Enhancing the activity and stability of Fe/Co-based nitrogen-doped carbon with richer nitrogen and metal-N active sites towards oxygen reduction reactions

Abstract

Transition metal and nitrogen co-doped carbon (M–N–C) materials possess a broad spectrum of applications in electrocatalysis, particularly in oxygen reduction reactions (ORRs). However, enhancing their stability and further improving their activity continue to be challenging. Herein, based on nitrogen enrichment and coordination effects with Fe/Co of polydopamine (PDA), an efficient and straightforward method was applied to induce dopamine (DA) under alkali-free conditions to generate a PDA shell on a Fe/Co-based zeolitic imidazolate framework (ZIF) surface, and subsequent one-step pyrolysis yielded Fe/Co-based nitrogen-doped carbon with richer nitrogen and metal-N active sites (denoted as Fe/Co-NC RN x, x is the mass ratio of dopamine and Fe/Co-ZIF). Excitingly, the nitrogen content (11.23%) and metal-N active sites content (32.05%) of the resulting material Fe/Co-NC RN III (III refers to 30%, the mass ratio of dopamine and Fe/Co-ZIF) exceeded that of the material Fe/Co-NC without PDA coating (10.11%, 26.36%). The metal-N active site density (SD) of Fe/Co NC RN III was calculated to be 32.51 μmol g⁻¹, which was nearly twice that of Fe/Co-NC (16.48 μmol g⁻¹). Fe/Co-NC RN III achieved a half-wave potential (E_1/2) of 0.90 V in alkaline electrolytes, significantly surpassing that of Pt/C catalysts. Moreover, it retained 91% of its current after 30 000 s of chronoamperometric response (i–t) testing, with a mere reduction of 16 mV in E_1/2 after ten thousand cyclic voltammetry (CV) cycles. In addition to richer nitrogen and metal-N active sites, the excellent ORR activity and outstanding stability can be related to its unique hollow core–shell structure and hierarchical porous structure. The results indicated that this strategy can successfully boost the performance of Fe/Co-based nitrogen-doped carbon with richer nitrogen and metal N active sites, providing a convenient and feasible idea for developing high-activity, long-life M–N–C catalysts.

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Introduction

For advancing sustainable energy sources,¹ clean energy installations are prioritized, exemplified by fuel cells and metal–air batteries as they are green, non-polluting and environmentally friendly.^2–6 However, slow cathode ORR kinetics constrains the advancement of battery devices.⁷ Currently, Pt/C serves as a commercially employed catalyst for ORRs.⁸ However, it is plagued by high costs and poor stability.^9,10 Therefore, numerous researchers have explored efficient non-precious metal materials as substitutes for Pt/C,^11–13 such as perovskite,^14–16 transition metal oxides,^17–19 transition metal nitrides,^20–22 and M–N–C.^23–25 Among these materials, researchers extensively focus on M–N–C materials because of their good crystallinity, electrical conductivity, and structural stability, and their efficient adsorption and desorption of ORR intermediates have gained extensive interest.^26–32

Metal–organic frameworks (MOFs) represent a category of materials generated by assembling metal sites with multifunctional organic ligands.³³ Within this category, ZIFs stand out as an extraordinary type of precursors and immolated templates for the synthesis of M–N–C materials;^34,35 their high surface area promotes mass transport and exposes more active sites, which are crucial for catalytic processes.^36,37 To further enhance the catalytic activity, recent advancements have introduced binary metal sites within M–N–C structures, which inherits the superiority of M–N–C and explores the potential to enhance the catalytic activity by exploiting the synergistic effects between different metal atoms.^38–40

However, immediate carbonization of ZIFs usually results in electrocatalysts with inferior electrical conductivity, numerous micropores and gathering of metal atoms, which leads to poor stability and mediocre electrocatalytic activity.^41–43 For this reason, researchers have worked to improve the stability and activity of materials derived from ZIFs, which encompasses both the premodification of ZIFs precursors^44–48 and the post-functionalization of electrocatalysts derived from ZIFs.^49,50 Excitingly, Li et al.⁵¹ discovered that M–N–C catalysts with a core–shell structure exhibited significantly reinforced stability. They posited that this surprising result could be due to stripping or etching of the external active sites in the ORR process, thereby continually exposing additional internal active sites. Subsequently, Zhang et al. validated this mechanism,⁵² which showed that the active sites beneath the shell remained active. Therefore, it may be beneficial to utilize a carbon-coated shell to boost the performances of M–N–C electrocatalysts. Recently, extensive research has focused on PDA as a protective coating in catalysts and batteries due to its rich nitrogen content, abundance of active groups, ease of polymerization, low cost, and wide availability.^53–56 Moreover, PDA may induce the generation of more metal-N sites on the material surface during pyrolysis, which is significantly beneficial for the electrocatalytic process.^57–59

In this work, Fe/Co-based nitrogen-doped carbon materials with richer nitrogen and metal-N active sites were synthesized (denoted as Fe/Co-NC RN x, x is the mass ratio of dopamine and Fe/Co-ZIF), which were derived from Fe/Co-ZIF@PDA particles using different amounts of PDA to coat Fe/Co-ZIF. Among them, Fe/Co-NC RN III showed impressive ORR catalytic activity with excellent stability, attributed to its abundant nitrogen content, ample metal-N active sites, special hollow core–shell structure, hierarchical porous structure and metal and nitrogen co-doped within the carbon substrate. It achieves an E_1/2 value of 0.90 V within alkaline electrolytes, significantly surpassing that of the commercial Pt/C (20 wt%) catalyst. Moreover, it maintained 91% of the current after 30 000 s i–t test, with a 16 mV mere reduction in E_1/2 after ten thousand CV cycles. XPS analysis showed that the nitrogen content (11.23%) and metal-N active sites content (32.05%) of Fe/Co-NC RN III exceeded that of the material Fe/Co-NC devoid of PDA coating (10.11%, 26.36%). The metal-N SD of Fe/Co-NC RN III was computed to be 32.51 μmol g⁻¹, which is nearly two times that of Fe/Co-NC (16.48 μmol g⁻¹). Therefore, our discovery offers a valuable insight for the construction of high-activity long-life M–N–C catalysts by providing feasible insights.

Results and discussion

The synthetic route for Fe/Co-NC RN is shown in Scheme 1a. ZIF-8 doped with Fe³⁺ and Co²⁺ (i.e., Fe/Co-ZIF) was first synthesized, and then the methanol mixture of Fe/Co-ZIF with different amounts of DA added was stirred at room temperature. It is well known that DA needs to be polymerized in a weakly alkaline environment, and thus researchers usually add alkali (e.g., Tris solution). Interestingly, even though we did not add alkali to this step of DA polymerization, the PDA shell layer was successfully coated onto Fe/Co-ZIF to form Fe/Co-ZIF@PDA by the mechanism shown in Scheme 1b. The catechol of DA in a methanol solution will coordinate with Fe³⁺ and Co²⁺ of Fe/Co-ZIF (Co²⁺ can coordinate with catechol better than Fe³⁺), which induces the dissociation of Fe/Co-ZIF and releases 2-methylimidazole (2-MeIM). In addition, 2-MeIM is alkaline, causing DA polymerizing to form PDA, which will ulteriorly coordinate with metal ions of Fe/Co-ZIF, thereby continuing promoting Fe/Co-ZIF to dissociate.⁶⁰ Eventually, DA was further polymerized on Fe/Co-ZIF to generate Fe/Co-ZIF@PDA, which was pyrolytically carbonized to obtain Fe/Co-NC RN at 900 °C. The reason for choosing 900 °C as the carbonization temperature is that Zn will be evaporated at high temperatures, which, in turn, facilitates the creation of sparsely distributed metal sites.⁶¹ Fe/Co-NC without any nitrogen-doped carbon (N–C) shell layer was obtained by the pyrolysis of Fe/Co-ZIF without coated PDA at the same temperature and analyzed in comparison with Fe/Co-NC RN.

Scheme 1 (a) Schematic of the synthesis pathway to Fe/Co-NC RN. (b) Probable principle for the generation of Fe/Co-ZIF@PDA (the blue sphere coordinating with DA represents Fe³⁺ or Co²⁺).

As shown in Fig. 1a and Fig. S1a (ESI†), both Fe/Co-NC RN III and Fe/Co-NC maintain a good ZIF dodecahedral morphology, but compared with the smooth surface of Fe/Co-NC, we can observe that N–C coatings generated by the pyrolysis of PDA uniformly cover the surface of Fe/Co-NC RN III, which can be further proved using the TEM (Fig. 1b). As depicted in the HRTEM image displayed in Fig. 1c, Fe/Co-NC RN III formed a hollow core–shell structure, with a N–C shell layer measuring approximately 20 nm in thickness. The hollow gap between the core and the shell in Fe/Co-NC RN III is clearly observed, while the TEM images of Fe/Co-NC show that it has no shell layer (Fig. S1b and c, ESI†). As depicted in Fig. S2 (ESI†), both Fe/Co-NC RN I and Fe/Co-NC RN V were also successfully coated by the N–C shell layer, although Fe/Co-NC RN I has only a thin N–C coating, and the coating of Fe/Co-NC RN V is significantly thicker than that of Fe/Co-NC RN III. The relevant elemental mapping pictures indicate that C, N, Fe and Co exhibit uniform distribution in Fe/Co-NC RN III (Fig. 1d).

Fig. 1 Structural characterizations of Fe/Co-NC RN III: (a) SEM image. (b) TEM image. (c) HRTEM image. (d) Elemental mapping images (scale bar: 1 μm).

The XRD patterns of both Fe/Co-NC RN III and Fe/Co-NC show a couple of broad peaks at 2θ = 26.5° and 44.4°, corresponding to the graphitic carbon (002) and (101) planes, respectively (Fig. 2a). There is no peak associated with crystalline Fe or Co species (e.g., Fe, Co, Fe₃N or Co₂N) detected, indicating that the M–N–C structure was successfully synthesized. Similarly, there are also a couple of broad peaks of graphitic carbon in Fe/Co-NC RN I and Fe/Co-NC RN V XRD patterns (Fig. S3, ESI†). The degree of graphitization of Fe/Co-NC RN was further researched using Raman spectra (Fig. 2b). For all the Fe/Co-NC RN species and Fe/Co-NC, they all have two peaks at 1345 cm⁻¹ and 1590 cm⁻¹, which are consistent with the D band and G band, respectively. The ratio of disordered carbon to graphitic carbon (I_D/I_G) of Fe/Co-NC is 1.00, larger than that of Fe/Co-NC RN I (0.99), Fe/Co-NC RN III (0.97) and Fe/Co-NC RN V (0.96). We can find that I_D/I_G decreases with the increase in the thickness of the N–C shell layer. A lower I_D/I_G ratio signifies enhanced graphitization, resulting in increased conductivity during electrochemical kinetics, thereby augmenting electrocatalytic activity.⁶²

Fig. 2 (a) XRD patterns, (c) N₂ adsorption–desorption isotherms and (d) pore-size distribution of Fe/Co-NC and Fe/Co-NC RN III. (b) Raman spectra of Fe/Co-NC and all the Fe/Co-NC RN species.

As illustrated in Fig. 2c, type-I/IV isotherms occur in both samples, which suggest that micropores and mesopores are present, but the almost vertical tails of the isotherm of Fe/Co-NC RN III at higher relative pressures (∼1.0) indicate the existence of macropores. As depicted from Fig. 2d, there are a large number of mesopores smaller than 10 nm and micropores in Fe/Co-NC, whereas Fe/Co-NC RN III has a certain amount of mesopores larger than 10 nm and even macropores. Although the Fe/Co-NC Brunauer–Emmett–Teller (BET) surface area measures 750.9 m² g⁻¹, a bit higher than that for Fe/Co-NC RN III (724.3 m² g⁻¹), the pore volume for Fe/Co-NC RN III of 0.456619 cm³ g⁻¹ is much larger than that of 0.390546 cm³ g⁻¹ for Fe/Co-NC. It is well known that electrocatalytic reactions such as ORRs involve the transfer of gases and electrolytes between active sites, while mesopores and macropores boost transfers of charge and mass to improve the electrocatalytic activity.^63–66 The Fe/Co-NC pore size is generally small, lacking macropores and having very few mesopores larger than 10 nm. Therefore, it is reasonable to assume that Fe/Co-NC RN III with a hollow core–shell structure and hierarchical porous structure ensures that it has more active sites at the interface accessible to itself, which effectively increases the utilization rate of active sites. This is one of the factors contributing to its superior electrocatalytic activity to that of Fe/Co-NC, which will be demonstrated in the following.

The surface chemical constitutions and elemental states of Fe/Co-NC and Fe/Co-NC RN III were researched utilizing XPS. As illustrated in Fig. S4 (ESI†), C, N, O, Fe, and Co elements exist within both Fe/Co-NC and Fe/Co-NC RN III. Notably, the nitrogen content of Fe/Co-NC RN III (11.23%) exceeds that of Fe/Co-NC (10.11%) (Fig S5 and Table S1, ESI†), ascribed to the N–C shell derived from PDA. As depicted from Fig. 3a, C 1s can be deconvoluted to three peaks, C=C (284.8 eV), C–N (285.7 eV) and C–O/C=O (288.8 eV), similarly observed in Fe/Co-NC (Fig. S6, ESI†), indicating the successful incorporation of nitrogen into a carbon substrate. N 1s is deconvoluted into four peaks at 398.4 eV, 399.2 eV, 400.7 eV, and 401.5 eV, corresponding with pyridinic-N, metal-N, pyrrolic-N, and graphitic-N, respectively (Fig. 3b), which demonstrates that Fe–N and Co–N species exist within Fe/Co-NC RN III.⁶⁷ The N 1s spectra of Fe/Co-NC is shown in Fig. S7 (ESI†). The metal-N content of Fe/Co-NC RN III (32.05%) exceeds that of Fe/Co-NC (26.36%) (Fig. S8 and Table S2, ESI†). Therefore, we can reasonably speculate that the PDA shell layer probably induces more metal-N sites to generate within the carbonization procedure. Researchers found that metal-N is an effective active center of ORRs,^68–70 and therefore, Fe/Co-NC RN III has superior electrocatalytic activity over Fe/Co-NC, which can be attributed to richer nitrogen content and metal-N active sites. Fe 2p can be fitted to two peaks at 710.3 eV and 713.2 eV, which can be attributed to Fe²⁺ and Fe³⁺, respectively (Fig. 3c). Fe/Co-NC Fe 2p XPS spectra are depicted at Fig. S9 (ESI†). Two peaks deconvoluting at 780.4 eV and 783.9 eV for Co 2p correspond to Co–N and Co²⁺, respectively (Fig. 3d), and the Co–N content of Fe/Co-NC RN III is clearly observed to surpass that in Fe/Co-NC (Fig. S10, ESI†). No metallic iron or cobalt species are found in the Fe 2p and Co 2p spectra of Fe/Co-NC RN III, which excludes metallic iron or cobalt nanoparticles from existing. This finding corresponds with the XRD analysis.

Fig. 3 High-resolution XPS spectra of Fe/Co-NC RN III: (a) C 1s, (b) N 1s, (c) Fe 2p and (d) Co 2p.

All the ORR electrochemical experiments were conducted under oxygen-saturated conditions. The ORR activities of Fe/Co-NC RN III and contrast catalysts were measured using rotating disk electrodes with all potentials referenced to versus reversible hydrogen electrodes. As depicted in Fig. 4a, Fe/Co-NC RN III exhibits the most extraordinary activity, reaching an E_1/2 value of 0.90 V, which far exceeds those of Pt/C (0.86 V), Fe/Co-NC (0.87 V), Fe/Co-NC RN I (0.88 V) and Fe/Co-NC RN V (0.84 V). Comparative analysis reveals that a moderately thick N–C shell coating can indeed effectively boost the activity. However, an excessively thick shell layer may mask catalytic active sites and greatly hinder activity.

Fig. 4 Electrocatalytic performance of Fe/Co-NC RN III towards ORRs. (a) LSV polarization curves with a rotational speed of 1600 rpm and (d) Tafel plots of Pt/C, Fe/Co-NC and all the Fe/Co-NC RN species. (b) ORR LSV curves before and after 10 000 CV cycles, (e) K–L plots at diverse potentials and (f) calculated n value and peroxide yield during ORRs for Fe/Co-NC RN III. (c) Normalized chronoamperometric curves of Fe/Co-NC and Fe/Co-NC RN III.

The stability of Fe/Co-NC RN III was investigated. After 10 000 CV cycles, the E_1/2 value of Fe/Co-NC RN III merely reduced by 16 mV, whereas the E_1/2 value of Fe/Co-NC decreased significantly by 32 mV (Fig. S11a, ESI†), and Pt/C even decreased by 49 mV (Fig. S11b, ESI†). In addition, the long-period operational life of Fe/Co-NC RN III was further evaluated by chronoamperometry (Fig. 4c). Excitedly, Fe/Co-NC RN III maintained 91% of its current after 30 000 s continuously operating in a condition of oxygen-saturated alkaline solution, while the current of Fe/Co-NC dropped to 65%. In contrast to Fe/Co-NC, the stability of Fe/Co-NC RN III is particularly remarkable. Therefore, we can reasonably hypothesize that this strategy is able to significantly boost the stability of Fe/Co-based nitrogen-doped carbon.

The Tafel plots of these catalysts were further analyzed to reveal ORR kinetics within them (Fig. 4d). Not surprisingly, Fe/Co-NC RN III has the lowest Tafel slope (62.01 mV dec⁻¹), lower than that for Pt/C (79.78 mV dec⁻¹), Fe/Co-NC (82.31 mV dec⁻¹), Fe/Co-NC RN I (72.56 mV dec⁻¹) and Fe/Co-NC RN V (84.56 mV dec⁻¹), proving that Fe/Co-NC RN III possesses the fastest ORR kinetics among them. Fe/Co-NC RN III ORR LSV curves with different rotational speeds are depicted in Fig. S12 (ESI†). The Koutecky–Levich (K–L) plots of Fe/Co-NC RN III were plotted using the K–L equation (Fig. 4e), yielding an average electron transfer number (n) of 3.93 for Fe/Co-NC RN III, proving the four-electron transfer process for ORRs.⁷¹ Furthermore, the peroxide yield and n value of Fe/Co-NC RN III during ORRs were researched by rotating ring-disk electrodes. The n value ranges from 3.86 to 3.92 and the peroxide yield remains below 6.9% within the potential range of 0.2 V to 0.7 V (Fig. 4f), further demonstrating that the ORR occurring at Fe/Co-NC RN III is a four-electron transfer procedure, which is necessary for fuel cells.⁷²

Since nitrite ions (NO₂⁻) can specifically adsorb onto metal-N sites, the active site density for the ORR is able to quantify by the charge stripped through in situ electrochemical nitrite poisoning experiments (Fig. S13, ESI†).⁷³ As depicted in Fig. S14 (ESI†), the reduction peak of NO₂⁻ for Fe/Co-NC RN III is significantly larger than that of Fe/Co-NC, with the corresponding strip charge for Fe/Co-NC RN III being 15.66 C g⁻¹, which is substantially greater than that for Fe/Co-NC (7.94 C g⁻¹). The SD of Fe/Co-NC RN III was calculated to be 32.51 μmol g⁻¹, which is nearly twice that of Fe/Co-NC (16.48 μmol g⁻¹). It can be concluded that the higher SD of metal-N active sites determines the excellent activity of Fe/Co-NC RN III. Moreover, the methanol tolerance of Fe/Co-NC RN III was evaluated. After methanol was added, the current drop of Fe/Co-NC RN III was essentially negligible, whereas Pt/C showed a severe current loss (Fig. S15, ESI†). Hence, Fe/Co-NC RN III is more methanol tolerant than Pt/C. These outcomes clearly demonstrate that Fe/Co-NC RN III is an efficient ORR catalyst with excellent catalytic activity and extraordinary stability within alkaline electrolytes. Therefore, constructing a hollow core–shell structure by coating Fe/Co-based nitrogen-doped carbon with a N–C shell layer is a feasible method to significantly promote its electrocatalytic performances.

The ORR property of Fe/Co-NC RN III ranks among the most active advanced ORR catalysts (Table S3, ESI†). In addition to its rich nitrogen content, unique hollow core–shell and hierarchical porous structure, it may be attributed to the abundant metal-N active sites of Fe/Co-NC RN III, which significantly optimize the adsorption energy of oxygen-containing intermediates during ORRs,⁷⁴ thereby endowing Fe/Co-NC RN III with exceptional catalytic activity.

Conclusions

In summary, a feasible and efficient strategy was exploited to construct a hollow core–shell structure to enhance ORR electrocatalytic performances of Fe/Co-based nitrogen-doped carbon catalysts. XPS analysis manifested that the nitrogen content (11.23%) and metal-N active site content (32.05%) of Fe/Co-NC RN III surpass that of Fe/Co-NC (10.11% and 26.36%). The metal-N SD of Fe/Co-NC RN III was calculated to be 32.51 μmol g⁻¹, nearly double that of Fe/Co-NC (16.48 μmol g⁻¹). Ascribed to its distinct hollow core–shell structure, abundant nitrogen content, hierarchical porous structure, ample metal-N active sites and co-doping of metal and nitrogen within the carbon substrate, Fe/Co-NC RN III exhibits an E_1/2 value of 0.90 V in an alkaline medium, which far exceeds that of Fe/Co-NC and Pt/C, and maintained 91% of the current after 30 000 s of chronoamperometry method testing, and the E_1/2 value only decreased by 16 mV after 10 000 CV cycles. The results indicated that this strategy successfully enhanced the performances of Fe/Co-based nitrogen-doped carbon with richer nitrogen and metal-N active sites, and these findings are anticipated to offer valuable insights for the advancement of high-activity, long-life M–N–C catalysts.

Author contributions

Zhenlu Zhao and Zeqi Wu jointly conceived and designed the experiment. Zeqi Wu synthesized the catalyst materials and conducted electrochemical testing. Zhenlu Zhao and Zeqi Wu collectively analyzed and discussed the experimental data along with characterization results, culminating in the findings. Zeqi Wu drafted the manuscript, which was subsequently revised by Zhenlu Zhao.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and/or its ESI.†

Conflicts of interest

The authors declare that they have no conflicts of interest.

Acknowledgements

We acknowledge the financial support from the Natural Science Foundation of Shandong Province (No. ZR2021MB017), Foundation of State Key Laboratory of Electroanalytical Chemistry (SKLEAC201907), and Study Abroad Fund.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nj04380d

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