Cobalt-Based Coordination Polymer for Oxygen Reduction Reaction

Abstract

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 [{Co 3 (μ 3 -OH)(BTB) 2 (BPE) 2 }{Co 0.5 N(C 5 H 5 )}], 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 O 2 to H 2 O by a nearly four-electron process. The crystal structure of 1 shows that the trimeric unit {Co 3 (μ 3 -OH)(COO) 5 N 3 } and monomeric unit {Co(COO) 2 (NC 5 H 4 ) 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 O 2 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.

Cobalt‐based coordination polymer as high activity electrocatalyst for oxygen reduction reaction: Catalysis by novel active site CoO 4 N 2

“…From the Figure B, Co‐3d states show much stronger hybridization with N‐2p states and O‐2p states from −6 to 3 eV, and it can be also observed that N‐2p states and O‐2p states overlap in the vicinity of the Fermi level, which can also enhance the stability of Co(C 6 H 5 CO 2 ) 2 (C 5 H 5 N) 2 . These results clearly demonstrated that Co(C 6 H 5 CO 2 ) 2 (C 5 H 5 N) 2 has good stability, which is consistent with the experimental results …”

“…The decreased energy of these two pathways are both higher than −0.41 eV, indicating the 4e − reduction mechanism via Pathway I should be more energetically favorable. This result is in good agreement with the experimental observations, in which the ORR process is proceeded via 3.6e − pathway . Therefore, we can predict that the whole ORR on the catalyst is mainly carried out through the Pathway I: O 2 → *OOH (T Co ) → *O (B Co‐N(2) ) → *OH (T Co ) → H 2 O.…”

“…The calculated free energy changes of each electron transfer step for Pathway I and II are shown in Table . Therefore, the η ORR of Pathway I is 0.43 V, which is consistent with experimental observation (0.42 V) . Moreover, the overpotential of ORR on Co(C 6 H 5 CO 2 ) 2 (C 5 H 5 N) 2 is lower than the theoretical value (0.45 V) as well as the experimental value (0.44 V) on the Pt(111), indicating it is indeed an effective ORR electrocatalyst.…”

“…These results clearly demonstrated that Co(C 6 H 5 CO 2 ) 2 (C 5 H 5 N) 2 has good stability, which is consistent with the experimental results. 26…”

“…Li et al have synthesized a conductive cobalt‐based MOF which is derived from a ZnCo bimetallic MOF, showing favorable ORR catalytic activity . Quite recently, Mandal et al have designed and synthesized a novel CP, which possesses three‐dimensional structure composed of trimeric unit {Co 3 (μ 3 ‐OH)(COO) 5 N 3 } and monomeric unit {Co(COO) 2 (NC 5 H 4 ) 2 } 2+ , linked by 1,3,5‐tris(4‐carboxyphenyl)benzene (BTB) and 1,2‐bis(4‐pyridyl)ethane (BPE). This CP structure has good conductivity, and can catalyze ORR through 3.6e − pathway.…”