Influence of Fe on electrocatalytic activity of iron-nitrogen-doped carbon materials toward oxygen reduction reaction

Abstract

The development of highly active nitrogen-doped carbon-based transition metal (M-N-C) compounds for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) greatly helps reduce fuel cell cost, thus rapidly promoting their commercial applications. Among different M-N-C electrocatalysts, the series of Fe-N-C materials are highly favored because of their high ORR activity. However, there remains a debate on the effect of Fe, and rare investigations focus on the influence of Fe addition in the second heat treatment usually performed after acid leaching in the catalyst synthesis. It is thus very critical to explore the influences of Fe on the ORR electrocatalytic activity, which will, in turn, guide the design of Fe-N-C materials with enhanced performance. Herein, a series of Fe-N-C electrocatalysts are synthesize and the influence of Fe on the ORR activity are speculated both experimentally and theoretically. It is deduced that the active site lies in the structure of Fe-N₄, accompanied with the addition of appropriate Fe, and the number of active sites increases without the occurrence of agglomeration particles. Moreover, it is speculated that Fe plays an important role in stabilizing N as well as constituting active sites in the second pyrolyzing process.

Graphical abstract

Keywords

oxygen reduction reaction / Fe-N-C / active sites / Fe addition / second heat treatment

1 Introduction

Nowadays, the establishment of sustainable and green energy systems has become definitely vital to humans’ development due to sustained growth in energy demand and environmental crisis. Proton exchange membrane fuel cell (PEMFC) is one of the most promising energy conversion devices considering its advantages of high energy density, high power density as well as zero-emission environmental friendliness, and fuel cell vehicles (FCVs) are expected to have a promising future in the field of electrical automobiles. The oxygen reduction reaction (ORR) takes place at the cathode of PEMFCs and plays a crucial role in fuel cell performance during operation because a large overpotential loss of about 300–400 mV is attributed to its sluggish kinetics [1]. Although Pt-based electrocatalysts can greatly promote ORR and have been extensively explored and successfully employed in the cathode of PEMFCs [2–4], the development of non-precious metal catalysts [5–7] is imperative in the long run. According to the report from Annual Merit Review and Peer Evaluation Meeting, the catalyst cost occupies the largest part (approximately 41%) in a PEMFC stack due to the precious and scarce Pt [8,9]. So far, the fuel cell cost has realized a 60% reduction in the past 10 years. However, to meet the requirement for commercially available FCVs in the future, the cost of an 80 kW_net PEMFC system should be decreased to $40/kW_net in 2025 and further to the ultimate target of $30/kW_net. It is thus of great interest to develop highly active and cost affordable non-precious metal eletrocatalysts as an alternative to Pt-based electrocatalysts.

Among various types of non-precious metal materials, nitrogen-doped carbon-based transition metal M-N-C (where M means transition metal like Fe and Co, N means nitrogen as a heteroatom doping) compounds have been well proven to show excellent performances toward the ORR with both a high activity and four-electron selectivity. Zhang et al. [10] synthesized a single-atomic-Fe doped catalyst by directly bonding Fe ions to 3D zeoliticimidazolate frameworks (ZIFs) followed by one-step thermal activation, and a respectable activity with a half-wave potential of 0.85 V versus reversible hydrogen electrode (RHE) was achieved, which was only 30 mV lower than that of commercial Pt/C. After 10000 times of potential cycling (0.6–1.0 V) in O₂ saturated acid, an enhanced stability with only a loss of 20 mV was presented. Wang et al. [7] constructed a (Fe, Co)/N-C electrocatalyst with Fe-Co dual sites embedded into nitrogen-doped porous carbon and it demonstrated an onset potential of 1.06 V and a half-wave potential of 0.863 V, respectively, which were better than those of commercial Pt/C (1.03 V and 0.858 V). The (Fe, Co)/N-C sample provided a kinetic current larger than 550 mA/cm² at 0.6 V, a peak power density more than 505 mW/cm² at 0.42 V and a negligible loss after 100 h operation at 600 mA/cm² and 1000 mA/cm². Although the as-developed Fe-N-C electrocatalysts are highly efficient and have been regarded as potential alternatives to high-cost Pt-based catalysts, there remains a debate on their active sites as well as the effect of Fe on the ORR activity, which really goes against a further development of high performance Fe-N-C electrocatalysts. First of all, Fe is of the essence for the series of Fe-N-C electrocatalysts toward ORR in acidic media because metal-free electrocatalysts usually have a worse performance. On the one hand, it is believed that Fe becomes a part of the active site in the pyrolysis process and is embedded in carbon supports. In this case, the active site can be mainly ascribed to metal/metal oxide/metal nitride/metal carbide [11,12] or nitrogen-coordinated transition metal complex like Fe-N_x [13–16]. On the other hand, Fe functions as an assistant element to facilitate the doping of pyridinicnitrogen/graphitic nitrogen into the carbon matrix, to help expose more edges as well as to catalyze the growth of carbon nanostructures and improve the graphene degree [17–21]. It is thus assumed that the active site is the nitrogen group like pyridinic nitrogen and/or graphitic nitrogen [22–24], or C-N structure even carbon atom next to nitrogen [25–27]. Moreover, it is noted that rare investigations have been conducted on the effect of Fe addition in the second heat treatment which is usually employed after acid leaching in catalyst synthesis. It is also critical to further comprehend the final composition of the as-obtained Fe-N-C electrocatalysts, which can further amend the active sites, thus enhancing the ORR activity.

In this paper, a series of Fe-N-C electrocatalysts was prepared with different amounts of Fe addition which was added either in the first or/and second heat treatment for the ORR. The delicate structures of the active site were identified and the effect of Fe on the ORR performance was explored by different advanced physicochemical characterization, especially X-ray absorption fine structure (XAFS), which was related to the local geometry of nitrogen to judge the coordination number. It is also essential to clarify the effect of Fe on the ORR performance through resolving its chemical states and composition transformation. Furthermore, the density functional theory (DFT) was applied to reveal the behavior of different reaction site models toward ORR through the deformation charge of O₂, bond length of O-O and free energy change in the elementary reaction of ORR.

2 Experiment

2.1 Chemicals and materials

Ethanol (C₂H₆O,≥99.5%, Sinopharm Chemical Reagent), Iron(II) acetate anhydrous (C₄H₆FeO₄, 90%, Energy Chemical), 1,10-Phenanthroline (C₁₂H₈N₂,≥99%, Sigma-Aldrich), Ketjenblack EC 600JD (AkzoNobel), and Nafion solution (20 wt%, Dupont) were used as received. Ultrapure water (Millipore,>18.2 MW cm) was used in the experiment.

2.2 Synthesis of electrocatalyst

2.2.1 Synthesis of Fe-N-C-X electrocatalysts

Commercial Ketjenblack EC 600JD was applied as carbon support without further treatment. The series of Fe-N-C electrocatalysts with different amounts of iron addition were synthesized as follows: Typically, 500 mg of EC 600JD, 500 mg of 1,10-phenanthroline, and some amount of iron(II) acetate (5 mg, 15 mg, 20 mg and 30 mg, respectively) were mixed homogeneously by planetary ball milling (350 r/min and 7 times with 15 min per time) to form uniform mixtures. Then the precursors were pyrolyzedina nitrogen atmosphere maintaining at 900°C for 2 h to obtain the final electrocatalyst, which was labeled Fe-N-C-X, where X represents the mass of added iron(II) acetate. For comparison, the metal free electrocatalyst Fe-N-C-0 was synthesized without addition of iron precursor.

2.2.2 Synthesis of Fe-N-C-X-II-Y electrocatalysts

To investigate the effect of iron addition, another addition of iron precursor in the second heat treatment was applied to gain the series of electrocatalysts Fe-N-C-X-II-Y, where II means the second annealing procedure and Y stands for the second addition amount of iron(II) acetate.

2.3 Physicochemical characterizations

An X-ray diffractometer (XRD, Bruker D8 Advance) with Cu Kα radiation (l = 0.1541 nm) was applied to identify the composition and crystal structure of the electrocatalysts. A transmission electron microscopy (TEM, JEM-2100F, JEOL, 200 kV) was employed to explore the microstructure morphology and distribution. An X-ray photoelectron spectroscopy (XPS, Kratos AXIS ULTRA DLD) with a standard Al Kα source was adopted to realize the chemical state and component of elements. A Raman spectra (Renishaw inVia, wavelength at 532 nm) was used to examine the defect and graphene degree of carbon. The specific surface area of the electrocatalysts was recorded on a Beishide Instrument (3H-2000PS1). Inductively coupled plasma optical (ICP, Thermal, iCAP7600) was taken to judge the metal content in the electrocatalysts. XAFS measurements were conducted at Fe K-edge (7112 eV) at the beamline BL14W1 of Shanghai Synchrotron Radiation Facility (SSRF), using Si(III) double-crystal monochromator. The XAFS data were analyzed using the Athena and Artemis programs with the theoretical standards calculated using FEFF8.

2.4 Electrochemical measurements

Electrochemical experiments (cyclic voltammetry (CV) and linear sweep voltammetry (LSV)) were conducted on a CHI 760e electrochemical analyzer (CH Instruments) using the rotating ring disk electrode (RDE, Pine Instruments). A conventional three-electrode system, including a catalyst-deposited glass carbon (F = 5 mm) as the working electrode, a saturated calomel electrode (SCE, saturated KCl) as the reference electrode, and a graphite rod as the counter electrode, was applied to evaluate the electrochemical performance of the electrocatalysts toward ORR. To prepare the working electrode, 5 mg of catalyst was dispersed in 0.5 mL of ethanol solution containing 0.25 wt% of Nafion and sonicated for 30 min to get a homogeneous ink. Afterwards, 12 µL of the ink was dropped on the glass carbon and dried at room temperature to fabricate the working electrode. The electrolyte solution was 0.1 M HClO₄, the potentials described were relative to reversible hydrogen electrode (RHE) by hydrogen calibrating, and the current density was normalized to the geometrical area (0.19625 cm²). All the electrochemical tests were performed with a scan rate of 5 mV/s. Durability tests was performed by cycling between 0.6 V and 1.0 V at a scan rate of 50 mV/s.

2.5 Computation methods

The spin-unrestricted DFT calculations were performed with the DMol3 8.0 code package from Accelrys [28]. The generalized gradient-corrected Perdew-Burke-Ernzerh of functional was used in all calculations [29]. The van der Waals interactions were accounted with Dispersion-corrected DFT scheme using the Tkatchenko-Scheffler (TS) method [30]. DFT semi-core pseudo potentials were applied in core treatment with the double numerical plus polarization (DNP) basis set.

Models were sampled with the 5 × 5 × 1 Monkhorst-Pack grid. The different defects were modeled with the monolayer graphene in the 6 × 6 prismatic super cell and the lattice parameters were a = 14.760 Å, b = 14.760 Å, and c = 20.0 Å. The nitrogen-doped graphite was created by substituting N atom for C atom. The Fe-N_x-C was modeled on the basis of pyridinic N_x which was created by graphite double vacancy. The gas-phase geometry optimization was carried out with the adsorptions of O₂, OOH, O, and OH.

The adsorption energy was calculated as E_ad = G_state–G_species–G_structure, where G_state,G_species, and G_structure are the total free energy of the system with adsorbed species, the dissociative species (O₂, OOH, O, and OH), and the catalyst model, respectively.

The free energy was given as G = E + ZPE–T·S–n·e·U [29], where E is the total energy of the structure, ZPE is the zero-point energy of the absorbed species and equal to ∑(hv_i/2) (h is the Planck constant and v_i is the vibrational frequency), T is the temperature, S is the entropy of the structure, n is the number of electrons transferred, e is the charge constant, and U is the applied potential. The vibrational frequency and entropy were calculated according to the theoretical method [31].

The free energy of 2(H⁺ + e^–) is equal to H₂ on the standard condition that U = 0 and pH= 0. The free energy of O₂ was calculated according to the free energy of 4.92 eV released by the reaction of H₂O that 2H₂ + O₂→2H₂O. G_{{\mathrm{H}}_2\mathrm{O}}(l) = G_{{\mathrm{H}}_2\mathrm{O}}(g) + R·T× In(P/P₀) was used to get the free energy of H₂O, where R is gas constant, P₀ = 1 bar, and P = 0.035 bar.

3 Results and discussion

3.1 Exploration of active site in Fe-N-C electrocatalysts

To investigate the active site in Fe-N-C electrocatalysts, a series of Fe-N-C electrocatalysts with different amounts of Fe addition (0 mg, 5 mg, 15 mg, 20 mg, and 30 mg) were synthesized and their corresponding compositions and structures were carefully characterized.

As seen in Fig. 1(a), the XRD patterns display that when the amount of Fe addition is 20 mg or less, only two diffraction peaks corresponding to C (002) and C (101) are presented. Along with the increase in the amount of Fe to 30 mg, the diffraction peak corresponding to Fe (PDF#06-0696) emerges, indicating the formation of metallic iron in the heat treatment. The TEM images of Fe-N-C-X electrocatalysts when X≤20 only expresses carbon structure without metallic particle (Fig. 1(c) and Fig. S1(Electronic Supplementary Material, ESM ) while metallic particles are clearly observed in Fe-N-C-30 (Fig. 1(d)). This agrees well with the XRD result that Fe aggregates to form particles when the addition amount of Fe reaches up to 30 mg. The Raman spectra (Fig. 1(b)) show the graphitic structure, where the G band and D band represent the graphitic carbon and disordered carbon like defect or amorphous structure, respectively. The ratios of intensity ratios of D band and G band (I_D:I_G) for the series of Fe-N-C-X electrocatalysts are similar.

The ICP result of different Fe-N-C-X electrocatalysts (Table S1 (ESM)) identifies the existence of Fe and it is observed that the Fe content becomes higher with the increase in the amount of Fe precursor added. However, the composition of Fe is not interpretable by XRD when its amount is lower than 30 mg. To resolve the structure and understand the formation of Fe in the as-obtained Fe-N-C-X electrocatalysts, the XAFS was recorded for the Fe-N-C-20 sample (Fig. 2). The white line (around 7133 eV) in Fe K-edge X-ray absorption near edge structure (XANES) (Fig. 2(a)) implies that the Fe-N-C-20 electrocatalyst has a higher oxidation state of Fe, as compared to the referred Fe foil, which indicates that the component of Fe in Fe-N-C-20 is not metallic Fe and has a valence above zero. The fingerprint peak at 7115 eV [32] corresponding to the Fe-N₄ square-planar structure was detected in the Fe-N-C-20 electrocatalyst. Meanwhile, the large difference between Fe-N-C-20 and Fe foil spectra confirms the absence of Fe nanoparticles in the Fe-N-C-20 electrocatalyst, which is in well accordance with the XRD result. The intrinsic structure was further corroborated by the Fourier transformed extended X-ray absorption fine structure (EXAFS) spectra. As shown in Fig. 2(b), the Fe-N-C-20 electrocatalyst exhibits the first strong peak at about 1.5 Å, which implies that Fe-N-C-20 is mainly comprised of the Fe-N scattering path, while that for Fe foil displays at 2.25 Å, which can be assigned to the Fe-Fe scattering path [7]. To further determine the coordination of Fe, the EXAFS fitting for both Fe-N-C-20 and Fe foil was performed as illustrated in Figs. 2(c) and 2(d) and the fitting result is listed in Table S2 (ESM). As a result, the Fe-N-C-20 electrocatalyst is well fitted with approximately four N atoms, confirming the presence of Fe-N₄ moieties.

The ORR activities (Fig. 3(a) and Fig. S2 (ESM)) on the series of Fe-N-C electrocatalysts with different Fe contents were examined. The Fe-free sample of Fe-N-C-0 presents an obviously inferior activity, and with the increment of Fe content, the electrochemical performance becomes better. However, when the Fe amount increases to the value higher than 30 mg, the agglomeration of Fe particles emerges, which leads to a worse ORR performance. Based on the composition verified by XANES and EXAFS, it is inferred that Fe-N_x forms in the pyrolysis process when small amount of Fe is added and it plays a critical role in the ORR activity. To further analyze the surface chemical states of Fe-N-C electrocatalysts, XPS was applied and the spectra are illustrated in Fig. 3(b). As seen from the full spectra, when increasing the amount of Fe added up to 30 mg, the characteristic peak of Fe emerges, indicating an agglomeration of Fe particles on the catalyst surfaces. The overall N 1s spectra are demonstrated in Fig. 3(c) and the calculated nitrogen species are summarized in Fig. 3(d) and Table S1 (ESM). The overall content of doped nitrogen is not in direct proportion to the ORR activity, indicating the fact that the key in the ORR performance should not be the total N content [10,33,34]. Obviously, the ORR activities on the series of Fe-N-C electrocatalysts is directly correlated with the content of Fe-N_x, suggesting a critical role for the ORR performance. The stability of the Fe-N-C-20 electrocatalyst was evaluated using an accelerated durability test (ADT). Its ORR polarization curves at 1600 r/min before and after 5000 cycles are shown in Fig. S3(a) (ESM). The half-wave potential of the Fe-N-C-20 electrocatalyst shows a slight negative shift of 28 mV. The TEM image of Fe-N-C-20 after 5000 cycles (Fig. S3(b) (ESM)) shows no obvious difference with the initial catalyst.

To further understand the function of Fe and N in the Fe-N-C electrocatalysts, DFT calculation has been performed to describe elementary reactions in the ORR. Different absorption sites on the models (Fig. S4 (ESM)) including monolayer graphite, pyridinic N-doped graphite, quaternary N-doped graphite, Fe-N₂-C, Fe-N₄-C, and metallic Fe₄-N-C have been tested and the Fe center shows the best O₂ affinity. Meanwhile, Fe-free models show seldom O₂ chemisorption, thus leading to an inefficient activation of O₂ and poor ORR performance, which is in accordance with the fact that the metal-free electrocatalyst usually has a poor performance in acidic medium [35]. The bond length and as-obtained deformation charge of O₂ on different models are listed in Table 1. Owing to the fact that O₂ gains more electrons from Fe-based models, the bond length of O₂ gets more elongated, indicating an easier reduction in further reactions. In particular, Fe-N₄-C and Fe-N₂-C behave much better in the O₂ absorption process. It is worth mentioning that when being adsorbed on Fe particle, O₂ molecules gain the most electrons in the catalyst models and are well dissociated (Fig. S5(a) (ESM)).

The elementary reactions of ORR in acidic medium can be described as

(1){\mathrm{O}}_{\mathrm{2}}{\mathrm{+4H}}^{\mathrm{+}}{\mathrm{+4e}}^{-}\to {{\mathrm{O}}_{\mathrm{2}}}^{\mathrm{*}}{\mathrm{+4H}}^{\mathrm{+}}{\mathrm{+4e}}^{-}

(2){{\mathrm{O}}_{\mathrm{2}}}^{\mathrm{*}}{\mathrm{+4H}}^{\mathrm{+}}{\mathrm{+4e}}^{\mathrm{-}}\to {\mathrm{OOH}}^{\mathrm{*}}{\mathrm{+3H}}^{\mathrm{+}}{\mathrm{+3e}}^{\mathrm{-}}

(3){\mathrm{OOH}}^{\mathrm{*}}{\mathrm{+3H}}^{\mathrm{+}}{\mathrm{+3e}}^{-}\to {\mathrm{O}}^{\mathrm{*}}{\mathrm{+H}}_{\mathrm{2}}{\mathrm{O+2H}}^{\mathrm{+}}{\mathrm{+2e}}^{-}

(4){\mathrm{O}}^{\mathrm{*}}{\mathrm{+H}}_{\mathrm{2}}{\mathrm{O+2H}}^{\mathrm{+}}{\mathrm{+2e}}^{-}\to {\mathrm{OH}}^{\mathrm{*}}{\mathrm{+H}}_{\mathrm{2}}{\mathrm{O+H}}^{\mathrm{+}}{\mathrm{+e}}^{-}

(5){\mathrm{OH}}^{\mathrm{*}}{\mathrm{+H}}_{\mathrm{2}}{\mathrm{O+H}}^{\mathrm{+}}{\mathrm{+e}}^{-}\to {\mathrm{2H}}_{\mathrm{2}}\mathrm{O}

The free energy changes in the adsorption process on different structures are also listed in Table 1. As seen, the O₂ chemisorption is endothermic on monolayer graphite, pyridinic N-doped graphite, and quaternary N-doped graphite, suggesting that O₂ tends to keep away from metal-free structures, which is unfavorable for ORR. On the contrary, O₂ tends to be chemisorbed on Fe-N_x-C, which contributes greatly to the ORR performance. Figure 4 plots the free energy change on Fe-N₂-C and Fe-N₄-C during ORR. It is observed that the final step of ORR on the Fe-N₂-C electrocatalyst is endergonic, while all the elementary reactions of ORR on the Fe-N₄-C electrocatalyst are exergonic. It seems more difficult for OH* to react with H⁺when being adsorbed on Fe-N₂-C. According to the d band center theory [36], Fe-N₄-C with a lower d band center (–1.5596 eV) expresses a weaker adsorption of O compared to Fe-N₂-C (–0.9844 eV), thus leading to an easier process for desorption of OH* and formation of H₂O. Particularly, when O₂ is adsorbed on the metallic sites of Fe₄-N-C, it is revealed that the adsorption of oxygen is too strong to desorb OH* (Fig. S5(b) (ESM)), which is not favorable for the ORR. Overall, it is anticipated that Fe-N₄-C should be the most likely active site in the series of Fe-N-C electrocatalysts, which consists with the experimental results.

3.2 Influence of Fe addition in the second heat treatment

In most cases, the M-N-C electrocatalysts were synthesized by mixing carbon supports, nitrogen precursors, and metal salts followed by the heat treatment. To ensure the maximum content of Fe doping without inactive species like metallic particles, an acidic leaching and a second pyrolysis are usually applied to obtain the final catalyst [37–39]. Great efforts have been made to realize the formation of active site in the first pyrolysis process. However, it is also necessary to have a thorough understanding of the relation between the ORR performance and the structure of elelctrocatalysts in the second pyrolysis process.

As seen in Fig. 5(a) and Fig. S6 (ESM), a tremendous decay on the ORR activity occurs after the second heat treatment, while an increase in the ORR activity is observed when comparing Fe-N-C-5-II-10 to Fe-N-C-5-II-0. The difference lies in the existence of an extra Fe addition for the Fe-N-C-5-II-10 electrocatalyst in the second annealing process. It is thus believed that a pure second heat treatment does not do the best to promise the ORR performance and the addition of Fe in the second annealing also plays a critical role in the ORR. To examine the influences of Fe addition in the second heat treatment on the ORR performance, various physicochemical characterizations were conducted. Both XRD patterns and TEM images indicate that no metallic iron or iron oxide particles are formed in the second heat treatment (Fig. S7 (ESM)). In Fig. S7(b) (ESM), the Raman spectra indicate that the extra addition of iron salt does not change the graphene degree since there exist similar values for the ratios of I_D: I_G in Fe-N-C-5-II-0 (1.08) and Fe-N-C-5-II-10 (1.11). XPS was employed and the N 1s evolution as well as different nitrogen species are demonstrated in Figs. 5(b)–5(d). It is found that Fe-N-C-5-II-0 possesses a small amount of nitrogen compared to Fe-N-C-5-II-10, indicating the fact that an extra addition of iron salt in the second heat treatment will help to stabilize the nitrogen existed in the electrocatalysts. Moreover, it is also detected that the Fe-N_x specie is absent in Fe-N-C-5-II-0, thus leading to an inferior ORR performance to some extent. On the contrary, Fe can also be doped into the catalyst to form Fe-N_x in the second heat treatment, for the Fe-N_x specie occupies 12% of the total nitrogen content in Fe-N-C-5-II-10, thus resulting in an enhanced ORR activity.

4 Conclusions

In summary, the structure of active site as well as influence of Fe doping on the ORR performance in the second heat treatment were explored via synthesizing a series of Fe-N-C electrocatalysts followed by analyses from electrochemical measurement, physicochemical characterization as well as DFT calculation. It was found that along with the increase in the iron salt amount, the Fe-N₄ structure were formed before it aggregated to iron particles, which was identified by XAFS characterization. It is thus declared that the Fe-N₄ structure should be the active site rather than Fe-N₂ based on the experimental and theoretical results. Moreover, it is believed that a pure second heat annealing usually leads to a decay in the ORR activity while the addition of iron helps stabilize nitrogen and constitute active site again to increase the ORR activity. It is believed that this paper may provide a unique insight on efficiently improving the ORR activity of M-N-C electrocatalysts by increasing the density of Fe-N₄ structure and stabilizing it by further adding transition metals in another heat annealing in the future.

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