Coordinately unsaturated metal–organic framework as an unpyrolyzed bifunctional electrocatalyst for oxygen reduction and evolution reactions†
The development of energy storage and conversion systems, such as the unitized regenerative fuel cells and the rechargeable metal–air batteries, heavily relies on the effective bifunctional electrocatalysts for oxygen reduction/evolution reactions (ORR/OER). We herein report a Co metal–organic framework (MOF, denoted as Co-BTC-IMI), which was synthesized from Co ions, 1,3,5-benzenetricarboxylic acids and imidazoles through a hydrothermal reaction. The MOF was studied as an unpyrolyzed bifunctional electrocatalyst in an alkaline half-cell, where it delivered an ORR half-wave potential of 0.80 V vs. the reversible hydrogen electrode (RHE) and an OER potential of 1.59 V vs. RHE at 10 mA cm−2. Co-BTC-IMI possessed good structural stability against hydroxide corrosion and great π–π stacking and hydrogen bonding interactions and, therefore, excellent bifunctional catalytic durability. Structure characterizations demonstrated that one of the Co(II) ions in Co-BTC-IMI was connected by two O atoms from two different deprotonated 1,3,5-benzenecarboxylic acids, leaving the coordination of Co(II) largely unsaturated. Further modeling simulations shed light on the advantage of the unsaturated coordination by significantly promoting the interactions with molecular water and oxygen and improving the electronic conductivity, leading to enhanced mass and charge transfer properties. The moderate affinity of oxygen and higher valent Co after water adsorption contributed to the enhanced bifunctional catalytic activity of Co-BTC-IMI as compared to Co-BTC. The overall bifunctional activity of Co-BTC-IMI is promising as compared to the recently reported unpyrolyzed MOFs, as well as the commercial platinum-group metals.