The sluggish oxygen reduction kinetics and insufficient durability of cathode catalysts restrict the practical application of proton exchange membrane fuel cells. This study focuses on the structural transformation of carbon-supported Pt3Cr from a disordered to an ordered phase and on the effect of such structural transformation on oxygen reduction reaction (ORR) activity and durability. X-ray diffraction and transmission electron microscopy results confirm the formation of carbon-supported Pt3Cr intermetallic nanoparticles with a mean particle size of ca. 7.2 nm. Line scanning EDX reveals that the practical Pt–Cr atomic ratio is approximately 3:1. X-ray photoelectron spectroscopy results indicate that the proportion of metallic Pt increases while the binding energy of Pt 4f decreases with such structural transformation. The Pt3Cr/C intermetallic nanoparticles exhibit enhanced mass and specific activities toward the ORR compared with commercial Pt/C but slightly lower mass activity than the disordered Pt3Cr/C alloy nanoparticles. After the accelerated durability test for 5000 cycles, the Pt3Cr intermetallic nanoparticles displayed negligible decay in ORR mass activity; however the ORR mass activity on the isordered Pt3Cr alloy decreases to ca. 50%. Much enhanced durability of the Pt3Cr/C intermetallic nanoparticles toward the ORR is definitely caused by the much higher structural and compositional stabilities of the Pt3Cr/C intermetallic nanoparticles than that of the disordered Pt3Cr/C alloy nanoparticles, suggesting that the Pt3Cr intermetallic nanoparticles may serve as highly active and durable ORR electrocatalysts for practical application.

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.Keywords: bifunctional electrocatalysts; lithium−oxygen batteries; nickel ferrite; ordered mesoporous structure; tunable pore size

•Pd flower-like nanostructured networks was prepared via a novel CO-assistant method.•The size and morphology of the materials are temperature depended.•The novel materials indicated enhanced activity for formic acid electrooxidation.Novel Pd flower-like nanostructured networks are synthesized via a simple CO-assisted reduction. The morphology and size of the Pd nanostructures are found to strongly depend on the temperature and solvent during the synthesis process. Such Pd flower-like nanostructured networks exhibit a much enhanced activity of about 3 times of that on conventional Pd nanoparticles towards the electrocatalytic oxidation of formic acid. The specific activity of formic acid oxidation on Pd nanostructures is also greatly improved, indicating that the formation of flower-like nanostructured networks is beneficial for the electrooxidation of formic acid. Thus, it could be served as highly active catalyst for formic acid electrooxidation although the stability needs to be greatly improved.Novel Pd flower-like nanostructured networks, synthesized via a simple CO-assisted reduction, exhibit enhanced electrocatalytic activity of about 3 times of that on Pd nanoparticles for formic acid oxidation.