Electrochemical determination of the diffusion coefficient of cations into Chevrel phase-based electrochemical transfer junction by potential step chronoamperometry and impedance spectroscopy

The molybdenum chalcogenides Mo6X8 (X = S, Se) offer the possibility of intercalation/de-intercalation processes by chemical or electrochemical way. Besides the different applications of so-called Chevrel phases, we have proposed an electrochemical transfer junction for selective recovery of metallic cations in the perspective of recycling of industrial liquid mineral wastes. Thus, the knowledge of the diffusion properties of cations in the Chevrel phases is essential. Here we report on the electrochemical determination of diffusion coefficients of Co2+, Ni2+, Fe2+, Cd2+, Zn2+, Mn2+ and Cu2+ for Mo6S8 and Mo6Se8 matrices. Experiments were realized on samples with compactness of 50% and 96–98%. They point out that the lower compactness is unfavorable to the mobility of the cobalt ions. From potential step chronoamperometry and electrochemical impedance spectroscopy, the diffusion coefficients were found around 10−9 cm2 s−1, even 10−6 cm2 s−1 for copper. These results confirm the high mobility of transition metal ions in studied phases and complete the data for Co, Fe or Mn–Mo6S8 system and Mn–Mo6Se8 system. For the sulfide phase, the following sequence for D˜ is observed Ni < Co < Fe < Cd < Zn < Mn ≪ Cu and can be explained in regards with structural considerations and repulsion effects for copper.