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Engineering the electronic structures of transition metal oxides via vacancy defects is an efficient approach to enhance their catalytic performances, while facilely creating vacancy defects and deeply exploring the effects still remain challenging. Herein, a facile reduction method was applied to introduce oxygen vacancy defects into metal-organic frameworks (MOFs)-derived cobalt oxide (Co 3 O 4) polyhedron. Using zeolitic imidazolate framework-67 (ZIF-67) as Co source and self-sacrificial template, Co 3 O 4 with polyhedron morphology and porous structure was synthesized, and then oxygen vacancy defects were introduced into the as-prepared Co 3 O 4 through one-step NaBH 4 reduction treatment. Strikingly, the obtained electrocatalyst (denoted as Co 3 O 4-x) exhibited a larger current density (112.9 mA cm−2 vs. 1.75 V), a lower overpotential (335 mV at 10 mA cm−2) and a smaller Tafel slope (83 mV dec−1) than the Co 3 O 4 , which is even superior to the state-of-the-art IrO 2 catalyst. Moreover, experiment results combined with density-functional theory (DFT) calculations revealed that the existence of oxygen vacancies could improve the conductivity, promote the activation of H 2 O and reduce the potential of potential-limited step (PLS), thus greatly boosting electrocatalytic oxygen evolution reaction (OER) performances. This work reported a strategy for preparing defect-rich OER electrocatalysts, and also provided a deep understanding of the defect effects on catalytic performances. The electronic structures of metal-organic framework-derived Co 3 O 4 polyhedron have been regulated by oxygen vacancy defects and as-prepared Co 3 O 4-x electrocatalysts exhibited enhanced performances for OER. [Display omitted] • Oxygen vacancy defects have been introduced into MOFs-derived Co 3 O 4 polyhedron to enhance OER performances. • As-Prepared Co 3 O 4-x exhibited much better performance than Co 3 O 4 , which is even superior to the state-of-the-art IrO 2. • DFT revealed oxygen vacancies could improve the conductivity, promote H 2 O activation and reduce PLS potential. [ABSTRACT FROM AUTHOR]

Abstract The design of inexpensive, efficient, durable and self-supported bifunctional electrodes for simultaneously catalyzing the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in the same electrolyte is essential for renewable energy conversion processes and future sustainable development, however, it still remains a challenging issue. Reported herein is the first example of selenium phosphorus co-doped cobalt oxide nanosheets on highly conductive Co foil collector (CoOSeP@Co foil), fabricated by successively selenylation and phosphorization of a pre-oxidized Co foil, as a novel bifunctional electrode for water electrolysis. Due to the improved intrinsic catalytic activity of cobalt oxide derived from doping and the strong interaction between active CoOSeP nanosheets and highly conductive Co foil collector, the designed CoOSeP@Co foil integrated electrode displays favorable catalytic activities towards both HER and OER in strongly alkaline media, achieving an electrocatalytic current density of 10 mA cm−2 at a low overpotential of 155 mV for HER and 347 mV for OER, respectively. Beyond that, the electrode could keep working and maintain its catalytic activity for at least 30 h. Furthermore, an alkaline electrolyzer constructed by employing CoOSeP@Co foil as an integrated bifunctional electrode in both the cathode and anode can generate 10 mA cm−2 at a cell voltage of 1.74 V and shows good long-term stability of more than 30 h. This work not only opens a new avenue to improve the intrinsic catalytic activity of materials by co-doping with heteroatoms, but also brings us a new idea for designing other cost-effective, self-supported, efficient and robust overall water splitting electrodes. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]