To solve electric double layer between parallel plate

Hi.

I'm trying to solve electric double layer between charged parallel plate using Nernst-Planck eqn & Poisson eqn

rather than Poisson-Boltzmann eqn. Because I want to research double layer overlap and flow in nano channel.

At first step, I tried to solve that system. left end is charged wall.

________zero charge________



---------------------------------------------
zero charge
Using 'transport of dilute species' and 'electrostatics' modules, I can solve electric double layer in that system. There

was no problem. However, see the next domain.

________charged wall________



---------------------------------------------
charged wall
In above situation which is corresponding charged nanochannel, I failed to solve electric double layer.

If COMSOL gives me some error message, I agree that my model is wrong. But there's no error, convergence is very nice.

In spite of that, result is very strange comparing to EDL structure in text book.

Why do I encounter this type of failure? My BCs are wrong?

Followings are my BCs;

at charged wall, surface charge density, qs (electrostatics)
no flux (transport of dilute species)

at ends of channel, ground (electrostatics)
bulk concentration, c0 for cation and anion (transport of dile species)

for initial condition, potential = 0 & concentration = c0 for cation and anion

Maybe I think there's no problem in my model, so I confused.

Please let me know how can I solve EDL in that system using NP eqn & Poisson eqn rather than Poisson-Boltzmann eqn.

Thanks.