A Versatile, Robust, and Facile Approach for in Situ Monitoring

Publication Date (Web): January 4, 2019. Copyright © 2019 American ... were performed to confirm the capacity of this device for liquid in situ EC-NM...
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A Versatile, Robust, and Facile Approach for in Situ Monitoring Electrocatalytic Processes through Liquid Electrochemical NMR Spectroscopy Shuo-Hui Cao, Shuo Liu, Hui-Jun Sun, Long Huang, Zu-Rong Ni, WenLong Jiang, Mei Zhan, Zhi-You Zhou, Shi-Gang Sun, and Zhong Chen Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04006 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019

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Analytical Chemistry

A Versatile, Robust, and Facile Approach for in Situ Monitoring Electrocatalytic Processes through Liquid Electrochemical NMR Spectroscopy Shuo-Hui Cao†, Shuo Liu‡, Hui-Jun Sun†, Long Huang‡, §, Zu-Rong Ni†, Wen-Long Jiang†, Mei Zhan‡, 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 of Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, P. R. China ‡State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China §Sino-Precious Metals Holding Co., Ltd., Kunming 650106, P. R. China. Supporting Information Placeholder ABSTRACT: Since the strength of liquid nuclear magnetic resonance (NMR) to noninvasively and specifically realize the structural elucidation and quantitative analysis of small organic molecules, in principle, liquid in situ electrochemical NMR (ECNMR) possesses great advantages for detecting dissolved species during electrochemical process. However, the intrinsic incompatibilities between the coupling techniques, as well as the sophisticated setups modification still limit the applications towards a wide range. To overcome these bottlenecks, herein we propose an easy-to-construct design with the good compatibility, presenting improved electrochemical and NMR performances. As proof of concepts, model experiments of alcohols electrooxidation were performed to confirm the capacity of this device for liquid in situ EC-NMR study. The temporal evolution of both the product and the current distributions can be reliably recorded to aid mechanistic and kinetic understanding of electrocatalysis. The depiction of the selective electrooxidation reveals the surface structure-catalytic functionality. This work demonstrates the universality and effectivity of the proposed platform to develop liquid in situ EC-NMR technique as a useful tool for the dynamic analysis of electrochemical processes at a molecular level.

Due to the high mass energy density and the great production yield from biomass, alcohols electrooxidation has been an attractive topic for fuel cell applications to generate electric energy.1, 2 To engineer better catalysts and materials for the service of fuel cells, the electrooxidation processes should be comprehensively understood,3, 4 which thus calls for reliable and simultaneous measurements of both electrical transfer and molecular transform.5, 6 Fourier transform infrared spectroscopy (FTIRS) and differential electrochemical mass spectrometry (DEMS) have been reckoned as two representative in situ spectroelectrochemical techniques to acquire real-time information on the nature of species involved in alcohols

electrooxidation.7, 8 However, DEMS is limited to the detection of volatile species,9 while FTIRS suffers from serious intensity loss as the infrared light is strongly absorbed by solution.10 And they both require careful quantitative correction. Since liquid nuclear magnetic resonance (NMR) is powerful in structural elucidation and quantitative analysis of soluble small organic molecules, electrochemical NMR (EC-NMR) has been proposed to assist the identification of electrochemically generated species.11-13 Exhausts of methanol and ethanol electrooxidation have been successfully investigated by ex situ EC-NMR.14-17 However, to monitor dissolved molecules of fuels, intermediates, and products under conditions which allow direct and fast detection of electrochemical reactions in NMR sensitive volume, there is still a huge challenge in overcoming the relatively low sensitivity and low spectral resolution of in situ EC-NMR spectroscopy.18 It is worth noting that in situ measurements intrinsically cause incompatibility and adverse effects to NMR detection, where the magnetic field and the radio frequency (RF) field will be greatly interfered by conducting electrode materials.19 A series of works have focused on how to design appropriate in situ EC-NMR devices for liquid electrochemical reactions.20, 21 For example, the concept of skin depth22 has been imported into EC-NMR to guide the fabrication of a thin conducting film electrode with a cylindrical symmetry to decrease the line width of NMR spectra.23-28 Our recent work proposes a two-chamber thin-layer design to improve the ability of potential control, electrolysis acceleration, and interference elimination.27 Nevertheless, the large resistance, the easy exfoliation, the complex film coating technique, and the specialized construction of electrolytic cells or probe modification may limit the accessibility of the cylindrical thin film electrode device to wide applications.20, 21 Another kind of in situ device with carbon-fiber filaments as the working electrode can support large current,29 whereas NMR spectra broadening accompanied with reduced signal-to-noise becomes a significant issue.20, 30 As a compromise, some researchers including us have removed the microfiber

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electrode from the NMR detection region, constructing the so called in probe or on line design to improve the line width, but the ex situ electrochemical transformation upstream the NMR measurement would be encountered.20, 31-33 Very recently, we have employed the intermolecular zero-quantum based purechemical-shift proton NMR technique for in situ detection through carbon fibers, however specialized pulse sequence designs should be involved.34 As a result, it is still desirable to develop new in situ EC-NMR setups with high resolution and sensitivity for reliable molecular elucidation, easy implementation, and convenient popularization. Herein, with fluorinated tin oxide (FTO) as a promising electrode to satisfy EC-NMR combination, we propose a simple yet efficient device for in situ EC-NMR measurement in liquid. It is an easy-to-construct system through directly utilizing the availably commercial materials for establishment. Its feasibility and performance on both electrochemical operation and NMR detection have been demonstrated by in situ studies of alcohols electrooxidation. This work could hold the promise to create a new universal platform to further promote and popularize liquid in situ EC-NMR as a powerful approach for deeply understanding of electrochemical reactions at a molecular level. The liquid in situ EC-NMR design was schematically shown in Figure 1a. A three-electrode system was structured inside a 5 mm NMR tube. The working electrode was fabricated through tightly stacking two rectangular conducting glass slices which were single face FTO coated. The working electrode was coaxially inserted into the NMR detection region, with two FTO faces symmetrically exposed to the solution, while the counter electrode (Pt electrode) and the reference electrode (Pd/H electrode) were located above this part to avoid destroying the homogeneity of the magnetic field and RF field. It was worth mentioning that this device was easily assembled and operated by utilizing the commercial materials, which would be beneficial to popularize this technique. Herein, the comparison to the other two dominant liquid in situ EC-NMR configurations in literatures23-30 was presented. The appropriate materials property and configuration arrangement of the FTO based electrode contributed to its better capacity for in situ EC-NMR measurement, as a useful tool for electrochemical analysis. Firstly, voltammetric scanning was tested to assess electrochemical capacity of electrodes. The closer peak-to-peak separation in the FTO electrode (green curve), compared to that in the cylindrical gold film electrode (red curve) and in the carbon fiber electrode (blue curve), demonstrated its better performance to transfer electrons (Figure S1) which is the important basis for reliable electrocatalysis analysis. Secondly, according to nutation curves tests (Figure 1b, details seen supporting information), this FTO based electrode system did interfere with RF field little. In contrast, even through careful tuning and shimming, the prolonged pulse lengths and the continuous amplitude attenuation in the cylindrical gold film electrode and the carbon fiber electrode indicated the absorption loss and the homogeneity distortion of the RF field, which would potentially decrease detection sensitivity in practice. Thirdly, as shown in Figure 1c, comparable high-resolution NMR spectra could be obtained in the absence (black) and the presence (green) of the FTO electrode system. But the deteriorative line width and line shape was observed (after optimized tuning and shimming) when the cylindrical gold film electrode (red) or the carbon fiber electrode (blue) was utilized for in situ measurement, which would limit the capacity of precise molecular elucidation in electrochemical reactions. In this study, we consider that (1) symmetrically positioned FTO conducting sides parallel to the direction of the magnetic field would reduce the distortion to the homogeneity of the magnetic field;24 (2) the stack of single-facecoated FTO slices natively leave the bilateral sides uncoated by

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Figure 1. (a) Schematic of the liquid in situ EC-NMR device. Bottom inset shows the device relative to the direction of magnetic field and the RF field. (b-c) 1H NMR nutation curves (b) and 1H NMR spectra (c) with and without electrodes in NMR detection region for evaluation of in situ EC-NMR devices. The partial enlarged 1H NMR spectra are indicated by the dashed line in (c). Test samples were 0.15 M ethanol in solution with 0.1 M H2SO4. the conducting layer, avoiding RF shielding caused by an inclosed conducting configuration;28 (3) the thin FTO film (Figure S2) could be helpful to reduce the attenuation of RF penetration.23 And the intrinsic lower electromagnetic shielding effectiveness of oxides than metallic enclosures an carbon materials have an advantage.35, 36 More detailed investigations of the properties of FTO materials in NMR measurements have been underway. To prove the feasibility and universality of this platform, typical alcohol fuels including methanol, ethanol, 1-propanol, and 2-propanol were successively tested, both in acid medium by loading Pt as the electrocatalysts, and in alkaline medium with Pd as the electrocatalysts. Firstly, due to the chemical stability of the FTO electrode,37 continuous electrochemical monitoring could be well operated, without acid/base corrosion induced recording interrupt (Figure S5), demonstrating the robustness of this ECNMR device in electrochemical researches. Secondly, because of the low-loss in both sensitivity and resolution of this protocol, in situ 1H NMR spectroscopy could give well-resolved spectra, providing a useful tool in directly identifying and quantifying liquid products during electrolysis. It thus overcome the limits of high cost level and complex quantifying procedures in previous in situ EC-NMR studies where isotopically enriched fuels were usually required.18, 26, 33 Under test conditions, it was identified that, for 2-propanol, acetone was the exclusive liquid product either in acid or in alkaline medium. And for methanol, ethanol, and 1-propanol, the oxidation products in the acid medium included the corresponding aldehydes and acids, while in the alkaline medium,38, 39 formate, acetate, and propionate were distinguished during the oxidation, respectively (Figure S8-S31). It was worth noting that the aldehydes in the acid solution were in a condensation form, which would cause line-broadening40 to reduce the resolution. Even though, the signals addressed to the condensation were clearly distinguished by the device in this work, whereas by using other in situ EC-NMR configurations, those signals could hardly be recognized (Figure S-33). Thirdly, attributed to this in situ protocol, the drawbacks of the inherent delays of observation in the ex situ configurations were efficiently overcome (Figure S35), providing a good opportunity to synchronously record the current profiles and the NMR signals in a reliable way (Figure S36-S43). As a result, the time-dependent

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Analytical Chemistry current distributions9, 41 were able to be measured to dynamically reveal the molecular energy activation in alcohols electrooxidation (Figure S44), through making a real-time comparison between the electrochemical data and the NMR results. The sensitivity evaluated to probe equally 0.1% current efficiency guaranteed the measurement (Figure S45). For clarity, methanol electrooxidation was picked out as an example to illustrate the in situ analysis. As shown in Figure 2, the chronoamperometric currents reflected the electrocatalytic activity at different potentials (Figure 2a and 2e), while the in situ NMR spectra (insets in Figure 2a and 2e) definitely revealed the chemical nature of liquid products under different reaction conditions, with formaldehyde, in fact generated but readily derived as a form of methanediol, and formic acid on the Pt electrocatalysts (H2SO4), yet formate on the Pd electrocatalysts (NaOH). It was considered that the complete oxidation products in acid and alkaline medium would be CO2 and CO32- respectively, which were not directly measured by 1H NMR but could be reasonably estimated based on Faraday's law9, 16, 17, 41. Herein, the dynamic product distributions during electrochemical processes could be well quantified by in situ EC-NMR spectroscopy, and the kinetic information of reactions could thus be recorded, showing the increased methanol conversion rates at higher

Figure 2. In situ EC-NMR measurements of methanol oxidation on commercial Pt catalysts in 0.15 M methanol+0.1 M H2SO4 solution (a-d) and on commercial Pd catalysts in 0.15 M methanol+0.1 M NaOH solution (e-h). (a, e) Current density-time curves. Insets show the representative 1H NMR spectra at different potentials. Potentials were quoted against the reversible hydrogen electrode (RHE). (b-d, f-h) Time-dependent quantity changes of methanol (MeOH), formaldehyde (FAL), formic acid (FA), formate (FAT), CO2, and CO32- recorded by in situ ECNMR. Insets separately show the current efficiencies (E-) for electro-generated products varying with time.

potentials (Figure 2b-2d and 2f-2h). Beyond that, it was the first time that in situ current efficiency monitoring was realized in ECNMR, which clearly revealed the molecular mechanism for origin of current. The results manifested that the molecular energy activation varied as a function of time at different potentials (insets in Figure 2b-2d and 2f-2h). In the acid medium, it was interesting to find that although the formaldehyde (as the form of methanediol) showed the higher yields compared to other products, its real-time current efficiency (E-FAL) experienced an increase at 550 mV, and decayed since at 650 mv to even lose the domination, while the current contribution from formic acid generation raised faster at higher potentials (insets in Figure 2b2d); in the alkaline medium, the current distributions maintained more stable at 550 mV and 650 mV, until presented a decline for formate at 750 mV (insets in Figure 2f-2h). The time dependent behaviors indicated the continuous evolutions of the electrocatalytic properties as the electrochemical reactions were proceeding. In situ EC-NMR spectroscopy should contribute to monitoring reaction processes and evaluating electrocatalytic performances in a real-time condition. We also demonstrated that this device was useful to describe structure-selectivity relationship in alcohols electrooxidation. As an example, 1,2-propanediol oxidation was studied to reveal insight into the mechanism and the pathway of selective polyol electrooxidation. Benefiting from the stability of FTO, electrochemical square-wave potential method could be combined to fabricate nanocrystals with defined structures, even if the synthesis was under harsh conditions.42, 43 As shown in the inset of Figure 3a, the high-index faceted Pt nanocrystals of tetrahexahedra (THH) were successfully synthesized on the FTO electrode. Illustrated through the cyclic voltammograms, Pt THH exhibited enhanced catalytic activity for 1,2-propanediol oxidation, with the peak current density in the forward scan 2.8 times higher than that of the commercial Pt catalysts (Figure 3a). By in situ EC-NMR spectroscopy, the comparative study of the electrooxidation reactions on Pt THH and commercial Pt catalysts was depicted in Figure 3b and 3c. For commercial Pt catalysts, the oxidation products of lactic acid (the transformation to lactic acid was considered to perform too fast to observe the formation of lactaldehyde)44, 45 and hydroxyacetone were identified together at the tested potentials, corresponding to the oxidation of the primary and the secondary alcohol groups of 1,2-propanediol, respectively. This result implied that both two alcohol groups could be oxidized using commercial Pt as catalysts. On the contrary, for Pt THH catalysts, the selective oxidization of the secondary alcohol group was found. Because the high-index facets of Pt THH were composed of low-coordinated Pt atoms,42, 43 the simultaneous adsorption of the vicinal two alcohol groups’ sites was expected. The more active secondary alcohol group46 could thus be oxidized preferentially, promoting the transformation to hydroxyacetone. The favor of C-C cleavage in hydroxyacetone44 caused the generation of acetic acid and formic acid with the subsequent oxidation to CO2. The higher proportion of the C-C bond breaking also accounted for the higher current density on Pt THH. Considering that it is possible to synthesize a series of nanocrystals with defined structures and facets through the electrochemical modulation,47, 48 the systematic evaluation of surface structure−catalytic functionality can be expected based on this in situ EC-NMR platform. In summary, we have developed a simple yet efficient liquid in situ EC-NMR system and demonstrated its capacity through the tests of alcohols electrooxidation. The improved electrochemical and NMR performances make this device suitable for the ECNMR combination, to reliably acquire real-time information on the chemical nature of the electrochemical reactions involved solution species. As model applications, the temporal evolutions

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ACKNOWLEDGMENT The authors would like to acknowledge the supports from National Natural Science Foundation of China (U1632274, 21505109, 11475142, 21621091, and 21706222), Fundamental Research Funds for the Central University (20720160074 and 20720150109), Natural Science Foundation of Fujian Province (2018J01008), and Key Science and Technique Project of Fujian Province (2017H0040).

REFERENCES

Figure 3. (a) Cyclic voltammograms of commercical Pt and tetrahexahedra (THH) Pt in 0.1 M 1,2-propanediol+0.1 M H2SO4 at 10 mV/s. Potentials were quoted against the reversible hydrogen electrode (RHE). Inset shows the scanning electron microscope imaging of Pt THH nanoparticles and the scale bar is 100 nm. (b) Representative 1H NMR spectra for monitoring electrooxidation of 1,2-propanediol under different potentials respectively utilizing commerical Pt (green) and Pt THH (red) as the catalysts. (c) Schematic representation of the potential electrooxidation pathways for 1,2-propanediol according to the in situ EC-NMR detection. The structure units to mark peaks in (b) indicate the molecules in (c) with the corresponding colors. of product and current distributions have been recorded, aiding mechanistic and kinetic understanding of the electrocatalytic processes. Combining the electrochemical crystal fabrication technique, the special structure-selectivity relationship in nanocrystals has been proved. Promisingly, this work provides a universal platform to popularize liquid in situ EC-NMR analysis at a molecular level. Although right now this design is focused on the constant potential electrolysis, it has the potential to be extended in the future through combining the thin-layer electrolysis configuration.27 In next steps, we are trying to combine in situ solid state NMR techniques,19 so that not only can high-resolution NMR spectra of solutes be measured, but also surface adsorbed species are complementally investigated, for the better understanding of electrooxidation comprehensively, and the rational designing of high-performance catalysts.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The description of experimental procedures and complementary data (PDF).

AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected]

Notes The authors declare no competing financial interests.

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