New Insights into Fundamental Electron Transfer from Single

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New Insights into Fundamental Electron Transfer from Single Nanoparticle Voltammetry Xiuting Li, Chuhong Lin, Christopher Batchelor-McAuley, Eduardo Laborda, Lidong Shao, and Richard G Compton J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00448 • Publication Date (Web): 11 Apr 2016 Downloaded from http://pubs.acs.org on April 11, 2016

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The Journal of Physical Chemistry Letters

New Insights into Fundamental Electron Transfer from Single Nanoparticle Voltammetry Xiuting Li,†,¶ Chuhong Lin,†,¶ Christopher Batchelor-McAuley,† Eduardo Laborda,‡ Lidong Shao§, and Richard G. Compton*,† †

Department of Chemistry, Physical & Theoretical Chemistry Laboratory, Oxford University,

Oxford, OX1 3QZ, United Kingdom ‡

Departamento de Química Física, Facultad de Química, Universidad de Murcia, 30100, Murcia,

Spain §

Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power,

Shanghai University of Electric Power, 2103 Pingliang Road, Shanghai 200090, P.R.China Corresponding Author *Correspondence to: [email protected]. Telephone number: +44(0) 1865 275957

ACS Paragon Plus Environment

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ABSTRACT

The reductive redox behavior of oxygen in aqueous acid solution leading first to adsorbed superoxide species at single palladium coated multiwalled carbon nanotubes (of length ca. 5 µm and width 130 nm) is reported. The small dimensions of the electroactive surface create conditions of high mass-transport permitting the resolution of electrode kinetic effects. In combination with new theoretical models it is shown that the physical location of the formed product within the double layer of the electrode profoundly influences the observed electron transfer kinetics. This generically important result gives new physical insights into the modelling of the many electrochemical processes involving adsorbed intermediates.

TOC GRAPHICS

KEYWORDS Electron Transfer Theory, Electrocatalysis, Double Layer Effects, Oxygen Reduction Reaction


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We have established a procedure for nanoscale voltammetry1: an electrode is inserted in an aqueous solution containing the reactants of interest and a dilute suspension of catalytic nanoparticles introduced. Over time a single nanoparticle impacts the electrode and adheres to it for a sufficiently long time for cyclic voltammetry to be conducted on the single adsorbed entity. If the latter is electrocatalytic then voltammetry occurs exclusively on the catalytic nanoparticles and the recorded current-voltage curves give insight into the electron transfer at the nanoparticles/solution interface under mass transport conditions reflecting the single adsorbed particles. This presented technique allows the study of new nanomaterials, not just the bulk material as commonly found for ultramicroelectrodes,2 under very high mass transport regimes. To seek new insights into electron transfer most generally, we apply the ‘nano-impacts’ method to the reduction of oxygen at palladium in aqueous acid. We use nitrogen-doped-multiwalled carbon nanotubes, N-CNT-Pd, (length ca. 5 µm; width 130 nm) covered in Pd nanoparticles (diameter ca. 1.4 nm), as reported in the SI and elsewhere.3 The electronic structure of multiwalled carbon nanotubes is semi-metallic and their conductivity sufficient that no significant potential drop occurs along the length of the nanotube.4 Herein, the role of the nanotube is assumed to be effectively graphitic in nature and acts solely as a conductive substrate, facilitating electrical connection to the palladium nanoparticles. The oxygen reduction reaction (ORR) mechanism on metal electrode surfaces is much studied5-10 but still under debate. Two common system dependent overall routes have been proposed: either a direct four electron reduction to H2O or a series pathway via two electron transfer to form hydrogen peroxide (H2O2) and subsequent reduction of H2O2 to H2O.11 The ORR on a palladium surface in H2SO4 proceeds at least partially via a series pathway.12-14


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The ability of the N-CNT-Pd to catalyse the ORR was evaluated by drop-casting onto a glassy carbon (GC) macroelectrode (ca. 3.0 mm in diameter). As shown in Figure 1(A) (black line), no voltammetric signal from ORR was observed at the bare GC electrode in an O2-saturated solution. Then the GC electrode was drop-cast with a 20 μL suspension of N-CNT-Pd (4.6 × 1012

M) and dried under a N2 atmosphere. Assuming homogeneous coverage over the electrode

surface constitutes ~5 layers of carbon nanotubes on the surface. Figure 1(A) shows the CV profiles of the N-CNT-Pds modified GC electrode in 0.5 M H2SO4 saturated with Ar (red line) and with O2 (blue line). It was concluded that N-CNT-Pd in combination with a carbon electrode allows the investigation of the O2 reduction at single N-CNT-Pd tubes via the nano-impact method to which we turn next.

Figure 1. (A) Voltammogram of the bare GC electrode (d = 3.0 mm) in 0.5 M H 2SO4 saturated with O2 (black line); voltammogram of the N-CNT-Pd modified GC electrode in 0.5 M H2SO4 saturated with Ar (red line) and with O2 (blue line), 50 mV/s. (B) Voltammogram in 0.5 M H2SO4 saturated with O2 of the carbon microwire electrode before (black line) and after (red line) collision and immobilized with NCNT-Pd.

Chronoamperometry was carried out with an carbon fiber microwire electrode (of length ca. 1 mm and diameter ca. 7 µm) by recording the current in an O2-saturated solution containing 0.5


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M H2SO4 and 1.4 × 10-13 M N-CNT-Pd at a potential of 0.0 V (vs. SCE). Individual impacting single N-CNT-Pd particles were detected and two example chronoamperometric scans with clear steps are shown in Figure 2. The increased current as shown in Figure 2 (A) is due to the catalysis of ORR at single N-CNT-Pd which collided with and became immobilized on the microwire electrode as evidenced elsewhere.1 The carbon nanotube transport from solution to the carbon microwire electrode is by virtue of Brownian motion, leading to the random and stochastic collision of the N-CNT-Pds with the electrified interface. Once in electrical contact with the electrode, the N-CNT-Pds were relatively stable and remained immobilized for ca. tens of seconds. After an initial impact event the catalytic current is observed in subsequent chronoamperograms. Figure 2 (B) show the chronoamperometric profile in which the current is observed to step-off, indicating the departure of the catalytic material N-CNT-Pd from the electrode surface. Figure 2 (C) provides some steady-state currents measured for ORR, which are found to be close to the value of the diffusion-limited current (the red dashed line). Therefore, except the diffusion, other mass transport effect can be ignored in this experiment.

Figure 2. Chronoamperometric currents showing catalytic reductive Faradaic steps of N-CNT-Pd in 0.5 M H2SO4 saturated with O2 at 0.0 V (vs. SCE). (A) an example of the collision of N-CNT-Pd with the carbon microwire electrode; (B) an example of the falling off of N-CNT-Pd from the wire electrode; (C) experimental steady-state currents for ORR on the collided N-CNT-Pd, where the red dashed line


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represents the theoretical diffusion-controlled peak current for a cylindrical electrode Idiff,15 where two electrons are transferred in ORR and other calculation conditions are the same with our experiment.

The current steps observed on the chronoamperograms indicate a residence time of the N-CNTPd long enough to permit the recording of a cyclic voltammogram at individual CNTs. Figure 1(B) shows the CV signal of ORR during the initial impact event of N-CNT-Pd which lands on the electrode within a few minutes of adding a known concentration of N-CNT-Pd suspension into an O2-saturated solution containing 0.5 M H2SO4. Note that for the recorded CV profiles the sizes of the current fluctuations are variable between different adsorbed carbon nanotubes. An example of a CV with similar wave as Figure 1(B) but distinct Faradaic current fluctuations is shown in Figure S3. The fluctuation of such Faradaic currents observed is explained in detail in our recent paper.16 As shown in Figure S4, the transfer coefficient was measured by plotting ln|I| for the forward sweep versus E with the data directly from CV experiments with linear fitting to get the slopes. For more accurate measurement, two background correction strategies including baseline correction and blank subtraction are employed. The resulting value of transfer coefficient varies from 0.31 to 0.56 for the N-CNT-Pd modified GC electrode and from 0.32 to 0.53 for the single N-CNT-Pd depending on the background technique used. This transfer coefficient is consistent with the reported Tafel slopes in the literature where a value of 120 mV dec-1 or greater (equivalent transfer coefficient α ≤ 0.49) is reported for the ORR on Pt(111) supported Pd film electrodes.12 For an irreversible electron transfer, the mass transport constant of a planar GC macroelectrode (of ca. 3.0 mm in diameter) and a single N-CNT-Pd acting as a cylindrical ‘nanoelectrode’ (of


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ca. 5 µm in length and ca. 130 nm in diameter) can be estimated as 2 × 10-5 m s-1 and 4 × 10-3 m s-1 respectively, which according to the Butler-Volmer equation would result in a higher overpotential of single N-CNT-Pd than the modified GC macroelectrode by ca. 0.32 V (see SI). The CVs of S2O82- reduction on the above electrodes exhibit the expected large potential shifts relating to the huge difference of the electrode size, as shown in Figure S5: The CV of single NCNT-Pd does not show a clear reduction wave because of the high overpotential of S2O82reduction on it. Comparing Figure 1(A) and Figure 1(B), it is observed that the voltammetric waves shifts negatively about 0.1 V (the quarter-wave potential as the characteristic potential17: ~0.3V for modified GC electrode and ~0.2V for the single CNT) when the size of the electrode drops from millimeter scale of GC to sub-micrometer scale of the CNTs. This slight potential shift might reflect the “reversibility” of the first electron transfer step (reduction of O2 to the superoxide ion). However, this step is usually thought to be the rate-determining step and the low transfer coefficient seen in the CV signal of ORR is commonly believed to signal “electrochemical irreversibility”.18 Under reversible conditions the apparent transfer coefficient of a one-electron transfer is one, and the wave-shape reflects a redox process under Nernstian control not any underlying electron transfer kinetics. Over the potential range used for Tafel analysis neither transport effects nor substantial changes in k0 between dropcast ensembles and single nanotubes are expected, therefore the observed kinetic limitation must have a more “complicated” cause. It is necessary to revisit the fundamentals of electron transfer theory in the light of these unexpected data. In particular for reactions where the product of the electron transfer is an adsorbed species, the electrical double layer is expected to have a significant effect on the kinetics of the electron transfer. A model is


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proposed here to study the effect of the electrical double layer on the kinetics of the electron transfer in which solution phase oxygen is reduced to adsorbed superoxide ions.19 To understand the contradiction, the first electron transfer step in ORR is studied in the simulation. The Gibbs energy surface for the electrochemical reduction of a neutral species (O2(sol)) to a negatively charged adsorbate (O2-· (ads)) in eq.(1). is calculated and hence the corresponding reaction current can be obtained. Details about the simulation can be found in the SI.

  X   ads  X  sol  + e   .

(1)

Figure 3 provides an example situation for reaction (1). Figure 3(A) is a Gibbs energy surface at the equilibrium potential Eeq and the white dashed line shows the minimum energy path. The Gibbs energy surface describes both the adsorption process from X(sol) to X-· (ads) and the change of the charge carried by the redox species. Figure 3(B) shows the minimum energy path, varying as a function of the overpotential E − Eeq. As discussed, neither the mass transport process nor the kinetics is the rate-determining factor for ORR in experiment. It is reasonable to assume that the small value of the transfer coefficient is caused by other factors, such as the electrical double layer effect. It is found in Figure 3(B) that the charged species X-· (ads) and the transition state (TS) are more potential-dependent than the neutral reactant X, indicating the sensitivity of these charged species to the electric field within the double layer. Furthermore, the calculated position of X-· (ads) and TS in Figure 3(B) are significantly inside the electrical double layer, which signals the possibly dominant importance of the electrical double layer for the first electron transfer step of ORR.


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Figure 3. Example of the Gibbs energy surface of reaction (1): (A) the Gibbs energy surface at the equilibrium potential; (B) reaction paths with different overpotentials on the d direction. q, d are reaction coordinates. -q relates to the charge carried by the reacting species and d is the distance from the electrode surface. The Debye length (see text) κ-1 = 2.0 Å. Other simulating parameters are common for all the simulations in this paper and can be found in the Supporting Information.

Figure 4. Electrochemical responses to the change of Debye length: (A) the effective transfer coefficient calculated at low overpotentials; (B) the hypothetical Gibbs energy difference between X(sol) and X-· (ads). The dashed lines represent the Nernstian condition. The Debye lengths are 0.2, 0.5, 1.0, 2.0, 3.3 Å and the standard heterogeneous rate constant k0 is 1 m s-1.

Figure 4 shows the response of reaction (1) to the change of the Debye length κ-1. Figure 4(A) shows the effective transfer coefficient as a function of κ-1 with a fast standard heterogeneous rate constant k0. Under reversible Nernstian condition, an ideal effective transfer coefficient should equal 1 as the relatively large k0 leads to reversibility of the reaction. However, it is found


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that even for an artificially fast k0 (in order to achieve full electrochemical reversibility) as applied in Figure 4, the presence of the electrical double layer leads the effective transfer coefficient deviating from 1. Figure 4(B) shows the Gibbs energy difference G

.

X   ads 

− GX(sol)

varying as a function of the Debye length, demonstrating that the apparent “irreversibility” inferred from the transfer coefficient can be caused by the decrease of the driving force as a consequence of the interaction between the electric field within the double layer and the charged species participating in the reaction. Similar conclusions were reported that the electrical double layer can affect the driving force and the transfer coefficient based on the classical Frumkin correction.20-22 However, the charge transfer in this model does not occur at a fixed distance to the electrode, which provides advancement on the classical Frumkin correction.

In conclusion, the data obtained at single nanoparticle catalysts has under conditions of extreme rates of mass transport coupled with theoretical modeling, allowed the key insight that the voltammetric characteristics of the ORR at Pd are likely dominated by double layer effects. As a consequence of the proposed model even for fast electron transfer reactions the illusion of slower kinetics is induced. The reduction of oxygen on Pd involves the initial formation of absorbed superoxide O2-· (ads) with fast electron transfer kinetics but with an apparent transfer coefficient of 0.3~0.5 due to double layer effects arising from the coulombic interactions of the adsorbed superoxide species inside the double layer. Future work will seek to re-evaluate some of the many electrode processes, many of technological significance, which are thought to include adsorbed species.


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ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Telephone: +44(0) 1865 275957 Author Contributions ¶

X.L. and C.L.: These authors contributed equally to this work.

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The research is supported by the European Research Council (ERC) under the EuropeanUnion’s Seventh Framework Programme (FP/2007-2013), ERCGrant Agreement no. 320403. The China Scholarship Council is gratefully acknowledged for funding X. Li’s PhD course. E. Laborda thanks the funding received from the Fundación Séneca de la Región de Murcia under the III PCTRM 2011-2014 Programme (Project 18968/JLI/13). L. Shao thanks the support from the Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power, and the National Natural Science Foundation of China (No. 21403137). Supporting Information The experimental materials, methods and the characterisation via the TEM and SEM of the NCNT-Pd are shown in the Supporting Information. Also included is the electrochemical data of the cyclic voltammetry for ORR on the N-CNT-Pds modified GC electrode and the single NCNT-Pd, the Tafel plots for ORR, the cyclic voltammetry for Na2S2O8 reduction reaction. The analysis and calculation of the potential shift for one-electron-transfer process with irreversible


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kinetics and the simulation approach are further detailed in the Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. REFERENCES 1. Li, X.; Batchelor-McAuley, C.; Whitby, S. A. I.; Tschulik, K.; Shao, L.; Compton, R. G. Single Nanoparticle Voltammetry: Contact Modulation of the Mediated Current. Angew. Chem., Int. Ed. 2015, 55, 4296-4299. 2. Heinze, J. Ultramicroelectrodes in Electrochemistry. Angew. Chem., Int. Ed. Engl. 1993, 32, 1268-1288. 3. Xin, Z.; Wang, J.; Huang, X.; Yao, Y.; Shao, L. Interactions between Low-Loading Pd Nanoparticles and Surface N-Functionalities and Their Effects on Hcooh Oxidation. J. Electrochem. Soc. 2015, 162, H898-H902. 4. Kanghyun, K.; Doyoung, J.; Kangho, L.; Haeyong, K.; Byung Yong, Y.; Jung Il, L.; Gyu Tae, K. Influence of Electrical Contacts on the 1/ F Noise in Individual Multi-Walled Carbon Nanotubes. Nanotechnology 2010, 21, 335702. 5. Steele, B. C. H.; Heinzel, A. Materials for Fuel-Cell Technologies. Nature 2001, 414, 345-352. 6. Šljukić, B.; Banks, C. E.; Compton, R. G. An Overview of the Electrochemical Reduction of Oxygen at Carbon-Based Modified Electrodes. JICS 2005, 2, 1-25. 7. Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Li-O2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2012, 11, 19-29. 8. Hartmann, P.; Bender, C. L.; Vračar, M.; Dürr, A. K.; Garsuch, A.; Janek, J.; Adelhelm, P. A Rechargeable Room-Temperature Sodium Superoxide (NaO2) Battery. Nat. Mater. 2013, 12, 228-232. 9. Nie, Y.; Li, L.; Wei, Z. Recent Advancements in Pt and Pt-Free Catalysts for Oxygen Reduction Reaction. Chem. Soc. Rev. 2015, 44, 2168-2201. 10. Katsounaros, I.; Schneider, W. B.; Meier, J. C.; Benedikt, U.; Biedermann, P. U.; Cuesta, A.; Auer, A. A.; Mayrhofer, K. J. J. The Impact of Spectator Species on the Interaction of H2o2 with Platinum - Implications for the Oxygen Reduction Reaction Pathways. Phys. Chem. Chem. Phys. 2013, 15, 8058-8068. 11. Damjanovic, A.; Genshaw, M. A.; Bockris, J. O. Distinction between Intermediates Produced in Main and Side Electrodic Reactions. J. Chem. Phys. 1966, 45, 4057-4059. 12. Climent, V.; Marković, N. M.; Ross, P. N. Kinetics of Oxygen Reduction on an Epitaxial Film of Palladium on Pt(111). J. Phys. Chem. B 2000, 104, 3116-3120. 13. Gara, M.; Laborda, E.; Holdway, P.; Crossley, A.; Jones, C. J. V.; Compton, R. G. Oxygen Reduction at Sparse Arrays of Platinum Nanoparticles in Aqueous Acid: Hydrogen Peroxide as a Liberated Two Electron Intermediate. Phys. Chem. Chem. Phys. 2013, 15, 1948719495. 14. Schneider, A.; Colmenares, L.; Seidel, Y. E.; Jusys, Z.; Wickman, B.; Kasemo, B.; Behm, R. J. Transport Effects in the Oxygen Reduction Reaction on Nanostructured, Planar Glassy Carbon Supported Pt/GC Model Electrodes. Phys. Chem. Chem. Phys. 2008, 10, 19311943.


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