Highly Stable Bismuth-Based Layered Oxides Modified by Phytic Acid for Anhydrous and Water-Assisted Proton Conductivity

Abstract

Proton-conducting materials serving as key components in various electrochemical and energy conversion devices have attracted a great deal of attention. The creation of superior proton-conducting materials in a wide temperature range under both humidified and anhydrous conditions is of great significance for practical applications. In this paper, we develop a coprecipitation synthesis approach for a new proton-conducting nanocomposite, [Bi₂O₂]H_x-PA_0.28, which consists of a layered framework with [Bi₂O₂]²⁺ and phytic acid (C₆H₁₈O₂₄P₆, abbreviated as PA) existing as interlayer-embedded charge-balancing anions. Infrared spectroscopy (FTIR), solid-state NMR, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) suggest that oxygen atoms on the surface of the [Bi₂O₂]²⁺ layer bridge the uncoordinated oxygen atoms on the phosphate group to form a strong hydrogen bonding network. This leads to a water-assisted proton conductivity of 1.76 × 10^–2 S cm^–1 at 90 °C under 98% RH and even a stable anhydrous proton conductivity of 5.24 × 10^–3 S cm^–1 at 150 °C. Isotope effect tests have verified the selective transmission of hydrogen ions by the composite electrolyte materials. [Bi₂O₂]H_x-PA_0.28 as electrolyte was further assembled into an ion-selective electrode for the recognition of hydrogen ions. Furthermore, open circuit potential time (OCPT) testing further confirms that solid-state electrolytes composed of [Bi₂O₂]H_x-PA_0.28 exhibit a good electrochemical stability. This work broadens the possibilities for the development of solid-state proton conductors in a wide temperature range, even under anhydrous conditions.