Ultrastable Polymolybdate-Based Metal–Organic Frameworks as Highly Active Electrocatalysts for Hydrogen Generation from Water

Two
novel polyoxometalate (POM)-based metal–organic frameworks
(MOFs), [TBA]3[ε-PMoV8MoVI4O36(OH)4Zn4]­[BTB]4/3·xGuest (NENU-500, BTB
= benzene tribenzoate, TBA+ = tetrabutylammonium ion) and
[TBA]3[ε-PMoV8MoVI4O37(OH)3Zn4]­[BPT] (NENU-501, BPT = [1,1′-biphenyl]-3,4′,5-tricarboxylate),
were isolated. In these compounds, the POM fragments serving as nodes
were directly connected with organic ligands giving rise to three-dimensional
(3D) open frameworks. The two anionic frameworks were balanced by
TBA+ ions residing inside the open channels. They exhibit
not only good stability in air but also tolerance to acidic and basic
media. Furthermore, they were employed as electrocatalysts for the
hydrogen evolution reaction (HER) owing to the combination of the
redox activity of a POM unit and the porosity of a MOF. Meanwhile,
the HER activities of ε­(trim)4/3, NENU-5, and HKUST-1 were also studied
for comparison. Remarkably, as a 3D hydrogen-evolving cathode operating
in acidic electrolytes, NENU-500 exhibits the highest
activity among all MOF materials. It shows an onset overpotential
of 180 mV and a Tafel slope of 96 mV·dec–1,
and the catalytic current density can approach 10 mA·cm–2 at an overpotential of 237 mV. Moreover, NENU-500 and NENU-501 maintain their electrocatalytic activities after
2000 cycles.