Solution Resistivity, Ohmic Drop and Oxygen Reduction Rate at High Temperature Pressurized Water

The solution resistivity (Rsol) and ohmic drop (ΔEΩ) of a three electrode high temperature electrochemical autoclave system containing a water + 11550 ppb NH4OH (pH(25 °C) 10) solution were analyzed in the temperature range of 25 °C to 285 °C using electrochemical impedance spectroscopy (EIS) and galvanostatic potentiometry combined with automatic current interrupt iR compensation technique. The determined values were compared with the conductivity values calculated by a pH & Conductivity Calculator software. The results showed that when the temperature of the solution is increased the magnitude of the Rsol and therefore ohmic drop of the system decrease and after a defined temperature (150 °C for EIS and 180 °C for current interrupt) increase. The variations in the conductivity of the solution versus temperature calculated by the pH & Conductivity Calculator software were in more agreement with the impedance based Rsol values determined in this study. While investigation of the oxygen reduction reaction on platinum versus temperature showed that exactly at 180 °C (the temperature corresponding to minimum Rsol and ohmic drop values determined using the current interrupt technique) the oxygen reduction reaction reached its maximum rate and by further increasing the temperature the oxygen reduction rate decreased and the reaction mechanism switched from a mixed mass-charge transfer controlled process to a charge transfer controlled process. The experiment results revealed that the Rsol values determined by the current interrupt technique are more accurate than the values obtained by EIS.