Microwave-Assisted Ultrafast Synthesis of Bimetallic Nickel-Cobalt Metal-Organic Frameworks for Application in the Oxygen Evolution Reaction.

Herein, a series of monometallic Ni-, Co- and Zn-MOFs and bimetallic NiCo-, NiZn- and CoZn-MOFs of formula M 2 (BDC) 2 DABCO and (M,M') 2 (BDC) 2 DABCO, respectively, (M, M'=metal) with the same pillar and layer linkers 1,4-diazabicyclo[2.2.2]octane (DABCO) and benzene-1,4-dicarboxylate (BDC) were prepared through a fast microwave-assisted thermal conversion synthesis method (MW) within only 12 min. In the bimetallic MOFs the ratio M:M' was 4 : 1. The mono- and bimetallic MOFs were selected to systematically explore the catalytic-activity of their derived metal oxide/hydroxides for the oxygen evolution reaction (OER). Among all tested bimetallic MOF-derived catalysts, the NiCoMOF exhibits superior catalytic activity for the OER with the lowest overpotentials of 301 mV and Tafel slopes of 42 mV dec -1 on a rotating disk glassy carbon electrode (RD-GCE) in 1 mol L -1 KOH electrolyte at a current density of 10 mA cm -2 . In addition, NiCoMOF was insitu grown in just 25 min by the MW synthesis on the surface of nickel foam (NF) with, for example, a mass loading of 16.6 mg MOF /g NF , where overpotentials of 313 and 328 mV at current densities of 50 and 300 mA cm -2 , respectively, were delivered and superior long-term stability for practical OER application. The low Tafel slope of 27 mV dec -1 , as well as a low reaction resistance from electrochemical impedance spectroscopy (EIS) measurement (R far =2 Ω), confirm the excellent OER performance of this NiCoMOF/NF composite. During the electrocatalytic processes or even before upon KOH pre-treatment, the MOFs are transformed to the mixed-metal hydroxide phase α-/β-M(OH) 2 which presents the active species in the reactions (turnover frequency TOF=0.252 s -1 at an overpotential of 320 mV). Compared to the TOF from β-M(OH) 2 (0.002 s -1 ), our study demonstrates that a bimetallic MOF improves the electrocatalytic performance of the derived catalyst by giving an intimate and uniform mixture of the involved metals at the nanoscale.