Graphite Wrapped Fe Core-shell Nanoparticles Anchored on

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Graphite Wrapped Fe Core-shell Nanoparticles Anchored on Graphene as pH-Universal Electrocatalyst for Oxygen Reduction Reaction Jing Zhao, Ning Fu, and Rui Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06153 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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ACS Applied Materials & Interfaces

Graphite Wrapped Fe Core-shell Nanoparticles Anchored on Graphene

as

pH-Universal

Electrocatalyst

for

Oxygen

Reduction Reaction Jing Zhao, Ning Fu and Rui Liu*

Ministry of Education Key Laboratory of Advanced Civil Engineering Materials, School of Materials Science and Engineering and Institute for Advanced Study, TongjiUniversity, 201804, Shanghai, China.

ABSTRACT: Development of high-performance non-precious metals and their nanocomposites as oxygen reduction electrocatalysts is critical but still challenging for fuel cells and metal-air batteries. Here, we introduce a synthetic strategy to fabricate graphite-wrapped Fe core-shell nanoparticles anchored on graphene as an efficient and stable pH-universal electrocatalyst for oxygen reduction reaction (ORR). The coordination among Fe3+, ellagic acid and graphene oxide would drive the formation of a sandwich-like assembly, which was subsequently converted into graphene supported Fe@graphitic carbon core-shell nanoparticles (denoted as GEF). The obtained GEFs exhibited remarkable ORR activity and durability in a wide range of pHs. Most notably, GEF pyrolized at 900 °C showed that the onset and half-wave potential at 1.01 and 0.90 V were more positive than commercial Pt/C (onset and half-wave potential: 0.94 and 0.84V) in alkaline media. Meanwhile, it also demonstrated comparable or even better activity as compared with the commercial Pt/C catalysts in acidic and neutral electrolytes. Keywords: graphene, Fe, core-shell, pH-universal, oxygen reduction reaction

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■INTRODUCTION Boosting the sluggish kinetics of the cathodic ORR is a key in optimizing electrochemical energy conversion efficiency of fuel cells and metal-air batteries.1,2 Precious metal Pt and its alloys loaded on carbon are excellent electrocatalysts for ORR,

but

suffer

from

high

cost

and

poor

stability.3,4Cost-effective

non-precious-metals and their carbon nanocomposites have attracted intense attention as electrocatalysts due to their abundance and high capability.5-7 Metal-organic coordination materials, such as metal-organic frameworks (MOFs) and infinite coordination polymers, have been regarded as ideal precursors for metal-carbon nanocomposites. A series of MOFs, assembly from metal ions and organic ligands (such as trimesic acid and imidazole) have been successfully converted into metal (e.g. Co, Fe, Ni)-carbon composites with diverse structures/morphologies and demonstrated excellent performance for ORR and other electrochemical processes.8-10 Bio-inspired catechol-based compounds, including dopamine and polyphenol (e.g. tannic acid or ellagic acid), represent another type of organic ligands for metal-coordination.11-13 For example, Dai and the coworkers have synthesized a mechanochemical assembly of TA-cobalt with surfactants, and then converted it into ordered mesoporous carbon.14Wang et al. have prepared Co (or Fe)-TA coordination crystals for the metal/carbon composites as a low-cost oxygen electrocatalyst with a high catalytic ability.15 In order to enhance the structural stability and conductivity of catalysts, two-dimensional (2D) conjugated materials have been usually incorporated into the


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construction of metal-carbon composites for energy storage and conversion.16-18 Among them, graphene, a monolayer of carbon atoms arranged in a honeycomb network, stands out as an excellent supporting material owing to its intriguing physicochemical features, such as high surface area and electric conductivity as well as chemical/mechanical stability.19-22 More interestingly, the easy chemical manipulation and altering of the electronic properties of graphene through post-modification or heteroatom doping could cause chemical/electron modulation to provide a desirable surface electronic structure for both the catalytic process and the active sites with practical significance. For example, a series of graphene supported highly dispersed Co or Fe or metal alloy nanoparticles (NPs) have been prepared for ORR.23-25 In addition, other graphene-based hybrid nanostructures, such as core-shell NPs supported graphene or sandwich-structured graphene composites, have been reported and bestowed the hybrid catalysts additional functionality and enhanced ability in the electrochemical process of lithium ion batteries or fuel cell.26-28 However, most of non-precious metal catalysts and their composites can only display comparable ORR catalytic performance with Pt-based catalysts in alkaline electrolyte. It is highly challenging and desirable to develop a pH-universal electrocatalyst with remarkable activity and durability for practical applications. Herein, we reported a facile and easily scalable method to prepare a graphene supported metal@graphite electrocatalyst. Starting from graphene oxide (GO), ellagic acid (EA) and Fe3+, an assembled coordination complex was formed and then converted into a well-defined nanostructure with graphite wrapped Fe core-shell NPs



anchored on graphene. The obtained catalyst with the best ORR performance exhibited a superior catalytic activity with a half-wave potential E1/2 of 0.90 V (vs. RHE) in 0.1 M KOH solution, which was 60 mV higher than that of commercial Pt/C. In addition, it also showed comparable or even better catalytic performance for ORR compared with commercial Pt/C catalysts in acidic and neutral electrolytes. ■RESULTS AND DISCUSSION Figure 1 illustrated a typical synthesis process. EA and Fe3+ were firstly dissolved in a GO dispersion to form a homogenous mixture. Then, a hydrothermal treatment led to the formation of a black powder (named as GO/EA-Fe). After a pyrolysis process in the presence of urea as N-doping agent, Fe nanoparticles encased in a graphitic layer anchored on graphene was obtained (named GEF-T, where T represented the carbonization temperature). EA, a typical bio-inspired polyphenol, has been reported to have strong coordination ability with metal ions.29 FTIR in Figure S1a demonstrated a successful coordination between EA and Fe3+ in GO/EA-Fe, indicated by the vanishing of O-H vibration peaks from EA and the similar characteristic peaks from EA-Fe, a direct coordination assembly between EA and Fe3+.12 The chelation ability of catechol groups in EA would lead to the formation of a stable EA-Fe3+ coordination structure (Figure S1b).30 Meanwhile, the C=O and O-H stretching vibration of GO decreased remarkably, indicating a simultaneous coordination of Fe3+ GO/EA-Fe on the surface of GO. The simultaneous coordination among Fe3+, EA and GO was further confirmed by the fact that the intensity of the characteristic diffraction peaks of EA and Fe(NO3)3was largely decreased in the XRD pattern of GO/EA-Fe


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(Figure S2). Additionally, the XPS spectra of GO-Fe and GO/EA-Fe were investigated and compared in Figures S3, S4 and Table S1-S3. Compared to EA-Fe, the proportion of C-O increased while the binding energy of Fe 2p decreased in GO/EA-Fe, which might be ascribed to the EA participation in the coordination of GO/EA-Fe.31-33 Transmission electron microscopy (TEM) image in Figure S5a showed that GO/EA-Fe remained a sheet structure as that of GO.34 After a heat-treatment under N2 atmosphere, GO/EA-Fe was successfully transformed in Fe-carbon nanocomposites with a conversion yield of~ 42 % (TGA, Figure S5b).TEM images of GEF-900 in Figure 2a and b clearly showed that core-shell NPs with overall 20-50 nm size were anchored on the graphene sheet. High-resolution TEM image (Figure 2c) illustrated an individual core-shell NP. The continuous lattice distance from the core to the shell at 0.203, 0.204 and 0.337 nm were well consistent with the d-spacing of Fe (110), Fe3C (220) and graphite (002), indicating that a predominantly Fe core was shielded within a Fe3C-C shell. Scanning transmission electron microscopy (STEM) with elemental mapping (Figure 2d-g) further confirmed a Fe@Fe3C-C core-shell NP was anchored on graphene. The loading amount of Fe in GEF-900 was estimated 6.41wt% as determined by ICP-AES. In Figure 3a, the XRD pattern of GEF-900 showed the Fe peaks at 44.7, 65.0 and 82.3° corresponding to Fe (110), Fe (200), and Fe (211), respectively and the characteristic peaks of iron carbide at37.63, 43.74, 44.99, 48.58 and 49.11° corresponding to Fe3C(121), Fe3C (102), Fe3C (031), Fe3C(131) and Fe3C (221), respectively. The diffraction peak of GO at 10.4° (Figure S6a) completely disappeared



in GEF-900 as a result of GO fully reduced into graphene.35 At the same time, a 26.4° peak of graphite (002) and d-spacing of 0.337 nm appeared, which corresponded to a typical van der Waals distance of graphitic layers. Raman spectra in Figure S7 also supported the conclusions from XRD measurements. Specifically, G-band at 1580 cm-1 in GEF-900 was left-shifted compared to 1594 cm-1 in GO (Figure S6b), indicating a recovery of the hexagonal network of carbon atoms with defects. The intensity ratio of ID/IG significantly decreased from 0.98 (GO) to 0.78 (GEF-900). Furthermore, a sharp 2D band peak was observed in GEF-900, in which high quality multilayer graphene was formed. The peak intensity of S3 at ~2910 cm−1 showed negligible change with that of GO, attributing to charge transfer caused by nitrogen doping in GEF-900.36 Two controlled experiments were carried out to study the formation mechanism of GEF. In one experiment, GF-900 was synthesized using the same procedure in the absence of EA. GF-900 possessed nanosheet morphology while Fe-based NPs without carbon coating were presented on the graphene sheet (Figure S8a and b). The characteristic diffraction peaks of Fe and Fe3C along with a 21.08° peak (d-spacing of 0.421 nm) were illustrated in the XRD pattern of GF-900 (Figure 3a). The peak at 21.08° of GF-900 showed a dramatic shift to a lower and broader 2θ angle than GEF-900, suggesting that GF-900 was disorderly two-dimensional sheets with larger interlayer spacing. Corresponding Raman spectra (Figure S7) exhibited a larger ID/IG ratio and a weaker 2D peak than that of GEF-900. In the other experiment, EF-900 was synthesized from EA-Fe without the presence of GO. TEM images of EF-900


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(Figure S8c and d) exhibited a large amount of NPs with the diameter of 20-120 nm embedded in carbon materials, while typical peaks of Fe, Fe3C and Fe2C were observed in XRD pattern (Figure 3a). However, no obvious characteristic peak of graphitized carbon was observed in EF-900, mainly resulting from that the larger iron particles disorderedly dispersed within carbon matrix were not able to efficiently catalyze the graphitization. Raman spectra (Figure S7) showed that the ID/IG value of EF-900 was located between those of GF-900 and GEF-900. Hence, we envision that formation process of GEF as follows: GO and EA simultaneously coordinate and immobilize Fe3+. EA intervention and the accompanied steric hindrance reduce the disordered graphene stacking so that a sandwich-like structure was assembled as shown in Figure 1. After a heat-treatment under inert atmosphere, intermediated Fe3+ was in-situ reduced and at the same time, EA is converted into carbon and embraces the reduced Fe to form core-shell NPs. Fe further catalyzes carbon into graphite as well as the formation of Fe3C. TEM images and XRD patterns of GO/EA-Fe carbonized at different temperatures were illustrated in Figure 2 and Figure S9. Fewer NPs in the absence of core-shell structure was observed in GEF-700 while XRD pattern indicated characteristic peaks indexed to Fe2.5C and Fe. GEF-800 showed a bunch of core-shell NPs with the diameter in the range of 18-30 nm (Figure 2j). HRTEM image in Figure 2k disclosed that the core-shell NPs in GEF-800 consisted of crystallized Fe encased with a thin graphite layer. These Fe@C core-shell NPs further converted into Fe@Fe3C-C when the temperature was elevated to 900 °C as mentioned above. In GEF-1000,



[email protected] core-shell NPs increased in size as a result of aggregation of iron precursors at high temperature as shown in Fig. 2l-m. It was worth noted that as the temperature changed from 700 to 900 °C, a broad peak at 21.1° in the reduced graphite oxide shifted to a sharp peak of 26.4°,confirming that GO was fully reduced and graphene was in ordered arrangement at 900 °C. However, when the temperature further rose to 1000℃, the degree of graphitization of GEF-1000 decreased, which might be ascribed to the excessive reaction of iron and carbon at extreme high temperature.37Similarly, the intensity ratio of ID/IG (Figure S10) significantly decreased from 1.38 (GEF-700), 0.97 (GEF-800) to 0.78 (GEF-900) and then increased to 1.16 (GEF-1000). The calculated Brunauer-Emmett-Teller (BET) surface areas of GEF-700, GEF-800, GEF-900 and GEF-1000 are 274, 375, 422 and 231 m2 g-1, respectively while both the micropore and the mesopore were observed in all GEFs (Figure S11). XPS survey of GEF-900 (Figure S12a) possessed the characteristic spectra of C1s (286.1 eV), N1s (402.9 eV), and Fe 2p (713.9 and 723.9 eV) and Fe0 (713.2 eV), confirming the successful doping of nitrogen and iron into the carbon. The high resolution C1s XPS spectrum (Figure S13b) of GEF-900 presented the characteristic peaks including C=C (284.46 eV), C-N (284.80 and 285.27 eV), C=O (287.34 eV) and O-C=O (290.07 eV).38,39 Deconvoluted N 1s peaks (Figure 3b) corresponding to graphitic N (400.68 eV) and pyridinic N (398.40 eV) indicated a successful N-doping from urea during pyrolysis.40,41 The Fe survey spectrum (Figure 3c) showed Fe (720.20, 707.10 eV), Fe3+2p1/2 (724.02 eV), and 2p3/2 (709.91 eV), suggesting the


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coexistence of pure Fe core and Fe3C shells.39 XPS surveys of GEF-800 (Figure S13) showed C, N, O and Fe in GEF-800 while the detailed comparison between GEF-800 and GEF-900 was summarized in Table S4 and S5. First, we systematically explored the ORR electrocatalytic properties of GF-900, EF-900 and GEF-900 in 0.1 M KOH solution using the RDE and RRDE techniques at room temperature. Cyclic voltammetry (CV) curves revealed apparent oxygen reduction peaks of all the samples. As shown in Figure S14a, the oxygen reduction peaks of GF-900, EF-900 and GEF-900 were located at 0.78, 0.81 and 0.98 V (vs. RHE), respectively, indicating that GEF-900 possessed higher ORR catalytic activity. To further evaluated the ORR activity of these samples and commercial Pt/C, rotating disk electrode (RDE) measurements were performed (Figure 4a). GEF-900 exhibited the onset potential and half-wave potential (E1/2) of 1.01 and 0.90 V with a limiting current density of approximately -5.97 mA cm-2 as shown in Figure 4b and c. These electrocatalytic performance parameters of GEF-900 were obviously better than that of GF-900 and EF-900. In additional, the ORR activity of nitrogen-doped graphene (NG-900) was also evaluated (Figures 4a and S14a). The performance of GEF-900 was significantly higher than NG-900, reflecting the positive effect of iron NPs in the ORR process. Notably, the onset potential and half-wave potential of GEF-900 were 70 and 60 mV higher even compared to the commercial Pt/C. LSV curves of the prepared GEF-900 electrode were obtained at different rotation speed (Figure 4d). Koutecky-Levich (K-L) plots suggested the first-order reaction kinetics toward dissolved O2concentration and the similar number of electron transfer at different



potentials for ORR.42 GEF-900 followed an almost 4e- transfer pathway toward catalyzing ORR according to the calculated results of K-L equation (Figure 4e). Further studied on ORR mechanism of GEF-900 catalyst by using rotating ring-disk electrode (RRDE) technique, the low percentage (

Graphite Wrapped Fe Core-shell Nanoparticles Anchored on

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