Electrochemical surface reconstruction of nickel cobalt pyrophosphate to Ni/Co-hydroxide-(oxy)hydroxide: an efficient and highly durable battery-type supercapacitor electrode material

Abstract

Developing transition metal-based electrode materials with high charge storage ability and excellent cycle life is a substantial challenge for supercapacitor applications. This report demonstrates an improved specific capacitance and excellent cycling stability for a Ni/Co-hydroxide-(oxy)hydroxide electrode material obtained from the electrochemical reconstruction of a Ni/Co-pyrophosphate precursor. Herein, we have prepared a series of nickel/cobalt pyrophosphate (Ni_xCo_100−x 600) (x = 90, 80, 70, 60, 50, 0) materials through a simple co-precipitation method followed by thermal annealing. Upon electrochemical treatment, the developed nickel/cobalt pyrophosphate materials were transformed into porous Ni/Co-hydroxide-(oxy)hydroxides. This is due to the etching of pyrophosphate anions with electrochemical cycling, confirmed by rigorous physical characterization. Among them, Ni-rich electrodes showed higher specific capacitance values due to the dominant intercalation redox behavior (battery-type storage), yet they suffer from poor cycle stability. In contrast, Co-rich electrodes display low specific capacitance and excellent capacitance retention as the charge storage mainly occurs through surface redox processes. Interestingly, the incorporation of an adequate amount of cobalt into the nickel system has dramatically improved the charge storage ability as well as cycling stability owing to the synergistic effect between Ni and Co redox centers and the balanced charge storage phenomenon contributed by both the surface and intercalation redox processes. Especially, the material developed from Ni₆₀Co₄₀ 600 has exhibited a maximum specific capacitance of 566 F g⁻¹ at 1 A g⁻¹ current density and achieved an excellent capacitance retention of 84% (∼99% coulombic efficiency) up to 10 000 cycles. Further, the corresponding hybrid device (Ni₆₀Co₄₀ 600//AC) has displayed an operational voltage window of 1.6 V and achieved a maximum energy density of 14.9 W h kg⁻¹ at 794.5 W kg⁻¹ power density and could retain ∼86% of its initial capacity even after 25 000 cycles, thereby indicating the suitability of the electrode for practical applications.