Kinetic and Mass-Transfer Parameters for Ce(III) Electro-oxidation in Nitric Acid with∕without Anion Impurities

Abstract

In this study, Koutecky–Levich ͑K-L͒ and cyclic voltammetry ͑CV͒ techniques were used to measure the kinetic and diffusion parameters for Ce͑III͒ electro-oxidation on glassy carbon in 4 M HNO 3 with/without SO 4 2− , Cl − , or Cr 2 O 7 2− impurities. It was found that the apparent diffusion coefficients ͑D͒ of Ce͑III͒ determined using the K-L and CV approaches were 1.65 ϫ 10 −5 and 1.12 ϫ 10 −5 cm 2 s −1 , respectively, while the D values varied by −5 to +15% and −2 to −26%, respectively, for the single-impurity cases. For the three-impurity case, the D value ͑1.59 ϫ 10 −5 cm 2 s −1 ͒ from K-L estimation was approximately three times that from the CV approach. The standard rate constants ͑k ° ͒ for Ce͑III͒ oxidation estimated from the K-L and CV approaches were 2.63 and 1.80 ϫ 10 −3 cm s −1 , respectively, whereas the k ° values were lowered by 35–91% and 38–80%, respectively, for the SO 4 2− case and by 46–49 and 33–58%, respectively, for the Cl − case. The multi-impurity case had the lowest rate constant around limiting current for Ce͑III͒ oxidation. The obtained kinetic and diffusion data are useful for the design of Ce͑III͒ to Ce͑IV͒ electro-regenerating systems and the evaluation of using spent Cr-etching solutions as candidate electrolytes in zinc–cerium redox flow batteries.