Metal (Ni, Co)-Metal Oxides/Graphene Nanocomposites as Multifunctional Electrocatalysts

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 5799 wileyonlinelibrary.com issues associated with energy security and environmental pollution. [ 1–5 ] Oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) are the most crucial electrochemical reactions to realize energy storage and conversion in these technologies. Although Pt-, Ir-, and Ru-based materials exhibit the highest activity for these electrochemical reactions, these precious-metal catalysts cannot be largely used for these clean energy because of their scarcity on earth and high cost. [ 6,7 ] Therefore, high-active, low-cost, and durable precious-metalfree catalysts from earth-abundant elements have been attracted considerable attention since last decade. [ 8–11 ] Among studied nonprecious metal catalysts, [ 12 ] nickel and cobalt are earth-abundant, low-cost, and environment-friendly materials that have widely been explored as electrocatalysts for the oxygen or hydrogen reactions in energy conversion and storage devices. [ 13–15 ] However, the pure cobalt and nickel oxides usually show insuffi cient electrical conductivity and low reactive surface areas, resulting in limited kinetics during these electrochemical reactions such as ORR, OER. [ 16 ] Oppositely, standalone metallic Ni or Co has good electrical conductivity, but is less active than Pt, because the formation energies of Ni–H or Co–H is lower than that of Pt–H for the HER. [ 17 ] Furthermore, combining graphene with metal or metal oxide is an effective way to improve catalytic activities due to the high surface area and excellent electrical conductivity of graphene, [ 18–20 ] thereby increasing number of active sites and promoting the charge transfer in electrodes. [ 21–23 ] For example, Co 3 O 4 /graphene, [ 24 ] Ni/graphene fi lm, [ 25 ] and NiO/rGO [ 26 ] composite catalysts have been explored showing enhanced catalytic activity, relative to single metals or metal oxides. Based on previous studies on the nickel or cobalt electrocatalysts, in this work, we synthesized a new family catalyst including Co-CoO/N-rGO and Ni-NiO/N-rGO via a pyrolysis of graphene oxide-supported cobalt and nickel salts, respectively. The possible synergetic effect among transition metals, metal oxides, and graphene was systematically studied, making them simultaneously highly active for the OER, ORR, or HER. Metal (Ni, Co)-Metal Oxides/Graphene Nanocomposites as Multifunctional Electrocatalysts