In Situ Electrosynthesis of Peroxydicarbonate ... - ACS Publications

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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In Situ Electrosynthesis of Peroxydicarbonate Anion in Ionic Liquid Media Using Carbon Dioxide/Superoxide System Ahmed Halilu,†,‡ Maan Hayyan,*,‡,§ Mohamed Kheireddine Aroua,*,∥,⊥ Rozita Yusoff,*,† and Hanee F. Hizaddin†,‡

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†

Department of Chemical Engineering, Faculty of Engineering and ‡University of Malaya Centre for Ionic Liquids (UMCiL), University of Malaya, Kuala Lumpur 50603, Malaysia § Department of Chemical Engineering, Faculty of Engineering, Sohar University, P. O. Box 44, Sohar P.C. 311, Sultanate of Oman ∥ Centre for Carbon Dioxide Capture and Utilisation (CCDCU), School of Science and Technology, Sunway University, Bandar Sunway, 47500 Petaling Jaya, Malaysia ⊥ Department of Engineering, Lancaster University, Lancaster LA1 4YW, U.K. S Supporting Information *

ABSTRACT: Climate engineering solutions with emphasis on CO2 removal remain a global open challenge to balancing atmospheric CO2 equilibrium levels. As a result, warnings of impending climate disasters are growing every day in urgency. Beyond ordinary CO2 removal through natural CO2 sinks such as oceans and forest vegetation, direct CO2 conversion into valuable intermediaries is necessary. Here, a direct electrosynthesis of the peroxydicarbonate anion (C2O62−) was investigated by the reaction of CO2 with the superoxide ion (O2·−), electrochemically generated from O2 reduction in bis(trifluoromethylsulfonyl)imide [TFSI−] anion derived ionic liquid (IL) media. This is the first time that the IL media were employed successfully for CO2 conversion into C2O62−. Moreover, the charge transfer coefficient for the O2·− generation process in the ILs was less than 0.5, indicating that the process was irreversible. Voltammetry experiments coupled with global electrophilicity index analysis revealed that, when CO2/O2 was contacted simultaneously in the IL medium, O2·− was generated in situ first at a potential of approximately −1.0 V. Also, CO2 was more susceptible to attack by O2·− before any possible interaction with the IL except for [PMIm+][TFSI−]. This was because CO2 has a higher global electrophilicity index (ωCO2 = 0.489 eV) than those for the [EDMPAmm+][TFSI−] and [MOEMMor+][TFSI−]. By further COSMO-RS modeling, CO2 absorption was proven feasible at the COSMO-surface of the [TFSI−] IL-anion where the charge densities were σ = −1.100 and 1.1097 e/nm2. Therefore, the susceptible competitiveness of either IL cations or CO2 to the nucleophilic effects of O2·− was a function of their positive character as estimated by their electrophilicity indices. As determined by experimental attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and DFT-FTIR computation, the reaction yielded C2O62− in the ILs. Consequently, the presence of O=O symmetric stretching FTIR vibrational mode at ∼844 cm−1 coupled with the disappearance of the oxidative cyclic voltammetry waves when sparging CO2 and O2 confirmed the presence of C2O62−. Moreover, based on DFT/B3LYP/6-31G, pure C2O62− has symmetric O=O stretching at ∼805 and ∼844 cm−1 when it is in association with the ILcation. This was the first spectroscopic observation of C2O62− in ILs, and the O=O symmetric stretching vibration has peculiarity for identifying C2O62− in ILs. This will open new doors to utilize CO2 in industrial applications with the aid of reactive oxygen species. KEYWORDS: climate engineering solutions, CO2 capture, CO2 conversion, quantum chemistry, COSMO-RS, reactive oxygen species

1. INTRODUCTION According to the future estimate on global CO2 levels, growing anthropogenic activities have potential to increase the terrestrial abundance of CO2 to surpass 550 ppm in the next 3−8 decades.1−3 This projection supports a NASA report, which strongly advocated previous and current atmospheric CO2 levels to be in the tune of approximately 60 mV is a representation of an electrochemically irreversible process. In another sense, further analysis of the CVs at various potential sweep rates showed that the separation between the cathodic peak (Epc) and the half-peak potential (Ep/2c) was not 56.5 mV as in Table S1, indicating that electrochemical generation of O2·− herein followed the irreversible pattern.20,55 Also, there is a linear variation between the peak current and square root of sweep rate v1/2 (Figure 2 and Figure S2). This kind of linear trend was also seen between current density and sweep rate according to Figure S3, which confirmed, for the two scenarios, a diffusioncontrolled process.68,69 As an extension, by following eq 2,70,71 comparing the peak current density with the square root of sweep rate v, there is still a linear relation that confirmed the linear-diffusion-controlled process.72 In general, this implied that the reduction of O2 commences in the bulk phase of the

(2)

(3) (4)

where Ip is the cathodic peak current of cyclic voltammetry (A), Iss is the steady-state current of chronoamperometry (A), α is the charge transfer coefficient, A is the surface area of the macroworking electrode (cm2), Co is the bulk concentration of O2 (mol/mL), Do is the diffusion coefficient of O2 (m2/s), v is the potential sweep rate (V/s), Ep is the peak potential for cathodic current (V), E0 is the formal potential of the reaction, R is the universal gas constant (J/ mol·K), T is the absolute temperature (K), F is Faraday’s constant (96485 C/mol), k0 is the standard heterogeneous rate constant (cm/ s), n is the number of electrons, and ro is the radius of the ultramicroelectrode (cm). 2.3.2. Fourier Transform Infrared (FTIR) Analysis of Post Reaction Medium. About 5 mL of stock of the post CO2 and O2·− reaction products was instantly sampled, and ∼0.1 mL was fetched and immediately analyzed on a Bruker Optics attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) IFS 66 V/S instrument. The FTIR machine can achieve up to 100 spectra per second, and this is a powerful visible, near IR, mid-IR, and far-IR vacuum spectrometer. This implies that it can record several IR D

DOI: 10.1021/acsami.9b05962 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Table 2. Diffusion Coefficient (Do) × 10−10 (m2/S), Solubility of O2 (C) (mM), CA, Steady-State Current (nA), Viscosity (η) (mPa S), and α Charge Transfer Coefficient parameter

[EDMPmm+][TFSI−]

[MOEMMor+][TFSI−]

[PMIm+][TFSI−]

Do Co α CA η

19.600 3.216 0.441 13.400 63.59

0.133 14.135 0.414 0.398 183.35

34.400 2.902 0.425 21.200 32.99

Figure 3. CVs of CO2 conversion by O2·− in (a) [EDMPAmm+][TFSI−], (b) [MOEMMor+][TFSI−], and (c) [PMIm+][TFSI−] after sparging with N2 (background), CO2, O2 separately, and CO2/O2 simultaneously at 27 °C, GC vs Ag/AgCl and v = 100 mV/s.

when O2 was sparged into the ILs before CO2 and when O2 was sparged into the ILs after sparging CO2. The evaluated ratios of peak currents (Ipa/Ip) were all less than unity for the CVs in ILs before and after sparging CO2 and varied linearly with the sweep rate (Figure S4). This also indicated the irreversible process for O2 reduction and confirmed that the electrochemical process was diffusion-controlled. It is worthwhile to also know that there is a precathodic hump accruing at approximately −0.7 V of Figure 1c, in addition to the redox peaks of O2 reduction to O2·−. These are usually observed especially when the IL contains impurity, which are inactive in the presence of N2 but active in the presence of O2. In many studies, the presence of an unknown trace of impurity in IL, in which systematic purging with argon or under vacuum conditions could not remove, is responsible for the precathodic hump.23,76,77 Also, the precathodic hump can be due to adsorption of the IL cation on the GC electrode surface, resulting in initiation of infinitesimally small electrochemical activity. This view is similar to the observation made by Islam et al., where they recorded the precathodic hump in their cyclic voltammetry.23 Another reason is the possible slight increase of

IL; as a result, there is no reduction of O2 at the surface of the GC electrode. Also, this observation strongly suggested that the electrochemical process was kinetically rapid, and in the presence of reversed oxidation CV waves, the O2·− generated was stable in [EDMPAmm+][TFSI−], [MOEMMor+][TFSI−], and [PMIm+][TFSI−].57 By comparing Figure 1a−c and Figure S1a−c, a contacting pattern of CO2 and O2 in the IL becomes immaterial; hence, the two figures show similar CV waves for O2 reduction. This was most likely due to the phenomenon of kinetic diameter (KD) common to gaseous molecules. KD of a gaseous molecule is an estimate of the size of the gaseous molecule’s electron conformer as an influencer of scattering, and it is different from the atomic or molecular diameter.73−75 For instance, KD of O2 (0.346 nm) relative to CO2 (0.33 nm) indicated that O2 might lead to scattering events near CO2. In the affirmative, this was applicable to N2 displacing CO2, O2, and H2O during the background experiment as shown from the CV scan procedure. More so, O2 with large KD can always displace CO2 with less KD such that the overall IL in question was saturated timely with O2. Consequently, the CV wave remained the same for scenarios E


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structurally different from the collection of other ILs in the literature that have been used so far for CO2 capture.27−29,86,87 Therefore, the chemical processes involving the captured CO2 in the ILs and the electrogenerated O2·− were rationalized into four characteristic CV waves. In the first scenario as in Figure 3a−c showing a background, no faradaic current appears after sparging the ILs with N2 and with CO2. The second scenario according to Figure 3 represents proof of O2·− generation owing to the presence of redox faradaic currents after sparging only O2 into the ILs. Similarly, the third scenario shown in Figure 3a−c reveals the disappearance of a reverse oxidation wave for O2·− when simultaneously sparging O2 and CO2 in the IL, whereas the forward peak appears at ± −1 V. These observations confirmed that, at the onset, the ILs contained no electrochemically active impurities and CO2 was electrochemically inactive in the ILs before generating O2·−. In contrast, following the generation of O2·−, there was a spontaneous CO2 consumption evidenced by the disappearance of oxidation faradaic current. This strongly suggested that an in situ C2O62− was produced as the main product, which is in accordance with a previous study.24 3.3. Interfacial Phenomena of CO2/O2·− System. Charge transfer coefficient (α) enabled the quantification of the interfacial phenomena between IL/O2 and the GC electrode material employed for O2·− generation. Accordingly, Table 2 shows that the charge transfer coefficients for O2 reduction in [MOEMMor+][TFSI−], [EDMPAmm+][TFSI−], and [PMIm + ][TFSI − ] were 0.441, 0.414, and 0.425, respectively; all are less than 0.5. These values implied that the transition state of O2 reduction to O2·− as influenced by the variation of applied voltage did not behave midway on an energy/reaction coordinate profile. This is in contrast with when α equals 0.5, and according to Bard et al., it measures the symmetry of the energy barrier for an electrochemical reaction where the transition state behaves midway between reactant and product.68 In addition, the charge transfer magnitude is not zero because of the existence of the driving force between the GC electrode material and O2 molecule in the ILs. The driving force is ∼7.6845 eV based on DFT calculations that considers −6.958 eV LUMO energy of O2 and 0.727 eV HOMO energy of the GC electrode material. In this case, the principal interaction of the molecular orbitals that results to electron transfer from the GC electrode material to O2 was based on GC electrode HOMO and O2 LUMO. 3.4. Quantum Chemical Reactivity Descriptor. The two unresolved curiosities associated with CO2 conversion by O2·− are the possibilities of O2·− participating in the chemical reaction with CO2 following a cumulative step involving twoelectron reduction of O2 and O2·− interaction with the ILcations earlier than CO2. Accordingly, Figure 4 first presents the electron distributions of the [EDMPAmm+][TFSI−], [MOEMMor+][TFSI−], [PMIm+][TFSI−], O2·−, and CO2 from their frontier molecular orbital (FMO). The energies of FMO enable estimation of the global electrophilicity index (ω) as a chemical reactivity descriptor using eq 1.88,89 The ω of IL/ CO2 is compared in Figure 4, and among the components in the IL/CO2/O2·− system, CO2 has a higher positive magnitude of ω = 0.489 eV relative to [MOEMMor+][TFSI−] (ω = 0.188 eV) and [EDMPAmm+][TFSI−] (ω = 0.212 eV). The higher electrophilicity index of [PMIm+][TFSI−] (ω = 0.989 eV) showed the IL to be slightly more positively mannered than CO2. In another insight, O2 is more competitive for reduction to O2·− than CO2 owing to its high electrophilicity index. By

current, which is associated with electrical double layer charging.78 However, the precathodic hump is absent in CVs shown in Figure 1a,b. 3.1.1. Mass Transfer Phenomena of O2·− Electrochemical Generation. The mass transfer phenomena of O2 in the IL medium are concerned mainly with the interpretation of diffusion coefficient (Do) and solubility (Co). According to Table 2, Dos of O2 in the [PMIm+][TFSI−], [EDMPAmm+][TFSI−], and [MOEMMor+][TFSI−] are evaluated to be 34.4, 19.6, and 0.133, respectively. The comparison between these Do values and the IL viscosity (η) ([PMIm+][TFSI−] = 34.4 m2/s-32.99 mPa s, [EDMPAmm+][TFSI−] = 19.6 m2/s-63.56 mPa s, and [MOEMMor+][TFSI−] = 0.133 m2/s-183.35 mPa s) confirms the fact that, when viscosity of the IL increases, diffusion of electroactive species through ILs decreases.79 In turn, these viscosity values were measured at 25 °C by setting the speed of the spindle, which was immersed into the ILs, to 5 rpm at 6.6 s−1 shear rates on a DV-II+ Pro Extra Brookfield viscometer, and their magnitudes herewith are consistent with the literature.79,80 In addition, the Do values implied, for a constant flux of O2 into a spatial domain within the IL to another, within the electrochemical cell, diffusion of O2 in [PMIm+][TFSI−] experienced the least concentration gradient because it has the lowest viscosity. Equally, the concentration gradient of O2 was slightly increased in [EDMPAmm+][TFSI−] because its Do is less than that in [PMIm+][TFSI−], and this is brought about because the viscosity of [EDMPAmm+][TFSI−] is greater than that of [PMIm+][TFSI−]. Also, the concentration gradient became highest in [MOEMMor+][TFSI−] whose Do for O2 is far less than those in [EDMPAmm+][TFSI−] and in [PMIm+][TFSI−]. This is because [MOEMMor+][TFSI−] has a relatively high viscosity. Moreover, the Cos of O2 in the ILs were estimated to be [PMIm+][TFSI−] = 2.902 mM, [EDMPAmm+][TFSI−] = 3.216 mM, and [MOEMMor+][TFSI−] = 14.135 mM. As seen, the Co values did not define a pattern with IL viscosity, and hence, it was strongly suggested to be influenced by the nature of the IL-cation. The [MOEMMor+][TFSI−] has the highest Co of 14.135 mM because, based on the strong suggestion, the IL-cation has an ether-like extension (R−O− R) that can create a hydrogen bond with dissolved O2, thereby increasing its solubility values. This observation is similar to the report of increasing O2 solubility in 1-alkyl-3-methylimidazolium cation derived ILs where the occurrence of C−F and C−H bonds induces hydrogen bonds with O2.81 In [MOEMMor+] IL-cation, there is an occurrence of C−O and C−F bonds, and either O or F has a strong electronegative point charge that is capable of creating a local dipole in the ILcation ends as a rationale for hydrogen bond creation with O2. Therefore, the absence of C−O or rather ether-like groups in [EDMPAmm+] and [PMIm+][TFSI−] IL-cations strongly suggested the reason why the magnitudes of their Co are 3.21 and 2.902 mM, respectively. In addition, the distinctive differences of Do and Co among the ILs were brought about by the role of viscosity and the nature of the cation, respectively.79,82 Their magnitudes confirmed molecular transport of O2 in the system and the absence of eddy diffusion due to turbulence.83−85 3.2. CO 2 Conversion by O 2 ·− . Figure 3 shows experimental evidence of CO2 conversion by O2·− in [MOEMMor + ][TFSI − ], [EDMPAmm + ][TFSI − ], and [PMIm+][TFSI−]. To begin with, this is the first time these ILs are being used successfully for CO2 capture, and they are F


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orbitals (HOMOs), the following increasing order of energy gap among the IL was observed: [PMIm+][TFSI−] (6.8 eV) < [MOEMMor+][TFSI−] (12.3 eV) < [EDMPAmm+][TFSI−] (15.3 eV). The energy gap of CO2 (10.3 eV) was higher than that of [PMIm + ][TFSI − ] while less than those of [EDMPAmm+][TFSI−] and [MOEMMor+][TFSI−]. By implication, the band gap energy information of these moieties indicated the decrease in optical properties as O2·− < [PMIm + ][TFSI − ] < CO 2 < [MOEMMor + ][TFSI − ] < [EDMPAmm+][TFSI−]. Therefore, the CO2 conversion by O2·− is more preferable before any possible [EDMPAmm+][TFSI−] and [MOEMMor+][TFSI−] IL-cation interaction with O2·−. This rationale strongly supported CO2 consumption by O2·− but stresses on the possible influence of the reacting medium on the process. In retrospect, Roberts et al.24 reported that CO2 consumption by O2·− usually has a doubling of the reduction peak as evidence for C2O62− formation. Other studies followed suite by indirectly showing evidence of C2O62− formation by carboxylating other groups directly in addition to the doubling of the reduction peak.24,25,90,91 In cognizance to the perspective of ω, the doubling effect of the faradaic reduction peak is not tenable especially when the electrophilicity index of the reacting medium is greater than that of CO2. For instance, the global electrophilicity index of tetraethylammonium perchlorate (TEAP) (i.e., the supporting medium usually used for O2·− generation24) is ω = 3.82 eV. This value is greater than the combined global electrophilicity indices of [MOEMMor+][TFSI−], [EDMPAmm+][TFSI−], [PMIm+][TFSI−], and CO2. The high value of the TEAP global electrophilicity index would make it attractive to be

Figure 4. Global electrophilicity indices as chemical reactivity descriptors for [EDMPAmm+][TFSI−], [MOEMMor+][TFSI−], [PMIm+][TFSI−], CO2, O2·−, and O2.

inference, [PMIm+][TFSI−] has more potential to interact with O2·−. Moreover, the O2·− global electrophilicity index according to Figure 5 is −14.764 eV. This indicated that O2·− did not have any positive character rather it is highly nucleophilic. Comparing the ILs and CO2, Figure 5 shows the global electrophilicity index to increase in the order [MOEMMor + ][TFSI − ] < [EDMPAmm + ][TFSI − ] < [PMIm+][TFSI−] < CO2. Also, by evaluating the IL energy gap from the difference in energy of their lowest unoccupied molecular orbitals (LUMOs) and highest occupied molecular

Figure 5. COSMO-RS analysis of (a) [EDMPAmm+][TFSI−], (b) [MOEMMor+][TFSI−], and (c) [PMIm+][TFSI−]. (d) Comparison among CO2, O2, O2·−, and ILs. G


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ACS Applied Materials & Interfaces reduced by O2·−, subsequently inducing the doubling effect of reduction peak current. On the contrary, when 1-butyl-3methylimidazolium hexafluorophosphate [BMIm+][HFP−] was used for CO2 conversion by O2·− as evidenced by AlNashef et al., the CV did not show the doubling effect of reduction current.15 The [BMIm+][HFP−] has a global electrophilicity index of 1.728 eV, which is roughly doubled that for [PMIm+][TFSI−]. 3.5. COSMO-RS Analysis of O2·−/CO2 System. The conductor-like screening model for realistic solvation (COSMO-RS) has a unique combination of quantum chemical treatment with an efficient statistical thermodynamic procedure for molecular surface interactions.92 This implementation assessed and confirmed realistic treatment of O2·−, CO2, and IL interaction using their mapped electron distribution of the molecules to their potentials. The σ-profiles were classified based on charge density (σ; e/nm2) within three possible sections: hydrogen bond donor (HBD) region (σ < −0.82e/ nm2), nonpolarity region (−0.8.2 e/nm2 < σ < +0.82 e/nm2), and the region of hydrogen bond acceptor (HBA) (σ > 0.8.2 e/nm2).63 From Figure 5a−c and the data from Table S5, it can be observed that the IL anion [TFSI−] has a polar repulsive end within all ILs because one of the σ-profiles centers at σ = −0.1998 e/nm2. The anion polar end located at the HBA region where σ = 0.1097 e/nm2 was due to the presence of oxygen or fluorine in the anion. According to Figure 5a, the [EDMPAmm+] cation has an HBD end where σ is equal to −0.8966 e/nm2. This σ-surface is likely to interact with [TFSI−] at σ = 0.1097 e/nm2. The [EDMPAmm+] cation also has a polar repulsive end at σ = −0.4967 e/nm2. Similarly, Figure 5b confirms that the [MOEMMor+] cation has both HBD and HBA ends at σ = −1.114, −0.497 e/nm2 and σ = 0.497, 1.366 e/nm2, respectively. This according to the COSMO-RS computation showed evident self-interaction of the cation at the σ-surface at σ = −0.497 and 0.497 e/nm2 ends, in a situation when more than one of them existed. Equally, the cation can also interact with the [TFSI−] anion at the σ-surface where σ = −1.114 and σ = 1.097 e/nm2. Besides, the cation also has polar repulsive ends at σ = −0.497 and 0.497 e/nm2. Generally, [EDMPAmm+] and [MOEMMor+] cations have HBD ends centered at σ = −0.897 and −0.906 e/ nm2 due to the positive charge contribution from their nitrogen atoms, respectively. Therefore, this strengthened their capacity to form hydrogen bonding with the [TFSI−] anion. Uniquely from Figure 6c, [PMIm+] has three typical polar repulsive ends centered at σ = −0.757, −0.497, and −0.000776 e/nm2. This cation did not have HBD or HBA ends, and this implicated any other form of potential noncovalent interaction between [PMIm+] and [TFSI−] anion but hydrogen bond interaction. The consequences of this on CO2 conversion was not presently clear and thus motivated further investigations. Furthermore, Figure 5d shows that O2·− has two polar attractive ends characterized by HBA, which center at σ = 1.71 and 2.34 e/nm2. This confirms that O2·− will interact with [EDMPAmm+] and [MOEMMor+] cations via hydrogen bonding. As to O2·− interaction with [PMIm+], it can be via any other noncovalent interaction but hydrogen bond interaction. O2·− can interact with CO2 whose charge density is quantified at the HBD region where σ = −1.10 e/nm2. These results suggested that the sp3 carbon in CO2 has a positive character and appeared to be hydrogen-like to avail interaction with O2·−. Moreover, the CO2 COSMO surface whose charge density was in the HBD region at σ = −0.906 e/nm2 and the

Figure 6. FTIR spectra of (a) ATR-FTIR observation of C2O62− generated in [PMIm+][TFSI−] at 27, 35, and 45 °C compared with blank IL ATR-FTIR and (b) DFT/B3LYP/6-31G-IR simulation of C2O62−.

[TFSI−] anion COSMO-surface at the HBA region σ = 1.097 e/nm2 confirmed the stance. O2 only has its charge density centering at the nonpolar regions where σ = −0.102 and −0.639 e/nm2. As a result, the absence of any form of noncovalent hydrogen bond interaction between O2 and ILs was indicated but there can be possible van der Waals interaction of the IL cation. These analyses strongly suggested that the IL absorbed CO2 while O2 diffused toward the electrode so that the diffusion-controlled electrochemical process was prevalent. 3.6. Spectroscopic Identification of C2O62− in ILs. Figure 6 shows a comparison between the experimental ATRFTIR (Figure 6a) the DFT-FTIR (Figure 6b) spectra and for characteristic harmonics of the C2O62− anion in [PMIm+][TFSI−]. 3.6.1. DFT-FTIR Identification. Table S6 shows estimation of the Mülliken charges of the C2O62− anion to confirm the presence of dipole moment initiators that enable the anion to be an IR-active asymmetric top molecule. An asymmetric top molecule did not possess an axis of symmetry, and hence, there was no preferred direction for it to carry out a simple rotation around the total angular momentum.46,93 In accordance with the DFT-FTIR spectrum of the C2O62− anion (Figure 6b) taking into consideration its symmetry operations, there are five visible characteristic harmonics centered at 576.7, 730.9, 805, 1267, and 1708 cm−1. Specifically, the harmonic, which centered at 805 cm−1, was attributed to O=O symmetric stretching vibration in the C2O62− anion. Other characteristic vibrations mentioned earlier are peculiar to O=C=O rocking and wagging (734 cm−1) and also the scissoring of O=C=O at 1705 cm−1. Comparing the characteristic vibration of the C2O62− anion at 734 and 805 cm−1 as much as others, the O=C=O harmonic may not have the singularity for C2O62− anion identification in IL. This is due to the occurrence of more than one particular harmonic at a given wavenumber. This was unlike the vibration at 805 cm−1, which only showed O=O symmetric stretching with no any other visible harmonic in the anion molecular structure. 3.6.2. Experimental ATR-FTIR Identification. The experimental ATR-FTIR shown in Figure 6a reveals many vibrations such as those centered at 510, 571, 608, 653.6, 742.1, 792.7, 844, 1050.6, 1133.03, 1175.8, 1346, and 1577.9 cm−1. In H


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numerous applications. Fortunately, with the use of ILs as potential media for O2·− generation and controlling stability, as well as for CO2 capture, a new dimension of research has emerged. This will certainly meet the demands of reducing global warming in the 21st century. Moreover, the proof of concept to this study can be extended in the real world upon realistic completion of process design and controls.

addition, the blank experiment (Figure 6a) has additional ATR-FTIR peaks centered at 885.8 and 952.42 cm−1. Among these frequencies, of particular interest herein was the correspondence of O=O symmetric stretching, which was centered at 844 cm−1 (Figure 6a, 27, 35, and 45 °C). This frequency of vibration for C2O62− generated in [PMIm+][TFSI−] at the different temperatures remained similar throughout. By comparing the DFT-FTIR and ATR-FTIR with the blank ATR-FTIR, it became evident that the [PMIm+][TFSI−] per se contained no electrogenerated C2O62− by showing no vibrational energy for symmetric O=O stretching vibration at 844 cm−1. This further confirmed the absence of the C2O62− anion in the prereaction IL medium. Furthermore, between the DFT-FTIR spectrum and the experimental ATR-FTIR spectrum, there was an evident blue shift of energy of the O=O harmonics to the tune of 39 cm−1. This blue shift was usual especially as an evidence of C2O62− coordination on the IL-cation. Otherwise, the blue shift deviation can mainly be brought about by the complexity to involve all influencing factors existing in the ATR-FTIR analysis of post electrosynthesized C2O62− in DFT-FTIR for [PMIm+][TFSI−]/C2O62− during the theoretical DFT analysis. Consequently, the energy for harmonics based on DFT- FTIR calculation was less than that of the experimental ATR-FTIR analysis. Also, the experimental O=O symmetric stretching has a broadband peak unlike the DFT-FTIR spectrum whose peak was sharp. Usually electronic transition in liquids or gases is not resolved because it is accompanied by either rotational transition that causes energy fluctuation with the broadband peak as the relic.94−96 Resolving this energy fluctuation was complex to be involved in DFT calculation; more so, the DFTFTIR spectrum, which simulated the C2O62− anion in an ideal vacuum environment, possessed sharp FTIR peaks as though only vibrational and no rotational transition existed. In previous studies, C2O62− was identified in substances like carbonate, and these blue shift observations were common attributes of C2O62− when coordinated to other groups such as K, (Me2N)2, Rb, Cs, and (Me4N). Accordingly, the value of the O=O stretching vibration frequency occurring at 844 cm−1 in the IL is comparable with the literature value of the ∼851 or 868 cm−1 IR band assigned to O=O stretching vibration of C2O62− in media involving K, (Me2N)2, Rb, Cs, and (Me4N).24,52,97,98

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05962. ·

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Electrochemical data for O2 − generation after sparging the IL with CO2; Gaussian 09 optimization, convergence criteria, and thermochemistry data for C2O62−; contact probability for [EDMPAmm+][TFSI−], [MOEMMor+][TFSI−], and [PMIm+][TFSI−] based on COSMO-RS implementation in COSMOthermX (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.H.). *E-mail: [email protected] (M.K.A.). *E-mail: ryusoff@um.edu.my (R.Y.). ORCID

Ahmed Halilu: 0000-0003-0381-2365 Maan Hayyan: 0000-0002-2179-0801 Mohamed Kheireddine Aroua: 0000-0002-9388-5439 Rozita Yusoff: 0000-0003-1654-4478 Funding

The Newton Fund Institutional Links Project (IF013-2015) supported this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to express their gratitude to the Newton Fund Institutional Links Project (IF013-2015) for financial support throughout this research.

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4. CONCLUSIONS The direct electrosynthesis of C2O62− from the CO2 reaction with O2·− in [EDMPAmm+][TFSI−], [MOEMMor+][TFSI−], and [PMIm+][TFSI−] media was carried out. The CO2 reactivity with O2·− was established to involve the disappearance of oxidative faradaic current. Moreover, the slight increase in reduction faradaic current and peak potential was an indication of in situ C 2 O 6 2− generation. The study spectroscopically identified the electrosynthesized C2O62− for the first time in an isolated [PMIm+][TFSI−] through a singularity symmetric O=O stretching vibration, which occurred at ∼844 cm−1. It was also established that symmetric O=O stretching vibration was a peculiar harmonic attributed to C2O62− in the IL. It is expected that this investigation will inspire further research proliferation on electrosynthesis of CO2-derived products in ILs as suitable media that can hold stable O2·−. Although O2·− was discovered in the last century, its continual usage ever since has been limited due to shortterm stability. Nevertheless, it is still a promising agent for

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