Mixed Ionic and Electronic Conduction in Li_3PO_4 Electrolyte for a CO_2 Gas Sensor
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Abstract
An electrochemical CO_2 gas sensor using Li_2CO_3 and Li_2TiO_3+TiO_2 as sensing and reference electrodes, respectively, and Li_3PO_4 as the electrolyte is the subject of this paper. The sensor response to CO_2 gas showed a systematic deviation from the prediction of the Nernst equation at low pCO_2. Based on the electromotive force (emf) measurement, the transference numbers of Li_3PO_4, a lithium-ion conductor, were estimated for different pCO_2 values, and the conduction domain boundary for Li_3PO_4 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 Li_3PO_4 electrolyte to a mixed (n-type electronic and ionic) conduction region at low pCO_2. Hebb-Wagner dc polarization measurements also indicate n-type electronic conduction in Li_3PO_4 with a mixture of Li_2CO_3 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 CO_2 sensor is due to the lithium mass transport from the Li_2CO_3-sensing electrode to the Li_3PO_4 electrolyte, resulting in nonstoichiometry of Li_3PO_4 at temperatures above 500°C.