Tailored metallacarboranes as mediators for boosting the stability of carbon-based aqueous supercapacitors

Abstract

Expanding the operating voltage of aqueous-based electrolytes by using neutral electrolytes and advanced cell designs is a promising strategy for the development of greener and safer supercapacitors. However, solvent decomposition and the oxidation of carbon electrodes are issues that still need to be resolved. Herein, we propose a novel protection strategy for stabilizing aqueous electrolytes at high voltages by using metallacarboranes with tuned redox potentials specifically selected for matching those of the electrolyte decomposition. Such metallacarboranes are strongly adsorbed in the micropores of conventional activated carbons without compromising their capacitance or their power capabilities. As a proof of concept, supercapacitors with optimized electrode weight ratios in 0.5 M Na₂SO₄ were constructed using a highly stable commercial activated carbon with the aim of operating them at 2.2 V. While this device malfunctioned after several hundreds of cycles, the addition of small amounts of the Na[Co(C₂B₉Cl₂H₉)₂] metallacarborane (redox pair at −0.98 V vs. Ag/AgCl) dramatically increased its durability. The supercapacitor prepared using 0.15 mmol g⁻¹ of Na[Co(C₂B₉Cl₂H₉)₂] retained 80% of its original capacitance and an energy density of 10.67 W h kg⁻¹ at 1 kW kg⁻¹ after 5000 cycles at 2.2 V. This strategy has the potential to be extended to different electrolytes, enabling the development of more durable supercapacitors that operate at voltages close to those of organic electrolytes while using safer and greener aqueous electrolytes.

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Supercapacitors are energy storage devices that are able to achieve high power input and delivery while maintaining outstanding durability.¹ They are complementary devices for rechargeable batteries, and both of them can be paired or even hybridized for servicing applications with highly demanding requirements, such as those of electric vehicles.² These features also make them promising storage systems for solving the grid stability problems derived from intermittencies in electricity production from renewable energy sources³ and for servicing in wearable, bendable or microsized electronic devices.^4–6

The main drawback of supercapacitors comes from their low energy density, which explains why they need to be coupled to a battery for the majority of their intended applications. Consequently, extensive research is being done regarding the increase of stored energy in supercapacitors.^7–11 The energy density of these devices can be estimated from the voltage, ΔV, and capacitance, C_g, from the equation¹²E = 1/2C_gΔV². The maximum voltage is limited by the stability of the components of the supercapacitor, which is mostly driven by the electrochemical stability window of the solvent and the electrolyte. Thus, voltages below 1.2 V are used in aqueous based electrolytes, while more than 2.5 V can be used in organic systems, and values higher than 3 V are employed when ionic liquids serve as the electrolyte.⁸ Even though organic solvents and ionic liquids offer a higher voltage and therefore larger energy storage, the use of aqueous electrolyte has important economic, safety and power performance advantages, and the development of novel procedures is required for increasing their allowable voltage.

In this regard, two of the most interesting strategies to achieving this objective consist of the combination of electrolytes and electrodes providing a high overpotential for water decomposition reactions,^13,14 and the use of unequal electrode capacitances in order to maximize the use of the available working potential window of each electrode.^15–17 For the first of these strategies, it has been proposed that the use of neutral electrolytes provides a larger stability window, which has been shown in the literature to be beneficial for reaching outstanding operating voltages up to 2 V.^16,18,19 The second strategy aims to expand the voltage by adapting the working potential windows of each electrode into their electrochemical stability limits. This objective can be achieved by balancing the weight ratio between electrodes.^15,20 The surface chemistry of porous carbons also plays an important role in stability. For instance, the introduction of certain heteroatoms, such as nitrogen or phosphorus, can delay ageing of the electrodes caused by the electrochemical oxidation reaction.^21,22 Unfortunately, doping of carbons can be an economically prohibitive process or can produce important changes in the porosity.

It is also possible to use additives within the electrolyte in order to enhance the energy density of supercapacitors. However, rather than increasing the operating voltage, these redox-active additives are usually envisaged for providing a higher capacitance, since they allow the storage of energy through faradaic reactions.⁸ Species based on the quinone/hydroquinone redox pair^23,24 and halogens,^25,26 especially iodine, have been used with this purpose.^25,27,28 As for the voltage increase, the use of additives that can improve the stability of the devices by protecting the current collectors has also been demonstrated recently in the literature,²⁹ allowing the loading of the devices to high voltages while avoiding corrosion of the collectors. Unfortunately, none of these additives reduce the electrochemical decomposition of water or avoid the electrochemical degradation of the carbon electrodes (which is usually related with the former). A long shelf-life, which is a critical issue for all supercapacitors, is a specially determining parameter in applications like energy storage from renewable sources, where minimal maintenance levels are required. Achieving a high stability is especially challenging when using aqueous electrolytes, since working voltages up to 2 V, well above the theoretical thermodynamic limit of water decomposition, are required for maximizing their energy densities.

In this study, we report a novel strategy for improving the durability of supercapacitors by minimizing the electrochemical reactions that are responsible for supercapacitor degradation at the stability limits of the electrodes via the addition of electrochemically redox reversible anionic species with tailored redox pairs. The concept might have some similarity with the zinc protection of aluminium boat hulls. For the purpose of stabilizing carbon electrodes, we wanted to use anionic molecules that are water soluble, able to be strongly adsorbed onto carbon materials, highly resistant to harsh conditions, able to perform electron transfer in a reversible way and whose redox potential could be tuned in a practical way to get the wanted potential compatible with carbon electrode stability. There are not too many molecules that allow electron transfer without major alteration of the molecular structure. Remarkable examples are C₆₀, ferrocene, Ru(bpy)₃²⁺, and some polyoxometalates, among others. Most of them are restricted, however, to having only one easily accessible redox potential. In other words, the structure does not allow easy functionalization to tune the redox potential. The redox potential of ferrocene can, for instance, be tuned by dehydrohalogenation but the resulting molecule is neutral or cationic, not anionic.

There are other sandwich complexes available that are octahedrally coordinated, very stable, anionic and in which the redox potential can be tuned by substituting hydrogen atoms by halogens in a stepwise manner, starting from a single platform. These are based on metallacarboranes, the best known and one of the most studied being the cobaltabisdicarbollide anion, [Co(C₂B₉H₁₁)₂]⁻.³⁰ For the sake of simplicity we will name it [COSANE]⁻, and its chlorinated derivatives [Cl_x-COSANE]⁻. The redox potential in aqueous NaCl (0.1 M) of these [Cl_x-COSANE]⁻ anions depends on x. For x = 0, E_1/2 = −1.27, when x = 2, E_1/2 = −0.98, for x = 4, E_1/2 = −0.86, and for x = 6, E_1/2 = −0.66 V vs. Ag/AgCl.

Thus, by wise choice of metallacarboranes with redox couples having potentials which are close to those of carbon degradation or hydrogen or oxygen evolution reactions of the electrodes of supercapacitors, we hypothesize that it would be possible to decrease the degradation rate of supercapacitors when they are loaded to high voltages. Metallacarboranes would act as electron consuming agents in their reversible redox processes avoiding the reactions associated with the degradation of the carbon material and the electrolyte. In addition, they would also contribute to pseudocapacitance, which is good for increasing or at least maintaining the high capacitance of the pristine porous carbon.

To do these studies we needed to test the suitability of Na[COSANE],³⁰ Na[Cl₂-COSANE],³¹ Na[Cl₆-COSANE]³² and Na[Cl₄-COSANE] that were prepared. The first three were prepared following reported procedures for the synthesis of their Cs salts and later converted into the sodium salts.^33,34 For [Cl₄-COSANE]⁻, Na[Co(C₂B₉Cl₂H₉)₂], we have developed a practical procedure of synthesis that is reported in the experimental section of this paper. The first screening indicated that [Cl₄-COSANE]⁻ was the one performing better; thus all further studies were done with this material. Its molecular structure is presented in Fig. 1a. The calculated molecular dimensions of the anion are 1.10 × 0.68 nm, and a molecular volume of 238.06 cm³ mol⁻¹ has been determined.^31,35 In addition, Na[Cl₄-COSANE] shows a redox potential near the potential region where the electrochemical hydrogen storage (which can be regarded as the first step of the HER in carbon materials) takes place in carbon materials at neutral pH in aqueous electrolyte.^14,19,36

Fig. 1 (a) Molecular structure of Na[Cl₄-COSANE]. (b) B 1s XPS of bare Na[Cl₄-COSANE] (black line) and activated carbon loaded with 0.15 (Y-015) (red line) and 0.45 (Y-045) (blue line) mmol g⁻¹ of Na[Cl₄-COSANE].

For testing our approach, we have selected a commercial activated carbon, named Y-000 in this work, which is specifically designed for supercapacitors (YP50F, Kuraray Chemical, Japan). The N₂ adsorption isotherm at −196 °C, Fig. S1,† of this activated carbon reveals a high N₂ uptake at very low relative pressures, a closed knee in the low 0–0.2 relative pressure region and a very small N₂ uptake from 0.2 up to 1, which are characteristics of microporous solids. Table 1 summarizes the textural parameters obtained from the N₂ adsorption isotherm.

Table 1 Textural parameters and 3-electrode gravimetric capacitance of activated carbon with different Na[Cl₄-COSANE] loadings

Sample	S BET (m^2 g^−1)	V DR (cm^3 g^−1)	C ^OCP/0.9 Vg (F g^−1)	C ^OCP/−1.3 Vg (F g^−1)
Y-000	2050	0.86	149	135
Y-015	1530	0.60	125	150
Y-045	1140	0.47	130	162
Y-100	60	0.02	120	168 (not stable)

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Along with porosity, the surface chemistry of activated carbons dictates their electrochemical behavior in aqueous electrolyte.^37–40 Temperature-Programmed Desorption (TPD) of Y-000 showed minor amounts of CO and CO₂ evolving from the surface (0.52 and 0.16 mmol g⁻¹; profiles are shown in Fig. S2†). This technique is highly valuable for assessing the amount and the nature of surface functional groups.^41–43 The surface concentration of oxygen (calculated from the total oxygen amount evolved during TPD and divided by S_BET) is only 0.42 μmol m⁻², revealing that Y-000 has a low amount of surface oxygen groups, much lower than other high surface area activated carbons.³⁷ As a general approximation, S_BET is related to gravimetric capacitance, while surface functional groups, especially those evolving as CO₂ during TPD, are proposed as the starting sites for the electrochemical carbon degradation reactions.^21,44,45 Thus, it is clear that this carbon has great potential as an electrode in supercapacitors.

Then, solutions of Na[Cl₄-COSANE] in distilled water were prepared at different concentrations ranging from 10⁻⁴ to 2 × 10⁻³ M and brought into contact at three different temperatures with the activated carbon for at least one day in order to obtain the adsorption isotherms (see Fig. S3a†). It is worth noting the strong affinity between the activated carbon and the Na[Cl₄-COSANE], as manifested in the adsorption isotherms by the high uptake at very low Na[Cl₄-COSANE] equilibrium concentrations, and how it develops an almost horizontal plateau when the amount of adsorbed Na[Cl₄-COSANE] reaches 1.25 mmol g⁻¹. This behavior indicates a strong interaction between the Na[Cl₄-COSANE] and the activated carbon. Interestingly, this interaction seems to be temperature-independent in the range used, as pointed out by the similar shapes of the isotherms and adsorption capacity in the monolayer measured at 15 and 35 °C. An adsorption test conducted at 50 °C also shows a similar adsorption capacity (see grey circular dot in Fig. S3a†). Similar tendencies in the relationship between adsorption and temperature in activated carbons have been reported for the removal of heavy metals.^46–48

The evaluation of the pH at the point of zero charge of Y-000 has given a value of 8.5, and consequently the surface should be positively charged at neutral pH, what favors the interaction between the [Cl₄-COSANE]⁻ anion and the activated carbon. The origin of the positive charge on the activated carbon surface has been assigned to the presence of basic surface groups (i.e., CO-desorbing groups)^49,50 and to the graphene layers that can act as Lewis bases,⁵¹ resulting in the formation of acceptor–donor complexes with anions.⁵² In addition, since metallacarboranes are known to have a great tendency to produce hydrogen and dihydrogen bonds with basic groups in a protonated way,^53,54 the favorable interaction with the positive sites on the carbon surface can promote Na[Cl₄-COSANE] adsorption. In order to check the effect of the presence of electrolyte (Na₂SO₄) on the adsorption of Na[Cl₄-COSANE], 7.9 mg of activated carbon loaded with 1.30 mmol g⁻¹ (Y-130) of Na[Cl₄-COSANE] were brought into contact with 10.0 mL of 0.5 M Na₂SO₄ at 25 °C for 24 hours. Remarkably, UV-VIS of the recovered solution only showed a small absorption band at 319 nm (Fig. S3b†) related to the presence of ca. 2 ppm of Na[Cl₄-COSANE], which corresponds to 0.3% of the total amount initially loaded on the activated carbon, confirming the strong and preferential adsorption of [Cl₄-COSANE]⁻ anions in the presence of sulfate anions.

The determination of the adsorption isotherm at 25 °C allowed the adsorption of different Na[Cl₄-COSANE] loadings on Y carbon electrodes employing different volumes of a 10⁻⁴ M solution. This method was used for achieving loadings of 0.15, 0.45 and 1.00 mmol g⁻¹ (carbons, Y-015, Y-045 and Y-100, respectively). The N₂ adsorption isotherms of these materials are compared in Fig. S1.† Assuming that the molecular volume of the [Cl₄-COSANE]⁻ anion is 238 cm³ mol⁻¹, theoretical micropore volumes of 0.76, 0.61 and 0.41 cm³ g⁻¹ should be expected for Y-015, Y-045 and Y-100, respectively. Since the experimental micropore volumes are much lower (see Table 1), it is clear that Na[Cl₄-COSANE] adsorption hinders the accessibility of N₂ to micropores and supports the encountered strong adsorption. PSDs confirmed that the Na[Cl₄-COSANE] is preferentially located in pores with sizes between 0.7 and 1.5 nm (Fig. S4†), which seems to be in close agreement with the molecular dimensions of the [Cl₄-COSANE]⁻.

X-ray diffraction patterns provided further evidence of the porosity filling achieved in Y-100 (see Fig. S5†). The X-ray scattering at low angles (i.e. large intensity values at 2θ below 10°) that is characteristic of microporous solids is clearly observed in the bare activated carbon. However, this feature disappears in the activated carbon saturated with Na[Cl₄-COSANE], Y-100. The diffraction pattern of this carbon does not show any of the peaks detected on the pure Na[Cl₄-COSANE], confirming that the adsorption interaction of Na[Cl₄-COSANE] with the carbon surface is strong and it impedes the self-arrangement of Na[Cl₄-COSANE] molecules³³ within the activated carbon pores.

XPS analysis of Y-045 and Y-015 detected the presence of boron (5.1 and 1.8% at., respectively) in Na[Cl₄-COSANE] within the pores of the activated carbon (Fig. 1b). It is important to note that the pristine activated carbon used is not doped with boron; the boron is present in the Na[Cl₄-COSANE] compound which is adsorbed on its surface. Interestingly, a new peak at higher binding energy than the main boron peak observed for the free Na[Cl₄-COSANE] salt (ca. 189.2 eV) that accounts for ca. 20% of the overall intensity of the spectrum is found in the B 1s spectra of adsorbed Na[Cl₄-COSANE] (located at 192.1 eV for Y-015 and 192.8 eV for Y-045), as shown in Fig. 1b. The main peak is also slightly shifted to lower binding energies on adsorbed Na[Cl₄-COSANE] (188.8 eV and 189.0 eV for Y-015 and Y-045, respectively). This clear modification of the electron density of boron confirms the strong interaction between the adsorbed Na[Cl₄-COSANE] and the surface of the activated carbon.

The activated carbons loaded with different Na[Cl₄-COSANE] amounts were processed into carbon electrodes using PTFE as the binder and a commercial carbon black as the conductivity promoter. In the first place, the cyclic voltammetry (CV) of Y-100 was obtained in neutral electrolyte (0.5 M Na₂SO₄) from the open circuit potential (OCP) to lower potential values (similar to the values that the negative electrode of a supercapacitor would face during the operation of the device) using a 3-electrode cell configuration; see the red line in Fig. 2a. A broad reduction peak is observed during negative scan and a well-defined oxidation peak centered at ca. −1.1 V vs. Ag/AgCl appears during the positive scan. This redox process is associated with Na[Cl₄-COSANE] and it demonstrates the charge transfer in the Na[Cl₄-COSANE]/activated carbon electrode. Electrodesorption of Na[Cl₄-COSANE] was indicated by a slow decline in the redox process upon successive cycling and the development of a yellowish colour in the electrolyte.

Fig. 2 Steady state cyclic voltammograms of Y-000 (black line) and Y-100 (red line) electrodes at (a) negative, (b) positive, and (c) full potential windows in 0.5 M Na₂SO₄. Scan rate: 2 mV s⁻¹.

The redox charge measured from the CV after cycling Y-100 is 51.4 C g⁻¹. Considering that the charge transferred by 1 mmol g⁻¹ of Na[Cl₄-COSANE] should be 96.5 C g⁻¹, it is possible to conclude that 53% of the Na[Cl₄-COSANE] initially present in Y-100 remains in the electrode after cycling. It is also worth noting that the amount of CO-evolved groups from TPD is 0.52 mmol g⁻¹, which is in agreement with the amount of remaining Na[Cl₄-COSANE] in this electrode after the electrochemical measurements. Interestingly, the double layer capacitance, measured from the voltammogram after cycling between −0.9 and −0.4 V in both electrodes, is only reduced 15% in the presence of Na[Cl₄-COSANE] (i.e. 136 F g⁻¹vs. 120 F g⁻¹ at −0.55 V for Y-000 and Y-100, respectively), whereas gravimetric capacitance in the more negative potential window (C^{OCP/−1.3 V}_g in Table 1) increases, thanks to the contribution of the Na[Cl₄-COSANE] redox process.

When the same potential range is explored in the bare activated carbon (Fig. 2a), a large reduction current related to the hydrogen electroadsorption process is observed from −1.0 V, which is the starting step of the hydrogen evolution reaction.^14,55,56 It must be noted that the intensity of this process decreases in the presence of Na[Cl₄-COSANE], as indicated by the lower current at −1.3 V. Similar conclusions can be drawn from the CV comparison at positive potentials from the OCP to 1 V (Fig. 2b). First, a 20% reduction of the capacitance of the electrode in the presence of Na[Cl₄-COSANE] is observed (Table 1), and second, a lower oxidation current during the positive scan and a lower contribution of the redox processes associated with the presence of electroactive CO functionalities are seen in the positive windows after the addition of Na[Cl₄-COSANE]. When the CV is recorded in the full potential window between −1.3 V and 1 V (Fig. 2c), slightly different voltammograms are observed in the presence and absence of Na[Cl₄-COSANE]. The presence of Na[Cl₄-COSANE] reduces the redox processes associated with surface oxygen groups and the oxidation peak at higher positive potentials decreases.

All these results confirm that the presence of Na[Cl₄-COSANE] modifies the electrochemical behavior of the electrode/electrolyte interface, especially for those processes related with electroactive CO functionalities and water decomposition reactions. These changes can be related to the preferential adsorption of Na[Cl₄-COSANE] on sites which would otherwise act as active sites for these reactions.^21,39 Since the currents registered at the positive and negative potential limits are lowered in the presence of this molecule, it seems that its adsorption inhibits the activity of such sites. This inhibiting effect, along with the concomitancy between the Na[Cl₄-COSANE] redox process and the hydrogen evolution reaction,⁵⁷ which drives part of the charge out of the undesirable water decomposition reaction, suggest that the adsorption of the necessary amount of Na[Cl₄-COSANE] in the activated carbon could protect the aqueous-based supercapacitors when exposed to high voltages. Since the protecting effect has been found to occur at both electrodes in the presence of Na[Cl₄-COSANE], we opted for the evaluation of supercapacitors where both electrodes are loaded with it.

In view of the above findings, we have set a value of 0.45 mmol g⁻¹ as the maximum Na[Cl₄-COSANE] loading for the construction of supercapacitor cells in order to ensure that most of the Na[Cl₄-COSANE] molecules have a strong interaction with the activated carbon. Thus, carbon electrodes with loadings of 0.15 and 0.45 mmol g⁻¹ (Y-015 and Y-045 electrodes) have been prepared and electrochemically characterized. The capacitance of the electrodes in positive (from the OCP to 0.9 V) and negative (from the OCP to −1.3 V) potential windows has been determined and the results are compiled in Table 1. Interestingly, the capacitance of Y-015 and Y-045 electrodes at positive potentials (C^{OCP/0.9 V}_g, Table 1) decreases only ∼15%, whereas the surface area decreases 25% and 40%, respectively. When the metallacarborane is loaded in the activated carbon, part of it could be adsorbed in pores that are initially inaccessible to the electrolyte, what could mitigate the impact on the double layer capacitance, but it could also increase the wettability of the surface owing to the polar character of the compound. Both these effects could compensate for the loss of total surface area of the electrodes loaded with Na[Cl₄-COSANE]. It is also remarkable that the Y-000 electrode showed a high reduction current at −1.3 V and a low coulombic efficiency (determined as the coefficient between anodic and cathodic charge) of only 92% under those conditions, while those loaded with Na[Cl₄-COSANE] retained efficiencies above 95%.

Next, symmetric cells (i.e. conventional design with equal mass of activated carbon in both electrodes) have been constructed and characterized in a 2-electrode cell. GCD measurements at 1 A g⁻¹ and at 1.8 V demonstrated that both Y-000 and Y-015 positive electrodes were working close to the positive potential limit, whereas around 250 mV were wasted on the negative electrode (Fig. S6a†), including the potentials where the redox processes of Na[Cl₄-COSANE] have been observed (Fig. 2a). Thus, under these conditions, the benefits of adding Na[Cl₄-COSANE] are reduced. Similar results were obtained for the Y-045 capacitor. In order to overcome this limitation, supercapacitors with unequal electrode mass were constructed following optimization through a charge balance protocol.^15,17,19 The operating voltage of the supercapacitors was set to 2.2 V, which is the highest value reached in aqueous electrolyte (and particularly for Na₂SO₄) when using conventional activated carbons,¹³ and the optimized design allowed us to expand the working potential window of the negative electrode to values lower than −1.2 V (see as an example the potential vs. time profiles during GCD of the optimized Y-000 capacitor, Fig. S6b†).

The advantages of using Na[Cl₄-COSANE] as an additive for avoiding degradation of supercapacitor electrodes working at high voltage can be observed in Table 2. It compiles the capacitance values of mass optimized capacitors, along with the energy density and capacitance retention after a GCD durability test of 5000 cycles conducted at 2.2 V. Initially, all the devices showed a similar energy and capacitance density, even though the micropore volume and surface area of Y-015 and Y-045 are somewhat lower than those of Na[Cl₄-COSANE]-free Y-000 (Table 1). CVs recorded at increasing cut-off voltage for these capacitors show a lower current density at the upper voltage limit in the presence of the metallacarborane, which is probably connected to a higher stability of the cell (Fig. S7†). In addition, the frequency dependence of capacitance in asymmetric capacitors as well as their Ragone plots shows only minor differences in the rate performance of the capacitors constructed with Y-015 (Fig. S8†), which demonstrates that the presence of Na[Cl₄-COSANE] does not hinder ion mobility within the pores of the electrodes. Consequently, the energy density and the power capability of the device remain unaltered.

Table 2 Performance of asymmetric supercapacitors constructed with different Na[Cl₄-COSANE] loadings. Current density: 1 A g⁻¹ voltage: 2.2 V

Sample	m +/m−	C g (F g^−1)	E (W h kg^−1)	C g retained^a (%)	E retained^a (%)
a After 5000 GCD cycles.
Y-000	1.3	33	16.3	23	8
Y-015	1.8	30	15.7	79	68
Y-045	1.9	29	13.0	32	21

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However, the best overall performance is attained with the Y-015 cell, thanks to its much higher capacitance and energy retained after a cyclability test consisting of 5000 GCD cycles at 1 A g⁻¹ and 2.2 V (see the superior C_g and E retained by Y-045 and especially Y-015 in Table 2). Fig. 3a and b report the voltage profiles of Y-000 and Y-015 capacitors recorded over the duration of a cyclability test. It is clear that the GCD profile of the Na[Cl₄-COSANE]-free supercapacitor is severely distorted with increasing number of GCD cycles, showing an increase in the ohmic drop and a lowered discharge time (i.e. lower capacitance and energy). These facts indicate the degradation of the device. Remarkably, the capacitor with the Na[Cl₄-COSANE] loaded electrodes is able to operate with only a minor impact on performance (see the similar shapes of the GCD profiles in Fig. 3b). Thus, the retention of capacitance and energy during the test shown by the Y-015 supercapacitor is clearly superior (Fig. 3c). The lower occurrence of degradation processes in Y-015 is confirmed by the coulombic efficiency evolution.⁸ This value is over 98% for most of the test, whereas the faradaic processes degrading the Y-000 device deliver a much lower efficiency for the first 2000 cycles (Fig. S9†). It is worth noting that the same cyclability test conducted at 2.0 V did not show relevant differences in capacitance retention, owing to the high electrochemical stability of Y-000 activated carbon under such conditions (Fig. S10†).

Fig. 3 Durability test on Y-000 and Y-015 capacitors with an optimized mass ratio. Voltage–time profiles from different GCD cycles (j = 1 A g⁻¹, ΔV = 2.2 V) of the cyclability test for (a) Y-000 and (b) Y-015 supercapacitors. (c) Energy density and gravimetric capacitance of Y-000 (black lines) and Y-015 (red lines) capacitors during the cyclability test. (d) 4^th GCD cycle recorded after increasing the floating time at 2.2 V for the Y-015 capacitor. The floating test was carried out right after the cyclability test.

Further evidence of the stability improvement in the presence of Na[Cl₄-COSANE] was collected by performing a floating test right after completing the cyclability one (Fig. 3d). The potential was maintained at 2.2 V for 10 hours, with 4 GCD cycles being recorded every two hours. The capacitor built using Y-015 electrodes was able to withstand the first 2 hours, and its degradation was complete after 10 hours (Fig. 3d), while the Y-000 capacitor was destroyed within the first 2 hours of the test (see the comparison between the Y-000 and the Y-015 GCD profile after 2 h, Fig. S11†).

It must be noted that this is the first time that such durability has been reached for a capacitor built with conventional activated carbons and Na₂SO₄ as the electrolyte.¹³ It is remarkable that, even though capacitance values are quite as expected for conventional activated carbons, the energy density achieved is ca. 16 W h kg⁻¹ at 1.2 kW kg⁻¹, thanks to the expanded operating voltage. This value is certainly higher than that achieved using acid or alkaline electrolytes,¹ with the additional advantage of using an environmentally friendly electrolyte, and it is comparable to those achieved using highly toxic and unsafe organic electrolytes,¹⁰ redox additives,²⁴ or hybrid asymmetric designs where very expensive materials, such as carbon nanotubes and conducting polymers,¹⁵ are employed. It is important to emphasize that this strategy can be extended to different electrodes and even to different electrolytes, allowing the improvement of the stability of supercapacitors with maximized energy densities. In this regard, we are currently implementing this strategy in electrodes of activated carbons with higher porosity, what may achieve higher energy density.

These results were replicated using coin cells with higher amounts of electrode in order to analyze the surface chemistry of the electrodes after the cyclability test. TPD of the wasted electrodes revealed the formation of a lower amount of surface oxygen groups in the Na[Cl₄-COSANE] loaded electrode (Fig. S12†). Since surface oxygen groups, especially those evolving as CO₂,^21,45,57 are known to be the initiators of the electrochemical carbon gasification, we propose that the strong interaction of adsorbed Na[Cl₄-COSANE] with some of the initially found CO-evolving functional groups is inhibiting the oxidation of the activated carbon. In order to illustrate this hypothesis, a supercapacitor with a mass ratio of 1.3 using Y-015 as the negative electrode and Y-000 as the positive electrode was constructed and subjected to the same cyclability test depicted in Fig. 3c. The cyclability test revealed a capacitance fading higher than that achieved using Y-015 (Fig. S13†). In addition, the capacitance increased in the first 500 cycles, probably as the outcome of the higher wettability and pseudocapacitance developed by the generation of oxygen functionalities in the positive electrode (which is free of Na[Cl₄-COSANE]). These results along with the lower amount of surface oxygen groups on the wasted Y-015 electrode confirmed that the presence of Na[Cl₄-COSANE] in the positive electrode protects the activated carbon from electrochemical oxidation.

At this point, it should be stated that boron has been employed in the past as an inhibitor of carbon gasification in the gas phase,^58–60 and, since the parallelism between carbon gasification and electrochemical oxidation has already been proposed in the literature,²¹ it would not be unreasonable to suppose that metallacarboranes could provide a similar effect.

Conclusions

We have demonstrated that the addition of Na[Cl₄-COSANE] is beneficial for maintaining the integrity of the electrolyte solution and the carbon electrodes, allowing us to reach unusually high voltages that enhance the energy density of supercapacitors while using an environmentally friendly aqueous electrolyte. The metallacarboranes can act as inhibitors of carbon oxidation and water decomposition. This novel strategy is certainly different to the introduction of surface functionalities on carbon materials, which could preserve them from oxidation by blocking or reducing the number of active sites.^21,22 Moreover, the huge tunability of such compounds could also be beneficial in the future for the preparation of tailored metallacarboranes for achieving an even larger stability boost and a possibly higher energy density. In summary, the combination of their properties as degradation inhibitors and active redox clusters with tuneable electroactivity makes metallacarboranes highly attractive additives that provide an impressive enhancement of supercapacitor stability in aqueous electrolyte.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors want to thank the Ministry of Economy and Competitiveness of Spain (MINECO) and FEDER (CTQ2016-75150-R, CTQ2015-66080-R and MAT2016-76595-R), the Generalitat Valenciana (PROMETEOII/2014/010) and the Generalitat de Catalunya (2014/SGR/149) for the financial support. I. Fuentes is enrolled in the PhD program of the UAB.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7se00503b

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