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C: Surfaces, Interfaces, Porous Materials, and Catalysis
Computational Screening of Electrocatalytic Materials for Hydrogen Evolution: Pt Monolayer on Transitional Metals Xi-Bo Li, Teng-Fei Cao, Feipeng Zheng, and Xiaobo Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09301 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018
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Computational Screening of Electrocatalytic Materials for Hydrogen Evolution: Pt Monolayer on Transitional Meta Xi-Bo Li*1, Teng-Fei Cao2, Feipeng Zheng1, , Xiaobo Chen*1 1. Siyuan Laboratory, Guangzhou Key Laboratory of Vacuum Coating Technologies and New Energy Materials, Guangdong Provincial Engineering Technology Research Center of Vacuum Coating Technologies and New Energy Materials, Department of Physics, Jinan University, Guangzhou, Guangdong, 510632, P. R. of China. 2. Beijing Computational Research Center, Beijing, 100084, P. R. of China. *Corresponding Email:
[email protected] (Pro. Li);
[email protected] (Pro. Chen).
ABSTRACT: Searching for none or less Pt contained electrocatalytic materials for hydrogen evolution is a promising way to reduce the cost of hydrogen production. Thus, a computational screening of electrocatalytic materials for hydrogen evolution reaction (HER), Pt monolayer on transitional metals (Pt/TM), are carried out by DFT in this work. According to several prerequisites, including difficulty of synthesis, stability in electrochemical environment, HER activity and economical cost, several stable, high activity and low cost surfaces for HER are chosen. In detail, the Pt/TM(001)[TM = Fe, Mo, V, W or Nb], Pt/TM(111)[TM = Ag, Pd or Au], Pt/Re(0001) are found to have predicted high activity comparable to pure Pt. Especially, the five Pt/TM(001) surfaces are economical and seldom reported. Even more, Pt/TM(001)[W, Nb, V, Fe] may be even much better than pure Pt, as both the suitable hydrogen adsorption ability and larger exchange current. Further DFT analysis indicates the HER activity on Pt/TM is related with the d-band center of the Pt layer, which could be tuned by the transition metal in substrate. Our results are expected to contribute to the understanding of the HER on Pt/TM and give theoretical evidences for corresponding experiments. 1. Introduction Hydrogen is a clean and efficient energy carrier, which has great potential to replace fossil fuels in human activities. The hydrogen evolution reaction (HER) is a critical electrochemical process for hydrogen production, which requires the assistance of catalysts. Pt-group noble metals are the most effective HER catalyst, but their application are limited by the high cost1,2. A great deal of efforts have been devoted to explore non-Pt catalysts, such as transition metal dichalcogenides3-10, carbides11-13 and phosphides14-15. However, these materials present much lower catalytic performance than that of Pt. Thus, alloys containing Pt or Pt cluster for HER have attracted great researcher16-29. Among them, a high-throughput DFT calculations have been also carried out to screen over 700 bimetallic catalysts involving Pt, and a BiPt surface alloy with a relative high HER activity has been derived19. However, Pt usage in this surface alloy system couldn’t be reduced too much, and also both acceptable structural stability and high catalytic activity are not well stratified. An alternative approach to retain the high performance of Pt and meanwhile to reduce Pt 1
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usage is to explore composite catalysts that compose of cost-effective transition metals covered by a monolayer of Pt atoms18-19, 30-31. Similar to Pt monolayer, by including a monolayer of Pd on the (0001) surface of Re, the HER activity could be significantly enhanced relative to the case of pure Pd or Re32. Similarly, the oxygen evolution reactivity of Pt(111) could also be improved by including earth-abundant transition metals as the subsurface layers31. The reason for those work could be ascribed to the modified chemical properties of the top layer metal due to the electronic modulation of inner atoms31, 33-34. In particular, the d-band center of the surface atoms, which correlate with the surface hydrogen adsorption energy, could be modulated by using different transition metal in the sublayers. This provides a mean of engineering the catalytic activity by composite catalysts. However, there are no systemic theoretical or experimental reports on this kind of composite catalysts hitherto. In this work, we carried out a large scale of DFT computational screening of Pt/TM(hkl) composite catalysts, in which the top layer TM atoms in P phase (FCC, BCC or HCP as below) on the (hkl) facet are substituted by a monolayer of Pt. We considered the (111) facet for a series of face center cubic( FCC) metals, such as Pt, Au, Pd, Ag, Ir, Ni, Rh, Co and Cu, and the (0001) facet for several hexagonal closely packed (HCP) metals, such as Os, Re, Ru, Ti, Zn and Co. For bulk center cubic (BCC) metals, such as Mo, Nb, W, Fe and V, we considered both (001) and (110) facets. In order to screen out suitable electrocatalytic materials for HER, the difficulty of synthesis, water environment, the variation of exchange current and the HER activity dependence on H coverage, preference of Heyrovsky and Tafel pathway are discussed in detail. In the end, the most suitable surfaces may own comparable HER activity to Pt(111) are screened out, including Pt/TM(001)[TM=Fe, Mo, W, V or Nb], Pt/TM(111)[TM=Ag, Pd or Au], Pt/Re(0001). The first five surfaces are economical and seldom reported, and especially, Pt/TM(001)[ W, Nb, V, Fe] is even much better than Pt(111). Furthermore, in order to uncover relationship between the HER activity and electronic properties of the surface, the relationship between d-band center and H adsorption ability are also discussed. 2. Computational Details DFT calculations were performed using the Vienna ab initio simulation package (VASP)3536 and the Vanderbilt ultra-soft pseudo-potential37. The cut-off energy of plane-wave basis set was set to be 400 eV. The generalized gradient approximation (GGA) parameterized by the RPBE functional was used for the exchange-correlation interaction38. A k-point mesh with a distance of 0.02 /Å based on the Gamma-centered Monkhorst-Pack scheme39 was applied for all calculations. Structural optimization was performed until all forces on each atom were less than 0.02 eV/Å. Spin polarization was considered for systems containing Fe, Co, or Ni atoms only. A Pt/TM(hkl) composite catalyst was simulated using a (2 × 2) slab model, in which the top layer TM atoms were replaced by Pt atoms. We used five layers of metal atoms for a slab, in which the bottom two layers were fixed at the bulk sites while the top three layers were allowed to relax. Neighboring slabs are separated by a vacuum region of 15 Å in thickness. Totally three closely-packed and one less closely-packed (110) surfaces were considered, namely the (111) facet of FCC-TMs, the (0001) facet of HCP-TMs, and the (001) and (110) facet of BCC-TMs. The exchange current is used to evaluate the HER activity of a catalyst1, 2
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i0= −ek0/(1+exp(−|G|/kT) (1) where T = 298.15 K and G is the reaction free energy of the rate-limiting step. The rate constant k0 is taken as 200s-1site-1. 3. Results and Discussions 3.1 Stability of Pt/TM(hkl) The energy stability of a Pt/TM(hkl) surface is evaluated by calculating the formation energy Ef, Ef = (EPt/TM(hkl) – mEpt–ETM(hkl))/m (2) where EPt/TM(hkl) and ETM(hkl) are the total energy of the Pt/TM(hkl) and TM(hkl) slab, respectively, while EPt is the chemical potentials of Pt in bulk, m is the number of Pt atoms. Obviously, a negative value of Ef indicates the Pt prefers to form monolayer rather than bulk on TM(hkl), or versa vice. As shown in Figure. 1, all the composite catalysts of Pt/TM(111) have negative formation energies except Pt/TM(111) [TM = Au, Ag, Co and Ni] in FCC and Pt/Co(0001) in HCP. Those results indicate all of Pt/TM(hkl) are energetically stable and easily to be synthesized except the five ones with positive formation energies. According to our DFT calculations, the Pt atoms prefer to form Pt bulk rather than monolayer on TM(111) [TM = Au, Ag, Co and Ni] and Co(0001). Although the DFT calculations show that the Pt prefers to form bulk state on TM(111) [TM = Au, Ag], previous experiments indicate Pt could form monolayer on Au(111) and Ag(111)40-41. Thus, Pt/TM(111) [TM = Au, Ag] could be synthesized. And also, many other Pt/TM(hkl), including Pt/Au(111), Pt/Rh(111), Pt/Pd(111), Pt/Ru(0001), Pt/Ir(111) and Pt/W(110) are also reported in previous experiments 41-43.
Figure 1. The stability of Pt/TM(hkl) is evaluated by formation energy. The light green and light magenta areas indicate the formation energy of Pt/TM(hkl) is smaller and larger than 0 eV/atom, respectively. The former one and the latter one indicate the stable and unstable area basing on DFT results, respectively. Those data are in Table. S1. 3
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3.2 Thermodynamics for HER HER takes place through either the Volmer–Heyrovsky (V–H) mechanism or the Volmer– Tafel (V–T) mechanism, depending on which one is energetically more favorable. A simple approach for quick judgement for the desorption manner is to compare the hydrogen adsorption free energies of the reaction intermediates of these two reaction pathways. For the V-H process, the differential hydrogen adsorption free energy is defined as ΔGm= ΔEm + ΔEZPE – TΔS (3) ΔEm = E[mH] – E[(m – 1)H] – 1/2 E[H2] (4) where ΔEZPE and TΔS are zero-point energy and entropy corrections, respectively. For most metal surfaces, ΔEZPE – TΔS is determined to be ca. 0.24 eV1, which is thus used throughout this work. E[mH] is the total energy of a surface with m hydrogen atoms adsorbed, while E[H2] is the energy of a hydrogen molecule. For the V–T pathway, the reaction intermediate has at least two hydrogen atoms adsorbed on the surface. Consequently, the sum hydrogen adsorption energy is defined by Eq 5 and 6. ΔG'm = ΔEm + ΔEZPE – TΔS (5) ' (6) ΔE m= E[mH] – E[(m – 2)H] –E[H2] ' Obviously, -ΔGm and -ΔG m represent the reaction energies of a Heyrovsky reaction and a Tafel reaction, respectively. By comparison of |-ΔGm| and |-ΔG'm|, one can simply evaluate the reaction pathway of HER on a metal surface. In additional, all our calculated data are in standard condition, pH = 0 and T = 298.15 K. 3.3 HER activity on different surfaces Figure 2 shows the categories of active sites on the surfaces considered in this work. In overall, four kinds of surface sites, which are marked with top, fcc, hcp and bridge, could be identified. For the (110) facet of BCC metals (or BCC(110)), an additional site labelled by bridge-top also calculated. But structural optimization shows that bridge-top sites are unstable for hydrogen adsorption. We have examined the most favorable adsorption sites for all Pt/TM surfaces by calculating ΔGm (m = 1) at 1/4 hydrogen coverage (1/4 H ML), as shown in Table S2. For all considered surfaces, the hydrogen adsorption energy of the most stable adsorption site is used to represent the HER catalytic activity.
Figure 2 Adsorption sites for H atom on Pt/TM(hkl). (a) FCC(111), (b) HCP(0001), (c) 4
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BCC(001), (d) BCC(110) facet. The green, blue and white balls indicate the transitional metal atom in sub-layers, the Pt atoms of top layer, the adsorption sites, respectively. The possible adsorption sites are also labelled by fcc, bcc, hcp and bridge in each slab model. Based on the hydrogen adsorption ability, we could obtain the exchange current densities for each surfaces from kinetic model. Figure 2a shows most favored ΔG1 of 1/4 H ML for all considered Pt/TM(hkl) surfaces, and Figure 2b exhibits the corresponding exchange current densities (j0). Apparently, j0 follows a volcano-shape dependence on ΔG. When ΔG approaches zero, j0 reaches the vertex of the volcano and the catalytic activity achieves the best level of this structure. We screen out the Pt/TM(hkl) surfaces with ΔG in between -0.1 and 0.1 eV in Figure 2a since they have nearly thermoneutral (ΔG = 0) hydrogen adsorption. Among these selected structures, Pt/Co(111), Pt/TM(0001) [TM = Co and Re] and Pt/TM(001) [TM = Fe, Mo and W] exhibit smaller |ΔG| and larger j0 than those of Pt, implying that they probably have better HER activity than the latter. Note that Pt/Co(111) and Pt/Co(0001) are unstable upon hydrogen adsorption, and thus they should be rejected. For all considered surfaces, only V–H mechanism could occur under this hydrogen coverage (1/4 H ML).
Figure 3 (a) Free energy diagram for hydrogen evolution and (b) exchange current on Pt/TM(hkl) at equilibrium (U=0 eV). All data are from Table S1 and Eq 1 at 1/4 H ML. In order to explore the HER activity dependence on H coverage, the ΔGm (ΔG'm) of 2/4, 3/4 and 1ML on each kind of surfaces are also calculated according to the most stable adsorption site at 1/4ML, as shown in Figure 4. The calculated results clearly show the ΔGm (ΔG'm) has a positive relationship with H coverage at a given surface (except Pt/Co(111), Pt/Ni(111) and Pt/Co(0001)). The increasing trend is due to the interaction between adsorption hydrogen atoms as H coverage increases, similar trends also happen on Mo2S systems44, and also indicating the tunability of HER activity by H coverage. In additional, the abnormal trend of ΔGm (ΔG'm) of Pt/Co(111) and Pt/Ni(111) in Figure 4a-b, and of Pt/Co(0001) Figure 4c-d is very clear. Their abnormal behaviors are related to the unstable of hydrogen atom adsorption: the *H will distort the top layer of the three configurations, making the surface unstable. Thus, the three configurations are not suitable for HER activity and will not discussed in following. Meanwhile, their alloying systems may show high HER activity as previous work24, 26, but they are beyond discussions in this work. As mentioned above, only the surfaces with |ΔG|
