Understanding the electrical response of electrochemical convertors, such as fuel cells or batteries, involves elucidating the effect of the macroscopic voltage on the microscopic charge distribution at the electrode-electrolyte interface. I will present a quantum/classical model which couples a quantum molecular description of the electrode-electrolyte interface with a polarizable-continuum representation of the long-range effects of the ionic solvent. I will mainly focus on the mathematical and numerical aspects. In the last part of my talk, I will present numerical calculations of the vibrational Stark effect for chemisorbed CO, which demonstrate the efficiency of this approach. This is a joint work with I. Dabo, Y. Li and N. Marzari.