Molecular Examination of Ion-Pair Competition in Alkaline Aluminate Solutions Using In Situ Liquid SIMS

Properties and composition of the spent electrolyte for premium circulation mediated by Al-air batteries

Al-air batteries can serve as a bridge for high-quality cyclic utilization of aluminum resources. However, limited insights into the spent electrolyte are challenging to accurately adjust the recovery process to obtain premium Al-containing products. Herein, the properties and composition of the spent electrolyte were explored through experiments and theoretical calculations. The results demonstrate that the viscosity of the spent electrolyte increased with the rise in discharge current density, time and temperature under highly alkaline conditions, while the ionic conductivity and causticity obviously decreased. Al(OH)₄^- was the prime and balanced aluminate species when the battery was discharged at 25 °C and coexisted with a bit of [Al₂O(OH)₆]^2-, [Al₂O₂(OH)₆]^4- and Al(OH)₆^3- ions. Especially, the characteristics of the spent electrolyte were mainly dominated by the discharge time and temperature when the current density was continuously increased. There was only Al(OH)₄^- in the electrolyte at a higher discharge temperature. The DFT results also reveal that the polynuclear aluminate ions were produced by the interaction between the mononuclear aluminate ion Al(OH)₄^- and OH^-. This work manifests a profound insight into the spent electrolyte from Al-air batteries for the efficient recycling of aluminum resources.

Uncovering Aging Chemistry of Perovskite Precursor Solutions and Anti-aging Mechanism of Additives

The aging of precursor solutions is the major stumbling block for the commercialization of perovskite solar cells (PSCs). Herein, for the first time we used the state-of-the-art in situ liquid time-of-flight secondary ion mass spectrometry to molecularly explore the perovskite precursor solution chemistry. We identified that the methylammonium and formamidinium cations and the I^- anion are the motivators of the aging chemistry. Further, we introduced two kinds of Lewis bases, triethyl phosphate (TP) and ethyl ethanesulfonate (EE), as new additives in the solution and unraveled that both of them can protect the reactive cations from aging through weak interactions. Significantly, TP is superior to EE in enhancing long-term solution stability as it can well-maintain the internal interaction structures within the solution phase. The PSC derived from a fresh TP-doped solution delivered a high power conversion efficiency of 23.06 %, 92.23 % of which remained in that from a 21-day-old solution.

An amorphous sodium aluminate hydrate phase mediates aluminum coordination changes in highly alkaline sodium hydroxide solutions

A newly identified intermediate phase containing tetrahedral Al is formed incipient to the crystallization of sodium aluminate hydrates.

Crystallization and Phase Transformations of Aluminum (Oxy)hydroxide Polymorphs in Caustic Aqueous Solution

Gibbsite, bayerite, and boehmite are important aluminum (oxy)hydroxide minerals in nature and have been widely deployed in various industrial applications. They are also major components in caustic nuclear wastes stored at various U.S. locations. Knowledge of their crystallization and phase transformation processes contributes to understanding their occurrence and could help optimize waste treatment processes. While it has been reported that partial conversion of bayerite and gibbsite to boehmite occurs in basic solutions at elevated temperatures, systematic studies of factors affecting the phase transformation as well as the underlying reaction mechanisms are nonexistent, particularly in highly alkaline solutions. We explored the effects of sodium hydroxide concentrations (0.1-3 M), reaction temperatures (60-100 °C), and aluminum concentrations (0.1-1 M) on the crystallization and transformation of these aluminum (oxy)hydroxides. Detailed structural and morphological characterization by X-ray diffraction (XRD), scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) spectrometry revealed that these processes depend largely on the reaction temperature and the Al/OH^- ratio. When 1 ≤ Al/OH^- ≤ 2.5, the reactions favor formation of high-crystallinity precipitates, whereas at an Al/OH^- ratio of ≥2.5 precipitation ceases unless the Al concentration is higher than 1 M. We identified pseudoboehmite, bayerite, and gibbsite as intermediate phases to bayerite, gibbsite and boehmite, respectively, all of which transform via dissolution-reprecipitation. Gibbsite transforms to boehmite in both acidic and weak caustic environments at temperatures above 80 °C. However, a "bar-shaped" gibbsite morphology dominates in highly caustic environments (3 M NaOH). The findings enable a robust basis for the selection of various solid phases by tuning the reaction conditions.