Structural and Reactivity Effects of Secondary Metal Doping into Iron-Nitrogen-Carbon Catalysts for Oxygen Electroreduction

doi:10.21203/rs.3.rs-2464372/v1

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Structural and Reactivity Effects of Secondary Metal Doping into Iron-Nitrogen-Carbon Catalysts for Oxygen Electroreduction

https://doi.org/10.21203/rs.3.rs-2464372/v1

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While improved activity was recently reported for bimetallic iron-metal-nitrogen-carbon (FeMNC) catalysts for the oxygen reduction reaction (ORR) in acid medium, the nature of active sites and interactions between the two metals are poorly understood. Here, FeSnNC and FeCoNC catalysts were structurally and catalytically compared to their parent FeNC and SnNC catalysts. While CO cryo-chemisorption revealed a twice lower site density of M-N_x sites for FeSnNC and FeCoNC relative to FeNC and SnNC, the mass activity of both bimetallic catalysts is 50–100% higher than that of FeNC, due to a larger turnover frequency in the bimetallic catalysts. Electron microscopy and X-ray absorption spectroscopy identified the coexistence of Fe-N_x and Sn-N_x or Co-N_x sites, while no evidence was found for binuclear Fe-M-N_x sites. ⁵⁷Fe Mössbauer spectroscopy revealed that the bimetallic catalysts feature a higher D1/D2 ratio of the spectral signatures assigned to two distinct Fe-N_x sites, relative to the FeNC parent catalyst. Thus, the addition of the secondary metal favored the formation of D1 sites, associated with the higher turnover frequency.

Physical sciences/Chemistry/Electrochemistry/Electrocatalysis

Physical sciences/Energy science and technology/Fuel cells

Physical sciences/Materials science/Materials for energy and catalysis/Electrocatalysis

Polymer Electrolyte Membrane fuel cell (PEMFC) has promising applications in future mobility and stationary power generation, ^1–2 for which the catalysis of the electrochemical oxygen reduction reaction (ORR) plays a key role. Over the past decade, the broad class of metal-doped carbon-nitrogen materials (MNC) emerged as the most promising alternative to platinum for catalyzing the ORR,^3–7 and single metal atom sites of the type M-N_x identified as the most active site.^8–15 Among 3d transition metals, Fe, Co, and Mn-based monometallic MNC catalysts have long shown promising catalytic ORR activity in acid environments, while SnNC recently joint as another promising candidate.^{7, 9–11, 16–17} Despite dramatic advances in understanding the ORR reaction mechanisms of MNC materials, their activity still needs to be increased in order to favorably compete with platinum.^18–20 Their stability in acid medium has long been a concern, yet a promising stabilization approach was recently discovered.²¹ While there is still some room for improvement in the site density (SD) of M-N_x sites in MNC catalysts, this strategy for increasing the activity will be limited by the competing formation of metal clusters (with low or no ORR activity in acid medium) during pyrolysis. Novel MNC materials comprising M-N_x sites with enhanced turnover frequency (TOF) is therefore a promising path.

While bimetallic Pt-M catalysts have been developed and the mechanisms leading to enhanced ORR activity identified,^22–23 bimetallic MNC catalysts have hitherto been under-explored and their activity descriptors remain unclear.^24–27 In particular, there is a lack of detailed understanding of the metals’ coordination, nature of active sites and interactions between the two metals.^{9, 28–31} Bogdanoff’s group reported higher ORR activity for CoSnNC and CoFeNC compared to the parent CoNC, which was attributed to changes in the carbon specific surface area.²⁹ Bimetallic FeCoNC catalysts have been the most studied metal combination for ORR catalysis among MNC materials.^{9, 31–35} Zelenay’s group reported lower initial ORR activity but improved durability in PEMFC for FeCoNC vs. the parent FeNC catalyst.⁹ Atanassov’s group reported improved four-electron ORR selectivity for FeMNC (M = Co, Cu, Ni, Mn), catalysts relative to FeNC, but similar or lower ORR activity.³⁰ Dai’s group claimed the presence of binuclear FeCo-N_x active sites in FeCoNC, on the basis of EXAFS analysis.³¹ More recently, FeNiNC revealed lower ORR activity and four-electron selectivity compared to FeNC.³⁶ Thus, previous studies on bimetallic MNC catalysts yielded largely conflicting results, which calls for more systematic multi-techniques analysis to decipher their reactivity trends. This study addresses this unmet need.

Electrocatalyst synthesis and structural characterization

Metal precursors FeCl₃ & SnCl₂ or FeCl₃ & CoCl₂ were introduced during the polymerization of aniline in 0.5 M HCl solution together with the carbon support (Ketjen EC 600JC), resulting in a homogenous polyaniline-carbon network (Supplementary Fig. 1a-c). These catalyst precursors were pyrolyzed in N₂ at 900°C, and then acid-washed and re-pyrolyzed several times (Methods in S.I). FeSnNC and FeCoNC have a partially graphitized structure and similar carbon morphologies, as shown by TEM images (Supplementary Fig. 1d,e). This agrees with X-ray diffraction (XRD) patterns, showing two broad peaks at 26.2° and ca 44°, corresponding to the (002) and (101) reflections of graphite (Supplementary Fig. 2a,b). The XRD pattern for FeSnNC revealed also peaks at 33.8 and 53.0°, tentatively assigned to two main diffraction lines of SnO₂, or Fe-doped SnO₂³⁷ (Supplementary Fig. 2c). A precise identification of the metal speciation was obtained from Mössbauer and X-ray absorption spectroscopy (XAS), discussed later. For FeCoNC, its XRD pattern shows an intense peak at 44.7°, and two other minor peaks at 65.1° and 82.4°, that can be assigned to α-FeCo (Supplementary Fig. 2d). α-FeCo has often been observed in previously reported FeCoNC materials.^{28, 38} Other minor peaks at 43.0-44.4° and 64.5° support the presence of a small amount of γ-Fe and α-Fe. Combined with the fact that the parent FeNC material showed no diffraction peak related to Fe (Figure S1d in Ref¹⁷), these facts suggest that the addition of cobalt enhances the formation of iron-rich clusters during pyrolysis.

FeSnNC and FeCoNC show similar specific surface area (381–391 m² g^− 1) as well as mesoporous and microporous volumes, as quantified from nitrogen sorption measurements (Supplementary Fig. 3). These values are all significantly lower than the corresponding values measured for the parent FeNC material, with e.g. 665 m² g^− 1 specific surface area (Supplementary Table 1, and Supplementary Table 7 of Ref¹⁷). The lower specific surface area for these bimetallic catalysts may in particular result from the enhanced formation of bimetallic particles during the first pyrolysis, compared to FeNC. Fe- or Co-rich particles can catalyze graphitization, leading to lower porous volumes and specific surface area. In the case of FeSnNC, the minor signal assigned to Sn-based particles in the final catalyst does not preclude the existence of a high amount of (Fe)Sn based particles after the first pyrolysis, and that would have been acid-washed later on. The latter hypothesis is supported by the final metal contents, measured with inductively coupled plasma (Supplementary Table 1, Supplementary Note 1). The chemical composition of the catalysts was analyzed with X-ray photoelectron spectroscopy (XPS) (Supplementary Fig. 4, Supplementary Tables 2 and 3). The N 1s XPS spectrum was fitted using eight components that correspond to the expected N-species in (M)NC materials.^{17, 39} They can further be grouped into four chemically-distinct groups. There is no significant difference in the relative concentration of each N-species in FeSnNC and FeCoNC, nor in the total N content (5.4–6.5 at %) (Supplementary Tables 2 and 3). We note that this N-species assignment is not free of ambiguities, yet a more accurate assignment is currently elusive^39–40.

To identify the metal speciation in these catalysts, we resorted to ⁵⁷Fe and ¹¹⁹Sn Mössbauer spectroscopies, and to XAS at the Fe, Sn and Co K-edges. ⁵⁷Fe Mössbauer spectra were recorded both at 5 K and at room temperature, since this can help distinguishing if a spectral component shows a superparamagnetic transition (e.g. for nanometric metallic Fe clusters). For ¹¹⁹Sn Mössbauer spectroscopy, the acquisition was performed only at room temperature, since Sn does not form superparamagnetic compounds.

The ⁵⁷Fe Mössbauer spectrum of FeSnNC at 5 K was fitted with four components (Fig. 1a). Two doublets were evident, usually labelled D1 and D2 in the FeNC literature and associated with different Fe-N₄ sites. Recent in situ and post mortem ⁵⁷Fe Mössbauer studies coupled with DFT concluded that D1 is likely a high-spin (HS) Fe(III)-N₄ site, with an oxygenated species adsorbed on Fe^{3+ 41–42,43} (Supplementary Note 2). D1 in FeSnNC accounts for 37% of the signal, while D2 accounts for only 14% (Supplementary Table 4a), much lower than the relative contribution of D2 in the parent FeNC (ca 60%).¹⁷ Combined ⁵⁷Fe Mössbauer-DFT studies assigned D2 to LS or MS Fe(II)-N₄ sites^41–42. In addition, two sextets were identified, and their high IS-values of 0.52–0.66 mm·s^− 1 prevents us from assigning them to zero-valent Fe species. The ⁵⁷Fe Mössbauer spectrum of FeSnNC acquired at room temperature was fitted with three doublets (Supplementary Fig. 5a and Supplementary Table 4a), demonstrating the superparamagnetic behavior of the Fe species associated with the two sextets that were observed at 5 K. These components can be assigned to nanometric or amorphous oxidized iron particles, explaining also why they were not identified by XRD. Overall, only ~ 50% of the signal is assigned to Fe-N₄ sites in FeSnNC (37% D1 and 14% D2). The ¹¹⁹Sn Mössbauer spectrum of FeSnNC at 300 K was fitted with three components, two of which can be assigned to Sn(IV) sites (D1 and the singlet) and the third one (D2) to Sn(II) sites, on the basis of their distinct IS values (Fig. 1b and Supplementary Table 4a). The parameters of these components are comparable to those recently reported by us for the parent SnNC material with Sn-N_x sites¹⁷. On the basis of calculated QS-values for different Sn-N_x moieties, we assigned D1 to O₂-Sn(IV)-N_x sites¹⁷ and D2 to Sn(II)-N_x sites¹⁷. In all, the low Sn/Fe ratio in FeSnNC strongly suggests that the ORR activity of FeSnNC can mainly be ascribed to Fe-N_x sites.

The ⁵⁷Fe Mössbauer spectrum of FeCoNC at 5 K was fitted with five components (Fig. 1c). The detailed discussion of these components can be found in Supp. Note 3, and a summary is given here. D1 accounts for 40% of the signal (Supplementary Table 4b), assigned to HS Fe(III)-N₄. The D2 component associated with LS or MS Fe(II)-N₄ is totally absent from FeCoNC, while it represented ca 60% of the absorption area in the FeNC parent material (Figure S2 in Ref¹⁷). The D3 component is assigned to FeCl₂·4H₂O ⁴⁴. A first sextet is assigned to α-Fe or α-FeCo alloy ⁴⁵, while sextet 2 is assigned to FeCoO_x superparamagnetic nanoparticles (Supplementary Fig. 5b). Overall, only ca 40% of the signal is assigned to FeN₄ sites (D1) in FeCoNC. In contrast, the parent FeNC material was shown to comprise all Fe atoms in the form of Fe-N₄ sites with a 40%/60% split for D1 and D2 sites (Figure S2 in Ref¹⁷). Thus, the Mössbauer results reveal the promotion of Fe clustering after the addition of cobalt, but also the absence of the D2 component in FeCoNC.

The local environments of Fe and Sn in FeSnNC were further investigated by modeling the ex situ EXAFS spectra at both the Fe and Sn K-edges. The result of the fitting is shown in Fig. 1e-f, and Supplementary Figs. 6–7, in the Fourier transformed (FT) and K-space, respectively. The first coordination shell around iron is well described by a Fe-N₄ moiety with a Fe-N bond length of 2.01 Å (Fig. 1f and Supplementary Table 5). A second shell contribution is given by a Fe-C signal, with a fitted bond distance of 3.04 Å (Supplementary Table 5). This analysis is in good agreement with our previous structural characterization of PANI-derived FeNC catalysts,⁴⁶ but the higher Debye-Waller factor related to the carbon shell, indicates a disordered carbon structure around iron (Supplementary Table 5).

The experimental EXAFS spectrum of FeSnNC at the Sn K-edge was also fitted assuming a variable number of light elements (N or C, not distinguishable by EXAFS) in the plane containing the Sn atom, and a variable number of oxygen atoms in the axial position. The result of the fitting is shown in Fig. 1e and Supplementary Fig. 7, revealing the dominant contribution of a first-shell peak associated with four in-plane N or C atoms at 2.03 Å, and of two axial oxygen atoms at 2.07 Å (Supplementary Table 6). The reminder of the FT-EXAFS spectrum is well reproduced by second-shell carbon atoms, and a minor Sn-Sn contribution with a fitted bond distance of 3.33 Å, suggesting that a minor amount of SnO₂ is present but it lacks long-range order or is (sub)nanometric (Supplementary Fig. 7, Supplementary Table 6). The X-ray absorption near edge structure (XANES) analysis is also in line with the presence of Sn(IV) species, with the threshold energy of the Sn K-edge XANES spectrum of FeSnNC being very close to that for Sn(IV)O₂ (Supplementary Fig. 8). This is in line with Sn(II) and Sn(IV) oxidation states and the major contribution of Sn(IV) species in the ¹¹⁹Sn Mössbauer spectrum of FeSnNC (D1 and the singlet representing 83% of the signal, Supplementary Table 4a). Since there is no strong Sn-Sn contribution in the EXAFS signal corresponding to the second coordination shell (Fig. 1f) it suggests that a minor amount of SnO₂ is present, which lacks long-range order or is nanometric. Such nano-SnO₂ is compatible with the singlet component in the ¹¹⁹Sn Mössbauer spectrum. The Sn-N in-plane bond distance of 2.03 Å is comparable to that determined by us in the parent SnNC material, 2.06 Å (Table S4 in Ref¹⁷), while the Sn-O axial distance is shorter in FeSnNC vs. SnNC (2.07 and 2.13 Å, respectively). This is however probably a bias due to the coexistence of a larger amount of nano-SnO₂ (with Sn-O bond as low as 2.05 Å⁴⁷) in FeSnNC relative to SnNC. The larger amount of nano-SnO₂ is suggested from Mössbauer data and also supported by a higher Sn-O average coordination number in FeSnNC than in SnNC (2.0 and 1.0, respectively, Supplementary Table 6, and Table S4 in Ref¹⁷). Finally, it is important to note that the EXAFS analysis reveals the absence of Fe-Sn bonds in FeSnNC, thereby supporting that Fe-N_x and Sn-N_x moieties are separate and do not form binuclear Fe-Sn-N_x sites.

Regarding the ex situ EXAFS analysis of FeCoNC, the Fe and Co K-edge FT-EXAFS spectra shown in Fig. 1d reveal first-shell peaks at 1.3 and 1.4 Å (not corrected for phase shift), assigned to Fe-N_x and Co-N_x moieties, respectively^10–11. Both the Fe and the Co K-edge spectra also reveal the presence of metal-based nanoparticles, evidenced by significant metal-metal interactions (Fe-Fe, Co-Co or Fe-Co), with bond distances compatible with a metallic structure. This agrees with XRD and ⁵⁷Fe Mössbauer results.

The carbon morphology and metal dispersion in FeSnNC and FeCoNC was further investigated with scanning transmission electron microscopy. Aberration-corrected annular dark-field (ADF) STEM images reveal the homogeneous dispersion of single metal atom sites based on Fe, Sn and Co within graphene planes (Fig. 2). FeSnNC and FeCoNC consist of a primary carbon matrix and secondary few-layer graphene sheets (Supplementary Fig. 9–10). Graphene-like structures were previously reported for PANI-derived MNC catalysts comprising Fe.^{9, 48–49} The presence of single metal atoms (dots exhibiting bright contrast) was confirmed in the ADF-STEM images, with no apparent metal clusters or nanoparticles. Having a significantly larger atomic number, the brighter Sn atoms could be distinguished from Fe in the ADF-STEM images (Supplementary Fig. 9), which was confirmed by electron energy loss spectroscopy (EELS) (Fig. 2a-d). Fe and Co cannot be distinguished from each other with ADF-STEM, but EELS detected both elements at the single atom level (Fig. 2e-h). Due to the weak EELS signal combined with the instability of the single atoms under the electron beam, it was impossible to determine the valence state of the individual metal atoms. EEL spectra obtained from the single atoms also routinely contained a nitrogen peak, suggesting coordination of N with Fe, Sn and Co (Fig. 2).

Electrochemical Oxygen-reduction activity, selectivity and stability

The electrocatalytic activity and selectivity for ORR were measured using a Rotating Ring Disk Electrode (RRDE) in 0.1 M HClO₄ electrolyte (Fig. 3). Representative polarization curves shown in Fig. 3a reveal that both bimetallic catalysts show enhanced activity compared to their parent FeNC material.^{17, 50} The catalytic ORR activity was quantified using a Koutecky-Levich analysis. Representative Tafel plots are shown in Supplementary Fig. 11. The beginning-of-life mass activity at 0.8 V vs. RHE averaged over multiple experiments is shown in Fig. 3b, suggesting a two-fold higher initial activity of FeSnNC vs. FeNC, and a 50% enhancement of FeCoNC vs. FeNC. The enhancement in ORR activity of FeSnNC and FeCoNC is even higher when compared to SnNC and CoNC (Fig. 3b). These trends can be rationalized by the similar TOF of Sn-N_x and Fe-N_x sites, yet lower SD of SnNC compared to FeNC, and by the lower TOF of Co-N_x moieties vs. Fe-N_x moieties.^{17, 51–52} Note that the bimetallic FeSnNC and FeCoNC catalysts showed a clearly higher mass activity than four benchmark PGM-Free FeNC catalysts⁵⁰ (see Fig. 6a from reference⁵⁰) and was equally active to other recent reports on advanced MNC materials.^{31, 52–53} The high selectivity was confirmed with RRDE (Fig. 3a), FeSnNC being slightly more selective than FeCoNC and FeNC. As the accelerated stress test (AST), we applied 10,000 rectangular-wave cycles between 0.6 and 1.0 V vs. RHE in N₂-saturated acidic electrolyte at room temperature. Both bimetallic catalysts exhibited a slightly higher mass activity after the AST (Supplementary Fig. 12a), which is favorable compared the activity loss of other FeNC and CoNC catalysts.⁵⁴ The high stability results observed here with N₂-saturated acid medium must however be taken with caution, since it was shown recently for FeNC catalysts that load-cycling AST in N₂-saturated acidic electrolyte is less aggressive than in O₂-saturated condition.^55–56 The selectivity toward four-electron ORR was also unmodified after the AST or even slightly improved, as shown in Supplementary Fig. 12b.

In summary, FeSnNC and FeCoNC are significantly more active than the parent FeNC material. For FeSnNC, the strong increase in mass activity relative to FeNC is surprising in view of the low amount of Sn. The increased mass activity of FeCoNC is surprising, as well, given the presence of Fe and/or Co clusters versus the absence of such clusters in the FeNC parent material. To better understand the reasons for the increased mass activity, we moved to quantify the SD and TOF of the bimetallic catalysts.

Deconvolution of the mass activity into SD and TOF and reactivity maps

The surface site density, SD, was measured using the previously developed and validated CO cryo-chemisorption technique. CO adsorbs to single metal active sites of pyrolyzed Fe, Co, and SnNC catalysts at 193 K.^{17, 50, 57–58}. The quantification of the amount of adsorbed CO per mass of MNC allows estimation of SD, assuming one CO molecule binds per M-N_x active site. The CO cryo-chemisorption method was successfully applied to quantify Fe-N_x, Co-N_x, Mn-N_x, Sn-N_x, Cu-N_x sites in monometallic MNC materials^{17, 59}. This implies that for bimetallic FeSnNC and FeCoNC materials, the SD measured corresponds to the sum of Fe-N_x and either Sn-N_x sites or Co-N_x sites. In turn, this implies that the TOF (obtained from the ratio between the overall mass activity and the SD (Supplementary equations 2 and 6) corresponds to a mean TOF averaged over all M-N_x sites. Both FeSnNC and FeCoNC adsorb significant amount of CO, as evidenced by the lower signal for the first pulses (Supplementary Fig. 13). The analysis shows that FeSnNC adsorbs more CO than FeCoNC (93·10^− 6 and 85·10^− 6 mol·g^− 1, respectively) (Fig. 3c). Compared to the parent monometallic catalysts, both FeSnNC and FeCoNC adsorb significantly less CO than FeNC (162·10^− 6 mol·g^− 1), but more than CoNC (71·10^− 6 mol·g^− 1) and SnNC (62·10^− 6 mol·g^− 1) (Fig. 3c, Supplementary Table 7a). This is in qualitative agreement with the EXAFS and Mössbauer spectroscopy characterization, showing that the addition of Sn, and especially Co, promoted single metal atom aggregation during pyrolysis.

Next, average TOF values at 0.8 V_RHE were derived from experimental ORR catalyst mass activity and experimental SD values (Supplementary Fig. 14). FeSnNC showed a slightly higher TOF than FeCoNC, while both bimetallic materials had a significantly higher TOF than their parent FeNC, SnNC, or CoNC catalysts (Fig. 3d, Supplementary Table 7a). The TOF of FeSnNC was more than 3-fold higher than that of FeNC. This is particularly intriguing and interesting, since the FeSnNC mainly comprises Fe-N_x active sites, and only a minute amount of Sn-N_x sites. The total number of Fe and Sn atoms in FeSnNC is 3.42·10²⁰ atoms·g^− 1 (Supplementary Table 7a) and in this, only about 2%, i.e. 0.07·10²⁰ atoms·g^− 1, are Sn. We note that Sn-N_x sites in monometallic SnNC showed comparable TOF to Fe-N_x sites in monometallic FeNC.¹⁷ Therefore, our present analysis suggests an enhancement in the TOF of the Fe-N_x sites in the bimetallic FeSnNC catalyst relative to those in FeNC, which can hardly be explained by a direct synergistic effect between the Fe-N_x and the 2% Sn-N_x sites. Moreover, EXAFS did not support the presence of binuclear Fe-Sn-N_x sites. The analysis is more difficult for FeCoNC, due to balanced amounts of Fe and Co and due to the fact that the material also comprises a significant amount of metallic Fe, Co or FeCo particles. Nevertheless, there is no experimental support for the presence of binuclear Fe-Co-N_x sites in FeCoNC either, and the TOF of Co-N_x sites in CoNC is much lower than that of Fe-N_x sites in FeNC.¹⁷ Therefore, the present analysis likewise suggests that the TOF of Fe-N_x sites in FeCoNC was increased via the addition of cobalt in the synthesis. We also assessed the overall utilization factor, defined as the ratio of SD to the sum of all metal atoms present in the materials (Supplementary Table 7a). The overall utilization factors for FeSnNC and FeCoNC are similar (0.14–0.16), and only slightly lower than those of the parent FeNC and SnNC catalysts (0.19–0.23). The slight decrease in overall utilization for the bimetallic catalysts is accounted for by the presence of metal clusters, unlike in FeNC and SnNC. The higher overall utilization factor for CoNC compared to all others is ascribed to the distinct synthesis from ZIF-8, while all other materials in Supplementary Table 7a were prepared via a polyaniline approach.

Overall, the favorably high mass activity of FeSnNC and FeCoNC over monometallic reference catalysts is caused by enhancements of the TOF (2.7-3.4x), despite lower SD (0.52-0.57x). The possibility to raise the TOF of Fe-N_x sites by the addition of a second metal is of fundamental and practical importance, but the results also show the importance of re-optimizing the synthesis to mitigate the formation of iron clusters. The SD-TOF reactivity map (Fig. 3e) provides a snapshot of the progress achieved with the bimetallic FeSnNC and FeCoNC catalysts compared to (i) the parent FeNC and SnNC materials, (ii) other benchmark FeNC catalysts prepared (Supplementary Table 7b). The map demonstrates the enhanced activity of FeCoNC and FeSnNC as a result of an increased TOF at acceptable SD. In particular, the FeSnNC material reaches an activity close to the target of 6.6 A·g^− 1, as defined by the ElectroCat network funded by the US Department of Energy and the EU projects CRESCENDO and PEGASUS funded by the Fuel Cells and Hydrogen Joint Undertaking^60–62.

Single-cell PEMFC results

The activity and performance of FeSnNC and FeCoNC were finally evaluated in single-cell PEMFC, and compared to that of the reference FeNC material. In addition, we also evaluated the PEMFC performance of these three catalysts after a short treatment at 750°C under flowing NH₃ (labelled as MNC-NH₃). Such an NH₃ treatment did not alter the metal speciation of monometallic MNCs, while boosting their ORR activity and performance in PEMFC¹⁷. Some work reported that NH₃ treatments increased the micropore surface area as well as the Lewis basicity of the MNC surface^{11, 42}. Using ex situ XAS measurements, we verified that the Fe, Sn and Co speciation of FeSnNC and FeSnNC-NH₃ as well as of FeCoNC and FeCoNC-NH₃ remained essentially identical (Supplementary Fig. 15). This suggests carbon matrix Lewis basicity as the main origin of the ORR improvements of NH₃-treated samples, while the nature and number of active sites was not affected. Figure 4a shows a moderately improved ORR activity of FeSnNC vs. FeCoNC at 0.8 V and above, in line with RDE results. The higher cell performance of FeSnNC vs. FeCoNC was most evident at larger current densities, where the cell performance was controlled by both ORR kinetics and ohmic or mass-transport losses. The single-cell performance based on FeSnNC-NH₃ and FeCoNC-NH₃ cathodes showed significantly enhanced kinetic performance below 600 mA cm^− 2 compared to the untreated catalysts. The higher performance is mainly due to improved ORR kinetics, as an analysis of the TOF in PEMFC suggests. The TOF value of FeSnNC-NH₃ was 2.5 e site^− 1 s^− 1, compared to 0.6 e site^− 1 s^− 1 for FeSnNC catalyst, as derived from mass-normalized cell current densities at 0.8 V iR-corrected cell voltage in PEMFC experiments and from the average SD derived using CO cryo-chemisorption (Supplementary Eq. 7). Figure 4b shows the iR-corrected Tafel plots, demonstrating a kinetic improvement of at least five-fold at 0.8 V cell potential, thanks to the NH₃ treatment. The comparison to the reference FeNC material is shown in Supplementary Fig. 16. FeSnNC and FeCoNC showed higher ORR activity and cell performance than FeNC and FeNC-NH₃. In conclusion, our PEMFC data demonstrate the impressive kinetic benefits of the bimetallic catalysts in a cell environment. The liquid electrolyte RDE data obviously transfer into the PEMFC environment.

Operando XANES signature of Fe-N_x, Sn-N_x, and Co-N_x moieties in bimetallic MNCs

Operando XAS experiments were conducted in N₂-saturated electrolyte on FeSnNC, FeSnNC-NH₃, FeCoNC and FeCoNC-NH₃ at the Fe, Sn and Co K-edges. This is the first operando report on Sn K-edge XANES spectra of single-metal-atom Sn-N_x sites. As the electrode potential was lowered, spectral changes at the Fe K-edge became evident for FeSnNC (Fig. 5a), which resembled Fe-N_x sites trends in aqueous acidic electrolyte.¹¹ The change in XANES threshold energy with potential indicated a Fe(III)/Fe(II) redox transition, while changes in spectral features revealed a structural modification of a significant fraction of Fe-N_x sites in the region 0.2–0.9 V_RHE. A similar trend of XANES spectral changes with electrochemical potential was observed for the parent FeNC material (Supplementary Fig. 17a). A detailed comparison shows that the Fe K-edge XANES spectra of FeSnNC and the parent FeNC are identical at 0.9 V_RHE (Supplementary Fig. 17b), while a small shift to higher energy was observed for FeSnNC vs. FeNC at 0.4 and 0.2 V_RHE (Supplementary Fig. 17c-d). This suggests a higher mean Fe oxidation state in FeSnNC than in FeNC in the low potential regime, while no difference was observed at 0.9 V and also at any potential above 0.6 V vs. RHE (not shown). In contrast, the Sn K-edge XANES spectra of FeSnNC showed no dependence on the electrode potential over the entire potential range (Fig. 5b), and the same was found true for SnNC (Supplementary Fig. 18). The potential-independence evidences that Sn cations in Sn-N_x sites do undergo no change in structure and oxidation state in the ORR potential range, akin to Co-N_x sites in CoNC¹⁰.

After NH₃-treatment, similar operando XANES trends were observed. With varying electrode potentials, the FeSnNC-NH₃ catalyst showed electronic and structural changes at the Fe K-edge that were similar to those of untreated FeSnNC (compare Fig. 5a and Supplementary Fig. 19). A deeper inspection of the operando Fe K-edge XANES spectra showed a remarkable positive shift in the edge position of FeSnNC-NH₃ with respect to FeSnNC (0.25 eV at 0.9 V vs. RHE), which is even more distinctive at low potentials (0.4 eV at 0.2 V vs. RHE) (Supplementary Fig. 20). This indicates a higher mean Fe oxidation state in FeSnNC-NH₃ under applied potentials, quite similar to the findings of FeSnNC above.

Since the trend of ORR activity observed in PEMFC is FeSnNC-NH₃ > > FeSnNC > FeNC (Fig. 4), the operando XANES data therefore suggests a positive correlation between ORR activity (TOF of Fe-N_x sites) at 0.8 V vs. RHE and the edge position of operando Fe K-edge XANES spectra measured at a given potential. This is confirmed by plotting the current density measured at 0.8 V iR-free PEMFC voltage vs. the energy shift \(\varDelta E\) of the operando threshold XANES spectra measured at 0.4 V vs. RHE (Supplementary Fig. 21). The current density of FeSnNC-NH₃ and FeSnNC at 0.8 V_{iR−corrected} were 90 and 20 mA cm^− 2, respectively, exceeding the 2.7 mA cm^− 2 value for FeNC measured under the same conditions¹⁷. Similar correlations were observed if the operando XANES energy shift \(\varDelta E\) at the Fe K-edge was considered at varying electrode potentials (see the Table inside Supplementary Fig. 21a). The trend also holds when the TOF is considered, rather than the overall ORR activity (Supplementary Fig. 21b). From these findings we conclude that the enhanced intrinsic catalytic reactivity (TOF) is associated, possibly even originates, from the higher mean oxidation state of the active Fe-N_x sites in the bimetallic catalysts.

The XANES spectra of FeCoNC and FeCoNC-NH₃ at the Fe K-edge showed potential-dependent spectral changes that were quantitatively very similar to those observed for the parent FeNC and the FeSnNC materials (Fig. 5c and Supplementary Fig. 22a). By contrast, no spectral Co K-edge changes with potential were observed for FeCoNC and FeCoNC-NH₃ (Fig. 5d and Supplementary Fig. 22b). This is consistent to the parent CoNC catalyst, the Co-N_x spectra of which behaved independent of potential.¹⁰ On the down side, the significant presence of low valent Fe particles in FeCoNC and FeCoNC-NH₃ revealed by EXAFS prevents the accurate detection of electronic effects on Fe-N_x sites (if any) induced by the presence of cobalt centers (Supplementary Fig. 23–24, Supplementary note 4).

Finally, we discuss how the addition of the secondary metal Sn or Co may lead to a higher ratio of D1/D2 species (ex situ ⁵⁷Fe Mössbauer) or higher average oxidation state of Fe at a given potential (operando XANES), both phenomena associated with the higher TOF. The selective formation of D1 vs. D2 sites may originate from (i) a modified carbon structure triggered by the presence of the secondary metal dopant, (ii) competition between iron and the secondary metal dopant for the ‘D2 sites’, or (iii) different pyrolysis trajectories induced by the presence of the secondary metal. The hypothesis (i) is related to increased disorder in the carbon matrix in presence of the second metal dopants. The D1 Fe motif is typically associated with an FeN₄C₁₂ porphyrinic site, and its formation requires more disorder in the carbon matrix than the D2 motif, which is associated with a FeN₄C₁₀ structure. The hypothesis (ii) is related to the possibility that the apparent effective selective formation of D1 sites in these bimetallic materials is actually an indirect effect resulting from different affinities of the MN₄C₁₀ and MN₄C₁₂ structures for Fe versus Sn or Co cations. The hypothesis (iii) is related to the transient species formed during pyrolysis. Catalytic precursors containing FeCl₂ evolve to Fe-N_x active sites via the formation of α-Fe₂O₃ at relatively low temperature, which subsequently transforms into tetrahedral Fe(II)-O₄ oxide between room temperature and 600°C.⁶³ The presence of an additional metal could trigger the formation of Sn- and Co-doped Fe₂O₃ with distinct structural and magnetic properties,^64–65 thus modifying the Fe-N_x synthesis pathway, possibly leading to the preferential formation of Fe-N_x moieties associated with the D1 signal (Supplementary Note 5).

The present work establishes that the TOF of Fe-N_x sites is significantly increased by the addition of a secondary metal atom, such as Sn or Co, and that binuclear sites with adjacent M-M or M-N-M motifs were absent in these bimetallic catalysts. ⁵⁷Fe Mössbauer spectroscopy identified a significantly larger ratio of Fe(III)-N_x/Fe(II)-N_x sites (D1/D2 ratio) in both bimetallic catalysts, relative to FeNC. The increased intrinsic TOF of bimetallic catalysts originates from this enhanced relative abundance of Fe D1 site.

Acknowledgements

The authors would like to acknowledge Dr. Sören Dresp, Dr. Shuang Li, Dr. Huan Wang and Dr. Tobias Reier for assistance with RRDE, XPS, nitrogen physisorption and CO pulse chemisorption experiments. We also thank Prof. Ulrike I. Kramm for help with Mössbauer spectroscopy measurements. Aberration-corrected STEM-EELS was conducted at the Center for Nanophase Materials Sciences, which is a US Department of Energy Office of Science User Facility, at Oak Ridge National Laboratory. This project received partial financial support by the Graduate School of Excellence Energy Science and Engineering (GRC1070). The research leading to some of these results has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 779366. This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation program, Hydrogen Europe and Hydrogen Europe research. A. Z and A. K acknowledge support from the French ANR project ANR-19-CE05-0006 (SPECTROSCOPE), and Synchrotron SOLEIL (Gif-sur Yvette, France) for provision of synchrotron radiation facilities at beamline SAMBA (proposal number 20201432). F. L., M. P. and P. S. acknowledge financial support by the Bundesministerium für Bildung und Forschung (BMBF) under The German Israeli Battery and Fuel Cell program, FKZ 03XP0251.

Supporting Information

Additional figures for TEM, XRD, N₂ sorption, XPS, Mössbauer spectra, XAS, CO adsorption and TPD data, supporting electrochemical characterization and additionnal Tables of pore size distribution, elemental content analysis, Mössbauer spectra fittings, EXAFS fittings, TOF and SD data.

Author Information

Corresponding authors: [email protected]; [email protected]; [email protected]

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Structural and Reactivity Effects of Secondary Metal Doping into Iron-Nitrogen-Carbon Catalysts for Oxygen Electroreduction

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