Atomically thin Co-doped nickel oxide/hydroxide superstructure toward high-energy supercapacitors

Atomically thin nanosheets can expose almost all electroactive sites, which deliver outstanding electrochemical performances. However, the direct fabrication of atomically thin nanosheets array on conductive substrates as binder-free electrodes remains a challenge. Herein, atomically thin (ca. 1.4 nm) Co-doped nickel oxide/hydroxide (CoNiOHx) nanosheets superstructure is grown on Ni foam via an alanine-induced in-situ synthesis strategy. High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) verifies the formation of Co8Ni16O32 mesophase between Co-doped Ni(OH)2 and NiO in the optimized sample. Density functional theory (DFT) calculations demonstrate an improved electrical conductivity and low deprotonation energy in Co8Ni16O32, contributing to elevated electrochemical performances. As expected, the electrode delivers a super-high specific capacity of 2244 mC cm−2 (ca. 1268 C g−1) at 2 mA cm−2 and prominent long-life span with no capacity decay after 5000 cycles. Moreover, the asymmetric supercapacitor constructed by coupling the above cathode and activated carbon (AC) anode displays a high energy density of 73.3 Wh kg−1 at the power density of 2250 W kg−1, as well as meritorious cycling stability. This work provides a new strategy for the construction of atomically thin superstructure on conductive metal substrate with high energy storage capability.