Surface conductivity of binary carbonate as a performance-governing parameter of an electrochemical CO2 gas sensor

In electrochemical CO2 gas sensor, the chemical potential of electrolyte changes on adsorption of CO2 molecules as the process involves catalytic electron transfer. In addition, it is the rate-determining step that decides sensor’s response. In this study, in-situ bulk AC, DC and surface electronic conductivities of CaCO3 + Li2CO3 binary solid electrolyte were investigated at different temperatures and CO2 gas partial pressures using complex impedance spectroscopy, Wagner’s DC polarization technique and four-probe method, respectively. For the four-probe conductivity measurements with crucial requirement of high temperatures and test gas variations, a customized sample holder was designed and fabricated having gold-plated equidistant, spring-loaded electrodes and localized heating system (maximum 593 K). The AC bulk conductivity was found to decrease with rise in CO2 gas concentration (from 0.1 to 100%) by about two orders and one order of magnitudes at lower and higher temperatures, respectively. Similarly, surface conductivity variation with temperature also showed Arrhenius behaviour for both the concentrations of CO2 viz. 0.04 and 10%, giving lower value of activation enthalpy for lower CO2 concentration. The surface conductivity change in the presence of different concentrations of CO2 gas is justified by comparing with AC bulk conductivity measurements at different CO2 partial pressures and DC conductivity along with sensing response. The mechanism is explained using activated charge transfer data. The range of Ea values on adsorption of CO2 gas was found to be in the electronic excitation window, suggesting involvement of a new parameter to be investigated for non-Nernstian response of EC sensors.