Construction of hydrogel composites with superior proton conduction and flexibility using a new POM-based inorganic–organic hybrid

Abstract

Polyoxometalates (POMs) have attracted extensive interests as solid-state proton conductors due to their high conductivity and good thermal stability. However, the low humidity stability and poor mechanical property of crystalline POMs greatly hinder their practical application. Herein, on the basis of the remarkable improvement of the humidity stability that can be achieved by hybridizing POMs with organic components, we prepared a new POM-based inorganic–organic hybrid, H[Zn(Hapca)₃][PW₁₂O₄₀]·7H₂O <strong>1</strong> (Hapca = 3-aminopyrazine-2-carboxylic acid), composed of Keggin-type PW₁₂O₄₀³⁻ anions and Zn(Hapca)₃²⁺ cations. Compound <strong>1</strong> exhibits a proton conductivity of 1.16 × 10⁻⁴ S·cm⁻¹ at 55 °C, which remains stable at a relative humidity of 95%. To construct POM-based conductive materials with good mechanical property, we introduced <strong>1</strong> into a poly(vinyl alcohol) (PVA)/glycerol (Gly) hydrogel and obtained the hydrogel composites <strong>1@PVA/Gly-</strong><i><strong>x</strong></i> (<i>x</i> = 20%, 30%, and 40%, where <i>x</i> is the percentage of the mass of <strong>1</strong> to the total mass of <strong>1</strong> and PVA<strong></strong>). Remarkably, the hydrogel composites <strong>1@PVA/Gly-</strong><i><strong>x</strong></i> combine the merits of their components. The proton conductivity of <strong>1@PVA/Gly-40%</strong> reaches 1.11 × 10⁻² S·cm⁻¹ at ambient humidity and 75 °C, outperforming most POM-based composite materials reported to date. Moreover, <strong>1@PVA/Gly-40%</strong> displays good elasticity and toughness and high strain sensitivity. Additionally, we assembled <strong>1@PVA/Gly-40%</strong> into a strain sensor for the effective monitoring of various human activities. This work provides a new platform for the transformation of crystalline POM-based inorganic–organic hybrids into materials with good mechanical properties.