Component-Dependent Electrocatalytic Activity of Ultrathin PdRh Alloy

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Component-Dependent Electrocatalytic Activity of Ultrathin PdRh Alloy Nanocrystals for the Formate Oxidation Reaction Juan Bai, Qi Xue, Yue Zhao, Jia-Xing Jiang, Jing-Hui Zeng, Shibin Yin, and Yu Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06193 • Publication Date (Web): 26 Dec 2018 Downloaded from http://pubs.acs.org on January 2, 2019

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ACS Sustainable Chemistry & Engineering

Component-Dependent Electrocatalytic Activity of Ultrathin PdRh Alloy Nanocrystals for the Formate Oxidation Reaction Juan Bai, † Qi Xue, † Yue Zhao, † Jia-Xing Jiang, † Jing-Hui Zeng,* † Shi-Bin Yin, ‡ and Yu Chen*† †Key

Laboratory of Macromolecular Science of Shaanxi Province, Key Laboratory of Applied Surface and Colloid Chemistry (MOE), Shaanxi Key Laboratory for Advanced Energy Devices, School of Materials Science and Engineering, Shaanxi Normal University, 199 South Chang’an Road, Xi'an 710062, PR China. ‡Collaborative Innovation Center of Sustainable Energy Materials, Guangxi Key Laboratory of Electrochemical Energy Materials, State Key Laboratory of Processing for Non-Ferrous Metal & Featured Materials, Guangxi University, 100 Daxue Road, Nanning 530004, China. * Corresponding authors Email: [email protected] (Yu Chen) and [email protected] (Jing-Hui Zeng).

ABSTRACT: The direct formate fuel cells in alkaline medium is attracting more and more attention due to their reasonable power density, facile power-system, and low formate crossover. Herein, we successfully prepare the XC-72 carbon supported PdRh alloy nanocrystals (PdRh/XC-72 nanocomposites) with different Pd/Rh atomic ratios by precipitation-reduction strategy. The as-prepared Pd5Rh1/XC-72 nanocomposites show the excellent electrocatalytic mass activity (4.5 A mg−1) for the formate oxidation reaction (FOR) in KOH medium, which is 2.3-fold mass activity enhancement over commercial Pd/C anodic catalyst (1.9 A mg−1) for the FOR in KOH medium. The obvious activity enhancement can be ascribed to the high dispersibility, electronic effect, and the suitable OHad species. Meanwhile, the Rh atoms in the PdRh alloy also can weaken the CO accumulation during the FOR and simultaneously enhance the electrochemical self-stability of catalyst, resulting in excellent electrocatalytic durability of Pd5Rh1/XC-72 nanocomposites for the FOR. This work demonstrates that the Pd5Rh1/XC-72 nanocomposites are promising high-performance electrocatalysts for the FOR. KEYWORDS: fuel cells, PdRh alloy, precipitation-reduction, formate oxidation reaction, electrocatalysts

INTRODUCTION Up to now, great concerns have been paid to the fuel cells which can effectively convert chemical energy of fuel molecules into electricity with high energy efficiency using hydrogen gas or organic molecules as fuel sources.1-11 Among them, direct formic acid fuel cells (DFAFCs) have preferential commercial applications in portable electronic products due to the low fuel crossover and high energy density.12-16 Compared with DFAFCs in acid medium, the direct formate fuel cells (DFFCs) in alkaline medium have accelerated electrode reaction kinetics.17-18 At the anode, the formate oxidation reaction (FOR) undergoes the possible electrode reaction of HCOO- + 3OH- → CO32- + 2H2O + 2e–.19 The thermodynamic voltage of the DFFCs can reach up to 1.45 V, which is much higher than that of the hydrogen-oxygen, methanol and ethanol fuel cells.20-21 At present, Pd-based and Pt-based catalysts have been considered to be the two types of catalysts with the most performance advantages for the DFFCs. Due to the different reaction mechanisms, no CO intermediates poisoning issue exists at Pd surface but CO intermediates poisoning occurs at Pt surface during the FOR.22-23 Compared to Pt catalysts, Pd catalysts can present more efficient catalytic activities for FOR. And recent studies have demonstrated that the FOR current at Pd anodic catalyst surpasses that at Pt anodic catalyst.23 So improving

catalytic performance of Pd-based anodic catalyst is important for the commercial promotion of the DFFCs. Till now, lots of efforts are committed to improving the activity and durability of Pd-based anodic catalysts.24-25 At present, designing Pd-based alloy catalysts has been proved to be an efficient approach for improving activity and stability.26-29 However, the preparation of bimetallic Pd alloy catalysts still faces challenge because of the discrepancy in standard potentials of the different metal precursors. Therefore, we developed a simple and effective precipitation-reduction strategy to prepare the alloy nanocrystals, which not only could improve the alloying degree but also prevent the nanoparticles aggregation.30 Recent investigations demonstrated that the introduction of Rh could provide aboudant spectator species (OHad) for the small organic molecules electrooxidation and simultaneously ehance the electrochemical stability due to the oxophilic character of Rh. 31-36 Thus, we expect that PdRh alloy catalyst may show the high activity and stability for the FOR. To our knowledge, no investigations about the FOR on PdRh alloy catalyst is reported till now. The electrocatalytic performance not only relates to their chemical composition of bimetallic catalysts but also highly depends on supporting materials.37-44 Generally, supporting bimetallic alloy nanocrystals on carbon nanomaterials can reduce the particle size of nanoparticles, obstruct the agglomeration of nanoparticles and raise the utilization rate of noble metals.45-46 At

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present, XC-72 carbon has been regarded as one of ideal support materials because of the high electrical conductivity, big surface area, and extensive commercialization.47 Here, we successfully synthesized the XC-72 carbon supported PdRh alloy nanocrystals (PdRh/XC-72 nanocomposites) with different atomic ratios using an effective precipitation-reduction strategy. For the FOR, the as-prepared Pd5Rh1/XC-72 nanocomposites with Pd/Rh atomic ratio of 5:1 exhibited notably enhanced catalytic performance compared to other PdRh/XC-72 nanocomposites and commercial Pd/C anodic catalyst.

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electron microscopy (TEM) were performed from a TECNAI G2 F20 microscopy. X-ray photoelectron spectroscopy (XPS) spectra were obtained by AXIS ULTRA spectrometer. Power X-Ray diffraction (PXRD) was investigated from a DX-2700 X-ray diffractometer. The surface charge of sample was measured by Nano ZS90 zeta potential analyzer. Thermogravimetric analysis (TGA) tests were performed on Q600 thermoanalyzer under air atmosphere.

RESULTS AND DISCUSSION Characterization of Pd5Rh1/XC-72 nanocomposites. The Pd5Rh1/XC-72 nanocomposites were prepared by a simple homogeneous precipitation reduction method (Scheme 1). Due to the weak alkalinity of Na2CO3, PdCl2 and RhCl3 transform slowly into the PdO and Rh(OH)3·4H2O precipitate, which anchor uniformly on the XC-72 carbon surface (Step A in Scheme 1). Then, PdO and Rh(OH)3·4H2O nanocrystals are reduced to PdRh alloy with NaBH4, which produces the highly dispersed Pd5Rh1/XC-72 nanocomposites (Step B in Scheme 1).

EXPERIMENTAL SECTION Reagents and Chemicals. Commercial Pd/C (30 wt% of Pd) catalyst was obtained from Johnson Matthey Corporation. XC-72 carbon was purchased from USA Massachusetts. Rhodium (III) chloride hydrate (RhCl3·3H2O), potassium hydroxide (KOH), and palladium (II) chloride (PdCl2) were obtained from Aladdin Industrial Co. Sodium carbonate anhydrous (Na2CO3) and potassium formate (HCOOK) were obtained from Sinopharm Chemical Reagent Co., Ltd.

Synthesis of Pd5Rh1/XC-72 nanocomposites. Typically, 0.17 mL of RhCl3 solution (0.05 M) and 0.83 mL of PdCl2 solution (0.05 M) were slowly dripped into 10 mL of XC-72 carbon suspension (1.24 g L-1). Then, the pH value of the mixture was adjusted to 9.0 by using Na2CO3 solution, and the mixture was heated at 60 °C for 5 hours. After that, the reductive reagent NaBH4 (0.189 g) was added into the reaction suspension and stirred for 1 hour. Finally, Pd5Rh1/XC-72 nanocomposites were obtained by centrifugation at 10000 rpm 15 minutes, washed three times by ultrapure water and dried. For comparison, Pd/XC-72 nanocomposites, Rh/XC-72 nanocomposites, Pd3Rh1/XC-72 nanocomposites, and Pd7Rh1/XC-72 nanocomposites were also fabricated by using the same method. The conventional Pd5Rh1/XC-72 nanocomposites (Pd5Rh1/XC-72-conventional) were synthetized by traditional NaBH4 reduction method, which are obtained by directly adding NaBH4 into the mixed solution of PdCl2, RhCl3, and XC-72 carbon. Electrochemical Characterization. The cyclic voltammetry (CV) tests were gathered to evaluate the electrocatalytic activity of the catalysts for the FOR. Chronoamperometry tests were then used to examine the electrocatalytic durability of the catalysts. Both the CV and chronoamperometry tests were performed on a electrochemical workstation (CHI 760 D) at room temperature with a three-electrode system of a carbon rod as auxiliary electrode, a saturated calomel electrode as reference electrode and an catalyst modified glassy carbon electrode (GCE) as working electrode. The catalyst suspension was obtained by mixing 4 mg of catalyst and 2 mL water containing 5 μL of 5 wt % Nafion under strong sonication for 30 min. Then, 4 μL catalyst suspension was dropped carefully on GCE surface and dried at 30 oC. All electrode potentials were converted to the reversible hydrogen electrode (RHE). Instruments. Energy-dispersive X-ray (EDS) elemental maps, selected area electron diffraction (SAED), and transmission

Scheme 1 The synthetic route of Pd5Rh1/XC-72 nanocomposites. The morphology of Pd5Rh1/XC-72 nanocomposites was measured by TEM (Figure 1A and B). TEM images present the highly dispersed nanocrystals are highly uniform distributed on the XC-72 carbon surface. The distribution histogram also displays that the narrow size distribution characteristic of the nanoparticles with an average particle size of about 2.4 nm (Inset in Figure 1B). The EDS pattern shows that the products contain the Pd and Rh elements (Figure 1C), and the atomic ratio of Pd/Rh is calculated to be Pd5Rh1. The metal loading at Pd5Rh1/XC-72 nanocomposites was determined by TGA (Figure S1), which shows the Pd and Rh loading at Pd5Rh1/XC-72 nanocomposites is ca.30 wt%. The HRTEM image reveals that the Pd5Rh1 nanospheres are well fixed onto the XC-72 carbon surface. The amplified HRTEM image reveals the lattice distance of Pd5Rh1 nanocrystals is 0.220 nm (Figure 1D), slightly smaller than that of the (111) facets of pure Pd metal (0.223 nm), which suggests that Rh atoms have entered in Pd crystal lattice, resulting in the formation of PdRh alloy. The EDS elemental maps were taken to visualize the distribution of the elements to be detected. The patterns of Pd and Rh element are very similar, which confirms the formation of alloy phase (Figure 1E), again.

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two doublets of Pd(0) species and Pd(II) species. The Pd 3d XPS analysis reveals that the percentage of metallic Pd at Pd5Rh1/XC-72 nanocomposites is 85.2%, indicating most of Pd exist as Pd(0). Similarly, the Rh 3d XPS spectrum shows that the Rh(0) in Pd5Rh1/XC-72 nanocomposites is ca. 87.5%, demonstrating the successful reduction of Rh (III) precursor.48-49 Compared with the Pd 3d binding energies of Pd/XC-72 nanocomposites (Figure S3), the Pd 3d binding energy in Pd5Rh1/XC-72 nanocomposites positively shift ca. 0.1 eV, clearly indicating the charge transfer charge transfer between Rh and Pd.

Figure 3. XPS spectra of Pd5Rh1/XC-72 nanocomposites (A) Pd3d and (B) Rh3d.

Figure 1. (A, B) TEM images of Pd5Rh1/XC-72 nanocomposites. (C) EDS spectrum of Pd5Rh1/XC-72 nanocomposites. (D) HRTEM image of Pd5Rh1/XC-72 nanocomposites. (E) STEM image and corresponding element maps of Pd5Rh1/XC-72 nanocomposites. Inset in Figure 1B: Histogram of particle size distribution.

Formation mechanism of Pd5Rh1/XC-72 nanocomposites. The [PdCl4]2− and Rh3+ ions are highly stable in acidic medium. After adjusting the pH value of the [PdCl4]2− and the Rh3+ ion solutions from 1.25 to 9 by using Na2CO3 solution, slowly hydrolysis processes then take place and constantly to form the yellow Rh(OH)3·4H2O precipitate (Figure 4AB) and brownish-red PdO precipitate (Figure 4CD) due to the hydrolysis, respectively. If the solution pH value is less than 9, [PdCl4]2− and Rh3+ ions can’t be hydrolyzed and turn into PdO and Rh(OH)3·4H2O precipitates.50-51 Thus, the pH value of reaction solution is selected as 9. XC-72 carbon is positive charge (zeta potential: −21.3 mV), which results in the anchorage of PdO (zeta potential: +10.1 mV) and Rh(OH)3·4H2O (zeta potential: +18 mV) precipitates due to the electrostatic interaction. TEM images display that ultrafine PdO/Rh(OH)3·4H2O nanocrystals uniformly anchor on the XC-72 carbon surface (Figure 4EF). And EDS elemental maps are also taken to demonstrate the elemental distribution. The patterns of Pd and Rh element are similar to that of C element, further confirming the uniform dispersion of PdO/Rh(OH)3·4H2O nanocrystals on the XC-72 carbon surface (Figure 4G). Evidently, the uniform anchorage of PdO/Rh(OH)3·4H2O nanocrystals on the XC-72 carbon surface contributes to the high dispersion of Pd5Rh1/XC-72 nanocomposites. As comparison, Pd5Rh1/XC-72-conventional nanocomposites seriously aggregate (Figure S4), which in turn indicates the precipitation-reduction strategy is practicable for the synthesis of high-quality Pd5Rh1/XC-72 nanocomposites.

PXRD measurement was used to determine the structure and crystal phase of Pd5Rh1/XC-72 nanocomposites (Figure 2A). PXRD pattern of Pd5Rh1/XC-72 nanocomposites display four obvious diffraction peaks at 40.2, 46.2, 68.1 and 82.4 (2θ angles) associated with the (111), (200), (220) and (311) planes of the face center cubic structured Pd, respectively. Meanwhile, there are no separated Pd and Rh reflection peaks. Additionally, the diffraction peaks of Pd5Rh1/XC-72 nanocomposites shift a little to higher angles according to the standard PDF card of Pd (PDF No. 87-0643) (Figure 2B). Thus, PXRD measurement reveals the Rh atoms have entered in the Pd lattice to generate PdRh alloy instead of heterostructures, which is coincident with the result of lattice-spacing of HRTEM analysis.

Figure 2. (A) PXRD pattern of Pd5Rh1/XC-72 nanocomposites. (B) Enlarged PXRD pattern. The surface composition and electronic structure of Pd5Rh1/XC-72 nanocomposites were measured by XPS (Figure 3). XPS data reveals that molar ratio of Pd/Rh at Pd5Rh1/XC-72 nanocomposites surface is 5:1 (Figure S2), in consistent with EDS results. For Pd5Rh1/XC-72 nanocomposites, two strong peaks of Pd emerge from Pd 3d3/2 and pd 3d5/2, which are divided into 3



The CV curve of Rh/XC-72 nanocomposites and Pd/XC-72 nanocomposites show that peak of Pd oxide reduction of is in 0.63 V, but there is no characteristic reduction peaks of Rh oxide (Figure S10). Thus, the reduction peak of PdRh/XC-72 nanocomposites is skewed towards Pd standard peak with the increase of Pd content in Figure 5A. The FOR activities of different catalysts were inspected by CV in an mixed aqueous solution with 1 M KOH and 1 M HCOOK (Figure 5B). Mass activity displays that the onset potential of Pd5Rh1/XC-72 nanocomposites for the FOR is similar to that of Pd3Rh1/XC-72 and Pd7Rh1/XC-72 nanocomposites. At 0.68 V, the FOR current of Pd5Rh1/XC-72 nanocomposites is 4.5 A mg−1, which is 1.25 times higher than that at Pd3Rh1/XC-72 nanocomposites (3.6 A mg−1) and 1.61 times higher than that at Pd7Rh1/XC-72 nanocomposites (2.8 A mg−1). The electrochemically active surface areas (ECSAs) of PdRh/XC-72 nanocomposites were mearsured by CO-stripping method based on a hypothetical single layer adsorption (Figure 5C).52 ECSA value of Pd3Rh1/XC-72 nanocomposites, Pd5Rh1/XC-72 nanocomposites and Pd7Rh1/XC-72 nanocomposites are 52.6, 55.4 and 57.4 m2 g-1 according to the equation ECSA= QCO/mC (C = 0.42 mC cm−2, QCO: the oxidation charge for CO, m: the loading amount). After upgrading mass activity to specific activity (Figure 5D), the FOR current density at Pd5Rh1/XC-72 nanocomposites are still greater than that at Pd5Rh1/XC-72 and Pd7Rh1/XC-72 nanocomposites, indicating that Pd5Rh1/XC-72 nanocomposites have the best FOR electrocatalytic activity among these PdRh/XC-72 nanocomposites. We further investigated the FOR activity of Rh/XC-72 nanocomposites and Pd/XC-72 nanocomposites by CV (Figure S11). Rh/XC-72 nanocomposites do not show obvious catalytic activity for the FOR. However, Pd5Rh1/XC-72 nanocomposites show better catalytic activity for the FOR than Pd/XC-72 nanocomposites (Figure S12), demonstrating that the introduction of Rh improve catalytic activity of Pd. The FOR on Pd/C electrode undergoes a series of reactions in alkaline medium, including HCOOad− → Had+ COOad-, COOad- → CO2+ e−, Had+OHad-→H2O + e−, and so on. The formation of PdRh alloy can modulate the electronic property of metal Pd. XPS spectra reveal that the Pd 3d binding energy of Pd5Rh1/XC-72 nanocomposites has a slight positive shift compared to Pd/XC-72 nanocomposites, implying the electronic charge transfer between Pd atoms and Rh atoms. The tuned electronic structure can the reduce the affinity of oxygen containing species on Pd surface.53 The researcher have proved that the weak adsorption of HCOOad− facilitates the FOR.54 Consequently, the obvious activity enhancement of Pd5Rh1/XC-72 nanocomposites partly originates from the electronic effect. According to reaction equations, the FOR activity of catalyst also relates to OHad- species. CV measurements have revealed the hydroxyl desorption peak potential of PdRh/XC-72 nanocomposites negtively shifts as the Rh content increases (Figure 5A), indicting that introduction of Rh can lower the formation potential of OHad- species. Thus, the enhanced interaction between Had and OHad- species also makes a special contribution for the FOR activity enhancement.

Figure 4. (A) Optical digital images of Rh3+ ion solution (a) before and (b) after adjusting pH value to 9.0. (B) PXRD pattern of Rh(OH)3·4H2O in Figure 4Ab. (C) Optical digital images of the [PdCl4]2− ion solution (a) before and (b) after adjusting pH value to 9.0. (D) PXRD pattern of PdO in Figure 4Cb. (EF) TEM images of PdO- Rh(OH)3·4H2O/XC-72 nanocomposites. (G) STEM image and corresponding element maps of PdORh(OH)3·4H2O/XC-72 nanocomposites.

Electrocatalytic activity of Pd5Rh1/XC-72 nanocomposites. Considering the activity of catalysts is strongly relates to their chemical composition, PdRh/XC-72 nanocomposites with different atomic ratios have been prepared by the same precipitation-reduction strategy. TEM images show the average particle sizes of Pd3Rh1/XC-72 nanocomposites and Pd7Rh1/XC-72 nanocomposites are very similar to Pd5Rh1/XC-72 nanocomposites (Figure S5). PXRD patterns that the diffraction peak of Pd3Rh1/XC-72, Pd5Rh1/XC-72 and Pd7Rh1/XC-72 nanocomposites is skewed towards Pd standard peaks with the increase of Pd content (Figure S6), confirming alloy formation. Taking Pd3Rh1/XC-72 nanocomposites as an example, it is observed that the diffraction peak of Pd3Rh1/XC-72 nanocomposites is much higher than Pd3Rh1/XC-72-conventional (Figure S7), which suggests the present precipitation-reduction strategy facilitates the generation of PdRh alloy. The as-prepared Pd3Rh1/XC-72 nanocomposites, Pd5Rh1/XC-72 nanocomposites and Pd7Rh1/XC-72 nanocomposites were used as catalysts for the FOR in KOH medium. Initially, the electrochemical properties of Pd3Rh1/XC-72, Pd5Rh1/XC-72 and Pd7Rh1/XC-72 nanocomposites were investigated by CV in KOH medium. CV curves show that the reduction peak area of Pd oxide increases with the Pd content and reduction peak potential of Pd oxide at PdRh/XC-72 nanocomposites positively shifts as the Pd content increases (Figure 5A). In order to understand this phenomenon, Pd/XC-72 nanocomposites and Rh/XC-72 nanocomposites were also fabricated by using the same synthetic process. TEM images show the highly monodisperse Pd and Rh nanoparticles are uniformly fixed on the surface of XC-72 carbon (Figure S8). The crystalline structure of the Pd/XC-72 nanocomposites and Rh/XC-72 nanocomposites were analyzed by PXRD (Figure S9). 4


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Figure 5. (A) CV curves of Pd3Rh1/XC-72 nanocomposites, Pd5Rh1/XC-72 nanocomposites and Pd7Rh1/XC-72 nanocomposites in N2-saturated 1 M KOH electrolyte. (B) Mass activity of Pd3Rh1/XC-72 nanocomposites, Pd5Rh1/XC-72 nanocomposites and Pd7Rh1/XC-72 nanocomposites in N2-saturated 1 M KOH +1 M HCOOK electrolyte. (C) CV curves of the adsorbed CO in 1 M KOH electrolyte at Pd3Rh1/XC-72 nanocomposites, Pd5Rh1/XC-72 nanocomposites and Pd7Rh1/XC-72 nanocomposites catalyst. (D) Specific activity of Pd3Rh1/XC-72 nanocomposites, Pd5Rh1/XC-72 nanocomposites and Pd7Rh1/XC-72 nanocomposites in N2-saturated 1 M KOH +1 M HCOOK electrolyte.

Figure 6. (A) CV curves of Pd5Rh1/XC-72 nanocomposites and Pd/C catalyst in N2-saturated 1 M KOH electrolyte. (B) Mass activity of Pd5Rh1/XC-72 nanocomposites and Pd/C catalyst in N2-saturated 1 M KOH +1 M HCOOK electrolyte. (C) CV curves of the adsorbed CO in 1 M KOH electrolyte at Pd5Rh1/XC-72 nanocomposites and Pd/C catalyst. (D) Specific activity of Pd5Rh1/XC-72 nanocomposites and Pd/C catalyst in N2-saturated 1 M KOH +1 M HCOOK electrolyte. The durability of Pd5Rh1/XC-72 nanocomposites and Pd/C catalyst for the FOR were performed by chronoamperometry measurements. Throughout the entire process, FOR current at Pd5Rh1/XC-72 nanocomposites are higher than that at Pd/C anodic catalyst, demonstrating that Pd5Rh1/XC-72 nanocomposites have superior activity than Pd/C catalyst. After 6000s, FOR current at Pd5Rh1/XC-72 nanocomposites maintain 408.2 A g−1. In contrast, EOR current at Pd/C catalyst only preserve 15.7 A g−1. This result indicates Pd5Rh1/XC-72 nanocomposites have better durability for the FOR. Indeed, CO-stripping measurements have revealed the onset potential and peak potential of COads oxidation on Pd5Rh1/XC-72 nanocomposites negatively shift 200 and 70 mV compared to those on Pd/C catalyst (Figure 6C), indicating Pd5Rh1/XC-72 nanocomposites have better anti-poison performance than Pd/C catalyst. The improved anti-poison performance can be ascribed to the higher oxophilicity of Rh relative to Pd, which accelerates the COads oxidation.55 Indeed, we also performed the accumulated COads-stripping CV tests after the chronoamperometry tests (Figure 7B). The oxidation charge value of accumulated COads at Pd5Rh1/XC-72 nanocomposites is 1.65 times lower than that at Pd/C catalyst, further confirming Pd5Rh1/XC-72 nanocomposites have improved anti-poison performance. Consequently, the anti-poison capability enhancement contributes to superior durability of Pd5Rh1/XC-72 nanocomposites for the FOR. After chronoamperometry tests, TEM image shows that the morphology of Pd5Rh1/XC-72 nanocomposites is maintained well (Figure S13). And PXRD (Figure S14) and EDS (Figure S15) measurements further demonstrate the alloy structure and chemical composition of Pd5Rh1/XC-72 nanocomposites retain constant after chronoamperometry test. As reported on previously work, the extreme chemical inertness of Rh can remarkably suppress the Rh dissolution and Ostwald ripening effect of

In order to evaluate the practical application of Pd5Rh1/XC-72 nanocomposites, we further compare its electrochemical performance with commercial Pd/C catalyst. CV curves show the hydroxyl desorption peak potential of Pd5Rh1/XC-72 nanocomposites negatively shifts by 76 mV compared to Pd/C catalyst in KOH medium (Figure 6A), which can be stemed from the addition of Rh. The FOR performance of catalysts was evaluated by CV in KOH electrolyte containing HCOOK (Figure 6B). The mass activity reveals Pd5Rh1/XC-72 nanocomposites achieve 2.3 times higher FOR current (4.5 A mg−1) at 0.68 V than Pd/C catalyst (1.9 A mg−1). Meanwhile, the FOR peak potential at Pd5Rh1/XC-72 nanocomposites negatively shifts 30 mV relative to Pd/C catalyst. Thus, the lower FOR onset potential and higher FOR current reveal that Pd5Rh1/XC-72 nanocomposites have better electrocatalytic performance than Pd/C catalyst. To obtain the ECSA value of Pd5Rh1/XC-72 nanocomposites and Pd/C catalyst, CO-stripping measurements were performed. The ECSA of Pd5Rh1/XC-72 nanocomposites is determined to be 55.4 m2 g-1, which is 1.45 times larger than that of commercial Pd/C catalyst (40.2 m2 g-1). Furthermore, the specific activity (81.2 A·m-2) of Pd5Rh1/XC-72 nanocomposites is still higher than that of Pd/C catalyst (48.8 A m-2), indicating the intrinsic activity enhancement.

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ACKNOWLEDGMENT

Rh-based nanoparticles.56 Thus, the excellent self-stability of Pd5Rh1/XC-72 nanocomposites also contributes to their superior durability for the FOR.

This research was sponsored by National Natural Science Foundation of China (21875133), the Fundamental Research Funds for the Central Universities (GK201602002, 2018CSLZ010 and GK201701007) and the 111 Project (B14041).

REFERENCES 1. Schmies, H.; Bergmann, A.; Drnec, J.; Wang, G.; Teschner, D.; Kühl, S.; Sandbeck, D. J. S.; Cherevko, S.; Gocyla, M.; Shviro, M.; Heggen, M.; Ramani, V.; Dunin-Borkowski, R. E.; Mayrhofer, K. J. J.; Strasser, P. Unravelling Degradation Pathways of Oxide-Supported Pt Fuel Cell Nanocatalysts under In Situ Operating Conditions. Adv. Energy Mater. 2018, 8, 1701663, DOI 10.1002/aenm.201701663. 2. Feng, Y.; Liu, H.; Yang, J. A Selective Electrocatalyst–Based Direct Methanol Fuel Cell Operated at High Concentrations of Methanol. Sci. Adv. 2017, 3, e1700580, DOI 10.1126/sciadv.1700580. 3. Stephens, I. E. L.; Rossmeisl, J.; Chorkendorff, I. Toward Sustainable Fuel Cells. Science 2016, 354, 1378-1379, DOI 10.1126/science.aal3303. 4. Zhang, Q.; Jiang, L.; Wang, H.; Liu, J.; Zhang, J.; Zheng, Y.; Li, F.; Yao, C; Hou, S. Hollow Graphitized Carbon Nanocage Supported Pd Catalyst with Excellent Electrocatalytic Activity for Ethanol Oxidation. ACS Sustainable Chem. Eng. 2018, 6, 7507−7514, DOI 10.1021/acssuschemeng.8b00208. 5. Duan, C.; Kee, R. J.; Zhu, H.; Karakaya, C.; Chen, Y.; Ricote, S.; Jarry, A.; Crumlin, E. J.; Hook, D.; Braun, R. Highly Durable, Coking and Sulfur Tolerant, Fuel-Flexible Protonic Ceramic Fuel Cells. Nature 2018, 557, 217, DOI 10.1038/s41586-018-0082-6. 6. Fu, G.; Yan, X.; Chen, Y.; Xu, L.; Sun, D.; Lee, J. M.; Tang, Y. Boosting Bifunctional Oxygen Electrocatalysis with 3D Graphene Aerogel-Supported Ni/MnO Particles. Adv. Mater. 2018, 30, 1704609, DOI 10.1002/adma.201704609. 7. Cano, Z. P.; Banham, D.; Ye, S.; Hintennach, A.; Lu, J.; Fowler, M.; Chen, Z. Batteries and Fuel Cells for Emerging Electric Vehicle Markets. Nat. Energy 2018, 3, 279, DOI 10.1038/s41560-018-0108-1. 8. Wang, H.; Yin, S.; Xu, Y.; Li, X.; Alshehri, A. A.; Yamauchi, Y.; Xue, H.; Kaneti, Y. V.; Wang, L. Direct Fabrication of Tri-metallic PtPdCu Tripods with Branched Exteriors for the Oxygen Reduction Reaction. J. Mater. Chem. A 2018, 6, 8662-8668, DOI 10.1039/c8ta01698d. 9. Ghobadi, S.; Şanlı, L.; Bakhtiari, R.; Gürsel, S. Green Composite Papers via Use of Natural Binders and Graphene for PEM Fuel Cell Electrodes. ACS Sustainable Chem. Eng. 2017, 5, 8407−8415, DOI 10.1021/acssuschemeng.7b02207. 10. Zhang, Y.; Li, X.; Li, K.; Xue, B.; Zhang, C.; Du, C.; Wu, Z.; Chen, W. Novel Au Catalysis Strategy for the Synthesis of Au@Pt Core-Shell Nanoelectrocatalyst with Self-Controlled Quasi-Monolayer Pt Skin. ACS Appl. Mater. Interfaces 2017, 9, 32688-32697, DOI 10.1021/acsami.7b08210. 11. Li, Q.; Wang, T.; Havas, D.; Zhang, H.; Xu, P.; Han, J.; Cho, J.; Wu, G. High-Performance Direct Methanol Fuel Cells with Precious-Metal-Free Cathode. Adv. Sci. 2016, 3, 1600140, DOI 10.1002/advs.201600140. 12. Joo, J.; Uchida, T.; Cuesta, A.; Koper, M. T.; Osawa, M. Importance of Acid-Base Equilibrium in Electrocatalytic Oxidation of Formic Acid on Platinum. J. Am. Chem. Soc. 2013, 135, 9991-9994, DOI 10.1021/ja403578s. 13. Liu, H.; Feng, Y.; Cao, H.; Yang, J. Pt-Containing Ag2S-Noble Metal Nanocomposites as Highly Active Electrocatalysts for the Oxidation of Formic Acid. Nano Micro Lett. 2014, 6, 252-257, DOI 10.5101/nml140027a. 14. Lu, Y.; Chen, W. PdAg Alloy Nanowires: Facile One-Step Synthesis and High Electrocatalytic Activity for Formic Acid Oxidation. ACS Catal. 2012, 2, 84-90, DOI 10.1021/cs200538g. 15. Qiu, X.; Zhang, H.; Wu, P.; Zhang, F.; Wei, S.; Sun, D.; Xu, L.; Tang, Y. One-Pot Synthesis of Freestanding Porous Palladium Nanosheets as Highly Efficient Electrocatalysts for Formic Acid Oxidation. Adv. Funct. Mater. 2017, 27, 1603852, DOI 10.1002/adfm.201603852.

Figure 7. (A) Chronoamperometric curves of Pd5Rh1/XC-72 nanocomposites and Pd/C catalyst in N2-saturated 1 M KOH +1 M HCOOK electrolyte at 0.68 V potential. (B) COads-stripping CV curves of Pd5Rh1/XC-72 nanocomposites and Pd/C catalyst in the 1 M KOH electrolyte after running chronoamperometry tests in 1 M HCOOH + 1 M KOH at 0.68 V for 6000s.

CONCLUSIONS We developed a precipitation-reduction strategy to successfully prepare Pd5Rh1/XC-72 nanocomposites with ultrafine size, clean surface and high dispersion. During the synthesis, the electrostatic interaction between XC-72 carbon and PdO/Rh(OH)3·4H2O precipitates lead to well-distributed anchorage of Pd2+ and Rh3+ precursors, which contributed to the high dispersion of PdRh alloy nanoparticles on XC-72 carbon surface. When as-prepared PdRh/XC-72 nanocomposites with different Pd/Rh atomic ratios were explored as the FOR electrocatalysts in KOH medium, Pd5Rh1/XC-72 nanocomposites reveled the best electrocatalytic activity for the FOR due to the high dispersibility, electronic effect, and the suitable OHad species. Meanwhile, the addition of Rh also resulted in less CO accumulation during the FOR due to bifunctional mechanism, which effectively enhanced the durability of Pd5Rh1/XC-72 nanocomposites for the FOR. Overall, Pd5Rh1/XC-72 nanocomposites were a highly promising electrocatalyst in DFFCs.

ASSOCIATED CONTENT Supporting information Additional information for TEM images of Pd3Rh1/ XC-72, Pd5Rh1/ XC-72, Pd7Rh1/XC-72, Pd/ XC-72, Rh/ XC-72 and Pd5Rh1/ XC-72-conventional nanohybrids. PXRD patterns of Pd3Rh1/ XC-72, Pd5Rh1/ XC-72, Pd7Rh1/XC-72, Pd/ XC-72, Rh/ XC-72 and Pd5Rh1/ XC-72-conventional nanohybrids. Metal mass-normalized of Pd5Rh1/XC-72, Rh/ XC-72 and Pd/XC-72 in N2-saturated 1 M KOH + 1 M HCOOK solution. TGA curve and XPS survey scan of Pd5Rh1/XC-72 nanohybrids. TEM images, XRD patterns and EDS spectrum of Pd5Rh1/XC-72 nanohybrids after chronoamperometry tests.

AUTHOR INFORMATION Corresponding Authors *

Email: [email protected] [email protected] (J.-H. Zeng)

(Y.

Chen)

and

Notes The authors declare no competing financial interests. 6


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Table of Contents artwork

Pd5Rh1/XC-72 nanocomposites are prepared by precipitation-reduction strategy, which reveal the enhanced electrocatalytic activity and stability for the formate oxidation reaction.


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Component-Dependent Electrocatalytic Activity of Ultrathin PdRh Alloy

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