Markedly Enhanced Oxygen Reduction Activity of Single-Atom Fe

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Markedly Enhanced Oxygen Reduction Activity of SingleAtom Fe Catalysts via Integration with Fe Nanoclusters Xiang Ao, Wei Zhang, Zhishan Li, Jian-Gang Li, Luke Soule, Xing Huang, WeiHung Chiang, Hao Ming Chen, Chundong Wang, Meilin Liu, and Xiao Cheng Zeng ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b05913 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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Markedly Enhanced Oxygen Reduction Activity of Single-Atom Fe Catalysts via Integration with Fe Nanoclusters Xiang Ao,†,§,‡ Wei Zhang, $,||,‡ Zhishan Li,† Jian-Gang Li,† Luke Soule,§ Xing Huang,*,∆ Wei-Hung Chiang,# Hao Ming Chen,¶ Chundong Wang,*,† Meilin Liu,§ and Xiao Cheng Zeng*,|| †

School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China e-mail: [email protected] || Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, United States e-mail: [email protected] § School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ∆ Scientific Center for Optical and Electron Microscopy, ETH Zürich, Otto-Stern-Weg 3, Zürich 8093, Switzerland e-mail: [email protected] #

Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan $ Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China ¶

ABSTRACT: Single-atom catalysts (SACs) have emerged as one of the most promising alternatives to noble metal-based catalysts for highly efficient oxygen reduction reaction (ORR). While SACs can offer notable benefits in terms of lowering overall catalyst cost, there is still room for improvement regarding catalyst activity. To this end, we designed and successfully fabricated a ORR electrocatalyst in which atomic clusters are embedded in atomically dispersed Fe-N-C matrix (FeAC@FeSA-N-C), as shown by comprehensive measurements using aberration-corrected scanning transmission electron microscope (AC-STEM) and X-ray absorption spectroscopy (XAS). The half-wave potential of FeAC@FeSA-N-C is 0.912 V (versus reversible hydrogen electrode (RHE)), exceeding that of commercial Pt/C (0.897 V), FeSA-N-C (0.844 V), as well as the half-wave potentials of most reported non-precious-group metal catalysts. The ORR activity of the designed catalyst stems from single-atom active centers but is markedly enhanced by the presence of Fe nanoclusters, as confirmed by both experimental measurements and theoretical calculations. KEYWORDS: Fe nanoclusters, electrocatalysts, oxygen reduction, single-atom catalyst, DFT computation.

The development of highly efficient and durable electrocatalysts for the oxygen reduction reaction (ORR) is exceedingly desired in various electrochemical energy technologies, such as fuel cells and metal-air batteries.1,2 At present, Pt-based nanomaterials are widely considered to be the stateof-the-art electrocatalyst for the ORR.3,4 Nevertheless, their large-scale implementation is hampered by high costs, limited natural abundance, and relatively low durability.5 Great efforts have been devoted to developing highly efficient and cost-effective non-precious-group metal catalysts (NPMCs) as alternatives to Pt-based catalysts.6,7 To date, a broad-range NPMCs have been developed, among which Fe-N-C nanostructures have been considered as one of the most promising candidates.8,9 However, the nature of the active sites of most Fe-N-C catalysts is still under debate and the

activity still needs further improvement for practical applications.10,11 Given that single-atom Fe-N-C catalysts, where Fe-N4 moieties formed by coordination of single-atom Fe with nitrogen are regarded as the active sites for ORR, could realize the utmost utilization of metal sites while accelerate the catalytic efficiency, this class of catalysts has been regarded as a frontier of electrocatalytic research to understand the underlying reaction mechanism of ORR.12,13 However, due to thermodynamic instability, Fe atoms are prone to migration and agglomeration into nanoparticles during the pyrolysis. This makes high Fe loading in the single-atom catalysts difficult to achieve, while resulting in deteriorated performance.12,14 Thus, further improvement of Fe-N-C catalyst performance necessitates the better design of stable structures with higher activity. Wang et al. reported a design of

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Fe-Co dual sites embedded in nitrogen-doped carbon, which can reduce the cleavage barrier of O-O bond to achieve high activity for ORR, as well as high selectivity to the four-electron reduction path.15 Xiao et al. found that the binuclear Co2N5 sites were more active than the single center sites for ORR.16 A recently reported electrocatalyst with Zn-Co atomic pairs also exhibits outstanding activity for ORR.17 These findings motivated us to explore catalytic behavior of multi-atom Fe-N-C catalysts, which may surpass singleatom Fe-N-C catalysts. Along with a design of highly active structures, the optimized pore structure and large surface area, which would facilitate mass transfer and easy accessibility of active sites, are also desirable to improve activity.18,19 Covalent organic frameworks (COFs), emerging as a class of crystalline porous materials like metal-organic frameworks, feature high surface areas, tunable compositions, and diverse structural topologies.20,21 COFs can be an ideal precursor for producing porous carbon-based materials via pyrolysis.22 Importantly, when COFs are used as a precursor to produce Fe-N-C catalysts, Fe ions can easily diffuse into the cavities of COFs, while the N sites on COFs may serve as a ligand for anchoring Fe cations. 23,24 Moreover, COFs’ periodic structures with separated units can inhibit agglomeration of Fe atoms during the pyrolysis process.25 Herein, by employing the structure of TAPB-PDA COF with a cavity diameter of ~2.5 nm, we prepared the Ndoped porous carbon that anchors both atomically dispersed Fe-N4 sites and Fe atomic clusters consisting of a few atoms (marked as FeAC@FeSA-N-C). The as-obtained catalyst exhibits excellent electrocatalytic performance for ORR with a half-wave potential (E1/2) of 0.912 V in 0.1 M KOH, which is 68 and 15 mV higher than that of the single-atom Fe-N-C counterpart (FeSA-N-C) and commercial Pt/C (Johnson−Matthey, 20 wt % Pt/C), respectively. In addition, the catalyst shows greater tolerance to methanol and higher durability than the commercial Pt/C. Both experimental measurements and DFT calculations indicate that the improved ORR performance is primarily ascribed to the formation of atomic Fe nanocluster, compared with the single-atom FeN-C catalyst. Our work may provide a concept to design high-performance single atom based catalysts.

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N-C), N-doped porous carbon that anchors both atomically dispersed Fe-N4 sites and Fe nanoparticles (marked as FeNP@FeSA-N-C), as well as N-doped porous carbon that anchors atomically dispersed Fe-N4 sites (marked as FeSA-NC) were also prepared by similar procedures. The experimental details are provided in the Experimental Section. The overall morphology of the FeAC@FeSA-N-C catalyst was initially examined by scanning electron microscopy (SEM, Figure 1b) and transmission electron microscopy (TEM, Figure 1c). Similar to the sample without the introduction of Fe (N-C) (Figure S1), it features homogeneous conjoint hollow spheres with a diameter of ~100 nm and a wall thickness of ~20 nm, which indicates that the structure of the original COF (Figure S2) was well inherited. However, no Fe-containing particles can be identified at this magnification, indicating that there was no severe agglomeration of Fe atoms during the pyrolysis process. In the highresolution TEM (HRTEM) image of FeAC@FeSA-N-C (Figure S3), again, no Fe-containing particles can be discerned; and only an amorphous carbon structure with numerous micropores consisting of randomly orientated graphitic domains was observed, similar to the case of FeSA-N-C and NC (Figure S4). Furthermore, no obvious bright spots were observed in the high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image recorded at medium-magnification (Figure S5), further confirming the absence of the Fe-containing large particles. However, as indicated by the corresponding EDX elemental mapping (Figure S5), Fe, N and C elements are uniformly distributed over the whole sample. Next, high-resolution HAADF-STEM measurement was further performed to elucidate the existing form of Fe atoms in FeAC@FeSA-N-C. As shown in Figures 1d and S6, numerous highly dispersed dots with heavier image contrast were observed (partially highlighted by green dashed circles), attributed to the stronger scattering of Fe, clearly showing the existence of ultrafine Fe clusters.26 The absence of the clusters in HRTEM images (Figures S3 and S7) is due to their low phase contrast as a result of their small size. Closer observation (Figures 1e) further shows that both mononuclear Fe species (highlighted by red dashed circles) and multinuclear nanoclusters (highlighted by green dashed circles) were present on the N-doped carbon. In order to validate the co-existence of the mononuclear and multinuclear Fe species, more HAADF-STEM images were collected on different regions of the samples (Figures S8-10). Additionally, an enlarged atomic-resolution HAADF-STEM image was shown in Figure 1f, from which a cluster consisting of a few atoms with ill-defined structure was identified. Energy dispersive X-ray (EDX) mappings at higher magnification (Figures 1g and S11) were also utilized to verify the distribution of different elements. The superposition of the elemental mappings on the nanoscale suggests possible bonding between the heteroatoms.27 After acid washing, mononuclear Fe species are preserved, but multinuclear nanoclusters can hardly be found (Figures S12-14), suggesting that the acid-

RESULTS AND DISCUSSION To synthesize the FeAC@FeSA-N-C catalyst, we employ the size controlled strategy based on periodic skeletons and separated building blocks of COF, as illustrated in Figure 1a. TAPB-PDA COF was prepared by a condensation polymerization reaction using 1,3,5-tris(4-aminophenyl)benzene and terephthaldehyde as monomers through a solvothermal method. By immersing the obtained COF in a Fe(NO3)3· 9H2O aqueous solution for 10 h, the iron precursor was adsorbed and confined within the separated cavities, in which the weakly adsorbed and excess ions were removed by washing with distilled water. FeAC@FeSA-N-C catalyst was then obtained by directly pyrolyzing the COF containing Fe precursors. For comparison, N-doped porous carbon (marked as


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Figure 1. (a) Illustration of the synthesis process of FeAC@FeSA-N-C. (b) SEM image, (c) TEM image and (d-f) HAADF-STEM image of FeAC@FeSA-N-C. (g) HAADF-STEM image and corresponding EDS element mapping images (C red, N blue, Fe green) of FeAC@FeSA-NC.

unstable Fe clusters were effectively removed, while the isolated Fe atoms should be coordinated with adjacent N atoms to form acid-resistant Fe-N configurations.28 X-ray diffraction (XRD) was conducted to analyze the crystalline phase of iron and carbon. As shown in Figure 2a, two broad peaks at around 23o and 44o were observed in the patterns of all the samples, assigned to the (002) and (101) planes of partially graphitized carbon, indicating TAPBPDA COF was fully carbonized after pyrolysis.29 Similar to other reported cases of single-atom Fe-N-C catalysts,30,31 no diffraction peaks related to crystalline Fe species were identified in XRD patterns. Notably, even for the sample of FeAC@FeSA-N-C that contains atomic Fe clusters, crystalline components were still absent in the XRD patterns, implying the size of the clusters are very small and the catalyst is predominantly composed of the carbon framework, being consistent with the aforementioned electron microscopy characterization and some previous works.32-34 Furthermore, Raman measurements were performed to characterize the carbon structure of the catalysts (Figure 2b). The nature of carbon, as characterized by the intensity ratio of D band and G

band (ID/IG), suggested that the sample FeAC@FeSA-N-C and FeSA-N-C have lower ID/IG values (0.90 and 0.91) than N-C (0.95), i.e., a higher degree of graphitization and conductivity. These features can be attributed to the iron-catalyzed graphitization mechanism during the high temperature pyrolysis.31,35 It is noteworthy that this mechanism is more obvious in FeNP@FeSA-N-C since cleaner graphitic carbon layers can be seen (Figure S15). The N2 adsorption-desorption isotherms and pore size distribution curves of Fe AC@FeSAN-C and N-C are presented in Figure S16. Interestingly, after Fe-species involved, the N2 uptakes in the isotherms evidently increased, showing the larger specific surface area (from 471 m2 g-1 to 558 m2 g-1). Besides, the pore size distribution curves also show that the catalyst modified with Fe has a larger pore volume (0.67 cm 3 g-1 vs. 0.39 cm3 g-1). These increases are mainly due to the activation effect of carbon, caused by metal salts.36 The larger surface area and pore volume are beneficial to increasing active sites exposed and growing mass transport capacity.37 X-ray photoelectron spectroscopic (XPS) measurements were performed to probe the surface chemical state of the


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Figure 2. (a) XRD patterns, (b) Raman spectra (the shaded areas are for D and G bands) and (c) N1s XPS spectra of FeAC@FeSA-N-C, FeSAN-C and C-N. (d) N bonding configurations and the corresponding contents in FeAC@FeSA-N-C, FeSA-N-C and C-N. (e) XANES spectra (inset: model of FeAC@FeSA-N-C. Fe red, N blue, C gray spheres) and (f) FT-EXAFS curves of FeAC@FeSA-N-C, FeSA-N-C and reference materials at Fe K-edge.

catalysts. The XPS survey spectra (Figure S17) portray the presence of C, N and O for all the three samples. After Fe introduction, a weak peak centered ~711 eV was observed for both FeAC@FeSA-N-C and FeSA-N-C, which is attributed to Fe. The low peak intensity of Fe is mainly due to the low content of Fe. The high-resolution Fe 2p XPS spectra further confirmed the presence of Fe, even after acid-etching (Figure S18), informing that the residual Fe should be coordinated by N to form acid-resistant Fe-N configuration. This conclusion is consistent with the N 1s spectra of FeAC@FeSAN-C and FeSA-N-C (Figure 2c), which present a peak centered at around 399.0 eV, attributed to Fe-coordinated nitrogen (Fe-N). Due to the removal of Fe clusters, the content of Fe decreased from 0.77 at. % for FeAC@FeSA-N-C to 0.47 at. % for FeSA-N-C (Table S1). XPS cannot detect the Fe species under the carbon shell, so inductively coupled plasma mass spectrometry (ICP-MS) was also employed, showing that the Fe contents in FeAC@FeSA-N-C and FeSA-N-C were 3.98 wt. % and 1.33 wt. %, respectively. The four other peaks observed in the N 1s spectra of the samples (Figure 2c) are ascribed to pyridinic N (398.2 eV), pyrrolic N (399.9 eV), graphitic N (400.8 eV) and oxidized N (402.1 eV), respectively. Notably, the N doping levels of FeAC@FeSA-N-C (4.38 at. %) and FeSA-N-C (4.27 at. %) were higher than that of N-C (3.72 at. %), which can be described by the enhanced N incorporation in the carbon framework due to involvement of the Fe species.38 The amount of nitrogen in the three asprepared samples was shown in Figure 2d and Table S1. In view of the fact that no obvious decrease of Fe-N content is seen upon acid-washing, the acid-resistance nature of Fe-N configurations is validated.28 Besides forming Fe-N, the Fe

also promotes transformation of pyrrolic N and oxidized N into graphitic N and pyridinic N, due to possible metal catalysis, which can be beneficial to the ORR performance.39 To understand the local atomic coordination and electronic structure of FeAC@FeSA-N-C and FeSA-N-C, X-ray absorption spectroscopy measurement, including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were recorded. For comparison, measurements were also performed on several reference samples, such as Fe foil, Fe2O3 and iron phthalocyanie (FePc). As shown in Figure 2e, the near-edge absorption threshold of Fe K-edge of FeSA-N-C was similar to that of FePc and between those of Fe2O3 and Fe foil, implying that the single Fe atoms carried positive charges and could have been stabilized by N atoms.12 The adsorption edge of FeAC@FeSA-N-C shifted towards lower energy with respect to FeSA-N-C, indicating co-existence of Fe clusters and positively charged Fe single atoms (Figure 2e, inset). In the preedge region, a weak peak at ~7113 eV was shown in both FeAC@FeSA-N-C and FeSA-N-C spectra. It was assigned to the 1s → 3d transition along with simultaneous charge transfer of ligand-to-metal, indicating that the dominant coordinated geometry around Fe was close to a square structure.40,41 This pre-edge peak was commonly regarded as the fingerprint of square-planar Fe-N4 configuration with a porphyrin-like structure, which can be found in FePc reference.42-46 However, the Fe2O3 reference also has a similar pre-peak at ~7113 eV due to its octahedral Fe structure caused by C3v symmetry along with two sets of three Fe-O bonds (Figure 3e).47 Although pre-edge features cannot


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clearly distinguish square-planar Fe-N4 and Fe2O3, the existence of Fe2O3 is less likely since the TAPB-PDA COF does not contain any oxygen element and the pyrolysis was conducted under an inert atmosphere. In addition, at such a high temperature (900 oC), iron oxide, if present, would be easily reduced to Fe by carbon.48 The Fourier transform (FT) EXAFS spectra of both FeAC@FeSA-N-C and FeSA-N-C present a primary peak located at ~1.5 Å, corresponding to the Fe-N(O) scattering path (Figure 2f). It is worth noting that an obvious Fe-Fe peak (~2.2 Å) was detected in FeAC@FeSAN-C but not in FeSA-N-C, suggesting that Fe clusters exist in FeAC@FeSA-N-C, but can be effectively removed upon acidwashing (Figure 2f). The second shell Fe-Fe bond at ~4.4 Å is not obvious in the spectra of FeAC@FeSA-N-C, implying that the size of the clusters is very small and has reduced structural periodicity.49,50 Additionally, no Fe-Fe bond at ~2.5 Å was detected, further confirming non-existence of Fe2O3.47,51 Due to the close location of Fe-N and Fe-O peaks in the EXAFS spectra, we also resort to wavelet-transform (WT) EXAFS analysis to discriminate the backscattering atoms even when they overlap in R-space. As shown in Figure S19, both FeAC@FeSA-N-C and FeSA-N-C show a WTmaximum with a k value of ~4.8 Å-1 for the Fe-N path, which can be distinguished from the Fe-O (Fe2O3) path (~5.3 Å-1). Thus the Fe-N(O) scattering path (~1.5 Å) for FeAC@FeSAN-C and FeSA-N-C observed in R space is unambiguously ascribed to the Fe-N scattering path. EXAFS fitting was further carried out to quantitatively unveil the chemical environment of typical Fe single atoms in the catalysts. Since the Fe-N configurations in the two samples were the same (Figure 2f), and Fe clusters in sample FeAC@FeSA-N-C would

result in too many fitting parameters. We chose to fit the EXAFS of FeSA-N-C for the first shell. The fitting parameters were listed in Table S2 and the fitting curves were given in Figure S20. The fitting result shows that one Fe atom is coordinated with four N atoms at ~2.01 Å, forming an Fe-N4 moiety (Figure S20a, inset). The ORR activity of the designed FeAC@FeSA-N-C was comprehensively evaluated by using various techniques. We carried out cyclic voltammetry (CV) measurements in N2/O2-saturated alkaline electrolytes (0.1 M KOH) in comparison with N-C and FeSA-N-C (Figure 3a). In contrast to the virtually featureless CV curves in N2-saturated solution, well-defined cathodic peaks appeared in the CV curves when measured in an O2-saturated solution. Among them, the FeAC@FeSA-N-C electrode gives rise to the highest catalytic activity with the most positive ORR peak at 0.82 V (versus reversible hydrogen electrode (RHE), the same below), while the N-C electrode shows a much lower one (0.66 V), reflecting the significant role of Fe in boosting ORR kinetics. Linear sweep voltammetry (LSV) curves recorded on rotating disk electrode (RDE) also show the superior performance of the FeAC@FeSA-N-C catalyst with an E1/2 value of 0.912 V, which is higher than that of commercial Pt/C (0.897 V) as well as most reported Fe-based catalysts (Figure 3b, Figure S21 and Table S3). The superior activity of FeAC@FeSA-N-C is further verified by the high kinetic current density (𝐽𝑘 ) at 0.85 V (61.1 mA cm-2), calculated from the equation 𝐽𝑘 = 𝐽𝐿 ∙ 𝐽/(𝐽𝐿 − 𝐽), which is 1.84-fold higher than that of Pt/C (Figure S21).

Figure 3. (a) CV curves of FeAC@FeSA-N-C, FeSA-N-C and N-C in O2-saturated (solid line) or N2-saturated (dash line) 0.1 M KOH solution with a sweep rate of 50 mV s-1. (b) LSV curves and (c) the corresponding Tafel plots of FeAC@FeSA-N-C, FeSA-N-C, N-C and Pt/C in O2saturated 0.1 M KOH solution with a rotation rate of 1600 rpm and a sweep rate of 10 mV s -1. All currents were corrected by deducting the background current and measured in N2-saturated electrolyte, and all potentials were corrected for ohmic loss. (d) Peroxide yield and electron transfer number calculated from RRDE measurements of FeAC@FeSA-N-C, FeSA-N-C, N-C and Pt/C. (e) Stability evaluation and methanol crossover effect test of FeAC@FeSA-N-C and Pt/C. (f) Discharging polarization curves and the corresponding power density plots of FeAC@FeSA-N-C-based and Pt/C-based ZABs.


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Figure S22 demonstrates the RDE polarization curves of FeAC@FeSA-N-C at different rotation rates ranging from 400 to 2500 rpm in O2-saturated 0.1 M KOH and the corresponding background current measured in N2-saturated electrolyte. Figure 3c shows the corresponding Tafel curves of these catalysts, again signifying that the FeAC@FeSA-N-C with the smallest Tafel slope (61 mV dec-1) is an efficient electrocatalyst. Furthermore, rotating ring-disk electrode (RRDE) tests were conducted to clarify the ORR pathway and H2O2 yield (Figure S23). As shown in Figure 3d, FeAC@FeSA-N-C and FeSA-N-C demonstrate a near fourelectron ORR pathway with low H2O2 yields, similar to the case of commercial Pt/C. However, the Fe-free N-C sample shows a dominant two-electron ORR pathway, which is consistent with the fact that the Tafel slope of N-C is much higher than those of FeAC@FeSA-N-C , FeSA-N-C and commercial Pt/C.52 Furthermore, the electrochemical active surface area (ECSA) was measured to evaluate the intrinsic activity of the active sites. As shown in Figure S24, the measured ECSA for FeAC@FeSA-N-C (113 cm2) is slightly larger than that of FeSA-N-C (100 cm2), indicating that the Fe clusters in FeAC@FeSA-N-C do not provide more active sites. As shown in Figure S24, after normalizing the polarization curves by using the measured ECSA, FeAC@FeSA-N-C still exhibits higher ORR activity than FeSA-N-C, showing that the existence of Fe nanoclusters brings higher intrinsic activity of single-atom Fe active sites. In addition to the activity, the durability and methanol tolerance were also investigated. Notably, FeAC@FeSA-N-C exhibits higher long-term stability than commercial Pt/C, as confirmed by the slower decay rate in the chronoamperometry measurement (Figure 3e) and the negligible decay in E1/2 after the accelerated durability test (ADT) (Figure S25). Moreover, the TEM and XRD measurements of the FeAC@FeSA-N-C show that the Fe nanoclusters and single-atom Fe atoms still remain, while no aggregation of Fe are observed (Figures S26-S28). It is known that Pt has a propensity to adsorb methanol and then oxidize it to yield carbon monoxide (CO), while simultaneously the oxygen reduction also takes place. The generated CO can poison the catalyst by occupying the Pt surface. 53 Compared with Pt-based catalysts, NPMCs seem to have much better selectivity towards ORR.54 As shown in Figure 3e, after the introduction of methanol into the electrolyte cell, the current density of Pt/C drops sharply, whereas no obvious change occurs for FeAC@FeSA-N-C, suggesting its excellent methanol tolerance. It is worthy of noting that although FeSA-N-C exhibits relatively poor activity, it still entails higher long-term stability and tolerance to methanol than the commercial Pt/C (Figure S29). Also, FeAC@FeSAN-C exhibits better ORR activity than that of FeSA-N-C and N-C when tested in acidic solution (Figure S30). To further clarify the effect of the atomic Fe nanoclusters on the catalysis activity, FeNP@FeSA-N-C catalyst was also prepared. The co-existence of Fe nanoparticles and single atoms can be confirmed by XRD, TEM, HAADF-STEM and EDX mapping (Figures S31-S33). As shown in Figures S34 and S35, the FeAC@FeSA-N-C catalyst displays a much more positive E1/2, higher electron transfer number, and lower

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H2O2 yield compared to FeNP@FeSA-N-C, validating the ineffectiveness of Fe nanoparticles towards improving the ORR activity. To examine potential application of the FeAC@FeSA-N-C catalyst, two home-made Zn-air batteries (ZABs) were assembled by using FeAC@FeSA-N-C and commercial Pt/C as an air cathode, respectively (Figure S36). As shown in Figure S37, the open circuit voltage of the ZAB based on FeAC@FeSA-N-C is 1.411 V, which is higher than that of Pt/C based cell (1.370 V). Figure 3f shows the polarization and power density plots of the batteries. The discharge voltage of FeAC@FeSA-N-C based ZAB is more positive than that of Pt/C based one at the same current density. Moreover, power density peak of FeAC@FeSA-N-C-based ZAB reaches 115 mW cm-2 at 184 mA cm-2, higher than that of the Pt/C-based one (96 mW cm-2 at 158 mA cm-2), indicating excellent performance of the FeAC@FeSA-N-C as the air cathode for ZABs. To evaluate the stability of FeAC@FeSA-N-C, a rechargeable ZAB was assembled by using a mixture of FeAC@FeSA-N-C and IrO2 as the air cathode. As a comparison, Pt/C mixed with IrO2 was also assembled in another ZAB. Expectedly, long-term cycling tests suggest that the over-potential (difference between charging and discharging potentials) of FeAC@FeSA-N-C/IrO2-based battery did not change significantly, while there was an obvious increase in over-potential of the Pt/C/IrO2-based one, indicating excellent stability of FeAC@FeSA-N-C catalyst in a ZAB (Figure S38). To further demonstrate promise for future practical application, two home-made ZABs fabricated using the FeAC@FeSA-N-C catalyst were connected in series to power a commercial light-emitting diode (LED, ~2.1 V) (Figure S39). To further understand the role that atomic Fe nanoclusters play in the superior ORR activity of the FeAC@FeSA-NC, density functional theory (DFT) calculations were performed using the computational hydrogen electrode model. As shown in Figure 4a, four different models, namely, Fe1@FeSA-N-C, Fe13@FeSA-N-C, FeNP@FeSA-N-C, and FeSA-N-C were constructed. Table S4 and Figure 4b show the diagrams of adsorption configurations and the corresponding adsorption Gibbs free energies (eV) of the reaction intermediate O* (ΔGO), OH* (ΔGOH), and OOH* (ΔGOOH). Next, we computed the d-band center of the active Fe atom, which coupled to four N atoms, in the Fe-N-C systems. We find that the interaction between Fe atom in Fe-N4 and Fe1 (or in Fe-N4 and Fe13) leads to a shift of the d-band center to a lower energy level, thereby lowering the binding strength of OH* while increasing the ORR activity of Fe1@FeSA-NC and Fe13@FeSA-N-C. As shown in Figure S40, from Fe1@FeSA-N-C to FeNP@FeSA-N-C, however, the binding strength of OH* increases with the shifting of the d-band center to a higher energy level. Both trends are consistent with d-band center theory of Hammer and Nørskov.55,56 Moreover, both the computed onset potentials of FeAC@FeSA-N-Cs (Fe1@FeSA-N-C (926 mV) and Fe13@FeSA-N-C (910 mV)) are higher than those of FeSA-NC without the Fe nanocluster (839 mV) and FeSA-N-C with


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Figure 4. (a) Computational models of Fe1@FeSA-N-C, Fe13@FeSA-N-C, FeNP@FeSA-N-C and FeSA-N-C. (b) Configurations of adsorbates on Fe-N4 site. (c) Free-energy path and (d) calculated overpotentials of the ORR for the Fe1@FeSA-N-C, Fe13@FeSA-N-C, FeNP@FeSA-N-C and FeSA-N-C models.

Fe nanoparticles instead of Fe nanoclusters (FeNP@FeSA-NC) (745 mV). As a result, the rate determining step for the Fe-N-C systems is the desorption of OH* intermediate (Figure 4c and Table S5). The corresponding computed overpotentials for the ORR are shown in Figure 4d. In addition, we compared the activity of other active sites in the Fe1@FeSAN-C and Fe13@FeSA-N-C systems (Figure S41). Due to the excessively strong adsorption of OH*, all of these active sites exhibited lower onset potential than the Fe-N4 site (Table S5). These comparative studies not only confirm the importance of the Fe site coupled to four N coordination to give rise to the superior activity of the FeAC@FeSA-N-Cs, but also show that the extra Fe atom/cluster, e.g., either Fe1 and Fe13, can play an important role in enhancing the activity of Fe-N4 sites, even though these clusters themselves are not robust active sites.

boost its activity. Our findings illustrate a way of incorporating nanoclusters onto single-atom catalysts toward enhancing the oxygen reduction activity.

EXPERIMENTAL SECTION Materials. 1,3,5-Tris(4-aminophenyl)benzene (93.0%), terephthaldehyde (98%), dioxane (99.5%), mesitylene (98%), glacial CH3CO2H (99.5%) and Iron(III) nitrate nonahydrate (98%) were purchased from Aladdin. Hydrochloric acid (37%) was purchase from Sinopharm. All chemicals were used without further purification and the used water was deionized (18.2 MΩ, Milli-Q pore). Preparation of TAPB-PDA COF. 1,3,5-tris(4-aminophenyl)benzene (55 mg, 0.16 mmol) and terephthaldehyde (31 mg, 0.23 mmol) were mixed into a dioxane/mesitylene solution (6.3 mL, v/v=4:1) and heated to 70 oC until the two precursors were completely dissolved. Next, distilled H2O (1.2 mL) and glacial CH3CO2H (1.8 mL) were successively added to the solution under sonication. The resulting suspension was sealed and heated to 70 oC for 72 hours. The resultant yellow product was collected by filtration, rinsed with toluene several times, and dried under vacuum. Preparation of N-C. The obtained TAPB-PDA COF powder was placed in a ceramic boat, which was subsequently transferred in a tube furnace and annealed at 800 oC for 2 h with a heating rate of 5 oC min-1 under flowing N2 gas. After that, the sample was naturally cooled to room temperature to eventually obtain the N-C catalyst.

CONCLUSIONS In conclusion, this comprehensive experimental/theoretical study shows that the Fe nanoclusters can serve as a promoter to the single-atom Fe-N-C catalyst to efficiently catalyze ORR. The coexistence of Fe single atoms and Fe nanoclusters was achieved through spatial isolation strategy using a COF template, and it was further confirmed by the measurements of aberration-corrected HAADF-STEM and XAS. The superior activity of the FeAC@FeSA-N-C was demonstrated by RDE tests in both alkaline and acidic media, as well in a ZAB. Finally, DFT calculations show that Fe-N4 site is the main active site but the Fe nanocluster can further


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Preparation of FeAC@FeSA-N-C. TAPB-PDA COF (50 mg) was dispersed in a 20 mL of Fe (NO3)3·9H2O (4 M) aqueous solution under ultrasonication for 30 min and vigorously stirred for 10 h. Then, the impregnated TAPB-PDA COF was separated by centrifugation and washed with distilled H2O up to four times to remove weakly adsorbed ions. After drying overnight, the dried material was placed in a tube furnace and then annealed at 800 oC for 2 h with a heating rate of 5 oC min-1 under flowing N2 gas and then naturally cooled to room temperature. Preparation of FeNP@FeSA-N-C. FeNP@FeSA-N-C was prepared by the same method with FeAC@FeSA-N-C except the material was only washed once after impregnation in the Fe (NO3)3·9H2O solution.

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The ECSA was determined based on the following equation: 𝐶𝑑𝑙 𝐸𝐶𝑆𝐴 = 𝐶𝑠 where 𝐶𝑑𝑙 is the double layer capacitance value which can be estimated from the linear slope of the fitting line of capacitive currents versus scan rates; and the 𝐶𝑠 value is adopted as 0.04 mF cm-2.57 Durability tests were evaluated by chronoamperometry at a potential of 0.6 V vs. RHE with a rotation speed of 1600 rpm in 100mL O2-saturated 0.1 M KOH. And the ADT was carried out by potential cycles between 0.6 V to 1.0 V vs. RHE in N2-saturated 0.1 M KOH solution for 5000 cycles at a scan rate of 50 mV s-1. The methanol crossover effect tests were conducted in the similar condition with an addition of 5ml of methanol. To assemble Zinc-Air battery, 6.0 M KOH solution containing 0.2 M zinc acetate was used as the electrolyte and a zinc plate served as the anode. The air cathode was prepared by loading (Pt/C)-(RuO2) (1:1) or (FeAC@FeSA-N-C)-(RuO2) (1:1) onto carbon paper. The mass loading of Pt/C, RuO2 and FeAC@FeSA-N-C catalyst is 1mg cm-2. Materials Characterization. Scanning electron microscope (SEM) was performed on a JEOL JSM 7500F electron microscope with a beam voltage of 20 kV. Transmission electron microscopy (TEM) was taken on a Hitachi-7700 with an acceleration voltage of 300 kV. High resolution TEM (HRTEM) was carried out by using a JEOL JEM2100F operated at 300 kV. Aberration-corrected HAADFSTEM was conducted on a Hitachi HD2700C. Powder Xray diffraction patterns of samples were recorded using an Empyrean diffractometer with Cu Kα radiation (λ = 1.5406 Å). Raman spectra were recorded with a LabRAM HR800 with an Ar laser excitation (514.5 nm) and a power of 10 mW. N2 sorption isotherms were measured using an ASAP 2020. X-ray photoelectron spectroscopy (XPS) were recorded with ESCALAB250Xi and all the reported binding energy data were calibrated by C1s (284.6 eV). Inductively coupled plasma mass spectrometry (ICP-MS) measurements were performed with an ICP analyzer (agilent 8900). The Xray absorption fine structure (XAFS) at the Fe K-edge was collected in a fluorescence mode at beamline 01C1 at the National Synchrotron Radiation Research Center in Taiwan. The electron storage ring was operated at 3.0 GeV with a constant current of ~ 400 mA. The incident beam energy was monochromatized using a Si (111) double crystal monochromator. Fe foil, FePc and Fe2O3 were used as references. The acquired data were normalized to the incoming incident photon flux and processed according to the standard procedure using the ATHENA module implemented in the IFEFFIT software packages. To obtain quantitative structural parameters around central atoms, least-square curve parameter fitting was performed using the ARTEMIS module of IFEFFIT software packages.

Preparation of FeSA-N-C. The FeAC@FeSA-N-C catalyst was treated with HCl (3M) followed by washing with distilled H2O to remove the Fe clusters. After that, the acidwashed FeAC@FeSA-N-C was subjected to a second heat treatment with the same condition as the first one to repair the damaged carbon structure and obtain the FeSA-N-C. Electrochemical Measurement. All the electrochemical measurements were carried out on a CHI 760E electrochemical workstation (CH Instruments, Inc., Shanghai) with a standard three-electrode system. A rotating disk electrode (RDE) with a glassy carbon (GC) disk of 5 mm in diameter and a rotating ring-disk electrode with a Pt ring (6.5 mm inner diameter and 8.5 mm outer diameter) and a GC disk of 5.5 mm diameter were used as the substrate for the working electrode. Ag/AgCl (saturated KCl solution) and a graphite rod were used as reference and counter electrode, respectively. The electrolyte was 0.1 M KOH or 0.1 M HClO4 was saturated using N2 or O2 flow for 30 min before testing. 5 mg of the samples and 40 uL Nafion solution (5 wt. %) were dispersed in 1 mL of solution containing 0.5 mL of water and 0.5 mL of ethanol and then sonicated for 1h to form a homogeneous catalyst ink. Then a certain volume of the catalyst ink was pipetted onto the GC substrate giving a catalyst loading of 0.37 mg cm-2. Both CV and LSV tests were carried out in N2/O2-saturated electrolyte. The scan rates for CV and LSV are 50 mV s -1 and 10 mV s-1, respectively. For hydrogen peroxide yield tests, the disk electrode for RRDE was scanned at a rate of 10 mV s -1 and the ring electrode potential was set to 1.5 V vs. RHE. Four-electron selectivity of catalyst was evaluated based on peroxide yield, which was calculated from the following equation: 𝐼𝑅 ⁄𝑁 𝐻2 𝑂2 (%) = 200 × (𝐼𝑅 ⁄𝑁) + 𝐼𝐷 The electron transfer number (n) was calculated from the following equation: 𝐼𝐷 n = 4× (𝐼𝑅 ⁄𝑁) + 𝐼𝐷 where 𝐼𝐷 is the disk current, 𝐼𝑅 is the ring current and N is the ring collection efficiency, equal to 0.37.


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Computational Method. Computational Methods. The density functional theory (DFT) calculations were conducted by the Vienna ab initio Simulation Package (VASP) code58-60 with electron correlation treated within the generalized gradient approximation (GGA)61 using the PerdewBurke-Ernzerhof (PBE)62 exchange-correlation functional. The projector-augmented-wave (PAW) pseudo-potentials63 are chosen to describe ionic cores. Considering the magnetic Fe atoms in the catalyst model, our calculations are done by DFT+U method.64 OH and OOH can form H-bonds with H2O due to the solvent effect, so 0.3 eV energies (Gsolv) are used to correct the total free energy of the state. In addition, the theoretical overpotential for the ORR by DFT calculations using the computational hydrogen electrode model. 65

ASSOCIATED CONTENT Supporting Information. Additional SEM, TEM, EDX, XPS, BET, theoretical calculation, electrochemical performance and a performance comparison table. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected] * [email protected]

Author Contributions ‡ These authors contributed equally to this work.

ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China (Grant No. 2017YFE0120500), the National Natural Science Foundation of China (Grants No. 51502099), and the Fundamental Research Funds for the Central Universities (HUST 2018KFYYXJJ051, 2019KFYXMBZ076). C.D.W. acknowledges the Hubei “Chu-Tian Young Scholar” program. X.C.Z. is supported by UNL Holland Computing Center. X.A. acknowledges the financial support of a scholarship from the China Scholarship Council (CSC). The authors also thank the technical support from the Analytical and Testing Center of Huazhong University of Science and Technology.

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TOC Graphic

12 Environment ACS Paragon Plus

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Markedly Enhanced Oxygen Reduction Activity of Single-Atom Fe

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