Electrochemical Investigations of Water and Hypochlorite Reduction on a-and y-FeOOH

Abstract: The cathodes in the electrochemical cells used for production of sodium chlorate process are made from mild steel. Due to the harsh environment in the chlorate cell, with oxidative species such as chlorate and hypochlorous acid, the cathodes corrodes. This corrosion might be beneficial, leading to surface enlargement and lower potential. It may however also have negative consequences, such as reduction of reaction intermediates and products, hypochlorite and chlorate, and the physical deterioration of the cathode. Since the electrochemical production of sodium chlorate is highly energy intensive, up to about 5.5 MWh is used per ton produced, it is of interest to reduce the energy consumption as much as possible. One way to lower the energy consumption is to understand how the cathode can be used as efficiently as possible in the process. Cathodes were taken from two different chlorate plants and were characterised with SEM, EDX and XRD. It was found that goethite was present on one of them while lepidocrocite was the corrosion product present on the second. It had also previously been established that the electrode showing lepidocrocite had longer starting period, i.e. before reaching sufficient current efficiency. Electrocatalytical properties were studied on pure phases of goethite and lepidocrocite electrodeposited onto titanium rotating discs and on carbon paste electrodes which were produced from pure powders. From Tafel plots it was seen that the activity towards hypochlorite reduction was lower for the iron oxyhydroxides than for a polished mild steel electrode. It was found that the activity for hypochlorite reduction followed the order of mild steel > goethite > lepidocrocite. It was determined, for all species, that the first electron transfer was the rate determining step and the transfer coefficient was 0.5. The study of hydrogen evolution on the species showed that the first linear sweeps had much lower activity compared to mild steel. However the activity changed during potentiostatic polarisation to almost the same as for the mild steel. Tafel slopes of 200-250 mV/decade showed that the first electron transfer was rate limiting but the transfer coefficient was small (0.3). It is presumed that the hydrogen evolution on all three different electrodes takes place on Fe(OH)2 which must first be formed on the surface. Since the transfer coefficient is low, the transition state is closer to the adsorbed hydrogen than to the water in solution. Hence the active site for water reduction is proposed to be the protonated surface group Fe(II)-OH2+ independent if the starting material is α-FeOOH, γ-FeOOH or mild steel.