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Fig.1
Interfacial layers between metal contact and electrolyte
Fig.2
Impedance of a hypothetical metal electrode (A = 1 mm2) in contact with 1 mM KCl (Rbulk = 100 Ω, Rc = 500 Ω, Cdl = 30 mF/cm2, D = 5.10-8 m2/s, T = 296.15 K) in a frequency range from 100 Hz < f < 100 MHz with undisturbed diffusion layer
Fig.3
Debye screening length depending on the concentration of a 1:1 electrolyte. Note, the symbol λD was chosen rather than к as found in literature in order not to disturb with the conductivity where к is used as well.
Fig.4
Cyclic voltammogram of two different gold electrodes in 140 mM PBS (phosphate buffered saline). The electrode area was 2 cm2 and the voltage slope was set to 50 mV/s.
Fig.5
Schematic of a stray field sensor with electric modeling. These sensors with interdigitated metallic electrodes with distances between 1 μm and 150 μm are built on a silicon wafer with silicon nitride as passivation layer. Sensors like these are useful as galvanically decoupled impedance sensors. (CiS Forschungs-institut für Mikroelektronik und Photovoltaik GmbH, Erfurt, Germany).
Fig.6
Tetrapolar interface consisting of four stainless steel rings with a diameter of 6 mm. The electrode system is molded in PMMA (poly-methylene-methacrylate).
Fig.7
Tetrapolar electrode made from a CD-R. The dimension of the inner electrodes is 100 μm. For electrical characterization of the electrodes, measurements in bulk electrolytes were done with the leads passivated using Tesa tape (Bayersdorf AG, Hamburg, Germany).
Fig.8
Interdigitated electrodes with different structural dimension a) 3 μm (enlarged vie in insert image), b) 20 μm and c) 150 μm, developed by CiS Forschungsinstitut für Mikroelektronik und Photovoltaik GmbH, Erfurt, Germany. These electrodes are arranged in blocks for the small electrode distance (up to 50 μm) while larger electrodes are made of one block (Fig.8c).
Fig.9
Geometry coefficient calculated over the frequency range for an electrode shown in Fig.6.
Fig.10
Molar conductivity of KCl with varying concentration. The conductivity in the frequency range of the confidence region was corrected by the conductivity of pure water (кw = 6.41 μs/m at 25oC).
Fig.11
Potential of surface modified electrodes with respect to an Ag/AgCl-electrode immersed in 3 M KCl. The chamber was filled with 140 mM PBS. A magnetic stirrer was used with a speed of about 500 rpm. Both test electrodes were made by plating silver onto gold with subsequent anodic oxidation in HCl. While one electrode was left bare, the other one was covered using a polyurethane gel.
Fig.12
Surface defect at a Ag/AgCl covered gold electrode. The electrode was made from a CD-R carrying a 24-karat gold layer. The surface was scanned using EDX (energy-dispersive X-ray spectroscopy). The rough surface is AgCl while the pure gold is found around the impurity which consist of organic dust. The stripe structure of the gold is a feature of the CD. (Image by Holger Rothe, iba Heiligenstadt)
Fig.13
Impedance spectrum (real and imaginary part) of a 0.125 mm2 – gold electrode made from a CD-R. (o - 0.055 mM PBS, * - 2.18 mM PBS).
Fig.14
Impedance of gold electrodes in PBS of different concentration and different surface area: a) A = 1 mm2, concentration from left to right: 140 mM, 2.18 mM 0.55 mM b) A = 4 mm2 , c = 140 mM, 35 mM, 8.75 mM, 2.18 mM, 0.55 mM and c) A = 16 mm2, c = 140 mM, 35 mM, 8.75 mM, 2.18 mM. The red circles show the resistance estimated from the conductivity of the electrolyte and the geometry of the chamber inclusively the spreading resistance in series with the charge transfer resistance got from cyclic voltammetry. The resistance of the counter electrode was taken into account as well but it was small with only few percent of the charge transfer resistance.
Fig.15
Cyclic voltammogram (CV) for gold electrodes of different area in 140 mM PBS (solid: 1 mm2, dotted: 4 mm2, dashed: 16 mm2), a) -100 mV < U < 100 mV, b) -300 mV < U < 300 mV. The ramp steepness was set to 10 mV/s. An Ag/AgCl – electrode served as reference while a massive platinum electrode was used as counter electrode.
Fig.16
Electric field strength near the electrode: a) Øelectrode = 0.2 mm and b) Øelectrode = 4 mm in a cylindric chamber with a diameter of 10 mm with a voltage of 100 mV applied against a distant electrode at 10 mm. Only the plane in the middle of the chamber is shown.
Fig.17
Simple approach for calculating the spreading resistance of a small disk shaped electrode
Fig.18
Comparison between conductivity measured with a conductometer (LF-300, WTW Weinheim) and by impedance measurement using gold electrodes of different size (ᐃ 0.12 mm2, o 1mm2, ◊ 4 mm2, and * 16 mm2).
Fig. 19
Mean value of the geometry factor k (1 kHz – 10 kHz) for gold electrodes with different surfaces area. This different behavior for the very small electrode suggests an increasing influence of parasitic elements. Compensating for parasitic elements requires that they are independent on the experimental condition and that a suitable model is used which needs to be tested individually for all electrode geometries.
Fig.20
Molar conductivity vs. square root of the concentration for gold electrodes of different size (◊ 0.125 mm2, x 1 mm2, o 4 mm2, ᐃ 16 mm2)
Fig.21
Locus diagram for the impedance of a 3 μm interdigitated electrode immersed in KCl-solution of different concentration. (along the arrow: 1 μM, 10 μM, 100 μM ,1 mM, 10 mM, 100 mM, 1 M). The frequency ranged from 500 Hz to 40 MHz. Note the different scaling of the axes.
Fig.22
Functional dependence of the measured real part of the impedance at 10 kHz with respect to the concentration of the KCl-solution
Fig.23
Change in impedance magnitude at 46 kHz (a) and 4.5 MHz (b) of a segmented flow of paraffine oil and KCl – solution in a 0.5 mm tube. The flow was set to about 2 droplets per second.
