RESUMO
Single-atom catalysts (SACs) open up new possibilities for advanced technologies. However, a major complication in preparing high-density single-atom sites is the aggregation of single atoms into clusters. This complication stems from the delicate balance between the diffusion and stabilization of metal atoms during pyrolysis. Here, we present pressure-controlled metal diffusion as a new concept for fabricating ultra-high-density SACs. Reducing the pressure inhibits aggregation substantially, resulting in almost three times higher single-atom loadings than those obtained at ambient pressure. Molecular dynamics and computational fluid dynamics simulations reveal the role of a metal hopping mechanism, maximizing the metal atom distribution through an increased probability of metal-ligand binding. The investigation of the active site density by electrocatalytic oxygen reduction validates the robustness of our approach. The first realization of Ullmann-type carbon-oxygen couplings catalyzed on single Cu sites demonstrates further options for efficient heterogeneous catalysis.
RESUMO
Rechargeable zinc-air batteries (RZAB) have gained significant attention as potential energy storage devices due to their high energy density, cost-effectiveness, and to the fact that they are environmentally safe. However, the practical implementation of RZABs has been impeded by challenges such as sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), including poor cyclability. Herein, we report the preparation of cobalt- and nitrogen-doped porous carbon derived from phloroglucinol-formaldehyde polymer networks with 2-methyl imidazole and cobalt phthalocyanine as precursors for nitrogen and cobalt. The CoN-PC-2 catalyst prepared in this study exhibits commendable electrocatalytic activity for both ORR and OER, evidenced by a half-wave potential of 0.81 V and Ej=10 of 1.70 V. Moreover, the catalyst demonstrates outstanding performance in zinc-air batteries, achieving a peak power density of 158 mW cm-2 and displaying excellent stability during charge-discharge cycles. The findings from this study aim to provide valuable insights and guidelines for further research and the development of hierarchical micro-mesoporous carbon materials from polymer networks, facilitating their potential commercialisation and widespread deployment in energy storage applications.
RESUMO
The present study designates the heteroatom (N,F)-doped various graphitic carbon nanofibers (GNFs) viz. GNF-linear segmented platelets, antlers, herringbone type, and their structural deformations from pristine fiber with many open-edge active centers as metal-free, cost-effective electrocatalysts for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs). Introduction of heteroatoms to GNF frameworks enlarges the lattice spacing between graphene platelets and leads to structural modulation. The developed GNF/N-F catalysts show excellent ORR activity with insensitivity to CH3OH and demonstrated outstanding electrochemical potential cycling stability of 10,000 cycles with well-retained ORR kinetics without much loss in the activity. X-ray photoelectron spectroscopy investigation of GNF/N-F catalysts explicitly shows the highly active forms of N (pyridinic, pyrrolic, and graphitic-N) and semi-ionic, ionic C-F of F in the catalysts. The deep-rooted synergistic effect among N and F atoms creates more active centers entrenched with extensive C-C bond polarization and larger charge delocalization with larger spin density differences accomplished in GNF/N-F catalysts. Wide open-edge cavities, opened tips, and many extensively accessible facets collectively enhance the ORR activity of the GNF-H/N-F catalyst. The present study provides a deep insight into the understanding of advanced metal-free electrocatalysts for efficient ORR in PEMFCs and metal-air batteries.
