Warburg element

The Warburg diffusion element is an equivalent electrical circuit component that models the diffusion process in dielectric spectroscopy.

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 (Z_W) 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:

{Z_{W}}={\frac {A_{W}}{\sqrt {\omega }}}+{\frac {A_{W}}{j{\sqrt {\omega }}}}

{|Z_{W}|}={\sqrt {2}}{\frac {A_{W}}{\sqrt {\omega }}}

where A_W 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,^[1] 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^[2] is defined as:

{Z_{O}}={\frac {1}{Y_{0}}}\tanh(B{\sqrt {j\omega }})

where $B={\frac {\delta }{\sqrt {D}}}$ ,

where $\delta$ is the thickness of the diffusion layer and D is the diffusion coefficient.

There are two special conditions of finite-length Warburg elements: the Warburg Short (W_S) for a transmissive boundary, and the Warburg Open (W_O) for a reflexive boundary.

Warburg Short (W_S)

This element describes the impedance of a finite-length diffusion with transmissive boundary.^[3] It is described by the following equation:

Z_{W_{S}}={\frac {A_{W}}{\sqrt {j\omega }}}\tanh(B{\sqrt {j\omega }})

Warburg Open (W_O)

This element describes the impedance of a finite-length diffusion with reflexive boundary.^[4] It is described by the following equation:

Z_{W_{O}}={\frac {A_{W}}{\sqrt {j\omega }}}\coth(B{\sqrt {j\omega }})

References

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