The Enhancement on Proton Conductivity of Stable Polyoxometalate-Based Coordination Polymers by the Synergistic Effect of MultiProton Units

Two novel polyoxometalate (POM)-based coordination polymers, namely, [Co(bpz)(Hbpz)][Co(SO₄)_0.5(H₂O)₂(bpz)]₄[PMo^VI ₈Mo^V ₄V^IV ₄O₄₂]⋅13 H₂O (NENU-530) and [Ni₂(bpz)(Hbpz)₃(H₂O)₂][PMo^VI ₈Mo^V ₄V^IV ₄O₄₄]⋅8 H₂O (NENU-531) (H₂bpz=3,3′,5,5′-tetramethyl-4,4′-bipyrazole), were isolated by hydrothermal methods, which represented 3D networks constructed by POM units, the protonated ligand and sulfate group. In contrast with most POM-based coordination polymers, these two compounds exhibit exceptional excellent chemical and thermal stability. More importantly, NENU-530 shows a high proton conductivity of 1.5×10⁻³S cm⁻¹at 75 °C and 98 % RH, which is one order of magnitude higher than that of NENU-531. Furthermore, structural analysis and functional measurement successfully demonstrated that the introduction of sulfate group is favorable for proton conductivity. Herein, the syntheses, crystal structures, proton conductivity, and the relationship between structure and property are presented.