Integrating hetero-coordinated FeN4 and NiP4 single-atomic sites in one system for oxygen reduction/evolution

doi:10.21203/rs.3.rs-1542920/v1

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Integrating hetero-coordinated FeN4 and NiP4 single-atomic sites in one system for oxygen reduction/evolution

https://doi.org/10.21203/rs.3.rs-1542920/v1

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The intrinsic activity of single-atom catalysts highly relies on their coordination configuration. However, selecting heterogenous ligands with larger electronegativity difference to severally customize the coordination and tune the electronic structure of dual-atom catalysts still remains challenge. Herein, a layer-by-layer separation and anisotropic coordination strategy is designed to perform the hetero-coordinated FeN₄ and NiP₄ sites anchored to the inner and outer surfaces of hollow carbon sphere (FeN₄-C-NiP₄), revealing a ligand-determined electronic properties of metal sites and their selectivity for oxidative/reductive reaction. Specifically, FeN₄ sites with lower spin and deficient electrons are responsible for oxygen reduction reaction (ORR, E_1/2=0.89 V), while four rich-electronic P ligands upshift the Ni center towards a higher spin, making the NiP₄ sites more favorable for oxygen evolution reaction (OER, η₁₀=218 mV_iR-free). Thus the FeN₄-C-NiP₄ could balance the competition between rate-limiting steps of ORR and OER, manifesting a small potential gap of 0.59 V. Moreover, above concept can be extended to achieve hetero-coordinated CoN₄-C-NiP₄ dual-atom sites, enhancing the activity and selectivity for hydrogen evolution reaction (HER) and OER.

Synchronously oxidative and reductive reactions are generally proceeded in one circulation system for various renewable energy conversion devices. For instance, the discharging and charging processes in Zinc-air battery are driven by reversible oxygen reduction reaction (ORR, O₂ + 2H₂O + 4e^- → 4OH^-) and oxygen evolution reaction (OER, 4OH^- → 2H₂O + O₂ + 4e^-)^1-3. Electrochemical water splitting employs hydrogen evolution reaction (HER) as cathodic-half cell reductive reaction and OER as anodic-half cell oxidative reaction⁴. In an effort to guarantee the output power and meet the compatibility, fabricating bifunctional electro-catalysts that simultaneously possess high oxidative and reductive activity at wide potential range is essential^5-7. Very recently, dual-atom catalysts (DACs) without direct bonding have emerged as a novel frontier as bifunctional electro-catalysts⁸, which combine the intrinsic activity of two atomic sites to break through the limitation of single atom catalysts (SACs)^{9, 10}. In addition, compare to directly bonded dual-atoms pair, DACs with separated spatial distribution possess more tunability in geometric structure, metal-metal distance and electronic property, which could broaden the regulation scope for key intermediates adsorption and proton-electron transfer pathway^{11, 12}. More importantly, a relative far metal-metal distance has been proven to form two non-interfering catalytic interfaces, which could simultaneously promote the seemingly incompatible oxidative and reductive reactions in one system¹³.

The intrinsic activity of SACs highly relies on the selection of ligands. Based on the discrepancy in size and electronegativity, ligands determine the local coordination environment and electronic structure of metal center¹⁴. In particular, P ligand with larger atom radius and lower electronegativity (2.01) can enhance the local charge density through its 3p lone-pair electrons, thereby tuning the adsorption strength of oxygen-contained species¹⁵. Meanwhile its unoccupied 3d orbitals could accommodate the valence electrons from metal center, accelerating the electron migrations¹⁶. However, metal center usually prefers to coordinate with large-electronegative N (3.04) and C (2.55) ligands due to the effortless formation of M-N/M-C bonds with shorter bond length and stronger bond energy, leaving difficulty for achieving pure M-P_x single-atom configuration¹⁷. On above basis, when extending the ligand regulation strategy into DACs domain, it is more challenging to use heterogeneous ligands with larger electronegativity difference to severally customize the electronic structure of two metal centers. This is because in most cases for the synthesis of DACs, two metal precursors are inevitably stabilized by similar organic linkers and underwent co-pyrolysis condition, hindering their selective coordination with different ligands¹⁸.

Herein, based on the layer-by-layer separation strategy and anisotropic coordination capacity of metal ions, we realize the fabrication of hetero-coordinated FeN₄ and NiP₄ atomic sites that successively anchored to inner and outer surfaces of hollow carbon sphere (denoted as Janus-distributed FeN₄-C-NiP₄). X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) verify the heterogeneous configurations of Fe-N₄ and Ni-P₄ sites, which are spatial isolation with negligible interaction. Compare to N-coordinated FeN₄-C-NiN₄, the FeN₄-C-NiP₄ strengthen the charge polarization and widen the spin density gap, generating higher spin NiP₄ sites with electron accumulation and relatively lower spin FeN₄ sites with electron deficiency. By employing mono-component FeN₄, NiP₄, and NiN₄ as references, electrochemical tests and theoretical calculations verify that the FeN₄-C-NiP₄ could provide separated FeN₄ and NiP₄ sites with different selectivity for reversible oxygen catalysis, which responsible for both high ORR and OER activity, small potential gap, and superior charge-discharge performance in Zn-air battery. Particularly, four rich-electronic P ligands upshift the Ni center towards a higher spin to modulate the energy barrier of O* adsorption and optimize the G_OOH*-G_OH* value to 2.75 eV, which are more favorable for boosting OER than conventional N ligands. Above concept could be extended to perform Janus-distributed CoN₄-C-NiP₄ dual-atom sites, which offer separated CoN₄ and NiP₄sitesto enhance the activity and selectivity for HER and OER, further verifying the role of heterogeneous ligands for oxidative/reductive catalysis.

Synthesis and structural features of hetero-coordinated FeN₄-C-NiP₄. The synthetic route of FeN₄-C-NiP₄ is schematic illustrated in Fig. 1a (SEM images and color change of intermediates shown in Supplementary Fig. 1 and Fig. 2), which involves four steps. Step I: forming SiO₂⁻@Fe³⁺ spheres. SiO₂ nanospheres (150 nm) were pre-synthesized and underwent surface modification to be negatively charged. Then Fe³⁺ were readily adsorbed on SiO₂⁻ to form the SiO₂⁻@Fe³⁺ spheres through electrostatic attractions, accompanied by color change from white to dark orange. Step II: adding excess adenosine monophosphate (AMP⁺) to wrap on the outer surface of SiO₂⁻@Fe³⁺. AMP⁺ molecule was consisted of phosphate functional group, pentose nucleo-saccharide and nitrogenous base adenine, as structure shown in Supplementary Fig. 3). Selecting cheap and sustainable biomaterial of AMP⁺ as N, P, and C source was due to following three reasons: i) -NH₂ from APM⁺ could provide lone pairs of electrons to strongly bind with Fe³⁺ via coordination interactions, as confirmed by color change and UV-vis in Supplementary Fig. 4. The resultant Fe³⁺-AMP⁺ complex tended to form atomic Fe sites after pyrolysis through spatial confinement effect; ii) AMP⁺ with large molecular weight could intertwine on the surface of SiO₂⁻@Fe³⁺, forming a thick AMP⁺ layer to avoid the contact of inner Fe³⁺ with other metal precursors in subsequent steps. As confirmed by TEM and SEM images in Supplementary Fig. 5, it indicates the completely coverage of AMP⁺ layer with thickness of 25.0 nm on the outer surface of SiO₂⁻@Fe³⁺; iii) the AMP⁺ was positively charged under isoelectric point, beneficial for further electrostatic adsorption. Step III: forming SiO₂⁻@Fe³⁺@AMP⁺@Ni(CN)₄²⁻ spheres. The electronegative [Ni(CN)₄]²⁻ could further adsorb on the outer surface of SiO₂⁻@Fe³⁺@AMP⁺. Specifically, the cyano groups in Ni(CN)₄²⁻ could disperse the central Ni²⁺ to avoid the aggregation after pyrolysis¹⁹. The precise element distribution of SiO₂⁻@Fe³⁺@AMP⁺@Ni(CN)₄²⁻ spheres was examined by EDX mapping (Supplementary Fig. 6), which verifies Janus-like structure consisted of inner Fe³⁺ and outer [Ni(CN)₄]²⁻. Step IIII: pyrolysis and leaching to acquire FeN₄-C-NiP₄. The SiO₂⁻@Fe³⁺@AMP⁺@Ni(CN)₄²⁻ powder was carbothermally reduced to form SiO₂@FeN₄-C-NiP₄ due to anisotropic coordination capacity of Fe³⁺ and Ni²⁺. After pyrolysis, AMP⁺ layer was transformed into N, P-doped carbon spheres, meanwhile the inner Fe³⁺-AMP⁺ and outer AMP⁺-Ni(CN)₄²⁻ were transformed into atomic Fe-N_x sites and Ni-P_x sites, respectively. Particularly, the post transition metal Ni²⁺, which contains much d-orbital electrons and exerts lower charge, preferred to form the Ni-P bonds because the unoccupied 3d orbitals of P could share the electrons pair of Ni²⁺ to form extra back donating bonds²⁰. The increase of bonding number leads to preferential formation of Ni-P than Ni-N. Finally, after leaching to remove the SiO₂, Janus-distributed and hetero-coordinated FeN₄-C-NiP₄ could be yielding.

The FeN₄-C-NiP₄ dual-atom sites were first investigated by X-ray diffraction (XRD) and Raman spectra. The XRD pattern exhibits two broad peaks at 25.7° and 44°, assigned to the (002) and (100) planes of graphitic carbon (Fig. 1b). No diffraction peak ascribed to Fe or Ni could be detected, demonstrating the absence of crystalline Fe and Ni-based species. The Raman spectra shows defects-related D band at 1325 cm^− 1 and sp²-hybridized carbon-related G band at 1590 cm^− 1 (Supplementary Fig. 7). The relative high I_D/I_G value (1.11) for FeN₄-C-NiP₄ reveals the defective structure of carbon after pyrolysis. The morphology of FeN₄-C-NiP₄ was investigated by large-scale HRTEM images (Supplementary Fig. 8) and high angle annular dark field scanning TEM (HAADF-STEM, Fig. 1c). Large number of interconnected and hollow nanospheres with diameter of 150 nm and wall thickness of 3.8 nm could be observed, manifesting the structural robustness. Meanwhile, no visible nanoparticles or clusters could be observed, implying the atomic dispersion of both Fe and Ni. The Brunauer-Emmett-Teller surface area of FeN₄-C-NiP₄ was calculated to be 290.71 m² g^− 1, with a hysteresis loop related to type-IV isotherm (Supplementary Fig. 9), indicative a high specific surface area with mesoporous characteristics. The mono-dispersion of both Fe and Ni atoms was directly monitored by HAADF-STEM (Fig. 1d). A number of bright dots with size of 2.0 Å are randomly dispersed, corroborating that both Fe and Ni atoms are isolated without the formation of Fe-Ni dual-atom pairs. The energy-dispersive X-ray (EDX) mapping reveals the co-existence of Fe, Ni, C, N, and P elements over the entire architecture (Fig. 1e), confirming the successful doping of P and well reservation of Fe and Ni sites. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) indicates that the weight fraction of Fe and Ni in FeN₄-C-NiP₄ is 1.10 at% and 1.05 at%, respectively.

The valence states and chemical environment of FeN₄-C-NiP₄ were elucidated by XPS (Supplementary Fig. 10). The wide-scan spectrum displays obvious signals of Fe, Ni, C, N, and P (content: 4.64 at%) in FeN₄-C-NiP₄. The high-resolution Fe 2p and Ni 2p spectra show main peaks for Fe²⁺ (709.8 and 723.1 eV), Fe³⁺ (715.2 and 728.9 eV) and Ni²⁺ (854.9 and 872.6 eV), respectively, manifesting the high-valence state of Fe and Ni in FeN₄-C-NiP₄. Theoretically, when electronic effect is existed between Fe and Ni, the Ni atoms could donate partial electrons to Fe atoms, leading to the positive shift of Ni region and correspondingly negative shift of Fe²¹. Importantly, the FeN₄-C-NiP₄ demonstrate standard location of Fe²⁺/Fe³⁺ and negative shift of Ni²⁺, suggesting a non-existent interaction between Fe and Ni. This is possibly due to a relative far distance between Fe and Ni sites, providing evidence for the spatially separated location of Fe and Ni sites in FeN₄-C-NiP₄. The N 1s spectrum reveals the coexistence of pyridinic N (398.1 eV, 36.8%), pyrrolic N (399.4 eV, 32.4%), and graphitic N (400.6 eV, 30.8%). The P 2p spectra could be fitted into three peaks at 131.9 eV (P 2p_3/2 for M-P), 132.9 eV (P-C), and 133.8 eV (P-O), respectively. The obvious M-P signal implies the direct bonding between metal and P atoms.

In order to further confirm the Ni-P bond in FeN₄-C-NiP₄, the Ni₂P-doped C nanosheets and mono-component NiN₄ single-atoms were fabricated. Importantly, through directly calcining the mixture of K₂[Ni(CN)₄] and AMP powders, keeping the same ratio used for FeN₄-C-NiP₄, Ni₂P-doped C nanosheets with high purity could be obtained, indicating the strong binding ability between Ni and P (structural features confirmed by HRTEM, XRD, and XPS in Supplementary Fig. 11). Particularly, both the XPS Ni 2p and P 2p spectra for Ni₂P-doped C nanosheets show similar location with those for FeN₄-C-NiP₄, providing evidence for the possible Ni-P bonding in FeN₄-C-NiP₄. Furthermore, mono-component NiN₄ with pure N coordination were synthesized according to previous work²², using the histidine to eliminate the P source. The formation of NiN₄ was confirmed by XRD, HRTEM, HAADF-STEM, EDX mappings, and XPS, as displayed in Supplementary Fig. 12. Supplementary Fig. 13 compares the XPS Ni 2p_3/2 spectra for FeN₄-C-NiP₄, Ni₂P-doped C nanosheets, and NiN₄. Obviously, the Ni 2p_3/2 peak of FeN₄-C-NiP₄ and Ni₂P-doped C nanosheets show a negative shift of 0.8 eV than that of the NiN₄, implying non-formation of Ni-N bond in FeN₄-C-NiP₄. Due to the weaker electronegativity of P (2.01) than N (3.04), the charges of Ni adjacent to P atom are less positive, resulting in more negative location of Ni-P than Ni-N.

Atomic structure analysis. XAFS analysis were conducted to decipher the coordination configuration of FeN₄-C-NiP₄ at atomic scale (Supplementary Fig. 14). The pre-edge peak for Fe K edge is situated near the Fe₂O₃ and shifts to higher energy region than Fe foil and Fe₂P, indicating that the Fe valence in FeN₄-C-NiP₄ is near + 3 (Fig. 2a). The pre-edge peak for Ni K-edge is located between Ni₂P and NiO, demonstrating the average oxidation state of Ni in FeN₄-C-NiP₄ is between + 1 and + 2 (Fig. 2b). It is worth noting that the wide-range line-type of Fe K-edge for FeN₄-C-NiP₄ is similar to that of the Fe₂O₃ with a salient peak at 7132 eV, while the line-type of Ni K-edge for FeN₄-C-NiP₄ is similar to that of the Ni₂P with mild ridge type. This suggests that the Fe and Ni for FeN₄-C-NiP₄ might possess similar coordination with Fe₂O₃ and Ni₂P, respectively. The Fourier transform (FT) EXAFS spectra of Fe K-edge and Ni K-edge for FeN₄-C-NiP₄ are illustrated in Fig. 2c (fitting details shown in Supplementary Figs. 15 and 16). For Fe moiety, the predominant peak located at 1.41 Å is assigned to the backscattering of Fe-N in the first shell, meanwhile no related peak corresponding to Fe-Fe, Fe-P, or Fe-Ni could be detected. For Ni moiety, the sole peak located at 1.81 Å is attributed to the scattering of Ni-P first-shell coordination, which is consisting with the location of Ni-P bonds for Ni₂P. Meanwhile, no Ni-Ni, Ni-N, or Ni-Fe contribution could be observed, confirming the entirely Ni-P bonding in FeN₄-C-NiP₄. EXAFS wavelet transform (WT) analysis present single intensity maximum at 3.5 Å⁻¹ for Fe K-edge and 5.4 Å⁻¹ for Ni K-edge, respectively (Supplementary Fig. 17). These could be assigned to the Fe-N and Ni-P path, further verifying the isolated feature of FeN₄ and NiP₄ in FeN₄-C-NiP₄. Quantitatively, the coordination numbers of Fe-N and Ni-P were extracted by EXAFS fitting. Due to the relative large difference in bond length, the coordination parameters of M-N and M-P could be well distinguished. As listed in Supplementary Tables 1 and 2, the coordination numbers for Fe-N were calculated to be 4.3 with bond length of 2.02 ± 0.01 Å. The coordination numbers for Ni-P were calculated to be 3.8 with a much longer bond length of 2.25 ± 0.02 Å. Thus the hetero-coordinated Fe₁-N₄ and Ni₁-P₄ configuration are determined to be the dominating structural motifs.

In order to investigate the role of ligands on tuning the electronic structure, the charge and spin density distribution, and projected density of states (PDOS) of FeN₄-C-NiP₄ were analyzed by DFT calculations, in comparison with N-coordinated FeN₄-C-NiN₄. Since the XPS and EXAFS verify negligible interaction between FeN₄ and NiP₄ sites, a relatively far Fe-Ni distance was simulated to avoid the interference. As shown in Fig. 2d, the charge density distribution of FeN₄ and NiN₄ sites for FeN₄-C-NiN₄ show similar intensity and scope due to the same N coordination. While introducing four P ligands to Ni center, the charge distribution scope of NiP₄ is largely pronounced with increased electron density around Ni center, resulting in electron-rich feature of NiP₄ sites. Comparatively, the FeN₄ sites is electron-deficient with depressed charge distribution. Thus the combination of FeN₄ and NiP₄ sites for FeN₄-C-NiP₄ shows an obviously enhanced surface charge polarization than N-coordinated FeN₄-C-NiN₄. Similarly, the spin gap in FeN₄-C-NiP₄ also becomes wider in comparison with FeN₄-C-NiN₄. As shown in Fig. 2e, the NiN₄ indicates a higher spin state (0.12) compare to that of the FeN₄ (-0.09). While introducing four P ligands into Ni center, the maximum spin density of NiP₄ further uplifts to 0.19, due to the contribution of 3p lone-pair electrons from P. As a result, the FeN₄-C-NiP₄ possess higher spin NiP₄ sites and relativity lower spin FeN₄ sites, illustrating that the heterogeneous coordination could further enlarge the spin difference of dual-metal sites. Moreover, the d band center of FeN₄ and NiP₄ sites in FeN₄-C-NiP₄ exhibit a larger difference in comparison with those in FeN₄-C-NiN₄. As shown in Fig. 2f, the PDOS location follows the order of FeN₄ (-0.208 eV) > NiN₄ (-1.956 eV) > NiP₄ (-2.951 eV). The FeN₄ with electron-deficient feature is endowed with an up-shifted d band center that closer to the Fermi level, while the d band center of NiP₄ is much lower than NiN₄, which is far away from Fermi level. This indicates the P ligands could further downshift the d band center of Ni center, resulting in enlarged difference in d band center of FeN₄ and NiP₄ sites for FeN₄-C-NiP₄. Overall, above results demonstrate the asymmetrical charge, spin, and PDOS deployment in FeN₄-C-NiP₄, which could simultaneously provide two atomic sites with electron accumulation and deficiency, or two atomic sites with high and low spin, as well as two atomic sites with close and far d band center towards Fermi level. Since these electronic properties are closely related to the adsorption behavior of oxygen-contained intermediates and thus determine the intrinsic activity of metal center^{23, 24}, the hetero-coordinated FeN₄-C-NiP₄ with stronger charge polarization, wider spin gap, and larger PDOS difference is promising for enlarging the difference in catalytic activity of atomic Fe and Ni sites.

Experimental and theoretical oxygen catalytic mechanism on of FeN ₄ -C-NiP ₄. Since the MN₄-C moiety has been affirmed to follow the activity order of Fe > Co > Ni for ORR, Ni > Co > Fe for OER, and Co > Ni > Fe for HER in alkaline medium^25–28, the FeN₄-C-NiP₄ dual-atom sites were applied to bifunctional ORR and OER in 0.1 M KOH. To identify the role of separated FeN₄ and NiP₄ sites and shed lights on the hetero-coordination for enhancing electro-catalytic selectivity, mono-component FeN₄, NiP₄, and NiN₄ were employed as references (structural details shown in Supplementary Figs. 18, 19, 12, respectively). The FeNi/Ni₂P nanoparticles synthesized without using SiO₂ template (Supplementary Fig. 20) and hollow C spheres without any metal doping were also used for comparison. As the linear sweep voltammetry (LSV) curves in Fig. 3a, the FeN₄-C-NiP₄ display similar onset potential (E_onset=1.02 V) and half-wave potential (E_1/2=0.89 V) compare to those of the FeN₄ (1.01, 0.89 V), which are much better than those of Pt/C (0.95, 0.83 V) and NiP₄ (0.85, 0.72 V). At 0.9 V, the FeN₄-C-NiP₄ exhibit the j_k of 1.44 mA cm^− 2, which is similar to that of the FeN₄ (1.53 mA cm^− 2) and much better than Pt/C and NiP₄ (Fig. 3b). The trend of Tafel slope is consistent with activity, that the FeN₄-C-NiP₄ and FeN₄ show faster ORR kinetics than Pt/C and NiP₄ (Supplementary Fig. 21a). Besides ORR, the FeN₄-C-NiP₄ also present outstanding OER activity, while the activity trend of reference samples are reversed. As shown in Fig. 3c, to reach a current density of 10 mA cm^− 2, FeN₄-C-NiP₄ require an overpotential (η) of 247 mV, which is slightly higher than NiP₄ (218 mV) and much lower than NiN₄ (313 mV), RuO₂ (350 mV), and FeN₄ (406 mV). At η = 300 mV, the FeN₄-C-NiP₄ reach a current density of 53.29 mA cm^− 2, which is similar to that of the NiP₄ (49.24 mA cm^− 2) and much higher than the other samples (Fig. 3d). The trend of Tafel slope is consistent with activity, that the FeN₄-C-NiP₄ and NiP₄ show much lower Tafel slope than NiN₄, RuO₂, and FeN₄, revealing the fast OER kinetics over FeN₄-C-NiP₄ and NiP₄ (Supplementary Fig. 21b). In addition, the hollow C spheres without any metal doping exhibit feeble activity for both ORR and OER, as shown in Supplementary Fig. 22. This illustrates that the hollow carbon spheres in FeN₄-C-NiP₄ are inactive for ORR and OER, verifying the pivotal role of Fe-N₄ and Ni-P₄ as active sites. The FeNi/Ni₂P nanoparticles synthesized without using SiO₂ template exhibit obviously degenerative activity for ORR and OER, in comparison with FeN₄-C-NiP₄ (Supplementary Fig. 23). This verifies the importance of atomic metal sites for enhancing the ORR and OER activity.

Overall, the ORR activity follows the order of FeN₄-C-NiP₄ ≈ FeN₄ > Pt/C > FeNi/Ni₂P > NiP₄ > hollow C spheres, while the OER activity follows the order of NiP₄ ≈ FeN₄-C-NiP₄ > FeNi/Ni₂P > NiN₄ > RuO₂ > FeN₄ > hollow C spheres. Accordingly, following conclusions could be deduced: i) the mono-component FeN₄ sites are highly active for ORR but poor for OER, meanwhile the mono-component NiP₄ sites are highly active for OER but poor for ORR. This illustrates that the FeN₄-C-NiP₄ could provide highly active FeN₄ and NiP₄ sites for ORR and OER, respectively; ii) the ORR activity of FeN₄-C-NiP₄ and FeN₄ is similar, so as the OER activity of FeN₄-C-NiP₄ and NiP₄. This illustrates no obviously synergistic effect is existed between FeN₄ and NiP₄ sites, which is possibly due to a relative far metal-metal distance; iii) the OER activity of NiP₄ atomic sites are much better than NiN₄ atomic sites, testifying the prominent role of four P ligands for promoting OER than conventional N coordination.

Inspired by the high ORR and OER activity, the FeN₄-C-NiP₄ exhibit a small potential gap of 0.59 V, surpassing most of the state-of-art bifunctional catalysts (Supplementary Fig. 24 and Fig. 3e). The ORR and OER stability of FeN₄-C-NiP₄ were evaluated by accelerated durability tests (ADTs) and chronoamperometry (Fig. 3f). The FeN₄-C-NiP₄ dual-atom sites manifest a remarkable long-term stability with negligible decay of E_1/2 (5 mV) for ORR and η_{10 mA cm−2} (8 mV) for OER after 5,000 cycles. Chronoamperometry also confirms the durability of FeN₄-C-NiP₄, exhibiting a small degradation of 3.4% and 5.8% after 18,000 s for ORR and OER, respectively. The practicability of FeN₄-C-NiP₄ as bifunctional catalysts was examined by Zn-air battery (Fig. 3g and Supplementary Fig. 25). The FeN₄-C-NiP₄-assemblied Zn-air battery could drive the LED light array for more than 24 h and exhibit a small charge-discharge voltage gap (1.18 V@10 mA cm^− 2). After 600 cycles (~ 200 h), it shows up an even better voltage gap to 1.02 V, outperforming the Pt/C + RuO₂ air-cathode (Supplementary Fig. 26).

DFT calculations were carried out to explain the origin of high ORR and OER activity on FeN₄-C-NiP₄ and unravel the explicit role of hetero-coordination. Figure 4a shows the DFT-optimized adsorption configuration of reaction adsorbates on FeN₄ and NiP₄ sites. The adsorption of O₂* is stronger at FeN₄ sites with bonding length of 2.337 Å, which is much shorter than NiP₄ sites (2.539 Å). On the contrary, the adsorption of OH* is stronger at NiP₄ sites with bonding length of 1.901 Å, which is shorter than FeN₄ sites (1.947 Å). These declare that the FeN₄-C-NiP₄ dual-atom sites possess two metal centers with contrasting adsorption behavior. Figure 4b displays the Gibbs free energy profiles on FeN₄ and NiP₄ sites, in an effort to identify the main active sites in FeN₄-C-NiP₄ that determine the ORR and OER, respectively. From left to right, the FeN₄ sites have more significantly exothermic proton-electron transfer steps than NiP₄ sites, indicating a favorable ORR pathway at FeN₄ sites. Moreover, the FeN₄ sites show a more favorable protonation of O₂* (ΔG_O2*→_OOH*= -0.82 eV) than NiP₄ (-0.59 eV), further authenticating a faster ORR kinetics on FeN₄ sites. For OER, the free energy difference between G_OOH* and G_OH* (ΔG_OOH*-_OH*)is found to be a key reaction descriptor, with the ideal value of 2.46 eV²⁹. The value of G_OOH*-G_OH* for NiP₄ sites (2.75 eV) is closer to 2.46 eV than that of FeN₄ sites (3.16 eV), indicating a favorable OER pathway at NiP₄ sites. Overall, above results suggest the oxygen catalytic selectivity of separated FeN₄ and NiP₄ sites in FeN₄-C-NiP₄, which are highly active for ORR and OER, respectively, thus their combination could balance the competition between rate-limiting steps of oxygen reduction/evolution, responsible for both high ORR and OER activity.

To shed lights on the role of hetero-coordination for enhancing the catalytic selectivity of atomic Fe and Ni sites, the ORR pathway at FeN₄ and FeP₄ sites (Fig. 4c and Supplementary Fig. 27), and OER pathway at NiN₄ and NiP₄ sites (Fig. 4d and Supplementary Fig. 28) were compared. For ORR, the FeN₄ and FeP₄ sites show very similar proton-electron transfer pathways, suggesting that for Fe center, selecting four N or four P coordination cause unconspicuous effect for promoting ORR. On the contrary, for OER, the NiP₄ and NiN₄ sites show disparate proton-electron transfer steps, wherein the first three electron-transfer steps at NiN₄ sites are too fast. As a result, the ΔG_OOH*-_OH* for NiN₄ (3.13 eV) is much far away from 2.46 eV than NiP₄ (2.75 eV). This verifies that for Ni center, four P coordination are more favorable for optimizing OER pathway than N coordination. Combining with the electronic properties illustrated in Fig. 2d-f, the correlation between ligands, electronic structure and oxygen reduction/evolution activity could be clarified. The PDOS is used to explain the improved ORR activity experimentally observed in FeN₄, as shown in Fig. 4e. According to the d band center theory, a closer d-band center to the Fermi level represents a strong binding ability for surface adsorbed O₂* species^{30, 31}. Accordingly, the electron-deficient FeN₄ sites with up-shifted d-band center are more favorable for adsorbing O₂* as reactant and possess a faster O₂* protonation pathway (as evidenced by the short O₂* bonding length of 2.337 Å and ΔG _O2*→_OOH*= -0.82 eV) compare to both NiN₄ and NiP₄ sites, resulting in higher ORR activity. The spin density is served to explain the strongly enhanced OER activity experimentally observed in NiP₄, as shown in Fig. 4f. For OER, the ΔG_OOH*-_OH* is largely restricted by the stability of O* adsorption, that either too strong or too weak O* adsorption leads to an increase in ΔG_OOH*-_OH*³². Significantly, the high spin NiP₄ sites could stabilize the unpaired electron of O* through the exchange interactions, balancing the too fast and too slow transformation to OOH* at NiN₄ and FeN₄ sites, respectively, resulting in an ideal ΔG_OOH*-_OH* of 2.75 eV and thus responsible for the outstanding OER activity. Overall, above results indicate the role of heterogeneous N, P coordination for respectively customizing the electronic structure of atomic Fe and Ni sites and thus enhancing the selectivity for oxygen catalytic reaction than conventional N coordination. Specifically, the FeN₄ sites with closer d-band center towards the Fermi level are responsible for ORR, while the NiP₄ sites with higher spin are more favorable for OER than both FeN₄ and NiN₄ sites, as schematic shown in Fig. 4g.

Generality of the synthesis of hetero-coordinated dual-atom sites. Above concept of using heterogeneous ligands to severally customize the electronic structure of dual atom sites could be extended to perform Janus-distributed CoN₄-C-NiP₄. The structural features was confirmed by HADDF-STEM image (Fig. 5a-b), EDX-mapping (Fig. 5c), HRTEM, XRD, and XPS analysis (Supplementary Figs. 29, 30). The dominated active sites for HER and OER in CoN₄-C-NiP₄ were first analyzed by DFT calculations (Fig. 5d). The free energy of hydrogen adsorption at CoN₄ is as low as 0.07 eV, which is much closer to 0 eV than that of the NiP₄ (1.35 eV), indicating more favorable HER kinetics at CoN₄ sites. The other side, the Gibbs free energy profiles for OER show a smaller ΔG_OOH*-_OH* value and a faster proton-electron transfer pathway at NiP₄ sites, in comparison with CoN₄ sites, suggesting a more favorable OER kinetics at NiP₄ sites. Thus the combination of separated CoN₄ and NiP₄ sites in CoN₄-C-NiP₄ is theoretically promising for generating high HER and OER activity, simultaneously. The experimental HER and OER performance of CoN₄-C-NiP₄ were examined by polarization curves obtained in 0.1 M KOH (Fig. 5e-f), and the trends were summarized in Fig. 5g. The η_{10 mA cm−2} for HER follows the order of Pt/C (96 mV) > CoN₄-C-NiP₄ (117 mV) ≈ CoN₄ (124 mV) > NiP₄ (249 mV). Meanwhile the η_{10 mA cm−2} for OER follows the order of CoN₄-C-NiP₄ (206 mV) ≈ NiP₄ (218 mV) > RuO₂ (350 mV) > CoN₄ (370 mV). These experimentally confirm that the separated CoN₄ and NiP₄ sites in CoN₄-C-NiP₄ possess different selectivity for HER and OER, well consistent with the DFT calculation. As a result, the CoN₄-C-NiP₄ dual-atom sites achieve a small voltage difference of 1.53 V to reach 10 mA cm^− 2 (Supplementary Fig. 31), surpassing most of the state-of-art non-precious bifunctional catalysts (Supplementary Table 4). The practicability of CoN₄-C-NiP₄ was evaluated by applied to bifunctional electrodes for water splitting. As shown in Fig. 5h, the CoN₄-C-NiP₄ electrodes require a cell potential of 1.544 V to afford 10 mA cm^− 2, which is even equal to that of the Pt/C//RuO₂, as well as exhibit a long-range stability operated for 24 h (Supplementary Fig. 32).

In summary, we realize the integration of two spatial-separated atomic sites in one system and select heterogeneous ligands to severally regulate their electronic structure, aiming at simultaneously achieving high oxidative and reductive activity. Through layer-by-layer electrostatic absorption, space-confinement pyrolysis and anisotropic coordination, Janus-distributed FeN₄-C-NiP₄ (or CoN₄-C-NiP₄) with hetero-coordination could be fabricated. The heterogeneous N, P coordination enlarges the difference in electronic structure of atomic Fe and Ni sites, generating higher spin NiP₄ with electron accumulation but lower spin FeN₄ with electron deficiency, providing an ideal model to systematically study the structure-function relationship towards oxidative/reductive reaction. Electrochemical tests and DFT calculation verify that the FeN₄sites with upshifted d-band center are dominated active sites for ORR, while the NiP₄ sites with higher spin are more active for OER, enabling FeN₄-C-NiP₄ to balance the competition between rate‐limiting steps of ORR and OER. This work realizes the heterogeneous ligands design on dual metal sites, which opens an avenue for simultaneously catalyzing various complex oxidative/reductive reactions in one system.

Acknowledgements

X.Q. acknowledges the financial supports from National Natural Science Foundation of China (21902078). Y.T. acknowledges the financial supports from National Natural Science Foundation of China (21875112). We also acknowledge the supports from National and Local Joint Engineering Research Centre of Biomedical Functional Materials and a project sponsored by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

Author contributions

X.Q. conceived the idea. X.Q., K.S., and A.Z. performed the experiments. X.Q. and Y.T. analyzed the data and supervised the project. X.Q. wrote the manuscript with contributions from all coauthors. All authors discussed the results.

Competing interests

The authors declare no competing interests.

Additional information

Supplementary information The online version contains supplementary material available at https://

Synthesis of hetero-coordinated FeN₄-C-NiP₄ and CoN₄-C-NiP₄. Firstly, SiO₂ nanospheres with a diameter of 150 nm were synthesized via a conventional stöber method³³. Then a layer-by-layer self-assembly approach was served to prepare PSS/PDDA/PSS/PDDA-modified SiO₂ according to our previously study.³⁴ This step could make the SiO₂ to be strongly negative-charged (denoted as SiO₂^-), in an effort to further adsorbing species with positive charges.

For a typical synthesis of Janus-distributed FeN₄-C-NiP₄, 100 mg of SiO₂^- nanospheres and 160 mg of Fe(NO₃)₃·9H₂O were added into 10 mL of deionized water and ultrasonically dispersed for 10 min. Then the suspension was centrifuged to remove the excess Fe³⁺ in the supernateand the precipitated SiO₂^-@Fe³⁺ spheres was picked up and re-dispersed in 10 mL of deionized water for further use. Subsequently, 500 mg of AMP was added into above solution and the pH value was adjusted to 4~5 (lower than isoelectric point), stirring for 1 h to obtain the SiO₂^-@Fe³⁺@AMP⁺ suspension. Then the solution was centrifuged to remove the excess AMP⁺ in the supernateand the precipitated SiO₂^-@Fe³⁺@AMP⁺solid was picked up and re-dispersed in 10 mL of deionized water. Afterwards, 80 mg of K₂[Ni(CN)₄] was added into above mixture and stirred for 1 h to form the SiO₂^-@Fe³⁺@AMP⁺@[Ni(CN)₄]^2-. After centrifuge to remove the excess [Ni(CN)₄]^2-, vacuum drying, and grinding, the resultant SiO₂^-@Fe³⁺@AMP⁺@[Ni(CN)₄]^2- powder was placed in a porcelain boat, and calcined at 700 ℃ under N₂flow for 3 h (ramp rate: 3 ^oC min^-1) to form the black SiO₂@FeN₄-C-NiP₄powder. At last, the products were leached in 10 mL of HF solution (40%) overnight and then washed to neutral to acquire the hetero-coordinated FeN₄-C-NiP₄ hollow spheres (yielding: ca. 0.358 g).

The synthesis of hetero-coordinated CoN₄-C-NiP₄ was similar to that of the FeN₄-C-NiP₄, except for using 200 mg of Co(NO₃)₃·9H₂O to substitute the Fe(NO₃)₃·9 H₂O.

Synthesis of mono-component FeN₄, NiP₄, NiN₄, and hollow C spheres. A series of mono-component single-atom catalysts were prepared to authenticate the main active sites in FeN₄-C-NiP₄ and investigate the role of hetero-coordination for bifunctional catalysis. The FeN₄ was synthesized by using the standard procedure of FeN₄-C-NiP₄ except for without adding K₂[Ni(CN)₄], as schematic illustrated in Supplementary Fig. 18. Likewise, the NiP₄ was synthesized by using the standard procedure except for without adding Fe(NO₃)₃·9H₂O, as schematic illustrated in Supplementary Fig.19. The synthesis of NiN₄ with pure nitrogen ligands was similar to that of the NiP₄, except for using the His⁺ to substitute AMP⁺, in an effort to eliminate the P source (as shown in Supplementary Fig.12). The C hollow spheres without any metal sites was synthesized by directly coating the AMP⁺ onto the surface of SiO₂^- to form the SiO₂^-@ AMP⁺ spheres. Subsequently, the calcining and leaching process was served to obtain the hollow C spheres.

Characterizations. HRTEM were performed on JEOL JEM-2100F transmission electron microscopy. AC-HAADF-STEM images were recorded on JEOL JEM-ARM 200F at an accelerating voltage of 200 k eV. SEM and EDX analysis were performed with a Hitachi S5500 SEM/STEM. XRD were recorded on Rigaku MiniFlex 600 I diffractometer with Cu K α radiation (λ= 1.5406 Å). ICP-MS was analyzed on Thermo X Series II ICP/MS quadrupole system, Thermo Fisher Scientific. BET surface area was analyzed on Micromeritics ASAP 2050 system at 77 K. Raman spectrum was recorded on Raman spectrometer (LabRAM HR800, λ=514 nm). UV-vis spectra were measured on a Shimadzu UV3600 spectrophotometer at 25 ^oC equipped with 1.0 cm quartz cells. XPS measurements were conducted on a Thermo VG Scientific ESCALAB 250 spectrometer with an Al K α radiator. The binding energy was calibrated by means of the C 1s peak energy at 284.6 eV. XAFS spectra were measured at the beamline BL14W1 station of the Taiwan Synchrotron Radiation Facility. The obtained XAFS data was processed in Athena (version 0.9.26) for background, pre-edge line and post-edge line calibrations. Then Fourier transformed fitting was carried out in Artemis (version 0.9.26). For Wavelet Transform analysis, the χ(k) exported from Athena was imported into the Hama Fortran code. The parameters were listed as R range of 1-4 Å, k range of 0-13 Å^-1, k weight of 2, and Morlet function with κ=10, σ=1 was used as the mother wavelet to provide the overall distribution.

Electrochemical measurements. All electrochemical measurements were performed on a CHI 760D electrochemical analyzer (Shanghai Chenhua Co.) by using a standard three electrode system, which was consisted of the rotating ring-disk electrode (RRDE) covered with the catalysts as the working electrode (0.196 cm²), a saturated calomel electrode (SCE) as the reference electrode, and a graphite rod as the auxiliary electrode. All potentials were calibrated into the reversible hydrogen electrode (RHE), using the following equation: E_RHE = E_SCE + 0.0592 pH + 0.242. The catalyst ink was prepared by ultrasonically dispersing the mixture of 5 mg of catalyst, 0.9 mL ethanol, and 0.1 mL of Nafion (5 wt.%). Then 10 μL of the catalysts was casted on electrode and dried at room temperature. The ORR measurements were performed on the RRDE voltammograms in 0.1 M KOH electrolyte at a rotation rate of 1600 rpm and a scan rate of 5 mV s^-1. Before testing, O₂ flow was purged into the electrolyte for at least 30 min to saturate the electrolyte. The OER and HER measurements were evaluated by linear sweep voltammetry (LSV) in N₂-saturated 0.1 M KOH electrolyte with a scan rate of 5 mV s⁻¹. The linear region of the plots were fitted using the Tafel formula Z = b log(j) + a, where Z referred to overpotential, j referred to current density, and b referred to the Tafel slope.

Zn-air battery and overall water splitting tests. The Zn-air battery tests were performed on a homemade Zn-air cell. The air cathode was consisted of a hydrophobic carbon paper with catalyst layer on the electrolyte-facing side and a gas-diffusion layer on the air-facing side. The catalyst ink was dropped and painted onto the carbon paper (1.0 cm²) to form the catalyst layer with loading of 10 mg cm^-2. A well-polished Zn plate with a thickness of 0.3 mm was served as anode and the 0.2 M ZnCl₂ + 6 M KOH solution was used as electrolyte. The cycling stability tests were performed on Land CT2001A system with 5 min rest time between each discharge and charge at a current density of 5 mA cm^-2. Each discharge and charge period was set to be 10 min.

The overall water splitting measurements was conducted in a two-electrode system composed of two pieces of nickel foams (3 cm ×1 cm). The pre-cleaned nickel foams were wrapped with Teflon tape at the 2 cm ×1 cm area from one end, leaving the rest 1 cm × 1 cm area exposed for the deposition of catalyst with loading mass of 2 mg cm^-2. The overall water splitting tests were performed in 0.1 M KOH electrolyte at a scan rate of 5 mV s^-1 by using the catalyst-modified nickel foams as cathode and anode, respectively.

Theoretical calculation. The Vienna Ab Initio Package (VASP) was employed to perform all the density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the PBE formulation. The projected augmented wave (PAW) potentials were served to describe the ionic cores and took valence electrons into account using a plane wave basis set with a kinetic energy cutoff of 400 eV. Partial occupancies of the Kohn-Sham orbitals were allowed using the Gaussian smearing method and a width of 0.05 eV. The electronic energy was considered self-consistent when the energy change was smaller than 10⁻⁵ eV. A geometry optimization was considered convergent when the force change was smaller than 0.05 eV/Å. During structural optimizations, the 3×3×1 Monkhorst-Pack k-point grid for Brillouin zone was used for k-point sampling for structures. The Fe-N₄ and Ni-P₄ structures were modelled as the Fe single-atom doped at the carbon layer with four N ligands and Ni single-atom doped at the carbon layer with four P ligands. All the atoms were allowed to relax during geometric optimization. The free energy (ΔG) for elemental reaction step were calculated as: ΔG = ΔE + ΔE_ZPE- TΔS, where ΔE is the difference between the total energy, ΔE_ZPE and ΔS are the differences in the zero-point energy and the change of entropy, T is the temperature respectively.

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Integrating hetero-coordinated FeN4 and NiP4 single-atomic sites in one system for oxygen reduction/evolution

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