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Single-crystalline hollow CoO nanooctahedra were successfully synthesized by the combination of self-gas-leaching and Ostwald ripening processes through a template-free route. Elaborate microscopy evidence discloses that hollow CoO nanocrystals (NCs) evolve from the channeled CoO NCs, which is an essential intermediate state to construct the hollow structure. Electrochemical measurements indicate that hollow CoO NCs exhibit three times higher specific capacitance than the solid CoO and show excellent cycling stability with the specific capacitance retaining more than 97% after 4500 cycles at a current density of 2 A g−1. Meanwhile, as a catalyst for oxygen evolution reaction and hydrogen production from hydrolysis of sodium borohydride, the hollow and channeled CoO NCs exhibit much better performances than their solid counterparts. The hollow CoO NCs afford a current density of 10 mA cm−2 at a small overpotential of merely 0.375 V and a small Tafel slope of 55 mV decade−1, and they also possess more steady catalytic activity than the channeled CoO. [ABSTRACT FROM AUTHOR]