Direct Molecular Evidence of Proton Transfer and Mass Dynamics at

Subscriber access provided by TULANE UNIVERSITY

Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Direct Molecular Evidence of Proton Transfer and Mass Dynamics at Electrode-Electrolyte Interface Jun-Gang Wang, Yanyan Zhang, Xiaofei Yu, Xin Hua, Fuyi Wang, Yi-Tao Long, and Zihua Zhu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03282 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Letters

Direct Molecular Evidence of Proton Transfer and Mass Dynamics at Electrode-Electrolyte Interface Jun-Gang Wang,a,b,† Yanyan Zhang,b, c, d, † Xiaofei Yu,b Xin Hua,a Fuyi Wang, c, d, Yi-Tao Long,a,* and Zihua Zhub,* a Key Laboratory for Advanced Materials & School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China b Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99354, USA c Beijing National Laboratory for Molecular Sciences, National Centre for Mass Spectrometry in Beijing, CAS Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China d University of Chinese Academy of Sciences, Beijing 100049, P. R. China

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Proton transfer has been widely regarded as a key step in many electrochemical and biological processes. However, direct molecular evidence has long been lacking. In this work, we chose the electrochemical oxidation of acetaminophen (APAP) as a model system and utilized in situ liquid time-of-flight secondary ion mass spectroscopy (ToFSIMS) to molecularly examine proton solvation and transfer in this process. In addition, we successfully captured and identified the transient radical intermediate, providing solid molecular evidence to resolve an important debate in electron transfer-proton transfer oxidation mechanism of APAP. Moreover, the potential-dependent behaviors of both inert ions and electroactive species during the dynamic potential scanning were chemically monitored in real time and the mass diffusion mechanism regarding the electroactive and non-electroactive species was revealed under polarized conditions. The results are consistent with our computer simulations. The observations in this work greatly improved our understanding of proton transfer and mass dynamics occurring at the electrode-electrolyte interface in complex electrochemical processes.

2


Page 2 of 23

Page 3 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC

3



Page 4 of 23

Proton transfer plays an important role in fundamental material science, chemical, electrochemical and biochemical processes occurring in either homogeneous or inhomogeneous aqueous environments.1-3 In electrochemistry, proton transfer has long been of great interest in both experimental and theoretical researches. Protons are transferred from or to the redox species accompanied by electrons leaving or entering an electrode during proton coupled electrochemical reactions. Gaining knowledge of these processes is critical in understanding of organic electrochemistry.4 In order to elucidate the proton transfer mechanism, traditional electrochemical approaches such as polarography5, cyclic voltammetry6 and scanning electrochemical microscopy7 have been extensively used. However, these methods are limited in providing direct molecular information for the mechanism study of hydrated proton transfer. In addition, a considerable number of computational efforts have been performed, including BornOppenheimer potential energy surface theory8, density functional theory (DFT)9,10, many-body perturbation theory11, Car-Parrinello molecular dynamics (CPMD)1, multistate empirical valence bond approach (MS-EVB)12, and ab initio MD13-15. Although significant progress has been made in understanding of proton transfer processes, direct experimental evidence for the dynamics of the hydrated protons near the electrode-electrolyte interface during electrochemical reactions has been rarely reported, yet which is the key to the fundamental understanding of proton transfer mechanism in proton coupled electrochemical processes. At the electrode-electrolyte interface, mass dynamics of the electro-inactive supporting electrolyte usually accompanies the proton transfer, a sufficient concentration of which is necessary in traditional electroanalytical experiments such as cyclic voltammetry to ensure high ionic strength of the solution.19-22 Experimentally, it has long been of significant interest to obtain direct evidence of the dynamic behavior of the supporting electrolyte during electrochemical reactions. Numerical simulations have been employed to extract interfacial information such as rate constants, transfer coefficients and diffusion coefficients near the electrode surface which relied upon accurately modeling both electrode kinetics and mass transport.23-25 For example, Compton

and

co-workers

applied

Nernst-Planck-Poisson 4


equation

from

Page 5 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


chronoamperometry to cyclic voltammetry and demonstrated the dependence of the voltammetric wave shape on the properties of inert electrolytes.20 However, no direct molecular evidence for the dynamics of inert ions at the electrode-electrolyte interface is provided. The challenges for experimental evidence arise from the following two possible reasons. Proton diffusion is fast and protons tend to be easily hydrated in aqueous environments.16,17 More importantly, proton transfer occurs in the electric double layer at the electrode-electrolyte interface, and most traditional analysis tools cannot provide real-time molecular information in such a confined space (generally sub nanometers to tens of nanometers).18 Recently, in situ liquid SIMS interfaced with electrochemistry has been developed to investigate complex chemistries occurring at the electrodeelectrolyte interfaces.26 It has been successfully used to study the solid electrolyte interphase (SEI) layer in lithium-ion battery and monitor the short-lived radical intermediates during electro-oxidation of ascorbic acid.27,28 This new development allows molecular monitoring of intermediates and products in electric double layers during dynamic potential scanning. Moreover, as SIMS detects charged ions, it is a perfect technique to monitor positively charged proton and related solvated species.29,30 These studies demonstrated that in situ liquid SIMS coupled with electrochemistry has great potential to provide molecular evidences for proton transfer, radical reaction intermediates, as well as dynamics of other electroactive/non-electroactive species at the electrode-electrolyte interface. A classic system, the electrochemical redox of acetaminophen (APAP) was chosen as a model reaction system to study proton transfer (Figure 1). APAP known as paracetamol is a commonly used analgesic and antipyretic agent which is typically metabolized by cytochrome P450 2E1 system to form electrophile N-acetyl-pbenzoquinone imine (NAPQI).31 The electrochemical oxidation of APAP can be used to mimic oxidative metabolism reaction catalyzed by enzymes from the cytochrome P450 (CYP450) family.32 Although this classical system has been extensively investigated using traditional electrochemical approaches,33,34 its reaction mechanism remains uncertain. For example, Getek et al. combined electrochemistry and 5



electrospray ionization mass spectrometry to investigate this reaction and hypothesized that NAPQI was formed with a mechanism of two-electrons and two-protons transfer.35 However, Lohmann et al. studied the electrochemical dehydrogenation of APAP and proposed a two-step oxidation mechanism.36-38 Due to its significance in organic redox electrochemistry and enzymatic catalysis,35,39 direct molecular verification of electrooxidation reaction mechanism of APAP is highly desired. In this study, we utilized in situ liquid SIMS coupled with a vacuum-compatible electrochemical cell to directly examine the releasing and recombination of hydrated protons at the electrode-electrolyte interface during electrochemical redox of APAP at a molecular level. With direct molecular evidences, we successfully identified the shortlived radical intermediate and elucidated the electron transfer-proton transfer oxidation mechanism for APAP. Moreover, the potential-dependent mass transfer behaviors of both electrochemical active and non-active species at the electrode-electrolyte interface were monitored in real time during dynamic potential scan, which were obviously different from that of solvated proton species. Our study offers fundamental insights into the mechanism of electrochemical reactions and mass transfer at the electrodeelectrolyte interface under polarized condition.

6


Page 6 of 23

Page 7 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. (A) Schematic of in-situ liquid ToF-SIMS coupled with an electrochemical workstation for probing the electrode-electrolyte interface during an electrochemical reaction. Detailed electrochemical characterizations are shown in Figures S1-3. (B) Schematic of proton releasing and recombination during an electrochemical reaction. Applied dynamic potential (C), the corresponding electrochemical current during electrochemical redox of APAP (D), and representative dynamic ion (H+(H2O)n) signal acquired by ToF-SIMS (E) in a microelectrochemical cell.

At the open circuit potential, a series of hydrated protons (H+(H2O)n, 1 ≤ n ≤ 30) were clearly observed in liquid SIMS spectra (Figure S4), revealing their specific abundance with increase of the cluster size. These results demonstrated the feasibility of liquid SIMS to provide molecular information of the proton-solvent molecular structures at the electrode-electrolyte interface (Figure S5). The protonated water trimer (H+(H2O)3) as the dominated form of protonated water clusters was verified experimentally, indicating that H+(H2O)3 has the strongest binding per water molecule (Figure S6). Then, we performed five consecutive CV scans (Figure 2A) in the microelectrochemical cell under high vacuum and conducted in situ liquid SIMS to monitor the electrochemical redox of APAP in real time. Figures 2B-D show time-resolved variation of hydrated proton (H+(H2O)), inert ion (K+) and active electro-species (C6H5NO+) under dynamic potential scan of APAP solution, respectively. The 7



intensities of these ions from liquids after punching through the SiNx membrane were relatively stable under the open-circuit potential (0-50 s in Figure 2), indicating that the system reached a steady state due to the self-renewable liquid surface. Interestingly, as soon as a negative potential was applied on the electrode, a sharp decrease of ion intensities was observed for all (H+(H2O)n) ions (Figures 2 and S7), which could be attributed to the enrichment of K+ at the electrode-electrolyte interface (i.e., an electric double layer was formed,40 as shown in Figure 2C), which is due to electrostatic attraction of positive ion species to cathodic polarized electrode. Enrichment of alkali cations can decrease ionization yield of other positive ion species, such as hydrated protons.41,42 The potential-dependent behaviors of solvated protons (H+(H2O)n (1 ≤ n ≤ 30)) are similar, and the relative abundances of these hydrated protons barely changed during dynamic potential scan (Figure S7B and 7C).

Figure 2. Consecutive CV curves (50-430 s, 5 cycles) obtained in a homemade microelectrochemical cell in 2 mM KNO3 solution containing 0.1 mM APAP, at a scan rate of 0.01 V s-1 (A). Dynamic intensity of (B) solvated H+ species (H+(H2O)), (C) inert ion (K+) and (D) active species (C6H5NO+) analyzed by ToF-SIMS during cyclic voltammetry in potential range from - 0.20 to 0.18 V. Open circuit potential at 0-50 s and 430-480 s. Applied triangle wave type potential versus time during scan (E). (F) The representative dynamic ions intensity of H+(H2O)n during dynamic potential scan in presence of 0.1 mM APAP (a) and absence of APAP (b) in 2 mM KNO3 solution, at a scan rate of 0.01 V s-1. For simplification, only H+(H2O) was shown here. The fitting lines of signal intensity indicated in (F) were fitted to a polynomial function to visualize the trends. (G) Schematic illustration of dynamic of protons at the electrode-electrolyte interface during oxidation process (I) and reduction process (II).

8


Page 8 of 23

Page 9 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A possible explanation is that the relative abundances depend mainly on the energy of global minima needed for stabilization as well as the hydrogen-bond interaction between the proton and the surrounding water molecules.14,43,44 As shown in Figure 2F, the fluctuation of hydrated protons at the electrodeelectrolyte interface was monitored in real-time during dynamic potential scans with or without Faradaic process. During proton coupled electrochemical redox of APAP (a Faradaic process) which involved the proton releasing or recombination, the protonated species exhibited potential dependent behavior during these dynamic processes. The protons produced in the oxidation process interact strongly with surrounding water molecules due to the extremely high charge density of protons, resulting in the formation of a series of pronated water clusters (H+(H2O)n) and cooperative motion.4547

Based on the behavior of protons mentioned above, one can study detailed reaction

processes near the electrode surface. At - 0.20 V during anodic polarization, which was far away from the oxidation potential of APAP, the current was low because no electrochemical reaction takes place. As the polarization potential increased to the oxidation potential of APAP (+ 0.12 V), the signal intensity of solvated proton species increased (Figure 2F, a). As the APAP transferred an electron to the electrode, a proton was released from APAP to surrounding water molecules driven by solvent fluctuations to stabilize the charge redistribution in the aqueous solution (Figure 2G, I). Subsequently, the released protons experienced the dissociation and cage escape processes to form a solvated proton (H+(H2O)n) and thus completed the proton transfer reaction.48 Therefore, the concentration of protons near the electrode-electrolyte interface will increase, causing the “effective pH” near the electrode to be lower than that in the bulk.49 With the enhancement of the anodic polarization, the signal intensities of solvated proton species decreased, corresponding to the diffusion-controlled electrochemical oxidation of APAP (Figure S2). During the cathodic polarization, the signal intensity of solvated proton species decreased companied by the negative reduction current (Figure 2G, II). These results demonstrated that protons’ recombination occurred during the reduction of NAPQI. With increase of the cathodic polarized potential, both the minimum signal intensity of solvated proton species and 9



the maximum negative current at - 0.02 V observed, indicating that the protons near the electrode surface were consumed during the reduction process, whereas NAPQI recombined with protons to form APAP at the electrode-electrolyte interface. The dynamics of solvated proton species with the redox current demonstrated the capability of in situ liquid SIMS to capture proton solvation and transport at the electrodeelectrolyte interface, which is unobtainable from conventional electrochemical approaches. In the case of non-Faradic process, three distinct regions can be clearly identified from the dynamics of solvated proton species (Figure 2F, b) which is different from that in the presence of Faradaic process (Figure 2F, a). With the increase of anodic polarization, the signal intensity of solvated proton species quickly increased from 0.20 to - 0.07 V which is opposite to the behavior of solvated K+ species (Figure S8). After that, it reached to a plateau (from - 0.07 to + 0.18 then reversed to - 0.14 V), indicating a balance between diffusion effect and electrostatic interaction on the solvated proton species. The third region was located in the cathodic polarization from - 0.14 to - 0.20 V companied by the fast decrease of the signal intensity. These observations can be interpreted as follows: during cathodic polarization, the inert electrolyte (K+) and related solvated species (K+(H2O)n) were enriched at the electrodeelectrolyte interface due to electrostatic interaction, resulting in decreasing of ionization yields of other positive secondary ions. During anodic polarization, the K+-related species were repulsed away from the electrode-electrolyte interface, and (H+(H2O)n) signals recovered. Our results provided direct molecular evidence for the dynamic behaviors of protons at the electrode-electrolyte interface during the electrochemical redox of APAP. Since protons are part of the overall redox process, the analysis of the proton dynamics during the redox electrochemical reaction provides fundamental insights into the mechanism of the redox processes. The mass dynamics of representative electrochemical active species versus the applied potential in the presence of Faradaic process were shown in Figures 2D and S9. A typical ion is C6H5NO+, which is a characteristic fragment ion from both the reactant 10


Page 10 of 23

Page 11 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and product (APAP and NAPQI, respectively) during cyclic voltammetry of APAP. It has a series of solvated forms, e.g. [C6H5NO+H2O]+, [C6H5NO+(H2O)2]+ and [C6H5NO+(H2O)3]+. Periodic potential-dependent behaviors of these ions were observed during this process, which provided the evidence for the dynamics of electroactive species on the electrode-electrolyte interface. First, the signal intensities of these active ions increased during the anodic polarization and subsequently decreased during the cathodic polarization, which showed an opposite trend compared to that of K+ and solvated K+ species (Figures 2C and S9). In the case of anodic polarization from - 0.20 V to + 0.07 V, the intensities of these active ions increased with increasing of the potential, which can be attributed to the enrichment of the electrochemically active species at the electrode-electrolyte interface. A plateau region (with some slight increase) for these active species was observed during anodic scan from + 0.07 to + 0.18 V and subsequent cathodic scan from + 0.18 to - 0.10 V, demonstrating that the mass dynamic equilibrium was reached at the electrodeelectrolyte interface for these ions of active species. By contrast, the signal intensity of these active species decreased under cathodic polarization from - 0.10 to - 0.20 V. With the decrease of the potential, K+ and related hydrated species were attracted and accumulated at the electrode-electrolyte interface, which was associated with the repulsion of active ion species from the electrode-electrolyte interface. As mentioned before, enrichment of K+ at the electrode-electrolyte interface may cause decrease of ionization yields of other positive secondary ions. Therefore, it is necessary to check negative secondary ion signals to reconfirm transfer behavior of the active species. The molecular signal of APAP in negative ion mode is weak, and the most characteristic ions are CN- and CNO-. The periodic oscillation of CN- and CNO- during the electrochemical reaction showed similar potential-dependent behaviors (Figure S10) as that of the characteristic positive ions, e.g., C6H5NO+ series, further confirming the behavior pattern of active species. An interesting observation is that the ion intensities of active species, represented by C6H5NO+ in the positive mode and CN- or CNO- in the negative mode, respectively, seem to reach a plateau at the maximum oxidation current with slight increase until the 11



maximum reduction current appears, and then decreases with increasing of K+ (Figures 2D and S11). As shown in Figure S2, both the cathodic and anodic peak currents were proportional to the square root of the scan rate, indicating a typical diffusion-controlled process. However, from previous studies, very little evidence for “diffusion limit” on a molecular level has been reported. Our data suggests that such a “diffusion limit” does exist, i.e., the total amount of active species at the electrode-electrolyte interface can reach a plateau from the maximum of oxidation current to the maximum reduction current. This plateau can be easily explained by the limitation in diffusion of both reactants and products during oxidation and reduction processes. Hence, these results provided solid molecular evidence to reveal the mass dynamics of electrochemically active species at the electrode-electrolyte interface. In this work, K+ and NO3- served as two major non-active ions in the electrolytes. The mass voltammetry of K+ in the presence of Faradaic process has been described in Figure 2C. Solvated K+ species as a function of the applied dynamic potential scan is shown in Figure S11. The signal intensities of both K+ and related solvated species increased sharply under initially applied cathodic polarization (- 0.20 V), which was attributed to the accumulation of K+ and related solvated cluster ions driven by the Coulombic force. When the anodic polarization was applied, K+ and related solvated cluster ions were repelled by the electric field force. The signals decreased gradually and reached a minimum value at + 0.18 V. Compared to those at - 0.20 V, a 61% decrease in signal intensities of K+ and related solvated cluster ions was observed at + 0.18 V, indicating that the concentrations of K+ and related solvated cluster ions at the electrode-electrolyte interface were significantly influenced by the applied polarization potential. All K+ and K+(H2O)n ions showed similar potential dependent behavior during CV scan, and the relative abundances of K+(H2O)n ions did not show any obvious change. These results revealed that the distribution of K+ and related solvated cluster ions was mainly affected by the near field effect between K+ and surrounding water clusters rather than the long range field effect induced by the external electric field, which is similar with the behavior of H+(H2O)n ions. Owing to the long range electrostatic effects (induced by polarized electrode) on 12


Page 12 of 23

Page 13 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the ions, ions are attracted to or repelled by the polarized electrode. In the presence of Faradaic process where electrochemical redox of APAP occurred, one can observe three different regions between the anodic and cathodic polarizations (Figure 3). Firstly, before the oxidation current reached its maximum, i. e., during the anodic scan from 0.20 to + 0.11 V, the intensity of K+ and K+(H2O) continually decreased. Their repulsion from the electrode-electrolyte interface was caused by the Coulombic force from the anodic polarized electrode (Figure 3B, I). This process is associated with increasing of electro-active species at the electrode-electrolyte interface. Secondly, a plateau region for K+ and related solvated species was found from the maximum of oxidation current to the maximum reduction current, i.e., during anodic scan from + 0.11 to + 0.18 V and subsequent cathodic scan from + 0.18 to - 0.03 V. This plateau was caused by a dynamic equilibrium between a Coulombic force (FC) and a mass diffusion effect (FD) on K+ as well as related solvated species at the electrode-electrolyte interface (Figure 3B, II). Moreover, the steady intensity of inert K+ and related solvated species resulted in a weak non-Faradaic current, which contributed a small portion to the overall current intensity. This provides the molecular evidence for the function of the inert ions which can eliminate the charging current during the Faradaic process. Thirdly, after the maximum of reduction current was reached, i.e., during the cathodic potential range from - 0.03 to - 0.20 V, the signal intensity of K+ and related solvated species quasilinearly increased. In this case, the Coulombic force played a predominant role to attract these ions close to the electrode-electrolyte interface and resulted in their concentration increase (Figure 3B, III). By contrast, when no Faradic process occurred, the dynamics of K+ and solvated K+(H2O)n species showed a totally different behavior. The current increased rapidly from 0-10 s (- 0.20 to - 0.10 V) during the anodic scan, and signal intensities of K+ and K+(H2O) ions were dramatically decreased. These observations can be attributed to the static repulsion from the anodic polarized electrode. Signal intensities of K+ and K+(H2O) kept relatively constant during the CV scan from - 0.10 to + 0.18 V then reversed to - 0.10 V. However, the current continued increasing to reach its maximum at the highest potential (+ 0.18 V) and then decreased sharply. This finding is a little 13



surprising because a constant charging mode during non-Faradic process was expected. Such an observation may possibly be due to the oxidation of the Au electrode or contaminations in the system. At the last 10 s (- 0.10 to - 0.20 V) during the cathodic scan, we can see a rapid increase of the reduction current, which is associated with increasing of K+ and K+(H2O) signals. These results indicated that without the disturbance from the electroactive species, the establishment of balance between diffusion effect and electrostatic interaction on the inert ions in the absence of Faradaic process was easier compared to that in the presence of Faradaic process. NO3- is another interesting non-active ion in the system. Mass voltammetry of NO3is shown in Figures S10, S13 and S14. Basically, its behavior is opposite from the potential dependence behavior of K+. This result is understandable because it has opposite polarity with K+. Moreover, Cl- is a contamination in APAP and it shows very interesting behavior during dynamic potential scan, which is described in details in Figure S12.

Figure 3. The representative dynamic ion intensities of K+ and K+(H2O) during dynamic potential scan in the presence (a) and absence (b) of 0.1 mM APAP in 2 mM KNO3 solution, at a scan rate of 0.01 V s-1. The fitting line of signal intensity indicated in (A) fitted to a polynomial function to visualize the trends. (B) Schematic illustration of dynamic behavior of K+ at the electrode-electrolyte interface under polarization.

To get more insights into the mass dynamics of inert ions at the electrode-electrolyte 14


Page 14 of 23

Page 15 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


interface as a function of potential, finite element method (FEM) simulation was performed. The simulated potential field demonstrated that different locations (distance normal to the polarized electrode surface) experienced different polarized potentials (Figure 4A). With increase of the anodic polarized potential, K+ was repelled from the electrode surface and the concentration of K+ at the polarized electrode surface decreased (Figure 4B). This is in agreement with the well-known Gouy-Chapman-Stern modelling of the electric double layer.40 In order to evaluate the probing location during in situ liquid ToF-SIMS experiments, we modulated the K+ concentration profile at different locations within 0 to 40 nm away from the polarized electrode, as a function of the applied polarized potential (Figure 4C). As the distance from the polarized electrode increased, the concentration of K+ was less influenced by

Figure 4. (A) FEM simulated potential distribution along the polarized electrode (- 200, - 50, 50 and 180 mV). (B) Simulation of K+ concentration profile along the polarized electrode (potential scan from - 200 to 180 mV, distance from 0 to 68 nm). (C) Probing locations with different distance (from 0 to 40 nm) away from the polarized electrode. (D), (E) and (F) FEM simulated distributions of K+ concentration profile at various probing locations during dynamic potential scan. (G) Comparison between the experimental K+ mass intensity and the simulated K+ concentration trends at various probing locations as a function of the applied potential.

Coulombic force. The trend of K+ concentration at different locations as a function of 15



the applied potential exhibits representative U-shape (Figures 4D-F). When the location is close to the electrode-electrolyte interface, K+ experiences intense Coulombic force and exhibits an obvious concentration drop during the anodic polarization. A comparison between the experimental K+ intensity trend and the simulated K+ concentration profile shows that the best agreement can be achieved when the modeled location is closer to the electrode (d < 3 nm) (Figure 4G). This agreement provides the estimation of probing location of in situ liquid SIMS and reflects the linkage between the K+ intensity and the chemical information near the electrode-electrolyte interface during dynamic potential scan. Therefore, a direct correlation between the mass transfer in electric double layer and a mass spectrometric signal was successfully established. It provides the capability for further investigation of mass transfer, species-specific adsorptions, and their interaction on Faradaic and non-Faradic processes at the electrode-electrolyte interface. The lack of detailed knowledge of dynamic mass transfer process at the electrodeelectrolyte interface is a common limitation on traditional electrochemical theory, especially in building of mass transfer models under polarized condition. Our results revealed the non-Faradaic processes at molecular level corresponding to the diffusion of inert ions at the electrode-electrolyte interface, which has not been obtained before. Moreover, the radical intermediate (APAP·+, m/z=151.0) was molecularly identified during dynamic potential scan and further verified by potential step experiments. These results suggest the existence of transition state species during APAP electro-oxidation and support a multi-step oxidation mechanism of APAP (More details are shown in Supporting Information, Figures S15-19). Based on the above observations, the complete profile of mass dynamics including electroactive species (reactants and products), inert ions, and electrochemical reaction intermediates at the electrodeelectrolyte interface during the electrochemical reaction was portrayed (Figure 5). At open circuit potential, these ions are physically adsorbed at the electrode interface (Figure 5, A-I). During the cathodic polarization, the inert ions K+ and related solvated species can accumulate on the electrode interface, resulting in less locations available

16


Page 16 of 23

Page 17 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. (A) Schematic of the mass dynamics of both electrochemical active and non-active species at the electrode-electrolyte interface during dynamic potential scan. (B) The electrochemical interface processes during an electrochemical reaction.

for the electro-active species (Figure 5, A-II). With increasing of anodic polarization, inert ions NO3- can be attracted onto the electrode interface. Meanwhile, the reactants start to be oxidized at the electrode interface which is accompanied by the generation of a small amount of reaction intermediates and products (Figure 5, A-III). Then the preferential adsorption of reactants occurs during anodic polarization, resulting from the high electrochemical affinity for the reactants at the anodic polarized electrode interface (Figure 5, A-IV). These results provided direct molecular evidence for the traditional mass transfer theory and revealed the capability of in situ liquid SIMS to real-time monitor the mass dynamics of whole components which participated in both Faradaic and non-Faradaic processes (Figure 5B). This work provides direct molecular evidence for proton solvation and transfer during electrochemical reaction at the electrode-electrolyte interface, which has not been reported before. Real-time movement of electrochemical active and non-active and inert species in electric double layer was recorded, which allows us to molecularly verify the diffusion limit. In addition, the transient radical intermediate during oxidation of APAP was successfully identified, supporting a multi-step oxidation mechanism. Furthermore, our computational simulation results of K+ movement during CV scans not only agree very well with our in situ liquid SIMS data, but also suggest that the signals from liquid in in situ liquid SIMS measurement mostly represent the chemical 17



change of a very thin layer of the liquid within 3 nm from the electrode-electrolyte interface. The above observations facilitate an unprecedented level of comprehensive understanding of complex chemistries occurring at the electrode-electrolyte interface, and we expect a wide application of this novel strategy in electrochemistry field.

18


Page 18 of 23

Page 19 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SUPPORTING INFORMATION This material is available free of charge on the ACS Publications website at DOI Detailed

electrochemical

characterization

of

micro-electrochemical

cell,

electrochemical property of APAP, potential step experiments of APAP, positive and negative secondary ion mass voltammetry in the absence of APAP (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]. Author Contributions †J.W.

and Y.Z. contributed equally to this work.

ORCID Jun-Gang Wang: 0000-0002-1938-3991 Yanyan Zhang: 0000-0002-2048-145X Xin Hua: 0000-0003-1064-083X Fuyi Wang: 0000-0003-0962-1260 Yi-Tao Long: 0000-0003-2571-7457 Zihua Zhu: 0000-0001-5770-8462 Notes The authors declare no competing financial interest. Acknowledgements This work was supported by LDRD fund of the Pacific Northwest National Laboratory 19



(PNNL). Y.-T. L., J.-G. W. and X. H. thank the National Natural Science Foundation of China (21421004, 21327807, 21705046), the Program of Introducing Talents of Discipline to Universities (B16017). J.-G. W. appreciates the financial support from the China Scholarship Council (Grant No. 201600090072). This work was performed at EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at PNNL.

20


Page 20 of 23

Page 21 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) Marx, D.; Tuckerman, M. E.; Hutter, J.; Parrinello, M. The Nature of the Hydrated Excess Proton in Water. Nature 1999, 397, 601-604. (2) Asmis, K. R.; Pivonka, N. L.; Santambrogio, G.; Brümmer, M.; Kaposta, C.; Neumark, D. M.; Wöste, L. Gas-Phase Infrared Spectrum of the Protonated Water Dimer. Science 2003, 299, 1375-1377. (3) Mohammed, O. F.; Pines, D.; Dreyer, J.; Pines, E.; Nibbering, E. T. Sequential Proton Transfer through Water Bridges in Acid-Base Reactions. Science 2005, 310, 83-86. (4) Costentin, C. Electrochemical Approach to the Mechanistic Study of Proton-Coupled Electron Transfer. Chem. Rev. 2008, 108, 2145-2179. (5) Gupta, N.; Linschitz, H. Hydrogen-Bonding and Protonation Effects in Electrochemistry of Quinones in Aprotic Solvents. J. Am. Chem. Soc. 1997, 119, 6384-6391. (6) Chung, T. D.; Anson, F. C. Electrochemical Monitoring of Proton Transfer across Liquid/Liquid Interfaces on the Surface of Graphite Electrodes. Anal. Chem. 2001, 73, 337-342. (7) Zhang, J.; Unwin, P. R. Scanning Electrochemical Microscopy (SECM) Feedback Approach for Measuring Lateral Proton Diffusion in Langmuir Monolayers: Theory and Application. Phys. Chem. Chem. Phys. 2002, 4, 3814-3819. (8) Mielke, S. L.; Garrett, B. C.; Peterson, K. A. A Hierarchical Family of Global Analytic BornOppenheimer Potential Energy Surfaces for the H+H2 Reaction Ranging in Quality from Double-Zeta to the Complete Basis Set Limit. J. Chem. Phys. 2002, 116, 4142-4161. (9) Hammer, N. I.; Shin, J.-W.; Headrick, J. M.; Diken, E. G.; Roscioli, J. R.; Weddle, G. H.; Johnson, M. A. How Do Small Water Clusters Bind an Excess Electron? Science 2004, 306, 675-679. (10) Bryantsev, V. S.; Diallo, M. S.; Goddard Iii, W. A. Calculation of Solvation Free Energies of Charged Solutes Using Mixed Cluster/Continuum Models. J. Phys. Chem. B 2008, 112, 9709-9719. (11) Christie, R.; Jordan, K. D. Theoretical Investigation of the H3O+(H2O)4 Cluster. J. Phys. Chem. A 2001, 105, 7551-7558. (12) James, T.; Wales, D. J. Protonated Water Clusters Described by an Empirical Valence Bond Potential. J. Chem. Phys. 2005, 122, 134306. (13) Tuckerman, M.; Laasonen, K.; Sprik, M.; Parrinello, M. Ab Initio Molecular Dynamics Simulation of the Solvation and Transport of Hydronium and Hydroxyl Ions in Water. J. Chem. Phys. 1995, 103, 150-161. (14) Knight, C.; Voth, G. A. The Curious Case of the Hydrated Proton. Acc. Chem. Res. 2011, 45, 101109. (15) Voth, G. A. Computer Simulation of Proton Solvation and Transport in Aqueous and Biomolecular Systems. Acc. Chem. Res. 2006, 39, 143-150. (16) Wolke, C. T.; Fournier, J. A.; Dzugan, L. C.; Fagiani, M. R.; Odbadrakh, T. T.; Knorke, H.; Jordan, K. D.; McCoy, A. B.; Asmis, K. R.; Johnson, M. A. Spectroscopic Snapshots of the Proton-Transfer Mechanism in Water. Science 2016, 354, 1131-1135. (17) Rini, M.; Magnes, B.-Z.; Pines, E.; Nibbering, E. T. Real-Time Observation of Bimodal Proton Transfer in Acid-Base Pairs in Water. Science 2003, 301, 349-352. (18) Gerlits, O.; Wymore, T.; Das, A.; Shen, C. H.; Parks, J. M.; Smith, J. C.; Weiss, K. L.; Keen, D. A.; Blakeley, M. P.; Louis, J. M. Long-Range Electrostatics-Induced Two-Proton Transfer Captured by Neutron Crystallography in an Enzyme Catalytic Site. Angew. Chem. 2016, 128, 5008-5011. (19) Polcari, D.; Dauphin-Ducharme, P.; Mauzeroll, J. Scanning Electrochemical Microscopy: A 21



Comprehensive Review of Experimental Parameters from 1989 to 2015. Chem. Rev. 2016, 116, 1323413278. (20) Dickinson, E. J.; Limon-Petersen, J. G.; Rees, N. V.; Compton, R. G. How Much Supporting Electrolyte Is Required to Make a Cyclic Voltammetry Experiment Quantitatively “Diffusional”? A Theoretical and Experimental Investigation. J. Phys. Chem. C 2009, 113, 11157-11171. (21) Oldham, K. B.; Zoski, C. G. Mass Transport to Electrodes. Compr. Chem. Kinet. 1986, 26, 79-143. (22) Amatore, C.; Oleinick, A.; Svir, I. Theoretical Analysis of Microscopic Ohmic Drop Effects on Steady-State and Transient Voltammetry at the Disk Microelectrode: A Quasi-Conformal Mapping Modeling and Simulation. Anal. Chem. 2008, 80, 7947-7956. (23) Compton, R. G.; Banks, C. E. Understanding Voltammetry; World Scientific: London, 2011. (24) Ouyang, W.; Saven, J. G.; Subotnik, J. E. A Surface Hopping View of Electrochemistry: NonEquilibrium Electronic Transport through an Ionic Solution with a Classical Master Equation. J. Phys. Chem. C 2015, 119, 20833-20844. (25) Willard, A. P.; Reed, S. K.; Madden, P. A.; Chandler, D. Water at an Electrochemical Interface-a Simulation Study. Faraday Discuss. 2009, 141, 423-441. (26) Liu, B.; Yu, X.-Y.; Zhu, Z.; Hua, X.; Yang, L.; Wang, Z. In Situ Chemical Probing of the ElectrodeElectrolyte Interface by ToF-SIMS. Lab Chip 2014, 14, 855-859. (27) Zhu, Z.; Zhou, Y.; Yan, P.; Vemuri, R. S.; Xu, W.; Zhao, R.; Wang, X.; Thevuthasan, S.; Baer, D. R.; Wang, C.-M. In Situ Mass Spectrometric Determination of Molecular Structural Evolution at the Solid Electrolyte Interphase in Lithium-Ion Batteries. Nano Lett. 2015, 15, 6170-6176. (28) Wang, Z.; Zhang, Y.; Liu, B.; Wu, K.; Thevuthasan, S.; Baer, D. R.; Zhu, Z.; Yu, X.-Y.; Wang, F. In Situ Mass Spectrometric Monitoring of the Dynamic Electrochemical Process at the ElectrodeElectrolyte Interface: A Sims Approach. Anal. Chem. 2017, 89, 960-965. (29) Lu, J.; Hua, X.; Long, Y.-T. Recent Advances in Real-Time and in Situ Analysis of an ElectrodeElectrolyte Interface by Mass Spectrometry. Analyst 2017, 142, 691-699. (30) Liu, P.; Lu, M.; Zheng, Q.; Zhang, Y.; Dewald, H. D.; Chen, H. Recent Advances of Electrochemical Mass Spectrometry. Analyst 2013, 138, 5519-5539. (31) Heard, K. J. Acetylcysteine for Acetaminophen Poisoning. N. Engl. J. Med. 2008, 359, 285-292. (32) Wienkers, L. C.; Heath, T. G. Predicting in Vivo Drug Interactions from in Vitro Drug Discovery Data. Nat. Rev. Drug Discov. 2005, 4, 825-833. (33) Miner, D. J.; Rice, J. R.; Riggin, R. M.; Kissinger, P. T. Voltammetry of Acetaminophen and Its Metabolites. Anal. Chem. 1981, 53, 2258-2263. (34) Nematollahi, D.; Shayani-Jam, H.; Alimoradi, M.; Niroomand, S. Electrochemical Oxidation of Acetaminophen in Aqueous Solutions: Kinetic Evaluation of Hydrolysis, Hydroxylation and Dimerization Processes. Electrochim. Acta 2009, 54, 7407-7415. (35) Getek, T.; McRae, T.; Hinson, J. Utility of Solution Electrochemistry Mass Spectrometry for Investigation the Formation and Detection of Biologically Important Conjugates of Acetaminophen. J. Chromatogr. A 1989, 474, 245-256. (36) Lohmann, W.; Dötzer, R.; Gütter, G.; Van Leeuwen, S. M.; Karst, U. On-Line Electrochemistry/Liquid Chromatography/Mass Spectrometry for the Simulation of Pesticide Metabolism. J. Am. Soc. Mass. Spectrom. 2009, 20, 138-145. (37) Lohmann, W.; Karst, U. Simulation of the Detoxification of Paracetamol Using on-Line Electrochemistry/Liquid Chromatography/Mass Spectrometry. Anal. Bioanal. Chem. 2006, 386, 17011708.

22


Page 22 of 23

Page 23 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Bussy, U.; Boujtita, M. Advances in the Electrochemical Simulation of Oxidation Reactions Mediated by Cytochrome P450. Chem. Res. Toxicol. 2014, 27, 1652-1668. (39) Dahlin, D. C.; Miwa, G. T.; Lu, A. Y.; Nelson, S. D. N-Acetyl-P-Benzoquinone Imine: A Cytochrome P-450-Mediated Oxidation Product of Acetaminophen. Proc. Natl. Acad. Sci. 1984, 81, 1327-1331. (40) Allen, J. B.; Larry, R. F. Electrochemical Methods: Fundamentals and Applications. John Wiley & Sons, Inc: New York, 2001. (41) Ming, L. Y. Work-Function Dependence of Negative-Ion Production During Sputtering. Phys. Rev. Lett. 1978, 40, 574-577. (42) Ming, L. Y. Velocity Dependence of the Ionization Probability of Sputtered Atoms. Phys. Rev. Lett. 1981, 47, 1325-1328. (43) Natarajan, S. K.; Morawietz, T.; Behler, J. Representing the Potential-Energy Surface of Protonated Water Clusters by High-Dimensional Neural Network Potentials. Phys. Chem. Chem. Phys. 2015, 17, 8356-8371. (44) Shi, Z.; Ford, J.; Wei, S.; Castleman Jr, A. Water Clusters: Contributions of Binding Energy and Entropy to Stability. J. Chem. Phys. 1993, 99, 8009-8015. (45) Swanson, J. M.; Maupin, C. M.; Chen, H.; Petersen, M. K.; Xu, J.; Wu, Y.; Voth, G. A. Proton Solvation and Transport in Aqueous and Biomolecular Systems: Insights from Computer Simulations. J. Phys. Chem. B, 2015, 119, 9212–9218. (46) Petersen, M. K.; Iyengar, S. S.; Day, T. J.; Voth, G. A. The Hydrated Proton at the Water Liquid/Vapor Interface. J. Phys. Chem. B 2004, 108, 14804-14806. (47) Tarbuck, T. L.; Ota, S. T.; Richmond, G. L. Spectroscopic Studies of Solvated Hydrogen and Hydroxide Ions at Aqueous Surfaces. J. Am. Chem. Soc. 2006, 128, 14519-14527. (48) Dongare, P.; Maji, S.; Hammarström, L. Direct Evidence of a Tryptophan Analogue Radical Formed in a Concerted Electron-Proton Transfer Reaction in Water. J. Am. Chem. Soc. 2016, 138, 21942199. (49) Quan, M.; Sanchez, D.; Wasylkiw, M. F.; Smith, D. K. Voltammetry of Quinones in Unbuffered Aqueous Solution: Reassessing the Roles of Proton Transfer and Hydrogen Bonding in the Aqueous Electrochemistry of Quinones. J. Am. Chem. Soc. 2007, 129, 12847-12856.

23


Direct Molecular Evidence of Proton Transfer and Mass Dynamics at

Recommend Documents