Article pubs.acs.org/JPCC
Electrocatalytic Activity and Design Principles of Heteroatom-Doped Graphene Catalysts for Oxygen-Reduction Reaction Feng Li,† Haibo Shu,*,†,‡ Xintong Liu,† Zhaoyi Shi,† Pei Liang,† and Xiaoshuang Chen‡ †
College of Optical and Electronic Technology, China Jiliang University, 310018 Hangzhou, China National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Science, 200083 Shanghai, China
‡
S Supporting Information *
ABSTRACT: Heteroatom-doped graphene materials have emerged as highly efficient and inexpensive and variations of graphene doping structures; however, there is still a lack of fundamental understanding of the trend and mechanisms in their ORR activity, which greatly hinders the development of highly active graphene-based catalysts. Here we use densityfunctional calculations to study the ORR activity and mechanism of nonmetal-element doped graphene catalysts with different doping configurations. Our results demonstrate that binding energies of ORR intermediates (i.e., *OH) on the catalysts can serve as a good descriptor for the ORR activity, attaining the optimal value at the vicinity of ∼2.6 eV. The analysis of electronic structures indicates that the ORR activity of doped graphene catalysts depends on the abundance of electronic states at the Fermi level, which dominates the charge transfer between ORR intermediates and the catalysts. Using binding energy as a descriptor, we predict the realization of highly active graphene-based electrocatalysts by the dual-doping scheme, which is supported by recent experimental reports. Moreover, we find that the catalytic activity of graphene basal planes can be activated by the B−Sb and B−N codoping approaches. This work elucidates the inherent correlation between the ORR activity of nonmetal-doped graphene catalysts and the dopant type and doping configurations, opening a route to design highly active graphene-based ORR electrocatalysts. nitrogen (N),15−19 phosphorus (P),20−23 sulfur (S),16,18,24,25 selenium (Se),26 antimony (Sb),27 and their mixtures.28−34 These studies have demonstrated that introducing nonmetal dopants into the graphene matric can effectively modify the electronic structure of graphene, consequently leading to the improvement of catalytic activity. Despite the great experimental achievements, a fundamental guiding principle for assessing the ORR activity of heteroatomdoped graphene is still lacking, which becomes a bottleneck for further optimizing graphene-based catalysts toward the practical applications. The difficulty for evaluating the ORR activity in experiment originates from two main limitations: (i) the ORR activity is affected by multiple factors, including of geometric structure, dopant density and configuration, and chemical environment and (ii) the ORR mechanism is associated with the reactivity of catalysts that is difficult to be directly detected by currently available experimental methods, including various electron-based, spectroscopy, and electrochemical technologies. Taking N-doped graphene as an example, there are three typical doping configurations: graphitic N, pyridinic N, and pyrrolic
1. INTRODUCTION The oxygen reduction reaction (ORR), as a fundamental step of energy conversion in fuel cells and metal-air batteries, highly requires favorable catalysts to obtain fast reaction kinetics for practical applications.1−4 Platinum (Pt)-based materials are still regarded as the most efficient ORR catalyst due to their outstanding catalytic performance.4,5 Unfortunately, the high cost, scarcity, and weak durability of Pt-based catalysts have hampered the widespread commercialization of renewable energy technologies.6,7 Hence, the development of costeffective and efficient catalysts with earth-abundant elements is strongly desirable. In this respect, carbon-based nanomaterials, such as carbon nanotubes and graphene,8−11 possess hugely potential as the alternative to Pt-based catalysts due to their great abundance, large surface area, superior electrical conductivity, high chemical stability, and strong tolerance to acid/alkaline environments. However, perfect graphene and carbon nanotubes are chemically inert, and thus it requires the introduction of defects and dopants to improve their surface activity.12 For instance, the formation of Stone−Wale defects and zigzag edge defects in graphene can greatly enhance their ORR activity.13 In recent years, tremendous efforts have been devoted to the exploration of graphene catalysts doped with various p-block nonmetal heteroatoms, such as boron (B),14−16 © XXXX American Chemical Society
Received: March 31, 2017 Revised: May 17, 2017 Published: June 13, 2017 A
DOI: 10.1021/acs.jpcc.7b03093 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C N.17,18,35 In the doping process, it is inevitable to bring these N dopants into the graphene materials, but it is difficult to identify which type of N dopant creates active sites for the ORR. Hence, developing a simple and efficient ORR descriptor is critical for guiding the design of graphene-based catalysts in experiment. In general, monitoring the ORR intermediates on catalytic surfaces is an effective way for the understanding of ORR pathways and mechanisms. According to Sabatier principle, the interactions between the catalysts and the ORR intermediates (e.g., *O2, *OOH, *O, and *OH, here the prefix of * denotes adsorbed state) govern the adsorption/activation of oxygen molecules and the release of products (H2O2, H2O, and OH−).36 The principle has been developed into the d-band center theory. A large number of studies have indicated that the d-band center theory is a good descriptor to evaluate the ORR activity of transition-metal and transition-metal-oxide catalysts,37,38 but it is invalid for evaluating the catalytic activity of metal-free graphene catalysts due to the lack of d electrons in the materials. Recently, some new descriptors have been proposed, such as the coordination number (CN) of catalytic sites,39 Fermi softness,40 and electron affinity and electronegativity.41 However, these descriptors are not adequate for evaluating the ORR activity of heteroatom-doped graphene materials due to the complexity and diversity of graphene doping structures. In this work, we employ first-principles calculations within the framework of density functional theory (DFT) to elucidate quantitatively the relationship between electrocatalytic activities of various heteroatom-doped graphene materials and the dopant type and doping configuration. We find that binding energies of ORR intermediates (i.e., *OH) can become a good descriptor to evaluate the ORR activity of the doped graphene electrocatalysts. We begin with this study by making a volcano activity plot to establish the relationship between ORR activities of single-doped graphene materials and the descriptor, and obtain the optimal binding energy to maximize catalytic activity. Then the calculations of electronic structures are performed to understand the inherent mechanism of activity descriptor. Finally, using the binding energies of *OH as a descriptor, we predict ORR activities of dual-doped graphene structures, and a theoretical scheme for activating the catalytic activity of graphene basal plane is proposed.
is evaluated by calculating their formation energies (Ef) as follows: Ef = E D − E P + ΔnCμC + ΔnHμH − ΔnX μ X
(1)
where ED and EP are total energies of doped and perfect graphene sheet (or nanoribbons) respectively. μC, μH, and μX are chemical potentials of C, H, and dopant X (X = N, P, As, Sb, and S), and ΔnC, ΔnH, and ΔnX are the difference between the number of C, H, and dopant atoms X in doped and perfect graphene sheet (nanoribbons), respectively. Here μC, μH, and μX are referred to the energy of C atom in graphene, the energy of H atom in H2 molecule, and the energy of dopant atom in its bulk crystal (or gas molecule), respectively. Then the most stable two configurations in each doping type are selected as the catalysts for the ORR based on the comparison of formation energies. The calculated formation energies of various graphene doping structures are listed in Table S1. The binding strength of ORR intermediates on the catalysts are evaluated by calculating their binding energies as follows: E b = ED + EI − ET
(2)
where ET and EI are the energies of an adsorbed system and isolated ORR intermediate, respectively. The ORR scheme includes two possible reaction pathways: four-electron (4e−) and two-electron (2e−) transfer pathways.42 The four-electron reaction was proved to be much more efficient than the two-electron one. In both acid and alkaline mediums, the 4e− reduction reaction includes two reaction mechanisms: (i) associative mechanism that involves the formation of *OOH during the ORR and (ii) direct O2 dissociation mechanism. To gain insight into the ORR pathway and mechanism, the free-energy diagram on various doped graphene catalysts has been calculated using a computational hydrogen electrode (CHE) model proposed by Nørskov et al.43 The CHE model defines that the chemical potential of a proton/electron (H+ + e−) in solution is equal to half of the chemical potential of a gaseous H2. The change of free energies (ΔG) in each a reaction step is calculated as follows:43 ΔG = ΔE + ΔZPE − T ΔS + ΔG U + ΔGpH
(3)
where ΔE is the reaction energy of reactant and product molecules adsorbed on catalytic surface in each ORR step, ΔZPE is the change in zero-point energies (ZPE), T is temperature which refers to the room temperature (T = 298.15 K), and ΔS is the change in entropy. The effect of an external bias (ΔGU) is shifted by −eU in each (H+ + e−) transfer step, where e is the number of electrons transferred and U is the applied bias. ΔGpH = kBTln 10×pH where kB is the Boltzmann constant, T is the room temperature (T = 298.15 K), and the pH value is set to 0 for the acidic medium and 14 for the alkaline medium, respectively. Zero-point energies (ZPE) and entropies of ORR intermediates are calculated from vibrational frequencies and those of gas phase molecules are obtained from the standard thermodynamic database.44 The calculated details for the determination of various energies and entropies of ORR intermediates have been indicated in S2 and S3 of the SI. All DFT calculations are performed using the projector augmented wave (PAW) method45 as implemented in the Vienna ab initio simulation package (VASP).46,47 The exchange-correlation energy is treated by the spin-polarized generalized-gradient approximation (GGA) with the version of Perdew−Burke−Ernzerhof (PBE).48 The kinetic energy cutoff for the plane-wave expansion is set to 400 eV. The k-point
2. COMPUTATIONAL DETAILS As reported by a large number of previous studies, the nonmetal dopants (X), such as N, P, As, Sb, and S, can be incorporated onto both graphene edges and basal plane.15−25,27 To identify the active sites of the doped graphene catalysts, three different doping types have been established (see Figure S1 in the Supporting Information (SI)), including X-doping onto graphene basal plane (G-X), zigzag edge (Z-X), and armchair edge (A-X), respectively. For the G-X, we consider four potential doping configurations (G-Xn, n = 1−4), and they are created by introducing nonmetal dopants into an in-plane graphene supercell consisting of 96 C atoms. For the doping of graphene edges, we consider five potential doping configurations at the zigzag edge (Z-Xn, n = 1−5) and the armchair edge (A-Xn, n = 1−5), respectively. The doping models of Z-Xn and A-Xn are established based on hydrogenated ribbons with the slab size of 20 Å × 11.45 Å and 9.26 Å × 17.04 Å, respectively. The stability of the potential doping configurations B
DOI: 10.1021/acs.jpcc.7b03093 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
graphene catalysts, the nearest-neighboring C atoms of dopant atoms exhibit the stronger interaction with ORR intermediates than the dopant atoms. Take G-N1 (N doped graphene basal plane, see Figure S1) as an example, where binding energies of *OOH and *OH are 0.10 and 0.89 eV at the N dopant and 0.40 and 1.85 eV at its nearest neighboring C atoms, respectively. Thus, the ORR active sites on N- and S-doped graphene catalysts locate at the nearest-neighboring C atoms of dopant atoms (see the left panel of Figure 1b). It originates from unpaired electrons of dopant atoms delocalized to its adjacent C atoms.10 In contrast, the active sites on P-, As-, and Sb-doped graphene catalysts situate at the dopant atoms due to the strong interaction of ORR intermediates with the dopant atoms (see the right panel of Figure 1b). It should be mentioned that vdW interaction plays an important role for describing the weak interaction on graphene-based catalysts. We make a comparison of binding energies of *OH on various graphene catalysts calculated using the DFT-D2 and PBE-GGA methods. As listed in Table S6, two methods lead to smaller energy difference (
