(667g) Designing Single Site Catalysts for the Two-Electron Oxygen Reduction Reaction By Tailoring Environments Beyond the Binding Site

The direct synthesis of H2O2 through the two-electron oxygen reduction is limited by further reduction of OOH* to H2O through the four-electron pathway. Single atom catalysts embedded in nitrogen-doped-carbon are promising materials for the two-electron reduction because of their exquisitely tuneable active sites. Using characterisation studies, electrochemical measurements, and first principles calculations, we elucidate how tailoring environments beyond the binding site of Fe, Ni, and Co single atom catalysts alters selectivity. Isolated Ni sites confined by amino anthraquinone demonstrate an 80% faradaic efficiency to the two-electron reduction across a wide-range of potential. Similar faradaic efficiencies are also obtained on the Ni-N₃S₁ motif. First principles calculations indicate that amino anthraquinone physisorbs on Ni sites, creating a ~3.2 Å wide nano-channel within which reaction intermediates are confined. These confinement effects weaken the OOH* adsorption strength, thus favouring desorption over further reduction. For the sulfur modified Ni sites, we leverage XPS, XAS, and TEM characterisation studies to create a computational library of active site motifs having progressively increasing extents of site oxidation. First principles calculations reveal that while replacing nitrogen with sulfur within the first coordination shell strengthens OOH* adsorption energies, progressive oxidation of the active site has a weakening effect. The net effect results in OOH* desorption being favoured over further dissociation, thus confirming the experimentally observed selectivity on the sulfur-modified catalysts. The weakening of OOH* adsorption strengths through sulfur modification and site oxidation also results in Co sites that are ordinarily selective towards the four-electron pathway, becoming more selective towards two-electron reduction. Despite inducing structural changes beyond the binding site such as confinement effects, sulfur modification, and site oxidation, a linear scaling relationship between the adsorption energies of OOH* and OH* is observed. This scaling relationship indicates that even such structural changes cannot circumvent reactivity limitations imposed by the Sabatier volcano.