Elsevier

Applied Surface Science

Volume 471, 31 March 2019, Pages 445-454
Applied Surface Science

Full Length Article
Adsorption properties of O2 on the unequal amounts of binary co-doped graphene by B/N and P/N: A density functional theory study

https://doi.org/10.1016/j.apsusc.2018.12.019Get rights and content

Highlights

  • The positively charged dopants P and B act as active sites for O2 adsorption.

  • PN3-G and NP3-G exhibit considerably high catalytic activity.

  • The O2 molecule prefers to form triangle or O2 with the active-site atom.

  • The O atoms easier to migrate towards the P atom and deviate away from the N atom.

  • Excessive adsorption making further dissociation difficult.

Abstract

Graphene acts as an advanced substrate material, and it has been used as an electrode in fuel cells because it can efficiently adsorb oxygen molecules. In this study, density functional theory (DFT) calculation has been performed to exactly simulate the adsorption and dissociation of oxygen molecules on the XY3-co-doped graphene. The results show that the positively charged P and B atoms act as active sites for oxygen adsorption on the surface of graphene. In addition, PN3-G and NP3-G exhibit higher catalytic activity than the other samples because of the facile transfer of electrons from the highest occupied molecular orbital (HOMO) of O2 to the orbital above the Fermi level in the samples. More interestingly, excessive adsorption leads to the direct dissociation of oxygen molecules and making further dissociation difficult. In conclusion, the adsorption of oxygen molecules at the hollow sites on the PN3-G and NP3-G occurs via the formation of a triangular ring or transformation into O2 is proposed as the optimal strategy because these routes are thermodynamically favorable and the associated energy barrier is low. Furthermore, after dissociation, it was found that the oxygen atoms preferably approach the P atom. This work may be useful as a reference for future experimental studies to develop effective metal-free catalysts for fuel-cell cathodes.

Introduction

Fuel cells directly convert chemical energy to electrical energy with zero emission and high efficiency. Thus, they are considered effective for controlling the greenhouse effect caused by substantial CO2 emissions. Among the many factors affecting the fuel cell performance, the oxygen reduction reaction (ORR) [1], [2] occurring on the cathode is the most significant, and it can be improved by controlling the thermal effects and pH, or by using a nanomaterial-based cathode [3], [4], [5], [6], [7], [8]. Recently, graphene, because of its large active surface area [9] and tunable morphology [10], has emerged as a promising electrode material.

A number of studies have demonstrated by theoretical analysis and experiments that graphene can be employed as a metal-free catalyst [11] with doping nonmetallic atoms, defect structure, and compound constructed to possess appropriate bandgap. Doping with foreign atoms has been confirmed to be the most promising substitute for a noble metal catalyst which opening up a new material platform toward low cost and high catalytic efficiency [12], [13]. Substitutional doping with adventitious nitrogen and boron atoms into the carbon framework can create Lewis basic sites, increase interaction with the underlying substrate, and dramatically enhance the electron density of states (DOS) near the Fermi level [14], [15], [16]. Theoretically, the interactions between the surface of doped graphene and oxygen molecule have been discussed extensively for various adsorption sites and thermodynamically favorable surfaces [17]. Hou et al. [18] investigated that the energy barrier of ORR on both sides of the Si-doped carbon nanotubes (CNTs) is different. Xia et al. [19] reported that the edge of S-doped grapheme show high electrocatalytic activity. Tsetseris et al. [20] simulated the change of Fermi level in the valence band substitute by phosphorus could remain stabilized of electronic properties. As reported by Kang et al. [21], different doping position effect remarkably with the electronic structure for the O doped graphene.

Recently, researchers show that metal free dual doping like B, N, P, O, and S could synergistic affect considerably the reactive sites because of the changes in charge redistribution in doped graphene [22], [23], [24]. Importantly, the catalytic activity of N, O co-doped graphene for the high efficiency of hydrazine oxidation process has been reported experimentally [25] and double doping by P, N could extremely increase chemical adsorption capacity [26]. Furthermore, previous DFT calculations showed that the lowest adsorption energy is calculated for G-BN3 sites and predicted that these sites could strengthen chemisorption of the surface for the oxygen molecule in non-equivalent co-doped graphene catalyst [27]. Besides, it has been suggested through the different shape of substitution atoms that change atoms electronegativity has a significant influence on the adsorption properties of the oxygen molecule on the surface [28].

In the present study, we have used density functional theory (DFT) to study the adsorption of oxygen molecules on the surface of a fuel cell cathode based on doped-graphene, and the electron transfer mechanism in XY3 type co-doped graphene electrocatalysts. The effects of electronegativity of the doped atom and the nature of oxygen adsorption sites on the adsorption intensity have been determined. Our study demonstrates the adsorption mechanism of oxygen molecules on the surface of doped-graphene and provides a useful reference for the design of co-doped graphene cathode.

Section snippets

Computational methods

The density functional theory (DFT) calculations in this study are basing on the Dmol3 package which considered as a generalized ab-initio method are widely used in quantum mechanics calculation by replacing wave function with electron density [29]. Generalized gradient approximation (GGA) [30] by Perdew-Burke-Ernzerh (PBE) [31] functional is used for the computation of electron exchange-correlation effect. Graphene supercell of the single layer structure with a 5 × 5 × 1 unit cell contains 50

Stability of doped graphenes

As a preliminary test, it is necessary to consider the bond length and charge density distribution before the oxygen molecule is adsorbed on the surface of pristine and doped-graphene. The initial structure of the graphene under different doping conditions is shown in Fig. 1, the “X” atom of XY3 doping is in the site of “A”, and “Y” atoms are dispersed at “1”, “2” and “3” sites. In this paper, the N, B, and P atoms are chosen by different electronegativity and atomic radius by four kinds of XY3

Conclusions

In this study, the effects of varying the amounts of dopants in binary co-doped graphene on the structural stability, electronic properties, and adsorption-dissociation behavior of graphene were investigated by density functional theory (DFT) calculations. The results show that the positively charged dopants P and B act as active sites for O2 adsorption. After the oxygen adsorption, the electron transfer was determined from the charge density difference and Mulliken population analyses. A

Acknowledgements

This work was supported by the National Natural Science Foundation of China [grant numbers 51676037] and Postgraduate Research & Practice Innovation Program of Jiangsu Province [grant numbers KYCX18_0086].

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