(322b) Impedance Analysis of Pt Microelectrodes in Phosphate-Buffered Saline Electrolyte

Electrochemical impedance spectroscopy (EIS) was used to explore the charge capacity and faradaic reactions for Pt electrodes which are the foundation of the Enlite^® glucose sensor produced by Medtronic Diabetes. This work supports an effort to understand electrode impedance in deep-brain electrical stimulation, used to treat patients with movement disorders such as Parkinson’s disease. In the early 1990s, a sequence of Voigt components and an ohmic resistance called the measurement model was developed to regress impedance data of electrochemical systems. A measurement model analysis[1] was used to identify the stochastic error structure of repeated measurements, to determine the frequency range that was consistent with the Kramer-Kronig relations,[2] and to determine the frequency at which the nonuniform current and potential distribution influenced the impedance.[3]

Polarization curves were used to determine that the dominant faradaic reactions in the presence of air were oxygen reduction and evolution. A process model was developed that accounted for oxygen evolution, mass-transfer influenced oxygen reduction, and the nonideal capacitance of the electrode, represented as a constant-phase element. Surprisingly, the same reactions seemed to play a role when the electrolyte was de-aerated by sparging of nitrogen. Frequencies above 20 Hz were found to be influenced by ohmic impedance associated with nonuniform current and potential. Several methods were used to estimate the capacitance of the electrode. This analysis showed that, when the data influenced by nonuniform current and potential was truncated, the capacitance obtained by the measurement model was in good agreement with the capacitance obtained by the application of the Brug formula,[4] derived under assumption of a surface distribution of time constants. When all frequencies were included, the measurement model analysis yielded a capacitance in good agreement with the extrapolation of a complex capacitance to high frequency.

[1] W. Watson and M. E. Orazem, EIS: Measurement Model Program, ECSArXiv, 2020, https://ecsarxiv.org/kze9x/.

[2] P. Agarwal, M. E. Orazem, and L. H. García-Rubio, “Application of Measurement Models to Electrochemical Impedance Spectroscopy: 3. Evaluation of Consistency with the Kramers-Kronig Relations,” Journal of The Electrochemical Society, 142 (1995), 4159-4168.

[3] H. Liao, W. Watson, A. Dizon, B. Tribollet, V. Vivier, and M. E. Orazem, “Physical Properties Obtained from Measurement Model Analysis of Impedance Measurements,” Electrochimica Acta, 354 (2020), 136747.

[4] G. J. Brug, A. L. G. van den Eeden, M. Sluyters-Rehbach, J. H. Sluyters, The Analysis of Electrode Impedances Complicated by the Presence of a Constant Phase Element, Journal of Electroanalytical Chemistry, 176 (1984), 275-295.