Revelation of Multiple Underlying RuO2 Redox Processes Associated

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Revelation of Multiple Underlying RuO2 Redox Processes Associated with Pseudocapacitance and Electrocatalysis Chong-Yong Lee and Alan M. Bond* School of Chemistry, Monash University, Clayton, Victoria 3800, Australia Received June 19, 2010. Revised Manuscript Received August 20, 2010 Advances in basic knowledge relevant to the pseudocapacitive and electrocatalytic properties of RuO2 materials require a detailed understanding of the redox chemistry that occurs at the electrode interface. Although several redox processes have been identified via dc cyclic voltammograms derived from surface-confined RuO2 materials, mechanistic details remain limited because the faradaic signals of interest are heavily masked by the background current. Here, it is shown that the underlying electron transfer reactions associated with the VI to II oxidation states of surface-confined RuO2 materials in acidic medium are far more accessible in the background current free fourth and higher harmonic components available via large-amplitude Fourier transformed ac voltammetry. Enhanced resolution and sensitivity to both electron transfer and protonation processes and discrimination against solvent and background capacitance are achieved so that the Ru(V) to Ru(VI) process can be studied for the first time. Thus, kinetic and thermodynamic information relevant to each ruthenium redox level is readily deduced. The relative rate of electron transfer and the impact of protonation associated with Ru(VI) to Ru(II) redox processes are found to depend on the nature of the RuO2 materials (extent of crystallinity and hydration) and concentration of sulfuric acid electrolyte. In the electrocatalytic oxidation of glucose in alkaline medium, access to the underlying electron transfer processes allows ready detection of the redox couple associated with the catalysis. Thus, application of an advanced ac electroanalytical technique is shown to provide the methodology for enhancing our understanding of the charge transfer processes of RuO2, relevant to pseudocapacitance and electrocatalysis.

Introduction Ruthenium dioxide (RuO2) is a practically important material that possesses desirable properties such as metallic conductivity, high chemical and thermal stability, excellent catalytic activity, and extensive redox activity. In catalysis, RuO2 is employed as an active component in dimensionally stable anodes for chlorine generation in the chlor-alkali industry,1 oxygen or hydrogen evolution in water electrolysis,2 and CO oxidation in sensors.3 This metal oxide electrode also has contributed to the electrochemical energy storage technology based on the identification of “pseudocapacitance” by Trassati in the 1970s associated with the dc voltammetry of surface-confined RuO2.4,5 Now, hydrous forms of RuO2 provide a model system and benchmark for pseudocapacitive materials.5 In the context of pseudocapacitance applications, for technological reasons, most electrochemical investigations related to the use of RuO2-based materials are targeted at achieving elevated levels of specific capacitance. A major breakthrough was achieved by the introduction of hydrated ruthenium dioxide, RuO2 3 nH2O, prepared via the sol-gel route.6,7 When combined with advances in nanomaterials, this strategy led to an architecture for *To whom correspondence should be addressed. E-mail: alan.bond@sci. monash.edu.au. Telephone: þ61 3 9905 1338. Fax: þ61 3 9905 4597. (1) Trasatti, S. Electrochim. Acta 2000, 45, 2377. (2) Borgarello, E.; Kiwi, J.; Relizzetti, E.; Visca, M.; Gratzel, M. Nature 1981, 289, 158. (3) Over, H.; Kim, Y. D.; Seitsonen, A. P.; Wendt, S.; Lundgren, E.; Schmid, M.; Varga, P.; Morgante, A. G.; Ertl, G. Science 2000, 287, 1474. (4) Trasatti, S.; Buzzanca, G. J. Electroanal. Chem. 1971, 29 (Appendix 1) . (5) Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications; Kluwer: Dordrecht, The Netherlands, 1999. (6) Zheng, J. P.; Jow, T. R. J. Electrochem. Soc. 1995, 142, L6. (7) Zheng, J. P.; Cyang, P. J.; Jow, T. R. J. Electrochem. Soc. 1995, 142, 2699. (8) Sugimoto, W.; Iwata, H.; Yasunaga, Y.; Murakami, Y.; Takasu, Y. Angew. Chem., Int. Ed. 2003, 42, 4092.

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RuO2 3 nH20 nanosheets that exhibit high capacitance.8,9 The introduction of hybrid systems involving the deposition of RuO2 onto a carbon support also contributed to improved supercapacitor performance.10-12 However, despite these advances, a detailed understanding of the faradaic processes associated with pseudocapacitance remains limited, which restricts prospects for systematically planned breakthroughs. In particular, the roles of the various faradaic processes that participate in the electrocatalytic activity of RuO2 need to be better defined. To date, the primary research tool used to study faradaic activity associated with the different redox levels of ruthenium in oxidized and reduced forms of RuO2 is dc cyclic voltammetry. However, in this approach, the very large background current masks the much smaller faradaic signal that needs to be measured. Nevertheless, changes in ruthenium redox states in acidic medium, believed to vary from Ru(II) to Ru(VI), have been identified or proposed. These electron transfer processes are thought to be accompanied by adsorption of protons. Equation 1 summarizes the reactions taking place in the context of Ru(IV) to Ru(II) reduction: RuO2 þ δHþ þ δe - TRuO2- δ ðOHÞδ

ð1Þ

where 0 e δ e 2. An analogous equation summarizes the Ru(IV) to Ru(VI) redox chemistry. The continuous change in δ accompanying proton insertion and deinsertion occurs within the (9) Sugimoto, W.; Iwata, H.; Murakami, Y.; Takasu, Y. J. Electrochem. Soc. 2004, 151, A1181. (10) Miller, J. M.; Dunn, B.; Tran., T. D.; Pekala, R. W. J. Electrochem. Soc. 1997, 144, L309. (11) He, X. J.; Geng, Y. J.; Oke, S.; Higashi, K.; Yamamoto, M.; Takikawa, H. Synth. Met. 2009, 159, 7. (12) Min, M.; Machida, K.; Jang, J. H.; Naoi, K. J. Electrochem. Soc. 2006, 153, A334.

Published on Web 09/16/2010

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Figure 1. (a) dc and (b-h) FT ac cyclic voltammograms obtained from an anhydrous RuO2 thin film-modified glassy carbon electrode in contact with a 0.5 M H2SO4 aqueous electrolyte solution. (b) dc aperiodic current and (c-h) first to sixth ac harmonic components, respectively. FT-ac voltammetry experimental conditions: Estart = 1.15 V, f = 9.0 Hz, and ΔE = 0.08 V. When Eswitch = 0.00 V (black), v = 0.043 V/s, while when Eswitch = -0.10 V (red), v = 0.047 V/s. dc voltammetric parameters (Estart, Eswitch, and v) are the same as in the FT-ac case. Reversible potentials associated with each process are designated by a reference line.

potential window of ∼1.2 V available between the evolution of hydrogen and oxygen and leads to phenomena that induce capacitive behavior that follows a Frumkin-type isotherm.5 In an alkaline medium, redox transitions are reported to involve the even higher Ru(VII) oxidation state.13-15 To fully identify the essential thermodynamic and kinetic aspects of the underlying faradaic process associated with RuO2 materials, we now apply the technique of large-amplitude alternating current (ac) cyclic voltammetry.16-18 Conventionally, ac voltammetry, as is the case with the related technique of impedance spectroscopy,19,20 is utilized in the small amplitude format.21 The extension to large amplitudes enhances the nonlinearity of the faradaic response, thereby facilitating access to fourth- and higher-order harmonic components that are devoid of background current. When coupled with Fourier transform (FT) data processing,16-18,22,23 new and enhanced information associated with faradaic processes becomes available. This feature enables us to more readily interpret faradaic processes that (13) Burke, L. D.; Healy, O. J. J. Electroanal. Chem. 1980, 109, 199. (14) Burke, L. D.; Healy, J. F. J. Electroanal. Chem. 1981, 124, 327. (15) Kumar, A. S.; Zen, J.-M. J. Mol. Catal. A: Chem. 2006, 252, 63. (16) Bond, A. M.; Duffy, N. W.; Guo, S.-X.; Zhang, J.; Elton, D. M. Anal. Chem. 2005, 77, 186A. (17) Engblom, S. O.; Myland, J. C.; Oldham, K. B. J. Electroanal. Chem. 2000, 480, 120. (18) Gavaghan, D. J.; Bond, A. M. J. Electroanal. Chem. 2000, 480, 133. (19) Nahir, T. M.; Bowden, E. F. Langmuir 2002, 18, 5283. (20) Yoo, H. D.; Jang, J. H.; Ka, B. H.; Rhee, C. K.; Oh, S. M. Langmuir 2009, 25, 11947. (21) Macdonald, J. R. Impedance Spectroscopy: Emphasizing Solid Materials and Systems; Wiley-Interscience: New York, 1987. (22) Smith, D. E. Anal. Chem. 1976, 48, 221A. (23) Smith, D. E. Anal. Chem. 1976, 48, 517A.

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participate in the underlying RuO2 redox electrochemistry associated with the pseudocapacitance and electrocatalytic effects. In this study, the FT-ac voltammetric method is applied in an investigation of a surface-confined process associated with the electrochemistry of anhydrous RuO2 and hydrated RuO2 3 nH2O materials, with the latter also being examined after heat treatment. A report on the application of RuO2 material in the electrocatalytic oxidation of glucose also is provided, based on data analysis using FT-ac voltammetry. For the study described here, a working electrode configuration is employed in which RuO2 particles are immobilized onto a glassy carbon surface,24-27 which is then placed in contact with an aqueous sulfuric acid or sodium hydroxide electrolyte.

Experimental Section Chemicals. Anhydrous and hydrated ruthenium dioxide and D-(þ)-glucose

were used as supplied by the manufacturer (Aldrich). Other chemicals employed were of analytical grade and also used without further purification. Deionized water (resistivity of 18 MΩ cm) derived from a Milli-Q-MilliRho purification system was used to prepare all solutions. Apparatus and Procedures. Large-amplitude FT-ac cyclic voltammetric experiments were conducted with instrumentation (24) Schmidt, T. J.; Gasteiger, H. A.; Stab, G. D.; Urban, P. M.; Kolb, D. M.; Behm., R. J. J. Electrochem. Soc. 1998, 145, 2354. (25) Sugimoto, W.; Ohnuma, T.; Murakami, Y.; Takasu, Y. Electrochem. SolidState Lett. 2001, 4, A145. (26) Sugimoto, W.; Kizaki, T.; Yokoshima, K.; Murakami, Y.; Takasu, Y. Electrochim. Acta 2004, 49, 313. (27) Sugimoto, W.; Iwata, H.; Yokoshima, K.; Murakami, Y.; Takasu, Y. J. Phys. Chem. B. 2005, 109, 7330.

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Figure 2. (a) dc (v = 0.045 V/s) and (b and c) FT-ac fifth harmonic cyclic voltammograms obtained from an anhydrous RuO2 thin filmmodified glassy carbon electrode in contact with designated H2SO4 concentrations (0.25-3.00 M). (d) Potential and peak current dependence of processes A0 -D0 on the concentration of H2SO4. Other experimental conditions are as described in the legend of Figure 1 except in 0.25, 0.5, and 1.0 M H2SO4 (Estart = 1.20 V, and v = 0.045 V/s); 2.0 M H2SO4 (Estart = 1.25 V, and v = 0.047 V/s); and 3.0 M H2SO4 (Estart = 1.30 V, and v = 0.048 V/s). described elsewhere.16 Conventional dc cyclic voltammetric experiments were conducted with a BAS model 100B electrochemical workstation (Bioanalytical Systems). All voltammetric experiments were undertaken at 20 ( 1 °C. A standard threeelectrode cell was employed, with Ag/AgCl (3 M NaCl) as the reference electrode and a platinum wire as the auxiliary electrode. The working electrode was a BAS glassy carbon electrode (geometric area of 0.07 cm2) modified with RuO2 or RuO2 3 nH2O.24-27 For experiments in acidic media, 10 mg of RuO2 or RuO2 3 nH2O was dispersed in 5 cm3 of distilled water and subjected to ultrasonification for 30 min. Subsequently, 5 μL of the magnetically stirred RuO2 or RuO2 3 nH2O dispersion was then pipetted onto a polished glassy carbon surface. After the sample had been dried, 1 μL of a 1 wt % Nafion ionomer solution was drop-cast onto the electrode surface to immobilize the RuO2 or RuO2 3 nH2O particles. In some experiments, RuO2 3 nH2O was heat-treated in the presence of air for 5 h at 250, 300, and 500 °C prior to being immobilized on the glassy carbon electrode. For experiments in alkaline media, 5 mg of RuO2 was dispersed in 2.5 cm3 of a 1 wt % Nafion ionomer solution and subjected to ultrasonication until homogeneous dispersion was obtained. Five microliters of the nafion/RuO2 dispersion was then pipetted onto a polished glassy carbon surface. This approach allowed a larger amount of RuO2 to be confined on the glassy carbon surface. X-ray powder diffraction data were obtained over 2θ ranges from 20° to 80° using a Phillips 1729 X-ray diffractometer (XRD) with Cu KR radiation (λ = 1.54056 A˚).

Results and Discussion Anhydrous RuO2 in Acidic Media. Figure 1a provides examples of dc cyclic voltammograms obtained from an anhydrous RuO2/glassy carbon electrode configuration in contact with an aqueous 0.5 M H2SO4 electrolyte solution (Figure S1 of the Supporting Information provides details of the scan rate dependence). The potentials given in the text and figures are referenced to Ag/AgCl (3 M NaCl). The dc voltammograms exhibit a typical “rectangular” background double-layer charging Langmuir 2010, 26(20), 16155–16162

current on which are superimposed faradaic currents much smaller in magnitude. The barely detectable peak currents at ∼0.32 and ∼0.52 V, designated A0 and B0 , respectively, have been assigned to Ru(III) to Ru(II) and Ru(IV) to Ru(III) reduction processes,1 respectively, while the even weaker peak at ∼1.0 V, designated C0 , is commonly assigned to a Ru(IV) to Ru(VI) oxidation process.1 The Ru(V) to Ru(VI) process (D0 ), identified later under ac conditions, is not resolved from the background current under the conditions of Figure 1a. Panels b-h of Figure 1 provide examples of the ac harmonic components available from large-amplitude FT-ac cyclic voltammetry. The data processing strategy employed to access the higher harmonics is illustrated schematically in Figure S2 of the Supporting Information. In the FT-ac voltammetric method, a sinusoidal waveform of known amplitude (ΔE) and frequency (f) is superimposed onto the dc ramp. In the experiment displayed in Figure 1, a sinusoidal waveform with a ΔE of 80 mV and f of 9.00 Hz was superimposed onto a dc ramp having a scan rate of 43.00 mV/s. The amplitude of 80 mV introduces significant nonlinearity and enables the sixth ac harmonic to be detected, while use of a very low frequency of 9.00 Hz ensures the voltammetric response is close to reversible. Figure 1b, the aperiodic dc component, resembles the dc cyclic voltammogram presented in Figure 1a but contains a small contribution from the presence of the sinusoidal modulation. Panels c-h of Figure 1 represent the fundamental to sixth ac harmonic components derived from FT analysis, respectively. They are presented for convenience in the envelope, rather than other formats.15 The essentially scan rate-independent behavior for the harmonics is shown in Figure S3 of the Supporting Information. The fundamental harmonic (Figure 1c) component contains a significant ac background current that is derived from the double-layer capacitance associated with the RuO2-modified electrode-electrolyte interface. Background ac current is still evident in the second and third harmonics (Figure 1d,e), with DOI: 10.1021/la102495t

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p p p p Table 1. I4ωt /I6ωt :I5ωt /I6ωt Ratios for Redox Centers A0 [Ru(III)/Ru(II)], B0 [Ru(IV)/Ru(III)], C0 [Ru(IV)/Ru(V)], and D0 [Ru(V)/Ru(VI)] Obtained from an Anhydrous RuO2 and Hydrated RuO2 3 nH2O (500 °C) Thin Film-Modified Glassy Carbon Electrode as a Function of H2SO4 Concentration (from 0.5 to 3.0 M)a

sample

[H2SO4] (M)

redox center A0 [Ru(III)/Ru(II)]

redox center B0 [Ru(IV)/Ru(III)]

redox center C0 [Ru(IV)/Ru(V)]

redox center D0 [Ru(V)/Ru(VI)]

0.50 19:4.4b 3.6:2.0 8.7:2.7 4.6:1.8 1.00 22:5.9 3.7:2.2 8.1:2.5 3.2:1.5 2.00 17:4.4 3.4:2.0 8.8:2.9 3.2:1.6 3.00 19:5.1 3.4:2.1 9.6:3.2 3.0:1.6 0.50 11:3.4 4.4:2.3 6.1:2.3 5.6:2.5 RuO2 3 nH2O (500 °C) 1.00 10:3.1 5.3:2.0 7.5:2.3 5.9:2.3 2.00 10:2.7 4.8:2.2 7.5:2.7 5.4:2.4 3.00 8.4:2.5 4.8:2.3 8.3:3.1 6.0:2.2 p p a p I4ωt, I5ωt , and I6ωt represent the peak current magnitudes for fourth, fifth, and sixth ac harmonic components, respectively. b The individual current p p p , I5ωt , and I6ωt used to determine the ratios are available in Table S1 of the Supporting Information. values for I4ωt

RuO2

only fourth and higher ac harmonics (Figures 1f-h) providing close to purely faradaic current responses. Thus, details of the charge transfer processes associated with the reduction (processes A0 and B0 ) and oxidation (processes C0 and D0 ) of RuO2 are best elucidated from analysis of the fourth- and higher-order ac harmonic components. The faradaic currents associated with processes A0 -D0 arise from coupled electron transfer and proton transfer. The higherorder ac harmonics exhibit high sensitivity to only processes with fast rates of electron transfer.28 The rapid decrease in the current associated with redox center A0 [Ru(III) to Ru(II) process] when progressing from the fourth to the sixth harmonics implies that this couple exhibits a relatively slow rate of electron transfer, although gating by proton coupling also could contribute to the apparently low level of reversibility. Redox center B0 [Ru(IV) to Ru(III) process] exhibits an exceptionally well-defined sixth harmonic component and has a shape close to that predicted for a fully reversible surface-confined one-electron charge transfer process with proton coupling being diffusion controlled. Redox centers C0 and D0 become progressively better resolved as the order of the harmonic becomes higher. These processes are assumed to be derived from one-electron charge transfer processes and hence are assigned to Ru(IV) to Ru(V) and Ru(V) to Ru(VI) oxidation-based transitions, respectively. The detection of process D0 , not seen under dc voltammetric conditions, is facilitated by the ability of the ac method to achieve significant discrimination against the irreversible overlapping solvent limiting-associated process that occurs at very positive potentials. When the potential is switched to a less positive value (red curves in Figure 1), the least reversible A0 [Ru(III)/Ru(II)] redox center exhibited a notable dependence on scan direction. In contrast, only a negligible influence on scan direction was evident for redox center B0 [Ru(IV)/Ru(III)] or centers C0 [Ru(IV)/Ru(V)] and D0 [Ru(V)/Ru(VI)]. The dependence of the potentials for processes A0 -D0 (see reference lines for definitions in Figure 1) derived from ac higher harmonics on H2SO4 concentration over the range of 0.25-3.0 M (Figure 2b,c) is consistent with proton transfer being coupled to electron transfer (eq 1) at each oxidation level. That is, peak potentials in dc (Figure 2a) and equivalent parameters in ac higher harmonics shift to more positive values as the H2SO4 concentration is increased. The dc faradaic current and double-layer capacitance also depend on H2SO4 concentration, but the background current always remains dominant in dc cyclic voltammograms. The fifth harmonic ac component (Figure 2b,c) also reveals an increase in the faradaic current magnitude as the concentration of H2SO4 becomes higher, particularly with respect (28) Lee, C. Y.; Bond, A. M. Anal. Chem. 2009, 81, 584.

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to processes C0 and D0 . These two processes also exhibit the strongest potential dependence with respect to H2SO4 concentration. Figure 2d summarizes the details of the effects of H2SO4 concentration on potential and current magnitude. Details of the method of measurement of peak potential and current magnitude data are given in Figure S4 of the Supporting Information. Information about the relative rates of electron transfer can be p p p p /I6ωt :I5ωt /I6ωt ratios (Table 1). For a deduced from the I4ωt reversible process, the ratio calculated from the simulations15 is p p p p /I6ωt and I5ωt /I6ωt indicate slower 3.47:2.15. Larger values of I4ωt electron transfer rates. On this basis, redox centers B0 and D0 are close to reversible and the order of the rates of electron transfer rate is as follows: B0 ≈ D0 >C0 >A0 . Hydrous RuO2 and Its Heat-Treated Form in Acidic Media. Amorphous hydrous RuO2 3 nH2O is a poorer electronic conductor but a better ionic conductor than crystalline anhydrous RuO2. The charge storage mechanism for reduction of RuO2 3 nH2O has been attributed to the reaction5-7 RuOx ðOHÞy þ δHþ þ δe - TRuOx - δ ðOHÞy þ δ

ð2Þ

As reflected in dc cyclic voltammograms derived from a RuO2 3 nH2O-modified glassy carbon electrode (Figure 3a), a much slower electron transfer rate is evident than with anhydrous RuO2 because the peak-to-peak separation, ΔEp, become larger as the scan rate is increased. In contrast, ΔEp is almost independent of scan rate for anhydrous RuO2. Also noteworthy is the fact that the dc and FT-ac fundamental harmonics (Figure 3b1) exhibit a much larger background current, which is consistent with enhanced double-layer capacitance. Because the electron transfer rate is slower, well-defined faradaic current is now only detectable up to the fourth harmonic (Figure 3), but not for the fifth- or higher-order harmonic, as found for the faster electron transfer situation prevailing in the case of the anhydrous RuO2modified electrode. Slow electron transfer in the case of RuO2 3 nH2O is also indicated by the nonoverlapping of curves in the oxidative and reductive scan directions, and the rapid decay in the relative magnitudes of the higher-order harmonic currents. In another series of experiments, amorphous hydrous RuO2 3 nH2O samples were heat-treated at 250, 300, and 500 °C. Figure 4 shows the intensity of the powder X-ray diffraction patterns increases when hydrated RuO2 3 nH2O samples are heat-treated. The peaks also become sharper, which is indicative of an increase in the sample crystallinity accompanying the removal of water. The (110), (101), and (211) planes are the most intense. Therefore, as expected, heating decreases the water content of the RuO2 3 nH2O sample and improves the crystallinity. dc cyclic voltammograms derived from heated samples (Figure 5a and Figure S5d of the Supporting Information) in contact with 0.5 M H2SO4 now Langmuir 2010, 26(20), 16155–16162

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Figure 3. (a) dc and (b) FT-ac fundamental, second, third, and fourth harmonic cyclic voltammograms obtained from a RuO2 3 nH2O thin film-modified glassy carbon electrode in contact with a 0.5 M H2SO4 aqueous electrolyte solution. For FT-ac experiments, Estart = 1.10 V, Eswitch = -0.10 V, f = 9.0 Hz, ΔE = 0.08 V, and v = 0.045 V/s. In the dc case, the scan rate ranges from 0.010 to 0.500 V/s and Estart and Eswitch are the same as for the FT-ac case.

Figure 4. XRD powder patterns for (a) amorphous RuO2 3 nH2O and (e) anhydrous RuO2. (b-d) Data for the RuO2 3 nH2O samples annealed in air for 5 h at (b) 250, (c) 300, and (d) 500 °C.

regain their “rectangularly” shaped background and also exhibit substantially lower double-layer capacitance. ac harmonic data (Figure S5b of the Supporting Information) indicate heat treatment at g300 °C is required to achieve well-defined fifth harmonic faradaic processes. Figure 5a shows the dc cylic voltammograms of the amorphous hydrous RuO2 3 nH2O sample after it had been heat-treated at 500 °C. The presence of close to reversible type Ru(IV) to Ru(V) and Ru(III) to Ru(II) processes akin to that associated with crystalline anhydrous RuO2 is now evident, but the Ru(IV) to Ru(III) process is barely detected above background current at ∼0.53 V; the Ru(V) to Ru(VI) process is not detectable. In contrast, ac higher harmonic components (Figure 5b,c) now provide well-defined Ru(IV) to Ru(III), and also Ru(V) to Ru(VI) processes. Figure 5d summarizes the peak current and potential dependence on the H2SO4 concentration (see Figure S6 of the Supporting Information for details of the ac harmonic responses). Clearly, peak current magnitudes derived Langmuir 2010, 26(20), 16155–16162

from all the redox centers after heat treatment are significantly larger than found with anhydrous RuO2. Ru(IV)/Ru(V) and Ru(V)/Ru(VI) redox centers show the strongest protonation effect, with Ru(IV)/Ru(III) and Ru(V)/Ru(VI) centers again exhibiting faster rates than the Ru(III)/Ru(II) and Ru(IV)/Ru(V) centers (see Table 1). Electrocatalytic Application of Anhydrous RuO2 in Alkaline Media. RuO2-modified electrodes give rise to electrocatalytic oxidation of several organic compounds such as benzaldehyde, benzyl alcohol, and glucose.14,29,30 Here, we provide an example of electrocatalytic oxidation of glucose. Experiments were performed in alkaline media.29 Figure 6a provides conventional dc cyclic voltammograms derived from an anhydrous RuO2-modified electrode in contact with 0.5 M NaOH in the presence and (29) Dharuman, V.; Pillai, K. C. J. Solid State Electrochem. 2006, 10, 967. (30) Lyons, M. E. G.; Fitzgerald, C. A.; Smyth, M. R. Analyst 1994, 119, 855.

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Figure 5. (a) dc and (b and c) FT-ac fourth and fifth harmonic cyclic voltammograms obtained from hydrated RuO2 3 nH2O heat-treated at 500 °C before being immobilized on the glassy carbon electrode in contact with a 0.5 M H2SO4 aqueous electrolyte solution. FT-ac voltammetry experimental conditions: Estart = 1.20 V, Eswitch = 0.00 V, v = 0.045 V/s, f = 9.0 Hz, and ΔE = 0.08 V. DC voltammetric parameters (Estart, Eswitch, and v) are the same as the FT-ac case. (d) Potential and peak current dependence of processes A0 -D0 on the concentration of H2SO4 (0.5-3.0 M).

absence of 15 mM glucose. In a pure (glucose-free) 0.5 M NaOH electrolyte, a well-defined redox couple is observed at ∼0.4 V, which is consistent with the literature report14,29 of a Ru(VI) to Ru(VII) highly reversible one-electron oxidation process (designated process H0 ) in this alkaline medium. Another three barely detectable peaks are designated processes E0 -G0 . In alkaline media, it has been reported13,14 that processes associated with oxidation state III to oxidation state VII transformation arise from the reactions given in eqs 4-6. We detected another process at approximately -0.7 V that is assigned as in eq 3. RuIII 2 O3 þ H2 O þ e - T 2RuII O þ 2OH -

ð3Þ

2RuIV O2 þ H2 O þ 2e - T RuIII 2 O3 þ 2OH -

ð4Þ

RuIV O2 þ 4OH - T RuVI O4 2 - þ 2H2 O þ 2e -

ð5Þ

RuVI O4 2 - T RuVII O4 - þ e -

ð6Þ

Thus, eq 3 describes the origin of process E0 [Ru(III) to Ru(II)] and eq 4 that of process F0 [Ru(IV) to Ru(III)]; eq 5 represents process G0 [Ru(IV) to Ru(VI)], and eq 6 refers to the major process H0 [Ru(VI) to Ru(VII)]. The potential regions for found processes F0 -H0 of approximately -0.4, ∼0.0, and ∼0.4 V (vs Ag/ AgCl) are in excellent agreement with the literature values.29 The introduction of 15 mM glucose into the 0.5 M NaOH solution produces a sigmoidal dc wave shape expected for a catalytic reaction, at potentials around ∼0.4 V14 where process H0 was found to give the following overall catalytic reaction scheme. 2RuVII O4 - þ glucose f 2RuVI O4 2 - þ gluconolactone 16160 DOI: 10.1021/la102495t

ð7Þ

Figure 6. dc voltammograms obtained at a v of 0.200 V/s from an anhydrous RuO2-modified glassy carbon electrode in contact with 0.5 M NaOH in the absence (black) and presence (red) of 15 mM glucose. Reversible potentials associated with each process are designated by a reference line.

The ac method (Figure 7) provides a clearer illustration of the underlying faradaic processes. In the fundamental harmonic, the voltammograms are again dominated by the background current. At the second and third harmonics, the background current is minimal. However, the purely faradaic processes are best revealed with ac fourth and higher harmonics. Across all the ac harmonic components, a well-defined ac voltammogram is observed for a highly reversible RuVIO42- to RuVIIO4- one-electron transfer process (H0 ) at ∼0.4 V. Processes E0 -G0 exhibit much smaller ac currents and are slower with respect to electron transfer rate. Interestingly, a new process (I0 ) is now observed at ∼0.55 V in the higher ac harmonics that lies beyond the solvent limit region in the Langmuir 2010, 26(20), 16155–16162

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Figure 7. FT-ac (a)-(h) first to eight harmonic voltammograms obtained from an anhydrous RuO2-modified glassy carbon electrode in contact with 0.5 M NaOH in the absence and presence of 15 mM glucose. FT-ac experimental conditions: Estart = -0.90 V, Eswitch = 0.80 V, v = 0.073 V/s, f = 9.02 Hz, and ΔE = 0.08 V. Reversible potentials associated with each process are designated by a reference line.

dc experiment. In alkaline medium, it is tempting to suggest that this process is associated with the RuVIIO42-/RuVIIIO4 redox couple. However, ruthenium tetraoxide, RuO4, is reduced to ruthenium perruthenate in alkaline media according to the reaction given in eq 8.31 Furthermore, RuO4 is soluble in water,32 but no dissolution is detected. Thus, we propose process I0 may be due to the one-electron electrochemical oxidation of RuVIIO4to RuVIIIO4 (eq 9), followed by reaction 8, to give a catalytic process. 4RuVIII O4 þ 4OH - f 4RuVII O4 - þ 2H2 O þ O2

ð8Þ

RuVII O4 - TRuVIII O4 þ e -

ð9Þ

Processes E0 -G0 exhibit far smaller ac currents at approximately -0.7, -0.4, and 0.0 V than do processes H0 and I0 found at approximately 0.4 and 0.55 V, respectively, which indicates they all exhibit significantly slower overall rates. This is in agreement with eqs 3-5, which involve a complicated chemistry accompanying electron transfer. The influence of the catalytic reaction on each redox process can also be identified in ac methods.33 In ac voltammetry, catalysis may lead to a decrease in peak current, rather than the increase found in dc methods. The strongest influence is on the process at ∼0.55 V (I0 ), with respect to peak current magnitude. Processes E0 -G0 are unaffected. Process H0 also is weakened. These new data therefore suggest the oxidation of glucose may be mediated by both the RuVIO42-/RuVIIO4- and RuVIIO4-/RuVIIIO4 processes. We propose the following overall equation for the catalytic process involving electrochemically generated ruthenium perruthenate: 2RuVIII O4 þ glucose f 2RuVII O4 - þ gluconolactone

ð10Þ

(31) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; John Wiley & Sons: New York, 1988. (32) Connick, R. E.; Hurley, C. R. J. Am. Chem. Soc. 1952, 74, 5012. (33) Zhang, J.; Bond, A. M. J. Electroanal. Chem. 2007, 23, 34.

Langmuir 2010, 26(20), 16155–16162

Conclusions This study demonstrates that unprecedented accessibility to the underlying faradaic processes associated with RuO2 materials is achieved via higher-order harmonic large-amplitude FT-ac voltammetry. When an anhydrous RuO2-modified electrode is in contact with an acidic electrolyte, the Ru(V) to Ru(VI) transition in the ac voltammetry is now well-resolved from the background oxidation process. Enhanced resolution and sensitivity to both electron transfer and protonation reactions now enable a detailed evaluation to be provided for faradaic processes associated with each specific redox center. In view of their faster electron transfer and/or coupled protonation rates, the Ru(IV) to Ru(III) and Ru(V) to Ru(VI) processes have been identified as primary charge storage centers. The less crystalline and less electronically conductive hydrous form of RuO2 exhibits less well-defined ac faradaic processes than the anhydrous form of RuO2. At present, the hydrous form of RuO2 is commonly employed as a material for electrochemical supercapacitors, while the anhydrous form is employed in electrocatalytic applications. However, it has been proposed34 that the next generation of pseudocapacitive materials may require a smoother pathway of both electrons and electrolyte protons to achieve extremely rapid charge-discharge processes. The higher-harmonic approach should be an excellent tool for monitoring both the electron transfer and protonation processes of any new materials, relative to the use of the background dominated dc technique described here. We anticipate that this powerful electroanalytical tool could be routinely applied to probe the efficacy of a wide range of pseudocapacitive materials and hence facilitate a more systematic approach to the design of new materials capable of achieving higher energy and power density. In alkaline medium, as expected, the Ru(VI) to Ru(VII) process is highly reversible. However, discrimination against the irreversible solvent limiting current allows detection of an additional of Ru(VII) to Ru(VIII) (34) Chang, K.-H.; Hu, C.-C.; Chou, C.-Y. Chem. Mater. 2007, 19, 2112.

DOI: 10.1021/la102495t

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faradaic processes that cannot be observed via dc methods. Even the underlying electron transfer processes present under catalysis conditions remain readily detected by the ac approach. To fully quantify the reaction mechanisms, sophisticated simulations that included the coupling of proton and/or electron transfer are needed to model the inherently complex Ru-based faradaic processes. Essentially, it has now been shown that access to detailed information relevant to underlying redox processes provides a critical preliminary step toward the goal of unlocking the mechanistic insights needed to more fully understand the relationship between the redox chemistry and pseudocapacitance or electrocatalytic properties. Acknowledgment. The financial support of the Australian Research Council (ARC) and the award of Monash University postgraduate scholarships (MIPRS, MGS, and PPA) to C.-Y.L. are gratefully acknowledged. We thank Rod Mackie for the assistance in the XRD analysis. Access to sample heating facilities available in the ARC Centre of Excellence for Electromaterials Science also is gratefully acknowledged.

16162 DOI: 10.1021/la102495t

Lee and Bond

Supporting Information Available: Figure S1 illustrates the scan rate dependence of dc cyclic voltammograms and the calculated differential capacitance of an anhydrous RuO2 thin film, while Figure S3 presents this dependence based on a FTac voltammetric approach. Figure S2 provides details of the data processing strategy used in large-amplitude FT-ac voltammetry. The methods used to identify analogues of dc voltammetric midpoint potentials and ac harmonic peak current magnitudes are shown in Figure S4. Figure S5 shows dc and fifth harmonic FT-ac cyclic voltammograms obtained from a RuO2 3 nH2O (heat-treated at 250 or 300 °C) thin filmmodified glassy carbon electrode. The details of H2SO4 concentrations of ac and fifth harmonic FT-ac cyclic voltammograms obtained from a hydrated RuO2 3 nH2O (500 °C) thin film-modified glassy carbon electrode are shown in Figure S6. Table S1 provides experimental values of Ip4ωt, Ip5ωt, and Ip6ωt and their corresponding peak potentials for redox centers A0 -D0 as a function of H2SO4 concentration derived from anhydrous RuO2 and hydrated RuO2 3 nH2O (500 °C) thin film-modified glassy carbon electrodes. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(20), 16155–16162