Chinese Journal of Chemical Physics  2018, Vol. 31 Issue (5): 649-654

The article information

Jing-jing Lin, Hai-feng Lv, Xiao-jun Wu

Enhanced Oxygen Reduction on Graphene via Y$_5$Si$_3$ Electride Substrate: a First-Principles Study

Chinese Journal of Chemical Physics, 2018, 31(5): 649-654

http://dx.doi.org/10.1063/1674-0068/31/cjcp1804071

Article history

Accepted on: May 10, 2018
Enhanced Oxygen Reduction on Graphene via Y$_5$Si$_3$ Electride Substrate: a First-Principles Study
Jing-jing Lin, Hai-feng Lv, Xiao-jun Wu
Dated: Received on April 18, 2018; Accepted on May 10, 2018
Hefei National Laboratory for Physical Science at the Microscale, School of Chemistry and Materials Sciences, CAS Key Laboratory of Materials for Energy Conversion, and CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei 230026, China; Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei 230026, China
*Author to whom correspondence should be addressed. Xiao-jun Wu, E-mail: xjwu@ustc.edu.cn
Abstract: Manipulating the chemical reactivity of graphene toward oxygen reduced reduction (ORR) is of particular interest for both fundamental research and practical application in fuel cell. Deposing graphene on selected substrate provides a structure-intact strategy to enhance its chemical reactivity due to substrate-induced charge and interface effect. Here, we report the graphene deposited on one-dimensional electride Y$_5$Si$_3$ as an effective ORR catalyst in acidic media. Thermodynamic calculations suggest that depositing graphene on electride materials can facilitate the protonation of O$_2$, which is the rate-determining step based on the four-electron reaction pathway and thus promote the ORR activity. Further electronic calculations reveal that low work function (3.5 eV), superior electrical conductivity and slight charge transfer from substrate to graphene result in the enhanced ORR performance of graphene. These findings shed light on the rational design of ORR catalysts based on graphitic materials and emphasize the critical role of substrates for energy-related electrochemical reactions.
Key words: First-principles calculations    Graphene    Oxygen reduced reduction    Electrides
Ⅰ. INTRODUCTION

Fuel cells are promising candidates for clean energy conversion in the quest for alternatives to conventional fossil fuel technology [1-3]. Oxygen reduction reaction (ORR), which occurs at the cathode of fuel cells, is critically fundamental in energy conversion process [4]. Noble metals, i.e. Pt or its alloys, have been considered as the best-known ORR catalysts. Despite intense research, the sluggish kinetic, high-cost and instability have impaired the large-scale practical applications of the above renewable-energy technologies [5-8]. It is highly desirable to explore new ORR catalysts with high performance and low cost.

Since the discovery of graphene, significant efforts have been devoted to developing graphene based ORR catalysts as substitute for Pt due to its superior electrical conductivity, long-term durability, environmental benignity, and unique physical and chemical properties [9-13]. However, the high overpotential of graphene, which is the key ORR activity descriptor, is still incomparable with those of Pt-based catalysts. How to improve the chemical reactivity of graphene is a big challenge. Previous experiments together with theoretical studies have shown that chemical modification via morphology control, additional dopants, or substance hybridization, can enhance the catalytic activity of graphene [14]. For instance, graphene substitutional doped with nitrogen atoms can act as an efficient metal-free electrode [15], where the lone pair of N can induce negative charge in graphene and form a positively charged neighboring carbon atom [16]. But, the control of doping types, distribution, and concentration of nitrogen are technique challenges for experimental research and practical applications [17]. Chemical modification of reduced graphene oxides with Co$_3$O$_4$ nanoparticles can also be used for a hybrid functional catalyst for ORR [18]. However, the damage to the perfect lattice of graphene in these strategies usually reduces their electric conductivity. It is of particular interest to enhance the ORR reactivity of graphene without breaking its lattice structure.

Manipulation of chemical reactivity of graphene via substrate provides an intact strategy, which is a key to realize many important nanotechnologies of electronic and catalyst. Previously, theoretical investigations reveal that depositing N-doped graphene on Co(111) or Fe(110) surface can further promote their ORR reactivity [19, 20], while the enhancement of pure graphene via substrate is seldom reported. Herein, we propose to enhance the chemical reactivity of pure graphene toward ORR by deposing graphene on water-durable electrides materials. Electrides, which exhibit delocalized anionic electrons in outer space, have attracted great research attention for their active catalytic ability of direct ammonia synthesis [21-24]. Y$_5$Si$_3$ is the first discovered water stable electride with a formula of [Y$_5$Si$_3$]$^{0.79+}$:0.79e$^-$, which exhibits strong orbital hybridization between yttrium 4d and anionic electrons [25]. Experimentally, Ru-loaded Y$_5$Si$_3$ is highly efficient for direct ammonia synthesis. Our results demonstrate that electride material Y$_5$Si$_3$ with rich anionic electrons on surface can significantly promote the ORR reactivity of graphene by facilitating the protonation of O$_2$, which is the rate-determining step based on the four-electron reaction pathway. The enhanced ORR performance mainly originates from low work function (3.5 eV), superior electrical conductivity and charge transfer from substrate to graphene. The development of graphene-based metal-free ORR catalyst using the anionic-rich electrides as substrate opens a brand-new avenue to other catalytic reactions and help to find applications in other fields.

Ⅱ. COMPUTATIONAL DETAILS

All density functional theory calculations were performed by the Vienna ab initio Simulation Package (VASP) package [26, 27]. Electronic exchange and correlation effects were described within the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functional [28]. Projector augmented wave method was used to describe nuclei-electron interactions [29, 30]. The plane-wave kinetic energy cutoff is set to be 450 eV. Account for the van der Waals interactions between graphene and substrate, we employed dispersion correction of Grimme DFT-D3 method [31]. The integration of the Brillouin zone was sampled with gamma-centered 6$\times$6$\times$1 and 9$\times$9$\times$1 $k$-points for geometry optimizations and electronic calculations, respectively [32]. The vacuum layer is set as 20 Å to isolate neighboring atomic layer and the structure is optimized until total energy is converged to 1.0$\times$10$^{-5}$ eV/atom and the force on each atom is less than 0.01 eV/Å.

Y$_5$Si$_3$ belongs to Mn$_5$Si$_3$-type structure and the anionic electron exists in the quasi one-dimensional void. A 1$\times$1 unit-cell was used for Y$_5$Si$_3$ (001) surface and for graphene, we adopted a 2$\sqrt{3}$$\times2\sqrt{3} supercell with lattice vectors of {\bf{b}}_1=2({\bf{a}}_1-{\bf{a}}_2) and {\bf{b}}_2=2({\bf{a}}_1-2{\bf{a}}_2) ({\bf{a}}_1, {\bf{a}}_2 are the primitive vectors) giving a mismatch for the lattice parameters of 0.7% and a twisted angle of 60^\circ. In this case, the overlayered graphene is slightly stretched below 1%. Here, we use the same graphene with or without substrate and the infinitesimal strain is tested to have no change to the ORR performance. The sequential electron-transfer process proposed by Nørskov et al. is employed to evaluate the free energy change from gaseous O_2 molecule dissociates into oxygenated intermediates [33, 34]. For the thermodynamic calculations, the electrocatalytic ORR can be decomposed into four one-electron oxidation steps, absorbing one proton and one electron at each step based on the standard hydrogen model, which is described as below in acidic media:  ^*+{{\rm{O}}_2}\left( {\rm{g}} \right) + {{\rm{H}}^ + } + {{\rm{e}}^ - } = {\rm{OOH}}^* (1)  {\rm{OOH}}^* + {{\rm{H}}^ + } + {{\rm{e}}^ - } = {\rm{O}}^* + {{\rm{H}}_2}{\rm{O}} (2)  {\rm{O}}^* + {{\rm{H}}^ + } + {{\rm{e}}^ - } = {\rm{OH}}^* (3)  {\rm{OH}}^* + {{\rm{H}}^ + } + {{\rm{e}}^ - } = {{\rm{H}}_2}{\rm{O}}\left( {\rm{l}} \right) (4) where ^* represents the active sites on graphene, "g" and "l" denote the gas phase and liquid phase, and OOH^*, O^*, OH^* are the intermediates during the ORR process. Thus, we can summarize the whole process as:  {{\rm{O}}_2}\left( {\rm{g}} \right) + 4{{\rm{H}}^ + } + 4{{\rm{e}}^ - } = 2{{\rm{H}}_2}{\rm{O}}\left( {\rm{l}} \right) (5) the Gibbs free energy of formation for each step can be given as:  \Delta G = \Delta {E_{{\rm{DFT}}}} + \Delta {\rm{ZPE}} + T\Delta S (6) where \Delta {E_{{\rm{DFT}}}}, \Delta {\rm{ZPE}}, and \Delta S are the changes in total energy from DFT, zero-point energy, and entropy from the initial state to the final state, respectively. T is temperature and U is electrode potential. The overpotential, which is crucial parameter for activity, is calculated by:  \eta^{\rm{ORR}}= \Delta G_{\max}/\textrm{e} + U_0 (7) where \Delta G_\max is the maximum Gibbs free energy of formation of the four reaction steps given by Eqs.(1)-(4); U_0 is the equilibrium potential for pH=0 at T=298 K and it gives a zero Gibbs free energy of formation for the overall reaction by Eq.(5). The binding energies of the oxygenated intermediates are all related to the energies of H_2 and H_2O including \Delta E_{\rm{OH^*}}, \Delta E_{\rm{OOH^*}}, and \Delta E_{\rm{O^*}} which are defined as:  \Delta E_{{\rm{OH}}^*} = E( {\rm{OH}}^*) - E( ^* ) - E[ ( {\rm{H}}_2{\rm{O}} ) - (1/2){\rm{H}}_2 ] (8)  \Delta E_{{\rm{OOH}}^*} = E( {\rm{OOH}}^* ) - E( ^* ) - E[ 2( {\rm{H}}_2{\rm{O}}) - (3/2){\rm{H}}_2] (9)  \Delta {E_{{\rm{O}}^*}} = E( {{\rm{O}}^*} ) - E( ^*) - E[ {( {{{\rm{H}}_2}{\rm{O}}} ) - {{\rm{H}}_2}} ] (10) where the E(^*), E(\textrm{OH}^*), E(\textrm{OOH}^*) and E(\textrm{O}^*) are the DFT energies of bare surface and with adsorbates of OH^*, OOH^*, O^* species. In order to avoid the error of high spin state of O_2 molecule, the O_2 is derived from H_2O\rightarrow(1/2)O_2+H_2. The activation energy of every step is not considered here, but the overpotential is the prerequisite characterization of catalytic performance. Ⅲ. RESULTS AND DISCUSSION Y_5Si_3 belongs to Mn_5Si_3 family with a symmetric space group of P6_3/mcm (No.193). The optimized crystal structure of Y_5Si_3 is shown in FIG. 1(a). The optimized lattice constant is a=b=8.46 Å and c=6.39 Å, respectively, consistent with the experimental value [24, 25]. The bulk phase features a one-dimensional channel along the (001) directions (c-axis) denoted by the red dashed circle. The one-dimensional hole of Y_5Si_3 material can accommodate hydrogens, forming Y_5Si_3H, which has been studied as a hydrogen storage material [25, 35]. The calculated electronic band structure and density of states (DOS) are shown in FIG. 1 (b) and (c). Y_5Si_3 exhibits typical metallic behavior, where the states near the Fermi energy level are mainly contributed by yttrium's 4d orbitals, and the hybridization of yttrium's d orbitals and silicon's p orbital occurs below the Fermi energy level within the range of -3.7 eV to -0.8 eV.  FIG. 1 (a) The optimized structure, (b) calculated band structure, and (c) density of state of Y_5Si_3 bulk are plotted. Yttrium and silicon atoms are depicted using cyan and orange balls. Unit cell is the black diamond and red dashed circle denotes the 1D channel. The highlighted character of electride is the loosely bound electrons, mostly distributed in the structural cavities of a lattice. The electron localization function analysis is a powerful and efficient tool to determine the form of electride. FIG. 2(a) displays the calculated electronic density distribution of Y_5Si_3, which exhibits nearly free one-dimensional electron. In contrast, when one hydrogen atom is inserted in one-dimensional channel, the delocalized electrons in channel disappear, as shown in FIG. 2(b), suggesting that the free electron is transferred to hydrogen atoms. The contribution of free electrons in Y_5Si_3 is labelled with green dashed line in FIG. 2(c), which is absent in Y_5Si_3H system, as shown in FIG. 2(d). This difference further confirms that Y_5Si_3 is electride material. Detailed electron distribution through Bader analysis indicates that the inserted hydrogen atom can adsorb about 0.76 e^-, so the chemical formula can be described as [Y_5Si_3]^{0.76+}:0.76e^-, consistent with previous theoretical and experimental study [25]. The loosely bound electrons endow Y_5Si_3 novel catalytic performance for ammonia synthesis reaction [23].  FIG. 2 Electronic density map of (a) Y_5Si_3 and (b) Y_5Si_3H. Electronic band structure of (c) Y_5Si_3 and (d) Y_5Si_3H. Electron density map is calculated in the energy range of -0.5 eV < E-E_{\rm{F}} < 0.0 eV. Isosurface is 0.7 e/bohr^3. The delocalized free electrons provide an opportunity to tune the chemical reactivity of graphene by deposing graphene on Y_5Si_3 surface. Y_5Si_3(001) surface with a 2\sqrt{3}$$\times$2$\sqrt{3}$ supercell is chosen as substrate to match the lattice of graphene, as shown in FIG. 3 (a) and (b). The lattice constant of graphene is 8.52 Å, which has a commensurate in-plane lattice with Y$_5$Si$_3$(001). The vertical distance between Y$_5$Si$_3$(001) surface and on-top graphene is optimized to be about 3.27 Å, as shown in FIG. 3(c). The adsorption energy of graphene on Y$_5$Si$_3$(001) surface is defined as $E_{\rm{ads}}$= $E$(graphene/Y$_5$Si$_3$)-$E$(Y$_5$Si$_3$)-$E$(graphene), where $E$(graphene/Y$_5$Si$_3$), $E$(Y$_5$Si$_3$) and $E$(graphene) are the total energies of graphene/Y$_5$Si$_3$, Y$_5$Si$_3$, and graphene, respectively. The calculated adsorption of graphene on Y$_5$Si$_3$(001) surface is about -1.91 eV, indicating that there is strong interaction between graphene and Y$_5$Si$_3$ surface.

 FIG. 3 (a) The (001) facet of Y$_5$Si$_3$. (b) Hybrid structure of Y$_5$Si$_3$(001) and graphene. (c) 2$\sqrt{3}$$\times$2$\sqrt{3}$ supercell of graphene.

Next the oxygen reduction process on pure graphene and graphene deposited on Y$_5$Si$_3$ substrate is investig- ated. There are six non-equivalent C atoms in supercell of graphene on Y$_5$Si$_3$ substrate, illustrated in FIG. 4(a). Following the four-electron steps of oxygen reduction, the intermediates states of O$_2$, OOH, O, and OH species adsorbed on site "2" of Y$_5$Si$_3$ supported graphene are displayed in FIG. 4(b), labelled with $^*$O$_2$, $^*$OOH, $^*$O, and $^*$OH, respectively. Table Ⅰ and FIG. 4(c) summarize the calculated Gibbs free energies of each state for ORR on the freestanding graphene and Y$_5$Si$_3$ supported graphene. It is clear that the protonation of O$_2$ is the rate-limit step of ORR on graphene substrate, and the rest three steps have a down-stairs Gibbs free energy. The calculated overpotential is about 0.63 eV for freestanding graphene, while this value is significantly reduced to 0.223 eV on the Y$_5$Si$_3$-supported graphene, comparable with Pt based materials. To further confirm the ORR performance in the basal plane of graphene, we choose site "3" and the adsorption energy of species O$_2$ and OOH on site "3" is approximately same as site "2", which shows a maximum difference of only 0.004 eV. The reduced overpotential of ORR on Y$_5$Si$_3$ supported graphene implies a promotion of ORR activity of graphene by Y$_5$Si$_3$ substrate.

 FIG. 4 (a) The top view of hybrid structure. (b) Optimal adsorption structure of intermediate species. (c) Free-energy diagram for the ORR on pristine graphene and Y$_5$Si$_3$ supported graphene.
Table Ⅰ The calculated Gibbs free energy changes $\Delta G$ (in unit of eV) of graphene and Y$_5$Si$_3$ supported graphene (graphene/Y$_5$Si$_3$) are summarized.

To understand the enhanced oxygen reduction activity on Y$_5$Si$_3$ supported graphene, the working function of Y$_5$Si$_3$(001) surface is calculated, as shown in FIG. 5(a). The calculated working function of Y$_5$Si$_3$(001) surface is about 3.5 eV, which is significantly smaller than that of graphene (4.5 eV). Therefore, charge is transfer from Y$_5$Si$_3$(001) surface to graphene, which is confirmed with the Bader charge analysis. The Bader charge analysis result indicates that there are about 0.48 electrons transferring from Y$_5$Si$_3$ to graphene, slightly changing the charge distribution of on-top graphene. The calculated electronic band structure of graphene on Y$_5$Si$_3$ substrate, as shown in FIG. 5(b), reveals that graphene's p$_z$ orbital strongly hybridized with Y's 4d orbitals, partly destructing the conjugated $\pi$ on graphene. The electrons transferred from substrate to graphene's p$_z$ orbital, preventing the over-binding of oxygenate species and resulting in the reduction of overpotential value. This is confirmed with the calculated binding energies of oxygen species, as summarized in Table Ⅱ. The adsorption energy of O$_2$ molecule on graphene is significantly reduced with Y$_5$Si$_3$ substrate.

 FIG. 5 (a) Work function of substrate Y$_5$Si$_3$ along (001) direction. (b) Band structure of hybrid structure and the decomposed p$_z$ orbital of graphene and 4d orbital of Y atom.
Table Ⅱ Binding energy of each species on Gr and Gr/Y$_5$Si$_3$ in ORR pathway.
Ⅳ. CONCLUSION

In conclusion, we have investigated the oxygen reduction process on the freestanding graphene and graphene supported by a novel one-dimensional electride of Y$_5$Si$_3$. Our results demonstrate that Y$_5$Si$_3$ has one-dimensional free electrons with the form of [Y$_5$Si$_3$]$^{0.76+}$:0.76e$^-$. Deposing graphene on Y$_5$Si$_3$(001) surface significantly reduces the Gibbs free energy of O$_2$ adsorption, which is the rate-limit step in oxygen reduction process. The calculated overpotential of graphene on Y$_5$Si$_3$ substrate is about 0.223 eV, comparable with that of Pt. The enhanced oxygen reduction activity of graphene via Y$_5$Si$_3$ substrate originates from the charge transfer from substrate to graphene due to the low work function of Y$_5$Si$_3$. The hybridization of Y's d orbital and C's p$_z$ orbital implies the charge occupied the C's p$_z$ orbital, which weaken the interaction of O$_2$ and graphene. The present work shed light on the substrate hybridization enhances ORR activity of graphene and provides an opportunity of rational design for ORR catalyst with graphene.

Ⅴ. ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China (No.21573204 and No.21421063), Ministry of Science and Technology of China (No.2016YFA0200602), Anhui Initiative in Quantum Information Technologies, Fundamental Research Funds for the Central Universities, National Program for Support of Top-notch Young Professional, Chinese Academy of Sciences Interdisciplinary Innovation Team, and Super Computer Center of USTC supercomputing center and CAS supercomputing center.

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