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Palladium Nanoparticles Anchored on the Three-Dimensional NitrogenDoped Carbon Nanotubes as a Robust Electrocatalyst for Ethanol Oxidation Honglei Yang, Xueyao Zhang, Hai Zou, Zhounan Yu, Shuwen Li, Jianhan Sun, Shengda Chen, Jun Jin, and Jiantai Ma ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01157 • Publication Date (Web): 05 May 2018 Downloaded from http://pubs.acs.org on May 6, 2018

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ACS Sustainable Chemistry & Engineering

Palladium Nanoparticles Anchored on the Three-Dimensional Nitrogen-Doped Carbon Nanotubes as a Robust Electrocatalyst for Ethanol Oxidation Honglei Yang*, Xueyao Zhang, Hai Zou, Zhounan Yu, Shuwen Li*, Jianhan Sun, Shengda Chen, Jun Jin and Jiantai Ma State Key Laboratory of Applied Organic Chemistry, Gansu Provincial Engineering Laboratory for Chemical Catalysis, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, P. R. China *

Corresponding authors.

E-mail addresses: [email protected] (Shuwen Li) [email protected] (Honglei Yang) Address (all the authors): Tianshui South Road NO. 222, Lanzhou 730000, PR China.

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Abstract Palladium nanoparticles were uniformly anchored on the nitrogen-doped carbon nanotubes with three-dimensional network structure (denoted as Pd/3DNCNTs) through a facile, surfactant-free and green approach with ethanol as reducing agents. As a robust catalyst for ethanol electrocatalytic oxidation reaction (EOR), Pd/3DNCNTs exhibits superior improved electrocatalytic activity, accelerated kinetics and robust stability, mainly attributed to the unique architecture features of 3DNCNTs. The results of this part of work reveal that the Pd/3DNCNTs with infusive electrochemical property for EOR is promising in direct ethanol fuel cells (DEFCs) and various other applications in electrochemistry. Additionally, the green approach probably provides some new ideas for the design of other new catalysts for fuel cells. Keywords: Three-Dimensional Carbon Nanotubes Nitrogen-Doping Palladium Nanoparticles Ethanol Oxidation Electrocatalyst

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INTRODUCTION Direct ethanol fuel cells (DEFCs) are the potential energy sources to achieve low pollution and environmentally friendly sustainable development owing to ethanol possesses distinct superiority of high energy density, low toxicity, easy storage and transportation, and large scale of production from biomass.1-3 Presently, one of the major hurdles for the commercialize production of DEFCs is the low activity and stability for the anode catalysts. It is reported that complete oxidization is difficult to achieve in the Ethanol electrocatalytic oxidation reaction (EOR), which is usually used as the anode reaction for DEFCs, because the C-C bond needs to be broken and 12 electrons will be released during the process of this reaction.3-5 As a promising candidate for anode catalyst, palladium (Pd) and Pd-based catalysts have exhibited superior activities for catalyzing EOR in basic media.6-11 However, Pd nanoparticles (NPs) often easily agglomerate to large size because of being partly oxidized, dissolved, or detached from supports during the DEFCs operation, which causes to degradation in EOR performance.12 Moreover, the support materials can make effects for the activity of the catalyst by influencing the size distribution, valence state, dispersion and stability of Pd NPs.13-14 Currently, carbonaceous supports are commonly used as supports for fuel cells. For instance, carbon nanotubes (CNTs) with a unique one-dimensional (1D) structure are widely deployed as supports for precious electrocatalysts due to their high surface area, good conductivity, and outstanding chemical, thermal and structural stability.15 However, integrated CNTs usually present chemical inert nature. The carrier is bound to be functionalized by varies of physical or chemical methods to achieve higher mass activity. It has been showed by previous literature that amorphous carbon blacks undergo corrosion under fuel cell startup/shutdown protocols.16-17 Therefore, seeking and developing novel catalyst 3



supports, which can enhance the performance and durability of the catalysts is still an urgent task. Doping nitrogen into carbon materials can improve the electrocatalytic property in the fuel cells reactions, which has been demonstrated by other researchers.18 It may ascribe that nitrogen atoms, which is electron-rich, incorporating with the carbon materials can promote π-binding ability, supply more defects and strengthen the interaction between active sites and supports.19 Many works have investigated how to construct the nitrogen doped carbon supports and claimed good activities. However, they have showed various limitations such as time-consuming processes and/or high operating temperatures. Based on the above considerations, herein, Pd NPs were uniformly anchored on the nitrogen-doped carbon nanotubes (NCNTs) with three-dimensional (3D) network structure (denoted as Pd/3DNCNTs) through a facile, surfactant-free and green approach with ethanol as reducing agents. Figure 1 shows the schematic synthetic route of Pd/3DNCNTs. The 3DNCNTs is easy to achieve by hydrothermal method at only 160 oC. Then the NCNTs can be linked by covalent bonds and hydrogen bonds and form the 3DNCNTs with 3D network structure. Figure 2 shows the possible formation mechanism of 3D structure for 3DNCNTs (the specific formation mechanism was displayed in Supporting Information).20-24 And “clean” Pd NPs are anchored on 3DNCNTs after being reduced by ethanol solution without any surfactant. The EOR activity in alkaline media was tested in the half-cell conditions. EXPERIMENTAL SECTION Materials The information of the materials used in the experiments is listed in the Supporting Information. 4


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Preparation of Pd/CNTs and Pd/3DNCNTs Preparation of 3DNCNTs. The acid oxidized carbon nanotubes (AO-CNTs) was obtained on the basis of our previous reports.25 The 3DNCNTs was synthesized according to the modified method our reports (Figure 1).20 The AO-CNTs (50 mg) was ultrasonically dispersed in H2O (20 mL). And then the addition of urea (1.5 g) needed to be completed. The obtained mixture was transferred to a Teflon-lined autoclave (50 mL) after being violently stirred for half an hour, and calculated for 5 h at 160 oC in a muffle furnace. After cooling, the solid was separated through membrane by vacuum filtration and washed with water repeatedly. The product was vacuum dried at 60 oC over night. Preparation of Pd/CNTs and Pd/3DNCNTs. The CNTs or 3DNCNTs (20 mg) was ultrasonically treated in ethanol solution (VH2O:VC2H5OH = 1:1, 20 mL). After being added with K2PdCl4 solution (0.01 mol L-1, 3.4 mL) under stirring, the system was refluxed for 4 hours. Then the mixture was by filtered to get black solid, which was washed by H2O and C2H5OH, and then vacuum dried at 60 oC for 12 h. Sample characterization and Electrochemical measurements The relevant information about Sample characterization and Electrochemical measurements is displayed in Supporting Information. RESULTS AND DISCUSSION As shown in Figure S2, two diffraction peaks approximately at 25° and 43°, typically assigned to (002) and (101) respectively, are distinctly observed without any change for CNTs and 3DNCNTs, suggesting that the graphic crystal structures were retained after nitrogen-doping.20 The nitrogen-doping effect on the CNTs is detected by Raman spectroscopy (Figure S3). The D band deriving from disordered sp2 hybridization carbons at graphene edges is found at about 1330 5



cm-1. Meanwhile, the G band also appears at about 1570 cm-1, demonstrating the existence of the tangential stretching vibration of the graphene hexagonal ring.26 The intensity ratio of the D and G band (ID/IG) slightly increases from 1.38 (CNTs) to 1.40 (3DNCNTs). Figure S4 demonstrates nitrogen adsorption-desorption isotherms and the pore size distribution calculated by the Barrett-Joyner-Halenda (BJH) method of CNTs and 3DNCNTs. The Brunauer-Emmett-Teller (BET) surface area of 3DNCNTs is 195 m2 g-1, which is higher than that of CNTs (101 m2 g-1). Compared with CNTs, there are more mesopores in 3DNCNTs. Moreover, the cumulative pore volume of 3DNCNTs is 0.96 m2 g-1, dramatically larger than that of CNTs (0.57 m2 g-1). The morphology and micro-structure of the as-prepared catalysts were characterized by scanning electron microscope (SEM) and transmission electron microscopy (TEM). As shown in Figure 3A and Figure S5A, the CNTs loosely stack. In contrast, the complex three dimensional 3D structures for 3DNCNTs is clearly observed in Figure 3B and Figure S5B, with stacked and overlapped structures and the walls cross-linking with each other. As illustrated in Figure 3C and Figure S6A, it is markedly observed that Pd NPs desultorily distribute on the disorderly CNTs. In contrast, Pd NPs, whose size distribution are narrow, are uniformly anchored on the relatively tight 3DNCNTs (Figure 3D and Figure S6B). What’s more, the 3DNCNTs possesses a clear three-dimensional network structure. TEM images with more detail structure are similar with SEM images (Figure 3E and Figure 3F). Obviously, Pd NPs with a wide size distribution tends to aggregate on the CNTs, while the homogeneous Pd NPs size equally distributes on the 3DNCNTs. The mean diameter for Pd NPs in Pd/3DNCNTs is approximately 20 nm (Figure 3D Inset). High-resolution TEM (HRTEM) was used for confirming the detailed structural features of the Pd NPs, and the results are 6


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displayed in Figure 3 (Inset). The lattice spacing of the Pd NPs for Pd/CNTs and Pd/3DNCNTs is measured to be around 0.23 nm, corresponding to the d-spacing vale of the (111) plane for Pd.27 The results of SEM and TEM exhibit the 3DNCNTs is a superior carrier for anchoring NPs. It is likely to due to that the role of unique structure and composition of 3DNCNTs. The nitrogen-containing functional groups existing in 3DNCNTs can act as Pd NPs crystal nucleating and anchoring points, which is conducive to the formation and homodisperse for equably Pd NPs. Moreover, the 3D network structure facilitates the uniformly dispersion and prevents the aggregation of Pd NPs. The crystal structure of the obtained catalysts is analyzed by wide-angle X-ray diffraction (XRD, Figure 4A). The diffraction peaks at about 25° in all the catalysts are ascribed to the crystal structure of CNTs. The other strong peaks appear at 39.9°, 46.6°, 68.0°, 82.0° and 86.4°, relating to the diffraction planes of (111), (200), (220), (311) and (222) and well corresponding with the diffraction of face-entered-cubic (fcc) crystal structure of pure Pd (JCPCDS NO. 46-1043).17 The Pd NPs are supported in the CNTs and 3DNCNTs have good crystal form illustrated by the XRD patterns. Based on the inductive coupled plasma optical emission spectrometer (ICP-OES) analysis, the mass contents of Pd in Pd/CNTs and Pd/3DNCNTs is 14.42 wt% and 14.50 wt%, respectively (Table S1). The analysis of catalyst element composition and element valence was by means of X-ray photoelectron spectroscopy (XPS). The nitrogen has incorporated successfully into CNTs evidenced by the clear N signal in the survey XPS spectrum of Pd/3DNCNTs (Figure 4B). The N mass contents for Pd/CNTs and Pd/3DNCNTs are 0.00 wt% and 2.21 wt% through the combustion method (Table S2). Meanwhile, the high resolution N 1s spectra (Figure 4C) are deconvoluted into three peaks at 398.9 eV, 399.9 eV and 401.1 eV, respevtively for the graphitic N, pyridinic 7



N and pyrrolic N.18 From Table S2, the pyrrolic N is the highest atomic type of N. Figure 4D shows that the Pd 3d signal of Pd/CNTs and Pd/3DNCNTs is related to two pairs of doubles (Pd 3d3/2 and Pd 3d5/2) and contains two valence states of Pd (Pd and PdO).13, 28 It is noteworthy that the content of PdO (Table 1) increases from 10% (Pd/CNTs) to 40% (Pd/3DNCNTs). As known, PdO which has the ability to donate oxygen, can promote the oxidation of CO to CO2, which can facilitate the EOR activity.29-30 Under half-cell conditions, the catalysts’ electrocatalytic activity and durability for EOR in alkaline media were systematically tested. Figure 5A presents the cycle voltammetry curves (CVs) in a deaerated KOH solution. The well defined peak appearing in -0.10 V to -0.40 V results from the reduction of PdO on the reverse sweep.6 According the coulombic charge summation during the reduction progress of PdO/Pd, the value of electrochemical active surface area (ECSA) can be calculated (Equation S1)6, 31. The value of ECSA in Pd/C, Pd/CNTs and Pd/3DNCNTs is 10.9, 12.0 and 20.1 m2 g-1, respectively (Table 2). The maximal value of ECSA implies that Pd/3DNCNTs are electrochemically more accessible.12 The catalyst activity for EOR was estimated by CV in N2-saturated in 1 M KOH and 1 M C2H5OH solution at a scan rate of 50 mV s-1 (Figure 5B). Two well-defined peaks are observed for all the tested catalysts. The broad peak appearing in the forward sweep is due to C2H5OH oxidation, while the other peak in the reverse sweep comes from the carbonaceous intermediates, which is not completely oxidized in the forward sweep.5 Onset potential (Eonset) and peak current density (If) in the forward sweep are common to estimate reaction dynamic property and catalyst activity. The current density was normalized with respect to the mass of Pd loading on the electrode. As listed Table 2, the Eonset of Pd/C, Pd/CNTs and Pd/3DNCNTs is -0.444 V, -0.541 V and -0.579 V, and the If is 287.8 mA 8


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mg-1 (Pd/C), 443.8 mA mg-1 (Pd/CNTs) and 775.6 mA mg-1 (Pd/3DNCNTs), respectively. The maximal If and the minimal Eonset exhibit that the Pd/3DNCNTs possesses higher electrocatalytic activity and the significant enhancement of the kinetics for EOR than those of Pd/C and Pd/CNTs.32-33 The CV experiments of the catalysts with increasing cycle number were carried out in 1 M KOH and 1 M C2H5OH solution for 1000 cycles for comparing the stability of Pd/C, Pd/CNTs and Pd/3DNCNTs. The If of the 1000th cycle is compare with the initial one and the results are shown in Figure 5C with the formation of histogram. Surprisingly, the percent of Pd/3DNCNTs is as high as 92.2%. In the contrast, the percent of Pd/C and Pd/CNTs is only 26.6% and 49.1%, respectively. It indicates that the Pd/3DNCNTs is a robust catalyst for EOR. In addition, the morphological structures of the Pd/3DCNTs after 1000 cycles dose not notably alter (Figure S7). Chronamperometry (CA) was also employed to judge catalyst long-term stability during the progress of EOR (Figure 5D). At the beginning, the limit current deceases rapidly for the accumulation of toxic species.34 As displayed, the CA curve of Pd/3DNCNTs decays the slowest current decay over time among the tested catalysts. The residual current density (Is) of Pd/3DNCNTs at 7200 s is 55.2 mA mg-1, which clearly overtakes Pd/C (5.2 mA mg-1) and Pd/CNTs (15.9 mA mg-1). It proves that the 3DNCNTs significantly promotes the tolerance and resistance of Pd NPs toward the poisoning effect of intermediate species of the EOR.27 Figure 5E shows linear sweep voltammetry (LSV) curves which were recorded in 1 mol/L KOH and 1 mol/L ethanol solution at a scan rate of 1 mV s-1. Based on LSV curves, the value of Tafel slope of catalyst Pd/C, Pd/CNTs and Pd/3DNCNTs is obtained and listed in Table 3. The Tafel slope of Pd/3DNCNTs is 156 mV dec-1, which is lower than that of Pd/CNTs (185 mV dec-1) and Pd/C (218 mV dec-1), attesting its 9



well kinetic property.35 The ability of resistance to toxic intermediates is investigated by CVs stripping in CO saturated 1 M KOH solution as shown in Figure 5F. As summarized in Table 3, the onset potential of CO oxidation (Ecoonset) for Pd/3DNCNTs is -0.168 V and lower than that of Pd/CNTs (-0.134 V) and Pd/C (-0.085 V). Additionally, the oxidation peak potential (Ecof) of CO oxidation for Pd/3DNCNTs (-0.065 V) is also lower than that of Pd/CNTs (-0.044 V) and Pd/C (-0.015 V). The lower of Ecoonset and Ecof implies that CO-like species is separated conveniently. Consequently, the anti-toxic ability of Pd/3DNCNTs is powerful.36 It is believed that the unique architecture and composition of 3DNCNTs is the role of outstanding catalytic performance and long-term durability of Pd/3DNCNTs towards EOR. The nitrogen-containing functional groups and three-dimensional network structure in 3DNCNTs is beneficial to the uniformly dispersion of homogeneous Pd NPs, resulting in the large ECSA. In addition, the 3D architecture of the electrodes not only improves electron transport, but also facilitates mass transfer from the outer surface to the inner structure. Moreover, the Pd NPs are firmly anchored on the 3DNCNTs owing to the interaction between Pd NPs and nitrogen-containing functional groups and the three-dimensional (3D) network structure of 3DNCNTs. Simultaneously, the properly sized of Pd NPs (~20 nm) can be trapped in the 3DNCNTs. Hence, the phenomenon that Pd NPs detached from supports and agglomerated to large size during the EOR operation is effectively arrested. CONCLUSION In conclusion, Pd/3DNCNTs consisting of Pd NPs uniformly anchored on the three-dimensional nitrogen-doped carbon nanotubes is fabricated through a facile, surfactant-free and green approach with ethanol as reducing agents. As a robust 10


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catalyst for EOR, Pd/3DNCNTs exhibits improved electrocatalytic activity, accelerated kinetics and superior stability, which is mainly attributed to the unique architecture features of 3DNCNTs. The results of this part of work reveal that the Pd/3DNCNTs with infusive electrochemical property for EOR is promising in direct ethanol fuel cells (DEFCs) and various other applications in electrochemistry. Additionally, the green approach probably provides some new ideas for the design of other new catalysts for fuel cells. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Detailed

information

of

Sample

characterization

and

Electrochemical

measurements. Additional figures, descriptions and tables. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Shuwen Li) *E-mail: [email protected] (Honglei Yang) ORCID Shuwen Li: 0000-0001-9511-2555 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of 11



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Prepared from NaOH-boiled Graphene and its Usage as a Support of PdO for Ethanol Oxidation Reaction. Int. J. Hydrogen Energy 2017, 42 (15), 9766-9774, DOI: 10.1016/j.ijhydene.2017.01.210. (31) Kumar, R.; Savu, R.; Singh, R. K.; Joanni, E.; Singh, D. P.; Tiwari, V. S.; Vaz, A. R.; da Silva, E. T. S. G.; Maluta, J. R.; Kubota, L. T.; Moshkalev, S. A. Controlled Density of Defects Assisted Perforated Structure in Reduced Graphene Oxide Nanosheets-palladium Hybrids for Enhanced Ethanol Electro-oxidation. Carbon 2017, 117, 137-146, DOI: 10.1016/j.carbon.2017.02.065. (32) Ma, J.; Wang, J.; Zhang, G.; Fan, X.; Zhang, G.; Zhang, F.; Li, Y. Deoxyribonucleic Acid-Directed Growth of Well Dispersed Nickel–Palladium–Platinum Nanoclusters on Graphene as an Efficient Catalyst for Ethanol Electrooxidation. J.

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Figure 1 Schematic synthetic route of Pd/3DNCNTs.

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Figure 2 Illustration of the possible formation mechanism of 3D structure for 3DNCNTs.

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Figure 3 SEM images of (A) CNTs, (B) 3DNCNTs, (C) Pd/CNTs and (D) Pd/3DNCNTs. TEM images of (E) Pd/CNTs and (F) Pd/3DNCNTs. (Size distribution histograms and HRTEM images of the NPs crystal structure were displayed in inset.)

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Figure 4 (A) XRD patterns and XPS spectra of (B) survey, (C) N 1s and (D) Pd 3d for (a) Pd/CNTs and (b) Pd/3DNCNTs.

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Figure 5 (A) CVs of the catalysts in N2-saturated 1 M KOH solution. (Scan rate: 50 mV s-1) (B) CVs of different catalysts in N2-saturated 1 M KOH and 1 M C2H5OH solution. (Scan rate: 50 mV s-1) (C) Percent of the oxidation peak current of the 1000th cycle compared to the initial cycle. (D) CA curves at -0.25 V in N2-saturated 1 M KOH and 1 M C2H5OH solution. (E) LSV in nitrogen-saturated 1 M KOH and 1 M C2H5OH solution. (Scan rate: 1 mV s-1, inset is Tafel plot based on LSV). (F) CO stripping curves in 1 M KOH solution at a scan rate of 50 mV s-1. (Catalysts: (a) Pd/C, (b) Pd/CNTs and (c) Pd/3DNCNTs)

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Table 1 Pd analysis of as-prepared catalysts by XPS. EB(eV) Pd

EB(eV) PdO

Pd to PdO ratio

Samples Pd 3d5/2

Pd 3d3/2

Pd 3d5/2

Pd 3d3/2

Pd/CNTs

335.5

340.7

340.3

343.8

90 : 10

Pd/3DNCNTs

335.4

340.6

337.8

342.6

60 : 40

EB: binding energy

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Table 2 Comparison of electrocatalytic activity with different catalysts. ECSA (m2 g-1)

Eonset (V vs. Hg/HgO)

If (mA mg-1)

Is (mA mg-1)

Pd/C

10.9

-0.444

287.8

5.2

Pd/CNTs

12.0

-0.541

443.8

15.9

Pd/3DNCNTs

20.1

-0.579

775.6

55.2

Catalysts

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Table 3 Parameters obtained from Tafel plot and CO-stripping curves. Catalysts

Tafel slope (mV dec-1)

Ecoonset (V vs. Hg/HgO)

Ecof (V vs. Hg/HgO)

Pd/C

218

-0.085

-0.015

Pd/CNTs

185

-0.134

-0.044

Pd/3DNCNTs

156

-0.168

-0.065

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Graphical Abstract

Aiming to design a robust ethanol electrocatalytic oxidation reaction catalyst, the Pd/3DNCNTs with three-dimensional network structure was fabricated through a facile, surfactant-free and green approach.

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Palladium Nanoparticles Anchored on the Three ... - ACS Publications

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