Enhancement of electrochemical hydrogen storage in NiCl_2-FeCl_3-PdCl_2-graphite intercalation compound effected by chemical exfoliation

摘要

In the present work, a quaternary NiCl_2-FeCl_3-PdCl_2-graphite intercalation compound (NiCl_2-FeCl_3-PdCl_2-GIC) was successfully synthesized by molten salts method. A part of this compound was subsequently subjected to chemical exfoliation to obtain expanded compound (NiCl_2-FeCl_3-PdCl_2-EGIC). The changes created in crystalline structure, morphology and chemical composition of GIC due to exfoliation were examined by XRD, SEM and EDS techniques and then related to electrochemical behaviour of electrodes made of the original and exfoliated compound. The results of electrochemical studies carried out by the cyclic voltammetry (CV) method in 6M KOH solution showed that current charges of all the cathodic and anodic peaks recorded for NiCl_2-FeCl_3-PdCl_2-EGIC are considerably higher already in the first two cycles as compared to those observed for the original NiCl_2-FeCl_3-PdCl_2-GIC. This improvement is ascribed to chemical exfoliation leading to a tremendous development of surface area of the compound due to the splitting and wrinkling of graphite flakes followed by easier access of hydroxyl ions of the electrolyte to active species of intercalates preserved between the graphene interspaces as well as expelled from the graphite interspacing. A large anodic peak was recorded on CV curves after the potentiostatic polarization of electrodes at the potential of -1.2 V where the reaction of hydrogen sorption/evolution occurs and intercalates highly dispersed in the graphite matrix are reduced to a metal form. This peak mainly corresponding to the recovery of hydrogen stored in the electrode appeared to be over five times higher for electrode made of exfoliated compound. This significant enhancement of the hydrogen storage capacity is attributed to electrochemically active Pd nanoparticles highly dispersed in porous structure of exfoliated compound and likely functioning in synergy with Ni/Fe clusters.