Electrochemical effect manifests itself as an unfavorable nonlinear response to an applied electric field in electrochemical devices and links intimately to the configuration of the molecular ions in the electric double layer (EDL) formed at the electrolyte/electrode interface. In this work, we probe the origin of the electrochemical effect using a double-gate graphene field effect transistor (GFET) of ionic liquid DEME-TFSI (or BMP-TFSI) top-gate, paired with ferroelectric Pb_0.92La_0.08Zr_0.52Ti_0.48O₃ (PLZT) back-gate of compatible gating efficiency. By measuing the GFET Dirac point shift in response to the bi-polar charge doping of the EDL molecular ions, the configurations of the interface molecular ions can be extracted and their dynamic response to visible light and gate electric field, quantified. We have observed that a strong electrochemical effect sets in as a consequence of the TFSI molecular anions self-organizing on a treated GFET surface, resulting in the Dirac point shift to a large positive voltage. Moreover, a reversible order-disorder transition of this molecular ion monolayer can be triggered using light illumination, revealing the critical role of the surface ion configuration on the overall performance of electrochemical devices.