Investigation of the Structure of Atomically Dispersed NiN x Sites in Ni and N-Doped Carbon Electrocatalysts by 61 Ni Mössbauer Spectroscopy and Simulations.

Ni and nitrogen-doped carbons are selective catalysts for CO ₂ reduction to CO (CO ₂ R), but the hypothesized NiN _x active sites are challenging to probe with traditional characterization methods. Here, we synthesize ⁶¹ Ni-enriched model catalysts, termed ⁶¹ NiPACN, in order to apply ⁶¹ Ni Mössbauer spectroscopy using synchrotron radiation ( ⁶¹ Ni-SR-MS) to characterize the structure of these atomically dispersed NiN _x sites. First, we demonstrate that the CO ₂ R results and standard characterization techniques (SEM, PXRD, XPS, XANES, EXAFS) point to the existence of dispersed Ni active sites. Then, ⁶¹ Ni-SR-MS reveal significant internal magnetic fields of ∼5.4 T, which is characteristic of paramagnetic, high-spin Ni ²⁺ , in the ⁶¹ NiPACN samples. Finally, theoretical calculations for a variety of Ni-N _x moieties confirm that high-spin Ni ²⁺ is stable in non-planar, tetrahedrally distorted geometries, which results in calculated isotropic hyperfine coupling that is consistent with ⁶¹ Ni-SR-MS measurements.