Hi All,
I am working on bipolar electrodes immersed in solutions. As the electron density is different through the length of these electrodes, I was wondering if you could guide how to calculate the thickness of double layer capacitance over it in different areas.
Best,
Meisam
Double layer thickness is usually discussed in terms of the screening length (the Debye length, 1 / K) which depends on ionic strength and relative permittivity. This is calculated using Gouy-Chapman theory which has similar assumptions but different geometry to Debye-Hueckel theory. In aqueous PBS, the Debye length will be around half a nanometre, i.e. a little larger than the radius of a hydrated potassium ion (330 pm). Consequently, the theory here is not a huge amount of help since the granularity of the electrolyte solution is obviously evident and continuum assumptions can no longer be justified. However, at these ionic strengths and for typical applications where double layer capacitance is more of a nuisance rather than the core topic of investigation, it is reasonable to adopt a Helmholtz descriptions (rather than the more detailed Parsons model) of the double layer- the electrode potential is effectively screened by the first layer or two of hydrated ions and there is no diffuse layer. However, specifically adsorbed species, typically anions, can complicate matters and any unsuspected inert insulating spectator species that adsorb will also decrease the capacitance below typical values of 30-50 uF/ cm^2 on gold or platinum.
Dear Danny,
Many thanks for your comprehensive guidance. It provided useful information specially in terms of distances for me.
Hello@ I have a question about How can I calculate double layer thickness of ca-bentonite without any total dissolved cations? In the other hands I have to get concentration's cations of double layer or pore water? Thanks
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