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HIGHLIGHTS • The 2.27-nm-thick hybridized quasi-2D structure of La 2 O 3 crystalline nanoparticles embedded in La 2 O 3 amorphous nanosheets (La 2 O 3 @NP-NS) exhibited a low overpotential of 310 mV at 10 mA cm −2. • The mass activity of La 2 O 3 @NP-NS reached as high as 6666.7 A g −1 at overpotential of 310 mV. Such a high mass activity was more than three orders of magnitude higher than that of benchmark IrO 2 (4.4 A g −1) and RuO 2 (2.05 A g −1) and five orders of magnitude higher than that of commercial La 2 O 3 (0.048 A g −1).