On the various types of uncontrolled potential increase in electrochemical reactors—The anode effect

Various types of uncontrolled potential increase must be distinguished at gas-evolving electrodes. A theoretical analysis on the basis of fundamental relationships shows that the increase in potential – commonly associated with the so-called anode effect at cells with constant or varied current – is caused in some cases by limitation of mass transfer. In other cases it is the result of reaching a summit current value created by the presence of gas bubbles in contact with the electrode surface. Which of the causes is the controlling one depends on the bulk concentration of the reacting substance and on the state of the electrode surface, particularly on material and roughness directly acting on the wettability. It is not sufficient to attribute any uncontrolled potential increase solely to mass transfer control as hitherto done. Several types must be taken into consideration.