  The device of electric double-layer transistor (EDLT) with ionic liquid has
been employed as an effective way to dope carriers over a wide range, which can
induce metallic state, magnetic reconstruction and superconducting transition.
However, the induced electronic state can hardly survive in the materials after
releasing the gate voltage, strongly restricting the experimental study for
discovery of physics. Here, we show that a permanent superconductivity with
transition temperature $T_c$ of 24 and 15 K is realized in single crystals and
polycrystalline samples of HfNCl and ZrNCl upon applying proper gate voltage
$V_{\rm G}$, respectively. Reversible change between insulating and
superconducting state can be obtained through applying positive and negative
$V_{\rm G}$ at low temperature such as 220 K, whereas $V_{\rm G}$ applied at
high temperatures ($\geq$ 250 K) could induce partial deintercalation of Cl
ions and result in irreversible superconducting transition. The present results
clarify a connection between traditional chemical synthesis and the
electrochemical mechanism of the EDLT induced superconductivity. Such a
technique shows a great potential to systematically tune the bulk electronic
state in the similar two-dimensional (2D) systems.
