Improving proton conduction of the Prussian blue analogue Cu₃[Co(CN)₆]₂·: N H₂O at low humidity by forming hydrogel composites

In this study, we have synthesized Prussian blue analogue nanocrystals of copper hexacyanocobaltate (CuHCC), and their derivatives via a facile stepwise encapsulation of 1,3-diaza-2,4-cyclopentadiene (Im; Im@CuHCC) and acetic acid (HAc; Im-HAc@CuHCC) into the CuHCC framework. These materials were characterized by powder X-ray diffraction, thermogravimetric analysis, infrared spectroscopy, N2 adsorption/desorption and scanning electron microscopy. Im-HAc@CuHCC shows humidity dependent proton conduction with conductivity (σ) in the 0.933-8.55 × 10-2 S cm-1 range at 98% relative humidity (RH) while σ ≈ 10-6-10-5 S cm-1 at 25% RH in the 298-353 K range. We further prepared and characterized a series of composites of Im-HAc@CuHCC with a polyvinyl alcohol (PVA) hydrogel, labeled Im-HAc@CuHCC-PVA-X (X = 0, 2, 5 and 10%, and represents the mass ratio of Im-HAc@CuHCC and the hydrogel). Regarding PVA hydrogel, the tensile strength of composites is enhanced at least 1.5 times. Importantly, the X = 10% composite shows superior proton conduction with σ = 0.81-6.5 × 10-2 S cm-1 at 25% RH in the 298-353 K range, confirming that the formation of the hydrogel composite is a facile but efficient strategy to enhance the water-retention capacity and proton conduction at low humidity for porous proton conductors.