Joint Electroreduction of Chromium and Boron Ions and Electrochemical Synthesis of Chromium Borides in Halide–Oxide Melts

Abstract

The electroreduction of oxygen-containing chromium ions $${\text{CrO}}_{{\text{4}}}^{{{\text{2}}-}}$$ and $${\text{C}}{{{\text{r}}}_{{\text{2}}}}{\text{O}}_{{\text{7}}}^{{{\text{2}}-}}$$ and their joint electroreduction with fluoroborate ion $${\text{BF}}_{{\text{4}}}^{ - }$$ in the equimolar KCl–NaCl melt at 1073 K are studied by cyclic voltammetry. The electroreduction of $${\text{CrO}}_{{\text{4}}}^{{{\text{2}}-}}$$ and $${\text{C}}{{{\text{r}}}_{{\text{2}}}}{\text{O}}_{{\text{7}}}^{{{\text{2}}-}}$$ proceeds via two steps to form chromium oxide Cr2O3 in the first step. In the second step, Cr2O3 also precipitates in the case of the electroreduction of $${\text{CrO}}_{{\text{4}}}^{{{\text{2}}-}}$$ , and a mixture of Cr2O3 and metallic chromium is formed due to the electroreduction of $${\text{C}}{{{\text{r}}}_{{\text{2}}}}{\text{O}}_{{\text{7}}}^{{{\text{2}}-}}$$ . Against the background of the KCl–NaCl melt, fluoroborate ion can interact with the $${\text{C}}{{{\text{r}}}_{{\text{2}}}}{\text{O}}_{{\text{7}}}^{{{\text{2}}-}}$$ ion to form chromium ( $${\text{Cr}}{{{\text{O}}}_{{\text{2}}}}{\text{F}}_{{\text{4}}}^{{{\text{2}}-}}$$ ) and boron ( $${\text{BOF}}_{{\text{2}}}^{-},$$ BOF) oxyfluoride complexes. The possibility that electrochemical synthesis of chromium boride phases occurs in a halide–oxide melt is shown by the joint electroreduction of chromium and boron oxyfluoride complexes.