Ultrasmall Ni/NiO nanoclusters on thiol-functionalized and-exfoliated graphene oxide nanosheets for durable oxygen evolution reaction

The demand of high anodic potential for oxygen evolution reaction (OER) because of its sluggish kinetics limits the overall efficiency and practical applications of electrochemical water splitting process. Though metal oxides are envisioned as the potential contenders in this quest due to their high redox potential, nevertheless their low conductivity and instability are among the formidable challenges that need to be addressed. Here, we demonstrate the synthesis and electrochemical applications of covalently linked ultrasmall Ni/NiO NCs (about ∼2 nm) with the exfoliated thiol-functionalized graphene (G-SH) nanosheets as a highly efficient and durable electrocatalyst for OER. Ni/NiO@G-SH nanohybrid showed a very sharp onset potential of 1.46 V, Tafel slope of 46 mV/dec, turnover frequency (TOF) of 245 s^-1@1.72 V and a steady-state current response at 10 mA/cm² for more than 3 days in 0.1 M KOH solution. We believe that the active redox couple of Ni^2+/3+ in nanoscale Ni/NiO at equilibrium fluctuates periodically for the expected sustained OER process. Moreover, the synergistic effect between NCs-GO-SH nanosheets together with the slightly reducing environment due to the strong electron donor thiol groups facilitate the dynamics of the released O₂ as a final product and thus encourage the recycling potential of such nanohybrid materials at low anodic bias.