Rhodium and Nitrogen Codoped Graphene as a Bifunctional

Subscriber access provided by Washington University | Libraries

C: Energy Conversion and Storage; Energy and Charge Transport

Rhodium and Nitrogen Codoped Graphene as Bifunctional Electrocatalyst for Oxygen Reduction Reaction and CO2 Reduction Reaction, Mechanism Insights Yanan Meng, Xiaochun Qu, Kai Li, Yuewen Yang, Ying Wang, and Zhijian Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10153 • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 13, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Rhodium and Nitrogen Codoped Graphene as Bifunctional Electrocatalyst for Oxygen Reduction Reaction and CO2 Reduction Reaction, Mechanism Insights

Yanan Menga,b, Xiaochun Qua,c,*, Kai Lia, Yuewen Yanga, b, Ying Wanga,*, Zhijian Wua,b*

aState

Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China. bUniversity

of Science and Technology of China, Hefei, Anhui 230026, PR China.

cDepartment

of Chemistry, College of Science, Yanbian University, Yanji 133002, PR China.

*Corresponding authors. [email protected]; [email protected]; [email protected].

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

Developing cost-effective and highly active heteroatom doped carbon-based bifunctional electrocatalysts for advanced energy conversion and storage devices are attracted worldwide attention. In this work, the reaction mechanisms for oxygen reduction reaction and CO2 reduction reaction on rhodium and nitrogen codoped graphene, i.e., RhNx-Gra (x=2-4), are studied by using the density functional method. The calculated formation energies show that RhNx-Gra (x=2-4) is thermodynamically stable. For oxygen reduction reaction, the energy barrier is 1.08, 0.54 and 0.24 eV for RhNx-Gra (x=2-4) at the rate-determining step, respectively. Thus, RhNx-Gra (x=3,4) have lower energy barrier compared with 0.80 eV for pure Pt. The working potentials are calculated to be 0.33 and 0.34 V for RhNx-Gra (x=3,4). For CO2 reduction reaction, CH4 is the preferred product for RhN2-Gra with the limiting potential of -0.71 V. HCHO and CH3OH are competitive products for RhN3-Gra and the limiting potentials are -0.60 and -0.68 V, respectively. For RhN4-Gra, HCOOH is the most favorable product with the limiting potential of -0.39 V and small overpotential of 0.14 V. These results suggest that RhNxGra (x=2-4) have high catalytic activity and selectivity toward both oxygen reduction reaction and CO2 reduction reaction, in particular for RhNx-Gra (x=3,4).


2

Page 2 of 37

Page 3 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.

INTRODUCTION The growing energy crisis, environmental pollution and global warming are serious problems

for human society. To solve these problems, efficient and stable electrochemical energy conversion and storage devices are required. As a clean energy device, fuel cells (FCs) have been studied extensively and have long been thought to be promising candidates for applications in energy conversion storage devices1-3 due to the high energy conversion efficiency, low emission and non-pollution properties. In FCs, the oxygen reduction reaction (ORR) has a great effect on the energy conversion efficiency4. However, the slow kinetics of ORR at the cathode severely degrade the performance of the FCs5,6. On the other hand, the CO2 reduction reaction (CO2RR) is one of the essential processes in both achieving the carbon recycling and alleviating the environmental issues resulting from the excessive emission of CO2. The main challenge for CO2RR to form diverse hydrocarbons is the large overpotential and low selectivity, which degrades the efficiency and catalytic activity of the catalysts7. At present, Pt and Pt-based alloys8,9 are the best catalysts for ORR. Au, Ag and Cu are well known catalysts for CO2RR10,11. However, the low durability, low efficiency, poor selectivity and large overpotential limit the practical applications of these metal catalysts. Therefore, it is essential to find the catalysts with high efficiency, high stability, high selectivity and low overpotential. Graphene, a two-dimensional one-atom-thick carbon crystal, has drawn great attention in the field of electrocatalysts since its discovery in 200412. Due to its huge specific surface area and superior electrical conductivity, graphene is becoming a promising electrocatalyst for energy conversion and storage in the various devices13. Recently, due to the low cost, good durability


3


and high catalytic activity, the transition metal and nitrogen (MNx) co-doped graphene have drawn great attention and provide new strategies for developing the catalysts for ORR and CO2RR. For ORR, Ghosh et. al concluded that transition metal doped graphene showed good ORR activity14. By analyzing more than 45 M-N-C electrocatalysts, the researchers found that Fe coordinated to N (FeNx) serves as an active site for four-electron direct reduction of O2 to H2O15. In another study, FeNx catalyst shows high ORR activity and low HOOH yield16. For CoNx doped graphene, large surface area, positive onset and half wave potentials as well as low HOOH yield are obtained17. Mn modified glycine derivative carbon (MnNx) shows better catalytic activity than the only nitrogen doped case and has lower half-wave potential than that of commercial Pt-C catalyst18. On the theoretical aspect, graphitic materials that have been codoped by four nitrogen atoms and transition metal atoms of groups 7 to 9 in the periodic table are found to be active for ORR19. FeN3 doped graphene possesses good catalytic activity and its catalytic mechanism is a four-electron process20. The favorable pathway is the OOH hydrogenation to form O+H2O, and the rate-determining step is the formation of the second water molecule20. FeN4 doped graphene has high ORR activity that is comparable to the Pt catalyst21. For MnN4 doped graphene, both OOH dissociation and the O2 direct dissociation pathways are probable for ORR22. For CoNx (x=2,4) doped graphene, the intermediate HOOH can be adsorbed steadily on the CoN4 surface, and tends to be dissociated on the CoN2 surface23,24, leading to a dual site 2x2 e- ORR mechanism for CoNx doped graphene23. For CO2RR, experimentally, the studies showed that the transitional metal and nitrogen codoped carbon materials could reduce CO2 into CO with high selectivity and catalytic activity25-30. FeN4 doped graphene (FeN4-C10) is more active than CoN4 doped graphene (CoN4-C10) for the


4

Page 4 of 37

Page 5 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reduction of CO2 to CO, with more positive onset potential and larger partial current densities27. CoN5 doped carbon material could reduce CO2 to CO with conversion rate of almost 100% and high stability28. For the FeNx doped graphene, the product ratio of CO/H2 can be tuned by the pH of the electrolyte during the process of CO2RR, and at high local pH value, the CO2 is reduced into CO with high selectivity29. Experimental and density functional calculations demonstrated that NiNx doped porous carbon materials exhibit high intrinsic activity towards the CO2RR with high current density30. Recently, the density functional calculations indicated that the single transition metal (Co, Rh and Ir) atoms doped porphyrin-like graphenes could be good catalysts for CO2RR, and the CoN4 doped porphyrin-like graphene exhibits high catalytic activity for CH3OH generation via a six-electron reaction pathway31. In another study, the synthesized FeNx co-doped carbon material can be served as bifunctional electrocatalyst for both ORR and CO2RR, which has high ORR performance and high selectivity for reduction CO2 into CO32. Previous studies indicated that Rh has high catalytic activity and selectivity, high oxidation resistance and stable chemical properties, for instance, in ORR study, RhN4 doped graphene exhibits good catalytic activity19, while it prefers to reduce CO2 to HCHO in CO2 reduction reaction31. Motivated by the previous studies,

in this work, we have studied the reaction

mechanism of RhNx (x=1-4) doped divacancy graphene (RhNx-Gra). Both ORR and CO2RR have been investigated. The possible reaction pathways are presented for the studied compounds, and the most favorable pathway is indentified. We hope this study could provide a useful guidance for the future experimental study.

2.

COMPUTATIONAL METHODS AND MODELS


5


In this work, a 4×5 orthorhombic supercell graphene with RhNx (x=2-4) coordinated structure was modeled in all the simulations. A vacuum space of 10Å in the z direction was used to avoid interactions between adjacent layers. All the geometry optimizations and total energy calculations were calculated at the spinpolarized density functional theory (DFT) level using the Vienna Ab Initio Simulation Package (VASP)33-35. The projector augmented wave (PAW)36 was used to describe the interactions between the ion cores and valence electrons. The GGA-PBE37 exchange-correlation functional was adopted. The plane wave kinetic energy cutoff was set to be 400 eV. The Brillouin zone in reciprocal space was sampled by the Monkhorst-Pack scheme38 using a 4x4x1 k-point grid. The transition states of the different chemical reactions were searched by using the climbing image nudged elastic band (CI-NEB) method39, in which the minimum energy pathway was optimized with the force-based conjugate-gradient method34 until the maximum force was relaxed to be less than 0.05 eV/Å. For all the calculated reactions energies, the zero point energy corrections were considered. The van der Waals interactions between the reactants and the substrates was corrected by the semi-empirical dispersion-corrected density functional theory (DFT-D2)40,41. The adsorption energy (ΔEads) is calculated as:

Eads  Eadsorbate / RhNx  Gra  ( Eadsorbate  ERhNx  Gra )

where the

Eadsorbate / RhNxGra Eadsorbate E , and RhNxGra are the total energies of the RhNx-Gra with

adsorbate, the isolated adsorbate, and clean RhNx-Gra, respectively.


6

Page 6 of 37

Page 7 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Based on a computational hydrogen electrode (CHE) model which introduced by Nørskov et al.42, the change in free energy (ΔG) of each reaction pathway for ORR and CO2RR was studied. The free energy change (ΔG) is determined as: ΔG =ΔE +ΔZPE – TΔS +ΔGU +ΔGpH where ΔE is the reaction energy gained from the DFT calculations, ΔZPE is the change of zero point energy, T is the temperature (298.15 K) and ΔS is the change of entropy. ΔGU and ΔGpH are the contributions to the free energy due to the change of the electrode potential U and pH value, respectively. ΔGU = -neU, in which n is the number of transferred electrons and U is the applied potential. ΔGpH = kBT x ln10 x pH, where kB is the Boltzmann constant and pH=0 in the acid medium. The entropies and vibrational frequencies of the gas molecules (including O2, H2, H2O, CO2, HCOOH, CH4, HCHO, CH3OH etc.) are taken from the experimental study43. The zero point energies and entropies of the intermediates were obtained by calculating the vibrational frequencies. The limiting potential (UL) is defined as the maximum free energy change (ΔGmax) among all elementary steps along the most favorable pathway by using the relation of UL = -ΔGmax/e. The overpotential is defined as η=U0−UL, where U0 is the equilibrium potential.

3.

RESULTS AND DISCUSSION

3.1 The Stability of RhNx-Gra (x=1-4) To check the stability of RhNx-Gra, the formation energy is calculated as:


7


E f  ERhNx  Gra  yC  ( EGra  x N   Rh )

where

ERhNxGra

and

EGra

are the total energies of RhNx-Gra and the perfect graphene,

respectively. 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 taken as one-half of the total energy of the N2 molecule in the gas phase. μRh is computed for an isolated Rh atom. x is the number of added N atoms, y is the number of removed C atoms. In this work, x=1-4, y=x+2.

Figure 1. The most stable structures and formation energies of RhNx-Gra (x=1-4). In the Figure, the gray, cyan and blue balls represent C, Rh and N atoms, respectively. Our calculations indicated that except for RhN1-Gra, the formation energies of RhNx-Gra (x=2-4) are negative, which indicates that RhNx-Gra (x=2-4) are stable (Figure 1). For the three possible structures in RhN2-Gra, the structure with the two N atoms next to each other in the sixmembered ring (Figure S1 and Figure 1) is the most stable. Thus, in the following work, we have studied the reaction mechanisms of ORR and CO2RR for RhNx-Gra (x=2-4).


8

Page 8 of 37

Page 9 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.2 Electronic Properties

Figure 2. Partial density of state (PDOS) and the d band center (εd) of Rh atom for (a) RhN2Gra, (b) RhN3-Gra and (c) RhN4-Gra. The d band center εd is marked by magenta dashed line, and the Fermi level is set at zero. The partial density of states in Figure 2 showed that there is a hybridization between Rh 4d and N 2p orbitals, as well as C 2p and N 2p orbitals on the RhNx-Gra (x=2-4) catalyst surface. This implies that the interaction between Rh and N as well as C and N is strong. The d band center (εd) is known to describe the binding strength of the adsorbates on the different surfaces4447.

From Figure 2, it is seen that the εd moves to lower energy with the increase of N, i.e., -2.19

for RhN2-Gra, -2.27 for RhN3-Gra and -2.39 for RhN4-Gra. This implies that the binding strength


9


of the reaction intermediates with the catalyst surface is the largest for RhN2-Gra, followed by RhN3-Gra and RhN4-Gra, and similar trend gained from previous study48. This is indeed true from the calculated adsorption energies of the reaction intermediates for ORR (Table 1) and CO2RR (Table 2).

3.3 ORR Activity 3.3.1 Adsorption of the ORR intermediates During the ORR process, all the intermediates (O2, O, H, OH, OOH, H2O and HOOH) adsorbed on RhNx-Gra (x=2-4) are analyzed. The most stable adsorption structures and adsorption energies are given in Figure S2, S3 and S4, respectively. For the most stable structure, the adsorption energy is also listed in Table 1. It is known that the adsorption of O2 is the first step in ORR process. For O2 adsorption, there are two different configurations, i.e., side-on and end-on. Our calculation showed that for RhN2Gra and RhN4-Gra, the side-on configuration can not be located on the surface and it tends to transform into the end-on configuration after the geometry optimization (Figure S2a for RhN2Gra, Figure S4a for RhN4-Gra). For RhN3-Gra, both side-on (Figure S3a) and end-on configurations (Figure S3b) are stable on the surface. From Table 1, it is seen that the adsorption energy of -0.74 eV on RhN4-Gra, close to -0.69 eV on Pt (111)49. For *H, the most stable site on RhNx-Gra (x=2,4) is at the top of Rh. For RhN3-Gra, however, *H atom is the most stable on the bridge site between Rh and C atoms. For RhNx-Gra (x=2,3), the most stable site of *O is the bridge site between Rh and C atoms, while for RhN4-Gra, the top


10

Page 10 of 37

Page 11 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of Rh is the most stable site for *O atom. For *OOH and *OH, the most stable site on RhNx-Gra (x=2-4) is the top of Rh. The adsorption energy of H2O is -0.72 eV for RhN2-Gra, -0.44 eV for RhN3-Gra. For RhN4-Gra, the adsorption of H2O is very weak with an energy of -0.26 eV due to the long Rh-O distance of 3.39 Å. In addition, HOOH can be adsorbed on the RhNx-Gra (x=3,4) surface with the adsorption energy of -0.58 eV and -0.47 eV, respectively, while it does not exist on RhN2-Gra surface. Table 1. The most stable adsorption energies of the ORR intermediates on RhNx-Gra (x=2-4). TRh and BriRh-C refer to the top site of Rh and the bridge site between Rh and C atoms, respectively. * indicates that the species is in adsorbed state. RhN2-Gra

RhN3-Gra

RhN4-Gra

side-on

-

-0.76/TRh

-

end-on

-1.81/TRh

-0.96/TRh

-0.74/TRh

*O

-5.49/BriRh-C

-5.14/BriRh-C

-3.20/TRh

*H

-3.56/TRh

-2.86/BriRh-C

-2.65/TRh

*OH

-3.57/TRh

-2.72/TRh

-2.55/TRh

*OOH

-2.41/TRh

-1.65/TRh

-1.38/TRh

H2O

-0.72/TRh

-0.44/TRh

-0.26/TRh

HOOH

-

-0.58/TRh

-0.47/TRh

3.3.2 ORR mechanism On the catalyst surface, the adsorbed O2 molecule would follow two different pathways, namely, dissociation of O2 and hydrogenation of O2. The possible reaction pathways for RhNxGra (x=2-4) are shown in Figure S5, S6 and S7, respectively, while the most favorable pathways are shown in Figure 3.


11


Our calculations showed that O2 dissociation needs to overcome a very high energy barrier of 1.78 eV, 1.10 eV and 1.69 eV for RhNx-Gra (x=2-4) (Figure S5a1, Figure S6a1 and Figure S7a1). This indicated that the O2 dissociation is not favorable for the ORR process on RhNx-Gra (x=24). On the contrary, O2 hydrogenation to form *OOH is easy with a relatively low energy barrier of 0.77 eV, 0.35 eV and 0.11 eV for RhNx-Gra (x=2-4) (Figure 3), respectively. Thus, in the following, only the pathway following *OOH is discussed in detail. RhN2-Gra. From the Figure S5, one can see that there are three possible reaction pathways after OOH, i.e., *OOH → *O+*OH, *OOH+*H → *OH+*OH and *OOH+*H → *O+H2O. The calculated results indicated that *OOH dissociation and *OOH hydrogenation into *OH+*OH have high energy barriers of 0.68 eV (Figure S5b2) and 0.74 eV (Figure S5b1), respectively. While *OOH hydrogenation to form *O+H2O is relatively easy with a small energy barrier of 0.13 eV and the reaction is exothermic by a very large energy of -2.79 eV (Figure 3). After the first water molecule is released from the catalyst surface, the remaining *O atom is hydrogenated to form *OH with a very large energy barrier of 1.08 eV. This is also the rate-determining step for RhN2-Gra. The formation of the second water molecule needs to overcome an energy barrier of 0.51 eV. RhN3-Gra. For RhN3-Gra, *OOH dissociation is difficult due to the high energy barrier of 1.10 eV (Figure S6b1). For the hydrogenation of *OOH, however, it has three competitive pathways, i.e., *OOH+*H→*OH+*OH, HOOH and *O+H2O (Figure 3). *OOH hydrogenation into HOOH has an energy barrier of 0.54 eV and a small exothermic energy of -0.28 eV (Figure 3). Due to the weak adsorption of HOOH on the catalyst surface (-


12

Page 12 of 37

Page 13 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.58 eV, Table 1), it would be desorbed from the catalyst surface, leading to a two-electron pathway. *OOH hydrogenation into *O+H2O has an energy barrier of 0.56 eV, similar to the HOOH formation (0.54 eV), but the reaction is exothermic by a large energy of -1.55 eV (Figure 3). For the remaining *O atom, it forms the second water molecule through two sequential hydrogenation steps. The formation of *OH needs an energy of 0.36 eV, while the formation of the second water molecule needs an energy of 0.37 eV (Figure 3). Finally, *OOH hydrogenation into *OH+*OH requires to overcome an energy barrier of 0.54 eV (Figure 3), same as HOOH formation (0.54 eV). Meanwhile, the formation of *OH+*OH is a highly exothermic reaction with an energy of -1.93 eV, which is more exothermic than the formation of HOOH (-0.28 eV) and *O+H2O (-1.55 eV). Thus, *OOH hydrogenation to *OH+*OH is slightly favored. After *OH is formed, the hydrogenation of one of *OH gives the first water molecule with a small energy barrier of 0.05 eV and a larger exothermic energy of 2.22 eV (Figure 3). The formation of the second H2O molecule needs to overcome an energy barrier of 0.37 eV (Figure 3). The above results indicated that for RhN3-Gra, both two-electron and four-electron pathways are competitive, with the four-electron pathway being slightly favored. RhN4-Gra. For RhN4-Gra, the geometrical structures for the most favorable reaction pathway are shown in Figure S8. The energy calculations suggested that the breaking of O-OH bond is difficult to occur due to the high energy barrier of 1.52 eV (Figure S7b1), implying that *OOH dissociation is impossible for RhN4-Gra. Compared to *OOH dissociation, *OOH hydrogenation is relatively easy. There are three final products after *OOH hydrogenation, i.e., *O+H2O, 2*OH


13


and HOOH. Among the three products, the most favorable pathway is the formation of *O+H2O (Figure S7), and is discussed in the following.

Figure 3. The favorable reaction pathway for ORR on RhNx-Gra (x=2-4). The numbers in parenthesis are the energy barriers (left) and reaction energies (right) in units of eV. * denotes that the ORR species is adsorbed on the catalyst surface. For *OOH hydrogenation into *O+H2O, it is easy to proceed with a negligible energy barrier of 0.01 eV and a larger exothermic energy of -2.13 eV (Figure 3). For the remaining *O atom, it


14

Page 14 of 37

Page 15 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


forms the second H2O molecule through the two sequential hydrogenation steps. The energy barrier is low for the two processes, i.e., 0.003 eV for the formation of *OH and 0.24 eV for the formation of the second water molecule (Figure 3). The rate-determining step is the formation of second H2O molecule with an energy barrier of 0.24 eV. For HOOH, although it is easy to form with the energy barrier of 0.23 eV (Figure S7), it will decompose into *OH+*OH immediately due to the negligible energy barrier of 0.04 eV (Figure S7). Meanwhile, the pathway following *OH+*OH is not favorable due to the high energy barrier of 0.64 eV. Thus, ORR on RhN4-Gra is a four-electron process. In a word, as seen from Figure 3, for RhNx-Gra (x=2-4), the most favorable pathway is the fourelectron process. RhN2-Gra and RhN4-Gra have the same reaction pathway, but with different rate-determining steps. As the increase of the number of N atoms, the energy barrier of the ratedetermining step for ORR decreases. RhN2-Gra has the highest energy barrier (1.08 eV), followed by RhN3-Gra (0.54 eV) and RhN4-Gra (0.24 eV), which indicated that the more number of N atoms, the better catalytic activity for ORR. This observation agrees with the previous study on FeNx doped carbon materials that there is an overall increase in catalytic activity with the amount of atomic N15. On the other hand, previous studies showed that the energy barrier for the rate-determining step is 0.79 eV for Pt (111)49, 0.80 eV for Pt (100)50, 0.56 eV for FeN4-Gra21, and 0.51 eV for MnN4-Gra22. This demonstrated that RhN4-Gra with an energy barrier of 0.24 eV is a promising catalyst for ORR with the most favorable pathway of O2 →*OOH→*O+H2O → *OH → H2O. Meanwhile, RhN3-Gra with an energy barrier of 0.54 eV is also a good ORR catalyst.


15


3.3.3 Effect of electrode potentials on the ORR

Figure 4. Free energy diagrams for the favorable reaction pathways on (a) RhN2-Gra, (b) RhN3Gra and (c) RhN4-Gra at different electrode potentials U. Considering the practical applications, the effect on the free energy for the most favorable pathways on RhNx-Gra (x=2-4) are analyzed at different external potentials (U) based on the method developed by Nørskov et al.42. The free energies change for all possible proceeding of ORR on RhNx-Gra (x=2-4) are shown in Table S1, S2 and S3, respectively. The free energy diagrams for the most favorable pathway are shown in Figure 4. From Figure 4, it is seen that all the elementary reactions are downhill at U=0 V, which indicated that the four-electron ORR


16

Page 16 of 37

Page 17 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reaction pathways on RhNx-Gra (x=2-4) surfaces are preferred thermodynamically under zero electrode potential. With the increase of the external potentials, the free energies start to be uphill in the second H2O molecule formation for RhN2-Gra (Figure 4a), in the formation of *OOH for RhN3-Gra (Figure 4b) and RhN4-Gra (Figure 4c). The calculated working potential is 0.33 V for RhN3-Gra and 0.34 V for RhN4-Gra, which is close to 0.41 V for FeN4-Gra51, higher than 0.18 V for FeN2-Gra52, and also higher than 0.30 V for CoN2-Gra24. These results indicated that RhNxGra (x=3,4) would exhibit high catalytic activity for ORR.

3.4 CO2RR Activity 3.4.1 The structure and adsorption CO2RR intermediates CO2RR is a quite complicated process, which can proceed through multiple pathways that involve numerous intermediates and different configurations. In this study, the final products of CO2RR contain only one C atom and range from HCOOH/CO (two electron reduction pathway), HCHO (four electron reduction pathway), CH3OH (six electron reduction pathway), and CH4 (eight electron reduction pathway). For the C2+ hydrocarbons and intermediates, LangmuirHinshelwood mechanism would be required to account for coupling of C1 intermediates to form C-C bond, which is difficult for the single active site in the present model. Therefore, we only consider the formation of C1 intermediates and products. During the CO2RR process, all the possible intermediates adsorbed on RhNx-Gra (x=2-4) are analyzed, both the possible adsorption sites and different structures of intermediates are taken


17


Page 18 of 37

into account. The calculated adsorption energies were shown in Table 2 and the most stable structures for the favorable pathways are shown in Figure 5. Table 2. The calculated adsorption energies (Eads, eV) and the Rh-C (O) bond distance (d, Å) for the CO2RR intermediates on RhNx-Gra (x=2-4). # refers to Rh-O bond distance, while the remaining is the Rh-C bond distance. * indicates that the species is in adsorbed state. RhN2

RhN3

Eads

d

Eads

RhN4 d

Eads

d

CO2

-0.67

2.08(2.27#)

-0.38

2.18

-0.26

3.35

*COOH

-3.65

1.96

-2.87

1.99

-2.65

2.00

*HCOO

-3.12

2.08#

-2.18

2.09#

-2.01

2.08#

*HOCOH

-3.12

1.92

-2.53

1.90

-1.84

1.90

HCOOH

-0.79

3.58

-0.57

3.18

-0.45

3.16

*H2COO

-0.27

1.97#(2.02#)

0.75

1.99#(1.99#)

-

-

*CO

-2.40

1.83

-1.88

1.85

-1.10

1.88

*COH

-4.27

1.81

-3.18

1.85

-2.62

1.93

*CHO

-3.63

1.94

-2.86

1.96

-2.61

1.96

HCHO

-0.73

3.64

-0.59

2.84

-0.26

3.60

*HCOH

-3.95

1.87

-3.30

1.87

-2.52

1.88

*CH

-5.50

1.86

-

-

-

-

*CH2OH

-3.07

2.04

-2.29

2.07

-

-

*OCH3

-

-

-2.14

1.98

-

-

CH3OH

-0.81

2.06

-0.53

3.35

-

-

*CH2

-4.02

1.82

-3.45

2.07

-

-

*CH3

-3.03

2.06

-

-

-

-

CH4

-0.61

3.92

-

-

-

-


18

Page 19 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. The most stable structures for the most favorable pathway. (a) the formation of CH4 via *COOH on RhN2-Gra; (b) the formation of CH3OH via *COOH on RhN3-Gra; (c) the formation of HCOOH via *COOH on RhN4-Gra; (d) the formation of HCHO via *COOH on RhN4-Gra. The numbers on top of each column are the transferred numbers of (H+ + e−) pairs. In the Figure, the grayness, cyan, blue, red and white balls represent C, Rh, N, O and H atoms, respectively. For CO2RR, the adsorption of CO2 on RhNx-Gra (x=2-4) is the first step. The adsorption energies are -0.67 eV and -0.38 eV, respectively, for RhNx-Gra (x=2,3) (Table 2). After the


19


adsorption on the active site, the linear structure of CO2 in the free gas phase becomes bent. On the RhN2-Gra surface, CO2 binds to Rh atom through bidentate C-Rh-O configureuration and the angle ∠ OCO is 143.7º (Figure 5a). For RhN3-Gra, the CO2 binds to Rh atom through a monodentate C-Rh configuration and the angle ∠OCO is 147.8º (Figure 5b). The net charge on CO2 is -0.42e for RhN2-Gra, -0.37e for RhN3-Gra. Thus, the large charge transfer in RhNx-Gra (x=2,3) induces large geometry deformation with the deviation of the angle ∠OCO from linear structure. For RhN4-Gra, the interaction between CO2 and Rh is weak with the adsorption energy of -0.26 eV, and the linear CO2 structure is bent slightly with the angle ∠OCO of 174.8º (Figure 5c). The net charge on CO2 is also small, which is -0.06e. Nonetheless, it is known that under realistic electrochemical conditions, the local electric field exerted by solvated cations and their corresponding image charges at the electrode/electrolyte interface can remarkably enhance the adsorption of CO2 molecule. Thus, we have studied the CO2RR on the three catalysts RhNx-Gra (x=2-4). As we all known, after the adsorption of CO2 on the catalyst surface, the first hydrogenation (H+ + e- pair) step would form two intermediates, *COOH and *HCOO. For *HCOO, both oxygen atoms bind to Rh atom on RhN2-Gra surface (schematic structure is shown in Figure 6a) with the adsorption energy of -3.12 eV, while for RhNx-Gra (x=3,4), only one O atom binds to Rh atom (schematic structures are shown in Figure 6b and Figure 6c), with the adsorption energy of -2.18 eV and -2.01 eV, respectively. Compared to *HCOO, for *COOH, only the structures with the C binding to Rh are obtained (Figure 5), which has strong adsorption energies of -3.65 eV, -2.87 eV and -2.65 eV, respectively (Table 2), for RhNx-Gra (x=2-4). For the hydrogenation of *HCOO and *COOH, CO and HCOOH will be generated. Whether CO and HCOOH act as final products or undergo further reduction is principally determined by their adsorption


20

Page 20 of 37

Page 21 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


energies. If the adsorption energy is small, CO and HCOOH will be desorbed from the catalyst surface as the final product. On the contrary, the strong adsorbed CO and HCOOH will continue to be hydrogenated. From Table 2, one can see that the adsorption energies of HCOOH are -0.79 eV, -0.57 eV and -0.45 eV, respectively, for RhNx-Gra (x=2-4), indicating that HCOOH is more likely to be desorbed as liquid product considering the weak binding strength. However, CO will be reduced further on RhNx-Gra (x=2-4) surfaces, due to strong adsorption energies of -2.40 eV, -1.88 eV and -1.10 eV, respectively (Table 2). CO prefers to adsorb on the top site of Rh by forming Rh-C bond with the bond distance in the range of 1.83Å-1.88Å (Table 2) and the molecule is tilted relative to the catalyst surface by 23.6º, 5.5º and 0.0º, respectively for RhNxGra (x=2-4) (Figure 5). On the other hand, *H2COO is formed after *HCOO hydrogenation on RhNx-Gra (x=2,3) surfaces with the two oxygen atoms binding to Rh (schematic structures are shown in Figure 6a and 6b). For RhN4-Gra, *H2COO is not located because it tends to transform into HCOOH after the geometry optimization. Following the hydrogenation of CO, many oxygenated groups will be produced, such as *CHO, *COH, *HCOH, *CH2OH, *OCH3 and so on. For these oxygenated groups, C binding to Rh is more stable (schematic structures are shown in Figure 6a, 6b and 6c). For all the intermediates and products, the free energy changes are collected in Tables S4 for RhN2-Gra, S5 for RhN3-Gra and S6 for RhN4-Gra.

3.4.2 Reaction mechanism for CO2RR RhN2-Gra, Figure 6a. It is seen from Figure 6a that CO2 hydrogenation would form *HCOO and *COOH. For *HCOO, the free energy is downhill by -0.38 eV. After the formation of *HCOO,


21


there are two possible products, i.e., HCOOH and *H2COO. Our results showed that HCOOH is more favorable than *H2COO. Compared with *HCOO, the formation of *COOH is more favorable due to the large downhill free energy of -0.97 eV. Following *COOH, there are three possible pathways, i.e., *COOH hydrogenation into HCOOH, *HOCOH and *CO+H2O. The calculated results indicated that the formation of *CO is favored due to the downhill free energy of -0.30 eV. As mentioned above, since *CO adsorbs strongly on the catalyst surface, it will be hydrogenated further to form either *COH or *CHO. Our results revealed that the formation of *CHO is favorable than *COH. *CHO hydrogenation forms *HCOH which is more favorable than HCHO. The intermediate *HCOH hydrogenates further to form either *CH2OH or *CH+H2O with the free energy changes of -0.46 eV and 0.88 eV, respectively. Clearly, *CH2OH is the preferred intermediate. Its hydrogenation would give either *CH2+H2O or CH3OH, with the uphill free energy of 0.55 eV and 0.86 eV, respectively. Thus, *CH2 is preferred. After the two sequential hydrogenation steps, CH4 is formed. The free energy change is -1.35 eV for *CH3 and 0.41 eV for CH4, respectively. The formed CH4 would desorb from RhN2-Gra surface due to the small adsorption energy. Thus, on RhN2-Gra surface, CO2 can be reduced to CH4 via eight-electron reduction pathway with the uphill free energy of 0.71 eV. The minor pathway is the formation of HCOOH from *HCOO with the uphill free energy of 0.81 eV. RhN3-Gra, Figure 6b. Similar to RhN2-Gra, the formation of *COOH has downhill free energy of -0.17 eV, more favorable than *HCOO with uphill free energy of 0.72 eV. After *HCOO, HCOOH is formed with a downhill free energy of -0.29 eV.


22

Page 22 of 37

Page 23 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



23


Figure 6. The free energy change of CO2RR (a-c) and HER (d) on RhNx-Gra (x=2-4). For each intermediate on (a-c), the schematic structure is also shown.


24

Page 24 of 37

Page 25 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Following *COOH, the formation of *CO is preferred with a downhill free energy of -0.59 eV. The formation of *CHO from *CO is more favored compared with the formation of *COH. Different from RhN2-Gra, on RhN3-Gra surface, *CHO reacts with hydrogen to form formaldehyde (HCHO) adsorbed on the top of Rh with uphill free energy of 0.60 eV. Another intermediate from *CHO hydrogenation is *HCOH, which lies 0.08 eV above HCHO. Following *HCOH, *CH2OH is formed with a downhill free energy of -0.26 eV. For *CH2OH, since both C and O are possible sites for hydrogenation, two products are possible, i.e., *CH2+H2O and CH3OH. The results indicated that the formation of CH3OH costs nothing with zero energy change, while it needs 0.28 eV for the formation of *CH2+H2O. Thus, on RhN3-Gra surface, the formation of CH3OH and HCHO are competitive pathways with the uphill free energy of 0.68 and 0.60 eV, respectively. The minor pathway is the formation of HCOOH from *COOH with the uphill free energy of 0.60 eV. RhN4-Gra, Figure 6c. Different from RhNx-Gra (x=2,3) in which the formation of *COOH has a downhill free energy, *COOH is slightly uphill with free energy of 0.04 eV for RhN4-Gra, while *HCOO has much higher uphill free energy of 0.71 eV. The formation of HCOOH from *HCOO has downhill free energy of -0.28 eV. Following *COOH, the formation of HCOOH needs an uphill free energy of 0.39 eV, while the formation of *CO is slightly downhill by -0.04 eV. After the formation of *CO, *CHO is formed with a downhill free energy of -0.08 eV. The further reduction of *CHO gives formaldehyde (HCHO) with an uphill free energy of 0.73 eV (Figure 6c). On the other hand, the formation of HCOOH via *COOH is easier than via *HCOO. Therefore, CO2 reducing to HCOOH via two-electron reduction pathway is the most favorable. The formation HCHO is the


25


Page 26 of 37

secondary pathway with an uphill free energy of 0.73 eV. *CHO would be the main intermediate on the catalyst surface due to its strong adsorption (-2.61 eV). Table 3 Potential limiting steps (PLS), limiting potentials (UL/V) and overpotentials (ƞ/V) for CO2RR on RhNx-Gra (x=2-4). U0 is the equilibrium potential. Comparison is made with the previous studies. * indicates that the species is in adsorbed state. PLS

UL

U0

ƞ

Product

RhN2-Gra

*CHO+H+ + e-→*HCOH

-0.71

0.17

0.88

CH4

RhN2-Gra

CO2+ H+ + e-→*HCOO

-0.81

-0.25

0.56

HCOOH

RhN3-Gra

*CHO+ H+ + e-→*HCHO

-0.60

-0.07

0.53

HCHO

RhN3-Gra

*CHO+ H+ + e-→*HCOH

-0.68

0.02

0.70

CH3OH

RhN4-Gra

CO2+ H+ + e-→*HCOO

-0.71

-0.25

0.46

HCOOH

RhN4-Gra

*COOH+ H+ + e-→HCOOH

-0.39

-0.25

0.14

HCOOH

RhN4-Gra

*CHO+ H+ + e-→HCHO

-0.73

-0.07

0.66

HCHO

Cu(111)

CO2+ H+ + e-→*COOH

-0.90

CH453

Cu(211)

*CO+ H+ + e-→*CHO

-0.80

CH454

RhN4-Gra

*CHO+ H+ + e-→HCHO(l)

-0.50

HCHO31

Cu-C3N4

*CO+ H+ + e-→*CHO

-0.75

CH3OH55

PyrroN3

*COOH+ H+ + e-→HCOOH

-0.44

HCOOH57

In a word, as seen from Figure 6 and Table 3, for RhN2-Gra, the most favorable pathway is CO2 →*COOH→*CO→*CHO→*HCOH→*CH2OH→*CH2 →*CH3 →CH4. The potential limiting step is the *CHO→*HCOH with the potential of -0.71 V . This value is lower than the formation of methane on Cu (111) (-0.90 V)53 and Cu (211) (-0.80 V)54. For RhN3-Gra, CO2 →*COOH → *CO →*CHO →*HCOH →*CH2OH →CH3OH is the one of favorable pathways. *CHO →


26

Page 27 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


*HCOH is the potential limiting step with the potential of -0.68 V, lower than the formation of methanol for Cu-C3N4 (-0.75 V)55 and Ni doped Cu (111) (-1.08 V)56. For RhN4-Gra, CO2 → *COOH →HCOOH is the most favorable pathway. *COOH hydrogenation into HCOOH is the potential limiting step with the potential of -0.39 V, lower than the formation of formic acid for N doped graphene (-0.44 V)57. The above results indicated that RhNx-Gra (x=2-4) has high catalytic activity and selectivity for CO2RR.

3.4.3 Hydrogen evolution reaction The hydrogen evolution reaction (HER) is the competitive reaction for CO2RR, which has great effect on the Faradaic efficiency of CO2RR at low pH region. To understand the effect, the free energies of HER on the RhNx-Gra (x=2-4) have been calculated (Figure 6d). As the distance between the two metal atoms in the RhNx-Gra (x=2-4) is about 10Å, the Volmer-Heyrovsky mechanism has been taken into consideration. From Figure 6d, it is shown that the free energy barriers of HER are 1.05 eV, 0.23 eV and 0.16 eV, respectively for RhNx-Gra (x=2-4). Thus, for RhN2-Gra, HER is suppressed. For RhNx-Gra (x=3,4), however, CO2RR would be suppressed by the active HER. It is known that the activation energy of HER can be increased by changing the pH of the electrolyte. For instance, ΔGpH for H species can be increased from 0.0 for pH=0.0 to 0.42 eV for pH=7.042. Thus, for RhN4-Gra, the activation energy for HER can be increased from 0.16 eV (pH=0.0) to 0.58 eV (pH=7.0), leading to an increased Faradaic efficiency of CO2RR in neutral environment. Therefore, choosing a suitable electrolyte to ensure a neutral environment near the electrode surface could be an effective method to hinder the HER and improve the CO2RR.


27


CONCLUSIONS The ORR and CO2RR mechanisms for RhNx (x=2-4) doped graphene have been studied by using the density functional method. Our calculations indicated that they are thermodynamically stable due to the negative formation energies. For each reaction, the possible reaction pathways are presented and discussed. For ORR, with the increase of the doped N atoms, the energy barrier of the rate-determining step for ORR decreases, which are 1.08, 0.54 and 0.24 eV for RhNx-Gra (x=2-4), respectively. For RhNx-Gra (x=3,4), these values are smaller than 0.80 eV for pure Pt, 0.56 eV for FeN4-Gra, in particular for RhN4-Gra. The predicted working potentials are 0.33 and 0.34 V for RhNx-Gra (x=3,4). For CO2RR, the catalytic activity and selectivity are sensitive to the number of the doped N atoms. CH4 is the most favorable product for RhN2-Gra. The potential limiting step is *CHO+H+ + e-→*HCOH with the free energy barrier of 0.71 eV. For RhN3-Gra, both HCHO and CH3OH are the competitive products. For HCHO, *CHO+H+ + e-→*HCHO is the potential limiting step with a free energy barrier of 0.60 eV, while *CHO+H+ + e-→*HCOH is the potential limiting step for CH3OH with a free energy barrier of 0.68 eV. For RhN4-Gra, it shows good selectivity and the most favorable product is HCOOH. The potential limiting step is *COOH+H+ + e→HCOOH with a low free energy barrier of 0.39 eV and small overpotential of 0.14 V. The above results indicated that RhNx-Gra (x=2-4), especially RhNx-Gra (x=3,4), have good catalytic activity and selectivity toward both ORR and CO2RR and are promising bifunctional electrocatalysts. Thus, future experimental study on rhodium and nitrogen codoped graphene should be rewarding.


28

Page 28 of 37

Page 29 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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),

Department

of

Science

and

Technology

of

Sichuan

Province(2017GZ0051) and Jilin Province Natural Science Foundation (20150101012JC). Part of the computational time is supported by Jilin University and the High Performance Computing Center of Jilin Province.

SUPPORTING INFORMATION Figure S1 is the three possible structures and formation energies for RhN2-Gra. Figure S2, S3 and S4 are the most stable adsorption structure for ORR species on RhN2-Gra, RhN3-Gra and RhN4-Gra, respectively. Figure S5, S6 and S7 are the possible reaction pathways for ORR on RhN2-Gra, RhN3-Gra and RhN4-Gra, respectively. Figure S8 is the atomic structures of the initial state, transition state, and final state for the most favorable reaction pathway for RhN4-Gra. Table S1, S2 and S3 are the Gibbs free energies for all possible adsorption processes of ORR on RhNx-Gra (x=2-4), Table S4, S5 and S6 are the free energy change for all the intermediates during CO2RR on RhNx-Gra (x=2-4).

AUTHOR CONTRIBUTIONS The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally.


29


REFERENCES (1) Steele, B. C. H.; Heinzel, A., Materials for fuel-cell technologies. Nature 2001, 414, 345 352. (2) Brumfiel, G., Hydrogen cars fuel debate on basic research. Nature 2003, 422, 104. (3) Bashyam, R.; Zelenay, P., A class of non-precious metal composite catalysts for fuel cells. Nature 2006, 443, 63 - 66. (4) Nie, Y.; Li, L.; Wei, Z. D., Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction. Chem. Soc. Rev. 2015, 44, 2168 - 2201. (5) Tang, C.; Wang, H. F.; Chen, X.; Li, B. Q.; Hou, T. Z.; Zhang, B. S.; Titirici. M. M.; Wei, F., Topological defects in metal-free nanocarbon for oxygen electrocatalysis. Adv. Mater. 2016, 28, 6845 - 6851. (6) Ghosh, S.; Kar, P.; Bhandary, N.; Basu, S.; Sardar, S.; Maiyalagan, T.; Majumdar, D.; Bhattacharya, S. K.; Bhaumik, A.; Lemmensg, P., et al. Microwave-assisted synthesis of porous Mn2O3 nanoballs as bifunctional electrocatalyst for oxygen reduction and evolution reaction. Catal. Sci. Technol. 2016, 6, 1417 - 1429. (7) Dong, C. K.; Fu, J. Y.; Liu, H.; Ling, T.; Yang, J.; Qiao, S. Z.; Du. X. W., Tuning the selectivity and activity of Au catalysts for carbon dioxide electroreduction via grainboundary engineering: A DFT study. J. Mater. Chem. A 2017, 5, 7184 - 7190. (8) Gasteiger, H. A.; Panels, J. E.; Yan, S. G., Dependence of PEM fuel cell performance on catalyst loading. J. Power Source 2004, 127, 162 - 171.


30

Page 30 of 37

Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9) Debe, M. K., Electrocatalyst approaches and challenges for automotive fuel cells. Nature 2012, 486, 43 - 51. (10) Bhowmik, A.; Vegge, T.; HHansen, H. A., Descriptors and thermodynamic limitations of electrocatalytic carbon dioxide reduction on rutile oxide surfaces. ChemSusChem 2016, 9, 3230 - 3243. (11) Pan, F. P.; Deng, W.; Justiniano, C.; Li, Y., Identification of champion transition metals centers in metal and nitrogen-codoped carbon catalysts for CO2 reduction. Appl. Catal. B 2018, 226, 463 - 472. (12) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigoroieva, I. V.; Firsov, A. A., Electric field effect in atomically thin carbon films. Science 2004, 306, 666 - 669. (13) Li, M. T.; Zhang, L. P.; Xu, Q.; Niu, J. B.; Xia, Z. H., N-doped graphene as catalysts for oxygen reduction and oxygen evolution reactions: Theoretical considerations. J. Catal. 2014, 314, 66 - 72. (14) Ghosh, S.; Basu, R. N., Multifunctional nanostructured electrocatalysts for energy conversion and storage: current status and perspectives. Nanoscale 2018, 10, 11241 - 11280. (15) Artyushkova, K.; Serov, A.; Carbonell, S. R.; Atanassov, P., Chemistry of multitudinous active sites for oxygen reduction reaction in transition metal-nitrogen-carbon electrocatalysts. J. Phys. Chem. C 2015, 119, 25917 - 25928. (16) Wang, Q.; Zhou, Z. Y.; Lai, Y. J.; You,Y.; Liu, J. G.; Wu, X. L.; Terefe. E.; Chen, C.; Song, L.; Rauf, M., et al. Phenylenediamine-based FeNx/C catalyst with high activity for oxygen


31


reduction in acid medium and its active-site probing. J. Am. Chem. Soc. 2014, 136, 10882 10885. (17) Fu, X. G.; Choi, J. Y.; Zamani, P.; Jiang, G. P.; Hoque, M. A.; Hassan, F. M.; Chen, Z. W., Co-N decorated hierarchically porous graphene aerogel for efficient oxygen reduction reaction in acid. ACS Appl. Mater. Interfaces 2016, 8, 6488 - 6495. (18) Kang, J.; Wang, H.; Ji, S.; Key, J. L.; Wang, R. F., Synergy among manganese, nitrogen and carbon to improve the catalytic activity for oxygen reduction reaction. J. Power Source 2014, 251, 363 - 369. (19) Federico, C. V.; Jose, I. M.; Jan, R., Density functional studies of functionalized graphitic materials with late transition metals for oxygen reduction reactions. Phys. Chem. Chem. Phys. 2011, 13, 15639 - 15643. (20) Meng, Y. N.; Li, K.; Yang, Y. Y.; Wang, Y.; Wu, Z. J., The oxygen reduction reaction mechanism on FeN3 doped divacancy graphene: A theoretical perspective. J. Electrochem. Soc. 2018, 165, F145 - F151. (21) Kattel, S.; Wang, G. F., Reaction pathway for oxygen reduction on FeN4 embedded graphene. J. Phys. Chem. Lett. 2014, 5, 452 - 456. (22) Lu, Z. S.; Xu, G. L.; He, C. Z.; Wang, T. X.; Yang, L. Z.; Yang, Z. X.; Ma, D. W., Novel catalytic activity for oxygen reduction reaction on MnN4 embedded graphene: A dispersion corrected density functional theory study. Carbon 2015, 84, 500 - 508.


32

Page 32 of 37

Page 33 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23) Sun, X. X.; Li, K.; Yin, C.; Wang, Y.; Jiao, M. G.; He, F.; Bai, X. W.; Tang, H.; Wu, Z. J., Dual-site oxygen reduction reaction mechanism on CoN4 and CoN2 embedded graphene: Theoretical insights. Carbon 2016, 108, 541 - 550. (24) Kattel, S.; Atanassov, P.; Kiefer, B., Catalytic activity of Co-Nx/C electrocatalysts for oxygen reduction reaction: A density functional theory study. Phys. Chem. Chem. Phys. 2013, 15, 148 - 153. (25) Varela, S. A.; Sahraie, N. R.; Steinberg, J. L.; Ju, W.; Oh, H. S.; Strasser, P., Metal-doped nitrogenated carbon as an efficient catalyst for direct CO2 electroreduction to CO and hydrocarbons. Angew. Chem. Int. Ed. 2015, 54, 10758 - 10756. (26) Su, P. P.; Iwase, K.; Nakanishi, S.; Hashimoto, K.; Kamiya, K., Nickel-nitrogen-modified graphene: An efficient electrocatalyst for the reduction of carbon dioxide to carbon monoxide. Small 2016, 12, 6083 - 6089. (27) Pan, F. P.; Zhang, H. G.; Liu, K. X.; Cullen, D.; More, K.; Wang, M. Y.; Feng, Z. X.; Wang, G. F.; Wu, G.; Li, Y., Unveiling active sites of CO2 reduction on nitrogen-coordinated and atomically dispersed iron and cobalt catalysts. ACS Catal. 2018, 8, 3116 - 3122. (28) Pan, Y.; Lin, R.; Chen, Y. J.; Liu, S. J.; Zhu, W.; Cao, X.; Chen, W. X.; Wu, K. L.; Cheong, W. C.; Wang, Y., et al. Design of single-atom Co-N5 catalytic site: A robust electrocatalyst for CO2 reduction with nearly 100% CO selectivity and remarkable stability. J. Am. Chem. Soc. 2018, 140, 4218 - 4221. (29) Varela, A. S.; Kroschel, M.; Leonard, N. D.; Ju, W.; Steinberg, J.; Bagger, A.; Rossmeisl, J.; Strasser, P., pH effects on the selectivity of the electrocatalytic CO2 reduction on graphene-


33


embedded Fe-N-C motifs: Bridging concepts between molecular homogeneous and solid state heterogeneous catalysis. ACS Energy Lett. 2018, 3, 812 - 817. (30) Yan, C. C.; Li, H. B.; Ye, Y. F.; Wu, H. H.; Cai, F.; Si, R.; Xiao, J. P.; Miao, S.; Xie, S. H.; Yang, F., et al. Coordinatively unsaturated nickel-nitrogen sites towards selective and highrate CO2 electroreduction. Energy Environ. Sci. 2018, 11, 1204 - 1210. (31) Wang, Z. X.; Zhao, J. X.; Cai, Q. H., CO2 electroreduction performance of a single transition metal atom supported on porphyrin-like graphene: A computational study. Phys. Chem. Chem. Phys. 2017, 19, 23113 - 23121. (32) Li, E. L.; Yang, Y.; Wu, Z. M.; Wang, Y.; Ruan, M. B.; Song, P.; Xing, W.; Xu, W. L., A bifunctional highly efficient FeNx/C electrocatalyst. Small 2018, 14, 1702827 - 1702833. (33) Kresse, G.; Hafner, J., Ab initio molecular-dynamics simulation of the liquid-metalamorphous-semiconductor transition in germanium. Phys. Rev. B 1994, 49, 14251 - 14269. (34) Kresse, G.; Furthmuller, J., Iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169 - 11186. (35) Kresse, G.; Furthmuller, J., Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comp. Mater. Sci. 1996, 6, 15 - 50. (36) Blcchl, P. E., Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953 - 17979. (37) Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865 - 3868.


34

Page 34 of 37

Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Monkhorst, H. J.; Pack, J. D., Special points for brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188 - 5192. (39) Henkelman, G.; Uberuaga, B. P.; Jonsson, H., A climbing image nudged elastic band method for finding saddle points and minimum energy paths. J. Chem. Phys. 2000, 113, 9901 - 9904. (40) Grimme, S., Semiem pirical GGA-type density functional constructed with a long-range dispersion correction. J. Comput. Chem. 2006, 27, 1787 - 1799. (41) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H., A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements HPu. J. Chem. Phys. 2010, 132, 154104. (42) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H., Origin of the overpotential for oxygen reduction at a fuel-cell cathode. J. Phys. Chem. B 2004, 108, 17886 - 17892. (43) http://webbook.nist.gov/chemistry/. (44) Mavrikakis, M.; Hammer, B.; Nørskov, J. K., Effect of strain on the reactivity of metal surfaces. Phys. Rev. Lett. 1998, 81, 2819 - 2822. (45) Hammer, B.; Nørskov, J. K., Theoretical surface science and catalysis-calculations and concepts. Adv. Catal. 2000, 45, 71 - 129. (46) Kitchin, J. R.; Nørskov, J. K.; Barteau, M. A.; Chen, J. G., Role of strain and ligand effects in the modification of the electronic and chemical properties of bimetallic surfaces. Phys. Rev. Lett. 2004, 93, 156801.


35


(47) Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Nørskov, J. K., Changing the activity of electrocatalysts for oxygen reduction by tuning the surface electronic structure. Angew. Chem. Int. Ed. 2006, 45, 2897 - 2901. (48) Ling, C. Y.; Shi, L.; Quyang, Y. X.; Zeng, X. C.; Wang, J. L., Nonosheet supported singlemetal atom bifunctional catalyst for overall water splitting. Nono Lett. 2017, 17, 5133 - 5139. (49) Duan, Z. Y.; Wang, G. F., A first principles study of oxygen reduction reaction on a Pt (111) surface modified by a subsurface transition metal M (M = Ni, Co, or Fe). Phys. Chem. Chem. Phys. 2011, 13, 20178 - 20187. (50) Duan, Z. Y.; Wang, G. F., Comparison of reaction energetics for oxygen reduction reactions on Pt (100), Pt (111), Pt/Ni (100), and Pt/Ni (111) surfaces: A first-principles study. J. Phys. Chem. C 2013, 117, 6284 - 6292. (51) Zhang, J.; Wang, Z.; Zhu, Z. P.; Wang, Q., A density functional theory study on mechanism of electrochemical oxygen reduction on FeN4-graphene. J. Electrochem. Soc. 2015, 162, F796 - F801. (52) Yang, Y. W.; Li, K.; Meng, Y. N.; Wang, Y.; Wu, Z. J., A density functional study on the oxygen reduction reaction mechanism on FeN2-doped graphene. New J. Chem. 2018, 42, 6873 - 6879. (53) Durand, W. J.; Peterson, A. A.; Studt, F.; Abild-Pedersen, F.; Nørskov, J. K., Structure effects on the energetics of the electrochemical reduction of CO2 by copper surfaces. Surf. Sci. 2011, 605, 1354 - 1359.


36

Page 36 of 37

Page 37 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(54) Shi, C.; Chan, K.; Yoo, J. S.; Nørskov, J. K., Barrier of electrochemical CO2 reduction on transition metals. Org. Process Res. Dev. 2016, 20, 1424 - 1430. (55) Jiao, Y.; Zheng, Y.; Chen, P.; Jaroniec, M.; Qiao, S. Z., Molecular scaffolding strategy with synergistic active centers to facilitate electrocatalytic CO2 reduction to hydrocarbon/alcohol. J. Am. Chem. Soc. 2017, 139, 18093 - 18100. (56) Ou, L. H.; Long, W. Q.; Huang, J. X.; Chen,Y. D.; Jin, J. L., Theoretical insight into effect of doping of transition metal M (M = Ni, Pd and Pt) on CO2 reduction pathways on Cu (111) and understanding of origin of electrocatalytic activity. RSC. Adv. 2017, 7, 11938 - 11950. (57) Liu, Y. J.; Zhao, J. X.; Cai, Q. H., Pyrrolic-nitrogen doped graphene: A metal-free electrocatalyst with high efficiency and selectivity for the reduction of carbon dioxide to formic acid: A computational study. Phys. Chem. Chem. Phys. 2016, 18, 5491 - 5498.

TOC Graphic


37

Rhodium and Nitrogen Codoped Graphene as a Bifunctional

Recommend Documents