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Enlarging Surface/Bulk Ratios of NiO Nanoparticles toward High Utilization and Rate Capability for Supercapacitors

Reasonable design and delicate control of microstructures are critical to achieve high energy density of active materials for pseudocapacitors that seriously depend on usable reaction interface. This work shows the effect of ultrasmall particle size on enhancing utilization and rate performance of active materials. Three types of NiO nanocrystals with different sizes of 3.36, 6.24, and 7.18 nm in average diameter are uniformly distributed on mesoporous carbon nanosheets derived from corn straw piths. The nanosheets with 3.36 nm NiO particles present an extremely high NiO utilization of 93.4% (2404 F g−1 at 0.5 A g−1), which is 2–2.5‐fold higher than materials with large sizes (6.24 and 7.18 nm). This enhancement is ascribed to more complete conversion and higher ionic/electronic conductivity from a preferable surface/bulk ratio of NiO. By coupling with commercial activated carbon, the asymmetric supercapacitors present high energy and power densities (28.53 Wh kg−1 at 375 W kg−1), with 78.3% capacitance retention after 10 000 cycles at 10 A g−1.