AC impedance characteristics of solid-state planar electrochemical carbon monoxide sensors with Nafion® as solid polymer electrolyte

Abstract

The AC impedance characteristics of planar solid-state electrochemical carbon monoxide (CO) sensors with Nafion® as the solid polymer electrolyte are reported. For a re-cast Nafion® sensor design, Nyquist plots indicated very high impedances (10 7–10 8 Ω), which slightly decreased with an increase in the working electrode surface area (0.25–0.50 cm 2). Only one enlarged semi-circle was observed, relating to an equivalent circuit with a geometrical capacitance ( C g) in parallel with a bulk resistance ( R b). In contrast, a sensor design using a commercial membrane produced a system with a much lower impedance (<10 3 Ω), with Nyquist plots showing a flattened semi-circle at high frequencies (due to C g and R b in parallel), followed by the start of a large semi-circle (due to the double-layer capacitance, C dl, in parallel with the charge-transfer resistance, R ct) over a lower frequency range. On addition of CO, the second circular arc reduced in size, indicating that the interfacial impedance of the electrochemical system had decreased.