in-situ deposition of platinum nanoparticles on bacterial cellulose membranes and evaluation of pem fuel cell performance

in-situ deposition of platinum nanoparticles on bacterial cellulose membranes and...

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Accepted Manuscript

Title: In-situ deposition of platinum nanoparticles on bacterial cellulose membranes and evaluation of PEM fuel cell performance

Authors: Jiazhi Yang, Dongping Sun, Jun Li, Xujie Yang, Junwei Yu, Qingli Hao, Wenming Liu, Jianguo Liu, Zhigang Zou, Jun Gu

PII: S0013-4686(09)00774-9 DOI: doi:10.1016/j.electacta.2009.05.073 Reference: EA 14719

To appear in: Electrochimica Acta

Received date: 23-9-2008 Revised date: 20-5-2009 Accepted date: 24-5-2009

Please cite this article as: J. Yang, D. Sun, J. Li, X. Yang, J. Yu, Q. Hao, W. Liu, J. Liu, Z. Zou, J. Gu, In-situ deposition of platinum nanoparticles on bacterial cellulose membranes and evaluation of PEM fuel cell performance, Electrochimica Acta (2008), doi:10.1016/j.electacta.2009.05.073

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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In-situ deposition of platinumnanoparticles on bacterial cellulose membranes and evaluation of PEM fuel cell performance

Jiazhi Yang , Dongping Sun 1, Jun Li , Xujie Yang , Junwei Yu, Qingli Hao School of Chemistry and Chemical Engineering, Nan Jing University of Science and technology, Nanjing 210094, PR China

Wenming Liu, Jianguo Liu*2, Zhigang Zou, Jun Gu, Eco-materials and Renewable Energy Research Center, Department of Materials Science and Engineeringand

National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P R China

Abstract

In-situdeposition of platinum(Pt) nanoparticles on bacterial cellulose membranes (BC) for a fuel cell application was studied. The platinum / bacterial cellulose (Pt/BC) membranes under different experimental conditions were characterized by using SEM (Scanning electron microscopy), TEM (Transmission electron microscopy), EDS (energy dispersive spectroscopy), XRD (X-ray diffractometry) and TG (thermo- gravimetric analysis) techniques. TEM images and XRD patterns both lead to the observation of spherical metallic platinum nanoparticles with mean diameter of 3-4 nm well impregnated into the BC fibrils. TG curves revealed these Pt/BC composite materials had the high thermal stability. The electrosorption of hydrogen was investigated by CV

corresponding author: Tel / fax : +86-25-84315256, +86-25-83621219 E-mail addresses:dongpingsun@163.com (D.P. SUN), jianguoliu@nju.edu.cn

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(cyclic voltammetry). It was found thatPt/BC catalysts have high electrocatalytic activity in the hydrogen oxidation reaction.The single cell performance of Pt/BC was tested at 20 oC, 30 oC, and 40 oC under non-humidified conditions. Preliminary tests on a single cell indicate that renewable BC is a good prospect to be explored as membrane in fuel cell field [1].

Keywords:Bacterial cellulose; Platinum nanoparticles; Membrane electrode assembly; Cyclic voltammetry; Proton exchange membrane

1. Introduction

There is a growing interest over the past several decades in the development of proton exchange membrane fuel cells (PEMFC) due to their advantages of high power density,, simplicity of operation, high energy conversion efficiency and low harmful emissions [2-3].Proton exchange membrane (PEM), a key component of PEMFC, essentially serves for transportation of protons and prevention of fuel crossover. Up to now, perfluorinated ionomer membranes (PFIM) such as Nafion have been considered to be the predominant choice for polymer electrolyte membranes due to high proton conductivity and chemically and physically stability. However, the PFIM still have some shortcomings such as high cost, limited operation temperature and high fuel permeability strongly hindering the commercialization of fuel cells [4]. To improve the performance of

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PEM, considerable efforts have been devoted to modify Nafion membrane or to develop alternative new hydrocarbon-based polymer membranes [5].

Bacterial cellulose (BC), which is synthesized by Acetobacter xylinum, is a natural and low-cost biopolymer[6]. It has many excellent properties such as high water holding capacity, biodegradability, and high tensile strength, etc. [7]. Meanwhile, BC membranes can retain its chemical and thermal stability up to 275oC with high mechanical strength [8]. Furthermore, the hydroxyl groups on its backbone can provide BC with a high hydrophilicity, which is crucial for the operation of polymer electrolyte membrane fuel cells [9].

A recent study showed that BC possessed reducing groups capable of initating the precipitation of palladium, gold, and silver from aqueous solution [1, 10]. In contrast, sodium hexachloroplatinate was not reduced to platinum by the action of BC. Platinum is considered the best electrocatalyst for the fourelectron reduction of oxygen to water in acidic environments as it provides the lowest overpotentials and the highest stability [1-12]. In this work platinum nanoparticles were deposited on the BC membrane surface through the in-situ chemical-reduction method. This development of new methods of membrane electrode assemblies (MEA) fabrication is expected to boost the commercialization of fuel cells.

2. Experimental

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2.1. Preparation of BC pellicles

Acetobacter xylinum NUST4.2 was grown in a static culture containing 20g/l

D-glucose, 21g/l sucrose, 10g/l yeast extract, 4g/l (NH4)2SO4, 2g/l KH2PO4, and 0.4g/l

MgSO4 dissolved in deionized water (DI) at 29 oC for 7 days. The pH of the medium was adjusted to 6.0-6.2 by 2.5M NaOH. BC pellicles were purified by soaking in DI at 70 oC for 3h and then 1M NaOH in DI at 70 oC for 90 min. Samples were rinsed with DI to pH=7 and stored in refrigerator at 4 oC prior to use.

2.2. In-situ preparation of Pt/BC nanocomposite membranes

For in-situ preparation of platinum nanoparticlesin 3D network structure of BC membrane was conducted through liquid phase chemical deoxidization method. Firstly, the BC pellicles were cut into small pieces, and comminuted by high speed homogenizer. Secondly, the BC homogenateswere soaked in a 5mM solution of hexachloroplatinic acid (H2PtC16·6H2O) dissolved in 50mM sodium citrate, pH5.0 and incubated at 40 oC for 12h, the hexachloroplatinate is not spontaneously reduced inside the cellulose. Thirdly to induce platinum nanoparticles precipitation, the soaked BC homogenates were then rinsed with DI and reduced by 1.5M solution of NaBH4 or HCHO into the cellulose matrix at 45 oC for 24h, under vigorous stirring, when the pellicleswere completely black

Page 5 of 29 Accepted Manuscript in appearance. This material was termed Pt/BC nanocomposite. The corresponding samples are denoted as BH-Pt/BC or HC-Pt/BC. Finally, the obtained platinum/BC composites were rinsed with DI and freeze dried.

2.3. Chemical modification of BC membranes

In the present study, doping with proton acid or inorganic acid on the BC pellicles was performed in an attempt to enhance the capability of proton exchange [13-14]. Modification of matrix was prepared by equilibrating dehydrated BC composites in 5% solution of H3PW12O40·29H2O (PWA), for 12h. The modification of BC membranes was frozen at -40 oC and dried in a vacuum at -52 oC.

2.4. Characterization

The morphology and composition of the obtained composite were analyzed by using scanning electron microscopy (SEM, JEOLJSM-6380LV) transmission electron microscopy(JEM-2100), and energy dispersive spectroscopy (EDS, ISIS30). Thermogravimetric analysis (TG) and differential thermal analysis (DTA) were carried by using a TGA/SDTA85 instrument. The samples were kept in a Platinum crucible and heated in a furnace, flushed with air at the rate of 200 mlmin−1, from 30 oC to 700 oC, at a heating rate of 10 oC·min−1. Platinum/BC nanocomposite were crystallographically

Page 6 of 29 Accepted Manuscript characterized by X-ray diffractometry (Bruker D8 ADVANCE) with an area detector using a CuKα source (λ = 1.54056 A) operating at 40kV and 40mA.

2.5. Electrochemical measurements

The electrochemical characterization was carried out by the cyclic voltammetry(CV) using a potentiostat (CHI630B) connected to a three electrode test cell. The working electrode was a thin layer of BH-Pt/BC or CH-Pt/BC cast on a piece of PTFE hydrophobizedcarbon paper (0.5 X 1.0 cm2 ). The loading of Pt on the carbon paper was 0.5mgcm-2. Platinum plate and a saturated calomel electrode (SCE) were used as the counter and the reference electrode. The Pt surface areas of the catalysts were estimated fromhydrogen adsorption charges in cathodicvoltammograms, which were obtained between-0.2V and +1.2V versusSCEat a scan rate of 50mV/s in N-purged electrolytes. For cyclic voltammetry of hydrogen adsorption, the electrolyte solution which is 0.5M

H2SO4 was de-aerated with high-purity nitrogen for 2h prior to measurement.

A single fuel cell was assembled from a MEA, two copper net plates on the supply sides for gas, and two Teflon gaskets. The construction of single fuel cell is shown in Fig.1a. The PEM is hydrated native or chemical modification of BC. The BH-Pt/BC (20 wt%) or Pt/C (E-TEK 20wt%Pt/C) and the acetylene black (3wt%) were ultrasonicated and stirred in the distilled water-isopropanol (1:3 volume ratio) solution for 12 h to obtain a

Page 7 of 29 Accepted Manuscript homogeneous black suspension solution.The catalyst ink was brushed onto both side of the membranewith the Pt loading of 0.5 mg/cm2, and then this assembly was rapidly dried applying a vacuum dryer.The drying step caused the assembly to become dehydrated to MEA. The MEA may be assembled together by H-bonding between the fibrils without adhesives or glues, so the catalyst layers can prevent the bonding or the catalysts can be destroyed due to the bonding process [15]. Current generated by application of H2 to the anode of the MEA, was measured by an ammeter connected to the current collectors. The total thickness of the MEA was approximately 100 um and the active area of MEA was 6.25 cm2. The flow rate of H2 and O2was regulated by a flow meter at 10 and 20 cm3/min. The single fuel cell testing system is shown in Fig.1b.

3. Result and discussion

3.1. SEM observation and EDS analysis

The SEM images of bare BC nanofibers and the TEM images of Pt/BC hybrid nanofibers are presented in Fig.2. The SEM image of Fig.2a shows a side view of the BC nanofibers, with an average diameter of about 30 nm and a length ranging from micrometers up to dozens of micrometers. The well-organized three-dimensional network structure is synthesized by Acetobacter xylinum during culture. As can be seen from Fig.2a, the BC is porous with interconnecting pores. The pore size varies in a 5-10 um range. With this structure, BC own the ability to incorporate fine divided metals [16].

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The TEM images of Pt/BC nanocomposite membranes show that nanoparticles are discrete in BC (Fig.2b, Fig.2c). The migration of platinum nanoparticles to the inner structure of BC can be explained by a mass diffusion driven process. In fact, bacterial cellulose contains 95% of water, which in this experiment corresponds to have a porous membrane separating an aqueous colloid from bulk water. For comparison, when NaBH4 and HCHO were used to reduce Pt4+ to Pt metal (Reaction1 and Reaction2), we found that there are some differences in platinum nanoparticles scale. The effects of the liquid phase chemical deoxidization method on the morphology of resulting platinum nanoparticles were strongly dependent on the reducibility of reductants. Typical TEM images of the BH-Pt/BC presented in Fig.2b show remarkably more uniform and higher dispersion of the metal particles than TEM images of the CH-Pt/BC presented in Fig.2c. The average diameters of 3.3 nm for BH-Pt/BC and 3.9 nm for CH-Pt/BC were observed though these images.

2622226HPtClHCHOHOPtHCOOHHCl(Reaction1)
26422323212HPtClNaBHHOPtNaHBOHCl(Reaction2)

The platinum/BC nanocomposite membranes were determined by SEM-EDS. Platinum atoms were detected on both BH-Pt/BC and CH-Pt/BC samples, confirming that through liquid phase chemical deoxidization method is an effective method of in-situ preparation of platinum nanoparticles on the surface of BC membrane. The synthesis of the Pt

Page 9 of 29 Accepted Manuscript catalyst imbedded in BC can be explained by the following in-situ deposition mechanism (shown in Fig.3). At first diffusion of Pt ions into BC matrix leads to coordination with the cellulose nonionic hydroxyl groups. NaBH4 or HCHO is then added to the solutionto reduce the platinum. The average Pt contents of BH-Pt/BC samples surface is 19.2 wt%

(Table 1), which is higher than the CH-Pt/BC (14.4%). The high platinum contents of BH-Pt/BC surface possible due to reaction of in-situ preparation of platinum in strong reducing environments faster than in weak reducing environments, the faster reaction leads to increase the total deposited platinum on membrane surface and to lower the platinum penetration in the membrane.

3.2. XRD analysis

The XRD patterns of Pt/BC under different reductant agent conditions are given in

Fig.4.In the case of two broad peaks located at 14.6, and 23.2 are attributed to the BC. The two catalysts exhibited characteristic diffraction peaks of Pt (1) at 2θ of 39.8◦, Pt (200) at 2θ of 46.9◦, and Pt (220) at 2θ of 67.5◦. The peaks can be indexed to the [1], [200], [220] reflections of a Pt face-centered cubic (f c c) crystal structure. The broader diffraction peaks for the two catalysts also lead to smaller average particle size as calculated by the Scherrer equation [17]. The calculation results, which estimated the average size of 3.1 nm for BH-Pt/BC and 3.5 nm for HC-Pt/BC, are in good agreement with the TEM measurements.

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The relative contents of the Pt(1), Pt(200), and Pt(220) crystal faces can be calculated by the equation [18]:

IIIIIlllhklhkl(1)

Where I h k l is relative values of diffraction. The relative content of crystal faces of the samples was shown in table 2. XRD analysis showed that the relative content of the Pt (1) crystal face in the BH-Pt/BC is 63.1%, which is higher than the normal value of 54.35%. This benefited the acceleration of the hydrogen reaction in PEMFC [19]. The relative content of the Pt (1) crystal face in the HC-Pt/BC is 49.5 %.

3.3. Electrochemical performances

The morphology, contents, and the relative content of the Pt (1) crystal face of the obtained platinum/BC evaluated shown that BH-Pt/BC may be more suitable for catalysts lay of fuel cell than CH-Pt/BC.So we characterized BH-Pt/BC and CH-Pt/BC catalysts electrochemically active by cyclic voltammetry in an electrolyte of 0.5M H2SO4 and the resulting voltammograms are shown in Fig.5.It can be seenfrom Fig.5 that the hydrogen adsorption and desorption peaksare located at about -0.15V and the Pt redox peaks are at

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