Structural and electrochemical stability of CoAl layered double hydroxide in alkali solutions

CoAl layered double hydroxides (LDHs) have been found excellent supercapacitive behavior, however, little is known on their stability in alkali solutions. Here, a series of CoAl LDHs with molar ratios of Co to Al between 1 and 9 synthesized by homogenously precipitation with urea were characterized. When the LDH materials are either soaked in alkali solutions or electrochemically charged/discharged on the electrodes, they transform into β-Co(OH)2. The transformation reaction accelerates as the Al content in the LDH materials decreases, or the ambient temperature and/or the alkali concentration for the soaking treatment increases. When the KOH concentration goes up from 6.0 mol l−1 to 7.0 mol l−1, there is a much greater decrease in mass of the residual solid, indicating that the transformation leaps in the 7.0 mol l−1 KOH solution. Elemental analyses on both the residual solid and the filtrate support those observations. Electrochemical characterization shows that the transformation reaction has great effects on the supercapacitive behaviors and cycling charge/discharge performances. For example, the specific capacitance of Co3.85Al(OH)8.7(OCN)0.84(CO3)0.58·2.5H2O gives the maximum of 549.0 F g−1 at the 5th cycle under a specific current of 800 mA g−1, however, that of its residual solid after the soaking in 7.0 mol l−1 KOH for 5 h decreases to less than 80 F g−1 within the 200 cycles. In addition, the dissolution of Al(OH)3 in the electrolyte and/or surface modification of Y, Er or Lu can improve the cycling charge/discharge performances because they can retard the transformation from LDH into β-Co(OH)2.