Graphdiyne-Supported Single-Atom-Sized Fe Catalysts for the Oxygen

Oct 2, 2018 - Single-atom-sized catalysts (often called single atom catalysts) are highly desired for maximizing the efficiency of metal atom use. How...
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Graphdiyne-Supported Single-Atom-Sized Fe Catalysts for the Oxygen Reduction Reaction: DFT Predictions and Experimental Validations Yuan Gao, Zhewei Cai, Xingchen Wu, Zhilie Lv, Ping Wu, and Chenxin Cai ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02360 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 2, 2018

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Graphdiyne-Supported Single-Atom-Sized Fe Catalysts for the Oxygen Reduction Reaction: DFT Predictions and Experimental Validations Yuan Gao,†1 Zhewei Cai,‡1 Xingchen Wu,† Zhilie Lv,† Ping Wu,*† and Chenxin Cai*†

† Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, College of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210097, P.R. China. ‡Department of Chemical and Biomolecular Engineering, Clarkson University, Potsdam, NY 13699, United States.

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Y. Gao and Z. Cai contributed equally this work.

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Abstract: Single-atom-sized catalysts (often called single atom catalysts) are highly desired for maximizing the efficiency of metal atom use. However, their synthesis is a major challenge that largely depends on finding an appropriate supporting substrate to achieve a well-defined and highly dispersed single atom. This work demonstrates, based on density functional theory (DFT) predictions and experimental validations, that graphdiyne is a good substrate for anchoring Fe atoms through the formation of covalent Fe‒C bonds to produce graphdiyne-supported single-atom-sized Fe catalysts (Fe– graphdiyne catalysts); moreover, this catalyst shows high catalytic activity to oxygen reduction reactions (ORRs) similar to or even slightly better than the precious metal benchmark (commercial 20 wt.% Pt/C catalyst). DFT predicts that the O2 molecule can bind with an Fe atom, and the electron transformation process of ORRs occurs through a 4e‒ pathway. To validate the theoretical predictions, the Fe‒graphdiyne catalyst was then synthesized by a reduction of Fe3+ ions adsorbed on a graphdiyne surface in aqueous solution, and its electrocatalytic activities toward ORR were experimentally evaluated in alkaline electrolytes (0.1 M KOH). The electrochemical measurements indicate that the Fe‒ graphdiyne catalyst can facilitate the 4e‒ ORR while limiting the 2e‒ transfer reaction, showing a high 4e‒ selectivity for ORRs and a good agreement with DFT predictions. The results presented here demonstrate that graphdiyne can provide a unique platform for synthesizing well-defined and uniform single-atom-sized metal catalysts with high catalytic activity towards ORRs.

Keywords: single-atom-sized metal catalysts; non-noble metal catalysts; carbon materials; graphdiyne; oxygen reduction reactions.

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INTRODUCTION The oxygen reduction reaction (ORR), a fundamental step for energy conversion in fuel cells and metalair batteries, requires favorable catalysts to obtain fast reaction kinetics for practical applications. Pt and Pt-based alloy materials have long been regarded as the most efficient catalysts for the four-electron (4e‒) ORR.1–14 However, the slow kinetics of the ORR on Pt-based catalysts, and the poor durability and high cost of these catalysts have hampered the widespread commercialization of renewable energy technologies.1–3 To address these issues with Pt, one promising strategy is to design and synthesize high activity non-noble-metal catalysts to substitute Pt-based catalysts.15–24 To achieve this goal, two key aspects must be considered: increasing the exposed active sites of the catalysts and finding a suitable substrate to anchor the catalysts.25–27 Downsizing the catalyst particles to a nanocluster and even to a subnanometer cluster is one of the most effective ways to enhance the exposure of active sites and increase the catalytic efficiency.28,29 Further decreasing the size of subnanometer clusters to the well-defined atomic level (i.e., single-atom-sized catalysts, often called single atom catalysts30) affords an opportunity to expose the most active sites and obtain maximum atom use and should be the ultimate goal in the field of nanocatalysis.18,23,31 Unfortunately, these highsurface-energy, small-sized nanoclusters or single atoms are too mobile and easy to sinter under realistic reaction conditions, and they usually suffer from serious aggregation, resulting in a drastic decrease in their catalytic performance.31,32 Thus, another significant point in the synthesis of non-noble-metal catalysts is finding an appropriate and efficient substrate to anchor the catalyst atom, facilitate the masstransport properties of ORR-relevant species, and promote the electron-transfer kinetics of ORRs. In this respect, carbon-based nanomaterials, for example graphene, have been extensively studied as the substrate material of catalysts due to their large surface-to-volume ratio, superior electrical conductivity, and strong tolerance of acid/alkaline environments.33–35 However, pristine graphene is not the ideal substrate for those highly dispersed metal catalysts because the nature of the strong sp2 hybridized chemical bonds formed in the graphene does not readily permit stable immobilization of metal atoms.36 ACS Paragon Plus Environment

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The migration barrier of graphene-supported metal atoms has been estimated as 0.2–0.8 eV,37 which is too low to prevent their migration even at an ambient temperature. Thus, the metal atoms on a pristine graphene surface usually migrate to edge locations where free dangling bonds are available, resulting in particle agglomeration and a poor catalyst distribution.36 The small migration barrier is believed to be a result of the uniform distribution of charge on graphene sheet.38 The construction of vacancies or creation of dangling bonds can destroy or alter the uniformly distributed charge on the graphene sheet and thus improve the stability of metal atoms.36 However, atom-scale control of vacancy formation is experimentally quite difficult. As a result, searching new substrate materials that can anchor the single catalyst atoms uniformly yet avoid creating the evenly distributed vacancies is a feasible alternative way and is also highly desirable. Graphdiyne, a new two-dimensional periodic carbon allotrope with a one-atom-thick layer of carbon built from triple- and double-bonded units of sp- and sp2-hybridized carbon atoms,39–41 is expected to be a good substrate candidate for anchoring the individual metal atoms because the charge distribution on a graphdiyne sheet is not even.32,42,43 Moreover, it is anticipated that the interaction between metal atoms, such as Fe atoms, and the alkyne and aryl π-conjugated networks will help to stabilize metal atoms from aggregation.32,44–46 Our previous work has demonstrated that, based on density functional theory (DFT) calculations, the Fe atom can be tightly anchored on a graphdiyne sheet (Figure S1) by forming Fe‒C bonds to produce graphdiyne-supported single-atom-sized Fe catalysts (Fe‒graphdiyne catalysts).32 This work will demonstrate, based on DFT predictions and experimental validations, that Fe‒ graphdiyne is a good catalyst for the 4e‒ ORR with a high catalytic activity. We first calculated the adsorption of O2 and its intermediates along with the pathways and the free energy diagrams of the ORR on the catalyst surface. The results showed that an O2 molecule can bind with an Fe atom, and the electron transformation process of ORRs occurs through a 4e‒ pathway. To validate the theoretical predictions, we then synthesized the Fe‒graphdiyne catalyst by a facile reduction of Fe3+ ions adsorbed on a graphdiyne surface in aqueous solution (using NaBH4 as a reducing reagent) and evaluated its electrocatalytic activities toward the ORR in alkaline electrolytes. The electrochemical measurements ACS Paragon Plus Environment

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indicated that the synthesized Fe‒graphdiyne catalyst can facilitate the 4e‒ ORR while limiting the 2e‒ transfer reaction, showing a good agreement with DFT predictions. The results presented here will inspire the study of carbon nanomaterials to improve their catalytic efficiency and provide a theoretical framework to analyze their catalytic activity. RESULTS AND DISCUSSION Free Energy Diagrams. We first performed the theoretical DFT calculations to predict the possible intermediates formed during the ORR catalyzed by Fe‒graphdiyne catalysts and evaluated their stabilities on the catalyst surface in terms of the adsorption energy (Ead). The detailed calculation setups are presented in the calculations and experimental methods section. The overall 4e‒ ORR to OH‒ in alkaline medium is: O2 + 2H2O + 4e‒ → 4OH‒

(1)

with two possible reaction pathways: dissociative and associative.47–49 In the dissociative pathway (Scheme S1), an O2 molecule is first adsorbed onto the catalyst surface from the electrolyte (step S1), and then is dissociated into two *O (* refers to an active site on the catalyst surface) atoms adsorbed on the catalyst surface (step S2). The formed *O atom is reduced by H2O together with one electron transfer to form an adsorbed *OH intermediate and a solvated OH‒ anion (step S3). The surface adsorbed *OH is finally dissolved as OH‒ as a result of a direct one electron reduction (step S4) or by one electron reduction with H2O (step S5). Generally, carbon materials, including the doped carbon materials such as doped graphene, etc., feature a relatively high energy barrier (usually ~1.2 eV)48 in the dissociative pathway (our calculation estimates that the energy barrier for the ORR on the surface of the Fe‒graphdiyne catalysts via the dissociative pathway exceeds 1.1 eV); therefore, the following associative mechanism (eqns. 2‒5) is dominant and considered in our calculations: * + O2(g) + H2O + e‒ → *OOH + OH‒

(2)

*OOH + e‒ → *O + OH‒

(3)

*O + H2O + e‒ → *OH + OH‒

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(5)

where *O, *OH, and *OOH are adsorbed intermediates. The associative and dissociative pathways only differ in the first and second steps.49 The first step in the associative pathway is direct reduction of the O2 molecule to *OOH via H2O and one electron transfer (eqn. 2), the second step is cleavage of the O‒ O bond in *OOH, which is driven by a second electron transfer, to form *O (eqn. 3). The steps for reduction of *O (eqn. 4) and *OH (eqn. 5) are the same as those in dissociative pathways.

Figure 1. (a) Top side views of the atomic configurations of *OOH, *O, and *OH adsorbed on a Fe– graphdiyne surface. Atomic color code: pale blue, carbon in the C6 ring with sp2 hybridization; green, carbon in the acetylenic-like rods with sp hybridization; orange, Fe; red, oxygen; and white, hydrogen atom. (b) and (c) Calculated free energy diagrams of 4e– pathways of the ORR at the Fe–graphdiyne (black line) and Pt(111) (red line) catalyst surfaces at the equilibrium electrode potential U4e0 (=0.455 V vs. NHE, with η = 0 V) (a) and at the experimental measured onset potential Uonset (=0.21 V vs. NHE, with η = 0.25 V) (b).

The catalytic sites of the ORR are closely related to the spin and charge distributions of the Fe‒ graphdiyne framework. We screened three possible active sites, i.e. the sp2 hybridized carbon atoms in the C6 ring, the sp hybridized carbon atoms in the acetylenic-like rods, and the Fe atom, to determine the energetically most stable configuration for each intermediate by evaluating the charge distribution, ACS Paragon Plus Environment

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spin, and Ead of *OH. It was found that *OH prefers to adsorb on the top of an Fe atom because this site has a large spin and a high positive charge and thus has a stronger interaction with *OH compared to the sp2 and sp hybridized carbon atoms in the C6 ring and the acetylenic-like rods, respectively. This means that only the Fe atom in a Fe‒graphdiyne catalyst can be a potential active site for the ORR. Subsequently, we calculated the adsorption of the ORR intermediates (*OOH, *O, and *OH) on this site. The most stable configuration and the Ead value for each intermediate are depicted in Figure 1a, which indicates that *O and *OH can be strongly adsorbed on the top of the Fe atom with an Ead of –2.10 and –1.77 eV, respectively. The adsorption of the intermediate *OOH is relatively weak with an Ead of –0.13 eV. This relatively weak adsorption of *OOH will favor the breakage of the O‒O bond in *OOH and facilitate the ORR.42 We then constructed the free energy diagrams to theoretically evaluate the possible catalytic characteristics of the Fe‒graphdiyne catalyst to the ORR and to predict the reaction pathway selectivity. For the associative 4e‒ reaction pathways depicted in eqns. 2‒5, the free energy diagram at the equilibrium potential U4e0 (= 0.455 V vs. normal hydrogen electrode, NHE, in 0.1 M KOH solution, pH=13. At U4e0, the reactant and product have the same energy),48 shown in Figure 1b (black line), indicates that all electron transfer steps on the Fe‒graphdiyne catalyst surface are endothermic reactions except for the second electron transfer step (the cleavage of *OOH to *O and OH‒, eqn. 3). The free energy change at U4e0 for each step is 0.54 (∆G1, eqn. 2), ‒1.23 (∆G2, eqn. 3), 0.04 (∆G3, eqn. 4), and 0.65 eV (∆G4, eqn. 5), respectively. Therefore, ∆G4, corresponding to desorption of *OH (eqn. 5), is the most endothermic reaction step, implying that this is the slowest step in the mechanism for the ORR on the surface of the Fe‒graphdiyne catalyst. Therefore, this step can be considered the rate-determiningstep (RDS). Similar results are obtained on the Pt(111) surface (red line, Figure 1b), which is usually used as a benchmark for the ORR,50 under same conditions (at U4e0). The free energy change for each electron transfer step on Pt(111) is 0.72 (∆G1, eqn. 2), ‒1.69 (∆G2, eqn. 3), 0.14 (∆G3, eqn. 4), and 0.83 eV (∆G4, eqn. 5), respectively. The largest free energy change (0.83 eV), which is similar to those reported previously for *OH desorption from the Pt(111) surface (usually in the range of 0.65 to 0.85 ACS Paragon Plus Environment

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eV50), also occurs at the *OH desorption step. These results imply that the mechanism and RDS for the ORR on a Fe‒graphdiyne surface are identical to those on a Pt(111) catalyst, which will be verified by the Tafel analysis (see the related discussion presented in the next section). However, the largest free energy change obtained on the Fe‒graphdiyne surface (0.65 eV) is lower than that on Pt(111) (0.83 eV), suggesting that the Fe‒graphdiyne catalyst has a higher ORR catalytic activity than the Pt(111) catalyst at U4e0. The free energy diagram can be adjusted by applying a certain overpotential (η, = 0.455 ‒ U, vs. NHE). To simulate real conditions, the choice of the value of η needs to consider the electrochemically measured polarization curves.50 We chose η = 0.25 V to correct the free energy diagram for the ORR. This is the overpotential at the onset potential (Uonset, defined as the potential at a background-subtracted current density of ‒0.1 mA/cm2)51 for the ORR on the Fe‒graphdiyne catalyst (Uonset = 0.21 V vs. NHE, which is obtained from the electrochemical measurements). Under such an η, the diagram (black line, Figure 1c) indicates that the free energy change of each electron transfer step decreases in comparison with those obtained at U4e0. The first electron transfer step to form *OOH and the fourth electron transfer step involving *OH desorption to form OH‒ are endothermic reactions with free energy changes of ∆G1(at Uonset) = 0.29 eV and ∆G4(at Uonset) = 0.40 eV, respectively, whereas the second electron transfer step to form chemisorbed *O and the third electron transfer step to form *OH are exothermic reactions with free energy changes of ∆G2(at Uonset) = ‒1.48 eV and ∆G3(at Uonset) = ‒0.21 eV, respectively. At U4e0 (with η = 0), the third electron transfer step is an endothermic reaction (Figure 1b). It changes, however, to an exothermic reaction under an η of 0.25 V (Figure 1c). The fourth electron transfer step still has the largest free energy change among all four reactions steps, implying that this step is still the RDS even when applying η (0.25 V). However, the free energy change under such an η is significantly smaller than that at U4e0 (0.40 eV vs. 0.65 eV). Although the free energy change of each electron transfer step on Pt(111) also decreases when applying η (= 0.25 V, red line, Figure 1c) in comparison with those at U4e0, the free energy change for the RDS on the Pt(111) surface still remains higher than that on the Fe‒graphdiyne surface (0.58 eV vs. 0.40 eV), suggesting that the catalytic ACS Paragon Plus Environment

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activity of the Fe‒graphdiyne catalyst to the ORR is higher than that of the Pt(111) catalyst at a given η, which will be confirmed from electrochemical measurements. Synthesis and Characterization of the Fe‒graphdiyne Catalysts. To experimentally validate the above theoretical predictions, we synthesized the Fe‒graphdiyne catalysts via a chemical reduction of the Fe3+ ions, which were adsorbed on the surface of graphdiyne, in an aqueous solution of NaBH4 (see the detailed synthesis procedures described in the calculations and experimental methods section). The as-synthesized catalyst contains a large amount of 4‒5 nm diameter metallic Fe particles wrapped in the carbon layers (Figure S2). These metallic Fe particles can block the O2 molecules (reactant) from the active sites and further obstruct the identification of the real active sites or lead to a change in the catalytic mechanism of the ORR.52,53 Thus, these metallic Fe particles must be removed by purification (acid leaching). After purification, we only observed the loosely and uniformly dispersed catalyst from transmission electron microscopy (TEM) images (Figure 2a), which exhibit the typical graphdiyne-like structure (Figure 2b). We do not find the presence of any metallic Fe particles even in the highresolution TEM (HRTEM) images (Figure 2c). X-ray diffraction (XRD) measurements also confirm that the metallic Fe particles have been completely removed by acid leaching because no XRD patterns corresponding to Fe particles are observed (curve b in Figure S3. Of note, before acid leaching, the assynthesized catalysts show strong diffraction peaks of the (002), (102), and (103) planes of the metallic Fe particles at ~44.7, 65.1, and 82.4 degrees, respectively, as shown in curve a in Figure S3). There are only the broad XRD patterns at ~26.0 and 43.7 degrees, which correspond to the (002) and (100) carbon planes,6 respectively, being observed. However, the mapping of the element distribution using HAADF‒ STEM (high-angle annular dark-field scanning transmission electron microscopy) indicates the presence of the Fe element (Figure S4); moreover, the Fe element uniformly distributes on the catalyst surface, implying that these Fe species are chemically coordinated to the carbon, most likely in the form of Fe‒C bonds, to form single-atom-sized Fe on the graphdiyne surface. This can be confirmed by the subÅngström resolution, aberration-corrected HAADF‒STEM image (Figure 2d), which contains many bright dots corresponding to individual heavy atoms with a size of ~0.1 nm distributed on the ACS Paragon Plus Environment

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graphdiyne surface that are attributed to the single-atom-sized Fe components in the synthesized catalyst. No formation of metal aggregates is observed, agreeing well with the results from XRD measurements. The histogram analysis shows very narrow size distributions of Fe (1.05 ± 0.40 Å, Figure 2e) and typical sizes of Fe atoms.

Figure 2. TEM (a–c) and HAADF–STEM images (d) of the synthesized Fe–graphdiyne catalyst. The red circles in image (d) indicate single-atom-sized Fe components anchored on a graphdiyne surface. (e) The histogram analysis of the size distribution of Fe atoms counted 200 Fe atoms from HAADF–STEM images. (f) Fe K-edge Fourier transformed R-space EXAFS spectra of the Fe–graphdiyne catalyst (red line) and the Fe foil reference sample (blue line).

To further verify that the synthesized Fe–graphdiyne contained only atomically dispersed Fe atoms, extended X-ray absorption fine structure (EXAFS) spectra were measured (Figure S5). As shown in the Fourier transformed Fe K-edge EXAFS (Figure 2f), there is only one notable peak at ~1.5 Å, corresponding to Fe–C scattering paths, and no Fe–Fe scattering paths (should be at ~2.2 Å) are detected, confirming the sole presence of atomically dispersed Fe atoms in the synthesized catalyst. These results demonstrate that we have successfully synthesized the Fe–graphdiyne catalysts.

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Figure 3. (a) Raman spectra of graphdiyne (black line) and Fe–graphdiyne (red line). (b–d) The survey XPS spectrum of the Fe–graphdiyne catalyst (b), high-resolution XPS spectra of C1s (c), and Fe2p (d), respectively, and their related curve-fitted components. The circles in (c) represent the experimentally measured data.

The surface chemical information of the catalyst was further characterized using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), which can provide insightful chemical bond information. As depicted in Figure 3a, the Raman spectra of the graphdiyne display four prominent bands at ~1357, 1608, 1932, and 2173 cm‒1 (black line in Figure 3a), respectively. The band at ~1357 cm‒1 corresponds to the breathing vibration of sp2 carbon domains in aromatic rings (D band), and the band at 1608 cm‒1 can be assigned to the first-order scattering of the E2g mode observed for an in-phase stretching vibration of the sp2 carbon lattice in aromatic rings (G band). The bands at 1932 and 2173 cm‒1 can be attributed to a vibration of conjugated diyne links (‒C≡C‒C≡C–).44,54 After formation of the Fe– graphdiyne catalyst (anchoring Fe atoms on the graphdiyne surface), the G band exhibits an ~2 nm shift toward a lower wavenumber (red line), indicating the chemical interaction between graphdiyne and an Fe atom. Similar shifts are observed in the sp carbon peak (from 1932 to 1928 cm–1 and from 2173 to ACS Paragon Plus Environment

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2171 cm–1). These results suggest the formation of the Fe‒C bonds in the Fe‒graphdiyne catalysts, which is consistent with the theoretical predictions based on DFT calculations, the conjectures from TEM observations, and EXAFS analysis. The XPS spectrum shows the existence of carbon, oxygen, and iron elements (Figure 3b). The presence of oxygen is a typical result because the surface of carbon materials tends to be oxidized when handled under ambient conditions.52 Figure 3c displays the high-resolution XPS C1s spectrum of the catalyst, which consists of four subpeaks at 284.4, 285.1, 286.2, and 288.3 eV, corresponding to carbon in sp2 (C=C), sp (C≡C), C‒O, and C=O moieties,44 respectively. The Raman and XPS results indicate that we have synthesized the Fe–graphdiyne catalyst. Of note, we also tried to record the high-resolution XPS spectrum of Fe2p and then analyze the states of the Fe atoms in the catalysts; however, the XPS signal of the Fe is too low to be quantitatively analyzed (Figure 3b, 3d). This is probably due to the low amount of the Fe component in the synthesized catalyst because quantitative determination of the Fe amount in the catalyst via ICP-OES (optical emission spectroscopy using inductive coupled plasma as an ionization source) indicates a Fe loading of only ~0.63 wt.% on the catalyst. Electrocatalytic Performances of the Fe‒graphdiyne Catalysts to the ORR. After the synthesis of the Fe‒graphdiyne catalyst, its electrocatalytic activity toward the ORR was experimentally measured and compared to a precious metal benchmark (commercial 20 wt.% Pt/C catalyst) to validate the theoretical predications. The ORR performance was evaluated by comparing the values of the Uonset, half-wave potential (U1/2), kinetic current density (iK, in mA/cm2) at 0.1 V (vs. NHE), and the rate constant (k, in cm/s) for the ORR. Of note, the detailed procedures for electrode fabrications, electrochemical measurements, and the data analysis are presented in the section on calculations and experimental methods. The cyclic voltammetry (CV) results indicate that a cathodic peak for O2 reduction is observed at ~53 mV (vs. NHE) for the Fe‒graphdiyne catalyst (upper panel in Figure 4a). The cathodic peak potential is close to that obtained for the 20 wt.% Pt/C catalyst, which shows the O2-reduction peak at ~56 mV (lower panel in Figure 4a). The peak current (in mA/cm2) obtained at the Fe‒graphdiyne catalyst is ACS Paragon Plus Environment

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~0.42 mA/cm2, which is even slightly higher than that measured at the Pt/C catalyst (~0.25 mA/cm2). These results suggest that the Fe‒graphdiyne catalyst has a high ORR activity in alkaline solution. a

b Fe−graphdiyne Commercial Pt/C

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d Before ADTs After ADTs

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Figure 4. (a) CV responses of the Fe–graphdiyne catalyst (upper panel) and the commercial Pt/C catalyst (lower panel) in N2- (blue line) and O2-saturated (red line) 0.1 M KOH solution at ambient temperature. The scanning rate was 50 mV/s. The loading of the Fe–graphdiyne catalyst was 0.49 mg/cm2, and the loading of the Pt/C catalyst was 20 µgPt/cm2. (b) RDE measurements in O2–saturated 0.1 M KOH solution for the Fe–graphdiyne catalyt (orange), and the commercial 20 wt.% Pt/C catalyst (violet). The measurements were performed at a rotating speed of 1600 rpm and a scanning rate of 5 mV/s. (c) Tafel ORR plots obtained at the Fe–graphdiyne (orange), and commercial 20 wt.% Pt/C catalyst (violet). (d) The stability of the Fe–graphdiyne catalyst to the ORR. The RDE responses were recorded in O2–saturated 0.1 M KOH solution at a rotating speed of 1600 rpm and a scanning rate of 5 mV/s before (wine) and after ADTs (magenta).

To further evaluate the ORR activity of the Fe‒graphdiyne catalyst, rotating disk electrode (RDE) measurements were conducted on the Fe‒graphdiyne catalyst and the commercial Pt/C catalyst, and the results are depicted in Figure 4b. The Fe‒graphdiyne catalyst shows a Uonset of ~0.21 V (vs. NHE) and a diffusion limited current density of ~5.7 mA/cm2 in O2-saturated 0.1 M KOH solution. The wide current ACS Paragon Plus Environment

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plateau starting from ~0 to at least –0.5 V (vs. NHE) indicates an efficient 4e‒ dominated ORR pathway, which is confirmed by the calculated electron transfer number (n) of the ORR on the Fe‒graphdiyne catalyst (n = 3.96, which is calculated from the analysis of the slope of the Koutecky-Levich plot, Figure S6. See eqn. 9 in the calculations and experimental methods section) and has been further verified by the rotating ring-disk electrode (RRDE) measurements (see the next section). The value of U1/2 is ~0.10 V, and iK (at 0.1 V vs. NHE), which was also calculated from the Koutecky-Levich plot (Figure S6), is ~6.70 mA/cm2 with a corresponding k of ~1.47 × 10–5 cm/s for the ORR (Table 1). These data indicate that the kinetics of the ORR on the surface of the Fe‒graphdiyne catalyst are fast. Moreover, the performance of the Fe‒graphdiyne catalyst is very comparable with or even slightly better than the commercial Pt/C catalyst for the ORR, judging by the values of Uonset, U1/2, iK, and k, as listed in Table 1. This conclusion is consistent with the theoretical prediction.

Table 1. Comparison of the ORR Kinetic Parameters for the Fe–graphdiyne and Commercial Pt/C Catalysts in 0.1 M KOH Solution a Uonset (V vs. NHE)

U1/2 (V vs. NHE)

iK b (mA/cm2)

k (cm/s)

Fe–graphdiyne

0.21

0.10

6.70

1.47×10–2

Pt/C

0.20

0.10

6.40

1.40×10–2

catalyst

a

The loading of the Fe–graphdiyne and commercial Pt/C catalysts on the electrode surface was 0.49 mg/cm2 and 20 µgPt/cm2, respectively. b The values of iK (at 0.1 V vs. NHE) were calculated based on the Koutecky-Levich plot (eqn. 9 in the calculations and experimental methods section).

The Tafel slope was measured to evaluate the ORR kinetic features of the catalysts (Figure 4c). The Tafel plots show two distinct slope regions. In comparison to the well-established “dual” Tafel slope for the ORR on Pt (60 and 120 mV/dec at a potential higher and lower than 0.03 V vs. NHE, respectively),55 a Tafel slope value of ∼63 mV/dec is obtained with the Fe‒graphdiyne catalyst at a high potential region, which is almost identical to that obtained with commercial Pt/C catalysts (62 mV/dec). These results imply that the ORR mechanisms on the Fe‒graphdiyne surface and the Pt catalyst surface

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are identical, giving a clear validation of the theoretical predictions based on the free energy pathway diagrams (Figures 1b and 1c). The stability of the Fe‒graphdiyne catalyst is an important issue and was evaluated by recording the ORR response of the catalyst before and after accelerating durability tests (ADTs), which were carried out by potential cycling of the catalyst between –0.15 to 0.2 V (vs. NHE) at 50 mV/s in a N2-saturated 0.1 M KOH solution for 5000 cycles, and the response was compared with that for a Pt/C catalyst. The results are depicted in Figure 4d, which shows almost no changes in the values of Uonset, only a slightly negative shift of U1/2 (∼11 mV) and a decrease in iK (by ∼8%), suggesting that the Fe‒graphdiyne catalyst is stable during the ADTs in alkaline media. However, for commercial Pt/C catalyst, negative shifts of 40 and 20 mV are obtained for U1/2 and Uonset, respectively, after the ADTs (Figure S7); moreover, iK decreases by ∼15%. These results indicate that the Fe‒graphdiyne catalyst has a better durability than the Pt/C under these test conditions. The Fe‒graphdiyne catalyst is more durable possibly because the active sites in the catalyst are not easily oxidized or reduced. The Fe is highly dispersed and coordinated to carbon atoms, which appears to stabilize the Fe atoms in the catalyst against corrosive dissolution. However, Pt particles on carbon are likely to dissolve in solution, aggregate into larger particles, and then detach from the support, resulting in poor durability.52 Consequently, the synthesized Fe‒graphdiyne catalyst is more stable than the Pt/C catalyst. 4e‒ Pathway Selectivity. After confirming the high electrocatalytic activity of the Fe–graphdiyne catalyst to the ORR, its selectivity was evaluated. Theoretically, ORR can also proceed through twoelectron reactions (2e‒) with the generation of an OOH‒ intermediate. Understanding the nature of the 2e‒ pathway is important for designing appropriate catalysts for the ORR that possess high 4e‒ pathway selectivity to enhance the electrocatalytic efficiency. In alkaline solution, the first step in the 2e‒ pathway and the 4e‒ pathway of the ORR is direct reduction of the O2 molecule to *OOH via the H2O molecule with one electron transfer (eqn. 2); the following step in the 2e‒ pathway is desorption of *OOH (eqn. 6), not cleavage of the O‒O bond in *OOH as in the 4e‒ pathway (eqn. 3). *OOH + e‒ → OOH‒ + *

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Figure 5. (a) Calculated free energy diagrams of the 2e– pathway of the ORR on the Fe–graphdiyne catalyst surface at the equilibrium electrode potential U2e0 (= –0.08 V vs. NHE) and at the experimental measured onset potential Uonset (=0.21 V vs. NHE). (b) Experimental ring and disk current densities for the Fe–graphdiyne catalyst on RRDE at 1600 rpm in an O2-saturated 0.1 M KOH solution (of note, the current scales for the ring and disk electrodes are different). The Pt ring electrode was held at a constant potential of 0.5 V (vs. NHE) when the GC disk potential was scanned from 0.3 to –0.5 V at a rate of 5 mV/s. The inset is a schematic illustration of the oxidation OOH– species, which are generated at the GC disk electrode and at the Pt ring electrode. (c) The measured n values and the corresponding 2e– pathway selectivity (expressed by OOH%) for the Fe–graphdiyne catalyst based on RRDE measurements. The values of n and OOH% were calculated using eqns. 13 and 14, respectively, which are given in the calculations and experimental methods section.

The previous studies have indicated that the 2e‒ and 4e‒ pathway selectivity is associated with desorption of *OOH (eqn. 6) and *OH (eqn. 5) on a catalyst surface,56 respectively; therefore, at a given potential U, the probability of the 2e‒ reaction pathway depends on the value of ∆G5 (for desorption of *OOH, eqn. 6) relative to that of ∆G4 (for desorption of *OH, eqn. 5). Our calculations indicate that desorption of OOH* (eqn. 6) on the Fe‒graphdiyne catalyst surface is an exothermic reaction with a free energy change of ∆G5 (at U2e0) = –0.01 eV (black curve, Figure 5a. U2e0, the equilibrium electrode potential for the 2e‒ pathway, is ‒0.08 V vs. NHE in 0.1 M KOH solution, pH=13),57 implying that the 2e‒ pathway is unavoidable at U2e0 (of note, the free energy change for the desorption of *OH on the 4e‒ pathway is ∆G4 = 0.65 eV at U4e0 = 0.455 V vs. NHE). At Uonset (0.21 V vs. NHE), the free energy diagram for the 2e‒ pathways shows that desorption of *OOH is more exothermic compared to that at ACS Paragon Plus Environment

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U2e0 with a ∆G5 (at Uonset = 0.21 V vs. NHE) = ‒0.30 eV (red curve, Figure 5a). However, the desorption of *OH on the 4e‒ pathway is ∆G4 = 0.40 eV at Uonset(= 0.21 V vs. NHE). Therefore, from the point of view of thermodynamics (free energy changes), desorption of *OOH is easier than that of *OH because of the negative value of ∆G5 (for desorption of *OOH), implying that the 2e‒ pathway is unavoidable for the ORR on the surface of the Fe‒graphdiyne catalyst. The same conclusion is also reached regarding the Pt(111) catalyst.50 Of note, although the first reaction step in the 4e‒ pathway and the 2e‒ pathway is the same, i.e., the reduction of the O2 to *OOH, the calculated free energy diagrams for 4e‒ pathway (black curve, Figure 1b, 1c) and 2e‒ pathway (Figure 5a) at both equilibrium electrode potential and the Uonset (0.21 V vs. NHE) have significantly different. This is due to that (i) their equilibrium electrode potentials are significantly different (the equilibrium electrode potentials for 4e‒ and 2e‒ pathways are U4e0 = 0.455 and U2e0 = ‒0.08 V vs. NHE, respectively, in 0.1 M KOH solution, pH=13), and (ii) their η are also different (see red curve in Figure 5a, and black curve in Figure 1c) because of the different equilibrium electrode potentials again for different pathways, even these free energy diagrams are calculated under the same electrode potential of Uonset (0.21 V vs. NHE). Experimentally, the RRDE technique was employed to verify this theoretical prediction on selectivity by recording the formation of peroxide species (i.e., OOH‒) under different electrode potentials, as shown in Figure 5b. The current densities observed at the ring electrode increase with the potential of the disk electrode scanning in a negative direction and reach a relatively stable value of 0.1 mA/cm2 when the disk potential is negative instead of 0 V (vs. NHE), suggesting that the amount of the peroxide species generated at the disk electrode increases with negative scanning of the disk potential. This result indicates that the 2e‒ pathway for the ORR at the Fe‒graphdiyne surface occurs even at more negative electrode potentials, which is in agreement with the theoretical predictions. The same behavior (mixed 2e‒ and 4e‒ pathways for the ORR) has always been observed at all other carbon-based catalysts, such as graphene-based carbon materials.47,48 However, the observed current densities of the ring electrode here are very low compared with that measured at the disk electrode; for example, the ring current densities are only ~2% of the disk current densities (0.1 mA/cm2 to 5.4 mA/cm2) at a disk potential of –0.1 V (vs ACS Paragon Plus Environment

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NHE). This percentage is similar to that experimentally observed on commercial Pt/C catalysts (Figure S8), and is much lower than those previously reported at doped graphene materials (it exceeds 30%),48 implying that the 4e‒ selectivity of the ORR at the Fe‒graphdiyne catalyst surface is high. Although the 2e‒ pathway proceeds much easier than that of the 4e‒ pathway from the point view of thermodynamics because of the easier desorption of *OOH (for 2e‒ pathway) than *OH (for 4e‒ pathway), the high 4e‒ selectivity of the Fe‒graphdiyne catalyst to the ORR is probably due to the fast kinetic of the desorption of *OH from the surface of the Fe‒graphdiyne catalyst. We have tried to calculate the kinetics of desorption of *OOH and *OH and find their activated energy barriers (Ea), however, we did not obtain the values of Ea of these reactions since we cannot find their reasonable transition states. The reason is that their desorption reactions are complicated because they include the electron transfer and desorption steps simultaneously. This conclusion is further verified by the measured values of n and the percentage of the peroxide species (OOH%) at different disk potentials (Figure 5c). The calculated value of n in the potential range of interest is ~3.96, which is similar to that estimated from the Koutecky-Levich plot; it corresponds to a mixed ~2% 2e‒ pathway selectivity and ~98% 4e‒ pathway selectivity, which can be verified by the calculated value of OOH% as shown in Figure 5c. These results suggest that, although the 2e‒ pathway cannot be totally eliminated, the 4e‒ selectivity of the Fe‒graphdiyne catalyst for the ORR is very high, exhibiting a nearly 100% 4e‒ pathway selectivity, similar to the case of the Pt/C catalyst. To completely eliminate the 2e‒ pathway, it is required that a catalyst surface binds *OOH strongly enough to induce an endothermic *OOH desorption process. However, this criterion is generally very difficult to meet: at a certain potential, *OOH desorption is endothermic whereas OH* desorption is exothermic, which therefore avoids the 2e‒ reduction pathway to exhibit a totally 100% 4e‒ pathway selectivity. CONCLUSIONS In summary, by combining DFT calculations and experimental studies, we have demonstrated that graphdiyne is a good substrate for anchoring an Fe atom to form an Fe‒graphdiyne catalyst. Moreover, ACS Paragon Plus Environment

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the formed catalysts have shown high catalytic activity to the ORR in alkaline medium, with activity similar to or even slightly better than the precious metal benchmark (commercial 20 wt.% Pt/C catalyst). DFT predicts that the O2 molecule can bind with an Fe atom through the chemisorption mode, and the electron transformation process of the ORRs occurs through a 4e‒ pathway. To validate the theoretical predictions, the Fe‒graphdiyne catalyst has been synthesized by a reduction of Fe3+ ions adsorbed on a graphdiyne surface in aqueous solution, and its electrocatalytic activities toward the ORR have been evaluated in alkaline electrolytes. The electrochemical experimental results indicate that the Fe‒ graphdiyne catalyst can facilitate the 4e‒ ORR while limiting the 2e‒ transfer reaction, showing a high 4e‒ selectivity of the ORR, and there is good agreement between the DFT predictions and electrochemical measurements. The results presented here demonstrate that graphdiyne can provide a unique platform for fabricating well-defined and uniform graphdiyne-supported single atom catalysts with high catalytic activity towards the ORR and will open a new avenue to developing carbon nanomaterial-based single-atom-sized catalysts for use in environment- and energy-related fields. It should be pointed out that this work evaluated the electrocatalytic activity of the Fe‒graphdiyne catalyst to ORR in alkaline solution. The electrolytes that are used in evaluating the ORR activity have significant impacts on the performance of the catalysts. The electrocatalytic performance of the Fe‒ graphdiyne catalyst to ORR in acidic medium may significantly differ from those obtained in alkaline medium. Fundamental understandings on the catalytic characteristics of the Fe‒graphdiyne catalyst in different medium are of importance to find the key factors that affected its electrocatalytic performance and to its practical applications. Therefore, the ORR activity of this catalysis in acidic solution, such as HClO4 solution, should be studied in the future and compared with those obtained in this work. CALCULATIONS AND EXPERIMENTAL METHODS Calculation Setups. The calculations were conducted using plane-wave spin-polarized density functional theory (DFT) as implemented in DmoL3 code.58,59 The Perdew-Burke-Ernzerhof (PBE)60 exchange-correlation energy functional, which is a generalized gradient approximation (GGA) ACS Paragon Plus Environment

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functional with Grimme methods for DFT–D correction, was used in the calculations. The convergence tolerance of energy, maximum force, maximum displacement, and Gaussian electron smearing width for the geometry optimization were set to 1.0 × 10‒5 Hartree, 0.002 Hartree per Å, 0.005 Å, and 0.005 eV, respectively. We used a 4 × 4 × 1 Monkhorst–Pack k-point mesh to sample the Brillouin zone.61 The primitive cell of graphdiyne, consisting of 18 carbon atoms, was used as a model system (Figure S9). In this configuration, only one Fe atom per super cell was bound on one side of the slab to reduce the lateral interactions between adsorbates. To evaluate whether the constructed cell is large enough for our calculations, we also calculated the binding one Fe atom on a larger cell (2 × 2, containing 72 carbon atoms. Figure S10a) and the adsorption of the *OH intermediate on the formed Fe‒graphdiyne catalyst (on Fe atom. Figure S10b). The relevant Ead was found to be about ‒1.81 eV, which is similar to that calculated on the primitive cell (1 × 1, ‒1.77 eV), implying that the primitive cell consisting of 18 carbon atoms is large enough for our calculations. For calculation of the adsorption of O2 and the related intermediates involved in the ORR, the adsorption energy (Ead) is defined in eqn. 7: Ead = Et – E0 – Ex

(7)

where Et, E0, and Ex are the total energy of the total adsorbed system including the adsorbed species (O2, and the related intermediates) and Fe‒graphdiyne, the energy of the Fe‒graphdiyne catalyst, and the energy of the isolated adsorbed species, respectively. In this definition, a negative value of Ead represents an exothermic adsorption process, suggesting the adsorption is energetically favorable. The free-energy diagram of the ORR was calculated based on a computational hydrogen electrode (CHE) model,62 which declares that the chemical potential of a proton/electron (H+ + e−) in solution is equal to half the chemical potential of a gaseous H2. The reference electrode was set up to be the NHE. The change in free energy (∆G) of each reaction step is calculated based on eqn. 8: ∆G = ∆E + ∆ZPE − T∆S + ∆GU + ∆GpH

(8)

where ∆E is the reaction energy associated with the reactant and product molecules adsorbed on the catalytic surface in each ORR step, ∆ZPE is the change in zero-point energies (ZPE), T is the ACS Paragon Plus Environment

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temperature which refers to room temperature (T = 298.15 K), and ∆S is the change in entropy. The ZPEs and entropies of the ORR intermediates are calculated from vibrational frequencies and those of gas phase molecules are obtained from the standard thermodynamic database. The effect of an external bias (∆GU) is to shift ∆G by ‒eU in each (H+ + e−) transfer step, where e is the number of electrons transferred and U is the applied bias. ∆GpH = kBTln 10×pH where kB is Boltzmann’s constant and the pH value is set to 13 because the electrochemical measurements were performed in 0.1 M KOH solution. Synthesis of Fe‒graphdiyne Catalysts. To synthesize the Fe‒graphdiyne catalysts, we first synthesized the graphdiyne using the reported procedures.41,46,54 Then, ~50 mg of graphdiyne was dispersed in 10 mL of water and mixed with 100 mg of FeCl3 by vigorously stirring overnight in an Ar environment. During this process, the Fe3+ ions can be adsorbed on the surface of graphdiyne. Subsequently, the adsorbed Fe3+ ions were reduced to Fe by adding NaBH4 to the system, forming Fe‒graphdiyne catalysts. The products were collected by filtering and further purified by overnight refluxing in 100 mL of H2SO4 solution (2 M) in air at 110 °C to remove any metallic Fe particles produced during the reduction. After refluxing, the suspensions were filtered and washed with distilled water until the filtrate exhibited a pH value of ~7. Characterizations. XPS was measured with an ESCALAB 250 XPS spectrometer (VG Scientifics) using a monochromatic Al Kα line at 1486.6 eV. Binding energies were calibrated with respect to the C1s peak at 284.6 eV. Raman spectra were measured on a Labram HR 800 microspectrometer (Jobin Yvon) with an excitation source of 514 nm. XRD patterns were recorded on a Rigaku/Max-3A X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm). The morphologies of the synthesized Fe‒ graphdiyne catalysts were observed on a JEOL JEM-2100F TEM. The sub-Ångström-resolution, aberration-corrected HAADF–STEM images were recorded on a JEOL JEM 2200FS STEM/TEM, equipped with a CEOS (Heidelburg, Ger) probe corrector, with a nominal image resolution of 0.07 nm. The amount of Fe in the catalysts was quantitatively determined via ICP-OES using an Optima 7300 DV (PerkinElmer). X-ray absorption spectra (XAS) were measured with the 1W1B beamline of the

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Beijing Synchrotron Radiation Facility. The data were analyzed and Fourier transformed by standard procedures with default parameters determined by Athena software. The data were fitted in R-space. Electrochemical Measurements. The electrochemical measurements were performed with an RRDE technique in a three-electrode cell at an ambient temperature. An RRDE (Pine Research Instrumentation) was used as the working electrode. The RRDE electrode consisted of a catalyst-coated glassy carbon (GC) disk (with a diameter of 5.61 mm and an electrode area of 0.2472 cm2) surround by a Pt ring (0.1856 cm2 in area, with inner and outer ring diameters of 6.25 and 7.92 mm, respectively). A platinum wire and an Ag/AgCl electrode (saturated with KCl solution) were used as the counter electrode and reference electrode, respectively. The data were recorded using a Pine Wave Driver (bipotentiostat, Pine Research Instrumentation). The supporting electrolyte was 0.1 M KOH aqueous solution (AR grade) prepared with ultrapure water (18.2 MΩ cm). All electrode potentials in this work are referenced to NHE by adding a value of 0.197 V to the potentials measured versus the Ag/AgCl reference electrode. To immobilize the catalyst on the electrode surface, 12 mg of the synthesized catalyst powder was ultrasonically dispersed in a mixed solvent containing 450 µL of water, 500 µL of ethanol, and 50 µL of 5 wt.% Nafion solution, and then ten microliters of the ink was cast onto the GC disk surface, and the solvent was allowed to evaporate in air at an ambient temperature, resulting in a catalyst loading of 0.12 mg (0.49 mg/cm2) on the electrode surface. The ORR activity was measured in the O2-saturated 0.1 M KOH solution. Before the data were recorded, the GC disk electrode was cycled in the potential range from 0.2 to –0.8 V (vs. NHE) at a scan rate of 100 mV/s for at least 20 cycles until a reproducible CV response was obtained. Then, the CVs were recorded on the disk electrode at 50 mV/s. CV collected in an argon-saturated electrolyte serves as a blank. The linear sweep voltammetry (LSV) polarization curves were recorded via cathodic scans of the GC disk electrode potential from 0.3 to –0.5 V at 5 mV/s with varying rotating speed from 400 to 3600 rpm. Current densities (mA/cm2) were normalized using the geometric electrode area. The ik and, subsequently, the k for the ORR were derived from Koutecky-Levich plots (eqn. 9). ACS Paragon Plus Environment

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ACS Catalysis 













=  +  =  +  /

(9)

⁄

= 0.62    ⁄

(10)

 = 

(11)

where i is the measured current density, iK and iL are the kinetic- and diffusion-limiting current densities, ω is the angular velocity of the disk (ω = 2πf, where f is the linear rotation speed), n is the number of electrons transferred per O2 molecule, F is the Faraday constant (96485 C/mol), C0 is the bulk concentration of O2 (1.18 × 10–6 mol/mL), ν is the kinematic viscosity of the electrolyte (0.01 cm2/s), D0 is the O2 diffusion coefficient (1.93 × 10–5 cm2/s), and k is the electron-transfer rate constant. The value of n and ik can be obtained from the slope and intercept of the Koutecky-Levich plots, respectively. A Tafel analysis was performed between the iK and η (in V) for the ORR polarization on the Fe‒ graphdiyne catalyst surface using eqn. 12. η = a + b log iK

(12)

where a (in V) is the Tafel constant and b (in V/dec) is the Tafel slope. The selectivity for water formation at the Fe‒graphdiyne catalyst was measured using two different techniques: (i) the Koutecky-Levich technique where half-cell measurements are performed at different rotating speeds to obtain n, the total number of electrons passing during the ORR reaction, and (ii) RRDE experiments where the Pt ring was held at a potential of 0.5 V (vs. NHE), which is high enough to oxidize any OOH‒ intermediates produced on the GC disk electrode during the ORR process. The value of n (a value of n = 4 corresponds to a complete 4e‒ reduction of O2 to water, whereas a value of n = 2 is a 2e‒ reduction of O2 to H2O2) and the OOH− intermediate production percentage (OOH−%, which serves as the 2e− pathway selectivity) were determined via eqn. 13 and eqn. 14, respectively.  = 4 ×

#

(13)

% # $ & '



OOH % = 200 ×

%& ' % # $ & '

(14)

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where iD is the disk current, iR is the ring current, and N is the collection efficiency of the RRDE (which is 37% in this work as specified by the manufacturer). AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (P. W.). *E-mail: [email protected] (C. C.). Notes The authors declare no competing financial interest. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Configurations of the primitive cell of graphdiyne for calculations and adsorption of the Fe atom on the graphdiyne sheet; TEM images and XRD patterns of the as-synthesized Fe–graphdiyne catalyst before purification; the mapping analysis of the element distribution; EXAFS spectroscopy; KouteckyLevich plot for the ORR at the Fe–graphdiyne catalyst; and the stability and the experimental RRDE data of the ORR at the commercial Pt/C catalyst. ACKNOWLEDGMENTS This work is supported by NSFC (21335004 and 21675088), NSF of Jiangsu Province, and the Priority Academic Program Development of the Jiangsu Higher Education Institutions. REFERENCES (1) Wagner, F. T.; Lakshmanan, B.; Mathias, M. F. Electrochemistry and the Future of the Automobile. J. Phys. Chem. Lett. 2010, 1, 2204–2219. (2) Nie, Y.; Li, L.; Wei, Z. Recent Advancements in Pt and Pt-Free Catalysts for Oxygen Reduction ACS Paragon Plus Environment

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Reaction. Chem. Soc. Rev. 2015, 44, 2168–2201. (3) Xia, W.; Mahmood, A.; Liang, Z.; Zou, R.; Guo, S. Earth-Abundant Nanomaterials for Oxygen Reduction. Angew. Chem., Int. Ed. 2016, 55, 2650–2676. (4) Li, Y.; Li, Y.; Zhu, E.; McLouth, T.; Chiu, C.-Y.; Huang, X.; Huang, Y. Stabilization of HighPerformance Oxygen Reduction Reaction Pt Electrocatalyst Supporting on Reduced Graphene Oxide/Carbon Black Composite. J. Am. Chem. Soc. 2012, 134, 12326–12329. (5) Wu, P.; Zhang, H.; Qian, Y.; Hu, Y.; Zhang, H.; Cai, C. Composition- and Aspect-Ratio-Dependent Electrocatalytic Performances of One-Dimensional Aligned Pt-Ni Nanostructures. J. Phys. Chem. C 2013, 117, 19091–19100. (6) Hu, Y.; Zhang, H.; Wu, P.; Zhang, H.; Zhou, B.; Cai, C. Bimetallic Pt-Au Nanocatalysts Electrochemically Deposited on Graphene and Their Electrocatalytic Characteristics towards Oxygen Reduction and Methanol Oxidation. Phys. Chem. Chem. Phys. 2011, 13, 4083–4094. (7) Tan, X.; Prabhudev, S.; Kohandehghan, A.; Karpuzov, D.; Botton, G. A.; Mitlin, D. Pt-Au-Co Alloy Electrocatalysts Demonstrating Enhanced Activity and Durability toward the Oxygen Reduction Reaction. ACS Catal. 2015, 5, 1513–1524. (8) Zhu, H.; Zhang, S.; Guo, S.; Su, D.; Sun, S. Synthetic Control of FePtM Nanorods (M = Cu, Ni) To Enhance the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2013, 135, 7130–7133. (9) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G.; Ross, P. N.; Markovic, N. M. Trends in Electrocatalysis on Extended and Nanoscale Pt-Bimetallic Alloy Surfaces. Nat. Mater. 2007, 6, 241–247. (10) Li, H.-H.; Ma, S.-Y.; Fu, Q.-Q.; Liu, X.-J.; Wu, L.; Yu, S.-H. Scalable Bromide-Triggered Synthesis of Pd@Pt Core-Shell Ultrathin Nanowires with Enhanced Electrocatalytic Performance toward Oxygen Reduction Reaction. J. Am. Chem. Soc. 2015, 137, 7862–7868. (11) Gan, L.; Heggen, M.; Cui, C.; Strasser, P. Thermal Facet Healing of Concave Octahedral Pt-Ni Nanoparticles Imaged in Situ at the Atomic Scale: Implications for the Rational Synthesis of Durable High-Performance ORR Electrocatalysts. ACS Catal. 2016, 6, 692–695. ACS Paragon Plus Environment

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