particular, this architecture suggests the strong potential of hollow nanoparticles as catalysts since reactions typically occur at the active sites on the catalyst surface. Additionally, hollow nanoparticle catalysts can effectively maximize the usage of materials by preserving the active surface while eliminating the inactive bulk interior. [ -] For metal-based catalysts, combining multiple elements of precious and nonprecious metals in a hollow structure can further reduce the usage of precious metals, which generally feature higher activity but also higher cost. [ -] However, most studies on metal-based hollow nanoparticles report compositions not exceeding three elements due to the difficulties of mixing immiscible elements. [ , ] Multimetallic particles combining at least five dissimilar elements in a single structure, also called high-entropy-alloy (HEA) nanoparticles, have recently received significant attention due to their advantageous physicochemical properties, including a broad selection of elements, high corrosion resistance, high thermal and chemical stability, enhanced mechanical strength, and increased catalytic activity for broad applications. [ -] Yet there is no report on the synthesis of hollow HEA nanoparticles, where precious metal usage and catalytic activity could be maximized. This is mainly because the fabrication of HEA particles typically relies on fast high-temperature processes and rapid cooling rates to ensure uniform elemental mixing. [ ] Such harsh conditions make it difficult to control the particle morphology and particularly challenging to achieve hollow structures, which are conventionally produced via template-based wet chemistry methods. [ , -] As a result, it is an even greater challenge to achieve the scalable production of such hollow HEA particles in high quantity for real-world applications.

In this work, we report for the first-time a continuous "droplet-to-particle" method of synthesizing hollow HEA nanoparticles, made possible by the introduction of a gas-blowing agent and transient high-temperature heating to successfully synthesize hollow HEA particles with uniform mixing of up to eight dissimilar elements. In a typical synthesis (Figure and Figure S , Supporting Information), we generate an aerosol stream of droplets as small as µm in diameter that contains 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-O battery operations with a record-high current density per catalyst mass loading of mA g cat.

-, 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.

Hollow nanoparticles are an important class of nanomaterials with a structure consisting of a shell encapsulating a large inner void. The unique hollow structure of such nanoparticles features various advantageous properties, including high specific surface area, low density, reduced path lengths for mass and charge transfer, and efficient material usage. [ -] In

metal chloride salts homogeneously dissolved in ethanol along with citric acid, which serves as a blowing agent. The droplets are then carried by argon through the heating zone of a tube furnace with a flow rate of L min -. Upon heating (below °C), ethanol evaporates rapidly from the droplet, leading to the instantaneous formation of solid condensed particles containing uniformly mixed precursors and citric acid. When the temperature further increases to ≈ °C, citric acid rapidly decomposes to produce CO x and H O, which puffs the particle into a hollow structure, as suggested by thermogravimetric analysis (TGA; Figure S , Supporting Information). As the temperature further increases up to °C, the metal salt precursors decompose into metals, which alloy into one phase on the shell to complete the synthesis of hollow HEA nanoparticles. The entire process takes place in less than s.

These hollow HEA nanoparticles have great potential for energy and catalysis applications, as the hollow structure should enable high activity per mass loading. To demonstrate the utility of these materials, we investigated the ability of hollow HEA nanoparticles composed of RuIrFeCoNi as a cathode catalyst for Li-O batteries and demonstrated a record-high current density per catalyst mass loading, as well as good stability and durable catalyst activity. This technique provides strong potential for producing hollow nanomaterials at large-scale for broad applications in catalysis and energy.

A wide variety of hollow HEA nanoparticles can be synthesized via this droplet-to-particle method. As a proof-of-concept demonstration, we synthesized and characterized hollow RuIrFeCoNi HEA nanoparticles. Equal amounts of MCl x H y (M = Ru, Ir, Fe, Co, Ni) precursors (total concentration of . m in ethanol) with citric acid ( . m in ethanol) were atomized into droplets (see the Experimental Section for more details). After passing through the heating zone of a tube furnace via Ar as the carrier gas, the product can be easily and continuously collected in-line using a poly nm and a shell thickness of ≈ nm. Therefore, the ratio of the shell thickness to particle radius (shell thickness/radius) is ≈ %, which corresponds to more than % mass reduction compared with solid HEA particles. In other words, when providing the same amount of active sites on the surface, the hollow RuIrFeCoNi HEA particles can save % in catalytic materials compared to solid morphologies.

Scanning transmission electron microscopy (STEM) also reveals the walls of the nanoparticles are comprised of small nanocrystals (≈ -nm) that feature pores (Figure e). Adv. Mater.

, ,

for the collective bulk RuIrFeCoNi HEA nanoparticles, the five elements are entropy-stabilized in a single FCC structure with a lattice constant of ≈ . Å (Figure h). The lattice constant calculated from SAED is ≈ . Å, consistent with that obtained by XRD. In all syntheses, we observed no aggregation between particles since the residence time (< s) is much shorter than the characteristic coagulation time (≈ s). [ ] We further studied the relationship between the shell thickness and particle size to explore the puffing mechanism for hollow HEA particle formation. A plot between the ratio of the shell thickness to particle radius (shell thickness/radius) as a function of the particle diameter is shown in Figure a. Since the droplets contain the same concentration of citric acid, the amount of gas produced during thermal decomposition is proportional to the volume of the droplet. If we assume all gases produced are employed to expand the particles, the shell thickness should always be proportional to the initial droplet radius and thus we would expect a fixed shell thickness/radius. However, we did not observe this, as Figure a shows the shell thickness/radius decreased with the particle Adv. Mater.

, ,

diameter, with larger particles tending to have a smaller ratio (i.e., thinner shells). This suggests not all gases produced from the decomposition of citric acid were able to expand the particles. Instead, some of this gas must have escaped from the surface to the environment. Since larger particles have smaller surface to volume ratios, a smaller fraction of gas would escape from the surface compared to smaller particles. Therefore, with lower relative gas loss, the large particles have a comparatively higher amount of gas for hollow particle expansion, leading to a smaller shell thickness/radius. We also conducted kinetic control experiments to understand how the concentrations of citric acid affect the shell thickness since the addition of the blowing agent in the precursor is critical for the hollow structure formation. We studied different concentrations of citric acid ( , . , . , . , . , . , . , and . m) . m did we observe the particle to become partially hollow (Figure S , Supporting Information). However, at this concentration, the shell thickness/radius was % with a void volume ratio of only ≈ . %, indicating the citric acid was still too low for sufficient gas to be produced and expand the droplet. The shell thickness/radius of the hollow particle is further decreased when we increase the concentration of citric acid to . m (shell thickness/radius ≈ %) and .

m (shell thickness/ radius ≈ %), showing an evident blowing effect (Figure b). Adv. Mater. , ,

When the concentration of citric acid is higher than that of the metal salt, e.g., . m, the shell thickness/radius decreased to ≈ % with a void volume ratio of ≈ % (Figure S , Supporting Information). Once the concentration of citric acid was increased to . -. m (twice that of the metal salts), we indeed observe a hollow particle with a thin shell (shell thickness/radius of less than % with a void volume ratio of more than %), indicating a large amount of expanding gas was generated. The relationship between the shell thickness/radius and the concentration of citric acid is plotted in Figure c. If we aim to achieve a hollow particle with a shell thickness/radius of %, the minimum concentration of citric acid is ≈ . m. These findings show that the shell thickness of the hollow HEA particles can be effectively controlled by adjusting the concentration of citric acid. As a result, the mass loading of the nanoparticles can be well tuned to achieve the desired performance in catalysis or energy applications.

We also explored the synthesis of hollow HEA nanoparticles with more complex compositions. For demonstration, we selected an octonary composition with eight dissimilar elements (Cr, Mn, Fe, Co, Ni, Pd, Ru, Ir). We note that although there are significant physicochemical property differences in these elements as well as their metal precursors (Table S , Supporting Information), octonary hollow HEA nanoparticles can be synthesized, as demonstrated by the homogenous elemental distribution on the shell of the resulting materials (Figure d). The CrMnFeCoNiPdRuIr HEA particle shown in Figure d (≈ nm in diameter) features a shell thickness of ≈ nm. The shells of these octonary hollow particles feature uniform elemental mixing and are composed of -layers of nanocrystals, the size of which are fixed (≈ -nm) regardless of the particle size (Figure S , Supporting Information). A HAADF-EDS line scan across the particle also shows the double peaks characteristic of hollow structures (Figure S , Supporting Information), in which the elements concentrate at the shell.

These hollow HEA materials can serve in a broad range of applications, such as catalysis, energy, and portable electronics. [ , , ] As a proof-of-concept demonstration, we aimed to exploit the catalytic activities of these materials for energy conversion and storage devices, such as metal-air batteries and fuel cells. Two immediate advantages can be expected from these high-entropy hollow particles. For one, such a unique structure effectively maximizes the material usage efficiency by significantly reducing the catalyst mass loading that is required to maintain a reasonable performance (i.e., rate capability). For another, the high-entropy nature of these particles should provide superior catalyst stability under prolonged reaction conditions. [ ] To explore these merits, we used an aprotic Li-O battery as a study platform. The Li-O technology holds great promise as a post-Li-ion contender, however its development is hindered by low energy efficiency and poor cyclability. [ -] Li-O batteries employ the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the cathode, with both incurring high overpotentials during discharge and recharge, respectively, as well as serious stability concerns with all the cell components. [ -] The overarching goal by the research community is therefore to find an efficient and stable catalyst to promote highly reversible ORR and OER during cycling. Since the catalytic activity is often monotonically correlated with the catalyst loading, minimizing the usage of expensive catalytic elements while providing improved rate performance is highly desired for the development of Li-O batteries.

Based on previous reports, we selected a desirable combination of elements (RuIrFeCoNi) to produce hollow HEA nanoparticles, among which the noble metals (Ru and Ir) ensure the material's catalytic activity while the transition metals (Fe, Co, Ni) enhance the stability of the catalyst by increasing the overall entropy. [ ] The hollow HEA nanoparticles were directly sputtered onto a carbonaceous cathode substrate as a catalyst, as shown in Figure S (Supporting Information). We measured the rate performance with this hollow HEA catalyst for Li-O battery operation and showed the results in Figure a. In comparison to the pristine carbonaceous cathode, the overpotential with increased current density is significantly suppressed after applying the hollow HEA catalyst (Figure S , Supporting Information). Note that in these experiments, the applied current densities were all normalized to the mass loading of the cathode catalyst. As mentioned previously, a crucial benefit we can expect from a hollow structure is reduced catalyst loading while continuing to maintain the desired rate and cycling performance. We found this to be indeed the case, with the specific mass loading not only appearing to be among the lowest in the literature to promote cell operation with a lifetime of over cycles, but the operational current density of the hollow HEAs also outperformed reported cathode catalysts for Li-O battery operations, to the best of our knowledge (Figure b). [ , , , , -] This result indicates that with the use of the hollow nanoparticle design, the required total mass loading of the catalyst for the operation of electrochemical devices can be significantly reduced.

We then chose a record-high current density ( mA g cat.

-) for cycling tests to evaluate the catalyst stability, which is another key feature that such high-entropy materials should offer. [ , ] With a capacity cutoff at mAh g cat.

-, the test cells utilizing RuIrFeCoNi hollow HEA nanoparticles lasted more than cycles (Figure c; detailed cycling profiles are shown in Figure S in the Supporting Information). Importantly, despite repeated formation and decomposition of Li O (as schematically depicted as the typical toroidal morphology shown in Figure S in the Supporting Information), as well as frequent attack by reactive oxygen species, the morphology of the hollow HEA catalysts remained perfectly intact, even after cycles and over h of cycling (Figure d). [ -] This extremely efficient catalyst usage and good stability is not limited to Li-O batteries. The strong promise of these hollow HEA particles can be extended to a broad range of applications that go beyond oxygen catalysis.

In this study, we demonstrate a continuous droplet-toparticle technique to synthesize hollow HEA nanoparticles by aerosolizing metal salt precursors and a gas-blowing agent, followed by transient high-temperature heating. The heatinduced decomposition of the gas-blowing agent expands the precursor droplets into hollow structures. Furthermore, these structures are converted into hollow HEA nanoparticles due to the added conditions of high-temperature heating, short residence time, and high cooling rates, which enable the alloying of up to eight dissimilar elements featuring drastically different physical and chemical properties. With these conditions, we are Adv. Mater.

, , able to synthesize various kinds of hollow HEA nanoparticles with uniform elemental mixing in a continuous and supportfree manner. The obtained HEA nanoparticles possess ≈ % interior void space for a decrease of % mass loading compared with solid HEA nanoparticles. The shell thickness of the materials can be kinetically tuned by adjusting the concentration of citric acid, which allows us to control the mass loading of the nanoparticles when used for catalysis toward desired performance. Besides free powders, the hollow HEA nanoparticles can also be deposited on different supports for broad applications. As a proof-of-concept for Li-O battery operations, we showed that hollow RuIrFeCoNi HEA nanoparticles can achieve a record-high current density of mA g cat.

-when used as the cathode catalyst compared with the traditional solid catalyst. Upon cycles of reversible Li-O battery operation, neither apparent morphological decomposition nor degradation was observed for the hollow HEA catalyst, indicating its superior stability. This low-cost, low-waste, and simple technique demonstrates strong potential for large-scale, high-temperature, hollow nanomaterial manufacturing.

Material Synthesis: All HEA precursors were purchased from Sigma Aldrich, including palladium (II) chloride (≥ . %), ruthenium (III) chloride hydrate ( . %), iron (III) chloride hexahydrate (≥ %), nickel (II) chloride hexahydrate (≥ %), iridium (III) chloride hydrate ( . %), cobalt (II) chloride hexahydrate ( %), manganese (II) chloride monohydrate (≥ %), and chromium (III) chloride (≥ %). Citric acid (> . %) was also purchased from Sigma Aldrich.

All hollow HEA nanoparticles were synthesized using the desired MCl x H y precursors ( . m) at equivalent ratios and citric acid ( . m) [ , , , , -] and this present work. The hollow HEA catalyst stands out with one of lowest mass loading but the highest current density for Li-O battery cycling due to its hollow structure. c) Cycling performance of a typical Li-O test cell using the hollow HEA RuIrFeCoNi catalyst, with a current density of mA g cat.

for h discharge/recharge. d) The morphology of the hollow HEA catalyst on the cathode of Li-O cell before and after the cycling.

Adv. Mater. , , dissolved in ethanol. As shown in Figure S (Supporting Information), the precursor solutions were atomized into small droplets by a collision nebulizer from CH Technologies, Inc. (USA) at a tunable flow rate with argon as the carrier gas. In general, µm droplets were generated, which were then flowed through a diffusion dryer, where most of the solvent was removed. The resulting solid precursor particles were then passed through a tube furnace (Thermolyne , set at °C) to produce the HEA particles. The final hollow HEA nanoparticles were collected on a Millipore PVDF membrane with a pore size of . µm installed in a mm in-line stainless-steel filter holder (PALL Corporation).

For the Li-O battery tests, we purchased lithium bis(trifluoromethane)sulfonimide (LiTFSI, ≥ . %, trace metals basis), , -dimethoxyethane (DME, anhydrous), lithium foil (≥ . %, trace metals basis, . mm thickness), and poly(tetrafluoroethylene) (PTFE, wt% aq.) from Sigma-Aldrich. The DME solvent was further dried using a molecular sieve ( Å) prior to Li-O cell assembly. Deionized water (DI) (H O, . MΩ cm) was acquired from a Barnstead Nanopure Diamond system. The DME-based electrolyte was prepared by mixing LiTFSI with purified DME at m concentration. Carbon paper gas diffusion layers (GDLs; purchased from Toray) were purchased from the FuelCellStore, which were cleaned in sequence using acetone, methanol, and isopropanol, followed by thoroughly drying under vacuum before use. D ordered mesoporous ( DOm) carbon was prepared using a template-assisted process reported by Fan et al., [ ] which was loaded on the GDL substrate (mass ratio : DOm:PTFE) as a pristine cathode. The hollow HEA catalyst was then deposited directly onto the pristine cathode during the aerosol spray process, as shown in Figure S (Supporting Information).

Material Characterization: The microstructure and morphology of the hollow HEA samples were measured by a Hitachi SU-FEG-SEM at kV and a JEOL TEM/STEM ARM CF equipped with HAADF and annular bright field (ABF) detectors. A mrad probe convergence angle was used to perform STEM imaging. HAADF images were acquired using the JEOL ARM CF with a mrad inner-detector angle. An Oxford Xmax TLE windowless X-ray detector was used to collect the EDS results. XRD was performed (Bruker AXS D Advanced, WI, USA) with a scan rate of ° min -.

Electrochemical Measurements: All electrochemical characterization was carried out using a Biologic VMP potentiostat and a homedesigned Swagelok-type cell. Li-O test cells were assembled in an Ar-filled/O -tolerant glovebox (Mbraun, H O < . ppm) under room temperature. The Li-O test cells used a -electrode configuration in which the Li foil was used as the anode, the catalyst-decorated DOm carbon was used as the cathode, and two pieces of polypropylene films were used as separators. All current density and capacity values were either normalized to the applied weight of the hollow HEA catalysts or the area of the total electrode so as to establish a fair comparison with literature results. In a typical experimental setup, µL DMEbased electrolyte ( m LiTFSI in DME) was applied to the Li-O test cell. After assembly, O (Airgas) with ultrahigh purity was filled into the head space of the Swagelok cell using a flow rate of ≈ sccm for min. The head space was then separated from the O gas line and equilibrated to ambient pressure. The rate capability measurements for the hollow HEA-catalyst-loaded DOm carbon cathodes were conducted at current densities ranging from to mA g cat.

with a cutoff time of min for both discharge and recharge stages. The galvanostatic cycles were tested at a constant current density of mA g cat.

-with a cutoff capacity of mAh g cat.

-(operated for h per cycle). Note that the total capacity of the Li anode was in excess compared to the applied capacities for all electrochemical measurements.