Polyoxometalate-driven assembly of inorganic–organic hybrid materials: Proton conductivity studies

Abstract

A zinc-containing, one-dimensional coordination polymer, [Zn(NH₂trz)₃](NO₃)₂ (1D-CP, NH₂-trz \(=\) 4-amino-4H-1,2,4-triazole) has been employed to synthesize two water-insoluble proton conductors, ZnL-PW₁₂ and ZnL-SiW₁₂ (L \(=\) 4-amino-4H-1,2,4-triazole), that are formulated as {[Zn(NH₂trz)₃](NO₃)}₂{HPW₁₂O₄₀}·4H₂O and [Zn(NH₂trz)₃][NH₃trz]₂[SiW₁₂O₄₀]·10H₂O, respectively. These proton conductors were prepared through simple room-temperature aqueous reactions involving [Zn(NH₂trz)₃](NO₃)₂ and phosphotungstic or silicotungstic acids, which proceed via aqueous disassembly of the CP, followed by recrystallization into a new phase driven by the addition of heteropolyanions. Here, the Keggin POMs, specifically H₃PW₁₂O₄₀ and H₄SiW₁₂O₄₀, impart enhanced stability to the overall structures of the resulting Zn-complex-POM adducts, ZnL-PW₁₂ and ZnL-SiW₁₂. Upon mixing the aqueous solutions of 1D-CP and POMs, immediate precipitations of the water-insoluble inorganic–organic hybrid products are obtained, the aqueous suspensions of which exhibit acidic pH. This observation prompted us to investigate proton conductivity behaviour of the resulting materials. As anticipated, the isolated products, ZnL-PW₁₂ and ZnL-SiW₁₂ exhibit significant proton conductivity values of 5.1 × 10⁻³ and 5.6 × 10⁻³ S cm⁻¹, respectively, at 80°C and 98% relative humidity, demonstrating excellent stability for about 30 h with decent cycling stability over four heating–cooling cycles.

Graphical abstract

A zinc-based one-dimensional coordination polymer, [Zn(NH₂-trz)₃](NO₃)₂ (1D-CP), was modified into two water-insoluble proton conductors by a process involving the disassembly and subsequent restructuring of the coordination polymer into more stable inorganic–organic hybrids by the addition of phosphotungstic and silicotungstic heteropolyanions. The resulting hybrid materials, ZnL-PW₁₂ and ZnL-SiW₁₂ (L = 4-amino-4H-1,2,4-triazole), exhibit significant proton conductivities of 5.1 × 10⁻³ and 5.6 × 10⁻³ S cm⁻¹, respectively, and demonstrate excellent stability for >30 h under the operational conditions.