In situ growth of polyoxometalate-based metal-organic framework nanoflower arrays for efficient hydrogen evolution

The conversion of traditional polymolybdate-based metal-organic frameworks (POMOFs) crystals to well-aligned nanoarrays are highly attractive for electrocatalytic hydrogen evolution but remains significant challenge. Herein, we demonstrated that the POMOFs nanoarrays as self-supported electrode toward hydrogen evolution with high catalytic activity and stability. Single-crystal X-ray analysis reveal the {ε-PMoV8MoVI4O37Zn4} (Zn-ε-Keggin) serve as secondary building blocks and directly connected to BPB organic ligands (BPB = 1,4-bis(pyrid-4-yl)benzene) to obtain novel [ε-PMoV8MoVI4O37(OH)3Zn4][BPB]3 (named as ZnMo-POMOF). Particularly, ZnMo-POMOF nanoflower arrays grown in-situ on a Ni foam substrate exhibiting excellent electrocatalytic hydrogen evolution performance of 180 mV at a current density of 10 mA/cm2 with the Tafel slope of 66 mV/dec, thus among one of the best POMOF-based electrocatalysts reported so far. DFT calculations reveal that the bridging oxygen active sites (Oa) significantly optimizes Gibbs free energy of H* adsorption for Zn-ε-Keggin polymolybdate units (−0.07 eV), thereby increasing the intrinsic activity of the ZnMo-POMOF.