Metal Effect\non the Proton Conduction of Three Isostructural\nMetal–Organic Frameworks and Pseudo-Capacitance Behavior of\nthe Cobalt Analogue

Three isostructural transition metal–organic frameworks,\n[M(bta)<sub>0.5</sub>(bpt)(H<sub>2</sub>O)<sub>2</sub>]·2H<sub>2</sub>O (M = Co (<b>1</b>), Ni (<b>2</b>), Zn (<b>3</b>), H<sub>4</sub>bta = 1,2,4,5-benzenetetracarboxylic acid,\nbpt = 4-amino-3,5-bis(4-pyridyl)-1,2,4-triazole), were successfully\nconstructed using different metal cations. These frameworks exhibit\na three-dimensional network structure with multiple coordinated and\nlattice water molecules within the framework, contributing to high\nstability and a rich hydrogen-bond network. Proton conduction studies\nrevealed that, at 333 K and 98% relative humidity, the proton conductivities\n(σ) of MOFs <b>1–3</b> reached 1.42 × 10<sup>–2</sup>, 1.02 × 10<sup>–2</sup>, and 6.82 ×\n10<sup>–3</sup> S cm<sup>–1</sup>, respectively. Compared\nto the proton conductivity of the initial ligands, the σ values\nof the complexes increased by 2 orders of magnitude, with the activation\nenergies decreasing from 0.36 to 0.18 eV for <b>1</b>, 0.09\neV for <b>2</b>, and 0.12 eV for <b>3</b>. An in-depth\nanalysis of the correlation between different metal centers and proton\nconduction performance indicated that the varying coordination abilities\nof the metal cations and the water absorption capacities of the frameworks\nmight account for the differences in conductivity. Additionally, the\npotential of <b>1</b> as a supercapacitor electrode material\nwas assessed. <b>1</b> exhibited a specific capacitance of 61.13\nF g<sup>–1</sup> at a current density of 0.5 A g<sup>–1</sup>, with a capacitance retention of 82.4% after 5000 cycles, making\nit a promising candidate for energy storage applications.