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Ravi Nandan, Hiroki Nara, Ho Ngoc Nam, Quan Manh Phung, Quynh Phuong Ngo, Jongbeom Na, [Joel Henzie](https://orcid.org/0000-0002-9190-2645), [Yusuke Yamauchi](https://orcid.org/0000-0001-7854-927X)

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[Tailored Design of Mesoporous Nanospheres with High Entropic Alloy Sites for Efficient Redox Electrocatalysis](https://mdr.nims.go.jp/datasets/a057c2cb-b2dc-4c9e-a7c4-97e16b3a52a5)

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Tailored Design of Mesoporous Nanospheres with High Entropic Alloy Sites for Efficient Redox ElectrocatalysisRESEARCH ARTICLEwww.advancedscience.comTailored Design of Mesoporous Nanospheres with HighEntropic Alloy Sites for Efficient Redox ElectrocatalysisRavi Nandan, Hiroki Nara, Ho Ngoc Nam, Quan Manh Phung, Quynh Phuong Ngo,Jongbeom Na,* Joel Henzie,* and Yusuke Yamauchi*High Entropy Alloys (HEAs) are a versatile material with unique properties,tailored for various applications. They enable pH-sensitive electrocatalytictransformations like hydrogen evolution reaction (HER) and hydrogenoxidation reactions (HOR) in alkaline media. Mesoporous nanostructureswith high surface area are preferred for these electrochemical reactions, butdesigning mesoporous HEA sis challenging. To overcome this challenge, alow-temperature triblock copolymer-assisted wet-chemical approach isdeveloped to produce mesoporous HEA nanospheres composed ofPtPdRuMoNi systems with sufficient entropic mixing. Owing to active siteswith inherent entropic effect, mesoporous features, and increasedaccessibility, optimized HEA nanospheres promote strong HER/HORperformance in alkaline medium. At 30 mV nominal overpotential, it exhibits amass activity of ≈167 (HER) and 151 A gPt−1 (HOR), far exceedingcommercial Pt-C electrocatalysts (34 and 48 A gPt−1) and many recentlyreported various alloys. The Mott-Schottky analysis reveals HEA nanospheresinherit high charge carrier density, positive flat band potential, and smallercharge transfer barrier, resulting in better activity and faster kinetics. Thismicelle-assisted synthetic enable the exploration of the compositional andconfigurational spaces of HEAs at relatively low temperature, whilesimultaneously facilitating the introduction of mesoporous nanostructures fora wide range of catalytic applications.R. Nandan, J. Henzie, Y. YamauchiResearch Center for Materials NanoarchitectonicsNational Institute for Materials Science (NIMS)1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanE-mail: HENZIE.Joeladam@nims.go.jp; y.yamauchi@uq.edu.auH. NaraWaseda Research Institute for Science and EngineeringWaseda University3-4-1 Okubo, Shinjuku, Tokyo 169-8555, JapanThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/advs.202402518© 2024 The Author(s). Advanced Science published by Wiley-VCHGmbH. This is an open access article under the terms of the CreativeCommons Attribution License, which permits use, distribution andreproduction in any medium, provided the original work is properly cited.DOI: 10.1002/advs.2024025181. IntroductionMulti-metallic nanomaterials have gainedsignificant attention due to their di-verse compositional and configurationalpossibilities.[1–3] The inherent hetero-geneity allows for increased functionalityowing to multi-elemental synergy, makingthem desirable for various applicationsincluding energy storage and conver-sion, biomedicine, catalysis, sensing,electronics, and photonics.[4–8] The re-cent introduction of high entropy alloy(HEA) nanostructures incorporating fiveor more principal elements has expandedthe multi-metallic material library, creat-ing unique opportunities to explore andoptimize the structure-property correlationfor targeted complex applications.[2,3,5,9–15]The higher configurational entropy in HEAsystems overrides the enthalpy penalties,preventing phase segregation by boostingthe elemental solubility and resulting inhigher phase purity.[3,16] Achieving phasepurity in HEA results in the confinementof multiple atomic species into the samelattice through entropic contributions.[9,17]H. N. Nam, Y. YamauchiDepartment of Materials Process EngineeringGraduate School of EngineeringNagoya UniversityNagoya 464-8603, JapanQ. M. PhungDepartment of ChemistryGraduate School of ScienceNagoya UniversityFuro-cho, Chikusa-ku, Nagoya 464-8602, JapanQ. M. PhungInstitute of Transformative Bio-Molecules (WPI-ITbM)Nagoya UniversityFuro-cho, Chikusa-ku, Nagoya 464-8601, JapanQ. P. Ngo, J. NaMaterials Architecturing Research CenterKorea Institute of Science and Technology (KIST)5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of KoreaE-mail: jongbeom@kist.re.krAdv. Sci. 2024, 11, 2402518 2402518 (1 of 14) © 2024 The Author(s). Advanced Science published by Wiley-VCH GmbHhttp://www.advancedscience.commailto:HENZIE.Joeladam@nims.go.jpmailto:y.yamauchi@uq.edu.auhttps://doi.org/10.1002/advs.202402518http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/mailto:jongbeom@kist.re.krhttp://crossmark.crossref.org/dialog/?doi=10.1002%2Fadvs.202402518&domain=pdf&date_stamp=2024-07-19www.advancedsciencenews.com www.advancedscience.comPhase purity confers two significant benefits: i) the differingsizes of the atomic species cause pronounced lattice distortion,which not only lowers the overall system energy but also slowsdown interatomic diffusion, resulting in higher overall structuralstability.[17] Besides, lattice distortion affects the local interatomicinteractions, which regulate the adsorption energies of activesites.[5,9,17,18] And ii), the confinement of multiple atomic speciesgenerates “cocktail effects” that broaden the catalytic range andactive site ability beyond what can be realized with monometallicor traditional alloying systems.[5,9,17,18]The oxygen evolution reaction (OER)/oxygen reduction reac-tion (ORR) is a complex multi-electron transfer process that playsan essential role in electrochemical energy devices like water elec-trolyzers/fuel cells.[19–23] It is essential to note that this process ismore favorable in alkaline mediums, while the complementaryhalf-cell reaction, hydrogen evolution reaction (HER)/hydrogenoxidation reaction (HOR), is more kinetically favorable in acidicmediums.[20,24–27] Unfortunately, integrating these reactions inthe same electrochemical device presents practical difficulties,which makes it challenging for electrolyzers and fuel cells tofunction efficiently. It is also worth noting that when it comesto HER/HOR, Pt/C-based electrocatalysts are considered state-of-the-art in acidic mediums.[26,27] However, they fall short whenused in alkaline conditions.[26,27] This is because the nature ofthe intermediate species as well as their concomitant adsorptionenergy changes depending on the pH of the overall reaction, andthe mono-metallic or traditional alloying systems have active sitesthat only offer discrete adsorption energy choices.[9,24,26,27] Theycannot effectively counter the changes in a dynamic reactionenvironment, leading to overall decreased efficiency of electro-catalysts. This, in turn, requires higher energy consumption inelectrolyzers and reduced working/operating voltage (open cir-cuit voltage) windows in fuel cells. HEAs are a potential strategyto address the above limitations.[9,17,28] This approach leveragesfour critical effects: high entropy, cocktail, sluggish interatomicdiffusion, and lattice distortion, leading to a near-continuousrange of adsorption energies.[9,17,28] HEA sites are expectedto offer an expanded catalytic range and greater tolerance to-ward variations in reaction pH without compromising catalyticefficacy.[9,18]In general, electrochemical reactions occur mainly on cat-alytic surfaces, and nanoparticles (≤10 nm) are favorable dueto their high surface-to-volume ratio, but they are susceptibleJ. NaKHU-KIST Department of Converging Science and TechnologyKyung Hee UniversitySeoul 02447, Republic of KoreaJ. Na, Y. YamauchiSchool of Chemical Engineering and Australian Institute forBioengineering and Nanotechnology (AIBN)The University of QueenslandBrisbane, QLD 4072, AustraliaY. YamauchiDepartment of Plant & Environmental New ResourcesKyung Hee University1732, Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 17104,Republic of Koreato dissolution and agglomeration during aggressive operatingconditions.[10,29,30] These effects combine to reduce the longevityand overall effectiveness of electrocatalysts. Additional catalyticsupport platforms are needed to induce stability; however, theymay decrease the specific capacity of the catalysts, trigger diffu-sion limitation, and complicate material synthesis.[31] These is-sues can be addressed by adopting the mesoporous approach incatalyst design. Mesoporous architectures enhance the effectivesurface area, exposing more active atomic sites to the environ-ment while increasing the effective utilization of materials. Atthe same time, porous channels boost the effective transporta-tion of reactant/intermediate species.[4,5,32–34] Adopting the highentropic alloying strategy with embedded mesoporous featureshas the potential to open a new avenue for the rational designand development of next-generation electrocatalysts. However,current synthetic approaches like carbothermal, electrosynthesis,solvothermal pyrolysis, mechanical milling, reactive sputter de-position, and laser ablation have limited ability to controllably im-pose mesoporosity in HEA systems.[10,12,17,28,35–39] Although post-synthesis acid leaching has effectively created meso-porosity, itleads to material loss and adds complexity to the synthesis andhandling process. An alternative approach could involve explor-ing a wet chemical synthesis protocol in combination with softtemplating methods to avoid material loss.[40]In this work, we successfully developed an effective designprotocol for creating mesoporous HEA nanospheres of PtPdRu-MoNi. This is achieved using a single-pot wet chemical strat-egy that utilizes the soft templating feature of Pluronic F-127(F127), a triblock copolymer (as shown in Figure 1a) with apolypropylene glycol as the central hydrophobic block flankedwith two hydrophilic blocks of polyethylene glycol (PEG). By ad-justing conditions including the solvent composition and reac-tion time, the morphology and reactivity of the final HEA sys-tems are tuned effectively. Importantly, the overall process doesnot require/demand any sophisticated and dedicated instrumen-tation facility and hence is immediately scalable. The optimizedmesoporous PtPdRuMoNi nanosphere demonstrates its superiorcatalytic efficiency through electrocatalytic hydrogen evolutionand oxidation studies, HER and HOR, in an otherwise kineti-cally less favored alkaline medium. At a nominal overpotentialof 30 mV, the PtPdRuMoNi nanosphere exhibits a specific activ-ity of approximately 167 (HER) and 151 A gPt−1 (HOR), whichis significantly much higher than that of state-of-the-art Pt-C-based electrocatalysts (34 and 48 A gPt−1, respectively) and variousother recently reported multi-metallic systems (Table S1, Sup-porting Information). The computational insights demonstratethe high entropic alloying effect of PtPdRuMoNi nanospheres,resulting in an increased inherent catalytic ability, as evidencedby its higher exchange current density of 2.83 mA cm−2 com-pared to Pt-C (1.67 mA cm−2), an intrinsic material property. Themesoporous structure of the PtPdRuMoNi nanosphere furtherfacilitates the exposure of numerous HEA sites, enabling the mi-gration of reacting species and contributing to its excellent cat-alytic performance. The methodology used to design the PtPdRu-MoNi nanosphere is simple and effective for developing a varietyof catalytically active mesoporous HEA nanostructures and canbe further explored to develop a wide range of mesoporous HEAsystems for diverse fields of applications.Adv. Sci. 2024, 11, 2402518 2402518 (2 of 14) © 2024 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2024, 35, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202402518 by National Institute For, Wiley Online Library on [23/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 1. a) Schematic illustration of mesoporous PtPdRuMoNi HEA nanospheres formation using a one-pot wet chemical reduction process for overallhydrogen evolution and oxidation reactions in alkaline media. The representative b) SEM, c) TEM, and d) HAADF-STEM images of HEA10 (mesoporousPtPdRuMoNi HEA nanospheres). The associated e) HRTEM image, f) FT pattern, deduced from (e), g–j) corresponding set of planes and concomitantinterplanar spacing, deduced from (e) and (f) for HEA10.2. Results and Discussion2.1. Structural Studies of Mesoporous HEA NanospheresThe process for designing mesoporous PtPdRuMoNi HEAnanospheres is illustrated in Figure 1a. This result is achievedthrough a simple one-pot wet chemical reduction process thatutilizes the F-127 triblock copolymer as a pore-instituting agent.F-127 consists of a hydrophobic block at the center, flanked bytwo hydrophilic blocks. When the copolymer interacts with anaqueous medium such as water, it forms micelles with the hy-drophobic branch as the core.The dynamic light scattering (DLS) study reveals that the meandiameter of the micelles is ≈14 nm, as shown in Figure S1 (Sup-porting Information). The hydrophilic exterior surface of the mi-celles can accommodate metal ion complexes through either hy-drogen bonding or electrostatic interactions, depending on thenumber of water molecules present in the coordination sphereof the metal ion complexes in an aqueous solution.[33,41–43] Toprobe the possible interaction of metal ions with F127-basedAdv. Sci. 2024, 11, 2402518 2402518 (3 of 14) © 2024 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2024, 35, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202402518 by National Institute For, Wiley Online Library on [23/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.commicelles, if any, we measured the UV–vis absorption spectraof metal ions containing precursor solution with and withoutcopolymer micelles (Figure S2, Supporting Information). Theabsorption spectra of the respective metal salts containing so-lutions exhibit characteristic typical absorption peaks. Interest-ingly, the metal salt solution with micelles exhibit similar peakswith a noticeable hyperchromic effect. This hyperchromic effect,where the absorption spectra shifted to increased absorbance in-tensity values without affecting the position of the peaks, indi-cates an intimate interaction between metal ion complexes andthe micelles in the solution (please see the Supporting Informa-tion for details).[33] F127 polymer was first dissolved entirely inwater, creating a transparent solution containing micelles, as evi-denced by the Tyndall effect (Figure S3, Supporting Information).Afterward, aqueous metal salt solutions of Ni, Mo, Ru, Pd, Pt,and a reducing agent, L-ascorbic acid (L-AA), were added in acontrolled manner. The HCl solution was used to regulate theL-AA-reducing power.[44] The reaction solution was then placedin an oil bath at 95 °C for varying reaction times. The final prod-uct, mesoporous PtPdRuMoNi HEA nanospheres, was collectedusing centrifugation and treated with multiple washes of ace-tone/ethanol and water to remove the polymeric micelles beforeundergoing structural characterization. The mesoporous HEAnanospheres are named HEAX, where X represents the individ-ual metal salt concentration used for the synthesis. For example,HEA10 indicates a concentration of 10 mm for each metal saltsolution of Pt/Pd/Ru/Mo/Ni used during synthesis. The repre-sentative scanning electron microscopic (SEM) image (Figure 1b)of HEA10 shows spherical nanospheres with mesoporous exteri-ors. This is further confirmed in the bright field transmissionelectron microscopic (TEM) and the high angle annular darkfield (HAADF) scanning TEM (STEM) images (Figure 1c,d) dueto contrast differences. The high-resolution TEM (HRTEM) im-age near the edges (Figure 1e) suggests a crystalline nature withvisible lattice fringes. A randomly selected area in the HRTEMimage, indicated by the red box (Figure 1e), was processed togenerate a fast Fourier transform (FT) pattern, as shown inFigure 1f. The spots in the FT pattern indicate the presence of(111), (200), and (220) sets of planes corresponding to the face-centered cubic (fcc) crystal arrangement.[45] The inverse FT (IFT)processing was used to obtain the lattice fringes and concomi-tant interplanar spacings associated with (111), (200), and (220)sets of planes (Figure 1g–j). The interplanar spacing for (111),(200), and (220) sets of planes were found to be 0.22, 0.19, and0.13 nm, respectively (Figure 1g–j), similar to standard fcc-phasePt systems.[45,46] The overall TEM study suggests that these Pt-PdRuMoNi nanospheres are mesoporous and crystalized in thefcc phase. Subsequently, we used energy dispersive spectroscopy(EDS) study with HAADF-STEM to analyze elemental distribu-tion in the HEA10 system. The study reveals that the randomlyselected mesoporous HEA10 nanospheres (Figure 2a) containedmetallic Pt, Pd, Ru, Mo, and Ni, which are distributed throughoutthe structure (Figure 2b–f). This observation is further confirmedin a STEM-EDS line scan of a single particle (Figure S4, Support-ing Information). The inductive coupled plasma-optical emis-sion spectroscopy (ICP-OES) further confirms that the HEA10mesoporous nanospheres contain Pt, Pd, Ru, Mo, and Ni metals(Table S2, Supporting Information). The combined data suggeststhat although the starting precursor solution contains equivalentatomic ratios, the final HEA mesoporous nanospheres are com-paratively rich in Pt and Pd atoms likely due to the favorable re-duction kinetics of Pt and Pd metal ions among PtPdRuMoNi ele-mental grouping. The estimated configurational entropy (ΔSmix)based on ICP-OES measurement for HEA10 is 1.5R, makingthem HEA materials.[2]The combined evaluation of elemental mapping and line scan-ning profile suggests that though all five elements are distributedthroughout the HEA10 nanospheres, the core seems to be richin palladium, while the exterior is rich in platinum. This in-fers that, though the standard reduction potential of Pt com-plexes ([PtCl4]2−/Pt ≈ +0.76 V vs SHE) is higher than Pd com-plexes ([PdCl4]2−/Pd ≈ +0.59 V vs SHE), the process starts withthe preferable reduction of Pd species. This anomalous behav-ior has been observed previously, even in a bimetallic system ofPtPd-based nanostructures where the core was rich in Pd whilePt was preferentially concentrated at the exterior.[40] In a mul-tielement co-reduction process, the reduction of metal speciesis very complex and cannot be governed solely by their stan-dard reduction potentials found in the reference literature.[44]The reduction kinetics become more complicated due to the pres-ence of multiple metal ion complexes, their coordination envi-ronment, organic additives, choice of reducing agent, and reac-tion ambiance.[4,44,47,48]To further understand the high entropy alloying feature ofactive sites in mesoporous HEA10 nanospheres, we have em-ployed a surface-sensitive probing technique, X-ray photoelec-tron spectroscopy (XPS). The intense peaks associated with Pt,Pd, Ru, Mo, and Ni moieties (Figure S5, Supporting Informa-tion) suggest the multi-elemental character of the HEA10 exte-rior. In high entropy alloying systems, the inherent heterogene-ity and cocktail effects cause very complex interatomic interac-tion and hybridizations between the neighboring atoms, whichmodify the valance state giving rise to the core level shift.[9,17]Hence, we note the shift of ≈0.5 eV for Pt0 4f7/2 peak in HEA10toward the lower energy side compared to standard monometal-lic Pt systems (Figure S5b, Supporting Information). This ob-servation suggests that the interaction of Pt with other neigh-boring heteroatoms results in an electron transfer from othermetallic moieties to Pt. Pt is more electronegative than Pd, Ru,Mo, and Ni, indicating the electron-withdrawing tendency of Ptduring interatomic neighboring interactions, which could leadto regulating the binding energy (adsorption/desorption) of reac-tant/intermediate species.[5,11,17,18,45,49–51] The wave functions ofPt 4f and Pt 5d levels overlap significantly, suggesting the increasein electron density, which can cause an downward shift of the d-band center, thus modifying the adsorption/desorption energy asthe difference between the d-band center and Fermi level used asa matrix of binding energies for reactant species.[45,51] Moreover,the Pt-rich exterior with mesopores and intrinsically supportedwith high entropy alloying features is expected to benefit ouraimed application of HER and HOR in an alkaline medium.[26,27]To shed some light on the stable solid solution phase of HEA10,its powder X-ray diffraction (XRD) pattern is shown in Figure 2g,which unveils the fcc-phase with characteristic fcc peaks centeredat 39.4, 46, 67.4, 81.3, and 85.7 degrees correspond to the (111),(200), (220), (311), and (222) sets of planes, respectively.[52] Theinterplanar spacing obtained from XRD peaks using Bragg’s lawfor (111), (200), and (220) sets of planes (Figure 2g) are found toAdv. Sci. 2024, 11, 2402518 2402518 (4 of 14) © 2024 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2024, 35, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202402518 by National Institute For, Wiley Online Library on [23/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 2. a) HAADF-STEM image, b–f) corresponding elemental distribution patterns of Pt, Pd, Ru Mo, and Ni, respectively. g) The XRD pattern ofHEA10. h) The HAXPES spectrum showing the valance band of HEA10 and Pt using a Cr K𝛼 source. i) The SAXS pattern of HEA10 mesoporousnanospheres. (The scattering vector q for SAXS is given by q = 4𝜋 sin𝜃/𝜆, where 𝜆 is the wavelength of X-ray radiation).be 0.22, 0.19, and 0.13 nm, respectively, which matches TEM ob-servation (Figure 1g–j). The lattice constant of the mesoporousPtPdRuMoNi HEA10 nanospheres, deduced from the XRD pat-tern, is 0.393 nm. This lattice constant is larger than the indi-vidual lattice constants of the comprising mono-metals (i.e., Mo(0.315 nm), Ni (0.349 nm), Ru (0.271 nm), Pd (0.385 nm), andPt (0.391 nm)), which can be attributed to the phase purity ofHEA10. The comparatively larger lattice constant can account forthe differing sizes of the atomic species getting accommodatedin a single unit cell, causing pronounced lattice distortion tolower the overall system energy for global structural stability.[17]Thus, the XRD measurements indicate that the PtPdRuMoNiHEA mesoporous nanospheres crystalize predominantly in thefcc crystal lattice system (Figure 2g) which agrees with TEMmeasurements (Figure 1f–j). The valence band spectrum was ex-amined using hard X-ray photoelectron spectroscopy (HAXPES)with a Cr K𝛼 source to probe the impact of entropic effects on theelectronic structure of the material. The high energy of the Cr K𝛼source (5.4 keV) enables non-destructive analysis deeper insidethe material, beyond the surface. Figure 2h shows the valenceband (VB) spectrum of monometallic Pt system and HEA10. Themonometallic Pt-system exhibits distinct peaks as expected.[11]In contrast, the HEA10 spectrum is broad and featureless, char-acteristic of high entropy alloys due to the complex orbital hy-bridization that reflects electronic structure modification.[11,53,54]The overall diameter of the HEA10 mesoporous nanospheres is≈125± 10 nm with a pore size of 10 to 12 nm. To evaluate the peri-odicity of the pores, a small-angle X-ray scattering (SAXS) patternwas recorded on HEA10, which shows a peak at q = 0.374 nm−1corresponding to a pore-to-pore distance of ≈16 nm (Figure 2i).The combined structural and morphological studies suggest thatwe have successfully employed the wet chemical approach withassisted triblock copolymers to design PtPdRuMoNi mesoporoushigh entropy alloy nanospheres.2.2. Structural and Morphological Optimization StudiesIn the process of creating inorganic nanocrystals through wetchemical synthesis, adding a reducing agent to a solution ofmetal salt causes nucleation to occur, resulting in a colloidalsolution.[4,55,56] Subsequently, the nuclei in this solution growAdv. Sci. 2024, 11, 2402518 2402518 (5 of 14) © 2024 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2024, 35, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202402518 by National Institute For, Wiley Online Library on [23/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 3. a) SEM and b) bright-field TEM images of HEA10NF. c) TEM image of a magnified portion of HEA10NF. d) The HAADF-STEM image and e–i)corresponding elemental distribution patterns in HEA10NF. g) N2 adsorption-desorption isotherms, and j) BET surface areas for HEA10 and HEA10NF.through continuous growth or coalescence, or a combinationof both.[4,55–60] In our study, we examined this process by syn-thesizing a PtPdRuMoNi HEA system using the same methodas for HEA10 mesoporous nanospheres but without the use ofF127 copolymer, leading to the creation of HEA10NF (Figure3, the NF stands for no F127 was used during synthesis).HEA10NF has an agglomerated structure consisting of bulksheets and nanospheres (Figure 3a). The bright field TEM im-ages (Figure 3b,c) in combination with the HAADF-STEM im-age (Figure 3d), show that the structure is made up of individualsmall nanoparticles (≈4 nm) that coalesced together. This can beexplained by the multiple coalescence events that randomly occurto reduce the high surface energy caused by the large surface-to-volume ratio and high collision frequency, which drives theensemble growth of the final structure instead of the individualgrowth of the small nanoparticles.[56–58,61]The EDS elemental distribution studies in HAADF-STEMmode on HEA10NF suggest the distribution of Pt, Pd Ru, Mo,and Ni throughout the structure (Figure 3e–i, Table S2, Support-ing Information), like that of in HEA10. This further infers thatthe use of a copolymer like F127, a soft templating approach, dur-ing synthesis brings twofold effects, evident from the morpho-logical appearance of HEA10 and HEA10NF. First, it institutesthe porous feature in the final structure (Figures 1b–d and 3a–d),which was manifested from the physicochemical studies of ni-trogen adsorption/desorption isotherms and deduced Brunauer–Emmett–Teller (BET) specific surface area (Figure 3j,k, HEA10≈ 170 m2 g−1 and HEA10NF ≈ 19.6 m2 g−1). Second, the hy-drophilic exterior of micelles engages with the metal ions com-plexes (Figure S2, Supporting Information), thus localizing thenucleation sites. It is also expected to reduce the overall mobil-ity of nuclei and, hence, the collision frequency. This can be re-flected in terms of increased nanospheres dispersion as well asthe smaller size of HEA10 nanospheres (Figure 1b), while the ab-sence of micelles-supported synthesis results in the overall struc-tural agglomeration in HEA10NF (Figure 3a).We further studied the time dependence and concentration de-pendence of the synthesis protocol. For over 16 h, we examinedhow the structure of HEA20 evolved using SEM (Figure 4a–d),XRD (Figure 4e–h), and physisorption measurements (FigureS6, Supporting Information). The associated time-dependentXRD studies on HEA20 (Figure 4e–h) show the characteristicAdv. Sci. 2024, 11, 2402518 2402518 (6 of 14) © 2024 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2024, 35, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202402518 by National Institute For, Wiley Online Library on [23/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 4. a–d) Time-dependent SEM studies and concomitant e–h) XRD patterns of HEA20.(111), (200), (220), (311), and (222) fcc peaks matching HEA10(Figure 2g). No additional diffraction peaks for monometal-lic/intermetallic or oxides-based systems are observed in XRD,suggesting that mesoporous HEA20 nanospheres crystallized inpredominantly fcc phase. In addition, the physisorption mea-surements (Figure S6, Supporting Information) show that theeffective surface area and pore volume are noticeably high formesoporous nanospheres obtained for a reaction time of 16 h.The EDS elemental mapping and line scans show that Pt, Pd,Ru, Mo, and Ni are uniformly distributed in the HEA20 meso-porous nanospheres (Figures S7–S9, Supporting Information).Thus, the time-dependent studies indicate that the optimum re-action time for mesoporous HEA nanospheres is ≈16 h. Further-more, to study the effect of nucleation on overall structure as-sembly and growth, the metal salt concentration in precursor so-lution was varied (1, 2, 5, and 10 mm), keeping other reactionconditions unchanged (i.e., F127 copolymer concentration, reac-tion time ≈16 h, amount of L-AA and HCl). Accordingly, the fi-nal products were named HEA1, HEA2, HEA5, and HEA10, re-spectively. In HEA1, the low concentration of metal ions in theprecursor solution results in irregular nanostructures with arbi-trary sizes (Figure S10a,b, Supporting Information). Based on theXRD pattern, HEA1 is amorphous, however, spherical nano fea-tures emerge as the concentration of metal ions in the precursorsolution increased from 1 to 2 mm (Figure S10c,d, SupportingInformation). A small peak in the XRD pattern at 39.5° indicatedoverall poor crystallinity. This peak becomes more prominent asthe concentration of metal ions increases to 5 mm for HEA5Adv. Sci. 2024, 11, 2402518 2402518 (7 of 14) © 2024 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2024, 35, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202402518 by National Institute For, Wiley Online Library on [23/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.com(Figure S10e,f, Supporting Information). Another peak at 46°,characteristic of the fcc structure, also begins to emerge (FigureS10g, Supporting Information).[52] The final structure of HEA5has more prominent spherical features than HEA1 and HEA2.These experimental studies suggest that low metal ion concen-trations in the precursor solution may not produce enough nu-cleation and collision events, hindering the growth and evolu-tion of the overall structure. The lack of metal ion concentrationin the precursor solution and the subsequent nucleation eventsmay adversely affect the crystalline health of the overall struc-ture. Effectively, with a concentration of 10 mm metal ions in thesolution, the final structure of HEA10 is well-dispersed, meso-porous nanospheres with good crystallinity and a predominantsingle-phased fcc system (Figures 1 and 2). This methodology canbe useful in designing and developing high-entropy alloy meso-porous systems at mild reaction conditions. These mild reac-tion conditions are unique compared to typical high-temperaturemethods and could thus offer an opportunity to further expandthe compositional space of HEA nanostructures.2.3. Electrocatalytic CharacterizationThe electrochemical hydrogen evolution from water, HER, is sen-sitive to pH and is sluggish in an alkaline environment.[26,27]High pH adversely affects virtually all aspects of HER, includ-ing water dissociation, the adsorption/desorption energy of in-termediates, and recombination strength to H2, making the HERprocess energy intensive.[24,26,27] Similarly, the HOR is an impor-tant half-cell reaction in hydrogen-based fuel cells and is alsokinetically unfavorable in an alkaline media.[25] Therefore, de-signing efficient catalysts for HER/HOR applications in an al-kaline medium is crucial.[26] High entropy alloys with diverse ac-tive surface sites offering near-continuum adsorption/desorptionenergies and mesoporous features could be suitable catalysts forHER/HOR in an alkaline medium.[1,17] In this study, HER/HORreactions are used to evaluate the multifunctional electrocatalyticability and versatility of mesoporous HEA nanospheres in al-kaline media. Before electrochemical measurements, the sur-face physisorption features of HEA10, HEA10NF, and HEA20were studied using nitrogen adsorption-desorption isotherms, asthese electrochemical processes depend sensitively on the sur-face area of materials (Figure S11, Supporting Information). Theisotherms profiles for HEA10 and HEA20 exhibit an H1 typehysteresis loop and type IV adsorption isotherm, indicating theinherited presence of mesopores with uniform open geometry,which is in good agreement with SEM observations (Figures 1band 4d).[62] The isotherm profile of HEA10NF indicates it is non-porous, matching SEM and TEM observations (Figure 3). Thecorresponding BET surface area, pore volume, and pore area forHEA10 (≈170 m2 g−1, 0.231 cc g−1, and ≈102 m2 g−1) are foundto be superior to HEA20 (≈148 m2 g−1, 0.19 cc g−1, and ≈74 m2g−1) and nonporous HEA10NF (≈20 m2 g−1, 0.109 cc g−1, and≈8.5 m2 g−1) (Figure S10, Supporting Information). The valuesfor HEA10NF are markedly lower, illustrating the role of poly-meric pore-directing agents in increasing the effective surfacearea. Physisorption studies indicate that the HEA10 mesoporousnanospheres have optimal surface features. To further examinethese surface features from an electrochemical point of view,we utilized cyclic voltammetry (CV) in non-Faradic regions, aswell as electrochemical impedance spectroscopy (EIS), hydrogenunderpotential deposition (HUPD), and carbon monoxide (CO)stripping to evaluate the double-layer capacitance (Cdl, FiguresS12–S15, Supporting Information)).[63] This matrix is directlyproportional to the effective electrochemical active surface areaof the catalyst.[63] The experimental findings demonstrate thatHEA10 has the highest Cdl values (Tables S3–S6, Supporting In-formation) among HEA10, HEA10NF, and HEA20, which alignswith the trend observed in our physisorption studies (Figure S11,Supporting Information). The Cdl values for Pt-C and HEA20,as deduced from HUPD studies, are similar (Table S5, Support-ing Information), indicating the similar electroactive nature ofcatalytic sites. This similarity is reflected in the VB spectrumof HEA20, which exhibits peak-type features similar to those ofmonometallic Pt system (Figure S16, Supporting Information).This suggests that the high entropic effect in HEA20 is not veryprominent in regulating the overall electronic structure. In con-trast, the VB spectrum of HEA10 is featureless as demonstratedin Figure 2h. Although HEA10 and HEA20 have similar XRD andSTEM EDS data, the low catalytic activity of HEA20 is attributedto minimal entropic effects, as demonstrated in the HAXPESspectrum (Figure S16, Supporting Information).[11] These experi-mental results suggest that HEA10 mesoporous nanospheres arethe optimal catalytic material for HER/HOR processes in an alka-line medium. We also included the state-of-the-art commerciallyavailable Pt-C electrocatalyst for HER/HOR processes for com-parison.Figure 5a displays the normalized polarization curves con-cerning the overall catalyst loading for HER-HOR in an alka-line medium (0.1 M KOH). The results reveal that HEA10 meso-porous nanospheres outperform the Pt-C electrocatalyst undersimilar experimental conditions. The plateau region observed inthe HOR branch (the positive potential side) is due to the lim-ited transport of H2 from the electrolyte solution to the catalystsurface.[25,27] To showcase the superiority of HEA10 over Pt-C,Figure 5b displays a selected portion of the polarization curves(±100 A gcat−1). HEA10 requires an overpotential of only 9 and25 mV for HOR and HER, respectively, to achieve a specific cur-rent density of 50 A gcat−1 (Figure 5b). These values are muchbetter than those of Pt-C (33 mV and 36 mV), indicating theinherently bifunctional active nature of HEA10. To understandthe comparative HOR kinetics, the respective HOR polarizationcurves were normalized with their maximum plateau value cur-rent (i.e., diffusion-limited current density, Ilim, Figure 5c) to neu-tralize the respective structural/geometrical features of Pt-C andHEA10. Figure 5c shows the starting point of the HOR plateau,where the overall polarization curves are shifted positively by≈40 mV for Pt-C. This indicates that Pt-C has less favorable re-action kinetics under the same experimental conditions versusHEA10.[27] This is further supported by the fact that HEA10 hasa higher kinetic current and smaller Tafel slopes than Pt-C, asshown in Figure 5d. The exchange current value is an intrin-sic material property directly correlating to the catalyst’s inher-ent electrochemical active nature, and it is higher for HEA10(2.83 mA cm−2) than for Pt-C (1.67 mA cm−2).[26] The improvedreaction rate, better kinetic current, low Tafel slopes, and high ex-change current for HEA10 can be attributed to its inherent highentropic alloying feature. This is well reflected in its ultrahighAdv. Sci. 2024, 11, 2402518 2402518 (8 of 14) © 2024 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2024, 35, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202402518 by National Institute For, Wiley Online Library on [23/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 5. a) LSV-polarization curves for HER-HOR on Pt-C and HEA10 in an alkaline medium (0.1 m KOH). b) The magnified portion of LSV curvesfrom (a) is in the range of ± 100 A gcat−1. c) HOR polarization curves of Pt-C and HEA10 normalized to their respective maximum limiting currentd) Tafel curves of respective kinetic current densities, e) HER-HOR polarization curves normalized concerning the Pt weight for Pt-C and HEA10. f)Overpotential comparative study (@J10) for HER in an alkaline medium on HEA10 with some of the recently reported HEA, multi-metallic, and Pt-basedelectrocatalysts (tabulated in supporting information Table S1, Supporting Information).mass activity, which is ≈167 (HER) and 151 A gPt−1 (HOR) at anominal overpotential of 30 mV. These values are significantlyhigher than those of state-of-the-art Pt-C-based electrocatalysts,which are only 34 and 48 A gPt−1, respectively (Figure 5e).The HEA10 material has both high entropy alloying and meso-porous features, making it highly effective for HER-HOR activi-ties. Nonporous HEA10NF nanospheres show inferior HER per-formance compared to mesoporous HEA10 nanospheres (FigureS17a, Supporting Information). This is because HEA10NFnanospheres have limited active sites available, as they were pre-pared without using the micelle templates (Figure 3; Figure S11,Supporting Information). This was confirmed by physisorptionand electrochemical studies using CV, EIS, HUPD, and CO-stripping (Tables S3–S6, Supporting Information). The HEA10mesoporous nanospheres show better HER activity than manyrecently reported HEAs, multi-metallic, and Pt-based alloys inAdv. Sci. 2024, 11, 2402518 2402518 (9 of 14) © 2024 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2024, 35, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202402518 by National Institute For, Wiley Online Library on [23/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.coman alkaline medium (Figure 5f; Table S1, Supporting Informa-tion). The potentiometric study also shows that HEA10 canmaintain stable performance when subjected to HER for morethan 10 h (Figure S17b, Supporting Information). The XRD andSEM studies suggest that HEA10 nanospheres retain their struc-tural and mesoporous features in post-electrochemical studies(Figure S18, Supporting Information). The double-layer capaci-tance, i.e., Cdl, is also nearly comparable (Table S7, Supporting In-formation), indicating the operational stability of HEA systems.Inspired by promising HER and HOR performance, we alsoperformed the oxygen reduction reaction (ORR) on HEA10mesoporous nanospheres in an alkaline medium (Figure S19,Supporting Information) under identical reaction conditions.The comparative ORR polarization curves in terms of mass activ-ity (A) on Pt-C and HEA10 are shown in Figure S19 (SupportingInformation). The diffusion-limited current density at 0.4 V ver-sus RHE for HEA10 (−780 A gpt−1) is nearly three times higherthan Pt-C (−260 A gpt−1). Throughout the working potential win-dow of HEA10, peroxide (HO2−) generation and average electrontransfer number (n) involved in per oxygen molecule reductionare below 5% and 3.91, respectively. These observations suggestthat mesoporous HEA10 nanospheres can function as electrocat-alysts in both fuel cells and metal-air battery systems.2.4. Mott–Schottky (MS) and Computational AnalysisTo gain a deeper understanding of electrocatalysts’ behavior atthe electrode-electrolyte interface, Mott-Schottky (MS) analysiswas employed in an alkaline medium.[64,65] Figure 6a,b showthe MS plots (1/Ccs2 vs V, where Ccs is the space charge capac-itance and V is the applied potential) for Pt-C and HEA10, pro-viding valuable insights into the inherent catalytic ability andelectron transfer characteristics of electrocatalysts. The positiveslopes of the MS plots indicate that both electrocatalysts are n-type.[64,65] Moreover, the slope of the MS plot is inversely pro-portional to the charge carrier density, which, in this case, is theelectron.[65,66] HEA10 displays a slope value that is an order ofmagnitude smaller than commercially available Pt-C electrocat-alyst (5.12 × 108 compared to 7.12 × 109), indicating a highercharge carrier density in HEA10. In nanostructured electrocata-lyst systems, the charge carriers extend into the electrode up toa distance of 100–10 000 Å from the surface, creating a spacecharge distribution.[65] When exposed to electrolytes, these elec-trocatalysts experience space charge redistribution due to an elec-tron transfer from the electrode to the electrolyte in an n-typesystem.[64–66] This transfer of charge carriers creates a capacitiveregion known as the space charge double layer, with an asso-ciated electric field that facilitates facile charge transfer duringreactions.[65] This space charge double layer is an intrinsic mate-rial property and differs from the interfacial electrode-electrolytedouble layer, which is proportional to the effective surface area orelectroactive site density— an extrinsic feature that depends onthe material design (Tables S3–S6, Supporting Information).[31,65]The Mott–Schottky equation measures the space charge capac-itance (Ccs) as a function of applied potential.[66] The HEA10,with its high charge carrier density, was expected to exhibit betterspace charge redistribution, generating a stronger built-in elec-tric field in the space charge region. The resulting enhanced elec-tronic interactions at the electrode-electrolyte interface offer opti-mal adsorption/desorption of intermediate species and compara-tively fast charge transfer kinetics.[65–67] This effect is further sup-ported by the more favorable position of the flat band potential(Vfb, Figure 6a,b) for HEA10.The magnitude of Vfb represents the upward band bendingfor an n-type system when exposed to electrolyte at thermody-namic equilibrium, hence representing the magnitude of the ki-netic barrier for charge transfer, as illustrated in Figure 6c.[65–67]The Vfb for HEA10 (−0.223 V, Figure 6c) is found to be positivelyplaced concerning that for commercial Pt-C (Vfb = −0.246 V),representing a comparatively reduced energy barrier for bet-ter charge transfer owing to smaller upward band bending forHEA10. Overall, the MS analysis of HEA10 shows it has n-typebehavior, high charge carrier density, comparatively minor bandbending owing to a more positive flat band potential, and asmaller charge transfer barrier during reactions in an alkalinemedia versus Pt-C. These observations show that HEA10 has en-hanced catalytic activity and concomitant fast charge transfer ki-netics, as evidenced by its superior electrochemical performance(Figure 5; Figures S17 and S18, Supporting Information).To further support and gain insights into the enhanced elec-trochemical performance of HEA10, density functional theory(DFT) calculations were performed. In Figure 6d, the HER andHOR processes[68] were calculated for the Pt(111) and represen-tative HEA10 surfaces at an electrode potential of 0.0 V versusRHE. We first consider the HER under alkaline conditions, start-ing with the Volmer step H2O + e− + * → *H + OH−.[69] Thisconsists of three elementary reactions; one can readily observedistinct differences between HEA and Pt surfaces. In the firstreaction, H2O is adsorbed, which is exergonic on the HEA sur-face (−0.52 eV) and endergonic on Pt(111) (0.1 eV). The ad-sorbed H2O molecule dissociates into *H and *OH in the sec-ond reaction. The activation energy of this reaction is ≈0.9 eVon the Pt(111), while on the HEA surface, it is notably lower(only 0.45 eV). The reduced activation energy on the HEA sur-face, consistent with the Evans–Polanyi principle, is attributed toits enhanced ability to strongly bind *H and *OH species. Thisreduction of activation energy, indicative of improved water dis-sociation, should directly contribute to the higher HER activityobserved on the HEA surface. At the end of the Volmer step, the*OH species is converted into OH−. This conversion is down-hill on Pt(111) by 0.95 eV, whereas on the HEA surface, it goesuphill by 0.27 eV. In the subsequent Tafel step (*H → ½H2),the activation free energy on the HEA surface is calculated to be0.58 eV, whereas the Pt(111) surface exhibits better H2 desorptionwith a slightly smaller activation free energy (0.38 eV). However,for a comprehensive comparison of the catalytic activity betweenPt and HEA, it is essential to identify the rate-determining step(RDS) of the entire Volmer-Tafel mechanism. On the Pt(111) sur-face, the RDS is determined to be the water dissociation step,characterized by an activation energy of 0.9 eV, whereas the RDSon the HEA surface is predicted to be the Tafel step with an acti-vation energy of 0.58 eV. Overall, among three factors governingthe alkaline HER activity[1]—*H binding, *OH binding, and wa-ter dissociation—the latter appears to explain the superior HERactivity observed on the HEA10 surface.Similarly, we briefly discuss the Gibbs free energy diagramfor the HOR (as the reverse process of the HER shown inAdv. Sci. 2024, 11, 2402518 2402518 (10 of 14) © 2024 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2024, 35, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202402518 by National Institute For, Wiley Online Library on [23/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 6. Mott–Schottky plots for a) Pt-C and b) HEA10 in nitrogen purged 0.1 m KOH aqueous solution (X- intercept in MS plot shows the respectiveflat band potential (Vfb)). c) An illustration showing an upward bending of the band edges for an n-type material at the electrode-electrolyte interfaceat thermodynamical equilibrium. (Flat band potential determines the magnitude of band bending.) d) Calculated Gibbs free energy profiles (in eV) forHER and HOR on the (111) facet of HEA and Pt systems at 0.0 V versus RHE. The brown and teal numbers correspond to the activation free energy ofthe rate-determining step of HER and HOR, respectively.Figure 6d). Although the HEA surface exhibits a rather highaffinity toward hydrogen, its surface also demonstrates goodability in adsorbing OH−, which can readily combine with *Hto form *H2O. Accordingly, this combination step turns out tobe the RDS with an activation energy of 0.78 eV. In contrast, theRDS on the Pt(111) surface involves the *H + OH− → *H-*OHreaction, with an activation free energy of up to 0.95 eV. Thelower activation energy on the HEA surface obtained in ourcalculations is consistent with the observed higher HOR kineticsthan Pt-C-based electrocatalysts.3. ConclusionIn this study, a wet-chemical method utilizing a triblock copoly-mer has been developed to create mesoporous HEA nanospheresconsisting of PtPdRuMoNi. The surface electronic structure andoverall morphology of these HEA nanospheres can be modu-lated by varying reaction conditions. The growth of these HEAnanospheres is governed by multiple coalescence events, result-ing in ensemble-driven overall growth of the structure. The studyalso finds that the crystalline health of the final product dependsAdv. Sci. 2024, 11, 2402518 2402518 (11 of 14) © 2024 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2024, 35, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202402518 by National Institute For, Wiley Online Library on [23/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comon the metal ion concentration in the precursor solution. Theuse of polymeric micelles successfully creates mesopores in theHEA nanospheres, increasing their effective surface area, porevolume, and accessibility to reactant species. The optimizedmesoporous HEA nanosphere (HEA10) demonstrates excellenthydrogen evolution and oxidation reaction activities with fasterreaction kinetics in an alkaline medium. HEA10 exhibits a massactivity of approximately 167 (HER) and 151 A gPt−1 (HOR) ata nominal overpotential of 30 mV, which is significantly higherthan that of state-of-the-art Pt-C-based electrocatalysts (34 and48 A gPt−1). This high activity is attributed to HEA10’s intrinsiccatalytic active nature and tendency for faster charge transferowing to its high entropy alloying features, as revealed by ex-perimental outcomes, MS analysis, and computational insights.Our strategy can open an avenue for copolymers-assisted designof a wide range of multifunctional mesoporous high entropynanoalloys with rational morphologies for a diverse range oftargeted applications.4. Experimental SectionMaterials: All the chemicals used in the synthesis were received with-out further purification. The metal salts used were Potassium tetrachloro-platinate(II) (K2PtCl4, 98%), Sodium tetrachloropalladate(II) (Na2PdCl4,98%), Ruthenium(III) chloride hydrate (RuCl3.xH2O, 99.98%), Molyb-denum (V) chloride (MoCl5, 95%) and Nickle(II) chloride hexahydrate(NiCl2.6H2O, 99.9%), purchased from Sigma-Aldrich. The triblock copoly-mer Pluronic F-127 (C3H6O.C2H4)x, BioReagent grade), L-ascorbic acid(C6H8O6, MW. 176.12, Reagent grade), Nafion perfluorinated resin so-lution (5 wt. %) were purchased from Sigma Aldrich. The carbon blackVulcan (XC-72R) used during catalyst ink preparation was purchased fromThe Fuel Cell Store. Ethanol and acetone were purchased from NacalaiTesque, Inc. For all synthesis, washing, and electrochemical measure-ments Milli-Q water with an ionic purity 18.2 MΩ was used.Materials Characterization: The structural and morphological obser-vation and analysis were carried out using FE-SEM+EDX [SU8000] scan-ning electron microscope (SEM, Hitachi High Technologies/Bruker, modelno.- SU8000/Quantax FQ5060) and 200 kV field emission transmissionelectron microscope (TEM, JEM-2100F1) JEOL Ltd.). Surface elementalcomposition was probed using X-ray photoelectron spectroscopic (XPS)analysis device (Quantera SXM) from ULVAC-PHI Co., Ltd. Monochro-matic Al K𝛼 (1486.6 eV) focused X-ray source was used for excitation.The C 1s binding energy (284.6 eV) was used for calibration. The sameinstrument was used to obtain valance band (VB) spectra with Cr K𝛼(5417 eV) focused X-ray source for excitation. The elemental compositionwas obtained using an induced binding plasma emission optical analy-sis device (ICP-OES, high resolution type) (SPS3520UV-DD) from HitachiHigh-Tech Science Co., Ltd. The powder X-ray diffraction patterns wererecorded using MiniFlex600 system (X-ray wavelength: Cu K𝛼) from RigakuCo., Ltd. The small-angle X-ray scattering (SAXS) pattern was recordedusing Rigaku NANO-Viewer for pore-to-pore distance measurement. Ni-trogen adsorption–desorption isotherms were obtained using BELSORP-mini (BEL, Japan) at 77 K, and the specific surface area and porous fea-ture analysis were carried out using Brunauer–Emmett–Teller (BET) andBarrett‒Joyner‒Halenda (BJH) models. Particle size analyzer from OtsukaElectronics Co., Ltd. (Model No. ELSZ-2000ZS, particle size analysis spec-ification: 0.6 nm to 8 μm) was used to evaluate the average size of micellesdispersed in aqueous medium. The UV–vis spectroscopic system fromTECAN (model no.- M200 PRO) was used to record the absorption spec-tra of metal salt solution with and without polymer micelles in an aqueousmedium.PtPdRuMoNi-HEA Nanospheres Synthesis: The PtPdRuMoNi high en-tropy alloys (HEA) nanospheres were synthesized by the assembly of atriblock copolymer (F127) micelles using a wet-chemical approach. In atypical synthesis, 200 mg of Pluronic F127 triblock copolymer was mixedwith aqueous solutions of K2PtCl4 (10 mm, 6 ml), Na2PdCl4 (10 mm, 6 ml),RuCl3.xH2O (10 mm, 6 ml), MoCl5 (10 mm, 6 ml), NiCl2.6H2O (10 mm,6 ml) under sonication to completely dissolved the F127. Furthermore,0.8 ml of 6 m HCl was added to this solution. This precursor solution wasgently stirred for 30 min for homogeneous mixing and transferred to oilbath kept at 95 °C. After 10 min, 8 mL L-asocrbic acid (L-AA, 0.1 m) so-lution was carefully introduced into the precursor solution under gentlestirring conditions for 16 h reaction time under reflux. The nonporous Pt-PdRuMoNi (HEA10NF) was prepared using a similar method without us-ing F127 copolymer. To explore the insights of HEA nanospheres synthesisprocess, the metal salt solutions concentration (1, 2, 5, 10, 20 mm) andreaction time (1, 2, 4, 16 h) were varied accordingly (Figure 4, main textand Figure S10, Supporting Information). The final product, post-reaction,was collected by centrifuging (14 000 rpm, 25 min) and washed multipletimes with acetone/ethanol/water to remove soft polymer templates.Electrochemical Characterization: The high entropy alloys (HEAs)nanospheres were grinned by a mortar. The HEAs (10 mg) were sonicatedin 5 ml of hexane before adding 40 mg carbon black (XC-72) and 30 mLhexane, followed by sonication for 1 h at room temperature. The resultingpowder was collected by centrifugation (12 000 rpm) and washed threetimes with ethanol. The powder was dried in a vacuum oven (DP-200, Yam-ato Scientific co., ltd.) at 60 ˚C for 12 h. To prepare the working electrode,5.0 mg of the carbon-supported catalysts was dispersed in a 950 μL mixtureof DI and isopropanol (1:2 vol, v/v) and a 50 μL of 5 wt.% Nafion solution.The suspension was ultrasonicated for at least 60 min to obtain a homo-geneous ink (catalyst concentration of 5 mg mL−1). The ink was drop-casted on a 3 and 4 mm diameter glassy carbon disk electrode for RDEand RRDE, respectively, to obtain the catalyst density of 0.25 mg cm−2.An aqueous alkaline solution (0.1 m KOH) media was used to study thehydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR),and oxygen reduction reaction (ORR).Computational Details: Plane-wave DFT calculations employing thePerdew–Burke–Ernzerhof (PBE) function[70] were performed with the Vi-enna Ab initio Simulation Package (VASP) package.[71,72] The projectoraugmented wave (PAW) method[73,74] with wave functions expanded toan energy cutoff of 450 eV. Grimme’s D3 correction Becke-Johnson damp-ing function was applied to account for weak dispersion interactions.[75]A Gaussian smearing width of 0.2 eV was applied to all calculations. Theirreducible Brillouin zone was sampled using a Γ-centered k-point mesh of2× 2× 1. The metal surfaces were simulated using a 5× 5 slab cell consist-ing of 100 atoms (four atomic layers). The two bottom layers were fixed,whereas the remaining atoms were relaxed with thresholds of 10−4 eV and5 × 10−2 eV Å−1 for the energy and residual force, respectively. In all cal-culations, a vacuum of 15 Å along the z-direction was used to minimizeCoulombic interactions with periodic self-images.To obtain the Gibbs free energy profile for HER and HOR, thewidely used Nørskov’s computational hydrogen electrode model wasadopted,[76–78] in which the Gibbs free energy of a proton–electron pairwas estimated as G(H+ + e−) = 1/2 G(H2) and the Gibbs free energy ofOH− was calculated as GOH– = GH2O − 1/2G(H2). The Gibbs free energyof other reaction intermediates was computed as G = E + EZPE + TS whereE, EZPE, and S represent the DFT total energy, zero-point energy, and vibra-tional entropy of adsorbates (calculated under standard conditions of p =1 atm and T = 298.15 K), respectively.[79] Transition states (TS) of H2Oand H2 desorption processes were calculated using the climbing imagenudged elastic band (CINEB) method.[80] In particular, the activation bar-rier was defined by the difference between the TS and the correspondingstable configurations of reactants.Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsR.N. and Y.Y. acknowledge financial support from the Japan Society forthe Promotion of Science (JSPS, No. P22063). J.H. acknowledges financialAdv. Sci. 2024, 11, 2402518 2402518 (12 of 14) © 2024 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2024, 35, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202402518 by National Institute For, Wiley Online Library on [23/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comsupport from the JSPS Grants-in-Aid for Scientific Research Kakenhi Pro-gram (20K05453). The authors acknowledge the JST-ERATO Yamauchi Ma-terials Space-Tectonics Project (JPMJER2003), the KIST institutional pro-gram (2E33241 and 2V10082), the National Research Foundation of Ko-rea (NRF) grant funded by the Korean government (MSIT) (No. RS-2023-00259920 and No. RS-2024-00350423), and the ES Program (until May2024) via Nagoya University. 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Geng, Comput. Phys. Commun.2021, 267, 108033.[80] G. Henkelman, B. P. Uberuaga, H. Jónsson, J. Chem. Phys. 2000, 113,9901.Adv. Sci. 2024, 11, 2402518 2402518 (14 of 14) © 2024 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2024, 35, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202402518 by National Institute For, Wiley Online Library on [23/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.com Tailored Design of Mesoporous Nanospheres with High Entropic Alloy Sites for Efficient Redox Electrocatalysis 1. Introduction 2. Results and Discussion 2.1. Structural Studies of Mesoporous HEA Nanospheres 2.2. Structural and Morphological Optimization Studies 2.3. Electrocatalytic Characterization 2.4. Mott9040�Schottky (MS) and Computational Analysis 3. Conclusion 4. Experimental Section Supporting Information Acknowledgements Conflict of Interest Data Availability Statement Keywords