Phase-controlled synthesis of Co-Mg hydroxide for high-performance hybrid supercapacitors

Two-dimensional Mg-substituted cobalt hydroxides (Co x Mg y (OH) z ) with different crystal phases were synthesized via a facile reaction of magnesium (Mg) powder and cobalt aqueous solution. The phases of Co x Mg y (OH) z (amorphous phase and β phase) were controlled by adjusting the cobalt salt precursors. The obtained hybrid amorphous Co x Mg y (OH) z (a-Co x Mg y (OH) z ) showed excellent specific capacitance (662.2 F g −1 , 1 A g −1 ) than β phase Co x Mg y (OH) z (β-Co x Mg y (OH) z ) and pure Co(OH) 2 (p-Co(OH) 2 ). Additionally, the hybrid supercapacitor assembled with a-Co x Mg y (OH) z and the activated carbon showed high specific capacitance (77.8 F g −1 , 1 A g −1 ) and ultrahigh cycling stability with 99.7% retention of the first capacitance at 10 A g −1 after 5000 cycles.