Insights into the electrolyte changes during the electro-deoxidation of niobium oxide cathode

Abstract

In the last two decades, tremendous work has been investigating the electro-reduction of metal oxide cathodes in what is known as the FFC-Cambridge process. This paper explores the electrolyte changes that occur during the electro-deoxidation of metal oxides. The investigation takes niobium oxide as a case study and subjects a Nb2O5 cathode to a constant potential difference of 2.5V between the cathode and carbon anode during the in-situ recording of the cyclic voltammetry of the electrolyte on a glassy carbon electrode. The deoxidation of the cathode supplies oxygen ions that increase the oxygen content of the electrolyte, even in the very early stage of the process. We could detect many non-Faradic processes and increase the electrolyte electronic and ionic conductivity a few hours after starting the electro-deoxidation due to the reduction of CaO. It was also possible to detect partial dissolution of the cathode in the electrolyte and back deposition of Nb on the current collector. The dissolved niobium ions can combine with the oxide ion in the melt to form a negatively charged complex which can be discharged on the anode to form a thick ceramic layer. The electrolyte changes were also investigated using electrochemical impedance spectroscopy.