Enhancing the Supercapacitive Behaviour of Cobalt Layered Hydroxides by 3D Structuring and Halide Substitution

Among the two-dimensional (2D) materials, layered hydroxides (LHs) stand out due to their chemical versatility, allowing the modulation of physicochemical properties on demand. Specifically, LHs based on earth-abundant elements represent promising phases as electrode materials for energy storage and conversion. However, these materials exhibit significant drawbacks, such as low conductivity and in-plane packing that limits electrolyte diffusion. In this work, we explore the synthetic flexibility of α-Co^II hydroxides (Simonkolleite-like structures) to overcome these limitations. We elucidate the growth mechanism of 3D flower-like α-Co^II hydroxyhalides by using in situ SAXS experiments combined with thorough physicochemical, structural, and electrochemical characterization. Furthermore, we compared these findings with the most commonly employed Co-based LHs: β-Co(OH)₂ and CoAl layered double hydroxides. While α-Co^II LH phases inherently grow as 2D materials, the use of ethanol (EtOH) triggers the formation of 3D arrangements of these layers, which surpass their 2D analogues in capacitive behavior. Additionally, by taking advantage of their anion-dependent bandgap, we demonstrate that substituting halides from chloride to iodide enhances capacitive behavior by more than 40 %. This finding confirms the role of halides in modulating the electronic properties of layered hydroxides, as supported by DFT+U calculations. Hence, this work provides fundamental insights into the 3D growth of α-Co^II LH and the critical influence of morphology and halide substitution on their electrochemical performance for energy storage applications.