Cobalt-Based Coordination Polymer for Oxygen Reduction Reaction

Lack of control over the structure and electrically nonconductive properties of coordination polymers (CPs) creates a major hindrance to designing an active electrocatalyst for oxygen reduction reaction (ORR). Here, we report a new semiconductive and low-optical band gap CP structure [{Co₃(μ₃-OH)(BTB)₂(BPE)₂}{Co_0.5N(C₅H₅)}], 1, that exhibits high-performance ORR in alkaline medium. The electrical conductivity of compound 1 was measured using impedance spectroscopy and found to be 5 × 10^-4 S cm^-1. The Ketjenblack EC-600JD carbon used as a support for all the electrochemical methods such as cyclic voltammetry, rotating disk electrode, rotating ring-disk electrode and Koutecký-Levich analysis. The as-synthesized Co-based catalyst has the ability to reduce O₂ to H₂O by a nearly four-electron process. The crystal structure of 1 shows that the trimeric unit {Co₃(μ₃-OH)(COO)₅N₃} and monomeric unit {Co(COO)₂(NC₅H₄)₂}²⁺ are linked with BTB and BPE linkers to form a three-dimensional structure. Theoretical calculations predict that the monomeric center acts as an active catalytic site for ORR. This could be due to the efficient overlap of highest occupied molecular orbital-lowest unoccupied molecular orbital between monomer and O₂ molecule. This CP, 1, shows facile 3.6-electron ORR, and it is inexpensive compared with widely used Pt catalysts. Therefore, this CP can be used as a promising cathode material for fuel cells in terms of efficiency and cost effectiveness.