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

Abstract

AbstractHerein, a series of monometallic Ni‐, Co‐ and Zn‐MOFs and bimetallic NiCo‐, NiZn‐ and CoZn‐MOFs of formula M2(BDC)2DABCO and (M,M’)2(BDC)2DABCO, 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 mgMOF/gNF, 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 (Rfar=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.