Chunhui Xiao‡,
Abdul Rehman and
Xiangqun Zeng*
Department of Chemistry, Oakland University, Rochester, Michigan 48309, USA. E-mail: zeng@oakland.edu
First published on 12th March 2015
The dynamic interactions between the electrochemically generated superoxide radical (O2˙−) and three structurally different ionic liquids (ILs) were characterized using an electrochemical quartz crystal microbalance (EQCM). We established the long-term stability of the interface (on the scale of hours and weeks) against the most ubiquitous interferent in electrochemical energy devices and sensors: oxygen. Oxygen potentially limits the application of IL electrolytes in these devices. In particular, the electrochemical behavior of the O2/O2˙− couple and the ion pair formed between the cation of an IL and O2˙− were evaluated. O2˙− tends to form ion pair complexes with the cation of an IL, subsequently abstracting a proton to form different products depending on the cationic structure of the IL used. The reversibility of the O2/O2˙− electrode reaction depends on the subsequent chemical reactions between O2˙− and the IL, which are more pronounced at slow scan rates. It was found that O2˙− was significantly more stable in the IL with the [BMPY] cation than in ILs with imidazolium salts. The stability of the ILs towards O2˙− attack follows the order [BMPY][NTf2] > [BdMIM][NTf2] > [BMIM][NTf2] as evaluated on a time-scale of a few seconds to minutes and up to 3 weeks. It was found that the formation of the [Cation]⋯O2˙− ion pair complex lowers the local viscosity of the IL near the electrode, reflected in the change in the oscillating frequency of the quartz crystal electrode. As much as is the feasibility for the formation of this ion pair, that much is the tendency of the IL to lose its electrochemical stability. A combined analysis can provide a quick indication of the dynamic stability of the electrode–electrolyte interface for any IL in the presence of certain electrochemical reactions using the EQCM technique.
Unlike in conventional organic solvents, the redox peaks of the O2/O2˙− couple in ILs are asymmetrical in shape, with the characteristics of a typical steady-state condition. This is a result of the specific interactions of O2˙− with the cation of the ILs.20–22 As superoxide has a negative charge and the electronic spin density is delocalized between the two oxygen atoms, it can act as a powerful nucleophile in aprotic solvents.23–25 This property has also been observed in ILs. For example, the superoxide generated in situ in ILs based on the imidazolium cation tends to attack positively charged organic cations, leading to strong ion pairing of the O2˙−⋯[Cation]+.26,27 The ion pairing phenomenon during the dioxygen reduction reaction in EMI+-based ILs has been thoroughly investigated.28 It was observed that, in DMSO solution, the degree of association of O2˙− with the EMI+ cation is comparable with that of O2˙− with H2O. However, as the degree of association in pure ILs is much stronger, such an ion pairing may ultimately lead to the abstraction of a proton from the cation to form HO2˙− and carbine, or other types of compounds,29–31 thus hampering the stability of the O2˙− species in the ILs. The cation structure in an IL plays an important part in this regard. Islam et al.32 reported that O2˙− can attack the 2-position of the imidazolium ring to form an ion pair complex. Eventually, the ion pair complex of the imidazolium cation and the O2˙− species undergoes a ring-opening reaction to form hydrogen peroxide. AlNashef et al.33 reported that O2˙− can react with alkyl imidazolium cations to yield 2-imidazolones. Hayyan et al.22 studied the generation of O2˙− in pyridinium-, morpholinium-, ammonium- and sulfonium-based ILs by cyclic voltammetry (CV) and chronoamperometry. They found that O2˙− is not stable in the pyridinium-based ILs, but is stable in the other ILs for the duration of the experiments. However, most experiments were performed to observe the stability of the superoxide at very short time-scales (from seconds to minutes) depending on the methods and techniques used. However, none of these studies estimated the stability of the ILs involved at the longer time-scales that are important in the real-world applications of these systems. In many of these studies, simulation models were used to fit the experimental parameters to obtain mechanistic information.
In contrast, we and others have shown19,34,35 that the use of an in situ electrochemical quartz crystal microbalance (EQCM) provides a better and powerful method to characterize the structure and dynamics of the properties of the IL–electrode interface, allowing real-time monitoring of variations in the surface populations during electrochemically driven redox reactions. The electrified interface causes the IL ions to rearrange, thereby generating different kinds of solvent polarization effects depending on the IL used, which determines the reaction rate. The high viscosity and resulting slow relaxation dynamics of ILs36 allow the time-scale of these polarization effects and the electron transfer processes to be at similar levels, thus allowing the measurement of subtle variations in the structure of the IL solvent during the redox reaction. The results can then be used to quantify the ionic interactions via adsorption on the electrode surface, ion pairing within the IL or with the reaction products, and the ionic arrangements. The data generated in this way (Scheme 1) enable us to account for different types of structural orientations under potential control during the electrochemical perturbation of the IL–electrode interface. Therefore it is thought that this approach may give an insight into various fundamental aspects of IL stability against certain species such as the O2˙− radical and the dynamics of redox reactions in ILs.
In this work, the dynamic interactions between the electrochemically generated superoxide radical (O2˙−) and ILs with different structures were characterized by the EQCM approach to quantitatively estimate their stability against this radical. Specifically, we studied the interactions between the electro-generated superoxide and the ILs with two new focuses. First, we were motivated to characterize the reactivity of the in situ generated superoxide anion radicals in ILs at much longer time-scales (seconds to hours) by multiple cycles of CV and at various scan rates to quantitatively determine the stability of ILs in the presence of superoxide anion radicals. Second, we intended to demonstrate that the EQCM technique can measure the dynamics of the potential-induced subtle variations at an IL–electrode interface in the absence and presence of oxygen redox processes. The key emphasis is to understand how the IL structural heterogeneity of the interface contributes to the reaction kinetics by selectively interacting with the superoxide radical. Cumulatively, these analyses can provide new quantitative information regarding the oxygen reduction reaction (ORR) in ILs. This is important for the selection and design of new and stable electrolytes and for the utilization of O2˙− in a wide range of applications, such as developing highly selective electrochemical sensors and electrochemical energy storage devices using ILs.
O2 + e− ⇌ O2˙− | (1) |
The presence of the O2˙− oxidation peak provides evidence that the O2˙− generated in situ is chemically stable within the time-scale of experiments in the three ILs studied. These results are consistent with those reported previously.22,26,37–40
It is necessary to determine the concentration of O2 in each IL for quantitative evaluation. In this study, the concentration of O2 (C) and its diffusion coefficient (D) were determined by a chronoamperometric technique with both macro- and ultra-micro disk electrodes. For the macro-disk electrode, when a sufficient over-potential is applied, the current density is inversely proportional to the square root of time, as represented by Cottrell's equation:
![]() | (2) |
iss = 4nFDCr0 | (3) |
It is clear from Table 1 that the order of magnitude for the oxygen concentration is [BdMIM][NTf2] > [BMPY][NTf2] > [BMIM][NTf2]. However, as can be seen from the results of the EQCM experiments, [BdMIM][NTf2], with the highest oxygen concentration, has the lowest peak current for superoxide oxidation. The frequency change in this IL was also the smallest among the three ILs studied. This indicates that, in this viscous electrolyte system, D might play a more significant part in the ORR than the concentration. For this reason, we addressed the contributions from D in more detail. As shown in Fig. 1, the cathodic reduction peak current (ipc) in [BdMIM][NTf2] is lower than that in the other two ILs, as it has an extremely small D value, close to one-quarter of that in [BMIM][NTf2]. The similarity of ipc in both [BMPY][NTf2] and [BMIM][NTf2] is a result of their close values of D and C. The solubility of oxygen in [BMPY][NTf2] is higher than that in [BMIM][NTf2], but the diffusion of O2 in [BMIM][NTf2] is faster. Note that the order of the anodic superoxide oxidation peak current (ipa) in the three ILs is different from ipc, with the following order: ipa,[BMPY][NTf2] > ipa,[BMIM][NTf2] > ipa,[BdMIM][NTf2]. The anodic peak potential (Epa) also shows a significant difference. For example, Epa in [BMIM][NTf2] is shifted by about 300 mV positive compared with other two ILs. The change in the superoxide oxidation processes can be explained as an ECrev mechanism in which one-electron oxygen reduction is followed by the reversible formation of an ion pair O2˙−⋯[Cation]+ as proposed by Compton and coworkers:26,27
O2 + e− ⇌ O2˙− + [Cation]+ ⇌ O2˙−⋯[Cation]+ | (4) |
The rapid ion pairing of O2˙−⋯[Cation]+ hampers the superoxide oxidation process, causing a positive shift in the potential and leading to an increased peak-to-peak separation (ΔEp), making it more electrochemically irreversible than expected for a simple E process. The extent of irreversibility depends on the kinetics of the coupled chemical processes.
Experiments with the O2/O2˙− couple were performed in three ILs at varying scan rates (detailed data are given in Fig. S1†) to further investigate the relationship between the stability of the superoxide and the structure of the ILs. Fig. 2 shows the variations in ipc, ipa, Epc, Epa and ΔEp when the scan rates were varied. The peak currents were proportional to the square root of the scan rate for all three ILs. This shows that the reduction of O2 and the oxidation of O2˙− in the ILs are diffusion-controlled. Epc shifted to a more negative value and Epa shifted to a more positive value with increasing scan rates. This is characteristic of a quasi-reversible system. The Epa shows notable differences among the three different ILs studied, but Epc remains at a relatively constant value, reflecting the different degrees of association between the electro-generated superoxide and the cation. The uncompensated resistance (iRu) and surface reaction kinetics at the metal electrodes were used to explain the variation in the ΔEp of the O2/O2˙− redox couple in different aprotic solvents.37,43 However, the ΔEp values of the oxygen redox processes in these three ILs follows the order: ΔEp,[BMIM][NTf2] ≫ ΔEp,[BdMIM][NTf2] > ΔEp,[BMPY][NTf2]. The broadest ΔEp peak can be observed in [BMIM][NTf2], but the uncompensated resistance (iRu) in [BMIM][NTf2] should be the lowest as its conductivity is the highest of the three ILs. Thus the variation in ΔEp is mainly attributed to the variation in the electrode reaction kinetics in different ILs and there may be an effect from iRu, although this is too small to have a significant effect. The variation in electrode reaction kinetics were the results of the (bond-forming) ion paring O2˙−⋯[Cation]+, which leads to a positive shift in Epa as well as an increase in ΔEp. The stronger the interactions between O2˙− and the cation, the larger the value of ΔEp. The formation of the ion pair is probably a result of both electrostatic interactions and hydrogen bonding between the O2˙− and the [Cation]+. It has long been known that all three of the imidazolium ring protons (C2, C4 and C5) are acidic, although the proton in the C2 position is the most acidic. [BdMIM]+ has fewer protons at the C2 position of the ring, which makes it a weaker hydrogen bond donor than [BMIM]+. As the ability to donate hydrogen bonds increases with the increasing s-character of the C–H bond, imidazolium-based ILs commonly have a higher hydrogen bond acidity than pyrrolidinium- and alkyl ammonium-based ILs.
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Fig. 2 Plot of (A) ipc and ipa and (B) Epc, Epa and ΔEp versus the scan rate in three ILs in the presence of oxygen (40 vol%) at the second potential cycle. The dilution gas was nitrogen. |
![]() | (5) |
![]() | (6) |
Note that these reactions are irreversible, thus the long-term stability of ILs against this radical anion still needs to be investigated. For this purpose, the reversibility of the O2/O2˙− redox couple in the ILs was further investigated by multiple cycles of CV at various scan rates.
Fig. 3 shows the data for the continuous cycling (20 cycles) of the oxygen redox process in three ILs at fast (500 mV s−1) and slow (10 mV s−1) scan rates (other scan rates are shown in Fig. S2†). The ipc gradually decreased with increasing numbers of scan cycles. This is explained based on two factors: (1) the decrease in the double-layer charging current in subsequent cycles; and (2) the depletion of oxygen as a result of the redox processes of O2/O2˙−, as oxygen cannot be rapidly supplied from the bulk ILs to the electrode interface. Several interesting trends were observed for ipa. For example, ipa remains almost constant in both [BMIM][NTf2] and [BMPY][NTf2] at 500 mV s−1. It decreases with each subsequent scan cycle in [BdMIM][NTf2]. A small intermediate peak at about −0.7 for the oxidation of O2˙− and hydrogen peroxide (H2O2) to O2 is observed; a similar phenomenon has been reported by other workers.32 At a scan rate of 10 mV s−1, the ipa in [BMIM][NTf2] and [BdMIM][NTf2] dramatically decreases, whereas ipa decreases very slowly in [BMPY][NTf2]. In addition, compared with a scan rate of 500 mV s−1, a new small anodic peak at about −0.4 V at a scan rate of 10 mV s−1 was observed in [BMIM][NTf2] (Fig. 3D), which is assigned to the oxidation of HO2−.32 The enlarged view of this potential region in three ILs is shown in Fig. S3.† A smaller peak at about −0.4 V can be observed in [BdMIM][NTf2], but is hardly seen in [BMPY][NTf2], re-confirming that the abstraction of protons from [BMIM][NTf2] is easiest among the three selected ILs, followed by [BdMIM][NTf2] and [BMPY][NTf2].
Therefore the reversibility of the O2/O2˙− redox couple in the three ILs is different under different conditions (scan rate, cycle number), suggesting that the stability of the superoxide depends on the structure of the ILs. The significant irreversibility observed in the multiple CV experiments at slower scan rates indicates that the kinetics of the ion pairing and subsequent chemical reactions are slow, which can be observed more clearly at slower scan rates. At slow scan rates some of the O2˙−⋯[Cation]+ ion complex can form H2O2 and other compounds (e.g. 2-imidazolones). Thus a mechanism of homogeneous chemical reactions (including reversible ion pair formation and an irreversible protonation process) coupled to the electron transfer process of oxygen can be summarized as follows:
![]() | (7) |
The quantitative diagnosis of chemical reactions coupled with electrode reactions is often based on the relative current heights of the anodic and cathodic peaks. If the conversion of the ion pair complex to H2O2 is a slow process, at fast potential scan rates most of the O2˙− will be oxidized back to O2, thus the value of ipa/ipc will be close to 1. If the conversion of the ion pair complex to H2O2 is fast, some of the O2˙− will be lost to this process. Less O2˙− will be available for oxidation back to O2 on the backward potential scan and therefore the anodic peak will be smaller and the value of ipa/ipc will be <1. The ipa/ipc values of three ILs at the fifth cycle were plotted as a function of scan rate (Fig. 4). Note that the ipa and ipc values were achieved using the CHI 1040A electrochemical analyzer software after subtraction of the background current. In addition, the main anodic peak of about −1.1 V is about 13 times larger than that of the peak at about −0.7 V in [BdMIM][NTf2]. Therefore we believe that the effect of this intermediate peak is very small to be negligible. The trends of the plots are similar for [BdMIM][NTf2] and [BMIM][NTf2] in which ipa/ipc decreases at fast scan rates with increasing scan rate, although, at slow scan rates, ipa/ipc increases with increasing scan rates. The value of ipa/ipc showed the least change in [BMPY][NTf2], but a slight decrease in ipa/ipc can still be observed at scan rates <20 mV s−1, even in this IL.
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Fig. 4 Variations in the ipa/ipc for the fifth cycle with different scan rates in three ILs in the presence of oxygen (40 vol%). Nitrogen was used as the diluting gas. |
Nicholson and Shain44 reported that the general trend followed by the peak current ratio as a function of scan rate can serve as a quick diagnosis for electrochemical mechanisms involving coupled chemical reactions. At fast scan rates the oxygen reduction process is an ErCr mechanism with a reversible electron transfer (Er) followed by a reversible chemical reaction (Cr) (eqn (4)). At slow scan rates the reaction is changed to a mechanism of a reversible electron transfer (Er) followed by an irreversible chemical (Ci) reaction (ErCi mechanism), corresponding to ion pair formation and further protonation processes during the following superoxide reaction in the IL (eqn (7)). Fig. 5 compares the change in ipa/ipc at various scan rates with the increase in the number of cycles in three ILs. The change in the ipa/ipc value in ILs at different scan cycles is an indicator of the loss in the reversibility of the redox process with time. The smallest change in ipa/ipc observed in [BMPY][NTf2] indicates that the stability of the O2˙− in this IL is more than 100 min. The most striking result is in [BMIM][NTf2], where ipa/ipc shows a large change at 30 min at scan rates of 20 and 10 mV s−1 (if we convert the CV scans to a time-scale, the 20 cycles for these two scan rates are equivalent to 50 and 100 min, respectively). The change in ipa/ipc could be seen in [BdMIM][NTf2], although it is much smaller than that of [BMIM][NTf2], showing the varying stability of the superoxide radical in different ILs. In summary, the rate of reaction between the O2˙− and the cation of the ILs follows the order [BMIM][NTf2] > [BdMIM][NTf2] > [BMPY][NTf2]. This order is consistent with the result obtained from the ΔEp value.
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Fig. 5 Variations in the values of ipa/ipc in each CV cycle at different scan rates in three ILs in the presence of oxygen (40 vol%). Nitrogen was used as the dilution gas. |
Martin et al.49 reported a series of equivalent circuit parameters and a modified Butterworth–Van Dyke equivalent electrical circuit for the characterization of a QCM with simultaneous mass and liquid loading. The values of |Δf0/ΔR1| allow the separation of the mass and viscosity effects for the frequency signals. For a 10 MHz quartz crystal, if |Δf0/ΔR1| is larger than 11.6 Hz Ω−1, then the frequency changes can be ascribed predominantly to the mass effect. In the case of oxygen reduction, for both the waves for a single cycle and the overall trend after three cycles, the |Δf0/ΔR1| ratios are smaller than 11.6 Hz Ω−1, confirming that the QCM response is mainly due to viscosity changes at the interface.
The O2˙− generated couples with the cation of an IL near the electrode surface to form an ion pair complex, which is not completely reversible, as discussed earlier, and is hypothesized to lower the local viscoelasticity near the electrode, leading to the change in oscillating frequency (Scheme 2). In general, by consideration of the similar structures in ILs, the differences in viscosity are mainly influenced by hydrogen bonding and van der Waals interactions.50,51 In this case, O2˙− as a hydrogen bonding acceptor interacts with the cation and competes with the anion for the proton. This leads to [Cation]⋯O2˙− ion pairing, thereby suppressing the hydrogen bonding in ILs. This is expected to reduce the viscosity as a result of weakened interactions between the cation and anion.50
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Scheme 2 Schematic illustration of the ionic arrangement with (A) increasingly negative potential and (B) the formation of ion pairs in ILs. |
The decay length, δ, which describes the approximate thickness of the electrolyte and electrode interface, the effective change of which is sensed by the EQCM, is limited by a function of the density and viscosity of the solution as follows:45
δ = η1/2/(πf0ρ)1/2 | (8) |
The EQCM response appears to be governed essentially by the cation in the diffusion layer near the negatively electrified electrode surface and the ion pairing force between the cation and O2˙−. Fig. 7A demonstrates simultaneously the recorded frequency changes over a total of 20 CV cycles, sweeping in the selected ILs in the presence of oxygen (40 vol%) at a scan rate of 20 mV s−1. In the range of selected ILs, we observed distinct variations in the changes in f0 generated by potential cycling. It is clearly seen that the order of magnitude for the total change in Δf0 is [BMIM][NTf2] (602.4 Hz) > [BMPY][NTf2] (303.4 Hz) > [BdMIM][NTf2] (178.4 Hz). A flat frequency curve was observed in [BMIM][NTf2] in the absence of oxygen, as in the other two ILs (data not shown), indicating that the QCM response can be ascribed to the ORR. [BMIM][NTf2] presents the highest change in f0 as it combines the maximum diffusion coefficient with the strongest intensity of ion pairing. [BdMIM][NTf2] presents the lowest f0 response because of its minimum diffusion coefficient dominates over the moderate ion pairing force. The numerical values of the resulting Δf0 before and after CV sweeping in each IL with different potential cycling scan rates are shown in Fig. 7B. Surprisingly, when the scan rate was higher than or equal to 100 mV s−1, suggesting that the oxygen reduction as well as the reactivity of the superoxide with ILs was reversible, the frequency was essentially unchanged. However, at slower scan rates, when there is a longer period of negative potential polarization causing more and more cation layers to be gathered near the electrode surface, an increase in the local concentration of the cation is expected, which can lead to a higher possibility of forming ion pairs and thus a higher EQCM response. This was exactly the case when the scan rates were less than or equal to 50 mV s−1, with significantly higher values of Δf0 as a result of the stronger and irreversible reactivity of the superoxide with the ILs. These results further confirmed our previous hypothesis that the QCM response during oxygen reduction comes from the viscosity and density changes resulting from the formation of the ion pair complex. The smaller dependence between the QCM response and the scan rate (>50 mV s−1) denies another possibility that QCM response comes from the mass loading, such as the adsorption of superoxide or other intermediate compounds, as more adsorbate will be produced at slower scan rates and therefore the frequency due to mass loading should decrease rather than increase. The other type of variations seen in the frequency curves is the kind of frequency wave produced during each cycle of the measurement, which provides an indication of the stability of the superoxide anion as related to the electrochemical data. This wave is clearly visible in the case of the ferrocene couple (Fig. S4†), where there is no overall increase in frequency and the direction of the frequency shift changes fairly coherently with the direction of the potential scan. This frequency wave is also observable for [BMPY][NTf2], even for the 20th cycle (Fig. 7A). This indicates that the superoxide is consistently being re-oxidized, lessening the ion pairing effect during the anodic scan and reversing the direction of the shift in frequency. In contrast, this kind of wave is only visible in the first few cycles for [BMIM][NTf2], after which only an increase in the overall frequency is seen. This shows that the O2˙− generated has more irreversible interactions within this IL. [BdMIM][NTf2] lies between these two, further supporting the results obtained via electrochemical measurements for the long-term stability of O2˙− in different ILs. Thus, by combining the two results, the long-term stability of the O2˙− in ILs can be determined in situ in several electrochemical reactions.
The principles obtained from this work can be extended to other redox processes in ILs. The results from this study allow the relationship between the structure of ILs and their chemical stability towards O2˙− to be characterized, which is important for the selection and design of new and stable electrolytes and for further utilization of O2˙− in many applications. This study also established that EQCM is an effective tool to investigate the redox reaction at the IL–electrode interface. Work is in progress to use the EQCM technique to study different types of electron transfer reaction in various ILs.
Footnotes |
† Electronic supplementary information (ESI) available: Fig. S1–S6 as noted in the text are provided. See DOI: 10.1039/c5ra00396b |
‡ Present address: School of Science, Xi'an Jiaotong University, Xi'an, China. |
This journal is © The Royal Society of Chemistry 2015 |