ABSTRACT: Electrochemical impedance spectros- ... consists solution resistance Rs, a constant phase element ... Atlas of Electrochemical Equilibria in Aqueous.
European Cells and Materials Vol. 6. Suppl. 1, 2003 (page 105)
ISSN 1473-2262 Poster ID: 427
IMPEDANCE OF ELECTRODES FOR STIMULATION AND RECORDING OF NATURAL NEURAL NETWORKS I.Schenker1, W.Franks1, P.Schmutz2, A.Hierlemann1 & H.Baltes1 1
Physical Electronics Laboratory, ETHZ, Zurich, Switzerland. Institute of Materials Chemistry and Corrosion, ETHZ, Zurich, Switzerland.
ABSTRACT: Electrochemical impedance spectroscopy was used to study the impedance of an electrode-electrolyte system. For the stimulation and recording of natural neural networks a low impedance is required; hence the need for a full impedance characterization. The electrode materials investigated were bright Pt, Pt-black and TiN; the electrolyte was 0.9% aqueous NaCl. An equivalent circuit model has been established. The effect of surface roughening has been investigated. Cyclic voltammetry was used to obtain the exchange current density of bright Pt. MEASUREMENTS & RESULTS: The measurements were performed using a three electrode apparatus: calomel reference electrode and Pt counter electrode with a commercially available potentiostat (Autolab PGSTAT 30) and frequency response analyzer (ECO Chemie B.V., NL). Both Ptblack and TiN have dendritic surface structures, and are used here to explore the effect of surface roughness. ZCPA
Solution
Metal
RS R ct
Fig. 1: Equivalent circuit model. The equivalent circuit model, shown in Fig. 1, consists solution resistance Rs, a constant phase element ZCPS, representing the interface capacitance, and the charge transfer resistance Rct [1]. Fig. 2 and 3 show the modulus and the phase of the measured impedance of bright Pt, Pt-black and TiN electrodes and the modeled values. 7
Bright Pt Pt−Black TiN
6
Log |Z| [Ohms]
5 4 3 2 1 0 −3
−2
−1
0
1 2 Log f [Hz]
3
4
5
Fig. 2: Impedance modulus experimental results for 1 cm2 bright Pt, Pt-black and TiN electrodes. Modeled results given by solid lines.
The exchange current density, J0, of Pt was measured using cyclic voltammetry and a value of 42.5 PA/cm2 was obtained. 90
Bright Pt Pt−Black TiN
80 70
− Phase [degrees]
2
60 50 40 30 20 10 0 −3
−2
−1
0
1 2 Log f [Hz]
3
4
5
Fig. 3: Phase experimental results for the 1 cm2 bright Pt, Pt-black and TiN electrodes. Modeled results given by solid lines. DISCUSSION & CONCLUSION: The close fit of the modeled and experimental results indicates that a model has been experimentally verified, and can therefore be used to optimize future electrode designs. For frequencies below 2 Hz the impedance modulus for Pt-black is 65 r 6 times lower than for bright Pt; similar results were obtained for TiN. Thus, as expected, roughening the electrode surface results in a lower impedance modulus. The measured exchange current density of 42.5 PA/cm2 is closer to that of the O2 reaction (Jo = 4.5 PA/cm2) than to that of the H2 reaction (J0 = 794 PA/cm2) [2]. In fact, the H2 reaction does not take place given the open circuit potential of 0.35 V [3]. This finding leads to the omission of a Warburg impedance element in the model over the measurement frequency range used here. REFERENCES: 1W. Franks, et al (2003) Transducers‘03: 963 – 966. 2D.A. Stenger, et al (1994) Enabling Technologies for Cultured Neural Networks, Academic Press. 3M. Pourbaix (1974) Atlas of Electrochemical Equilibria in Aqueous Solutions, Nat. Assoc. of Corrosion Engineers. ACKNOWLEDGEMENTS: Funding is gratefully acknowledged from the Information Societies Technology (IST) European Union Future and Emerging Technologies program, contract number IST 2000-26045.
