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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.