III. Robust fused aromatic pyrazine-based two-dimensional network for stably cocooning iron
3.3 Results and Discussion
As shown in the Figure 3.1, the FA-PON consists of uniformly distributed holes. The hole-to-hole distance in the structure is 0.994 nm. Each hole contains six aromatic nitrogen atoms facing center. The
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chemical formula of the repeating unit is C24N6, and hence is designated as C4N1. The C4N1 structure was synthesized by the polycondensation between triphenylene hexamine (TPH) and hexaketocyclohexanone (HKH) octahydrate in N-methyl-2-pyrrolidone (NMP) in the presence of a few drops of sulfuric acid (H2SO4) as an acid catalyst. The energy gain due to the formation of the fused aromatic pyrazine ring (approximately -89.7 kcal mol-1)31 induces a spontaneous reaction between TPH and HKH, resulting in a layered two-dimensional (2D) fused aromatic porous organic network (FA- PON). The black product was Soxhlet extracted with water and methanol for three days each to completely remove any impurities and freeze-dried at −120 °C under reduced pressure (0.05 mmHg) for three days.
Figure 3.1. Schematic representation of the synthesis of C4N. The dotted line shows the extension of the periodic structure into two-dimensional space.
To analyze the long-range ordering of the C4N1 structure, theoretical simulations and powder x-ray diffraction (PXRD) experiments were performed. To clarify the lattice packing, lattice modeling and Pawley refinement of the C4N1 were conducted using material studio software, and two possible structures, of eclipsed (AA) and staggered (AB) stacking, were modeled. As shown in Figure 3.2a, C4N1 shows three intense peaks at 7.80° (100), 26.40° (101) and 45.60° (141). The experimental PXRD matches the simulated pattern of the AA stacking structure after Pawley refinement. Pawley refinement showed unit cell parameters of a = b = 13.06 Å, c = 3.49 Å, α =β = 90 and γ = 120 ο with refinement results of Rp = 4.40 % and Rwp = 7.72%. The difference between the experimental and refined pattern shows that they match each other very well (Figure 3.2a, b).
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The chemical structure of the resultant C4N1 was further investigated by solid-state 13C cross- polarization magic-angle-spinning (CP-MAS) NMR spectroscopy. The solid-state CP-MAS 13C NMR spectrum revealed four major peaks with chemical shifts of 109.9, 124.7, 132.1 and 140.7 ppm (Figure 3.2c), which are, respectively, assigned to aromatic carbon atoms (a, b, c, d) in the phenyl and pyrazine rings. The minor peak at 176 ppm is associated with the edge carbonyl carbon (C=O) in the C4N1
structure40. These results indicate the formation of a layered 2D C4N1 structure. Thermogravimetric analysis (TGA) was performed to investigate the stability of the C4N1. The C4N1 framework was stable up to 450 °C in air (Figure 3.2d). The result suggests that good thermo-oxidative stability is because the aromatic rings are fused, except for the edge termini.
Figure 3.2. Structural characterization of C4N1. (a) Power XRD pattern of C4N1. The dark red curve is an experimental result, the purple curve represents a Pawley-refined pattern and the pink curve is the difference between the experimental and refined curve. (b) Simulated structure of C4N, showing 9 unit cells in a box with eclipsed (AA) stacking structure. (c) Solid state CP-MAS 13C NMR (the * indicates side band peaks). (d) TGA curves obtained with a ramping rate of 10 °C min−1 in air and nitrogen atmospheres.
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The chemical composition of the C4N1 was investigated by X-ray photon spectroscopy (XPS), showing only C 1s, N 1s and O 1s peaks (Figure 3.3a). This result agrees well with the elemental analysis (EA, Table 3.1). The C 1s peak can be deconvoluted into three peaks at 284 (C-C sp2), 285 (C- N sp2) and 286 eV (C-Heteroatom) (Figure 3.3b), which correspond to C=C, C=N and C-heteroatom bonding in the structure. The deconvoluted N 1s peak shows two main peaks centered at 398.2 and 399.8 eV, which can be attributed to aromatic (sp2) pyrazine (C=N) and edge primary amine (C-NH2) nitrogen atoms, respectively (inset, Figure 3.3c). The oxygen peak can be deconvoluted into two peaks at 531.8 and 533.08 eV, which are, respectively, associated with mostly edge carbonyl (C=O) oxygen and physically adsorbed moisture (Figure 3.3d).
Figure 3.3. XPS analysis of C4N. (a) XPS survey spectrum of C4N. (b) deconvoluted C 1s peak of C4N.
(c) deconvoluted N 1s peak of C4N. (D) deconvoluted O 1s peak of C4N.
Elemental analysis (EA) found that the C, H, N and O contents were 66.57 (C), 3.81 (H), 19.52 (N) and 10.20 wt% (O), respectively. The values are in close agreements with the theoretical ones (Table
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3.1). The microscopic morphology of the C4N1 was investigated with field-emission scanning electron microscopy (FE-SEM). The SEM image shows a layered morphology with smooth and large grain sizes of a few tens of microns (Figure 3.4a). The transmission electron microscopy (TEM) image shows a transparent thin layered morphology (Figure 3.4b).
Table 3.1. Elemental composition of C4N network from different analytical techniques
Technique C H N O Total
Theoretical (wt%) 69.56 2.43 20.28 7.72 100
EA (wt%) 66.57 3.81 19.52 10.20 100
XPS (at%)* 76.67 --- 13.80 8.58 100
SEM (at%)* 71.22 --- 19.95 8.83 100
SEM (wt%)* 67.03 --- 21.90 11.07 100
* These techniques are known to be more sensitive to the surface chemical composition of sample due to their beam depth.
Figure 3.4. The microscopic morphology of C4N. (a) FE-SEM image and (b) TEM image.
An indirect-contact oxygen reduction reaction (ORR) electrocatalyst comprised of Fe/Fe3C nanoparticles encapsulated in nitrogen-doped graphitic layers was designed and synthesized by polycondensation between TPA and HKH in the presence of iron chloride (FeCl3). FeCl3 not only works as a Lewis acid catalyst for the polycondensation (aromatization) between TPA and HKH, but also as
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a Fe precursor for the formation of the Fe/Fe3C nanoparticles. The Fe/Fe3C nanoparticles were formed by subsequent chemical reduction with sodium borohydride (NaBH4) and thermal annealing at 800 °C under argon for 3 h. Heat-treatment was optimized to convert the nanoparticles into crystalline Fe/Fe3C nanoparticles, as well as to catalyze the conversion of C4N1 into well-ordered nitrogenated (N-doped) graphitic layers on the surface of the Fe/Fe3C nanoparticles. As a result, the process leads to Fe/Fe3C nanoparticles encased in N-doped graphitic C4N1 layers, designated Fe/Fe3C@C4N1.
The resulting C4N1 framework is a porous 2D layered structure consisting of six nitrogen atoms pointing toward the center of each hole. As such, the C4N1 matrix plays a pivotal role by providing rich- coordination sites to anchor the iron (Fe3+) precursor between layers and prevent the leaching of Fe3+
ions during work-up procedures. After chemical and thermal reduction, the resultant Fe/Fe3C@C4N1
catalyst (black colored) shows a strong magnetic response indicating the formation of Fe/Fe3C nanoparticles (Figure 3.5).
Figure 3.5. Schematic illustration of the structural development of the Fe/Fe3C@C4N1 catalyst, showing the formation of Fe3O4@C4N1 by reduction of FeCl3 with NaBH4 and the subsequent annealing of Fe3O4@C4N1 into Fe/Fe3C@C4N1. Inside view of the structure shows that the Fe/Fe3C@C4N1 catalyst consists of Fe nanoparticle cores encased in well-ordered nitrogenated graphitic shells, which are evenly distributed in the C4N1 matrix.
To investigate the morphology of the Fe/Fe3C@C4N1 catalyst, FE-SEM and TEM analyses were performed. The TEM images of the Fe/Fe3C@C4N1 catalyst showed Fe/Fe3C nanoparticle cores in N- doped graphitic shells were well distributed in the C4N1 matrix (Figure 3.6).
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Figure. 3.6. Low resolution TEM image of Fe/Fe3C@C4N catalyst, showing Fe/Fe3C nanoparticles encased in graphitic layers and uniformly distributed in C4N matrix.
To get more insight into the structure, atomic-resolution TEM (AR-TEM) was also conducted. The result revealed that the Fe/Fe3C nanoparticles were well encapsulated within the graphitic layers (Figure 3.7a-d, Figure 3.8).
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Figure 3.7. Characterization of the Fe/Fe3C@C4N1 electrocatalyst. (a) AR-TEM image of catalyst at low magnification. Scale bar: 100 nm. (b, c) AR-TEM image at high magnification, showing lattice pattern. Inset is the SAED pattern from the square in (c). Scale bar: 5 nm. (d) High-angle annular dark- field (HAADF) scanning TEM (STEM) image and corresponding element mappings of carbon, nitrogen and iron. Scale bar: 10 nm.
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Figure 3.8. (a) Atomic-resolution TEM (AR-TEM) image at low magnification. Scale bar: 100 nm.
(b,c). High magnification (AR-TEM) image showing the encasulation of Fe/Fe3C nanoparticles inside the nitrogenated graphatic shells. Inset is fast fourier transform (FFT) of Fe3C nanoparticle core. Scale bar: 5 nm. (d) High-angle annular dark-field (HAADF) scanning TEM (STEM) image and EDS element mappings of carbon, nitrogen and iron. Scale bar: 10 nm.
The encapsulating graphitic layers are highly crystalline, showing clear stripes. The interlayer spacing (0.34 nm) corresponds to the [002] diffraction plane of the graphitic carbon layers. The lattice fringes of some of the nanoparticles show crystalline structure, assignable to orthorhombic Fe3C and cubic Fe. The spacing of the crystal lattice is 0.204 nm, which can be attributed to the {110} plane of Fe viewed along the <001> zone axis (Figure 3.7c). For Fe3C, the lattice spacing is 0.204 nm, which is
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assigned to the {111} plane of Fe3C (Figure 3.8a-c) viewed along the <132> zone axis. High-angle annular dark-field (HAADF) scanning TEM (HAADF-STEM) and elemental mapping images (Figure 3.7d) of the Fe/F3C@C4N1 catalyst confirmed the presence of C, N and Fe in the nanoparticles and its uniform distribution over the selected area (Figure 3.8d). The presence of C and N around the nanoparticle in the elemental mapping further confirms the perfect encapsulation of the Fe/Fe3C nanoparticles (Figure 3.7d).
The structure of the Fe/Fe3C@C4N1 catalyst was further characterized with PXRD (Figure 3.9a). A strong peak at 26.13°, corresponding to the (002) facet of the graphitic layers was observed. The peaks at 37.74, 39.79, 40.63, 44.99, 45.86, 49.15, 51.81, 54.39, 57.99, 70.82, 77.90 and 86.17° are related to the characteristic diffraction of orthorhombic Fe3C (JCPDS no. 35-0772). The diffraction peaks at 44.7, 65.0 and 82.3° can be indexed to cubic Fe. Considering the TEM results, the d-spacing of 0.220 and 0.209 nm is in good agreement with the d-spacing of the (110) and (111) planes of Fe and Fe3C (Figure 3.7c, 3.8). The above results confirm the co-existence of Fe/Fe3C nanoparticles and graphitic carbon layers.
Next, XPS analysis was performed to assess the bonding nature and elemental composition. The XPS survey spectrum of the Fe/Fe3C@C4N1 catalyst showed the presence of C, N, O and Fe (Figure 3.9b). The XPS survey spectrum showed a relatively weak Fe signal, and the Fe content was calculated to be 0.56 at% (2.54 wt%). Compared to the TGA results (vide infra), such a low Fe content suggests that most of the Fe/Fe3C nanoparticles are concealed deep within the graphitic layers. The deconvoluted C 1s spectrum can be resolved into four major peaks at 284.46 (C=C), 285.93 (C-N), 286.55 (C-O) and 288.08 eV (C=O) (Figure 3.9c). The N 1s spectrum of nitrogen can be further deconvoluted into two peaks at 398.5 and 400.7 eV (Figure 3.9d), which correspond to aromatic and graphitic nitrogen, respectively.
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Figure 3.9. (a) Powder XRD patterns of Fe/Fe3C@C4N1 catalyst and Fe, Fe3C references. (b) XPS survey spectrum of Fe/Fe3C@C4N catalyst, displaying C 1s, N 1s, O 1s and Fe 2p peaks. (c) high resolution C1s XPS spectrum of Fe/Fe3C@C4N. (d) deconvoluted N1s peak of Fe/Fe3C@C4N.
The O 1s spectrum of oxygen can be resolved into two peaks at 532.38 and 533.58 eV (Figure 3.10a), which are related to C=O and moisture, respectively. The Fe 2p spectrum of the Fe/Fe3C@C4N1
catalyst showed two major peaks at 706.68 and 712.68 eV corresponding to Fe0 and Fe3+ respectively (Figure 3.10b), confirming the coexistence of Fe/Fe3C in the material. The ratio of Fe/Fe3C was found to be 1/1.4 from the XPS.
To investigate the quantitative content of Fe, thermogravimetric analysis (TGA) was also conducted (Figure 3.10c). The TGA curve of the Fe/Fe3C@C4N1 catalyst recorded in air atmosphere showed a residual amount of 9.60 wt% above 800 °C. The residual amount can be attributed to Fe3O4 formation after TGA in air condition, and hence the Fe contribution is 6.9 wt%.
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Figure 3.10. (a) deconvoluted N1s peak of Fe/Fe3C@C4N. (b) deconvoluted Fe2p peak of Fe/Fe3C@C4N. Deconvulated spectra of Fe/Fe3C@C4N catalyst. (c) TGA thermogram of Fe/Fe3C@C4N1 catalyst obtained under air atmosphere at a ramping rate of 10 °C min−1.
Given the structural information about the Fe/Fe3C@C4N catalyst, its ORR activity was evaluated in both acidic and alkaline media. For comparison, other samples and commercial Pt/C were also evaluated under identical conditions. Four samples of Fe/Fe3C@C4N catalyst were prepared at different annealing temperatures to investigate the effect of temperature on their catalytic activity. Among them, Fe/Fe3C@C4N catalyst Fe/Fe3C@C4N1 catalyst obtained at 800 °C showed the highest catalytic activity (Figure 3.11a-d, 3.12a).
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Figure 3.11. (a) CV curves of Pt/C and Fe/Fe3C@C4N samples prepared at different annealing temperatures. (b) Polarization curves for Fe/Fe3C@C4N catalysts at different annealing temperatures in O2-saturated 0.1 M aq KOH solution. (c) Tafel plots for Fe/Fe3C@C4N catalysts at different annealing temperatures in O2-saturated 0.1 M aq. KOH solution. (d) Electron transfer numbers (n) of Fe/Fe3C@C4N catalysts at different annealing temperatures in the potential range of 0.5-0.7 V (vs. RHE) in O2-saturated 0.1 M aq. KOH solution.
As seen in Figure 3.12b, the Fe/Fe3C@C4N1 catalyst exhibited the highest ORR performance with a half-wave potential of 0.884 V, even higher than that of commercial Pt/C catalyst (38 mV) and other Pt free catalysts (Table 3.2). To further evaluate the catalytic activity for ORR, Tafel plots were also derived from the polarized curves (from Polarization curve, Figure 3.12b). The Tafel slope of the Fe/Fe3C@C4N catalyst was smaller (92 mV dec−1) than that of commercial Pt/C (96 mV dec−1) (Figure 3.12c), indicating its excellent catalytic activity. To better understand the ORR mechanism of the catalyst, the electron transferred number (n) was investigated using a rotating ring-disk electrode (RRDE) experiment. The electron transferred number was calculated to be 3.92-3.98 in the potential
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range of 0.5-0.7 V (Figure 3.12d), demonstrating that the Fe/Fe3C@C4N1 catalyst exhibits a more efficient four electron ORR pathway compared to that for Pt/C (3.91-3.93).
Figure 3.12. (a) Half wave potential of Fe/Fe3C@C4N catalysts at different annealing temperatures in O2-saturated 0.1 M aq. KOH solution. (b) Polarization curves for Fe/Fe3C@C4N catalysts and Pt/C in O2-saturated 0.1 M aq KOH solution. (c) Tofel plot for Fe/Fe3C@C4N catalysts and Pt/C in O2-saturated 0.1 M aq KOH solution. (d) Electron transfer numbers (n) of Fe/Fe3C@C4N catalysts at different annealing temperatures in the potential range of 0.5-0.7 V (vs. RHE).
Table 3.2. Comparison of recently reported non-precious metal catalysts for the ORR in alkaline media (0.1 or 1 M KOH, rotation speed; 1,600 r.p.m.)
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Methanol crossover is an important issue in direct methanol fuel cells (DMFCs), where the penetration of methanol through the proton membrane damages the cathode catalyst. To evaluate this tendency, the ORR activity of the Fe/Fe3C@C4N1 catalyst and commercial Pt/C were assessed in the presence of methanol in O2-saturated alkaline medium. The cathodic current did not fluctuate much for the Fe/Fe3C@C4N1 catalyst, while that of the commercial Pt/C significant declined (Figure 3.13a).
These results further verified that the Fe/Fe3C@C4N1 catalyst is more suitable than commercial Pt/C in DMFCs.
For practical application, the durability of a catalyst is vital. To investigate the durability of the Fe/Fe3C@C4N1 catalyst, chronoamperometric measurements were conducted in O2-saturated alkaline medium and the stabilities of the samples before and after 10,000 cycles were recorded. The normalized current-time curve of the Fe/Fe3C@C4N1 catalyst revealed 92% current retention after 10,000 cycles, while commercial Pt/C retained 80% current (Figure 3.13b). These results show that the superior activity and better durability of the Fe/Fe3C@C4N1 catalyst compared to commercial Pt/C can be
Catalysts
Electrolyt e (M)
E1/2
(V vs RHE)
Onset potential (V vs RHE)
References
Fe@C4N 0.1 0.884 1.017 This work
Pt/C 0.1 0.844 1.021 This work
Co1-xS/RGO 0.1 0.87 Angew Chem Int Edit 2011,
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Fe−N/C−800 0.1 0.81 0.92 J Am Chem Soc 2014,
136(31): 11027-11033.
Co0.50Mo0.50
OyNz/C 0.1 0.76 0.92 Angew Chem Int Ed Engl
2013, 52(41): 10753-10757.
Fe−N4/C 0.1 0.87 J Am Chem Soc 2013,
135(41): 15443-15449.
N-CG–CoO 1 0.81 0.90 Energ Environ Sci 2014,
7(2): 609-616.
N–Fe–
CNT/CNP 0.1 0.87 Nat Commun 2013, 4.
FePhen@M
OF-ArNH3 0.1 0.86 1.03 Nat. Commun. 2015, 6,
7343.
NT-G 0.1 0.87 1.05 Nat. Nanotechnol. 2012, 7,
394.
Co3O4/N-
rmGO 0.1 0.83 Nat. Mater. 2011, 10, 780.
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attributed to the synergetic effects of the polar conductive C4N1 matrix and stably cocooned Fe/Fe3C nanoparticles in the N-doped graphitic layers. The former provides efficient oxygen diffusion and electron pathways. The latter is due to the structure of the indirect-contact catalyst, which provides catalytic active sites on the stable surface of the N-doped graphitic layers rather than the unstable surface of the Fe/Fe3C nanoparticles, allowing efficient electron tunneling through the graphitic layers.41
Electrocatalytic performance was also investigated in an acidic medium (0.1 M aq. HClO4 solution), and the onset and half wave potential were more negative and larger, respectively (Figure 3.13c). The limited current density of the Fe/Fe3C@C4N1 catalyst was higher than that of Pt/C (Figure 3.14d).
These results indicate that the Fe/Fe3C@C4N1 catalyst also works in acidic medium. Although the overall catalytic performance for ORR was inferior to commercial Pt/C, the Fe/Fe3C@C4N1 catalyst is one of the rare indirect-contact catalysts that displays appreciable activity with stability in an acidic medium.
Figure 3.13. (a) Amperometric i-t curves of Fe/Fe3C@C4N1 and Pt/C in methanol tolerance tests. (b) Current variation with respect to cycle number for the Fe/Fe3C@C4N (800 °C) and Pt/C catalysts in O2-
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saturated 0.1 M aq. KOH solution. (c) Polarization curves of Fe/Fe3C@C4N (800 °C) and Pt/C catalysts in O2-saturated 0.1 M aq. HClO4 solution. (d) Tafel plots of Fe/Fe3C@C4N (800 °C) and Pt/C catalysts in O2-saturated 0.1 m aq HClO4 solution.
To test its potential practicability, its full cell performance was evaluated using hybrid Li-air and Zn- air batteries (Figure 3.14a, b). Figure 2.14a shows the initial discharge-charge profiles of a hybrid Li- air battery using the Fe/Fe3C@C4N1 catalyst and a mixture of state-of-the-art catalysts (Pt/C+IrO2) at the current range from 0.5 to 2.0 A g-1. For the Fe/Fe3C@C4N1 catalyst, the obtained discharge plateaus were at the same potential as the Pt/C+IrO2 catalyst over the entire current range, revealing its exceptional ORR properties. At the same time, the Fe/Fe3C@C4N1 catalyst exhibited superior oxygen evolution reaction (OER) activities during the charging process. The charging plateaus were found to be similar to the benchmark Pt/C+IrO2 catalyst, verifying its efficient OER activities.
Cyclic discharge-charge performance was evaluated at a current density of 0.5 A g-1 to demonstrate reversibility and reproducibility (Figure 2.14c). Cyclic performance was highly reproducible and obtained in invariant tendency for 100 h, indicating the Fe/Fe3C@C4N1 electrode was highly durable without clogging or damage during the repeated redox process.
Zn-air cells were also fabricated using the Fe/Fe3C@C4N1 catalyst-based air electrodes. Figure 3.14b presents the polarization and power density curves of Zn–air cells using Fe/Fe3C@C4N1 and Pt/C+IrO2 catalysts. The open-circuit voltages (OCVs) were determined to be 1.46 and 1.45 V for the Fe/Fe3C@C4N1 and Pt/C+IrO2,respectively, close to the theoretical value of Zn-air cells. The peak power density achieved by the Fe/Fe3C@C4N1 was 109 mW cm-2, showing even better performance than that of Pt/C+IrO2 (108 mW cm-2).
The cyclic performance of the rechargeable Zn-air battery with Fe/Fe3C@C4N1 was also evaluated (Figure 3.14d). As similarly confirmed by the Li-air cell test, Fe/Fe3C@C4N1 exhibited identical performance to that of the mixed state-of-the-art catalyst (Pt/C+IrO2) at 10 mA cm-2 for 20 h, confirming its high stability and excellent oxygen-related electrochemical redox activities. Based on these results, the Fe/Fe3C@C4N1 catalyst can be considered an efficient bifunctional catalyst with performance comparable to the benchmark Pt/C+IrO2 in the practical applications of hybrid Li and Zn-air batteries.
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Figure 3.14. Full-cell performance of Fe/Fe3C@C4N1. (a) Discharge-charge curves of Fe/Fe3C@C4N1
and Pt/C in 0.5 M aq. LiOH with 1.0 M aq. LiNO3 solution with respect to various current densities of 0.5 A g-1. (b) Polarization and power density curves of Zn-air cell using the Fe/Fe3C@C4N1 catalyst and Pt/C in 6 M aq. KOH as an electrolyte. (c) Cyclic performance of three-electrode hybrid Li-air cells using the Fe/Fe3C@C4N1 catalyst and Pt/C. (d) Discharge profiles of Zn-air batteries for Fe/Fe3C@C4N1
and Pt/C at a current density of 10 mA cm-2.