title: Polyoxometalate-based metal organic frameworks (POMOFs): structural trends, energetics, and high electrocatalytic efficiency for hydrogen evolution reaction.
abstract: The grafting of the triangular 1,3,5-benzene tricarboxylate linkers (denoted trim) on tetrahedral ε-Keggin polyoxometalates (POMs) capped by Zn(II) ions, formed in situ under hydrothermal conditions, has generated three novel POM-based metal organic frameworks (POMOFs). (TBA)(3)[PMo(V)(8)Mo(VI)(4)O(36)(OH)(4)Zn(4)][C(6)H(3)(COO)(3)](4/3)·6H(2)O (ε(trim)(4/3)) is a 3D open-framework built of molecular Keggin units connected by trim linkers, with channels occupied by tetrabutylammonium (TBA) counterions. ε(trim)(4/3) is a novel (3,4)-connected net, named ofp for open-framework polyoxometalate, and computer simulations have been used to evaluate its relative stability in comparison with ctn- and bor-like polymorphs, showing the stability of this novel phase directly related to its greatest density. A computational study was also undertaken with the aim of locating TBA molecules, the positions of which could not be deduced from single crystal X-ray diffraction, and further rationalizes their structure directing role. In (TBA)(3)[PMo(V)(8)Mo(VI)(4)O(37)(OH)(3)Zn(4)][C(6)H(3)(COO)(3)] (ε(2)(trim)(2)), the building unit is not the molecular Keggin but a dimerized form of this POM. Their connection via trim linkers generates a 3D framework with channels filled by TBA cations. In (TBA)(3)[PMo(V)(8)Mo(VI)(4)O(37)(OH)(3)Zn(4)][C(6)H(3)(COO)(3)]·8H(2)O ([ε(trim)](∞)), zigzag chains are connected via the organic linkers, forming 2D grids. Modified electrodes were fabricated by direct adsorption of the POMOFs on glassy carbon or entrapment in carbon paste (CPE). A remarkable electrocatalytic hydrogen evolution reaction (HER) was detected with a yield greater than 95%, and a turnover number as high as 1.2 × 10(5) was obtained after 5 h. The reported POMOF-based electrodes are more active than platinum, with a roughly 260 mV anodic shift. Finally, the electrocatalytic activities of ε(trim)(4/3)/CPE electrodes in various XCl (X = Li, Na, K, Cs) media have been studied. This allowed us to detect a cation effect and propose an electrocatalytic mechanistic pathway for the HER.
authors.affiliation: Institut Lavoisier de Versailles, UMR 8180, Université de Versailles Saint-Quentin en Yvelines, 45 Avenue des Etats-Unis, 78035 Versailles cedex, France.
authors.forename: Brigitte
authors.initials: B
authors.lastname: Nohra
authors.forename: Hani
authors.initials: H
authors.lastname: El Moll
authors.forename: L Marleny
authors.initials: LM
authors.lastname: Rodriguez Albelo
authors.forename: Pierre
authors.initials: P
authors.lastname: Mialane
authors.forename: Jérôme
authors.initials: J
authors.lastname: Marrot
authors.forename: Caroline
authors.initials: C
authors.lastname: Mellot-Draznieks
authors.forename: Michael
authors.initials: M
authors.lastname: O'Keeffe
authors.forename: Rosa
authors.initials: R
authors.lastname: Ngo Biboum
authors.forename: Joël
authors.initials: J
authors.lastname: Lemaire
authors.forename: Bineta
authors.initials: B
authors.lastname: Keita
authors.forename: Louis
authors.initials: L
authors.lastname: Nadjo
authors.forename: Anne
authors.initials: A
authors.lastname: Dolbecq