Revisited insights into charge storage mechanisms in electrochemical capacitors with Li2SO4-based electrolyte

Energy Storage Materials 22 (2019) 1–14

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Revisited insights into charge storage mechanisms in electrochemical capacitors with Li2SO4-based electrolyte
n
ski 1, Krzysztof Fic *, 1, Anetta Płatek 1, Justyna Piwek 1, Jakub Menzel 1, Adam Slesi 1 1 1 Paulina Bujewska , Przemysław Galek , Elz_ bieta Frąckowiak Poznan University of Technology, Berdychowo 4, 60-965, Poznan, Poland

A R T I C L E I N F O

A B S T R A C T

Keywords: Electrochemical capacitors Aqueous electrolyte Activated carbon Operando techniques

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
1 Li2SO4 solution, 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 the operando approach; 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, a post-mortem analysis of carbon electrodes (N2 adsorption) 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.

1. Introduction Electrochemical capacitors (ECs), often called supercapacitors (SCs), ultracapacitors (UCs) or electric double-layer capacitors (EDLCs), are electrochemical devices for fast and efficient energy storage [1–6]. Their unique properties, such as high power rate, high efficiency, reliability, and long cycle life, result from their charge accumulation mechanism, which in principle exploits electrostatic interactions at the electrode/electrolyte interface, accompanied—in certain cases—by redox-based processes [7–9]. Thus, ECs can be considered a functional compromise between electrolytic capacitors (high power but low energy density) and conventional batteries (moderate power, high energy density) [4,6, 10–14]. The vast selection of electrode materials, electrolytes and constructions allows for designing a device perfectly suitable for specific operating conditions. To date, several materials have been proposed as promising electrode or electrolyte components for ECs; however, carbon-based materials, with their well-developed surface area, versatile porosity, and variety of surface functionalities, appear to be the most promising candidates [15–40]. In addition to carbon-based materials, selected transition metal

oxides (MnO2 and RuO2), electrically conducting polymers (ECPs) and metal-organic frameworks (MOFs) have been proposed as alternative components for electrode materials, demonstrating pseudocapacitive effects and thus improving the specific energy of the device [24,31,37, 41–63]. Another interesting group that has emerged recently is reduced graphene oxide (rGO) and its composites with metal and metal oxide nanoparticles [52,64–77]. Since the capacitive mechanism of charge storage is limited by either the accessible surface area of the interface or the transport of the interacting species from the electrolyte bulk to the interface, the majority of the research currently being conducted in the EC field is focused on the optimization of electrode properties [7,9,17–19,78–87]. In addition to the electrode materials, one cannot neglect the evitable role of the electrolyte (in liquid, gel or solid form) in the charge storage process. As the interfacial interaction involves both components (electrode and electrolyte), improvements in capacitor performance always relate to electrode/electrolyte “matching”. Currently, the development of EC technology is focused on finding solutions that allow the energy density (or specific energy) to be increased. In general, this parameter (E) might be improved by either

* Corresponding author.. E-mail address: krzysztof.fi[email protected] (K. Fic). 1 Authors declare equal contribution to this work. https://doi.org/10.1016/j.ensm.2019.08.005 Received 17 February 2019; Received in revised form 28 June 2019; Accepted 5 August 2019 Available online 8 August 2019 2405-8297/© 2019 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).

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Liþ cations and SO2
4 anions might shift the hydrogen and oxygen evolution potentials towards higher values [125]. The concepts are not mutually exclusive and have several aspects in common. Nearly one decade later, neutral aqueous solutions are still mysterious and intriguing electrolytes for electrochemical capacitors and are worth closer insight. In this paper, we provide comprehensive insight into the activated carbon/neutral electrolyte interface. In addition to standard electrochemical measurements, we performed post-mortem and operando studies to elucidate the intriguing performance.

capacitance (C) enhancement or operating voltage (U) increase: 1 E ¼ C  U2 2

(1)

The intrinsic capacitance of the electric double layer varies from 20 to 50 μF cm
2 [31,88]; therefore, the gravimetric properties of this parameter might be adjusted by the selection of a suitable electrode material with a well-developed specific surface area [7,38,89–95] and excellent wettability [96–98]. However, one should note that the development of the specific surface area typically increases the micropore volume and therefore might aggravate charge propagation. The capacitance itself might still be “boosted” by faradaic current. The pseudocapacitance phenomenon has been widely discussed elsewhere [21,31,43,46,50–55,88,99–104]. Here, it is worth noting that one should clearly distinguish between the pseudocapacitive and non-capacitive battery-like performance of materials (mainly transition metal oxides, including CoOx, NiOx, and FeOx). Both pseudocapacitive and non-capacitive materials might be applied in ECs; however, once a battery-like material is incorporated into a “capacitor”, the system should be considered a hybrid one [105]. In addition to the capacitance improvement obtained by tailoring a material to have desired features (surface area, pore size distribution, and surface functionalities), one cannot neglect the critical role of the electrolytic solution. In fact, the maximum operating voltage is governed by the kind of electrolyte applied. In commercial applications, electrolytes based on organic solvents such as acetonitrile or propylene carbonate are applied because their wide electrochemical stability (up to 3.0 V) results in a high specific energy of the device. Ionic liquids, i.e., liquids composed entirely of ions, are quite often considered interesting alternatives, but their high viscosity and moderate conductivity impede broad application [31,71,106–109]. Organic electrolytes, despite their advantages, also display numerous disadvantages. One of the major drawbacks is their low conductivity and frequently high viscosity, which affects electrode penetration and charge propagation. Furthermore, thorough drying of the electrode and an oxygen and moisture-free assembling atmosphere remarkably elevate the cost of the final device. Environmental aspects, as well as user safety issues (e.g., the toxicity of acetonitrile decomposition products), cannot be neglected. Thus, water-based electrolytes are interesting alternatives to organic electrolytes and ionic liquids. Their advantages include high conductivity, low price, and eco-friendly and non-toxic character. Certainly, ECs with aqueous electrolytes cannot compete with their organic-based counterparts in terms of energy, at least when purely capacitive charge storage is the only mechanism. This shortfall is caused by the water decomposition voltage at ca. 1.2 V. Interestingly, over the years, it has appeared that the water decomposition voltage on porous carbon electrodes strongly depends on the electrolyte pH. For alkaline and acidic electrolytes, the maximum operating voltage does not exceed 1.2 V. This is most likely caused by low overpotentials for hydrogen and oxygen evolution in alkaline and acidic electrolytes, respectively. The porosity and conductivity of the carbon electrode seem to play an important role as well [31,82,110–112]. Initially, pH-neutral electrolytes, such as Li2SO4, Na2SO4, K2SO4 and LiNO3 solutions, have been applied in asymmetric capacitors, either with an unequal mass of electrodes or based on MnO2/carbon composites. The high voltage achieved (above 1.2 V) in these devices is attributed to the configuration of the system [25,31,43,45,48,113–123]. Later, application in symmetric carbon/carbon systems demonstrated that the operating voltage could reach 1.6 V over thousands of cycles [124]. Further works have mostly focused on 1 mol⋅L
1 Li2SO4 aqueous solution as the most beneficial electrolyte since the maximum voltages reported in the literature have gradually increased [119,120,125–129]. The mechanism responsible for the high decomposition voltage has not yet been fully confirmed. It has been assumed that either the hydrogen stored in statu nascendi on the negative electrode [124,126] or the strong solvation of

2. Materials and methods All analytical-grade chemical reagents (Li2SO4, carboxymethylcellulose (CMC) sodium salt, and tocopherol) were purchased from SigmaAldrich. Two activated carbons (ACs) with similar specific surface areas (SBET~1800 m2 g
1) and pore size distributions, i.e., self-standing electrodes from the activated carbon cloth Kynol® ACC 507-20 and composite electrodes from Kuraray YP50F activated carbon powder, were used. The electrode diameter was 10 mm, and the average mass was 9.5 mg if not stated otherwise. The electrochemical capacitors were assembled in two-electrode Swagelok® cells; electrodes of similar mass were separated by a Whatman™ GF/A membrane with a 0.26 mm thickness and 1.6 μm pore size. Selected capacitors were equipped with a reference electrode (saturated calomel electrode, denoted as SCE). The electrochemical analyses included galvanostatic charging/discharging at various current loads (0.2–10 A g
1), cyclic voltammetry (1–100 mV s
1) and electrochemical impedance spectroscopy (100 kHz–1 mHz), performed on a VMP3 multichannel potentiostat/ galvanostat (BioLogic, France). The capacitance values are expressed per electrode, if not stated otherwise. 2.1. Viscosity and conductivity measurements The viscosity of the electrolytic solutions was measured with a Brookfield DV2T viscometer in a plate-cone set-up with controlled and adjustable temperatures of the investigated fluid. The conductivity of the solutions was measured with a Mettler-Toledo SevenCompact conductometer with controlled temperature. 2.2. Operando Raman spectroscopy measurements Raman spectra were recorded with a ThermoFisher® DXR Raman microscope equipped with a 532 nm laser. A three-electrode electrochemical cell with a self-standing activated carbon electrode (5 mm diameter), Pt wire as the counter electrode and SCE as a reference was used. The electrochemical data were collected with the VMP3. A reasonable distance between the working electrode (WE) and the counter electrode (CE) was maintained to avoid any impact of oxygen evolution (at the CE) on measurements. 2.3. Electrochemical quartz crystal microbalance measurements Activated carbon powder (YP50F) was mixed with an NMP-based suspension of PVdF and coated on a quartz crystal resonator (9 MHz standard frequency) equipped with a stainless steel current collector. The resonator was placed in a three-electrode electrochemical cell with an electrolyte volume of 0.4 mL (excess). Pt wire was used as the counter electrode, while SCE served as the reference electrode. In this case, the electrolyte concentration was reduced to 0.1 mol L
1. Higher concentrations of the electrolyte did not allow EQCM analysis to be performed as the crystal resonator was overloaded. However, the electrochemical response of the carbon electrode operating in 0.1 mol L
1 electrolyte in a typical three-electrode cell indicates that there is no remarkable 2

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difference between the capacitance value and electrolyte stability. However, the rate capability is slightly affected, which is attributed to the reduced conductivity of the electrolyte in the case of 0.1 mol⋅L
1 Li2SO4 solution. A VMP3 potentiostat/galvanostat connected to a QCA922 frequency source device (Seiko, Japan) was used for data collection.

The porous texture and the specific surface area of the electrodes were analysed by nitrogen adsorption/desorption as indicated above.

2.4. Internal pressure measurements

It has already been demonstrated that among capacitors operating with sulphate-based electrolytes, the highest capacitance is observed for devices with 1 mol⋅L
1 Li2SO4 solution as the electrolyte [125]. The high capacitance achieved by Li2SO4-based devices is attributed to the dimensions of solvated or partially desolvated lithium ions, which match the porosity of the carbon electrode. It is well known that alkali metal ions are strongly solvated in water, and their solvation degree increases with the diameter of the ion-solvent complex in the order Kþ
3. Results and discussion 3.1. General properties of Li2SO4 aqueous solution

Pressure measurements were conducted in a specially designed, leakproof polycarbonate pouch-like electrochemical vessel. The vessel was equipped with a valve connected to a pressure transducer (Keller 33X). The system was kept in a thermostatic chamber at 30  C. The indicated pressure was standardized based on the mass of the active material of one electrode and expressed in bar⋅g
1. The pressure evolution rate was expressed on an hourly basis (bar⋅g
1⋅h
1). The values were further converted from bars into millilitres of gas in standard pressure conditions, based on the calibration procedure. The electrodes (20 mm in diameter, ca. 30 mg each, equal mass of positive and negative of electrodes) were positioned facing each other and placed on stainless steel current collector foils (316L grade) painted with conductive adhesive. The electrodes were produced using Kuraray YP50F activated carbon (97%) as an active material and polytetrafluoroethylene (PTFE) as a binder (3%). The electrodes were prepared via dispersing an aqueous solution of PTFE in an isopropanol-based dispersion of AC, followed by evaporation and subsequent mechanical processing to obtain a thin film. 2.5. Scanning electrochemical microscope measurements The changes of the electrode thickness were monitored by scanning electrochemical microscope (SECM150, BioLogic, France). A selfstanding electrode (diameter of 5 mm) was placed in a customized electrochemical cell equipped with Pt wire as a counter electrode and SCE as a reference electrode. Redox shuttling based on ferricyanide anion [Fe(CN)6]3- was used to record the ‘approach curve’ data in a feedback mode. 2.6. Nitrogen adsorption/desorption measurements The electrode materials were flushed at 100  C by He flow and subsequently outgassed under vacuum. An ASAP 2460 analyser (Micromeritics, US) was used to determine the differences in specific surface area (SSA) calculated from the Brunauer–Emmett–Teller equation in the relative pressure range 0.01–0.05. The procedure followed the ISO 9277 standard. The SBET values, as well as the pore size distribution, were compared by using the 2D NL-DFT model [130–132]. 2.7. Experimental set-up for research on vitamin E as an additive for longterm performance improvement Electrodes in the form of pellets were prepared from activated carbon (Kuraray YP50F, 97 wt %) and polytetrafluoroethylene (PTFE – Sigma Aldrich, 60 wt % dispersion in water, 3 wt%) as a binder. The mixture was subdivided into four smaller parts. Vitamin E (tocopherol, SigmaAldrich®) was added to three samples at different concentrations: 1, 5 and 10%. Isopropanol was added to each sample and thoroughly mixed. The slurry was stirred at elevated temperature (50  C) until complete solvent evaporation. The obtained dough was rolled into a thin film ca. 200 μm in thickness. A glassy microfibrous material (Whatman™ GF/A) was used as a separator. The carbon electrodes were denoted as YP50F with information concerning the amount of vitamin E. Li2SO4 solution (1 mol L
1) was used as the electrolyte. Moreover, to verify how the modification method influences the electrochemical performance, vitamin E was also added to the electrolyte solution. First, vitamin E was dissolved in methanol (1 wt%), and then a water/organic electrolyte was prepared (in a volume ratio of 9:1).

3.2. Fundamental investigations of carbon electrodes operating in Li2SO4 solution The electrochemical performance of carbon electrodes operating in 1 mol⋅L
1 Li2SO4 is presented in Fig. 3. Fig. 3 shows cyclic voltammetry profiles recorded in a three-electrode cell with a 5 mV⋅s
1 sweep rate at room temperature (22  2  C). Initially, when the potential range is maintained between
0.75 V and þ0.75 V vs. NHE, a rectangular shape corresponding to electric double layer charging/discharging is observed (olive-green curve). Broadening of the potential range from
1.0 V to þ0.8 V vs. NHE leads to the formation of a reduction peak during negative polarization, identified as the beginning of reversible hydrogen storage [110,136–139]. The desorption 3

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Fig. 1. Viscosity and conductivity of the most commonly applied aqueous electrolytes.

Fig. 3. Cyclic voltammetry curves at a scan rate of 5 mV s
1 for activated carbon operating in 1 mol⋅L
1 Li2SO4.

Fig. 2. Conductivity vs. viscosity of 1 mol∙L
1 Li2SO4 solution.

(oxidation) peak is formed during the positive run at approximately þ0.5 V vs. NHE. Such a difference between reduction and oxidation potentials results from the fact that on porous electrodes, the mass and charge transfer values vary remarkably. Cathodic polarization with water decomposition generates OH
species that locally change the pH and shift the oxidation potential. Furthermore, the kind of redox process in the case of hydrogen storage is slightly different; in this case, a surface reaction is considered and might occur with significant overpotential. As shown by D. Lozano-Castello et al., the desorption peak at high potential values depicts the desorption of strongly bound hydrogen in the neutral medium [140]. A further increase in the potential range leads to an increase in both the reduction and oxidation current. For reduction potentials lower than - 1.25 V vs. NHE, two desorption peaks can be observed: the first peak is for weakly adsorbed hydrogen at approximately
0.75 V vs. NHE, and the second peak is for strongly adsorbed hydrogen at higher potentials. The change in adsorption “strength” may correspond to local changes in pH at the electrode/electrolyte interface, according to the following reaction:

C þ x H2O þ x e
↔ þ OH
A progressive change in the electrolyte pH leads to a change in the hydrogen sorption/desorption mechanism towards weakly adsorbed hydrogen. The influence of pH on the hydrogen desorption process in 1 mol⋅L
1 Li2SO4 can be observed via the galvanostatic process, as shown in Fig. 4. During this experiment, the galvanostatic intermittent titration technique (GITT), the system was subjected to 10 periods of charging (reduction) at
50 mA g
1 current density for 1 h each, followed by a 1 h rest period between each galvanostatic cycle. After 10 ‘reductive’ cycles, the electrode was discharged by 10 oxidation cycles with a current density of þ50 mA g
1. As for the reductive cycle, each oxidation period was separated by 1 h of relaxation time. During the first cycle, the reduction potential reached
1.0 V vs. NHE, while the equilibrium potential for the charged electrode was
0.84 V vs. NHE. The progressive change in electrolyte pH can be 4

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voltage of 1.8 V. Furthermore, the redox processes occurring within the voltage window seem to be reversible and non-destructive for the electrode (confirmed by Raman spectra in Fig. 5). However, such a situation is expected if the potential distribution between electrodes is symmetric. Unfortunately, as shown in Fig. 6, this is often not the case in real twoelectrode systems. The potential distribution is not symmetric and is shifted towards positive values, i.e., closer to the oxidation region. For the cell charged to 1.5 V, the positive electrode potential reaches the oxygen evolution potential. This most likely triggers the first step of system degradation (carbon oxidation) and indicates the voltage limit for devices working with 1 mol⋅L
1 Li2SO4 solution. A further increase in cell voltage results in a shift in positive electrode potential towards carbon oxidation and provokes a change in degradation mechanism from oxygen evolution towards fast carbon degradation. Interestingly, as indicated by operando mass spectrometry measurements, there is no direct oxygen evolution [135,147,148]. It appears that oxygen generated in statu nascendi immediately reacts with the carbon surface and produces CO and CO2. This process is strictly voltage dependent—the reaction rate is proportional to the electrode potential [134,135]. Indeed, gas evolution has a detrimental impact on capacitor performance since the internal pressure increases rapidly (as discussed later in the manuscript) and causes a dramatic increase in internal resistance [148]. Moreover, an increase in CO and CO2 partial pressure might provoke their dissolution in the electrolyte and Li2CO3 precipitation. Such deposition is expected to block the porosity, resulting in a dramatic capacitance fade [146,148]. Meanwhile, a negative electrode potential for cell voltages higher than 1.0 V is maintained above the hydrogen evolution equilibrium potential. In this case, H2 was detected during mass spectrometry measurements [134]. This confirms that most of the degradation processes during capacitor operation are a direct result of oxidation processes observed on the positive electrode. Furthermore, an increase in the negative electrode capacitance from the pseudocapacitive effect of hydrogen electrosorption enforces the shift in the positive electrode potential towards higher values and accelerates the oxidation process. It should be noted that this might also be a reason for the initial capacitance increase, which has often been observed during long-term tests. Most likely, the initial capacitance increase comes from hydrogen electrosorption and is balanced at the positive electrode by the potential increase. After a certain time, the capacitance starts to decrease since oxidation of the positive electrode occurs and the corresponding capacitance loss dominates over hydrogen electrosorption.

Fig. 4. Galvanostatic charge/discharge of an activated carbon electrode in 1 mol⋅L
1 Li2SO4 aqueous solution with a periodic relaxation period; current 50 mA g
1.

observed from the change in potential plateau towards lower values (ca. 30 mV from the first cycle to the last cycle). After changing the polarization, a non-linear curve for the discharge process with hydrogen desorption until þ0.75 V vs. NHE is observed. It should be noted here that neither process was limited by potential; hence, the electrode always reached the maximum potential possible for the given current conditions. Further oxidation led to oxygen evolution at electrode potentials above 1.0 V vs. NHE. Indeed, carbon oxidation preceded oxygen evolution and was found to occur at the potential 0.975 V vs. NHE. Finally, the potentials established during the relaxation periods (dashed lines) were identified as the optimal electrolyte operating window. To confirm these findings, the hydrogen sorption/desorption phenomenon was also investigated by operando Raman spectroscopy performed on the same experimental schedule. During the reduction process, the intensity increases progressively for characteristic carbon bands, i.e., the D-band (1350 cm
1) and G-band (1585 cm
1). The change in G-band intensity is most likely a result of the charge transfer reaction (hydrogen electrosorption) at the electrode/ electrolyte interface [140]. Simultaneously, the changes in intensity of the 1110 cm
1 and 1500 cm
1 bands correspond to carbon structure changes during hydrogen insertion [140,141]. A small change in the Raman shift is observed for the G- and D-band maximum intensities (2 cm
1) and confirms that hydrogen is being ‘inserted’ into the carbon matrix [110,142–145]. Of course, the specific adsorption of hydrogen cannot be neglected. At the structural level, this process impacts the electrode volume; operando studies with scanning electrochemical microscopy confirm that the electrode thickness changes noticeably. Additionally, the specific surface area of the electrode measured after 1000 cycles of galvanostatic charging increases by 300 m2 g
1 [146]. Surprisingly, the most significant changes in the intensity of specific bands are observed after the polarization change. During the first step of electrode oxidation, extensive hydrogen desorption can be observed. The dramatic increase in the D- and G-band intensities is clearly connected with the charge transfer reaction in potentials ranging from
1.2 V to 0.0 V vs. NHE. This means that most of the hydrogen is oxidized during the first oxidation cycle and is rather weakly adsorbed. In subsequent cycles, the decrease in D- and G-band intensity indicates the end of the hydrogen desorption reaction. Analysis of the galvanostatic curve recorded for the carbon electrode in 1 mol⋅L
1 Li2SO4 and comparison of the equilibrium potentials for positive and negative polarization allow for the approximation of the theoretical potential window (voltage) for the device. Considering the equilibrium potentials in Fig. 4, one should expect a maximum capacitor

3.3. Internal pressure measurements To verify the gas evolution processes, the electrochemical capacitors were subjected to internal pressure measurements. One of the first attempts was made in a paper by Hahn et al., in 2005, where the gases that evolved during the operation of a capacitor operating in organic electrolyte were identified [149]. Another article by the same authors quantitatively described the evolution of pressure in organic-based electrolyte [150]. However, no articles are devoted to capacitors operating with aqueous electrolytes. Moreover, the volume of the evolved gases was estimated using a calibrated pressure-measuring device. Fig. 7A presents 2-h floating periods at different voltage levels followed by 1 h of open circuit conditions (self-discharge). This kind of experimental scheme allows observation of the internal pressure behaviour during imposed voltage stress as well as during the subsequent rest period. No remarkable pressure increase is observed at 1.0, 1.2 and 1.4 V; instead, a small decline is found. This behaviour is assigned to the wetting behaviour of the porous electrodes, where no electrolyte decomposition into gaseous products occurs. The pressure starts to increase steadily by 0.8 bar g
1 at 1.6 V and continues at this rate for 2 h. The pressure increase rate at 1.8 V is approximately three times as high as that at 1.6 V. In general, the pressure increase rate appears to be constant upon application of a given constant voltage. No voltage higher 5

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Fig. 5. Raman spectra for an ACC 507-20 activated carbon electrode operating with 1 mol⋅L
1 Li2SO4 electrolyte. Selected Raman profiles (right part of the heat map) correspond to the last points before the polarization change.

increases to 1.6 bar g
1 h
1 in 2 h. This trend most likely means that the system did not reach steady state and several parallel parasitic reactions were triggered. Importantly, leakage current measurements appear to be a very sensitive indicator for all processes occurring at the electrode/electrolyte interface, as they perfectly correlate with pressure changes. Briefly, the leakage current values become higher as the voltage increases, as shown in Fig. 8. As expected, the most significant increase is observed for the capacitor voltage of 1.8 V, where the gas generation rate is the highest. The secondary y-axis depicts the corresponding gas generation rates expressed in volume under standard conditions. The generation of gas starts at approximately 1.5 V with a rate of approximately 1 mL g
1 h
1 and increases steeply at higher voltages. 3.4. Long-term performance of capacitors with 1 mol⋅L
1 Li2SO4 electrolyte Before the long-term electrochemical test, determination of the maximum voltage was performed. Comparing the efficiencies calculated from cyclic voltammetry (at 2 mV s
1) and the galvanostatic mode (at 0.1 A g
1), one can see that the values are very similar (Fig. 9). The applied regime, i.e., 2 mV s
1 and 0.1 A g
1, was selected to observe all side effects, i.e., electrolyte decomposition, parasitic reactions, corrosion or oxidation of the electrode material. For the capacitor operating with 1 mol⋅L
1 Li2SO4 solution, the efficiency is higher than 90%, i.e., 94% up to 1.7 V. Therefore, no differences are observed between the efficiency from the potentiodynamic mode and coulombic efficiency from the galvanostatic mode. Given that, it is impossible (or at least very difficult) to draw conclusions about the maximal voltage window. However, the profile for the energetic efficiency is different from the other profiles. After reaching 1.1 V, the efficiency drops below 90%. This could suggest that electrolyte

Fig. 6. Electrode potential vs. voltage profile for a capacitor operating with 1 mol⋅L
1 Li2SO4.

than 1.8 V was tested because 1.8 V is the operating voltage limit in aqueous electrolyte. When the potentiostatic mode is terminated, the pressure immediately decreases, and after a short time, a small decline is observed. This behaviour is assigned to the dissolution of the gas in the electrolyte. Fig. 7B presents the calculated pressure increase rates. No significant changes are observed at low voltages (up to 1.4 V). The increase rate at 1.6 V is equal to 0.4 bar g
1 h
1 and remains fixed during the potentiostatic hold stage. An interesting phenomenon is observed at 1.8 V, where a continually increasing rate is revealed: the rate starts at 1 and linearly 6

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Fig. 7. (A) Pressure variation during potentiostatic conditions and under opencircuit conditions, (B) corresponding pressure increase rates.

Fig. 9. Determination of the maximum voltage window for the EDLC based on Kynol 507-20 and 1 mol⋅L
1 Li2SO4. (A) Efficiency of the charging/discharging process as a function of cell voltage; (B) cyclic voltammetry with a scanning rate of 2 mV s
1 for various voltages.

decomposition occurs. Usually, the limit of 90% efficiency has been established as a criterion of voltage determination. However, the results of 3-electrode measurements (Figs. 3 and 4) allow for the assumption that in a full device, the maximum voltage will be higher than 1.1 V. Therefore, it was further assumed that low energetic efficiency might result from non-optimal electrode material selection for 1 mol⋅L
1 Li2SO4 electrolyte: carbon cloth, Kynol 507-20, is a highly microporous material with a very narrow pore size distribution. Hence, SO2
4 anions might be unable to fully penetrate the micropore volume. Nevertheless, the goal of this study was to understand and describe the ageing mechanism of a capacitor with sulphate-based electrolytic solution, not to optimize the electrode and electrolyte parameters themselves. Despite being strictly microporous, carbon cloth reveals several advantages in comparison to powdered activated carbons: this material can be used to construct a selfstanding electrode entirely of carbon without any additives, such as polymer binder or conductive agents. As reported elsewhere, such additives might aggravate the carbon electrode performance [151]. For these reasons, even though the energy efficiency was not excellent, it was decided to perform the study with activated carbon cloth. Thus, such an approach allowed for monitoring the state of health of the carbon material during the electrochemical tests and neglect several side reactions

Fig. 8. Leakage current values after 2 h of potentiostatic holding and evolution of gases at various voltages.

7

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resulting from the decomposition or reactivity of polymer binders or any other components. Fig. 9B presents cyclic voltammetry profiles recorded at a scan rate of 2 mV s
1. At a capacitor voltage of up to 1.1 V, the cyclic voltammograms display an ideal rectangular shape. This observation is in accordance with assumptions from the energetic efficiency calculated in galvanostatic mode (>90%). Above 1.1 V, the CV curves start to deviate from the ideal EDL shape, demonstrating an increase in current during the charging process at approximately 0.6 V. The deviation from a linear response with respect to voltage indicates that specific side reactions occur in the system. However, a non-linear current profile results in a high charging/ discharging efficiency value, calculated from the potentiodynamic mode. Thus, cyclic voltammetry must not be used as a quantitative indicator. Of course, such measurement provides valuable information about the “quality” of the charging/discharging process, but it “overestimates” the amount of charge accumulated and released during the charging and discharging process. Coulombic efficiency compares de facto the time of charging and discharging and does not provide any direct information about the “quality” of these processes, as the resistance is not included in the calculations (moreover, correction by ohmic drop might overestimate the capacitance). In this case, the optimal method is to integrate the galvanostatic profile (Fig. 10). The galvanostatic charging/discharging data are in accordance with the cyclic voltammograms presented in Fig. 9. Above a 1.1 V capacitor voltage, distortion from a linear discharge curve is observed. Therefore, the amounts of charge accumulated and released are considerably different; however, the times of both processes are similar. Moreover, the hydrogen stored on the negative electrode could lead to disproportionality in the charge accumulated and released. Therefore, not only is the overall performance of a symmetric cell essential but the behaviour of individual electrodes should also be considered. In fact, without 3-electrode measurements, a full explanation of the phenomena observed in a 2-electrode system cannot be reliably elucidated. Thus, coulombic efficiency is recommended only if the charge storage is purely capacitive and the galvanostatic curve is triangular. The cyclability test (Fig. 11) consisted of subsequent galvanostatic charging/discharging cycles (0.5 A g
1) up to an 80% initial capacitance retention. This limit has been established by the International Standard (IEC 62391–1:2015) on the basis that a 20% initial capacitance loss or two-fold increase in equivalent circuit resistance (ESR) is an end-of-life criterion [152–159]. Surprisingly, for a high capacitor voltage (1.6 V), a 20% capacitance fade was observed after 120 000 cycles (248 days of continuous charging/discharging). Such long-term performance was unexpected, as normally a much lower number of cycles is presented/reported; based on

Fig. 11. Long-term cycling (0.5 A g
1) test for a capacitor with 1 mol⋅L
1 Li2SO4: (A) relative capacitance vs. cycle number; (B) cyclic voltammetry profiles (5 mV s
1) for fresh and aged cells.

both 120 000 cycles and 248 days of constant operation, the system demonstrated a very long time for “destruction”, keeping in mind that 80% of the initial capacitance was still available. Thus, it could be predicted that the system will work much longer until reaching total fade conditions (0% relative specific capacitance). Interestingly, after aging to an 80% relative capacitance, the system maintained good charge propagation (Fig. 11B). The increase in current recorded at low capacitor voltage is attributed to the pseudocapacitive effect of hydrogen adsorption/desorption. However, negligible contributions from the quinone/ hydroquinone redox couple cannot be excluded. To investigate the reasons for performance fade, after long-term performance tests, the system was disassembled, and the components were tested via post-mortem analysis. As observed in Fig. 12A, the specific surface area (SSA) of the positive electrode drops dramatically to 1371 m2 g
1 (
75%), in comparison to 1841 m2 g
1 (100%) for pristine carbon cloth. The percentage decrease in SSA (25%) is similar to the specific capacitance fade (20%); therefore, a loss of microporosity results in a reduced amount of charge accumulated. Furthermore, the positive electrode displays a diminished micropore volume; thus, some oxidation products, i.e., functional groups, are probably blocking the entrances to the pores. The porous structure of the negative electrode remained unchanged. In this case, SSA dropped only by 120 m2 g
1. Moreover, the pore size distribution (PSD) of the negative electrode almost overlaps the profile for pristine carbon cloth. This suggests that only a negligible number of pores are blocked (Fig. 12B).

Fig. 10. Scheme of efficiency calculation from the galvanostatic charging/discharging curve. 8

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Energy Storage Materials 22 (2019) 1–14

Fig. 13. Cyclic voltammetry profiles recorded with the frequency (mass) change monitored by EQCM.

The frequency change was converted to mass change with the Sauerbrey equation. It has been claimed that in such systems, the viscoelastic aspects of resonant media cannot be neglected [160,161]. However, we sought to keep the experimental conditions in line with best practices (e.g., a very thin and rigid electrode and flat resonator). First, the mass changes recorded during positive and negative polarization are substantially different. Most likely, bare lithium ions (mass 6.9 g mol
1) and solvated species ([Li][H2O], m ¼ 23.9 g mol
1; [Li] [2H2O], m ¼ 40.9 g mol
1; [Li][3H2O], m ¼ 57.9 g mol
1; or [Li][4H2O], m ¼ 74.9 g mol
1) interact actively at the interface once an electric double layer (EDL) is formed. The slope of the negative electrode mass change profile during both polarization steps is the same, which suggests that the same kind of ions participate in EDL formation. Nonetheless, during reversible charging/discharging, the indirect mass of the electrode steadily changes, while the direct mass change remains stable. As this result was reproducible during several experimental runs under various configurations, we assumed that it might be caused by deposition on the electrode surface (Li2CO3 or corrosion products) or the generation of functional groups on the carbon surface. In this case, a well-defined redox response from oxygen-based functionalities might not be observed, as the electrode loading is very small, the oxygen content might be less than 10 wt% and there is a large excess of electrolyte at neutral pH. The redox activity of the quinone/hydroquinone couple is promoted in acidic or alkaline media, while in neutral media, the activity is remarkably diminished. Nonetheless, small humps observed during positive electrode ‘discharging’ (near þ0.75 V vs. NHE) might suggest that mild oxidation of the carbon surface occurs. Finally, ions from the electrolyte bulk can move towards the polarized electrode and change in density in the close vicinity of the piezoelectric crystal. During positive polarization, the mass changes were assumed to be due to the mass of the bare sulphate anion (m ¼ 96.1 g mol
1) being much higher than that of all solvated species of the Liþ cation. However, as shown in Fig. 13, the recorded mass change is significantly smaller than expected. The cyclic voltammogram displays a rectangular shape, indicating typical electrostatic attractions and capacitive current. Even after exceeding 1.5 V vs. NHE, when an increase in current is observed, the slope of the mass change curve does not change. Therefore, recalculation of the possible species responsible for such a mass change was performed iteratively, taking into account all species present in the solution. It appears that the mass change corresponds to OH
transport. Clearly, the mass change recorded in this experiment is the average of observable ionic fluxes towards the electrode and in the opposite direction, but one cannot definitively exclude the specific adsorption of OH
species. Interestingly, this observation is in perfect agreement with the results obtained for pH measurement after long-term galvanostatic

Fig. 12. N2 adsorption/desorption isotherms at 77 K (a) and pore size distribution (b) for pristine carbon cloth, positive electrode and negative electrode.

The results of post-mortem analysis confirmed that the positive electrode is always oxidized during the long-term tests and might be responsible for the overall performance fade. 3.5. EQCM and SECM investigations Despite the insightful findings provided by post-mortem analyses, a precise description of the ongoing processes at the electrode/electrolyte interface requires a more sophisticated approach. To observe the interfacial interactions, fundamental electrochemical techniques were conducted in operando mode with an electrochemical quartz crystal microbalance (EQCM) and scanning electrochemical microscopy (SECM). Such an approach allows direct monitoring of the system response to electrochemical signals. In our case, we focused on the gravimetric (monitored with EQCM) and volumetric (monitored with SECM) changes in the activated carbon electrodes. Both techniques should provide comparable results since changes in mass should impact the electrode volume. In Fig. 13, cyclic voltammograms recorded at a potential scan rate of 5 mV s
1 for negative and positive polarization are presented. A very small electrode mass on the piezoelectric resonator (required by EQCM investigation principles) allowed for scanning within a wide potential range, i.e., >2.0 V. Interestingly, the current response indicates that for both electrodes, charge is accumulated via electrostatic attractions without any charge transfer reactions. For both electrodes, the current profile is typically capacitive, as no well-defined redox peaks are observed. However, for the positive electrode, oxidation of surface impurities can be observed during the first cycle. 9

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Energy Storage Materials 22 (2019) 1–14

cycling and voltage floating. In both cases, the pH of the electrode eluate (solution used for electrode flushing) is always alkaline (~10). Furthermore, a local increase in pH in the electrode vicinity accelerates the oxidation process since the oxygen evolution potential is negatively correlated with pH. This observation might also explain the positive impact of the electrolyte (buffering effect) on long-term performance [162–166]. Similar findings were obtained for scanning electrochemical microscopy investigations. The carbon electrode was subjected to the same scanning procedure, but in this experiment, the change in the electrode height was monitored (Fig. 14B). Fig. 14A presents the mass change recorded by EQCM vs. charge exchanged. The profile could be divided into three main parts: anion adsorption, ion exchange, and cation adsorption. As predicted from the CV profiles (Fig. 13), the species adsorbed during negative polarization are much heavier than the anions or species adsorbed during positive polarization (mLiþ*2H2O > mOH-). Therefore, either sulphate anions do not actively participate in EDL formation or their concentration is constant at the interface; only OH
species move actively during

polarization. Fig. 14B presents the electrode height change vs. potential applied. The profile perfectly resembles the mass change vs. charge profile recorded with EQCM. It seems that there is a significant change in the volume of the negative electrode, while the volume of the positive electrode remains almost unchanged until þ1.5 V vs. NHE. Further changes are clearly related to electrode oxidation since the potential values are well beyond the oxygen evolution potential. 3.6. Vitamin E for long-term performance improvement Since oxidative stress has been recognized as the main destroyer of capacitor performance, a natural next step was to find an agent with antioxidative properties to prolong the capacitor lifetime. Given the biochemical role of tocopherol (vitamin E, Fig. 15), this compound appeared to be a very promising candidate in this application. To find the optimal application route, tocopherol was used as an additive in the electrode material and electrolytic solution (in methanol, as tocopherol is insoluble in water). The typical electrochemistry of tocopherol has not been widely elaborated; however, for certain organic solvents (CH2Cl2/0.1 mol⋅L
1 Bu4NBF4), the redox activity has been described, with an oxidation potential at þ0.276 V vs. Pt wire. First, the activated carbon electrodes were soaked with vitamin E, and their SSA was verified. Fig. 16 presents the nitrogen adsorption/ desorption isotherms for YP50F non-modified and vitamin E-modified materials. All electrode materials exhibit microporous characteristics with a small number of mesopores. However, the higher the amount of vitamin E added is, the smaller the value of the specific surface area of the material. This suggests that vitamin E might block the pores of activated carbon. This process was confirmed by the pore size distribution (Fig. 16B), which reveals the volumes of different sizes of pores. The volume of pores with a diameter from 0.8 to 1 nm decreases with an increasing amount of vitamin E. No significant differences were observed for pores in the range of 1–2 nm, which suggests that tocopherol is not large enough to block the pores in this diameter range. Based on the information obtained from the physical characterization of the electrode materials, in further experiments, YP50F with 1% vitamin E was chosen as the most promising material. As stated above, vitamin E was expected to prevent oxidation during the electrochemical experiments and, consequently, prolong the lifetime of the electrochemical capacitors. Before the ageing tests, the influence of vitamin E on the electrochemical behaviour of the cells was determined. Three different configurations were tested: 1 mol⋅L
1 Li2SO4 solution with non-modified YP50F electrodes (as a reference), 1 mol⋅L
1 Li2SO4

Fig. 14. Operando studies of the ongoing processes on the electrode/electrolyte interface performed using (A) electrochemical quartz crystal microbalance (mass change vs. charge exchanged); and (B) scanning electrochemical microscopy (electrode thickness change vs. electrode potential).

Fig. 15. The structures of tocopherol (vitamin E) in regular and activated state [167]. 10

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Energy Storage Materials 22 (2019) 1–14

Fig. 17. Capacitance vs. charge/discharge current, calculated from the integral under the discharging curves.

density. It is worth mentioning here that the character of these changes seems to be the same for all cells. The long-term performance was evaluated with floating tests. Each system was charged up to maximum voltage (1.6 V) with a current density of 0.5 A g
1 and held at this voltage for 60 min. After this time, the cell was discharged, and the procedure was repeated. This kind of test allowed the lifetime of the device to be compared. As previously discussed, the experiments were conducted until a 20% capacitance loss was reached or until the equivalent distributed resistance (EDR) increased by two-fold. It should be noted that the energetic efficiency in all cases was moderate, started from 80% and then continuously decreased. This suggests that a high polarization current appears at higher voltages. This might be caused by several reasons, with electrolyte decomposition as the major cause. However, taking into account that the lifetime of all systems exceeded more than 100 h at 1.6 V, it seems that the contribution of the ‘decomposition current’ in each cycle is incremental. However, the coulombic efficiency is satisfactory and oscillates around 90%. The reference cell with non-modified components was operated for 184 h, and after this time, the EDR increased by two-fold (Figs. 18A and 19A). During long-term tests, another component with a different time constant appeared in all cases, as the shape of the semi-circle changed. Since the spectra were recorded for two-electrode cells, equivalent circuit models cannot be reliably applied to resolve this issue. However, considering the fact that this change occurs in high-frequency regions, one might assume that this is an adsorption component. However, it is still unclear whether the additional component concerns the adsorption of vitamin E isolated from the electrolyte solution or deposits resulting from the corrosion of cell components. For the system with the modified electrolyte, the lifetime was longer—242 h. In this case, the two-fold increase in EDR as the end-of-life criterion was crucial (Figs. 18B and 19B). Neither system reached a 20% capacitance loss. Taking into account the charge/discharge efficiency, it is slightly higher for the cell with the modified electrolyte. For the cell with both components modified with vitamin E, the lifetime was the shortest—only 133 h during the floating test (Fig. 18C). This supports the idea that vitamin E might be preferably adsorbed at the interface and aggravates effective ion adsorption. In this context, a high polarization current near the maximum voltage might contribute to a shorter lifetime of the cell. In contrast to previous cases, the end-of-life criterion of 20% capacitance loss was reached. Efficiency was retained until 90 h during the test. However, after that, a significant drop in efficiency is observed. The EDR observed during the floating test increased

Fig. 16. N2 adsorption/desorption isotherms (A) and pore size distributions (B) of different YP50F-based carbon electrode materials.

solution with vitamin E (1%) and non-modified YP50F electrodes and 1 mol⋅L
1 Li2SO4 solution with vitamin E (1%) coupled with vitamin E (1%)-modified YP50F electrodes. The cyclic voltammetry technique was used to determine whether there was any influence of vitamin E on charge propagation. The experiments were performed at different scan rates—from 10 mV⋅s
1 to 500 mV s
1. The recorded CV profiles were almost the same, although some deviation from the rectangular CV profile was observed, especially for slower and medium scan rates (10–100 mV s
1). For scan rates higher than 200 mV s
1, the recorded voltammograms were distorted. Interestingly, the cell with the modified electrolyte and electrodes did not demonstrate any remarkable changes under any of the scan rates applied in comparison to the non-modified reference. The galvanostatic charge/discharge technique was used to quantitatively describe of the influence of vitamin E on the capacitance values (Fig. 17). In accordance with the results obtained by cyclic voltammetry, the highest capacitance was reached for the non-modified system. Nonetheless, for the cell with the electrolyte modified with vitamin E, the capacitance values were lower (~10%). This might be caused by the specific adsorption of vitamin E at the carbon surface, which either competes with ion adsorption and blocks ion access to the micropores or makes the surface more hydrophobic and thus aggravates the electrode wettability. The situation is slightly worse for the cell where both components were modified – the difference in capacitance is almost 30 F g
1 for 0.5 A g
1. For all cases, the capacitance values decrease with increasing current 11

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Fig. 18. Capacitance retention and charge/discharge efficiency during floating tests.

by approximately 1.5 times for this cell. 4. Conclusions Carbon-based electrochemical capacitors with 1 mol⋅L
1 Li2SO4 aqueous solution demonstrate several interesting features.

Fig. 19. EDR evolution during floating tests for the capacitors with vitamin E added.

12

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1. The optimal operating voltage is estimated to be 1.6 V. Higher operating voltages are feasible; however, the capacitor lifetime will be significantly reduced. 2. The viscosity of the electrolytic solution seems to play a negligible role in the charge propagation process. The wettability of the electrode appears to be more influential. 3. The long-term performance is governed by the positive electrode. Three-electrode experiments demonstrated that the positive electrode operates near the oxygen evolution potential at capacitor voltages up to 1.2 V. For higher voltages, oxygen evolution has a detrimental impact on the electrode and thus capacitor performance: the specific surface area drops dramatically, and the lifetime is therefore aggravated. Interestingly, nascent oxygen immediately reacts with the carbon surface and provokes CO and CO2 generation. 4. Hydrogen storage on the negative electrode only has a positive impact during initial cycles. The pseudocapacitive effect of the electrosorption process contributes to the capacitance increase until the charge balance causes the positive electrode to operate above the oxygen evolution potential. 5. The internal pressure does not change remarkably for capacitor voltages up to 1.2 V. At elevated voltages, the internal pressure starts to increase dramatically with a detrimental impact on capacitor performance. 6. Electrochemical quartz crystal microbalance analysis and scanning electrochemical microscopy confirmed that the negative electrode changes the mass and volume noticeably during capacitor operation. The mass and volume changes for the positive electrode remain unchanged. 7. The local pH change and electrode oxidation might be provoked by the increase in OH
species in the vicinity of the positive electrode. 8. Tocopherol, as an antioxidant, has a positive impact on the long-term performance, but an optimal application route should be used. In this case, the optimal approach seems to be electrolyte modification. The capacitor lifetime was prolonged from 184 to 232 h in a voltage (floating) test.

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Future development of electrochemical capacitors operating in neutral aqueous electrolyte should be oriented towards effective protection of the positive electrode against oxidation. Preliminarily, careful selection of the separator to prevent OH
migration from the negative electrode compartment, a tailored electrolyte with buffering properties or the selection of activated carbons with an asymmetric configuration and designed functionalities should be considered. Taking into account the sustainability of such systems, this approach is clearly worth following. Conflicts of interest Authors do not declare any conflict of interest. Acknowledgement Authors acknowledge the funding received from European Research Council within the Starting Grant project (GA 759603) under European Unions’ Horizon 2020 research and innovation programme. AP and JM acknowledge National Science Centre for financial support in 2017/25/ N/ST4/01738 and 2015/17/N/ST4/03828 project. AS and PB acknowledge Polish Ministry of Science and Higher Education for financial support received as 03/31/DSMK/0368 project. References [1] [2] [3] [4] [5]

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