Metal-macrocyclic amine borate-derived Co and Cu nanocrystals embedded in a N-doped carbon matrix for highly efficient Al-air batteries.

Two metal-complex borates with open frameworks, [Co(Cyclen)CO3]·[B5O6(OH)4]·H2O (1, Cyclen = 1,4,7,10-tetraazacyclododecane) and [Cu(Me3tacn)]·[B6O7(OH)6]·7H2O (2, Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane), were synthesized via a mild solvent evaporation method. Their boron-oxygen clusters and metal-complex cations assemble through hydrogen bond interactions into ordered supramolecular pore structures. These structures provide sufficient space for the in situ growth of metal nanocrystals. Subsequent pyrolysis of these borates yielded a series of catalysts. Among them, the well-performing catalysts Co-750 and Cu-900 feature metal nanocrystals embedded in an N-doped carbon matrix abundant in electrocatalytically active N species. Co-750 and Cu-900 exhibit excellent ORR performance, with half-wave potentials of 0.83 V and 0.81 V, respectively. When applied as a cathode in liquid Al-air batteries, Co-750 achieves a higher power density (186 mW cm-2), demonstrating performance comparable to that of reported Co-based catalysts. Moreover, the Co-750-based quasi-solid-state Al-air battery delivers an open-circuit voltage of 1.554 V and sustains stable discharge for over 15 hours at 1 mA cm-2.