Confined Pyrolysis within a Nanochannel to Form a Highly Efficient

Publication Date (Web): September 14, 2018. Copyright © 2018 American Chemical Society. *E-mail: [email protected]. Cite this:ACS Energy Lett. 3, XX...
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Confined Pyrolysis within a Nanochannel to Form a Highly Efficient Single Iron Site Catalyst for Zn−Air Batteries Zheng Kun Yang,†,‡ Cheng-Zong Yuan,† and An-Wu Xu*,†

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Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, China ‡ Department of Chemistry, University of Science and Technology of China, Hefei 230026, China S Supporting Information *

ABSTRACT: Structural engineering of atomic-scaled metal/N−C catalysts is crucial yet challenging in enhancing their performance for the oxygen reduction reaction (ORR). Herein, we demonstrate exclusive single iron sites in periodic mesoporous nitrogen-doped carbon through an efficient template-directed pyrolysis strategy. The channel in SBA-15 was introduced to give a large specific area and uniform mesopore, providing abundant exposed active sites for the ORR. As a result, the optimal single-atom Fe catalyst exhibits a more positive onset potential (1.03 V vs RHE) and half-wave potential (0.902 V vs RHE) than those of a commercial Pt/C catalyst in an alkaline medium. For the single-atom Fe catalyst after 20000 s, 93.6% of the current at 0.8 V vs RHE can be retained, demonstrating its good stability. Additionally, when utilized to assemble the air electrode of a primary zinc−air battery, the obtained Fe single-atoms catalyst provides high voltage and durability, indicating its practical applicability.

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features of periodic MC can provide a large reaction interface area, more accessible active sites, and many electron transport paths during the ORR process. To prepare the mesoporous M/ N−C catalysts, the template method is the most widely employed approach. However, major challenges in developing active M/N−C catalysts still remain: (1) the poor solubility of commonly used precursors,20,21 restricting their infiltration into the voids of the template; (2) the need for further improvement of the activity and stability of M/N−C catalysts. A previous report proved that the strong interaction between M, N species and the C support can lead to significant enhancement in activity of the ORR catalysts. Thus, the design and synthesis of a single precursor at the molecular level containing M, N, and C sources could endow enhanced activity and durability after pyrolysis. Here we report a versatile and robust template-directed approach to synthesize the iron SAs on nitrogen-doped periodic MC (Fe SAs/MC) through pyrolysis of a novel water-soluble N-rich Fe-based coordination complex. SBA-15 was utilized as a hard template,22 ferrous sulfate (FeSO4) was employed as an Fe source, and 5-amino-1,10-phenanthroline (phen−NH2) was selected as a bidentate nitrogen-donor

he oxygen reduction reaction (ORR) is a crucial process occurring in many renewable energy conversion systems, such as proton exchange membrane fuel cells (PEMFCs), metal−air batteries, and low-temperature alkaline fuel cells. Traditionally, platinum-group noble metals are the best ORR catalysts,1−3 taking into consideration their high activity by four-electron reduction of oxygen to water as the final product. However, high cost, low abundance, and poor stability hinder their broad use. In this regard, considerable research efforts have been devoted to exploring non-platinum-group metal (non-PGM) electrocatalysts. A variety of alternative cost-effective materials were investigated extensively to facilitate the ORR on the cathode.4−9 Particularly, metal−nitrogen dual-doped carbon (M/N−C) catalysts, in which the metal is either Fe or Co, have been found to possess good activity and durability.10−17 It has been widely suggested that the active sites of these catalysts are closely related to the N-coordinated Fe single atoms (SAs) in the form of MN4 structures. Furthermore, the large specific surface area (SSA),18 highly porous structure, and good electrical conductivity of the carbon matrix enable enhancement of their ORR performance. Well-defined mesoporous carbon (MC) architectures, a family of porous material,19 have received extensive research interest by virtue of their high porosities, large SSA, excellent conductivity, and strong resistance to corrosion. As a vital configuration of M/N−C catalysts, these unique structural © XXXX American Chemical Society

Received: August 16, 2018 Accepted: September 12, 2018

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DOI: 10.1021/acsenergylett.8b01508 ACS Energy Lett. 2018, 3, 2383−2389

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Scheme 1. Schematic Illustration of the General Procedures for the Synthesis of Mesoporous Fe SAs/MC Catalysts (A) and Synthetic Process of the Typical Fe SAs/MC(950) Catalyst (B): (a) Chemical Structure and Photograph of phen−NH2, (b) Chemical Structure of the Fe/phen−NH2 and Photograph of the Precursor of the Fe/phen−NH2 Coordination Compound and SBA-15 Template, and (c) Photograph of the As-Prepared Fe SAs/MC(950) Catalyst

coordination interaction was applied to fabricate the Fe/ phen−NH2 complex, which could be formed in 30 s at ambient temperature. UV−vis analysis indicates the formation of the Fe/phen−NH2 compound (Figure S1). Importantly, the resulting N-enriched Fe-based complex is soluble in water/ ethanol solutions, which can guarantee its uniform adsorption and immobilization into the SBA-15 pore channels during solvent evaporation. In addition, the scaffold in the phen−NH2 ligand is highly thermally stable, which enhances the graphitization degree and electrical conductivity; this is beneficial for improvement of the catalysts ORR performance. The crystalline phase purity and structural order of the Fe SAs/MC(950) product were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis. The wide-angle XRD pattern of the Fe SAs/MC(950) sample displays a broad peak at around 24° (Figure S2), which can be ascribed to the (002) planes of carbon species.23 No peaks of metallic iron or iron nitrides were identified. The small-angle XRD measurements were conducted on both the Fe SAs/ MC(950) and SBA-15 template (Figure 1a). Three wellresolved peaks of the SBA-15 hard template are indexed to the diffractions of (100), (110), and (200) planes of the hexagonal p6mm mesostructure,24 while the Fe SAs/MC(950) sample exhibits the diffraction peak of the (110) plane, indicating that an ordered mesoporous structure is formed for Fe SAs/ MC(950). It is noted that the position of the (100) diffraction peak for Fe SAs/MC(950) is shifted toward a higher angle. Figure 1b exhibits the TEM image of the Fe SAs/MC(950) catalyst, in which mesopores are clearly visible. When viewed along the pore axis direction, it explicitly reveals a honeycomblike uniform mesoporous structure (inset in Figure 1b), similar to the TEM image of the SBA-15 silica template (Figure S3). This result demonstrates that mesopores and walls of the silica template were successfully replicated into carbon frameworks and periodic mesopores of Fe SAs/MC(950). The high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image (Figure 1c) and electron energy-loss

ligand and carbon source. Moreover, the as-prepared ironbased chelation complex is also soluble in the mixed solvents of ethanol and water, guaranteeing intimate contact with the extraneous template and completely filled in pore volumes. Aberration-corrected high-angle annular dark field scanning transmission electron microscopy (AC HAADF-STEM) and X-ray absorption fine structure (XAFS) spectroscopy demonstrated that Fe is dispersed as SAs in the N-doped MC matrix. Due to the highly ordered mesoporous structure and large SSA, the high-density single-atom sites can be exposed to highly diffused reactive species. The optimal catalyst Fe SAs/ MC(950) pyrolyzed at 950 °C shows excellent ORR activity with a more positive onset potential (1.03 V vs RHE) and halfwave potential (0.902 V vs RHE) than those of a commercial Pt/C catalyst in 0.1 M KOH and is also superior to those of most reported nonprecious ORR catalysts. Moreover, when used to assemble the air electrode of the primary zinc−air (Zn−air) battery, the obtained Fe SAs/MC(950) catalyst provides high voltage and durability, indicating its practical applicability. Our work will open a new way to design highly active single-atom catalysts. Mesoporous Fe SAs/MC catalysts toward the ORR were prepared via a simple yet versatile template strategy, as shown schematically in Scheme 1A. In brief, a mixture of the Fe/ phen−NH2 complex and a mesoporous SiO2 (SBA-15) template was first prepared through a coordination reaction at room temperature by adding Fe(II) into a phen−NH2 ligand-containing template suspension under stirring (step I). Then, the as-obtained red powder was pyrolyzed at selected temperatures in an argon atmosphere (step II). Upon dissolution of the SiO2 template using 6 M KOH, followed by acid washing to eliminate inactive and unstable iron species (step III), the final Fe SAs/MC electrode materials were fabricated. Figure 1B illustrates the chemical structure of the phen−NH2 ligand and Fe/phen−NH2 complex, the synthetic process of the typical Fe SAs/MC(950) catalyst, and the corresponding digital images. The relatively strong metal 2384

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Figure 1. (a) Small-angle XRD patterns of the SBA-15 template and Fe SAs/MC(950) sample. (b) TEM of the as-prepared mesoporous Fe SAs/MC(950) catalyst. (c) HAADF-TEM image and corresponding EELS mapping images of carbon (d), nitrogen (e), and iron (f) of the mesoporous Fe SAs/MC(950) catalyst. (g) Nitrogen adsorption−desorption isotherm and pore size distribution. (h) AC HAADF-STEM image of the Fe SAs/MC(950) catalyst, showing isolated single atomic Fe sites.

The local structure information on Fe SAs/MC(950) was further investigated by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectroscopy. The Fourier transform (FT) k3weighted χ(k) function of the EXAFS spectrum for Fe SAs/ MC(950) presents dominant Fe−N coordination with a peak at about 1.5 Å (Figure 2a). No obvious Fe−Fe interactions were detected, indicating that Fe atoms are isolated dispersions and stabilized by nitrogen. As shown in Figure 2b, the absorption edge position of Fe SAs/MC(950) is located between those of Fe foil and Fe2O3 reference samples, elucidating that the atomically dispersed Fe atoms carry partially positive charges and the valence state of Fe is situated between Fe(0) and Fe (III); this is in good agreement with EELS and Mössbauer spectroscopy analysis (Figure S5). X-ray photoelectron spectroscopy (XPS) measurement was performed to investigate the chemical composition of the asprepared catalyst. As expected, the XPS survey spectrum of Fe SAs/MC(950) reveals the presence of C, N, Fe, and O peaks (Figure S6), indicating N and Fe co-doping of carbon. The occurrence of an O peak possibly arises from oxygen implantation by the SBA-15 template or integration of physically adsorbed O2. The high-resolution C 1s spectrum can be deconvoluted into three peaks centered at 284.7, 286.2,

spectroscopy (EELS) elemental mappings (Figure 1d−f) confirm that the Fe and N elements are homogeneously distributed in the MC matrix. The textural parameters and mesoporosity ordering of the Fe SAs/MC(950) material were evaluated by nitrogen isothermal adsorption−desorption measurements. As shown in Figure 1g, a distinct hysteresis loop of a type-IV isotherm characteristic is revealed. The high and sharp capillary condensation discloses the existence of uniform and well-ordered mesopores in the Fe SAs/MC(950) catalyst. The Barrett−Joyner−Halenda (BJH) analysis exhibits that the mesopore size distribution in the Fe SAs/MC(950) is centered mainly at 3.5 nm (the inset in Figure 1g). The Brunauer−Emmett−Teller (BET) SSA for the Fe SAs/ MC(950) was measured to be 1130 m2 g−1, with a total pore volume of 0.98 cm3 g−1. The SSAs of Fe SAs/MC (850) and Fe SAs/MC (1050) are 1032 and 1390 m2 g−1, respectively (Figure S4). Therefore, all of these results clearly suggested that the well-ordered mesostructure is definitely formed. The AC HAADF-STEM image shown in Figure 1h reveals that the Fe SAs/MC(950) catalyst has a number of single atomic Fe sites across the carbon surface (bright dots marked with yellow circles). These isolated single atomic sites in the catalyst could increase the density of active sites for enhanced ORR performance. 2385

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Figure 2. (a) Fourier transforms of the Fe K-edge EXAFS spectra of Fe SAs/MC(950) and reference samples of Fe foil and Fe2O3. (b) Kedge XANES spectra of Fe SAs/MC(950) and reference samples (Fe foil and Fe2O3). (c) N 1s XPS spectra of the Fe SAs/MC(950) catalyst and (d) N K-edge NEXAFS spectrum of Fe SAs/MC(950).

Figure 3. (a) LSV curves of Fe SAs/MC catalysts and Pt/C at a rotation rate of 1600 rpm in O2-saturated 0.1 M KOH solution. (b) Corresponding half-wave potentials and onset potentials of Fe SAs/MC(950) and Pt/C samples. (c) Tafel plots for Fe SAs/MC(950) and Pt/C obtained from the LSV curves in O2-saturated 0.1 M KOH. (d) Hydrogen peroxide yield and calculated electron transfer number of Fe SAs/MC(950) and Pt/C in O2-saturated 0.1 M KOH solution. (e) LSV curves of Fe SAs/MC(950) in O2-saturated 0.1 M KOH at various rotation speeds. The inset represents the corresponding K−L plots. (f) Durability test of the Fe SAs/MC(950) and Pt/C catalyst in O2saturated 0.1 M KOH solution.

coordinated, pyrrolic, graphitic, and oxidized N,26−28 respectively. For the N K-edge spectrum (Figure 2d), peak A′ (398.6 eV), peak A (399.7 eV), and peak B (401.5 eV) can be ascribed to splitting pyridinic π*, pyridinic π*, and graphitic π* transitions in Fe SAs/MC(950);29,30 the appearance of the C

and 287.5 eV, corresponding to sp2 CC, sp3 C−O, and sp3 C−N,25 respectively (Figure S7). The high resolution of the N 1s spectrum in Figure 2c reveals the presence of five types of N atoms, whose components have peaks at 398.6, 399.1, 399.8, 400.9, and 402.9 eV, generally recognized as pyridinic, iron2386

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Figure 4. (a) Schematic configuration of the primary Zn−air battery. (b) Photograph of an as-assembled Zn−air battery with an open-circuit voltage of 1.521 V. (c) Galvanostatic discharge curves of the primary Zn−air batteries with Fe SAs/MC(950) and Pt/C as cathode catalysts at different current densities. (d) Photograph showing a LED panel powered by three Zn−air batteries. (e) Specific capacities for the Zn−air batteries using Fe SAs/MC(950) and Pt/C as ORR catalysts, which were normalized with the mass of consumed Zn. (f) Long-term stability of the primary Zn−air battery with the Fe SAs/MC(950) cathode at a current density of 5 mA cm−2. The battery was recharged by refilling the Zn anode and electrolyte.

Fe SAs/MC samples. The peak potential of Fe SAs/MC(950) is ∼30 mV higher than that of Pt/C (0.86 V). From this result, it is concluded that 950 °C is the optimal pyrolysis temperature for balancing various impacts to obtain a singleatom Fe catalyst for high ORR activity. The ORR performance of Fe SAs/MC catalysts was further investigated on a rotating disk electrode (RDE) in an O2-saturated 0.1 M KOH solution (Figure 3a). The onset potential (Eonset, defined as the current density reaches 0.1 mA cm−2) and half-wave potential (E1/2) of the Fe SAs/MC(950) catalyst are 1.03 and 0.902 V vs RHE, respectively (Figure 3b). Remarkably, both values are more positive than those of Pt/C (Eonset: 0.99; E1/2: 0.86 V). Such a positive onset potential and E1/2 have rarely been achieved for previously reported noble-metal-free electrocatalysts (Table S2). The superior ORR catalytic activity of the Fe SAs/ MC(950) is further supported by a Tafel slope of 63 mV/ decade (Figure 3c), which is smaller than the value of 66 mV/ decade for the Pt/C catalyst, indicating a similar catalytic reaction mechanism for O2 electroreduction. The kinetic current densities of Fe SAs/MC(950) and Pt/C normalized to both the geometrical electrode surface area (jk,s) and mass (jk,m) of the catalyst at 0.85 V are plotted in Figure S10. Notably, the values of jk,s and jk,m for the Fe SAs/MC(950) are 8.2 mA cm−2 and 41 mA g−1, respectively, which are higher than those of Pt/C (jk,s: 7.0 mA cm−2; jk,m: 35 mA g−1). Rotating RDE (RRDE) measurements were performed to determine whether the ORR over Fe SAs/MC(950) occurs by four-electron (4e) reduction of oxygen.32 Figure S11 presents the disk and ring currents measured at 1600 rpm in a 0.1 M KOH medium. The H2O2 yield over the Fe SAs/MC(950) catalyst remains below 3% at the measured potentials (0−0.9 V

peak (407.5 eV) suggests the formation of a C−N−C or C−N σ* bond. The pyridinic and graphitic N species are predominant, which leads to enhancement of the electrocatalytic activity. After structural and compositional examination, the asobtained catalysts were subjected to electrocatalytic property measurements. To investigate the influence of sintering temperatures on the ORR activity of the resulting singleatom Fe catalyst, the precursor was heated at different temperatures. Table S1 displays the atomic percentage of C, N, Fe, and O elements in Fe SAs/MC catalysts pyrolyzed at different temperatures. It is found that the atomic percentages of both N and Fe are decreased with increasing pyrolysis temperature, whereas an increase tendency is observed for the content of C. Figure S8 compares the Raman spectra of Fe SAs/MC catalysts prepared at different temperatures. The ratio values of ID/IG calculated from the peak intensity in Raman spectra are 0.99, 0.985, and 0.98 for Fe SAs/MC(850), Fe SAs/MC(950) and Fe SAs/MC(1050), respectively. Therefore, all three samples possess a high graphitization degree. The shrink ratios of ID/IG associated with increasing temperature suggest that the higher-temperature pyrolysis leads to formation of more ordered graphitic domains,31 which is in favor of high electrical conductivity in electrocatalytic processes. The electrocatalytic ORR activities of three Fe SAs/MC samples were initially evaluated and compared by cyclic voltammetry (CV) in a N2- or O2-saturated 0.1 M KOH solution and a commercial 20% Pt/C catalyst as the reference sample. As shown in Figure S9, the Fe SAs/MC(950) has the highest cathodic peak, corresponding to ORR among all three 2387

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ACS Energy Letters vs RHE), and n is the range of 3.95−3.99 (Figure 3d), similar to that of Pt/C; this result confirms that the ORR on the Fe SAs/MC(950) catalyst in an alkaline medium follows the effective 4e pathway. In order to get further insight into the ORR kinetics and 4e selectivity, RDE tests with rotating speeds ranging from 400 to 1600 rpm were conducted (Figure 3e). On the basis of the Koutecky−Levich (K−L) equation,33 five J−1 versus ω−1/2 plots were obtained (inset in Figure 3e). Good linear relationships of K−L plots for Fe SAs/MC(950) are observed, indicating first-order reaction kinetics toward the dissolved oxygen. The average value of n calculated from the slope of each line is 4.02 in 0.1 M KOH electrolyte, suggesting a direct 4e ORR process, which is consistent with the result of the RRDE experiment. Chronoamperometric (CP) response was carried out to evaluate the stability of Fe SAs/MC(950) catalysts. As displayed in Figure 3f, Fe SAs/MC(950) exhibits pronounced stability with 93.6% retention over 20000 s of continuous operation, whereas Pt/C exhibits a faster current loss (79.5% retention). Furthermore, our developed catalyst Fe SAs/MC(950) shows superior immunity to methanol crossover (Figure S12). By addition of 3 M methanol to the solution during the CP test, the current remains almost unchanged, while that for Pt/C suffers a sharp decrease. These electrochemical data reveal that Fe SAs/MC(950) can work as a desirable catalyst with superior activity and durability for ORR, making it a promising air cathode electrocatalyst in a two-electrode primary Zn−air battery. To evaluate the performance of Zn−air with Fe SAs/ MC(950) as an air cathode, a liquid primary Zn−air battery was first constructed, in which the Fe SAs/MC(950) ORR electrocatalyst loaded on the carbon fiber paper electrode was used as the air cathode, a polished Zn foil was used as the anode, and a 6 M KOH solution was employed as the electrolyte (Figure 4a). A Zn−air battery fabricated from a Pt/ C catalyst with the same amount of catalyst loading was also measured for comparison. As shown in Figure 4b, the opencircuit voltage of the Zn−air battery with Fe SAs/MC(950) catalyst was observed to be as high as 1.52 V. The galvanostatic discharge measurements reveal that the voltage plateaus decreased with increasing current densities (Figure 4c). In addition, the Zn−air battery made from Fe SAs/MC(950) catalyst shows a higher voltage compared with that from Pt/C catalyst, which can be attributed to its higher ORR activity as well as richer mesoporosity for improved mass transport. Figure 4d shows that three series-connected Zn−air batteries based on the Fe SAs/MC(950) catalyst can easily power up a light-emitting diode (LED, 3.7 V) panel displaying “Zn−air”. At a discharge current density of 5 mA cm−2, the specific capacity of the battery with the Fe SAs/MC(950) air cathode was calculated to be ∼739 mAh g−1, which was normalized to the mass of consumed Zn electrode (Figure 4e). The corresponding energy density was 960 Wh kg−1. These values are higher than that with the Pt/C-based cathode battery (706 mAh g−1, 882 Wh kg−1). Notably, the battery can be recharged by refilling the Zn anode and electrolyte periodically, and no obvious voltage drop was observed after three cycles (Figure 4f), highlighting again the excellent durability of the Fe SAs/ MC(950) catalyst toward ORR in Zn−air batteries. In this work, a facile and versatile nanochannel-confined pyrolysis strategy was developed to synthesize SAs Fe anchored on periodic mesoporous N-doped carbon electrocatalysts. By combining AC HAADF-STEM characterization and EXAFS and XANES analyses, atomically dispersed Fe

active sites have been identified. Meanwhile, our results display that the pyrolysis temperature largely affects the ORR activity and the optimal catalyst exhibits excellent ORR activity superior to that of commercial Pt/C in a 0.1 M KOH solution, along with outstanding durability. We consider that rich porosities and high SSAs for exposing abundant Fe sites are responsible for its outstanding catalytic activity. Furthermore, when our newly developed electrocatalyst is employed as an air cathode in a liquid primary Zn−air battery, the Fe SAbased battery exhibits high performance and prolonged operation durability. Taken together, our study provides an avenue to design and prepare efficient single-atom oxygen reduction catalysts used in electrochemical energy storage and conversion devices.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b01508. Experimental details, electrocatalytic measurement details, XRD patterns, Raman spectra, elemental compositions from XPS, nitrogen adsorption/desorption isotherms, and electrochemical analysis (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

An-Wu Xu: 0000-0002-4950-0490 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Special funding support from the National Natural Science Foundation of China (51572253, 21771171, 21802132), the Scientific Research Grant of the Hefei National Synchrotron Radiation Laboratory (UN2017LHJJ), the China Postdoctoral Science Foundation (2018M632536), Fundamental Research Funds for the Central Universities, and cooperation between NSFC and the Netherlands Organization for Scientific Research (51561135011) is acknowledged.

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ACS Energy Letters

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DOI: 10.1021/acsenergylett.8b01508 ACS Energy Lett. 2018, 3, 2383−2389