Understanding the rate performance of microporous carbons in aqueous electrolytes

Electrochimica Acta 350 (2020) 136408

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Understanding the rate performance of microporous carbons in aqueous electrolytes ~ ez c, J.M. Rojo a, * I. Aldama a, M.A. Lillo-Rodenas b, M. Kunowsky b, J. Iban Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científica (CSIC), Sor Juana In es de la Cruz, 3, Campus de Cantoblanco, E-28049, Madrid, Spain b nica-IUMA, Universidad de Alicante, San Vicente del Raspeig S/N, E-03080, Alicante, Spain Departamento de Química Inorga c Centro Nacional de Investigaciones Metalúrgicas (CENIM), CSIC, Avda. Gregorio del Amo, 8, E-28040, Madrid, Spain a

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 January 2020 Received in revised form 16 April 2020 Accepted 4 May 2020 Available online 11 May 2020

Variation of specific capacitance versus current density is studied for microporous carbons. Although literature states that capacitance retention is higher for macro/mesoporous than for microporous carbons, the results reported here show that high capacitance retention can be reached for microporous carbons in combination with aqueous electrolytes (2M H2SO4, 1M KOH and 6M KOH). Six carbon monoliths are studied; three pristine ones and those three heat-treated, so as to reduce their content of surface oxygen groups and develops porosity. The capacitance retention is analyzed based on five parameters: electronic conductivity, surface chemistry and porosity of the monoliths, ionic conductivity and type of electrolyte. The capacitance retention is higher for the monoliths working as negative (H3Oþ  and Kþ) electrodes than as positive (HSO 4 and OH ) ones, being these results of interest for the use of carbon monoliths in asymmetric and hybrid supercapacitors. The highest capacitance retention is obtained by combining (i) monolith electronic conductivity of 11e14 Scm1 and micropore size of 0.6  e0.8 nm for H3Oþ, Kþ and HSO 4 , and of 0.85e0.95 nm for OH ; (ii) electrolyte ionic conductivity above 1 600 mScm and 6M KOH electrolyte, since this electrolyte performs better than 2M H2SO4 and 1M KOH. © 2020 Elsevier Ltd. All rights reserved.

Keywords: Capacitance retention Rate performance H2SO4 electrolyte KOH electrolyte Microporous carbon monoliths

1. Introduction The capacitance retention is a parameter usually studied in supercapacitors by analyzing the dependence of the specific capacitance of the active electrode material as a function of the current density. In all cases, the specific capacitance decreases with the increase of the current density and this trend is associated with kinetic limitations, in which formation of the double layer and/or pseudocapacitive redox reactions are involved. In general, a significant decrease in specific capacitance with the increase of the current density implies low capacitance retention. In contrast, a slight decrease in specific capacitance means high capacitance retention. In carbon-based supercapacitors, the capacitance retention seems to depend on the electric conductivity and porosity of the carbon electrodes, but also on the electrolyte chosen [1]. Regarding the electronic conductivity of the electrode, it is known that high electric conductivity of the carbon-based

* Corresponding author. E-mail address: [email protected] (J.M. Rojo). https://doi.org/10.1016/j.electacta.2020.136408 0013-4686/© 2020 Elsevier Ltd. All rights reserved.

electrodes leads to an enhancement of the capacitance retention on current density. For increasing the electric conductivity of the electrodes, a conductor material, e.g. carbon black, with high electronic conductivity and negligible specific capacitance is usually added. Then, the electrodes, consisting of powder carbon material and powder carbon black, are usually processed together with a binder as pellets or films. Another possibility for increasing the electronic conductivity of the carbon electrode is through its preparation as a carbon monolith, i.e. as a binder-free and selfstanding monolithic piece of carbon. Compared to pellets and films, the monoliths show higher electronic conductivity, which is derived from better contact of the carbon particles [2]. The main disadvantage of the carbon monoliths is their fragility. While pellets and films are ductile, monoliths are fragile [3]. The carbon monoliths are obtained by carbonization of: (i) gels obtained from several carbon precursors and catalysts [4], (ii) gels having a soft template or hard template that is removed during or after the carbonization, respectively [5,6], (iii) mixtures of carbon precursors and binders, both conforming the monolith [3] and (iv) biomass precursors showing monolithic shapes [7e10]. Other approaches for enhancing the electronic conductivity of the carbon-based

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electrodes are: (i) Graphitization treatment of the powdered carbon materials at high temperature in inert atmosphere. (ii) Preparation of the powdered carbons with specific morphologies, e.g. carbon spheres, capsules or cages [11e18], carbon sheets [19e25], onion-like carbons [26], self-standing carbon nanofibers [27e30], self-standing carbon foams [31] and carbon mats [32]. (iii) Preparation of core-shell composites, in which the core component provides high electronic conductivity and the shell component large surface area [33e36]. (iv) Preparation of common composites, in which a component provides high electronic conductivity and another component large surface area [37e41]. Regarding porosity of the carbon materials working as active electrode materials, literature states that micropores (<2 nm) do not lead to sufficiently high capacitance retention. For getting high capacitance retention on current density, mesopores (2e50 nm) and/or macropores (>50 nm) seem to be needed [18,30,31,35]. A hierarchical arrangement of interconnected pores, with mesopores for enhancing the capacitance retention and micropores for obtaining high specific capacitance has been proposed as the best option for optimizing carbon-based electrodes [42e50]. Carbon materials with mesopores and micropores show higher capacitance retentions than those with micropores only [46]. Another approach for improving the capacitance retention on current density consists of the presence of voids of micrometer size in electrodes based on microporous carbons [51]. Pseudo capacitance, which is due to reversible redox reactions associated with the presence of heteroatoms, such as oxygen groups, nitrogen groups, phosphorous groups, etc., all of them placed at the surface of the carbon electrodes, leads to an increase of the specific capacitance [52e57]. Combining pseudocapacitance and porosity (double layer capacitance), higher capacitance rates have been reported for oxidized mesoporous carbons than for oxidized microporous ones [58]. The decrease of the specific capacitance with the increase of the current density involves both the kinetics of pseudocapacitance, due to the presence of surface heteroatoms, and the kinetics of the double layer capacitance, due to the presence of pores with different sizes, micro-, meso-, macropores. Regarding the electrolyte, in particular for aqueous electrolytes, those studied in this paper, concentrations lower than 1 M do not provide sufficient amounts of ions to form the double layer and to give up pseudocapacitive reactions [59,60]. Those low concentrations could also be insufficient to reach high capacitance retention on current density, probably due to the low ionic conductivity of the electrolytes. The ionic conductivity increases with the increase of the concentration, and reaches its highest value for a certain concentration. For aqueous electrolytes, the common concentrations chosen are 0.5 and 2M for H2SO4, 1M for Na2SO4, 1M and 6M for KOH and 3M for LiNO3. As far as we know, there are scarce papers dealing with systematic studies based on a given carbonbased electrode and several concentrations of the electrolyte [61], and few reports in which the same carbon-based electrode is studied in combination with more than one aqueous electrolyte [52,55,62e67]. In this work, we have chosen H2SO4 and KOH aqueous electrolytes, with concentrations of 2M for H2SO4 and 1 or 6M for KOH, for comparing same and different ionic conductivity and different electrolyte type. Moreover, it is known that the ions, cations and anions, can contribute to the total capacitance of the carbon electrode in a different manner. In relation to the double layer capacitance, the minimum size of pore needed for the electroadsorption of the cation and anion at the electric double layer is different, below 0.5 nm for the Kþ and H3Oþ cations and above 0.5 nm for the OH and HSO 4 anions [3,68e73]; these sizes being compatible with the pore sizes of microporous carbons. Cations and anions show different partial desolvations as they electro-adsorb into the micropores [74]. Then, the accessible surface area to

cations and anions is different, and their double layer capacitance is different as well. With respect to the pseudocapacitance, cations can show a significant contribution, but anions do not [75]. Taking into account the differences observed for cations and anions, their capacitance retention on current density can be very different, an aspect that merits further research. Because the capacitance rate performances reported in literature dealt with carbon-based electrodes processed with different binders and conducting additives (different electronic conductivities) and carbon materials showing different porosities and surface chemistries on the one hand and aqueous electrolytes of different types and concentrations (different ionic conductivities) on the other hand, the aim of the present work is to understand the effect of five parameters on the capacitance retention: the electronic conductivity, surface chemistry and microporosity of the carbon material and the ionic conductivity and type of the aqueous electrolyte. Three microporous carbon monoliths with different electronic conductivity, surface chemistry (different contents of surface oxygen groups) and porosity (different pore size distributions in the microporous range) are transformed into three derived monoliths. The thermal treatment to reduce oxygen content leads to some porosity increase with respect to the pristine monoliths while maintaining the electronic conductivity. Two aqueous electrolytes, 2M H2SO4 and 1M KOH, the former with higher ionic conductivity than the latter and both commonly used in combination with carbon-based electrodes, are studied. Moreover, the 6M KOH electrolyte, which shows higher ionic conductivity than 1M KOH one and similar ionic conductivity as 2M H2SO4 electrolyte, is also studied in combination with some particular monoliths. The results reported in this work show the parameters that lead to the highest capacitance retention. The knowledge of those parameters is important for the preparation of new carbon-based electrodes with tailored porosity, surface chemistry and electronic conductivity and for the adequate combination of the carbon materials with aqueous electrolytes of different type and ionic conductivity. Moreover, the results here reported are useful for the design of asymmetric and hybrid supercapacitors, being the capacitance retention on current density clearly different for the carbon monoliths functioning as negative electrodes (cations involved) and as positive electrodes (anions involved). 2. Experimental The pristine samples were three commercial carbon monoliths from Takeda Ltd., hereafter labelled as T3, T4 and T5. The monoliths prepared by heat-treatment of T3, T4 and T5 are labelled as T3t, T4t and T5t, respectively. This thermal treatment, performed to reduce the surface oxygen groups’ content, was carried out under N2 flow (100 ml/min) at 750  C, and held for 3 h. The heating rate from room temperature to 750  C was 5  C/min. After heating, the oven was switched off and the N2 flow was kept during cooling to room temperature. All the monoliths were cylindrical pieces of carbon, with 2e3 mm in diameter and ca. 20 mg weight for T3, T3t, T4 and T4t, and 4 mm in diameter and ca. 50 mg weight for T5 and T5t. Their length was in the 2e4 mm range. Textural properties of the monoliths were characterized by N2 and CO2 adsorption at 196 and 0  C, respectively. N2 adsorption/ desorption isotherms were obtained in a Quantachrome AUTOSORB-iQ-XR-2 from very low relative pressures. Prior to nitrogen adsorption, the monoliths were outgassed at 250  C for 8 h. CO2 adsorption isotherms were obtained using an Autosorb-6 by Quantachrome. Prior to CO2 adsorption, the monoliths were outgassed at 250  C for 6 h. Apparent BET surface areas, SBET, and total micropore volumes, VDR(N2), were determined from N2 adsorption data applying BrunauereEmmetteTeller and Dubinin-

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Radushkevich equations, respectively. The volumes VDR(N2) correspond to pores with sizes below 2 nm. The volumes VDR(CO2) correspond to narrow micropores, i.e. the volumes due to pores with sizes below 0.7e0.8 nm, and were determined applying the Dubinin-Radushkevich equation to the CO2 adsorption data [76]. An estimation of the mesoporous volume (Vmeso) was done as the difference between the volume (expressed as liquid) of N2 adsorbed at P/P0 ¼ 0.9 and that adsorbed at P/P0 ¼ 0.2 [77]. In order to obtain the pore size distributions (PSDs), the surface areas (SDFT) and the cumulative surface areas, the Non-Local Density Functional Theory (NLDFT) was applied to the nitrogen adsorption results. For the calculations, a model for porous carbons with heterogeneous surfaces and a lambda value of 0 were used [76]. Temperature-programmed desorption (TPD) was used for characterizing the surface chemistry of the monoliths. The measurements were performed in TGA equipment (TA Instruments SDT Q600), which was coupled to a quadrupole mass spectrometer (Balzers Instruments, Thermostar GSD 300 T3). After an equilibration step of 2 h, approximately 15 mg of each crashed monolith were heated with a ramp of 10  C min1 up to 920  C. The experiments were carried out under a helium flow of 100 ml/min. Scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), FEG HITACHI S-4800 instrument, was used for the microstructural characterization of the monolithic pieces. The monoliths were embedded in a resin; then, they were polished for analyzing their bulk phase. The SEM images were obtained in the secondary electron (SE) mode and analyzed by the Image-Pro Plus software. Mechanical measurements were carried out by compression tests of the monoliths in a conventional 10T-SERVOSIS machine. The cross-head speed was 0.005 mm s1. Three tests were carried out for each type of monolith. Electric conductivity of the monoliths was measured at room temperature by the two-probe method using a 1260 Solartron impedance/gain phase analyzer. The circular areas of the monoliths were painted with a commercial silver paint. The resistances of the cell and wires were previously measured. Once the resistance of the monoliths was determined, the electronic conductivity was calculated taking into account the geometric area of the circular surfaces and the length of the monoliths. Ionic conductivity of the electrolytes was measured at room temperature by a 912 Metrohm Conductometer. The equipment was previously calibrated with a reference KCl solution. Electrochemical measurements were performed in threeelectrode cells. The monoliths were the working electrodes, in contact with a gold rod acting as current collector. A Pt wire was the counter electrode. The reference electrode was Hg/Hg2SO4 for the 2M H2SO4 electrolyte and Hg/HgO for the 1M and 6M KOH ones. Prior to the electrochemical measurements, the electrolytes were infiltrated into the monoliths; first by immersion under primary vacuum (ca. 101 Torr) for 4 days, and then by immersion under atmospheric pressure for 2 additional days. This treatment was carefully chosen to assure good infiltration of the electrolytes. Cyclic voltammetry and galvanostatic charge/discharge experiments were performed by Autolab PGSTAT 302N equipment. 3. Results and discussion 3.1. Microstructural characterization, density, electronic conductivity and mechanical properties of the monoliths and ionic conductivity of the electrolytes SEM images obtained for the pristine monoliths and for the heat-treated ones show voids that appear at the border of carbon particles (Fig. 1). By linking the voids, it is possible to guess the

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Fig. 1. SEM images for the T3, T4 and T5 monoliths.

shape and size of the individual carbon particles, marked in red for some of them. The size of the individual particles is in the 1e10 mm range for the T3, T3t, T4 and T4t monoliths, but in the 10e100 mm range for the T5 and T5t ones. The size of the voids is in the 1e5 mm range for all the monoliths. In spite of the fact that good packing of the carbon particles is observed for all the monoliths, the individual particles are better sintered for the T3, T3t, T4 and T4t monoliths than for T5 and T5T ones. Indeed, the borders between individual carbon particles are observed with difficulty for the former monoliths, but clearly observed for the latter ones. In addition to

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the voids already discussed, T5 and T5t monoliths show (i) bigger voids, in the 10e100 mm range, that are marked by blue lines and (ii) some other voids coming from the release of carbon particles during polishing, marked by green lines. The presence of these two types of voids agrees with a worse sintering of the carbon particles in those monoliths. It is worth to mention that all the voids, smaller and bigger, are connected to the external surface of the monoliths. This is deduced from the similar contents of K and S (measured by EDS after infiltration with the KOH and H2SO4 electrolyte, respectively) for the voids placed on the external surface and inside the monoliths, the latter being analyzed after polishing the monoliths. Therefore, the voids favor the access of the electrolyte to the carbon particles; those voids remember the induced voids in compacted electrodes based on microporous carbons [51]. The thermal treatment at 750  C does not have significant effects on the microstructure of the monoliths, as deduced from comparison of the SEM images of the pristine and heat-treated monoliths. Densities of the pristine and heat-treated monoliths are compiled in Table 1. The densities are in all cases close to 1 g cm3, a value higher than the one reported for carbon monoliths obtained by sol-gel procedures, with densities usually below 0.5 g cm3, and only comparable to the densities reported for highly densified carbon monoliths [3]. The good packing of the individual carbon particles accounts for the high values of the monoliths’ densities. The slightly higher densities for the T3, T4, T3t and T4t monoliths than for the T5 and T5t ones agree with the better sintering of the carbon particles in the former monoliths and the presence of big voids in the latter ones, both as deduced from the SEM results. The thermal treatment under N2 flow has not significant effects on the monoliths’ densities; it also agrees with the similar microstructures observed by SEM on the pristine and heat-treated monoliths. Electronic conductivities of the pristine and heat-treated monoliths are also shown in Table 1. They show very high values, 4e28 Scm1, in agreement with the values reported for other carbon monoliths, usually in the 1e10 Scm1 range [4e10]. However, those conductivities are much higher than the ones reported for processed pellets and films of carbon powder with a binder, usually on the order of magnitude of 0.1 Scm1 [2]. The high electronic conductivity found for the pristine and heat-treated monoliths agrees with the good sintering of the carbon particles, as deduced from SEM. Comparing the monoliths’ electronic conductivity, it is higher for the T3, T3t, T4 and T4t monoliths than for the T5 and T5t ones; this agrees with the better sintering of the carbon particles in the former monoliths than in the latter ones. The thermal treatment does not have an appreciable effect on the electronic conductivity; this agrees with the similar microstructures observed by SEM for the pristine and heat-treated monoliths. All the monoliths behave as fragile materials from the point of view of their mechanical properties. For stresses higher than certain values the monoliths break. The maximum stress measured from the compression tests is shown in Table 1. The values obtained for the T3, T3t, T4 and T4t monoliths are similar, and clearly higher than those measured for the T5 and T5t ones. Therefore, the former

Table 1 Density (d), electric conductivity (s) and maximum stress (sM) for all the monoliths. Monolith T3 T4 T5 T3t T4t T5t

d (gcm3)

s

1.07 1.01 0.97 1.00 0.96 0.93

26 11 4 28 14 7

(Scm1)

sM (MPa) 53 50 14 61 51 12

monoliths are better performing than the latter ones. The worse behavior of the T5 and T5t monoliths can be explained by the presence of the bigger voids, which can locally concentrate the stress, leading to failure of the monoliths. The poorer mechanical properties observed for the T5 and T5t monoliths also agree with the fact that some carbon particles were removed while polishing the monoliths (Fig. 1). The thermal treatment does not have an appreciable effect on the mechanical properties, as deduced from comparison of the pristine and heat-treated monoliths (Table 1). In any case, when measuring the monoliths’ properties, such as density, electric conductivity and mechanical properties, the monoliths could all be easily handled, without breaking, and it was also the case when assembling the monoliths into electrochemical cells. Ionic conductivities of the electrolytes showed 686, 214 and 640 mS cm1 values for the aqueous solutions of 2M H2SO4, 1M KOH and 6M KOH, respectively. Then, the first and third solution show similar ionic conductivity values, being clearly higher, 3.20e2.99 times higher, than the ionic conductivity of 1M KOH solution. 3.2. Surface chemistry and textural characterization The analysis of evolved gases during TPD experiments was used to characterize the surface chemistry of pristine monoliths and heat-treated ones. These results are compiled in Table 2, where the content of CO2 comes from the presence of acidic oxygen groups and the content of CO from the presence of neutral and basic oxygen groups, all of them present on the surface of the carbon materials [78]. The two contents vary from monolith to monolith although, as expected, the values are much lower for the heattreated monoliths than for those pristine ones. Note that in the presence of aqueous electrolytes the CO groups contribute to the pseudo capacitance [52]. Paying attention to these groups, and focusing on the pristine monoliths, the CO content is lower for T3 and T4 (742 and 719 mmol CO/g, respectively) than for T5 (1192 mmol CO/g). The CO content is remarkably lower, and similar, for T3t, T4t and T5t monoliths (153e159 mmol CO/g). Therefore, the thermal treatment at 750  C under N2 flow led to a significant removal of the surface oxygen groups from the pristine monoliths. The N2 adsorption/desorption isotherms for all the monoliths are shown in Fig. 2a. With the exception of the T3 isotherm, which shows very low adsorbed volume, the other isotherms show a sharp increase in volume at very low relative pressures, below 0.05 P/P0. Such sharp increase is characteristic of the presence of micropores. The slope in the adsorption/desorption isotherms at relative pressures in the 0.2e0.9 range and the hysteresis cycles observed in this pressure range are characteristic of the presence of mesopores. Table 2 shows that the mesoporous volumes are much lower than the microporous ones and all the monoliths can be considered essentially microporous carbons. The small volume adsorbed at P/P0 values above 0.95 can be associated with macropores, in agreement with the voids observed by SEM. Comparing, for pristine and heat-treated monoliths, the total microporous volume, VDRN2, follows the trends T5>T4>T3 and T5t > T4t z T3t. Moreover, VDRN2 is higher for the heat-treated monoliths than for the pristine ones. This is especially remarkable when comparing the T3t monolith with the T3 one. Therefore, the removal of the surface oxygen groups leads to an increase in microporosity, in agreement with previously published papers [79]. The specific BET and DFT surface areas are higher for the heat-treated monoliths than for the pristine ones (Table 2). From the CO2 adsorption/ desorption isotherms (not shown) the narrow microporous volumes, VDRCO2, were determined. It is observed that the VDRCO2 values are slightly higher than the VDRN2 ones, both for the pristine and heat-treated monoliths (Table 2); this highlights that the

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Table 2 CO and CO2 contents deduced from TPD experiments. Micropore volume (VDRN2), mesopore volume (VMesoN2), specific BET surface area (SBET) and specific DFT surface area (SDFT) deduced from the N2 adsorption isotherms. Micropore volume (VDRCO2) deduced from the CO2 adsorption isotherms. Monolith

CO content (mmol g1)

CO2 content (mmol g1)

VDRN2 (cm3g1)

VMesoN2 (cm3g1)

SBET (m2g1)

SDFT (m2g1)

VDRCO2 (cm3g1)

T3 T4 T5 T3t T4t T5t

742 719 1192 159 153 156

433 426 316 37 20 87

0 0.17 0.19 0.19 0.19 0.22

0.01 0.08 0.03 0.02 0.02 0.01

10 383 444 420 410 489

8 477 599 582 526 708

0.18 0.21 0.21 0.23 0.23 0.24

average micropore size is below 0.7e0.8 nm in all the monoliths [80]. The pore size distributions (PSDs) in the microporous range (<2 nm) were deduced from the application of the DFT model to the N2 adsorption results (Fig. 2b). The T3 monolith shows a flat pattern, in accordance with the lack of N2 adsorption. The T4t monolith presents two peaks, the main peak at ca. 0.67 nm and the low-intensity peak at ca. 0.55 nm; because of the low intensity of the latter peak, it can be ruled out. The main peak for all the monoliths shows a full width at half maximum of ca. 0.1 nm. These narrow PSDs contrast with the broad PSDs usually observed for common activated carbons. Also, the main peak position changes from monolith to monolith. The maximum appears at 0.60e0.62 nm for T5, T5t and T3t monoliths and at 0.66e0.68 nm for T4 and T4t ones. The minimum pore size, i.e. the size at the starting of the PSD peak, also changes from monolith to monolith. That size is 0.50e0.55 nm for T5, T5t and T3t monoliths, and 0.6 nm for T4 and T4t monoliths. The maximum pore size, i.e. the size at the end of the PSD peak, is 0.70 nm for T5, T5t and T3t monoliths and 0.80 nm for T4 and T4t monoliths. Therefore, the micropores are slightly bigger for T4 and T4t monoliths than for T5, T5t and T3t ones. These differences in size have significant effects on the rate performance of the monoliths, as discussed below. The variation of the cumulative surface area vs. the pore size is shown in Fig. 2c. The surface areas are negligible for pores with sizes below 0.55 nm. The cumulative surface area increases with the increase of the pore size in the 0.55e0.8 nm range, reaching a maximum value that changes from monolith to monolith. This value is lower for T4 and T4t monoliths than for T5, T5t and T3t ones. The cumulative surface area is negligible for T3 monolith.

3.3. Electrochemical characterization. Study of the rate performances Cyclic voltammograms (CVs) recorded on the T4 monolith in combination with 2M H2SO4 electrolyte and on the T5t monolith in combination with 1M KOH electrolyte have been chosen as examples (Fig. 3). In both cases, the CVs were obtained from the open circuit potential (OCP) to negative potentials and, then, from the OCP to positive potentials. At negative potentials (cations involved), the broad hump at ca. 0.2 V observed for the T4 monolith, with high content of surface oxygen groups (Table 2) points out a pseudocapacitive contribution in addition to the double layer capacitance. At those potentials, the rectangular shape observed for T5t monolith suggest that the double layer capacitance dominates the electrochemical response, being the pseudocapacitive contribution very low; it agrees with the low content of surface oxygen groups in that monolith, as deduced from the TPD measurements (Table 2). At positive potentials (anions involved), the CVs show rectangular shapes for the two monoliths, in agreement with the only existence of double layer capacitance [75]. Below/above certain potentials, the current decreases/increases sharply,

indicating electrolyte decomposition with hydrogen and oxygen evolutions at the lowest and highest potentials, respectively; hydrogen can reversibly be adsorbed on the carbon material for the KOH electrolyte [81]. Although the potential of zero charge (PZC) is the reference potential and values of PZC and OCP can differ around 0.1e0.15 V [75], in this paper the OCP is taken as the reference potential for comparison of the working potential ranges for the negative (cations involved) and positive (anions involved) electrodes. It is observed that the working potential range is wider for the negative electrode (0.7e0.8 V) than for the positive electrode (0.2e0.3 V) not only for the T4 and T5t monoliths, but also for all the monoliths in combination with the acidic and alkaline electrolyte. The currents measured from the CVs are higher for the negative electrodes than for the positive ones, evidencing different capacitances for the two electrodes. Therefore, the carbon monoliths show different electrochemical features as they function as negative and positive electrodes. The specific capacitances of the monoliths were determined from galvanostatic charge/discharge (GCD) measurements performed at several current densities. The measurements were carried out from the OCP to negative and positive potentials, within the working potential ranges determined from the CVs, and in the total working potential range. The three types of GCD plots obtained for the T4 monolith in combination with 2M H2SO4 electrolyte are shown as examples (Fig. 4aec). The total specific capacitance, Cs total, which agrees with the specific capacitance determined from symmetric two-electrode cells [75], is compared with the specific capacitance of the negative electrode, Cs (H3Oþ) þ and positive electrode, Cs (HSO 4 ) in Fig. 4d. The values of Cs (H3O ) are higher than those of Cs (HSO 4 ), indicating a preference of the T4 monolith for functioning as negative electrode than as positive electrode. The working potential range is wider for that monolith functioning as negative electrode than as positive electrode, as already discussed. Then, the charge stored is higher for the monolith working as negative electrode. The T4 monolith and also the other monoliths studied in this paper should preferably work as negative electrodes in asymmetric supercapacitors and hybrid ones (supercapatteries) [82]. Variations of the specific capacitance (Cs) vs. the current density (J) for all the monoliths working as negative electrodes (H3Oþ and  Kþ) and as positive ones (HSO 4 and OH ) are shown in Figs. 5 and 6, respectively. The experimental Cs (H3Oþ), Cs (Kþ), Cs (HSO 4 ) and Cs (OH) followed linear dependences on J, the R2 fitting parameter showing values in the 0.898e0.999 range, with an average value of 0.971 for all the fittings done. It is worth noting that the linear fittings dealt with the J ranges shown in those figures for each monolith/electrolyte ion combination. Out of those J ranges the dependences of Cs vs. J could depart from the straight line, perhaps for Cs (H3Oþ) and Cs (Kþ) of the monoliths showing a pseudocapacitive contribution in addition to the double layer capacitance, i.e. for the pristine T4 and T5 monoliths with high content of surface

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Fig. 3. Cyclic voltammograms recorded at 0.2 mV s1 for the T4 monolith in the presence of 2M H2SO4 electrolyte (a) and for the T5t monolith in the presence of 1M KOH electrolyte (b).

Fig. 2. Nitrogen adsorption/desorption isotherms (a), incremental pore volume vs. pore size (b) and cumulative surface area vs. pore size (c) for all the monoliths.

oxygen groups. In this paper, the slope of the linear fittings is taken as a measurement of the capacitance rate, and this parameter is used for comparison of the monolith/electrolyte ion combinations. As all the slopes show negative values, low and high slopes mean low and high capacitance rates, respectively. For Cs (H3Oþ) and Cs (Kþ) two groups of capacitance rates can be distinguished (Fig. 5). A group with higher values for T4 and T4t monoliths. Another group

with lower values for T3t, T5t and T5 monoliths. For Cs (HSO 4 ) and Cs (OH) a similar pattern, with higher slopes for T4 and T4t monoliths than for T3, T3t and T5 ones is observed (Fig. 6). Therefore, higher capacitance rates (above 0.3 F mA1) are observed for T4 and T4t monoliths as they function as negative or positive electrodes. The higher capacitance rates cannot be explained on the basis of the electronic conductivity of the monoliths. Indeed, the electronic conductivities of T4 and. T4t monoliths are intermediate between those measured for T3 and T3t monoliths and for T5 and T5t ones (Table 1). However, the higher capacitance rates found for T4 and T4t monoliths indicate that electronic conductivities of 11e14 Scm1 are sufficient to reach such high rates. Regarding porosity, the BET and DFT surface areas of T4 and T4t monoliths are smaller than those of T3t, T5t and T5 ones (Table 2 and Fig. 2c). The voids of micrometer size (macropores) that favor the access of the electrolyte to the carbon particles are found in all the monoliths. However, the size of micropores is larger for T4 and T4t monoliths than for T3t, T5t and T5 ones, as deduced from their narrow PSDs (Fig. 2b). Moreover, the higher capacitance rates observed for T4 and T4t monoliths point out that micropores with sizes in the 0.6e0.8 nm range are sufficiently big to reach high mobility of H3Oþ, Kþ and HSO 4 at the electric double layer. Those micropores, however, are small for permitting high mobility of OH; the capacitance rate shows values lower than 0.47 F mA1 for that anion in combination with T4 and T4t monoliths.

I. Aldama et al. / Electrochimica Acta 350 (2020) 136408

7

Fig. 5. Dependence of the specific capacitance due to the H3Oþ (a) and Kþ (b) as a function of the current density for all the monoliths in combination with 2M H2SO4 and 1M KOH, respectively. The results for the T5t monolith in the presence of 6M KOH electrolyte, blue stars in (b) are also shown. The slopes of the linear fittings are shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4. Galvanostatic charge/discharge plots obtained at 3 mA g1 for the T4 monolith functioning as negative electrode (a), as positive electrode (b) and in the total potential range, negative plus positive electrode (c). Variations of the specific capacitance of the T4 monolith working as negative electrode (H3Oþ), as positive electrode (HSO 4 ) and the total specific capacitance versus the current density (d).

Comparing the capacitance rates of the cations with their respective anions for all the monoliths, the former rates are higher than the latter ones. These results evidence the preferable choice of the monoliths as negative electrodes for asymmetric and hybrid supercapacitors from the point of view of the capacitance rate performance. The higher capacitance rates of the cations agree with the fact that the minimum pore size required for the electro-

adsorption of the cations at the electric double layer (0.4e0.5 nm for H3Oþ and 0.37e0.42 nm for Kþ) [3,68,69,71] is smaller than that  required for the anions (0.55 nm for HSO 4 and 0.65 nm for OH ) [3,69,71]. Moreover, the fact that pores with sizes of 0.6e0.8 nm allow high rates for H3Oþ, Kþ and HSO 4 indicates that the pore size needed for reaching high capacitance rate is about 0.2e0.3 nm larger than the minimum pore size required just for electroadsorption of the ions at the double layer. Comparing the two anions, the better capacitance rates found for HSO 4 agree with the fact that the minimum pore size required for the electro-adsorption of HSO 4 at the double layer (0.55 nm) [3,69] is smaller than that required for OH (0.65 nm) [71]. For reaching high capacitance rate of OH, pore sizes of 0.85e0.95 nm could be adequate, as deduced from the fact that the pore size should be 0.2e0.3 nm larger than the minimum pore size needed for the OH electro-adsorption at the double layer. Comparing the two cations, the minimum pore size needed for electro-adsorption of H3Oþat the double layer, of 0.4e0.5 nm [3], is equal or bigger than that needed for the electro-adsorption of Kþ, of 0.37e0.42 nm [68,69,71]. The better rate performance of H3Oþ as compared to Kþ suggests that another parameter different from the cation size is involved, perhaps the ionic conductivity of the electrolyte. To check this point, a 6M KOH solution with ionic

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I. Aldama et al. / Electrochimica Acta 350 (2020) 136408

rate follows the trend 6M KOH>2M H2SO4>1M KOH.

4. Conclusions

 Fig. 6. Dependence of the specific capacitance due to the HSO 4 (a) and OH (b) as a function of the current density for all the monoliths in combination with 2M H2SO4 and 1M KOH, respectively. The results for the T5t monolith in the presence of 6M KOH electrolyte, blue stars in (b) are also shown. The slopes of the linear fittings are shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

conductivity of 640 mS cm1, which is higher than that for the 1M KOH solution, of 214 mS cm1, was chosen as electrolyte in combination with the T5t monolith (blue stars in Figs. 5b and 6b). Using the 6M KOH electrolyte, it is observed that the Cs (Kþ) rate, of 0.27 F mA1 and the Cs (OH) rate, of 0.48 F mA1 are much higher than the rates measured with 1M KOH electrolyte, of 0.69 F mA1 and -1.2 F mA1, respectively. Therefore, the capacitance rates increased with the increase of the electrolyte ionic conductivity. The increase in the Cs (Kþ) rate, by 2.55, and in the Cs (OH) one, by 2.50, agree with the increase in the ionic conductivity, by 2.99. Values of Cs (Kþ) and Cs (OH) obtained with the 6M KOH electrolyte are higher than those measured with 1M KOH electrolyte. Therefore, the 6M KOH electrolyte is performing better, both from the point of view of capacitance rate and capacitance value than the 1M KOH one. Comparing the 6M KOH electrolyte with the 2M H2SO4 one, both with similar ionic conductivities, and higher than 600 mScm1, for the same T5t monolith (Figs. 5 and 6), the capacitance rate is higher for Cs (Kþ) than for Cs (H3Oþ), and higher for Cs (OH) than for Cs (HSO 4 ). Therefore, the rate performance also depends on the type of electrolyte chosen, performing the 6M KOH electrolyte better than the 2M H2SO4 one. Comparing the three electrolytes, the capacitance

Pristine monoliths and heat-treated ones consist of individual carbon particles that are sintered with their neighbors. This feature accounts for the high density, ca. 1 g cm3, and high electronic conductivity, 4e28 S cm1, measured. The voids placed at the border of the particles are connected to the external surface of the monoliths and favor the access of the electrolyte to the micropores of the carbon particles. The monoliths made from bigger carbon particles and showing bigger voids are more fragile; lower maximum stress value. The pristine monoliths have surface oxygen groups that are mostly removed by heating at 750  C under N2 flow. This treatment increases the accessible surface area but maintains the electronic conductivity. Unlike activated carbons with broad PSDs, all the monoliths studied in this work are microporous carbons with narrow PSDs. This fact has permitted the finding of differences in the rate performances associated with the ions of the H2SO4 and KOH electrolytes. The carbon monoliths show different electrochemical features as they work as negative and positive electrodes. The working potential range is wider and the specific capacitance is higher for the negative electrodes (H3Oþ and Kþ involved) than for the posi tive electrodes (HSO 4 and OH involved). Regarding variation of the specific capacitance as a function of the current density, linear  dependences for H3Oþ, Kþ, HSO 4 and OH are found in the J ranges studied. However, out of these J ranges the dependences could be different from linear for the monoliths showing a pseudocapacitive contribution in addition to the double layer capacitance. In this paper, the slope of the linear fittings has been taken as a measurement of the capacitance rate for comparison purposes. The rate performance is higher for the monoliths functioning as negative  electrodes (H3Oþ, Kþ) than as positive ones (HSO 4 and OH ). For obtaining high capacitance rates, it is needed a combination of: (i) electronic conductivity of the carbon electrode, of 11e14 S cm1, which is higher than that of common pellets and films, (ii) micropore sizes around 0.2e0.3 nm bigger than the minimum size of pore required for the electro-adsorption of the electrolyte ions at the double layer; pores ranging 0.6e0.8 nm are suitable for H3Oþ, Kþ and HSO 4 , but bigger than 0.8 nm, maybe 0.85e0.95 nm, are needed for OH, (iii) ionic conductivity of the electrolyte equal or higher than 600 mScm1; the capacitance rate increases with the increase of the electrolyte conductivity as deduced from comparison of the 1M and 6M KOH electrolyte, (iv) adequate choice of the electrolyte; the 6M KOH electrolyte is performing better than the 2M H2SO4 one, in spite of the fact they show similar ionic conductivities. Finally, it is worth noting that micropores with sizes below 1 nm allow high capacitance rates in combination with aqueous electrolytes. This is in contrast to the opinion that only meso-/macropores allow high rate performances. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement I. Aldama: Investigation. M.A. Lillo-Rodenas: Investigation, ~ ez: Writing - original draft. M. Kunowsky: Investigation. J. Iban Investigation. J.M. Rojo: Writing - original draft.

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Acknowledgements Authors thank Prof. A. Linares-Solano, now retired, for providing the pristine carbon monoliths and for helpful discussions about carbon materials. Authors also thank Prof. G.Z. Chen from Univ. Nottingham (UK) for helpful discussions about OCP and PZC. Javier Toro and Edurne Laurín are acknowledged for technical assistance of electrochemical and SEM measurements, respectively. Funding through the PID2019-104717RB-I00 project is acknowledged to Spanish MICINN. References [1] F. Beguin, E. Frackowiak (Eds.), Carbons for Electrochemical Energy Storage and Conversion Systems, Taylor and Francis Group, Boca Raton (USA), 2010. [2] A. Garcia-Gomez, P. Miles, T.A. Centeno, J.M. Rojo, Why carbon monoliths are better supercapacitor electrodes than compacted pellets, Electrochem. Solid State Lett. 13 (8) (2010) A112eA114. ~ ez, J.M. Rojo, [3] G. Moreno-Fernandez, M. Kunowsky, M.A. Lillo-Rodenas, J. Iban New carbon monoliths for supercapacitor electrodes. 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