In this study, we employ Ni Mössbauer spectroscopy of Ni-N-C catalysts to provide additional depth in our understanding of the structure of NiNx CO2R active sites. To perform this study, we devised a synthesis strategy to make isotopically-enriched Ni-N-C catalysts which could be tested using Ni synchrotron-radiation Mössbauer spectroscopy (SR-MS) which has been recently developed at the SPring-8 synchrotron (Hyogo, Japan). To supplement these Ni Mössbauer spectra, we also performed a Density Functional Theory (DFT) simulation to calculate hyperfine coupling parameters on Ni-Nx moieties. In addition, extensive characterization including CO2R testing, XAS, XPS, and PXRD was used to corroborate the conclusions about the structure of NiNx sites. This holistic approach, employing the emerging Ni SR-MS technique alongside complementary structural characterization methods, provides a self-consistent picture of the structure of Ni-N-C catalysts which can inform future CO2R catalyst design efforts.