In Situ Monitoring Potential-Dependent Electrochemical Process by

Mar 20, 2017 - Bruna Ferreira Gomes , Pollyana Ferreira da Silva , Carlos Manuel Silva Lobo , Maiara da Silva Santos , Luiz Alberto Colnago. Analytica...
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In Situ Monitoring Potential-Dependent Electrochemical Process by Liquid NMR Spectroelectrochemical Determination: A Proof-ofConcept Study Shuo-Hui Cao,† Zu-Rong Ni,† Long Huang,‡ Hui-Jun Sun,† Biao Tang,† Liang-Jie Lin,† Yu-Qing Huang,† Zhi-You Zhou,‡ Shi-Gang Sun,*,‡ and Zhong Chen*,† †

Department of Electronic Science, Fujian Provincial Key Laboratory of Plasma and Magnetic Resonance, State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, P. R. China ‡ Department of Chemistry, State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, P. R. China S Supporting Information *

ABSTRACT: We report the design and the performance of a two-chamber thin-layer electrochemical device for in situ potential-dependent liquid NMR measurement. Liquid NMR spectra, simultaneously recorded with cyclic voltammetry (CV), have been obtained to reveal molecular changes with potentials scanning. As a proof of concept, redox properties of 1,4benzoquinone based systems have been investigated, and a π dimerization has been identified by combining both in situ and ex situ NMR analyses. This work provides a new approach for spectroelectrochemistry, which will contribute to developing electrochemical NMR (EC-NMR) as an important tool for the analysis of electrochemical process at a molecular level.

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However, it should be noted that in these reports, most liquid NMR spectra are limited to record products under constant potential electrolysis,6,7 and potential-dependent methods are rarely adopted, which is reasonable due to the fact that the low sensitivity of NMR detection requires large amounts of electro-generated products but limited diffusion inside the NMR tube costs time to complete reactions.12 Here, we report a proof-of-concept study to acquire the potentialdependent NMR spectra during cyclic voltammetry (CV) experiments, through a NMR-adapted electrochemical device to improve electrolytic analysis capacity and reduce measurement interference. Redox behaviors of quinone based systems are investigated to demonstrate how this in situ EC-NMR technique can provide dynamic structural characterization and mechanism elucidation in an electrochemical process.

he developments of in situ spectroelectrochemistry will greatly benefit the electrochemical research, because not only the electrical signal but also the molecular structure information can be simultaneously observed during electrochemical processes.1 Nuclear magnetic resonance (NMR) has been playing an important role in the chemical structure elucidation. Therefore, the combination of NMR and electrochemical devices has attracted considerable interest.2−4 Liquid NMR has been utilized to develop the first in situ electrochemical NMR (EC-NMR) paradigm.5 Because of its intrinsic high-resolution feature, liquid NMR has become a promising approach in the elucidation of electrochemical reaction products. 6,7 Although the incompatibility between the introduction of conducting electrodes and the requirement of homogeneous magnetic field inevitably exists,8 sustaining efforts have been devoted to developing this approach and promoting its applications. Pioneering investigations on thin metal film electrode9,10 with reduced interference to NMR have attracted follow-up studies8,11 to make further improvements. Subsequently, carbon microfibers have been utilized as electrodes to achieve higher current.12 The accelerated 13C NMR and 2D NMR methods have been introduced to enrich EC-NMR measurements.13,14 Recent progresses from oxidative metabolism studies15 to fuel-cell electrocatalysis research16,17 demonstrate the activity and the feasibility of the in situ liquid NMR spectroelectrochemistry with a wide application prospect. © XXXX American Chemical Society

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EXPERIMENTAL SECTION

1,4-Benzoquinone (0.1 M) was dissolved in deuterated acetonitrile (or the mixtures with water) with LiClO4 (1 M) as the supporting electrolyte. Electrochemical experiments were conducted with a CHI 660C electrochemical workstation (Shanghai CHI Instrument Co. Ltd., China). CV was performed in a NMR-adapted electrochemical design at a Received: January 20, 2017 Accepted: March 20, 2017 Published: March 20, 2017 A

DOI: 10.1021/acs.analchem.7b00249 Anal. Chem. XXXX, XXX, XXX−XXX

Letter

Analytical Chemistry

3.8 mm outer diameter of the coaxially inserted Au film tube, a spacer approximately 200 μm to be filled with analytes solution (no more than 80 μL) was thus yielded, which was near a typical cell thickness in thin-layer electrochemical works to accelerate bulk electrolysis.18 Second, in this two-chamber design, the RE was positioned closely to the WE in one chamber, separated from the CE, so that the ohmic polarization during electrolysis could be partly reduced. Under a low scanning rate to 5 mV/s, IR drop was controlled to an acceptable level (Figure S5); on the other hand, the time scale of NMR detection could thus match that of the voltammetric measurement. In addition, Nafion membrane was used to act as the ion exchange member, preventing electro-generated species in the WE (in the NMR detection region) from diffusing to the CE with suffering the reversed consumption. As a result, this NMR adapted electrochemical design would benefit bulk electrolysis in a fairly short time to obtain enough reaction products for potential-dependent NMR characterization. As a proof of concept, we took quinone, one of the most important organic redox systems,19,20 to test the power of this method. An attractive aspect concerning the electrochemical pathways regulated by the environmental factors21−24 would be verified. As shown in Figure 2 and Figure 3, cyclic

slow scan rate of 5 mV/s to reduce IR drop. NMR spectra were recorded by a 500 MHz NMR instrument (Agilent Technologies, Santa Clara, CA). Careful probe tuning, shimming of the magnetic field homogeneity, and 90° pulse calibration were performed before the test. Deuterated acetonitrile provided the signal for the lock channel to further support field stability. The standard 1D pulse sequence was used to obtain in situ 1H spectra with a spectral-width of 10 ppm. The full width at half-maximum of the typical spectrum of 1,4-benzoquinone maintained to be 4 Hz in the presence of the Au film electrode. Each spectrum was recorded with a single scan and an experiment time of ∼2 s (delay time 1 s + acquisition time 1 s). As a result, when NMR and electrochemical measurements were simultaneously performed, the in situ 1H NMR spectra were able to continuously record potential-dependent electrochemical reactions with a step of 10 mV. In ex situ NMR experiments, electrochemical treatments and NMR detections were performed successively. Electrochemical treatments were carried out in a normal electrolysis cell. Heteronuclear singular quantum correlation (HSQC) was used to record 1H−13C correlations of samples under electrochemical treatments. Diffusion ordered spectroscopy (DOSY-1H NMR) was used to measure diffusion coefficients of concerned components. The basic sequences included in the software package of the instrument were used to record 2D spectra. All the experiments were performed at 25 °C.

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RESULTS AND DISCUSSION The in situ EC-NMR design was schematically shown in Figure 1. A 50 nm thin Au film coated glass tube, fabricated by a

Figure 2. (a) Cyclic voltammogram and (b) the corresponding in situ 1 H NMR spectra of 1,4-benzoquinone (Q) in deuterated acetonitrile. Colored squares in the cyclic voltammogram indicate selected potentials to show corresponding NMR spectra.

Figure 1. Schematic of the in situ EC-NMR setup.

sputtering deposition apparatus in a vacuum, was used as the working electrode (WE). A Pt foil with 100 μm thickness acted as the reference electrode (RE), attaching to the upper segment of the glass tube without Au coating. A Pt wire was inserted inside this glass tube to act as the counter electrode (CE). The glass tube was filled with supporting electrolytes and the bottom was enveloped by a Nafion membrane. This assembled three-electrodes-analogous system was inserted into a 5 mm NMR tube containing electroactive species to form an electrolysis cell. CE and RE were kept out of the NMR detection region. This device utilized thin-metal design8−11 for reducing interference to radio frequency (Figure S2), and the real-time potential-dependent determination was achieved through elaborate improvement and optimization in combining electrochemical device and NMR detection. First, we induced the thin-layer electrochemistry concept in this device. Taking into account the 4.2 mm inner diameter of the NMR tube and

Figure 3. (a) Cyclic voltammogram and (b) the corresponding in situ 1 H NMR spectra of 1,4-benzoquinone (Q) in deuterated acetonitrile containing 5 M water. Colored squares in the cyclic voltammogram indicate selected potentials to show corresponding NMR spectra.

voltammograms of 1,4-benzoquinone in nonaqueous and water-mixed media and a series of NMR spectra simultaneously collected with the CV were presented. In acetonitrile media, two well-separated voltammetric waves were observed (Figure 2a), indicating two successive one-electron transfer steps.22 The in situ NMR spectra (Figure 2b) showed the decreased signals of 1,4-benzoquinone (6.90 ppm) around the first reduction wave (Redn I), corresponding to the formation of paramagnetic semiquinone radicals.12 The appearance of a new signal at 6.73 ppm implied the formation of quinone dianion at potentials scanning to the second reduction wave (Redn II). And the B

DOI: 10.1021/acs.analchem.7b00249 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry NMR spectra recorded around the first and the second oxidation waves (Oxid I and Oxid II) experienced the signal decreasing at 6.73 ppm and the followed signal recovering at 6.90 ppm, indicating the generation of semiquinone radical and the further oxidation to quinone. With the addition of water (5 M) to acetonitrile, the cyclic voltammogram displayed potential shift, with two redox waves overlapped (Figure 3a). From in situ NMR spectra shown in Figure 3b, the alternately decreased and increased NMR signals at 6.90 and 6.73 ppm around the reduction (Redn I) and oxidation (Oxid I) waves indicated the transform between quinone and quinone dianion, confirming a single step two-electron process in the presence of high concentration water.22 The above descriptions demonstrate that, based on in situ NMR spectra recorded with potentials scanning, molecular information on real-time intermediates can be monitored and identified for the understanding of electrochemical processes in a reliable way. A special spectroelectrochemical behavior was observed when less water (1 M) was added (Figure 4). On the

scan, the singlet was widened and the intensity was greatly decreasing with potentials scanning the first wave (Oxid I), which can be explained by one-electron transfer induced radical species to influence both intensity and line width. At more positive potentials (Oxid II), NMR signal re-emerged as a singlet with intensity gradually recovered, which can be interpreted by the sufficient oxidation of the radical anions, leading to the form with detectable protons in NMR. Considering the larger current peak of wave Oxid II compared with that of wave Oxid I, another pathway except the oneelectron oxidation of radical anions would be involved, and the two-electron oxidation of the dimeric complex may contribute to the current intensity. In order to confirm our hypothesis that the π dimerization occurred in the process, ex situ NMR measurements were involved to provide multiparameters detection. Ex situ HSQC (1H-13C) measurements were performed to record electrochemical products under defined potentials, which can provide the information on the direct bond between 1H and 13C. The obvious emergence of a new 1H−13C correlation peak gave the direct evidence for the generation of new products (Figure S8). In addition, we mixed 1,4-benzoquinone (0.1 M), hydroquinone (0.1 M),and sodium hydroxide (0.2 M) to mimic the interactions between 1,4-benzoquinone and quinone dianion. As shown in Figure S9, before the addition of alkali, two distinct NMR lines separately addressed to 1,4-benzoquinone (6.90 ppm) and hydroquinone (6.77 ppm) were observed, while the spectrum presented one single peak when sodium hydroxide was mixed for deprotonation. The results demonstrated the generation of products with equivalent protons through the interaction between 1,4-benzoquinone and quinone dianion. Figure S10 shows the results of ex situ diffusion coefficient (D) measurement through DOSY-1H NMR: after reduction electrolysis performed at −800 mV for 60 min, the D value of the singlet around 6.90 ppm was decreased from 18.1 × 10−10 m2 s−1 to 10.8 × 10−10 m2 s−1 (while the D values of water were to be 20.7 × 10−10 m2 s−1 during the test) and was near to the one obtained from the singlet of the mixture of 1,4-benzoquinone, hydroquinone, and sodium hydroxide. The decrease in D value for the singlet implies the intermolecular interaction, providing the evidence to further support the production of the dimerization form with a symmetrical structure.

Figure 4. (a) Cyclic voltammogram and (b) the corresponding in situ 1 H NMR spectra of 1,4-benzoquinone (Q) in deuterated acetonitrile containing 1 M water. Inset shows the amplified spectra. Colored squares in the cyclic voltammogram indicate selected potentials to show corresponding NMR spectra.

negative-going scan, around the reduction wave (Redn I), the NMR signal of 1,4-benzoquinone (6.90 ppm) gradually decreased, while a new peak in the lower field emerged and become to be dominant (inset of Figure 4b shows the amplified spectra). Owing to the high energy resolution in liquid ECNMR, differences between spectra as minute as to 5 Hz could be clearly distinguished to monitor potential-dependent electrochemical processes. To exclude the possibility that spectra changes rooted in the distortion caused by the intrinsic inhomogeneous magnetic field, we examined spectra sections addressed to solvents which were electroinactive at the tested potentials. The consistent spectra for solvents at different potentials (Figure S7) demonstrated that spectral changes around 6.90 ppm should be due to potential-induced electrochemical processes. According to the singlet at 6.91 ppm presented at −800 mV, it indicated that the structure of the completely reduced product should be symmetrical with protons in the lower field. It has been reported in the literature that the dimerization stabilized by hydrogen-bonding in aqueous media can interpret the voltammogram of quinone in glassy carbon electrodes.25−27 We thus proposed the hypothesis that a π dimerization undergoing the face-to-face interaction between the quinone dianion and the starting 1,4benzoquinone occurred at the reduction potentials, by the aid of hydrogen-bonding interactions with water. This π dimeric structure can provide equivalent protons in chemical shift and account for the minute shift to the lower field, giving agreement with the features of experimental results. On the positive going-

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CONCLUSION In summary, we have developed an in situ approach for potential-dependent liquid EC-NMR determination. Because of the credit of the NMR adapted electrochemical device, not only can bulk electrolysis be accelerated but also interference reaction can be avoided, thus faithfully reflecting NMR characters during electrochemical processes. To the best of our knowledge, it is the first time that liquid NMR spectra along with a cyclic voltammetric scanning can be simultaneously monitored. Benefiting from the high-resolution feature, we demonstrate that liquid EC-NMR powerfully reveals subtle changes in molecular configurations with potentials scanning, and therefore mechanisms of electrochemical processes can be reasonably elucidated. In addition, owing to advantages of multiparameters detection in NMR measurements, more information can be obtained to assist analyses through combining ex situ NMR techniques. Hopefully, considering that ultrafast correlation spectroscopy (COSY) has been coupled to electrochemistry,14 our recent work on ultrafast C

DOI: 10.1021/acs.analchem.7b00249 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry multidimensional NMR techniques28,29 may be capable of synchronizing with the in situ detection and provide opportunities to develop more reliable characterization even in inhomogeneous magnetic fields. Furthermore, this EC-NMR platform holds potentials to be generalized to investigate other electroactive species, and our efforts have been underway.

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(18) Wang, T.; Zhao, D.; Alvarez, N.; Shanov, V. N.; Heineman, W. R. Anal. Chem. 2015, 87, 9687−9695. (19) Ma, W.; Long, Y.-T. Chem. Soc. Rev. 2014, 43, 30−41. (20) Costentin, C. Chem. Rev. 2008, 108, 2145−2179. (21) Quan, M.; Sanchez, D.; Wasylkiw, M. F.; Smith, D. K. J. Am. Chem. Soc. 2007, 129, 12847−12856. (22) Hui, Y.; Chng, E. L. K.; Chng, C. Y. L.; Poh, H. L.; Webster, R. D. J. Am. Chem. Soc. 2009, 131, 1523−1534. (23) Tessensohn, M. E.; Webster, R. D. Electrochem. Commun. 2016, 62, 38−43. (24) Jin, B.; Huang, J.; Zhao, A.; Zhang, S.; Tian, Y.; Yang, J. J. Electroanal. Chem. 2010, 650, 116−126. (25) Staley, P. A.; Newell, C. M.; Pullman, D. P.; Smith, D. K. Anal. Chem. 2014, 86, 10917−10924. (26) Astudillo, P. D.; Valencia, D. P.; Gonzalez-Fuentes, M. A.; DiazSanchez, B. R.; Frontana, C.; Gonzalez, F. J. Electrochim. Acta 2012, 81, 197−204. (27) Macias-Ruvalcaba, N. A.; Evans, D. H. J. Phys. Chem. C 2010, 114, 1285−1292. (28) Lin, L. J.; Wei, Z. L.; Lin, Y. Q.; Chen, Z. Chem. Commun. 2015, 51, 1234−1236. (29) Wei, Z. L.; Yang, J.; Chen, Y.; Chen, L.; Cao, S. H.; Cai, S. H.; Lin, Y. Q.; Chen, Z. Appl. Phys. Lett. 2016, 108, 084102.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.7b00249. Description of NMR adapted electrochemical device and the performance and ex situ NMR characterization (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhi-You Zhou: 0000-0001-5181-0642 Zhong Chen: 0000-0002-1473-2224 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge supports from NCSF (Grants 21327001, 21505109, and 21229301) and FRFCU (Grants 20720160074, 20720150109, and 20720150018).

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DOI: 10.1021/acs.analchem.7b00249 Anal. Chem. XXXX, XXX, XXX−XXX