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Characterisation of electrode materials for dielectric spectroscopy SUPPLEMENTAL MATERIALS D Malleo*§1^, JT Nevill*2±, A van Ooyen3, U Schnakenberg3, LP Lee2 and H Morgan1 1 Nanoscale Systems Integration Group, School of Electronics and Computer Science, University of Southampton, SO17 1BJ United Kingdom 2 Department of Bioengineering, 306 Stanley Hall #1762, University of California at Berkeley, Berkeley, CA, 94720, USA 3 Institute of Materials in Electrical Engineering, RWTH Aachen University, Sommerfeldstraße 24, D-52074 Aachen * These authors contributed equally and are listed alphabetically. §Corresponding author. Email address: [email protected] Tel: +44 (0)1865 856 855; Fax: +44 (0)1865 848 684; ^ Present address: Oxford Gene Technology, Begbroke Science Park, Oxford, OX5 1PF United Kingdom ± Present address: Fluxion Bioscience, 384 Oyster Point Blvd, Suite 6, South San Francisco, CA 94080, USA Supplemental Methods Electrode Fabrication Metal electrodes were patterned onto 100 mm diameter Pyrex wafers. Each electrode had dimensions of 8 mm x 8 mm and was patterned on 25 mm x 75 mm glass rectangles and connected by a 0.5 mm wide electrode line to a 15 mm x 10 mm metal contact pad at the opposite end of the slide, see Figure 1(a). All metal films were sputtered (Nordiko NS 2550 DC) with either Platinum or Iridium targets of 99.9% purity at 10-5 mbar using a Ti adhesion layer. The electrodes were patterned using a lift-off process. For the experiments, a pair of electrodes was held in a custom-made plastic measuring cell filled with 5 ml of electrolyte ensuring the electrodes were completely submerged (Figure 1(a)) at a fixed spacing of 5.8 mm. Wires were bonded to contact pads with conductive epoxy; electrodes were connected to the dielectric analyzer via coaxial lines. All measurements were taken using two identical electrodes. Serial dilutions of NaCl electrolyte were prepared from 1 M to 0.1 mM. 0.9% w/v physiological saline (154 mM NaCl) was also used for the time-dependent measurements. Dulbecco’s phosphate buffer saline modified solution (without calcium chloride and magnesium chloride) was bought from SigmaAldrich. Impedance spectra were acquired using a high-resolution dielectric analyzer (Novocontrol Alpha-N). The instrument was set to 0 V DC bias, and an AC excitation signal of 25 mV RMS amplitude was swept over the frequency range from 1 MHz to 1 Hz in 50 steps, logarithmically spaced. For the PPY films, impedance was measured to 0.01 Hz.(Figure 2 (a) to (e)) Platinum Black Bright platinum electrodes were converted to Pt black using published techniques 1. An electrode was connected to the negative terminal of a dc power supply and immersed in a solution of 1.4% v/v hexachloroplatinic acid diluted in DI water. To this solution, 0.02% w/v lead acetate was added. A platinum foil counter-electrode was also immersed in the solution and connected to the positive terminal of the power supply. A potential of 1.6 V was applied (giving a current density of 8 mA/cm2), for 120 seconds. The plated electrodes appear matteblack, they were thoroughly rinsed and stored in saline solution until use. Iridium Oxide Iridium oxide films were reactively sputtered in Ar/O2 plasma onto a Ti adhesion layer and Pt sub-layer. The sputtering chamber was evacuated to at least 10-5 mbar prior to sputtering by means of a cryogenic pump. A throttle valve was used to control the system pumping speed. The substrate was not heated to ensure the deposition of amorphous films. Film thickness was determined with a Tencor PA-10 profilometer. A DC voltage was applied to the target with a power of 180 W and the argon flow in the plasma was kept at 100 sccm. A sputtered film thickness of approximately 300 nm was reached. The electrode was activated using a CHI 750 electrochemical workstation (CH Instruments, Austin, Texas, US), operating in ‘cyclic voltammetry’ mode with a Pt foil counter electrode and an Ag/AgCl reference electrode in a physiological saline solution (154 mM NaCl). Activation parameters were: Vmin = -1.1, Vmax = 1.2, Vrate = 0.1 V/m. Fifty activation cycles were applied to the electrodes before performing impedance measurements reported here. Figure 3(a) shows the CV curves after several successive cycles; Figure 3(b) shows the charge delivery capacity, obtained by integrating the current along the CV curve, for each cycle, as well as the capacitance measured in 1 M NaCl solution after 10, 20 and 50 activation cycles. PPy/PSS The activation solution was prepared following the protocol of George et al 2. Briefly, 0.2 M Pyrrole reagent (98% Sigma) and 0.2 M of Poly (4-styrenesulfonic acid) solution (Sigma), were mixed and stirred for 15 minutes at room temperature. The electrodes were immersed in the polymer solution and connected to the positive terminal of a DC power supply. A Pt foil counter-electrode was also immersed in the solution and connected to the negative terminal of the power supply. A voltage of 1.24 V was applied giving a current density of 1.5 mA/cm2. The deposition was carried out for 180 seconds. Equivalent Circuit Analysis Equivalent circuits were used to model device behaviour. There is wide variability in equivalent circuit model used in impedance spectroscopy, and Macdonald has warned against the fallacies of arbitrary equivalent circuit modelling 3. Here, an equivalent circuit model with elements representative of the known physical elements, keeping the number of elements to a minimum.4 Impedance spectra were fitted to the equivalent circuit models (Figure 1(b) and 1(c)) using a linear least squares fitting algorithm in Zview (Scribner Associates). For the model in Figure 1(b), a single capacitor (Cdl) describes the combination of Stern layer and diffuse layer, with the total double layer capacitance equal to half the capacitance of one electrode. The bulk electrolyte was modelled by a single resistor (the electrolyte capacitance makes little contribution in this frequency range and was ignored). The applied voltage was low (25mV RMS) so that the charge transfer resistance (Rct) at the electrode interface was also ignored. For the model represented in Figure 1(c) a constant phase element (CPE) was used to model the double layer, defined as: ZCPE 1 j P T This reduces to an inductive impedance for P=-1, a resistive impedance for P=0 and a capacitive impedance for P=1. The origin and use of CPEs has been extensively discussed in the literature, see 5-10 Imaging Scanning Electron Microscope images were taken with a Hitachi S-5000 SEM. Samples were diced with a diamond scribe, and placed directly into the SEM without any further treatment, except for the iridium oxide sample which was coated with a thin layer of platinum and gold (< 2nm thick), (Tousimis Sputter Coater). Supplemental Results and Discussion Equivalent Circuit Modelling The results from fitting the impedance spectrum to the simple equivalent circuits shown in Figure 1(b) are found in the main manuscript in Table 1. Here, we present and discuss the fitting results for both equivalent circuit models (R-C and R-CPE). The values presented in Table 1 are repeated here with the addition of fit quality (represented as least square error in the following tables). For both models, the resistive elements represent the solution resistance. The double layer is modelled as either a capacitor or a CPE. The advantage of the R-C fits is that the capacitive values resultant from the fits to different solutions or even different materials can be directly compared. The advantage of fitting to R-CPE circuits is that the least square error of the fits is generally lower, since the charging and discharging of the double layer is not simply exponential and is better modelled by a constant phase element. Both fits are displayed here simply to offer more information to the reader. All values are normalized to the electrode area. The CPE fits can give some insight into the topology of the electrode surface if one accepts the traditional assumption that values of CPE-P lower than 1 indicate a surface with fractal features or otherwise rough. Interestingly, it can be seen (Table 1) that the CPE-P values found do increase with increasing values of salt concentration (e.g. P=0.74 for 0.1 mM NaCl and P=0.93 for 1 M NaCl) if not fixed, suggesting a more complex relationship between CPE-P and the surface. The values recorded for the platinum black electrodes, shortly after the electrochemical process had taken place are shown in Table 2. Values of double layer capacitance three orders of magnitude higher than for the plain platinum have been recorded, indicating a similar increase in the effective electrode-electrolyte interface area. The CPE-P values fit increased to a range of 0.81-0.94, contrary to the traditional assumption that a higher fractal index is reflected in lower CPE-P values. In this instance too, the CPE-P values increase with increasing salt concentration in the electrolyte. The double layer capacitance was too high to measure at the lowest NaCl concentration, as indicated by the dashes. The values recorded for the iridium oxide electrodes, activated by 50 cyclic voltammetry cycles are shown in Table 3. Values of double layer capacitance are slightly lower than the ones recorded with platinum black electrodes, but still three orders of magnitude higher than the plain platinum electrodes. A similar increase in the effective electrode-electrolyte interface area is seen. The CPE-P values fit decreased to a range of 0.61-0.70, considerably lower than the ones recorded for the platinum black electrodes. The CPE-P values increased with increasing salt concentration in the electrolyte. The values recorded for the PPy/PSS coated electrodes, shortly after galvanostatic deposition of the polymer are shown in Table 4. Values of double layer capacitance are four orders of magnitude higher than for the plain Pt. The CPE-P values fit ranged from 0.57 to 0.81. The fit quality of the CPE-P is noticeably worse than the other electrodes and may explain the wide range of values, which is attributed to the extremely high capacitance values, which are harder to measure. In this instance, the CPE-P values tend to increase with increasing salt concentration in the electrolyte, although an out-of- trend local maximum was measured with 100mM NaCl and PBS solutions. The trends associated with CPE-P fits related to apparent surface roughness and electrolyte concentration suggest that any inference about an electrode’s characteristics and the CPE-P value needs close scrutiny. Electrode ageing Iridium Oxide The stability of IrOx electrodes over time was assessed using two pairs of activated electrodes. Figure 4 (a) and (b) show the impedance data for electrodes stored dry, showing that the impedance changes over time and becomes more capacitive. The biggest change occurs within the first 24 hours, with smaller changes over the following 9 days. Electrodes stored wet exhibited similar behaviour; in both cases a re-activation procedure was sufficient to restore the electrodes. The changes in impedance were quantified using circuit analysis. Figure 5 (a) and (b) show the double layer CPE values (obtained from the series R-CPE circuit) plotted against time. Whereas the power factor (P) of the CPEs tends not to vary with time, its base value decreases with time in the same fashion as the capacitance. PPy/PSS Two pairs of PPy/PSS polyelectrolyte electrodes were stored in DI water or 154 mM NaCl solution. Measurements were performed in 154 mM NaCl, once a day, for 8 days. Unlike the IrOx, there was little change in the impedance or in the values of the fitted capacitance; less than 1.5% over the course of 8 days (Figure 6). Supplemental Tables Table 1. R-CPE fit results for Pt electrodes: As predicted by Gouy-Chapman-Stern theory, the double layer capacitance is found to increase with increasing electrolyte concentration. Unexpectedly, the power factor value in fitted CPE also increases with salt concentration. Table 2. R-CPE fit results for Pt black electrodes: The CPE-P value increased to a range of 0.81-0.94, contrary to the assumption that a higher fractal index is reflected in lower CPE-P values 7. The CPE-P values increase with increasing electrolyte concentration. (The double layer capacitance contribution was too low to obtain a good fit at the lowest NaCl concentration, as indicated by the dashes). Table 3. R-CPE fit results for activated IrOx electrodes:, the CPE-P values were lower than Pt black electrodes; in the range 0.61-0.70. Table 4. R-CPE fit results for PPy/PSS: The CPE-P values ranged from 0.57 to 0.81; the uncertainty in the fit was worse than for the other electrodes, due in part to the extremely high capacitance values which are difficult to measure without going significantly lower in frequency. Supplemental Figures Figure 1. (a) Sample cell used to keep two electrodes at a fixed distance. (b) and (c) Equivalent circuit models used to represent an electrode/electrolyte measurement cell. For the conditions presented here (low Vrms, low frequency, and simple geometry), the circuit can be simplified to an RC (or R-CPE) circuit. The resistance of the Stern layer and diffuse layer are assumed to be infinite; the Stern layer capacitance and diffuse layer capacitance are lumped into a ‘double layer’ capacitor (Cdl); and the solution capacitance does not play a role in the impedance spectrum at the frequencies measured. Figure 2. Impedance spectra acquired for platinum (a-b), platinum black (c-d), iridium oxide activated with 50 CV cycles (e-f) and PPy/PSS (g-h) electrodes in serially diluted solutions of NaCl. Error bars represent one standard deviation. From this data the circuit parameters were extracted using the equivalent circuit models shown in figure 1(b) and 1(c). Figure 3. (a) Current vs. voltage curves, scanned at 0.1 V/s. The colour refers to three successive activation steps (red, green and blue), each of which consisted of 10 complete cycles. The increased charge injection capacity is evident from the larger and larger area that is enclosed by the CV curves. The main feature of the CV curves is their symmetry along the potential axis, resulting in transfer of equal integral anodic and cathodic charges through the phase boundary electrode/electrolyte. Iridium changes its oxidation state repeatedly during the potential cycling, and from this aspect it can be stated that the activated iridium oxide show a reversible electrochemical behaviour. The shape of the curves is complicated; the existing current peaks are broad and not very well depicted, which suggests the existence of various active surface sites with different formal potentials participating in the redox processes. (b) (left axis, filled squares) Charge injection capacity evolution plotted against number of activation cycles. (right axis, filled circles) Capacitance measured in 1M NaCl. Figure 4 Effect of ageing on the impedance of iridium oxide electrodes. (a) Impedance magnitude and phase for electrodes stored dry, measured immediately after activation, then after one and ten days and finally after re-activation process. (b) Similar data for electrodes stored in saline solution. Figure 5 Effect of ageing/storage on the impedance of iridium oxide electrodes. Variation of CPE values recorded in the course of ten days after the initial activation and subsequent to reactivation for both wet- and dry- stored electrodes. The CPE values were obtained by fitting a series R-CPE circuit to the measured impedance spectra. Left: CPE-T values. Right: CPE-P values. Figure 6. Response of PPy/PSS electrodes over time. The extracted values for both R-C and R-CPE fits remained relatively constant over the course of 8 days. Supplementary References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. C. A. Marrese, Analytical Chemistry, 1987, 59, 217-218. P. M. George, A. W. Lyckman, D. A. LaVan, A. Hegde, Y. Leung, R. Avasare, C. Testa, P. M. Alexander, R. Langer and M. Sur, Biomaterials, 2005, 26, 3511-3519. J. R. Macdonald, Ann Biomed Eng, 1992. M. Schuettler, T. Doerge, S. L. Wien and S. Becker, Proceedings of the 10th Annual Conference of The International FES Society, 2005. H. P. Schwan, Annals of the New York Academy of Sciences, 1968, 148, 191-&. P. Zoltowski, Journal of Electroanalytical Chemistry, 1998, 443, 149-154. S. H. LIU, in Phys Rev Lett, 1985, pp. 529-532. L. Nyikos and T. Pajkossy, Electrochimica Acta, 1985, 30, 1533-1540. T. Pajkossy and L. Nyikos, Journal of Electroanalytical Chemistry, 1992, 332, 55-61. J. C. Wang, Electrochimica Acta, 1988, 33, 707-711.