Secondary-Atom-Assisted Synthesis of Single Iron Atoms Anchored on

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Secondary-atom-assisted synthesis of single iron atoms anchored on N-doped carbon nanowires for oxygen reduction reaction Jin-Cheng Li, Fei Xiao, Hong Zhong, Tao Li, Mingjie Xu, Lu Ma, Min Cheng, Dong Liu, Shuo Feng, Qiurong Shi, Hui-Ming Cheng, Chang Liu, Dan Du, Scott P. Beckman, Xiaoqing Pan, Yuehe Lin, and Minhua Shao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00869 • Publication Date (Web): 31 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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ACS Catalysis

Secondary-atom-assisted synthesis of single iron atoms anchored on N-doped carbon nanowires for oxygen reduction reaction Jin-Cheng Li1, 2, 3, Fei Xiao1, Hong Zhong2, Tao Li4, 5, Mingjie Xu1, 6, 7, Lu Ma4, Min Cheng3, Dong Liu2, Shuo Feng2, Qiurong Shi2, Hui-Ming Cheng3, Chang Liu3,* Dan Du2, Scott P. Beckman2, Xiaoqing Pan6, 7, Yuehe Lin2,* Minhua Shao1, 8,* 1Fok

Ying Tung Research Institute, Hong Kong University of Science and Technology, Guangzhou 511458, China. of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164, USA. 3Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China. 4Department of Chemistry and Biochemistry, Northern Illinois University, 1425 W. Lincoln Hwy., DeKalb, IL, 60115, USA. 5X-ray Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States. 6Department of Chemical Engineering and Materials Science, University of California Irvine, Irvine, CA, 92697, United States. 7Irvine Materials Research Institute (IMRI), University of California, Irvine, CA, 92697, United States. 8Department of Chemical and Biological Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China. 2School

ABSTRACT: The development of efficient Fe-N-C materials enriched with single-atom Fe sites towards oxygen reduction reaction (ORR) is still a great challenge since Fe atoms are mobile and easily aggregate into nanoparticles during the hightemperature treatment. Herein, we proposed a facile and universal secondary-atom-assisted strategy to prepare atomic iron sites with high density hosted on porous nitrogen-doped carbon nanowires (Fe-NCNWs). The Fe-NCNWs showed an impressive half-wave potential (E1/2) of 0.91 V and average kinetic current density (JK) of 6.0 mA cm-2 at 0.9 V. They also held an high ORR activity in acidic solution with the E1/2 of 0.82 V and average JK of 8.0 mA cm-2 at 0.8 V. Density functional theory calculations demonstrated that the high ORR activity achieved is originated from single-atom iron sites that decrease the energy barrier in reaction path efficiently.

KEYWORDS: single-atom catalysts, Fe-N, carbon nanowire, density functional theory, oxygen reduction Great efforts have been devoted to low- or no- pollutantemission energy conversion devices that have high energy densities, such as fuel cells and metal-air batteries.1-3 The sluggish kinetics of the cathodic oxygen reduction reaction (ORR) hinders the further improvement of these technologies.4-6 To achieve the practical application, Ptbased materials are currently utilized to expedite the ORR. However, they suffer from prohibitive cost and poor durability. Low-cost carbon-based nanomaterials have attracted great interests as Pt alternatives in catalyzing the ORR.7-10 Heteroatoms doping into carbon matrix change the electron structures of carbon, resulting in an enhanced ORR activity.11-15 Among all the carbon-based materials developed, Fe-N-C with nitrogen coordinated with single iron atoms (Fe-Nx) showed ORR activities comparable to that of Pt.16-17 Redox-induced Fe–Nx transition demonstrated as a determinant role for the ORR activity.1820 Such Fe-N-C materials were usually synthesized by the high-temperature pyrolysis of organic compounds

containing iron and nitrogen sources.21-23 It is acknowledged that at high temperature, iron atoms are highly mobile and easily aggregate into Fe nanoparticles or form compounds that have relatively lower ORR activity than atomic Fe-Nx moieties.24-28 Such competitive relationship between nanosized Fe species and Fe-Nx moieties during the preparation of Fe-N-C materials lead to the final catalysts with low-density of Fe-Nx moieties and undesirable ORR performance. To maximize the atomic utilization and catalytic performance, it is necessary to eliminate these inactive Fe-based nanoparticles and increase the number of exposed atomic Fe-Nx active sites in the Fe-N-C catalysts.29-31 Acid leaching has been commonly used to remove impurities while retain Fe-Nx moieties.32-33 However, such a strategy is unable to essentially increase the density of active sites and therefore cannot significantly improve the ORR performance. Herein, a facile and universal secondary-atom-assisted strategy was proposed to prepare single iron atoms hosted on porous nitrogen-doped carbon nanowires (Fe-NCNWs).

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Iron ions surrounded by secondary metal ions (Al, Mg, and Zn) were directly converted to the atomic Fe-Nx moieties rather than nanosized iron compounds. Furthermore, the secondary metal compounds produced in the hightemperature pyrolysis acted as templates for the formation of nanopores, increasing the surface area to host atomic FeNx moieties. Owing to the abundant atomic Fe-Nx active sites and pore structure, Fe-NCNWs showed outstanding ORR performance under both alkaline and acidic conditions. Density functional theory (DFT) calculations demonstrated that the high ORR activity was attributed to enriched singleatom iron sites decreasing the energy barriers in multi-step electron transfer process. The schematic synthetic process for the Fe-NCNW materials is shown in Figure S1. Typically, cetyltrimethyl ammonium bromide (CTAB) was dissolved in an aqueous HCl solution. After ammonium peroxydisulfate (APS) was added, white precipitates of oxidized CTAB templates were appeared. Pyrrole was drop-wise added in the above mixture, followed by adding Fe(NO3)3 and M(NO3)n (M = Al, Mg, or Zn). Polymerization of pyrrole occurred on the oxidized CTAB templates to form polypyrrole nanowires (Figure S2). After complete polymerization, the polypyrrole nanowires containing iron and secondary M ions were collected by vacuum filtration. The metal ion modified polypyrrole nanowires were then subject to pyrolysis, acid leaching and subsequent heat-treatment to obtain FeNCNWs (Fe-NCNW-Al, Fe-NCNW-Mg, and Fe-NCNW-Zn were prepared with the secondary metals of Al, Mg, and Zn, respectively). For comparison, Fe-N modified carbon nanowires were prepared without adding secondary metals, denoted as Fe-NCNW-w/o. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized to reveal the morphology and structure of Fe-NCNWs. As shown in Figure 1a, Fe-NCNWs inherit the one-dimensional nanostructure of the original polypyrrole nanowires. A typical TEM image (Figure 1b) shows that the nanowires have a wavy morphology and their widths are ~50 nm. Each of them is made of a thick body connecting with a thin wing. Such a feature can be beneficial for hosting more exposed active sites and improve the ORR performance. Highresolution TEM characterization results show distorted graphite layers in Fe-NCNWs (Figure 1c). Such a structure endows the catalysts with abundant defects and nanopores, which can host large numbers of atomic Fe-Nx moieties. Furthermore, no nanoparticle was observed in TEM images, suggesting that iron can embed into carbon matrix as a form of dispersive isolated atoms. To further estimate the degree of graphitization of Fe-NCNWs, Raman spectroscopy was performed. As shown in Figure 1d, different from PPy precursors, Fe-NCNWs hold an impressive D-band and Gband of carbon materials and their ratio is ~1.35, indicating that Fe-NCNWs have a high degree of graphitization and consequently a good electrical conductivity. Figure 1e shows N2 adsorption/desorption isotherm curves of FeNCNWs. Clearly, the Fe-NCNW materials have a strong adsorption in the low-pressure region (P/P0=0-0.1). Such a feature originates from abundant micropores. The pore distribution curves of Fe-NCNWs (Figure S3) were obtained by nonlocal density functional theory (NLDFT, suitable for

to micropore analysis) and Barrett-Joyner-Halenda (BJH, appropriate for mesopore evaluation), respectively. The FeNCNW materials prepared with different secondary metals of Al, Mg and Zn have similar pore distributions and large Brunauer–Emmett–Teller (BET) surface areas in the rage of 785~928 m2 g-1. Such high BET surface areas permit the loading of abundant active sites. X-ray diffraction (XRD) patterns show two obvious peaks at 25.4° and 43.4° corresponding to the (002) and (101) planes of graphite, which further confirms the high crystallinity of Fe-NCNWs and is in accordance with the TEM and Raman results. Furthermore, no XRD patterns from iron species were observed.

Figure 1. (a)) SEM, (b) low-magnification and (c) highresolution TEM images of the Fe-NCNW sample. (d) Raman spectra of Fe-NCNWs, Fe-NCNW-w/o and PPy nanowires. (e) Nitrogen adsorption-desorption isotherm curves and (f) XRD patterns of Fe-NCNWs and Fe-NCNW-w/o. To directly observe the status and distribution of iron species in Fe-NCNWs at the atomic level, aberrationcorrected scanning TEM (STEM) measurements were conducted and the result is shown in Figure 2a. Uniformly dispersive single-atom Fe sites were clearly demonstrated, which was disclosed by the bright dots marked with red cycles. No other nanoparticles were observed. Auxiliary energy-dispersive X-ray spectroscopy (EDS) elemental analysis was carried out to detect the elemental composition and distribution in the Fe-NCNWs. Figure 2b shows a STEM image and corresponding atomic carbon, nitrogen and iron element maps of Fe-NCNWs. All elements uniformly distribute in the Fe-NCNWs material, indicating that doped nitrogen coordinated with Fe atoms can be incorporated into the carbon matrix. To further reveal the chemical composition of Fe-NCNWs, X-ray photoelectron spectroscopy (XPS) characterization was adopted and the results were shown in Figure S4. Possibly owing to atomically dispersed iron form, the iron signal was very weak (iron content: 0.2~0.3 at.%). The nitrogen contents in the Fe-NCNW materials derived from different secondary metals were almost the same (6.2~6.5 at.%). Based on the binding energy, the N 1s spectrum of the Fe-NCNW materials can be fitted with three peaks at 398.7, 401.0 and 403.7 eV (Figure 2c).34 The first one (398.7 eV) may originate from pyridinic N or Fe-Nx because of the binding energies of them are quite similar,35-36 while the last two are attributed to graphitic N (401.0 eV) and oxidized N (403.7 eV). In general, the pyridinic N or Fe-Nx is more active for the ORR especially in acidic solutions.37 The Fe-NCNW materials prepared with different secondary metals held

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ACS Catalysis high pyridinic N (Fe-Nx) ratios of 33~39% (Figure 2c) and are expected to have high ORR performance. X-ray absorption near-edge structure (XANES) and Extended Xray absorption fine structure (EXAFS) were performed to further verify the iron species in Fe-NCNWs. The position of the absorption edge of Fe-NCNW is between those of standard FeO and Fe2O3 (Figure 2d), indicating that the oxidation state of Fe in Fe-NCNW are between 2+ and 3+. Fourier-transform EXAFS curve of Fe-NCNW in Figure 2e shows a Fe-N peak at 1.4 Å. The absence of the Fe-Fe scattering peak at 2.1 Å, and any other long-range order scattering peaks, further demonstrating that Fe is atomically dispersed in Fe-NCNW. All the characterization results demonstrated that abundant atomic Fe-Nx moieties have been successfully created in porous carbon nanomaterials.

Figure 2. (a) ADF-STEM image of the Fe-NCNW sample. (b) STEM image of Fe-NCNW and the corresponding EDS elemental mapping images of C, N, and Fe. (c) XPS N 1s spectrum of Fe-NCNWs. (d) Fe K-edge XANES spectrum of Fe foil, FeO, Fe2O3, and Fe-NCNW. (e) Fourier-transform EXAFS spectrum of Fe foil and Fe-NCNW. Cyclic voltammetry (CV) tests were performed in a N2-and O2-saturated 0.1 M KOH solution to preliminarily estimate the double layer capacitance by simply measuring the area within the CV curves and the ORR activity. Figure S5a shows Fe-NCNW catalysts hold larger areas within the CV curves and more positive ORR peak potentials than those of FeNCNW-w/o, which suggests the Fe-NCNW catalysts have higher electrochemical accessible surface area and better ORR activity. Furthermore, the ORR activity evaluation of Fe-NCNWs was performed in an O2-saturated 0.1 M KOH solution by the rotating ring-disk electrode (RRDE) technique. Figure 3a shows the disk current density of FeNCNWs，Fe-NCNW-w/o, and commercial Pt/C catalyst (28 wt.%, TKK). All the catalysts had a similar limiting current density of 5.7~5.8 mA cm-2, approaching the theoretical value of the four-electron-transfer-pathway at a rotation speed of 1600 rpm. Fe-NCNW-w/o prepared through traditional method without addition of secondary metal showed a good ORR activity with a half-wave potential (E1/2) of 0.88 V, which is comparable to Pt/C (0.875) and in good agreement with previous reports.38-40 Nevertheless, the ORR activities of Fe-NCNWs prepared with the secondary metals had an additional substantial improvement over Fe-NCNW-w/o. The E1/2 shifted from 0.88 V for Fe-NCNW-w/o to 0.90~0.91 V for Fe-NCNWs, which was larger than those of recently published singleatom Fe catalysts, including Fe/N/S decorated hierarchical

carbon/carbon nanotubes (0.85 V),30 Fe-N active sites within porphyrinic triazine-based frameworks (0.87 V),41 carbon nanotubes coated with Fe-doped porphyrinic carbon (0.88 V),42 and isolated single Fe atoms anchored on nitrogen-doped porous carbon (0.90 V).43 Figure 3b shows the corresponding HO2- yield and electron transfer number derived from disk and ring currents. The Fe-NCNWs had a very low HO2- yield (less than 5%) and an average electrontransfer-number (n) of 3.98 at all potentials, which were superior to Fe-NCNW-w/o (HO2- yield < 7%; n = 3.96). Figure 3c shows Tafel plots of Fe-NCNWs and reference samples. Clearly, Fe-NCNWs showed a smaller Tafel slope of ~60 mV/decade than that of Fe-NCNW-w/o (66 mV/decade), suggesting a better ORR kinetics. Furthermore, the kinetic current densities of Fe-NCNW-Al, Fe-NCNW-Mg, and Fe-NCNW-Zn at 0.9 V were 6.2, 6.5, and 5.2 mA cm-2 with the average value of ~6.0 mA cm-2, which are more than twice of that of Fe-NCNW-w/o (2.4 mA cm-2). These results demonstrated that the ORR activities of FeNCNWs in alkaline solutions far exceeded those of FeNCNW-w/o and commercial Pt/C catalysts. In addition to the activity, the durability of Fe-NCNWs was evaluated by cycling CV tests at a potential range of 0.6-1.0 V at 50 mV s1. As shown in Figure 3d, after 10000 continuous cycles, the half-wave potential of Fe-NCNW shows a small negative shift of ~10 mV, which is smaller than that of Pt/C (29 mV, Figure S6a).

Figure 3. (a)ORR polarization curves recorded from disk electrode of Fe-NCNWs, Fe-NCNW-w/o, and Pt/C in an O2saturated 0.1 M KOH solution. (b) HO2- yield and electron transfer number of Fe-NCNWs and Fe-NCNW-w/o. (c) Tafel plots of Fe-NCNWs and Fe-NCNW-w/o. (d) ORR durability evaluation of Fe-NCNW and Pt/C tested by chronoamperometric responses at 0.5 V. In addition to the alkaline ORR performance, we also measured the acidic ORR performance of Fe-NCNWs and the ORR polarization curves are shown in Figure 4a. Compared to Fe-NCNW-w/o, the Fe-NCNW materials held remarkably higher ORR activities with 30~40 mV shifts in the E1/2 (0.78 V versus 0.81~0.82 V). Such high ORR activities of Fe-NCNWs were superior to Fe-N-doped carbon nanofibers (0.74 V),44 active-site imprinted Fe-N-doped carbon (0.76 V),45 carbon nanotubes coated with irondoped porphyrinic carbon (0.79 V),42 but still inferior to

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Pt/C (0.88 V). These results suggested that Fe-NCNWs were one of the best noble-metal-free ORR catalysts in acidic solutions. As shown in Figure 4b, the H2O2 yield of FeNCNWs was lower than 3% at all potentials and the corresponding average n is ~3.98, much better than FeNCNW-w/o (H2O2 yield < 8%; n = 3.89). These results indicate that like Pt catalyst, Fe-NCNWs have a high catalytic efficiency and follow a highly efficient four-electrontransfer pathway in acidic solutions. Figure 4c shows that Fe-NCNWs hold a Tafel slope of ~63 mV/decade, smaller than that of Fe-NCNW-w/o (72 mV/decade), revealing the Fe-NCNW materials have a better ORR kinetics. Furthermore, Fe-NCNWs showed an impressive average kinetic current density of 8.0 mA cm-2 at 0.8 V, which is more than twice of that of Fe-NCNW-w/o (3.8 mA cm-2). Figure 4d shows the ORR durability evaluation result of Fe-NCNW. After 10000 continuous cycles, the Fe-NCNW material shows a half-wave potential negative shift of ~16 mV, slightly higher than that of Pt/C (Figure S6b).

Figure 4. ORR polarization curves recorded from disk electrode of Fe-NCNWs, Fe-NCNW-w/o, and Pt/C in an O2saturated 0.1 M HClO4 solution. (b) H2O2 yield and electron transfer number of Fe-NCNWs and Fe-NCNW-w/o. (c) Tafel plots of Fe-NCNWs and Fe-NCNW-w/o. (d) ORR durability evaluation of Fe-NCNW and Pt/C tested by chronoamperometric responses. According to the above experimental results, introduction of secondary metals in the synthesis has been demonstrated to be an effective strategy to achieve a substantial improvement on ORR activity in both alkaline and acidic solutions. To reveal the nature of the activity enhancement, the differences in structure and composition of Fe-NCNW and Fe-NCNW-w/o were analyzed. Similar to Fe-NCNW, Fe-NCNW-w/o had one-dimensional nanostructure (Figure S7). However, it also had some undesirable nanosized iron-containing species coated with graphite layers, preventing them from acid leaching (Figure S8).42 Based on the above results, we propsed the preparation principle of single-atom iron catalyst and traditional Fe-N-C catalyst. As shown in Figure 5a, the secondary metal ions converted into metal oxides during high-temperature pyrolysis can isolate and restrict Fe atoms, leading to achieving abundant atomic Fe-Nx moieties. These secondary metal oxides are almost inert for

carbon coating.46 Thus, the subsequent acid leaching can effectively remove these secondary metal oxides, leading to a number of additional nanopores and larger specific surface area. By contrast, in the synthesis of Fe-NCNW-w/o, some Fe atoms converted into Fe nanoparticles in the absence of constraints (Figure 5b and Figure S8b, c), which could catalyze the growth of carbon layers on their surfaces.47 The carbon layer could protect the inside Fe particle during acid treatment.48 Accordingly, the final catalyst had a relatively smaller porosity and BET surface area (Figure S3 and Figure 1e) compared with those prepared with the assistance of secondary metals. More importantly, due to the formation of Fe nanoparticles, the concentration of atomic Fe-Nx moieties in Fe-NCNW-w/o was lower than that in Fe-NCNWs, which was confirmed by nitrogen doping configurations (Figure S9 and Figure 2c).

Figure 5. Schematic for achieving (a) single-atom iron catalyst and (b) traditional Fe-N-C catalyst. Color code: nitrogen atom (blue), iron (red), and secondary metal atom (green). (c) Free energy diagrams of the ORR process on FeN4 site and Fe particle at U = 0, pH = 1, and T = 298 K. The insets in (c) are the configurations of adsorption of oxygenbased species on the Fe-N4 site. Finally, DFT calculations were performed to reveal the nature of Fe-NCNWs for highly efficient ORR. Atomic Fe-N4 moiety and iron nanoparticle were constructed. The computational details are presented in the Supporting Information. Free energy diagrams at applied potential U = 0 in Figure 5c show the first three steps are exothermic while the last two steps are endothermic, indicating that the rate-determining step (RDS) may be one of the last two steps. By comparing the free energetics, the reduction of O* to OH* is suggested to be the RDS for atomic Fe-N4 moiety, which has the highest endothermic energy of 0.94 eV. However, for iron particle, the reaction barrier is as large as 1.51 eV at the last electron-transfer step (i.e., OH* + H+ + e→ H2O + *). The Fe-N4 moiety has a remarkably lower energy barrier than that of the iron particle, indicating that Fe-N4 site is energetically more favorable for the ORR process, which is in good agreement with our experimental observations. Furthermore, similar results were also obtained when the applied potential U was 0.45 V and 0.83 V (Figure S10). In conclusion, a facile and universal secondary-atomassisted strategy was proposed to increase the number of

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ACS Catalysis single-atom Fe active sites in one-dimensional porous carbon. Fe ions surrounded by secondary metal ions (Al, Mg, Zn) directly converted to atomic Fe-Nx moieties rather than nanosized Fe-containing compounds. Aberration-corrected TEM and EXAFS results demonstrated that Fe exists in atomically dispersed form, coordinated by nitrogen. Simultaneously, the secondary metal oxides acted as templates to create nanopores and increase surface areas. Owing to the abundant atomic Fe-Nx active sites and porous structure, the final catalysts showed outstanding ORR performance. In alkaline media, E1/2 was ~0.91 V and average JK is 6.0 mA cm-2 at 0.9 V; in acidic media, E1/2 was ~0.82 V and average JK was 8.0 mA cm-2 at 0.8 V. Finally, DFT calculations demonstrated that the high ORR activity was attributed to highly efficient single-atom Fe sites decreasing the energy barriers in multi-step electron transfer process. This work provides a new insight in increasing the number of isolated metal atoms in carbon-based nanomaterials for highly efficient electrochemical reactions.

ASSOCIATED CONTENT Supporting Information. Material preparation, material characterization, electrochemical measurements, and computational methodology are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (C. Liu), [email protected] (Y. Lin), or [email protected] (M. Shao).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Key R&D Program of China (No 2017YFB0102900), the Research Grant Council (N_HKUST610/17) of the Hong Kong Special Administrative Region. It used resources of the Advanced Photon Source and the Center for Nanoscale Materials, Office of Science user facilities, supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DEAC02-06CH11357. T. Li is thankful for the NIU start-up fund.

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Secondary-atom-assisted synthesis of single iron atoms anchored on N-doped carbon nanowires showing excellent catalytic activity for oxygen reduction reaction

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