concentrations ranging from 0,001% to 1%. Analogous tests were also conducted for 0,1% addition of 6 chosen ILs. All of ILs used in the research were composed of 2,5-dihydroxybenzenesulfonic anion and alkyldimethylammonium cation. The alkyl substituent of this cation varied, including n-ethyl, n-butyl, n-hexyl, cyclohexyl, n-octyl and n-decyl groups. The performed analysis proved an inhibitive effect of investigated compounds on the corrosion processes. The highest effectivities were observed for the ILs with bulkier alkyl substituent (i.e. containing cyclohexyl, n-octyl and n-decyl groups). On the basis of potentiodynamic polarization tests, a mixed corrosion inhibition mechanism was identified, with a predominant influence on the anodic process. Moreover, particular ionic liquids with the best anti-corrosive properties were used as additives to the electrolyte solution in electrochemical capacitors at a concentration of 0.1%. The results of electrochemical tests indicate a significant effect of the addition of ionic liquids on the working parameters of the electrochemical capacitors systems. This work was financially supported by the National Science Centre of Poland granted on the basis of the decision number 2018/31/B/ST8/01619 and by European Research Council (ERC) within ERC-StG-2017 project (grant agreement No. 759603) under the European Union’s Horizon 2020 research and innovation programme. References 1) F. Béguin, V. Presser, A. Balducci, E. Frackowiak, Adv. Mater., 26, 2219 (2014). 2) T. Brousse, D. Bélanger, J.W. Long, J. Electrochem. Soc., 162, A5185 (2015). 3) M. Graś, Ł. Kolanowski, J. Wojciechowski, G. Lota, Electrochem. Commun., 68, 28 (2018). 4) K. Fic, G. Lota, M. Meller, E. Frackowiak, Energy & Environmental Science, 5, 842 5 (2012). 5) J. Wojciechowski, Ł. Kolanowski, A. Bund, G. Lota, J. Power Sources 368, 18 (2017). 6) E. McCafferty, Introduction to Corrosion Science, Springer, New York (2010). 7) R.W. Revie, H.H. Uhlig, Corrosion and Corrosion Control, fourth ed., John Wiley & Sons, New Jersey (2008). 8) C. Verma, E.E. Ebenso, M.A. Quraishi, J. Mol. Liq., 233, 403 (2017).

This paper reports on the performance of the supercapacitor operating in aqueous acetic acid salts. Lithium, sodium and magnesium acetate aqueous solutions at various concentrations have been selected as electrolytes. Maximum operational voltage and the overall capacitor performance have been determined by several electrochemical techniques. Floating and galvanostatic charge/discharge tests proved the promising performance at high voltages (1.5 V); the capacitance values have been retained at more than 80% of initial value for all tested electrolytes. Additionally, due to the the ability to operate at high voltages, the maximum energy obtained in the system with 0.5 mol L −1 CH 3 COONa is more than two times higher than with 6 mol L −1 KOH, i.e., conventional aqueous capacitor. Taking into account a mild character of the electrolytes used, a novel concept of eco-friendly energy storage device has been proposed.

Increasing operating voltage is a straightforward approach to increase the specific energy of aqueous electrolyte based electrical double-layer capacitors (EDLCs). A broader operating voltage window, however, comes at the expense of accelerated cell ageing processes. Two complementary in situ gas analysis techniques, i.e., internal cell pressure measurement and Online Electrochemical Mass Spectrometry (OEMS), were applied to study the influence of electrolyte salt concentration on the electrochemical degradation of the electrode and decomposition of electrolyte during cycling of symmetrical carbon based EDLCs. Higher concentrations of Li2SO4 aqueous electrolyte salt increase the coulombic efficiency of the cell and reduce the amount of volatile side reaction products (e.g., CO, CO2, and H2) arising from carbon corrosion and water electrolysis. The lower amount of degradation products when increasing the salt to solvent ratio is believed to result from a favorable stabilization of the increasingly coordinated electrolyte solvent-salt bonds, which in turn increases the energy barrier for H2O decomposition. Higher salt concentrations not only increase the electrochemical reversibility at high cell voltages but also increase cell charging capacities as a result of increased electrolyte conductivity, which all-in-all facilitates implementation of future high energy aqueous EDLCs.

This paper presents supercapacitors utilizing new redox-active electrolytes with bromine species. Two sources of Br specimen were investigated, i.e. dibromodihydroxybenzene dissolved in KOH and potassium bromide dissolved in KOH with hydroxybenzene additive. KOH-activated carbon, exhibiting a well-developed porosity, was incorporated as an electrode material. The tested systems revealed a capacitance enhancement explained by Br− and partial BrO3− redox activity. The optimisation of the electrolyte concentration resulted in a capacitance value of 314 F g−1 achieved at 1.1 V voltage range. Good cyclability performance (11% capacitance loss) combined with a high capacitance value (244 F g−1) were obtained for the system operating in 0.2 mol L− 1 C6H4Br2O2 in 2 mol L− 1 KOH electrolytic solution.

This paper reports on a primary study of the capacitor system that can successfully overcome the main drawback of aqueous electrolytes, i.e. low max. voltage and then low energy density of the device. Our idea employs hybridization of electrolytes, i.e. use of two separated electrolytes with different pH values, each for one electrode. In such a way, the capacitor voltage is extended to 2.1 V. 6 mol L−1 KOH plays the role of electrolyte for the negative electrode whereas 1 mol L−1 H2SO4 or 5 mol L−1 LiNO3 are electrolytic solutions for a positive one. This idea allows us to take advantage of very negative hydrogen evolution potential for KOH and high overpotential of oxygen evolution, especially for LiNO3 solutions. Accordingly, it was possible to maintain almost 100 F g−1 after 25 000 cycles at 1 A g−1, gaining the energy more than 10 Wh kg−1 along the average power of 1 kW kg−1.

High-voltage aqueous electrolyte based supercapacitors (U > 1.23 V) attract significant attention for next-generation high power, low cost and environmentally friendly energy storage applications. Cell ageing is however markedly pronounced at elevated voltages and results in accelerated overall performance fade and increased safety concerns. Online electrochemical mass spectrometry, combined with cell pressure analysis, is for the first time shown to provide a powerful means for in situ investigation of degradation mechanisms in aqueous electrolyte/carbon based supercapacitors. The activated carbon electrodes possess high specific surface area and oxygen-based surface functionalities (mainly phenol, lactone and anhydride groups), which are oxidized already at a cell voltage of 0.6 V to provoke the evolution of minor amounts of CO and CO2. Noticeable water decomposition starts at a high voltage of 1.6 V with the evolution of H2 on the negative electrode and carbon corrosion on the positive electrode with the generation of predominantly CO. In this paper we also report that short-term cycling leads to partly reversible gas evolution/consumption side-reactions giving negligible capacitance. On the other hand, long-term cycling causes irreversible side-reactions, deteriorates the electrochemical performance, and increases the internal pressure of the cell. Repeated cycling (U < 2 V) is confirmed as a more harmful technique for the electrode integrity compared to the voltage holding in a floating test. In situ gas analysis is shown to provide valuable insights into the electrochemical cell ageing aspects, such as the nature and potential onsets of side-reactions, hence paving the way for fundamental understanding and mitigating the performance and safety loss of high-energy aqueous supercapacitors.

Lithium-ion capacitors (LICs) are intermediate devices between lithium-ion batteries (LIBs) and electrical double-layer capacitors (EDLCs).1,2,3 Their design consists of an activated carbon positive electrode that stores charges by capacitive behavior, while lithium cations are inserted in the graphite negative electrode. Thanks to their high cell voltage as well as the capacity of their electrodes, they can address quite a wide range of applications. Indeed, their energy density approaches that of lead-acid batteries, and their power density is close to that of EDLCs. However, the interest in such hybrid design is mitigated by the need of an initial prelithiation step of the graphite anode which is mandatory to further operate the LIC device. This step is usually performed using a sacrificial metallic lithium foil.3 More recently, alternative strategies have been proposed such as taking advantage of specific charging profiles on the first cycle that provide lithium cations from the electrolyte4, or using lithiated compounds that serve on the first charging cycle as the lithium cations source5,6. All these strategies introduce in the device many drawbacks including potential hazards, difficult manufacturing process, or/and high CO2 footprint. In this communication, we will present strategies that take advantage of low CO2 footprint material. Lithiated organic molecules have been recently envisioned as electrode materials for all-organic Li-ion batteries.7 Some of these molecules do not show any cycling ability and only provide lithium cations in an irreversible manner during the first charge. This drawback can be turned into an advantage in LICs where sacrificial compounds used for the prelithiation must be the lithium cations source but only during the first charge. During the subsequent cycles, the delithiated compounds must remain electrochemically inactive. Subsequently, our strategy consists in the use of a lithiated organic material, namely 3,4–dihydroxybenzonitrile dilithium salt (Li2DHBN) that is mixed with activated carbon at the positive electrode of the LIC, and which can irreversibly provide lithium cations to the graphite electrode during an initial in-situ charging step without any negative effects with respect to further operation of the LIC.8 It paves the way toward simpler and safer LICs that are easily recyclable. References: 1. Amatucci, G.G., Badaway, F., Pasquier, A.D. & Zheng, T. An asymmetric hybrid nonaqueous energy storage cell. J. Electrochem. Soc. 148, A930 (2001). 2. Naoi, K., Naoi, W., Aoyagi, S., Miyamoto, J.-I. & Kamino, T. New generation “nanohybrid Supercapacitor”. Acc. Chem. Res. 46, 1075 (2013). 3. Aida, T., Yamada, K. & Morita, M. An advanced hybrid electrochemical capacitor that uses a wide potential range at the positive electrode. Electrochem. Solid State Lett. 9, A534 (2006). 4. Decaux, C., Lota, G., Raymundo-Piñero, E., Frackowiak, E. & Béguin F. Electrochemical performance of a hybrid lithium-ion capacitor with a graphite anode preloaded from lithium bis(trifluoromethane)sulfonimide-based electrolyte, Electrochim. Acta 86, 282 (2012). 5. Park, M.-S., Lim, Y.-G., Kim, J.-H., Kim, Y.-J., Cho J. & Kim, J.-S. A novel lithium-doping approach for an advanced lithium ion capacitor. Adv. Energy Mater. 1, 1002 (2011). 6. Jeżowski, P., Fic, K., Crosnier, O., Brousse, T. & Béguin, F. Use of sacrificial lithium nickel oxide for loading graphitic anode in Li-ion capacitors. Electrochim. Acta 206, 440 (2016). 7. Chen, H., Armand, M., Courty, M., Jiang, M., Grey, C.P., Dolhem, F., Tarascon, J.-M. & Poizot, P. Lithium salt of tetrahydroxybenzoquinone: toward the development of a sustainable Li-ion battery. J. Am. Chem. Soc. 131, 8984 (2009). 8. P. Jezowski, O. Crosnier, E. Deunf, P. Poizot, F. Béguin, T. Brousse, Safe and recyclable lithium-ion capacitors using sacrificial organic lithium salt, Nature Materials 17, 167 (2018).

This paper presents a unique strategy aiming at extending the operational voltage range in supercapacitor working in the neutral aqueous electrolyte. In-depth analysis of the equilibria formed at the carbon/electrolyte interface gives the possibility to reach 1.8 V with excellent reversibility during cycling. To reach the goal, the donor - acceptor nature of carbon electrode surface has been taken into consideration. Controlled oxidation of electrode materials coupled with ammonia adsorption have been realized in order to adjust the optimal equilibrium conditions. Nitric acid oxidation of carbon leads to the formation of electrochemically active acidic sites promoting adsorption of ammonia. Modification with ammonia results in the introduction of self-controlled pH gradient within the capacitor system and formation of protective layer on electrodes, responsible for higher overpotentials of solvent decomposition. This, in turn, allows capacitor operation at higher voltages. Furthermore, this configuration eliminates the need for physical separation of ions (such as an ion-exchange membrane) to decelerate their mixing and improves the power density of the device.

Highly tunable properties of materials used for the construction of electrochemical capacitors make them a perfect choice for a broad scope of applications with high power demand. The ability to design the system according to the expected power/energy profile allows them being considered as powerful alternatives to conventional capacitors and batteries. Carbon materials with the developed specific surface area are the most common electrode components of electrochemical capacitors because of their cost, versatile form, availability, easiness of processing, and eco-friendly character. Biomass is frequently used for carbon production, however, among many natural organic materials, only some of them should be regarded as a useful precursor. Ongoing research brings many novel concepts of using bio-derived materials in high-performance electrochemical capacitors. This review article summarizes the progress on the applications of abundant biomaterials and materials derived from biomass in the field. Various ‘green’ resources have been used as precursors for activated carbons, as binders, or as gel (gelating) agents for solid-state electrolytes. The authors attempt to critically evaluate a commercial potential of these materials upon ongoing trends in research & development of electrochemical capacitors. Pros and cons of utilizing the selected biomass materials are provided and perspectives for their advanced processing are discussed.