Investigation of Electrochemical Behavior of ...

Journal of The Electrochemical Society, 152 共7兲 J85-J92 共2005兲

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0013-4651/2005/152共7兲/J85/8/$7.00 © The Electrochemical Society, Inc.

Investigation of Electrochemical Behavior of Stimulation/ Sensing Materials for Pacemaker Electrode Applications II. Conducting Oxide Electrodes A. Norlin,a,b,* J. Pan,a,**,z and C. Leygraf a,** a

Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden b St. Jude Medical AB, SE-175 84 Järfälla, Sweden The electrochemical behavior, interfacial properties, and stability of RuO2-, IrO2-, 共Ru1−xMnx兲O2- and 共Ir1−xMnx兲O2-coated electrodes for pacemaker applications were investigated in a phosphate buffered saline solution, by electrochemical impedance spectroscopy and cyclic voltammetry 共CV兲. The psuedocapacitive properties of these conducting oxides and influence of coating roughness and porosity were examined, and accelerated aging of the electrode materials was simulated by fast sweep rate CV cycles between −3 to 1 V vs. Ag/AgCl. Changes in surface composition and structure due to the accelerated aging were investigated using X-ray photoelectron spectroscopy and scanning electron microscopy. The conducting oxides exhibit high interfacial capacitance. At high sweep rates, not all of total capacitance could be utilized due to voltage drop associated with resistance down the pores. Above a certain sweep rate, the charging/discharging mechanism changes from capacitive to resistive character. Showing the best performance among the investigated materials, the RuO2 exhibits capacitive characteristics at sweep rates up to 20 V/s and excellent stability under the accelerated aging. The IrO2 coating was not stable during the cycling. The mixed oxides experience limitations at high sweep rates due to the ohmic effects and some degradation due to the accelerated aging. © 2005 The Electrochemical Society. 关DOI: 10.1149/1.1933372兴 All rights reserved. Manuscript submitted December 7, 2004; revised manuscript received February 10, 2005. Available electronically June 17, 2005.

A more general introduction of stimulation/sensing electrodes has been described previously.1 Pacemaker electrodes transfer electric stimulation pulses from the pacemaker to the heart. The electrical current is converted to an ionic current through electrochemical processes taking place at the electrode/electrolyte interface. The transfer of electrical stimulation current is usually accomplished by three different mechanisms: 共i兲 nonfaradaic charging/discharging of the electrochemical double layer, 共ii兲 reversible, and 共iii兲 irreversible faradaic reactions. The first two mechanisms are considered to be safe, as they do not generate by-products. Charge transfer through irreversible faradaic reactions is undesirable, but as the pacing process involves high overpotentials, by-product formation cannot be completely ruled out. For stimulation purposes it is important to determine the safe charge injection capability, which is dependent on the capacitive properties of the electrode material and the reversible reactions taking place on the electrode surface. Conducting oxides are of interest for stimulation/sensing electrode application because they exhibit psuedocapacitive properties, which may contribute to the safe charge-transfer capability. RuO2 and IrO2 are used for many different applications, e.g., in electrochromic devices and as catalysts in alkali/chlorine processes and fuel cells, and their electrochemical properties are well documented.2-5 More recently, RuO2 has gained interest as a material for super capacitor applications,6-8 where its ability to act as a proton condenser is favorable for storing charge in the material. RuOx共OH兲y surface sites are reversiblely oxidized and reduced with the participation of a proton injection/ejection process,9,10 generally described by RuOx共OH兲y + ␦H+ + ␦e− RuO共x−␦兲共OH兲共y+␦兲

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The cost of electrochemical capacitors can been reduced by mixing the expensive RuO2 with cheaper oxides, such as MnO2, without decreasing the charge storage capacity significantly.11 IrO2 is currently used in some pacemaker electrodes and neural stimulation electrodes.12-16 Like RuO2, IrO2 also transfers the stimulation pulse by reversible faradaic reactions according to IrOx共OH兲y + ␦H+ + ␦e− IrO共x−␦兲共OH兲共y+␦兲

* Electrochemical Society Student Member. ** Electrochemical Society Active Member. z

E-mail: [email protected]

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The charge storage capacity of RuO2 and IrO2 is dependent on the amount of nonstoichiometric hydrous and oxy-hydroxide species in the oxide.9,17-21 The safe charge delivery capability of pacemaker electrodes also depends on the applied pulse polarity, amplitude and shape as well as the electrolyte. Pacemaker electrodes made of IrO2, produced by potential cycling, were reported to have a cathodic maximum safe charge delivery of 3-4 mC/cm2 共0.1 ␮s pulses16 and 0.2 ms pulses22-25兲, whereas the anodic safe charge delivery is much higher, around 35 mC/cm2 for 0.1 ms pulses. Combining electrochemical impedance spectroscopy 共EIS兲, cyclic voltammetry 共CV兲, and surface analysis, we have investigated electrochemical processes taking place on different kinds of electrodes, and degradation of the electrode materials due to the accelerated aging. The results for Pt, Ti, and TiN coated electrodes were reported in a preceding paper.1 This paper presents the electrochemical properties and stability of micrometer-thick conducting oxides RuO2, IrO2, 共Ru1−xMnx兲O2 and 共Ir1−xMnx兲O2 on Ti substrate exposed in a phosphate buffered saline solution. Experimental Electrode materials.— The RuO2 and IrO2 samples 共commercially available兲 were supplied by Heraeus. The oxides were deposited by PVD on pure Ti substrates. The oxide thickness was 3.5 ␮m for the RuO2 and 3.0 ␮m for the IrO2. Examples of scanning electron microscopy 共SEM兲 pictures of the surfaces are shown in Fig. 1a, 1b, 2a, and 2b, respectively. The 共Ru1−xMnx兲O2 and 共Ir1−xMnx兲O2 samples 共films on Ti substrate兲 were provided by St. Jude Medical, Inc., Sunnyvale, CA. The oxide thickness was 10 ␮m. SEM pictures of the surfaces are shown in Fig. 3a, 3b, 4a, and 4b. Electrolyte and electrochemical cell.— A phosphate-bufferedsaline solution, pH 7.4, was used as electrolyte for all electrochemical measurements. The solution has similar ion strength as blood, and a phosphate buffer to maintain the pH. A standard threeelectrode cell 共EG&G PARC flat cell兲 was used, with an Ag/AgCl 共saturated KCl兲 reference electrode, a Pt mesh as counter electrode and a Luggin capillary, to minimize the IR-drop between the reference and the working electrodes. The cell was equipped with a stirrer, set at 750 rpm during the experiments. All measurements were performed at room temperature. Instruments and measurements.— EIS measurements of the materials in the solution were performed to characterize the electrode/

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Figure 1. SEM photographs of the RuO2 electrode: 共a兲 surface view, 共b兲 high magnification, and 共c兲 after 1000 CV cycles at 1.7 V/s.

Figure 2. SEM photographs of the IrO2 electrode: 共a兲 surface view, 共b兲 high magnification, and 共c兲 after 1000 cycles at 1.7 V/s.

electrolyte interfaces. EIS spectra were also obtained for the materials after simulated aging 共see the following兲. All EIS spectra were acquired using an electrochemical interface 共Solartron 1287兲 and a frequency response analyzer 共Solartron 1250兲, controlled by a computer with ZPlot software 共Scribner Associates, Inc.兲. The measurements were performed at the open circuit potential 共OCP兲 over a frequency range from 1 ⫻ 104 to 1 ⫻ 10−3 Hz. The ac perturbation amplitude was 10 mV.

CV was used to study the electrochemical processes that may occur on the electrode surface. The measurements were performed by using either a Solartron 1287, controlled by a computer with CorrWare software 共Scribner Associates, Inc.兲, or an EG&G 273A, controlled by PowerCV software. The CV cycling was performed between −3 and 1 V vs. Ag/AgCl 共saturated KCl兲, the potential range used in electrostimulation. The cycling was performed with a



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Figure 3. SEM photographs of the 共Ru1−xMnx兲O2 electrode: 共a兲 surface view, 共b兲 high magnification, and 共c兲 after 1000 cycles at 1.7 V/s.

Figure 4. SEM photographs of the 共Ir1−xMnx兲O2 electrode: 共a兲 surface view, 共b兲 high magnification, and 共c兲 after 1000 cycles at 1.7 V/s.

sweep rate varying from 1 mV/s to 20 V/s 共upper limit of the instrument/software systems兲. The simulated ageing 共CV cycling兲 was performed with a sweep rate of 170, 1700, and 8500 mV/s, for 100, 1000, and 5000 cycles, respectively, to simulate aging of the electrode materials. The number of cycles was chosen so that the theoretical charge passing the electrode would be similar for different sweep rates. Roughly, based on the amount of charge passing the electrode calculated from the

CV curves, the simulated aging corresponds to 4 months of normal operation of the stimulation electrode. EIS was used to measure the aging of the material as follows: 共1兲 OCP for 1 h; 共2兲 EIS before CV; 共3兲 CV cycles at 170, 1700, or 8500 mV/s, after 100, 1000, and 5000 cycles, respectively; and 共4兲 EIS after the CV cycles. Surface analysis of the surface structure of the electrode materials and their degradation was performed using SEM before and after the accelerated aging. Following usual procedures, selected samples


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Journal of The Electrochemical Society, 152 共7兲 J85-J92 共2005兲 EIS characterization of the electrode/electrolyte interfaces.— Figure 5 shows the spectra in Bode format for the mixed oxides. The spectra exhibit essentially one time constant behavior, and the interfacial processes of the mixed oxides can be represented by the simplest equivalent circuit shown in Fig. 5, where Re is the electrolyte resistance, R p is the polarization resistance, and CPE is the constant phase element, which is used instead of a capacitance to account for the nonideal capacitive response. The CPE impedance is represented by ZCPE =

Figure 5. Bode plots of the 共Ru1−xMnx兲O2 and 共Ir1−xMnx兲O2 electrodes.

were analyzed by X-ray photoelectron spectroscopy 共XPS兲 before and after CV cycling at 1700 mV/s for 1000 cycles to obtain surface composition change due to the aging.

Results and Discussion Surface morphology and composition.— The RuO2 coating covers the substrate completely 共Fig. 1a兲, and the rough surface structure at high magnification 共Fig. 1b兲 indicates an enlarged surface area. The IrO2 coating 共from two production batches兲 has numerous pinholes and inhomogenities 共Fig. 2a兲. EDS analysis of the areas inside the pinhole showed large quantities of titanium originating from the substrate. At high magnifications the IrO2 shows a so-called fractal structure 共Fig. 2b兲, previously reported by several authors.26,27 The mixed oxide coatings cover the substrate completely as shown in Fig. 3a and 4a for 共Ru1−xMnx兲O2 and 共Ir1−xMnx兲O2, respectively. Fig. 3b and 4b show the surface features at high magnifications. The XPS spectra of 共Ru1−xMnx兲O2 and 共Ir1−xMnx兲O2 reveal all expected major peaks of Ru, Ir, Mn, and O, respectively.28 Judging from the peak areas, Mn accounts for about 10 at. % of the surface composition of the 共Ru1−xMnx兲O2, whereas it is up to 30 at. % Mn on the surface of 共Ir1−xMnx兲O2. The Ru peaks, shifted toward higher binding energies relative to the metallic Ru, indicate presence of higher oxidation states, such as RuO2 and Clbounded Ru compounds.29,30 The Ru content is around 14 at. % on the 共Ru1−xMnx兲O2, and the Ir content is around 8 at. % on the 共Ir1−xMnx兲O2. The double O peaks can be attributed to the oxide of Ru, Ir, or Mn, and some hydroxide or oxyhydroxides in the surface layer.8 The amount of hydroxide and oxyhydroxides is comparable to the amount of oxide, indicating a certain degree of hydration on the surface. Both 共Ir1−xMnx兲O2 and 共Ru1−xMnx兲O2 have detectable amounts of Cl-containing contamination, which is probably a residue from the production process. On the RuO2, three O peaks correspond to RuO2, RuO3, and hydroxides.28 The areas of the peaks indicate a high level of oxygen with a large amount of hydroxides in the surface layer of the oxide. For the IrO2, the content of hydroxides is lower compared to that on the RuO2.

1 Q共i␻兲␩

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where i is the imaginary number, ␻ the angular frequency, Q is a constant, and ␩ is a mathematic expression 共0 艋 ␩ 艋 1兲. For an ideal capacitor, ␩ = 1 and Q is the capacitance. The origin of CPE may be due to geometric factors such as the roughness and porosity of the electrode, or surface processes such as adsorption, surface reconstruction and diffusion.31-33 The value of CPE can be used to estimate the total interfacial capacitance of the electrode, which includes contributions from both double layer capacitance, and psuedocapacitance of the redox reaction 共caused by the ac perturbation兲. In this case, the redox reactions taking place on the conducting oxide electrodes also involve in the interfacial processes, and the double-layer capacitance and psuedocapacitance cannot be separated in the measurements. Here the R p represents an overall resistance in which the redox charge transfer reaction at the electrode-electrolyte interface is involved. The impedance spectra of the RuO2 and IrO2 electrodes are shown in Fig. 6a. Compared to the mixed oxides, these electrodes exhibit more complicated features in their impedance spectra, as evident from the rise in phase angle around 1 Hz for the IrO2 and around 0.01 Hz for the RuO2. For these oxides, the differentiation between fast and slow charging/discharging processes is important in order to determine the charge storage characteristics of the material. At high frequencies only the easily available redox sites, predominately located at the surface and grain boundaries of the oxide film, take part in the proton exchange reactions with the solution.9,10 At lower frequencies, charge carriers in the bulk oxide begin to contribute to the measured impedance,10,17-19 evidenced by the increasing capacitive impedance at decreasing frequency, Fig. 6b. The proton diffusion is probably dominating in the low frequency response.17 In this case, the simplest equivalent circuit 共Fig. 5兲 no longer can be used for the spectra fitting, and it is necessary to include some additional element in the equivalent circuit to account for the mass transport of the protons inside the bulk oxide.17 Satisfactory fit was obtained with a commonly used equivalent circuit shown in Fig. 6a, which includes a Warburg element 共W兲 to represent the diffusion process of charged species within the oxide. The Warburg element is described by19 ZW = RW coth关共i␶W␻兲1/2兴/共i␶W␻兲1/2

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where RW is the diffusional resistance, ␶W is the diffusion time constant 共␶W = RWCW, and CW is the internal capacitance of the oxide兲. In the diffusional interpretation ␶W = L2 /D 共L is the effective diffusion thickness and D the effective diffusion coefficient兲. The results from spectra fitting are summarized in Table I. The CPE values give a measure of the total interfacial capacitance, which includes contributions from both double layer capacitance and psuedocapacitance associated with the surface bound redox species. The results show that the total interfacial capacitance is higher for the mixed oxides than for the pure oxides, and it is higher for the RuO2 than for the IrO2. The internal capacitance available inside the oxide, CW, can be calculated from the diffusion time constant ␶W and the diffusional resistance RW 共CW = ␶W /RW兲. The CW values obtained are also shown in Table I. The available internal capacitance of the RuO2 is about 50 times higher than that of the IrO2. This may



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Figure 7. Cyclic voltammogram of RuO2, 共Ru1−xMnx兲O2 and 共Ir1−xMnx兲O2 electrodes, at sweep rate 50 mV/s.

Figure 6. 共a兲 Bode plots of the RuO2 and IrO2 electrodes, and 共b兲 Nyquist plot of the RuO2 electrode.

be explained by the higher ability of the RuO2 to inject and eject protons due to its higher content of oxygen and hydroxides in the oxide, as revealed by the XPS measurements. The diffusional time constant ␶W is about 70 times higher for the RuO2 than the IrO2, Table I, indicating slower diffusion processes in the RuO2 than the IrO2. The effective diffusion coefficient of the charge carrier in the oxides can be calculated from their ␶W values

Table I. Numeric values from spectra fitting. Rp CPE 共⍀ cm2兲共mF/cm2兲 RuO2 IrO2 共Ru1−xMnx兲O2 共Ir1−xMnx兲O2 a

2 30 15,000 ¯a

11 3 15 23

␩ 0.70 0.82 0.84 0.76

Cannot be determined from spectra fitting.

RW CW 共⍀ cm2兲共mF/cm2兲 1000 700

140 3

␶W 共s兲 140 2

through Deff = L2 /␶W, using the oxide thickness as the effective diffusion length L. It is 9 ⫻ 10−10 cm2 /s for the RuO2 and 5 ⫻ 10−8 cm2 /s for the IrO2. It was reported in literature that the proton diffusion coefficient of RuO2 was 10−11 - 10−12 cm2 /s for anodically and thermally produced oxides,17,34 and 10−8 cm2 /s for oxides produced by pulsed laser deposition.21 For sputtering deposited IrO2, it was estimated to be 10−7 - 10−8 cm2 /s.20 Likely proton is the main charge carrier in the oxides in this study, comparison between the results from this study with the literature values shows that the RuO2 has a proton diffusion constant in between the more hydrated oxides 共anodically and thermally produced兲 and the less hydrated 共sputter deposited兲. This indicates that the RuO2 obtains a relatively high level of hydration shortly after or even before immersion in the electrolyte 共see the XPS results兲, whereas the IrO2 maintains its low level of hydration. It is important to recognise that the interfacial parameters obtained by EIS gives information of the interface under a small perturbation. Other potential step or sweep experiments are necessary to determine the performance during fast charge/discharge conditions and at high over-potentials when transient processes dominate at the interface. Cyclic voltammetry.— When subjected to CV between −3 and 1 V vs. Ag/AgCl 共saturated KCl兲, the IrO2 was severely degraded during the measurements 共Fig. 2c兲. Therefore no results from IrO2 are included here. It has previously been reported that IrO2 exhibits metallic conductivity over a more limited potential range 共0.5 1.4 V vs. RHE兲 compared to RuO2.9 Moreover, Ir can form soluble complexes with chloride ions,35 which can affect its stability in the solution. The CV curves from the other conducting oxides at a sweep rate of 50 mV/s are shown in Fig. 7. Unless otherwise stated, the potential in all graphs is given relative to the Ag/AgCl 共saturated KCl兲 electrode. At this sweep rate, all materials showed typical characteristics of a capacitive response, where the current density is relatively constant over a wide potential range. The 共Ir1−xMnx兲O2 exhibits a peak at −0.8 V in the cathodic scan. No reverse peak is seen in the anodic scan, indicating some irreversible reduction such as partial reduction of Ir or Mn oxides. Between 0 and −1 V, the 共Ru1−xMnx兲O2 shows a rather higher current density, whereas the RuO2 shows a lower current density. The apparently higher charge capacity of the mixed oxides is most likely due to the higher oxide thickness and higher porosity, but additions from the Mn redox sites can not be ruled out. Below −1 V, the current density of all materials increases due to the hydrogen evolution.


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Figure 8. Cyclic voltammogram at 0.5, 5, and 20 V/s, 共a兲共Ru1−xMnx兲O2, 共b兲共Ir1−xMnx兲O2, and 共c兲 RuO2.

At high sweep rates, the microstructure of the electrode plays an important role in the current response. When the sweep rate increases, the interface charging or discharging current will increase according to icap = Cinterfaces

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where Cinterface is the interfacial capacitance, i.e., combined double layer capacitance and the psuedocapacitance of the surface bound

Figure 9. Cyclic voltammogram, expressed as capacitance vs. potential at 10, 15, and 20 V/s for 共a兲共Ru1−xMnx兲O2, 共b兲共Ir1−xMnx兲O2, and 共c兲 RuO2.

redox-sites, and s is the sweep rate. At sufficiently high sweep rates, owing to the effect of distributed resistance 共IR drop兲 inside the pores, only a fraction of the pores is accessible for the electrochemical processes due to a reduced penetration depth. Therefore the available capacitance will diminish with increasing sweep rate. This implies that, when the sweep rate is increased to a certain level, the current response in the CV eventually will change character from a near capacitive response to a near resistive response. This phenom-


Journal of The Electrochemical Society, 152 共7兲 J85-J92 共2005兲 enon has been described in the literature,36-38 and discussed for rough TiN coated electrodes in the preceding paper.1 At sweep rates above 5 V/s for 共Ir1−xMnx兲O2 and 10 V/s for 共Ru1−xMnx兲O2, the current practically does not increase further, Fig. 8a and 8b. Therefore, the capacitance available for charging and discharging of the double layer is limited by this condition. The high total capacitance of the mixed oxides measured by EIS cannot be fully utilized during the fast CV cycling. This is illustrated in Fig. 9a and 9b, where the capacitance is plotted versus applied potential for various sweep rates. The results imply that, for the mixed oxide electrodes under very fast potential sweep, the charge transfer processes will be almost completely reversible 共desirable for stimulation electrodes兲. However, not all the capacitance of mixed oxides will be available for transferring the stimulation pulse, indicating a certain limit in utilizing the capacitance of porous electrodes and psuedocapacitance of conducting oxides. Although the mixed oxides exhibit a near linear current-potential relationship, the RuO2 still shows some capacitive behavior at 20 V/s 共Fig. 8c and 9c兲. It is clear that a higher amount of charge can be transferred at high charge/discharge rates through the RuO2, compared to the mixed electrodes. This is advantageous for pacemaker electrode applications where the stimulation processes take place during a very short time period. Accelerated aging.— After 1000 cycles between −3 and 1 V at a sweep rate of 1700 mV/s, the cathodic charge of RuO2 decreased slightly whereas the anodic charge remained the same in the CV. The decrease in cathodic charge was mainly due to less H2 evolution, which led to more reversible response with increasing cycling numbers. The SEM examination of the RuO2 surface 共Fig. 1c兲 showed that the coating was basically unaffected by this accelerated aging. No cracks or pinholes were found but some areas of the coating appeared dark in the SEM micrographs. The EDS analysis revealed that the darker areas were salt deposits on the surface. The XPS analysis of the surface composition showed a small increase in the O content. After 1000 cycles at 1700 mV/s the IrO2 coating was severely damaged as shown in Fig. 2c. This was confirmed by SEM observation, and Ti 共substrate兲 was detected by EDS analysis of the areas where the coating had been removed. According to the manufacturer the coatings had passed the attachment test 共tape test兲, claiming adequate quality of the original coating. The mixed oxides were to a certain extent stable during the accelerated aging. The 共Ru1−xMnx兲O2 coating was not visibly degraded 共Fig. 3c兲, but XPS analysis did not detect any Mn in the outmost surface layer 共ca. 5 nm information depth of XPS兲. However, Mn was detected by EDS analysis 共information depth of micron level兲, which indicates some surface depletion due to selective dissolution of Mn during the CV cycling. The 共Ir1−xMnx兲O2 coating showed some degradation 共Fig. 4c兲, which was not as severe as for the pure IrO2, and Ti was detected by EDS analysis only in some of the deepest cracks. In all, the results suggest that the mixed oxides suffer certain degradation upon the accelerated aging. It should be emphasized that these results were obtained after the CV cycling at 1.7 V/s, which is very accelerated aging.

Conclusions In this work, the electrochemical behavior of RuO2, IrO2, 共Ru1−xMnx兲O2, and 共Ir1−xMnx兲O2 electrodes for electrostimulation applications was investigated in a phosphate buffered saline solution, and their stability was evaluated by accelerated electrochemical aging. The results from electrochemical impedance spectroscopy and the CV, combined with surface analysis of the oxides using scanning electron microscopy and X-ray photoelectron spectroscopy before and after the accelerated aging, lead to the following conclusions: The IrO2 and 共Ir1−xMnx兲O2 electrodes were not stable when subjected to CV with very low negative potentials. The 共Ru1−xMnx兲O2

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was stable after accelerated aging, but selective dissolution of Mn may occur. No degradation of the RuO2 was detected. For conducting oxides, the electrochemical double layer and easily available redox sites on the surface contribute to faster charging/ discharging processes, while slower charging processes take place in the bulk oxide. The fast charging/discharging processes are favorable for pacemaker electrode applications where they contribute to reversible charge transfer during stimulation pulses. Compared to the IrO2, the RuO2 electrode exhibited higher interfacial capacitance and also a higher internal capacitance, probably due to the higher level of hydration of RuO2. Diffusion of charge carriers was slower within the RuO2. During fast cyclic potential sweeps 共up to 20 V/s兲, the RuO2 still exhibits some capacitive characteristics in the charge transfer mechanism, whereas the 共Ru1−xMnx兲O2 and 共Ir1−xMnx兲O2 electrodes changes with increasing sweep rate until the current response appeared to be of near resistive character. Acknowledgments Swedish Research Council and St. Jude Medical AB are acknowledged for the financial support and Naixiong Jiang at St. Jude Medical, Sunnyvale, CA, for providing the mixed oxide electrodes. Many thanks to Eva Micski, St Jude Medical AB, Veddesta, for assistance with the SEM, and Associate Professor Inger OdnevallWallinder is gratefully acknowledged for the XPS measurements. The Royal Institute of Technology assisted in meeting the publication costs of this article.

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