Synthesis of MnM–NC (M = Ga, In, Sn) dual-single-atom catalysts for efficient electrocatalytic oxygen reduction

Abstract

The oxygen reduction reaction (ORR) is an important reaction in the field of energy conversion and storage. Manganese-based catalysts are expected to be alternative candidates to iron-based or platinum-group catalysts by virtue of their low cost, good stability and resistance to Fenton reactions. The easily tunable localized p-orbitals of p-block metals and the d-orbitals of manganese can hybridize, which facilitates the oxygen reduction process. Here, we have designed and synthesized a series of dual-single-atom catalysts that consist of Mn and p-block metal single atoms dispersed on N-doped carbon, denoted as MnM–NC (M = Ga/In/Sn). After introducing p-block metal single atoms, the half-wave potential of the catalysts increases to 0.85 V (vs. RHE), and their stability is also improved, even surpassing that of commercial Pt/C. Theoretical calculations show that the dual-single-atom sites could adjust the d-band center of the Mn site, weakening the adsorption strength of the intermediate *OH, thus improving the ORR catalytic activity. This work provides insights into the coupling of Mn–NC species with p-block metals to improve ORR performance and also inspires more structural and application innovations of p-block metals in the ORR research field.

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Hongfei Cheng

Hongfei Cheng is currently an assistant professor at Tongji University, Shanghai, China. She received her PhD in 2020 from Nanyang Technological University (NTU), Singapore, under the supervision of Prof. Hua Zhang. Then, she worked as a postdoctoral researcher at NTU and the Agency for Science, Technology and Research (A*STAR), Singapore. She joined Tongji University in November 2022 and established her research group, which is currently developing new nanomaterials for catalytic-related applications, such as electrocatalytic hydrogen generation, fuel cells, and nanozymes.

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1 Introduction

The electrocatalytic oxygen reduction reaction (ORR) is key to the overall efficiency of emerging energy storage and conversion devices, such as fuel cells^1–3 and metal–air batteries.⁴ Platinum group catalysts are still the best choice for the ORR,^5–7 but their high cost and low reserves hinder wide application. Therefore, single-atom catalysts (SACs), especially carbon nanomaterials doped with nitrogen and transition metal single atoms, have gained notable attention for the ORR,^8,9 among which Fe–N–C nanomaterials show good performance comparable to Pt/C under alkaline conditions. However, Fe–N–C nanomaterials suffer from poor stability due to the inevitable Fenton reactions.¹⁰ Alternatively, Mn-based materials, where Mn is also an abundant and inexpensive metal, have been demonstrated to be promising ORR catalyst candidates.^11–15 Mn–N–C materials have been proved by theoretical calculations to have comparable performance to Fe–N–C,¹⁶ with the absence of the Fenton reactions and better durability.^17,18 To improve the performance of Mn–N–C materials for commercial applications, introducing non-metallic atoms such as B,¹⁹ P,²⁰ S,^21,22 and O^23–25 and incorporating a second transition metal, such as Fe^26–29 and Co,³⁰ are two common strategies, which can break local structural symmetry and induce charge redistribution, thus optimizing the adsorption energy of oxygen intermediates.

In addition to the commonly studied d-block metals, recent studies have demonstrated the electrocatalytic feasibility of main-group elements for the ORR, such as p-block metal single atom sites.^31–37 The p–d orbital hybridization arising from p-metals and d-metals helps to adjust the adsorption strength of electrocatalytic intermediates,^38,39 based on which d–p dual-single-atom pairs such as Fe–Sn,^40,41 Fe–Sb⁴² and Fe–Al⁴³ have been developed. However, the coupling of p-block metals with Mn single atoms has not been reported.

Inspired by the above studies, we develop Mn–N–C SACs doped with atomically dispersed p-block metals, including Ga, In and Sn, and demonstrate that all of these p-block metals can improve the ORR activity. Theoretical calculations show that p-block metal single-atom sites can adjust the d-band center of the Mn single-atom site through p–d orbital hybridization, thus attenuating the over-adsorption of *OH.

2 Results and discussion

2.1 Morphology and structure characterization

The catalysts with a dual-single-atom structure were obtained by a two-step pyrolysis method,⁴⁴ which is shown in Fig. 1a. First, a ZIF-8 precursor with good crystallinity was synthesized (Fig. S1) and subjected to pyrolysis to obtain nitrogen-doped carbon (NC). Two very broad XRD peaks centered at ∼25° and ∼43° can be attributed to the (002) and (101) crystal planes of carbon⁴⁵ (Fig. 1b), indicating that the obtained NC has an ordered graphitic structure in the short range but is disordered in the long range. Subsequently, Mn ions and p-block metal ions were adsorbed onto NC, and then secondary pyrolysis was performed to obtain dual-single-atom catalysts, denoted as MnM–NC (M = Ga/In/Sn). Mn–N–C was also prepared by following the same procedures without adding p-block metal precursors. The XRD patterns of the catalysts only show the characteristic peaks of carbon when the p-block metal content is lower than the Mn content (Fig. 1b and S2–S4); therefore, it is speculated that all the metal sites in these catalysts are atomically dispersed. By tuning the content of the p-block metal precursors, it was found that when the p-block metal accounted for 15 wt%, the ORR activity reached the optimum (Fig. S5–S8). Hence, in the following discussions, the p-block metal content is 15 wt% for all the MnM–NC catalysts unless otherwise specified.

Fig. 1 (a) Schematic diagram of the preparation process of catalysts with a dual-single-atom structure. (b) XRD patterns of NC, Mn–NC, MnGa–NC, MnIn–NC and MnSn–NC catalysts. (c) SEM images of Mn–NC, MnGa–NC, MnIn–NC and MnSn–NC.

The SEM images show that ZIF-8 has a typical rhombic dodecahedron shape with good uniformity and NC shows a similar morphology (Fig. S9). The pore structure on the NC surface could be observed. After introducing the metal precursors and secondary pyrolysis, the morphology of the products was still well preserved (Fig. 1c). Transmission electron microscopy (TEM) images of MnGa–NC show a morphology similar to that of ZIF-8 (Fig. S10), and no Mn or Ga agglomerates were observed (Fig. 2a and S11). The high-resolution TEM image of MnGa–NC (Fig. S12) shows a worm-like characteristic of amorphous carbon. TEM-energy dispersive X-ray spectroscopy (EDS) mappings (Fig. 2b and S13) show a uniform distribution of the elements Mn, Ga and N.

Fig. 2 (a) TEM image of MnGa–NC. (b) EDS elemental mapping of the MnGa–NC catalyst. (c) HAADF-STEM image of Mn–NC. (d) The enlarged region in c, with single-atomic sites in the red dashed circle. (e) HAADF-STEM image of MnGa–NC. (f) The enlarged region in (e), with single-atomic sites in the red dashed circle and potential dual-single-atom sites in the orange dashed rectangle.

More in-depth fine characterization was carried out in order to visualize the presence of single atoms as well as the potential dual-single-atom structure. For this purpose, aberration-corrected high-angle annular dark-field scanning TEM (AC-HAADF-STEM) images were collected. Mn–NC and MnGa–NC were selected as representative samples for comparison. Due to the higher atomic number of Mn and Ga compared to carbon and nitrogen, they appear as bright spots on the images. Fig. 2c and d show the AC-HAADF-STEM images of the Mn–NC sample, where the monatomic Mn sites are indicated by red dashed circles. Fig. 2e and f show the AC-HAADF-STEM images of the MnGa–NC sample, where the potential dual-single-atom sites are marked by orange rectangular dashed boxes. Comparisons of more regions are provided in Fig. S14–S24 to illustrate the atomic dispersion of metal elements. Comparing the AC-HAADF-STEM images of Mn–NC and MnGa–NC, it can be visually observed that MnGa–NC has a higher density of atomic metal sites. This may be due to the interaction between Mn and Ga, which would facilitate their anchoring on the substrate. The typical regions #1 and #2 from Mn–NC and MnGa–NC were subsequently selected to be enlarged and more intuitively illustrated in 2D and 3D forms (Fig. S25). The bright spots in #1 are more widely spaced and are considered to be isolated monatomic sites. The bright spots in #2 are more compact and can be empirically considered as potential dual-single-atom sites, which may exist in the form of Mn–Ga, Ga–Ga or Mn–Mn based on the metal precursor casting. More brightness line profile analysis was performed to demonstrate the dual-single atom sites (Fig. S26). The metal loading in each single-atom sample was also measured by inductively coupled plasma emission spectroscopy (ICP-OES), as summarized in Table S1. The Mn and Ga contents in MnGa–NC were 2.41 wt% and 0.42 wt%, which is consistent with the Ga cast amount of 15 wt%. The Mn and In contents in MnIn–NC were 2.16 wt% and 0.37 wt%, and the contents of Mn and Sn in MnSn–NC were 2.33 wt% and 0.35 wt%, respectively. The loadings were similar and all of them were in accordance with the cast. The Mn content in Mn–NC was 1.80% and the metal loading was less than that of samples doped with p-block metals, which is also consistent with the AC-HAADF-STEM image comparison. In conclusion, monoatomic Mn and dual-single-atom structure catalysts were successfully prepared.

Subsequently, X-ray spectroscopy was used to probe the microstructure and chemical state information of the samples. First, the surface elemental valence and bonding information of the samples were investigated using X-ray photoelectron spectroscopy (XPS). In the C 1s high-resolution spectrum, the peaks at 284.8 eV, 285.6 eV, and 288.9 eV can be attributed to C=C bonding, C=N/C–N bonding, and C–O bonding, respectively (Fig. S27).^18,43 For the N 1s high-resolution spectra, the peaks at about 398.4 eV, 399.1 eV, 400.5 eV, 401.4 eV and 403.7 eV can be attributed to pyridine-N, metal-N, pyrrole-N, graphite-N and oxidative-N species, respectively (Fig. S28).^9,17,41 It can be found that the percentage of metal-N in the series of samples containing p-block metals is slightly enhanced relative to that of Mn–NC (Fig. S29), which, combined with the AC-HAADF-STEM results and the ICP-OES results, can be deduced to be the contribution from the increase of the metal single-atom sites. The pyridine-N is also thought to contribute to the ORR activity.⁴⁶ It can be found that all the MnM–NC catalysts have similar N and C compositions, indicating that the N-doped carbon matrix across different catalysts have similar structures. For the XPS spectra of Mn 2p, the 2p_3/2 peaks of the samples doped with p-block metals show a slight negative shift compared with Mn–NC, indicating the Mn in MnM–NC gains more electrons from coordinated N (Fig. S30).

To further reveal the atomic coordination structure of the catalysts, X-ray absorption near edge structure (XANES) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy were applied. In the Mn K-edge XANES spectra (Fig. 3a), the absorption edges of the Mn–NC and MnGa–NC samples are located between MnO(II) and Mn₂O₃(III) and are closer to MnO, suggesting that the Mn in the catalysts has a mixed valence state that is closer to Mn(II). The average valency of Mn in Mn–NC is estimated to be +2.45 and decreased to +2.38 for MnGa–NC (Fig. S31), which aligns with the XPS results. In the Fourier transformed (FT)-EXAFS spectra of the Mn K-edge in R-space (Fig. 3b), for the sample Mn–NC, a strong peak at about 1.3 Å is attributed to the coordination scattering of Mn–N as well as Mn–C. For MnGa–NC, in addition to the major Mn–N peak at 1.3 Å, there is a small peak at 2.4 Å, which can be assigned to dual-single-atom sites, such as Mn–Ga. For both samples, no Mn–O peak is detected at 1.5 Å, and no significant Mn–Mn scattering path is detected at 2.3 Å. This suggests the atomic dispersion of Mn in the samples without the formation of oxide particles or metal clusters, which is further confirmed by the wavelet transforms of FT k³-weighted EXAFS spectra at the Mn K-edge (Fig. S32). The R-space of the Mn K-edge was further fitted for Mn–NC and MnGa–NC. For Mn–NC (Fig. S33 and S34), the dominant scattering path is Mn–N, and the model (inset in Fig. S33) matches the fitting well; meanwhile for MnGa–NC (Fig. 3c and S35), the overall N coordination number is still about 4 but the coordinated N is a little different due to the asymmetric structure. It is worth noting that the Mn–Ga scattering paths make huge contributions to good fitting (Table S2). The XANES spectra of the Ga K-edge show that the adsorption edges of Ga–NC and MnGa–NC are close to that of the Ga₂O₃ reference, demonstrating a relatively high valency of Ga, and the absorption edge of MnGa–NC is also negatively shifted compared with Ga–NC (Fig. 3d). Referring to the R-space spectra of Ga₂O₃ and reported Ga foil,⁴⁷ in the R-space spectrum of the MnGa–NC catalyst, the Ga–Ga peak is absent, whereas the main peak appears at 1.4 Å and an additional small peak at around 2.4 Å can be found, which are attributed to Ga–N and Ga–Mn scattering, respectively (Fig. 3e). Since the main peak Ga–N is very close to the Ga–O peak (Fig. 3e), wavelet transforms for FT k³-weighted EXAFS spectra at the Ga K-edge were performed to distinguish them (Fig. 3g–i), which suggest that the Ga element in MnGa–NC is atomically coordinated with N atoms and no Ga oxides are formed. A fine fitting was also done for MnGa–NC and Ga–NC at the Ga K-edge (Fig. 3f and S36–S38), and the peak around 2.4 Å for MnGa–NC is similarly attributed to the scattering between Mn and Ga (Table S3). From the EXAFS results of both the Mn K-edge and the Ga K-edge, it can be concluded that the Mn–Ga scattering path plays an irreplaceable role in the fitting of the MnGa–NC sample, implying that such Mn–Ga dual-single-atom sites exist in the catalyst. In summary, the above analyses demonstrate that Mn and Ga are atomically dispersed in MnGa–NC and the dual-single-atom structure of Mn–Ga is indeed present in MnGa–NC.

Fig. 3 (a) The normalized Mn K-edge XANES spectra. (b) The Fourier transform of k³-weighted EXAFS spectra of Mn–NC, MnGa–NC and related references at the Mn K-edge. (c) EXAFS fitting curve in R-space of MnGa–NC at the Mn K-edge (inset shows the fitted model, with purple representing Mn atoms, proto-green Ga atoms, silver-grey N atoms, and brown C atoms). (d) The normalized Ga K-edge XANES spectra. (e) The Fourier transform of k³-weighted EXAFS spectra of Ga–NC, MnGa–NC and related references at the Ga K-edge. (f) EXAFS fitting curve in R-space of MnGa–NC at the Ga K-edge. WT for FT k³-weighted EXAFS spectra of (g) Ga₂O₃, (h) Ga–NC, and (i) MnGa–NC at the Ga K-edge.

2.2 Electrocatalytic ORR performance

The electrochemical ORR activity of all samples was evaluated in 0.1 M KOH electrolyte using a rotating disk electrode (RDE). Linear sweep voltammetry (LSV) curves (Fig. 4a) show that the NC catalyst exhibited the worst activity (E_1/2 = 0.753 V vs. RHE). In contrast, Mn–NC demonstrated significant performance enhancement (E_1/2 = 0.821 V vs. RHE), indicating the existence of highly active sites such as the Mn–N₄ moiety. After introducing p-block metal single atoms into Mn–NC, E_1/2 was further improved by around 30 mV, rivaling commercial 20 wt% Pt/C (E_1/2 = 0.844 V vs. RHE). It is noted that these three kinds of p-block meals induce similar activity enhancement. This suggests that synergistic coupling between p-block metals and Mn centers facilitates oxygen intermediate adsorption/desorption. Tafel slope analysis (Fig. 4b) reveals that Mn–NC already exhibits superior kinetics to 20 wt% Pt/C, with further kinetic enhancement upon p-metal incorporation. These catalysts containing p-block metals showed higher current densities at 0.8 V (J_k) than Pt/C, indicating accelerated reaction kinetics (Fig. 4c). Electrochemical double-layer capacitance (C_dl) measurements (Fig. 4d and S39) revealed that introducing p-block metals can improve the electrocatalytically active surface area, and the specific activities were also improved (Fig. S40), implying that the incorporation of p-block metals can expose more active sites as well as increase the intrinsic activity. Rotating ring disk electrode (RRDE) measurements (Fig. 4e and S41) showed that the Ga-optimized catalysts can maintain an electron transfer number (n) ≈ 3.95 and HO₂⁻ yield < 5% across a wide range of potentials, indicating that dual-single-atom configurations favor 4e⁻ pathways. LSV curves at different rotational speeds (625–2025 rpm) were also tested and the number of transferred electrons was calculated based on the K–L equation (Fig. S42–S46), the results of which were in general agreement with the RRDE tests.

Fig. 4 (a) Comparison of LSV polarization curves for various catalysts in 0.1 M KOH. (b) Tafel plots for various catalysts. (c) The kinetic current densities at 0.80 V vs. RHE and E_1/2 (half-wave potential) for various catalysts. (d) Electrochemical double-layer capacitance measurements of various catalysts. (e) Electron transfer number and HO₂⁻ yield in the ORR process of various catalysts. (f) The i–t test of MnGa–NC and 20 wt% Pt/C. (g–i) ADT (accelerated durability test) for MnGa–NC, MnIn–NC and MnSn–NC.

The stability of the catalyst was tested accordingly. MnGa–NC exhibited exceptional stability with about 90% current retention after a 40 000 s chronoamperometry test at 0.75 V vs. RHE (Fig. 4f), outperforming commercial 20 wt% Pt/C (87% retention). MnIn–NC and MnSn–NC also exhibited >90% current retention, outperforming Mn–NC and NC (Fig. S47). Moreover, accelerated cyclic voltammetry tests show that Mn–NC (Fig. S48) and MnM–NC (Fig. 4g–i) catalysts demonstrate superior stability to commercial 20 wt% Pt/C even under a high loading amount (Fig. S49 and S50). The methanol resistance test (Fig. S51), which is important for direct methanol fuel cells, shows that p-block metal doped catalysts exhibited good resistance to methanol compared with Mn–NC, while the oxygen reduction efficiency of commercial Pt/C significantly decreased in the presence of methanol.

2.3 Theoretical calculations

To gain a deeper theoretical understanding of the origins of the oxygen reduction activity of p-block metal-doped Mn–N–C catalysts, density functional theory (DFT) simulations were performed by modeling Mn–N₄, Ga–N₄ and the coupling-generated MnGa–N₆ on the graphene network structure (Fig. S52–S59). We use Ga as a representative p-block metal for preliminary theoretical analysis. Initially, both the association mechanism (Fig. 5a and S60) and the dissociative mechanism of the ORR process are considered (Fig. S61), but it was found the former is more likely to occur. Therefore, the following discussions are based on the association mechanism. The adsorption energies of each intermediate for the three structural models are calculated by setting the potential at U = 1.23 V (Fig. 5b). For Ga–N₄, its extremely high *OH adsorption strength hinders the H₂O generation in the last step, which in turn hinders the regeneration of active sites, resulting in low ORR performance. For the Mn–N₄ structure, similarly, the protonation desorption process of *OH needs to overcome the maximum energy barrier and thus is the rate determining step (RDS) of the overall reaction. Compared to Mn–N₄, the introduction of Ga, i.e., the MnGa–N₆ model, facilitates both the *O to *OH and *OH to H₂O transitions, and more importantly, weakens the *OH adsorption strength, thus facilitating the overall reaction process.

Fig. 5 (a) The diagram of the ORR mechanism on the MnGa–NC active site in an alkaline environment, in which brown atoms represent carbon atoms, silver-grey atoms represent nitrogen atoms, purple atoms represent manganese atoms and green represents gallium atoms. (b) Comparison of the step diagrams of the oxygen reduction process on the Mn–N₄, Ga–N₄, and MnGa–N₆ sites. (c) Calculation of the PDOS of the 3d orbitals of Mn and the 4p orbitals of Ga in the MnGa–N₆ model. (d) Comparison of the calculation of the d-band center of Mn in Mn–N₄ and MnGa–N₆. (e and f) Schematic diagrams of differential charge densities on the sites of Mn–N₄ and MnGa–N₆, with the yellow area representing the accumulation of electrons and the blue area representing the loss of electrons.

Subsequently, to reveal the specific origin of activity in the gallium-containing samples, we calculated the projected density of the p-orbitals of Ga and the d-orbitals of Mn. It is found that the d–p orbital hybridization originates from the partial overlap of energies between their d–p orbitals (Fig. 5c and S62). A comparison of the d-band center of metal Mn in Mn–N₄ and modified MnGa–N₆ reveals that the d-band center is downshifted from the original −2.156 eV to −2.238 eV after introducing Ga (Fig. 5d and S63). According to d-band center theory,⁴⁸ the downshift of the d-band center can alleviate the over-adsorption of oxygen species and facilitate the overall reaction process. Although In and Sn differ from Ga in their electronic structures, they also exhibit strong p–d orbital hybridization with Mn (Fig. S64 and S65). Further theoretical calculations indicate that all three p-block metal-doped catalysts have a reduced RDS (*OH → H₂O) energy barrier under p–d hybridization (Fig. S66). The charge density difference map can visually reveal the high-density electron enrichment on the Mn sites, which is favorable for the adsorption and activation of O₂. Through the quantitative calculation of Bader charge, Mn and Ga in MnGa–N₆ get more electrons compared to Mn in Mn–N₄ and Ga in Ga–N₄, respectively, which is consistent with the XAS and XPS results. The Mn and Ga atoms in MnGa–N₆ gain an additional 0.021 and 0.013 electrons, respectively, which are thought to come from the co-transfer of the surrounding nitrogen (Fig. 5e, f and Table S4). In summary, the introduction of p-block metals can adjust the d-band center of the Mn center and optimize the adsorption of oxygen species.

3 Conclusions

In summary, we have synthesized a series of Mn single-atom electrocatalysts for the ORR, which are characterized by the dual-single-atom structure composed of Mn sites and p-block metals (Ga, In, and Sn). Experimental characterization and theoretical modeling demonstrate the coupling between the d orbital of Mn and the p orbital of p-block metals. After introducing p-block metals, the ORR activity was improved and comparable with that of commercial Pt/C. DFT calculations show that the introduction of p-block metals can adjust the d-band center of Mn single-atom sites, modulate the adsorption and desorption strengths of oxygen species, and lower the RDS energy barriers, thereby improving ORR kinetics. All the MnM–NC catalysts exhibited good catalytic stability that is superior to commercial Pt/C. We expect that this work will inspire more innovative studies on p-block metals in the field of the ORR.

Author contributions

Yujie Cui and Yihong Liu contributed equally to this work. Yujie Cui did most of the experiments, analysed the data and wrote the original draft; Yihong Liu conducted DFT calculations; Jiayi Li, Haibin Ma, Tingting Pan, Hainan Wei, Xinyue Shi, Xiaoyan Zhou, Ping Zhang, Weixing Niu, Shengnan Sun, and Menghao Yang provided guidance on the DFT calculations; Wei-Hsiang Huang provided resources for X-ray absorption investigation and supervised the data analysis; Jiwei Ma provided guidance on the methodology and supervision, including funding acquisition; Hongfei Cheng conceived the idea and supervised the whole project. All authors contributed to the writing – review & editing.

Conflicts of interest

The authors declare that they have no conflicts of interest.

Data availability

The data supporting this article have been included as part of the SI. Supplementary information is available. See DOI: https://doi.org/10.1039/d5ta05397h.

Acknowledgements

This work was funded by the National Natural Science Foundation of China (22405191), the Natural Science Foundation of Shanghai Municipality (24ZR1468100) and the Fundamental Research Funds for the Central Universities to H. C. We thank the National Synchrotron Radiation Research Center (beamline TLS 17C and TPS 44A) for the allocation of synchrotron beam time under Proposal No. 2024-2-027–1.

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Footnote

† These authors contributed equally to this work.

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