Carbon nanosheet-supported CrN nanoparticles as efficient and robust oxygen reduction electrocatalysts in acidic media and seawater Zn–air batteries†

Yating Zhang^a, Haoming Wu^a, Jun Ma^c, Junming Luo*^a, Zhe Lu^a, Suyang Feng^a, Yijie Deng*^b, Hui Chen^a, Qi Wang^a, Zhengpei Miao^a, Peng Rao^a, Neng Yu^d, Yuliang Yuan^a, Jing Li^a and Xinlong Tian*^a
aSchool of Marine Science and Engineering, Hainan Provincial Key Lab of Fine Chemistry, Hainan University, Haikou 570228, China. E-mail: luojunming@hainanu.edu.cn; tianxl@hainanu.edu.cn
^bSchool of Resource Environment and Safety Engineering, University of South China, Hengyang 421001, China. E-mail: dengyijie19891009@163.com
^cSchool of Materials Engineering, Jiangxi College of Applied Technology, Ganzhou 341000, China
^dState Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang 330013, China

First published on 23rd July 2024

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Introduction

Developing clean and efficient renewable energy conversion technologies has become a global demand. As energy conversion technologies that convert chemical energy into electricity, fuel cells and metal–air batteries have drawn enormous attention for their advantages of low operating temperature, high energy conversion efficiency, and no pollution emission.¹ The oxygen reduction reaction (ORR) is the core reaction of fuel cells and metal–air batteries and suffers from slow kinetics due to multiple electron-transfer steps, which significantly confines their performance and hinders their practical applications. Pt is the best monometallic catalyst that accelerates the ORR kinetics.^2,3 Unfortunately, its high price and low reserves make it very difficult to realize the large-scale production of Pt-based catalysts.^4–6 Therefore, it is essential to explore non-precious metal materials to replace Pt as ORR catalysts.

In the past two decades, non-precious metal materials that are suitable for the ORR in alkaline media have made great progress. Many metal and N co-doped carbon (M–N–C) materials, metal-free carbon materials, and transition metal nitrides have successfully exhibited equal or even better ORR performance compared to commercial Pt/C catalysts in alkaline media.^7–16 Nevertheless, the ORR performance of non-precious metal catalysts in more corrosive media, such as acidic media and seawater, still needs to be improved.¹⁷ The high concentration of H⁺ in acidic media and the high concentration of Cl⁻ in seawater are corrosive to many materials. Therefore, exploring materials that can tolerate corrosive H⁺ and Cl⁻ ions is crucial to the development of ORR catalysts for polymer exchange membrane fuel cells (PEMFCs) and seawater-based metal–air batteries.

Transition metal nitrides have high melting temperature, high corrosion resistance, and electronic structure similar to transition metals, which makes them potential candidates for ORR catalysts in acidic media and seawater.¹⁸ Previous studies have revealed that obstacles like low conductivity, low exposure of active sites, and insufficient d electrons could significantly restrain the ORR activity of transition metal nitrides.^8,19 With appropriate strategies including doping metals with rich d electrons,^8,20,21 morphology control,^22–24 regulation of particle size,⁸ and combination with carbon supports,^8,19,25 their ORR activity can be improved to a large degree. Nevertheless, the ORR performance of transition metal nitrides in acidic media is still unsatisfactory for practical applications. Thus, efforts to improve the acidic ORR performance of transition metal nitrides are still needed. Moreover, the ORR performance of transition metal nitrides in seawater has rarely been reported. Despite the high corrosion resistance of transition metal nitrides, it is still a puzzle whether transition metal nitrides are able to render good ORR activity and stability in seawater.

Herein, in this work, we prepared carbon nanosheet-supported CrN nanoparticles (denoted as CrN/CNS) using a novel and environmentally benign method and investigated their ORR performance in acidic media and seawater. The optimal CrN/CNS material showed an outstanding ORR activity and robust stability in acidic media. Its ORR half-wave potential (E_1/2) reached 0.76 V vs. RHE in acidic media, which is one of the best among those reported for transition metal nitrides and comparable to those of M–N–C catalysts. Moreover, this material showed competitive ORR activity and extraordinary stability in simulated seawater. It had an E_1/2 of 0.50 V vs. RHE in a simulated seawater and showed no decay but 20 mV increase in E_1/2 after stability tests. More importantly, the optimal CrN/CNS material-based seawater Zn–air batteries (ZABs) exhibited good stability within 200 h of constant discharging.

Results and discussion

The CrN/CNS materials were prepared by pyrolyzing the mixture of dicyandiamine (DCDA) and Cr(acac)₃ in a N₂-saturated atmosphere via a two-step pyrolysis method. As illustrated in Fig. 1a, the mixture of DCDA and Cr(acac)₃ was first annealed at 650 °C to form Cr₂O₃/C powder, and then annealed at 750–850 °C to obtain the CrN/CNS-T catalyst (T represents the annealing temperature at the second step). The XRD pattern of Cr₂O₃/C in Fig. 1b shows a carbon diffraction peak at 20–30° and diffraction peaks corresponding to a Cr₂O₃ (JCPDS no. 238-1749) phase. However, in the XRD pattern of CrN/CNS-800 shown in Fig. 1c, apart from a carbon diffraction peak at 20–30°, there were clear diffraction peaks well corresponding to a CrN (JCPDS no. 76-2494) phase, confirming the presence of CrN in the CrN/CNS-800 sample. The morphology change at each pyrolysis step during the formation of CrN/CNS was monitored by scanning electron microscopy (SEM). As presented in Fig. S1a,† the SEM image of Cr₂O₃/C showed surfaces with many flakes. However, in Fig. S1b,† a large carbon nanosheet can be observed in the SEM image of CrN/CNS-800.

The morphology of CrN/CNS-800 was also characterized by transmission electron microscopy (TEM). As shown in Fig. 2a and b, CrN/CNS-800 had carbon nanosheets and nanoparticles with different diameters. The inset in Fig. 2a indicates that nanoparticles with diameters of 5–10 nm were in the majority. Fig. 2c and d show that nanoparticles with large and small diameters both had lattice fringes that well correspond to the (111) facet of CrN. The high-angle annular dark field (HAADF) image and energy dispersive spectrometry (EDS) elemental mapping images in Fig. 2e–g show that Cr elements were mostly distributed on CrN nanoparticles. These results verify the presence of CrN nanoparticles in CrN/CNS-800. The energy dispersive X-ray (EDX) spectrum in Fig. 2h indicates that the Cr content in CrN/CNS-800 was 28.5 wt%. The N₂ adsorption–desorption results in Fig. 2i and j show that CrN/CNS-800 had a Brunauer–Emmett–Teller (BET) surface area of 341 m² g⁻¹ and mesoporous structure with pore diameters of 3.6 and 10.9 nm.

Fig. 3 shows the ORR performance of the prepared catalysts in a 0.1 M HClO₄ solution. As shown in Fig. 3a, Cr₂O₃/C had very poor ORR activity, its ORR onset potential was only about 0.6 V vs. RHE and its ORR current density only reached 2.5 mA cm⁻². In contrast, CrN/CNS-800 had excellent ORR activity, indicating that the presence of CrN played a decisive role in the CrN/CNS-800 sample. The effects of the final annealing temperature on the ORR activity of CrN/CNS samples were also investigated, as can be seen in Fig. 3b, CrN/CNS-800 had the best ORR activity. The ORR E_1/2 of CrN/CNS-800 reached 0.76 V vs. RHE, only 100 mV lower than that of a commercial 20 wt% Pt/C catalyst. To the best of our knowledge, its ORR E_1/2 is one of the best among those reported for transition metal nitrides (Fig. 3c) and comparable to those of M–N–C materials (Table S1†). In addition, we also synthesized a CrN/CNS-800 #1 material via only one-step annealing the mixture of Cr(acac)₃ and DCDA. As shown in Fig. S2,† CrN/CNS-800 #1 had worse ORR activity than CrN/CNS-800, verifying the superiority of the two-step annealing procedure in the synthesis of CrN/CNS-800. Tafel analysis in Fig. S3a† shows that the cathodic Tafel slopes of CrN/CNS-750, CrN/CNS-800 and CrN/CNS-850 in acidic media were 54, 42 and 41 mV dec⁻¹, respectively, implying that the ORR rate-limiting steps of CrN/CNS-800 and CrN/CNS-850 in the acidic media may be the formation of H₂O from OH*.²⁶ The ORR mechanism of the optimal catalyst was analyzed by rotating ring disk electrode (RRDE) tests. CrN/CNS-800 had an extremely low ring current (Fig. S4†). Fig. 3d shows that the electron transfer number of CrN/CNS-800 was 3.95 at 0.1 V vs. RHE, and the H₂O₂ yield was as low as 3.5% at the same potential. These results indicate that the ORR mechanism of CrN/CNS-800 in acidic media was dominated by a 4-electron-transfer pathway, and most of the O₂ can be dissociated during the ORR process.

The ORR stability of the optimal catalyst was evaluated using a chronoamperometry method. As shown in Fig. 3e, compared to the commercial Pt/C catalyst that retained 64% of its ORR current, CrN/CNS-800 had better ORR stability and retained 82% of its ORR current after the stability test in acidic media. We also compared their ORR E_1/2 attenuation after the stability test. As shown in Fig. 3f, the E_1/2 attenuation of CrN/CNS-800 and the Pt/C catalyst was 27 and 45 mV, respectively, also confirming that CrN/CNS-800 had better ORR stability than the Pt/C catalyst. It should be mentioned that the ORR stability of CrN/CNS-800 outperformed many reported metal nitrides (Table S2†). To figure out why CrN/CNS-800 had excellent ORR stability in acidic media, we further analyzed the CrN/CNS-800 sample after the stability test. As shown in Fig. S5a–c,† the morphology of CrN/CNS-800 after the stability test remains intact, and severe aggregation of CrN nanoparticles was not detected. The EDX profile in Fig. S5d† shows that the Cr content in CrN/CNS-800 after the stability test was 27.4 wt%, only slightly lower than that before the stability test. Therefore, the excellent ORR stability of CrN/CNS-800 is probably due to its intact morphology and negligible loss of Cr content during the stability test.

Fig. 4 shows the ORR performance of CrN/CNS materials in a simulated seawater (0.5 M NaCl solution). As shown in Fig. 4a, CrN/CNS-800 had the best ORR activity in the simulated seawater among CrN/CNS materials. The ORR E_1/2 of CrN/CNS-750, CrN/CNS-800, and CrN/CNS-850 was 0.47, 0.50, 0.45 V vs. RHE in the simulated seawater, respectively. The ORR E_1/2 of the optimal catalyst was 150 mV lower than that of the commercial Pt/C catalyst. Tafel analysis in Fig. S3b† shows that the cathodic Tafel slopes of CrN/CNS-750, CrN/CNS-800 and CrN/CNS-850 in the simulated seawater were 123, 125 and 165 mV dec⁻¹, respectively, implying that the ORR rate-limiting steps of these materials in the simulated seawater may be the electron transfer to O₂.²⁶ We further investigated the ORR mechanism of the optimal catalyst in the simulated seawater using RRDE tests. As shown in Fig. S6,† the ring current of CrN/CNS-800 in the simulated seawater was very low. Fig. 4b shows that the electron transfer number of CrN/CNS-800 was higher than 3.7 and the H₂O₂ yield of CrN/CNS-800 was lower than 13% in the potential range of 0–0.6 V vs. RHE. These results indicate that the ORR mechanism of CrN/CNS-800 in the simulated seawater was dominated by a 4-electron-transfer pathway, and most of the O₂ can be dissociated during the ORR process in the simulated seawater.

Fig. 4c shows the ORR stability of CrN/CNS-800 and the Pt/C catalyst in the simulated seawater. Interestingly, the ORR current of CrN/CNS-800 and the Pt/C catalyst both showed no decay but increase during 40000 s stability tests. We further compared the ORR activities of CrN/CNS-800 and the Pt/C catalyst before and after stability tests in the simulated seawater. As presented in Fig. 4d, the ORR E_1/2 of CrN/CNS-800 had an improvement of 20 mV after the stability test, while that of the Pt/C catalyst had an improvement of 10 mV after the stability test. These results suggest that CrN/CNS-800 had outstanding ORR stability in the simulated seawater.

X-ray photoelectron spectroscopy (XPS) tests were conducted to reveal the electronic structure of CrN/CNS catalysts and figure out possible active species for the ORR. The XPS survey spectra in Fig. S7† indicate the presence of Cr, N, O, and C elements in all CrN/CNS catalysts. As shown in Fig. 5a, both the XPS spectra of Cr 2p in CrN/CNS-750 and CrN/CNS-800 can be divided into two valence states: Cr–N (Cr in CrN, 575.9 eV) and Cr–O (Cr in Cr₂O₃, 577.89 eV), while the XPS spectrum of Cr 2p in CrN/CNS-850 fits well into three valence states: Cr–C (Cr in CrC, 574 eV), Cr–N (Cr in CrN, 575.9 eV), and Cr–O (Cr in Cr₂O₃, 577.89 eV). The presence of the Cr–C valence state was also verified in Fig. S8,† in which we found CrN/CNS-850 had a clear Cr_6.2C_3.5N_0.3 (JCPDS no. 19-0326) phase. Compared to CrN/CNS-750 and CrN/CNS-800, the Cr–N valence state in CrN/CNS-850 had a negative shift of 0.24 eV. This negative shift is probably caused by the presence of Cr_6.2C_3.5N₀₃ in CrN/CNS-850 which changed the coordination environment of Cr atoms. Fig. 5b shows that CrN/CNS-800 had the highest Cr–N proportion and the lowest Cr–O proportion. Since CrN/CNS-800 had the best ORR activity, it can be deduced that the proportion of the Cr–N valence state in CrN/CNS catalysts is highly related to their ORR activity.

Since N species including pyridinic N, pyrrolic N, and graphitic N have been reported to be active sites for the ORR,^35,36 we also analyzed the XPS spectra of N 1s in CrN/CNS samples. Fig. 5c shows that the XPS spectra of N 1s of CrN/CNS samples fit well into four valence states: N in CrN, pyridinic N, pyrrolic N, and graphitic N. As presented in Fig. 5d, the total proportions of pyridinic N, pyrrolic N, and graphitic N in all CrN/CNS materials were almost the same, indicating that N species are not the key factor causing differences in the ORR activity of these three samples. Raman spectra in Fig. S9† show that the peak ratios of defect carbon and graphitic carbon in CrN/CNS-750, CrN/CNS-800, and CrN/CNS-850 were almost the same, implying that the carbon nanosheets in all CrN/CNS materials had nearly identical proportions of defect structure and graphitic structure. Thus, the carbon structure in CrN/CNS materials is not the key factor causing differences in their ORR activity either. Based on the XPS and Raman results, we can deduce that the proportion of the Cr–N valence state in CrN/CNS samples is the key factor determining their ORR activity.

To reveal the ORR active sites of CrN/CNS samples, we conducted O₂ temperature-programmed desorption (O₂-TPD) tests. As shown in Fig. S10,† the O₂-TPD curves of CrN/CNS-750 and CrN/CNS-800 had strong peaks at 375 °C, which was ascribed to desorption of atomic oxygen. Besides, the O₂-TPD curve of CrN/CNS-800 also had a weak peak at 121 °C that was related to desorption of molecular oxygen. CrN/CNS-850 had a strong peak and a weak peak at 370 and 112 °C, respectively. More interestingly, only CrN/CNS-850 had a weak peak at 618 °C. According to the XPS results, CrN/CNS-850 had a unique Cr–C valence state that had much lower binding energy than the Cr–N valence state. This means that Cr atoms in the Cr–C valence state have more d electrons than those in the Cr–N valence state, which makes them have stronger adsorption to oxygen. Thus, the weak peak at 618 °C in the O₂-TPD curve of CrN/CNS-850 is probably due to the presence of the Cr–C valence state. And the strong peaks at 370–375 °C in the O₂-TPD curves of all CrN/CNS samples are probably due to the presence of the Cr–N valence state, as they all had the Cr–N valence state.

The RRDE results confirm that the ORR mechanism of CrN/CNS was not a complete 4-electron-transfer mechanism. Under this circumstance, a stronger adsorption to oxygen will theoretically benefit O₂ dissociation and therefore results in higher ORR activity. However, CrN/CNS-850 had the strongest adsorption to oxygen but its ORR activity was not the best. This is probably because it had a much lower amount of Cr in its surface than CrN/CNS-750 and CrN/CNS-800. As evidenced in Table S3,† CrN/CNS-850 only had 1.66 at% of Cr in its surface, much lower than those of CrN/CNS-750 and CrN/CNS-800 that had 5.71 and 5.04 at% of Cr in their surfaces, respectively. The lower amount of Cr also explains why CrN/CNS-850 had lower ORR current density than CrN/CNS-750 and CrN/CNS-800. Compared to CrN/CNS-800, the lower ORR activity of CrN/CNS-750 is probably due to its lower proportion of the Cr–N valence state. As for CrN/CNS-800, it had not only a high Cr content in its surface but also a high proportion of the Cr–N valence state, these may be the reasons why it exhibited the best ORR activity among CrN/CNS materials.

To demonstrate the potential of CrN/CNS-800 in seawater metal–air batteries, we assembled ZABs using simulated seawater and natural seawater as electrolytes and CrN/CNS-800 as the air-cathode catalyst (Fig. 6a). The open-circuit voltages (OCV) of CrN/CNS-800-based ZABs in simulated seawater and natural seawater were 1.18 and 0.98 V, respectively (Fig. 6b). Fig. 6c shows that the CrN/CNS-800-based ZABs rendered maximum power densities of 9.7 and 7.3 mW cm⁻² in simulated seawater and natural seawater, respectively, which is comparable to those of other material-based seawater ZABs (Table S4†). Noticeably, the discharging polarization curve of the ZAB in natural seawater showed a sharp decrease in voltage when the current density approached 15 mA cm⁻². In contrast, the discharging polarization curve of the ZAB in the simulated seawater showed a steady decrease in voltage. This implies that ingredients other than Na⁺ and Cl⁻ in natural seawater have great influence on the discharging polarization of the ZAB at large current density. Fig. 6d shows that the CrN/CNS-800-based ZABs in the simulated seawater and natural seawater exhibited specific capacities of 662.9 and 1349.5 mA h g_Zn⁻¹ at 1 mA cm⁻², respectively. The lower specific capacity of CrN/CNS-800-based ZABs in the simulated seawater can be ascribed to the severer dissolution of the Zn anode in the simulated seawater. We found that the dissolution of the Zn anode in the simulated seawater was 0.24 g, while that in natural seawater was only 0.15 g. Apart from higher specific capacity, the CrN/CNS-800-based ZAB in natural seawater also had better stability. As shown in Fig. 6e, the CrN/CNS-800-based ZAB in natural seawater maintained constant discharging at 1 mA cm⁻² for 200 h with a slight decrease in voltage, while the CrN/CNS-800-based ZAB in the simulated seawater had an obvious decrease in voltage with constant discharging at 1 mA cm⁻² for 180 h. The better stability of the CrN/CNS-800-based ZAB in natural seawater may be attributed to less dissolution of the Zn anode in natural seawater. We also tested the stability of the CrN/CNS-800-based ZAB in natural seawater at a high discharging current density. As shown in Fig. S11,† at the discharging current density of 10 mA cm⁻², the CrN/CNS-800-based ZAB in natural seawater maintained a steady voltage for nearly 4 h and then showed an obvious decrease in voltage. The poorer stability at higher discharging current density may be caused by more generation of OH⁻ from the ORR, which may react with Ca²⁺ and Mg²⁺ to form Ca(OH)₂ and Mg(OH)₂ precipitates and cover the surfaces of electrodes. The above results indicate that CrN/CNS-800 has promising application as a robust cathode catalyst in natural seawater ZABs at low discharging current density.

Conclusion

In conclusion, using DCDA as the nitrogen and carbon source, we have successfully prepared CrN/CNS materials via a two-step pyrolysis method. The optimal CrN/CNS material had an ORR E_1/2 of 0.76 V vs. RHE in acidic media, which is one of the best among those reported for transition metal nitrides and comparable to those of M–N–C materials. The optimal CrN/CNS material only had 18% decay in ORR current density and 27 mV decay in ORR E_1/2 after stability tests in acidic media, outperforming a commercial Pt/C catalyst and many transition metal nitrides. In addition, the optimal CrN/CNS material had an ORR E_1/2 of 0.50 V vs. RHE and outstanding stability in simulated seawater, and its ORR performance showed no decay but increase after stability tests in simulated seawater. More importantly, the optimal CrN/CNS material-based ZABs maintained good stability in natural seawater within 200 h constant discharging. XPS and O₂-TPD results indicate that both the proportion of the Cr–N valence state and the Cr content in surfaces play a key role in the ORR activity of CrN/CNS materials. This work demonstrates the great potential of CrN as an efficient and robust ORR catalyst in acidic media and natural seawater ZABs.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Yating Zhang: data curation, formal analysis, investigation, methodology; visualization. Haoming Wu: data curation, formal analysis, validation. Jun Ma: data curation, formal analysis, software. Junming Luo: conceptualization, funding acquisition, investigation, methodology, project administration, supervision, writing – original draft, writing – review & editing. Zhe Lu: formal analysis, software, visualization. Suyang Feng: software, visualization. Yijie Deng: conceptualization, funding acquisition, methodology, writing – original draft, writing – review & editing. Hui Chen: data curation, validation. Qi Wang: formal analysis, validation. Zhengpei Miao: funding acquisition, writing – review & editing. Peng Rao: funding acquisition, methodology. Neng Yu: funding acquisition, resources. Yuliang Yuan: funding acquisition, visualization. Jing Li: funding acquisition, resources. Xinlong Tian: funding acquisition, project administration, resources, supervision, writing – original draft, writing – review & editing.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the Hainan Provincial Natural Science Foundation of China (project no. 221RC449, 221RC540, and 522QN281), the Research Fund Program of Key Laboratory of Fuel Cell Technology of Guangdong Province (project no. 202021), the Key Research and Development Project of Hainan Province (project no. ZDYF2023GXJS006, ZDYF2021GXJS207, ZDYF2020207, ZDYF2020037, and ZDYF2023SHFZ091), the Specific Research fund of the Innovation Platform for Academicians of Hainan Province (YSPTZX202315), the Research Project of Collaborative Innovation Center of Hainan University (project no. XTCX2022HYC06), the Start-up Research Foundation of Hainan University (project no. KYQD(ZR)-20008, 20082, 20083, 20084, 21065, 21124, 21125, 21170, 23169, and 23068), the National Natural Science Foundation of China (project no. 52162027, 22342006, 52304326, 22305055), the Opening Project of Key Laboratory of Electrochemical Energy Storage and Energy Conversion of Hainan Province (KFKT2021007), the Foundation of State Key Laboratory of Marine Resource Utilization in South China Sea (Hainan University, Grant No. MRUKF2021029), and the Jiangxi Provincial Natural Science Foundation (project no. 20224BAB203016).

References

1. M. K. Debe, Nature, 2012, 486, 43–51 CrossRef CAS PubMed .
2. J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. R. Kitchin, T. Bligaard and H. Jónsson, J. Phys. Chem. B, 2004, 108, 17886–17892 CrossRef .
3. X. Tian, X. Zhao, Y.-Q. Su, L. Wang, H. Wang, D. Dang, B. Chi, H. Liu, E. J. M. Hensen, X. W. Lou and B. Y. Xia, Science, 2019, 366, 850–856 CrossRef CAS PubMed .
4. Y. Nie, L. Li and Z. D. Wei, Chem. Soc. Rev., 2015, 44, 2168–2201 RSC .
5. Z. Xia, L. An, P. Chen and D. Xia, Adv. Energy Mater., 2016, 6, 1600458 CrossRef .
6. W. Xia, A. Mahmood, Z. B. Liang, R. Q. Zou and S. J. Guo, Angew. Chem., Int. Ed., 2016, 55, 2650–2676 CrossRef CAS PubMed .
7. Y. J. Deng, B. Chi, J. Li, G. H. Wang, L. Zheng, X. D. Shi, Z. M. Cui, L. Du, S. J. Liao, K. T. Zang, J. Luo, Y. F. Hu and X. L. Sun, Adv. Energy Mater., 2019, 9, 1802856 CrossRef .
8. J. M. Luo, X. C. Qiao, J. T. Jin, X. L. Tian, H. B. Fan, D. M. Yu, W. L. Wang, S. J. Liao, N. Yu and Y. J. Deng, J. Mater. Chem. A, 2020, 8, 8575–8585 RSC .
9. Y. W. Lou, J. J. Liu, M. Liu and F. Wang, ACS Catal., 2020, 10, 2443–2451 CrossRef CAS .
10. N. N. Wang, X. D. Chen, J. T. Jin, P. Zhang, X. C. Qiao and L. F. Cui, Carbon, 2020, 169, 82–91 CrossRef CAS .
11. K. Srinivas, D. Liu, F. Ma, A. Chen, Z. Zhang, Y. Wu, Q. Wu and Y. Chen, Small, 2023, 19, 2301589 CrossRef CAS PubMed .
12. X. Yao, Y. Zhu, T. Xia, Z. Han, C. Du, L. Yang, J. Tian, X. Ma, J. Hou and C. Cao, Small, 2023, 19, 2301075 CrossRef CAS PubMed .
13. J. Wang, H. H. Zhong, J. Yang, H. Y. Li, P. G. Tang, Y. J. Feng and D. Q. Li, J. Energy Chem., 2023, 82, 547–559 CrossRef CAS .
14. Y. Qin, C. Z. Guo, Z. H. Ou, C. L. Xu, Q. Lan, R. Jin, Y. Liu, Y. C. Niu, Q. Xu, Y. J. Si and H. L. Li, J. Energy Chem., 2023, 80, 542–552 CrossRef CAS .
15. L. Hao, Y. Gao, A. W. Robertson and Z. Sun, Innov. Mater., 2024, 2, 100050 CrossRef .
16. L. Xu, R. Iqbal, Y. Wang, S. Taimoor, L. Hao, R. Dong, K. Liu, J. Texter and Z. Sun, Innov. Mater., 2024, 2, 100060 CrossRef .
17. Y. Q. Wang, J. Y. Hao, Y. Liu, M. Liu, K. Sheng, Y. Wang, J. Yang, J. Li and W. Z. Li, J. Energy Chem., 2023, 76, 601–616 CrossRef CAS .
18. J. G. Chen, Chem. Rev., 1997, 96, 1477–1498 CrossRef PubMed .
19. J. Luo, H. Tang, X. Tian, S. Liao, J. Ren, W. Zhao and X. Qiao, Electrochim. Acta, 2019, 314, 202–211 CrossRef CAS .
20. J. M. Luo, X. L. Tian, J. H. Zeng, Y. W. Li, H. Y. Song and S. J. Liao, ACS Catal., 2016, 6, 6165–6174 CrossRef CAS .
21. X. L. Tian, J. M. Luo, H. X. Nan, Z. Y. Fu, J. H. Zeng and S. J. Liao, J. Mater. Chem. A, 2015, 3, 16801–16809 RSC .
22. H. B. Tang, J. M. Luo, X. L. Tian, Y. Y. Dong, J. Li, M. R. Liu, L. N. Liu, H. Y. Song and S. J. Liao, ACS Appl. Mater. Interfaces, 2018, 10, 11604–11612 CrossRef CAS PubMed .
23. X. L. Tian, L. Wang, B. Chi, Y. Xu, S. Zaman, K. Qi, H. Liu, S. Liao and B. Y. Xia, ACS Catal., 2018, 8, 8970–8975 CrossRef CAS .
24. Y. Z. Dong, Y. M. Wu, M. J. Liu and J. H. Li, Chemsuschem, 2013, 6, 2016–2021 CrossRef CAS PubMed .
25. B. Sarkar, A. Parui, A. K. Singh and K. K. Nanda, J. Mater. Chem. A, 2021, 9, 16575–16584 RSC .
26. A. Holewinski and S. Linic, J. Electrochem. Soc., 2012, 159, H864–H870 CrossRef CAS .
27. Q. C. Huang, S. Z. Chen, T. T. Ma, H. B. Zou and W. Yang, Int. J. Electrochem. Sci., 2017, 12, 6340–6351 CrossRef CAS .
28. M. E. Kreider, M. B. Stevens, Y. Z. Liu, A. M. Patel, M. J. Statt, B. M. Gibbons, A. Gallo, M. Ben-Naim, A. Mehta, R. C. Davis, A. V. Ievlev, J. K. Norskov, R. Sinclair, L. A. King and T. F. Jaramillo, Chem. Mater., 2020, 32, 2946–2960 CrossRef CAS .
29. M. E. Kreider, A. Gallo, S. Back, Y. Z. Liu, S. Siahrostami, D. Nordlund, R. Sinclair, J. K. Norskov, L. A. King and T. F. Jaramillo, ACS Appl. Mater. Interfaces, 2019, 11, 26863–26871 CrossRef CAS PubMed .
30. B. F. Cao, J. C. Neuefeind, R. R. Adzic and P. G. Khalifah, Inorg. Chem., 2015, 54, 2128–2136 CrossRef CAS PubMed .
31. Y. Yao, X. T. Lin and X. Zhang, J. Mater. Sci., 2018, 53, 7691–7702 CrossRef CAS .
32. T. Sun, Q. Wu, R. C. Che, Y. F. Bu, Y. F. Jiang, Y. Li, L. J. Yang, X. Z. Wang and Z. Hu, ACS Catal., 2015, 5, 1857–1862 CrossRef CAS .
33. S. Lai, L. Xu, H. Liu, S. Chen, R. Cai, L. Zhang, W. Theis, J. Sun, D. Yang and X. Zhao, J. Mater. Chem. A, 2019, 7, 21884–21891 RSC .
34. Q. Liu, S. Cao, Y. Fu, Y. Guo and Y. Qiu, J. Electroanal. Chem., 2018, 813, 52–57 CrossRef CAS .
35. D. H. Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo and J. Nakamura, Science, 2016, 351, 361–365 CrossRef CAS PubMed .
36. W. Ding, Z. Wei, S. Chen, X. Qi, T. Yang, J. Hu, D. Wang, L.-J. Wan, S. F. Alvi and L. Li, Angew. Chem., Int. Ed., 2013, 52, 11755–11759 CrossRef CAS PubMed .

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, disk and ring current in the RRDE test, additional TEM images, EDS elemental mapping images, HAADF images, XRD patterns, EDX and XPS analysis, Raman spectra, performance of representative metal nitride-based ORR catalysts. See DOI: https://doi.org/10.1039/d4ta04066j

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