The free energy of electron transfer (ET) is computed for the electrochemical Fe2+/Fe3+ reaction with a Pt electrode. The quantum mechanical effects of the solvent nuclei are included via the method of path integral quantum transition state theory (PI-QTST). The effect of isotopic substitution (deuterium for hydrogen) on the ET rate constant at zero overpotential is investigated by analyzing the change in the quantum free energy barrier for ET. The results exhibit an inverse isotope effect, and interestingly, the isotope effect is primarily due to changes in the second and third water solvation shells around the iron ion.