Tannic Acid-Mediated In Situ Controlled Assembly of NiFe Alloy Nanoparticles on Pristine Graphene as a Superior Oxygen Evolution Catalyst

Controlled assembly of small and highly dispersed earth-abundant transition metal-based oxygen evolution reaction (OER) catalysts on pristine graphene could greatly boost the OER efficiency while significantly reducing the cost, but this presents great challenges. Pristine graphene (rather than reduced graphene oxide or graphene with a damaged electronic structure) supported NiFe alloy nanoparticles (NPs) have been synthesized for the first time via tannic acid (TA)-mediated in situ controlled assembly, in which TA not only noncovalently modifies the graphene to strongly coordinate with Ni2+ and Fe2+ but also in situ reduces these metal ions to alloy NPs. The chemical composition of these alloy NPs can be tailored via changing the ratio of Ni2+ and Fe2+ in the reaction. These nanohybrids show ultrahigh and tunable OER catalytic activities. The optimum one obtained using the ratio of 9:1 achieves 10 mA cm(-2) at an oveipotential of 246 mV and shows a Tafel slope of 46 mV dec(-1), both of which are lower than those of most reported OER catalysts, including state-of-the-art Ru- and Ir-based ones. In addition, this catalyst exhibits little change of the overpotential after 20 h of chronopotentiometric measurement (from 246 to 258 mV at 10 mA cm(-2)) and negligible current loss after 1000 CV cycles (from 123.3 to 133.4 mA cm(-2) at 1.54 V). The superior catalytic activity and durability are ascribed to the in situ assembled small, highly dispersed, and highly conductive NiFe alloy NPs on pristine graphene significantly facilitating the electron transfer and increasing the electrochemically accessible active sites, the doped FeOOH remarkably promoting the intrinsic activity of Ni(OH)(2) on the surface of these alloy NPs, and the robust in situ growth greatly enhancing the durability during the OER testing. This work provides a green, facile, and economical strategy to controllably fabricate low-cost and high-performance OER catalysts and also sheds light on the performance enhancement mechanism from tunable OER catalytic activity.