Investigation of the Structure of Atomically Dispersed NiN_xSites in Ni and N-Doped Carbon Electrocatalysts by ⁶¹Ni Mössbauer Spectroscopy and Simulations

Ni and nitrogen-doped carbons are selective catalysts for CO₂reduction to CO (CO₂R), but the hypothesized NiN_xactive sites are challenging to probe with traditional characterization methods. Here, we synthesize ⁶¹Ni-enriched model catalysts, termed ⁶¹NiPACN, in order to apply ⁶¹Ni Mössbauer spectroscopy using synchrotron radiation (⁶¹Ni-SR-MS) to characterize the structure of these atomically dispersed NiN_xsites. First, we demonstrate that the CO₂R results and standard characterization techniques (SEM, PXRD, XPS, XANES, EXAFS) point to the existence of dispersed Ni active sites. Then, ⁶¹Ni-SR-MS reveal significant internal magnetic fields of ∼5.4 T, which is characteristic of paramagnetic, high-spin Ni²⁺, in the ⁶¹NiPACN samples. Finally, theoretical calculations for a variety of Ni-N_xmoieties confirm that high-spin Ni²⁺is stable in non-planar, tetrahedrally distorted geometries, which results in calculated isotropic hyperfine coupling that is consistent with ⁶¹Ni-SR-MS measurements.