Investigation on the decomposition process of sodium aluminate solution by spectroscopic and theoretical calculation

Self-protecting CoFeAl-layered double hydroxides enable stable and efficient brine oxidation at 2 A cm-2

Low-energy consumption seawater electrolysis at high current density is an effective way for hydrogen production, however the continuous feeding of seawater may result in the accumulation of Cl-, leading to severe anode poisoning and corrosion, thereby compromising the activity and stability. Herein, CoFeAl layered double hydroxide anodes with excellent oxygen evolution reaction activity are synthesized and delivered stable catalytic performance for 350 hours at 2 A cm-2 in the presence of 6-fold concentrated seawater. Comprehensive analysis reveals that the Al3+ ions in electrode are etched off by OH- during oxygen evolution reaction process, resulting in M3+ vacancies that boost oxygen evolution reaction activity. Additionally, the self-originated Al(OH)n- is found to adsorb on the anode surface to improve stability. An electrode assembly based on a micropore membrane and CoFeAl layered double hydroxide electrodes operates continuously for 500 hours at 1 A cm-2, demonstrating their feasibility in brine electrolysis.

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.

Ab Initio Molecular Dynamics Reveal Spectroscopic Siblings and Ion Pairing as New Challenges for Elucidating Prenucleation Aluminum Speciation

The characterization of prenucleation species is essential to understand crystallization mechanisms across many chemical systems and often involves the use of vibrational spectroscopy. Nowhere is this more evident than in the development of "green" aluminum processing technologies, where detailed understanding of the speciation of aluminum and its polynuclear analogues in highly alkaline, low water solutions is elusive. The aluminate anion Al(OH)₄^- predominates in alkaline conditions, yet equilibrium with dimeric species, either μ-oxo Al₂O(OH)₆^2- or di-μ-hydroxo Al₂(OH)₈^2-, can be assumed. Using ab initio molecular dynamics with full solvation and the presence of counterions, this work reconciles previous contradictory studies that had concluded only a single species under relevant solution conditions. We reveal that the two dimers are energetically separated by 2 kcal/mol in pure water but that the stability of each can be reversed by ion pairing expected in saturated salt solutions. Simulated Raman and IR spectra for each species (accounting for anharmonicity and the fluctuating solvating environment) provide the first proof that the considered species are "spectroscopic siblings", whose multiple overlapping bands prevent definitive assertions in terms of speciation when compared to the experimental spectra. These observations are likely to hold in higher order aluminate oligomers and as such present a massive challenge toward understanding the crystallization mechanisms relevant to aluminum processing.