Engineering d–p orbital hybridization of single-atom Fe sites via axial B-mediation for the oxygen reduction reaction

Abstract

Single-atom Fe–N–C catalysts have demonstrated promising potential in the oxygen reduction reaction (ORR), yet their intrinsic activity remains less than ideal. Orbital hybridization provides a versatile means to modulate the thermodynamic and kinetic properties during electrochemical processes. In this study, we adopt an “axial ligand boron-modulation” strategy to regulate the electronic structure of single-atom Fe sites through d–p orbital hybridization. The synthesized FeN₄–B/NC demonstrates exceptional ORR activity with a half-wave potential of 0.915 V, surpassing planar FeN₄/NC and commercial Pt/C. In situ XAS results reveal the dynamic stretching of Fe–N/O and Fe–B bonds during the ORR process, providing an intuitive confirmation that the single-atom sites undergo reversible structural changes to optimize the adsorption of reaction intermediates. Theoretical investigations combined with zero-field cooling temperature dependence analyses demonstrate that in the intermediate spin state, hybridization occurs between the central Fe's 3d orbitals and B's 2p orbitals, which results in increased e_g orbital occupancy and positions the d-band center closer to the Fermi level, which enhances charge transfer efficiency and O₂ adsorption capabilities. Furthermore, the newly developed FeN₄–B/NC catalyst shows remarkable performance in liquid and quasi-solid-state zinc–air batteries.

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Introduction

With the advancement of flexible and wearable electronic devices, zinc–air batteries (ZABs) with high energy density, low cost, and safety demonstrate significant potential for practical applications.^1,2 The oxygen reduction reaction (ORR) plays a pivotal role in ZAB systems. Although platinum-based catalysts are widely regarded as the optimal choice for the ORR due to their exceptional catalytic activity, the prohibitively expensive nature, scarce resources, limited stability, and poor tolerance to CO/methanol significantly hinder their large-scale implementation.³ Therefore, it is highly desirable to explore low-cost, non-precious metal-based catalysts with remarkable catalytic performance.

Single-atom catalysts (SACs), characterized by their atomically dispersed metal active sites, represent promising experimental and theoretical models for elucidating catalytic mechanisms and guiding the rational design of ORR catalysts.⁴ This is attributed to their high metal atom efficiency, unique electronic structure, and uniformly tunable active sites.^5,6 In this regard, Fe-based catalysts with square-planar Fe–N₄ centers are widely regarded as promising candidates for achieving high-efficiency ORR activity and replacing Pt-based catalysts.^7,8 However, the strong binding affinity of Fe–N₄ centers towards *OH intermediates hinders the desorption of OH⁻ or H₂O, thereby limiting their catalytic activity and kinetics.^9,10 Recent studies have demonstrated that the incorporation of foreign atoms at the fifth coordination position, such as Cl,^11,12 O,¹³ S,¹⁰ or I,¹⁴ can regulate the electronic properties of Fe-based active sites. This modulation occurs because the Fe–O bonds formed between ORR intermediates and Fe–N₄ sites involve axial orbital overlap relative to the Fe–N₄ plane, thereby significantly improving the ORR performance.¹⁵ In addition, Fe, as a transition metal with localized d orbitals, exhibits the unique ability to engage in d–p orbital hybridization with heteroatoms that possess p orbitals. Through this d–p orbital hybridization, Fe improves its charge transfer capabilities by modifying the electronic environment and optimizing the energy levels of relevant orbitals. This enhancement facilitates more efficient participation of electrons in electrochemical reactions.¹⁶ However, due to the ionic nature of their axial ligands binding to Fe–N₄ sites, such ligands tend to leach during the ORR process.¹⁷ Establishing stable charge distribution at Fe–N₄ sites is therefore fundamentally and practically significant. If such stability can be achieved, it would pave the way for progressive enhancement in the ORR activity of Fe–N₄ SACs.

Here, we introduce a novel d–p orbital hybridization approach facilitated by the axial coordination of B to enhance the intrinsic ORR activity. Utilizing a molecular-cage encapsulation approach combined with ligation strategies, we synthesized two Fe SACs with distinct active centers on N-doped carbon supports, including square-planar FeN₄ and quasi-octahedral geometry FeN₄–B configurations. Notably, the FeN₄–B catalyst demonstrated superior ORR performance in alkaline media, significantly outperforming FeN₄-based catalysts. Density functional theory (DFT) calculations revealed that the interactions between B's 2p orbitals and Fe's 3d orbitals (d_z², d_xz, and d_yz) establish both bonding and antibonding interactions. This orbital coupling induces an upshift of the d-band center, which facilitates *OH adsorption and consequently reduces the energy barrier for the rate-determining step (RDS) of the ORR. To further evaluate the practical application potential of FeN₄–B, we incorporated it as the cathode in a ZAB system with a solid-state configuration. The FeN₄–B/NC-based ZAB exhibited exceptional performance, delivering higher power density and energy density compared to the commercial Pt/C + RuO₂-based ZAB, demonstrating its immense potential for practical applications.

Results and discussion

Structure and composition analyses

The synthesis procedure for the carbon nanocage catalyst incorporating single-atom FeN₄–B sites is illustrated in Fig. 1a. To construct FeN₄–B/NC, we employed a molecular cage encapsulation strategy. Iron(II) phthalocyanine (Fe Pc), which possesses a condensed aromatic ring framework and well-defined molecular structure, was utilized as the metal precursor. 5-Borondiphenic acid (5-bop) served as the boron source. The synthesis proceeded through three key steps. First, Fe Pc and 5-bop underwent Lewis acid–base interactions to form Fe Pc@5-bop complexes (10.3 Å × 7.6 Å). These complexes were then immersed in a methanol solution containing Zn²⁺ ions and 2-methylimidazole to encapsulate them within ZIF-8 nanocages (11.6 Å). Finally, high-temperature annealing was conducted to obtain the final FeN₄–B/NC material.

Fig. 1 (a) Illustration of the synthetic pathway for FeN₄–B/NC. (b) TEM image, (c) HR-TEM image and (d) HAADF-STEM image of FeN₄–B/NC. (e) Elemental mapping images showing the distribution of Fe, C, N, and B. (f) AC-STEM image and (g) three-dimensional atomic overlap maps and intensity profile derived from the Gaussian function fitting of the yellow box in (f).

It was observed via scanning electron microscopy (SEM) in Fig. S1 that the annealed FeN₄–B/NC exhibited a consistent morphology with Fe Pc@5-bop@ZIF-8, indicating that no collapse occurred during the process. Transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS) mapping of Fe Pc@5-bop@ZIF-8 show that Fe Pc@5-bop is encapsulated in ZIF-8 and the dodecahedral appearance of ZIF-8 is not compromised, and Fe and B are uniformly distributed in ZIF-8 (Fig. S2). TEM reveals that FeN₄–B/NC retains the regular rhombic dodecahedral structure of ZIF-8 (Fig. 1b). No clusters or nanoparticles were observed in the high resolution-TEM (HR-TEM) image (Fig. 1c), while clear lattice fringes corresponding to graphitized carbon structures were evident. The HAADF-STEM image of the sample does not show visible iron-based nanoparticles (Fig. 1d). EDS mapping (Fig. 1e) demonstrates uniform distribution of Fe, C, N, and B elements within the FeN₄–B/NC polyhedral framework. The face-scanning analysis using HAADF-STEM combined with EDS shows that the elemental content of Fe in the FeN₄–B/NC catalyst is 0.49 wt% (Fig. S3). This further indicates that the encapsulation–thermal decomposition strategy successfully anchored the atomically dispersed Fe sites in the carbon substrate and achieved a considerable active site density. Aberration-corrected (AC) HAADF-STEM imaging allows atomic-level observation of the FeN₄–B/NC structure (Fig. 1f). Numerous isolated points are identified, which correspond to iron atoms as clearly visualized in the corresponding 3D representation (Fig. 1g).

X-ray diffraction (XRD) patterns of Fe Pc@ZIF-8 and Fe Pc@5-bop@ZIF-8 are consistent with those of ZIF-8, indicating that the incorporation of Fe Pc and Fe Pc@5-bop does not disrupt the structural integrity of ZIF-8 (Fig. S4a). Furthermore, no Fe metal signals were detected in the XRD patterns of these samples after thermal decomposition (Fig. S4b). Additionally, the graphitic characteristics of the catalysts were systematically investigated using Raman spectroscopy to evaluate their degree of graphitization. In the Raman spectra, the D peak corresponds to defects while the G peak represents the graphitization degree. As shown in Fig. S5, the FeN₄–B/NC sample exhibits a lower I_D/I_G ratio (1.01), indicating a higher degree of graphitization and consequently enhanced electrical conductivity.

To investigate the chemical states of FeN₄–B/NC and FeN₄/NC, X-ray photoelectron spectroscopy (XPS) analysis was conducted. The survey spectra reveal the presence of C, N, O, and Fe elements in both samples (Fig. S6a). Additionally, a distinct B peak was observed for FeN₄–B/NC, confirming the successful incorporation of B atoms within the carbon nanocage framework. As shown in Fig. S6b, the high-resolution C 1s XPS spectrum was deconvoluted into three characteristic peaks: C–C (284.8 eV), C–N (285.6 eV), and O–C=O (286.9 eV).¹⁸ The Fe 2p spectra for both samples are shown in Fig. 2a. The Fe 2p spectra exhibit four prominent peaks at binding energies of 708.4 eV (Fe²⁺ 2p_3/2), 714.7 eV (Fe³⁺ 2p_3/2), 722.9 eV (Fe²⁺ 2p_1/2), and 726.9 eV (Fe³⁺ 2p_1/2).¹⁹ Relative to FeN₄/NC, the binding energy shifts of both the Fe 2p_1/2 and 2p_3/2 orbitals in FeN₄–B/NC exhibited higher values. This observation suggests an electron transfer from Fe to B, accompanied by an increase in electron density around the Fe center.²⁰ As shown in Fig. 2b, the B 1s XPS spectrum of FeN₄–B/NC reveals four distinct boron species: Fe–B (∼190.3 eV), B–C (∼190.9 eV), B–N (∼191.8 eV), and B–O (∼193.1 eV).²¹ The presence of the Fe–B bond provides direct evidence of chemical bonding between boron and iron, ruling out the possibility of mere π–π stacking interactions. Based on the N 1s spectroscopy analysis (Fig. 2c), both FeN₄–B/NC and FeN₄/NC samples exhibit pyridinic N (398.4 eV), Fe–N (399.5 eV), pyrrolic N (400.9 eV), graphitic N (402.5 eV), and oxidized N (404.7 eV) species.²² However, significant differences in their nitrogen coordination environments are observed: FeN₄/NC predominantly features pyrrolic N, whereas FeN₄–B/NC shows a preference for pyridinic N. This indicates that FeN₄/NC adopts a wrinkled structure, while FeN₄–B/NC exhibits a planar tetragonal FeN₄ coordination geometry.²³ The structural modification arises from the incorporation of B, which strengthens the Fe–N interaction and facilitates the conversion of pyrrolic N to more stable pyridinic N and modulates the FeN₄–B/NC's electronic properties and optimizes the distribution of active sites. This structural optimization ultimately enhances both catalytic performance and operational durability.^24,25

Fig. 2 XPS spectra of (a) Fe 2p, (b) B 1s, and (c) N 1s. (d) Fe k-edge XANES spectra. (e) FT-EXAFS spectra for (d) Fe k-edge. FT-EXAFS of the R-space fitting curve for (f) FeN₄/NC and (g) FeN₄–B/NC (inset: model of FeN₄ and FeN₄–B). (h) WT-EXAFS plots.

To probe the intricate details of local coordination geometry and electronic states, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopic analyses were employed. As shown in Fig. 2d, the absorption edge of Fe in FeN₄–B/NC is positioned between those of FeO and Fe₂O₃, indicating that the average oxidation state of Fe is between +2 and +3.²⁶ Compared to FeN₄/NC, a noticeable positive shift in the Fe K-edge suggests a change in the electronic structure of Fe sites, which means an increase in the oxidation state of Fe. This is likely due to the B axial coordination leading to electron transfer from Fe to B,²⁷ consistent with previous XPS observations.¹¹ In the inset of Fig. 2d, after B incorporation, the intensity of the pre-edge peak decreases (related to the 1s–4p_z dipolar transition in a planar D_4h configuration), indicating that the local symmetry transitions from planar D_4h to C_4v.^28,29 Fourier-transformed k³-weighted EXAFS spectra were plotted. As shown in Fig. 2e, FeN₄ exhibits a prominent peak centered at 1.63 Å in the R-space, corresponding to the Fe–N scattering path. Notably, a shoulder peak at 2.1 Å is observed for FeN₄–B/NC, which corresponds to Fe–B backscattering, further confirming the presence of an Fe–B axial interaction.³⁰ The absence of Fe–Fe features in FeN₄–B/NC and FeN₄/NC samples indicates that Fe sites are atomically dispersed within the carbon framework. To elucidate the coordination environment around Fe sites, quantitative least-squares EXAFS curve fitting was performed. In the FeN₄/NC structure, iron centers are each bonded to four nitrogen atoms in the first coordination shell, with a mean bond distance of 1.92 Å (Fig. 2f and Table S3). In contrast, Fe in FeN₄–B/NC is bonded to four N atoms and one B atom (FeN₄–B), with an average bond length of 1.96 Å and 2.15 Å, respectively (Fig. 2g). Importantly, the FT k³-weighted EXAFS spectra match perfectly with the simulated EXAFS spectra obtained from DFT, further validating the proposed FeN₄–B structure. Additionally, in the wavelet transform (WT)-EXAFS plots, we observe minor peak shifts in the k-space between FeN₄/NC and FeN₄–B/NC (Fig. 2h), indicating subtle changes in their atomic coordination environments.²¹

Electrochemical performances

The ORR electrocatalytic performance was evaluated using a rotating disk electrode (RDE) in an O₂-saturated 0.1 M KOH solution. As shown by the comparative cyclic voltammetry (CV) curves, all synthesized catalysts exhibit distinct reduction peaks (Fig. 3a).³¹ Notably, FeN₄–B/NC achieves the most positive peak potential, indicating its potentially superior ORR activity. Linear scan voltammetry (LSV) curves (Fig. 3b) reveal that FeN₄–B/NC delivers an outstanding half-wave potential (E_1/2) of 0.915 V vs. RHE, surpassing those of FeN₄/NC (0.866 V), NC (0.748 V), and commercial Pt/C (0.850 V). This improvement demonstrates that the B-axial coordination in FeN₄ configuration enhances the ORR performance through synergistic electronic modulation. Furthermore, as shown in Fig. 3c, the Tafel slope of FeN₄–B/NC (44 mV dec⁻¹) is significantly lower than those of FeN₄/NC, NC, and Pt/C, suggesting that the incorporation of B-axial coordination at the Fe site effectively accelerates ORR kinetics. Fitting the LSV curves at various rotation speeds with the K–L equation reveals that the electron transfer number of FeN₄–B/NC is approximately 4.0, indicating a 4-electron ORR process (Fig. 3d and S7). RRDE tests further demonstrate that FeN₄–B/NC exhibits a higher electron transfer number closer to 4 than FeN₄/NC. The introduction of axial B-coordination reduces the H₂O₂ yield from 11% to 1%, suggesting enhanced O₂ dissociation and superior ORR activity for FeN₄–B/NC (Fig. 3e). Furthermore, the turnover frequency (TOF) of FeN₄–B/NC at 0.8 V is 0.982 O₂ s⁻¹, which is much higher than that of Pt/C (0.049 O₂ s⁻¹), indicating that the iron-related active sites in FeN₄–B/NC have high efficiency in the ORR.³²

Fig. 3 (a) CV plots. (b) LSV curves. (c) Tafel plots. (d) The polarization curves at different rotation speeds and the corresponding K–L plots (shown as inset) for FeN₄–B/NC. (e) H₂O₂ yields and the corresponding n. Chronoamperometric (i–t) responses of (f) before and (g) after the addition of 1 mL of methanol. (h) Comparative analysis of the ORR performance of FeN₄–B/NC against contemporary SACs and SACs with axial coordination.

In addition to its excellent ORR activity, stability of a catalyst is crucial for practical applications. The long-term durability of FeN₄–B/NC for the ORR was evaluated using chronoamperometric measurements. Notably, FeN₄–B/NC shows a minimal current decay of only 5.8% over 11 hours of continuous operation, significantly outperforming Pt/C, which exhibits an 11.4% current loss (Fig. 3f). Furthermore, in methanol tolerance tests conducted after 200 seconds of potentiostatic measurement (Fig. 3g), commercial Pt/C displays severe current fluctuations and a noticeable drop upon addition of 1 mL methanol, whereas FeN₄–B/NC maintains relatively stable current responses, demonstrating strong resistance to methanol poisoning.³³ Compared to other SACs reported previously, FeN₄–B/NC achieves top-tier activity (Fig. 3h and Table S3). The accelerated durability test (ADT) results reveal that after 5000 cycles of testing, the half-wave potential in alkaline electrolyte decreases by 22 mV for FeN₄–B/NC, whereas a more significant loss of 38 mV is observed for Pt/C (Fig. S8). These findings demonstrate the superior stability of FeN₄–B/NC during the ORR. We further evaluated the oxygen evolution reaction (OER) performance of FeN₄–B/NC in 1 M KOH solution. As shown in Fig. S9, the FeN₄–B/NC exhibited an overpotential of 370 mV at a current density of 10 mA cm⁻², comparable to that of RuO₂ and superior to that of FeN₄/NC (380 mV).³⁴

Study on the oxygen electrocatalysis mechanisms

To investigate the structural evolution of FeN₄–B/NC during the ORR process, in situ XAS experiments were conducted. The XAS spectra of FeN₄–B/NC were collected at different potentials in O₂-saturated 0.1 M KOH solution. As shown in Fig. 4a, a negative shift in the absorption edge energy was observed with decreasing applied potential, providing evidence for the occurrence of the ORR.³⁵ To further probe the structural changes at the active sites during the ORR, EXAFS oscillations were analyzed. The Fe K-edge FT-EXAFS spectra (Fig. 4b) reveal the structural stability of FeN₄–B/NC to retain its initial structural integrity under operating conditions. Specifically, as the applied voltage decreases, the Fe–N/O bond length shortens from 1.52 to 1.39 Å, while the Fe–B bond length extends from 2.11 to 2.21 Å, demonstrating that at different potentials, both Fe–N/O and Fe–B bonds undergo stretching due to intermediate adsorption at the active centers.³⁶ Additionally, variations in the intensities of Fe–N/O and Fe–B bonds were observed at different potentials, likely attributed to redox processes involving Fe atoms and structural rearrangements. These changes were further influenced by the adsorption and desorption of reaction intermediates, suggesting dynamic structural modifications of FeN₄–B during the ORR. The FT-EXAFS fitting results at various potentials showed an increase in the coordination number of Fe–N/O bonds (Fig. 4c–f and Table S2), indicating that Fe atoms serve as the active centers for the ORR. Furthermore, Fig. S10 displays the WT plots of FeN₄–B/NC at different potentials, showing partial shifts in Fe–N/O scattering paths, which further confirms that Fe atoms are the active sites for the ORR.

Fig. 4 (a) In situ XANES of the Fe k-edge of FeN₄–B/NC at various potentials. (b) FT-EXAFS spectra of the Fe k-edge at various potentials. (c–f) Fe k-edge FT-EXAFS of the R-space fitting curve at OCP, 0.9, 0.8 and 0.7 V, respectively.

Theoretical calculations revealed that the axial coordination of B to FeN₄ significantly influences both the electronic structure and ORR performance. The optimized structures of FeN₄ and FeN₄–B are presented in Fig. 5a. It can be seen that establishing the axial coordination of B at the Fe–N₄ locus causes the distortion of the square-plane field around Fe–N₄, which is transformed into a quasi-octahedral geometry. Bader charge analysis reveals the net charge on individual atoms, where positive values indicate electron loss (oxidation) and negative values reflect electron gain (reduction).³⁷ As evidenced by the calculations (Fig. 5a), the Bader charges for Fe in FeN₄ and FeN₄–B systems are determined to be −1.39 and −0.93, respectively. This suggests an electronic transfer from Fe to B, leading to an increase in Fe's valence state in agreement with the aforementioned XPS and XAS results.³⁸ As shown by differential charge density plots (Fig. 5b–d), electron transfer in symmetric FeN₄ occurs exclusively within the FeN₄ plane, with a symmetrical distribution of electron clouds. In contrast, the axial B atom in FeN₄–B interacts with the Fe metal center, inducing pronounced charge redistribution in the axial region and breaking the symmetry of charge distribution within the FeN₄ plane. Free energy diagrams for the ORR on both catalysts are plotted in Fig. S11 and S12. At U = 0 V (Fig. 5e), the RDS of FeN₄–B exhibits a higher energy barrier (ΔG = −0.84 eV) compared to FeN₄ (ΔG = −0.61 eV). Conversely, at U = 1.23 V (Fig. 5e), the ΔG for the rate-determining step in FeN₄ (−0.64 eV) is significantly higher than that of FeN₄–B (−0.24 eV).³⁹ These results indicate that introducing axially coordinated B atoms at the FeN₄ site can effectively reduce the energy barrier, thereby enhancing the intrinsic ORR activity.⁴⁰ Moreover, FeN₄–B shows a lower initial oxygen activation energy barrier compared to FeN₄, indicating enhanced oxygen adsorption capability.

Fig. 5 (a) Optimization model and Bader charge of FeN₄ and FeN₄–B. Electron density difference maps of (b) FeN₄ and (c) FeN₄–B. (d) 2D slice of differential charge. Yellow and blue regions indicate areas of increased and decreased electron density, respectively. The free energy step diagram at (e) U = 0 V and U = 1.23 V. (f) DOS. (g) Schematic of bonding status and adsorption behavior under FeN₄ (blue) and FeN₄–B (red). (h) χ–T plots and χ⁻¹–T plots. (i) PDOS of FeN₄ and FeN₄–B. (j) The orbital interaction between Fe and B.

As revealed by the density of states (DOS) analysis (Fig. 5f and g), the incorporation of an axial B atom causes a shift in the d-band center from −0.98 eV for the FeN₄ to −0.67 eV for the FeN₄–B.⁴¹ The elevation of the d-band center facilitates electron excitation to anti-bonding orbitals, increasing their occupation and thereby enhancing the O₂ adsorption capability at the Fe site.^42,43 This upward shift is attributed to the increased occupancy of e_g orbitals due to the introduction of a high-energy p-band, leading to enhanced unpaired electrons, which facilitate charge transfer and improve reaction kinetics.⁴⁴ To investigate the spin states of FeN₄–B/NC, we employed zero-field-cooled temperature-dependent (ZFC-T) magnetic susceptibility (χ_m) measurements. Through analysis of the temperature-dependent χ⁻¹ data (Fig. 5h), we determined that FeN₄–B exhibits an effective magnetic moment of 2.83 µB, with a t_2g⁴e_g¹ electron configuration, revealing a middle-spin state (M.S.).⁴⁵ The d-orbitals split into four distinct states: d_x²−y², d_xy, and two-fold degenerate d_xz/d_yz. The d_z² and d_xz/d_yz orbitals undergo hybridization with the p orbitals of B atoms, resulting in bonding (σ and π) and antibonding (σ* and π*) states.⁴⁶ In contrast, the horizontal d-orbitals (d_x²−y² and d_xy) remain non-binding due to their inactivity.⁴⁷ As shown in Fig. 5i, the projected density of states (PDOS) analysis reveals a strong orbital overlap between Fe 3d (d_z²/d_xz/d_yz) and B 2p orbitals in FeN₄–B, indicating a significant d–p coupling effect that generates both bonding (σ and π) and antibonding (σ* and π*) states (Fig. 5i).^48,49 Based on the orbital hybridization theory and the observed PDOS overlap, we determine the optimal electron occupancy across these bonding and antibonding states.⁵⁰ As illustrated in Fig. 5j, the vacant 2p_z orbital of B⁻ can accept an electron from the 3d_z² orbital of middle-spin Fe³⁺.⁵¹ This process involves the delocalization of electrons into σ orbitals, thereby preventing excessive occupancy of the antibonding σ* orbital and facilitating the reaction progress.⁵² Additionally, the half-filled 2p_x and 2p_y orbitals of B⁻ enable moderate π* coupling with the 3d_xz and 3d_yz orbitals of Fe³⁺.^53,54 This coupling stabilizes the Fe sites without fully occupying the d-electrons, thus optimizing the adsorption energies for *OOH and *OH species.⁵⁵

Application of FeN₄–B/NC in ZABs

Motivated by the remarkable ORR and OER performance of FeN₄–B/NC, liquid and flexible ZABs were developed by utilizing FeN₄–B/NC as the air cathode and Zn foil as the anode (Fig. 6a). As illustrated in Fig. 6b, the ZAB utilizing FeN₄–B/NC as the cathode demonstrates an open-circuit voltage (OCV) of 1.504 V, which exceeds that of the ZAB with a Pt/C + RuO₂ cathode (1.450 V) within a 50-minute period. This reduction is attributed to the enhanced charge and discharge capabilities of FeN₄–B/NC. Furthermore, the FeN₄–B/NC-based ZAB achieves a maximum power density of 186 mW cm⁻² (Fig. 6c), which considerably surpasses that of the commercial Pt/C + RuO₂-based ZAB (52 mW cm⁻²). The FeN₄–B/NC-based ZAB demonstrates a remarkable specific capacity of 790 mAh g_Zn⁻¹, which not only meets the required standards, but also surpasses that of Pt/C + RuO₂-based ZAB (744 mAh g_Zn⁻¹) (Fig. 6d). The FeN₄–B/NC-based ZAB demonstrates minimal decline in discharge/charge performance over 300-hour of cycling at 10 mA cm⁻², with a 10-minute discharge and 10-minute charge process. In contrast, the Pt/C + RuO₂-based ZAB experiences significant degradation within 65 hours, further validating the superior ORR/OER stability of the FeN₄–B/NC-based cathode (Fig. 6e). In addition, the FeN₄–B/NC-based ZAB demonstrates a notable and consistent round-trip efficiency, increasing from an initial 58.3% to a final 61.1%, highlighting its exceptional recharging performance (inset of Fig. 6e). According to Fig. 6f, the discharge voltages of FeN₄–B/NC-based batteries at current densities of 5, 10, 25, and 50 mA cm⁻² are 1.24, 1.20, 1.12, and 1.02 V, respectively. The outstanding stability of the FeN₄–B/NC catalyst is the result of the inherent rigidity of the FeN₄ structure, the additional stability provided by the axial B ligands, the efficient charge transfer ability, and the ideal adsorption strength ensured by the resulting intermediate spin state, all of which work together in synergy. Even after the current density is returned to 5 mA cm⁻², the system sustains a stable discharge voltage of 1.24 V, highlighting its remarkable rate performance. Compared with other Fe SACs, the FeN₄–B/NC-based ZABs exhibit superior performance and stability as evidenced in Table S4.

Fig. 6 (a) Schematic illustration of the self-assembly process of a ZAB with FeN₄-B/NC as air cathode. (b) OCV plots (inset: OCV recorded by a multimeter). (c) Polarization curve for discharge and corresponding power density. (d) Specific capacity at 10 mA cm⁻². (e) Charge–discharge cycling performance at 10 mA cm⁻². (f) Discharge curves under different current densities. (g) Schematic diagram of the homemade FZAB. (h) OCV plots. (i) Cycling tests of the FZAB with the FeN₄-B/NC cathode and (j) under various bending degrees.

To explore the feasibility of the FeN₄–B/NC electrocatalyst in flexible and wearable electronics, an all-solid-state flexible ZAB (FZAB) was assembled, as shown in Fig. 6g. To simplify the testing procedure, we assembled a sandwich-type FZAB using a piece of zinc foil, an electrolyte membrane with a hydrogel, and a cathode with FeN₄–B/NC through a layer-by-layer method. The flexible ZAB with the FeN₄–B/NC cathode achieves a stable OCV of ∼1.41 V, surpassing the performance of the Pt/C + RuO₂-based counterpart (Fig. 6h). Furthermore, the cycling stability of the FeN₄–B based FZAB reached 62 hours at a current density of 2 mA cm⁻² and a charge–discharge rate of 4 min per cycle (Fig. 6i), which exceeded that of the Pt/C based FZAB (26 hours). The FeN₄–B/NC-based flexible ZAB exhibited stable charge/discharge voltages at various bending states (0°, 90° and 180°), further demonstrating the practical application potential of the FeN₄–B/NC cathode (Fig. 6j).⁵⁶

Conclusions

In summary, a “B-mediation” strategy has been developed to transform planar FeN₄ into a pseudo-octahedral FeN₄–B configuration, where an axial B atom coordinates with the FeN₄ center. This approach modulates the electronic structure of Fe single atoms through d–p orbital hybridization, achieving an ORR half-wave potential of 0.915 V for FeN₄–B/NC and an OER overpotential of 370 mV at 10 mA cm⁻². In situ XAS confirmed the dynamic structural changes of the Fe sites during the reaction process, providing direct evidence for its outstanding performance. DFT calculations revealed that d–p orbital hybridization between the axial B atom and Fe centers shifts the d-band center closer to the Fermi level and optimizes *OH intermediate adsorption, thereby reducing the reaction energy barrier for the RDS and facilitating both ORR and OER processes. Experimental validation using a liquid ZAB setup demonstrated the practical potential of FeN₄–B/NC, delivering remarkable power density and capacity (186 mW cm⁻² and 790.8 mAh g_Zn^—1, respectively), along with robust stability over 300 hours of operation. Furthermore, FeN₄–B/NC showed promise for flexible ZABs with an open-circuit voltage of 1.41 V and stable charge–discharge cycling performance under various bending angles.

Author contributions

All of the authors contributed to the literature search, writing and editing of this review.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: experimental methods, electrochemical evaluation, Zn–air battery test, DFT methods, Fig. S1–S12 and Tables S1–S4. See DOI: https://doi.org/10.1039/d5sc07064c.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 22075099), and the Education Department of Jilin Province (No. JJKH20250070BS). The authors acknowledge the BL11B beamline (31124.02.SSRF.BL11B) of the Shanghai Synchrotron Radiation Facility for providing the XAS beamtime.

Notes and references

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