The following impedance spectrum was obtained for an electrochemical system.
Which of the following mechanisms can be considered to describe the electrochemical system?
A) $$\begin{array} { l } M \stackrel { k _ { 1 } } { \longrightarrow } M _ { a ds } ^ { + } + e ^ { - } \ \ M _ { a ds } ^ { + } + M \stackrel { k _ { 2 } } { \longrightarrow } M _ { a ds } ^ { + } + M _ { s o l } ^ { 2 + } + 2 e ^ { - } \end{array}$$
B) $$\begin{array} { l } M \stackrel { x _ { 1 } } { \longrightarrow } M _ { ads } ^ { + } + e ^ { - } \ \ M + M _ { a d s } ^ { + } \stackrel { k _ { 2 } } { \longrightarrow } M _ { s d } ^ { 2 + } + M + e ^ { - } \end{array}$$
C) $$\begin{array} { l } M \stackrel { k } { \longrightarrow } M _ { a ds} ^ { + } + e ^ { - } \ \ M _ { ads } ^ { + } \stackrel { k _ { 3 } } { \longrightarrow } M _ { ad s } ^ { 2 + } + e ^ { - } \ \ M _ { ads } ^ { 2 + } \stackrel { k _ { 3 } } { \longrightarrow } M _ { s o l } ^ { 2 + } \end{array}$$
D) $$\begin{array} { l } M \stackrel { k _ { 1 } } { \longrightarrow } M _ { a ds} ^ { + } + e ^ { - } \ \ M _ { a ds } ^ { + } \stackrel { k _ { 3 } } { \longrightarrow } M _ { s o l } ^ { 2 + } + e ^ { - } \ \ M \stackrel { k _ { 3 } } { \longrightarrow } M _ { so l } ^ { 2 + } + 2 e ^ { - } \end{array}$$

Question

The following impedance spectrum was obtained for an electrochemical system.
 Which of the following mechanisms can be considered to describe the electrochemical system?
A) $$\begin{array} { l } M \stackrel { k _ { 1 } } { \longrightarrow } M _ { a ds } ^ { + } + e ^ { - } \ \ M _ { a ds } ^ { + } + M \stackrel { k _ { 2 } } { \longrightarrow } M _ { a ds } ^ { + } + M _ { s o l } ^ { 2 + } + 2 e ^ { - } \end{array}$$ 
B) $$\begin{array} { l } M \stackrel { x _ { 1 } } { \longrightarrow } M _ { ads } ^ { + } + e ^ { - } \ \ M + M _ { a d s } ^ { + } \stackrel { k _ { 2 } } { \longrightarrow } M _ { s d } ^ { 2 + } + M + e ^ { - } \end{array}$$ 
C) $$\begin{array} { l } M \stackrel { k } { \longrightarrow } M _ { a ds} ^ { + } + e ^ { - } \ \ M _ { ads } ^ { + } \stackrel { k _ { 3 } } { \longrightarrow } M _ { ad s } ^ { 2 + } + e ^ { - } \ \ M _ { ads } ^ { 2 + } \stackrel { k _ { 3 } } { \longrightarrow } M _ { s o l } ^ { 2 + } \end{array}$$ 
D) $$\begin{array} { l } M \stackrel { k _ { 1 } } { \longrightarrow } M _ { a ds} ^ { + } + e ^ { - } \ \ M _ { a ds } ^ { + } \stackrel { k _ { 3 } } { \longrightarrow } M _ { s o l } ^ { 2 + } + e ^ { - } \ \ M \stackrel { k _ { 3 } } { \longrightarrow } M _ { so l } ^ { 2 + } + 2 e ^ { - } \end{array}$$

Quizplus · Accepted Answer

The impedance spectrum shows a depressed semicircle at high frequencies followed by a diffusion-controlled straight line at lower frequencies. This is indicative of a system where charge transfer across the electrode/solution interface is followed by diffusion of the reactants or products in solution. Choice C involves a reversible redox reaction followed by diffusion, which is consistent with the observed spectrum. Choice A involves a charge transfer followed by a chemical reaction, which would result in a different shape of the impedance spectrum. Choice B involves a charge transfer followed by adsorption, which also would result in a different shape of the impedance spectrum. Choice D involves an adsorption followed by a chemical reaction, which again would result in a different shape of the impedance spectrum.

The Following Impedance Spectrum Was Obtained for an Electrochemical System