MxCo(3-x)O(4) (M = Co, Mn, Fe) porous nanocages derived from metal-organic frameworks as efficient water oxidation catalysts

The design and development of efficient and robust water oxidation catalysts based on earth-abundant elements are crucial for energy conversion and storage technology. Herein, we report Co3O4 porous nanocages derived from simple metal-organic frameworks by a simple self-assembly method and a low temperature annealing process. This method can synthesize MxCo(3-x)O(4) (M = Co, Mn, Fe) porous nanocage materials and allow precise control of the ratio of substituted metal in Co3O4 catalysts. These catalysts were investigated for photochemical, chemical-driven (cerium(IV)-driven) and electrochemical water oxidation, and they presented a superior activity for water oxidation. A high turnover frequency (TOF) of similar to 3.2 x 10(-4) s(-1) per Co atom was obtained under neutral pH using Co3O4 porous nanocages in the photocatalytic water oxidation reaction, which is comparable with those of nanostructured Co3O4 clusters supported on mesoporous silica. Under cerium(IV)-driven water oxidation conditions, a high TOF of similar to 3.6 x 10(-3) s(-1) per Co atom was achieved, which was the highest among those of other known reported cobalt oxides. The overpotential of Co3O4 porous nanocages for the electrochemical water oxidation (eta - 0.42 V at 1 mA cm(-2)) is comparable to the reported overpotentials of catalysts based on cobalt. Multiple experimental results (e.g. XRD, TEM, HR-TEM and XPS) confirm that Co3O4 porous nanocages are highly stable. This study illustrates a guideline for the design and synthesis of inexpensive and highly active spinel catalysts for water oxidation.