Fe–N-doped graphene as a superior catalyst for H2O2 reduction reaction in neutral solution

doi:10.1016/j.jcat.2014.12.020

Journal of Catalysis

Volume 323, March 2015, Pages 55-64

https://doi.org/10.1016/j.jcat.2014.12.020 Get rights and content

Highlights

•
Fe–N-doped graphene containing 0.3 at% of Fe and 1.9 of N is synthesized.
•
Fe doping increases the active specific surface area of N-doped graphene.
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Fe doping does not change the mechanism of H₂O₂ reduction on N-doped graphene.
•
Fe–N-doped graphene shows higher catalytic activity than N-doped graphene.
•
An increase in catalyst loading decreases oxygen evolution from electrode surface.

Abstract

Fe–N-doped graphene containing 0.3 at% of Fe and 1.9 of N with an active specific surface area of 77.5 ± 4 m² g⁻¹ is synthesized in two steps: first N-doped graphene is produced by thermal dissociation of methane in an inductively coupled thermal plasma (ICP) reactor and subsequently the product is converted to Fe–N-doped graphene by a wet chemical method. The catalytic activity of this catalyst toward H₂O₂ reduction reaction (HPRR) is studied by rotating disk electrode for fuel cell applications. Although the mechanism of HPRR on Fe–N-doped graphene, N-doped graphene and graphene is similar, Fe–N-doped graphene shows the highest catalytic activity toward HPRR. The exchange current density based on the active surface area (j₀_A) of Fe–N-doped graphene in Na₂SO₄ solution is (4.4 ± 0.2) × 10⁻⁸ A cm⁻², which is 5 times greater than j₀_A of the N-doped graphene. Direct reduction of H₂O₂, H₂O₂ decomposition, O₂ reduction, and O₂ desorption are predicted as the reactions involved in the HPRR while O₂ reduction and O₂ desorption are negligible at low and high overpotentials, respectively. Investigation of the effect of the electrolyte and catalyst loading reveals that HPRR on Fe–N-doped graphene suffers from kinetic limitations especially at low catalyst lodgings, fast rotation rates, and strong acidic and basic solutions.

Graphical abstract

Introduction

Graphene is a two-dimensional form of carbon with atoms arranged in a honeycomb lattice structure. Its unique characteristics such as high specific surface area, chemical stability, and high conductivity make graphene a superior candidate for electrochemical energy systems such as fuel cells [1], [2], [3] and lithium ion batteries [4], [5]. Recently, there has been intense research effort to replace the conventional noble catalysts such as platinum and gold by graphene-based catalysts in the cathode side of fuel cells. Nitrogen doping increases the catalytic activity of graphene toward both the O₂ reduction reaction (ORR) [6], [7], [8], [9], [10] and the H₂O₂ reduction reaction (HPRR) [11], [12]. This increase in catalytic activity may be related to the high electronegativity (χ) of nitrogen species which create net positive charges on adjacent carbons facilitating oxygen adsorption [13]. Substitution of some carbon atoms in nitrogen-doped graphene lattice with heteroatoms such as boron (χ = 2) with a lower electronegativity than carbon (χ = 2.55) creates a unique structure that can be used for ORR [14]. Zheng et al. [14], for instance, showed that the current density produced through ORR on B–N-doped graphene is much higher than the single B- or N-doped graphene.

Iron with χ = 1.8 also seems to be a suitable chemical element for dual doping of graphene in combination with nitrogen. Fe–N-doped graphene shows high catalytic activity and stability toward ORR in both acidic and alkaline solutions [15], [16]. Using a periodic continuum solvation model based on the modified Poisson–Boltzmann equation, Sun et al. have recently shown that both 2e and 4e pathways with OOH as an intermediate product can occur on Fe–N-doped graphene through ORR in acid solution [17]. Although several works have reported the performance of this highly active catalyst toward ORR, the application of Fe–N-doped graphene toward HPRR has not been studied.

Hydrogen peroxide is a potential oxidant available in high concentrations in aqueous solutions. As such H₂O₂ has the potential of replacing O₂ to increase the power density of some fuel cells such as membraneless fuel cells and direct borohydride fuel cells with ceramic membrane [18], [19], [20]. The most important drawbacks of using H₂O₂, however, are the slow kinetics of HPRR on non-noble catalysts and the side reaction of the decomposition of H₂O₂. The O₂ produced through the decomposition reaction may be released into the solution leading to the loss of a part of the oxidant and also creating operational problems [12].

We have recently reported the use of graphene nanoflakes (GNFs) and nitrogen-doped graphene nanoflakes with 32% N (N-GNFs32) for the HPRR [12]. Although N-GNFs32 did catalyze HPRR to considerable extent, its catalytic activity was still noticeably lower than polycrystalline gold. Moreover, there was some O₂ evolution occurring when this catalyst was used.

In this work, three main objectives are pursued:

(1)
As a part of our effort to find a highly active catalyst for H₂O₂ reduction with minimum gas evolution, we investigate Fe–N-doped graphene with 0.3 at% of Fe and 1.9 at% of N (Fe–N-doped graphene) as a catalyst for HPRR. The reaction path and the kinetic parameters such as exchange current density and Tafel slope are determined in 0.1 M Na₂SO₄ solution using a glassy carbon rotating disk electrode (RDE). These results are then compared with N-GNFs32 in order to understand the effect of Fe atoms.
(2)
The effect of catalyst loading on the kinetic parameters and the intermediate product of HPRR in 0.1 M Na₂SO₄ solution is investigated. Catalyst loading directly affects the thickness of the catalyst film deposited on the glassy carbon RDE. As a result, the characteristics of the catalyst film such as the active surface area available for the reaction(s) may be changed. Catalyst loading may also affect the amount of intermediate products of electrochemical reactions. Bonakdarpour et al. [21], for instance, demonstrated that the amount of the intermediate product of ORR, that is, H₂O₂, on Fe–N–C catalyst is a function of the catalyst loading. While at high catalyst loadings H₂O₂ constituted less than 5% of the products, the fraction of H₂O₂ increased to more than 95% at low catalyst loadings. This result motivates the authors to study the effect of catalyst loading on the intermediate product of HPRR, that is, O₂, in order to minimize oxygen evolution from electrode surface.
(3)
HPRR is also studied on Fe–N-doped graphene in the basic and acidic electrolytes. Electrolyte can affect the mechanism and the kinetics of an electrochemical reaction. The ORR on gold, for instance, has higher catalytic activity in basic electrolyte in comparison with acidic electrolyte [22]. In the current work, the effect of the electrolyte on the mechanism, kinetic parameters such as the exchange current density, and the current density produced through HPRR is studied briefly in 0.1 M KOH and 0.5 M H₂SO₄ solutions.

Section snippets

Background

HPRR may occur through three different paths: (i) direct path (direct reduction of H₂O₂ by Eq. (1)), (ii) indirect path (H₂O₂ decomposition by Eq. (2) followed by the reduction of O₂ by Eqs. (3) and/or (4)), (iii) a combination of these two paths. Depending on the relative reaction rates of Eqs. (1), (2), (3), (4) and on the extent of O₂ desorption, seven different cases are expected for HPRR [23]. These cases and a short description of the reactions involved in each case are presented in Table

Catalyst synthesis

The catalyst, Fe–N-doped graphene, was synthesized in two steps: first nitrogen-doped graphene nanoflakes (N-GNFs) were produced, and then N-GNFs were converted to the Fe–N-doped graphene using a wet chemical method. The method to produce nitrogen-doped graphene nanoflakes involved the plasma decomposition of methane as the carbon source and nitrogen as the functionalization gas [25], [26], [27]. The equipments necessary for the fabrication of graphene nanoflakes were an inductively coupled

Catalyst characterization

The XPS results (Table 2 and Fig. S1 of the supporting information) indicate that the catalyst surface is composed of carbon, nitrogen, oxygen, and iron; carbon and oxygen combined represent over 97 at%. The signal for iron was faint and not immediately obvious when viewing the whole XPS survey spectrum, but further analysis of the signal showed that iron was present at the surface at a level of 0.3 at%. The de-convolution of the iron peak did not provide any substantial information as the signal

Conclusion

The results of this work indicate the superior catalytic activity of Fe–N-doped graphene toward H₂O₂ reduction reaction (HPRR) in comparison with graphene and N-doped graphene. Fe doping creates three important changes on the characteristics and catalytic activity of N-doped graphene: (1) increasing the active specific surface area of the catalyst, (2) increasing the heterogeneous reaction rate constant of H₂O₂ decomposition, and (3) increasing the exchange current density of H₂O₂ reduction

Acknowledgments

The authors would like appreciate the McGill Engineering Doctoral Award (MEDA) program, the Natural Sciences and Engineering Research Council of Canada (NSERC), and the Fonds de recherche Nature et technologie Québec (FRNTQ) for financial support.

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Fe–N-doped graphene as a superior catalyst for H2O2 reduction reaction in neutral solution

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Background

Catalyst synthesis

Catalyst characterization

Conclusion

Acknowledgments

Appl. Catal. B

J. Power Sources

J. Catal.

Appl. Catal. B

J. Power Sources

Adv. Energy Mater.

Electrochim. Acta

Catal. Today

Carbon

J. Electroanal. Chem. Interfacial Electrochem.

J. Electroanal. Chem. Interfacial Electrochem.

J. Electroanal. Chem.

Electrochem. Commun.

ChemElectroChem

Adv. Energy Mater.

Graphene-based composite anodes for lithium-ion batteries

Energy Environ. Sci.

Adv. Energy Mater.

ACS Nano

J. Mater. Chem.

Angew. Chem., Int. Ed.

Fe–N-doped graphene as a superior catalyst for H₂O₂ reduction reaction in neutral solution