Impact of Local Structure and Spin on the ORR Performance of Single Atom M-N-C Catalysts
Authors: Jung Hyeonjung , Hossain Md Delowar , Bajdich Michal 
Published: December 2024

Transition metal single-atom catalysts supported on N-doped graphene (M-N-Cs) offer significant advantages in metal utilization and uniformity of active sites compared to traditional heterogeneous catalysts. They are particularly notable for their superior electrocatalytic Oxygen Reduction Reaction (ORR) performance. Various modifications have been explored, including different metal combinations, different first shell elements (N, C, S, B), and coordination numbers, to enhance electrocatalytic ORR activities. Another emerging focus is the link between their local structure and spin state. The Zhenan Bao and Jaramillo groups recently found that the catalytic activity of the NiNx moiety stems from its tetrahedrally distorted structure because it stabilizes the high-spin Ni state more than the low-spin state.[1] The local structure and its distortion can be modified by adjusting the carbon nanotube diameter,[2] pore size,[3] layering,[4] or by adding ligands at the first or second shell.[5]

In this study, we have conducted a computational screening of M-N-Cs with 3d, 4d, and 5d metals and local structures including square planar, tetragonal pyramidal, and tetrahedral symmetries. Metals in their divalent states within the MN4 units exhibited a V-shaped trend in formation energy correlated with increasing atomic numbers across each series. This pattern appears to be linked to the interaction between the two electrons of the N4 ligand and the d electron count of the metals, and it also emerged in the *O/*OH adsorption energy plots. Additionally, the preferred spin state varied with the metal type and structural symmetry, influencing electron distribution and crystal field splitting. These factors collectively impacted the binding energies with adsorbents, subsequently altering oxygen reduction reaction (ORR) activity. Further, we explored the adsorption energies of various intermediates (*CO, *H, *Cl, and *N2H) relevant to carbon dioxide reduction reaction (CO2RR), hydrogen evolution reaction (HER), chlorine evolution reaction (ClER), and nitrogen reduction reaction (NRR), respectively. Our primary objective was to identify optimal metal-structure combinations for enhanced catalytic performance. To achieve this, we considered several descriptors including partial Integrated Crystal Orbital Hamilton Population (ICOHP), metal d-band center, Madelung energy, and metal ionization potential. Moving forward, we aim to extend these findings by incorporating distorted coordination environments and integrating experimental synthesis strategies to induce such distortions, potentially unlocking new pathways for catalytic efficiency enhancement.

[1] Koshy, David M., et al. "Investigation of the Structure of Atomically Dispersed NiNx Sites in Ni and N-Doped Carbon Electrocatalysts by 61Ni Mössbauer Spectroscopy and Simulations." Journal of the American Chemical Society 144.47 (2022): 21741-21750.

[2] Cepitis, Ritums, et al. "Surface curvature effect on dual-atom site oxygen electrocatalysis." ACS Energy Letters 8.3 (2023): 1330-1335.

[3] Glibin, Vassili P., et al. "Non-PGM electrocatalysts for PEM fuel cells: thermodynamic stability and DFT evaluation of fluorinated FeN4-based ORR catalysts." Journal of The Electrochemical Society 166.7 (2019): F3277.

[4] Wu, Yahui, et al. "Boosting CO2 electroreduction over a cadmium single‐atom catalyst by tuning of the axial coordination structure." Angewandte Chemie International Edition 60.38 (2021): 20803-20810.

[5] Choi, Daeeun, et al. "Bridging the Catalytic Turnover Gap Between Single‐Atom Iron Nanozymes and Natural Enzymes by Engineering the First and Second Shell Coordination." Advanced Materials (2023): 2306602.