Quatermetallic Pt-Based Ultrathin Nanowires Intensified by Rh Enable Highly Active and Robust Electrocatalysts for Methanol Oxidation

Journal Pre-proof Quatermetallic Pt-Based Ultrathin Nanowires Intensified by Rh Enable Highly Active and Robust Electrocatalysts for Methanol Oxidation Wei Wang, Xiaowei Chen, Xue Zhang, Jinyu Ye, Fei Xue, Chao Zhen, Xinyan Liao, Huiqi Li, Pingting Li, Maochang Liu, Qin Kuang, Zhaoxiong Xie, Shuifen Xie PII:

S2211-2855(20)30180-4

DOI:

https://doi.org/10.1016/j.nanoen.2020.104623

Reference:

NANOEN 104623

To appear in:

Nano Energy

Received Date: 7 January 2020 Revised Date:

9 February 2020

Accepted Date: 14 February 2020

Please cite this article as: W. Wang, X. Chen, X. Zhang, J. Ye, F. Xue, C. Zhen, X. Liao, H. Li, P. Li, M. Liu, Q. Kuang, Z. Xie, S. Xie, Quatermetallic Pt-Based Ultrathin Nanowires Intensified by Rh Enable Highly Active and Robust Electrocatalysts for Methanol Oxidation, Nano Energy, https://doi.org/10.1016/ j.nanoen.2020.104623. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Credit Author Statement Wei Wang, Xiaowei Chen and Shuifen Xie designed, performed the experiments, analyzed the data and wrote the manuscript. Xue Zhang performed the DFT calculations and wrote the manuscript. Jinyu Ye, Fei Xue, Chao Zhen, Xinyan Liao, Huiqi Li, Pingting Li and Maochang Liu provided assistances in experiments, characterization of materials, and manuscript preparation. Qin Kuang, Zhaoxiong Xie and Shuifen Xie supervised the work and revised the manuscript.

1.5-nm-thin quatermetallic PtCoNiRh NWs with high atomic-exposure and accessional interatomic Pt–Rh sites were successfully synthesized to serve as highly active and robust electrocatalysts toward methanol oxidation reaction (MOR). The anticorrosion effect of incorporated-Rh effectively stabilized the surrounding Pt atoms and modulated the intermediate CO binding, endowing the PtCoNiRh NWs with reinforced ultrathin features and superior MOR performances in acidic condition.

Revised ms #NANOEN-D-20-00072

Quatermetallic Pt-Based Ultrathin Nanowires Intensified by Rh Enable Highly Active and Robust Electrocatalysts for Methanol Oxidation Wei Wanga,1, Xiaowei Chena,1, Xue Zhangb,c,1, Jinyu Yeb, Fei Xued, Chao Zhena, Xinyan Liaoa, Huiqi Lib, Pingting Lia, Maochang Liud, Qin Kuangb, Zhaoxiong Xieb,*, Shuifen Xiea,*

a

College of Materials Science and Engineering, Huaqiao University, Xiamen, 361021, China

b

State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry,

College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China c

Institute of Advanced Materials Science and Engineering, Shenzhen Institutes of Advanced

Technology, Chinese Academy of Sciences, Shenzhen 518055, China d

International Research Center for Renewable Energy, State Key Laboratory of Multiphase

Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China

1

These authors contribute equally to this work.

*Corresponding author. E-mail addresses: [email protected] (Z. Xie), [email protected] (S. Xie).

1

Abstract: Inferior stability and anti-poisoning capacity of Pt-based ultrathin nanowires (NWs) are critical weaknesses under detrimental acidic running conditions for proton-exchange membrane fuel cell applications due to their energetic surface. Here 1.5-nm-thin quatermetallic PtCoNiRh NWs with high atomic-exposure are fabricated to serve as robust electrocatalysts for acidic methanol oxidation reaction (MOR). Surpassing Rh-free PtCoNi NWs and most of state-of-the-art catalysts, the PtCoNiRh NWs achieve extremely high MOR activity (1.36 A·mg-1Pt and 2.08 mA·cm-2) with substantially lowered onset-potential and improved CO-tolerance. The anticorrosion effect of incorporated-Rh can effectively stabilize the PtCoNiRh NWs in the corrosive MOR. Electrochemical in situ Fourier transform infrared spectroscopy and density functional theory simulation cooperatively reveal that the methanol dehydrogenation is inclined to occur at the interatomic Pt–Rh sites, where the intermediate COads prefers bridge binding mode rather than linear mode with facilitated removal. Integratedly, the complete 6e--transferred MOR process is reliably accelerated and stays efficient on the quaternary PtCoNiRh NWs.

Keywords: Pt-based ultrathin nanowire, interatomic Pt–Rh site, in situ FTIR, CO-tolerance, methanol oxidation reaction

TOC GRAPHICS

2

1. Introduction Platinum (Pt) based nanostructures have attracted intensive attention in the research field of surface electrocatalysis toward proton-exchange membrane fuel cells (PEMFCs), owing to their far ahead catalysis in both the anode and cathode reactions, such as methanol oxidation reaction (MOR), ethanol oxidation reaction (EOR) and oxygen reduction reaction (ORR), etc. [1–7]. However, critical issues, including high cost, Pt-scarcity, intermediate poisoning and poor durability, hamper the practical applications of Pt-based electrocatalysts and demand prompt solutions [2,3,8–10]. To elevate the atomic economy of Pt, both the specific activity and the Pt atomic-exposure ratio should be optimized on Pt-based nanostructures. One-dimensional (1D) Pt-based ultrathin nanowires (NWs) with only a few atomic-layer thickness offer an attractive platform to tackle these challenges [11–17]. For example, jagged Pt ultrathin NWs enable ultrahigh ORR mass activity owing to the extremely large electrochemical active surface area (ECSA) [12]. Besides that, incorporating other transition metals, such as Ni, Co, Ga, et. al., into ultrathin Pt NWs can dramatically boost their electrocatalysis activities through affecting the lattice stress and electronic effects derived from different atomic radius and electron orbital hybridization [15,16]. However, in acidic MOR, the dehydrogenated intermediate CO tends to firmly adsorb on the reactive Pt surface sites and poisons the catalysts [8,9,17]. In addition, the acid corrosion triggers the elemental leaching, leading to dreadful structural damage and serious reduction in MOR catalytic activity [1,2,8,9], which would be more serious for ultrathin Pt alloy NWs because of the only a few atomic layers thickness. Therefore, the stability and anti-CO poisoning capacity of Pt-based alloy ultrathin NWs are still particularly critical issues for their MOR catalysis. Incorporating an additional anticorrosive metal into Pt-based alloy ultrathin NWs, arguably, could be a feasible strategy for stabilizing the surrounding metal atoms and thus intensify the 1D ultrathin structure. In addition, the incidental generated interatomic surface site on the Pt-based ultrathin NWs may also affect the intermediate CO binding and removal complexity. Herein, we report a one-pot synthesis of atomically thin (~1.5 nm, about 8 atomic layers) quatermetallic PtCoNiRh NWs with interatomic Pt–Rh sites and modulated CO binding for 3

boosting and stabilizing the MOR electrocatalysis. Unlike other easily oxidized transition metals, Rh is highly anticorrosive [18,19]. The incorporation of Rh can construct accessional Pt–Rh sites stably on the surface of the ultrathin Pt-based NWs even during the strong acidic MOR catalysis process. Electrocatalytic studies show that, compared to Rh-free trimetallic PtCoNi NWs supported on carbon (PtCoNi NWs/C) and most of the state-of-the-art electrocatalysts, the quatermetallic PtCoNiRh NWs/C catalyst exhibits substantial enhancements in MOR mass activity and specific activity with distinctly negative MOR onset potential and prominent CO-tolerance and durability. Electrochemical in situ Fourier transform infrared (FTIR) spectroscopy and density functional theory (DFT) calculation illuminate that the intermediate CO binding is thermodynamically transferred from linear mode (COL) on the Pt sites of the PtCoNi NWs to bridge mode (COB) on the accessional Pt– Rh sites of the PtCoNiRh NWs with lowered barrier for the further oxidation of COads. Overall, the PtCoNiRh NWs/C effectively accelerate the kinetics and improve the durability for MOR catalysis. 2. Results 2.1. Characterizations of ultrathin PtCoNiRh NWs To prepare the atomically thin PtCoNiRh NWs, four metal precursors, including platinum(II) acetylacetonate [Pt(acac)2], cobalt(II) acetylacetonate [Co(acac)2], nickel(II) acetylacetonate simultaneously

[Ni(acac)2], added

and

into

rhodium(III) an

oleylamine

acetylacetonate (OAm)

[Rh(acac)3], solution

were

containing

cetyltrimethylammonium chloride (CTAC) and Mo(CO)6. The resulting mixture was stirred and ultrasonicated at room temperature for 20 min, and then heated in a 180 °C oil bath for 6 h (see the experimental section in supporting information for details). Fig. 1a and b show large area high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and TEM images of the obtained products, exhibiting uniform ultrathin NWs with a high-yield approaching 100%. The average diameter of the NWs is statistically measured to be only around 1.5 nm (Fig. S1), approximately only 8 atomic-layer thick, 4

revealing the atomic-level thin feature. Aberration corrected HAADF-STEM image shows a clear lattice spacing of 0.225 nm, ~2.2% smaller than that of the Pt(111) lattice plane (0.230 nm) (Fig. 1c). Compositional analysis by inductively coupled plasma mass spectrometry (ICP-MS) reveals an overall Pt/Co/Ni/Rh atomic ratio of 64:11:12:13 for these NWs, which is consistent with the result of scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDS) (Fig. S2). STEM-EDS elemental mapping images and line-scanning profile show a homogeneous distribution of Pt, Co, Ni and Rh elements throughout the whole NWs (Fig. 1d and e). Notably, the PtCoNiRh NWs are highly sensitive to irradiation by the electron beam during the STEM-EDS experiments, further implying the atomic-thin character. Powder X-ray diffraction (PXRD) pattern shows a single set of diffraction peaks indexed to the face-centered cubic (fcc) structure (Fig. 1f). The diffraction peaks have slight shifts to higher angles relative to pure Pt (JCPDS No. 65-2868), which can be ascribed to the shrunken lattice spacing observed in the HAADF-STEM image due to the involvement of smaller Co, Ni and Rh atoms. As a referential sample, Rh-free trimetallic PtCoNi NWs were also fabricated using this synthetic method merely without the addition of Rh precursors. The trimetallic PtCoNi NWs are also in fcc phase with an average diameter around 1.5 nm (Fig. S3) and a Pt/Co/Ni atomic ratio of 74:13:13 (determined by ICP-MS). X-ray photoelectron spectroscopy (XPS) spectra show the partially overlapped peaks of Rh 3d and Pt 4d for the PtCoNiRh NWs (Fig. 1g), whereas only the peak of Pt 4d can be observed on the Rh-free PtCoNi NWs in the same energy region (Fig. 1h). In addition, both the PtCoNiRh and PtCoNi NWs show the coexistence of XPS signals for Co and Ni elements (Fig. S4). Therefore, except the absence of Rh, the PtCoNi NWs have the similar structure and composition characters with the PtCoNiRh NWs, which paves a pathway for investigating the role of the extra Rh when applying these atomically thin NWs in electrocatalysis. 2.2. Growth mechanism of ultrathin PtCoNiRh NWs Engineering ultrathin 1D multimetallic NWs is an open challenge because of the different reduction potentials of metal cations and inherent isotropic growth feature of metal atoms. 5

For probing the formation mechanism of the quaternary PtCoNiRh NWs, we investigated the intermediates at different reaction time points by TEM and ICP-MS (Fig. S5). Initially, abundant small nanocrystals were formed at approximately 40 s. ICP-MS result shows that the nanocrystals at this stage were composed of only Pt, implying the preferential reduction of Pt cations. After 3 min, the nanoparticles grew into relatively short NWs, accompanied with trace reduction of other metals. As the reaction lasted for 60 min, uniform NWs had been the major products, of which the Pt/Co/Ni/Rh atomic ratio was 81:2:9:8. When the reaction time was prolonged to 8 h, the morphology of NWs was well maintained, and the metal molar ratio was consistent with that of the 6 h products. Additionally, the roles of a series of synthetic parameters were investigated systematically. As shown in Fig. S6, the reaction without addition of CTAC just yielded small nanoparticles, and the absence of Mo(CO)6 directly leaded to irregular nanoparticles with relatively large sizes. These results demonstrate that CTAC acts as a structure-directing agent and Mo(CO)6 works as a reduction agent, which control the anisotropic growth behavior and the reaction rate for inducing the formation of NWs, respectively [15,19]. Meanwhile, the reaction temperature plays a vital role on the uniformity and composition for the PtCoNiRh NWs (Fig. S7). Specifically, when the reaction was elevated from 180 oC to 200°C, poor quality NWs mixed with small nanoparticles were obtained. If the temperature was lowered to 150 °C, PtRh-composed NWs (with only trace Ni) were gained, which can be attributed to the lower reduction potentials of Co2+ and Ni2+ than that of Pt2+ and Rh3+ cations. 2.3. MOR catalytic performance MOR catalytic activity of these multimetallic ultrathin NWs were investigated to correlate the surface interatomic Pt–Rh sites with their electrocatalytic performance. Before the electrochemical tests, the PtCoNiRh NWs and PtCoNi NWs were loaded on carbon supports (20 wt% of Pt on Vulcan XC-72 carbon, determined by ICP-MS), which are denoted as PtCoNiRh NWs/C and PtCoNi NWs/C (Fig. S8). ECSAs of the PtCoNiRh NWs/C, PtCoNi NWs/C and commercial Pt/C were evaluated by CO stripping and calculated to be 65.3, 63.7, 55.4 m2 g-1Pt, respectively (Fig. 2a). It is notable that the peak potentials of CO stripping are 6

0.387, 0.485, and 0.535 V for the PtCoNiRh NWs/C, PtCoNi NWs/C and commercial Pt/C, respectively, indicating the introduction of Rh endows the PtCoNiRh NWs with substantially improved CO-tolerance. Fig. 2b and c show the MOR mass activities and specific activities, which are normalized to the Pt mass and the ECSA, respectively. Specifically, the mass activity of the PtCoNiRh NWs/C catalyst reaches 1.36 A mg-1Pt, which is 1.5 times and 3.9 times higher than that of the PtCoNi NWs/C (0.87 A mg-1Pt) and commercial Pt/C (0.35 A mg-1Pt), respectively. Meanwhile, the PtCoNiRh NWs/C exhibits the most superior specific activity of 2.08 mA cm-2, which is also much higher than those of the PtCoNi NWs/C (1.37 mA cm-2) and commercial Pt/C (0.63 mA cm-2). Remarkably, the MOR catalytic performance of the PtCoNiRh NWs/C is superior to most of the reported Pt-based catalysts (Fig. 2d and Table S1) [17,20–26] and the most active commercial bimetallic PtRu/C catalysts (Figure S9) tested in the similar acidic media. In addition, the onset potential (denoted at 0.1 A mg-1Pt) of the PtCoNiRh NWs/C exhibits obviously negative shifts of 60 and 130 mV, respectively, compared to those of the PtCoNi NWs/C and commercial Pt/C, indicating the involvement of Rh can effectively lower the anodic potential for MOR (insert in Fig. 2b). To further evaluate the CO-tolerance of those three catalysts, CO was purged into the electrolytes containing 0.1 M HClO4 and 3 M methanol during MOR test. Remarkably, the PtCoNiRh NWs/C retains 85.1% peak current density after the CO supply was cutted off, while the PtCoNi NWs/C and Pt/C only maintain 64.1% and 43.8% peak current (Fig. 2e and Fig. S10). This result is consistent with the CO stripping tests, clearly illustrating the excellent anti-CO poisoning property of the PtCoNiRh NWs/C. In addition, the neighboring Rh atoms can stabilize the surrounding Pt atoms [19], benefiting the MOR durability and the structural stability for the PtCoNiRh NWs. Electrochemical durability evaluated by chronoamperometry tests at 0.5 V (vs. SCE) shows that the PtCoNiRh NWs/C maintains the highest mass activity and most stable morphology after 10000 s test, whereas the PtCoNi NWs/C and commercial Pt/C exhibit obviously serious activity loss and aggregation (Fig. 2f and Fig. S11). Previous advances have clarified that the synchronous incorporation of Ni and Co into Pt nanostructures can effectively boost their electrocatalytic performances [4,27,28]. Evidently, 7

the referential PtCoNi NWs/C already shows substantial improvement in MOR catalysis compared to the Pt/C. This improvement can be ascribed to the compression stress and electronic ligand effects derived by the small foreign Ni and Co atoms, as well as the ultrathin 1D nanostructure [13,15–17,29]. Here the more distinct promotion of the MOR performance by the PtCoNiRh NWs/C, compared to the PtCoNi NWs/C, can be attributed to the incorporation of Rh, of which the role still needs to be in-deep depicted. A previous theory calculation indicated individual Rh atom on surface Pt(111) is identified to be more effective than the neighboring Rh atoms for MOR [30]. Accordingly, the Rh content in the PtCoNiRh NWs is a crucial factor for the MOR enhancements. Specifically, PtCoNiRh NWs with other Rh contents (Pt/Co/Ni/Rh = 66:14:14:6 and 57:10:10:23) can also be prepared using this synthetic approach by changing the addition of Rh precursors (Fig. S12). Through the MOR electrocatalytic tests, a volcano relationship between the MOR activity and the Rh content can be clearly established for these quatermetallic NWs (Fig. S13), showing a 13% Rh is the optimized atomic content among the inspected samples. 3. Discussion 3.1. Electrochemical in situ FTIR study In situ FTIR studies (Fig. S14) were further conducted on the PtCoNiRh NWs/C and PtCoNi NWs/C catalysts to get insight into the auxo-action of the Rh atoms for MOR at the molecular-level. Fig. 3a and b show the in situ FTIR spectra from -0.2 to 0.3 V at an interval of 0.1 V in 0.1 M HClO4 and 0.5 M methanol (reference potential ER = -0.25 V, vs. SCE). The negative IR characteristic peak at 2342 cm-1 is contributed to the asymmetric stretch vibration of CO2 [31–33]. The presence of CO2 reveals the complete oxidation of methanol by 6 electrons transferring process. A bipolar IR characteristic band at 2035 cm-1 and a unipolar band at 1891 cm-1 belong to the COL and COB [31,34–36], respectively. It is commonly accepted that MOR proceeds via a dual pathway mechanism by CO or non-CO reactive intermediates [8,9]. Therefore, the presence of COads on the in situ FTIR spectroscopy demonstrates that the MOR processes happened on those two ultrathin NWs 8

undergo the COads intermediate path, where COads oxidation is the last step for complete MOR. As shown in Fig. 3c, the integrated intensity of the COL and COB IR band exhibited on those two catalysts experience an up-down trend. The foregoing rise in peak intensity of both the COL and COB during the MOR is an exhibition of the accumulation of the surface-bound poisonous COads stemming from the methanol dehydrogenation. Later, the CO intensity decreased due to the higher oxidation rate than the formation rate of COads at high potentials [32,33,36]. It is significant that the COB characteristic peaks kept obviously higher on the PtCoNiRh NWs/C than that on the PtCoNi NWs/C in the potential window (Fig. S15). This unique COB peak can be attributed to the involvement of Rh atoms on the PtCoNiRh NWs as other possible structural and compositional difference between the two NWs have been ruled out. The presence of conspicuous COB configuration on the PtCoNiRh NWs is also verified by in situ CO adsorption experiment (Fig. 3d), conducted by bubbling CO in 0.1 M HClO4 solution at a potential of -0.2 V (vs. SCE). This spectral feature of COB was also observed in the study of overlayer Rh on Pt(111) and Pt(100) single crystal electrodes[37–39], confirming the existence of surface interatomic Rh–Pt sites on the PtCoNiRh NWs and their function for modulating the CO binding. 3.2. DFT calculation For further comprehensively understanding the role of surface interatomic Pt–Rh sites and the MOR catalysis mechanism on these PtCoNiRh NWs, we carried out simulative DFT investigations of CO adsorption and MOR on two model surfaces, i.e. Pt-skin Pt3Co0.5Ni0.5(111) and Rh-decorated Pt-skin Pt3Co0.5Ni0.5(111), respectively. Firstly, we evaluate the CO adsorption properties on the possible catalytic sites. The possible CO adsorption sites on the Rh-decorated Pt-skin Pt3Co0.5Ni0.5(111) surface include two top sites (Rh and Pt site), one bridge site (Rh–Pt site), and two types of hollow sites (Rh–Pt–Pt fcc and hcp sites). According to the results (Fig. 4a), it is interesting to note that, the adsorption energy (Eads) for CO on the interatomic Rh–Pt site with a bridge mode is more negative than all the other sites. The result is well in line with the above-mentioned in situ FTIR analysis, disclosing the root for the appearance of COB on the PtCoNiRh NWs. 9

Subsequently, the MOR pathways on both the Pt-skin Pt3Co0.5Ni0.5(111) and Rh-decorated Pt-skin Pt3Co0.5Ni0.5(111) surfaces were investigated by DFT calculations. According to the previous theoretical investigation [23,24,40–43], CH3OH* can go through either CH2OH* or CH3O* pathway by breaking the C–H bond or the O–H bond. In addition, CHO* and COH* are the mainly controversial intermediates [24,30,40,41]. Therefore, the overall reaction network including all the possible elementary steps is mapped out systematically, as depicted in Fig. S16. The reaction free energies of each electrochemical element step were calculated by using the computational hydrogen electrode (CHE) model. The energy profiles and optimized structures in methanol dehydrogenation on two surfaces are displayed in Fig. 4b and c. On the Rh-decorated Pt-skin Pt3Co0.5Ni0.5(111) surface, Rh atom is the most favorable site for all the intermediates, while the intermediates prefer to adsorb on the Pt atom that close to the Co and Ni atoms in the sublayer on the Pt-skin Pt3Co0.5Ni0.5(111) surface. It is notable that the Rh-decorated Pt-skin Pt3Co0.5Ni0.5(111) is more energetically favorable than the Pt-skin Pt3Co0.5Ni0.5(111) surface. For example, the Eads for CH3OH → CH3OH* on the Rh decorated Pt-skin Pt3Co0.5Ni0.5(111) is much stronger than on the Pt-skin Pt3Co0.5Ni0.5(111) (Table S2), suggesting that CH3OH on Rh-decorated Pt-skin Pt3Co0.5Ni0.5(111) are likely to have smaller barriers to be activated than that on the Pt-skin Pt3Co0.5Ni0.5(111) surface. This is due to the enhanced binding strengths between the intermediate adsorbates and the Rh atoms on the Rh-decorated Pt-skin Pt3Co0.5Ni0.5(111). These computational results convincingly support the experimental observations that the activity of MOR is improved after incorporating Rh in the Pt-based alloy NWs. Furthermore, the initial dehydrogenation of methanol on the Pt-skin Pt3Co0.5Ni0.5(111) surface prefers to break O–H bond generating CH3O* rather than C–H bond generating CH2OH*. While on the Rh-decorated Pt-skin Pt3Co0.5Ni0.5(111), it is easier to break the C–H bond and generate CH2OH*. The C–H bonds in CH3O* and CH2OH* break subsequently to form CH2O* and CHOH*, respectively. Furtherly, the CHO* can be generated by breaking one C–H bond in CH2O* and the O–H bond in CHOH*. CHO* generated from the C–H bond dissociation occurring in CH2O* is more energetic on the Pt-skin Pt3Co0.5Ni0.5(111) surface than that from 10

the O–H bond dissociation in CHOH* on the Rh-decorated Pt-skin Pt3Co0.5Ni0.5(111) surface. However, due to COH* is considerably stable on the later one, CHOH* prefer to break the C–H bond and resulting in the COH* formation. Therefore, MOR on the Rh-free Pt-skin Pt3Co0.5Ni0.5(111) surface undergoes path 2, i.e., CH3OH → CH3O* → CH2O* → CHO* → CO*, while MOR on the Rh-decorated Pt-skin Pt3Co0.5Ni0.5(111) surface prefers path 1, i.e., CH3OH →CH2OH* → CHOH* → COH* → CO* (Fig. S17 and Table S3 and 4). The CO* then interacts with surface hydroxyl (OH*) to form COOH. Notably, the formation of COOH* intermediates by CO* oxidation (CO*+OH* → COOH*) is the rate limiting step for both samples. The Rh-decorated Pt-skin Pt3Ni0.5Co0.5(111) have a lower COOH* formation energy than the pristine Pt-skin Pt3Ni0.5Co0.5(111), which can be ascribed to that the surface Rh atoms possess the ability to provide more surface OH* than Pt [30,44,45], favoring the further oxidation of CO*. The new formed COOH* prefers to adsorb on Rh site rather than Pt site with an H-down configuration, and releases a (H+ + e-) pair to form CO2* subsequently and release CO2 eventually. From above theoretical analysis, the interatomic Pt–Rh bridge sites on the Rh-decorated Pt-skin Pt3Co0.5Ni0.5(111) surface can act as active centers to accelerate the oxidation of COads more efficiently than the sole-Pt sites on the pristine Pt-skin Pt3Ni0.5Co0.5(111). Overall, the integral MOR activation energy barrier is reduced on the Rh-decorated Pt-skin Pt3Co0.5Ni0.5(111) surface with boosted electrocatalytic activity.

4. Conclusion In summary, 1.5-nm-thin quatermetallic PtCoNiRh NWs with high atomic exposure and interatomic Pt–Rh sites are successfully fabricated to serve as highly active and durable MOR electrocatalysts. The ultrathin 1D feature of these PtCoNiRh NWs can be well stabilized during the acidic MOR catalysis owing to the anticorrosion of Rh and its stabilizing effect towards surrounding Pt atoms. Compared to Rh-free PtCoNi NWs and most of the state-of-the-art electrocatalysts, the PtCoNiRh NWs exhibit a much superior MOR catalytic performance, achieving 1.36 A·mg-1Pt and 2.08 mA·cm-2 in mass activity and specific activity, respectively. Electrochemical in situ FTIR study certifies the existence of Rh can modulate 11

the intermediate CO binding from linear to bridge mode, facilitating the removal of COads and thus improving the MOR CO-tolerance. DFT calculation further identifies the interatomic Pt– Rh sites are the thermodynamically favored sites for CH3OH adsorption and dehydrogenation, as well as for the intermediate COB binding and removal. Integratedly, the PtCoNiRh NWs expressly facilitate the tandem MOR catalytic network with substantially improved stability. This work presents a powerful strategy to synthesize stable Pt-based alloy ultrathin NWs and demonstrates a reliable investigation from the fundamental understanding of active site and reaction mechanism to achievement of highly robust catalysts.

Conflict of Interest The authors declare no conflict of interest.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21771067 and 21931009), the Natural Science Foundation of Fujian Province (Distinguished Young Investigator, 2017J06005), the Program for New Century Excellent Talents in Fujian Province University and the Scientific Research Funds of Huaqiao University. We also thank the Instrumental Analysis Center of Huaqiao University for analysis support.

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:

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Figure 1. Structure and composition analysis of the PtCoNiRh NWs. (a) HAADF-STEM image; (b) TEM image; (c) atomic-resolution aberration corrected HAADF-STEM image; (d) STEM-EDS mapping; (e) STEM-EDS line-scanning profile; (f) XRD pattern; (g) Rh 3d and Pt 4d XPS of PtCoNiRh NWs; and (h) Pt 4d XPS of PtCoNi NWs.

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Figure 2. Electrocatalysis of the PtCoNiRh NWs/C, PtCoNi NWs/C, and Pt/C for MOR. (a) CO stripping performed between -0.25 and 0.95 V vs. SCE at a scan rate of 20 mV s-1 in 0.1 M HClO4 solution; (b) MOR CVs normalized to the Pt mass recorded in a 0.1 M HClO4 + 0.5 M CH3OH solution at 50 mV s-1, the insert showing the MOR onset potentials; (c) histograms of specific and mass activities; (d) comparison of the mass activity of the PtCoNiRh NWs with the representative reported Pt-based MOR catalysts; (e) comparison of the MOR mass activity before and after CO purging in 0.1 M HClO4 and 3 M CH3OH solution; and (f) chronoamperometric (i-t) measurements at 0.5 V vs. SCE for 10,000 s.

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Figure 3. In situ FTIR spectra of MOR on (a) PtCoNi NWs/C and (b) PtCoNiRh NWs/C; (c) plots of the integral intensities of the COL and COB species as a function of potential; (d) in situ FTIR spectra for CO adsorption on the PtCoNi NWs/C and PtCoNiRh NWs/C at -0.2 V (vs. SCE).

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Figure 4. DFT calculation of COads configuration and MOR mechanisms on the model catalyst surfaces. (a) COads configurations and energies on both Pt-skin Pt3Co0.5Ni0.5(111) and Rh-decorated Pt-skin Pt3Co0.5Ni0.5(111) surfaces. (b,c) Free energy diagrams and optimized structures of the intermediates in MOR on both Rh-free Pt-skin Pt3Co0.5Ni0.5(111) surfaces (b) and Rh-decorated Pt-skin Pt3Co0.5Ni0.5(111) surfaces (c) (Pt: yellow, Co: blue, Ni: green, Rh: brown, C: black, O: red, H: white).

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Wei Wang received his B.S. degree in chemical engineering at Qingdao University of Science and Technology in 2015, and M.S. degree in physics at Xiamen University under the supervision of Prof. Shuifen Xie in 2018. He is currently a Ph.D. candidate at National University of Singapore. His research interests include the design and fabrication of novel inorganic nanocatalysts.

Xiaowei Chen received his B.S. degree from department of materials chemistry, Huaqiao University in 2018. Currently, he is pursuing his M.S degree at Huaqiao University under the supervision of Prof. Shuifen Xie. He is interested in the design and synthesis of metallic nanostructures toward electrocatalysis.

Dr. Xue Zhang is currently an assistant researcher in Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences. She got her Ph.D. degree from Xiamen University in 2018 and she went to Eindhoven University of Technology as a visiting Ph.D. student in 2015. Her research is focused on the reaction mechanism of electrocatalysis through DFT calculations and the model catalysts in experiment.

Dr. Jin-Yu Ye received his B.S. degree in chemistry (2010) and Ph.D. degree in physical chemistry (2016) from Xiamen University. Currently, he works as a Research Assistant in Department of chemistry of Xiamen University. His research interests include electrocatalysis and spectro-electrochemistry.

Fei Xue is a currently a Ph.D. candidate in International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power

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Engineering, Xi’an Jiaotong University under the supervision of Prof. Liejin Guo and Prof. Maochang Liu. He acquired his B.E. degree from School of Materials Science and Engineering at Xi’an Jiaotong University in 2014. His current research focuses on fabricating and analyzing the materials for overall water splitting.

Chao Zhen received her B.S. degree from department of chemistry, Hainan Normal University in 2018. She is currently pursuing her M.S degree at Huaqiao University under the supervision of Prof. Shuifen Xie. She is interested in the synthesis of ultrathin 2D metallic nanostructures and their catalytic performances.

Xinyan Liao received her B.S. degree from department of applied chemistry, YiChun University in 2018. Currently, she is pursuing her M.S degree at Huaqiao University under the supervision of Prof. Shuifen Xie. She is interested in the synthesis of noble metal-based nanocrystals and their applications for hydrogen evolution reaction.

Huiqi Li received her B.S. degree from department of chemistry, Heilongjiang University in 2015. Then she continued pursuing her Ph.D. degree at Xiamen University under the supervision of Prof. Zhaoxiong Xie. She is interested in the synthesis of noble metal-based nanocrystals with excavated structures and their applications in catalysis.

Pingting Li received her B.S. degree from department of Polymer materials and engineering, Jilin Institute of Chemical Technology in 2017. She is currently pursuing her M.S. degree at HuaQiao University under the supervision of Prof. Shuifen Xie. Her research interest is focused on the synthesis of Pt-based core-shell nanomaterials and their applications in electrocatalysis and hydrogenation. 23

Dr. Maochang Liu is currently a Full Professor in International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, China. He received his Ph.D. degree in Power Engineering and Engineering Thermal Physics in 2014 from Xi'an Jiaotong University, Xi'an, China. From Sep. 2011 to Sep. 2013, he worked as a visiting scholar at Georgia Institute of Technology with Prof. Younan Xia. His research interest is focused on the controlled synthesis of metal and semiconductor nanoparticles and related application in photocatalysis and electrocatalysis.

Dr Qin Kuang received his B.S. (2001) and Ph.D. (2008) in Chemistry from Xiamen University. He worked as a postdoctoral research fellow at Hong Kong University of Science and Technology from 2011 to 2013 with Prof. Shihe Yang. He was promoted to a professor of Chemistry at Xiamen University in 2017. So far, he has published more than 100 peer-reviewed research journal publications and H index reached 47. His current research is focused on surface/interface engineering of inorganic functional nanomaterials and their applications in the energy and environmental fields.

Dr. Zhaoxiong Xie received his B.S., M. S. and Ph.D. degrees from department of chemistry, Xiamen University in 1987, 1990 and 1995, respectively. During 1997–1998, he was a postdoc in Centre d’Etudes de Saclay, France, and Ulm University, Germany. Since 2002, he holds the position of Professor of physical chemistry at Xiamen University. He won the National Distinguished Young Scientist Fund of China in 2007 and Chang Jiang Chair Professorship in 2014. So far, he has published more than 260 peer-reviewed research journal publications. His current research interests are focused on surface/interfacial chemistry of functional inorganic nanomaterials.

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Dr. Shuifen Xie received his B.S. degree in chemistry (2007) and Ph.D. degree in physical chemistry (2013) from department of chemistry, Xiamen University, China, under the supervision of Prof. Zhaoxiong Xie. From Sep. 2011 to Aug. 2013, he worked as a visiting scholar at Georgia Institute of Technology with Prof. Younan Xia. He is currently a full professor in College of Materials Science and Engineering, Huaqiao University, China. His research interest is focused on the design and synthesis of inorganic nanomaterials for catalytic applications and energy storage/conversion.

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Highlights 1.5-nm-thin quatermetallic PtCoNiRh NWs with high atomic-exposure and interatomic Rh-Pt sites were synthesized. PtCoNiRh NWs/C displayed high activity toward acidic MOR with remarkably improved CO-tolerance and durability. Anticorrosive Rh effectively stabilized the surrounding Pt atoms, intensifying the ultrathin features of PtCoNiRh NWs. Electrochemical in situ FTIR and DFT simulation identified the interatomic Pt–Rh sites as the MOR-active centers.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: