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Article

Core-Shell Fe3O4@NCS-Mn Derived from Chitosan-Schiff Based Mn Complex with Enhanced Catalytic Activity for Oxygen Reduction Reaction

by
Jinhui Tong
1,*,
Yuliang Li
1,
Lili Bo
2,
Wenhui Wang
1,
Tao Li
1 and
Qi Zhang
1
1
Key Laboratory of Eco-Environmental Polymer Materials of Gansu Province, Key Laboratory of Eco-Functional Polymer Materials of the Ministry of Education, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China
2
College of Science, Gansu Agricultural University, Lanzhou 730070, China
*
Author to whom correspondence should be addressed.
Catalysts 2019, 9(8), 692; https://0-doi-org.brum.beds.ac.uk/10.3390/catal9080692
Submission received: 25 July 2019 / Revised: 11 August 2019 / Accepted: 13 August 2019 / Published: 15 August 2019
(This article belongs to the Section Electrocatalysis)
Browse Figures
Versions Notes

Abstract

:
A core-shell type of Fe3O4/NCS-Mn composite was prepared by pyrolyzing a precursor fabricated by coating a chitosan-Schiff base Mn complex on Fe3O4 cores. For comparison purposes, the Fe3O4@NCS sample in the absence of Mn and the Fe3O4@NC sample derived from just chitosan coating Fe3O4 were also prepared. Among the three catalysts, Fe3O4@NCS-Mn demonstrates the best electrocatalytic activity compared to commercial Pt/C (20%) for oxygen reduction reaction (ORR). The average of the transferred electron number (n) approached 3.6 in the range of −0.3 to −0.8 V (vs. Ag/AgCl). Moreover, the catalyst exhibited high stability and durability against methanol and may potentially be a promising ORR catalyst for fuel cells.

1. Introduction

Based on their paramount importance in electrochemical energy conversion and storage devices, it is essential to develop electrocatalysts with high efficiencies that minimize the overpotential in the oxygen reduction reaction (ORR) [1,2]. Although it is known to be efficient, the application of platinum-based catalysts is limited by scarcity, high cost, and poor durability against methanol [3]. Therefore, efforts have been made to develop Pt alternative catalysts for ORR. In alkaline media, electrocatalysts with various categories have been developed, further, the corresponding mechanisms have been explored, including nonmetal-doped carbon materials [4,5,6,7,8], carbon-transition metal hybrids [9,10,11,12,13,14,15,16,17,18,19,20], metal organic framework-modified nitrogen-doped graphene [21,22,23], and transition metal oxides [24,25,26,27,28]. Nonmetal heteroatom doped carbon materials and transition metal oxides have especially been given attention due to their advantages of high electron conductivity and favorable redox reversibility, respectively [29,30].
In our previous work, we developed several non-metallic heteroatom-doped carbon catalysts, including N-doped carbon spheres [31], porous N-doped carbon/carbon nanotube [32], N, S-doped graphitic carbon [33], and N, P dual-doped graphitic biocarbons [34]. The catalysts demonstrated comparable ORR activities and higher durability against methanol, especially when compared with commercial Pt/C (20%) in alkaline media [31,32,33,34]. Moreover, spinel transition metal oxides, such as Fe3O4 and their hybrids with nanocarbon materials, exhibited excellent catalytic activities in ORR [35,36,37]. While the spinel structure often provides two or more catalyst surfaces, it makes possible for the oxygen spill over reaction couple with the reaction path of ORR and therefore enhances the ORR catalytic activity [38].
Independent of the materials mentioned above, combining the virtues of the aforementioned various materials, we designed a core-shell structure of Fe3O4 cores coated with N-doped carbon-Mn shells derived from a chitosan Schiff-base Mn complex coating on Fe3O4 cores. In the present study, we expected an enhanced compositional homogeneity, catalytic activity, chemical stability, and methanol durability for ORR.

2. Results and Discussions

2.1. Catalysts Characterization

The X-ray diffraction (XRD) peaks at 2θ ≈ 30.1, 35.5, 44.6, 47.7, and 56.9° can be attributed to (220), (311), (200), (−210), and (333) plane of Fe3O4 (JCPDS No. 79-0418), respectively. The peak at 2θ ≈ 62.7, 65.0, and 75.6° can be attributed to the diffraction of Fe2C (JCPDS No. 03-1022). The diffraction peaks at 2θ ≈ 41.3, 60.1, and 71.7° can be attributed to (201), (311), and (123) plane of Fe2.7Mn0.3C (JCPDS No. 73-1341), respectively. The results confirm that Fe2C and Fe2.7Mn0.3C is formed by the reaction of Fe3O4 with the chitosan Schiff-base Mn (II) complex during the pyrolysis process. It is also clearly shown that the as-prepared catalysts are dominated by Fe3O4 (Figure 1).
The images of transmission electron microscope (TEM) in Figure 2 indicate a rather similar morphology between Fe3O4@NC and Fe3O4@NCS-Mn, with a size between 13–27 nm for the cores of Fe3O4 embedded in carbon shells. There were also certain aggregations of the particles of Fe3O4 likely caused by their magnetism. In Figure 3, scan electron microscope (SEM) image reveal a porous structure for the sample Fe3O4@NCS-Mn, which was potentially formed in the pyrolysis process due to the release of a large amount of gases during the carbonation of chitosan Schiff-base. The corresponding energy dispersive X-ray spectroscopy (EDS) images (based on the region of the yellow squared area of the SEM image) indicate that C, N, O, Mn, and Fe elements were evenly distributed in the sample.
The specific surface areas and pore size distributions were investigated using N2 adsorption/desorption experiments for the three samples. As depicted in Figure 4, the samples exhibit type-II isotherms with H4 hysteresis loop. This indicated the coexistence of slit-like pores as well as irregular mesoporous structure. The Brunauer-Emmett-Teller (BET) surface area/pore volume were 87/0.11, 214/0.28, and 143/0.17 m2·g−1/cm3·g−1 for Fe3O4@NC, Fe3O4@NCS, and Fe3O4@NCS-Mn, respectively (Table 1). Furthermore, all samples were dominated by the mesopores, which account for higher than 80% of BET surface areas and 71% for the samples’ pore volumes (Table 1).

2.2. Electrochemical Tests

The catalytic activities of the as-prepared catalysts for ORR were tested in O2 saturated 0.1 M KOH. The commercial Pt/C (20 wt%) was used as a control catalyst. The linear sweep voltammetry (LSV) plots of different catalysts for ORR are displayed in Figure 5 and the characteristic parameters for ORR activity evaluation are listed in Table 2. As can be seen, the Fe3O4@NCS-Mn catalyst showed the highest onset potential and limiting current density of −0.02 V (vs. AgCl) and 3.8 mA/cm2, which is almost the same with the corresponding ones of commercial Pt/C (20%). The catalyst also exhibited the most positive half-wave potential of −0.205 V (vs. AgCl), which is only 66 mV more negative than that of Pt/C (20%). The results confirm that the addition of Mn can enhance the catalytic activity.
The kinetics of ORR that employed Fe3O4@NCS-Mn as a catalyst were further investigated. The LSV curves at different rotation speeds are shown in Figure 6. As can be seen, the limiting current density increased with the increase in the rotating speeds due to the enhanced oxygen flux to the electrode surface [38]. Figure 7a shows the plots of j−1 versus ω−1/2 at different potentials in the range of −0.3~−0.55 V. A great linear correlation between j−1 and ω−1/2 confirms that the reaction is first-order with regard to the concentration of dissolved oxygen. Based on the Koutecky-Levich (K-L) equation, Figure 7b presents the transferred electron number (n) in ORR under different potentials. The average n was found to be 3.6 in the potential range of −0.30 V to −0.60 V, which indicates an approximate 4e− ORR process. This is considered to be the most efficient ORR pathway.
In methanol-based fuel cells, one important feature for ORR catalyst is its durability and tolerance toward methanol. The durability of Fe3O4@NCS-Mn against methanol was tested using cyclic voltammetry (CV) measurements. Figure 8a indicates that there was no significant change in the CV curve upon the addition of 3 M CH3OH to O2 saturated 0.1 M KOH, except for a slight decrease in reduction current and peak potential. Within the entire scanning range, there was no observable current for the oxidation of methanol. Comparatively, CV curve for Pt/C (20%) electrode shows a peak identified as the oxidation current of methanol, and the O2 reduction peak completely disappeared under the same conditions (Figure 8b). Therefore, Fe3O4@NCS-Mn is a more selective ORR catalyst than Pt/C (20%) with much stronger durability against methanol.
The traditional ORR catalysts have been challenged in their stabilities, which is another important feature for ORR catalyst with great quality. The relative current was measured for both Fe3O4@NCS-Mn and commercial Pt/C (20%) at −0.3 V (vs. Ag/AgCl) in an O2 saturated 0.1 M KOH solution. Figure 9 indicates that the relative current of Fe3O4@NCS-Mn shows a decay of 53.2%, whereas the Pt/C (20%) catalyst exhibits a decay of 58.5% after a 36,000 s chronoamperometric test. Therefore, Fe3O4@NCS-Mn is more stable than the Pt/C (20%) catalyst in the alkaline medium.

3. Experimental Methods

3.1. Materials and Instruments

Iron(II) chloride tetrahydrate, chitosan, salicylaldehyde and manganese nitrate (50 wt% aqueous solution) were supplied by Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Pt/C (20 wt% Pt on carbon black) and Nafion (5 wt%) were supplied by Alfa Aesar (Haverhill, MA, USA). All reagents were of analytical grade and were used as received. A field-emission scanning electron microscope (JSM-6701F, FEOL, Japan), a transmission electron microscope (JEM-2010, Japan), a XRD-6000 diffractometer with Cu Kα radiation (λ = 1.54178 Å) (Shimadzu, Japan) and an ASAP2020 Micromeritics Instrument (TriStar II, USA) were used for characterization of the catalysts. A 760E electrochemical workstation (CH Instruments, Shanghai, China) was used for electrochemistry tests.

3.2. Preparation of the Catalysts

The Fe3O4@chitosan Schiff-base Mn (II) complex was prepared as described in our previous work [39]. The catalyst Core-shell Fe3O4@NCS-Mn was prepared by pyrolyzing Fe3O4@chitosan Schiff-base Mn (II) complex at 800 °C in N2 for 2 h with a heating rate of 5 °C/min. For comparison purpose, the other two catalysts were derived from different precursors and also prepared under the same conditions. One was derived from the precursor of Fe3O4@chitosan (denoted as Fe3O4@NC), while the other was derived from the precursor of Fe3O4@chitosan Schiff-base in the absence of Mn (denoted as Fe3O4@NCS).

3.3. Electrochemistry Tests

The electrocatalytic performances of the as-prepared samples were tested on the electrochemical workstation using an Ag/AgCl (in 3 M KCl) as the reference electrode and a graphite rod as the counter electrode and a 3 mm glassy carbon as the working electrode. A catalyst suspension to be tested was prepared using ultrasonically dispersed 2.5 mg of the catalyst into a mixture solvent of 980 µL ethanol and 20 µL water. The electrode to be tested was prepared by depositing 7 µL of the above suspension onto the working electrode and dried at 40 °C for 2 h with a catalyst loading of 0.25 mg·cm−2. Before the test, the electrolyte solution was saturated using O2. The working electrode was activated using the cyclic voltammetry (CV) method at 50 mV·s−1 for several cycles.

4. Conclusions

Combining the advantages of transition metal oxide and nitrogen-doped carbon material, the present work developed a feasible strategy to synthesize a core-shell structure based on a chitosan-Schiff base Mn complex coating on Fe3O4 cores (Fe3O4@NC-Mn). Used as the ORR catalyst, Fe3O4@NCS-Mn achieved nearly equivalent onset potential and maximum current density to the commercial Pt/C (20%) catalyst. Moreover, it demonstrated impressive chemical stability and stronger durability against methanol. Along with an average number of the transferred electron of 3.6, Fe3O4@NCS-Mn can be potentially used as the ORR catalyst in methanol based fuel cell.

Author Contributions

The experimental work was conceived and designed by J.T.; Y.L., L.B., and W.W. performed the experiments; T.L. and Q.Z. analyzed the data; J.T. drafted the paper. All of the authors have given approval for the final version of the manuscript.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 09 00692 g001 550
Figure 1. XRD patterns of Fe3O4@NC, Fe3O4@NCS, and Fe3O4@NCS-Mn catalysts.
Figure 1. XRD patterns of Fe3O4@NC, Fe3O4@NCS, and Fe3O4@NCS-Mn catalysts.
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Figure 2. Typical TEM morphology of (a) Fe3O4@NC and (b) Fe3O4@NCS-Mn catalysts.
Figure 2. Typical TEM morphology of (a) Fe3O4@NC and (b) Fe3O4@NCS-Mn catalysts.
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Figure 3. SEM image of Fe3O4@NCS-Mn and corresponding EDS elemental mapping images of C, N, O, Mn, and Fe based on the selected region.
Figure 3. SEM image of Fe3O4@NCS-Mn and corresponding EDS elemental mapping images of C, N, O, Mn, and Fe based on the selected region.
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Figure 4. N2 adsorption/desorption isotherms and the corresponding pore size distribution curves of (a) Fe3O4@NC, (b) Fe3O4@NCS, and (c) Fe3O4@NCS-Mn.
Figure 4. N2 adsorption/desorption isotherms and the corresponding pore size distribution curves of (a) Fe3O4@NC, (b) Fe3O4@NCS, and (c) Fe3O4@NCS-Mn.
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Figure 5. Polarization curves of Pt/C (20%), Fe3O4@NC, Fe3O4@NCS, and Fe3O4@NCS-Mn. Scan rate: 10 mV·s−1; rotation speed: 1600 rpm.
Figure 5. Polarization curves of Pt/C (20%), Fe3O4@NC, Fe3O4@NCS, and Fe3O4@NCS-Mn. Scan rate: 10 mV·s−1; rotation speed: 1600 rpm.
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Figure 6. Polarization curves of Fe3O4@NCS-Mn catalyst at different rotation speeds. Scan rate: 10 mV·s−1.
Figure 6. Polarization curves of Fe3O4@NCS-Mn catalyst at different rotation speeds. Scan rate: 10 mV·s−1.
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Figure 7. (a) Koutecky-Levich (K-L) plots of Fe3O4@NCS-Mn catalyst for ORR; (b) the dependence of the transferred electron number (n) on the potential.
Figure 7. (a) Koutecky-Levich (K-L) plots of Fe3O4@NCS-Mn catalyst for ORR; (b) the dependence of the transferred electron number (n) on the potential.
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Figure 8. CV curves of (a) Fe3O4@NCS-Mn and (b) Pt/C (20%) in N2 or O2-saturated 0.1 M KOH as well as O2-saturated 0.1 M KOH with 3 M CH3OH. Scan rate: 10 mV·s−1.
Figure 8. CV curves of (a) Fe3O4@NCS-Mn and (b) Pt/C (20%) in N2 or O2-saturated 0.1 M KOH as well as O2-saturated 0.1 M KOH with 3 M CH3OH. Scan rate: 10 mV·s−1.
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Figure 9. The chronoamperometric current-time curves of Pt/C (20%) and Fe3O4@NCS-Mn catalyst for 36,000 s. Rotation speed: 1600 rpm.
Figure 9. The chronoamperometric current-time curves of Pt/C (20%) and Fe3O4@NCS-Mn catalyst for 36,000 s. Rotation speed: 1600 rpm.
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Table
Table 1. The textural properties of Fe3O4@NC, Fe3O4@NCS, and Fe3O4@NCS-Mn.
Table 1. The textural properties of Fe3O4@NC, Fe3O4@NCS, and Fe3O4@NCS-Mn.
SampleFe3O4@NCFe3O4@NCSFe3O4@NCS-Mn
SBET [m2·g−1]Total87214143
Microporous173621
Mesoporous70178122
Pore volume [cm3·g−1]Total0.110.280.17
Microporous0.030.050.02
Mesoporous0.080.230.15
Table
Table 2. Comparison of catalytic activity of the catalysts towards oxygen reduction reaction (ORR).
Table 2. Comparison of catalytic activity of the catalysts towards oxygen reduction reaction (ORR).
SampleOnset Potential
(V vs. Ag/AgCl)		Half-Wave Potential
(V vs. Ag/AgCl)		Limiting Current Density (mA·cm−2)
Pt/C−0.02−0.1393.8
Fe3O4@NC−0.21−0.5612.0
Fe3O4@NCS−0.02−0.2922.8
Fe3O4@NCS-Mn−0.02−0.2053.8

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MDPI and ACS Style

Tong, J.; Li, Y.; Bo, L.; Wang, W.; Li, T.; Zhang, Q. Core-Shell Fe3O4@NCS-Mn Derived from Chitosan-Schiff Based Mn Complex with Enhanced Catalytic Activity for Oxygen Reduction Reaction. Catalysts 2019, 9, 692. https://0-doi-org.brum.beds.ac.uk/10.3390/catal9080692

AMA Style

Tong J, Li Y, Bo L, Wang W, Li T, Zhang Q. Core-Shell Fe3O4@NCS-Mn Derived from Chitosan-Schiff Based Mn Complex with Enhanced Catalytic Activity for Oxygen Reduction Reaction. Catalysts. 2019; 9(8):692. https://0-doi-org.brum.beds.ac.uk/10.3390/catal9080692

Chicago/Turabian Style

Tong, Jinhui, Yuliang Li, Lili Bo, Wenhui Wang, Tao Li, and Qi Zhang. 2019. "Core-Shell Fe3O4@NCS-Mn Derived from Chitosan-Schiff Based Mn Complex with Enhanced Catalytic Activity for Oxygen Reduction Reaction" Catalysts 9, no. 8: 692. https://0-doi-org.brum.beds.ac.uk/10.3390/catal9080692

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