Electrochemical Generation of Individual O2 Nanobubbles via H2O2

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Electrochemical Generation of Individual O2 Nanobubbles via H2O2 Oxidation Hang Ren, Sean R. German, Martin A. Edwards, Qianjin Chen, and Henry S. White* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, United States S Supporting Information *

ABSTRACT: Herein, we use Pt nanodisk electrodes (apparent radii from 4 to 80 nm) to investigate the nucleation of individual O2 nanobubbles generated by electrooxidation of hydrogen peroxide (H2O2). A single bubble reproducibly nucleates when the dissolved O2 concentration reaches ∼0.17 M at the Pt electrode surface. This nucleation concentration is ∼130 times higher than the equilibrium saturation concentration of O2 and is independent of electrode size. Moreover, in acidic H2O2 solutions (1 M HClO4), in addition to producing an O2 nanobubble through H2O2 oxidation at positive potentials, individual H2 nanobubbles can also be generated at negative potentials. Alternating generation of single O2 and H2 bubbles within the same experiment allows direct comparison of the critical concentrations for nucleation of each nanobubble without knowing the precise size/geometry of the electrode or the exact viscosity/ temperature of the solution.

U

concentration of O2 required for nucleation to be estimated. In this Letter, we demonstrate that the two-electron oxidation of H2O2 at a Pt nanoelectrode can be used to generate and study individual O2 nanobubbles. The critical concentration for nucleation of an O2 nanobubble was voltammetrically determined to be 0.17 M. This critical concentration corresponds to a threshold above which nucleation occurs very quickly and is ∼130 times higher than the equilibrium saturation O2 concentration at 1 atm. In acidic H2O2 solutions, a H2 bubble can be also generated through H+ reduction. Both bubbles are formed at the same Pt nanodisk electrode under the same experimental conditions. Thus, the well-characterized H2 bubble can act as an internal control for bubble formation, reducing the effect of experiment-to-experiment variability on quantification of the supersaturation required to nucleate an O2 bubble. A single O2 nanobubble is generated by electrochemical oxidation of H2O2 at a Pt electrode, as shown schematically in Figure 1A. During the anodic scan, the current transitions from cathodic to anodic at ∼0.75 V (vs Ag/AgCl) as H2O2 is oxidized to O2 (Figure 1B). The current rapidly increases as the potential becomes more positive until it reaches a peak current (ipO2 nb), beyond which it rapidly decreases. The precipitous drop in current, which occurs at ∼1.1 V in Figure 1B, is indicative of the fast nucleation and growth of an O2 bubble on the electrode. The bubble reduces transport of H2O2 to the electrode surface, as depicted in the right-hand part of Figure 1A.11 Following nucleation, the current decreases to a nonzero residual current, iOr 2 nb, that is essentially independent of the

nderstanding the dynamics of bubbles on the nanometer scale is of fundamental interest.1 Their nonclassical behavior (e.g., long lifetime,2 size-dependent contact angle3,4) has attracted considerable recent attention. In addition, the nucleation and stability of bubbles on electrodes is of practical importance as these gases are directly involved in many electrocatalytic processes.5,6 Particularly, the nucleation behavior of O2 bubbles is of significant interest as O2 is the reactant and product for many electrocatalytic processes for energy conversion/storage, including fuel cells and water splitting systems.7 On one hand, gaseous bubbles attached to the electrode can reduce the exposed surface and lower overall efficiency;5 on the other, the high diffusivity of O2 in the gas phase can significantly reduce the diffusional resistance to an active site.8 Electrochemical methods using nanoelectrodes provide a means to synthesize and study individual nanobubbles in situ. The kinetics of gas evolution at an electrode can be directly controlled via the applied potential, allowing the supersaturation of dissolved species to be precisely quantified and controlled. We previously described the electrochemical generation of single H2 and N2 nanobubbles at a Pt nanoelectrode by reduction of H+ and oxidation of N2H4, respectively.9−11 The nucleation supersaturations and lifetimes of these bubbles were also investigated.12,13 Generation of O2 nanobubbles by water oxidation has also been briefly described;14,15 however, the complex voltammetric behavior for water oxidation as well as restructuring of the Pt electrode surface at high positive overpotentials16 obscures analysis of the resulting O2 nanobubble. Hydrogen peroxide (H2O2) is electrochemically oxidized to O2 at Pt surfaces at potentials less positive than that for water oxidation.17 In addition to this milder condition, the H2O2 concentration can be readily controlled, allowing the critical © XXXX American Chemical Society

Received: April 11, 2017 Accepted: May 15, 2017 Published: May 18, 2017 2450

DOI: 10.1021/acs.jpclett.7b00882 J. Phys. Chem. Lett. 2017, 8, 2450−2454

Letter

The Journal of Physical Chemistry Letters

attain a sufficient value required for bubble nucleation. Similar voltammetric responses were also observed using different size nanoelectrodes. In each case, bubbles formed when the H2O2 concentration was ≥0.4 M, and for each electrode, the value of iOp 2 nb was independent of H2O2 concentration within this range. Voltammograms corresponding to bubble formation at electrodes with radii ranging from 4 to 88 nm are shown in Figure 3A, from which it is apparent that larger electrodes give larger values of iOp 2 nb.

Figure 1. Electrochemical generation of a single O2 nanobubble. (A) Schematic of O2 generation by electrochemical oxidation of H2O2 before (left) and after (right) nucleation of an O2 nanobubble at a Pt nanodisk electrode. (B) Cyclic voltammogram of an 88 nm radius Pt disk electrode in 0.6 M H2O2 and 1 M HClO4 at a scan rate of 200 mV/s. A background scan without H2O2 is shown as the blue dashed line. The arrows on the i−V curve indicate the direction of the scan. For clarity, electrogenerated protons are omitted from (A).

electrode potential (inset to Figure 1B). We ascribe this residual current to the dynamic equilibrium shown in Figure 1A, where electrogenerated O2 entering the bubble at or near the electrode/solution/gas three-phase boundary balances the outflux of O2 at the bubble/solution interface.18,19 The residual current displays variability,9 which we recently explained is due to variability in the depth of recession of the Pt disk electrode below the glass shroud.19 Analogous voltammetric responses are observed for the formation of single N2 (from N2H4 oxidation)10 and H2 nanobubbles (from H+ reduction).11 As shown in Figure 2, a peak-shaped voltammogram indicative of O2 bubble formation is only observed when the

Figure 3. Generation of O2 nanobubbles on electrodes of different radii (see the Supporting Information for details of electrode sizing). (A) Voltammetric responses of Pt nanoelectrodes with radii of 4, 17, 23, and 88 nm in 0.6 M H2O2 and 1 M HClO4. (B) Peak current of O2 nucleation, ipO2 nb, as a function of electrode radius. The red line is a linear fit through the origin (y [nA] = 0.31 [nA/nm] × x [nm], R2 = 0.94). Error bars indicate the standard deviation from triplicate measurements with the same electrode.

The peak current (iOp 2 nb ) corresponds to the instant immediately prior to O2 bubble nucleation as the nucleation and growth of the bubble occurs very rapidly (as indicated by the sharp decrease in current after iOp 2 nb is reached). Moreover, due to the rapid molecular transport at nanoelectrodes, the distribution of O2 in the solution corresponds to a dynamic steady state at the instant during the voltammetric scan (see Figure S2 for scan rate independence of voltammograms). Under these conditions, iOp 2 nb can be related to the critical surface concentration of O2 required for nucleation of a nanobubble, Ccrit O2 , by expressing the steady-state current at a disk electrode in terms of the O2 diffusion away from the electrode

Figure 2. Dependence of the i−V response of a 23 nm radius Pt electrode on the concentration of H2O2 in an aqueous solution containing 1 M HClO4. The potential was initially scanned toward positive potentials beginning at 0 V. The second voltammetric cycles are shown, and the scan rate was 200 mV/s.

H2O2 concentration is ∼0.4 M or greater. Moreover, above this critical concentration, the magnitude of iOp 2 nb is independent of the concentration of H2O2, as can be seen by comparing the blue and green curves (see further examples in Figure S1). The independence of iOp 2 nb on H2O2 concentration suggests that a critical concentration of electrogenerated O2 is required for nucleation of the O2 bubble (Ccrit O2 ). At concentrations less than ∼0.4 M, the oxidation of H2O2 yields a quasi-sigmoidal wave, indicative of H2O2 diffusion to an unblocked electrode. In this case, the O2 concentration at the electrode surface does not

iOp2 nb = 4naFDO2COcrit2

(1)

In eq 1, n is the number of electrons transferred to generate a single molecule of O2 (n = 2), a is the radius of the electrode, and DO2 is the diffusion coefficient for O2. The plot of iOp 2 nb vs the electrode radius in Figure 3B shows a positive correlation (R2 = 0.94). From the slope of the linear fit, slope we calculate Ccrit ) as 0.24 ± 0.04 M (95% O2 (equal to 4nFDO2

confidence interval from fitting). Note that this calculated value 2451

DOI: 10.1021/acs.jpclett.7b00882 J. Phys. Chem. Lett. 2017, 8, 2450−2454

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The Journal of Physical Chemistry Letters of Ccrit O2 is highly sensitive to the uncertainty in the value of DO2, which is strongly dependent on the nature/viscosity of the electrolyte. We use DO2 = 1.67 × 10−5 cm2/s, which was reported for a 0.1 M HClO4 solution.20 In addition to the uncertainty arising from the value of DO2, a nonideal electrode geometry (e.g., noncircular geometry, recession) and the fluctuation in temperature (gas solubility, nucleation, and diffusion are a function of temperature) contribute to the uncertainties in our analysis and to the deviation from the linearity in Figure 3B. However, if a second gas bubble is generated at the same electrode and in the same solution as an internal control, the nucleation process for bubbles of different gases can be compared directly. Such comparison can greatly reduce the uncertainties from the electrode geometry and diffusion coefficient, as discussed below. The voltammogram shown in Figure 4A shows that the reduction of H+ at negative potentials in a 1 M HClO4 solution

If the geometry of the electrode is not a perfect disk inlaid on a plane, for example, it is elliptical or somewhat recessed, then eq 1 can be generalized as p inb = GnFDC crit

where G is a geometric factor that accounts for the effect of electrode geometry on the steady-state current distribution (for a planar disk electrode, G = 4a). Taking the ratio of iOp 2 nb and iHp 2 nb eliminates this geometric factor, yielding

COcrit 2 crit CH 2

= slope ×

D H2 DO2

iOp 2 nb i Hp2 nb

=

nO2DO2COcrit2 n H2DH2C Hcrit2

,

where nO2 and nH2 are the numbers of electrons transferred corresponding to each molecule of H2 or O2 generated electrochemically (both equal to 2). This suggests that the critical nucleation concentration for one gas can be directly deduced if the critical nucleation concentration for the other gas and the ratio of the diffusion coefficients are known, regardless of the exact electrode geometry. Additionally, the uncertainty in the diffusion coefficient is also greatly reduced as the ratio of DO2 to DH2 cancels out factors of temperature and viscosity according to the Stokes−Einstein relation.22 A plot of iOp 2 nb vs iHp 2 nb for nanoelectrodes with radii between 4 and 88 nm is strongly correlated (R2 = 0.996; see Figure 4B), and from the slope of the linear fit (−0.365 ± 0.006), the critical concentration for O2 is estimated to be 0.17 ± 0.01 M based on 11 the previously reported Ccrit and DH2/DO2 = 1.85.23 H2 of 0.25 M crit This CO2 agrees well with that obtained from water oxidation (∼0.14 M)15 and corresponds to a supersaturation of ∼130. According to classical nucleation theory (CNT), nucleation and growth of a bubble requires nuclei to reach a critical radius, above which growth of the bubble is energetically favorable. Assuming equilibrium at the liquid/gas interface, the partial pressure of O2 of the critical nucleus, PO2, (the sum of the ambient pressure, Pambient(=1 atm), and the Laplace pressure, crit PLaplace) can be related to Ccrit O2 using Henry’s law (CO2 = KHPO2). From PLaplace, the radius of curvature of the bubble (r) can then 2γ be determined using the Young−Laplace equation (r = P ,

Figure 4. Generation of individual O2 and H2 nanobubbles in the same solution. (A) Voltammogram of a 17 nm radius Pt electrode in 0.6 M H2O2 and 1 M HClO4 at a scan rate of 200 mV/s. (B) Plot of ipO2 nb vs iHp 2 nb measured during the same voltammograms from different electrodes. From the slope of the linear fit (y [nA] = −0.365 × x crit [nA], R2 = 0.996), Ccrit O2 is 68% of CH2 (

(2)

Laplace

where γ is the surface tension of the gas−liquid interface).13 Using the macroscopic surface tension of 1 M HClO4 (γ = 0.071 N m−1)24 and Henry’s constant for O2 (KH = 1.3 mM atm−1)23 at room temperature, the measured critical concentration of O2 (0.17 ± 0.01 M) corresponds to a critical nucleus with a radius of curvature of 11 nm.15,25 While this radius of curvature is larger than the radius of the smallest electrode in this study (4 nm), such an electrode could support a nucleus with a sphere-cap geometry of any radius of curvature. Moreover, recession of the Pt disk electrode below the glass shroud would imply a geometric radius > 11 nm, while not affecting our conclusions regarding the radius of curvature of the bubble nucleus. The critical supersaturation leading to nucleation of O2 bubbles (∼130), as well as for other gases (∼310 for H2 and ∼160 for N2, obtained using a similar methodology),10,11,14 is much lower than CNT predicts for homogeneous nucleation (∼1400, based on γ = 0.071 N m −1 ). 26 Therefore, heterogeneous nucleation is the most likely mechanism, which reduces both the barrier and critical supersaturation for bubble nucleation. In addition, heterogeneous nucleation also explains the gas-type dependence because these bubbles are generated at different electrode potentials, corresponding to

). Error bars

indicate the standard deviation from triplicate measurements.

leads to formation of a H2 nanobubble on the negative-going scan, following the dissolution of the O2 nanobubble. The generation of single H2 nanobubbles from H+ reduction at a nanoelectrode has been reported11 and exhibits similar characteristics to the formation of O2 nanobubbles described above (a peak current for H2 nanobubble nucleation, iHp 2 nb, is observed). The cathodic current between 0.7 and −0.25 V is due to the electrochemical reduction of H2O2 with a negligible contribution from O2 reduction, as shown in detail in section 5 of the Supporting Information. Moreover, in the region of H2 formation (−0.2 V vs Ag/AgCl), the reduction of H2O2 at Pt is strongly inhibited by hydrogen adsorption,21 and the contribution of H2O2 reduction to the current in this region is also negligible and can be ignored in subsequent calculations (see Supporting Information section 6 for a detailed discussion and supporting experiments). 2452

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different chemical adsorbates at the Pt surface (Pt−O for N2 and O2 vs Pt−H for H2). However, we do not have direct evidence to rule out homogeneous nucleation because gases at the gas−liquid interface can serve as surfactant to reduce the nucleation barrier.27,28 The addition of electrolyte (from 0 to 0.2 M NaCl) or the change in the pH (from 0 to 7) has limited effect on the nucleation peak currents (relative deviation within 7%), indicating that the aqueous electrolyte composition has a near-negligible effect on Ccrit O2 (Figure S6). This suggests that none of these changes affects the energetics of the bubble nucleus. Invariance in Ccrit O2 is also predicted by CNT as none of the solution compositions noticeably change the surface tension. While the catalytic decomposition of H2O2 at Pt surfaces can also occur at open circuit to generate a background concentration of O2,29 this process appears unimportant in this study. Indeed, at anodic potentials sufficient to nucleate a bubble, O2 near the electrode surface is produced predominantly by H2O2 electrooxidation.17,30,31 Moreover, the presence of chloride, which is known to inhibit the decomposition of H2O2 at Pt,32 exhibits little effect on iOp 2 nb (Figure S6), suggesting that any background generation of O2 from H2O2 decomposition is not significant compared to the electrogenerated O2. Thus, in this work, the contribution of O2 from H2O2 decomposition can reasonably be ignored. In summary, oxidation of H2O2 at a Pt nanoelectrode can be used to generate a single O2 nanobubble and study its nucleation. A single H2 bubble can also be generated during the same voltammetric scan. Because the nucleation behavior of H2 has been previously reported extensively, its peak current serves as an internal control enabling determination of the critical nucleation concentration without the knowledge of the exact geometry of the electrode. Such an internal control also reduces the uncertainties from the effect of temperature and viscosity on the diffusion coefficient, which directly affects the accuracy of the measured critical concentration for gas nucleation. Using a H2 bubble as an internal control, the critical concentration for nucleation of an O2 nanobubble was measured as 0.17 M, which is 130 times higher than the equilibrium concentration of O2 at 1 atm. Given the size of the electrodes and the bubbles, the electrochemical studies of the nucleation of O2 nanobubbles presented here are particularly relevant to electrocatalytic processes involving Pt nanostructures.33

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hang Ren: 0000-0002-9480-8881 Henry S. White: 0000-0002-5053-0996 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Office of Naval Research (N00014-16-1-2541).

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EXPERIMENTAL METHODS Pt nanoelectrodes were fabricated and characterized according to a previously reported procedure.34 All potentials reported are referenced to a Ag/AgCl (3 M NaCl) electrode. Other experimental details are provided in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b00882. Experimental methods, voltammetry at different H2O2 concentrations, effect of scan rate on voltammetric response, voltammetric characterization of nanoelectrodes, effect of electrolyte on O2 nucleation, and further discussion of the voltammograms (PDF) 2453

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The Journal of Physical Chemistry Letters (18) Petsev, N. D.; Shell, M. S.; Leal, L. G. Dynamic Equilibrium Explanation for Nanobubbles’ Unusual Temperature and Saturation Dependence. Phys. Rev. E 2013, 88, 010402. (19) Liu, Y.; Edwards, M. A.; German, S. R.; Chen, Q.; White, H. S. The Dynamic Steady State of an Electrochemically Generated Nanobubble. Langmuir 2017, 33, 1845−1853. (20) Wakabayashi, N.; Takeichi, M.; Itagaki, M.; Uchida, H.; Watanabe, M. Temperature-Dependence of Oxygen Reduction Activity at a Platinum Electrode in an Acidic Electrolyte Solution Investigated with a Channel Flow Double Electrode. J. Electroanal. Chem. 2005, 574, 339−346. (21) Gómez-Marín, A. M.; Schouten, K. J. P.; Koper, M. T. M.; Feliu, J. M. Interaction of Hydrogen Peroxide with a Pt(111) Electrode. Electrochem. Commun. 2012, 22, 153−156. (22) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2000. (23) Tham, M. K.; Walker, R. D.; Gubbins, K. E. Diffusion of Oxygen and Hydrogen in Aqueous Potassium Hydroxide Solutions. J. Phys. Chem. 1970, 74, 1747−1751. (24) Neros, C. A.; Eversole, W. G. The Surface Tension of Aqueous Perchloric Acid at 15°, 25°, and 50°C. J. Phys. Chem. 1941, 45, 388− 395. (25) Battino, R.; Rettich, T. R.; Tominaga, T. The Solubility of Nitrogen and Air in Liquids. J. Phys. Chem. Ref. Data 1984, 13, 563− 600. (26) Blander, M.; Katz, J. L. Bubble Nucleation in Liquids. AIChE J. 1975, 21, 833−848. (27) Massoudi, R.; King, A. D. Effect of Pressure on the Surface Tension of Water. Adsorption of Low Molecular Weight Gases on Water at 25 Deg. J. Phys. Chem. 1974, 78, 2262−2266. (28) Lubetkin, S. D. Why Is It Much Easier to Nucleate Gas Bubbles Than Theory Predicts? Langmuir 2003, 19, 2575−2587. (29) Bianchi, G.; Mazza, F.; Mussini, T. Catalytic Decomposition of Acid Hydrogen Peroxide Solutions on Platinum, Iridium, Palladium and Gold Surfaces. Electrochim. Acta 1962, 7, 457−473. (30) Prabhu, V. G.; Zarapkar, L. R.; Dhaneshwar, R. G. Electrochemical Studies of Hydrogen Peroxide at a Platinum Disc Electrode. Electrochim. Acta 1981, 26, 725−729. (31) Solla-Gullón, J.; Rodríguez, P.; Herrero, E.; Aldaz, A.; Feliu, J. M. Surface Characterization of Platinum Electrodes. Phys. Chem. Chem. Phys. 2008, 10, 1359−1373. (32) Hall, S. B.; Khudaish, E. A.; Hart, A. L. Electrochemical Oxidation of Hydrogen Peroxide at Platinum Electrodes. Part V: Inhibition by Chloride. Electrochim. Acta 2000, 45, 3573−3579. (33) Guo, Y.-G.; Hu, J.-S.; Wan, L.-J. Nanostructured Materials for Electrochemical Energy Conversion and Storage Devices. Adv. Mater. 2008, 20, 2878−2887. (34) Zhang, B.; Galusha, J.; Shiozawa, P. G.; Wang, G.; Bergren, A. J.; Jones, R. M.; White, R. J.; Ervin, E. N.; Cauley, C. C.; White, H. S. Bench-Top Method for Fabricating Glass-Sealed Nanodisk Electrodes, Glass Nanopore Electrodes, and Glass Nanopore Membranes of Controlled Size. Anal. Chem. 2007, 79, 4778−4787.

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