RuN4 Doped Graphene Oxide, a Highly Efficient Bifunctional Catalyst

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RuN4 doped graphene oxide, a high efficient bifunctional catalyst for oxygen reduction and CO2 reduction from computational study Shize Liu, Lin Cheng, Kai Li, Cong Yin, Hao Tang, Ying Wang, and Zhijian Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05729 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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ACS Sustainable Chemistry & Engineering

RuN4 doped graphene oxide, a high efficient bifunctional catalyst for oxygen reduction and CO2 reduction from computational study Shize Liu,†,‡ Lin Cheng,*,† Kai Li,‡ Cong Yin,§ Hao Tang,*,§ Ying Wang,‡ Zhijian Wu*,‡ †

College of Chemical Engineering, Inner Mongolia University of Technology, Inner Mongolia Key Laboratory of Theoretical and Computational Chemistry Simulation, Hohhot 010051, P. R. China. ‡ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China. § School of Automation Engineering, University of Electronic Science and Technology of China, Chengdu 611731, P. R. China. Abstract The searching of high efficient bifunctional catalysts for both oxygen reduction reaction (ORR) and CO2 reduction reaction (CO2RR) for energy conversion and storage has caused particular attention in recent years. In this work, through computational design, we have studied both ORR and CO2RR for RuN4 doped graphene (RuN4-Gra) and modified RuN4-Gra by oxygen atom (RuN4/O-Gra). For ORR, our calculations indicated that RuN4/O-Gra has better catalytic performance both kinetically and thermodynamically. The calculated energy barrier is 0.39 eV at the rate-determining step, much smaller than 0.80 eV for pure Pt and 0.56 eV for FeN4-Gra. The predicted working potential is 0.54 V, larger than 0.38 V for FeN4-Gra. For CO2RR, the formation of HCOOH is refrained due to the strong adsorption of CO and strong hydrogen evolution reaction (HER) for RuN4-Gra. However, after the modification of RuN4-Gra with oxygen atom, the reaction pathway is changed and the formation of CO is prevented. RuN4/O-Gra has high selectivity toward HCOOH, and HER is also refrained. Therefore, RuN4/O-Gra is a high efficient bifunctional catalyst for both ORR and CO2RR.

Keywords: RuN4 doped graphene; RuN4 doped graphene oxide; Oxygen reduction reaction; CO2 reduction reaction; Density functional method; *Corresponding authors. [email protected] (LC); [email protected] (HT); [email protected] (ZJW). 1

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Introduction In the last few decades, the over-use of fossil fuels has brought many serious problems, such as energy crisis and global warming. To solve these problems, the clean sustainable energy sources and effective ways to reduce the concentration of CO2 in the atmosphere have been developed. Full cells (FCs) as high efficient energy devices without greenhouse gas emission have been studied extensively [1, 2]. The main challenge for FCs is the sluggish oxygen reduction reaction (ORR) at the cathode, which lowers the energy efficiency and hinders their commercial applications [3, 4]. On the other hand, CO2 reduction reaction (CO2RR) is an efficient method to convert the inert CO2 into either liquid fuels or other valuable chemical products (e.g., HCOOH, CH3OH, CH4, etc.) [5-12]. The main problem during the CO2RR is large overpotential, low selectivity and CO poisoning. In addition, the strong hydrogen evolution reaction also refrains the CO2RR. Up to now, Pt and Pt-alloys are still the most efficient catalysts for ORR [13], while the metal catalysts (e.g., Fe Cu, Pd, Ni, etc.) are studied for CO2RR [14-18]. However, these metal catalysts have also many disadvantages, such as high cost, low tolerance for CO poisoning, lower selectivity, etc. [19-22]. Therefore, the low dimensional carbon materials, such as graphene [23], have become the recent focus due to their low cost, high catalytic activity, low overpotential, etc. For ORR, several studies indicated that Co and N co-doped graphene has excellent catalytic performance for ORR [24, 25]. The Mn modified glycine derivative-carbon has lower half-wave potential and excellent durability, which is comparable to the commercial Pt/C catalyst [26]. Fe and N codoped graphene has good onset potential and high reduction current densities for ORR [27]. Further study showed that Fe and N codoped porous graphene is an efficient and cost-effective catalyst for ORR both in alkaline solutions and acidic solutions [28]. The carbon-supported Ru85Se15 chalcogenide shows better ORR catalytic activity than Pt/C catalyst in the methanol solution [29]. Theoretically, it has been found that CoN4 doped graphene can promote the ORR toward the four-electron reduction pathway with a low reaction barrier [30-32]. The MnN4 doped graphene could also promote the 2


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ORR along four-electron reaction pathway with the energy barrier of 0.51 eV at the rate-determining step [33]. FeN4 doped graphene is reported to have excellent catalytic active for ORR, and the favorable kinetically pathway is the OOH dissociation with the energy barrier of 0.56 eV at rate-determining step [34]. In addition, a series of MN4 (M = transition metals) doped graphene has been studied and the ORR activity is discussed based on the volcano plot [35]. For CO2RR, the experimental studies showed that Co-N5 site anchored on polymer-derived hollow N-doped porous carbon spheres shows nearly 100% CO selectivity for CO2RR [36]. Construction of transition metal cyclam-like moieties within N-doped graphene is reported to have high catalytic activity and CO selectivity for CO2RR [37]. The nickel-nitrogen-modified graphene has high catalytic active for reduction CO2 to CO, in which the selectivity reaches 90% [38]. On the theoretical aspect, the single Ru doped graphene is reported to have high selectivity and catalytic activity for reducing CO2 to CH4 [39]. The CoN4, RhN4 and IrN4 doped graphenes have excellent catalytic activity and low overpotential for CO2RR. For the CoN4 doped graphene, CH2O is the main product, while the main product is CH3OH for RhN4 and IrN4 doped graphenes [40]. Besides the separate studies for both ORR and CO2RR as mentioned above, there are also studies in which ORR and CO2RR are investigated in a single work [41-45]. For Fe and N codoped carbon material, it has been found that it shows not only high onset potential, superior stability and poisoning tolerance for ORR, but also nearly 100% selectivity for CO production during CO2RR [41]. Fe/Fe3C nanoparticles embedded in N-doped carbon nanotubes also exhibit multifunctional catalytic activity for ORR, OER (oxygen evolution reaction) and CO2RR [42]. In Fe and N codoped carbon black, it has been found that ORR and CO2RR occur at different active sites, and pyrrolic N is highly relevant to CO2RR [43]. For the heterojunction-assisted Co3S4@Co3O4 core-shell, ORR is a four-electron process, while the main product of CO2RR is formate with a high faraday efficiency of 85.3 % [44]. Highly dispersive Au nanoparticle on carbon black has high catalytic activity for ORR in basic solution, and it also has high CO selectivity for CO2RR without HER at the potential ≥ -0.725 3



V [45]. Motivated by the previous studies, in this work, we have studied the ORR and CO2RR for RuN4 doped graphene (RuN4-Gra). During our study, we found that RuN4-Gra is easily to be poisoned by CO due to its strong adsorption on the catalytic surface. To solve the issue, we have modified the RuN4-Gra by oxygen atom (RuN4/O-Gra, i.e., RuN4 doped graphene oxide) in order to reduce the binding strength of CO. Indeed, after the modification, CO poisoning effect is prevented and RuN4/O-Gra shows excellent catalytic activity toward both ORR and CO2RR. We hope this work could provide an effective strategy to enhance the catalytic activity by tailoring the geometric structures.

Computational methods All the geometry optimizations, charge analysis and frequency calculations were performed using the spin-polarized density functional theory (DFT) method [46, 47] implemented in Vienna Ab initio Simulation Package (VASP) [48, 49]. The projector augmented wave method [50] was used to describe the ion-electron interaction. The exchange correlation functional of Perdew-Burke-Ernzerhof (PBE) [51] within the generalized gradient approximation was used in all the calculations. The plane-wave energy cutoff was set to be 400 eV. The Brillouin zone was sampled with 551 Monkhorst-Pack-point grid [52]. The convergence criteria for the electronic and ionic iterations were set to be 10-5 eV and 0.05 eV/Å, respectively. The transition state and energy barrier were determined by the climbing image nudged elastic band method [53]. The Quantum Chemical Molecular dynamics (QM/MD) simulations were used to determine the stability of the studied compounds. The NVT ensemble at the temperature of 300K is simulated using the algorithm of Nosé. The van der Waals corrections from the semi-empirical dispersion-corrected density functional theory were considered in all calculations [54, 55]. A 55 surpercell graphene was used to build RuN4-Gra (Figure 1a) and RuN4/O-Gra (Figure 1b) with the lattice parameters a=b=12.3 Å. A 15 Å vacuum layer was set to prevent the interactions of periodic images. 4


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To determine the most stable adsorption configuration of the intermediates, the adsorption energy ΔEads is calculated as: ΔEads = Eadsorbate/sub − Eadsorbate − Esub (1) where the subscript "sub" represents the substrate RuN4-Gra or RuN4/O-Gra. Eadsorbate/sub and Eadsorbate refer to the energy of the substrate with adsorbate and the isolated adsorbate, respectively. The Esub is the energy of clean substrate. The formation energy ΔEf is calculated by the following formula: ΔEf = Esub + 6μC − (EGra + 4μN + μRu) (2) where EGra is the total energies of the perfect graphene. μC is the chemical potential of the carbon atom defined as the total energy per carbon atom in a perfect graphene. μN is the chemical potential of nitrogen which is obtained by the one-half of gaseous N2 molecule. μRu is the energy for isolated Ru. The calculation of the free energy change (ΔG) of each reaction step is based on the computational hydrogen electrode (CHE) model [56], which is defined as follows: ΔG = ΔE + ΔZPE − TΔS + ΔGU + ΔGpH (3) where ΔE refers to the energy from DFT calculations. ΔZPE and ΔS refer to the energy difference of zero point energy and entropy, respectively. T is the temperature (T = 298.15 K). ΔGpH = kBT  In10  pH, in which kB is the Boltzmann constant, and pH = 0 for the acid medium. In the CHE model [56], the free energy of H+ + e- can be replaced by the half of H2 (g) at standard conditions. The free energy of O2 is 4.92 eV, which is obtained from the reaction of O2+2H2→2H2O at the temperature of 298.15 K and the pressure of 0.035 bar. The entropies and vibrational frequencies of the gas molecules such as O2, H2, H2O, CO2, CO, etc. are taken from the NIST database [57]. The zero point energy and entropy of each intermediate are obtained by calculating the vibrational frequencies. According to CHE model, the limiting potential (UL) can be derived as UL = -ΔGmax/e, in which ΔGmax is the largest free energy change among all the elementary reactions in the reaction pathway.

Results and discussion The stability and electronic properties of RuN4-Gra and RuN4/O-Gra. 5



Figure 1. The optimized structure of RuN4-Gra (a) and RuN4/O-Gra (b). The charge density difference distribution of RuN4-Gra (c) and RuN4/O-Gra (d). The isosurface value is 0.0035. In (c) and (d), the yellow and cyan colors represent charge accumulation and depletion, respectively. The gray, blue, green and red colors represent C, N, Ru and O, respectively.

The optimized structures of RuN4-Gra and RuN4/O-Gra are shown in Figure 1a and Figure 1b, respectively. The calculated formation energy is -3.81 eV for RuN4-Gra and -5.74 eV for RuN4/O-Gra, respectively, indicating that they are thermodynamically stable. Quantum Chemical Molecular dynamics study also supports this conclusion (Fig. S1). The calculated partial density of states suggested that the large hybridization occurs between N-2p and C-2p orbitals below the Fermi energy level for both RuN4-Gra and RuN4/O-Gra. In addition, the strong hybridization between N-2p and Ru-4d orbitals occurs in the range of 0.0 to -10.0 eV for both RhN4-Gra and RuN4/O-Gra. This means that the interaction between C and N, as well as Ru and N are strong. The splitting of Ru 4d orbitals are seen clearly near the Fermi energy level for RuN4/O-Gra. The calculated d-band center is -2.59 eV for RuN4-Gra, -4.55 eV for RuN4/O-Gra. This revealed that the binding strength of the intermediates is weakened after RuN4-Gra is oxidized to form RuN4/O-Gra, as demonstrated from the adsorption energies in Tables 1-3. 6


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Figure 2. The calculated partial density of states for RuN4-Gra (a) and RuN4/O-Gra (b). The black and pink vertical dotted lines represent the Fermi energy level and d-band center (εd), respectively.

ORR activity. The adsorption of ORR species and active sites on RuN4-Gra and RuN4/O-Gra. For ORR, the adsorption of O2 is the first step. There are usually two configurations for the adsorption of O2 on the catalyst surface, i.e., side-on and end-on configurations. For RuN4-Gra, the most stable adsorption structures are shown in Fig. S2, Supporting information, and the corresponding energies are listed in Table 1. As seen from Table 1, both adsorption configurations are located for RuN4-Gra. The adsorption energy is -1.77 eV for side-on configuration, and Ru-O bond distance is 1.99 Å. The adsorption energy for end-on configuration is -1.85 eV with Ru-O bond 7



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distance of 1.88 Å. In addition, the atomic O has the strongest adsorption energy of -5.27 eV with the short Ru-O bond distance of 1.77 Å. OH shows stronger adsorption than OOH. H is also adsorbed on the catalyst surface strongly with the adsorption energy of -3.06 eV. Water molecule is easy to be desorbed due to the weak adsorption energy of -0.20 eV.

Table 1. The adsorption energies ΔEads (eV), Bader charges Q (e) and bond distances dRu-O and dO-O (Å) for the most stable adsorption configurations in RuN4-Gra. QRu, QN4, QC-total, and Qads refer to the charge of Ru, the total charge of four N atoms, the total charge of eight C atoms connecting to the four N atoms, and adsorbed species, respectively. NC is the net charge (NC = QRu+ QN4 + QC-total + Qads). ΔEads

dRu-O

dO-O

QRu

QN4

QC-total

Qads

NC

RuN4-Gra

-

-

-

0.89

-4.91

4.71

-

0.69

side-on O2

-1.77

1.99

1.41

1.30

-4.44

3.90

-0.70

0.06

end-on O2

-1.85

1.88

1.31

1.18

-4.65

4.00

-0.48

0.05

O

-5.24

1.77

-

1.36

-4.68

4.12

-0.59

0.21

H

-3.06

-

-

0.91

-4.62

3.61

-0.11

-0.20

OH

-3.58

1.94

-

1.25

-4.64

4.06

-0.40

0.27

OOH

-2.44

1.89

1.51

1.20

-4.46

3.45

-0.41

-0.22

H2O

-0.20

2.17

-

1.01

-4.81

4.39

0.12

0.71

The calculated charge indicated that for RuN4-Gra, RuN4 moiety and its eight C atoms have net charge of 0.69 |e| (Table 1). This implies that the charge transfer occurs mainly around RuN4 moiety and the eight C atoms connecting to N atoms, which should be the active sites for the catalytic activity. After the intermediates are adsorbed, even smaller net charge is obtained, demonstrating again that RuN4 moiety and the eight C atoms connecting to the N atoms are the catalytic active sites. This conclusion is also supported by the calculated charge density difference (Fig. 1c).

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Table 2. The adsorption energies ΔEads (eV), Bader charges Q (e) and bond distances dRu-O and dO-O (Å) for the most stable adsorption configurations in RuN4/O -Gra. QRu, QO, QN4, QC-total, and Qads refer to the charge of Ru, the charge of O at the bottom, the total charge of four N atoms, the total charge of eight C atoms connecting to the four N atoms and adsorbed species, respectively. NC is the net charge (NC = QRu+ QO+ QN4 + QC-total + Qads). ΔEads

dRu-O

dO-O

QRu

QO

QN4

QC-total

Qads

NC

RuN4/O-Gra

-

-

-

1.38

-0.60

-4.69

4.10

-

0.19

side-on O2

-0.47

2.10

1.35

1.51

-0.62

-4.61

4.90

-0.61

0.57

end-on O2

-0.64

2.10

1.30

1.44

-0.57

-4.55

4.32

-0.50

0.14

O

-3.88

1.79

-

1.71

-0.62

-4.53

4.43

-0.72

0.26

H

-1.60

-

-

1.33

-0.62

-4.54

3.53

-0.18

-0.47

OH

-2.71

2.03

-

1.51

-0.62

-4.53

4.35

-0.49

0.23

OOH

-1.48

2.05

1.45

1.52

-0.62

-4.56

3.86

-0.47

-0.27

H2O

-0.33

2.50

-

1.43

-0.63

-4.60

4.06

0.05

0.31

For RuN4/O-Gra, the most stable adsorption structures are shown in Fig. S3 and the corresponding energies are listed in Table 2. From Table 2, it is seen that similar trend for the adsorption energy is observed as in RuN4-Gra. However, we noted that after modified by atomic O, the adsorption energy is weakened for all the intermediates, except for water molecule which is slighted strengthened. This is in agreement with the lower d band center of RuN4/O-Gra compared with RuN4-Gra (Fig. 2). The catalytic active sites are again the RuN4 moiety and the eight C atoms connecting to the N atoms, as demonstrated by both the calculated net charge (Table 2) and charge density difference (Fig. 1d).

ORR mechanism. The complete ORR mechanism is studied for both RuN4-Gra and RuN4/O-Gra. All the possible reaction pathways in the gas phase and in solution are listed in Fig. 9



S4 for RuN4-Gra and Fig. S5 for RuN4/O-Gra. The most favorable pathways are summarized in Fig. 3.

Figure 3. The most favorable kinetic pathways for RuN4-Gra and RuN4/O-Gra. The numerical values in the parenthesis are the energy barriers (left) and reaction energies (right) in eV. The black and red data represent the gas phase and solution, respectively. The asterisk (*) indicates that the species is adsorbed.

To start the ORR, the first step is the O2 absorbed on the catalyst surface. Subsequently, O2 will either decompose into two separated O atoms, or hydrogenated to form OOH. Our calculations indicated that for both RuN4-Gra and RuN4/O-Gra, the direct O2 dissociation is difficult due to the high energy barriers of 1.23 eV (Fig. S4) and 0.90 eV (Fig. S5), while the formation of OOH is easy with the energy barriers of 10


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0.05 eV and 0.21 eV (Fig. 3), respectively. This means O2 hydrogenation to form OOH is easy. After the formation of OOH, the following reaction pathways would be either its decomposition to form O+OH, or its further hydrogenation to form O+H2O, OH+OH and HOOH. Our calculations suggested that HOOH is not stable on both RuN4-Gra and RuN4/O-Gra surfaces. Thus, both catalysts have a four-electron process. In the following, we will discuss these remaining pathways. For RuN4-Gra (Fig. 3), the most favorable pathway is the formation of O+H2O with the energy barrier of 0.12 eV and the reaction is exothermic by -3.08 eV. After the first water molecule is desorbed from the catalyst surface, the remaining O atom will be hydrogenated further to form the second water molecule. The formation of OH has an energy barrier of 0.35 eV. The formation of the second water molecule needs an energy barrier of 0.47 eV, which is the rate-determining step for RuN4-Gra. In addition, from Fig. 3 it is also seen that the decomposition of OOH to form O+OH and the formation of OH+OH are also competitive pathways with the energy barriers of 0.53 eV and 0.56 eV, respectively. After considering the solvent effects, the main conclusion remains the same as in the gas phase. In solution, the energy barrier is 0.51 eV, slightly larger than 0.47 eV in the gas phase (Fig. 3). For RuN4/O-Gra (Fig. 3), the formation of O+H2O is the favorable pathway. After the formation of OOH, its hydrogenation gives O+H2O immediately with zero energy barrier and large exothermic energy of -2.52 eV. The hydrogenation of O atom forms OH with an energy barrier of 0.39 eV, which is the rate-determining step for RuN4/O-Gra. Solvent effect does not have great influence on this pathway. However, for OOH dissociation pathway (i.e., f2→g2→h1→i1→j1), the energy barrier at rate-determining step drops significantly from 0.70 eV in the gas phase to 0.40 eV in solution, making the pathway is also a competitive pathway (Fig. 3).

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Figure 4. The free energy changes of the reaction pathway O2 → *O2→ *OOH → *O → *OH → H2O on RuN4-Gra (a) and RuN4/O-Gra (b) at different electrode potentials. The solid and dashed lines denote the gas phase and solution, respectively.

In a word, for RuN4-Gra and RuN4/O-Gra, the most favorable pathway is the same, i.e., OOH → O → OH → H2O (Fig, 3). However, the rate-determining step is different. For RuN4-Gra, the rate-determining step is the formation of the second water molecule with an energy barrier of 0.47 eV, while it is the formation of OH with an energy barrier of 0.39 eV for RuN4/O-Gra. Thus, RuN4/O-Gra has slightly lower energy barrier. This value is much smaller than 0.69 eV for CoN4-Gra [32], 0.56 eV for FeN4-Gra [34], 0.51 eV for MnN4-Gra [33], 0.79 eV for Pt (111) [58] and 0.80 eV for Pt (100) [59]. This indicates that kinetically, the two catalysts, in particular RuN4/O-Gra, would have good performance for ORR. On the other hand, the calculated working potential is 0.0 V for RuN4-Gra, 0.54 V for RuN4/O-Gra (Fig. 4), larger than 0.41 V for FeN4-Gra [34] for the latter. After considering the solvent effect, the trend of free energy change is similar to that in the gas phase. Therefore, RuN4/O-Gra would have better ORR performance both thermodynamically and kinetically than RuN4-Gra.

CO2RR activity. The structures and adsorption of CO2RR species. For CO2RR, the CO2 adsorption on the catalyst is the first step. For RuN4-Gra, the adsorption energy is -0.78 eV (Table 3), indicating a relatively strong adsorption. 12


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After CO2 is adsorbed on the catalyst surface, the angle ∠OCO becomes bent with the value of 141.1º (Fig. 5). The coupling of CO2 with (H+ + e−) would form COOH and HCOO. The adsorption energy is -3.11 eV for COOH, -3.09 eV for HCOO, close to each other. HCOOH has a weak adsorption energy of -0.92 eV, indicating that it will release from the catalyst surface as a liquid product. In addition, CO adsorbs strongly on the catalyst surface with an energy of -2.73 eV, which might poison the catalyst.

Table 3. The adsorption energies ΔEads (eV) and bond distances dRu-C (Å) of CO2RR intermediates on RuN4-Gra and RuN4/O-Gra. RuN4 -Gra

RuN4/O-Gra

ΔEads

dRu-C

ΔEads

dRu-C

*CO2

-0.78

2.04

-0.21

3.61

*COOH

-3.11

1.99

-1.43

2.21

*HCOO

-3.09

2.51

-2.41

3.14

*HCOOH

-0.92

2.92

-0.57

3.56

*CO

-2.73

1.81

-0.45

2.03

For RuN4/O-Gra, the modification of oxygen atom on the graphene weakens the adsorption of CO2RR species, similar to ORR species. The weak adsorption of CO2 (-0.21 eV) (Table 3) makes CO2 only slightly bent (∠OCO=179.5º, Fig. 5). Contrary to RuN4-Gra, HCOO shows much stronger adsorption energy than COOH (Table 3). Compared with RuN4-Gra, HCOOH is further weakened with the adsorption energy of -0.57 eV. The adsorption energy for CO is -0.45 eV, which is much weaker than -2.73 eV on RuN4-Gra (Table 3). Fig. 5 also shows that for HCOO, monodentate structure with only one Ru-O bond is formed for RuN4/O-Gra, while bidentate structure with two Ru-O bonds is observed for RuN4-Gra.

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Figure 5. The geometry evolution for the formation of HCOOH for RuN4-Gra (a) and RuN4/O-Gra (b). The number on the top of each configuration indicates transferred proton-electron (H+ + e-) pairs.

CO2RR mechanism. For RuN4-Gra, as seen in Fig. 6a, the hydrogenation of CO2 forms HCOO and COOH. For HCOO, the free energy nearly unchanged. The formation of HCOOH needs a free energy of 0.45 eV. For COOH, the free energy change is -0.43 eV. The formation of HCOOH from COOH needs a free energy of 0.85 eV. Thus, for the formation of HCOOH, the pathway from HCOO is preferred. On the other hand, From Fig. 6a, it is also seen that COOH hydrogenation also gives CO with free energy change of -1.11 eV, which is the preferred intermediate compared with HCOOH and HOCOH. After the formation of CO, its hydrogenation needs a very high free energy of 0.98 eV to form CHO, and even higher free energy to form COH. CO desorption is also difficult due to the strong adsorption. Therefore, for RuN4-Gra, HCOOH is the main product from HCOO pathway, while CO would be the main intermediate on the catalyst surface to refrain the formation of HCOOH. For RuN4/O-Gra, it is seen that the modification by oxygen atom changes the pathways dramatically (Fig. 6b). HCOO is preferred compared with COOH. Thus, CO from the hydrogenation of COOH is prevented. HCOOH is the only product for RuN4/O-Gra. The free energy change is 0.61 eV for HCOOH formation. The calculated limiting potential is -0.61 V. Since the equilibrium potential is -0.20 V for HCOOH, the obtained overpotential is 0.41 V. Hence, RuN4/O-Gra has a very high selectivity for HCOOH and the poisoning CO is prohibited. 14


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For both compounds, similar trend is observed after the solvent effect is considered (Fig. S8) and main conclusions are the same as in the gas phase.

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Figure 6. The free energy changes of the CO2RR on RuN4-Gra (a) and RuN4/O-Gra (b). HER is shown in (c).

Since hydrogen evolution reaction (HER) and CO2RR both need to consume the proton-electron (H+ + e-) pair, they are competing reactions. Thus, whether the HER are easy to occur on the two catalysts must be considered. Our calculations demonstrated that the free energy change of H is -0.55 eV for RuN4-Gra, implying a strong HER (Fig. 6c). For RuN4/O-Gra, however, HER is refrained due to the high H free energy change of 1.01 eV (larger than 0.61 eV for HCOOH formation). The weakened adsorption of H (-1.60 eV, Table 1) on RuN4/O-Gra compared with RuN4-Gra (-3.06 eV, Table 2) could be the reason for the refrained HER. This suggests again that RuN4/O-Gra is a good CO2RR catalyst.

Conclusions In this study, RuN4 doped graphene (RuN4-Gra) and RuN4 doped graphene oxide (RuN4/O-Gra) have been studied by using the density functional method. Our calculations revealed that RuN4 moiety and its eight adjacent carbon atoms are the catalytic active sites. The modification of RuN4-Gra by oxygen atom moves the d-band center to lower energy, making RuN4/O-Gra has less interaction energy with 16


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the adsorbates. This also makes the catalytic properties changed greatly. For ORR, the energy barrier is 0.47 eV at the rate-determining step for RuN4-Gra, and the working potential is calculated to be 0.0 V. For RuN4/O-Gra, the energy barrier at the rate-determining step is decreased to 0.39 eV, and the predicted working potential is 0.54 V. This suggested that RuN4/O-Gra is the preferred ORR catalyst both kinetically and thermodynamically. For CO2RR, the main product is HCOOH for RuN4-Gra. Nonetheless, since the intermediate COOH is favored over HCOO and the produced CO is strongly on the catalyst surface, the formation of HCOOH is refrained. In addition, HER is also strong for RuN4-Gra. For RuN4/O-Gra, however, HCOO pathway is favored over COOH. Thus, the formation of CO is prevented and HCOOH is the only product. Meanwhile, HER is also refrained. This implies that RuN4/O-Gra is a good CO2RR catalyst. In a word, RuN4 doped graphene oxide is a high efficient bifunctional catalyst for both ORR and CO2RR. We think that further experimental study on this catalyst should be rewarding.

Acknowledgements This work is supported by the National Key Research and Development Program of China (2016YFA0602900), National Natural Science Foundation of China (21503210, 21521092, 21733004, 21673220), Jilin Province Natural Science Foundation (20150101012JC), Program for Young Talents of Science and Technology in Universities of Inner Mongolia Autonomous Region, and Inner Mongolia Natural Science Foundation (2015MS0216). Part of the computational time is supported by the High Performance Computing Center of Jilin University and the High Performance Computing Center of Jilin Province.

Supporting Information Figure S1 is the energy of molecular dynamic simulations of RuN4-Gra and RuN4/O-Gra. Figure S2 and Figure S3 are the most stable configurations of ORR intermediates for RuN4-Gra and RuN4/O-Gra, respectively. Figure S4 and Figure S5 are the reaction pathways of ORR on the RuN4-Gra and RuN4/O-Gra, respectively. 17



Figure S6 and Figure S7 are the geometries of the initial state, transition state, and final state for reaction pathways of ORR on the RuN4-Gra and RuN4/O-Gra, respectively. Figure S8 is the free energy changes of CO2RR and HER on RuN4-Gra and RuN4/O-Gra in the gas environment and solution.

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Graphical Abstract

RuN4 doped graphene oxide is predicted to be a high efficient bifunctional catalyst for oxygen reduction and CO2 reduction.

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RuN4 Doped Graphene Oxide, a Highly Efficient Bifunctional Catalyst

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