Carbonate-free CoAl layered double hydroxides supercapacitors: Controlled precipitation via acid mediated decomplexation

Layered double hydroxides (LDH) belong to a class of highly versatile compounds used in a myriad of applications such as energy storage, catalysis, heavy metal adsorption, etc. Given the high tunability of its structure, ranging from different metal cations to different types of intercalations, LDH can be readily customised to suit the intended applications. One of the inherent problem of LDH lies in their carbonate contamination. The presence of CO2/3- anions in the intercalation greatly affects the versatility and tunability of LDH. Carbonate intercalation also poses as an interfering species for reactions involving carbonate as intermediates or final products. Unfortunately, existing synthesis procedures of LDH produce either carbonates during the precipitating process or poorly crystallised LDH that absorbs atmospheric carbon dioxide to form carbonates during the drying process; either way results in carbonate contaminated LDH. In this work, we address the problem with the use of ethylenediamine (EDA) complexation followed by controlled decomplexation to prepare CoAl LDH (LDH-EDA). It is shown that the obtained LDH-EDA is carbonate-free and outperforms the CoAl LDH obtained using conventional urea method (LDH-Urea) in terms of crystallinity, surface area and pseudocapacitive properties. Through a detailed experimental and computational study of the preparation mechanism, we propose that ammonium nitrate acts as an acid that mediates the decomplexation of the cobalt (II) trisethylenediamine complex, freeing Co2+ ion in a control manner for the second precipitation process. The multistep process has allowed a controlled rate of formation of LDH, resulting in the highly crystalline LDH-EDA microstructures with pure nitrate intercalation and a flower-like morphology.