Both NiNC-IMI and NiNC-PANI samples exhibit a pronounced diffraction peak at around 26°, assigned to the family of graphitic carbon planes. Additional Bragg peaks suggested the presence of a metallic Ni 0 phase, associated with (carbon-encapsulated) Ni nanoparticles in the bulk of the catalyst. It is well-documented that such Ni particles are chemically inert for the CO2 reduction electrocatalysis. The chemical state and elemental composition of the catalysts surface were obtained from the core level regions of C1s, N1s, and Ni2p(3/2) (Figure ). Carbon speciation is dominated by sp 2 hybridized moieties, showing a high carbonization degree. Following earlier analyses of MNC electrocatalysts, the N speciation was split into a few distinct fractions (Supplementary Note 1, Supplementary Table ). The Ni2p3/2 core level region (Figure in NiNx site motifs) were identified (Supplementary Table ). In both NiNC-IMI and NiNC-PANI, Ni prevails in the chemical states of Ni 2+ and Ni + . These two chemical Ni states are commonly found for the N-coordinated, catalytically active single Ni atom sites. The overall elemental composition of either catalyst is very similar, with metallic Ni particles adding extra Ni at% for NiNC-IMI (Supplementary Table ). The TEM micrographs in Supplementary Figure revealed the catalyst microstructures. Notably, NiNC-IMI exhibits a fiber-like backbone structure with homogeneously distributed diameters (~ 20 nm). In addition, carbon layer-embedded metallic Ni particles and pores (due to particle leaching) with constant size (5~10 nm) could be observed (Supplementary Figure ). By contrast, the NiNC-PANI shows a densely compacted layer-like backbone structure, and the encapsulated Ni particles vary from 10 to 50 nm diameters (Supplementary Figure ). However, no visible pores could be identified. These results are corroborated by the BET analysis. The NiNC-IMI showed a specific surface area of 220 m 2 g -1 with a large fraction of mesopores (~ 4-5 nm pore size).