We study the time-dependent Poisson-Nernst-Planck (PNP) system with different diffusion coefficients on ion species and electroneutral initial conditions. Assuming that the Debye length is small relative to the domain size, we derive an asymptotic formula for the dynamic solution by matching outer and Debye layer solutions. It is interesting to note that for the time-dependent problem, the outer solution has a diffusive layer that does not occur in time-independent problems. This talk is based on a joint work with Huaxiong Huang and Jonathan Wylie.