Pyridine polymer tubular structures connected with polyoxometalates as bifunctional electrocatalysts for water splitting

In this work, we successfully prepared four POM-based organic-inorganic hybrids, namely, [(C5H6N)2(C4H5N2)][PMo12O40] (1), [(C5H6N)3(C5H5N)][PMo12O40] (2), [(C3H6N8)3][PMo12O40]<middle dot>4H2O (3), and [(C2H5N4)3][PMo12O40] (4) (where C5H6N = pyridine, C4H5N2 = pyrazine, C3H6N8 = 2,7-diamino-1,3,4,6,8,9-hexaazaspiro[4.4] nonane, and C2H5N4 = 3-amino-1,2,4-triazole), using a hydrothermal method. Compounds 1 and 2 exhibited a lamellar three-dimensional structure. Compared to compound 1, compound 2 contained only one ligand, pyridine, which formed a pyridine polymer tubular structure that was further connected to a [PMo12O40]3- anion, creating a pyridine-PMo12-pyridine stacking-like structure. Compounds 3 and 4 showed a stereostructure, where organic ligands were wrapped around polyacid spheres. Unlike compound 3, compound 4 maintained a similar three-dimensional structure but had a hexagonal astral ligand configuration. However, ligands formed hexagonal boxes that were smaller than those in compound 3, with shorter distances between the ligands. The overpotential values for compound 2 were 143 mV (HER) and 136 mV (OER) at 10 mA cm-2, which were significantly lower than those of the other compounds, the H3[PMo12O40] precursor, and the organic ligands. Given the relatively outstanding HER/OER catalytic properties of compound 2, a dual-electrode water-splitting device was assembled. The compound 2/CC parallel to compound 2/NF system achieved a low cell voltage of 1.48 V at 10 mA cm-2, which was significantly lower than that of the commercial Pt/C/CC parallel to RuO2/NF setup (1.5 V). In addition, compound 2/CC parallel to compound 2/NF exhibited rapid response capabilities and showed no significant increase in voltage after 6000 s of operation.