Boosting oxygen reduction performances in Pd-based metallenes by co-confining interstitial H and p-block single atoms

Qiu, Yu; Sun, Mingzi; Wu, Jiandong; Chai, Chunxiao; Wang, Shengwei; Huang, Hong; Zhao, Xiao; Jiao, Dongxu; Xu, Shan; Wang, Dewen; Ge, Xin; Zhang, Wei; Zheng, Weitao; Song, Yujiang; Fan, Jinchang; Huang, Bolong; Cui, Xiaoqiang

doi:10.1038/s41467-025-60400-5

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Published: 06 June 2025

Boosting oxygen reduction performances in Pd-based metallenes by co-confining interstitial H and p-block single atoms

Nature Communications volume 16, Article number: 5262 (2025) Cite this article

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Electrocatalysis

Abstract

The efficiency of the oxygen reduction reaction (ORR) is limited by the scaling relationship in the conventional oxygen associative pathway. To break such limitations, we present an approach to effectively activate the oxygen dissociative pathway through co-confining single p-block (In, Sn, Pb) atoms and interstitial H atoms within Pd metallenes, leading to good ORR performance. PdPbH_x metallenes exhibit a high mass activity of 1.36 A mg⁻¹ at 0.95 V (vs. RHE), which is 46.9 times higher than that of the benchmark Pt/C. The minimal performance decay after 50,000 potential cycles confirms a good stability. In situ vibrational spectroscopy investigations and theoretical calculations highlight that interstitial H atoms facilitate the direct dissociation of O₂ while single Pb atoms enhance O₂ adsorption strength. The electroactive PdPbH_x metallenes is attributed to the up-shifted Pd-4d orbitals induced by H and Pb atoms. This research supplies critical inspiration for developing highly efficient ORR electrocatalysts.

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Introduction

Fuel cells hold immense potential as a pristine and efficient energy technology, offering zero carbon emissions, elevated energy efficiency, and reduced operating temperatures^1,2,3. However, the sluggish reaction kinetics of the cathodic oxygen reduction reaction (ORR) pose a significant obstacle in electrocatalysis, hindering the widespread adoption of commercial fuel cells^4,5. This challenge is primarily ascribed to the multiple proton-coupled electron transfer steps and corresponding oxygen-containing intermediates involved in the ORR process^6,7,8,9. Specifically, most electrocatalysts exhibit an oxygen-associative mechanism that involves the adsorption of three intermediates: superoxide (^*OOH), hydroxyl (^*OH), and oxygen atom (^*O)^{10,11,12,13,14,15,16,17}. These intermediates exhibit the scaling relationship of ΔG_*OOH ≈ ΔG_*OH + 3.2 eV and ΔG_*O ≈ 2ΔG_*OH^8,18. Consequently, optimizing the binding energy of each intermediate independently becomes a great challenge, resulting in a high theoretical overpotential required to drive the ORR process and subsequently diminishing ORR activitys^6,19,20. According to the well-established ORR free energy diagram, the rate-determining step (RDS) in ORR is either O₂ → ^*OOH or ^*OH → H₂O⁷. Considering that the^*OH → H₂O step is unavoidable during the ORR process, there is a critical need for a practical approach that bypasses ^*OOH formation and directly dissociates the adsorbed oxygen molecule (^*O₂) into two ^*O species to further enhance ORR performance^21,22. Nevertheless, achieving this objective entails the development of an efficient electrocatalyst with robust O₂ adsorption capability, which is a longstanding challenge that persists in the field.

Two-dimensional metallenes, distinguished by their large specific surface area, high electrical conductivity, and maximized atomic utilization, are promising candidates in the domain of ORR^{23,24,25,26,27}. Moreover, their adjustable microscopic structures make them an ideal platform for regulating the reaction mechanisms^28,29. Prior research has explored diverse strategies such as alloying^30,31, defect engineering^32,33, and surface strain^25,34 to modulate the d-band center of active sites in metallenes, thereby controlling their binding energies with oxygen-containing intermediates and improving ORR performances. However, it should be noted that the strong attractive interactions between surface atoms in metallenes inherently generate a compressed strain³⁴, which results in relatively low binding energy with ^*O₂. This induces a significant challenge in triggering the oxygen dissociation pathway within metallenes, as it requires overcoming a moderate energy barrier.

Herein, we present a series of highly efficient catalysts of Pd-based metallenes for alkaline ORR achieved by co-confining single p-block (In, Sn, Pb) atoms in the sub-surface and interstitial H atoms within the lattice of Pd metallenes. Among them, PdPbH_x metallenes exhibit a high mass activity of 1.36 A mg⁻¹ at 0.95 V (vs. RHE), which is 46.9 times higher than that of commercial Pt/C and shows a competitive performance with previous works on Pd-based catalysts. Notably, the PdPbH_x metallenes demonstrate negligible activity decay even after undergoing 50,000 potential cycles, exhibiting no observable changes in structure and composition. In situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy analysis reveals that interstitial H atoms induce the oxygen dissociative pathway, while the confined single Pb atoms further enhance the pathway by promoting stronger O₂ adsorption. Density functional theory (DFT) calculations elucidate that the improved electroactivity of Pd sites is potentially attributed to the introduction of interstitial H and single Pb atoms, which facilitates the adsorption of crucial intermediates and accelerates the electron transfer to promote the direct dissociation of O₂. Additionally, the improved p-d overlapping improves the selectivity of the oxygen dissociative pathway due to the decreased energy barriers, leading to high ORR performance.

Results

Morphology and structure characterizations

Building on the fact that p-block elements exhibit good miscibility with Pd and can form various solid solutions with Pd (Supplementary Fig. 1), a series of PdMH_x (M= In, Sn, Pb) metallenes were synthesized using a seed-mediated method, as illustrated in Fig. 1a and Supplementary Fig. 2. In this process, Pd and M ions were co-reduced epitaxially on the surface of Pd metallene seeds to form PdM metallenes with a defect-rich structure. The Pd atoms located at the defect areas of PdM metallenes exhibit higher energy and increased activity, making them susceptible to oxidation and etching by residual air³⁵. These Pd atoms are subsequently re-deposited on the surface and edges of the PdM metallenes, effectively burying the M atoms within the lattice. As the Pd atoms from the interior and edges of the defect areas migrate, the pores gradually expand. Meanwhile, hydrogen atoms generated in situ from the decomposition of N,N-dimethylformamide (DMF) permeate into the lattice of PdM during the etching process, resulting in the formation of PdMH_x nanorings (Supplementary Figs. 2–8).

The morphology of the PdMH_x metallenes was characterized using transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). As a typical structure, PdPbH_x metallenes show the interconnected ring-like shape (Fig. 1b, c), with a thickness of 3.93 ± 0.2 nm by analyzing the atomic force microscopy (AFM) images and corresponding height profiles (Fig. 1d). The observed reduction in the hole size of the nanorings in AFM images relative to TEM images is predominantly a consequence of the tip effect inherent to AFM measurements^36,37. Energy dispersive X-ray spectroscopy (EDS) elemental mapping demonstrated the uniform distribution of Pd and Pb elements in the PdPbH_x metallenes, confirming their alloy feature (Fig. 1e). The Pd/Pb atomic ratio, determined based on EDS results, was found to be 91.3/8.7 (Supplementary Table 1), which was consistent with the data obtained from inductively coupled plasma atomic emission spectroscopy (ICP-AES, Supplementary Table 2). Furthermore, atomic-resolution HAADF-STEM images and corresponding intensity profiles (Fig. 1f, g) revealed that the atomic structure of the PdPbH_x metallenes consisted of relatively bright spots (high peak intensity) and numerous relatively dark spots (low peak intensity). Given the higher atomic number of Pb (Z = 82) compared to Pd (Z = 46), the brighter spots were attributed to Pb atoms, while the darker spots were attributed to Pd atoms. Importantly, the Pb atoms were found to be atomically dispersed within the Pd lattice without any Pb-Pb bonds, indicating a single-atom alloy feature. Notably, the PdInH_x and PdSnH_x metallenes show a similar ring-like morphology and single-atom alloy structure to that of the PdPbH_x metallenes (Supplementary Figs. 9 and 10), demonstrating the universality of the synthetic method.

Structural and electronic properties

Figure 2a presents the X-ray diffraction (XRD) results, which show that the peaks of PdPbH_x metallenes shift to a lower angle compared to Pd metallenes. This shift is in line with the standard PdH_x phase (PDF#87-0637) with the H content of 0.32 (Supplementary Fig. 11), confirming the formation of metal hydride. In addition, we have plotted a standard curve of the relationship between the lattice constant and Pb content within fcc-Pd_xPb_1-x alloys, demonstrating that the XRD peak shift of PdPbH_x is mainly due to the incorporation of interstitial hydrogen atoms instead of Pb atoms (Supplementary Fig. 12). H solid-state nuclear magnetic resonance (ssNMR) was conducted to further investigate the presence of interstitial H atoms. Figure 2b shows a new peak at 26.1 ppm in the ¹H solid state nuclear magnetic resonance (ssNMR) spectrum of PdPbH_x metallenes, indicating the formation of the Pd-H bonds^38,39. The interaction between Pd and H atoms was also confirmed by electron energy loss spectroscopy (EELS) (Fig. 2c) and X-ray photoelectron spectroscopy (XPS) results (Supplementary Fig. 13), which demonstrate that PdPbH_x metallenes exhibit a lower EELS peak energy and a narrower valence band compared to Pd metallenes. These observations are consistent with the behavior observed in palladium hydride systems^40,41.

X-ray absorption spectroscopy (XAS) was employed to study the coordination environments and chemical states of Pd and Pb atoms in PdPbH_x metallenes. Figure 2d displays the X-ray absorption near-edge structure (XANES) spectra of Pd K-edge in PdPbH_x metallenes, in which the adsorption edge in PdPbH_x metallenes is close to that in the Pd foil, demonstrating that the Pd atoms in PdPbH_x metallenes are mainly in the metallic state. The conclusion was consistent with XPS results (Supplementary Fig. 14). The coordination situation of Pd atoms in PdPbH_x metallenes was further investigated by Fourier transform extended X-ray absorption fine structure (FT-EXAFS) and wavelet transform EXAFS (WT-EXAFS) (Fig. 2e, f and Supplementary Fig. 15). Figure 2e displays that the EXAFS at the Pd K-edge spectra of PdPbH_x metallenes is the same as that of Pd foil, meaning the crystal structure remains unchanged after doping. The Pd-Pd/Pb bond length in PdPbH_x metallenes was measured to be 2.82 Å (Fig. 2e, f and Supplementary Table 3), which is 0.08 Å longer than that in Pd foil (2.74 Å) due to the existence of interstitial H atoms²⁴. XANES spectra of Pb L₃-edge indicate that Pb atoms in PdPbH_x metallenes are primarily in the metallic state (Fig. 2g). The FT-EXAFS and WT-EXAFS results (Fig. 2h, i, Supplementary Fig. 16) show that the Pb component in the PdPbH_x metallenes only exhibits one peak of Pb-Pd bond, indicating that Pb atoms are atomically dispersed in the Pd lattice without aggregations. Additionally, the EXAFS fitting results of PdPbH_x metallenes reveal a coordination number of approximately 9.1 for the Pb-Pd bond, slightly higher than that for the Pd-Pd bond with a value of 7.2 (Supplementary Table 3). This suggests that Pb atoms are mainly located in the interior area with a large coordination number. Conversely, XAS analysis of PdPb metallenes indicates that Pb atoms are distributed throughout the entire structure, including both the internal and exterior regions. (Supplementary Figs. 17–19). These inferences are further supported by the galvanic replacement experiment, where Cu was used to replace surface Pb atoms (Supplementary Table 4)^23,33. XPS results (Supplementary Fig. 20) confirm that Pb atoms are confined within the internal region of PdPbH_x metallenes, whereas Pb atoms are located outside PdPb metallenes rather than being confined solely within the internal region.

Evaluation of ORR performance

The electrocatalytic performance of PdPbH_x metallenes for ORR was evaluated using a rotating disk electrode (RDE) measurement and compared to commercial Pt/C as the benchmark. Before the test, PdPbH_x metallenes, PdH_x metallenes (a comparison sample without the Pb element, Supplementary Fig. 21), and PdPb metallenes were deposited onto the carbon black support. Cyclic voltammetry (CV) curves were measured for these catalysts in a N₂-saturated 0.1 M KOH solution at a scan rate of 50 mV s⁻¹. The electrochemical active surface areas (ECSA), which provide an estimate of the available active sites for the ORR process, were calculated based on the integral calculation of the reduction peak areas of Pd-oxides. Figure 3a shows that PdPbH_x metallenes/C exhibits the highest ECSA value and the most positive Pd-O peak potential compared to the other three catalysts (Supplementary Table 5, Supplementary Figs. 22 and 23). This observation indicates that PdPbH_x metallenes possess a significant number of active sites, primarily located on steps and ledges within the nanoring structure^42,43,44. Additionally, the CV curves of all samples at a scan rate of 10 mV s⁻¹ showed both PdPbH_x and PdPb exhibited lower current densities at 1.3 V during the positive sweep compared to PdH_x (Supplementary Fig. 24), indicating that the introduction of Pb can reduce the oxidation degree at high potentials⁴⁵. Furthermore, the ORR polarization curves were measured in an O₂-saturated 0.1 M KOH solution at a scan rate of 10 mV s⁻¹. As shown in Fig. 3b, the half-wave potential (E_1/2) in PdPbH_x metallenes/C is 0.958 V, which is much more positive than that in PdH_x metallenes/C (0.910 V), PdPb metallenes/C (0.907 V), and commercial Pt/C (0.845 V). Noting that the LSV curves in Fig. 3b are the positive scan from 0.3 V to 1.1 V (Step I), which is higher to that observed during the negative scan from 1.1 V to 0.3 V (Step II) (Supplementary Fig. 25). This difference can be attributed to the reaction of Pd/Pt with OH⁻ at high potentials, forming inactive Pd/Pt-based oxides⁴⁶. However, the LSV curves (Step III) revert to the one observed in Step I, indicating that the Pd/Pt-based oxides may be electrochemically reduced back to their original active metal state under these test conditions. Additionally, quasi in situ XPS results show that the area ratio of Pd²⁺/Pd²⁺+Pd⁰ always keeps below 0.25 across the whole testing potential (Supplementary Fig. 26), indicating the Pd in PdPbH_x metallenes remained predominantly in its metallic state throughout the ORR process. The Tafel slope of PdPbH_x metallenes/C is 59.8 mV dec⁻¹, which is much lower than that of PdH_x metallenes/C with 68.5 mV dec⁻¹, PdPb metallenes/C with 88.9 mV dec⁻¹, and commercial Pt/C with 101.3 mV dec⁻¹ (Fig. 3c), suggesting the faster ORR reaction kinetics on PdPbH_x metallenes/C among the four samples¹⁰. By applying the Koutecky-Levich equation to analyze polarization curves, the catalytic kinetic currents were calculated. These values are subsequently normalized using both the metal loading mass and ECSA to determine the mass activity (MA) and specific activity (SA) (Fig. 3d). Since the universally chosen potential (0.9 V) is too close to the diffusion limit region to provide reliable dynamic current density due to the fast kinetic of PdPbH_x metallenes/C (Supplementary Fig. 27), we evaluated the activity at 0.95 V to eliminate the O₂ diffusion limit^23,47. As shown in Fig. 3d, at 0.95 V, PdPbH_x metallenes/C exhibited outstanding MA (1.36 A mg⁻¹_Pd) and SA (1.59 mA cm⁻²), which are 46.9-fold and 31.9-fold enhancements compared to commercial Pt/C, respectively. Optimization experiments for ORR performance with varying Pb doping amounts revealed that the synthesized PdPbH_x catalyst with a Pb atomic ratio of 8.7% exhibited better performance (Supplementary Fig. 28). Additionally, the ORR performance of PdPbH_x metallenes was evaluated in acidic electrolytes, also showing much better activity compared to commercial Pt/C (Supplementary Fig. 29 and Supplementary Table 6). These results indicate the superior electrocatalytic activity of PdPbH_x metallenes/C for the ORR compared to the other catalysts studied. Notably, when alloyed single atoms are changed into In and Sn, these hydride bimetallenes materials also exhibit good ORR performance (Supplementary Figs. 30, 31), highlighting the versatility of this strategy for enhancing ORR activity through the co-confinement of single p-block and interstitial H atoms in Pd-based metallenes.

**Fig. 3: Electrocatalytic performance of PdPbH_x metallenes/C, PdH_x metallenes/C, PdPb metallenes/C and commercial Pt/C catalysts.**

To assess the electrochemical durability of PdPbH_x metallenes/C, the accelerated stability test was conducted in an O₂-saturated 0.1 M KOH solution between 0.6 and 1.0 V versus RHE at a scan rate of 200 mV s⁻¹. Remarkably, after 50,000 cycles, the half-wave potential (E_1/2) of PdPbH_x metallenes/C exhibited a slight decrease of 8 mV, indicating good stability (Fig. 3e). ECSAs measured before and after 50,000 potential cycles remain at a high value without evident changes (Supplementary Fig. 32). The MA and SA of PdPbH_x metallenes/C also remained at 95% and 96% of their initial values after 50,000 cycles, respectively (Fig. 3f). In contrast, the ORR activity of PdH_x metallenes/C, PdPb metallenes/C, and commercial Pt/C decreased notably after 50,000 cycles (Supplementary Fig. 33). The long-term chrono-amperometry measurements were conducted at 0.6 V vs. RHE for PdPbH_x metallenes/C and commercial Pt/C (Supplementary Fig. 34). The results showed that the PdPbH_x metallenes/C maintains approximately 90.6% of its initial current density after testing for 30,000 s, whereas commercial Pt/C only can maintain 33.7%, demonstrating the extraordinary durability of PdPbH_x metallenes compared to commercial Pt/C. Furthermore, the morphology, single atom alloy feature, and hydride nature of PdPbH_x metallenes were retained after the accelerated stability measurement, as confirmed by imaging techniques (Supplementary Figs. 35–37, Table 1 and 2). The ORR performance of PdPbH_x metallenes, including the mass activity and long-term stability, surpasses those of the recently reported Pd-based ORR catalysts under alkaline media (Supplementary Table 7). Simultaneously, the half-wave potential and mass activity at 0.95 V vs. RHE of PdPbH_x metallenes also surpass those of most reported high-activity M-N-C materials. (Supplementary Table 8). In comparison, PdH_x metallenes/C displayed noticeable morphology changes and substantial aggregation after 50,000 cycles (Supplementary Fig. 38). This enhanced durability of PdPbH_x metallenes can be attributed to the unique nanoring structure, which exhibits a stronger binding force with the carbon support. The corresponding stability of the electrocatalysts has also been verified by molecular dynamics (MD) simulations at room temperature, where both PdPb and PdH_x show much reduced stability due to the strong structural distortions (Supplementary Fig. 39). In comparison, the PdPbH_x is much more stable. Importantly, the synthesis of PdPbH_x metallenes can be easily scaled up to produce larger quantities without significant changes in crystal structures and catalytic performances, supporting the practical applicability of PdPbH_x for future applications (Supplementary Fig. 40). To further validate the good ORR performances of PdPbH_x metallenes, practical Zn-air batteries (ZABs) and H₂-O₂ AEMFC tests were conducted with fuel cell devices containing PdPbH_x metallenes/C and commercial Pt/C as a cathode catalyst material. For ZABs performance, compared to the commercial Pt/C, PdPbH_x metallenes/C exhibit higher open-circuit voltage (1.55 V), power density (254.1 mW cm⁻²) and specific capacity (794.1 mAh per gram Zn) (Fig. 3g, h and Supplementary Fig. 41). We also examined the rate capability of our PdPbH_x metallenes/C at different current densities, further demonstrating the potential of PdPbH_x metallenes/C as air cathodes for practical applications (Fig. 3i). The H₂-O₂ fuel cell utilizing PdPbH_x metallenes/C achieved a peak power density of 722 mW cm⁻² at a loading of 0.3 mg cm⁻², surpassing the Pt/C-based systems (589 mW cm⁻²) (Supplementary Fig. 42).

Mechanistic studies via in situ ATR-FTIR spectroscopy

The four-electron ORR process involves two fundamental pathways as shown in Fig. 4a and summarized in Supplementary Table 9^48,49. The first pathway is the oxygen associative mechanism, where ^*OOH intermediates are formed through the protonation of ^*O-O. The second pathway is the oxygen dissociative mechanism, where ^*O-O^* directly dissociates into two ^*OH species, followed by continuous protonation. Consequently, different ORR mechanisms can be distinguished by specific reaction intermediates: ^*O-O^* species indicate the dissociative mechanism while the ^*OOH intermediate represents the associative mechanism. In situ ATR-FTIR spectroscopy was then performed to identify reaction intermediates and investigate the ORR mechanism on different metallene-based catalysts (Supplementary Fig. 43). In the wavenumber range of 1050–1200 cm⁻¹, a peak around 1175 cm⁻¹ corresponding to the ^*O-O^* species was observed on PdPbH_x metallenes/C and PdH_x metallenes/C (Fig. 4b and c)⁵⁰, while no obvious adsorption peak is observed in this region on Pd metallenes/C and PdPb metallene/C (Fig. 4d and Supplementary Fig. 44). The results indicate the dissociative pathway is exclusively occurred on the hydride metallenes, highlighting the unique role of interstitial H atoms in inducing the oxygen dissociation pathway. Notably, the peak around 1300 cm⁻¹ corresponds to the O-O stretching mode of ^*OOH intermediate^51,52 is observed on all metallene-based catalysts, indicating the presence of the oxygen associative pathways during the ORR process (Fig. 4e–g). To further elucidate the influence of doped Pb atoms, the wavenumber of the O₂ adsorption peak (within the range of approximately 1430–1470 cm⁻¹)^52,53 and the integral area ratio of the ^*O-O^*/^*O₂ peak was systematically analyzed for both PdPbH_x metallenes/C and PdH_x metallenes/C. As displayed in Fig. 4h, the wavenumber of the ^*O₂ peak in PdPbH_x metallenes/C is significantly lower than that in PdH_x metallenes/C under all applied potentials, suggesting a stronger O₂ adsorption energy on the former catalyst. Furthermore, the integral area ratio of the ^*O-O^*/^*O₂ peak in PdPbH_x metallenes/C is higher compared to that in PdH_x metallenes/C during the entire ORR process (Fig. 4i). In summary, Pb atoms in PdPbH_x metallenes enhance the adsorption strength of O₂ molecules on the catalyst surface and facilitate the activation process of ^*O₂ to ^*O-O^*, thereby promoting the oxygen dissociative process and improving ORR performances in PdPbH_x metallenes/C.

**Fig. 4: The exploration of ORR mechanism for catalysts via in situ ATR-FTIR spectra.**

Theoretical calculations

Detailed theoretical investigations regarding the electronic structures and reaction trends for ORR were carried out to explore the outstanding ORR performances of PdPbH_x metallenes. For demonstrating the electroactivity differences, the distributions of bonding and anti-bonding orbitals near the Fermi level (E_F) for PdPb, PdH_x, and PdPbH_x surfaces were first compared (Fig. 5a–c, Supplementary Fig. 45). For the (111) surface of the PdPb, the bonding orbitals only partially distribute on the surface while the anti-bonding orbitals are slightly more dominant, which supplies limited electroactive sites to guarantee efficient electron transfer during the ORR (Fig. 5a). For PdH_x, although the bonding orbitals become more evident on unsaturated Pd sites of the edge, the overall anti-bonding distributions are still dominant on the surface (Fig. 5b). With the introduction of Pb, the electronic distributions of PdPbH_x are evidently optimized, where the surface sites are mainly dominated by the bonding orbitals, offering a highly electroactive surface to enable efficient adsorption of oxygen and accelerate electron transfer (Fig. 5c). The projected partial density of states (PDOS) is further compared to understand the electronic structures of different electrocatalysts. For the PdPb surface, the Pd-4d orbitals exhibit a relatively broad distribution with continuous peaks, covering from E_V-6.0 eV to E_V (E_V denotes 0 eV) (Fig. 5d). For Pb sites, the 6 s orbitals show limited contributions, which are located far from the E_F. In contrast, the Pb-6p orbitals cross the E_F to increase the electron density, promoting the electron transfer of the electrocatalysts. Similar to the electronic distributions, the PDOS of PdH_x are different due to the electronic modulations induced by the formation of the hydride system (Fig. 5e). Notably, Pd-4d orbitals become sharper when compared to that of PdPb, leading to the overall upshifting band to the E_F with enhanced electroactivity. Meanwhile, the H-1s orbitals are well overlapping with the Pd-4d orbitals, supporting the stable structures of PdH_x. For the PdPbH_x, the Pd-4d orbitals become more concentrated and closer to the E_F, which results in a slightly increased valence state induced by the electronic modulations of introduced Pb (Fig. 5f). Compared to PdPb, the Pb-6s orbitals remain similar while the 6p orbitals also become more concentrated near E_F, which benefits the p-d overlapping with Pd-4d orbitals to enhance electron transfer and exchange efficiency. In addition, the H-1s orbitals are not strongly disturbed by the Pb, which guarantees the stability of the structures. In addition, we have included two additional compositions of 4.5% and 18% for Pb and supplied H contents of 0.2 and 0.4 as comparison samples. Both electronic structures and reaction trends have also been demonstrated. Compared to PdPbH_x, we notice that the higher Pb and H concentrations will slightly downshift the d orbitals of Pd, which demonstrates that Pb and H compositions also influence electron transfer (Supplementary Fig. 46a–d). From the site-dependent PDOS, we notice that edge Pd atoms demonstrate higher 4 d orbitals than the Pd sites in PdPb (Fig. 5g). In particular, the nearby H atoms significantly improve the electroactivity of Pd sites, which benefits the direct dissociations of O₂. For the Pb sites, the Pb-6s,6p orbitals are strongly different from the bulk metal, which are attributed to the isolated states of Pb atoms in PdPbH_x (Fig. 5h). Moreover, the surface and edge Pb sites display increased electron density since both 6 s and 6p orbitals shift closer to the E_F, which not only improves the overlapping with Pd-4d orbitals but also facilitates the electron transfer to enhance the adsorption of O₂. Then, we have summarized the d-band and p-band centers in Pd and Pb sites, respectively (Fig. 5i). From PdPb to PdPbH_x, the d-band center has gradually upshifted from E_V-2.05 eV to E_V-1.81 eV, revealing the increased electroactivity of Pd for stronger adsorption of O₂. In contrast, the downshifted p-band center in PdPbH_x is ascribed to the contributions of Pb-6p orbitals becoming higher near the E_F, which promotes the electron transfer from PdPbH_x to the O₂ and thus enhances the adsorption of O₂. For the introduction of Pb, we notice that further decrease or increase in the Pb compositions results in the decreases of d-band and p-band centers, indicating that a suitable Pb composition is critical to guarantee high electroactivity of Pd sites (Supplementary Fig. 46e, f). In comparison, the increasing H contents lead to the downshifted electroactivity of d-band and p-band centers towards decreasing electroactivity.

**Fig. 5: Theoretical investigations regarding the electronic structures and reaction trends for ORR.**

As shown in Fig. 5j and Supplementary Fig. 47, the PdPbH_x displays a lower adsorption energy of O₂ than PdPb and PdH_x, indicating the stronger adsorption ability with O₂, which can promote the oxygen dissociative pathway. The increases in Pb and H contents in PdPbH_x have been found to enhance the adsorption of O₂ for the ORR (Supplementary Fig. 46g). The reaction energy change of the ORR dissociative process is further investigated under both equilibrium (U = 1.23 V) and limiting potential (U_L). As shown in Fig. 5k, under the equilibrium potential of 1.23 V, both PdH_x and PdPbH_x demonstrate a spontaneous tendency for the generation of 2^*O, supporting the high selectivity towards the dissociative pathways. Specifically, PdPbH_x shows a smaller (RDS) energy barrier for the rate-determining step of ^*OH desorption, being 0.49 eV, compared to 0.56 eV of PdH_x. This demonstrates that the introduction of Pb atoms further promotes the proceeding of the oxygen dissociative pathway by reducing the dissociation barriers and desorption of *OH during the ORR. Meanwhile, the PdPb, Pd, and Pt(111) all meet large energy barriers at the initial conversion from O₂ to 2^*O, indicating the oxygen dissociative pathway was difficult to occur (Supplementary Fig. 48). For both associative and dissociative mechanisms, it is worth noting that the chemisorption of O₂ is the chemical step without any electron transfer involved, where the energy costs of O₂ dissociation cannot be alleviated by introducing the electrochemical potential⁴⁶. Based on the computational hydrogen electrode (CHE) model, the corresponding U_L of ORR under the dissociative mechanism is evaluated to be 0.98 V and 0.95 V for PdPbH_x metallenes and PdH_x metallenes, respectively. The high ORR performance of PdPbH_x metallenes is ascribed to the fast dissociation step, while the high barrier of the dissociation in PdPb metallenes significantly limits the kinetics of the ORR. The bulk Pt and Pd metallene have significantly higher RDS barriers for the dissociative ORR mechanisms compared to PdPbH_x (Supplementary Fig. 48). The dissociation barrier of O₂ further confirms that PdPbH_x exhibits the smallest kinetic barrier of 0.08 eV to guarantee the efficient dissociative ORR (Supplementary Fig. 49a). In order to more intuitively compare the difference of ORR pathways on three catalysts, the ΔG changes of the associative pathways are demonstrated (Supplementary Fig. 49b). The first protonation from chemisorbed *O₂ to *OOH is the potential limiting step for PdPbH_x metallenes, PdH_x metallenes, and PdPb metallenes. In contrast, the Pd and Pt(111) exhibit the last desorption step of ^*OH as the potential limiting step. For both PdPbH_x and PdH_x, the limiting potential of the dissociative pathway is much higher than that of the associative pathway, supporting the domination of the dissociative pathway during the ORR (Supplementary Fig. 49c). In comparison, the limiting potential for PdPb, Pd, and Pt(111) display a converse trend, where the associative pathways are more feasible with higher limiting potentials.

To compare the selectivity of different ORR pathways, the energy difference of O₂ → 2^*O and ^*O₂ → ^*OOH were summarized for PdPb, PdH_x and PdPbH_x (Fig. 5l). For PdPb, the ΔG of ^*O₂ → ^*OOH was 0.10 eV while the ΔG of ^*O₂ → 2^*O was 0.81 eV, demonstrating that oxygen was difficult to dissociate on PdPb. Different from PdPb, the ΔG of ^*O₂ → 2^*O of PdH_x and PdPbH_x were −0.17 eV and −0.87 eV, which is far below their ΔG of ^*O₂ → ^*OOH (0.55 eV and 0.64 eV, respectively). The huge energy difference illustrates that PdH_x and PdPbH_x prefer to occur in the oxygen dissociative pathway, proving that the oxygen was more easily dissociated on PdPbH_x. In addition, the RDS barrier comparisons further reveal the ORR mechanism preference. It was found that PdPbH_x exhibits the smallest overall RDS barriers, supporting its superior ORR performances (Supplementary Fig. 49d). For both PdPbH_x and PdH_x, the smaller difference of RDS barriers between dissociative and associative mechanisms lead to the co-existence of both ORR mechanisms as observed by experimental characterizations in Fig. 4. On the other hand, associative mechanisms strongly prevail in PdPb, Pd metallene, and Pt (111) owing to their significantly lower RDS barriers. With the modulations of Pb and H compositions, the corresponding ΔG of ^*O₂ → 2^*O and ^*O₂ → ^*OOH are also affected (Supplementary Fig. 46h–i). The Pb introduction is able to strengthen the adsorption of O₂ to accelerate the dissociative mechanism, where a suitable composition optimizes the RDS barrier to reach superior ORR. The H introduction modulates the reaction trends of dissociative mechanisms, while its impact on associative mechanisms is limited. It is essential to optimize both Pb and H contents to ensure the highest electroactivity of the surface Pd sites during electrocatalysis. Additionally, the oxygen coverage on PdPbH_x also affects the ORR trends (Supplementary Fig. 50a, b). Higher oxygen coverage results in higher energy barriers for direct dissociation mechanisms and favors the associative mechanisms (Supplementary Fig. 50c, d). As the oxygen coverage gradually increases, the surface electroactivity is weakened, especially for the adsorption of O₂, which limits the direct dissociation of adsorbed O₂, leading to increasing energy barriers for the dissociative mechanisms. These are attributed to the electronic modulations on the Pd sites by the O coverage (Supplementary Fig. 50e, f)⁵⁴. For Pd-4d orbitals, we notice the downshifting trend with the oxygen coverage increases, which will reduce the overall electroactivity and binding strengths. The broad O-2p orbitals cover from E_V-6.0 eV to E_V + 2.0 eV, which will overlap with Pb-6p and Pd-4d orbitals to further modulate the surface electroactivity towards ORR. The decreasing d-band centers of Pd lead to the reduced adsorption strengths of O₂ to promote the associative mechanism. These results indicate that the dissociative mechanism dominates the ORR at low oxygen coverage for PdPbH_x while the associative mechanism becomes more feasible at high oxygen coverage.

Discussion

In summary, the synthesis of PdMH_x (M = In, Sn, Pb) metallenes, with the inclusion of single p-block atoms and interstitial H atoms in the lattice of Pd metallenes, has led to the development of a highly efficient alkaline ORR catalyst. Among them, PdPbH_x metallenes demonstrated a high MA of 1.36 A mg⁻¹ under 0.95 V. This value is 46.9 times higher than that of commercial Pt/C and surpasses all previously Pd-based reported ORR catalysts. Additionally, the PdPbH_x metallenes exhibited good durability, with no significant reduction in activity even after 50,000 potential-scanning cycles. The in situ ATR-FTIR spectra revealed that the presence of interstitial H atoms triggered the oxygen dissociative pathway, while the inclusion of Pb atoms enhanced the initial adsorption capacity of O₂ on Pd metallenes, promoting the oxygen dissociative pathway. DFT calculations further demonstrated that the incorporation of hydride systems is able to optimize the electronic structures of PdPbH_x, and the confined single Pb atoms facilitated electron transfer through p-d orbital coupling. This resulted in enhanced O₂ adsorption and subsequent dissociation to ^*O with lower energy barriers when compared to the conventional conversion to ^*OOH, providing a more efficient pathway for the ORR. Our findings highlight the successful regulation of the ORR pathway through co-doping metal and non-metal atoms, offering valuable insights for the design of highly efficient electrocatalysts.

Methods

Materials

Palladium acetylacetonate (Pd(acac)₂, 99%) was purchased from Sigma-Aldrich (USA). Plumbum acetylacetonate (Pb(acac)₂, 99%), Indium acetylacetonate (In (acac)₃, 99%) and Tin acetylacetonate (Sn(acac)₂, 99%) were purchased from Aladdin (China). Commercial Pt/C catalyst (20 wt. %) was obtained from Johnson Mattkey (UK). Polyvinylpyrrolidone (PVP MW = 40000, K30) was obtained from Aladdin Co., Ltd. (China). Hexadecyl trimethyl ammonium bromide (CTAB, 99%) were obtained from Shanghai Macklin Biochemical Co., Ltd. (China). N,N-dimethylformamide (DMF, AR), potassium hydroxide (KOH, AR), isopropanol (CH₃CH₂CH₂OH) and ethanol (CH₃CH₂OH, AR) were all purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Carbon monoxide (ultra-purity) was purchased from Xin’guang Gas Co., Ltd. (Changchun, China). Nafion-ethanol solution was obtained from Adamas-beta Chemical (Switzerland). Milli-Q deionized water (DI water, 18.2 MΩ cm⁻¹) was used in all experiments. All reagents were used as received without further purification.

Synthesis of PdMH_x metallenes

The typical synthetic method of PdPbH_x metallenes: Pd metallenes to be used as the seed were synthesized by using a modified protocol previously reported work⁵⁵. First step: 50 mg Pd(acac)₂, 160 mg PVP, and 185 mg CTAB were added to the mixed solution with 10 mL DMF and 2 mL water with magnetic stirring. The resulting homogeneous yellow solution as transferred into a Schlenk flask, and charged with CO gas. Then the flask was immediately shifted into an oil bath that had been pre-heated to a temperature of 40 °C and held for 12 h. Second step: 20 mg Pd(acac)₂ and 10 mg Pb(acac)₂ were added to the above solution. After the solution is ultrasonically mixed, and charged with CO gas. Then the flask was immediately shifted into an oil bath that had been pre-heated to a temperature of 40 °C and held for 12 h again to obtain PdPb metallenes via epitaxial growth. The obtained PdPb metallenes were separated by centrifugation and washed three times with ethanol. Third step: PdPb metallenes were dispersed in the 15 mL DMF and transferred into a 25 mL Teflon-lined stainless steel autoclave. The autoclave was heated at 160 °C for 4 h. The final product was collected by centrifugation (4695 g, 10 min) and washed three times with ethanol. The PdInH_x and PdSnH_x metallenes were synthesized via a similar procedure, except that In(acac)₃ (10.0 mg) and Sn(acac)₂ (10.0 mg) were utilized instead, respectively.

Synthesis of PdH_x metallenes

The synthesis step was the same as PdPbH_x metallenes except that Pb(acac)₂ was removed from the second step.

Synthesis of PdPb metallenes

The synthesis step was the same as PdPbH_x metallenes except the third step was removed.

Material characterizations

Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed on a JEOL JEM-2100F transmission electron microscope (Japan). Scanning transmission electron microscopy (STEM) images and corresponding energy-disperse X-ray spectroscopy (EDS) mapping, and electron-energy loss spectroscopy (EELS) were obtained by a JEOL ARM200CF (Japan). For atomic force microscopy (AFM) characterization, samples were deposited onto mica wafers and analyzed in tapping mode under ambient air conditions via the ScanAsyst (Dimension Icon, Veeco Instruments/Bruker, Germany). X-ray photoelectron spectroscopy (XPS) data were collected using a Thermo Fisher Scientific ESCALAB-250 instrument (USA) with a monochromatic Al-Kα radiation source (1486.6 eV) and a hemispherical detector (0.1 eV energy resolution). Inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements and X-ray diffraction (XRD) patterns were conducted using an ELAN 9000/DRC system and a Bragg-Brentano diffractometer (D8-tools, Germany), respectively. X-ray absorption spectra of the Pd K-edge and Pb L3-edge were collected in both transmission (for Pd foil, Pb foil, PdO, and PbO) and fluorescence (for the sample) mode at Shanghai Synchrotron Radiation Facility (SSRF) on beamline BL14W1.

Electrochemical measurements

The as-prepared Pd-based metallenes are stored in an ethanol solution, with the concentration of Pd determined by ICP-AES to be approximately 1.0 mg_Pd/mL. For catalytic evaluation, 5 mL of metallene solution (containing 5 mg of Pd) and 45 mg Ketjen carbon were added in 45 mL ethanol and then sonicated for 20 min to achieve a homogeneous dispersion. Subsequently, the sonicated mixture is subjected to an additional minute of oscillation. The ultrasonic and oscillating processes were repeated three times to ensure the catalyst was fully loaded on the Ketjen carbon. Then the solution was centrifuged and poured off the supernatant, dried in a 60 °C vacuum drying oven for 12 h. For the preparation of the electrochemical test catalyst ink, 1 mg of the as-synthesized PdPbHₓ metallenes/C powder was dispersed into a 1 mL mixture consisting of 750 μL deionized water, 245 μL isopropanol, and 5 μL of 5 wt% Nafion solution. This mixture underwent 30 min of sonication in an ice-water bath to form a homogeneous dispersion. The resulting uniform catalyst ink was then utilized for fabricating the working electrode. As a comparative benchmark, the commercial 20 wt% Pt/C catalyst from Johnson-Matthey was employed.

Electrochemical measurements were conducted using a three-electrode configuration on an Autolab electrochemical workstation. The working electrode consisted of a catalyst-coated glassy carbon electrode (GCE, 5 mm diameter) mounted on a rotator, while a platinum sheet (1.0 cm²) served as the counter electrode and a Hg/HgO electrode acted as the reference electrode. The preparation method of 0.1 M KOH electrolyte is as follows: dissolving 2.8 g of KOH in 500 mL of deionized water, followed by ultrasonication to ensure a uniform solution. The 0.1 M KOH electrolyte was freshly prepared before electrochemical measurements and stored in a sealed container at 25 °C. The calibration potential was approximately equal to the calculated value based on the Nernst equation (the measured pH value of 0.1 M KOH solution was 13.3 ± 0.3 under the actual ORR testing process). All the potentials were converted to the potentials referring to the RHE. Prior to catalyst deposition, the glassy carbon electrode was sequentially polished with alumina polishing slurries, rinsed thoroughly with deionized water, and left to air-dry naturally. Subsequently, 10 μL of the ink was dropped on a GCE to form a smooth thin film, yielding a Pd loading of 1.0 μg (5.1 μg cm⁻²). For Commercial Pt/C, the Pt loading of 2.0 μg (10.2 μg cm⁻²) were added to the GCE to obtain a similar limited diffusion current with other catalysts. The ECSA (m²·g⁻¹_PGMs) of the catalysts was calculated according to the equation:

$${{{\rm{ECSA}}}}=\frac{{{{\rm{Q}}}}}{{mq}}$$

(1)

where Q represents the surface charge for oxygen desorption, q is the charge required for the reduction of a PdO monolayer (424 μC cm⁻²) and m is the Pd loading amount. For ECSA, the cyclic voltammogram (CV) was obtained in N₂-saturated 0.1 M KOH with a scan rate of 50 mV s⁻¹. ORR polarization curves were measured in an O₂-saturated 0.1 M KOH electrolyte, employing a scan rate of 10 mV s⁻¹ and a rotation speed of 1,600 rpm. iR-correction has been applied to the recorded ORR curves before determining the kinetic current (I_k) based on the Koutecký-Levich equation:

$${I}_{k}=\frac{{I}_{d}\,*\,I}{{I}_{d}-I}$$

(2)

where I_d is the limiting current density, and I is the measured current density at 0.9/0.95 V versus RHE. For each catalyst, the specific activity and mass activity were derived by normalizing the kinetic current against the corresponding electrochemical active surface area (ECSA) and the metal (platinum group metals, PGMs) mass, respectively. All activities were obtained by averaging at least three patches of materials and corresponding performances.

In the accelerated durability tests, a graphite rod was indeed used as the counter electrode, which could prevent Pt from the counter electrode dissolving and redeposition on the working electrode during the long hours of the test. The accelerated durability tests were conducted by cycling between 0.6 V and 1.0 V versus RHE at 200 mV s⁻¹ for 50,000 cycles. For comparison, the ORR performance of the commercial Pt/C was also measured through similar procedures.

EIS measurements were conducted at the open-circuit potential (OCP) by applying a sinusoidal AC perturbation with a frequency range from 100 kHz to 10 mHz and an amplitude of 10 mV. Prior to the measurement, the OCP was stabilized with a fluctuation of less than 1 mV/min. All recorded potentials were corrected for ohmic drop using iR compensation (R_s = 40 ± 5 Ω in 0.1 M KOH).

Zn-air battery test

The anode and cathode have used a polished zinc foil (0.5 mm thickness) and a catalyst-coated hydrophobic carbon paper (Spring-P2), respectively. The electrolyte uses a solution composed of 0.2 M Zn(AC)₂ and 6 M KOH. The catalyst ink was prepared by ultrasonically mixing 3 mg catalysts and 1 mg Vulcan XC 72 carbon in 600 μL dispersion solution (volume ratio of water/isopropanol/Nafion =44.2/55.2/0.6). To fabricate the cathode, the catalyst ink was then deposited onto the carbon paper via drop-casting, with a catalyst loading of 1.0 mg cm⁻² (0.1 mg_{Pd or Pt} per cm²). By using the LAND CT3002A instrument, the discharge polarization curve was recorded.

Membrane electrode assembly (MEA) fabrication and fuel cell tests

The catalyst at the cathode and anode is 20 wt% PdPbH_x/C or commercial 20 wt% Pt/C with a metal loading of 0.3 mg_Pd/Pt cm⁻² and commercial 60 wt% PtRu/C (Shanghai Hesen Electric Co., Ltd) with a metal loading of 0.6 mg PtRu mg_PtRu cm⁻², respectively. The cathode ink was obtained by mixing the catalyst powder with water, isopropanol (volume ratio of 1:10), and 30 wt% home-made ionomer⁵⁶ (m_ionomer/(m_ionomer+ m_catalyst) = 30 wt%). The anode slurry was also prepared using the same method mentioned above, except that the amount of ionomer is 23 wt% (m_ionomer/(m_ionomer+ m_catalyst) = 23 wt%).

A catalyst-coated membrane (CCM, ~4 cm²) was prepared by sequentially spraying cathodic and anodic slurry onto each side of the commercial membrane (Alkymer® W-25 alkaline membrane. Thickness: 25 μm). Afterwards, the CCM was immersed in 1 M NaOH aq. at 60 °C for 24 h (to promote the ion exchanging Cl⁻ with OH⁻ in alkaline conditions, ensuring the membrane’s suitability for alkaline environments), followed by washing 3 times with deionized water, and then MEA was fabricated by sandwiching the CCM between two carbon papers (Sunrise Power Co., Ltd.) at 25 °C and a gauge pressure of 0.2 MPa for 2 min.

The single cell performance was conducted on a home-made test system at 80 °C by feeding O₂/H₂ (100% relative humidity, RH) with a 400/400 mL min⁻¹ flow rate at the cathode and anode. A durability test was performed at 80 °C and 100 mA cm⁻².

XAFS data analysis

The collected XAFS data underwent systematic processing in Athena software (version 0.9.26), involving background subtraction, pre-edge linear correction, and post-edge linear calibration. Subsequent Fourier transform fitting analysis was conducted using Artemis (version 0.9.26). For Pd fitting, the parameters were configured as: k³-weighting, k-range from 3 to ~12 Å⁻¹, and R-range from 1 to ~3 Å; while for Pb fitting, the k-range was adjusted to 2 to ~10 Å⁻¹ (with the R-range maintained at 1 to ~3 Å). Four critical parameters- coordination number (CN), bond length (R), Debye-Waller factor (σ²), and E₀ shift (ΔE₀)- were optimized without imposing fixed constraints or correlation limitations. For Wavelet Transform (WT) analysis, the χ(k) data extracted from Athena was imported into the Hama Fortran code. The specific parameters for this analysis were defined as: R-range of 1–4 Å; k-range of 0– ~ 13 Å⁻¹ (for Pd) and 0– ~ 10 Å⁻¹ (for Pb); k-weight value of 2; and a Morlet mother wavelet with κ = 10 and σ = 1, which facilitated comprehensive distribution characterization.

XRD data refinement

The XRD data were refined using the Rietveld method with the HighScore software to optimize the reliability and structural parameters.

Quasi in situ XPS measurement

The PdPbH_x sample was dropped onto three carbon papers, each with a Pd loading of 1 mg. These carbon papers were then subjected to chronoamperometry (i-t) tests at constant potentials of 0.7 V, 0.9 V, and 1.1 V for 200 s, respectively. Upon completion of the tests, the carbon papers were promptly transferred into centrifuge tubes and sealed under an argon atmosphere to prevent oxidation. Subsequently, XPS characterization was conducted to analyze the chemical states of the samples.

In situ ATR-FTIR measurement

Surface-enhanced infrared absorption spectroscopy (SEIRAS) in attenuated total reflection (ATR) configuration was adopted for the measurements. Electrochemical ATR-SEIRAS characterization was performed using a Thermo Nicolet 8700 spectrometer, which was outfitted with a liquid nitrogen-cooled MCT detector. During the testing process, 0.1 M KOH was used as the electrolyte, which, due to the low alkalinity, will not cause the gold film to detach from the prism. Additionally, to avoid any impact from multiple tests on the gold film, we only performed one oxygen reduction test at full voltage on each gold film. 30 μL of the catalyst ink was applied onto the gold-film working electrode and left to air-dry. Subsequently, the ink-coated silicon column- functioning as the prism with dimensions of 10 mm radius and 25 mm height- was integrated into a custom-built spectro-electrochemical cell to serve as the working electrode. The Hg/HgO was used as the reference electrode, which was introduced near the working electrode via a Luggin capillary while the Pt mesh (1 cm × 1 cm) served as the counter electrode. All spectra were normalized using the differential reflectance formula:

$$\frac{\varDelta R}{R}=\frac{{E}_{s}-{E}_{R}}{{E}_{R}}$$

(3)

where Es and E_R represent the sample and reference spectra, respectively. For the spectra results, all the spectral resolutions were 4 cm⁻¹. The reference spectrum was recorded at 0 mV and the sample spectra were collected at the potentials from 1000 mV to 1600 mV (vs RHE) with the successive potential step of 100 mV.

Calculation setup

In this work, all the density functional theory (DFT) calculations are carried out by the CASTEP package to investigate the electronic structures and reaction trends of PdPb, PdH_x, and PdPbH_x for ORR⁵⁷. In particular, we have selected the generalized gradient approximation (GGA) and Perdew-Burke-Ernzerhof (PBE) functionals to supply accurate descriptions of the exchange-correlation interactions^58,59,60. We have selected the ultrasoft pseudopotentials with ultrafine quality for the plane-wave cutoff energy, which has been set to 380 eV by default. For further increases of the cutoff energy to 400 eV, the total energy of PdPbH_x test model only shifts a very minor scale of around 11 meV, which justifies the use of this cutoff energy setting. The Broyden-Fletcher-Goldfarb-Shannon algorithm with the selection of coarse k-point has been applied for the energy minimization processes in this work⁶¹. The PdPb surface has been cleaved from the (111) surface of Pd with a four-layer thickness in a 5 × 5 × 1 supercell, where 9% Pd atoms are replaced by the Pb atoms. For PdH_x, the metallene structure has been cleaved from the (111) surface of PdH with a three-layer thickness, where x is close to 0.3. The PdPbH_x is further constructed by replacing 9% Pd atoms with Pb atoms, which matches the experimental characterizations. The Pd metallene has been cleaved from the Pd(111) surface with four-layer thicknesses in a 5 × 5 × 1 supercell, where the PdPb metallene is constructed by replacing 9% Pd atoms with the Pb atoms. The bulk Pt surface is cleaved from the Pt(111) surface with an eight-layer thickness in a 5 × 5 × 1 supercell. The structures are supplied in Supplementary Data 1. For all the surfaces, 20 Å vacuum space has been introduced in the z-axis to guarantee complete relaxation.

To achieve sufficient geometry optimizations, a series of strict convergence criteria are applied, which include that the Hellmann-Feynman forces should be converged to less than 0.001 eV/Å, the total energy difference should be converged to smaller than 5 × 10⁻⁵eV/atom, and the maximum displacement for each atom should be smaller than 0.005 Å. In this work, we have performed DFT calculations based on the computational hydrogen electrode (CHE) model proposed by Nørskov et al. ¹⁸ The equilibrium potential on a reversible hydrogen electrode (RHE) is set to be the reference of applied potential. When an electrode potential U is applied, the chemical potential of the electron will be reduced by eU, which has been considered in all states involving an electron in the electrode. The limiting potential U_L is then determined when all the reaction steps become spontaneous. The free energy (∆G) is calculated based on

$$\Delta {{{\rm{G}}}}=\,\Delta {{{\rm{E}}}}+\Delta {{{\rm{ZPE}}}}-{{{\rm{T}}}}\Delta {{{\rm{S}}}}$$

(4)

where the ∆ZPE and ∆S are the changes of the zero-point energy and entropy, respectively at T = 298.15 K and 1 atm. The MD simulations have been carried out under 298 K and NVT mode in PdPbH_x, PdH_x, and PdPb metallene for 5 ps, where each step is 1 fs with a total of 5000 simulation steps (Supplementary Data 1).

Data availability

The data generated in this study are provided in the Supplementary Information and Source Data file. Source data are provided in this paper. Source data are provided with this paper.

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Acknowledgements

J. Fan and X. Cui acknowledged the support of the National Natural Science Foundation of China (12034002 (X.C.), 22279044 (X.C.), and 22402064 (J.F.)). B.H. acknowledged the support of the National Key R&D Program of China (2021YFA1501101), Research Grant Council of Hong Kong (15304023, 15304724, C1003-23Y), National Natural Science Foundation of China/Research Grant Council of Hong Kong Joint Research Scheme (N_PolyU502/21), the National Natural Science Foundation of China/Research Grants Council of Hong Kong Collaborative Research Scheme (CRS_PolyU504/22), the Shenzhen Fundamental Research Scheme-General Program (JCYJ20220531090807017), and the Natural Science Foundation of Guangdong Province (2023A1515012219).

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These authors contributed equally: Yu Qiu, Mingzi Sun.

Authors and Affiliations

School of Materials Science and Engineering, Key Laboratory of Automobile Materials of MOE, Electron Microscopy Center, Jilin University, 2699 Qianjin Street, Changchun, 130012, China
Yu Qiu, Jiandong Wu, Shengwei Wang, Hong Huang, Xiao Zhao, Dongxu Jiao, Shan Xu, Dewen Wang, Xin Ge, Wei Zhang, Weitao Zheng, Jinchang Fan & Xiaoqiang Cui
Department of Chemistry, City University of Hong Kong, Kowloon, Hong Kong SAR, China
Mingzi Sun & Bolong Huang
Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China
Mingzi Sun & Bolong Huang
Key Laboratory of Pathobiology of MOE, Nanomedicine and Translational Research Center, The Third Bethune Hospital of Jilin University, 126 Sendai Street, Changchun, 130033, China
Jiandong Wu
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian, 116024, China
Chunxiao Chai & Yujiang Song

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Yu Qiu
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Mingzi Sun
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Jiandong Wu
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Chunxiao Chai
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Shengwei Wang
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Hong Huang
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Xiao Zhao
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Shan Xu
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Xin Ge
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Yujiang Song
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Contributions

J.F., B.H. and X.C. supervised the execution of the overall project. Y.Q. designed and performed the synthetic and electrochemical experiments, characterized the catalyst, and analyzed the data. M.S. and B.H. conducted the DFT calculations. J.W. assisted with the idea design of catalysts. S.W. assisted with the data analysis. H.H. and X.Z. assisted with the electrochemical ORR tests. D.J. assisted with the calculations of the ORR mechanism. S.X. assisted with the drawing of the schematic diagram. D.W. assisted with the ssNMR tests. X.G. and Wei Z. performed the atomic-resolution HAADF-STEM tests. C.C. and Y. S. performed the alkaline H₂-O₂ fuel cell test. Weitao Z. assisted with the paper writing. The results of the manuscript were discussed by all authors.

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Correspondence to Jinchang Fan, Bolong Huang or Xiaoqiang Cui.

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Nature Communications thanks Zhao Cai and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

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Qiu, Y., Sun, M., Wu, J. et al. Boosting oxygen reduction performances in Pd-based metallenes by co-confining interstitial H and p-block single atoms. Nat Commun 16, 5262 (2025). https://doi.org/10.1038/s41467-025-60400-5

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Received: 06 November 2024
Accepted: 20 May 2025
Published: 06 June 2025
DOI: https://doi.org/10.1038/s41467-025-60400-5