Investigation of the Structure of Atomically Dispersed NiN <sub> <i>x</i> </sub> Sites in Ni and N-Doped Carbon Electrocatalysts by <sup>61</sup> Ni Mössbauer Spectroscopy and Simulations
David M. Koshy, Md Delowar Hossain, Ryo Masuda, Yoshitaka Yoda, Leland B. Gee, Kabir Abiose, Huaxin Gong, Ryan C. Davis, Makoto Seto, Alessandro Gallo, Christopher Hahn, Michal Bajdich, Zhenan Bao, Thomas F. Jaramillo
Abstract
Ni and nitrogen-doped carbons are selective catalysts for CO2 reduction to CO (CO2R), but the hypothesized NiNx active 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 NiNx sites. 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-Nx moieties 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.