Metal Effect on the Proton Conduction of Three Isostructural Metal–Organic Frameworks and Pseudo-Capacitance Behavior of the Cobalt Analogue

Three isostructural transition metal-organic frameworks, [M(bta)_0.5(bpt)(H₂O)₂]·2H₂O (M = Co (1), Ni (2), Zn (3), H₄bta = 1,2,4,5-benzenetetracarboxylic acid, bpt = 4-amino-3,5-bis(4-pyridyl)-1,2,4-triazole), were successfully constructed using different metal cations. These frameworks exhibit a three-dimensional network structure with multiple coordinated and lattice water molecules within the framework, contributing to high stability and a rich hydrogen-bond network. Proton conduction studies revealed that, at 333 K and 98% relative humidity, the proton conductivities (σ) of MOFs 1-3 reached 1.42 × 10^-2, 1.02 × 10^-2, and 6.82 × 10^-3 S cm^-1, respectively. Compared to the proton conductivity of the initial ligands, the σ values of the complexes increased by 2 orders of magnitude, with the activation energies decreasing from 0.36 to 0.18 eV for 1, 0.09 eV for 2, and 0.12 eV for 3. An in-depth analysis of the correlation between different metal centers and proton conduction performance indicated that the varying coordination abilities of the metal cations and the water absorption capacities of the frameworks might account for the differences in conductivity. Additionally, the potential of 1 as a supercapacitor electrode material was assessed. 1 exhibited a specific capacitance of 61.13 F g^-1 at a current density of 0.5 A g^-1, with a capacitance retention of 82.4% after 5000 cycles, making it a promising candidate for energy storage applications.