A hierarchical porous zirconium metal–organic framework (UiO-66) was prepared continuously through a microdroplet flow reaction strategy for the first time. The existence of metal-node defects arising from incomplete coordination in UiO-66 was found to be the main reason for the formation of mesopores. The dimensions of mesopores could be facilely tuned by adjusting the residence time, and the portion of mesopores was linearly correlated with the missing-nodes. The surface acidity was enhanced due to a large amount of pendant coordinated-free carboxylate groups in pores. With increasing residence time, the missing-nodes among the frameworks were sequentially repaired by self-healing of coordination spheres. Also, these hierarchical porous MOFs demonstrate superior storage capacities for CO₂ and CH₄. The method of constructing mesopores and producing surface acids presented in this work may open up a new avenue for developing novel hierarchical porous MOFs with special functionalities.

The use of manganese oxides as promising candidates for anode materials in lithium ion batteries has attracted a significant amount of attention recently. Here, we develop a general approach to synthesize hollow nanospheres of MnO₂, Mn₃O₄ and MnO, using carbon nanospheres as a template and a reagent. Depending on the calcination temperature, time and atmosphere, hollow nanospheres of MnO₂ assembled by randomly dispersed nanosheets, or hollow nanospheres of Mn₃O₄ and MnO composed of aggregated nanoparticles, are produced. The electrochemical properties of the three hollow nanoparticles are investigated in terms of cycling stability and rate capability. They deliver the specific capacities of 840, 1165 or 1515 mA h g⁻¹ after 60 cycles at 100 mA g⁻¹ for MnO₂, Mn₃O₄ and MnO. Even at 500 mA g⁻¹, the reversible capacities could be still kept at 637, 820, and 1050 mA h g⁻¹ after 150 cycles. The outstanding performances might be related with their hollow structure, porous surface and nanoscale size.

N-doped porous carbons have been considered as one of the most promising earth-abundant catalysts for the oxygen reduction reaction (ORR) owing to their high activity and excellent stability. Among various types of N-containing groups, pyridinic-N has been identified as the most effective catalytic sites for the ORR, as demonstrated by a recent study. However, the fabrication of porous carbons with a high density of exposed pyridinic-N sites has been rarely reported. In this work, the ORR catalytic properties of a series of pyridinic-N doped porous carbon (PNPC) which was derived by carbonization of a pyridyl-ligand based MOF were investigated. At different carbonization temperatures, this series of carbon exhibits different pyridinic-N contents and different degrees of carbonization. The ORR studies show that the graphitization degree of carbon has a significant impact on ORR catalytic activity besides N-groups. Electrochemical impedance spectroscopy (EIS) reveals that the electron transfer resistance in the ORR decreases significantly with the higher degree of graphitization, which gives rise to a higher ORR activity for these PNPCs. The synergistic effect of the high density of pyridinic-N sites and decreased electron impedance results in remarkably improved ORR activity which is comparable with that of the commercial Pt/C (10 wt%) catalyst. The result of this work could provide some guidance for designing or synthesizing highly efficient ORR catalysts.

This review provides an overview of crucial roles that MOFs have played in constructing non-noble ORR catalysts.

MnO nanorods encapsulated by N-doped carbon are prepared, using polypyrrole-coated MnOOH nanorods as both a template and a precursor. The resulting coaxial nanorods have a one-dimensional shape, nanoscale size and an N-doped carbon coating within one particle, which substantially improves their electrochemical performance. As an anode material for lithium-ion batteries, the coaxial nanorods of MnO/N-doped carbon deliver a specific capacity of 982 mA h g⁻¹ at a current density of 500 mA g⁻¹ after 100 cycles, higher than the values for pure MnO nanostructures and MnO/C nanocomposites. At a current density of 5000 mA g⁻¹, the reversible capacity of the coaxial nanorods could be as high as 372 mA h g⁻¹.