Metal- and nitrogen-doped carbons (M-N-Cs) represent a promising class of low cost electrocatalysts derived from nature-abundant elements for various electrochemical processes including CO₂RR [1,2]. Traditionally, M-N-Cs containing active metals (M = Fe, Co, Ni) are synthesized by direct pyrolysis of inorganic and organic precursors, a process that often results in the undesired formation of inorganic side phases through carbothermal reduction, impeding the effective integration of active metals like Fe, Co and Ni. Furthermore, comparing the intrinsic activities of different M-N-Cs can be complicated due to variations in catalyst morphology and active site concentration that arise during the pyrolysis.

To address these challenges, we developed an active-site imprinting strategy in which active metals are introduced post-pyrolysis via ion-exchange [3-5]. In this work, we employed the Mg imprinting strategy to produce Co-N-Cs and Ni-N-Cs with comparable morphology and metal dopant concentration. Our approach allows for a more direct comparison of the intrinsic activities that arise from the metal dopant. The Ni-N-Cs derived this way are consistently higher in activity and selectivity than the corresponding Co-N-Cs, exhibiting a CO Faraday efficiency of up to 95% at potentials between -0.5 to -0.8 V_RHE. The Ni-N-C catalyst maintains high stability at -0.65 V_RHE, with 92.5% retention of current density and 97.6% retention of CO selectivity after 100 hours of continuous operation.

While Ni-N-Cs are known to be relatively good CO₂RR catalysts, the origin of its activity remains rather elusive. Contrary to the widely accepted MN₄ active sites in M-N-Cs, NiN₄ sites are theoretically inert to both CO₂RR and HER due to the instability of the *COOH and *H intermediates respectively, often leading to the proposal of alternative but unproven NiN_x sites with various coordination structures to justify their performance. In this study, however, the absence of inorganic side phases allowed us to characterize the tetrapyrrolic NiN₄ coordination structure via Extended X-ray Absorption Fine Structure (EXAFS), suggesting that NiN₄ sites should in fact be CO­₂RR-active. Therefore, we performed mechanistic studies on the tetrapyrrolic NiN₄ sites using density functional theory (DFT), drawing inspirations from recent studies which elucidated the critical role of cations in enhancing CO₂RR activity on noble metal catalysts like Cu, Ag, and Au [6-7]. The presentation will further illustrate the influence of cations on the activity and selectivity of M-N-Cs towards CO₂RR, as well as well as the advantage of the pyrrolic nitrogen coordination in promoting the adsorption of these cations. 

Overall, this work not only highlights the potential of Mg-imprinted strategies in developing high-performance, morphological comparable electrocatalysts for sustainable energy applications and intrinsic activity studies, but also provides mechanistic insights on the involvement of cations in the valorization of the seemingly inert NiN₄ site.