(389i) Designing M-N-C Electrocatalysts with Inspiration from Thermal and Molecular Catalysis

The next generation of catalytic processes to meet the grand challenge of decarbonization of the energy and chemical industries will include technologies that use renewable electricity, different feedstocks from petroleum, and spatially distributed production modalities. In this context, atomically dispersed metals incorporated into nitrogen-doped carbon (“M-N-C”) are promising alternatives to precious-metal catalysts for fuel cell and (electro)chemical synthesisapplications. M-N-Cs have active site motifs reminiscent of molecular complexes and they catalyze both thermal and electrochemical reactions, which highlights opportunities for cross-disciplinary innovation. In this presentation, I will discuss two examples where approaches from the worldviews of thermal and molecular catalysis are leveraged to understand and design M-N-C electrocatalysts.

In the first example,¹ the mechanism of O₂ reduction mediated by hydroquinone (HQ) molecules is studied over M-N-Cs (M = Co, Fe) in the absence of an external electrochemical driving force. Although conventional wisdom describes this thermocatalytic system using coupled electrochemical half-reactions, our kinetic studies reveal a contrasting mechanism at M-N-C surfaces crowded by HQ molecules, which are poised to stabilize the transition state via a unique inner-sphere mechanism. Molecular crowding of the surface changes the dominant reaction pathway and circumvents the rate–potential relationship expected for electrocatalytic O₂ reduction, opening new opportunities to design fuel cell systems that reduce O₂ at lower overpotential by using mediators to drive thermocatalytic pathways.

In the second example,² an approach to synthesize Fe-N-C is developed using inspiration from routes for the metalation of molecular macrocyclic complexes. FeN_x moieties are formed without exposure to high temperatures (>200 °C) and their density is improved by increasing the availability of uncoordinated N_x binding sites in the N-doped carbon support. Aerobic HQ oxidation serves as a catalytic benchmark reaction to corroborate this improvement in Fe dispersion. I envision that the modularity of the synthetic approach will enable rational design of Fe-N-C catalysts through tailoring their underlying supports.

References

[1] Bates, J. S.; Biswas, S.; Suh, S.-E.; Johnson, M. R.; Mondal, B.; Root, T. W.; Stahl, S. S. J. Am. Chem. Soc. 2022, 144, 922–927.

[2] Bates, J. S.; Khamespanah, F.; Cullen, D. A.; Al-Zahrani, A.; Hopkins, M. N.; Martinez, J. J.; Root, T. W.; Stahl, S. S. Submitted.