The effect of DC polarization on the humidity sensing characteristics of bismuth iron molybdate

A pellet of bismuth iron molybdate, compressed to contain gold wire electrodes, was used to confirm protons or hydronium ions as the dominant carriers in a humid atmosphere, by DC polarization measurements. To do this a step voltage was applied to the electrodes for 50 min, followed by a short circuit for an additional 50 min. The time dependence of the charge and discharge currents were measured in both cases. This was done for a number of relative humidity values as set by the flow of humid air (5, 10, 19, 36, and 76% RH). At 76% RH the effect of bias was measured for 0.25, 0.5, 1, 2, 4, and 10 V, while only 0.5 V was used for the remaining humidity values. The charging curves were analyzed using the Logistic fitting routine in the software package Origin, while the discharging curves were analyzed using a simple power law function. The bias dependence showed a change over from a current dominated by DC polarization to one dominated by electrolyses at 2 V. The ratio of electronic current through the semiconductor bulk to the total current, which includes ionic polarization, varied from 56% at 5% RH to 0.68% at 76% RH at 0.5 V bias. The humidity dependent measurements confirmed the Grotthuss chain reaction, or at least proton migration, as the dominant conduction mechanism for porous pellet humidity sensors, even at low humidity. In most studies using DC polarization it is assumed that the current decay is exponential with time, but as clearly shown here this is not the case. The power law current discharge mechanism suggested by Jonscher is seen instead, with a log-log slope near 0.5 as is common with non-Debye capacitance.