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.

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.

Ultrasmall Ni₂P/rGO was synthesized using template-confinement strategy of MOFs and served as a highly efficient electrocatalyst for overall water splitting.