Electrochemical Impedance Spectroscopy (EIS) is a widely used technique to analyse the properties of electrode surfaces and bulk electrolytes in electrochemical sensors. At low frequencies (below 100 Hz), EIS provides insights into the electrode-electrolyte interface, particularly the double-layer capacitance. At higher frequencies (>1 kHz), the double-layer capacitance effects diminish, and the impedance is predominantly influenced by electrolyte resistance. Designing effective EIS electrodes requires a comprehensive understanding of both the electrode-electrolyte interface and bulk electrolyte properties. In this study, we developed a COMSOL-based time-dependent model to simulate and understand the EIS response of a planar two-electrode sensor. The intended application is to simulate the EIS response in a buffer solution used for biological cell perfusion in organ-on-chip systems.
Simplified models, such as the Randles circuit for parallel plate capacitors, offer basic estimations but become inadequate for complex geometries like planar interdigitated electrodes and electrolytes with bulk reactions. Numerical simulations offer a more accurate approach in these cases. While traditional analytical models provide steady-state analysis, time-dependent numerical models offer detailed insights into the dynamic processes at the electrode-electrolyte interface during electrode excitation.
Our COMSOL model captures the dynamic processes in the Debye layer, providing a comprehensive understanding of electrode-electrolyte interactions and transient behaviours. It employs the Nernst-Planck and Poisson equations to solve for ion concentration and electric potential, respectively. It includes practical parameters such as electrode sheet resistance and parasitic capacitances, enhancing its representational accuracy. The EIS responses from the model were validated against experimental results at various NaCl concentrations (1, 10, and 100 mM). The model results show a comparable impedance spectrum and values to experimental data, particularly at higher frequencies (>1 kHz), demonstrating its effectiveness in capturing the electrochemical behaviour of the system.
