Na-rich metal hexacyanoferrate with water-mediated room-temperature fast Na⁺-ion conductance

Besides of applications in the battery, fast Na⁺-ion conductors have important applications in electrochemical sensors, electrochromic devices, and oxygen separation membranes. However, investigations on room-temperature solid-state Na⁺-ion conducting materials were scarcely reported. A Na-rich metal hexacyanoferrate (Na–FeHCF) was synthesized via a facile precipitation method and characterized by EA (Elemental Analysis), ICP (Inductively Coupled Plasma spectrometer), IR (Infrared Spectroscopy), TGA (Thermogravimetric Analysis), PXRD (Powder X-ray Diffraction), and water sorption/desorption. Na–FeHCF exhibits water-mediated ion conductivity. The Na⁺-ion conductivity (σ) of Na–FeHCF is 1.27 × 10⁻⁸ S cm⁻¹ under anhydrous condition, and it increased to 2.52 × 10⁻³ S cm⁻¹ at 98% RH (relative humidity). As a room-temperature ion conductor, its application as an impedance moisture detecting device has been explored. Na–FeHCF coating with poly(vinylidene fluoride) film on interdigital electrodes exhibited good linearity, nice repeatability, small hysteresis, short response and recovery time.