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10SYNTHESIS METHOD OF CHLORINE-FREE FE/N/C CATALYST FOR OXYGEN REDUCTION REACTION
Nadirov R.K.
al-Farabi Kazakh national university, C.Sc., senior lecturer
Sabirov Ye.A.
al-Farabi Kazakh national university, master student
ABSTRACT
A new chlorine-free approach to obtain Fe/N/C catalysts was proposed in the present work. Replacement of iron (III) chloride with the iron (III) nitrate as a source of Fe 3+ cations in the precursors mixture before heat treatment leads to increase the activity of the final catalyst toward oxygen reduction reaction, as has been shown by using rotate disk electrode technique.
Keywords: Fe/N/C catalyst, synthesis, oxygen reduction reaction.
1. Introduction
Of the different types of the fuel cell under development at the present time, the Proton Exchange Membrane Fuel Cell (PEMFC) is attracting the most attention [1]. However, the PEMFC has a long way to go to become commercially viable for applications other than in niche markets. The use of the precious metal Pt as the preferred catalyst for the anode and cathode is one of the impediments to widespread PEMFC commercialization on account of its high cost and scarcity. This fact stimulates researchers worldwide to look for non-precious metal electrocatalysts as alternative materials. Among these materials, pyrolyzed iron nitrogen-containing complexes supported on carbon materials (Fe/N/C) are considered the most promising catalysts for application in PEMFC because they have demonstrated high oxygen reduction reaction (ORR) activity and stability [2,3].
Several approaches were proposed to obtain Fe/N/C catalysts, which include preparation of Fe-N-C precursors and its further heat treatment without oxygen access [4-6]. As it has been found, such type of catalysts is very sensitive toward impurities. Zelenay with co-workers of Los Alamos Laboratory proposed a simple synthesis method of sulfur -free approach to obtain a Fe/N/C catalyst [7]. At the same time, iron (III) chloride still remains as a reactant in this approach.
We speculated that chlorine containing compounds that present in the Fe/N/C catalysts (including original FeCl3) may have a negative impact on the activity of catalysts toward ORR. To test this assumption, two catalyst samples were synthesized; for the first sample, the way, described in [7] was used. For the second one, iron (III) nitrate nonahydrate was utilized as Fe+ cations source. The activities of both catalysts were compared toward ORR by using rotate disk electrode technique.
2. Experimental
2.1 PANI - Carbon -Iron Salt Composite Synthesis
Aniline (Beijing Chemical Industry, 99%) was distilled before experiments. Hydrogen peroxide (Sigma-Aldrich, 35%), nitric acid (Sigma-Aldrich, ACS reagent, 70%), iron (III) chloride hexahydrate (Sigma-Aldrich, 98%), as well as iron (III) nonahydrate
(Sigma-Aldrich, ACS reagent, >98%) were used as received.
Commercial Ketjen-black EC 300J (0.5 g), used as carbon support, was treated in 0.5 M HNO3 (50 ml) overnight, approximately 16 h, and then filtered following by washing with bi-distilled water and vacuum drying at 70 0C.
Two similar ways were used to synthesize PANI-Carbon-Iron Salt composite as a precursor for further obtaining Fe/N/C catalysts. For the first recipe (labelled as sample a), 0.25 g of the prepared carbon support was dispersed in 50 ml bi-distilled water and sonicated for 30 minutes. 1.86 g of aniline was dissolved in 50 ml bi-distilled water then added to the slurry of carbon support. The obtained mixture was stirred to obtain apparently homogeneous mass and then to a water bath maintained at 8-10 0C. Thereafter, 4.0 g of Fe(NOs)sx9H2O was added to the mixture; nitric acid was used to adjust the pH at the level of 1.5-2.0. 3 ml of 0.4 M H2O2 was added in drop wise to the mixture; the resulting mixture was kept under stirring for 24 h at 8 0C. After that, the mixture was filtered, washed and vacuum dried at 70 0C.
For the second recipe (labelled as sample b), 2.5 g of FeCl3*6H2O was used instead of Fe(NOs)sx9H2O; the remaining steps were carried out as described above.
2.2 Heat Treatment of PANI - Carbon -Iron Salt Composite and Final Product Characterization
The dry powders were annealed in a tube furnace under N2 flow at the temperature ramping rate of 30 0C/min and the annealing at 900 0C for 3 h. Ammonium carbonate was added to the powder before the annealing to create porosity in the final product, i.e. Fe/N/C catalyst.
X-ray diffraction (XRD) spectra were obtained using diffractometer DRON-3. SEM images were taken on a Quanta 3 D 200 I (USA) using 30 KeV field emission electron beam. Chemical analysis of the product was performed using optical emission spectroscopy with inductively coupled plasma (OPTIMA 8000, Per-kin Elmer).
2.3 Rotate Disk Electrode (RDE) Measurements Technique
RDE measurements were performed using poten-tiostat-galvanostat Ellins in a conventional three-electrode cell filled with 0.5 H2SO4 solution. Ag/AgCl (3.0 M NaOH, 0.235 V vs. RHE) was used as a reference electrode; a graphite rod were used as counter electrode.
In order to prepare working electrode (RDE), 10 mg of the catalyst was dispersed in 0.5 ml of isopropa-nol with 20 ^L of 5% Nafion suspension and sonific-sted for 1 h; then 30 ^L of the ink was dropped on the rod-end (0.25 cm2) of glassy-carbon electrode. Prior to
ORR measurements each electrode was potential cycled in 0.1 NaOH for 10 cycles until a stable voltam-mogram was obtained.
The ORR measurements were carried out with a disc rotating rate of 800 rpm at linear scan rate of 10 mV/s.
3. Results and Discussion
3.1 Catalysts Characterization
The SEM images of both samples (a) and (b) are shown in Fig. 1.
a b
Figure 1. SEM images of samples a and b
Samples a and b have similar images, indicating that the nature of iron (III) salt (either nitrate or chloride) has not significant affect on the morphology of a final product.
X-ray diffraction spectra of the sample a is presented in Fig.2.
Figure 2. XRD of sample a.
The spectra presented in Fig.2. includes phases typical for those materials obtained by thermal treatment of Fe-N-C precursors; similar spectra was obtained also for sample b.
Analysis of the chemical composition performed by using optical emission spectroscopy indicated the presence of 0.4% of chlorine in sample b. At the same time, no chlorine-containing phases were detected in
XRD which may be due to the sensitivity of this method.
3.2 RDE Measurement
RDE measurement results for samples a and b are shown in Fig.3. A higher value of half-wave potential of sample b can be seen. The linear parts of the curves presented in Fig.3. (from 0.90 to 0.95 V) were used to create Tafel plots for both samples; the values of Tafel
slope, exchange current density, as well as half-wave potentials, are presented in Table 1.
-4 J
Potential (V) vs. RHE
Figure 3. Current-potential curves measured on RDE coated with catalysts a or b
Table 1.
Half-wave potentials (E1/2), Tafel slopes and exchange current densities (ip) for the ORR on the catalysts
Sample E1/2 (V) Tafel Slope (mV/dec) Exchange Current Density (mA/cm2)
a 0.78 95.3 2.10X10-2
b 0.71 91.2 1.71x10-3
Such differences in the activity of both catalysts we associate with the method of their preparation, namely, with the replacement of iron salt.
Conclusion
For the first time, iron (III) nitrate was used instead of iron (III) chloride in Fe/N/C catalyst synthesis, to avoid contaminating the final product with chlorides. This replacement leads to an increase in activity of a catalyst toward oxygen reduction reaction.
Acknowledgement
This research was financially supported by the grant of the Ministry of education and science of the Republic of Kazakhstan 5715/GF 4.
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