A Hydrangea‐Like Superstructure of Open Carbon Cages with Hierarchical Porosity and Highly Active Metal Sites

Abstract

Carbon micro‐/nanocages have attracted great attention owing to their wide potential applications. Herein, a self‐templated strategy is presented for the synthesis of a hydrangea‐like superstructure of open carbon cages through morphology‐controlled thermal transformation of core@shell metal–organic frameworks (MOFs). Direct pyrolysis of core@shell zinc (Zn)@cobalt (Co)‐MOFs produces well‐defined open‐wall nitrogen‐doped carbon cages. By introducing guest iron (Fe) ions into the core@shell MOF precursor, the open carbon cages are self‐assembled into a hydrangea‐like 3D superstructure interconnected by carbon nanotubes, which are grown in situ on the Fe–Co alloy nanoparticles formed during the pyrolysis of Fe‐introduced Zn@Co‐MOFs. Taking advantage of such hierarchically porous superstructures with excellent accessibility, synergetic effects between the Fe and the Co, and the presence of catalytically active sites of both metal nanoparticles and metal–Nx species, this superstructure of open carbon cages exhibits efficient bifunctional catalysis for both oxygen evolution reaction and oxygen reduction reaction, achieving a great performance in Zn–air batteries.

Physically Adsorbed Metal Ions in Porous Supports as Electrocatalysts for Oxygen Evolution Reaction

“…Meanwhile, the OER properties of catalysts from the non‐carbon supports were relatively inferior to these of carbon‐based ones. The carbon‐based catalysts possess the high conductivity and excellent porosity, which affect the loading of the adsorbed metal ions and the electron and ion transfer, and therefore boost the OER activity of the catalysts . To confirm the in situ transformation process, the XPS was further performed to characterize a series of catalysts with different adsorbing substrates before and after the OER process.…”

“…N 2 adsorption isotherms of Co 0.7 Fe 0.3 CB indicate that its Brunauer-Emmett-Teller (BET) surface area is 241.9 m 2 g −1 and the pore size distribution exhibits its abundant micropore and mesopore features ( Figure S1, Supporting Information), which is beneficial for the exposure of the active sites and transport of ions and electrons. [32] The morphology of Co x Fe 1−x CB sample (taking the Co 0.7 Fe 0.3 CB as a representative sample) shows onion-shaped porous carbon, as depicted in Figure 1b,c, in which the metal Co and Fe are adsorbed in the porous carbon channels in the ionic form, so no metal nanoparticles can be observed from transmission electron microscopy (TEM) image. Furthermore, the Co 0.7 Fe 0.3 CB contains 10.7 wt% Co and 4.5 wt% Fe, based on the inductively coupled plasmaatomic emission spectroscopy (ICP-AES).…”

“…The carbon-based catalysts possess the high conductivity and excellent porosity, which affect the loading of the adsorbed metal ions and the electron and ion transfer, and therefore boost the OER activity of the catalysts. [32] To confirm the in situ transformation process, the XPS was further performed to characterize a series of catalysts with different adsorbing substrates before and after the OER process. As shown in the Figures 5c-f and S9 (Supporting Information), the cobalt in all samples experienced a course of changes from Co 2+ to Co 3+ , which demonstrates that PAMI in porous materials can directly convert into an active species in electrochemical activation process.…”

Single‐Atom Iron Catalysts on Overhang‐Eave Carbon Cages for High‐Performance Oxygen Reduction Reaction

“…[2] For instance, it is found that metal sites near the external surface of catalysts are better able to catalyze the oxygen reduction reaction (ORR), an important reaction in metal-air batteries and fuel cells, whereas those buried in dense carbon matrix are inactive. [3] The low utilization of metal sites gives insufficient performance due to the poor mass transport through catalyst layers. [4] On the account of most ORR catalysis occurring at the three-phase boundary (catalystelectrolyte-oxygen), engineering of the morphology of catalysts to both anchor and expose metal sites at three-phase boundaries to a large degree, promoting the mass transport of ORR-related species through the catalyst layers, is regarded as one of the most promising ways to improve the utilization of each active site, giving remarkable catalytic performance.…”

“…Furthermore, this structure also exhibits excellent methanol tolerance and stability, with scarcely noticeable current change in both alkaline (0.1m KOH) and acidic (0.5 m H 2 SO 4 ) electrolytes. [16] Fe/OES was prepared through a three-step method (Figure 1 a): 1) zeolitic imidazolate framework-8 (ZIF-8, Zn(MIM) 2 , MIM = 2-methylimidazolate) [17] with suitable nanocavities (11.6 ) and small apertures (3.4 ) was employed as a host for the encapsulation of iron sources (iron(III) acetylacetonate, Fe(acac) 3 , molecular diameter ca. 9.7 ) to form Fe/ZIF-8; 2) silicon oxide (SiO x ) was then used as a suit of "armor" to entirely cover the surface of Fe/ZIF-8 by a wet-chemistry method; [18] 3) the prepared Fe/ZIF-8@SiO x was calcined under Ar flow, followed by removing the SiO x shell and the generated metal nanoparticles with 12 % HF and diluted HCl.…”