Pyrolysis of melamine-treated vitamin B12 as a non-precious metal catalyst for oxygen reduction reaction

Abstract

Active, inexpensive non-precious metal substitutes for current Pt-based catalysis are needed to reduce the cost of proton exchange membrane fuel cells (PEMFC). In this work, pyrolysis of a carbon black-supported melamine-treated vitamin B12 (py-B12-M/C) catalyst for the oxygen reduction reaction (ORR) establishes that the surface nitrogen content and nitrogen–carbon ratio strongly affect ORR activity. Under optimal conditions, the number of transferred electrons in py-B12-M/C and the hydrogen peroxide yield of its ORR are 3.95 and 2.5%, respectively.

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Introduction

Clean, highly active and eco-friendly power sources are needed to meet the increased global demand for energy and to reduce the human impact on the environment. One such power source is the fuel cell. The proton exchange membrane fuel cell (PEMFC), which efficiently transforms chemical energy into electrical energy via electrochemical reactions, is considered an ideal future power source. However, cost and available resources are important development issues since most PEMFCs use expensive platinum catalysis. Accordingly, catalysis using non-precious metals must replace platinum catalysis.

The various factors that affect oxygen reduction reaction (ORR) activity include transition metal nitrogen-containing complexes, the species of the transition metal, the structure of the catalyst, the surface properties of the carbon support, the nitrogen content and others. The ligand–metal interaction has an important effect on ORR activity. Lalande et al. noted that the surface nitrogen content of the catalyst is the main factor in ORR activity.¹ Studies also show that increasing the surface nitrogen content directly improves ORR activity.^2–7 Nitrogen precursors used for non-precious metal catalyst include macrocyclic complexes, organic compounds, organic polymers, inorganic salts and gaseous precursors.^8–11 Nallathambi et al. used various nitrogen precursors with different nitrogen–carbon ratios (N/C ratio) for metal-nitrogen–carbon cathode catalyst and found that the N/C ratio of the nitrogen precursor increased the accessible active site density by reducing carbon deposition in the pores of the cathode during pyrolysis.¹²

In our earlier work, pyrolyzed cyanocobalamin (vitamin B12) supported by carbon black (py-B12/C) was used as an ORR catalyst in PEMFC. Cell performance was higher and hydrogen peroxide yield was lower compared to previous methods of Co-based catalysis in PEMFCs.^13,14 Here, we report the use of melamine, which has a high N/C ratio, as a nitrogen precursor mixed with vitamin B12 as a catalyst. Compared to other Co-based catalysis methods reported so far, the pyrolyzed nitrogen-incorporating vitamin B12, which is supported by carbon black (py-B12-M/C), provides stronger catalytic activity in the ORR via the direct four-electron reduction pathway.^13,14 Therefore, the proposed catalysis method improves fuel cell performance.

Experimental

1. Preparation of pyrolyzed nitrogen-incorporated vitamin B12 supported by carbon black

A 0.10 g mass of cyanocobalamin (99%, Aldrich) was dissolved in 10 mL of ethanol by stirring for 30 minutes at room temperature. Then, 0.40 g of carbon black (Vulcan XC-72R) and the specific weight of melamine (99%, Aldrich) were added to the cyanocobalamin solution and stirred for another 30 minutes at room temperature. The melamine added to the mixture in quantities of 50, 100, 150 and 200 mg yielded py-B12-M50/C, py-B12-M100/C, py-B12-M150/C, pt-B12-M200/C, respectively. The mixture was steam heated to 80 °C to eliminate the solvent. Slurry obtained by filtering the suspension through a paper filter was then dried at room temperature under a vacuum for 12 hours.

Pyrolyzed nitrogen-incorporating vitamin B12 supported by carbon black was prepared at 700 °C. Pyrolysis was achieved by loading the slurry into a fused aluminum oxide boat, which was introduced into a furnace in a quartz tube; the temperature was then increased to 700 °C at a rate of 20 °C per minute in a nitrogen atmosphere and then maintained at 700 °C for two hours. Following the pyrolysis, the furnace was cooled to room temperature by natural convection.

2. Material analysis

The Brunauer–Emmett–Teller (BET) surface area of catalysts was recorded by a ASAP 2020 Micromeritics Surface Area and Porosity Analyzer. An X-ray photoelectron spectroscope (XPS, VG ESCA Scientific Theta Probe using 1486.6 eV Al Kα source) was used to study N1 changes in pyrolyzed nitrogen-incorporating vitamin B12.

3. Electrochemical measurements

Electrochemical measurements of a three-compartment cell were performed with a potentiostat/galvanostat (Biologic Bi-stat). The working electrode was a rotating-ring disk electrode (RRDE, PINE AFE7R9GCPT) with a glassy carbon (GC) disk and a platinum ring. The counter electrode and reference electrode were Pt foil and a saturated calomel electrode (0.242 V vs. NHE), respectively. All potentials in this work are with reference to the reversible hydrogen electrode (RHE). The electrolyte in all electrochemical measurements was oxygen-saturated 0.1 M HClO₄ solution.

Catalyst ink was prepared by mixing 160 mg of catalyst with 20 mL deionized water. A 40 μL quantity of the ink and 5 μL of 0.1 wt% Nafion® solution were dropped onto the GC disk and then left to dry at room temperature. The catalyst loading was 630 μg cm⁻². The GC disk was polished to a mirror-like finish using 1.0 μm alumina slurry followed by 0.05 μm alumina slurry before depositing the catalyst ink onto the GC disk. The ORR curves for the GC were plotted at a low scan rate of 10 mV s⁻¹ to reduce non-Faradic current generated by the catalyst. To determine the hydrogen peroxide yield of the ORR catalyzed on the GC disk, 1.2 V vs. RHE was applied to the ring to produce a current that oxidized the hydrogen peroxide.

4. Fuel cell test

A membrane-electrode-assembly (MEA) with an area of 5 cm² was made by hot-pressing two electrodes on both sides of a Nafion® 212 (H⁺, DuPont) at 135 °C and 130 kg cm⁻² for 2 min. In the preparation of the specific cathode, the py-B12-M/C was dispersed in a 5 wt% Nafion® solution as a cathode catalyst ink, with a catalyst-to-dry Nafion® mass ratio of 1 : 2. The cathode catalyst ink was hand-painted onto the carbon cloth, giving a py-B12-M/C loading of approximately 6.0 mg cm⁻², and the cathode was then dried at room temperature in a vacuum for 6 hours. The anode of MEA was a commercial electrode (E-TEK) – Pt/C with a metal loading of 0.25 mg cm⁻². A polarization experiment was carried out on the PEMFC at 70 °C, using hydrogen and oxygen through the anode and the cathode at flow rates of 0.10 and 0.15 SLPM. Hydrogen and oxygen were passed through the humidifiers at 70 °C before they entered an MEA. The back pressure gauges on the anode and the cathode sides were set to 1 atm. The PEMFC performance was measured using a fuel-cell test station (Asia Pacific Fuel Cell Technologies, Ltd.) by recording the cell voltage and current after they had reached steady values. The measurements of electrochemical impedance spectroscopy at 20 kHz provided the cell resistance as a function of current density, which yielded a typical cell resistance of approximately 0.4 Ω cm².

Results and discussion

The main reactions in the ORR pathway are the following:

O₂ + 4H⁺ + 4e⁻ → 2H₂O…E⁰ = 1.229 V (r1)

O₂ + 2H⁺ + 2e⁻ → H₂O₂…E⁰ = 0.695 V (r2)

H₂O₂ + 2H⁺ + 2e⁻ → 2H₂O…E⁰ = 1.763 V (r3)

Chemical decomposition of H₂O₂ may also form O₂ and H₂O.

The direction reduction pathway of reaction (1) involves a four-electron transfer. The H₂O₂ pathway of reaction (2) involves a two-electron transfer. Since a direct ORR pathway yields a higher thermodynamically reversible potential compared to an H₂O₂ pathway, reaction (1) is the preferred ORR in the PEMFC.

Fig. 1 presents the ORR behaviors of py-B12-M50/C, py-B12-M100/C, py-B12-M150/C and py-B12-M200/C according to the RRDE results. The lower and upper parts of Fig. 1 plot the densities of the disk current (I_d) and the ring current (I_r), respectively, as functions of applied potential. The typical I_d curve obtained for acid media has three dominant potential regions – the kinetic range (>0.8 V), the mixed range (0.8–0.6 V), and the mass-transfer range (<0.6 V). The I_d curve for py-B12-M150/C, which is clearly above those of the other catalysis in all regions, indicates its much higher ORR activity. The total electron-transfer number (n) and the hydrogen peroxide yield (%H₂O₂) in the catalyzed ORR are

where N (0.37) is the RRDE collection efficiency. Fig. 1b and c plot the number of electrons transferred (n value) and the hydrogen peroxide yield (%H₂O₂) of the ORR, respectively, as functions of the potential of the glassy carbon (GC) disk. The figures show that py-B12-M150/C yields the highest n and the lowest % H₂O₂. When overpotential is high (0.3–0.7 V), the n values and %H₂O₂ for py-B12-M150/C remain at approximately 3.95 and 2.5%, respectively, indicating that catalysis of ORR by py-B12-M150/C occurs preferentially along the four-electron direct ORR pathway. To determine the ratio of the reaction rate via the direct pathway (reaction (1)) to that of the H₂O₂ pathway (reaction (2)), (k₁ over k₂, k₁/k₂), Damjanovic et al. proposed an analysis to evaluate k₁ and k₂ using the RRDE approach, based on measurements of the magnitudes of the ring current (I_r) and the disk current (I_d) at various rotation rates (ω) and at a fixed potential value.¹⁵

Fig. 1 (a) ORR curves for py-B12-M50/C, py-B12-M100/C, py-B12-M150/C and py-B12-M200/C; (b) the n values and (c) %H₂O₂ of the catalysts dependence on disk potentials. Rotating speed: 1600 rpm; scan rate: 10 mV s⁻¹, ring potential: 1.2 V.

Fig. 2 shows the range of k₁/k₂ ratios for py-B12-M150/C is 30–55, which exceeds the maximum k₁/k₂ ratio obtained by other catalysis. Compared to py-B12/C,¹³ which we used previously, py-B12-M150/C is also more effective in the ORR, with n values and %H₂O₂ approximating 3.90 and 5.0%, respectively.

Fig. 2 The k₁/k₂ ratios of py-B12-M50/C, py-B12-M100/C, py-B12-M150/C and py-B12-M200/C as a function of applied potential.

Fig. 3 shows the polarization curves of PEMFC using py-B12-M150/C and py-B12/C as the cathode catalysts. To eliminate the internal resistance, the cell potentials are corrected by iR-compensation.⁵ The anode polarization is assumed to be negligible at the low current density, which the cathode polarization dominates. Py-B12-M150/C exhibits a higher cell performance than py-B12/C in all regions. The 100-hour durability test of PEMFC using py-B12-M150/C was performed at 0.4 V with flowing H₂ and air through the anode and the cathode, respectively, as presented in Fig. 4. After the 100-hour durability test, it decays about 6.5% of the initial current density, which exhibits the excellent stability compared to our previous result.¹³

Fig. 3 Polarization curves of the H₂–O₂ PEMFC using py-B12-M150/C and py-B12/C as the cathodes. Operation temperature: 70 °C; back pressure of H₂ and O₂: 1 atm; anode catalyst: 0.25 mg cm⁻² of Pt/C; cathode catalyst: 6.0 mg cm⁻² of specific catalyst; electrolyte: Nafion® 212 (H⁺, DuPont).

Fig. 4 The 100-hour durability test of H₂–air PEMFC using py-B12-M150/C. Operation conditions: H₂–air, 70 °C and 1 atm of back pressure. Operation temperature: 70 °C; back pressure of H₂ and air: 1 atm; anode catalyst: 0.25 mg cm⁻² of Pt/C; cathode catalyst: 6.0 mg cm⁻² of py-B12-M150/C; electrolyte: Nafion® 212 (H⁺, DuPont).

Table 1 indicates the BET surface areas for py-B12-M50/C, py-B12-M150/C and py-B12-M200/C. As a result, the BET surface areas of catalysts are increased by the amount of melamine, which the additional melamine may change the surface characteristics. Nallathambi et al. also found that the BET surface area of catalyst was increased by N/C ratio of precuros.¹² They indicated that increased surface accessibility contributes to improved ORR activity.

Table 1 The BET surface of py-B12-M50/C, py-B12-M150/C and py-B12-M200/C

Catalyst	BET surface area m^2 g^−1
Py-B12-M50/C	42.08
Py-B12-M150/C	75.06
Py-B12-M200/C	121.81

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Fig. 5a–c present the XPS N1s spectra of py-B12-M50, py-B12-M150 and py-B12-M200, respectively, for which the corresponding fitting results are shown in Table 2. Comparison of the XPS spectra for py-B12-M50, py-B12-M150 and py-B12-M200 show that the peak values for of 2-methylbenzimidazole, pyridinic- and quaternary-type nitrogen, are 398.0, 400.4 and 401.4 eV, respectively.^16–18 Some studies of nitrogen-doped carbon catalysis have determined that quaternary-type nitrogen improves ORR activity.^19–22 Chung et al. claimed that the quaternary-type nitrogen should reduce the band gap energy of carbon and enhance catalytic activity.²³

Fig. 5 XPS showing N1s spectra of (a) py-B12-M50; (b) py-B12-M150 and (c) py-B12-M200.

Table 2 The fitting results from XPS N1s spectra of Fig. 5

N1s (atomic%)	Quaternary-type nitrogen (401.4 eV)	Pyridinic-type nitrogen (398.7 eV)	2-Methylbenzimidazole (398.0 eV)
Py-B12-M50	7.6%	45.6%	46.8%
Py-B12-M150	24.7%	60.5%	14.8%
Py-B12-M200	9.5%	56.8%	33.7%

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Table 3 presents the ratio of nitrogen to carbon (N/C ratio) for py-B12-M50, py-B12-M150 and py-B12-M200 according to the XPS fitting results. The N/C ratios of py-B12-M50, py-B12-M150 and py-B12-M200 are 0.159, 0.169 and 0.210, respectively. Jaouen et al. reported that increasing the doping of carbon blacks in catalysis with nitrogen increases catalysis activity.⁷ Marcotte et al. also showed that nitrogen concentration on the support surface correlates positively with catalytic activity.²⁴ With the increasing N/C ratio, py-B12-M150/C has higher ORR activity than py-B12-M50/C. However, py-B12-M200/C has higher N/C ratio but lower ORR activity than py-B12-M150/C. Since the former has less ratio of quaternary-type nitrogen than the latter, the activity of py-B12-M200/C in the ORR is lower than that of py-B12-M150/C.

Table 3 The N/C ratio for py-B12-M50, py-B12-M150 and py-B12-M200 from XPS fitting results

	N1s atomic%	C1s atomic%	N/C ratio
Py-B12-M50	8.61%	54.06%	0.159
Py-B12-M150	8.62%	50.91%	0.169
Py-B12-M200	9.94%	47.63%	0.210

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Conclusions

Pyrolyzed nitrogen-incorporating vitamin B12 supported by carbon black improves activity in the ORR and has potential for use in PEMFC. The RRDE measurements reveal that py-B12-M150/C favors a direct four-electron reduction pathway from O₂ to H₂O. The high activity of py-B12-M150/C in the ORR is attributable to its quaternary-type nitrogen and its high nitrogen-to-carbon ratio. Additionally, the PEMFC using py-B12-M150/C shows a high performance and a great stability. Although identifying this catalyst is an important advance in the use of non-precious metal metals for catalysis in PEMFC, the mechanisms of nitrogen precursors with central metal and surrounding ligands in ORR require further study.

Acknowledgements

The authors would like to thank financial supports from the Ministry of Education Top University Projects (100H451401) and the National Science Council of Taiwan (NSC 101-2221-E-011-047-MY3) & (NSC 102-3113-P-011-001).

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