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]·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∥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∥RuO₂/NF setup (1.5 V). In addition, compound 2/CC∥compound 2/NF exhibited rapid response capabilities and showed no significant increase in voltage after 6000 s of operation.