The existence of NiNx CO2R active sites is implied by the preceding analyses; we next used Ni synchrotron radiation-based Mössbauer spectroscopy ( 61 Ni-SR-MS) in the energydomain to further characterize the structure of these dispersed NiNx sites. These measurements used the 61 Ni-SR-MS instrumentation and methodology that was recently reported ; the three 61 NiPACN samples were measured along with a metallic Ni powder for verification of the experimental methods. The 61 Ni-SR-MS spectra of the metallic Ni powder showed a broad and symmetric peak characteristic of magnetic Zeeman splitting and could be fit to an internal magnetic field of 7.3 T (SI-Figure ). This value agrees well with reference values of ~7.5 T and validates the application of 61 Ni-SR-MS towards the analysis of the 61 NiPACN powders. 61 Ni-SR-MS spectra of all three 61 NiPACN powders showed a single symmetric, broad peak that is indicative of a magnetic hyperfine interaction that splits the spectra into 12 transitions (Figure ). Fitting each spectrum to a single site, magnetic Hamiltonian provides an estimate of the internal hyperfine magnetic field, Bfit, for the samples. Slight anisotropy can be observed on the 61 NiPACN-0.1 wt% sample which could indicate a relevant electric field gradient. Incorporation of a perturbative quadrupolar term in the Hamiltonian along the axis of the internal field did not yield significantly improved fits for any of the samples and thus only the single site magnetic Hamiltonian was used for subsequent analysis (SI-Figure , SI-Table ).