Atomically dispersed Zn–N–C nanomaterials are promising platinum‐free catalysts for the oxygen reduction reaction (ORR). However, the fabrication of Zn–N–C catalysts with a high Zn loading remains a formidable challenge owing to the high volatility of the Zn precursor during high‐temperature annealing. Herein, we report that an atomically dispersed Zn–N–C catalyst with an ultrahigh Zn loading of 9.33 wt % could be successfully prepared by simply adopting a very low annealing rate of 1° min−1. The Zn–N–C catalyst exhibited comparable ORR activity to that of Fe–N–C catalysts, and significantly better ORR stability than Fe–N–C catalysts in both acidic and alkaline media. Further experiments and DFT calculations demonstrated that the Zn–N–C catalyst was less susceptible to protonation than the corresponding Fe–N–C catalyst in an acidic medium. DFT calculations revealed that the Zn–N4 structure is more electrochemically stable than the Fe–N4 structure during the ORR process.

be made into well-faceted morphologies of rhombic dodecahedrons or cubic hexahedrons. [13,14] A critical step for finally getting the hollow structures by using the template method is to remove the templates, which normally requires that the template materials have poorer thermal/chemical stability than the desired species attached on the templates. ZIF8 templates can be easily etched in a solution system, [13,[15][16][17] owing to their chemical instability. But some literatures also show that an annealing process can also remove ZIF8, [18][19][20] which especially in our detailed investigation, is not due to the poor thermal stability.In particular, when we coated a layer of polymerized dopamine (PDA) on rhombic dodecahedron nanoparticles of ZIF8, and annealed the ZIF8@PDA composites at a temperature of 900 °C, the obtained product is nitrogen doped carbon (NC) hollow structured materials, indicating that ZIF8, just like a hard template, can be removed at high-temperature for constructing the hollow interiors. While interestingly, when ZIF8 was solely calcined at 900 °C, it results in same shape, solid NC dodecahedron nanoparticles, indicating ZIF8 itself can well retain the original structures during heating treatment. This observation implies that the hollow structures obtained by annealing the composites are absolutely not derived from poor thermal stability of the ZIF8 cores and the existence of PDA layer may also contribute to the hollowing procedure.Considering the unusual observation, we then gave this hollowing process a detailed investigation and proposed a "stresses induced orientation contraction" mechanism to address it. Particularly, pure ZIF8 rhombic dodecahedrons would gradually decompose, shrink, and transfer into NC along temperature increase, in which the uniform shrinkage stresses applied on different directions could be well balanced, resulting in NC with well inherited shapes but smaller sizes. While for ZIF8@PDA composites, due to the intense PDA-ZIF8 interface interactions, the PDA layer strongly drags the ZIF8 outward to restrain their contraction, thus leading to accumulated stresses in the central part of ZIF8 dodecahedron nanoparticles and gradually damaged interior cores for creating the hollow structures. Besides, the achieved NC materials demonstrate a very high catalytic activity to ORR owing to the nice hollow structure and very high content of active pyridinic N. Indeed this work gives an absolutely novel Hollow structured materials are widely applicable in various fields. Although many routes have been explored for getting such materials, a strategy mainly based on physical effect is still deficient. Herein, a "stresses induced orientation contraction" mechanism for preparation of hollow structures is reported. The composites constructed by zeolite imidazolate framework-8 (ZIF8) cores and polymerized dopamine (PDA) shells, upon annealing, form intensive interfacial interactions, which drag the ZIF8 cores outward to restrain their shrinkage. The gradually accumulated stresses in...

A 3D hierarchical Co/N-doped carbon nanotube/nanoparticle composite was successfully prepared as a high-performance ORR electrocatalyst, via pyrolysis of bimetallic Co, Zn-ZIF crystals coated on the surface of silica templates.

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