  The self-consistent field theory (SCFT) is used to study the mean potential
near a charged plate inside a $m:-n$ electrolyte. A perturbation series is
developed in terms of $g = 4 \pi b/\ell_{\rm {\scriptscriptstyle DB}}$, where
$b, \ell_{\rm{\scriptscriptstyle DB}}$ are Bjerrum length and {\em bare} Debye
length respectively. To the zeroth order, we obtain nonlinear Poisson-Boltzmann
theory. For asymmetric electrolytes ($m \neq n$), the first order (one-loop)
correction to mean potential contains a {\em secular term}, which indicates the
breakdown of regular perturbation method. Using a renormalizaton group
transformation (RG), we remove the secular term and obtain a globally
well-behaved one-loop approximation with {\em a renormalized Debye length} and
{\em a renormalized surface charge density}. Furthermore, we find that if the
counter-ions are multivalent, the surface charge density is renormalized
substantially {\em downwards}, and may undergo a change of sign, if the bare
surface charge density is sufficiently large.
