Highly disparate activity regions due to non-uniform potential distribution in microfluidic devices: Simulations and experiments

• Potential distribution is modeled in flow cell with Laplace equation.
• With increase of electrode size limiting current occurs.
• In the limiting region active and inactive electrode areas coexist.
• Limiting current depends on cell geometry and applied potential.
• Findings are confirmed experimentally in segmented electrode configuration.

In a microfluidic flow cell activity pattern can occur along a thin band electrode due to the potential distribution in the cell. For quantitative characterization of the pattern formation exclusively due to electric effect and for elimination of interactions of reaction sites from concentration distribution along the flow channel, a partial differential equation model is formulated for the spatiotemporal variation of electrode potential with Butler–Volmer kinetics limited by mass transfer. At constant applied circuit voltage, with increase of the electrode size a limiting current is achieved because of the spatial pattern formation. The limiting current arises due to the formation of high activity at the downstream edge, and low (nearly open circuit potential) activity at the upstream edge. The spatial pattern (e.g., ratio of active vs. inactive region) depends on the electrode size, the applied voltage, the conductivity of the electrolyte, and the distance from the downstream electrode edge to the reservoir. It is also shown that by placing equally spaced insulating stripes on the electrode much of the activity can be retained and the current does not decrease significantly due to the lessened surface area (as long as the surface area of the insulating stripes is less than about 50% of the entire electrode area). The model simulations are interpreted with a coupled ordinary differential equation model of segmented electrodes and the occurrence of the strong edge effects is confirmed with experiments of a four-electrode electrode array in a microfluidic flow cell with ferrocyanide oxidation.