Mixed Ionic and Electronic Conduction in Li[sub 3]PO[sub 4] Electrolyte for a CO[sub 2] Gas Sensor

An electrochemical gas sensor using and as sensing and reference electrodes, respectively, and as the electrolyte is the subject of this paper. The sensor response to gas showed a systematic deviation from the prediction of the Nernst equation at low . Based on the electromotive force (emf) measurement, the transference numbers of , a lithium-ion conductor, were estimated for different values, and the conduction domain boundary for separating n-type electronic conduction from ionic conduction was constructed. The conduction domain predicts that change in the Li activity in the sensing side of the cell drives the electrolyte to a mixed (n-type electronic and ionic) conduction region at low . Hebb-Wagner dc polarization measurements also indicate n-type electronic conduction in with a mixture of and gold as a reversible electrode. The transference numbers obtained from both the emf measurement and the Hebb-Wagner polarization measurements demonstrate that the origin of the non-Nernstian behavior of the sensor is due to the lithium mass transport from the -sensing electrode to the electrolyte, resulting in nonstoichiometry of at temperatures above .