ACS Cent Sci. 2022 Jul 27;8(7):975-982. doi: 10.1021/acscentsci.2c00509. Epub 2022 Jul 1.
Molecular materials must deliver high current densities to be competitive with traditional heterogeneous catalysts. Despite their high density of active sites, it has been unclear why the reported O2 reduction reaction (ORR) activity of molecularly defined conductive metal-organic frameworks (MOFs) have been very low: ca. -1 mA cm-2. Here, we use a combination of gas diffusion electrolyses and nanoelectrochemical measurements to lift multiscale O2 transport limitations and show that the intrinsic electrocatalytic ORR activity of a model 2D conductive MOF, Ni3(HITP)2, has been underestimated by at least 3 orders of magnitude. When it is supported on a gas diffusion electrode (GDE), Ni3(HITP)2 can deliver ORR activities >-150 mA cm-2 and gravimetric H2O2 electrosynthesis rates exceeding or on par with those of prior heterogeneous electrocatalysts. Enforcing the fastest accessible mass transport rates using scanning electrochemical cell microscopy revealed that Ni3(HITP)2 is capable of ORR current densities exceeding -1200 mA cm-2 and at least another 130-fold higher ORR mass activity than has been observed in GDEs. Our results directly implicate precise control over multiscale mass transport to achieve high-current-density electrocatalysis in molecular materials.