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-PMo^V₈Mo^VI₄O₃₆(OH)₄Zn₄]­[BTB]_4/3·xGuest (NENU-500, BTB = benzene tribenzoate, TBA⁺ = tetrabutylammonium ion) and [TBA]₃[ε-PMo^V₈Mo^VI₄O₃₇(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.