Nonmetal-Hybridized Interstitial High-Entropy Platinum Alloys for Oxygen Electrocatalysis in Practical Fuel Cells

doi:10.21203/rs.3.rs-4608536/v1

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Nonmetal-Hybridized Interstitial High-Entropy Platinum Alloys for Oxygen Electrocatalysis in Practical Fuel Cells

https://doi.org/10.21203/rs.3.rs-4608536/v1

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Discovering catalysts with both extraordinary activity and stability is a pivotal, yet highly challenging task for many modern electrocatalysis. High-entropy alloys (HEAs) have been considered promising electrocatalysts that could fulfill both requirements, in that their high configurational entropy could stabilize the catalyst independently of the enthalpic factor closely coupled to catalytic efficacy. It is hypothesized that incorporation of nonmetallic elements (N, B, F etc.) into conventional all-metallic HEAs might stimulate unique virtues complementing the roles of metallic constitutes. However, incorporating multiple light nonmetal elements into HEAs within a single synthesis is highly challenging, where distinctive and harsh synthetic conditions are typically required, due to the vastly differing properties of these nonmetals and their low affinities to many late transition metals. Here we introduce a generic synthesis that can simultaneously incorporate B, N, F into the interstitial sites of Pt-based HEA, and propose nonmetal-hybridized interstitial HEAs as material-exploration platforms for efficiently discovering exceptional electrocatalysts. As a first example, we demonstrate that these interstitial Pt HEAs can act as outstanding oxygen reduction electrocatalysts for fuel cells, particularly the N-incorporated catalysts exhibiting the-state-of-the-art activity and durability at both material and membrane-electrode-assembly levels. Given the distinct properties of B and F to N, such as electronegativity and polar hydrophobicity, we anticipate that B-/F-/multi-nonmetal incorporation could lead to more advanced compositions and elaborately designed catalyst layers, facilitating the construction of a catalytically more effective and robust mass-transport networks for ultralow-Pt fuel cells. The concept of nonmetal-hybridized interstitial HEAs presents tremendous opportunities for addressing challenging electrocatalysis involving complex tandem reaction steps.

Physical sciences/Materials science/Materials for energy and catalysis/Electrocatalysis

Physical sciences/Energy science and technology/Fuel cells

Physical sciences/Materials science/Materials for energy and catalysis/Fuel cells

Physical sciences/Chemistry/Electrochemistry/Electrocatalysis

Proton exchange membrane fuel cells represent a pillaring technology catalyzing the market of hydrogen energies. The widespread penetration of this technology, however, is seriously limited by high loadings of costly platinum catalysts^1,2, especially for the cathodic oxygen reduction reaction (ORR). Ultimately, the Pt loadings are expected to be cut down to 0.05 g/kW, whilst doubling the current power density to 9 kW/L^3,4. Simultaneously achieving these goals in turn imposes extreme requirements on the high activity and stability of the catalysts. In pursuit of this target, many novel strategies have emerged^5,6, such as lattice strain engineering^7,8, ordered intermetallics^9,10, enhancing substrate-particle interactions^11,12 and forming surface protective barriers^13-16. In practice, however, exceptionally active or robust catalysts respectively often rely on specialized synthesis for constructing delicate catalytic features^7,15,17; this practical constraint unfortunately tends to make ultrahigh activity and stability in a single system mutually exclusive tasks to achieve.

High-entropy alloys (HEAs) have been envisaged as promising electrocatalysts with a potential to fulfill both the activity and stability requirements^18-20. This is because their high configurational entropy could stabilize the catalyst independently of the enthalpic factor that is also closely coupled to activation of reaction intermediates^19-22. Thus far, most HEAs are confined to solely metallic elements. Non-metallic light elements, such as N, B, C, F possess distinct properties derived from their p-orbitals and potential interstitial lattice positions in alloys^23-25. The incorporation of nonmetals into conventional HEAs thus might stimulate unique catalytic virtues complementing the roles of metallic constitutes. Nevertheless, incorporating these light nonmetal elements is not straightforward because of their low affinities to many late transition metal elements, and usually demands distinctive synthetic routes due to their vastly differing properties. For instance, to incorporate F or N into binary Pt alloys, harsh conditions of fluorine-plasma etching treatment²⁶ and high-temperature annealing in an NH₃ atmosphere (even up to 80 bars) have been used^16,27-29, respectively. Therefore, establishing a simple, scalable and generic synthetic approach that can simultaneously incorporate a selection of these light elements presents tremendous challenges, but meanwhile of enormous value.

Here we extend the compositions of HEAs to nonmetals, furthering their local structural and chemical diversity and taking advantages of their electronic hybridization with metal constitutes. We propose constructing nonmetal hybridized interstitial HEAs as a novel strategy for discovering outstanding electrocatalysts (Fig. 1a). Through a facile mild-temperature solvothermal procedure, we have synthesized a series of Pt HEAs with interstitial nitrogen, boron and fluorine, and investigated in depth the N-hybridized interstitial Pt HEAs for practical fuel cells, as a first example. The introduction of interstitial nitrogen to Pt HEAs is essential to enhancing the ORR catalytic performance. The unique energy positions of N 2p orbitals and their hybridization with d-orbitals of surrounding metals have enhanced the modulation of intermediate energies and significantly hindered metal dissolution. These electrocatalysts are readily compatible with membrane-electrode-assembly (MEA) fabrication procedures for full cells, and the resulting MEAs have delivered record-high specific power and durability at ultralow Pt loadings.

All the fluorine/nitrogen/boron-incorporated Pt HEA electrocatalysts in this work have been synthesized using the same procedure, which involves solvothermal reactions (160°C) of metal acetylacetonates and fluorine/nitrogen/boron sources of perfluoroterephthalonitrile/pipemidic acid/phenylboronic acid, followed by annealing (400°C) under H₂/Ar (Fig. 1a, methods). Through this facile approach, we have discovered several outstanding electrocatalysts, including PtCuNiCoN, PtCuNiCoB, PtCuNiCoF, PtCuNiCoFN, PtCuNiCoFBN, PtCuNiCoFeN, PtCuNiFeMnN. The composition of a specific electrocatalyst, e.g., PtCuNiCoN, could be tunable (Supplementary Table 1) by adjusting the relative concentrations of precursors, and the Pt loadings in these catalysts are typically 10-25wt% (Supplementary Table 2). These electrocatalysts were found (Supplementary Fig. 2) to consist of 3 ~ 5 nm particles uniformly decorated on Ketjen black (KB) carbon, and they all had face-centered cubic (fcc) structures without the segregation of any other minor 3d-metal phases (Fig. 1b). Closed-packed {111} were the dominant surfaces, whose interplanar spacings showed clear contraction versus Pt. High-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) (Extended Data Fig. 1b-d) revealed that in general all elements were uniformly distributed within the particles without apparent long-range chemical ordering, as also supported by the diffraction results (Fig. 1b). Nonetheless, patches of brighter/darker atomic columns in the particles could be noticed, and subtle local displacements of atoms from the equilibrium lattice positions occurred frequently (Extended Data Fig. 1b), all illustrating high degrees of heterogeneity in local strains and coordination environments. In addition to the particles, metal single-atom sites coexisted in the substrate. In fact, both bright and dimmer single atoms have been observed (Extended Data Fig. 1b), suggesting Pt and at least one other 3d-metal single-atom site coexist^14,30. EELS measurements (Extended Data Fig. 1e) at the dimmer atoms on the N-incorporated sample further imply that all four types of metal-nitrogen (M-N) single-atom sites coexist.

One major next task would be determining the existence, quantity and exact crystallographic site of the incorporated nonmetal elements. We characterized these electrocatalysts using advanced tools of energy dispersive spectroscopic (EDS) mapping, atom probe tomography (APT)³¹ and electron energy loss spectroscopy (EELS). For HEA samples incorporated with either singular, dual or ternary nonmetals, EDS mappings (Fig. 1c) revealed that F/N/B signals appeared more intensely at the particles. APT measurements (Fig. 1d) on nitrogen-incorporated samples further supported that nitrogen was present in the core of the particles. But to completely rule out the possibility of F/N/B signals stemming from the carbon substrate, we synthesized these catalysts on CaCO₃, which was thoroughly removed after synthesis by acid. EDS mapping (Extended Data Fig. 2a,b) on these substrate-free catalysts once again showed clear signals for F/N/B, thereby corroborating that these nonmetal elements have been successfully incorporated into the bulk of these particles. Moreover, both APT and EDS line analysis (Extended Data Fig. 2b,c) consistently suggest that the atomic ratios of the incorporated fluorine, nitrogen and boron are typically of ~ 8.8, 6.4, 5.5%, respectively, similar to concentrations of doped N in binary Pt alloys via thermal annealing in NH₃^17,27,28.

In terms of the lattice positions of the nonmetals, the N 1s X-ray photoelectron spectrum (XPS) of PtCuNiCoN (Extended Data Fig. 3a) indicates that nitrogen (398.0 eV) species in the bulk likely situate at the tetrahedral/octahedral interstitial sites^32,33. Shoulder peaks signifying metal-nitrogen coordination appeared at 1.5–1.6 Å in FT-EXAFS; they have been fitted to extract information on local coordination (Extended Data Fig. 3b-f). We have also subjected the nitrogen-incorporated catalysts to annular bright field scanning transmission electron microscopy (ABF-STEM) measurements. Carefully examining the common zone axis ([100], [1\(\:\stackrel{-}{1}\)0]), we managed to directly observe (Fig. 1e) for the first time the interstitial location of nitrogen in Pt alloys: along the [100] zone axis, the nitrogen columns (arrows) lie at the square centers of the unit meshes, indicating their positions at the tetrahedral sites. Not all nitrogen columns were visible here due to the very low concentrations of nitrogen in the alloy²⁶. By contrast, the F atoms in PtCuNiCoF were observed to more likely situate at the octahedral sites, whereas the B atoms in PtCuNiCoB could take both octahedral and tetrahedral sites (Extended Data Fig. 3h,i). Taken together, our analysis has unambiguously shown that by using this highly facile and generic synthesis, we can successfully incorporate either singular, dual or ternary nonmetal elements of fluorine, nitrogen and boron into Pt HEAs at the interstitial sites.

How does this synthesis work is an intriguing question. To shed light on the mechanism, we have characterized the nitrogen-incorporated samples at various stages during synthesis. The HAADF-STEM images (Fig. 2a, Extended Data Fig. 5) illustrated that mixing KB with the precursor solution led to some of the precursor being absorbed in the porous carbon (but most still in the solution), where all metal elements remained in their oxidized states (Extended Data Fig. 4b-d and Supplementary Fig. 3). When the solvothermal reaction temperature reached 80°C, Pt²⁺ was the first reduced (Fig. 2b) due to its highest reduction potential³⁴, and bright Pt single-atoms clustering together was clearly observed (Fig. 2a). Further raising the reaction temperature to 160°C, other 3d metal ions, Cu²⁺, Ni²⁺ and Co²⁺ in sequence, began to reduce (Fig. 2c,d and Extended Data Fig. 4b-d), both directly on carbon as well as on the preexisting Pt clusters (Fig. 2a). During this phase, reduced metal atoms could react with the pyridinic N source to form M-N single atom sites in the carbon substrate (Extended Data Fig. 4a) and also catalytically on Pt alloy surfaces ^35–37. The alloy particle thus growing drastically during this period alongside with nitrogen incorporation into the lattice (Extended Data Fig. 4a). After only 4 hours of the reaction, the average particle size already reached 3.6 nm (Supplementary Fig. 4), approaching its ultimate value. We found that the pre-formation of Pt nuclei and its catalysis for 3d metal ion reduction was essential to successfully synthesizing a single-phase HEA. Removing platinum or replacing it by another noble metal element, such as ruthenium, led to issues of serious 3d metal segregation, inhomogeneous particle sizes and low yields.

The subsequent annealing in H₂/Ar was also indispensable to optimize the structure for good performance. During this thermally activated process, metals in their oxidized state (M-O and M-N) became further reduced (Fig. 2b-d, Supplementary Fig. 3a-d) converting to more Pt-M coordination, and increased nitrogen species in particles became registered at the interstitial sites (Extended Data Fig. 4a). Meanwhile, long-range structural ordering and lattice contraction (Supplementary Fig. 3f) also occurred. We note that 400°C is an optimal annealing temperature, and lower or higher annealing temperatures appear to deteriorate the performance. Finally, a scheme is used to illustrate the synthetic mechanism (Fig. 2e), which highlights the three key elements of the synthesis: prior nucleation of Pt, Pt-catalyzed 3d metal reduction and reactions with pyridinic nitrogen sources, and thermally activated reduction and alloying processes.

Due to limited scope of a single paper, here we mainly demonstrate the potential of nitrogen-hybridized interstitial Pt HEAs for practical fuel cells, as a first example. To reveal the effect of interstitial nitrogen, we also synthesized a reference catalyst without the interstitial nitrogen, i.e., PtCuNiCo (Supplementary Fig. 5). Linear sweep voltammograms (LSV) (Fig. 3a) showed that the activities of Pt/C, PtCuNiCo and PtCuNiCoN progressively increased, the catalyst with interstitial N being the best with mass/specific activities (MA/SA) at 7.78 A/mg_Pt and 12.1 mA/cm²; this result highlights the strong effect of N-incorporation on boosting the activity. We have also explored broadly in catalyst compositions (Supplementary Tables 1 and 3). Of the 24 representative electrocatalysts synthesized (Fig. 3b), it is evident that N-incorporated pentatonic and hexatomic HEAs are generally superior to the equivalent ternary and quaternary alloys, and that N-incorporated Pt alloys all exhibit much higher intrinsic activities than those without N. Of note, the multi-nonmetal incorporated HEA, PtCuNiCoNF also showed excellent MA at 4.6 A/mg_Pt and SA at 7.0 mA/cm² (Extended Data Fig. 6b,c). Compared to 50 best electrocatalysts in the literature, several catalysts out of the 24 explored in this work have reached the top 10 levels (Extended Data Fig. 6c). In particular, the specific activity of PtCuNiCoN and its mass activity retention after 30,000 cyclic voltammetry (CV) cycles rank at the top of all reported so far (Fig. 3d, Extended Data Fig. 6d,e). These results thus demonstrate the reliability of N-hybridized interstitial Pt HEAs as a material-exploration strategy for efficiently discovering outstanding ORR electrocatalysts.

More importantly, the superior properties of the catalysts can be readily expressed at MEA levels, leading to high mass activities and specific powers. At cathodic loading of 0.05 mg_Pt/cm², PtCuNiCoN (Fig. 3e, Extended Data Fig. 7a-e) showed an excellent MA of 1.15 A/mg_Pt, 2.6 times that of US DOE 2025 target (0.44 A/mg_Pt). In H₂-air tests, PtCuNiCoN achieved outstanding rated power densities of 1.1 and 0.85 W/cm² (Fig. 3g,h) at 0.1 and 0.06 mg_Pt/cm², equivalent to specific powers of 11.0 and 14.2 kW/g_Pt, which could potentially reduce the Pt loadings down to 7–9 g_Pt/100 kW, approaching the ultimate goal of 5 g/100 kW^3,4. Compared to other exceptional catalytic systems, the performance of our PtCuNiCoN catalyst represents the state of the art in both rated and peak powers by mass (Fig. 3f, Extended Data Table 1). In-situ electrochemical impedance spectroscopy measurements further revealed that this high power was deeply rooted in both the high catalytic activity and the fast mass-transport properties of our catalysts design (Extended Data Fig. 8).

Turning to the durability, PtCuNiCoN also showed better activity retention of ~ 80% after 30,000 CV cycles than those without interstitial nitrogen (Fig. 3c). A similarly exceptional stability was also recorded at MEA levels (Fig. 3e). These N-incorporated materials after 30,000 standardized accelerated stress test (AST) cycles (Methods) showed MA retentions (Extended Data Fig. 7e) of 70.4% and the peak power remained at 73.8% of its BOT value, even with ultralow loadings at 0.05 mg_Pt/cm². When operating potentiostatically under H₂-air, the MEA using PtCuNiCoN exhibited a fairly mild decaying rate over 100 hours, 0.0428%/hour at ~ 1.0 A/cm² (Fig. 3i). Even at higher currents of 1.2 and 1.4 A/cm², the MEA showed no decay at all for 40 hours (Extended Data Fig. 7f). These results thus unequivocally demonstrate the exceptional stability of the catalyst at both material and device levels.

Further characterization of the cycled catalysts revealed striking differences in their respective particle sizes (Fig. 4a,b, Extended Data Fig. 9). PtCuNiCoN managed to maintain a relatively sharp size distribution close to its original values even after 30,000 cycles, whereas issues of distribution-broadening and size-increasing seriously occurred to PtCuNiCo. For the heavily cycled PtCuNiCoN particles, the EDS signals of all elements, especially N (Fig. 4c,d), were still evident, confirming their robustness. We also performed further analysis on the cycled electrolyte to evaluate the leached metals (Fig. 4e): only small fractions of Ni and Co were dissolved, but the contents of Pt and Cu showed very little change after 30,000 cycles. These slight drops in Ni and Co contents were largely ascribed to the corrosion of 3d-metal-nitrogen single atom sites, whose proportion was observed to be reduced considerably after cycling (Fig. 4f, Supplementary Fig. 6). Nonetheless, no evidence of Pt band⁵⁴ formation within the Nafion membrane was observed after AST. We would like to note that, for the N-hybridized interstitial HEA nanoparticles, no formation of the reported stability-enhancing features, including surface nitrides¹⁴, Pt-skins^27,28, physical protective layers¹⁵, was observed (Fig. 4c,d). The above contrasting observations thus suggest a distinctly new mechanism likely bestowed the superior stability on these N-incorporated nanoparticles against dissolution, sintering and agglomeration.

To rationalize the superior catalytic behavior, density functional theory (DFT) simulations were applied to investigate ORR on (111) surfaces of Pt, PtCuNiCo and PtCuNiCoN, where nitrogen was modeled at the tetrahedral site (Methods, Supplementary Figs. 7 and 8).

From the reaction profiles of ORR (Fig. 5a), the highest energy cost for an elementary step on Pt, PtCuNiCo and PtCuNiCoN decreases in sequence, suggesting that ORR occurs fastest on PtCuNiCoN, consistent with the experimental results. The most prominent phenomenon is that the surfaces of both PtCuNiCo and PtCuNiCoN have stabilized the energies of *OOH and *OH compared to Pt(111), considerably decreasing the energy costs for potential rate-determining steps (RDS). Particularly, when interstitial nitrogen was present, the downshift for *OH was so substantial that the RDS switched from *O + H^་+e^-→*OH to *OH + H^་+e^།→*+H₂O. This crucial effect of interstitial nitrogen in PtCuNiCoN is profoundly linked to its ability to modulate the electronic structure of the binding site in a unique way. The N-coupled orbitals, formed through p-d hybridization of mainly Pt-N and Ni-N bonding, were found to lie within a distinct energy range from − 6.4 to -8.7 eV (Fig. 5c-e), thereby enabling the enhanced overlaps with the *OOH and *OH bonding peaks; this stablization effect is also clearly manifested by the wavefunction isosurfaces of *OOH/*OH bonding states below − 8.0 eV (insets, Fig. 5d,e, Supplementary Fig. 9), where strengthened N-M-O interactions have been verified by the electronic wavefunction continuously spanning along these atoms. These results highlight the unique ability of N 2p orbitals in tailoring the surface electronic structures for stabilizing the key oxygen intermediates, and thus optimizing the reaction landscape.

The other noteworthy feature of PtCuNiCoN is the ability to adjust the free energies of *O in a reversed direction to *OOH and *OH, which also considerably optimizes the reaction. This favorable effect is closely connected to the diverse local binding environments, which could permit spillages of reaction intermediates to find more favorable binding sites^5,19. For example (Fig. 5b), the reaction intermediates directly coordinated with two sets of atoms, Pt-Pt-Ni and Pt-Ni, on PtCuNiCoN (with three sets of atoms, Pt-Pt, Pt-Pt-Co, Pt-Co, on PtCuNiCo (Supplementary Fig. 8b)). The presence of dissimilar elements and interstitial N has led to the formation of chemically diverse and distorted binding sites, facilitating a much finer tuning in the positions and directions of the intermediate energies (Fig. 5a). Consequently, the energy modulations of intermediates become decoupled, somewhat breaking the scaling relation of intermediate energies that typically limits the catalytic efficacy of chemically resembling sites^55,56, i.e., Pt(111) or single-atom sites (Supplementary Fig. 8). These advantages of interstitial N in Pt HEAs are also exemplified at a higher voltage (Supplementary Fig. 10). In summary, the energies of p-orbitals and interstitial positions of nitrogen are highly distinct, such that p-d orbital hybridization and fine tuning of local geometry by nitrogen could induce enhanced optimization of intermediate energies that is not attainable through d-block metal alloying alone.

To determine the effect of nitrogen on the stability of local structures, we calculated the formation energies of surface single-atom vacancy to evaluate the initial step of surface corrosion. The stability was found to decrease in the order of PtCuNiCoN, PtCuNiCo and Pt (Extended Data Fig. 10a). In the case of PtCuNiCoN (Extended Data Fig. 10b), once a surface Pt atom was removed, the subsurface nitrogen tended to draw closer a nearby surface Pt to maintain its four-fold coordination, effectively blocking surface oxygen and preventing further metal corrosion. These results hence strongly suggest a close relationship bewteen the enhanced durability of PtCuNiCoN and the coordiation of N with metal atoms in the subsurface^[25,26].

This work has offered a powerful method for synthesizing a novel class of nonmetal-hybridized interstitial Pt HEAs and provided prospective directions for developing electrocatalysts with both outstanding activity and stability. The feasibility of incorporating different nonmetal elements into HEAs within a single synthesis considerably expand the compositional space and increase the local structural diversity for HEAs. This work highlights that the incorporation of p-block light elements can effectively complement and reinforce the catalytic virtues brought by Pt alloying with metals and that exceptionally high activity and stability for ORR can indeed be simultaneously achieved. Given the distinct properties of B and F to N, such as electronegativity, and polar hydrophobicity, it is expected that B-/F-/multi-nonmetal incorporation could generate more unique features on the catalyst and carbon surfaces, facilitating the construction of a catalytically more effective mass-transport network, as well as the maintenance of it. We thus believe that more advanced catalytic compositions and elaborately designed catalyst layers can be developed for even higher-power, more durable fuel cells at ultralow Pt loadings. The idea of nonmetal-hybridized Pt HEAs inspires novel solutions to persistent challenges, such as scaling-relation limitation and inadequate durability, facing other important multistep clean-energy electrocatalysis.

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Materials synthesis

Chemicals. All chemicals were used as received without further purification. Platinum (II) acetylacetonate (Pt(acac)₂, 97%), copper (II) acetylacetonate (Cu(acac)₂, 97%), nickel (II) acetylacetonate (Ni(acac)₂, 95%), cobalt (II) acetylacetonate (Co(acac)₂, 97%), iron (III) acetylacetonate (Fe(acac)₃, 98%), manganese (II) acetylacetonate (Mn(acac)₂, 97%), chromium (III) acetylacetonate (Cr(acac)₃, 98%), tungsten hexacarbonyl (W(CO)₆, 99%), Benzoic acid (99%), phenylboronic acid, perfluoroterephthalonitrile and pipemidic acid were purchased from Shanghai Macklin Biochemical Co., Ltd., China. Aluminum (III) acetylacetonate (Al(acac)₃, 98%), molybdenum (II) acetylacetonate (Mo(acac)₂, 97%), and N, N-dimethylformamide (DMF, 99.5%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., China. Iridium (III) acetylacetonate (Ir(acac)₃, 97%) was purchased from Shanghai Sigma-Aldrich Trading Co., Ltd., China. Ketjen Black ECP-600JD was purchased from Guangdong Canrd New Energy Technology Co., Ltd., China. Ethanol absolute (C₂H₅OH, 99.7%), acetone (C₃H₆O, 99.5%), perchloric acid (HClO₄, 70%), hydrochloric acid (HCl, 36%) and nitric acid (HNO₃, 65%) were purchased from Sinopharm Chemical Reagent Co., Ltd. China. DI water (18.2 MΩ⋅cm) used in all experiments was prepared by passing through an ultra-pure purification system.

Synthesis of nonmetal-hybridized interstitial high-entropy Pt alloy catalysts. Relatively abundant metallic elements (Cu, Ni, Co, Mn, Fe, Cr, V, Ti, Mo, W, Al, Zn) were preferentially investigated. In a typical synthesis process of PtCuNiCoN, 10 mg of platinum (II) acetylacetonate, 6.7 mg of copper (II) acetylacetonate, 6.5 mg of nickel (II) acetylacetonate, 6.5 mg of cobalt (II) acetylacetonate, 150 mg of benzoic acid, 30 mg of pipemidic acid, and 10 mg of carbon black (Ketjen black ECP-600JD) were added in a vial containing 5 mL of N, N-dimethylformamide (DMF). Then the suspension was sonicated for 10 min. The resulting mixture in the vial was subsequently placed in an oil bath which was heated to 160^oC, and maintained for 12 h. After that, the reactants in the vial were cooled to room temperature and subjected to centrifugation and being rinsed 3 times in mixed solutions of ethanol and acetone. The black powder retrieved from centrifuge tubes were dried in a vacuum oven overnight. The obtained material was then annealed under flowing H₂/Ar (10% H₂) gas at 400^oC (5^oC /min) for 2 h and cooled down to room temperature. Finally, an acid wash in 0.2 M HNO₃ for 15 minutes had been applied to the annealed material and then rinsed three times with deionized water. After drying overnight, the pristine sample of N-hybridized interstitial high-entropy Pt alloy catalysts have been prepared.

A series of catalysts with different compositions have been synthesized using the same method (described above) by adjusting the quantities of corresponding metal precursors. Similarly, the catalysts with incorporated boron or fluorine were prepared in the same way as for incorporating nitrogen, where a boron (phenylboronic acid) or fluorine source (perfluoroterephthalonitrile) have been used instead. Of note, before MEA tests, the pristine catalyst samples were further washed by 0.2 M HNO₃ at 85^oC in an Ar atmosphere within a sealed three-neck flask for 8 h and then dried overnight at 60^oC in vacuo.

Synthesis of (high-entropy) Pt alloy catalysts without interstitial nitrogen. The (high-entropy) Pt alloy catalysts without N-incorporation (such as, PtCuNiCoFe, PtCuNiFeMn, PtCuNiCo and so on) were also prepared following the same procedure as described above, but simply with no addition of nonmetal sources during the solvothermal reaction.

Characterizations

Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained with a JEM 2100 microscope. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images and energy dispersive X-ray spectroscopy (EDS) elemental mapping analysis were carried out by using FEI Themis Z with 300 kV acceleration voltage. Electron energy loss spectroscopy (EELS) was taken on the same microscope equipped with a GIF Quantum 1065, both EEL spectra in Extended Data Fig. 1e being processed by superimposing the signal from multiple test areas (nanoparticle areas or dimmer single atom areas) and being slightly smoothed. The annular bright field (ABF) STEM images were acquired on an aberration-corrected JEOL JEM-ARM300F2 microscope with camera length at 10 cm (operating at 300 kV). The integrated differential phase contrast (iDPC)-STEM images were acquired in a ThermoFisher Scientific Spectra 300 aberration-corrected TEM operating at 300 kV. A convergence semi-angle of 25 mrad and current of 5 pA were used for imaging.

X-ray diffraction (XRD) patterns were collected by using a DX-2700B powder diffractometer (Dandong Haoyuan Instrument Co., Ltd.), operated at 40 kV and 30 mA, using a Cu-K radiation source (λ = 1.5405 Å). X-ray photoelectron spectroscopy (XPS) was carried out on an America Thermo Scientific K-Alpha surface analysis system employing a monochromatic Al-K X-ray source. The actual metal contents were determined by the inductively coupled plasma-optical emission spectrometer (ICP-OES, Perkin Elmer Optima 8300).

To prepare the APT samples, an ethanol suspension with the nanoparticles was dropped onto an aluminum sample holder. After drying, a 150 nm Cr layer was deposited on the surface with a Leica EM ACE600 sputter coater to embed the nanoparticles. APT specimens were prepared via a standard lift-out procedure in a FEI G4 CX focused ion beam/scanning electron microscope (FIB/SEM). The APT experiments were carried out on a Cameca LEAP 5000 XR in a laser mode at a specimen temperature of 65 K, a pulse rate of 125 kHz, a 0.3% detection rate and a laser pulse energy of 30 pJ. Reconstruction and analysis of the APT data were performed using the commercial AP Suite 6.3.0.90 software.

X-ray absorption fine structure (XAFS) spectroscopy was carried out at synchrotron-based light source (BSRF 1W1B) and using the Rapid XAFS 2M (Anhui Absorption Spectroscopy Analysis Instrument Co., Ltd.) by transmission (or fluorescence mode) at 20 kV and 30 mA, and the Si (533) spherically bent crystal analyzer with a radius of curvature of 500 mm was used for Co, the Si (771) spherically bent crystal analyzer with a radius of curvature of 500 mm was used for Pt, the Si (553) spherically bent crystal analyzer with a radius of curvature of 500 mm was used for Cu, the Si (551) spherically bent crystal analyzer with a radius of curvature of 500 mm was used for Ni.

Data reduction, data analysis, and EXAFS fitting were performed and analyzed with the Athena and Artemis programs of the Demeter data analysis packages⁵⁷ that utilizes the FEFF6 program⁵⁸ to fit the EXAFS data. The energy calibration of the sample was conducted through standard Pt foil, which as a reference was simultaneously measured. A linear function was subtracted from the pre-edge region, then the edge jump was normalized using Athena software. The χ(k) data were isolated by subtracting a smooth, third-order polynomial approximating the absorption background of an isolated atom. The k3-weighted χ(k) data were Fourier transformed after applying a HanFeng window function (Δk = 1.0). For EXAFS modeling, the global amplitude EXAFS (CN, R, σ² and ΔE₀) were obtained by nonlinear fitting, with least-squares refinement, of the EXAFS equation to the Fourier-transformed data in R-space using Artemis software. EXAFS of the Pt foil were fitted and the obtained amplitude reduction factor S₀² value (0.772) was set fixed in the EXAFS analysis to determine the coordination numbers (CNs) in the Pt-N, Pt-Cu/Ni/Co and Pt-Pt scattering path in the samples. S₀² was fixed to 0.772, according to the experimental EXAFS fit of Pt foil by fixing CN as the known crystallographic value. Fitting range: 3.0 ≤ k (/Å) ≤ 13.0 and 1.0 ≤ R (Å) ≤ 3.2 (Pt foil); 3.0 ≤ k (/Å) ≤ 12.0 and 1.2 ≤ R (Å) ≤ 2.1 (PtO₂); 3.0 ≤ k (/Å) ≤ 12.0 and 1.0 ≤ R (Å) ≤ 3.3 (PtCuNiCo); 3.0 ≤ k (/Å) ≤ 12.0 and 1.0 ≤ R (Å) ≤ 3.3 (PtCuNiCoN). A reasonable range of EXAFS fitting parameters: 0.700 < Ѕ₀² < 1.000; CN > 0; σ² > 0 Å²; |ΔE₀| < 10 eV; R factor < 0.02.

Electrochemical measurements

Rotating disk electrode (RDE) measurements. All electrochemical measurements were performed at an electrochemical workstation (Vertex. 100 mA. EIS Bistat) at room temperature, using a three-electrode electrochemical setup with a rotating disk electrode (RDE) system. A working electrode of glassy carbon (5 mm inner diameter, 0.196 cm²), a counter electrode of platinum wire and a reference electrode of Ag/AgCl were used for the tests. In this work, all potentials were quoted with respect to a reversible hydrogen electrode (RHE). The RHE calibration was performed in a highly H₂-saturated 0.1 M HClO₄ electrolyte with a platinum disk as the working electrode.

For the preparation of RDE test electrode, 1.5 mg catalyst was mixed with 990 µL ethanol and 10 µL Nafion solution (5 wt%) and the mixture was ultrasonicated for 120 minutes to form the catalyst ink. Prior to the catalyst ink deposition, the GC disk was polished with 50 nm alumina powder and rinsed generously with high purity ethanol. Then the ink was deposited on glass carbon electrode to provide a uniform catalyst layer (CL). For comparison, the RDE electrode of commercial Pt/C (TKK 20%) was prepared with the same method.

Cyclic voltammetry (CV) measurement was typically carried out in the potential range of 0.05–1.1 V (vs. RHE) at a scan rate of 50 mV s^− 1 in a N₂-saturated 0.1 M HClO₄ solution. Linear sweep voltammetry (LSV) curves were tested in an O₂-saturated 0.1 M HClO₄ electrolyte at a rotation speed of 1600 rpm and a scan rate of 10 mV s^− 1. The activation test was operated with 150 cycles of CV in an O₂-saturated atmosphere. Accelerated durability test was performed with 30,000 cycles of CV in the voltage range of 0.6–1.1 V. After the activation or durability test, the catalysts could be collected by sonicating the GCE in ethanol for further structural and compositional analysis.

The electrochemical active surface area (ECSA) of the catalyst was derived from the average charges associated with the adsorption and desorption of hydrogen between 0.05 and 0.4 V_RHE, taking a value of 210 µC cm^− 2 for monolayer adsorption of hydrogen on the Pt surface. The MA was defined as the SA multiplied by the ECSA. For the calculation of the ECSA, we used the loading, as measured by ICP-OES.

Membrane electrode assembly (MEA) fabrication and single fuel cell test. The activity and durability of nitrogen-hybridized interstitial HEA catalysts as cathodes were evaluated at a fuel cell testing system (Dalian Yuke Innovation Technology Ltd.) and a single cell with serpentine flow fields. The cathode catalyst ink was prepared by ultrasonically mixing the HEA catalyst, a solvent (75 wt% n-propanol and 25 wt% H₂O) and 20% Nafion ionomer solution at an ionomer to carbon ratio of 0.8 for 40 min. Then the uniform ink was sprayed (Xiniu Ultrasonic Coating System 260E) on the one side of Nafion membrane (GORE 12 µm) to form the cathode catalyst layer with an active area of 5 cm². Extra care must be taken to avoid wrinkling of the membrane during catalyst coating. The anode catalyst layer was prepared by coating the commercial TKK 20% (TEC10E20E) Pt/C catalyst onto the other side of the Nafion membrane using the same method as above. The catalyst-coated membrane (CCM) containing cathode and anode catalyst layers, two gas diffusion layers (GDLs) including microporous layers (SGL 29BC) and two polyimide gaskets (used to better control the compression) were hot-pressed to obtain a MEA at 120^oC for 5 min under a pressure of 4 MPa. Both the anode and cathode Pt loadings of the prepared MEAs have been measured by ICP-OES and repeated for at least 3 times to ensure an accurate evaluation.

H ₂ -O ₂ single fuel cell tests: PtCuNiCoN was used as the cathode catalyst at 0.05 mg_Pt cm^− 2 and Pt/C was used for the anode at 0.05 mg_Pt cm^− 2. The temperature of cells was maintained at 80^oC. Pure hydrogen (99.999%) and O₂ (99.999%) were with 100% relative humidity (RH) and the back pressure of both the anode and cathode was 150 kPa_{abs, outlet}, while keeping the flow rate of H₂/O₂ at 200/500 mL min^− 1. Before recording the polarization curves, MEAs were activated via keeping the cell voltage at 0.5 V until the stable current value was obtained. Pt mass activity (MA) was determined at 0.9 V_iR−correct and H₂/O₂ flow rates of 200/500 mL min^− 1.

H ₂ -air single fuel cell tests: PtCuNiCoN cathodes with Pt loadings of 0.07 and 0.05 mg cm^− 2 were selected, while Pt/C of 0.03, 0.01 mg cm^− 2 was accordingly used for the anode. The cell temperature was kept at 80^oC at 100% RH, the anode/cathode flow rates were fixed at 500/1000 mL min^− 1, and the backpressure was 250 kPa_abs,outlet. Prior to the performance testing, an activation step by maintaining the cell voltage at 0.5 V was conducted for at least three hours. We also measured the polarization curves using constant current density holds, which showed very similar performance as obtained via constant voltage holds. The H₂-air polarization curves of TKK 20% Pt/C as cathode catalysts with Pt loadings of 0.07 and 0.05 mg cm^− 2_, respectively, were also measured using the same method.

Accelerated stress test (AST): In order to evaluate the durability of the catalysts, the AST was performed using 30,000 cycles of 0.6 to 0.95 V square-wave voltage with a duration of 3 s for each voltage, following DOE fuel cell test protocol. The fuel cell test condition in AST was set at 80^oC, 150 kPa_{abs, outlet}, and 100% RH with a H₂/N₂ flow rate of 200/75 mL min^− 1 for the anode/cathode. After the completion of voltage cycles, the H₂-O₂ current-voltage polarization curve was recorded again to measure the change of MA. Chronoamperometric measurements were conducted on MEAs fabricated using catalyst loadings of 0.03 mg/cm² TKK 20wt% Pt/C (anode), 0.07 mg/cm² PtCuNiCoN (cathode) and 12 µm Nafion membranes, under 150 kPa_{abs, outlet} H₂-air conditions.

All fuel cell impedance spectra were acquired with an electrochemical workstation (Gamry instrument, reference 3000). High-frequency resistance (HFR) was in-situ measured during the polarization curve test. The perturbation amplitude for the AC impedance was 5% of the direct current, and the frequency was 10 kHz, which would not disturb the electrochemical reaction of fuel cell. The EIS was tested from 20 kHz to 1 Hz, and 1 to 15 A under the conditions of H₂ (500 mL min^− 1) and air (1000 mL min^− 1) at 80ºC and 100% RH.

Calculation Methods

Spin-polarized density functional theory (DFT) calculations were performed using LASP (www.lasphub.com) program contained VASP 6.2.1 packages⁵⁹ with projected augmented wave (PAW) pseudo-potentials^60,61. The exchange-correlation energy was treated based on the generalized gradient approximation (GGA) by using Perdew-Burke-Ernzerhof (PBE) function⁶². The plane-wave cutoff energy was set to 450 eV. The DFT-D3(BJ) method of Grimme^63,64 was employed to describe long-range vdW interactions. In this work, it was uniformly replaced in Pt bulk according to the experimental element ratio (Pt: Cu: Ni: Co = 25:8:13:2) to optimize the structure for PtCuNiCo high entropy alloy (HEA) bulk. The 11% N-doped was distributed in lower energy tetrahedron sites depended on the comparison of tetrahedron, octahedron and replace sites. The surface models for PtCuNiCo and PtCuNiCoN HEAs were cut out 5 layers from the bulk, which ensure top layer was the lowest energy surface with 16 atoms. For Co, Ni, Cu or Pt single-atom model, the most stable structure^65,66 was selected. The Monkhorst-Pack scheme with a k-point separate on length of 0.05 Å⁻¹ was utilized for sampling the first Brillion zone⁶⁷. All atoms were fully relaxed in bulk optimizations, and 2 layers were relaxed while the rest layers were fixed in surface optimizations. The Quasi-Newton l-BFGS method was used for geometry relaxation until the maximal force on each degree of freedom less than 0.01 eV/Å.

To obtain the free energy profile, we have calculated Gibbs free energies for all the states based on the ZPE-corrected DFT total energy, which was set as the enthalpy at 0 K. It can be calculated with

where E_DFT is the total energy getting from DFT optimization, E_ZPE is the zero-point vibrational energy using the vibrational frequency calculations were performed via the finite-difference approach⁶⁸, C_V is the heat capacity, T is the kelvin temperature, and S is the entropy. The free energy for the gas-phase molecule was computed from thermodynamics by utilizing the standard thermodynamics data at the standard state.

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Acknowledgements:

The authors thanks Prof. Guangfeng Wei for his useful advice on interpreting the density functional theory calculations and the constructive feedbacks on the manuscript. Prof. Zhihua Zhong is thanked for the inspiring discussions and helpful suggestions on the project.

Funding:

Natural Science Foundation of China 21802102, 52372219 (TLiu)

Recruitment Program of China for global experts (TLiu)

National Science Foundation of Sichuan Province 23SYSX0008 (TLiu)

Science and Technology Commission of Shanghai Municipality 19DZ2271500 (TLiu)

National Key R&D Program of China 2020YFB2007400 (LZ)

National Natural Science Foundation of China T2321002 (LZ)

Natural Science Foundation of Jiangsu Province BK20230028 (LZ)

Fundamental Research Funds for the Central Universities of China (TLiu, LZ)

Deutsche Forschungsgemeinschaft 506711657-CRC1625 (TLi)

Author contributions:

Conceptualization and project administration: TLiu;

Funding acquisition: TLiu, TLi, LZ;

Supervision: TLiu;

Methodologies: PG, JL, XL, DZ, CF, YXC, LHZ, PFY, ZL;

Investigation: all authors;

Visualization: XL, PG, JL, DZ, ZG, CF;

Writing – original draft: TLiu;

Writing – review & editing: all authors.

Completing interests:

The authors declare that they have no competing interests.

Data and materials availability:

All data are available in the main text or the supplementary materials.

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Nonmetal-Hybridized Interstitial High-Entropy Platinum Alloys for Oxygen Electrocatalysis in Practical Fuel Cells

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