The Warburg diffusion element is an equivalent electrical circuit component that models the diffusion process in dielectric spectroscopy.
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A Warburg impedance element can be difficult to recognize because it is nearly always associated with a charge-transfer resistance (see charge transfer complex) and a double layer capacitance (see double layer (interfacial)), but is common in many systems. The presence of the Warburg element can be recognised if a linear relationship on the log of a Bode plot (log|Z| versus log(ω)) exists with a slope of value –1/2.
General equation
The Warburg diffusion element (ZW) is a constant phase element (CPE), with a constant phase of 45° (phase independent of frequency) and with a magnitude inversely proportional to the square root of the frequency by:
where AW is the Warburg coefficient (or Warburg constant), j is the imaginary number and ω is the angular frequency.
This equation assumes semi-infinite linear diffusion, that is, unrestricted diffusion to a large planar electrode.
Finite-length Warburg element
If the thickness of the diffusion layer is known, the finite-length Warburg element is defined as:
where
where
There are two special conditions of finite-length Warburg elements: the Warburg Short (WS) for a transmissive boundary, and the Warburg Open (WO) for a reflexive boundary.
Warburg Short (WS)
This element describes the impedance of a finite-length diffusion with transmissive boundary. It is described by the following equation:
Warburg Open (WO)
This element describes the impedance of a finite-length diffusion with reflexive boundary. It is described by the following equation: