ELECTROLYTE WETTING EFFECTS IN THE COMPARATIVE POLARIZATION BEHAVIOR OF THE HYDROGEN EVOLUTION REACTION AT NI-MO-CD ELECTRODES IN KF.2HF AND KOH.2H2O MELTS

In the electrolyte process of fluorine production from KF.2HF melts at 85-degrees-C, unusually high polarization arises both at the cell ano des (carbon) and cathodes (mild steel) at moderate current densities. The anomalous polarization behaviour in fluorine evolution at carbon a nodes has been extensively studied but much less is known about the or igin of hyperpolarization in hydrogen evolution at the cathodes. Here, the results of comparative polarization studies at a Ni-Mo-Cd composi te cathode material in a KF.2HF melt and a corresponding aquo-analogue melt of KOH.2H2O are reported. Large differences are observed which a re attributed to the different wetting characteristics of these two me lts at the interface of the Ni-Mo-Cd cathodes that are microporous. Su ch electrode material also offers a major improvement of cathode polar ization over that at mild steel surfaces. Conclusions on wetting effec ts are based on: (i) a comparative determination of hydrogen-bubble co ntact angles at the electrode in the two melts and (ii) an evaluation of the double-layer capacitances of the electrode interfaces with the two melts as determined by the fast potential-relaxation method, at sh ort times after current interruption. The apparent double-layer capaci tances at a given electrode are very different in the two melts.