Electrochemical impedance spectroscopy of synthetic pyrite doped with As, Co, and Ni

Electrochemical impedance spectroscopy (EIS) data were collected in pH 1.87 HCl solution with 0.0005 M ferric and 0.0005 M ferrous iron on a set of 19 synthetic pyrite electrodes previously studied with cyclic and ac voltammetry. The electrodes were either undoped or doped with As, Co, or Ni. Data were also collected on five natural pyrite samples. All the EIS spectra were modeled with variations of an equivalent circuit developed to represent impedance associated with charge transfer from a semiconductor to an oxidizing agent in an electrolyte solution via surface states. The electrodes were grouped according to their response to ac voltammetry at 100 Hz from an earlier study and according to dopant type for statistical comparison of the equivalent circuit element values. Previous evidence suggests that defect states in the bulk pyrite arising from impurity atoms become surface states at the interface and affect oxidation kinetics by mediating charge transfer. Charge transfer resistance is greatest for electrodes that responded with higher current to ac voltammetry. A two-step charge transfer mechanism with carrier recombination mediated by surface states is proposed. (C) 2008 The Electrochemical Society.