Controllable synthesis of ordered mesoporous NiFe₂O₄ with tunable pore structure as a bifunctional catalyst for Li-O₂ batteries.

Three-dimensional ordered mesoporous (3DOM) NiFe2O4 materials with tunable pore size ranging from 5.0 to 25.1 nm have been synthesized via a hard template and used as bifunctional electrocatalysts for rechargeable Li-O2 batteries. Characterization of the catalysts by X-ray diffraction and transmission electron microscopy confirms the formation of a single-phase 3DOM NiFe2O4 structure. Linear scanning voltammetry measurements reveal that Ketjen black (KB) carbon-supported 3DOM NiFe2O4 exhibits a decreased overpotential for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) than commonly used KB. A reduction in both the ORR and OER overpotentials increases with the mean pore size of 3DOM NiFe2O4 materials. Importantly, Li-O2 batteries with 3DOM NiFe2O4 materials as the cathode catalysts exhibit a significant enhancement in the discharge capacity, rate capability, and cyclability, and these performances increases with the mean pore size of 3DOM NiFe2O4 materials. For a Li-O2 battery equipped with a 3DOM NiFe2O4 catalyst with a maximum mean pore size of 25.1 nm, a long cycling life of up to 100 cycles under the limiting capacity of 1000 mAh gC(-1) is achieved, strongly indicating that the mesoporous size of the bifunctional catalysts plays a crucial role in enhancing the performance of Li-O2 batteries. The combined use of 3DOM NiFe2O4 with a maximal pore size of 25.1 nm and a poly(vinylidene difluoride hexafluoropropylene) separator with a tuned pore structure further improves the Li-O2 battery performance, highlighting the importance of the pore structure in the development of bifunctional catalysts and separators.