DFT analysis elementary reaction steps of catalytic activity for ORR on metal-, nitrogen- co-doped graphite embedded structure
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Metal-nitrogen coordinated graphite coordination structures are becoming more and more attractive for its novel catalytic activity in oxygen reduction reaction (ORR) at the fuel cells. In this work, single copper atom on graphitic carbon nitride acting as electrocatalyst for ORR have been investigated by using the density functional theory method. Our study results that the Cu site is the active center for all the possible elementary steps of the ORR. Further studies the elementary reaction steps are used to explore the underlying mechanisms to gain insights into ORR. Both the O2 dissociation and O2 hydrogenation paths are probably to ORR on the CuN4-Gra surface. All the possible elementary reaction steps for the ORR are exothermic with small reaction barriers (less than 1.98 eV) for O2 hydrogenation. Meanwhile, with large reaction barrier (3.16 eV) for O2 dissociation to go through the rate-limiting steps. The Gibbs free energy for each elementary step of ORR is used to clarify which path determine the ORR/OER on the CuN4 co-doped graphene. Scaling relation and surface phase diagram are obtained by calculated Gibbs free energy of intermediates at surface active sites with various adsorption species. The different working potentials are also considered for the studied catalysts, as the overpotential of ORR is also an important indexes of the catalytic activities of the catalyst, we calculated the overpotential for each active site on the structures and determined the minimum overpotential for ORR.
KeywordsCuN4-Gra coordinated structure Oxygen reduction mechanism Activity barrier Gibbs energy change Surface phase diagram
The metallic and N doped 2D materials embedded structure such as graphitic [1, 2, 3], pyridinic [4, 5, 6], and pyrrolic N [7, 8] and metal-N heterogeneous structures have been investigated to be as promising catalytic substitutes for oxygen reduction reaction (ORR) and initiate a command in an application to fuel cells in recent years [9, 10]. The metal and N heteroatoms of embedded structure could tune the charge distribution and change catalytic activity of graphite due to the difference in electronegativity between the contiguous N and C atoms. Currently their main usage is focused to as reliable energy storage or energy conversion device applications, [11, 12] the main challenges need to be overcome are related to the storage of O2, and energy conversion efficiency. Thanks to the high energy conversion efficiency and storage density of the ORR process for fuel cells,  it have been applied massively in portable electronic devices. Metal-N doped graphite catalytic materials have attracted immense attention, due to the high catalytic activity for ORR/OER reactions as power sources for electric vehicles . For example, there are considerable works which reveals metal-N graphite materials has exhibited a higher ORR activity than non-metallic N-doped graphene; [15, 16] Recently, some theoretical investigations have shown that nitrogen- coordinated transition metals (e.g., Mn, Fe, and Co) in graphene exhibit also good ORR catalytic activity [17, 18]. Bai et al.  studied the reaction mechanism for oxygen reduction reaction (ORR) on P-doped graphene through the DFT method, resulting that P-doped graphite could exhibit high electrocatalytic activity, the most favorable reaction pathway is the hydrogenation of O2 molecule to form OOH, then the hydrogenation of OOH gives H2O and atomic O in ORR. Liu et al.  found that Fe/N-doped graphene could create a promising catalytic activity for ORR, and it can be as guidance for catalytic material design. Lu et al.  has studied the formation energy for MnN4 embedded in graphene with dispersion-corrected density functional theory study for application in fuel cell devices, it is conducive to the design and improvement of catalyst ORR efficiency. Baran et al.  have investigated the ORR on metalloporphyrin combined with graphene based on metal MN4 structures, resulting that there is a sensible scaling relationships between the Gibbs free energies and overpotential of oxygenated species (such as O, OH and OOH) during the ORR process,  it implied a volcano plots within overpotential versus adsorption Gibbs free energy of oxygenated species of ORR activity. The free energy of the ORR intermediates can be calculated by using the procedures proposed by Nørskov et al.  the details about the calculation methods can be found elsewhere [25, 26] by calculation barrier energy can determine the most favorable reaction pathway of elementary reaction steps.
In this study we assume that the temperature is constant at 298.15 K with pO2 = 0.1 bar , thus the change in Gibbs free energy caused by the temperature effect is neglected and included only phase state, solvation and hydrogen bonds correction . The scope of this work is to explore the pathway of the reaction mechanism and the thermokinetics of the involved elementary reaction steps for ORR/OER on the CuN4 active site. To further identify the most favorable pathway for ORR was performed to locate transition states (TS) and obtain barrier energy by CI-NEB method. [29, 30] There are two possible reaction pathways for O2. One is O2 can capture one atomic H to form an OOH species and another is O2 dissociation. During ORR and OER process, it found that there are several possible elementary reaction steps, such as OH and OOH dissociation, atomic O hydrogenation and OH diffusion. According to the original plan, we shall describe these reaction pathways in detail by the theory developed by Nørskov et al.  It can correctly describe the activation energy and reaction energy of Elementary reaction processes . Moreover, the solvation and phase state effects are considered by a correction to the ORR from free energy in the calculations. It can be reflected in our calculations for the adsorption of intermediates, the DFT results show that the catalytic activity of the CuN4-graphite is comparable to that of Fe and Mn graphite embedded structure electrocatalysts catalyst, it demonstrates that the ORR on this class of catalysts can proceed via several reaction pathways with barrier and reaction energy.
1.1 Computational detail
2 Results and discussion
2.1 Various ORR involved species
It is found from Fig. 2 that the central Cu atom serves as the catalytic activity and the most stable adsorption sites for all ORR reaction intermediate species, which agree well with some relate previous work . The first stage, O2 tends to adsorbate on the top of the Cu atom and formed a Cu–O bond distance of 2.22 Å with adsorption energy of − 0.45/eV, and H2O absorbed on the top of Cu atom with a smaller adsorption energy (0.055 eV) than O2 molecule as show in Fig. 2a, e, respectively. For the electronic structure of bare surface (see Fig. 1d, e), the extra electrons compel the 2p states of N to Cu3d states which across to the Fermi energy and activating the adjacent carbon atoms, eventually resulting in the obvious enhanced adsorption properties of adsorbed species. In addition to the hybrid-ization of N2p* states with the C2p state around the Fermi energy, the hybridization of 2p states of C and N is also observed. This speculation is corroborated by the large binding energy of intermediates, see from Fig. 2, about − 2.18 eV for *OH, − 3.11/eV for *O, and for − 0.92/eV for *OOH, respectively.
2.2 ORR mechanism of O2
The ORR process of O2 may occur via two mechanisms. One is direct dissociation into two atomic O and another is associative to form OOH with one atomic H, all the two mechanisms make the adsorbed O2 molecule sequentially hydrogenates into the final product H2O. In the associative pathway, a O2 hydrogenation occurs to form OOH species and then it can be either hydrogenated to form H2O and one O atomic, and then the atomic O sequentially hydrogenates into the final product H2O with two atomic H. In addition, the overall possible reaction pathways are shown in Figure 1S (see from supporting information), where the individual possible reaction step of the dissociative pathways are shown. The relative reaction energy (ΔE) and reaction barrier energy (Eact) profiles of all reaction steps for the O2 molecule with atomic H ORR process are presented in Fig. 3a. As a rule, the total energy of the O2 adsorbed process on CuN4-graphene surface is used as the critical state, the adsorption energy of the states during the reduction steps is − 0.45/eV see in Fig. 2. The detailed data on all potential reaction step reaction energy and barrier energy are summarized in Fig. 3 in the supporting information. According to the calculated reaction barriers, the reaction pathway (as the red line in Fig. 8) is the most favorable pathway (reaction mechanism I) throughout the entire ORR. According to the reaction stages presented above, there are four possible stage for the ORR process on the CuN4-gra surface is first stage with the highest reaction barrier of 2.04/eV, it corresponding to the hydrogenation process of the O2 molecule with one atomic H to form the OOH species. Meanwhile, the rate-limiting stage of O2 dissociation mechanism process with a high reaction barrier energy of 3.16/eV, corresponding to the O2 change into two atomic O.
We carried out density functional theory (DFT) calculations of the active sites and the overall possible reaction pathways are illustrated in Fig. 8, where the individual microsteps of the dissociative pathway and associative pathway. In a word, it is seen from all the possible reaction pathways that for O2 dissociation, the favorable pathway is the process a O2 → 2*O → 2*OH → *O and H2O → *O → *OH → H2O form with the rate determining step of O2 dissociation (barrier activity 3.16/eV). It need note that *OH + *OH → *O + H2O with a barrier energy of 0.62 eV and *OH + *OH + H+→ *OH + H2O with a similar barrier energy (0.59/eV). It indicates that they are competitive pathways. Nonetheless, we also noted that the second atomic *O hydrogenation to form OH have the smallest energy barriers (0.26). On the other hand, the O2 hydrogenation is the process a O2 hydrogenation → *OOH dissociation → atomic *O and OH → *O hydrogenation → *OH hydrogenation → H2O form with the rate-determining step of O2 hydrogenation (barrier energy 1.98 eV) preferred due to a smaller energy barrier than O2 dissociation (3.16 eV). The formed OH species is stable on the Cu site, H atom would diffuse from the five ring site and form OH with a 1.48 barrier energy and high exothermic reaction energy (− 3.34/eV). For the OOH dissociation, producing an atomic O adsorbed on Cu with a exothermic reaction energy of 0.94/eV (See in Fig. 3e). The formed OH diffuses easily from the five-member ring to the Cu top site of the six-member ring with a reaction barrier of 0.69/eV and a small exothermic reaction energy of − 0.36, as presented in Fig. 4d.
2.3 Thermodynamics analysis
As showed in Fig. 7a, we presented the scaling relations between the Gibbs energy of the ORR intermediates, as a function of ΔGOH, and Fig. 7b presented the potential dependent surface phase diagram at T = 298 K and pO2 = 0.1 bar, corresponding to electron transfer steps (0.79, 2.34 V) is marked by circles, the Gibbs free energy between the voltage of the ORR intermediates. In Fig. 7, we observe a gap with about − 0.11 ~ 2.11 eV in the *O and *OH Gibbs free energy and 0.98–2.74/eV in the *O and *OOH Gibbs free energy. As a guide to the eye, we marked a dotted area between 2.26 and 2.93/eV in the x-axis of Fig. 7a which includes the gaps in the 3 lines. This area strictly separates active sites in each active site. Meanwhile, we sloped the lines *O versus *OH (purple) and *OOH versus *OH (orange) see in Fig. 7a, the value expected from the scaling relationships analysis and been recognized for work for Cu with oxides. These equations were also used as input to construct the volcano plot. We note that both the scaling relations are the same the results obtained for free energy analysis. This similarity is expected since the linear scaling relations are determined by the Gibbs free energy of binding, the intercept of these relations appears to be determined by the absorption strength of intermediates. All intermediates have only at top binding available, resulting in similar scaling relations. Nevertheless, graphite materials are probably more stable in acidic environments has great importance properties. Moreover, the fact that experimentally the active sites are created in the interstices between graphite layers in porous materials could, in principle improve the ORR/OER activities. 
Theoretical calculations revealed that the CuN4 co-doped graphene, it would improve the catalytic activity for the ORR process. This coordination structure has delivered a superior performance compared to conventional metal for catalyzing ORR in fuel cells. First, we calculated the adsorption energy of ORR intermediates, and then calculated reaction energy and barrier energy by using climbing image nudged elastic band (CI-NEB) method to search transition state of all of the ORR elementary reaction steps, indicating that the CuN4 co-doped graphene as efficient catalyst for ORR The intermediate molecules are always chemisorption at the top of Cu site on the CuN4 co-doped graphene, it implies that the N-chelated transition metal with carbon plays an important role in the ORR/OER process, the transition metal is the active center for all the possible elementary steps of the ORR. The most favorable pathway is the O2 hydrogenation to OOH form. OOH from the calculation of barrier energy for all elementary reaction steps, the hydrogenation of O2 is more easily than O2 dissociation pathways. According to the scaling relations between the Gibbs free energy and the separation activated site gap value expected from the scaling-relationships, the free energy of oxygenated intermediates (*OOH, *O, and *OH,) in ORR approximately linearly scale with each other, it can analysis and known how to work for metals and oxides of the ORR adsorbates and also has great influence on reaction pathways due to the different active sites. Indicating that the O2 hydrogenation is much easier than O2 dissociation. Therefore, our study revealed that the single atomic metallic Cu can improve the catalytic activity of graphite for the ORR without OER process. Since the hydrogenation of O2 molecule to form *OOH and the hydrogenation of *OOH to form H2O and atomic *O, it can be happened spontaneously with a negative reactions energy, this step plays an extremely important role in ORR. It predicted that the reduction and oxidation potential for UORR is 0.279 V and UOER is 2.76 V, respectively. Our theoretical research is helpful for designing novel efficiency catalysts for fuel cells.
Financial support comes from the China Scholarship Council (CSC: 201808440416) of China and Research and the Arts (HMWK) of the Hessen state in Germany. The Lichtenberg high performance computer is gratefully acknowledged, and we are also thankful to TU Darmstadt.
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Conflict of interest
The authors declare that they have no conflict of interest.
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