Experimental Validation of the Transmission Line Model Via Impedance Spectroscopy of an Ordered Array on Porous Carbon Electrode

Abstract

In this paper, the interface of a porous substrate with an arranged and well-defined geometry is investigated for the impedance response with different electrolytes by using the well known electrochemical technique of Electrochemical Impedance Spectroscopy (EIS). Based on De Levie's Transmission Line Model (TLM) for porous electrodes a modification has been done in the model by incorporating the CPE behavior to reflect the non-ideal behaviour for real electrodes at low frequencies. The geometry used to validate the proposed model was fabricated from a smooth Glassy carbon electrode (GCE) surface to correlate the EIS response to that obtained by the model. The desired array was obtained by using laser micromachining tool on a glassy carbon substrate with a goal to study the influence of different electrolytes, varying electrolyte concentrations, the morphological features of pore- depth and radii of pore as well as different pore shapes. The outcomes of the experiments carried out on the in-house setup are compared with the MATLAB simulated results of the impedance. The transition from resistive to capacitive behavior is found to be shifted to lower frequency side and TLM behavior at high frequencies is more apparent as the concentration is increased. For the same concentration, different electrolytes are found to behave differently due to varying conductivity and ion sizes. In all the experiments carried out the impedance response showed CPE (constant phase element) behavior at the low frequency end while for high frequencies the nature/appearance of TLM behavior is found to be dependent on the pore depth to radius ratio. In addition the concept of time constant is also dealt with which exhibits differences for different geometries of the pores on glassy carbon. The results suggest that the GC electrode engaged here could provide a tractable model for studying electrochemical impedance behavior of porous electrodes without using any surface area analysis or pore volume analysis technique. Keywords: Transmission Line Model, Glassy carbon, EIS, MATLAB