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Electrochemical oxygen reduction reaction in alkaline solution at a low overpotential on (220)-textured Ag surface Nan Zhang, Fuyi Chen, Danmin Liu, and Zhenhai Xia ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01009 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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ACS Applied Energy Materials

Electrochemical Oxygen Reduction Reaction in Alkaline Solution at a Low Overpotential on (220)-textured Ag Surface

Nan Zhanga, Fuyi Chen,*a Danmin Liub, Zhenhai Xia*c a

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xian

710072, China. E-mail: [email protected]. b

Key Laboratory of Advanced Functional Materials, Beijing University of Technology, Beijing

100022, China. c

Department of Materials Science and Engineering, Department of Chemistry, University of North

Texas, Denton, TX 76203, USA. E-mail: [email protected] Abstract: The oxygen reduction reaction (ORR) on textured silver surface is investigated through computational modelling and experimental measurement. The reaction paths in three possible mechanisms of ORR on pure silver surfaces are analyzed by the transition-state searching method, and the Ag(220) surfaces exhibits an activation energy barrier of O2 protonation reaction is 0.504 eV, which is the lowest activation energy barrier among the flat and stepped silver facets. Furthermore, the theoretical calculations show that the ORR overpotential of Ag(220) surfaces is 0.457 V, which is comparable to the Pt(111) catalyst with the overpotential of 0.441 V, indicating that the stepped Ag(220) facet is a high ORR active site on the textured silver surface. The ORR polarization curves of the textured silver surface are measured by the rotating disk electrode method and the experimental ORR activity trend is (220) facet > (111) facet > (200) facet for the textured silver surface. Both experimental and calculation results indicated that the stepped Ag(220) facet is one of high ORR activity origin for pure Ag materials with desirable morphologies. This new insight provides a fundamental understanding of the ORR reaction mechanism of monometallic Ag and to design advanced catalytic materials based on pure Ag. Keywords: Textured silver surface; oxygen reduction reaction; density functional theory; free energy ACS Paragon Plus Environment

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diagrams; adsorption energy; energy barrier; reaction energy 1. Introduction Oxygen electrocatalysis is one of the most studied topics in the fields of electrochemistry and catalysis because of its importance for electrochemical energy conversion and storage devices.1-3 The oxygen reduction reaction (ORR) is a one of the key chemical reactions in many portable energy devices including fuel cells and metal–air batteries.4-5 The structure-sensitivity of catalysts for ORR represents an important subject to adjust their performance by changing the surface/facet structures of catalyst materials.6-7 At present, the precious monometallic platinum (Pt) or platinum alloys exhibits the benchmark ORR activity in alkaline media.1,

8

Furthermore, the ORR activity on

different Pt facets may vary much in the same solution, and especially on the nanometer scale, the Pt(100) texured plane is higher than that on Pt(111) oriented nanoparticles in improving the electrocatalytic performance of ORR.6, 9 Nevertheless, due to the small reserves of Pt, for adapting the growing demand of large-scale applications in the commercialization of fuel cells, looking for a new alternative electrocatalyst to replace Pt-based catalysts become more and more impendent. It was found that Ag is a potential alternative Pt-free electrocatalyst for ORR, because of its naturally stable property in base, much more abundant and lower in cost than Pt.10-11 Recently, nanoporous silvers (np-Ag) have been reported to possess an overall ORR performance equivalent or better than that of the Pt/C catalyst.12-13 Furthermore, alloying of copper on the surface of Ag nanoparticles also promotes the catalytic activity of silver films in alkaline media.14-16 Xie et al. reported a unique nanoporous silver (np-Ag) structure with interlaced facets, which show that surface or grain-boundary like steps, kinks, and corner control has greater versatility for tuning the electrocatalytic properties of the Ag-based catalysts.12 While the trend of oxygen binding energy of metal Ag catalyst followed the order of {110} < {111} < {100} family of lattice planes as reported by Zhou et al.13, it was thought that the observed enhancement of np-Ag should be related to the (100) facets, however, as the number of Ag(100) facets had not increased in nanoporous silver catalysts,

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the author claimed that the origin of higher ORR activity in np-Ag catalysts were not due to an improved oxygen binding strength on Ag(100) facet, and the ORR activity origin was unknown for np-Ag catalyzed ORR. However, it is well-known that the pure silver single crystal catalysts have an experimental half-wave potential about 200 mV lower than that of platinum, the higher ORR activity in pure np-Ag catalysts need to be clarified. In this regard, in order to fully understand their catalytic properties for pure Ag nanomaterials with nanoporous morphologies, it is highly desirable to systematically investigate the effects of Ag surface structure on the catalytic performance from all three thermodynamic, dynamics and experimental aspects. In this work, firstly the energies of the stable adsorption states and transition states related to the progression of the ORR on flat and stepped facets of textured silver surface surfaces was calculated by the density functional theory (DFT) theory, secondly the reaction thermodynamics of the ORR on the Ag(111), Ag(200) and Ag(220) surfaces were compared, and discuss the structural effects on the ORR. The Ag(220) surfaces exhibits an activation energy barrier of O2 protonation reaction is 0.504 eV, which is the lowest activation energy barrier among the flat and stepped facets. Moreover, the Ag(220) surfaces exhibits an ORR overpotential of 0.457 V, which is comparable to the Pt(111) catalyst with the overpotential of 0.441 V. The ORR polarization curves of the textured silver foil are measured by the rotating disk electrode method and the experimental ORR activity trend is (220) facet > (111) facet > (200) facet for the textured surface. This computational and experimental efforts clearly elucidate that the Ag(220) facet is the catalytic active sites on the pure silver surfaces, and the high ORR activity in pure Ag comparable to Pt/C catalyst is possible due to the lower overpotential and the lower activation energy barrier of Ag(220) facets.

2. Method Calculation models. The silver facets were modelled by three slabs. The (111) (200) and (220) surfaces are obtained by cutting Ag bulk (fcc) along [111], [200] and [220] directions, respectively.



The surface atom arrangements on Ag(200) and Ag(220) plane are the same to the Ag(100) and Ag(110) plane as shown in Figure S1. The flat Ag surface were represented by Ag(200) and Ag(111) slabs. These slabs were represented by four Ag layers constructed from a (2×2) unit cell in which the top two layers of the flat surface were relaxed. The stepped Ag surfaces were modelled by Ag(220) slabs. Ag(220) slabs were represented by four Ag layers of repeated (2×2) unit cell, respectively. For the Ag(220) surface, top two layers were relaxed. Successive slabs in the z direction were separated by a vacuum equivalent to 15 Å. Adsorption was allowed on the relaxed side of the slab only. In order to confirm the reliability of the model, Eads(O) and Eads(O2) values on two different slab sizes were calculated as listed in Table S1. Compared with the three-layer slab, Eads(O) and Eads(O2) values on the two cases have deviations of 0.02, which do not change the results what we have obtained. Hence, the Ag(111) model with four-layer slab has reasonable accuracy. Calculation methods. The DFT calculations were carried out by using the Dmol3 17 package. The exchange-correlation potential was treated with the GGA-PBE18 functional. The orbital cutoff range was 5.0 Å. The convergence for total energy was set to be 1.0×10−5 Ha, maximum force was 0.002 Ha/Å, and maximum displacement was 0.005 Å. A double numerical plus polarization (DNP)19 basis set was used for our study. Comparable to the Gaussian 6-31 (d) basis20, while DNP basis set has shown excellent consistency with experiments in literatures when Ag element was included in any considered system21-22. The semicore pseudopotentials (DSPPs) have shown to describe ion–electron interaction. The self-consistent field convergence within 1.0×10−6, a Fermi smearing of 0.003 Ha (1 Ha=27.212 eV) was applied, and a 7×7×1 k-point sampling. The LST/QST23 method was applied to construct a dissociation path leading from the molecular precursor state to the coadsorption of two oxygen atoms in the unit cell by calculating the dissociation energy barrier of O2 molecular. We have compared the PBE (TS and Grimme correction) and RPBE functional by testing the lattice parameters of Ag and Eads(O) of Ag(111) as listed in Table S2. The calculated lattice parameter is 4.1248 Å for TS correction, which is more approaching to the experimental value of 4.09 Å24 and


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theoretical value of 4.14 Å25 than that of Grimma correction (4.1889 Å) and RPBE (4.2188 Å). In order to approach the experimental value, the PBE functional is chosen for our system. Furthermore, the Eads(O) of Ag(111) for TS correction is more approaching to the previous study26. Therefore, a TS approach is adopted for dispersion corrections. The conductor-like screening model (COSMO) is used to simulate a H2O solvent environment throughout the whole process27. The solvention energies of ORR intermediates on Ag(111) as listed in Table S3. For H, O, O2 and OH adsorption under solvent environment, Eads are stronger on Ag(111) surface while Eads(OOH) and Eads(H2O) are weaker, compared with gas environment. Free energy changed is calculated as follows: ∆G = ∆E + ∆ZPE − T∆S + ∆GU + ∆GpH + ∆Gfield, where ∆E is the DFT total energy, T is the temperature (298.15 K), ∆ZPE and ∆S are the change in zero-point energy and entropy, respectively. ∆GU = eU is the electrode potential, where U is the electrode potential with respect to the standard hydrogen electrode, and e is the transferred charge. ∆GpH = − kBTln10 × pH is the pH value of the electrolyte, where kB is the Boltzmann constant, and pH = 13 for an alkaline medium because of our experimental environment for ORR. ∆Gfield is the free energy correction caused by the electrochemical double layer and is neglected as in previous studies.28-30 The free energy of H2O(l) at standard conditions (p = 0.035 bar, p0 = 1 bar and T = 298.15 K) was obtained from GH2O(l) = GH2O(g) + RT × ln(p/p0), where R is the ideal gas constant and the gas-phase H2O(g) is calculated through DFT calculations. The free energy of O2(g) is derived as GO2(g) = 2GH2O(l) − GH2 − 4.92 eV at 298.15 K and a pressure of 0.035 bar.31 The free energy of OH− is obtained from the expression: GOH− = GH2O(l) − GH+, where GH+ = 1/2GH2 − kBTln 10 × pH suggested by Nørskov et al.28. The textured Ag(hkl) foil. The Ag(111), (200) and (220) foils were provided from Key Laboratory of Advanced Functional Materials, Beijing University of Technology. The textured Ag(hkl) foils were chemically polished before experiment in HCl solution per the procedure based on the work of Hamelin et al.11, 32, the foils were kept for about 15 min in concentrated H2SO4 before



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polishing. After rinsing with distilled water, the foils were dipped several times into concentrated HClO4. The foils were then transferred into the HCl solution. During this procedure, a white-yellow precipitate was formed on the foils, which makes the solution cloudy. After rinsing the foils in distilled water, the foils were immersed into a concentrated ammonia solution for several minutes (ca. 5-10 min). The texture coefficient (TC). The phase characterization of samples was studied by XRD. The preferred growth and quantitative information concerning the preferential crystallite orientation of the samples from the XRD data was obtained using the texture coefficient (TC), whose represents the texture of a particular plane.33 It was defined as: TC (hkl ) =

I ( hkl ) I 0 ( hkl ) ×100% ∑ ( I (hkl ) I 0 (hkl ) )

(1)

n

where I(hkl) was the measured relative intensity of a plane (hkl) and I0(hkl) was the standard intensity of the plane (hkl) taken from the JCPDS data. The value TC(hkl) = 1 represents films with randomly oriented crystallites, while higher values indicate the abundance of grains oriented in a given (hkl) direction. 33 Electrochemical measurements. The electrocatalytic activitiy of the catalysts was studied at room temperature in a three-electrode cell by rotating disk electrode (RDE) polarization curves. The Ag catalysts with a geometric area of 0.05 cm2 was directly used as a working electrode. The Ag catalysts were connected to a rotating disk electrode via Cu tape which was wrapped with parafilm to avoid contamination. The electrocatalytic ORR tests were performed in O2-saturated 0.1 M aq KOH with platinum foil and Hg/HgO (in 0.1 M KOH) electrode as counter and reference electrodes, respectively, within a three-electrode set-up. Linear sweep voltammetry (LSV) were performed at a scan rate of 5 mV s−1 and polarization curves were converted from LSV measured at 5 mV s−1. The current density was normalized to the geometric area of the catalyst layer and the measured potentials vs Hg/HgO were converted to a reversible hydrogen electrode (RHE) according to the


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Nernst equation (ERHE = EHg/HgO + 0.0591×pH + 0.164). All the electrochemical characterizations were carried out with an electrochemical analyzer (CHI 660C, CH Instruments).

3. Results and discussion 3.1 Adsorption of intermediates Due to the adsorption is a prerequisite of reactions proceeded on the catalyst, we start our study by investigating the adsorption characters of various ORR species on the Ag surface before exploring the reaction mechanism. Various possible sites on Ag(111), Ag(200) and Ag(220) surfaces are shown in Figure 1. Threefold sites are labelled sequentially from the edge inward as either the “f” (fcc) or the “h” (hcp) sites, furthermore, fourfold site in Ag(220) surface are labelled as the “h” site, two-fold sites labelled as the “b” (bridge) sites, and atop sites are labelled as the “t” (top) sites. Table 1 presents the most stable site and its respective binding energy for all the intermediates on the surfaces. Table 1 listed the intermediates species (H, O, OH, O2, OOH, H2O, and H2O2) for possible mechanisms of ORR. Firstly, we discuss the adsorption of O2 on the surfaces in detail because it was a necessary step to initialize the ORR. The Ag(111) facets have three adsorption configurations of nearly similar adsorption energies. Among them, the Mulliken charge of b site is 0.277, which is the largest O2 adsorption energies of −0.659 eV. The adsorption distances between the surface and two O atoms are 2.435 Å, as well as the elongation of the O-O bond to 1.283 Å. The Ag(200) facet has the largest O2 adsorption energies of −0.769 eV with the O−O bond length 1.299 Å and on b sites. There is an elongation in O−O bond relative to 1.225 Å in the gaseous phase with our DFT calculations. The adsorption distances between the surface and two O atoms are 2.388 Å. The Mulliken charge of O2 on Ag(200) surface are 0.361. The h site on Ag(220) facets have the largest O2 molecule adsorption energy of −0.841 eV among all sites on Ag(220) surface with the Mulliken charge of 0.438, an adsorption height i.e. Ag−O of 2.087 Å and the O−O bond extends to 1.312 Å. Compared



to the Ag(200) and Ag(111) surfaces, Ag(220) have more charge transfer from the Ag molecules to the adsorbed O2, shorter Ag−O distances and longer O−O bond length, suggesting a weaker O−O bond and stronger O2 adsorption. As listed in Table 1, the binding energies of molecular oxygen on Ag surface follow the order of Ag(111) < Ag(200) < Ag(220) facets. The properties of atomic O adsorbed on the surfaces are also studied. The h site of Ag(200) with the atomic oxygen adsorption energy of −4.059 eV is the best adsorption site, with the nearest adsorption distance between the surface and O atom is 0.834 Å and the largest Mulliken charge of O on Ag surface is 0.683. The f site is the best adsorption site of Ag(111) and Ag(220), with a adsorption energy of −3.661 eV and −3.821 eV, respectively. From Table 1, the binding energies of atomic oxygen on Ag surface follow the order of Ag(111) < Ag(220) < Ag(200). As compared with O, the H and OH is more strongly adsorbed on the surfaces. For Ag(111) surface, f site is more stable for H and OH adsorption, the adsorption energies of H and OH are −2.519 and −2.268 eV, respectively. For Ag(200) and Ag(220), H and OH prefer the bridge site. The OH adsorption energy for the Ag(200) is −2.547 eV, higher than on the Ag(111) surface (−2.268 eV) but lower than on the Ag(220) surface (−2.698 eV). Interestingly, the OH adsorption energies exhibit an obvious trend on three surfaces which is similar with the O2 adsorption energies. For OOH adsorption, only bridge sites are found to be stable on both Ag(111) and Ag(220) surfaces. The adsorption energies of OOH at b site on Ag(220) is larger than those on Ag(111) and Ag(200) surfaces (Table 1). For the adsorption of OOH, the O-O bond on Ag(200), Ag(111) and Ag(220) surfaces is 1.495, 1.494, 1.499 Å, respectively, which are slightly longer than that in the free H2O2 relative to 1.474 Å in the gaseous phase with our DFT calculations. This result implies that the adsorbed OOH may undergo either the decomposition reaction or direct hydrogenation and result in the formation of the H2O2 intermediate. Indeed, the chemical structure of H2O2 is not stable on Ag(200) surface suggested by our calculation. Unfortunately, the H2O2 molecules always decomposes into two OH molecules at the adsorption process. And the H2O has the weakest


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adsorption energies of all intermediates, it’s almost has no change before and after adsorption. Because of the H2O molecule always adsorbed at t site with its O end down, the whole H2O molecular plane nearly parallel to the surface. According to the d-band center theory,34-35 the closer the d-band center of the outmost metal to the Fermi level, the more stable for molecule adsorption. The calculated d-band center (εd) of Ag(111) Ag(200) and Ag(220) is −4.074, −4.030 and −3.838 eV, respectively. The εd of Ag(220) is more close to the Fermi level than those of Ag(111) and Ag(200). Table 1 shows that the binding energy of some species (OH, O2, OOH and H2O atoms) to the surfaces decreases with the lowing of location of their d-band centers. Previous studies have revealed that the ORR activity can be related to the oxygen binding energy on different surfaces.36 Clearly, the stepped facet is the key factor behind the high catalytic activity of Ag because they interact more strongly with oxygen molecule relative to close-packed (111) surfaces. However, the binding energies of H, O and H2O2 to different surfaces do not vary exactly corresponding to the change of their εd. In other words, the less stable adsorption of intermediates on Ag surfaces is not exactly consistent with the prediction of d-band center theory. Thus, besides the d-band center, there might be some other factors that affect the adsorption and activity for Ag surface. That means simply comparing specific surface plane to assess the relative activity of Ag can be misleading as it does not consider the free energy change of reaction and reaction barriers for various active surface sites. Hence, the activation barriers and free energy change of elementary ORR steps on various silver surfaces should be investigated.

3.2 Activation barriers of elementary ORR steps On the basis of previous obtained results,2, 31, 37-38 the ORR could be divided into three possible mechanisms: (Table 2), i.e. (1) dissociative 4e− path (oxygen dissociation), (2) associative 4e− path (peroxyl dissociation) and (3) associative 2e− path (hydrogen peroxide dissociation) mechanisms. The configurations of initial state (IS), transition state (TS) and final state (FS) for each elementary



step are shown in Figure 2~4. The energy barrier (∆Eact) and reaction energy. (∆E) for each dissociation pathway is presented in Table 2. 3.2.1 O2* → O* + O* and O2* + H2O + e− → OOH* + OH−. For O2 dissociation reaction on Ag(111), IS is O2 at the b site, FS is two O atoms at f sites (Figure 2a). After dissociation the O-O bond is elongated from 1.283 Å to 1.957 Å. Ag(111) surface has a higher energy barrier of 1.126 eV for O2 dissociation and the O2 dissociation process is exothermic by −0.516 eV. For O2 protonation reaction progress in Figure 2d, IS is the co-adsorption of O2 at f site and H2O at t site. In FS, OOH adsorption at b site. In TS, the H2O at t site is move close to the OO at b site, it's lead to the OO-H band length change to 1.473 Å. The reaction is exothermic by −0.453 eV with a barrier of 0.578 eV. This means that O2 prefers to protonate to form OOH on Ag(111) due to the lower barrier. Therefore, the oxygen dissociation mechanism is less competitive on Ag(111) compared with the O2 protonation mechanism. For O2 dissociation reaction on Ag(200), O2 will dissociate into two O atoms (Figure 3a). After dissociation the O-O bond is elongated from 1.312 Å to 1.819 Å. The reaction is exothermic by −0.409 eV with a barrier of 0.954 eV. TS of O2 protonation on Ag(200) is shown in Figure 3d, the H2O move closer to OO at the h site with an OO–H bond length change to 1.494 Å. The barrier is 0.651 eV and the reaction is exothermic by −0.27 eV. So that, the O2 is dissociate much difficult into two O atoms on Ag(200) than protonation on Ag(200). Since the formation of OOH is more competitive on Ag(200), the peroxyl dissociation and the hydrogen peroxide dissociation mechanism based on OOH are likely to occur. For O2 dissociation reaction on Ag(220), the molecular oxygen dissociation from a h site to two h sites (Figure 4a), the Ag(220) facet with the largest O2 adsorption energy of −0.841 eV has the weakest dissociation barrier of 0.557 eV among three Ag surface, and an exothermicity of −0.209 eV. In the TS of O2 protonation on Ag(220), the H2O at the t site will move closer to OO at the h site with an OO-H bond length of 1.502 Å (Figure 4d). −0.141 eV heat were release with a barrier of


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0.504 eV during the reaction. Analysis of the reaction paths concerning O2 protonation on pure silver surfaces clearly suggests that Ag(220) surfaces are more reactive than Ag(200) and Ag(111) surfaces. Thus it is clear that the stepped surface Ag(220) is the most favourable active surface for the O2 dissociation among all three Ag(hkl) surfaces. The O2 dissociation energy barrier is higher than the OOH formation barriers for the Ag(111), Ag(200) and Ag(220) surfaces. Thus, it is more difficult for the ORR to start from the direct O2 dissociation. 3.2.2 OOH* → O* + OH* and OOH* + H2O + e− → H2O2* + OH−. IS is OOH at the b site, FS is O atom at f site and OH at f site. In the TS, the O–OH is elongated from 1.499 Å in adsorbed OOH to 2.726 Å, during the OOH dissociation reaction on Ag(111) in Figure 2b. −1.137 eV heat were release with a barrier of 0.613 eV during the reaction. The H2O molecule at the t site will move closer to OOH at the f site with a HOO–H distance of 1.097 Å, during the OOH protonation reaction on Ag(111) (Figure 2e). 0.386 eV heat were absorbed with a barrier of 0.876 eV during the reaction. The barrier for OOH protonation reaction is slightly higher than the OOH dissociation reaction on Ag(111). The small difference in energy barrier indicates that the two reactions are competitive. The TS of OOH dissociation reaction on Ag(200) is shown in Figure 3b, the OOH moves closer to the Ag(200) surface with an elongated O–OH bond of 1.884 Å. −0.991 eV heat were release with a barrier of 0.520 eV during the reaction，the activity of the OOH dissociation on Ag(200) is higher than the Ag(111). Instead, it immediately decomposes into 2(OH) molecules during the adsorption process. This suggests that the hydrogen peroxide dissociation mechanism could not occur on Ag(200). For OOH dissociation reaction of Ag(220) (Figure 4b), the O-OH is elongated from 1.494 Å to 2.230 Å (in released). −1.435 eV heat were absorbed with a barrier of 0.391 eV during the reaction. Protonation reaction of OOH on Ag(220) is in Figure 4e, 0.196 eV heat were absorbed with a barrier of 0.556 eV during the reaction, compared with Ag(111) with a barrier of 0.876 eV. This suggests that the hydrogen peroxide dissociation mechanism could not occur on Ag(220).



3.2.3 H2O2* → OH* + OH*. For the TS of H2O2 dissociation reaction on Ag(111) is shown in Figure 2c, the HO–OH bond elongates to 2.093 Å at the b site. The barrier of H2O2 dissociation is 0.51 eV and the reaction is exothermic by −1.882 eV. Compared with the H2O2 formation reaction on Ag(111) (Figure 2e), i.e., OOH* + H2O + e− → H2O2* + OH− (Eact = 0.876 eV; ∆E = 0.386 eV), H2O2 dissociation reaction shows a slightly high reaction rate due to the large exothermic and low barrier. This indicates that the formed H2O2 will dissociate into OH rapidly rather than deposited on Ag(111). For H2O2*→OH*+OH* on Ag(220) is shown in Figure 4c, IS is H2O2 at the h site, FS is both OH at f sites. In the TS, the HO–OH elongates from 1.482 Å in adsorbed H2O2 to 1.786 Å at the h site. Similar to the result obtained on Ag(111), H2O2 dissociation reaction on Ag(220) has a large energy release of −2.536 eV with a low barrier of 0.533 eV. The higher barrier on Ag(220) will provide less activity for H2O2 dissociation than on Ag(111).

3.2.4 O* + H2O + 2e− → 2OH−. For O protonation reaction on Ag(111) is shown in Figure 2f, IS is H2O at t site and O at f sites, FS is OH at the f site. In the TS, O and H2O move closer to the f site with O–H bond length is 1.348 Å. −0.365 eV heat were released with a barrier of 0.321 eV during the reaction. For O* + H2O + 2e− → 2OH− on Ag(200) is shown in Figure 3f, H2O at t site and O at h site (IS) move closer to OH at the h site (FS). In the TS, the O and H move above the hollow site with an O–H bond length of 1.616 Å. The reaction rate of O protonation reaction on Ag(200) is improved due to the low barrier of 0.325 eV and −0.331 eV energy were released. The O protonation reaction has the lowest barrier in all elementary reactions for the ORR on Ag(200). For O* + H2O + 2e− → 2OH− on Ag(220) (Figure 4f), the IS is the co-adsorption of an O atom at f site and a H2O molecule at an adjacent t site while the final state is the adsorption of the formed OH


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molecule at a f site. In the TS, the O and H move above the h site and has a bond length of 1.835 Å. The reaction rate of O protonation reaction on Ag(220) is improved due to the low barrier of 0.302 eV and 0.463 eV energy is released. The reaction activity of O protonation reaction is also improved on Ag(220) with a lower barrier of 0.302 eV, lower than that of Ag(111) (0.321 eV). Therefore, Ag(220) shows more activity for O protonation reaction. −

It is obvious from the above that the ORR on Ag surface proceeds by the associative 4e path and the O2 protonation reaction is rate-limiting step of the peroxyl dissociation path. The Ag(220) facet with the weakest dissociation barrier of O2 protonation reaction 0.504 eV among three Ag surface,. This implies that the ORR catalytic activity obeys the following trend: Ag(220) > Ag(111) > Ag(200). To elucidate why the steped Ag facet is optimal to display a good catalytic behaviour, we address the electronic structure of these oxygen adsorption configurations and calculate the position of the d-band center (εd) relative to the Fermi level. Compared with Ag(200) and Ag(111) surfaces, εd of −3.838 eV in the Ag(220) surface is the closest to the Fermi level. It can be seen that orientation of crystallographic plane makes a great difference to the εd and the electronic structures of Ag surfaces. As proposed in the Hammer-Nørskov model, the interaction strength of atoms and molecules with metal surface to a large extent is controlled by the εd location, the h active site of the Ag(220) surface exhibits a superior surface reactivity than the Ag(111) surface. This enhanced activity comes from the bigger silver interatomic distance on the (220) surface, which results in decreased orbital overlap between Ag atoms, a less d-band width and a closer εd to the Fermi level.34 Therefore, Ag(220) exhibits a superior catalytic activity among three Ag textured surface.

3.3 Influence of electric potential on ORR The Gibbs free energy diagrams through by Nørskov28 was used to investigate the effects of the electric potential on the activity and mechanism of the ORR. To further illustrates catalytic performance, the free energy diagram for all three textured silver (200), (111), (220) surfaces and



Pt(111) surfaces in alkaline are illuminated in Figure 5 and Table 3. As shown in Figure 5, the ORR reactions at equilibrium electrode potential of 0.461 V in alkaline solution become exothermic except for the step of OOH* formation on three Ag surfaces, indicating that the electrode potential has a large influence for OOH* formation. The ORR reactions become exothermic until we reduce the electrode potential on the (200), (111) and (220) surface to −0.021, −0.005, and 0.004 V, respectively. These results are defined as the working potential of the electrocatalyst, which is a lowest electrode potential to keep all the elementary reactions to be exothermic. Therefore, the overpotential (η) for (200), (111) and (220) is 0.482, 0.466 and 0.457 V, respectively, And the overpotential (η) of the Pt(111) is 0.441 V, which in agreement with previous theoretically estimated η by Nørskov28, Compared to the overpotential (η) of the Pt(111), the low cost Ag is more suitable for electrocatalyst in alkaline solution and the stepped Ag(220) surface exhibits a superior catalytic activity in particular. We also provided the free energy profiles of direct O2 dissociation mechanism as shown in Figure S2. It reveals that the overpotential of Ag(111), Ag(200), Ag(220) and Pt(111) surfaces are 0.621, 0.679, 0.569 and 0.457, respectively, it is clearly that the dissociation mechanism of ORR is more unfavorable than the associative mechanism on Ag surfaces. The ORR activity were thermodynamically related to the oxygen adsorption properties and electronic structure for those catalysts, as illustrated in the Table 1 and Table 3, the Ag(220) surface exhibits the highest molecular oxygen adsorption energy, the largest Mulliken charge, the most upshifting d-band center, so the Ag(220) surface is the minimum overpotential among three Ag textured surface. Overall, we can say that the result obtained for the reaction thermodynamics mechanism generally agrees with that obtained for the reaction kinetics mechanism and does not affect our conclusions.

3.4 ORR activity of the textured Ag(hkl) surface

In order to directly observe the effect of textured Ag surface for a comparison with the DFT calculations, we measure the catalytic performance of the textured Ag(hkl) surfaces with


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experimental RDE methods. The grazing incidence X-ray diffraction (XRD) was performed to study the surface crystal features of each of the samples. XRD patterns in Figure 6a show that three wellcrystallized pure Ag catalysts have different textured crystal orientation. Table 4 lists the texture coefficient (TC) of the samples labelled as textured Ag(111), (200) and (220) surface. The TC for (111) plane, (200) plane and (220) plane for the textured Ag(111), (200) and (220) surface samples are 80.2%, 52.7% and 98.9%, respectively, while higher TC values indicate the abundance of grains oriented in a given (hkl) direction. The catalytic performance of the textured Ag(hkl) surface is measured by the RDE polarization curves, as shown in Figure 6b, the half-wave potentials (E1/2) of textured Ag(111), (200) (220) facet and Pt/C catalyst are 0.66, 0.64, 0.69 and 0.80 (±0.01) V, respectively, indicating that the Ag(220) is the most active surface for the ORR, (200) surface is less active with 50 mV lower relative to (220) surface. This is in excellent agreement with the DFT calculations, that is, the (220) surface favours the O2 protonation and dissociation. Therefore, we may expect that the ORR catalytic activity obeys the following trend: Ag(220) > Ag(111) > Ag(200), which allows us to consider the steped Ag facet materials as potential candidates for the ORR catalysts.

3.5 Discussion The previous volcano plots28 have revealed that the ORR activity was correlated to the atomic oxygen binding energy, i.e., the higher atomic oxygen adsorption energies would have positive effects on the ORR activity for metal Ag catalyst in order to climb the volcano plot. Blizanac et al.11 reported that the ORR activity followed the order of {100} < {111} < {110} family of lattice planes form their experimental work. Compared to the atomic oxygen binding energy order of {110} < {111} < {100} family of lattice planes by Zhou et al.13, it is clear that the biggest oxygen binding energy on (100) surface is not corresponding to the highest ORR activity observed on the (110) or (220) stepped surface, indicating that the theoretical volcano plot failed to interpret the ORR activity in



np-Ag catalysts. In this work, as shown in Figure 7, it is calculated that the overpotential (η) on Ag surface followed the order of (220) < (111) < (200), the molecular O2 adsorption energy (Eads) followed the order of (220) > (200) > (111) surface, the dissociation energy barrier of O2 molecular (Eact) on Ag surface followed the order of (220) < (200) < (111) surface, the dissociation energy barrier of O2 protonation reaction (Eact) on Ag surface followed the order of (220) < (111) < (200) surface, and the absolute value |εd| of Ag surface follows the order of (220) < (200) < (111). These η, Eads and Eact indices indicate that the stepped (220) surface have higher ORR activity in excellent agreement with the d-band center39 and the work of Blizanac et al.11, where the stepped (220) surface have its d-band center location more close to the Fermi level. Moreover, the experimental measurement demonstrates the most ORR active surface is Ag(220) surface, which are also in excellent agreement with those calculated indexes. It is well-known that the pure silver catalysts have an experimental half-wave potential (E1/2) about 200 mV lower than that of platinum in RDE polarization,12 which is consistent with the results from this work. However, the pure silver catalyst has a predicted ORR overpotential comparable to that of platinum as calculated by the free-energy diagram. Therefore, the ORR catalytic activity of pure silver is probably determined by the reaction kinetic energy barrier. So it is concluded that the enhanced ORR activity in pure Ag catalysts is related to the stepped Ag(220) surfaces, which is due to both low ORR overpotential and the low dissociation energy barrier of O2 protonation on the (220) surface of pure Ag catalysts.

4. Conclusions In summary, the DFT results of activation energy of O2 protonation and free energy diagrams well explained the experimental observation for the ORR on the Ag catalyst. The key role of O2 protonation on Ag surfaces was elucidated via theoretical results. The activation energy barrier for O2 protonation reaction on the stepped Ag(220) surface is 0.504 eV which is the lowest Eact among all


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the surfaces while the d-band centre is at −3.838 eV which is closest to the Fermi energy. We found the lowest overpotential is located at Ag(220) surface; the ORR activity of textured Ag(hkl) surface are evaluated in alkaline electrolyte, and it was also found that the Ag(220) surface had the best half-wave potential, as a result, the catalytic activity of Ag(220) surface agrees well with the calculation prediction. The stepped Ag(220) surface exhibits a superior catalytic activity, in particular, a high ORR activity origin of Ag materials with desirable morphologies. These new insights provide guidance for searching favorable structures for activity of electrocatalysts, and the theoretical data obtained here would be useful for the design of fuel cell catalysts with high efficiency.

Associated content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:. Details of reaction processes of alkaline solution, adsorption energies with different layers and different DFT correction, the solvent effects, silver surface structures, and the free-energy diagrams of the direct dissociation pathway (PDF)

Acknowledgements This work was supported by the National Natural Science Foundation of China (grant nos. 51271148 and 50971100), the Research Fund of State Key Laboratory of Solidification Processing in China (grant no. 150-ZH-2016), the Aeronautic Science Foundation Program of China (grant no.2012ZF53073), the Project of Transformation of Scientific and Technological Achievements of NWPU (grant no. 19-2017), the Doctoral Fund of Ministry of Education of China (grant no.20136102110013), and the Open Fund of State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology grant no. 2018-KF-18). We would like to thank the Center for High Performance Computing of Northwestern Polytechnical



University, China and the Analytical & Testing Center of Northwestern Polytechnical University for TEM and AFM characterizations.

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Table captions Table 1 The adsorption energies (eV) of different ORR chemical species on Ag(111) Ag(200), Ag(220) surface. Table 2 Calculated activation energy (Eact) and reaction energy (∆E) (eV) for various elementary steps of ORR on Ag(111), Ag(200) and Ag(220) surfaces. Table 3 The adsorption free energy of ORR species on Ag(111), Ag(200), Ag(220) and Pt(111) surfaces including the overpotential (η) and d-band center (εd) of clean surface. Table 4 Texture coefficient (TC) and the half-wave potentials (E1/2) of the textured Ag(hkl) surface.



Table 1 The adsorption energies (eV) of different ORR chemical species on Ag(111) Ag(200), Ag(220) surface. Ag(111) Ag(200) Ag(220) site Eads /eV site Eads /eV site Eads /eV f −2.519 b −2.035 b1 −2.139 H h −4.059 f −3.821 O f −3.661 OH f −2.268 b −2.547 b1 −2.698 O2 b −0.659 b −0.769 h −0.841 OOH b −1.011 h −1.213 b1 −1.259 H2 O t −0.204 t −0.211 t −0.257 H2O2 f −0.683 * * h −0.259 * Not stable

Table 2 Calculated activation energy (Eact) and reaction energy (∆E) (eV) for various elementary steps of ORR on Ag(111), Ag(200) and Ag(220) surfaces. Ag(111) Ag(200) Ag(220) Reactions Eact ∆E Eact ∆E Eact ∆E O—O bond scission a. O2* → O* + O* 1.126 −0.516 0.954 −0.409 0.557 −0.209 b. OOH* → O* + OH* 0.613 −1.137 0.520 −0.991 0.391 −1.435 c. H2O2* → OH* + OH* 0.510 −1.882 * * 0.533 −2.536 Protonation d. O2* + H2O + e− → OOH* + OH− 0.578 −0.453 0.651 −0.270 0.504 −0.141 − − e. OOH* + H2O + e → H2O2* + OH 0.876 0.386 * * 0.556 0.196 − − f. O* + H2O + 2e → 2OH 0.321 −0.365 0.325 −0.331 0.302 −0.463 − The 4e direct O2 dissociative path: a−f The 4e− O2 associative path: d−b−f The 2e− O2 associative path: d−e−c * note: H2O2 simultaneously decomposes into two OH molecules on Ag(200) surface during the adsorption process.


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Table 3 The adsorption free energy of ORR species on Ag(111), Ag(200), Ag(220) and Pt(111) surfaces including the overpotential (η) and d-band center (εd) of clean surface. Surface ∆GO* ∆GOH* ∆GOOH* η(eV) εd(eV) Ag(111) 1.030 0.101 1.84 0.466 −4.074 Ag(200) 1.082 −0.002 1.849 0.482 −4.030 Ag(220) 1.140 0.028 1.865 0.457 −3.838 Pt(111) −0.044 −0.048 1.824 0.441 −2.645

Table 4 Texture coefficient (TC) and the half-wave potentials (E1/2) of the textured Ag(hkl) surface. TC TC TC TC TC E1/2 Sample (111) (200) (220) (311) (222) (V vs. RHE) Textured Ag(111) surface 80.2% 19.8% 0.66 Textured Ag(200) surface 12.1% 52.7% 32.1% 0.64 Textured Ag(220) surface 98.9% 1.1% 0.69



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Figure captions Figure 1 The upper panels show the top view of the model of (a) Ag(200), (b) Ag(111), (c) Ag (220) surface with various possible adsorption sites, and the lower panels show a cross-sectional view. The blue balls denote Ag atoms. Figure 2 The geometry structures of (a) O2 dissociation reaction, (b) OOH dissociation reaction, (c) H2O2 dissociation reaction, (d) O2 protonation reaction, (e) OOH protonation reaction, and (f) O protonation reaction on Ag(111). IS, TS and FS denote the initial state, transition state and final state, respectively. The red and white balls denote O and H atoms, respectively. Figure 3 The geometry structures of (a) O2 dissociation reaction, (b) OOH dissociation reaction, (c) O2 protonation reaction, and (d) O protonation reaction on Ag(200). IS, TS and FS denote the initial state, transition state and final state, respectively. The red and white balls denote O and H atoms, respectively. Figure 4 The geometry structures of (a) O2 dissociation reaction, (b) OOH dissociation reaction, (c) H2O2 dissociation reaction, (d) O2 protonation reaction, (e) OOH protonation reaction, and (f) O protonation reaction on Ag(220). IS, TS and FS denote the initial state, transition state and final state, respectively. The red and white balls denote O and H atoms, respectively. Figure 5 Free-energy diagrams for the ORR associative mechanism at different electrode potentials U on (a) Ag(200), (b) Ag(111), (c) Ag(220) and (d) Pt(111) surfaces in alkaline solutions. Figure 6 (a) XRD patterns of the textured Ag surface. (b) ORR polarization curves of textured Ag surface at 1600 rpm. Figure 7 Half-wave potential (E1/2), d-band center (εd) (black line), molecular O2 adsorption energy (Eads) (blue line), energy barrier of O2 dissociation (Eact) (magenta line), energy barrier of O2 protonation (Eact) (violet line) and overpotential (η) (dark yellow line) of Ag surface.



Figure 1 The upper panels show the top view of the model of (a) Ag(200), (b) Ag(111), (c) Ag (220) surface with various possible adsorption sites, and the lower panels show a cross-sectional view. The blue balls denote Ag atoms. 311x191mm (300 x 300 DPI)


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Figure 2 The geometry structures of (a) O2 dissociation reaction, (b) OOH dissociation reaction, (c) H2O2 dissociation reaction, (d) O2 protonation reaction, (e) OOH protonation reaction, and (f) O protonation reaction on Ag(111). IS, TS and FS denote the initial state, transition state and final state, respectively. The red and white balls denote O and H atoms, respectively. 301x154mm (300 x 300 DPI)



Figure 3 The geometry structures of (a) O2 dissociation reaction, (b) OOH dissociation reaction, (c) O2 protonation reaction, and (d) O protonation reaction on Ag(200). IS, TS and FS denote the initial state, transition state and final state, respectively. The red and white balls denote O and H atoms, respectively. 286x111mm (300 x 300 DPI)


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Figure 4 The geometry structures of (a) O2 dissociation reaction, (b) OOH dissociation reaction, (c) H2O2 dissociation reaction, (d) O2 protonation reaction, (e) OOH protonation reaction, and (f) O protonation reaction on Ag(220). IS, TS and FS denote the initial state, transition state and final state, respectively. The red and white balls denote O and H atoms, respectively. 303x158mm (300 x 300 DPI)



Figure 5 Free-energy diagrams for the ORR associative mechanism at different electrode potentials U on (a) Ag(200), (b) Ag(111), (c) Ag(220) and (d) Pt(111) surfaces in alkaline solutions. 230x167mm (300 x 300 DPI)


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Figure 6 (a) XRD patterns of the textured Ag surface. (b) ORR polarization curves of textured Ag surface at 1600 rpm. 531x201mm (300 x 300 DPI)



Figure 7 Half-wave potential (E1/2), d-band center (εd) (black line), molecular O2 adsorption energy (Eads) (blue line), energy barrier of O2 dissociation (Eact) (magenta line), energy barrier of O2 protonation (Eact) (violet line) and overpotential (η) (dark yellow line) of Ag surface.

286x180mm (300 x 300 DPI)


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Electrochemical Oxygen Reduction Reaction in ... - ACS Publications

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