Boosting the Acidic Oxygen Reduction Activity of p‐Block Single‐Atomic Catalyst via p–p Orbital Coupling and Pore Engineering

Atomically dispersed main group element single‐atom catalysts (SACs) have recently attracted increasing attention in electrocatalysis. However, their performances in acidic oxygen reduction reaction (ORR) remain unsatisfactory owing to the suboptimal coordination environment, limited mass transfer, and active site exposure. Herein, a series of p‐block Sn SACs with hierarchical pore structures are prepared by a dual melting salt‐mediated soft template method. By deliberately regulating the pore structures, highly exposed Sn active sites with N/O coordination are obtained, which endow SnN3O‐50 with exceptional ORR performances, especially in acidic medium. The half‐wave potential of SnN3O‐50 is up to 0.816 V, with a loss of only 15 mV after 10 000 potential cycles. Furthermore, the peak power densities of the fuel cell and zinc–air battery assembled using SnN3O‐50 as cathodes reach 502 and 173.5 mW cm−2, respectively, demonstrating great potential for practical applications. Density functional theory (DFT) calculations reveal that the N/O coordination of Sn induces localization of 5p electrons, which leads to strong coupling with the p orbit of O2. Meanwhile, the presence of defects synergistically regulates the adsorption of reaction intermediates, thereby optimizing the free energy of the four successive ORR steps.


Introduction
The use of proton exchange membrane fuel cells (PEMFC) and metal-air batteries (MAB), offering the advantages of low carbon emission and environmental friendliness, is regarded as an efficient strategy for mitigating global warming and achieving carbon neutrality. [1,2]Boosting the oxygen reduction reaction (ORR) at the cathodic side is considered one of crucial steps As ORR occurs at the gas-liquid-solid three-phase boundary, the accessibility of the catalytically active sites and the mass transfer of O 2 and reaction species are crucial factors affecting the overall performance of ORR.Ideally, only single atoms at the three-phase interface are true active sites.Although pure micropores are capable of anchoring more metal single atoms, a large number of active sites are inevitably buried within the carbon matrix, leading to low site accessibility, [30] Taking this into consideration, the construction of mesoporous [31] and macroporous structures with optimal mass transfer channels is highly required. [32]The introduction of hierarchical pores through pore engineering is undoubtedly one effective means of boosting the electrochemical performance of SACs. [33,34]Generally, the formation of porous structures mainly depends on the synthesis strategy, and the structures obtained by direct pyrolysis of carbon precursors (metal-organic frameworks [MOFs [35][36][37] for instance) are mainly microporous.Although the formation of mesopores and macropores can be promoted by introducing templates (SiO 2 , etc.), [38] corrosive HF acid or strong alkaline solutions are required for subsequent template removal, which is an undesirably complex, dangerous, and environmentally unfriendly process.Hence, there is a need for "green" and easy-to-implement methods of forming hierarchical pore structures to promote mass transfer of ORR.Recently, the molten salt method has gained research attention in regulating the structure of carbon matrix, in which the salt systems play the dual role of a solvent/reaction medium and soft template at high temperatures. [39]n contrast with conventional templating methods, the pores are not generated at the expense of the carbon yield, and the removal of salts can be easily accomplished by washing with water and dilute acids.Moreover, the molten salts can act as protective agents to prevent the elimination of organic fragments, which reduces the loss of heteroatoms.
In this study, a p-block Sn SAC with hierarchical pore structures was prepared using a simple dual molten salt-mediated soft templating strategy, with high N-containing guanine as the carbon source.The Sn SAC has a specific surface area of up to 1628 m 2 g À1 and abundance of micro-and mesopores, allowing for a high utilization of the SnN 3 O active sites.The Sn SAC exhibits excellent ORR activity and stability in both acidic and alkaline solutions.The half-wave potentials in acidic and alkaline media are 0.816 and 0.905 V, respectively, with a loss of only 15 mV after 10 000 potential cycles, which exceeds the performance of most previously reported noble metal-free catalysts.The peak power densities of the fuel cell and zinc-air battery (ZAB) assembled using Sn SAC as cathodes are 502 and 173.5 mW cm À2 , respectively, indicating that Sn SAC is a very promising candidate for practical ORR applications.Density functional theory (DFT) calculations reveal that O 2 adsorption is greatly enhanced by p-p orbital coupling, which is a major rate limiting step for most of the non-noble metal catalysts.The coordination of N and O promotes the localization of the outer p-electrons, and the synergistic defect structure jointly optimizes the adsorption of oxygen-containing intermediates.Currently, studies of s/p-block SACs are mainly focused on alkaline ORR, and their performances under acidic conditions are still far from satisfactory.This work provides an effective approach to addressing this issue.

Results and Discussion
The process for preparing the Sn SAC is illustrated in Figure 1a.By varying the amount of Sn powder, heating temperature, and carbon precursor, a series of control samples were also prepared under the same conditions to investigate their effects on the structure and performances (details can be found in the Supporting Information).As an inexpensive nitrogen-rich compound, guanine has a characteristic bicyclic structure composed of carbon and nitrogen atoms, which decomposes at 360 °C.This is higher than the melting point of NaCl/ZnCl 2 dual salts used (260 °C, 1/1 by weight, Figure S1, Supporting Information).As a result, a similar sol-gel process occurs, in which the molten salts act as a solvent to achieve the atomic-scale mixing of reactants and prevent collapse of the cohesive carbon species (sol-carbon) formed. [40]The pore structure of the generated carbon is regulated by the ion pair and size of the salt cluster.Due to the strong polar effect of the molten salts, the Sn-Sn bonds of Sn powder gradually break with the increase of temperature, and the Sn atoms formed are trapped and anchored by the abundant N-and O-containing fragments that derived from guanine, forming Sn SAC. [41,42]he morphology of the prepared Sn SAC is shown in Figure 1b and S2, Supporting Information.SnN 3 O-50 has a porous structure consisting of many irregular sphere-like particles stacked together, while the sample from the direct pyrolysis of guanine at 900 °C (N-NC, N-NC-Sn) has a distinct lamellar shape with no obvious mesopores (Figure S3-S5, Supporting Information), showing the molten salts have a strong structure-directing effect on carbon.Transmission electron microscopy (TEM) images in Figure 1c and S6, Supporting Information, show that SnN 3 O-50 possesses a well-developed porous structure with no Sn nanoparticles or clusters.To further verify the existing form of Sn species, high-angle annular darkfield scanning transmission electron microscopy (HADDF-STEM) tests were performed.As shown in Figure 1d, an abundance of isolated bright spots is observed, as partially marked by red circles, demonstrating the presence of Sn single atoms.The strong polarization force of the dual molten salts destabilizes the Sn-Sn bonds and effectively inhibits aggregation of the Sn atoms (Figure S7, Supporting Information).Energy-dispersive X-ray (EDX) elemental distribution mappings indicate that N and Sn are uniformly distributed on the carbon substrate.The Sn content of Sn SAC, as determined by inductively coupled plasma optical emission spectrometer (ICP-OES), is 0.35%.
The X-ray diffraction (XRD) patterns of prepared Sn SACs are shown in Figure 2a.The two broad diffraction peaks at 23.7°and 43.7°correspond to the (002) and (101) crystallographic planes of carbon, respectively.No peaks correspond to Sn, Sn carbide, or Sn oxide is observed.This is in agreement with HADDF-STEM results.The defects of the sample were analyzed by Raman spectroscopy.As shown in Figure 2b and S8, Supporting Information, the peaks at 1350 and 1590 cm À1 correspond to disordered (D band) and graphitized (G band) carbon structures, respectively.The peak I centered at 1184 cm À1 is associated with sp 3 -C carbon located at edges and holes, while the peak D 00 located at 1510 cm À1 can be attributed to the presence of amorphous carbons such as C 5 ring, heteroatoms, or C-H vibrations in hydrogenated carbons.Comparison of the magnitude of the I D /I G ratios shows that SnN 3 O-50 is highly defective among the samples, which is favorable for mass transfer and exposure of active site during ORR.The specific surface area and pore structure of SnN 3 O-50 were determined by N 2 isothermal adsorption/ desorption.As present in Figure 2c, the isotherm of SnN 3 O-50 is classified as type IV(a).The significant H 2 -type hysteresis loop at p/p 0 of 0.5 indicates the existence of ink-bottle type mesopores with a relatively narrow pore size, while the steep N 2 adsorption at relatively low pressure (p/p 0 = 0-0.012)reflects the existence of micropores.The specific surface area of SnN 3 O-50 calculated by Brunauer-Emmett-Teller model (BET) is as high as 1628 m 2 g À1 .In sharp contrast, the BET-specific surface area of N-NC and N-NC-Sn obtained by direct pyrolysis of guanine is only 34 and 100 m 2 g À1 , respectively (Figure S9a, Supporting Information), highlighting the advantages of the molten salt method.The lack of micropores and mesopores of N-NC and N-NC-Sn can be clearly seen from the pore size distribution in Figure 2d and S9b, Supporting Information. [43,44]SnN 3 O-50 prepared by the dual molten salt strategy has a more developed micropore/mesopore structure, where formation of the abundant micropores is promoted by the high-temperature evaporation of ZnCl 2 , while the appropriate content of NaCl can promote mesopore and macropore formation. [45]The abundant micropores provided rich sites to anchor Sn single atoms, while the mesopores and macropores facilitate mass transfer and improve utilization of the SnN 3 O sites during the ORR.To further verify the synergistic effect of the hierarchical porous structure, a series of Sn SACs were prepared using different carbon  S15, Supporting Information).The pore size of the SnN 3 O-ZIF-8 is relatively narrow and is concentrated at %2 nm, indicating a predominantly microporous structure.In contrast, the pore size of SnN 3 O-ZIF-L is concentrated at %3 nm, and some pores with dimensions of 4-10 nm are also observed.The above results (Figure 2d and S15, Supporting Information) indicate that pore structures can be deliberately regulated by varying the carbon precursors and preparation methods, so that their effect on ORR performance can be investigated (as discussed in the electrochemical section).
The valence states and compositions of the Sn SACs were analyzed by X-ray photoelectron spectroscopy (XPS).SnN 3 O-50 is confirmed to contain four elements, Sn, N, C, and O (Figure 3a).The Sn 3d spectrum in Figure S16, Supporting Information, shows a Sn 3d 5/2 peak at 486.2 eV, indicating that the valence state of Sn in SnN 3 O-50 is between 0 (484.5-485.5 eV) and þ4 (486.3-487.3eV). [46]The highresolution N 1s spectrum shows a distinct Sn-N bond at 399.0 eV in addition to pyridine (398.4 eV), pyrrole (400.1 eV), and graphite N (401.4eV), indicating direct coordination of Sn to N atoms (Figure 3b). [47]The N configurations in NC, SnN To further investigate the coordination environment of the Sn SACs, X-ray absorption spectroscopy (XAS) measurement at the Sn K-edge was performed.The X-ray adsorption near edge structure (XANES) spectra of SnN 3 O-50 show that the absorption edge of Sn single atoms is located between that of Sn foil and SnO, indicating Sn transfers partial electrons to the surrounding carbon substrate due to the strong electronic interaction between them.The oxidation state of isolated Sn atoms in SnN 3 O-50 is less than þ2 (Figure 3d).The Fourier transformed extended X-ray absorption fine structure (FT-EXAFS) spectra of the samples are shown in Figure 3e.For SnN 3 O-50, the position of the main peak at %1.5 Å is very close to that of SnO 2 , which can be attributed to Sn-O/Sn-N coordination, and they are difficult to distinguish in R-space due to the close atomic number of N and O. [48] No characteristic signal of Sn-Sn bond is detected at %2.77 Å, further confirming the atomic dispersed state of Sn atoms.The wavelet transform (WT) of EXAFS is an effective method of distinguishing atomic neighbors surrounding a central atom because the weight of the atomic pair is positively correlated with the coordinates of K-space. [10]As shown in Figure 3g, the peak at about 8.4 Å À1 in K space belongs to the Sn-Sn pair in Sn foil, while the peak at about  S1, Supporting Information).Based on these results, a most probable structure model of SnN 3 O-50 is shown in the inset of Figure 3f.
The ORR activity of the Sn SACs was first evaluated using a rotating disc electrode (RDE) in O 2 -saturated 0.1 M HClO 4 solution (Figure 4a).It is clear from the linear sweep voltammetry (LSV) curves that SnN 3 O-50 has excellent ORR activity in acidic media, with an onset potential (Eonset) of 0.886 V and a halfwave potential (E 1/2 ) of up to 0.816 V (correspond to the potential at 3 mA cm À2 ), exceeding that of most reported noble metal free catalysts (Table S2, Supporting Information).This value is only 25 mV less than that of Pt/C (E 1/2 = 0.841 V) and is significantly higher than that of the NC (E 1/2 = 0.738 V), indicating that the activity of SnN 3 O-50 mainly originates from Sn related active sites.To further verify the true active sites of SnN 3 O-50,  S24, Supporting Information). [49]The reaction path of SnN 3 O-50 was analyzed by RDE at different rotational speeds and the associated K-L curves are plotted in Figure S25a, Supporting Information, yielding an electron transfer number of about 4. The yield of hydrogen peroxide (H 2 O 2 ) was further evaluated by using a rotating ringdisk electrode (RRDE).As shown in Figure 4b, the H 2 O 2 yield of SnN 3 O-50 in the range of 0.2-0.8V is only 0.5-1.7% with an  4c).Unlike Pt/C catalysts, which are susceptible to methanol toxicity, SnN 3 O-50 shows excellent methanol tolerance without any decay in the current density after methanol injection, showing it is a reliable cathodic catalyst for direct liquid fuel cells (Figure S26a, Supporting Information).Stability is an important parameter for assessing the ORR performance of the catalysts.After running in an O 2 -saturated 0.1 M HClO 4 solution at 1600 rpm and 0.7 V for 14 h, the current density of SnN 3 O-50 only decays by 18%, which is much better than that of the corresponding Pt/C control (61%) (Figure 4d).The durability of the catalysts was further evaluated by accelerated aging experiments.After 10 000 cyclic voltammetry (CV) cycles, the E 1/2 of SnN 3 O-50 declines by only 15 mV (Figure 4e).This impressive durability can be attributed to the inhibition of H  4f ).at lower potential range.Therefore, appropriate mesoporous and macroporous structures can facilitate the transport of reaction intermediates and improve accessibility of the active sites, which is one effective way to enhance the ORR activity in acidic media.
The ORR performance of Sn SACs is equally excellent under alkaline conditions.As shown in Figure 4g, in 0.1 M KOH solution, SnN 3 O-50 shows an E onset of 1.07 V and E 1/2 of up to 0.905 V, which is higher than that of the NC (0.850 V), SnN 3 O-30 (0.860 V), SnN 3 O-40 (0.890 V), SnN 3 O-60 (0.840 V), Pt/C (0.855 V), and exceeds that of most reported alkaline ORR catalysts (Table S3, Supporting Information).The Tafel slope of SnN 3 O-50 in alkaline solution is 66 mV dec À1 , which is slightly higher than its corresponding value under acidic conditions, but much lower than that of commercial Pt/C (110 mV dec À1 ) and NC (78 mV dec À1 ), indicating that SnN 3 O-50 also has the best reaction kinetics in alkaline media (Figure 4h).To determine the reaction path of SnN 3 O-50 under alkaline conditions, RDE tests were carried out at different rotational speeds and the associated K-L plots were constructed, yielding an electron transfer number of n = 4.08 (Figure S25, Supporting Information).It was further verified by RRDE tests that the electron transfer number of SnN 3 O-50 is 3.96-4 in the range of 0.2-0.8V and the H 2 O 2 yield is only 0.3-1.9%(Figure 4i).The data above show that SnN 3 O-50 also has excellent reaction path selectivity in alkaline solutions and the efficient four-electron path dominates.As shown in Figure S26b, Supporting Information, the current density of SnN 3 O-50 well maintains after the addition of methanol, whereas the current density of Pt/C decreases sharply, indicating that SnN 3 O-50 has good resistance to methanol poisoning under alkaline conditions.The current density of SnN 3 O-50 decays by only 3% after 12 h at 1600 rpm and 0.7 V, whereas that of Pt/C decays by 15% (Figure 4j).Subsequent antiaging experiments further demonstrated the excellent stability of SnN 3 O-50, even under conditions closer to those of practical use.After 10 000 CV cycles, the limiting current density of SnN 3 O-50 hardly changes, and the E 1/2 decreases by only 12 mV, much less than Pt/C (35 mV, Figure S28, Supporting Information).Notably, due to the contribution of carbon substrate to ORR, the performance difference of Sn SACs with different pore structures is relatively small under alkaline conditions (Figure S29, Supporting Information).However, SnN 3 O-50 still shows higher ORR activity than that of SnN 3 O-ZIF-8 and SnN 3 O-ZIF-L and the LSV slope of SnN 3 O-50 maintains the highest throughout the potential range, indicating pore engineering is a universal method to enhance the ORR performance of the catalysts (Figure 4k).In summary, SnN 3 O-50 demonstrates excellent electrochemical activity and stability in both acidic and alkaline media and is a very promising non-Pt catalyst for ORR.
DFT calculations were performed to theoretically interpret the origin of the superb ORR activity of SnN 3 O-50.The ORR activity of a given surface is related to the magnitude of adsorption energy of the reaction intermediates (OOH*, OH*), which scale roughly with that of O* (ΔE O ). [50] A smaller ΔE O will leads to strong adsorption of OOH* and difficult removal of OH* and vice versa.The optimum ΔE O is roughly 0.2 eV weaker than that of Pt (111). [51]To screen the most possible structure, six models with slight difference in Sn coordination were established.Furthermore, their corresponding defective models are also established to highlight the important role of pore structures (Figure S30, Supporting Information).As shown in Figure 5a The type and number of coordination atoms, and vacancy effect both determine the catalytic activity of the single metal center.Moreover, previous works have shown that the long-range interactions of the active sites with heterogeneous atoms on the substrate also influence the electronic structure and thus the catalytic activity of the single metal center. [23,29]As shown in Figure S31 5d). [27,52] Inspired by the outstanding ORR performance of SnN 3 O-50 in both acidic and alkaline media, PEMFC and ZAB were assembled to demonstrate its viability in real devices.As shown in Figure 6a, the Sn-based PEMFC demonstrates high activity in the kinetic region of E ≥ 0.7 V due to the high ORR activity of SnN 3 O sites, well consistent with the LSV results, which can be attributed to the enhanced mass transfer of the hierarchical pore structures.The peak power density is an important indicator to evaluate the PEMFC.The Sn-based PEMFC reaches a maximum power density of 502 mW cm À2 , which is superb among the state-of-the-art platinum group metal (PGM)-free cathode-based PEMFCs.The corresponding current density at 0.7 V is up to 201 mA cm À2 (Figure 6a).The performances of some representative PGM-free ORR catalyst-based PEMFCs are summarized and, compared in Table S3, Supporting Information, highlight the practical application potential of SnN 3 O-50 in acidic medium.
Furthermore, primary and rechargeable ZAB were assembled to further access the practical performance of SnN 3 O-50 in alkaline medium.The structure of the ZAB is illustrated in Figure 6b.As shown in Figure 6c, the open-circuit potential (OCP) of the Sn-based ZAB is as high as 1.57V, nearly 0.1 V higher than that of Pt-based ZAB and is very close to the theoretical voltage of 1.65 V, [53] calculated from the thermodynamics.The OCP of the Sn-based ZAB maintains at 1.57 V even after standing for 40 h, demonstrating excellent chemical stability.The current density of the Sn-based ZAB at 0.4 V is as high as 360 mA cm À2 under static atmospheric conditions, with peak power density of 176 mW cm À2 .This is 57.6% higher than that of Pt-based ZAB (112 mW cm À2 , Figure 6d).The rate capabilities of the ZABs are shown in Figure 6e.The discharge plateaus of the Sn-based ZAB at 2, 5, 10, 20, 40, and 80 mA cm À2 are 1.39, 1.35, 1.32, 1.28, 1.22, and 1.10 V respectively, much higher than those of Pt-based ZAB.At a current density of 5 mA cm À2 , the Snbased ZAB has a specific capacity of 802 mAh g Zn À1 , while the Pt-based ZAB is only 743 mAh g Zn À1 (Figure 6f ).The stability and reversibility of the ZABs were then evaluated by adding RuO 2 into the catalyst layer.As shown in Figure 6g, the Sn-based ZAB exhibits excellent cycling stability for 200 h, and the discharge and charge voltages maintain at 1.124 and 1.982 V, respectively, after an initial activation process.This undoubtedly stems from the high ORR activity and durability of SnN 3 O-50.
In sharp contrast, the voltage gap of Pt-based ZAB increases with cycling, and the battery fails after 125 h.As demonstrated in Figure 6h and S37-S38, Supporting Information, the Sn-based ZABs are able to light up small light bulbs and LED panels.

Conclusion
In conclusion, we have prepared a series of p-block Sn SACs through a facile molten salt method.Benefiting from the soft template effects of the molten salts, catalysts with high specific surface area and hierarchical pore structures were obtained, allowing for a high utilization of the SnN 3 O active sites and easy mass transfer during ORR.These unique structure merits enable SnN
3 O-30, SnN 3 O-40, SnN 3 O-60, SnN3O-ZIF-8, and SnN3O-ZIF-L are similar to SnN 3 O-50 (Figure S17, S18, Supporting Information), and the percentages of various N configurations in the samples are summarized in Figure 3c.A Sn-O bond is observed in the high-resolution O 1s spectra (Figure S19, Supporting Information), indicating that Sn is also trapped by O atoms.

Figure 2 .
Figure 2. a) XRD patterns of the Sn SACs, b) the normalized Raman spectra of the Sn SACs, c) N 2 adsorption/desorption isotherms, and d) DFT pore size distribution curves of SnN 3 O-50 and N-NC.
7.1 Å belongs to the Sn-O pair in SnO 2 .For SnN 3 O-50, no Sn-Sn pair is detected, and the smaller peak coordinates than SnO 2 in K-space indicates the presence of a lighter atomic pair than Sn-O.Combined with the XANES analysis, it can be concluded that both Sn-N and Sn-O coordination exists in SnN 3 O-50.The first shell coordination number and atomic bond distance of the central Sn atoms were then obtained by least squares fitting of the EXAFS (Figure S21, S23, Supporting Information).The average first shell N/O coordination number of Sn is closed to 4 with a bond length of 2.0 Å (Table

Figure 4 .
Figure 4. a) LSV curves of the Sn SACs and Pt/C.b) H 2 O 2 yield and n of SnN 3 O-50, NC, and Pt/C.c) Corresponding Tafel plots from (a). d) i-t curves of SnN 3 O-50 and Pt/C recorded at 0.7 V. e) LSV curves of SnN 3 O-50 before and after 10 000 potential cycles.f ) LSV curves of SnN 3 O-50, SnN 3 O-ZIF-8, and SnN 3 O-ZIF-L.g) LSV curves of the Sn SACs and Pt/C.h) Corresponding Tafel plots from (h). i) H 2 O 2 yield and n of SnN 3 O-50, NC and Pt/C, j) i-t curves of SnN 3 O-50 and Pt/C recorded at 0.7 V. k) Schematic illustrating the ORR kinetics for different pore sizes (a-f: 0.1 M HClO 4 , g-j: 0.1 M KOH).
2 O 2 production and the structural robustness of the SnN 3 O sites.The LSV curves of SnN 3 O-ZIF-8 and SnN 3 O-ZIF-L are shown in Figure 4f.Despite their large specific surface area and abundant SnN 3 O sites, the E 1/2 of SnN 3 O-ZIF-8 and SnN 3 O-ZIF-L is still 144 and 56 mV lower than that of SnN 3 O-50, respectively, indicating that in addition to the construction of highly active sites, adequate exposure of the active sites also has an important contribution to the ORR.Notably, because of the fast mass transfer process, the slope of the LSV curve of SnN 3 O-50 is obviously larger than that of SnN 3 O-ZIF-8 and SnN 3 O-ZIF-L at lower potential range (Figure , the adsorption energy difference (ΔE O -ΔE O Pt ) of SnN 4 is 0.6 eV, indicating a relatively weak adsorption to O* compared to the ideal catalytic surface.The modification of Sn site by O further weakens its adsorption to O*, and the ΔE O -ΔE O Pt of SnN 3 O and SnN 2 O 2 increase to 0.79 and 1.04 eV, respectively.The SnN 3 O-1 and SnN 3 OH models with out-of-plane bridging O are unstable and structural reconstruction occurs when O* is adsorbed.It is interesting that the introduction of defects greatly enhances the adsorption of Sn active sites to O.The ΔE O -ΔE O Pt of SnN 3 O-D (D represents the defect model) is reduced to 0.16 eV, very close to the theoretical optimum (0.2 eV).The above results show that ΔE O is closely related to the local structure of the active sites.
, Supporting Information, when a C atom is replaced by a N atom in defect, the ΔE O -ΔE O Pt is further enhanced to À0.23 eV, which leads to inferior ORR performance compared with SnN 3 O-D.The ORR free-energy paths of SnN 3 O-D were then calculated based on the structure screening results (Figure S32-S34, Supporting Information).As shown in Figure 5b, the free energy of SnN 3 O-D presents a consistent downhill tendency at U = 0 V.The free energy gaps of the four successive steps are 1.328, 1.198, 1.325, and 1.068 V respectively, with absolute difference from the ideal value (1.23 eV) less than 0.2 eV.At the equilibrium potential of 1.23 V, the energy uphill of the rate-determining step (RDS) of SnN 3 O-D (*OH desorption) is only 0.16 eV, approximately onethird that of SnN 3 O model without defects (0.46 eV), which endows SnN 3 O-D with high ORR activity even at lower overpotentials.It is worth noting that the adsorption of O 2 to generate OOH* is exothermic for SnN 3 O-D and endothermic for SnN 3 O.The differential charge density and density of state (DOS) analyses were then performed to dig out the reasons behind.As shown in Figure 5c, the strong electronic interaction between Sn and neighboring atoms leads to a decreased charge density of Sn atoms, consistent with the XPS and XANES results.The delocalized electrons tend to cluster around the Sn atoms.The DOS of p orbits of O 2 , SnN 3 O, and SnN 3 O-D are shown in Figure 5d and S35-S36, Supporting Information.The πp* orbital of O 2 moves downshift below the Fermi level after chemisorption on SnN 3 O-D, indicating a coupling of the p electrons of O 2 with that of Sn.The local peaks appeared after O 2 adsorption indicate that O 2 has very large contributions to the DOS of Sn 5p orbital (Figure Compared to SnN 3 O, the p orbits of SnN 3 O-D are more coincident with that of O 2 , leading to favorable adsorption to O 2 with an adsorption energy of À0.59 eV (Figure 5e,f ).The electron redistribution and strong p-p coupling between O 2 and SnN 3 O-D greatly promote the activation of O 2 , as evidenced by the elongated O-O bond (from 1.23 to 1.62 Å).This is beneficial for the rapid formation of OOH* and accelerates the reaction kinetics of ORR.

Figure 5 .
Figure 5. a) The adsorption energy difference (ΔE O -ΔE O Pt ) of various structure models.b) Free energy diagrams of SnN 3 O and SnN 3 O-D at U = 0, 1.23 V. c) Charge density difference map of SnN 3 O-D.d) DOS of Sn 5p orbitals, O 2 2p orbitals before and after O 2 chemisorption.e) O 2 absorption energies on SnN 3 O and SnN 3 O-D.f ) The structure models of SnN 3 O and SnN 3 O-D after O 2 adsorption.

Figure 6 .
Figure 6.a) Polarization and power density curves of the PEMFC operated at 80 °C and 100% RH. b) Structure mode of the ZAB.c) OCP, d) polarization and power density curves, e) rate capabilities, f ) galvanostatic discharge curves, and g) cycling stability of the ZAB.h) The photographs of LEDs lighted by the ZAB.
3 O-50 with outstanding ORR activity and stability, especially under acidic conditions.In 0.1 M HClO 4 , the half-wave potentials of SnN 3 O-50 reach 0.816 V, with a loss of only 15 mV after 10 000 potential cycles.Furthermore, the PEMFC assembled using SnN 3 O-50 as the cathode showed a peak power density of 502 mW cm À2 .DFT calculations revealed that N and O coordinates promote the localization of Sn 5p electrons, leading to strong coupling with the p orbit of O 2 .The presence of defects regulates the adsorption of O-containing intermediates at the SnN 3 O active sites, thereby optimizing the free energy gaps of the four successive ORR steps.Currently, the research on p-block SAC is very limited, and their ORR performances in acidic medium are still far from satisfactory.In this regard, this work provided a universal strategy to enhance the acidic ORR properties of SACs and is of great significance to the development the main-group element SACs in fuel cells.