Highly dispersed Pd-CeO2 Nanoparticles Supported on N-doped

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Highly dispersed Pd-CeO2 Nanoparticles Supported on N-doped Core-Shell Structured Mesoporous Carbon for Methanol Oxidation in Alkaline Media Qiang Tan, Cheng Yong Shu, Janel Abbott, Qinfu Zhao, Liting Liu, Ting Qu, Yuanzhen Chen, Haiyan Zhu, Yong-Ning Liu, and Gang Wu ACS Catal., Just Accepted Manuscript • Publication Date (Web): 04 Jun 2019 Downloaded from http://pubs.acs.org on June 4, 2019

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ACS Catalysis

Highly dispersed Pd-CeO2 Nanoparticles Supported on N-doped Core-Shell Structured Mesoporous Carbon for Methanol Oxidation in Alkaline Media

Qiang Tan†,‡, Chengyong Shu†, Janel Abbott‡, Qinfu Zhao┴, Liting Liu§, Ting Qu†, Yuanzhen Chen , Haiyan Zhu┴*, Yongning Liu†,*, Gang Wu‡,*

†

†

State Key Laboratory for Mechanical Behavior of Materials, School of Material Science and

Engineering, Xi'an Jiaotong University, Xi'an 710049, China ‡

Department of Chemical and Biological Engineering, University at Buffalo, The State University

of New York, Buffalo, NY 14260, USA ┴

Institute of Modern Physics, Northwest University, Shaanxi Key laboratory for Theoretical

Physics Frontiers, Xi’an, 710069, China §

Analytical and Testing Center, Northwestern Polytechnical University, Xi’an, 710072, China

*Corresponding Authors: [email protected] (Y. Liu); [email protected] (H. Zhu); [email protected] (G. Wu)

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ABSTRACT: Sluggish kinetics of the methanol oxidation reaction (MOR) at the anode of direct methanol fuel cells (DMFCs) is primarily due to adsorbed CO poisoning of precious metal catalysts. CeO2 is known to provide oxygen containing species to adjacent precious metal sites for facilitating CO removal during the MOR. In this work, highly dispersed Pd nanoparticles surrounding by CeO2 dots were deposited on a core-shell structured and nitrogen-doped mesoporous carbon sphere (NMCS) support, which exhibited encouraging electrocatalytic activity, CO tolerance, and stability for the MOR in alkaline media. The ratios of Pd to CeO2 were found crucial for overall catalytic performance enhancement. When compared to a commercial PtRu/C catalyst, an optimized Pd20%-CeO2 (20%)/NMCS catalyst presented a comparable CO stripping onset potential, ~6 times higher peak current density, and enhanced cyclic stability. The unique mesoporous carbon with nitrogen doping also benefits for uniform dispersion of Pd nanoparticles and CeO2 dots. In good agreement with experimental spectroscopy analysis, density functional theory (DFT) calculations suggest that the strong electronic interactions between Pd and surrounding CeO2 as well as nitrogen dopants in supports dramatically reduce the adsorption energy of CO at Pd surface, therefore enhancing CO tolerance of the Pd-CeO2/NMCS catalyst and further improving MOR activity. Using a polymer fiber membrane-based alkaline DMFC, the Pd20%-CeO2 (20%)/NMCS anode catalyst further demonstrated encouraging performance when a NiCo2O4 catalyst was used for the oxygen cathode. Keywords: methanol electrooxidation, Pd nanoparticles, CeO2 dots, mesoporous carbon


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TOC GRAPHICS

Alkaline leaching

Growth

Phenolic resin core

SiO2 template nucleation

SiO2 templated NMCS

NMCS CH3OH CO

Pd-CeO2 deposition

OH H2O CO2 Pd-CeO2/NMCS

CeO2 Pd

1. INTRODUCTION Methanol stored in a liquid form for direct methanol fuel cells (DMFCs) is a significant advantage when compared to hydrogen fuel cells especially for portable and stationary power source applications. According to pioneer work at Los Alamos National Laboratory,1 effective energy density of methanol can reach 2.25 kWh/kg, if DMFCs work at 0.5 V with 90% fuel efficiency, which is much higher than that of hydrogen (0.4 kWh/kg) stored as metal hydrides at 2 wt.%. However, a few challenges including the sluggish kinetics for both anode and cathode reactions, methanol crossover, and requirement of large amount precious metal catalysts still prohibit the large-scale applications of DMFCs.2 In particular, the methanol electrooxidation reaction (MOR) at the anode is essentially important to the overall performance of DMFCs.3-6 At present, Pt and Pd catalysts are identified as the most effective catalysts for the MOR in acidic and alkaline media, 3 ACS Paragon Plus Environment

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respectively. In addition to high cost, current catalytic performance of these precious metals is limited by severe carbon monoxide (CO) poisoning and degradation of metal nanoparticles during fuel cell operating conditions.4 Substantial efforts have been devoted to reducing the cost and improving the catalytic activity and durability of MOR catalysts. In previous reports, the synergistic effect was discovered between the oxides and precious metal particles, thereby playing an important role in improving the activity and stability of the hybrid catalyst during the MOR. The studied oxides include MnO27, CeO28-10, SnO211-12, MoO313, and TiO2.14 In particular, CeO2 attracts a great interest because cerium can alter its valence between Ce4+ and Ce3+, allowing oxygen to reversibly be added or removed from the crystal lattice.15 The unique feature in anode catalysts can mitigate CO poisoning during the MOR process and shows significant improvement in the kinetics and stability for the MOR.16-18 However, appropriate interactions between CeO2 and precious metal particles are very crucial to maximizing the promotional role for catalytic activity enhancement. In contrast, the possible agglomeration of CeO2 particles and poor contacts with metal particles are distinctly detrimental to catalyst performance due to poor electrical conductivity of CeO2. Traditional synthesis of CeO2 including co-precipitation or hydrothermal methods is not effective to control the size and dispersion of CeO2 particles. Because a subsequent hightemperature calcination process inevitably induces growth and agglomeration of CeO2 particles.1920

Therefore, preparing size-controlled, monodispersed CeO2 nanoparticles and integrating them

with appropriate precious metals are crucial for designing effective composite catalysts for the MOR. 4 ACS Paragon Plus Environment

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During catalyst design, appropriate carbon supports are also essential for conducting electrons and dispersing precious metal particles.21-22 Conventional carbon black supported catalysts often suffer from poor mass transport and insufficient stability due to undesirable surface chemistry, nanostructure, and porosity. Herein, we developed a core-shell structured and nitrogen-doped mesoporous carbon (NMCS) support through a simple template method, which is capable of improving precious metal dispersion and providing an abundant porosity for mass transfer, which will be beneficial for MOR electrocatalysis.23-24 The monodispersed CeO2 dots were introduced into porous structures of the NMCS as the initial seeding sites for depositing Pd nanoparticles. With the combined effects of mesopores and CeO2 dots, the highly-dispersed Pd nanoparticles anchored at the surface of CeO2 dots, leading to an integrated Pd-CeO2/NMCS composite catalyst with 3D porous structures. After further studying the effect of Pd to CeO2 ratios, an optimal Pd(20%)CeO2(20%)/NMCS catalyst showed excellent electrocatalytic activity and stability for the MOR. Well-dispersed Pd nanoparticles surrounding by CeO2 dots with enhanced electronic interactions along with the unique nitrogen-doped mesoporous carbon support likely contribute to catalytic performance enhancement for the MOR in alkaline media. 2. RESULTS AND DISCUSSION 2.1 Catalyst synthesis, morphology, and structure Typical synthetic routes of the Pd-CeO2/NMCS catalyst are illustrated in Scheme 1. The NMCS was first synthesized through a template method by using cetyltrimethylammonium bromide (CTAB) as a soft-template and nitrogen source, resorcinol-formaldehyde resin as carbon source, and


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tetraethyl orthosilicate (TEOS) as a pore-forming material.23 After a polymerization followed by pyrolysis and etching processes to remove SiO2 templates, the core-shell structured NMCS was obtained. Solid carbon spheres (SCS) with a smooth surface were synthesized with a similar process in the absence of pore-forming agent TEOS. The monodispersed CeO2 dots are prepared separately by a thermal-decomposition method, which were deposited on the NMCS first. Then a microwave-assisted Pd reduction method was developed to reduce Pd onto the NMCS to prepare the composite Pd-CeO2/NMCS catalyst. The ratios of Pd to CeO2 were carefully studied in terms of their catalytic activity and stability. As a comparison, other catalysts including Pd/NMCS and Pd-CeO2/SCS were prepared under identical conditions (Details are shown in the supporting information).

Scheme 1. Typical synthetic procedures of NMCS supports and Pd-CeO2/NMCS catalysts.


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Figure 1. Representative SEM (A, B) and TEM (C, D) images of the nitrogen-doped mesoporous carbon sphere supports. The morphology and nanostructures of the NMCS and CeO2 dots was investigated by using scanning electron microscopy (SEM) and transition electron microscopy (TEM) images. As shown in Figure 1A and 1B, the prepared NMCS presents a distinct spherical morphology with a narrow size distribution. Moreover, the surface of NMCSs is covered with coralline-like pore structures resulting from the pretreatment of the templates and framework shrinkage during carbonization (Figure 1B). The statistical results show that the average diameter of the NMCS is approximately 480 nm (Figure S1). As shown in Figure 1C & D, the NMCS presents a high-density carbon core and a porous shell. The unique core-shell structure originates from the disparity of reaction rates between the SiO2 template and the phenolic resin in the initial phase of reactions. The high-density 7 ACS Paragon Plus Environment

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carbon core is beneficial for stabilizing the core-shell structure, and the rough mesoporous shell plays a crucial role in uniformly dispersing Pd nanoparticles. The quasi-zero-dimensional sized CeO2 dots were deposited onto the NMCS via a facile thermal decomposition method (details are shown in the supporting information).9 The TEM images show that the as-prepared CeO2 dots are well dispersed with an average diameter of 3.5 nm on the support (Figure S2 and S3).

Figure 2. TEM images of Pd/NMCS (A-D) and Pd-CeO2/NMCS (E-H) catalysts with different magnifications. HAADF-STEM, secondary electron image and elemental mapping of Pd/NMCS (I) and Pd-CeO2/NMCS (J) catalysts.


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After depositing Pd nanoparticles on the NMCS by using microwave-assistant ethylene glycol reduction method, morphologies of Pd/NMCS and Pd-CeO2/NMCS were analyzed by using HR-TEM and the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images. Figures 2A-2D present the representative TEM and HR-TEM images of the Pd/NMCS catalyst. After Pd nanoparticle depositing, the Pd/NMCS catalyst maintains porous spherical morphology. More detailed observations in Figure 2B and 2C indicate that the Pd nanoparticles are successfully loaded at the surface of NMCSs without agglomeration, but the average diameter of Pd nanoparticle is around 13 nm (Figure S4). Figure 2I displays the HAADF-STEM images and elemental mapping of the Pd/NMCS catalyst, further verifying that the Pd2+ was reduced and loaded at the surface of NMCS, but showing irregular shapes and large sizes of Pd nanoparticles. In the presence of CeO2, Figures 2E-2H show the TEM and HRTEM images of the Pd-CeO2/NMCS catalyst, presenting more uniform dispersion and smaller sizes of Pd nanoparticles on the support. Moreover, all of Pd nanoparticles and CeO2 dots are uniformly filled into the furrows and pores of the NMCS, resulting in a smooth surface (Figure S5). At the nanoscale level, Pd nanoparticles and CeO2 dots are well integrated together as shown in Figure 2H. Their intimate contacts could provide an efficient electron exchange, which proved favorable for electrocatalysis associated with multiple electron transfer.16-18 In Figure 2J and Figure S6, the HAADF-STEM images and elemental mappings of the Pd-CeO2/NMCS catalyst further confirm that Ce and Pd are 9 ACS Paragon Plus Environment

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homogeneously dispersed all over the surface and the interior of the carbon sphere. On the other hand, the SCS support is hard to disperse CeO2 and Pd nanoparticles uniformly due to the lack of a porous structure. TEM images (Figure S7) of the Pd-CeO2/SCS catalyst showed significant agglomeration of Pd particles and CeO2 dots. Thus, the porous structure at the surface of the NMCS is crucial for the uniform dispersion of Pd nanoparticle and CeO2. The improved nanoparticle dispersion also likely builds up effective triple junctions among NMCS, CeO2, and Pd. The abundant porous volume is able to confine the Pd2+ during the reduction to control particle sizes of Pd nanoparticles. The pre-deposited CeO2 dots in the pore structure could also stabilize Pd nanoparticles and avoid their agglomeration.25 The highly dispersed and ultrafine Pd nanoparticles are beneficial for increasing the electrochemically active surface area (EASA) and enhancing mass activity for the MOR.

Pd(20%)-CeO2(40%)/NMCS CeO2[111]

Intensity / a.u.

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Pd(20%)-CeO2(20%)/NMCS

CeO2[220]


CeO2[200]

Pd(20%)-CeO2(6.7%)/NMCS

Pd[200] Pd[111]

Pd[220]

Pd/NMCS C[002]

NMCS 10

20

30

40

50

60

70

80

90

2 Theta / 

Figure 3. XRD patterns of NMCS, Pd/NMCS and Pd-CeO2/NMCS catalysts with different Pd to CeO2 ratios. 10 ACS Paragon Plus Environment

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X-ray diffraction (XRD) was conducted to investigate the crystalline structures of NMCS, Pd/NMCS, and Pd-CeO2/NMCS catalysts with different mPd: mCeO2 ratios (Figure 3). Before loading CeO2 and Pd, the NMCS shows a typical carbon diffraction pattern, revealing that the SiO2 templates in NMCS precursor was completely leached away by using concentrated KOH solution. After reducing Pd precursors through the microwave assisted ethylene glycol process, all Pd catalysts exhibit new diffraction peaks around 40.1°, 46.7°, and 68.1°, corresponding to the (111), (200), and (220) planes of face-centered cubic Pd.26 XRD analyses confirms that Pd2+ was successfully reduced and deposited onto the NMCS. Diffraction peaks of Pd for the PdCeO2/NMCS catalysts are weaker and broader than those for the Pd/NMCS catalyst. This implies that the particle size of Pd becomes smaller in the Pd-CeO2/NMCS catalyst.27 In addition, with an increase of the ratio of mCeO2 to mPd, these peaks of Pd became weaker and broader, suggesting a size reduction of Pd NPs due to increased content of CeO2. It should be noted that excess CeO2 in catalysts would compromise electron conductivity and block active sites of Pd.


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A

C

B

Figure 4. (A) N2 adsorption and desorption isotherms , (B) surface area and pore distribution plots, and (C) pore volume distribution graphs for NMCS, Pd/NMCS, Pd-CeO2/NMCS, commercial Pd/C, and Pd-CeO2/SCS catalysts.

The Brunauer–Emmett–Teller (BET) method was used to analyze N2 adsorption and desorption at 77K to determine surface areas and pore volume of studied catalysts. In Figure 4A, all of NMCS supported catalysts show typical type IV of mesoporous structure isotherms.28 On the contrary, the Pd-CeO2/SCS catalyst exhibits a similar isotherms with Pd/C, which indicates less porosity. As shown in Figure 4B and C, the NMCS has a high specific surface area (SSA, 489


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m2 g-1) with a hierarchical pore distribution, and the majority of the pore surface area is attributed to the mesopore. However, after loading Pd in the absence of CeO2, most of the mesopore disappeared, and the SSA of Pd/NMCS is decreased to 225.1 m2 g-1, indicating that the mesopores are blocked by the large Pd nanoparticles. Oppositely, the highly dispersed Pd nanoparticles and CeO2 dots in the Pd-CeO2/NMCS catalyst have no influence on the porosity of the NMCS, showing a comparable surface area of 464.7 m² g-1 (Figure 4B) and pore distribution to the NMCS. 2.2 Catalyst activity enhancement mechanisms


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Figure 5. CV (A) and CA (B) curves of Pd/C, Pd/NMCS, Pd-CeO2/NMCS, Pd-CeO2/SCS, and PtRu/C catalysts in N2 saturated solution of 1.0 M KOH + 1.0 M CH3OH. (C) CO stripping curves


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for different samples in N2 saturated 1.0 M KOH solution. The CA curves were tested by RDE with 200 rpm.

To evaluate MOR activity of these catalysts in alkaline media, cyclic voltammetry (CV) curves were recorded in N2 saturated solution of 1.0 M KOH + 1.0 M CH3OH at a slow scan rate (Figure 5A). All of studied catalysts, including commercial Pd/C, PtRu/C (morphology and structure are shown in Figure S8-S10), Pd/NMCS, Pd-CeO2/SCS, and Pd-CeO2/NMCS with various Pd to CeO2 ratios, exhibited the typical methanol electrooxidation potential-current profiles in alkaline media. The peak current density of the Pd-CeO2/NMCS catalyst shows a distinct increase than those of Pd/C and Pd/NMCS. When the ratio of mCeO2 : mPd is 1:1, the Pd(20%)-CeO2(20%)/NMCS catalyst in the forward peak exhibited the highest current density (1.5 A mg-1, at -0.11 V), which is larger than those of Pd/C (0.55 A mg-1, at -0.12 V), Pd/NMCS (0. 31 A mg-1, at -0.12 V), Pd-CeO2/SCS (0.90 A mg-1, at -0.07 V) and PtRu/C (0.26 A mg-1, at -0.17 V) catalysts, respectively. In Figure 5B, chronoamperometry (CA) tests were used to further evaluate electrocatalytic activity and stability of the MOR. Current density for each catalyst was recorded at -0.2 V for one hour. The measured current density of the Pd(20%)-CeO2(20%)/NMCS catalyst at 0.2 V after one hour is 109.4 mA·mg-1, which is the highest compared to Pd/C (11.4 mA·mg-1), Pd/NMCS (17.3 mA·mg-1), Pd-CeO2/SCS (40.6 mA mg-1), and PtRu/C (61.1 mA mg-1) catalysts. The tolerance to poisoning intermediate CO is a critical property of MOR catalysts.29 PtRu/C catalysts display enhanced CO poisoning tolerance when compared to Pt, which is due to the 15 ACS Paragon Plus Environment

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available OH at the relative low potentials via H2O dissociation on Ru sites. Hence, a commercial PtRu/C catalyst was used as a reference to evaluate poisoning tolerance for various studied catalysts. Figure 5C compares CO-stripping curves for studied catalysts in 1.0 M KOH solution. Among others, the Pd(20%)-CeO2(20%)/NMCS catalyst shows the largest CO-stripping peak and Pd reduction peak, indicating the highest EASA of Pd. The EASA of each catalyst was calculated according to the reduction peak area of Pd oxide using the Eq.1:9 𝑄

𝐸𝐴𝑆𝐴𝑃𝑑·𝑔−1 = 405μC·𝑐𝑚−2 ·𝑔

Eq.1

Where Q is calculated by integrating the charges associated with the PdO reduction peak, and the value of 405 μC cm-2 is assumed for the reduction of PdO monolayer.30 Thus, the EASA of the Pd(20%)-CeO2(20%)/NMCS catalyst was determined to be 47.2 m2·g-1, which is larger than those of of Pd/NMCS (28.6 m2·g-1), Pd/C (17.3 m2·g-1), and Pd-CeO2/SCS catalysts (19.6 m2·g-1 ). The increased EASA further confirms the uniform dispersion and the smallest size of Pd nanoparticles in the Pd(20%)-CeO2(20%)/NMCS catalyst. Importantly, the Pd(20%)-CeO2(20%)/NMCS catalyst shows much enhanced CO tolerance capability. The onset potential of CO stripping for the Pd(20%)CeO2(20%)/NMCS catalyst is negatively shifted for 111 mV relative to the Pd/C catalyst, which is comparable to the PtRu/C catalyst.31-32 The reduced onset potential suggests that the CO intermediate during the MOR can be oxidized more easily from the surface of Pd with assistance of surrounding CeO2.


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A

B

eV Pd03d3/2


Pd03d5/2

II

Graphitic N

Pyridinic N

Pd/NMCS

Intensity / a. u.

Pd 3d3/2

Intensity / a. u.

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PdII3d5/2

Pd(20%)-CeO2(20%)/NMCS Pd03d3/2 PdII3d3/2

Pd03d5/2 PdII3d5/2

Pd/NMCS Pd03d3/2 PdII3d3/2

NMCS

Pd03d5/2 PdII3d5/2

Pd/C

410

405

400

395

346

344

342

340

338

336

334

332

Binding Energy / eV

Binding Energy / eV

Figure 6. (A) N 1s XPS spectra of NMCS, Pd/NMCS, and Pd-CeO2/NMCS catalysts. (B) Pd 3d XPS spectra of Pd/C, Pd/NMCS, and Pd-CeO2/NMCS catalysts. To further elucidate the promotional role of CeO2 and NMCS supports, extensive characterization and theoretical simulation were carried out. At first, XPS analysis was employed to study surfaces species and electronic properties of studied catalysts. The XPS survey spectra (Figure S11) confirm the presence of N in the NMCS supports and Pd in various catalysts. The high-resolution N1s and Pd 3d spectra for different catalysts are compared in Figure 6A and 6B, respectively. The N 1s spectra (Figure 6A) revealed two dominant nitrogen peaks centered at 398.3 eV and 400.8 eV corresponding to pyridinic and graphitic nitrogen species.33-34 It means that the nitrogen species from CTAB are doped into the carbon structure during the pyrolysis process. The doped nitrogen could alter chemical and electronic environments of adjacent carbon atoms and improve their electrocatalytic activity compared to non-doped carbon.35-36 In Figure 6B, the highresolution Pd 3d spectra can be deconvoluted into two doublets assigned to metallic Pd0 and oxidized Pd(II), respectively.37-38 Pd possesses a higher oxidation state in the Pd-CeO2/NMCS


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catalyst than that in Pd/NMCS and Pd/C, suggesting that oxygen-containing groups are likely transferred from the adjacent CeO2 dots to the surface of Pd in the Pd-CeO2/NMCS catalyst.39-40 In addition, the Pd0 3d3/2 and Pd0 3d5/2 peaks in Pd-CeO2/NMCS appear at 340.7 eV and 335.5 eV, respectively, which are positively shifted by 0.6 eV from those of a monometallic Pd/C catalyst and 0.2 eV from those of the Pd/NMCS catalyst. Such shifts are a strong evidence of intense electron interactions between Pd and CeO2 or/and N dopant in supports, which would be beneficial for electrocatalytic properties.21, 41

Figure 7. Optimized structures of (A) Carbon, (B) C@Pd8, (D) N-doped carbon, (E) N-doped


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carbon@Pd8, (G) CeO2 and (H) CeO2@Pd8, as well as the structures of CO adsorption on (C) C@Pd8-CO, (F) N-doped carbon@Pd8-CO, and (I) CeO2@Pd8-CO.

To further elucidate the possible electron exchange between Pd and surrounding CeO2 or/and N dopants, DFT calculations were performed. The model structure of a Pd8 cluster was generated by the AB Cluster program.42 Then an optimization method with the PBE function of the PAW potentials within the VASP program43-44 was used to obtain the most stable (minimum-energy) Pd8 clusters on various carbon and CeO2 including C@Pd8 , N-doped C@Pd8, and CeO2@Pd8 structures (Figure 7). The optimized structures of CO adsorption on the above-structures were also simulated. More details of optimal structures are further described in the supporting information. The adsorption energies of CO on C@Pd8 (E1), N-doped C@Pd8 (E2), and CeO2@Pd8 (E3) were calculated with Eq.2 Eq.3, and Eq.4, respectively: 𝐸1 = 𝐸C@Pd8 -CO − 𝐸CO − 𝐸C@Pd8

Eq.2

𝐸2 = 𝐸N-doped C@Pd8 -CO − 𝐸CO − 𝐸N-doped C@Pd8

Eq.3

𝐸3 = 𝐸CeO2 @Pd8 -CO − 𝐸CO − 𝐸CeO2 @Pd8

Eq.4

Where the 𝐸CO@Pd8 -C , 𝐸CO@Pd8 -N-C , 𝐸CO@Pd8 -CeO2 are the total energies for the ground state of an optimized CO adsorption on various supported Pd8 clusters, respectively. The adsorption energy (E1) of CO on the Pd8 in C@Pd8 is -3.20 eV. With the nitrogen doping, the adsorption energy (E2) of CO on the Pd8 in N-doped C@Pd8 is reduced to -2.50 eV. Furthermore, with an addition of CeO2 dots, the adsorption energy (E3) of CO on the CeO2@Pd8 is reduced to -1.86 eV.


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These simulations indicate that both nitrogen doping into carbon and CeO2 could reduce the adsorption energy of CO on Pd and weaken the chemisorption of intermedia CO, which is advantageous for removing CO and mitigating poisoning of Pd sites during the MOR. It should be noted that compared to N dopant in supports, CeO2 is more dominant to weaken CO adsorption at the Pd surface. 2.3 Catalyst stability

Figure 8. The accelerated stability tests of (A) Pd(20%)-CeO2(20%)/NMCS, (B) Pd/C, and (C) Pd/NMCS catalysts in 1.0 M KOH+1.0 M CH3OH solution. (D) Changes of the forward peak current density of Pd/C, Pd/NMCS and Pd-CeO2/NMCS catalysts in 1.0 M KOH + 1.0 M CH3OH solution during the ADT tests.


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Stability of the Pd(20%)-CeO2(20%)/NMCS catalyst was evaluated by an accelerated durability test (ADT) by using a potential cyclic protocol (-0.8 to -0.3 V vs. Hg/HgO, 50 mV s-1 in Figure 8). As shown in Figure 8A, the Pd(20%)-CeO2(20%)/NMCS catalyst shows encouraging stability with a decrease of 7.0% in MOR peak current density after 1000 cycles. In contrast, the Pd/C (Figure 8B) and Pd/NMCS (Figure 8C) catalysts suffered from significant activity losses of 69.9% and 28.3%, respectively (Figure 8D). The Pd(20%)-CeO2(20%)/NMCS catalyst after the stability test was analyzed by using TEM images (Figure S12), showing nearly no morphology changes in the spherical carbon support and well-dispersed Pd nanoparticles. However, the Pd nanoparticles in these Pd/C and Pd/NMCS catalysts significantly aggregated after the ADT test (Figure S13 and S14). As for the NMCS supports, the solid carbon core sustains porous shells and avoids possible structural collapse. The corrosion resistant CeO2 dots, which are filled into the porous carbon structure, can further prevent Pd nanoparticles from aggregating. 2.4 Direct methanol fuel cell performance A

B

Figure 9. (A) The structure of polymer fiber membrane-based membrane-electrode assembly and 21 ACS Paragon Plus Environment

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a DMFC. (B) Polarization and power density plots of Pd-CeO2/NMCS (5 mg cm-2) and PtRu/C (5 mg cm-2) catalysts at 20 ℃ and ambient pressure. The cathode was a NiCo2O4 catalyst. The anode aqueous solution was 4.0 M KOH and 5.0 M methanol. The oxygen flow rate was 20 mL min-1.

To further evaluate anode performance of the Pd(20%)-CeO2(20%)/NMCS catalyst in DMFCs, we designed and fabricate a polymer fiber membrane (PFM, Figure S15)-based electrode assemblies as shown in Figure 9A. The cathode is a NiCo2O4 catalyst dispersed into a Ni foam for the oxygen reduction reaction in alkaline media.45 Prior to DMFC tests, the Pd(20%)CeO2(20%)/NMCS catalyst was studied in a series of KOH solutions in order to determine MOR activity dependence on KOH concentrations. As shown in Figure S16, MOR activity is enhanced gradually by increasing KOH concentrations, generating increasing peak current densities and negatively shifting potentials. This indicated OH- is participating into the MOR; higher concentration is favorable for improved catalytic activity. Therefore, 4.0 M KOH solution was chosen to evaluate DMFC performance. In Figure 9B, the cell with the Pd(20%)-CeO2(20%)/NMCS anode exhibited an open-circuit voltage of 0.86 V and generated current densities of 20 mA cm-2 at 0.5 V and 75 mA cm-2 at 0.33 V. A peak power density reached 25 mW·cm-2 at room temperature and ambient pressure, which is higher than that of a PtRu/C anode with the same precious metal loading (5.0 mgcm-2). It should be noted that, due to non-optimized electrode structures, membranes, and the use of inexpensive NiCo2O4 cathode, the measured DMFC performance is not


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one of the best. However, the Pd-CeO2/NMCS catalyst is able to outperform the state-of-the-art PtRu/C under identical conditions, which makes it promising for alkaline DMFCs.

3. CONCLUSIONS In summary, a core-shell structured and nitrogen-doped mesoporous carbon sphere was prepared via a template method for supporting uniformly dispersed Pd nanoparticles surrounding by CeO2 dots. Differing to traditional carbon black supports, the dominant mesopores and nitrogen doping of the NMCS provide abundant of seeding sites for Pd deposition. As a result, the optimized Pd20%CeO2

20%/NMCS

catalyst achieved significantly enhanced MOR activity and stability, which

reached ~6 times higher peak current density and insignificant degradation (7% after 1000 potential cycles), when compared to a commercial PtRu/C catalyst. Enhanced CO tolerance was evidenced by the negatively shift of onset potential (~110 mV) during the CO stripping on the Pd20%-CeO2(20%)/NMCS catalyst relative to a traditional Pd/C catalyst. Characterization of catalyst morphology, structure, and chemistry indicates that the well-integrated Pd and CeO2 nanoparticles with uniform dispersion onto the robust structure of NMCS are crucial for enhancing activity and stability. More importantly, strong electronic interactions between Pd and the surrounding CeO2 as well as the nitrogen dopants in supports enable significant reduction of the CO adsorption energy at the Pd surface, which is the underlying mechanism of enhanced CO tolerance of the Pd20%-CeO2(20%)/NMCS catalyst during the MOR. The catalyst was further incorporated into a homemade DMFC with a nonprecious metal cathode NiCo2O4 and demonstrated encouraging performance when compared to a commercial PtRu/C catalyst.


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Supporting Information

The details of the experimental section and supplementary figures are attached in the supporting information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.xxxxxxxxx.

Acknowledgments The research was supported by the National Natural Science Foundation of China (No. 21603171, No. 201803041 & No. 11204239) and the Postdoctoral Science Foundation of China (No. 2016 M592783). H. Z. acknowledges the Natural Science Foundation of Shaanxi Province of China (No. 2018JM1010). The research acknowledges the instrument analysis center of Xi’an Jiaotong University for material structure characterization. G.W. acknowledges the start-up funding from the University at Buffalo, SUNY along with U.S. DOE-EERE Fuel Cell Technologies Office. References 1. Ren, X.; Zelenay, P.; Thomas, S.; Davey, J.; Gottesfeld, S., Recent Advances in Direct Methanol Fuel Cells at Los Alamos National Laboratory. Journal of Power Sources 2000, 86, 111116. 2. 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. Advanced Science 2016, 3, 1600140. 3. Wang, Y.-C.; Huang, L.; Zhang, P.; Qiu, Y.-T.; Sheng, T.; Zhou, Z.-Y.; Wang, G.; Liu, J.24 ACS Paragon Plus Environment

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Highly dispersed Pd-CeO2 Nanoparticles Supported on N-doped

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