Implementation of hydrogen storage systems requires moderate bonding strength. However, this goal has remained a challenge, either due to the weak physisorption or extremely strong chemisorption. Here, we report on a new phenomenon, namely that H2 binding can be externally enhanced (or weakened) via superimposition of a positive (or negative) electric field. We demonstrate this concept using an 8-Li-doped carbon nanotube. The calculated adsorption energy E(ad) = -0.58 eV/H2 under F = +0.010 au is 93.33% lower than that under 0.000 au (F indicates the field intensity). This is because the positive field produces an extra dipole moment. In contrast, E(ad) increases from -0.30 to -0.20 eV/H2 when F = -0.010 au. In view of the fact that storage systems are insensitive to small unexpected field fluctuations, the application of the electric field as a reversible switch makes practical sense.