solving poisson equation in electroosmotic flow

hi everyone,i recently write a LBM FORTRAN code to solve Poisson-Boltzmann equation in a microchannel
does anyone know how to apply boundry condition for this equation?!

in the literatures there is a boundry condition model for this kind problem,but when i apply,it doesn’t work,i mean the code do not converge,and the error after some iterate become NaN.

if anyone knows another kind of boundry condition for poisson-Boltzmann equation ,please help me

please see this link,and tell me is this the correct boundry or not

thanks all