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Nickel and nitrogen co-doped carbon (Ni–N–C) has emerged as a promising catalyst for the CO 2 reduction reaction (CO 2 RR); however, the chemical nature of its active sites has remained elusive. Herein, we report the exploration of the reactivity and active sites of Ni–N–C for the CO 2 RR. Single atom Ni coordinated with N confined in a carbon matrix was prepared through thermal activation of chemically Ni-doped zeolitic imidazolate frameworks (ZIFs) and directly visualized by aberration-corrected scanning transmission electron microscopy. Electrochemical results show the enhanced intrinsic reactivity and selectivity of Ni–N sites for the reduction of CO 2 to CO, delivering a maximum CO faradaic efficiency of 96% at a low overpotential of 570 mV. Density functional theory (DFT) calculations predict that the edge-located Ni–N 2+2 sites with dangling bond-containing carbon atoms are the active sites facilitating the dissociation of the C–O bond of the *COOH intermediate, while bulk-hosted Ni–N 4 is kinetically inactive. Furthermore, the high capability of edge-located Ni–N 4 being able to thermodynamically suppress the competitive hydrogen evolution is also explained. The proposal of edge-hosed Ni–N 2+2 sites provides new insight into designing high-efficiency Ni–N–C for CO 2 reduction. 

Platinum group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) with atomically dispersed FeN 4 sites have emerged as a potential replacement for low-PGM catalysts in acidic polymer electrolyte fuel cells (PEFCs). In this work, we carefully tuned the doped Fe content in zeolitic imidazolate framework (ZIF)-8 precursors and achieved complete atomic dispersion of FeN 4 sites, the sole Fe species in the catalyst based on Mößbauer spectroscopy data. The Fe–N–C catalyst with the highest density of active sites achieved respectable ORR activity in rotating disk electrode (RDE) testing with a half-wave potential ( E 1/2 ) of 0.88 ± 0.01 V vs. the reversible hydrogen electrode (RHE) in 0.5 M H 2 SO 4 electrolyte. The activity degradation was found to be more significant when holding the potential at 0.85 V relative to standard potential cycling (0.6–1.0 V) in O 2 saturated acid electrolyte. The post-mortem electron microscopy analysis provides insights into possible catalyst degradation mechanisms associated with Fe–N coordination cleavage and carbon corrosion. High ORR activity was confirmed in fuel cell testing, which also divulged the promising performance of the catalysts at practical PEFC voltages. We conclude that the key factor behind the high ORR activity of the Fe–N–C catalyst is the optimum Fe content in the ZIF-8 precursor. While too little Fe in the precursors results in an insufficient density of FeN 4 sites, too much Fe leads to the formation of clusters and an ensuing significant loss in catalytic activity due to the loss of atomically dispersed Fe to inactive clusters or even nanoparticles. Advanced electron microscopy was used to obtain insights into the clustering of Fe atoms as a function of the doped Fe content. The Fe content in the precursor also affects other key catalyst properties such as the particle size, porosity, nitrogen-doping level, and carbon microstructure. Thanks to using model catalysts exclusively containing FeN 4 sites, it was possible to directly correlate the ORR activity with the density of FeN 4 species in the catalyst. 

Abstract FeN₄moieties embedded in partially graphitized carbon are the most efficient platinum group metal free active sites for the oxygen reduction reaction in acidic proton‐exchange membrane fuel cells. However, their formation mechanisms have remained elusive for decades because the Fe−N bond formation process always convolutes with uncontrolled carbonization and nitrogen doping during high‐temperature treatment. Here, we elucidate the FeN₄site formation mechanisms through hosting Fe ions into a nitrogen‐doped carbon followed by a controlled thermal activation. Among the studied hosts, the ZIF‐8‐derived nitrogen‐doped carbon is an ideal model with well‐defined nitrogen doping and porosity. This approach is able to deconvolute Fe−N bond formation from complex carbonization and nitrogen doping, which correlates Fe−N bond properties with the activity and stability of FeN₄sites as a function of the thermal activation temperature.