Electrochemical properties of layer-structured H_x(Ni_1/3Co_1/3Mn_1/3)O₂ for electrochemical capacitors in alkaline aqueous solutions

The electrochemical properties of layer-structured Hx(Ni1/3Co1/3Mn1/3)O2 were examined for the first time to evaluate its use as an electrode for an electrochemical capacitor in alkaline aqueous solutions. The protonated Hx(Ni1/3Co1/3Mn1/3)O2 (HNCM) was prepared by reacting Li(Ni1/3Co1/3Mn1/3)O2 (LNCM) with an HCl aqueous solution. The HNCM exhibited a large capacitance of 720 F g−1 (290 mAh g−1, −1 to 0.45 V vs. Hg/HgO) at a current density of 50 mA g−1 in 8 M KOH aqueous solution, which is the highest level for the bulk electrodes in aqueous solutions. High power density, that is a capacitance of 560 F g−1 at a large current density of 1500 mA g−1, was also confirmed. The HNCM exhibited good cycle stability, especially in 1 M KOH aqueous solution. The HNCM was found to be a promising electrode material for electrochemical capacitors because of its large capacitance, high cycle stability, and high rate capability.