Electrochemical behavior of exfoliated NiCl2–graphite intercalation compound affected by hydrogen sorption

The aim of this investigation was to examine the processes occurring in alkaline solution for electrodes made of graphite intercalation compound with nickel chloride (NiCl2–GIC) and exfoliated graphite intercalation compound with nickel chloride (NiCl2–EGIC). On the contrary to most acceptor graphite intercalation compounds (GICs), which are easily exfoliated upon heat treatment, NiCl2–GIC appeared to be thermally resistant even if the process was carried out at very high temperatures. Exfoliation of NiCl2–GIC was succeeded at room temperature by chemical method in the solution of hydrogen peroxide. The effects of exfoliation were examined by XRD and SEM techniques. As shown by these techniques, after exfoliation a part of NiCl2 intercalate persisted in the graphite lattice. Using the cyclic voltammetry method it was shown that the process of electrochemical reduction occurring for electrode made of NiCl2–EGIC differs significantly from that for the original NiCl2–GIC. The conversion of NiCl2–GIC to NiCl2–EGIC reveals new possibilities of practical use of the electrode, in which the Ni(II) → Ni reduction reaction and hydrogen sorption occur simultaneously. The obtained results have shown that the anodic charge increases on increasing the time of hydrogen sorption at a given potential and on decreasing the potential of hydrogen sorption. From comparison, one can conclude that the surplus charge measured during the anodic run is associated with the oxidation reaction of hydrogen stored in NiCl2–EGIC during the precedent reduction reaction resulting in the generation of electrochemically active nickel clusters in exfoliated graphite matrix. It was shown that the diffusion rate of hydrogen ions to/in the structure of NiCl2–EGIC exerts important influence on the kinetics of hydrogen storage.