nickel hydroxides nanoflakes on carbon cloth support as robust anode catalyst for electrocatalytic alcohol oxidation

Journal Pre-proof Preparation of palladium/nickel hydroxides nanoflakes on carbon cloth support as robust anode catalyst for electrocatalytic alcohol oxidation Xi Tao Yang, Ming Zhe Wen, Xiao Li, Jing Bing Wang, Li Xin Su, Xin Dong Fan PII:

S0254-0584(19)31362-8

DOI:

https://doi.org/10.1016/j.matchemphys.2019.122552

Reference:

MAC 122552

To appear in:

Materials Chemistry and Physics

Received Date: 2 February 2019 Revised Date:

13 October 2019

Accepted Date: 13 December 2019

Please cite this article as: Xi Tao Yang, Ming Zhe Wen, Xiao Li, Jing Bing Wang, Li Xin Su, Xin Dong Fan, Preparation of palladium or nickel hydroxides nanoflakes on carbon cloth support as robust anode catalyst for electrocatalytic alcohol oxidation, (2019), doi: 10.1016/j.matchemphys.2019.122552 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. © 2019 Published by Elsevier B.V.

1

Preparation of palladium/nickel hydroxides nanoflakes on carbon cloth support as robust anode catalyst for electrocatalytic alcohol oxidation Xi Tao Yang, † Ming Zhe Wen, † Xiao Li, † Jing Bing Wang, Li Xin Su and Xin Dong Fan* Department of Interventional Radiotherapy, Shanghai Ninth People's Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai 200011, China. †

X. T Yang, M. Z. Wen and X. Li contributed equally to this work

Corresponding author: Prof. Xin-dong Fan, [email protected]

Abstract Palladium (Pd) based catalysts are one of promising candidate to replace widely used Pt catalyst due to their lower cost and good electrocatalyst ability for ethanol oxidation reaction in alkaline media. However, most of Pd electrocatalysts suffer from limited operational durability and unsatisfying catalytic activity. Herein, a hybrid electrocatalyst consisting of Pd nanoparticles and Ni(OH)2 nanoflakes on the supported carbon cloth is prepared. Owe to the promoting effects of Ni(OH)2 nanoflakes on Pd nanoparticles for electrocatalytic ethanol oxidation, the prepared Pd/Ni(OH)2/CC exhibits high peak anode current of 161 mA/mg, and excellent operational durability in cycling and chronoamperometric testing. After 1 h of chronoamperometric testing, the mass-specific current on Pd/Ni(OH)2/CC hybrid reserves almost 80% of the original value and approaches to about 132 mA/mg. The electrocatalytic activity can remain via recycled cyclic voltammetry. Such high stability and current density is assigned to the synergistic effect between Pd nanoparticles and Ni(OH)2 nanoflakes.

Keywords ： Ni(OH)2 nanoflakes; Pd nanoparticles; Electrocatalysis; Alcohol oxidation.

1. Introduction Sustainable fuel cells based on the electrochemical oxidation mechanism have appealed increasing attention in decades of years because of their large power density and convenience in storage/delivery.[1, 2] Meanwhile, direct alcohol fuel cells (DAFCs) with low toxicity and abundant renewable biomass resources are of particular interest.[3] During the anode oxidation process of alcohol, the sequential transfer of electrons produce higher energy density. As a crucial component in the cell for chemical energy conversion, the electrocatalysts determine the performance of the cell.[4] Platinum (Pt) nanoparticles, a widely used anode catalyst materials for DAFC, have good electrocatalytic ability on alcohol oxidation.[5, 6] But their high cost and low poisoning resistance for carbonaceous species limit their application in large-scale manufacturing of DAFC.[7] Numerous efforts are devoted recently on the development of Pt-free electro-catalysts for DAFC.[8-10] Palladium (Pd) is one of promising candidates to replace Pt catalyst because of the lower cost and good electrocatalyst toward alcohol oxidation in the alkaline media.[11] Despite great potential, the practical application of Pd in DAFC has been impeded by the severe activity loss because of rapid poisoning by reaction intermediates.[10, 12] A feasible strategy to improve Pd catalyst stability is the rational design of multicomponent nanomaterials such as alloy catalyst and collocating with transition metal oxide hybrids.[10, 13] The synergistic interactions in such multicomponent systems are considered to alternate the electronic structure of Pd catalysts and weaken their interaction between reaction intermediates and Pd hybrid.[14, 15] For example, bimetallic Pd/Au alloy catalysts provided remarkable electro-catalysis in direct ethanol fuel cell as well as good stability.[16] The doping of transition metal nickel in Pd catalyst combined with hierarchical structures made the mass activity of PdNi hybrid is 5.6 fold higher than that of the commercial Pd/C catalyst.[15] The previous reported work presented the effects of nickel double hydroxides on promoting water dissociation to form OH adspecies in alkaline media.[17] The dopant of noble metals including Pd on nickel double hydroxides could reduce the oxygen evolution reaction

potential and enhance catalytic activity.[18] For the term of electrocatalytic alcohol oxidation, the significant interface between Pd nanoparticles and nickel hydroxides would maximize their electronic interactions and the synergistic effect, which is considered to promote catalytic effect of Pd.[19] This requires the preparation of unique Ni(OH)2 nanostructures with abundant active points. On the other hand, the dispersion and loading capacity of the hybrid material affect remarkably their practical catalytic activity.[20, 21] Highly dispersed Pd nanoparticles loaded on supported materials usually have higher overall catalytic activities.[22] Carbon fiber is a robust supporter for catalysts due to their excellent stability in alkaline media and high electrical conductivity. Besides, highly conductive carbon fiber also can offset the poor electrical conductivity of metal oxide materials in the hybrid catalyst.[23, 24] However, the specific surface area of carbon fiber is usually low, which impedes the efficient loading of catalyst. Recently, metal oxide nanomaterials with hierarchical nanostructures such as flake-shapes have been reported with high specific surface areas,[25, 26] which improves the catalyst or interfacial performance in battery and capacitor. If hierarchical nickel hydroxide (Ni(OH)2) can be prepared on the surface of supported carbon fiber, the achieved hybrid would possess of high specific surface areas and potential synergistic promotion for Pd nanocatalysts.[27] In present work, Ni(OH)2 nanoflakes are prepared by hydrothermal method and deposited on the surface of carbon fiber in the carbon cloth. Subsequently, the synthesized Pd nanoparticles are embed into Ni(OH)2 nanoflakes. The resultant Pd/Ni(OH)2 hybrid on carbon cloth can be used directly as catalyst-loaded anode materials. Due to the synergistic effects and hierarchical nanostrucutres, the electrocatalytic activity and stability of Pd hybrid are remarkably improved. This hybrid catalyst can generate continuously high current density in a long term of 50 min, showing good operational durability in practical application of highly catalytic activity for alcohol fuel cells. 2. Experiment Materials

All the reagents are used as received without further purification. All aqueous solutions are prepared with distilled water. Palladium chloride (PdCl2), nickel nitrate (Ni(NO3)2), sodium borohydride (NaBH4), urea and ammonium fluoride (NH4F) are of analytical grade and are provided by Sigma-Aldrich (Shanghai). Woven carbon cloth (CeTech, electrical resistivity < 13 mΩ/cm²) are cleaned in ethanol by sonication. All other solvents and common chemicals are purchased from Sinopharm Group Co., Ltd. (China) and used as received. Growth of Ni hydroxides nanoflakes on carbon cloth (Ni(OH)2/CC). Nickel hydroxides (Ni(OH)2) nanoflakes was prepared by a modified hydrothermal method and coated in-situ on the carbon cloth (CC).[25, 28] Briefly, to promote the growth of nanoflakes on CC, the CC was treated by nitric acid. A piece of clean CC (4 cm × 4 cm) was immerged into 50 mL of HNO3 (65%) and refluxed in an oil bath at 110oC for about at least 4 h. After washing with deionized water, the treated CC was dried at 50oC for 12 h before using. Subsequently, the treated CC was washed with deionized water and finally dried at 60 °C in an oven. The hydrothermal method was carried out to synthesize Ni(OH)2 nanoflakes by using nickel nitrate, ammonium fluoride and urea as starting precursors. Typically, 0.87 g (4.8 mmol) Ni(NO3)2, and 0.28 g (7.6 mmol) NH4F and 0.9 g urea were dissolved in 60 ml distilled water at room temperature to obtain a clear solution. A piece of treated CC with sizes of 1.5×2.5 cm was immerged into the above solution. Subsequently, this mixed solution was transferred into a 100 mL of Teflon-lined autoclave. The autoclave was sealed, inserted into an oven and maintained at 120 °C for 6 h to conduct the hydrothermal reaction. After naturally cooling down to room temperature, the prepared Ni(OH)2/CC was collected and rinsed four times into distilled water and subsequently two times into ethanol before being dried at 45 °C. Synthesis of Pd nanoparticles on Ni(OH)2/CC (Pd/Ni(OH)2/CC) The precursor Pd ions was reduced by NaBH4 and deposited in-situ on Ni(OH)2 nanoflakes coated CC.[29] 70 mg of PdCl2 was dissolved into 40 mL of deionized water and the pH of the resultant solution was adjusted to pH 10 using 0.1 M KOH solution. 0.2 mL of ice-cold NaBH4 aqueous solution (2.0 mg/mL) was added rapidly

and stirred for 5 min at room temperature. Then the prepared Pd/Ni(OH)2/CC was collected and rinsed into water three times before drying at 40oC for 12h. Characterization The structures of the prepared hybrid were observed by scanning electron microscope (SEM, Hitachi S4800) and transmission electron microscope (TEM, JEOL JEM 2010). The crystal lattices of nanostructures were characterized by X-ray diffraction (XRD, Bruker AXS D8, Germany) with Cu-Kα radiation. X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD) was used to analyse the composition of the composites. The content of nanocomposite on CC was tested by ICP-MS method. Electrochemical measurements were conducted using a three electrodes cell connected to an electrochemical workstation (CHI 660D, Chenhua Co., Shanghai). Pd/Ni(OH)2/CC served as working electrode to study the catalytic effects of Pd/ Ni(OH)2/CC on alcohol oxidation. Pt electrode and Ag/AgCl electrode were used as counter electrode and reference electrode, respectively. For cyclic voltammetry (CV) measurements, the electrode was immersed in a mixing solution of 1.0 M ethanol and 0.1 M KOH solution saturated with high purity nitrogen (99.99%). The potential window in CV measurements in the range of −0.9 V to 0.4 V versus Ag/AgCl at 20 mV/s. 3. Results and discussion The fabrication strategy of Pd/Ni(OH)2/CC hybrid is shown in scheme 1. An acid-treated CC is used as conductive support for the growth of Ni(OH)2 nanoflakes. The acid treatment introduces more active functional groups on the surface of CC, which is beneficial to the attachment of the Ni(OH)2. In the subsequent hydrothermal process, continuous and well-defined Ni(OH)2 nanoflakes are present on CC, which endows the carbon fiber with porous nanostructures. Owe to the reduction by excess NaBH4, large amount of small-sized Pd nanoparticles (Pd NPs) are produced and deposite on the porous Ni(OH)2/CC. The resultant Pd/Ni(OH)2/CC can be employed as anode electrocatalyst for alcohol oxidation. The microstructures of prepared Pd/Ni(OH)2/CC were characterized by SEM

observation (Fig. 1). The carbon nanofibers (CNF) in the carbon cloth are uniformly covered with a continuous layer of well-crystallized Ni(OH)2 nanoflake (Fig. 1A, B), forming a 3D hierarchical nanostructure. The inset high-resolution SEM image in Fig. 1A shows the typical carbon nanofibers have diameters of about 8 µm and their lengths are up to several micrometers. The nickel hydroxide nanoflakes are interconnected with each other and align vertically on the CC/CNF substrates. The unique hierarchical structure of the Ni(OH)2/CC substrate provides high surface area for the attachment of active materials.[30, 31] Through successive reduction reaction, the synthesized Pd nanoparticles attach densely on each CNF and develop into a porous Pd nanostructures on the Ni(OH)2/CC substrate (Fig. 1C, D). The hierarchical structures and active Pd/ Ni(OH)2 hybrid together generate unique synergetic effects and rich active sites, which would facilitate favorable electron transportation path for electrocatalytic behaviors.[32]

Scheme 1. The illustration of fabrication of Pd/Ni(OH)2/CC for electrocatalytic alcohol oxidation. To demonstrate the elemental composition, energy-dispersive spectroscopy (EDS) analysis was conducted. As shown in Fig. 2, the Pd/Ni(OH)2/CC hybrid consists of Pd and Ni as well as C and O elements. Besides, element mapping results of Pd indicate that Pd nanoparticles are homogeneously distributed throughout the Ni(OH)2/CC

substrates, which is consistent with its physical position observed from SEM. Owe to the spatial distribution of Pd nanoparticles on Ni(OH)2/CC, Pd nanoparticles have strong interaction with Ni(OH)2 nanoflakes.

Fig. 1. SEM images of Ni(OH)2/CC (A, B) and porous Pd/Ni(OH)2/CC (C, D). The inset in (A) are the image of carbon cloth coated with Ni nanoflakes.

Fig. 2. SEM image (A) and the EDX spectra (B) of porous Pd/NiNF/CC. The element mapping of Pd (C), Ni (D), C (E) and O (F). The chemical bonding environment of Ni and Pd on the surface of carbon fiber

were evaluated by X-ray photoelectron spectroscopy (XPS). The featured XPS spectrum of Pd/Ni(OH)2/CC reveals the presence of Pd, Ni, C, and O elements, which are consistent with the EDS results. The XPS spectrum of Ni 2p exhibites typical 2p3/2 peak at 855 eV and 2p1/2 peak at 873 eV, which are characteristic of β-Ni(OH)2 structures (Fig. 3A).[33] In the XPS spectrum of Pd, the typical double spin-orbit peaks located at 332 and 337 eV correlate to Pd 3d5/2 and Pd 3d3/2, respectively. Both of these characteristic peaks are ascribed to metallic Pd.[34] In the splitted spectra, the doublet peaks located at 333 and 338 eV are contributions from the adsorbed oxygen-Pd interaction and PdO species, attesting to surface oxidation and/or the strong interaction with the oxygen functional groups of carbon nanofibers.[35] Furthermore, the phases structures of Pd/Ni(OH)2/CC and Ni(OH)2/CC were analyzed by X-ray diffraction (XRD) technology. Intermediate Ni(OH)2/CC product exhibits diffraction peaks assignable to β-Ni(OH)2 (Fig. 3C, red line). In the pattern of Ni(OH)2/CC, three broad peaks appear at 2θ values of 34°, 40° and 58°, which respectively correspond to (100), (101) and (110) of Ni(OH)2 nanoflake.[36] A weak and broad diffraction peak localized around 43° is ascribed to carbon cloth.[37] In addition, the XRD pattern of Pd/Ni(OH)2/CC shows featured peaks at 38° and 46°. These diffraction peaks can be assigned to (111) and (200) facets of the face-centered cubic (fcc) crystalline Pd nanoparticles.[38]

Fig. 3. High-resolution Ni 2p (A) and Pd 3d (B) XPS spectra of Pd/Ni(OH)2/ CC. (C) The XRD pattern of Pd/Ni(OH)2/CC and Ni(OH)2/CC.

To determine the electrochemically effective catalytic surface area (ECSA) of Pd/Ni(OH)2/CC, the hydrogen adsorption/stripping process on electrode were studied by cyclic voltammetry (CV) measurement in KOH solution (0.5 M) (Fig. 4).[29] The cathodic peak between -0.6 and 0 V versus the Ag/AgCl

electrode is assigned to the electro-reduction of PdO to Pd during the reverse scan.[39, 40] The charges corresponding to the cathodic reduction peak area is used to determine the ECSA of the Pd/Ni(OH)2/CC. Based on the quotation of QPdO/(mPd · 0.405 mC cm-2),[41] ECSA of the Pd/Ni(OH)2/CC is about 133 cm2/mg in 0.5 M KOH. This value was larger than that of Pd nanoparticles directly grown on carbon cloth (Pd/CC, 75 cm2/mg) under the same condition. In the CV curves of carbon cloth and Pd/Ni(OH)2/CC, there are no obvious cathodic reduction peak, which indicates the synergistic effects between Pd and Ni(OH)2 nanoflakes lead to higher electrochemical activity of Pd/Ni(OH)2/CC hybrid.

Fig. 4. Cyclic voltammogram of carbon cloth, Ni(OH)2/CC, Pd/CC and Pd/Ni(OH)2/ CC in KOH (0.5 M). The scan rate is at 20 mV s−1.

The CV curves of Pd/Ni(OH)2/CC in 0.1 M KOH containing 1 M ethanol demonstrated different electrochemical features. As illustrated in Fig. 5, a broad anodic peak that typically assigned to the oxidation of ethanol to intermediate products appears between −0.3 and 0.15 V in the forward scan.[42] In addition, a smaller anodic peak in the reverse scan between −0.3 and 0 V is usually considered to be resulted from further oxidation of intermediate products.[43] The mass-specific current density during the forward scan can be employed to evaluate the catalystic activity. Compared with Pd/CC (97 mA/mg) and referenced Ni(OH)2/CC, Pd/Ni(OH)2/ CC hybrid shows the highest peak current of 161 mA/mg.

Fig. 5. Cyclic voltammogram of CC, Ni(OH)2/CC, Pd/CC and Pd/Ni(OH)2/CC in 1.0 M ethanol and 0.1 M KOH solution. The scan rate is at 20 mV/s.

In the alkaline solution, hydroxide ions are apt to attach on catalyst, which favor to reduce the barrier of ethanol oxidation reaction.[8] The alkaline-dependent electrocatalytic activity of Pd/Ni(OH)2/CC was studied by investigating the catalytic ethanol oxidation occurred in different concentrations of KOH solution (Fig. 6).[44] When the concentration of KOH increases from 0.2 M to 1.0 M, the current density on Pd/Ni(OH)2/CC slightly increases by 16% and approaches to a maximum value of 161 mA/mg (Fig. 6B). In addition, the onset electro-oxidation potential changes hardly when increasing the alkalinity. The above phenomenon indicates that the electrocatalytic ability of Pd/Ni(OH)2/CC is immune to the variation of alkaline. The electrocatalytic ethanol oxidation process proceeds stably in different content of KOH.[15] In addition to the good EOR activity, the advantages of Pd/Ni(OH)2/CC over previous Pd catalyst are its exceptional operation stability. The stability assessments were demonstrated in the repeated CV cycling process and long-term electrocatalysis. The Pd hybrid catalyst were continuously cycled over the potential range of −0.9 to 0.4 V at 50 mV/s (Fig. 7A). The anodic current density at −0.1 V during the forward scan was extracted for plotting against the cycle number as depicted in Fig. 7B. The Pd/Ni(OH)2/CC hybrid exhibits outstanding cycling stability and shows less than 10% reduction after even 100 cycles. The anodic current density on Pd/Ni(OH)2/CC approaches to 150 mA/mg. Compared with Pd/Ni(OH)2/CC, Pd/CC shows lower stability in CV cycling measurement. The electrocatalytic current

density on Pd/CC decay rapidly in the recycled CV process (Fig. 7C, D). After 50 cycles, the current density reduce gradually by about 39%.

Fig. 6. (A) Cyclic voltammogram of porous Pd/Ni(OH)2/CC in different concentration of ethanol in alkaline solution. (B) The plot of oxide peak current density against the concentration of ethanol. The scan rate is at 20 mV/s.

Fig. 7. The cyclic voltammogram of porous Pd/Ni(OH)2/CC (A) and Pd/CC (C) after different cycles and corresponding oxidation peak current variation (B, D). The scan rate is 20 mV/s.

Secondly,

to

investigate

the

long-term

electrocatalysis

stability,

chronoamperometric measurement (I ~ t) was conduct at a potential of 0.05 V for one hour in the mixed solution of 1.0 M ethanol and 0.1 M KOH (Fig. 8A). In the initial stage, the current on both electrode decays gradually, probably attributing to the accumulation of poisonous carbonaceous intermediates on the catalyst surface during

the ethanol oxidation.[45] The oxidation current density on Pd/Ni(OH)2/CC is much higher throughout the whole scan and decays slower than Pd/CC, indicating relatively higher catalytic activity and better stability of Pd/Ni(OH)2/CC for long-term electrocatalysis on ethanol oxidation. The current variation is further determined by the value of I/Io ratio, where Io represents the initial current density (Fig. 8B). Owe to the higher possibility of being poisoned by carbonaceous intermediates during continuous alcohol oxidation reaction, chronoamperometric stability of most catalysts are inferior to their corresponding cycling stability.[15] The referenced Pd/CC hybrid experienced remarkable activity decay and declined 80% of the initial activity within 1 h, which was agreement with previous Pd-based electrocatalysts.[46-48] By contrast, Pd/Ni(OH)2/CC exhibited flat activity decay and lost only 19.4% of the initial activity within 1 h. Finally, Pd/Ni(OH)2/CC still has a mass-specific current of 132 mA/mg, which is much higher than that of Pd/CC (23 mA/mg). It is worth highlighted that both the cycling stability and chronoamperometric stability of Pd/Ni(OH)2/CC exceeded general Pd or Pt-based electrocatalysts.[14, 15, 49-51] It is indicated Ni(OH)2 nanoflakes with abundant OH adspecies remarkably promoted the electrocatalytic performance of Pd for the ethanol oxidation in alkaline media.[52]

Fig. 8. Chronoampermetric curve (A) and corresponding I/I0 ratio (B) of Pd/CC and porous Pd/Ni(OH)2/CC in 1.0 M ethanol and 0.1 M KOH solution with potential held at 0.10 V.

Conclusion In summary, a Pd/Ni(OH)2/CC hybrid electrocatalyst for alcohol oxidation is developed. Interconnected Ni(OH)2 nanoflakes deposite uniformly on carbon cloth

that serve as a supporter for synthesized Pd nanoparticles. Compared with the individual Pd nanoparticles, Pd/Ni(OH)2/CC hybrid exhibits higher current density and stability in the electrocatalytic ethanol oxidation. It is indicated Ni(OH)2 nanoflakes with abundant OH adspecies remarkably promoted the electrocatalytic performance of Pd for the ethanol oxidation in alkaline media. The mass activity and operational durability of Pd are also enhanced. The Pd/Ni(OH)2/CC hybrid reserves almost 80% of the initial peak current and retains 132 mA/mg even after 1 h of chronoamperometric measurement. Such high stability is considered to be assigned to the synergistic effect between Pd nanoparticles and Ni(OH)2 nanoflakes.

Acknowledgement This research was supported by National Natural Science Foundation of China (No. 81871458), Clinical Research Program of 9th People's Hospital, Shanghai Jiao Tong University School of Medicine (No. JYLJ201801), the State Key Laboratory of Molecular Engineering of Polymers at Fudan University (No. K2017-03) and the China Postdoctoral Science Foundation (No. 2017M611585).

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1. A hybrid catalyst Pd/Ni(OH)2 nanoflake on the carbon cloth is prepared. 2. The Pd/Ni(OH)2 hybrid is efficient for electrocatalytic ethanol oxidation. 3. Ni(OH)2 nanoflake shows promoting effects on the catalytic activity of Pd. 4. Pd/Ni(OH)2 hybrid exhibits high peak current and excellent durability.

We declare that we have no conflicts of interest with any other people or organization.