Ultrathin Two-Dimensional Polyoxometalate-Based Metal–Organic\nFramework Nanosheets for Efficient Electrocatalytic Hydrogen Evolution

Abstract

The\nrational design of 2D polyoxometalate-based metal–organic\nframework (POMOF) nanosheets on a conductive substrate as a self-supporting\nelectrode is highly attractive but a great challenge. Herein is the\nfirst demonstration of POMOF nanopillar arrays consisting of 2D nanosheets\nas a self-supported electrode for the hydrogen evolution reaction\n(HER) in acidic conditions. Single-crystal X-ray analysis reveal that\nour as-prepared 2D [Co₂(TIB)₂(PMo₁₂O₄₀)]·Cl·4H₂O [named CoMo-POMOF;\nTIB = 1,3,5-tris­(1-imidazoly)­benzene] crystalline materials are connected\nby Co-α-Keggin polymolybdate units act as secondary building\nblocks and TIB as the organic ligands. The 2D CoMo-POMOF nanosheets\nwere successfully arrayed on a conductive nickel foam substrate by\na facile CoO nanorod template-assisted strategy. Remarkably, the CoMo-POMOF\nnanopillar arrays demonstrate superior electrocatalytic performance\ntoward the HER with an overpotential of 137 mV and Tafel slope of\n59 mV dec^–1 at 10 mA cm^–2, which\nare comparable to those of state-of-the-art POMOF-based electrocatalysts.\nDensity-functional theory (DFT) calculations demonstrate that the\nexposed bridging oxygen active sites (O_a) of Co-α-Keggin\npolymolybdate units in CoMo-POMOF optimize the Gibbs free energy of\nH* adsorption (Δ<i>G</i>_H* = −0.11\neV) and increase the intrinsic HER activity.