Breaking the Scaling Relationship for Oxygen Reduction Reaction Using Molecular Cobalt Complexes

Area of Science:

• Catalysis
• Electrochemistry
• Coordination Chemistry

Background:

• Developing efficient electrocatalysts for oxygen reduction reaction (ORR) is crucial for energy technologies.
• Modifying the secondary coordination sphere (SCS) offers a promising strategy to tune catalyst performance.
• Understanding structure-activity relationships is key to designing superior catalysts.

Purpose of the Study:

• To design and synthesize novel Cobalt(III) complexes with varied SCS for electrocatalytic ORR.
• To investigate the impact of SCS modifications on catalyst efficiency, turnover frequency (TOF), and overpotential (ηeff).
• To elucidate the mechanism by which SCS influences the rate-determining step (rds) of the ORR.

Main Methods:

• Synthesis of a series of Co(III) complexes (1-8) with bis-pyridine-dioxime framework and distinct SCS.
• Electrocatalytic evaluation of ORR in acetonitrile using cyclic voltammetry and rotating disk electrode techniques.
• Linear free energy relationship (LFER) analysis and kinetic studies to determine the rds and probe mechanistic details.

Main Results:

• All synthesized Co(III) complexes selectively catalyzed the 4e-/4H+ reduction of O2.
• A general trend of increasing TOF with ηeff was observed, consistent with LFER predictions.
• Complexes 2 and 3 exhibited exceptionally high TOF values, significantly deviating from the LFER trend.
• Kinetic analysis identified protonation of the Co(III)(O2•) adduct as the rds, with SCS in 2 and 3 facilitating proton transfer.

Conclusions:

• The secondary coordination sphere plays a critical role in enhancing electrocatalyst efficiency beyond conventional LFER expectations.
• Specific SCS modifications, like those in complexes 2 and 3, can act as proton relay sites, accelerating the rds.
• This work highlights the importance of rational SCS design for developing highly active and efficient electrocatalysts for ORR.