Ultrafast Synthesis of High Entropy Oxide Nanoparticles by Flame Spray Pyrolysis.

The synthesis of high entropy oxide (HEO) nanoparticles (NPs) possesses many challenges in terms of process complexity and cost, scalability, tailoring nanoparticle morphology, and rapid synthesis. Herein, we report the synthesis of novel single-phase solid solution (Mn, Fe, Ni, Cu, Zn)3(O)4 quinary HEO NPs produced by a flame spray pyrolysis route. The aberration-corrected scanning transmission electron microscopy (STEM) technique is utilized to investigate the spinel crystal structure of synthesized HEO NPs, and energy-dispersive X-ray spectroscopy analysis confirmed the high entropy configuration of five metal elements in their oxide form within a single HEO nanoparticle. Selected area electron diffraction, X-ray diffraction, and Raman spectroscopy analysis results are in accordance with STEM results, providing the key attributes of a spinel crystal structure of HEO NPs. X-ray photoelectron spectroscopy results provide the insightful understanding of chemical oxidation states of individual elements and their possible cation occupancy sites in the spinel-structured HEO NPs.

A room temperature rechargeable Li2O-based lithium-air battery enabled by a solid electrolyte.

A lithium-air battery based on lithium oxide (Li2O) formation can theoretically deliver an energy density that is comparable to that of gasoline. Lithium oxide formation involves a four-electron reaction that is more difficult to achieve than the one- and two-electron reaction processes that result in lithium superoxide (LiO2) and lithium peroxide (Li2O2), respectively. By using a composite polymer electrolyte based on Li10GeP2S12 nanoparticles embedded in a modified polyethylene oxide polymer matrix, we found that Li2O is the main product in a room temperature solid-state lithium-air battery. The battery is rechargeable for 1000 cycles with a low polarization gap and can operate at high rates. The four-electron reaction is enabled by a mixed ion-electron-conducting discharge product and its interface with air.

Scalable Synthesis of High Entropy Alloy Nanoparticles by Microwave Heating.

High entropy alloy nanoparticles (HEA-NPs) are reported to have superior performance in catalysis, energy storage, and conversion due to the broad range of elements that can be incorporated in these materials, enabling tunable activity, excellent thermal and chemical stability, and a synergistic catalytic effect. However, scaling the manufacturing of HEA-NPs with uniform particle size and homogeneous elemental distribution efficiently is still a challenge due to the required critical synthetic conditions where high temperature is typically involved. In this work, we demonstrate an efficient and scalable microwave heating method using carbon-based materials as substrates to fabricate HEA-NPs with uniform particle size. Due to the abundant functional group defects that can absorb microwave efficiently, reduced graphene oxide is employed as a model substrate to produce an average temperature reaching as high as ∼1850 K within seconds. As a proof-of-concept, we utilize this rapid, high-temperature heating process to synthesize PtPdFeCoNi HEA-NPs, which exhibit an average particle size of ∼12 nm and uniform elemental mixing resulting from decomposition nearly at the same time and liquid metal solidification without diffusion. Various carbon-based materials can also be employed as substrates, including one-dimensional carbon nanofibers and three-dimensional carbonized wood, which can achieve temperatures of >1400 K. This facile and efficient microwave heating method is also compatible with the roll-to-roll process, providing a feasible route for scalable HEA-NPs manufacturing.

Continuous Synthesis of Hollow High-Entropy Nanoparticles for Energy and Catalysis Applications.

Mixing multimetallic elements in hollow-structured nanoparticles is a promising strategy for the synthesis of highly efficient and cost-effective catalysts. However, the synthesis of multimetallic hollow nanoparticles is limited to two or three elements due to the difficulties in morphology control under the harsh alloying conditions. Herein, the rapid and continuous synthesis of hollow high-entropy-alloy (HEA) nanoparticles using a continuous "droplet-to-particle" method is reported. The formation of these hollow HEA nanoparticles is enabled through the decomposition of a gas-blowing agent in which a large amount of gas is produced in situ to "puff" the droplet during heating, followed by decomposition of the metal salt precursors and nucleation/growth of multimetallic particles. The high active sites per mass ratio of such hollow HEA nanoparticles makes them promising candidates for energy and electrocatalysis applications. As a proof-of-concept, it is demonstrated that these materials can be applied as the cathode catalyst for Li-O2 battery operations with a record-high current density per catalyst mass loading of 2000 mA gcat. -1 , as well as good stability and durable catalytic activity. This work offers a viable strategy for the continuous manufacturing of hollow HEA nanomaterials that can find broad applications in energy and catalysis.