Many CO 2 sensors proposed to date require high operation temperature (e.g., 400°C for solid electrolytes) to detect CO 2 with a high sensitivity, which restricts the wide application of CO 2 sensors.

We have found that composites consisting of the emeraldine base‐polyaniline (EB‐PAn) and poly(vinyl alcohol) (PVA) serve as a promising CO 2 sensor operating at room temperature with a high sensitivity. The logarithm of electrical conductivity of the EB‐PAn/PVA composite is proportional to the log of CO 2 concentration. For the composite with 13 wt % EB‐PAn and 87 wt % PVA, the linear relationship holds in the concentration range from 50 ppm to 5% at 30% relative humidity. This composite is insulating in a moist atmosphere without CO 2.

With the addition of CO 2, however, carbonate ions are formed by the hydrolysis of CO 2, and these ions equilibrate with the atmospheric CO 2. The carbonic acid is reversibly incorporated to and ejected from the EB‐PAn depending on the concentration of atmospheric CO 2, and the conductivity varies between conducting and insulating levels.