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Letter x

The Synergetic Contribution of Boron and Fe–N Species in Porous Carbons Towards Efficient Electrocatalysts for Oxygen Reduction Reaction Kai Yuan, Stavroula Sfaelou, Ming Qiu, Dirk Ferdinand Lützenkirchen-Hecht, Xiaodong Zhuang, Yiwang Chen, Chris Yuan, Xinliang Feng, and Ullrich Scherf ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01188 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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ACS Energy Letters

The Synergetic Contribution of Boron and Fe–Nx Species in Porous Carbons Towards Efficient Electrocatalysts for Oxygen Reduction Reaction Kai Yuan,*1,2 Stavroula Sfaelou,3 Ming Qiu,5 Dirk Lützenkirchen-Hecht,6 Xiaodong Zhuang,*3,7Yiwang Chen,*1 Chris Yuan,4 Xinliang Feng,3 and Ullrich Scherf2 1

College of Chemistry/Institute of Polymers, Nanchang University, 999 Xuefu Avenue, Nanchang

330031, China. 2

Macromolecular Chemistry Group (buwmakro) and Institute for Polymer Technology, Bergische

Universität Wuppertal, Gauss-Str. 20, D-42119 Wuppertal, Germany. 3

Center for Advancing Electronics Dresden (cfaed) and Department of Chemistry and Food

Chemistry, Technische Universität Dresden, Mommsenstrasse 4, 01062 Dresden, Germany. 4

Department of Mechanical Aerospace Engineering, Case Western Reserve University, Cleveland,

44106 Ohio, United States 5

Institute of Nanoscience and Nanotechnology, College of Physical Science and Technology,

Central China Normal University, 430079 Wuhan, China 6

Faculty of Mathematics and Natural Sciences-Physics Department, Bergische Universität

Wuppertal, Gauss-Str. 20, D-42119 Wuppertal, Germany 7

Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, School of Chemistry and

Chemical Engineering, Shanghai Jiao Tong University, Dongchuan Road 800, 200240 Shanghai, China

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ABSTRACT: The development of porous carbon materials as highly efficient, durable, and economic electrocatalysts for oxygen reduction reaction (ORR) is of great importance for realizing practical applications of many significant energy conversion and storage devices. Herein, we demonstrate a general approach to porous carbons decorated with boron centers and atomically dispersed Fe–Nx species (denoted as FeBNC). The as-prepared FeBNC can serve as efficient electrocatalysts for ORR in alkaline medium with ultra-low overpotential of down to 0.851 V vs. RHE, comparable to the state-of-the-art porous carbon catalysts and the benchmark system Pt/C. Theoretical calculation reveals that incorporating of boron dopant into traditional Fe–Nx species enriched porous carbons significantly lowers the energy barrier for oxygen reduction and therefore boost the overall performance. This work not only provides an easy method to synthesize B-doped Fe–Nx centers enriched porous carbons as high efficient electrocatalysts for ORR and Zn-air batteries, but also proves the origin of the catalytic performance from both B dopants and Fe–Nx sites.

TOC GRAPHICS


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ACS Energy Letters

Oxygen reduction reaction (ORR) is an important part of fuel cells and metal–air batteries, as well as many other significant energy conversion and storage devices.1, 2 However, the intrinsically sluggish kinetics of ORR limits the efficiency of these devices, which also presents great challenges in developing highly active ORR catalysts. Recently, carbon materials and transitionmetal-based materials have received considerable attention.3-6 Among various such catalysts developed, carbon materials doped with heteroatoms hold great potential as alternatives to noble metals for improving ORR due to their unique structural and electronic properties (e.g. charge polarization, difference in electronegativity, electron spin density between heteroatoms and carbon atoms, etc.) and their robustness as a matrix in different electrolytes.7-9 For example, studies have shown that nitrogen-doped carbon materials (such as graphene and carbon nanotubes) could be low-cost and efficient alternatives to Pt-based catalysts for ORR.10, 11 Incorporating a secondary heteroatom, such as B, P, or S, into nitrogen-doped carbon materials can adjust the surface polarities and electronic properties to further elevate ORR activity.5, 12-14 In particular, introducing the transition metals, e.g. Fe, Ni, or Co, into N-doped carbon materials can dramatically boost their performance in ORR, because the coordination of metal and nitrogen atoms to form highly active catalytic sites M–N–C (M=Fe, Ni, Co, etc.) and induce inhomogeneous charge distribution, resulting in significantly improved oxygen adsorption and reduction.15-22 Furthermore, the process affords the inherent merit of large specific surface area of N-doped carbon frameworks, leading to efficient use of active catalytic sites and high-throughput mass transportation. So far, it remains great challenge for both of the rational controlled preparation of porous carbons with atomically dispersed metal-nitorgen complexes and study the relationship between the ORR performance and metal-nitrogen complexes.



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Conducting polymers such as polyaniline and polypyrrole (PPy) are used frequently as precursors for the synthesis of M–N–C catalysts owing to their uniform distribution of heteroatoms and high carrier mobility among chemical structures.17, 23-25 Common methods for preparing M– N–C electrocatalysts by directly pyrolyzing combinations of heteroatom-containing compounds, transition metal precursors, and carbon are unsuitable for producing optimal compositions and uniform porous architectures, leading to a limited number of accessible active sites and relatively poor transport properties of electrochemical reactions.16,

17, 26

Hydrogels are physically or

chemically cross-linked polymer networks that have proved to be remarkable catalyst supports for electrochemical reactions and many other important applications.27 Therefore, heteroatom-rich conducting-polymer-based hydrogels should be preferred scaffolds for the preparation of heteroatom-doped noble-metal-free catalysts with controllable porous structures. Moreover, employing such polymers as heteroatom precursors would lead to uniformly distributed heteroatom sites on the surface and increased active-site density. In this study, we designed and synthesized a PPy-based hydrogel through template-free oxidative polymerization of pyrrole in the presence of readily available boric acid and FeCl3. After direct pyrolysis and acid leaching, the PPy-based hydrogel could be readily converted into B decorated porous carbon frameworks with atomically dispersed Fe–Nx species (denoted as FeBNC). The FeBNC with trace amount of iron (approximately 0.1 at%), mainly in the form of coordinated iron-nitrogen complex, could provide abundant and uniformly dispersed atomic Fe– Nx species acting as highly active sites. Moreover, the FeBNC catalyst was observed to have a highly porous structure and high specific surface area, thus ensuring large electrode–electrolyte interfaces, high-flux mass transportation, and easily accessible catalytic centers. Theoretical calculation demonstrates that the introducing of B into Fe–Nx enriched porous carbon matrix


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ACS Energy Letters

significantly decreases the overpotential for ORR due to the intrinsic unique electronic and chemical coupling effect. This remarkable combination of structural and compositional properties and their synergistic effects endow the as-prepared FeBNC catalyst with excellent activity in ORR in alkaline media with superior durability and strong tolerance to methanol crossover. The schematic preparation of FeBNC catalysts with hierarchical structure is shown in Figure 1a. The PPy hydrogel with a high nitrogen content was used as the carbon and nitrogen source, and boric acid and FeCl3 were used as readily available boron and iron sources, respectively (for details, see the experimental section in the Supporting Information). Previous studies have demonstrated that the pyrolysis temperature is critical to the catalytic performance of pyrolyzed carbon-based electrocatalysts. 12, 25, 28 To find the optimum condition for the preparation of FeBNC catalysts, various pyrolysis temperatures (600, 700, 800, 900, and 1000 °C) were apllied. The obtain FeBNC electrocatalysts were denoted as FeBNC-T, T=600, 700, 800, 900, and 1000. In addition, iron and nitrogen co-doped carbons (FeNC), boron and nitrogen co-doped carbon (BNC800), and nitrogen doped carbon (NC-800) were prepared as control samples from the corresponding composites by using the same procedure without using B-, Fe-, and Fe/B-contained precursors, respectively. Scanning electron microscopy and transmission electron microscopy (TEM) measurements indicated that the obtained FeBNC-800 showed interconnected coral-like nanofibers with diameters of 100–200 nm (Figures 1b–d). All FeBNC carbons showed similar coral-like morphology (Figures S1-3). Notably, after the carbons were leached with 1 M H2SO4 solution, no nanoparticles could be observed in all corresponding TEM images. Due to the similar morphology, FeBNC-800 will be discussed as typical example unless indicated otherwise. As indicated by highresolution TEM (HRTEM) images, FeBNC-800 was highly microporous and contained ordered



nanographite domains with an average d spacing of 0.36 nm (Figures 1e and S4). Such graphitic structures could facilitate the effective electron transports necessary for efficient electrocatalysis. FeBNC-800

was further investigated using high-angle annular dark-field scanning TEM

(HAADF-STEM) images (Figure 1f). As can be seen in the HAADF-STEM elemental mapping images, Fe, B, C, N, and O were uniformly dispersed over the entire coral-like structure. In the Xray diffraction (XRD) patterns of FeBNC (Figures S5 and S6a), only two broad characteristic peaks around 25.6°and 43.3°were found, which could be assigned to the (002) and (101) planes of graphitic carbon, and no clear crystalline phases of Fe3C and/or Fe were observed. The (002) peak of FeBNC shifted to a smaller Bragg-angle compared with that of graphite (2θ = 26.54°), indicating increased interlayer spacing compared with graphite. The XRD patterns along with the HAADF-STEM elemental mapping images consistently imply that the Fe species could be dispersed in FeBNC as subnanometer entities without the formation of large nanoparticles. The observed heteroatom-doped structure and subnanometer-dispersed Fe species, as well as the expanded interlayer, could provide numerous accessible active sites such as Fe–Nx for efficient electrocatalysis. To clarify the role of boric acid in the preparation of FeBNC, FeNC samples were synthesized without boric acid. The morphology of FeNC and FeBNC composites before acid leaching was checked by TEM. Due to the similar morphology, TEM images of FeNC-800 and FeBNC-800 before acid-treatment are typically presented in Figure S7 and Figure S8, respectively. Compared to FeNC-800, no Fe-based nanoparticles was observed in onion-like graphitic carbon for FeBNC800 before acid-treatment. After acid-treatment, nanoparticles with diameter up to a few tens of nanometers were formed in FeNC-800 compared to FeBNC-800 (Figure S9). The XRD patterns of the FeNC samples prepared without boric acid showed a few diffraction peaks associated with


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ACS Energy Letters

Fe-based carbides (Figures S5 and S6b). Moreover, the HAADF-STEM images and the elemental mapping images of Fe, C, N, and O further confirmed the nanoparticle-encapsulation structure (Figure S10). These results suggest that boric acid might facilitate the confinement of Fe atoms in the carbon layers in an atomically dispersed form, suppressing the formation of large nanoparticles. From the Raman spectra (Figures S11 and S12), broadening of the D-band, narrowing of the 2Dband, and increased ID/IG ratios were observed clearly in the FeBNC samples compared with the FeNC samples, indicating that introduction of boron creates plentiful defects in FeBNC.13, 29 Moreover, the addition of boric acid increased the surface area of the FeBNC samples considerably compared with that of the B-free FeNC samples. Significant differences in surface area and pore characteristics were observed between the FeBNC and the FeNC samples through N2 adsorption/desorption measurements (Figures S13-15, and Table S1). The Brunauer–Emmett– Teller (BET) surface areas of the FeBNC samples prepared at different temperatures ranged from 495 to 766 m2 g-1, which were considerably higher than those of the FeNC samples with values ranging from 338 to 489 m2 g-1. A larger surface area facilitates acid leaching of the unstable and catalytic-nonreactive phases of Fe-based particles, providing greater access to active sites. Furthermore, the distinct variations of the hysteresis loops of FeBNC and FeNC in the N2 sorption isotherms indicate the remarkable differences between the pore and channel distributions of these electrocatalysts. The FeNC samples showed typical type-IV curves with sharp capillary condensation steps in the relative pressure range of 0.85–0.95, indicating a porous structure with uniform mesopores, which was further verified by the profiles of nonlocal density functional theory pore size distribution and the plots of the cumulative pore volume versus relative pressure, as illustrated in Figures S13-15. The predominant mesopores of the FeNC catalysts were centered at around 4 nm. By contrast, the FeBNC catalysts displayed predominant micropores centered at



around 1.1 nm, in which the highly active Fe–Nx sites were easier to form.30, 31 Moreover, the FeBNC catalysts had typical mesopores at around 2.6 nm, which are beneficial for mass transfer during ORR. Thus, all the preceding results indicate that adding boric acid played a vital role in forming atomically dispersed Fe–Nx species in the FeBNC catalysts. Further composition and valence state data of the FeBNC and the FeNC catalysts were obtained by X-ray photoelectron spectroscopy (XPS). The XPS spectra (Figure S16) clearly confirmed that all samples consisted of Fe, C, N, and O. An extra peak corresponding to B was observed in the FeBNC catalysts, suggesting that B was doped successfully. The incorporation of B and N dopants in FeBNC catalysts could improve conductivity to facilitate electron transportation during electrochemical processes.32 The atomic surface concentrations of these catalysts are outlined in Table S2. The nitrogen doping levels of the FeBNC (3.69–7.78 at%) catalysts were higher than those of FeNC (2.54–3.74 at%). To further investigate the catalysts, high-resolution N 1s spectra were measured. The N 1s spectra of FeBNC could be deconvoluted into six bonding types centered at the binding energies of 397.6, 398.2, 399.0, 399.9, 400.8, and 402.1 eV, corresponding to B-N, pyridinic N, Fe–N, pyrrolic N, graphitic N, and oxidized N, respectively (Figures 2a and S17).24, 32-34 Notably, the relative ratios of the deconvoluted peak areas of the N 1s spectra clearly show that the FeBNC catalysts were dominated by pyridinic N and Fe– N (Figures 2a,c and Figure S17). By contrast, the FeNC catalysts were dominated by pyrrolic N and graphitic N (Figures 2a and S18). Pyridinic N was determined to be preferentially situated at the edges of carbon layers. A high content of pyridinic N has been proposed to serve as a descriptor of edge plane exposure and is positively correlated with the ORR catalytic activity.35 Moreover, the Fe–Nx moieties have been found to contribute to pyridinic N signal through XPS.33, 35 Therefore, the high-loaded pyridinic N in FeBNC catalysts was determined, indicating a high content of Fe–


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ACS Energy Letters

Nx moieties embedded in FeBNC carbon matrix. This result indicates that the FeBNC catalysts contained many Fe–Nx species, as corroborated by X-ray absorption spectroscopy (XAS) shown subsequently. The calcination temperature was observed to have a profound effect on the B-doping level of FeBNC, as shown in Figure 2b,d and Table S2. The B-doping content increased significantly from 1.42 at% for FeBNC-600 to 4.84 at% for FeBNC-800, and this increase can be attributed to the simultaneous presence of B-containing active species at temperatures higher than 700 °C.34, 36 Additionally, the high-resolution B 1s spectra of FeBNC catalysts could be deconvoluted into three peaks at 190.3, 191.2, and 192.3 eV, corresponding to the B–C, B–N, and B–O bonds, respectively, as presented in Figure 2b,d and Figure S19. The relative contributions of the B–C bonds decreased when the temperature was higher than 800 °C (Figures 2d and S19), because high temperatures can increase the strength of B–O bonds.34 The Fe contents in FeBNC and FeNC were approximately 0.1 and 0.4 at%, respectively, verified by the high-resolution XPS, TEM EDS and inductively coupled plasma optical emission spectrometry (ICP-OES) studies were performed (Figures S20-22 and Table S3). Nevertheless, a more detailed investigation such as valence state determination was impossible owing to the weak signal intensities. To further identify the status of the iron species dispersed in the FeBNC and FeNC catalysts, XAS at the Fe K-edge was performed. X-ray absorption near edge structure (XANES) and Fourier transform extended X-ray absorption fine structure (EXAFS) spectrometry are powerful and well-established techniques for determining the coordination environment and the chemical state of the center iron atoms in samples, even if their concentration is very low.15, 20, 29

Figure 3a shows the XANES spectra at the Fe K-edge of the FeBNC-800 and the FeNC-800

samples, as well as those of the commercial iron phthalocyanine (FePc), Fe3C, and iron foil



references. FeBNC-800 featured a weak pre-edge peak similar to that of FePc at around 7114.5 eV, which was recognized as a fingerprint of the Fe–N4 square-planar structure.20, 37 Furthermore, the Fe K-edge position of FeBNC-800 with a value of nearly 7119.9 eV was determined to be close to that of Fe in FePc. By contrast, the Fe K-edge XANES spectrum of the FeNC-800 catalyst was observed to be more similar to those of Fe3C and the metallic iron foil, suggesting the presence of iron-based crystalline structures (Fe3C and/or metallic iron) in FeNC-800. The magnitude of the Fourier transformation at the Fe K edge from the experimental EXAFS spectra of the samples was further analyzed (Figures 3b and S23). The EXAFS Fourier transforms revealed that FeBNC-800 was predominantly characteristic of N-coordinated iron. The main signal of FeBNC-800 at 1.48 Å was assigned to the Fe–N distance stem from a nitrogen shell surrounding iron atoms in reference to that of FePc, confirming the existence of Fe–N4 structures in FeBNC800.15,

18, 37

The minor signals at 2.07, 2.6, and 3.36 Å can be ascribed to Fe–Fe and Fe–C

interactions. By contrast, the spectrum of FeNC-800 with a strong peak at 2.07 Å was determined to be more like those of Fe3C and metallic Fe, which correspond to the Fe–Fe distance. Furthermore, clearly, the Fe–N peak in FeNC-800 was substantially weaker compared with that in FeBNC-800. This corroborates the existence of iron-based crystalline configurations in FeNC-800 and a less developed Fe–N environment than that in FeBNC-800. To further evaluate the valence bonds of the FeBNC and the FeNC samples, the C K-edge and the N K-edge XANES spectra of FeBNC-800 and FeNC-800 were measured as well. Compared with the FeNC-800 sample, the C K-edge XANES spectrum of the FeBNC-800 sample presented a sharply increased resonance peak at around 287.4 eV (Figure 3c). This change in the XANES spectrum indicates that a strong chemical interaction occurred between Fe and N atoms, corresponding to the Fe–N–C bond in the FeBNC-800 sample.13 The N K-edge of the XANES


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ACS Energy Letters

spectra of the FeBNC-800 and FeNC-800 samples (Figure 3d) presented an obvious resonance peak (peak a) assigned to the π*-transition in the aromatic C–N–C of the pyridinic site.38 Compared with the FeNC-800 sample, peak a in the XANES spectrum of the FeBNC-800 sample could be split into double peaks (a1 and a2), suggesting that partial pyridinic-N is bonded to Fe atoms to form Fe–Nx species.39 These results are consistent with the Fe K-edge XANES spectra and electron paramagnetic resonance spectroscopy (Figure S24),

19, 40

XRD, TEM, and STEM mapping

observations. Based on the preceding results, B and N co-decorated porous carbon materials FeBNC with an abundance of atomically dispersed Fe–Nx sites were prepared successfully. The electrocatalytic performance of FeBNC and the reference catalysts, as well as commercial Pt/C catalysts, for ORR was evaluated in 0.1 M aqueous KOH by using a three-electrode system. In a N2-saturated electrolyte, cyclic voltammogram (CV) of the catalysts were virtually featureless. By contrast, well-defined cathodic peaks corresponding to ORR were observed in the case of all catalysts in the O2-saturated electrolyte (Figures S25 and S26). Notably, among all FeBNC samples, FeBNC800 afforded the most positive peak potential in the cathodic sweep at 0.872 V vs. RHE, which was 59 mV higher than that of FeNC-800 (0.813 V, the best among all FeNC samples), suggesting superior ORR catalytic activity of FeBNC-800. The ORR activity levels of these catalysts were further investigated by linear sweep voltammogram (LSV) polarization curves recorded on a rotating disk electrode (RDE) at a speed of 1600 rpm (Figures 4a, S27, and Table S4). In agreement with the CV observations, FeBNC-800 exhibited the most positive ORR onset potential of 0.968 V, which was close to the value of Pt/C (1.012 V) and more positive than those of FeNC-800 (0.959 V), BNC-800 (0.697 V), and NC-800 (0.649 V), indicating once again that FeBNC-800 was more electrocatalytically active than the



control catalysts. Consequently, FeBNC-800 showed the highest ORR activity with a half-wave potential of 0.838 V, which was comparable to that of the commercial Pt/C catalysts (half-wave potential of 0.851 V). The half-wave potential of FeBNC-800 decreased remarkably by approximately 75 mV with a severe drop in the diffusion limiting current density after the addition of 10 mM KSCN (Figure S28), corroborating the formation of Fe–Nx active sites. The significant depression of catalytic activity can be ascribed to the blocking of Fe–Nx active sites by SCN-. SCNions are known to coordinate strongly to iron and, therefore, poison the iron-centered catalytic active sites for ORR.18 As shown in Figure 4b, the Tafel slope of FeBNC-800 (approximately 69 mV/dec) was very close to that of Pt/C (approximately 66 mV/dec). This means that similar to platinum, the ratedetermining step in ORR catalyzed by FeBNC-800 is probably the first electron reduction of oxygen. To gain insights into the kinetics parameters of ORR catalyzed by FeBNC-800, LSV curves at various rotational speeds (from 400 to 1600 rpm) were recorded on a RDE (Figure 4c) and fitted according to the Koutechy–Levich (K–L) equation (for details, see Supplementary Information). The electron transfer number (n) was found to be approximately 4.0, as determined from the K–L plots, indicating that the FeBNC-800 catalyst favored a four-electron pathway for ORR. By contrast, FeNC-800 displayed a lower electron transfer number of nearly 3.6, indicating inferior electrocatalysis selectivity in the presence of a less effective two-electron pathway. Additionally, FeBNC-800 achieved a kinetic-limiting current density of 23.6 mA cm-2, as calculated from the K–L plots, which was higher than that of FeNC-800 (17.8 mA cm-2, Figures S29-31). To further assess the ORR pathways of FeBNC-800, rotating ring-disk electrode (RRDE) measurements were performed (Figure S32). As shown in Figure 4d, FeBNC-800 exhibited a low


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ACS Energy Letters

ring current density of approximately 0.008 mA cm-2 for peroxide oxidation and a much higher disk current density of approximately 5.5 mA cm-2 for O2 reduction. Figure S33 and the inset of Figure 4d show the electron transfer number and the percentage of peroxide species for the electrocatalysts derived from the RRDE curves. As depicted in the figures, for FeBNC-800, the O2 molecules were directly reduced to H2O by means of an almost four-electron process (n ranged from 3.91 to 3.96) with a small ratio (less than 5%) of peroxide species for potentials ranging from 0.25 to 0.85 V. In addition to excellent ORR activity, FeBNC-800 exhibits better long-term durability and better tolerance to the methanol crossover effect than FeBNC-800, as verified by the chronoamperometric measurements (Figure 4e). In terms of practical applications, we explored the possibility of using FeBNC-800 as an air catalyst for a primary Zn–air battery. The Zn–air battery based on FeBNC-800 had an open-circuit voltage of approximately 1.3 V. Figure 4f shows its polarization curves and the corresponding power density plots. The Zn–air battery based on FeBNC-800 had a peak power density of 10.6 mW cm-2, which was 19.1% higher than that of the FeNC-800-based battery (8.9 mW cm-2), again suggesting improved catalytic performance of FeBNC-800. These results reveal that the as-prepared FeBNC-800 catalyst has potential for application in Zn–air batteries. Based on the preceding results, the superior catalytic activity and high stability of the FeBNC800 catalyst for ORR can be ascribed to the joint effects of the following key features: (i) Introduction of B and N atoms could induce uneven charge distribution in the carbon frameworks, generating positively charged C and B atoms, which are beneficial for the adsorption of oxygen species; (ii) B,N-decorated carbon frameworks boost electron and mass transfer ability, in addition to providing highly active B–C and N–C sites for ORR; (iii) a porous structure with high specific surface area provides large electrode–electrolyte interfaces, allows closed interactions between



oxygen, and creates fully accessible active sites for enhancing catalytic activity; and (iv) abundant and uniformly dispersed atomic Fe–Nx species act as highly active sites, and their synergistic effect significantly improves ORR activity. The combination of all these unique structural and chemical properties ensures numerous fully accessible and highly catalytic active sites, as well as expeditious mass transfer kinetics during ORR processes. The density functional theory (DFT) calculation was further used to understand the nature of the ultra-high ORR activity of FeBNC. The computational details are given in the Supporting Information (Figures S34-36 and Table S5). According to our experimental results, the iron atoms are coordinated to four nitrogen atoms via Fe–N4 configuration. The position of the boron atom is varied around the Fe-N4 square-planar structure. For simplifying, the first and second neighbor boron dopants to Fe-N4 sites are mainly considered. Then, five possible models, labeled FeN4-B1, FeN4-B2, FeN4-B3, FeN4-B4, and FeN4-B5 were built, geometry optimized and shown in Figure S27. Among these models, the FeN4-B2 and FeN4-B5 are thermodynamic stable models. The corresponding free-energy pathways of ORR in alkaline environment (pH = 13) are shown in Figure 5a and the detail reaction energetics at zero potential (U = 0 V) are listed in Table S4. The whole ORR catalytic cycle is composed of the following five elementary steps: (i) O2(g) + *

O2*

(ii) O2* + H2O(l) + e-

OOH* + OHO* + OH-

(iii) OOH* + e(iv) O* + H2O(l) + e(v) OH* + e-

OH* + OH-

OH- + *

where * in sign of the adsorption site and steps (ii)–(v) signify the four-electron transfer processes. There are two main possible ORR pathways for FeN4-B2, as presented in Figure 5b. One pathway


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ACS Energy Letters

is the O2 molecule captured by the Fe atom (denoted as Fe@FeN4-B2). The electron transferring steps (ii), (iv), and (iv) are endothermic at U = 0 V, indicating that external potential is needed to drive these reaction processes. The OH* detachment into OH- (last electron transferring step (v)) is the rate-determining step of ORR, which is endothermic by 0.964 eV. Another pathway is the O2 molecule captured by the B atom, denoted as B@FeN4-B2. Interestingly, at zero external potential all reactions are downhill, except for O2 adsorption, which has a tiny energy barrier (i.e., overpotential for B@FeN4-B2) of 0.521 eV. This overpotential is much lower than the value of 0.65 eV reported for Fe–N4.41 This result suggests that B atom in FeN4-B2 exhibit competitive reactivity in comparison with Fe–N4 active sites, resulting in the whole ORR process of B@FeN4B2 is more favorable than that of Fe@FeN4-B2. Similarly, there are also two possible ORR pathways for FeN4-B5, denoted as Fe@FeN4-B5 and B@FeN4-B5 for O2 molecule adsorbed by Fe and B atom, respectively. In Fe@FeN4-B5 pathway, all the elementary reaction steps are uphill except OOH* decomposition into O*. The rate-determining step of ORR is the last electron transfer step (v), and is endothermic under a high voltage of 1.188 V. During the B@FeN4-B5 pathway, the rate-determining step of ORR is O2 dissociation into OOH*, but when the potential increases to 0.878 V, all the elementary reaction steps become thermodynamically favourable. Again, these theoretically calculated results indicate that the B adsorption site is more favorable than Fe adsorption site in FeBNC, which explains the much higher ORR activity of FeBNC in comparison with boron-free FeNC catalyst. Overall, the positions of the B atom around the Fe–N4 square-planar structure seem critical to the adsorption of chemical species and catalytic activity. In summary, we demonstrated an easy approach towards atomically dispersing Fe–Nx species on B/N co-decorated porous carbons (FeBNC). The rich Fe-N coordination sites in FeBNC, which



were determined by XANES, high boron (4.84 at%) and nitrogen (6.63 at%) contents, and highly porous structure, synergistically contributed to the remarkable activity for ORR in alkaline media. Furthermore, the FeBNC catalysts show scope for use as the air cathode in practical Zn-air batteries. Theoretical calculations verify that the introducing of B into Fe–Nx enriched porous carbons triggered the outstanding electrocatalytic ORR properties. This work not only provides a general approach to porous carbons with rich heteroatom dopants and anchored Fe–Nx active sites towards superior performance for ORR and Zn-air batteries, but also offers new opportunity to boost energy devices by rational design electrocatalysts with integrated versatile active sites.


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ACS Energy Letters

Figure 1. (a) Schematic of overall synthetic procedure of Fe-, N-, and B-doped FeBNC catalysts. The blue and red boxes show digital photographs of the PPy hydrogel and the FeBNC-800 catalyst prepared by carbonization at 800 °C, respectively. (b) SEM, (c) TEM and (d) enlarged TEM images of FeBNC-800 sample. Inset: SAED pattern from (d). (e) High-resolution TEM image of the FeBNC-800 sample. Inset: contrast profile along the arrow indicates interlayer spacing. (f) HAADF-STEM image and corresponding EDS spectroscopy elemental mapping images of B, Fe, C, N, and O.



Figure 2. (a) High-resolution XPS N1s spectra of FeBNC-800 and FeNC-800 samples; (b) Highresolution XPS B1s spectra of FeBNC-800 sample; the contents of (c) nitrogen and (d) boron bonding configurations in FeBNC samples prepared at different temperatures.


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ACS Energy Letters

Figure 3. (a) Fe K-edge XANES spectra, and (b) Fourier transforms of the Fe K-edge EXAFS spectroscopy oscillations of FeBNC-800 and FeNC-800 samples, with FePc, Fe3C and iron foil as references; (c) C K-edge and (d) N K-edge XANES spectra of FeBNC-800 and FeNC-800 samples.



Figure 4. (a) LSV curves of FeBNC-800, FeNC-800, BNC-800, NC-800, and Pt/C at 1600 rpm; (b) Tafel plots of FeBNC-800, FeNC-800, and Pt/C; (c) LSV curves of FeBNC-800 at different rotation rates. The inset shows the corresponding K–L plots; (d) RRDE curve of FeBNC-800 (inset: calculated electron transfer number (n) and HO2- yield against the potential); (e) current–time chronoamperometric responses of FeBNC-800, FeNC-800, and Pt/C (top); chronoamperometric responses in O2-saturated 0.1 M KOH of FeBNC-800 and Pt/C electrocatalysts, where the arrow indicates the addition of 3 M methanol (bottom); (f) polarization curves and corresponding power density plots of Zn–air batteries using FeBNC-800 and FeNC-800 as ORR catalysts.


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ACS Energy Letters

Figure 5. (a) Free energy diagram for the optimized FeN4-B2 and FeN4-B5 models during the ORR under alkaline condition. The largest change in free energy determines the limiting step and overpotential. (b) The proposed reaction pathways for ORR of FeN4-B2 (side view and bond lengths).



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ASSOCIATED CONTENT Supporting Information. Additional experimental details, materials characterizations, computational details, and electrocatalytic performance of the catalysts (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (K.Y.); *E-mail: [email protected] (Y.C.); *E-mail: [email protected] (X.Z.) Author Contributions K.Y., S.S. and M.Q. contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is financially supported by the National Science Fund for Distinguished Young Scholars (51425304). K.Y. thanks the financial support from the National Natural Science Foundation of China (21704038, 51763018), the Natural Science Foundation of Jiangxi Province (20171ACB21009),

and

the

National

Postdoctoral

Program

for

Innovative

Talents

(BX201700112). X.Z. thanks the financial support from NSFC for Excellent Young Scholars (51722304). M.Q. thanks the support of self-determined research funds of CCNU from colleges’ basic research and operation of MOE (23020205170456). The authors also thank the financial support from EU Graphene Flagship, COORNET (SPP 1928) as well as the German Science Council, Center of Advancing Electronics Dresden(cfaed).


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ACS Energy Letters

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Species in Porous Carbons towar - ACS Publications - American

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