Synthesis of 61 NiPACN: In order to use 61 Ni SR-MS to probe Ni-N-C active sites, a suitable 61 Ni enriched catalyst sample needed to be synthesized. To achieve this, we followed a synthesis protocol used to make NiPACN, a polyacrylonitrile-derived Ni-N-C material, that was reported by our groups recently (Figure ). However, the 61 Ni precursor material could only be purchased in metallic form which necessitated a modification to the NiPACN synthesis which used acetone-soluble nickel nitrate as the Ni source. For 61 NiPACN, we first synthesized metal-free N-doped carbon by the traditional polymerization, oxidation, and pyrolysis steps and then deposited Ni onto the powder by dispersion in an acidic solution that could dissolve the metallic 61 Ni precursor. Then, after drying and a second pyrolysis step, the 61 Ni-enriched 61 NiPACN samples could be used for further testing and characterization (Figure ). In this study, three 61 NiPACN samples were synthesized and denoted 0.1 wt%, 0.5 wt%, and 1 wt% (based on Ni addition during synthesis) along with a control 0 wt% sample that was subjected to the acid treatment and 2 nd pyrolysis without addition of 61 Ni. We note that these denominations are significant overestimates of the true Ni loading measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, SI-Table ) but the naming convention based on the amount of Ni added during synthesis was kept for brevity. In addition, two batches of each of the 61 NiPACN catalysts were synthesized; the first batch was used for 61 Ni-SR-MS, Ni K-edge XAS, PXRD, and CO2R catalysis and the second for XPS. The ICP-AES showed qualitatively similar trends in Ni loading between the two batches but not quantitative matches, thus all conclusions herein are based on the qualitative Ni chemical states identified by physical characterization. These 61 NiPACN samples were representative of the NiPACN materials that have already been investigated by our group by a variety of methods including XAS, PXRD, XPS, HAADF-STEM, EELS, and ToF-SIMS. However, as the 61 NiPACN synthesis method diverges slightly from the NiPACN method we first aimed to demonstrate that there is evidence for NiNx active sites before proceeding to Ni SR-MS.