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Catalysts 2019, 9(2), 144; doi:10.3390/catal9020144
The Challenge of Achieving a High Density of Fe-Based Active Sites in a Highly Graphitic Carbon Matrix
Institut Charles Gerhardt Montpellier, UMR 5253, CNRS, Université Montpellier, ENSCM, Place Eugène Bataillon, CEDEX 5, 34095 Montpellier, France
Department of Chemistry and Chemical Biology, Northeastern University, Boston, MA 02115, USA
Department of Materials Chemistry, National Institute of Chemistry, 1000 Ljubljana, Slovenia
Synchrotron SOLEIL, L’orme des Merisiers, BP 48 Saint Aubin, 91192 Gif-sur-Yvette, France
Authors to whom correspondence should be addressed.
Received: 11 December 2018 / Accepted: 11 January 2019 / Published: 2 February 2019
As one of the most promising platinum group metal-free (PGM-free) catalysts for oxygen reduction reaction (ORR), Fe–N–C catalysts with a high density of FeNx moieties integrated in a highly graphitic carbon matrix with a proper porous structure have attracted extensive attention to combine the high activity, high stability and high accessibility of active sites. Herein, we investigated a ZnCl2/NaCl eutectic salts-assisted ionothermal carbonization method (ICM) to synthesize Fe–N–C catalysts with tailored porous structure, high specific surface area and a high degree of graphitization. However, it was found to be challenging to anchor a high density of FeNx sites onto highly graphitized carbon. Iron precursors with preexisting Fe–N coordination were required to form FeNx sites in the nitrogen-doped carbon with a high degree of graphitization, while individual Fe and N precursors led to a Fe–N–C catalyst with poor-ORR activity. This provides valuable insights into the synthesis-structure relationship. Moreover, the FeNx moieties were identified as the major active sites in acidic conditions, while both FeNx sites and Fe2O3 were found to be active in alkaline medium.
Keywords:oxygen reduction reaction; Fe–N–C; active site; graphitic carbon
The oxygen reduction reaction (ORR) is a cornerstone of various electrochemical energy storage and conversion devices [1,2,3], including metal–air batteries and fuel cells. Proton exchange membrane fuel cell (PEMFC) is a key technology to provide clean and sustainable energy conversion with potential applications in portable electronics, electric vehicles, and residential power generation. Recently, hydroxide exchange membrane fuel cells (HEMFCs) have attracted significant attention due to the tremendous progress in developing highly conductive and stable membranes and ionomers against hydroxide attack . However, the significant amount of Pt needed at the cathode side to accelerate the sluggish ORR kinetics remains a long-standing impediment for the widespread commercialization of PEMFCs and HEMFCs [4,5]. Thus, extensive efforts have been devoted to reducing or eliminating the use of precious metals at the cathode side in the past decades [6,7,8,9,10]. Fe(Co)–N–C catalysts represent the most promising precious metal-free (PGM-free) catalysts, with their ORR activity approaching or even surpassing that of commercial Pt/C in acidic and alkaline media [11,12,13,14,15], bringing us closer to empowering precious metal-free cathodes in practical H2/air PEMFCs and HEMFCs.
Fe(Co)–N–C catalysts are generally prepared via a high temperature pyrolysis of various metal, carbon, and nitrogen precursors, often leading to the coexistence of MNxCy moieties and metallic aggregates (carbides/nitrides/oxides) . After decades of efforts to understand the origin of the ORR activity of pyrolyzed Fe(Co)–N–C catalysts, a consensus has been reached that Fe(Co)Nx sites with the metal center coordinated with several nitrogen atoms located in the equatorial plane exhibit the highest intrinsic ORR activity, when normalized per atom of iron/cobalt [11,12,17,18,19]. However, the exact structure of such sites (e.g., pyrrolic vs. pyridinic nitrogen ligating the metal cation, edge vs. in-plane location of Metal-Nx moieties) , as well as the role of the carbon matrix in tuning the ORR activity and/or stability (e.g., basicity , graphitic or amorphous ordering ) is still under debate. In addition to the local structure of Fe(Co)Nx moieties, the porous structure of the carbon matrix in Fe(Co)–N–C catalysts plays a crucial role for their ORR activities. Micropores are generally believed to host Fe(Co)Nx moieties [17,21], while the meso/macropores are proposed to assist the mass transport of ORR-related species toward and away from active sites [22,23]. Thus, the preparation of Fe(Co)–N–C catalysts with a high density of Metal–Nx moieties integrated in a highly-graphitic carbon matrix integrating both micropores and larger pores (meso or macropores) seems attractive to combine high activity, high stability and high accessibility of active sites.
The ionothermal carbonization method (ICM) is an efficient strategy to synthesize nitrogen-doped carbons with tailored porous structure and ultra-high specific surface areas (>2000 m2/g) [24,25,26,27]. Particularly, the addition of eutectic salts ZnCl2/AMCl (AM=Li, Na, or K) to organic C and N precursors was demonstrated to be effective to modulate the porous structures of nitrogen-doped carbons prepared via pyrolysis . ZnCl2 with a low melting point acted as a porogen due to its early evaporation during pyrolysis . On the other hand, AMCl salt with high melting point remained in the carbon matrix during the entire pyrolysis process, being removed only after pyrolysis via acid washing, generating a specific porous structure through a templating effect . Moreover, the molten state of eutectic salt in a broad temperature range creates a “semiclosed system” with ionic-liquid-confined space, which reduces the loss of nitrogen and carbon during pyrolysis, and thus promotes the yield and nitrogen doping efficiency .
In this study, we therefore investigated a eutectic-salt (ZnCl2/NaCl)-assisted ICM to synthesize Fe–N–C catalysts (labelled FeNC-ICM hereafter) with high amounts of meso/macropores and high graphitization, and revealed the critical role of the nature of the iron precursor to obtain a high ORR activity for this approach. Individual iron and nitrogen precursors (e.g., Fe acetate and phenanthroline) were previously demonstrated to be efficiently transformed during pyrolysis into FeNx moieties in highly disordered N–C matrix (e.g., ZIF-8 derived Fe–N–C catalysts) [11,12,13,14,17]. However, the use of individual iron and nitrogen precursors were shown in this study to lead to poor ORR activity. Iron precursors with preexisting Fe–N coordination (Fe-phthalocyanine or Fe-phenanthroline complex) were required to obtain high ORR activity. Advanced material characterization showed that the fraction of Fe present as FeNx moieties and the absolute site density of FeNx moieties were low in the presently investigated materials, and that the N-doped carbon matrix was highly graphitic. A significant fraction of Fe was also present in the form of iron carbide and nanosized Fe oxide, suggesting it is more difficult to integrate a high density of FeNx moieties in a highly graphitic carbon matrix than in an amorphous one. These results provide valuable insights into the synthesis-structure relationships of pyrolyzed Fe–N–C catalysts. Moreover, the FeNx moieties were identified as the major active sites for the ORR in acidic medium, while both FeNx moieties and nanosized iron oxides were found to contribute to the ORR activity in alkaline medium.
2.1. ORR Activity
The FeNC-ICM catalysts were prepared via pyrolysis at 900 ˚C in inert atmosphere of a catalyst precursor including a dual carbon and nitrogen source (adenine), an iron source and the ZnCl2/NaCl eutectic salt. Three iron precursors were studied, namely iron(II) phthalocyanine (FePc), 1,10-phenanthroline iron(II) perchlorate (FePhen3) and iron(II) acetate (FeAc2), keeping all other synthesis parameters unchanged. The catalyst precursors were prepared with a fixed mass ratio of adenine/salts of 1/10, while the mass of each Fe precursor was adjusted to reach 4 wt.% Fe relative to the sum of (nitrogen + carbon mass) in the catalyst precursors. The detailed synthesis protocol is given in Materials and Methods. When using FeAc2 as Fe precursor, the amount of 1,10-phenanthroline necessary to reach a molar ratio Phen/Fe of 3 was also added, in order to ensure all three precursors possessed similar nitrogen contents. The catalysts obtained after pyrolysis and water-wash (to remove NaCl) are labelled as ICM-FePc, ICM-FePhen3 and ICM-FeAc2, the second part of the label identifying the iron precursor used.
The ORR activity of FeNC-ICM catalysts was evaluated with a rotating disk electrode (RDE) setup in both acidic and alkaline media. First, the effect of the nature of the iron precursor in the ICM approach on the ORR activity in acidic electrolyte was studied (Figure 1a). The ICM-FeAc2 catalyst showed a low onset potential for ORR in 0.1 M HClO4 and poorly-defined diffusion-limited current density. In contrast, the ICM-FePc and ICM-FePhen3 catalysts exhibited a half-wave potential (E1/2) of 0.75 and 0.77 V vs. a reversible hydrogen electrode (RHE), respectively. They also showed a well-defined redox peak at ~0.73 V vs. RHE in cyclic voltammograms (CVs) recorded in N2-saturated acidic medium, while no redox peak was observed for ICM-FeAc2 (Figure 1b). This redox peak has previously been attributed to the Fe2+/3+-N4 redox transition in pyrolyzed Fe–N–C materials [17,30]. This suggests that, with the present synthesis method, the FeNx moieties could only form at high temperature when using Fe precursors with pre-existing Fe–N4 or Fe–N6 coordinations, while the FeAc2 in the presence of Phen did probably not form the Fe(phen)3 complex during the dry ball-milling before pyrolysis, and could apparently not form FeNx moieties during the pyrolysis either. This result is different from what has been observed for other synthesis approaches, where separate FeAc2 and 1,10-phenanthroline precursors, mixed with ZIF-8 as a sacrificial carbon precursor, entirely converted into FeNx moieties in the ZIF-8 derived Fe–N–C [11,12,13,14,17]. Figure 1b further showed that the CV of ICM-FeAc2 was tilted and strongly deviated from a square signal, typical for samples with low electronic conductivity. This suggests that the nature of the iron precursor not only affected the formation of FeNx moieties, but also changed the nature of the carbon matrix obtained during pyrolysis. As will be shown later in this study, the situation is however complex here, with the presence of three different Fe-based species in ICM-FePhen3, and its redox assignment is further discussed after advanced spectroscopic characterization.
The most active ORR-catalyst in acid medium, ICM-FePhen3, was further characterized with X-ray diffraction (XRD) and Raman spectroscopy in order to identify the structure of the carbon matrix. Compared to the broad peaks (24° and 43°) associated with amorphous carbon in the XRD patterns of a ZIF-8 derived Fe–N–C catalyst (labelled Fe0.5NC, used here as reference Fe–N–C catalyst from our laboratory that was demonstrated to comprise only FeNx moieties ), ICM-FePhen3 exhibited narrower graphite-related peaks in XRD (peaks at 26°, 43° and 45°, Figure 2a). The first-order Raman spectrum of ICM-FePhen3 showed two peaks: the G band at ~1590 cm−1 and the disorder-induced D band at ~1360 cm−1 (Figure 2b). In the second-order Raman spectrum, three less intense peaks were observed, namely the G’ (2D) band at ~2710 cm−1 attributed to the D-band overtone, the peak at ~2918 cm−1 due to a combination of D and G band, and the peak at ~3240 cm−1 ascribed to the G-band overtone (Figure 2b) . ICM-FePhen3 showed a lower ID/IG ratio (0.67 vs. 0.84) and a much more intense peak at ~2710 cm−1 (widely observed in graphite and graphene)  compared to Fe0.5NC (Figure 2b), further revealing its higher degree of graphitization. Graphitic carbon is believed to have improved corrosion resistance relative to amorphous carbon, which could result in improved stability of Fe–N–C catalysts in PEMFCs or HEMFCs . However, a synthetic contradiction between the desire for a high density of FeNx sites and a high degree of graphitization of carbon might occur, as discussed later.
Then the ORR activity of ICM-FePhen3 was evaluated in 0.1 M HClO4 and 0.1 M KOH, as well as after acid wash (denoted as ICM-FePhen3-AW) (Figure 1c). The acid wash did not affect the ORR activity of ICM-FePhen3 in acidic medium (same on-set potential and half-wave potential as ICM-FePhen3), but resulted in a higher diffusion-limited current density, possibly due to the removal of poorly-active Fe species favoring the 2-electron reduction of ORR to H2O2. Moreover, ICM-FePhen3-AW exhibited a redox (attributed to Fe2+/3+N4) at similar potentials (but much lower intensity) than observed for ICM-FePhen3, suggesting that FeNx moieties survived the acid wash, and probably worked as the major active sites for ORR in acid (Figure 1d). In alkaline medium, ICM-FePhen3 and ICM-FePhen3-AW showed however different ORR activities, contrasting with the case in acidic medium. ICM-FePhen3-AW had an E1/2 value negatively shifted by 60 mV vs. ICM-FePhen3. These two observations strongly suggest that the acid-wash removed some Fe species that are ORR-inactive in acidic medium but ORR-active in alkaline medium. The iron speciation is reported and discussed further below. For the most active sample, ICM-FePhen3, its E1/2 value at a catalyst loading of 800 µgFeNC/cm2 in alkali was 0.86 V vs. RHE, surpassing that of the commercial Pt/C (TKK, 10 μgPt/cm2). This is among the best ORR activities reported for Fe–N–C catalysts in alkali [34,35,36,37].
2.2. Iron Speciation
In order to identify the Fe species present in ICM-FePhen3 and ICM-FePhen3-AW, XRD, 57Fe Mössbauer spectroscopy, and X-ray absorption spectroscopy (XAS) measurements were performed. The presence of Fe2O3 was detected by XRD in ICM-FePhen3 (main peak at ca 36° and secondary peaks at ca 30°, JCPDS #39-1346), but not detected in ICM-FePhen3-AW (Figure 2a). With room-temperature 57Fe Mössbauer spectroscopy, four major spectral components were detected in ICM-FePhen3 (Figure 3a and Table 1): a sextet assigned to iron carbide (Fe3C, 30%), two sextets assigned to iron oxide (ε-Fe2O3, 12%) , a broad singlet (12%) and a doublet (46%). The assignment of the singlet will be discussed later. The Mössbauer parameters of that doublet (Table 1) allow the following straightforward but ambiguous assignment: i) either the D1 component typical for low-spin ferrous FeN4 moiety in pyrolyzed Fe–N–C catalysts [11,17,39] or ii) a high-spin Fe3+ species such as amorphous or β-Fe2O3 with a small particle size [40,41]. It is known that the Mössbauer spectrum of nanometric ferric oxides changes from a doublet to a sextet spectral component below a certain critical temperature [42,43], while that of D1 (FeNx moiety) is independent of temperature, remaining a doublet even at 5 K . In order to distinguish between the two possible assignments, the 57Fe Mössbauer spectrum of ICM-FePhen3 was measured at 5 K (Figure 3b). In that spectrum, the relative area (RA) of the two sextets assigned to Fe2O3 (both ε + β) increased to 62%, while the doublet disappeared. This indicates that the well-defined narrow doublet observed at room temperature was arising from nanometric β-Fe2O3. This is also independently confirmed later with high-resolution transmission electron microscopy (HR-TEM) images.
For ICM-FePhen3-AW, the analysis of its room-temperature Mössbauer spectrum (Figure 3c) showed first that the magnitude of the γ-rays absorption was much decreased (synonym with much lower total Fe content in the sample) and second, that both the sextets of ε-Fe2O3 and the intense doublet of β-Fe2O3 observed in ICM-FePhen3 were no longer visible. Instead, the spectrum clearly showed only the sextet typical for Fe3C and the broad singlet component already observed in ICM-FePhen3 (Figure 3c). It however cannot be excluded that a small fraction of the narrow doublet assigned to nanosized Fe2O3 is still present in ICM-FePhen3-AW, embedded within the broad singlet. These results can be interpreted as the complete or nearly-complete removal of the Fe2O3 (nano)particles during the acid wash, while a significant fraction of Fe3C was not dissolved. Last, the singlet with unusually large line width (in green color in Figure 3) was present in all three Mössbauer spectra and is a spectral component previously identified in complex Fe–N–C materials comprising numerous Fe species . It represents only 12% of the Fe signal in ICM-FePhen3 but 55% in ICM-FePhen3-AW (Table 1), suggesting it corresponds to a species that is stable in acidic medium. As will be seen later, its assignment can only be made after discussing the XAS results.
In order to further study the coordination environments of iron, XAS measurements at the Fe K-edge were performed on ICM-FePhen3 and ICM-FePhen3-AW, as well as on selected reference compounds. The similar X-ray absorption near edge structure (XANES) spectra (Figure 4a) of ICM-FePhen3 and a Fe2O3 reference and their corresponding Fourier Transforms (FT) of the extended X-ray absorption fine structure (EXAFS) spectra (Figure 4b) confirmed that Fe2O3 is the dominant iron component in ICM-FePhen3, agreeing well with the Mössbauer results. The shoulder at 2.1 Å and the strong peak at 2.5 Å coincided with the Fe–Fe backscattering from Fe3C standards for the former, and Fe2O3 for the latter (Figure 4b). The removal of most of Fe2O3 by acid wash was confirmed by the much lower intensity of the peak at ca 2.5 Å in the FT-EXAFS signal of ICM-FePhen3-AW relative to ICM-FePhen3. Moreover, the FT-EXAFS spectrum of ICM-FePhen3-AW (Figure 4b) exhibited a major peak at 1.5 Å that can be attributed either to Fe-N contribution from FeNx moieties and/or Fe–O contribution from Fe2O3.
In order to go beyond the limitations of Mössbauer spectroscopy (unresolved broad singlet component), we then performed linear combination fitting (LCF) of the XANES spectra of ICM-FePhen3 (Figure 4c) and ICM-FePhen3-AW (Figure 4d) with three reference spectra, namely Fe2O3, Fe3C and Fe0.5NC (the latter comprising exclusively Fe as atomically dispersed FeNx moieties ). The XANES spectrum of ICM-FePhen3 was well fitted with a combination involving the above three components. Their relative amounts are shown in Table 2, and agree well with the RA % of Fe2O3, Fe3C and of the broad singlet quantified from the 5K Mössbauer-spectra fittings. Thus, for ICM-FePhen3, the broad singlet identified in the Mössbauer spectrum can be entirely assigned to FeNx moieties.
For ICM-FePhen3-AW, the three components of Fe2O3, Fe3C and Fe0.5NC were also needed to get the best LCF fit of its XANES spectrum (Figure 4d), including 30% of the reference Fe2O3 spectrum and 38% of the reference Fe0.5NC spectrum. LCF with only the Fe3C and Fe0.5NC references resulted in a clearly poorer fit quality (especially at the white line intensity region), confirming the presence of Fe2O3 in ICM-FePhen3-AW (Supporting Figure S1). This is in contrast with the Mössbauer analysis, which could not identify the presence of Fe2O3 or nano Fe2O3, probably due to the low signal-to-noise ratio. The presence of Fe2O3 in ICM-FePhen3-AW is also supported by the peak at ca 2.5 Å in the FT-EXAFS spectrum (Figure 3b) and by elemental mapping results (shown later).
The LCF XANES analysis and FT-EXAFS spectra thus indicate that the ICM-FePhen3-AW sample contains three Fe species: Fe3C, FeNx moieties and nanosized Fe2O3. Since only two spectral components have been used for the Mössbauer fitting of that sample, the comparison of RA% and XANES LCF % is less straightforward. The broad singlet used for the Mössbauer fitting of that catalyst probably includes both the signal from FeNx moieties and from nano-Fe2O3, and this indicates that the RA% for the singlet and for Fe3C indicated in Table 1 for ICM-FePhen3-AW is not reliable (as initially expected also from the high noise level of that spectrum). In conclusion, the combined Mössbauer, XANES and EXAFS analysis indicates that both ICM-FePhen3 and ICM-FePhen3-AW contain FeNx moieties. These sites were present already in ICM-FePhen3 and survived the acid wash, and probably worked as the major active sites for ORR in acidic medium. This hypothesis would explain the similar ORR activity of ICM-FePhen3 and ICM-FePhen3-AW in acidic medium.
2.3. Porous Structure and Morphology
Following the carbon bulk structure and Fe speciation characterization, the porous structure of ICM-FePhen3 was investigated with N2 adsorption/desorption isotherms. The hysteresis loop (assigned to small mesopores) and the slope of the N2 adsorption/desorption isotherm of ICM-FePhen3 (assigned to large mesopores) (Figure 5a), as well as the corresponding pore size distribution (calculated from density functional theory (DFT) method, Figure 5b) indicated the mesoporous structure of the carbon matrix, with a volume-average pore size around 3.4 nm (Table 3) .
Last, the morphology of ICM-FePhen3-AW was further studied with scanning electron microscopy (SEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Tubular-shaped, hollow carbon matrixes with diameters around 60 nm were observed from the SEM image (Figure 6a). The HAADF-STEM images (Figure 6c–e) identified a highly porous structure of the catalyst, agreeing well with the high double layer capacitance from CVs (Figure 1b,d) and the high BET surface areas (~1500 m2/g, Table 3). The obtained hierarchically porous structure was arisen from the ZnCl2/NaCl eutectic salts introduced in the catalyst precursor . The elemental compositions were investigated by energy dispersive X-ray spectroscopy (EDX). Carbon, oxygen and iron were present in the sample (Figure 6f). It should be noted that nitrogen was not analyzed due to the close energy between C and N, and the stronger signal of C. Moreover, trace amount of S (introduced from acid wash with H2SO4), Si (from the quartz tube and quartz boat used during pyrolysis) and Ni (from the TEM grid) were also detected by EDX. Aggregated Fe particles with different sizes were also observed in HAADF-STEM images for ICM-FePhen3-AW (Figure 6b–e). The EDX elemental mapping results suggested that these particles are iron oxides, or oxidized Fe3C (Figure 6g–j), and they survived the acid wash probably due to protection of the carbon shell. This is in line with the peak at ca 2.5 Å assigned to Fe-Fe backscattering from Fe oxides in the FT-EXAFS spectrum of ICM-FePhen3-AW, as well as the LCF of its XANES spectrum.
Based on the results and analyses, the ICM-FePhen3 catalyst comprises mainly Fe2O3 (60–68%) together with FeNx moieties (20–28%) and Fe3C (10–18%). After acid wash, the ICM-FePhen3-AW sample still comprises Fe in these three different environments, but now in comparable relative contents (26–34% Fe2O3, 35–41% FeNx moieties, 31–33% Fe3C). The majority of Fe2O3 particles in ICM-FePhen3 were thus removed by acid wash. The Fe2O3 particles still observed after acid wash may have been protected from the acid leaching by a surrounding carbon layer, and/or may have formed from the partial oxidation of Fe3C particles during the acid wash. The almost-superimposed ORR activity in acidic medium of ICM-FePhen3 and ICM-FePhen3-AW suggests that all or most of the FeNx moieties present in ICM-FePhen3 survived the acid wash, and are at the origin of the major part of the ORR activity of both samples, in line with the current consensus in the field that FeNx sites are the most active Fe-sites in acidic medium [11,14,17,47]. The slightly lower ORR activity of ICM-FePhen3-AW vs. ICM-FePhen3 may be assigned to a small contribution of Fe3C particles to the ORR activity of ICM-FePhen3, or to the removal by acid wash of a few FeNx moieties with low stability. For the former hypothesis, the acid wash is expected to remove Fe3C particles that are either unprotected by a carbon layer, or protected by a thin or porous N-doped carbon layer, while Fe3C particles with a thick graphite shell protection are expectedly stable to an acid wash. Since Fe3C particles surrounded by a thin N-doped carbon layer have been shown to possess some ORR activity in acidic medium, but also to be poorly stable due to defects in their carbon shell, it is possible that the small decrease in ORR activity of ICM-FePhen3-AW vs. ICM-FePhen3 comes from the removal of a fraction of the Fe3C particles. FeNx moieties were identified in both the ICM-FePhen3 and ICM-FePhen3-AW catalysts, with the former showing a clear redox peak in N2-saturated CV, while the latter exhibiting a redox peak at similar position with much lower intensity (Figure 1d). The redox signal of FeNx moieties in pyrolyzed Fe–N–C catalysts is often weak and broad, necessitating to resort to square-wave voltammetry to identify it. Thus, while the position of this redox peak fits well with that assigned to FeNx moieties, the overall redox signal of ICM-FePhen3 may originate from both FeNx moieties and, mostly, from Fe2O3 particles. This would also explain the reduced peak intensity of ICM-FePhen3-AW, as the majority of Fe2O3 was removed via acid wash. The residual redox signal after acid wash possibly originates from FeNx moieties and/or the residual Fe2O3.
In alkaline medium however, the ORR activity of ICM-FePhen3-AW is significantly lower than that of ICM-FePhen3 (Figure 1c), and this is paralleled by the removal of a large fraction of Fe2O3 during the acid wash. This suggests that one or several polymorphs of Fe2O3 contributed largely to the overall ORR activity of ICM-FePhen3 in alkaline. This is in line with the previously reported ORR activity of Fe oxide in alkaline medium . Other inorganic iron species (FeSn0.5 [49,50], [email protected]3C  and Fe2[email protected]4P ) have been reported to be ORR active as well, and they generally exhibit much superior ORR activity in alkali compared to that in acid, agreeing well with our results. Fe3C is also present in ICM-FePhen3 and may have contributed to the overall ORR activity in alkaline medium. While in acid medium, the ORR activity of ICM-FePhen3 is mostly assigned to FeNx moieties, the contribution to the overall ORR activity of Fe3C particles encapsulated in nitrogen-doped graphene may be higher in alkaline medium due to a stronger enhancement of the ORR turnover frequency with pH changing from 1 to 13 for N-doped carbon (with or without subsurface Fe3C) than for FeNx moieties. Both experimental results [37,53] and theoretical calculations  have proved the ORR activity of Fe3C particles encapsulated in nitrogen-doped graphene.
It should be noted that the major iron species in the present ICM-derived Fe–N–C catalysts is Fe oxides, and the site density of FeNx moieties is quite low. This explains the relatively low ORR activity in acidic medium of the present ICM-derived Fe–N–C catalysts compared to state-of-the-art Fe–N–C catalysts [11,14,17,39,47]. The low site density of FeNx moieties seems correlated to the high degree of graphitization of the carbon matrix derived from the ICM approach, as compared to the amorphous carbons usually obtained from the pyrolysis of ZIF-8. The present approach even requires the pre-existing Fe–N4 (FePc) or Fe–N6 (FePhen3) coordination as iron precursors to obtain reasonable ORR activity after pyrolysis. This suggests that there is a synthetic contradiction between the desires for a high density of FeNx sites (necessary to reach a high ORR activity) and a high degree of graphitization of carbon (possibly leading to improved stability). Highly graphitized carbons with few defects typically result in low nitrogen content, in turn limiting the maximum number of FeNx moieties that may be hosted in the carbon matrix.
4. Materials and Methods
Sample preparation. All catalyst precursors were prepared via the dry ball-milling of Zn(II) chloride, Na(I) chloride, adenine and an iron precursor. A mass of 100 mg of adenine, 825 mg of ZnCl2, 175 mg of NaCl and certain amount of iron precursor (iron(II) phthalocyanine (FePc), 1,10-phenanthroline iron (II) perchlorate (FePhen3), or iron (II) acetate (FeAc2) corresponding to 4 wt.% Fe in the total precursor (the inorganic ZnCl2/NaCl salts not being counted) were weighed and poured into a ZrO2 crucible with hundred zirconium-oxide balls of 5 mm diameter. When using FeAc2 as Fe precursor, 1,10-phenanthroline was added to reach the stoichiometry Fe:Phen of 1:3, to ensure all catalyst precursors possessed similar nitrogen contents before pyrolysis. The ZrO2 crucible was then sealed under air and placed in a planetary ball-miller (Fritsch Pulverisette 7 Premium, Fritsch, Idar-Oberstein, Germany). The powders were ball-milled for 30 min at 200 rpm milling speed, then pyrolyzed in flash mode at 900 °C in flowing Ar for 1 h. Then they were washed in deionized water for 48 hours at room temperature to remove NaCl and finally dried. The catalysts are labelled “ICM-Fe-precursor”. In some cases, the washing after pyrolysis was performed in 0.5 M H2SO4 rather than in water, in otherwise the same conditions. The catalysts are labelled in this case ICM-Fe-precursor-AW.
Electrochemical characterization. The catalyst powders were deposited on glassy carbon electrodes as working electrode. Catalyst ink was prepared by dispersing 10 mg of the catalyst powder in a mixture of Millipore water (36.5 μL, 18.2 MΩ cm) and ethanol (300 μL, Sigma-Aldrich, 99.8%, Lyon, France), into which 5 wt% Nafion solution (108.5 μL, Sigma-Aldrich, Lyon, France) was added as a binder phase. The resulting mixture was sonicated for 60 min, and then an aliquot of 7 μL was drop-cast onto the glassy carbon electrodes (0.196 cm2, Pine instrument, Grove City, PA, USA), resulting in a loading of 800 μg/cm2. The working electrode with the deposited catalyst layer was used in a three-electrode cell set-up connected to a bipotentiostat (Biologic SP 300, Bio-Logic Science Instruments, Isère, France) and rotator (Pine Instruments, Grove City, PA, USA). The graphite rod and reversible hydrogen electrode (RHE) were used as counter and reference electrodes, respectively. The ORR activity was measured in O2-saturated 0.1 M HClO4 or 0.1 M KOH from 0.05 to 1.05 V vs. RHE with a scan rate of 20 mV/s at 1,600 rpm. The cycle voltammetry (CV) was carried out between 0.05 to 1.05 V vs. RHE with a scan rate of 20 mV/s in N2-saturated 0.1 M HClO4.
XRD. X-ray diffraction patterns were conducted using a PANanalytical X’Pert Pro powder X-ray diffractometer (Malvern Panalytical Ltd, Malvern, UK) with Cu Kα radiation.
Raman. Raman spectra were collected using a LabRAM ARAMIS Raman microscope (HORIBA, Kyoto, Japan) with a 473 nm laser.
BET. N2 sorption analysis was performed at liquid nitrogen temperature (77 K) with a Micromeritics ASAP 2020 instrument (Micromeritics, Verneuil-en-Halatte, France). Prior to the measurements, FeNC-ICM catalysts were degassed at 200 °C for 5 h in flowing nitrogen to remove guest molecules or moisture. The pore size distributions were calculated by fitting the full isotherm with the quench solid density functional theory model with slit pore geometry from NovaWin (Quantachrome Instruments).
Mössbauer spectroscopy. 57Fe Mössbauer spectroscopy was used to obtain information on iron speciation. 120 to 300 mg samples were mounted in a 2-cm2 holder. Mössbauer spectra were measured at variable temperatures between 5 and 300 K in a helium flow cryostat (SHI-850 Series from Janis, Woburn, MA, USA). The Mössbauer spectrometer (Wissel, Germany) was operated in the transmission mode with a 57Co: Rh source at room temperature. The velocity driver was operated in the constant acceleration mode with a triangular velocity waveform. The velocity scale was calibrated with the magnetically split sextet of a high-purity α-Fe foil at room temperature. The spectra were fitted to appropriate combinations of Lorentzian profiles representing quadrupole doublets, sextets by least-squares methods. Isomer shifts are given relative to α-Fe at room temperature.
XAS. Fe K-edge X-ray absorption spectra were collected at room temperature in transmission mode at SAMBA beamline (Synchrotron SOLEIL, Saint-Aubin, France). The beamline is equipped with a sagittally focusing Si 220 monochromator and two Pd-coated mirrors that were used to remove X-rays harmonics. The catalysts were pelletized as disks of 10-mm diameter with 1-mm thickness using Teflon powder (1-μm particle size) as a binder. Data treatment and linear combination fitting (LCF) were performed with the Athena software .
SEM. Scanning electron microscopy micrographs were obtained with a Hitachi S-4800 apparatus (Hitachi, Tokyo, Japan).
TEM. Transmission electron microscopy micrographs were obtained with a JEOL ARM-200 CF probe Cs-corrected scanning transmission electron microscope (JEOL, Tokyo, Japan), operated at 80 kV and equipped with Centurio 100 mm2 SDD EDXS system and Gatan GIF Quantum ER Dual EELS system.
ZnCl2/NaCl eutectic salts assisted carbonization was demonstrated to be an effective approach to synthesize Fe–N–C catalysts when using iron precursors with pre-existing Fe–N coordination (FePc and FePhen3). The obtained catalysts exhibited a highly porous structure and a high degree of graphitization. However, it is a challenge to form a high density of FeNx sites in graphitized carbon, probably due to the low nitrogen content in the graphitic matrix. Iron precursors with pre-existing Fe-Nx coordination was required to form a small content of FeNx moieties at high temperature, while separate Fe and N precursors lead to an inactive Fe–N–C material. Even for the most active samples, a variety of iron species was detected in the FeNC-ICM catalysts, including FeNx moieties but also Fe carbides and, mostly, Fe oxides (different sizes and polymorphs). From a comparison of the ORR activity in acid and alkaline conditions for two catalysts (a fresh and an acid-washed version of the same starting material), the FeNx moieties were identified as the most active sites in acid medium, while both FeNx moieties and Fe2O3 oxides are proposed to promote ORR in alkaline electrolyte. The ORR activity of Fe3C particles encapsulated in nitrogen-doped graphene cannot be excluded in either acid or alkali, with different contributions to the overall activity.
The following are available online at https://www.mdpi.com/2073-4344/9/2/144/s1, Figure S1: Linear combination fitting of the Fe k-edge XANES spectra of ICM-FePhen3-AW with Fe3C and Fe0.5NC as references.
Conceptualization, J.L., F.J., Q.J. and S.M.; methodology, J.L., F.J. and Q.J.; electrochemical data acquisition and analysis, J.L.; XAS data acquisition and analysis, J.L., Q.J. and A.Z.; Mossbauer data acquisition and analysis, M.-T.S.; TEM data acquisition and analysis, G.D.; writing—original draft preparation, J.L.; writing—review and editing, J.L., F.J., Q.J., S.M., A.Z., M.-T.S., G.D.
This research was partially funded by the European Union’s Horizon 2020 research and innovation programme under the grant agreement number No. 721065 (project CREATE).
We acknowledge Synchrotron SOLEIL (Gif-sur Yvette, France) for provision of synchrotron radiation facilities at beamline SAMBA (proposal number 20180635). G.D. acknowledges the financial support from the Slovenian Research Agency (P2-0393).
Conflicts of Interest
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
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Figure 1. Electrochemical evaluation of ionothermal carbonization method (ICM)-derived Fe–N–C catalysts prepared from different iron precursors. (a) Oxygen reduction reaction (ORR) polarization curves measured in O2-saturated 0.1 M HClO4 with a scan rate of 20 mV/s at 1600 rpm. (b) Cyclic voltammograms (CVs) measured in N2-saturated 0.1 M HClO4 with a scan rate of 20 mV/s. (c) ORR polarization curves of ICM-FePhen3 catalyst before and after an acid wash (AW), measured in O2-saturated 0.1 M HClO4 or 0.1 M KOH with a scan rate of 20 mV/s at 1600 rpm. (d) CVs measured in N2-saturated 0.1 M HClO4 or 0.1 M KOH with a scan rate of 20 mV/s. Results obtained with a commercial Pt/C catalyst (Tanaka Kikinzoku Kogyo (TKK, Japan), 46.7 wt.% Pt, anodic scan) are presented as a reference. The catalyst loading for Fe-N-C is 800 μg/cm2. The Pt loading is 10 μgPt/cm2.
Figure 2. (a) X-ray diffraction (XRD) patterns, and (b) Raman spectra of ICM-derived Fe-N-C catalysts. The XRD and Raman spectra of Fe0.5NC (a ZIF-8 derived Fe-N-C pyrolyzed material ) are also presented as references for a material that comprises only FeNx moieties integrated in amorphous carbon.
Figure 3. 57Fe Mössbauer transmission spectra and their fittings with spectral components. ICM-FePhen3 measured at (a) 293 K and (b) 5 K, and (c) ICM-FePhen3-AW measured at 293 K.
Figure 4. X-ray absorption spectroscopy characterization of ICM-FePhen3 and ICM-FePhen3-AW. (a) Fe k-edge X-ray absorption near edge structure (XANES) spectra, (b) Fourier transforms of the extended X-ray absorption fine structure (EXAFS) spectra. The spectra of Fe2O3, Fe3C and Fe0.5NC (an Fe-N-C material comprising exclusively atomically dispersed FeNx moieties ) are also shown. Linear combination fitting (LCF) of the Fe k-edge XANES spectra of (c) ICM-FePhen3 and (d) ICM-FePhen3-AW with Fe2O3, Fe3C and Fe0.5NC as references. Error bars are given in parentheses.
Figure 5. (a) N2 adsorption/desorption isotherm, and (b) the corresponding pore size distribution from density functional theory (DFT) method of ICM-derived Fe–N–C catalysts.
Figure 6. (a) Scanning electron microscopy (SEM) image of ICM-FePhen3-AW; (b,e) bright-field scanning transmission electron microscopy (BF-STEM) and (c,d) high-angle annular dark-field (HAADF)-STEM images, (f) energy dispersive X-ray spectroscopy (EDX) spectrum measured in the same area as (d) and (e), and (g–j) the corresponding elemental mapping.
Table 1. Mössbauer parameters derived from the fittings: relative area (RA, %), isomer shift (IS, mm/s), quadrupole splitting (QS, mm/s), line width (LW, mm/s) and magnetic field (H, Tesla) of each component. Subscript number after the RA% number indicates the error bar, in %.
|Singlet||5512 (2)||0.17||-||2.6||-||Unresolved (1)|
(1) Unresolved from Mössbauer spectroscopy alone (see Table 2 for the assignment of that broad singlet); (2) Those RA % are only indicative, due to the low signal-to-noise ratio of that spectrum.
Table 2. Content of each Fe species as obtained from the fitting of 57Fe Mössbauer or Fe K-edge XANES spectra. Subscript number after the RA% or XANES % number indicates the error bar, in %.
|Sample||Mössbauer Component||Mössbauer RA % (300 K)||Mössbauer RA % (5 K)||LCF XANES Reference||XANES %|
(1) Both the sextet and doublet components assigned to Fe2O3 particles of different size and/or polymorphs; (2) Those RA % are only indicative, and poorly reliable due to the low signal-to-noise ratio of that spectrum.
Table 3. Surface area, pore volume and pore size of ICM-FePhen3 catalyst based on N2 physisorption isotherm analysis.
|Sample||BET Surface Area (m2/g)||Pore Volume (cc/g)||Pore Size (nm)|
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