MxCo3−xO4 (M = Co, Mn, Fe) porous nanocages derived from metal–organic frameworks as efficient water oxidation catalysts

Abstract

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 M_xCo_3−xO₄ (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 ∼3.2 × 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 ∼3.6 × 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 (η = 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.