I have used EIS to characterize carbon nanotube-based composite material in a 2-electrode configuration. It appears that the use of CPEs to fit the EIS spectra was very simple, and it could fit most of the data. However, how should I interpret the CPEs with respect to structure of my material? Any good sources for reading? Thanks.
"Constant Phase Elements" or CPEs are impedances following the equation:
Z = Zo(jω)-α
Some of the first to observe this behaviour were K.S. Cole and R.S. Cole. In his (excellent!!) book "Membranes, Ions and Impulses", K.S. Cole states that the apparent simplicity of this expression "..is an alternative expression of our ignorance of the mechanism"
You can find many different theories for the mechanisms causing this behaviour, but here are two facts, that you can store in the back of your mind when studying these mechanisms:
- iCPE is observed when there is a fixed ratio between the amount of energy stored and dissipated in each cycle.
- CPE is always observed only in a limited frequency interval.
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