An electrochemical CO2 gas sensor was studied which used Li2CO3 and Li2TiO3-TiO2 as sensing and reference electrodes, respectively, and Li3PO4 as the electrolyte. The sensor response to CO2 gas showed a systematic deviation from the prediction of the Nernst equation at low pCO2. Based upon electromotive force measurements, the transference numbers of Li3PO4, a Li-ion conductor, were estimated for various pCO2 values, and the conduction domain boundary for Li3PO4 separating n-type electronic conduction from ionic conduction was constructed. The conduction domain predicted that changes in the Li activity on the sensing side of the cell drove the Li3PO4 electrolyte into a mixed (n-type electronic and ionic) conduction region at low pCO2. Hebb-Wagner dc polarization measurements also indicated n-type electronic conduction in Li3PO4 with a mixture of Li2CO3 and Au as a reversible electrode. The transference numbers obtained from both the electromotive force measurements and the Hebb-Wagner polarization measurements demonstrated that the origin of the non-Nernstian behavior of the CO2 sensor was due to Li mass transport from the Li2CO3-sensing electrode to the Li3PO4 electrolyte; resulting in non-stoichiometry of Li3PO4 at above 500C.

Mixed Ionic and Electronic Conduction in Li3PO4 Electrolyte for a CO2 Gas Sensor. C.Lee, P.K.Dutta, R.Ramamoorthy, S.A.Akbar: Journal of the Electrochemical Society, 2006, 153[1], H4-14