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

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 (107–108 Ω), which slightly decreased with an increase in the working electrode surface area (0.25–0.50 cm2). Only one enlarged semi-circle was observed, relating to an equivalent circuit with a geometrical capacitance (Cg) in parallel with a bulk resistance (Rb). In contrast, a sensor design using a commercial membrane produced a system with a much lower impedance (<103 Ω), with Nyquist plots showing a flattened semi-circle at high frequencies (due to Cg and Rb in parallel), followed by the start of a large semi-circle (due to the double-layer capacitance, Cdl, in parallel with the charge-transfer resistance, Rct) 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.