Engineering Mesopores and Unsaturated Coordination in Metal-Organic Frameworks for Enhanced Oxygen Reduction and Oxygen Evolution Activity and Li-Air Battery Capacity

Metal-organic frameworks (MOFs) have undergone a rapid research expansion as unpyrolyzed oxygen electrocatalysts due to their flexible and variable compositions and structures. However, the most studied MOFs possess microporous nature, which limits the mass-transfer properties and reduce the space to store insoluble discharge product of aprotic metal-air batteries. We herein report a partial ligand removal approach to create mesopore defects and unsaturated coordination in primarily microporous Cu-BTC (BTC = 1,3,5-bezenetricarboxylic acid). The mesopores were engineered by introducing isophthalic acid as a partial ligand, which has one carboxylic group missing compared to BTC, to construct mesoporous Cu-BTC (MCu-BTC). MCu-BTC was further loaded by Co species (Co/MCu-BTC) to boost the bifunctional catalytic performance toward oxygen reduction and oxygen evolution reactions. The electrochemical characterizations indicate that the mesopores of MCu-BTC enhanced the discharging capacity (ca. 7000 mAh.g(-1)) of Co-10/MCu-BTC in an aprotic Li-air battery owing to the mesopores, which allowed electrolyte infiltration for both mass and charge transfer and provided more space to store the discharge product. Further modeling simulations shed light on the contribution of the unsaturated coordination by significantly promoting the electronic conductivity and generating adequate interactions with reactants, leading to enhanced mass- and charge-transfer properties.