Behind the Synergistic Effect Observed on PhosphorusâNitrogen

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Behind the Synergistic Effect Observed on Phosphorus-Nitrogen co-Doped Graphene during the Oxygen Reduction Reaction Eduardo Gracia-Espino J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09425 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 25, 2016

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The Journal of Physical Chemistry

Behind the Synergistic Effect Observed on Phosphorus-Nitrogen co-doped Graphene during the Oxygen Reduction Reaction Eduardo Gracia-Espino* Department of Physics, Umea University, 90187 Umea, Sweden.

ABSTRACT

Ab initio calculations are performed to investigate how the simultaneous introduction of phosphorus and nitrogen into graphene modifies the availability and spatial distribution of catalytic active sites for oxygen reduction reaction (ORR). A phosphoryl group (R3-P=O) is selected as a representative for the phosphorus doping, and the ORR is studied under alkaline conditions where a 4e- mechanism is used to determine the limiting step and overpotential (ηORR) along the entire graphene surface. A scanning procedure is used to construct ηORR maps for pristine-, N-, P-, and diverse PN co-doped graphenes. The results indicate that a single N (P) atom activates up to 17 (3) C atoms, while the simultaneous introduction of P and N activates up to 55 C atoms equivalent to 57% of the surface. Additionally, PN co-doped graphenes reveals that the relative location of both dopants has significant effects on the ORR performance, where a P-N separation distance of at least 4 Å minimize the localization of electronic states on the

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neighboring C atoms and improves the quantity and distribution of active sites. The results shows the importance of designing synthesis procedures to control the dopant concentration and spatial distribution to maximize the number of active sites. Furthermore, the ηORR maps reveal features that could be obtained by scanning tunneling microscopy allowing to experimentally identify and possibly quantify the catalytic active sites on carbon-based materials.

1. Introduction Nowadays, carbon-based materials are widely used as non-precious metal electrocatalysts for oxygen reduction reaction (ORR) and other energy-related electrochemical processes.1-2 Nonetheless, the reduced chemical reactivity of graphitic nanocarbons must be improved prior to the use, and it is commonly enhanced by introducing diverse heteroatoms into the graphitic lattice, where the most common dopants are B,3-5 N,6-9 S,10-12 and P.13-15 These doped nanocarbons exhibit improved catalytic activity towards ORR under alkaline conditions, and in some cases better onset potential than commercial Pt/C catalysts.7 Another important feature of doped nanocarbons is their high tolerance against methanol crossover and improved operational stability,8, 13, 15-18 thus being promising candidates as noble metal-free electrocatalysts. A more recent approach to increase the ORR performance involves the simultaneous introduction of two or more heteroatoms into the graphene lattice, where typically B-N,18-20 S-N,17, 21-22 or P-N16, 23-25 are used as co-dopants. The presence of two or more dopants often significantly boost the catalytic activity showing preferential 4e- reaction pathways, reduced overpotentials (ηORR), and increased current densities. The enhanced activity is usually attributed to synergistic effects


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produced by the dopants, but little is known about what is behind the observed improved catalytic activity. Theoretical studies of ORR on carbon-based materials have shown that the introduction of dopants significantly improve the interaction with oxygenated species,18, 26-27 and thus enhancing the ORR performance in agreement with experimental results. However, most theoretical studies only report the activity of few or sometimes a single catalytic site,28-30 and only limited reports actually identify changes in catalytic activity along certain paths,26-27 but without providing enough information of the newly activated C atoms. As a result, there are no detailed information in how the introduction of one or more heteroatoms increase the number of active sites, and how these sites are distributed along the catalyst surface. One of the most promising candidates as metal-free ORR catalyst is the PN co-doped graphene, where recent studies have reported simpler production methodologies, lower onset potentials, enhanced methanol crossover and operational durability.16, 23-25, 31 In this work I report how the spatial distribution of catalytic actives sites are affected by the simultaneous introduction of phosphorus and nitrogen atoms into graphene. The ORR is studied by means of density functional theory under alkaline conditions, where the associative ORR mechanism is used to identify and subsequently map the ηORR along the entire graphene surface. The ηORR maps reveal features that could be obtained by scanning tunneling microscopy (STM) allowing to experimentally identify and possibly quantify the catalytic active sites of carbonbased materials. 2. Theoretical Basis Ab initio calculations are performed using density functional theory (DFT).32-33 The generalized gradient approximation with the Perdew, Burke and Ernzerhof parametrization is


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chosen for the exchange-correlation functional34 as implemented in the SIESTA code.35 The wave functions for the valence electrons are represented by a linear combination of pseudoatomic numerical orbitals using a double-ζ polarized basis (DZP),36 while core electrons are represented by norm-conserving Troullier-Martins pseudopotentials in the Kleinman-Bylander non-local form.37-38 The real-space grid used for charge and potential integration is equivalent to a planewave cut-off energy of 300 Ry. Sampling of the 2D Brillouin zone is carried out with 1×4×4 Monkhorst-Pack grids during geometric optimization, however a k-sampling of 1×25×25 is used to determine the density of states. The systems are constructed by reproducing the 2D graphene unit cell 7×7 times resulting in a supercell containing 98 atoms. Periodic boundary conditions are used and the inter-layer distance is kept to a minimum of 30 Å to avoid interactions with their periodic images. A variable-cell structural relaxation is performed on all systems in order to include the strain effects introduced by the dopant atom. The geometry optimization is performed by conjugate gradient minimization until the maximum force is < 0.04 eV/Å. The doping/functionalization is performed by introducing a phosphoryl (R3-P=O) group at the center of the graphene, then 3 carbon atoms are replaced by nitrogen to form substitutional doping sites. The location of these N atoms is varied resulting in five different co-doped graphene surfaces labeled PN1-, PN2-, … PN5-Gr. The P and N concentration are ~1 and ~3 at%, respectively, the resulting configurations are depicted in Figure 1. A non-doped graphene (Pristine-Gr), a single nitrogen-doped (N-Gr) and a single phosphorus-doped graphene (P=O-Gr) are used as reference systems. Spin polarized calculations of ORR intermediates (*O, *OOH, and *OH) on the PN3-Gr system were carried out, and all systems registered changes in total energy of less than 0.10 eV when compared to their non-polarized counterparts. Therefore, a non-spin polarized scheme is used to study the rest of the doped graphene surfaces.


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The free energy of intermediates is calculated based on the computational hydrogen-electrode (CHE) model.39 The CHE model provides a way of including the effect of the applied electrical potential on reactions involving a proton-electron transfer. This technique has been used in several publications and has proved its feasibility to study diverse electrochemical reactions on metal and carbon based catalysts.18, 27, 40-41 The CHE model states that the reference potential is that of the standard hydrogen electrode (SHE), meaning that under standard conditions (pH = 0 in the electrolyte and 1 bar of H2 in the gas phase at 298.15 K) the chemical potential of a proton–electron pair (H+ + e-) in solution is equal to half of the chemical potential of a gas-phase H2 molecule at an electrode potential of U = 0. In this way, it avoids the explicit treatment of solvated protons, and instead it only requires the treatment of the gaseous hydrogen molecule easily computable using DFT. The Gibbs free energy change (∆G0) of intermediates is calculated by ∆G0 = ∆E + ∆ZPE − T∆S, where ∆E is the energy change obtained from DFT calculations, ∆ZPE is the change in zero-point energies, T is the temperature (298.15 K), and ∆S is the change in entropy. Standard values of zero-point energies and entropies for the ORR intermediates are obtained from Nørskov et al,39 see Table S1. Note that in the CHE model, the ∆G is computed using H2 and H2O as a reference states, hence a set of equivalent reactions for the ORR mechanism are used to determine ∆G0, see supporting information for further details. The effect of bias on states involving electron transfer results in a direct shift in the free energy of the electrons, and it is included by correcting the Gibbs free energy (∆GU) by –eU, where U is the applied bias (U=1.23 V) and e is the elementary charge. The pH correction is included as ∆GpH = kT×ln[H+], where k is the Boltzmann constant, T temperature, and pH = 14, resulting in a value of -0.828 eV. The total change in free energy is given by ∆G = ∆G0 + ∆GU + ∆GpH.


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Simulated STM images of the PN-doped graphene are obtained using the Tersoff-Hamann approximation42 as implemented in the STM utility available in the Siesta code. The charge density of the surfaces are calculated using a symmetric grids of 150 points. The resulting charge density are processed with the WSxM software43 to produce the STM images.

2. Results and Discussion 2.1. Structural characteristics and electronic properties The creation of PN co-doped graphene is performed by introducing a phosphorus atom at the center of the graphene super-cell. Owing to the large variety of chemical configuration that phosphorus can adopt, a phosphoryl group (R3-P=O) is selected to represent the phosphorus doping. Other studies have focused on substitutional doping where the P atom is simply bonded to three carbon atoms,44-45 however this configuration is highly reactive due the lone pair of electrons present in the phosphorus atom.46 Additionally, considering that the present material is intended to use as a catalyst for ORR in an aqueous environment, it is acceptable to assume that the P atom will adopt a phosphoryl (R3-P=O) configuration that is more energetically favorable than a R3-P doping.46 Afterward, the nitrogen doping is performed by adding three substitutional nitrogen atoms, in other words replacing three C atoms for N, where the P-N distance is varied by symmetrically placing the N atoms at different positions along the graphene sheet surrounding the phosphoryl group. In this way, five different systems are created with a P-N distance of 1.70, 2.75, 4.51, 7.35 and 8.78 Å, the PN-doped graphenes are labeled PN1-, PN2-, PN3-, PN4- and PN5-GR, respectively. The P and N concentrations on PN co-doped systems are kept constant at 1 and 3 At%, respectively (P:N ratio of 1:3). Diverse experimental reports have shown that PN co-doped graphenes tend to contain more nitrogen than phosphorus in their


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structure,16, 23, 25, 31 where the P:N ratio might strongly depend on the synthesis procedure. Here I selected a P:N ratio of 1:3 similar to those observed experimentally by Li, R., et al (P:N = 1:2.7),16 and Dong, L., et al (P:N = 1:2.1),23 where co-doped graphenes with this particular P:N ratio exhibited the best catalytic performance towards ORR. Figure 1 depicts the position of the studied co-doped configurations, the numbers indicate the position of the nitrogen atoms corresponding to each PN co-doped system, see also Figure S1 in the electronic supplementary information (ESI). Additionally, non-doped (pristine-Gr), nitrogen-doped- (N-Gr), and phosphorus-doped (P=O-Gr) graphenes are analyzed as reference systems.

Figure 1. Phosphorus-nitrogen doped graphene. The circles indicate the location of nitrogen atoms (three in each system) surrounding the phosphoryl (R3-P=O) group, shown as a letter P. The numbers in the circles correspond to specific systems (PN1-, PN2- … PN5-Gr). The blue numbers show N atoms in adjacent cells. The dashed line indicates the size and shape of the periodic graphene cell used during the computations.


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The introduction of a single N atom (N-Gr) generates marginal modification on the honeycomb structure with minor changes on the inter-atomic distances around the doping site, the registered expansion in the lattice parameter is ~0.13%. On the other hand, the introduction of a P atom (P=O-Gr) cause severe changes on the local structure, where the phosphorus-carbon bond length forces the R3-P=O group to adopt an out-of-plane configuration causing an increase of 0.56% in the lattice parameter, see Figure S1a in the ESI. The simultaneous introduction of P and N into the graphene makes no significant changes when compared to the P=O-Gr, see Figure S1(b-c). The electronic density of states of pristine- and doped-Gr are depicted in Figure 2. The introduction of a single N atom (N-Gr) induces a well-known n-type doping with a slight accumulation of states at the Fermi level, see Figure 2b. On the other hand, the R3-P=O group induces a p-type doping with the appearance of some localized states below the Fermi level and a large peak at -1.3 eV characteristic of the R3-P=O group, marked by an * in Figure 2h. More precisely the O atom contributes significantly to this peak as indicated by its partial density of states in Figure S2a. In both cases, N- and P=O-Gr, the enhanced density of states at the Fermi level suggest improved chemical reactivity when compared to pristine-Gr (Figure 2a). The electronic properties of PN co-doped systems are significantly affected by the relative position of the N atoms and the phosphoryl group. The density of states in Figure 2(c-g) exhibit two interesting features, one showing the effect of having the dopant atoms confined in a small region, and the other the effect of moving apart the N atoms from the R3-P=O group. The analysis of confining the dopants is first described, in this case, peaks located in the conduction band (marked by “+” in Figure 2) are moved towards the Fermi level depending on the dopants (P-N or N-N) relative position. Systems with small P-N or N-N separation distance (< 3 Å), namely systems PN1- and PN5-Gr, exhibit highly localized states at the Fermi level. The


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electronic state localization present on the PN1-Gr is attributed to the close vicinity of the N atoms and the phosphoryl group (P-N distance 1.70Å), on the other hand the states localization on the PN5-Gr is due the small separation distance between N atoms (2.4 Å). Note that this peak localization arise mainly from C atoms situated near the dopants, indicating an enhanced chemical reactivity (see Figure S2). The prominent state localization at the Fermi level observed on PN1- and PN5-Gr will have significant effects on the catalytic properties as discussed later. For those systems with larger P-N or N-N separation distance (PN2-, PN3-, PN4-Gr), the contribution to states at the Fermi level from C atoms around the dopants is minimized and its main contribution is shifted towards the conduction band as seen in Figure S2, a summary is presented in Table 1. The second one and more interesting feature is the shifting of the R3-P=O peak (identified by * in Figure 2) that is initially observed at -1.3 eV in P=O-Gr. After the introduction of N atoms close to the phosphoryl group, forming the PN1-Gr, the peak is shifted to lower energies (-4.3 eV) with significantly lower intensity. However, increasing the P-N separation shifts the peak to higher energy values, where eventually at sufficiently large distance the peak should returns to its original position at -1.3 eV as found in P=O-Gr, indicating that the effect of N on the phosphoryl group is minimized. From this study, a separation of ~8.8 Å between the P and N atoms shift the R3-P=O peak at -1.6 eV, only -0.3 eV from the single doped P=O-Gr, suggesting that the P-N interaction could vanish after a few nanometers. The evolution of the phosphoryl peak is clearly observed in Figure 2(c-h) with values of -3.1, -2.3, -2.0 and -1.6 eV for PN2-, PN3-, PN4- and PN5-Gr respectively, see also Table 1. The variation of the peaks position could be used to experimentally study the effect that N atoms have over the phosphoryl group, and possible extended to other dopants, by using STM and differential conductance dI/dV


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spectroscopy.47-49 Finally, the highly localized states and electronic gaps above and below the Fermi level present in systems with small P-N or N-N separation distance, PN1- and PN5-Gr, suggest deteriorated electron transport affecting the overall ORR.46, 50 On the other hand, those systems with larger P-N and N-N separation distances, such as PN2- and PN3-Gr, might exhibit the best electron transport characteristics that are highly desirable for ORR.

Table 1. P-N, and N-N separation distances, ηORR and peaks position of pristine and doped graphenes. Note, due the cell periodicity the shortest distance is reported.

distance (Å)

Sites with ηORR 1 eV. All atoms in pristine graphene are chemically equivalent and thus the analysis of a single site provides the description of the whole surface, therefore a map is not necessary. The main disadvantage of pristine graphene is the low chemical reactivity towards oxygenated species, thus exhibiting a large and unique ηORR equal to 1.0 eV, where the limiting step is the adsorption of molecular oxygen (Eq. 1), in agreement with Gong, Y, et al (ηORR = 1.18 eV).18 The complete analysis of the ORR mechanism is shown in Figure 4a, black line. Now by introducing a single nitrogen atom (N-Gr), the chemical reactivity of neighboring C atoms is significantly enhanced, improving the O2 interaction and reducing the ηORR.


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Figure 3. ηORR maps for N- and PN-doped graphene surfaces. (a) N-Gr. (b-f) PN1-, PN2-, PN3-, PN4- and PN5-Gr. The dashed line indicate the size and shape of the super-cell, the N (P) letters indicate the position of the nitrogen atoms (phosphoryl group). The “x” indicates the position where the lowest ηORR is observed and the corresponding free energy diagrams are plotted in Figure 4. All values are in eV.


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In this case, the best catalytic sites are the C atom adjacent to the N dopant with a reduced ηORR equal to 0.71 eV and the adsorption of O2 as the limiting step. This is in line with previous observations where the C atoms adjacent to N impurities are the most reactive sites.27, 30 Note that, it is not possible to determine the ORR activity directly on the N atom since the O2 molecule will preferentially adsorb on the adjacent C atoms, thus suggesting that the N atom is not catalytically active, at least not at low O2 coverage. This could be a consequence of the slightly positive charge observed on the adjacent C atoms,54 as well as the enhanced charge transfer towards the O2 molecule when adsorbed on the C atom and compared to non-doped graphene,55 and increased contribution of electronic states at the Fermi level as seen in Figure S2. The ORR free energy diagram of an adjacent C atom (indicated by an “x” in Figure 3a) is depicted in Figure 4a, and the optimized structures of the intermediates are shown in Figure S4 and S5. From Figure 3a, it is possible to observe that other C atoms (second and third neighbors) surrounding the N-dopant are also benefitted but with slightly larger ηORR (0.79 and 0.77 eV), as a result the introduction of a single N atom activates at least 17 C atoms with a ηORR < 0.81 eV, suggesting that larger current densities could be achieved at lower potentials when compared with non-doped carbon nanomaterials, as seen experimentally.6, 8 Interestingly, the map in Figure 3a shows that there are some preferential directions where the C atoms exhibit lower ηORR, resulting in a peculiar pattern seen as an upside-down triangle with the vertices located along the armchair directions of the graphene sheet (~4.3 Å away from the N atom). This preferential orientation could be artificially introduced due the geometry of the graphene super-cell, however, diverse experimental topography studies by STM have revealed a triangle shaped pattern around single doped N sites47-49 with vertices located along the armchair orientation as observed in Figure 3a. This opens the opportunity to experimentally observe and map the location of


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potential catalytic sites, especially since STM topography studies are already used to identify the dopant configuration.47-49 Continuing with the single-doped P=O-Gr, the introduction of a phosphoryl group generates local alterations in the structure, as well as changes in the electronic properties. However, these modifications are not beneficial at all for the ORR, where a weaker effect is observed on the neighboring C atoms than the N-Gr, and only the third neighbors exhibit an increment in ORR activity (ηORR = 0.78 eV, O2 adsorption is the limiting step), while the rest of the C atoms display a ηORR in the range of 0.84-0.98 eV. Interestingly, the first and second neighbors exhibit an overpotential of 0.84 and 0.92 eV, respectively, thus only the third neighbors located along the armchair direction exhibit an overpotential

Behind the Synergistic Effect Observed on PhosphorusâNitrogen

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