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[Display omitted] • Heterogenous electrocatalytic epoxidation of cyclooctene directly using water as oxygen source was achieved. • Co 4 P 2 W 18 shows higher Faradic efficiency (∼36 %) for epoxide than CoPW 11 and other typical electrocatalysts. • The O atom directly transfer from water to adsorbed cyclooctene on the surface of Co 4 P 2 W 18. • The adjacent terminal O to Co site plays key role in adsorption and activation of water and cyclooctene. The electrocatalytic epoxidation directly using water as oxygen source is considered as a sustainable strategy to achieve epoxide production under ambient conditions. However, it is challenging to develop non-noble metal based heterogeneous electrocatalysts for efficient epoxidation with a deep understanding of the structure–activity relationships. In this work, we report a Weakley-polyoxometalate Na 10 [Co 4 (H 2 O) 2 (PW 9 O 34) 2 ] (Co 4 P 2 W 18) with high efficiency toward electrocatalytic epoxidation of cyclooctene via directly transfer oxygen atom from water. A significantly higher faradaic efficiency of cyclooctene oxide was achieved over Co 4 P 2 W 18 than a Keggin type Co substituted polyoxometalate Na 5 [Co(H 2 O)PW 11 O 39 ] (CoPW 11) and some other typical electrocatalysts including RuO 2 , IrO 2 , and Co 3 O 4 in our system. Experimental results and theoretical calculation indicate that the oxygen atom directly transfers from water for the epoxidation of cyclooctene via the cleavage of O−H of pre-adsorbed water as the rate-determined step. Further calculation demonstrates that the water and cyclooctene were adsorbed by Co site and their adjacent terminal oxygen atom during the epoxidation reaction. With the assistance of this terminal oxygen atom for adsorption and as an electron capture center, lower energy barriers for activation of water and cyclooctene are observed on Co 4 P 2 W 18 than on CoPW 11. Our study provides atomic insight into how structure affects catalytic activity, which benefits designing efficient electrocatalysts. [ABSTRACT FROM AUTHOR]