Synergetic effect of C and Ni on hydrogen release from Mg–Ni-electrochemically synthesized reduced graphene oxide based hydride

Abstract

Mg–reduced graphene oxide (rGO), Mg–Ni and Mg–Ni–rGO nanocomposites were synthesized by ball milling. They were subsequently hydrogenated at P_H2 ≈ 15 bar, ∼320 °C. Isothermal (∼320 °C) and non-isothermal (room temperature – 450 °C, 5 °C min⁻¹) H release experiments were performed on the hydrogenated nanocomposites. The incubation time (dH/dt|_320°C ≈ 0) during isothermal H release drops from ∼15 min in Mg–rGO to ∼75 s in Mg–Ni and ∼60 s in Mg–Ni–rGO. The onset temperature during non-isothermal H release reduces from ∼350 °C in Mg–rGO to ∼275 °C in Mg–Ni–rGO. The reasons for the superior H release by Mg–Ni–rGO are investigated. Ball milling of Mg–rGO develops persistent Mg–C interaction at ∼283 eV from C1s X-ray photoelectron spectroscopy (XPS), involving electron transfer between Mg and C. In Mg–Ni, a non-stoichiometric Mg₂NiH_x phase and MgH₂ unit cell shrinkage develop upon H uptake resulting in improved H release. Mg–Ni–rGO possesses unreacted metallic Ni (X-ray diffraction) upon ball milling; Ni–C interaction (∼283.5–284.5 eV, C1s XPS) and red shift in G band (Raman) vis-à-vis Mg–rGO upon H uptake. Interestingly, Mg–Ni–rGO exhibits the individual effects of rGO and Ni addition. The Ni–C interaction from metallic Ni and possibly from Mg₂NiH_x lead to the superior H release by Mg–Ni–rGO. This work demonstrates the synergetic effect of adding multiple catalysts on H release.