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Abstract

Electrochemical reduction stands as an alternative to revalorize CO₂. Among the different alternatives, Ni single atoms supported on carbonaceous materials are an appealing catalytic solution due to the low cost and versatility of the support and the optimal usage of Ni and its predicted selectivity and efficiency (ca. 100 towards CO). Herein, we have used noble carbonaceous support derived from cytosine to load Ni subnanometric sites. The large heteroatom content of the support allows the stabilization of up to 11wt of Ni without the formation of nanoparticles through a simple impregnation plus calcination approach, where nickel promotes the stabilization of C₃NO_x frameworks and the oxidative support promotes a high oxidation state of nickel. EXAFS analysis points at nickel single atoms or subnanometric clusters coordinated by oxygen in the material surface. Unlike the well-known N-coordinated Ni single sites selectivity towards CO₂ reduction, O-coordinated-Ni single sites (ca. 7wt of Ni) reduced CO₂ to CO, but subnanometric clusters (11wt of Ni) foster the unprecedented formation of HCOOH with 27 Faradaic efficiency at -1.4V. Larger Ni amounts ended up on the formation of NiO nanoparticles and almost 100 selectivity towards hydrogen evolution.