11. ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY 

• EIS- The rate of an electrochemical process can be described by a time constant (τ/s)
  • Allows us to study processes with different time constants by using an ac signal within a broad frequency
• IS - impedance spectroscopy
  • can also mean immittance spectroscopy, which includes:
  • impedance: Z
  • admittance: Y
  • modulus: M
  • complex dielectric constant, dielectric permittivity
• Excitation signal
  • Can be described as a rotating vector E
  • Two vectors that rotate with the same rate (w) the impedance (Z) can be expressed as a complex number:
  • Z=Z’+jZ’’
  • Z’ = impedance at 0
  • Z’’= impedance at ϕ= pi/20=90 degrees
  • Impedance is measured at a number of different frequencies (ωx)
• Response
  • current sine-wave with amplitude = I

Electrochemical processes

• At interfaces:
  • electron transfer, charge-transfer resistance (Rct)
  • electrical double layer, double-layer capacitance (Cdl)
  • specific adsorption (RA , Ca)
• In bulk
  • ohmic resistance, solution resistance (RS)
  • mass transfer, diffusion impedance (ZW)
  • migration, convection

EIS

Summary

• Electrochemical impedance spectroscopy (EIS) gives information about kinetics and mechanisms of electrochemical processes
• EIS can be used to study and identify processes with different time constants (reaction rates)
• EIS is an important method in electrochemistry and material science