image: (A) Observation of pH as a function of time for the reaction process of NaOH-pour-4.55:1-CoSO4·7H2O, NaOH-pour-2.55:1-CoSO4·7H2O, NaOH-pour-2:1-CoSO4·7H2O and NaOH-pour-1:1-CoSO4·7H2O. (B) The relationship between pH of the final reaction solution, reaction rate, and the deintercalation/retention of tetrahedral Co2+. The regions shaded in blue and pink highlight the deintercalation and retention of tetrahedral Co2+, respectively. (C) Illustration of the proposed intercalation/deintercalation mechanism. (D) LSV curves and (E) Tafel slopes of different electrocatalysts.
Credit: ©Science China Press
Transition metal hydroxides (TMHs) are common in nature and widely used in catalysis, energy storage, and electronics. Their synthesis often relies on wet chemical methods, where water/anion-coordinated metal ions transform under increasing OH⁻ concentration. This process results in the formation of a complex network of anion-coordinated metal polyhedra, where unconventional polyhedra structures coexist with conventional octahedral structures. However, the dynamic behavior of these unconventional polyhedra in the formation process of Co(OH)₂ remains poorly understood due to limitations in traditional characterization techniques.
Recently, an international team, led by Prof. Minghua Huang from Ocean University of China, Dr. Saskia Heumann from Max Planck Institute, Prof. Heqing Jiang from Chinese Academy of Sciences and Prof. Helmut Cölfen from University of Konstanz, investigated the intercalation and deintercalation of tetrahedral Co²⁺ in the formation process of Co(OH)₂ using real-time in situ methods, including pH monitoring and UV-Vis spectroscopy. By tracking tetrahedral Co²⁺ incorporation and release, researchers discovered that in the early stages of Co(OH)2 formation, tetrahedral Co2+ is preferentially incorporated into the lattice. However, its retention is largely dictated by the effective OH- concentration in the reaction solution and the competitive ability of effective OH- is linked to its concentration, as well as other factors including the presence of reversible reactions involving OH-. These findings provide a deeper and more comprehensive understanding of the formation process of Co(OH)₂.
Beyond advancing fundamental understanding, these findings offer a strategy for tailoring TMH synthesis to optimize performance in applications like oxygen evolution reaction (OER) catalysis. The in situ approach also presents a powerful tool for studying hydroxide-based materials more broadly, paving the way for improved synthesis methods and enhanced material properties.
Journal
National Science Review