Electrochemical Impedance Spectroscopy For Electro-catalyzed Oxidation Of Two Anions On Platinum Electrode

Cyclic voltammetry and potentiostatic electrochemical impedance spectroscopy were uesed to study on the mechanism of electrochemical oxidation of two anions on platinum electrode.The effects of pH and chloride on the reaction were discussed.AC impedance spectrum of the reactions were fitted by the equivalent circuit and Faraday current formula.The mechanism of electro-oxidation of single anion on the platinum electrode was discussed.AC impedance spectrum shows capacitive-resistance behavior at low potential as OHads abosorbed on the platinum electrode.The radius of the capacitive reactance arc was dependant on the concentrations of OH" in solutions which related to the hydroxyl phase on Pt electrode surface.At high potential,adlayer of hydroxyl phase Pt-OHads changes to platinum oxide with further oxidation.Both membrane formation and dissolution occur on the electrode surface,which reflected capacitive and inductive resistance on the electrochemical impedance spectroscopy.The formation of platinum oxides was related to the pH value of reaction solutions attribute to the participate of OH-on the oxidaized Pt oxide species.Therefore,the pH of the reaction solutions lead to different Faraday behavior of the AC impedance spectrum.Chloride strongly adsorbed on the Pt electrode surface compete with the adsorption of OH",resulting in a spiral arc in the complex plane impedance diagram.The AC impedance spectrum illustrate that two capacitance-resistance arcs represent the adsorption of the two anions.Different adsorption capacity lead to the different arc radius of the capacitive reactance.Meanwhile,the adsorption of Cl-occupies the surface active site of the platinum electrode,the potential range of platinum oxide generation is shifted backwards.At high potential,Cl-can be adsorbed on the surface area of the platinum oxide passivation film and promote the dissolution of the passivation film.Therefore,there are three processes of dissolution of platinum hydroxide film,generation of platinum oxide film and dissolution of platinum oxide film on the surface of the electrode.The AC impedance spectrum shows corresponding capacitive and resistive behavior and two different types of inductive reactance.AC impedance of the electrode system was affected by the state variables that dependent with the intermediates such as Pt-OHads,Pt-Clads and Pt-O,through the analysis of the AC impedance spectrum and deducing the formula of Faraday current.The amount of adsorption and electrode potentials of different components as the state variables was calculated to obtain two constitute of Faraday impedance charge transfer resistors Rct and polarization resistors Zx in series.A series of the system Faraday behaviors could be performed with different electrode potential and the amount of anion adsorptions that related to polarization resistance.