activation energy as a function of the local electric field E

Dear all,
I'm simulating the movement of charged species using Nernst-Planck module.
Defining the Diffusion coefficient; I would like to define the activation energy for diffusion as a function of the local electric field. (lowering of the barrier due to applied voltage)
Because the electric field is not listed as a variable in the Nernst-Planck module; what do I have to do to define it in my expression.

Thanks,
Sebastian