C as ethanol tolerant oxygen reduction reaction catalyst for direct ethanol fuel cells

Accepted Manuscript Title: Spillover effect induced Pt-TiO2 /C as ethanol tolerant oxygen reduction reaction catalyst for direct ethanol fuel cells Author: S. Meenakshi K.G. Nishanth P. Sridhar S. Pitchumani PII: DOI: Reference:

S0013-4686(14)00902-5 http://dx.doi.org/doi:10.1016/j.electacta.2014.04.142 EA 22648

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

13-2-2014 21-4-2014 23-4-2014

Please cite this article as: S. Meenakshi, K.G. Nishanth, P. Sridhar, S. Pitchumani, Spillover effect induced Pt-TiO2 /C as ethanol tolerant oxygen reduction reaction catalyst for direct ethanol fuel cells, Electrochimica Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.04.142 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.

Spillover effect induced Pt-TiO2/C as ethanol tolerant oxygen reduction reaction catalyst for direct ethanol fuel cells S. Meenakshi, K.G. Nishanth, P. Sridhar*, S. Pitchumani

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CSIR-Central Electrochemical Research Institute -Madras Unit, CSIR Madras Complex, Taramani, Chennai, 600 113, India

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______________________________________________________________________________

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ABSTRACT

Hypo-hyper-d-electronic interactive nature is used to develop a new carbon supported HT-Pt-

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TiO2 composite catalyst comprising Pt and Ti in varying atomic ratio, namely 1:1, 2:1 and 3:1. The electro-catalysts are characterized by XRD, TEM, SEM-EDAX, Cyclic Voltammetry (CV) and Linear sweep voltammetry (LSV) techniques. HT-Pt-TiO2/C catalysts exhibit significant

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improvement in oxygen reduction reaction (ORR) over Pt/C. The effect of composition towards ORR with and without ethanol has been studied. The direct ethanol fuel cell (DEFC) with HT-Pt-

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TiO2/C cathode catalyst exhibits an enhanced peak power density of 41 mW cm-2, whereas identical conditions. Composite

materials;

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Keywords:

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21 mW cm-2 is obtained for the DEFCs with carbon-supported Pt catalyst operating under

chemical

synthesis;

electrochemical

techniques;

electrochemical properties.

_____________________________________________________________________________ *Corresponding author. Tel.: +91 44 2254 4554; fax: +91 44 2254 2456. E-mail address: [email protected] (P. Sridhar).

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1. Introduction Direct alcohol fuel cells fed by liquid fuels such as methanol and ethanol have the

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advantages of simple structure, portability, fuel availability and they have potentially extensive applications in power sources for mobile, stationary and portable devices [1-8]. Methanol has

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been considered the most promising fuel because it is more efficiently oxidized than other alcohols [9,10]. However, methanol is a toxic compound and its use on a large scale can cause

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some environmental problems. As an alternative fuel, ethanol is safer and compared with

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methanol (6.1 kWh kg-1), ethanol has higher energy density (8.2 kWh kg-1). In addition, it can be obtained in large quantity from biomass through a fermentation process of renewable resources

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such as sugarcane, wheat, corn or straw. These features make ethanol more attractive than methanol for direct alcohol fuel cells [11-14]. But Direct Ethanol Fuel Cells (DEFCs) are

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plagued with certain scientific and technological difficulties, namely the sluggish kinetics of

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anode and cathode reactions and crossover of ethanol from anode to cathode across the proton

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exchange membrane, which are precluding their commercialization. The crossover of ethanol results in parasitic ethanol-oxidation on the cathode leading to a mixed potential that not only lowers fuel utilization efficiency but also adversely affects the cathode performance and consequently the overall fuel cell efficiency [15,16]. Ethanol crossover in a DEFC can be mitigated either by using an ethanol impermeable membrane or by employing an ethanol tolerant-oxygen-reduction catalyst.

The most familiar oxygen reduction reaction (ORR) is based on noble metals, particularly platinum. Carbon supported platinum based materials are the most active, efficient and successful catalysts for electrochemical devices at the current technology stage. But the problem 2    Page 2 of 31

associated with Pt catalyst is the slowness of ORR which is due to the formation of –OH species at +0.8 V, which inhibits further reduction of oxygen and hence, results in loss of performance [17]. Research on alternative cathode catalysts is still necessary in order to find a material with

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an improved ORR activity and a higher ethanol tolerance than those of Pt. So researchers focused on alloys or composite catalysts such as that prevent the adsorption of –OH species,

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reduce oxygen selectively to water without being affected by the contaminants and tolerant to

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ethanol. The enhancement in the ORR activity observed when using supported Pt–M alloy electro-catalyst was ascribed to both geometric (decrease of the Pt–Pt bond distance) and

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electronic factors (increase of Pt d-electron vacancy) [18-20].

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Pt-based bimetallic catalyst (e.g Pt-Fe, Pt-Ni, Pt-Co, etc.) for ORR has been reported [21]. Pt-based alloys comprising Pt and other transition metals, namely Co and Pd, have been

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extensively investigated owing to their enhanced ORR activity. Pd, Pd alloys, Pt-Co, NiCoFe/C

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and Pt-Fe/C cathode catalyst have been studied in the presence of alcohols demonstrating high

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performances for ORR [22-28].

Recently, much of the research work is focused on the system of combining metal oxides, carbon and Pt. Strong d–d-Metal-Support Interaction (SMSI) of hyper-d-electronic metal with mostly hypo-d-oxide (TiO2, ZrO2, HfO2) or various hypo-d-hypo-f-oxide supporting composites (TiO2, CeO2). SMSI implies that the d–d-metal-oxide interaction (M/TiO2) for heterogeneous catalysis, in accordance with the bonding strength, results in substantial weakening and even suppression of intermediate chemisorptive bonds (M–H, M–CO). A new concept additionally implies the spillover of interactive primary oxides (M–OH) and their decisive interference in the overall catalytic process. However, while the dynamic spillover effect of the primary oxide 3    Page 3 of 31

(M–OH) dipoles repulsion takes place all over the available metallic surface regardless its nanosized magnitude, the stronger the hypo-hyper-d-interionic metal-oxide support interaction the

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higher the overall catalytic activity [29-35]. In particular transition metal oxides like TiO2 and WO3 have been used. The working

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principle of such metal oxide systems is based on the hydrophilic behavior due to water molecules trapped inside the oxide network turning to hydrous network, which substantially

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behaves as a continuous undisturbed reversible membrane mechanism of the altervalent changes

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resulting in OH− transfer within the system with consequent spillover of primary oxide over metallic catalyst particle [36]. Among the different metal oxides, TiO2 is more attractive due to

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its better electrochemical properties and stability under fuel cell operating conditions. In this study, hybrid TiO2-carbon material is proposed as a fuel cell catalyst support. The hybrid

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material possesses both electron conductivity of carbon and corrosion resistance of the oxide and

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even a synergistic effect. HT-Pt-TiO2/C (Pt: Ti in atomic ratio of 1:1, 2:1 and 3:1) and Pt/C

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electrocatalysts were prepared using ethylene glycol as the reducing agent. XRD and TEM characterizations were carried out to determine the particle size and distribution of the catalysts. ORR activity, cyclic voltammetry and single-cell performance of DEFCs comprising these catalysts were studied. 2. Experimental 2.1. Materials Dihydrogen hexachloroplatinate (IV) hexahydrate (H2PtCl6.6H2O) was procured from Johnson Matthey Chemicals India Pvt. Ltd. Titanium isopropoxide was procured from Aldrich. Commercial Gas Diffusion Layer (GDL) DC 35 series procured from Sigracet, SGL group. 4    Page 4 of 31

Vulcan XC-72 carbon was procured from Cabot Corporation. 5 wt.% Nafion ionomer was procured from DuPont, US. 2-Propanol and Ethanol were obtained from Merck-India and

water (18 MΩ cm) was used during the study.

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2.2. Preparation of carbon supported HT-Pt-TiO2 composite catalyst

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Perchloric acid (HClO4) from Rankem-India. All chemicals were used as received. De-ionized

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Pt-TiO2 (40 wt.%)/C catalyst was prepared by alcohol reduction process. In brief, titanium isopropoxide was diluted with isopropanol and the required amount of Vulcan XC-72

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carbon was added with continuous stirring. The resultant mixture was kept stirring for 24 h to form a uniform suspension, followed by drying at 80 oC. After that, the required amount of the

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above prepared support and chloroplatinic acid were dispersed with ethylene glycol. The suspension was then treated in an ultrasonic bath and refluxed for 5 h at 130 oC. The product was

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collected by filtration and washed well with copious DI water, then dried at 80 oC for 24 h. Pt-

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TiO2/C composite catalysts containing Pt and Ti in varying atomic weight ratios, namely 1:1, 2:1

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and 3:1, were prepared and subjected to heat-treatment at 750 oC  (temperature chosen based on our earlier study)  in a flowing mixture of 90% N2-10% H2 for 5 h and cooled to room temperature. Carbon-supported Pt was also prepared by the same procedure [37]. In the following text, carbon-supported platinum heat-treated at 750 oC is represented as HT-Pt/C and the as-received platinum catalyst is represented as Pt/C. The Pt–TiO2/C samples with varying atomic ratios of Pt to Ti, namely, 3:1, 2:1 and 1:1 are denoted as HT-PtTiO2(3:1)/C, HT-Pt-TiO2(2:1)/C and HT-Pt-TiO2(1:1)/C.

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2.3. Physical characterization of the catalyst Powder XRD patterns for the catalysts were obtained on a BRUKER-binary V3

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diffractometer using Cu Kα radiation (λ =1.5406 Å) between 20 and 80o in reflection geometry in steps of 5o min-1. For XRD characterization, about 30 mg of catalyst powder was pressed onto

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a quartz block using a glass slide to obtain uniform distribution. The crystallite size was

⎛ 0.9λ ⎞ ⎟ ⎝ B cos θ ⎠

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D= ⎜

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estimated from the (1 1 1) peak width according to the Scherrer equation as given below:

(1)                         

where λ is the X-ray wavelength, B is the peak width at half height in radians and θ is the

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diffraction angle.

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Surface scanning electron micrographs and Energy dispersive analysis by X-rays

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(EDAX) for electro-catalysts were obtained using JEOL JSM 35CF scanning electron

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microscope (SEM). The structure and morphology of electro-catalysts were examined under a 200 kV Tecnai-20 G2 transmission electron microscope (TEM). The samples were suspended in acetone with ultrasonic dispersion for 3 min. Subsequently, a drop of the suspension was deposited on a holey carbon grid followed by drying. TEM images for the samples were recorded with a Multiscan CCD Camera (Model 794, Gatan) using low-dose condition. 2.4. Electrochemical characterization of the catalyst Electrochemical measurements were carried out using a Biologic science instruments (VSP) with EC-Lab software. A glassy carbon (GC) disk with a geometrical area of 0.071 cm2 was used as a working electrode substrate for Cyclic Voltammetry (CV) and Linear Sweep 6    Page 6 of 31

Voltammetry (LSV) measurements. A saturated calomel electrode (SCE) and Pt foil were used as reference and counter electrodes, respectively. The working electrodes in the electrochemical experiments were prepared using an ink made of catalyst materials, prepared by dispersing

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required amount of catalyst in 1 mL of water and mixing for 10 min in an ultrasonic bath. After that, 10 μL aliquot of the dispersion was pipetted onto the glassy carbon substrate of the disc.

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After the evaporation of the water, 5 μL of a diluted Nafion solution (0.05 M) were pipetted onto

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the electrode surface to attach the catalyst particles onto the glassy carbon. The electrode was dried at room temperature. Prior to any electrochemical measurement, the working electrode was

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cycled between -0.25 and 0.8 V with respect to SCE at a sweep rate of 50 mV s-1 to activate the electrode. The rotating rate of the rotating disk electrode (RDE) was maintained at 1600 rpm

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throughout the experiment at a sweep rate of 5 mV s-1. CV and LSV were performed in solutions

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containing aq. 0.5 M HClO4 and 1.0 M ethanol saturated with N2 and O2.

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2.5. Preparation of membrane-electrode assemblies and their performance evaluation

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The aforesaid catalysts were performance evaluated in DEFCs by making membrane electrode assemblies (MEAs). The cathode catalyst layer comprising 2 mg cm-2 of HT-PtTiO2(2:1)/C or Pt/C or HT-Pt/C and 30 wt.% Nafion was coated on to the GDL, while Pt-Sn (3:1 atomic ratio) 2 mg cm-2, supported on Vulcan XC-72 carbon mixed with 10 wt.% Nafion coated on to one of the other GDL constituted the anode catalyst layer. The active area for the DEFC was 4 cm2. MEAs with Nafion 117 membrane was obtained by sandwiching the membrane between each of the above electrodes followed by hot-pressing at 130 oC for 3 min at a pressure of 20 kg cm-2. MEAs were evaluated using a conventional fuel cell fixture with parallel serpentine flow-field machined on graphite plates. The cells were tested at 70 oC with

2 M aq.

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ethanol at a flow rate of 2 mL min-1 at the anode and oxygen at a flow rate of 300 mL min-1 at atmospheric pressure at the cathode, respectively. Measurements for cell potentials with varying current densities were conducted galvanostatically using Model-LCN4-25-24/LCN 50-24

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procured from Bitrode Instruments (US). Stabilities of the HT-Pt-TiO2(2:1)/C catalyst was evaluated by conditioning the DEFCs at OCV for 10 h and recording the cell voltage with

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respect to time at 70 oC.

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3. Results and discussion

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3.1. XRD analysis for the catalysts

Fig. 1 presents XRD patterns of HT-Pt-TiO2/C catalysts, along with the Pt/C catalyst and

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HT-Pt/C. The diffraction peak at 25o is due to the Carbon (0 0 2) plane, while the peaks at 40o,

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46o and 67.5o represent the Pt (1 1 1), Pt (2 0 0) and Pt (2 2 0) planes, respectively. Absence of

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peak corresponding to titanium oxide indicates that the oxide deposited is very fine [38]. Crystallite size of the catalysts (based on the 111 plane) is summarized in Table 1. It clearly

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shows that HT-Pt-TiO2/C catalysts have smaller crystallite size in comparison with HT-Pt/C. 3.2. TEM and SEM analysis for the catalysts Fig. 2(a-c) shows the TEM images of Pt/C, HT-Pt/C and HT-Pt-TiO2(2:1)/C catalysts,

respectively. The corresponding histograms of the particle size distribution are shown in Fig. 3(a-c), which included analyses of different regions reflecting quantitatively the uniform distribution of the catalyst. The mean particle size obtained for the catalysts are presented in Table 1. This implies that the ethylene glycol synthesis method could lead to the formation of homogeneous and small particles on carbon. From the SEM image (Fig. 4(a)), it is clear that the 8    Page 8 of 31

HT-Pt-TiO2 particles are uniformly distributed on the carbon surface and the incorporation of TiO2 in the catalyst support is confirmed as seen in the typical image (Fig. 4(b)) given for HT-Pt-

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TiO2 (2:1)/C catalyst. 3.3. Electrochemical characterization

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Cyclic voltammetric technique has proven very useful in obtaining information on the

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stability in the reaction media and participation of the active sites on the electrode surfaces. Fig. 5 presents cyclic voltammogram of HT-Pt-TiO2(1:1)/C, HT-Pt-TiO2(2:1)/C, HT-Pt-

between 0 and 1.0 V (vs. NHE). All

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TiO2(3:1)/C, HT-Pt/C and Pt/C electro-catalyst in aq. 0.5 M HClO4 at a scan rate of 50 mV s-1 the catalysts show peaks associated with hydrogen

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adsorption/desorption between 0 and 0.3 V (vs. NHE) followed by the “double-layer” potential region; above 0.7 V (vs. NHE) oxide formation/reduction regions are observed. The

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electrochemical active surface area (ESA) is obtained using the charge corresponding to the

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reduction of oxide, which is formed due to adsorption of a monolayer of chemisorbed oxygen

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instead of using hydrogen adsorption/desorption peaks that may not give the exact charge as there is interference from hydrogen evolution reaction. ESA can be calculated from the following equation [39,40].

2

-1

ESA (cm g

Pt)

=

(

Q O μC cm

−2

−2

)

(

420 μC cm × electrode loading g Pt

cm

−2

)

=

(2)

The calculated ESA values of all the catalysts are presented in Table 1. HT-Pt-TiO2(2:1)/C catalyst shows higher ESA compared to all the other electro-catalysts. Increase in ESA is due to the formation of new sites at the interface between Pt and TiO2 [38,41].

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The experimental results pertaining to the ORR for HT-Pt-TiO2/C catalysts with varying Pt to Ti atomic ratios and Pt/C in the O2-saturated aq. 0.5 M HClO4 at a scan rate of 5 mV s-1 are presented in Fig. 6. The onset potentials for ORR on Pt/C, HT-Pt-TiO2(1:1)/C, HT-Pt-TiO2(2:1)

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and HT-Pt-TiO2(3:1)/C catalysts are 0.89, 0.91, 0.99 and 0.98 V, respectively confirming that HT-Pt-TiO2(2:1)/C and HT-Pt-TiO2(3:1)/C have higher activities than HT-Pt TiO2(1:1)/C and

The improved ORR is due to the ready adsorption and easy

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90 mV compared to Pt/C.

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Pt/C. The onset potential for oxygen reduction is shifted to more positive potential by about

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dissociation of O2 on the TiO2-modified Pt surface in relation to Pt/C.

Fig. 7 shows the ORR curves of Pt/C and HT-Pt-TiO2/C catalysts recorded in

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O2-saturated solution containing aq. 0.5 M HClO4 and 1 M ethanol at a scan rate of 5 mVs-1. Anodic currents of 17 mA cm-2 and 14.8 mA cm-2 are observed for Pt/C and HT-Pt-TiO2(3:1)/C

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catalysts at 0.8 V (vs. NHE) while lower anodic currents of 0.8 mA cm-2 and 1.84 mA cm-2 are

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obtained for HT-Pt-TiO2(2:1)/C and HT-Pt-TiO2(1:1)/C, respectively. The anodic current is due

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to the electrooxidation of ethanol. Therefore, it is evident that the HT-Pt-TiO2(2:1)/C catalyst possesses a lower catalytic activity towards the ethanol oxidation compared to all the other catalysts. However, HT-Pt-TiO2(2:1)/C exhibits a higher ORR activity with minimum ethanol oxidation. The onset potential for ORR in presence of ethanol on HT-Pt-TiO2(2:1)/C is shifted to more positive potential by about 271 mV compared with that on Pt/C. Ethanol adsorption and oxygen adsorption are competing with each other for the surface sites. These studies confirm that the HT-Pt-TiO2(2:1) /C has a higher ORR selectivity and better ethanol tolerance in relation to Pt/C. It is possible that HT-Pt-TiO2/C catalyst can be used as ethanol tolerant cathode catalyst for a DEFC.

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3.4. Performance evaluation for DEFCs Fig. 8 shows the performance of the DEFC for HT-Pt-TiO2(2:1)/C cathode catalyst,

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chosen based on the ORR studies, at various temperatures. It shows an enhancement of the cell performance with increase in temperature. Fig. 9 shows the performance of the DEFCs for Pt/C,

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HT-Pt/C and HT-Pt-TiO2(2:1)/C cathode catalysts at 70 oC. As can be seen, the open circuit voltage (OCV) of the fuel cell containing Pt/C is 0.65 V, while the corresponding value for HT-

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Pt-TiO2(2:1)/C increased to 0.7 V. The increase in OCV indicates that HT-Pt-TiO2(2:1)/C is

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poisoned less by adsorbed species from ethanol than the Pt/C. Further, the polarization curve for DEFC cathode containing HT-Pt-TiO2(2:1)/C clearly shows superior performance than that

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containing Pt/C, reflecting its higher ethanol tolerance. It is possibly due to the presence of TiO2 around Pt active sites that could block ethanol adsorption on Pt sites due to the dilution effect.

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The stability test carried out under OCV conditions for 10 h duration for the HT-Pt-TiO2(2:1)/C

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stability of the catalyst.

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catalyst is shown in Fig. 10. The constancy in the value during the test period clearly shows the

4. Conclusions

Carbon supported HT-Pt-TiO2 catalysts were synthesized by alcohol-reduction method. TEM and XRD results show that the HT-Pt-TiO2/C catalyst has uniform distribution. ORR studies in the presence and absence of ethanol on HT-Pt-TiO2/C catalyst containing Pt and Ti in varying atomic ratio show that HT-Pt-TiO2(2:1)/C exhibits higher ORR activity. DEFC employing HT-Pt-TiO2(2:1)/C as cathode catalyst exhibits better performance than that with Pt/C. Therefore, HT-Pt-TiO2/C catalyst is a good cathode catalyst for ethanol tolerant-oxygenreduction reaction in direct ethanol fuel cells 11    Page 11 of 31

Acknowledgements S. Meenakshi is grateful to CSIR, New Delhi, for a Senior Research Fellowship. The

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authors are thankful to Dr. Vijaymohanan Pillai, Director, CSIR-CECRI for his encouragement

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and support.

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References [1] B.D. McNicol, D.A.J. Rand, K.R. Williams, Direct methanol-air fuel cells for road

ip t

transportation, J. Power Sources 83 (1999) 15. [2] H. Dohle, H. Schmitz, T. Bewer, J. Mergel, D. Stolten, Development of a compact 500 W

cr

class direct methanol fuel cell stack, J. Power Sources 106 (2002) 313.

us

[3] X. Ren, P. Zelenay, S. Thomas, J. Davey, S. Gottesfeld, Recent advances in direct methanol

an

fuel cells at Los Alamos National Laboratory, J. Power Source 86 (2000) 111. [4] S.C. Thomas, X. Ren, S. Gottsfeld, P. Zelenay, Direct methanol fuel cells: progress in cell

M

performance and cathode research, Electrochim. Acta 47 (2002) 3741. [5] R. Dillon, S. Srinivasan, A.S. Aricò, V. Antonucci, International activities in DMFC R&D:

te

d

status of technologies and potential applications, J. Power Sources 127 (2004) 112. [6] H. Li, D. Kang, H. Wang, R. Wang, Carbon-supported Pt-RuCo nanoparticles with low-

Ac ce p

noble-metal content and superior catalysis for ethanol oxidization, Int. J. Electrochem. Sci. 6 (2011) 1058.

[7] C. Lamy, A. Lima, V. LeRhun, F. Delime, C. Coutanceau, J.M. L´eger, Recent advances in the development of direct alcohol fuel cells (DAFC), J. Power Sources 105 (2002) 283. [8] E. Antolini, Catalysts for direct ethanol fuel cells, J. Power Sources 170 (2007) 1. [9] K. Scott, A.K. Shukla, Direct methanol fuel cells: fundamentals, problems and persepectives, in: R.E. White, C.G. Vayenas, M.A. Gamboa-Aldeco (Eds.), Modern Aspects of Electrochemistry, Springer, New York, 2007. 13    Page 13 of 31

[10] A.S.  Aricò, V. Baglio, V. Antonucci, in: Electrocatalysis of Direct Methanol Fuel Cells, H. Liu, J. Jhang (Eds.), Wiley-VCH, Wienheim, 2009.

ip t

[11] S. Rousseau, C. Coutanceau, C. Lamy, J.-M. Léger, Direct ethanol fuel cell (DEFC): Electrical performances and reaction products distribution under operating conditions with

cr

different platinum-based anodes, J. Power Sources 158 (2006) 18.

us

[12] C. Lamy,  J.-M. Léger, Fuel cells-application to electric vehicles, J. Phys. IV 4 (1994) C1253.

an

[13] J.B. Xu, T.S. Zhao, Y.S. Li, W.W. Yang, Synthesis and characterization of the Au-modified Pd cathode catalyst for alkaline direct ethanol fuel cells, Int. J. Hydrogen Energy 35 (2010)

M

9693.

d

[14] F. Colmati, E. Antolini, E.R. Gonzalez, Effect of thermal treatment on phase composition

te

and ethanol oxidation activity of a carbon supported Pt50Sn50 alloy catalyst, J. Solid State

Ac ce p

Electrochem.12 (2008) 591.

[15] N.S. Suresh, S. Jayanti, Cross-over and performance modeling of liquid-feed polymer electrolyte membrane direct ethanol fuel cells, Int. J. Hydrogen Energy 36 (2011) 14648. [16] M. Neergat, D. Leveratto, U. Stimming, Catalysts for direct methanol fuel cells, Fuel Cells 2 (2002) 25.

[17] C.C. Liang, A.L. Juliard, The overpotential of oxygen reduction at platinum electrodes, J. Electroanal. Chem. 9 (1965) 390.

14    Page 14 of 31

[18] S. Mukerjee, S. Srinivasan, Enhanced electrocatalysis of oxygen reduction on platinum alloys in proton exchange membrane fuel cells, J. Electroanal. Chem. 357 (1993) 201.

ip t

[19] Q. Huang, H. Yang, Y. Tang, T. Lu, D.L. Akins, Carbon-supported Pt-Co alloy nanoparticles for oxygen reduction reaction, Electrochem. Commun. 8 (2006) 1220.

cr

[20] E. Antolini, R.R. Passos, E.A. Ticianelli, Electrocatalysis of oxygen reduction on a carbon

us

supported platinum-vanadium alloy in polymer electrolyte fuel cells, Electrochim. Acta 48 (2002) 263.

an

[21] W. Li, W. Zhou, H. Li, Z. Zhou, B. Zhou, G. Sun, Q. Xin, Nano-stuctured Pt-Fe/C as

M

cathode catalyst in direct methanol fuel cell, Electrochim. Acta 49 (2004) 1045. [22] O. Savadogo, F.J. Rodríguez Valera, Palladium-alloy catalysts as ethanol tolerant cathodes

d

for direct alcohol fuel cell (DEFC) applications, ECS Transactions 1 (2006) 247.

te

[23] F.J. Rodríguez Varela, O. Savadogo, Electroactive alcohol-tolerant Pt-alloys, ECS

Ac ce p

Transactions 3 (2006) 181.

[24] L. Jiang, G. Sun, S. Sun, J. Liu, S. Tang, H. Li, B. Zhou, Q. Xin, Structure and chemical composition of supported Pt-Sn electrocatalysts for ethanol oxidation, Electrochim. Acta 50 (2005) 5384.

[25] E. Antolini, J.R.C. Salgado, M.J. Giz, E.R. Gonzalez, Effects of geometric and electronic factors on ORR activity of carbon supported Pt-Co electrocatalysts in PEM fuel cells, Int. J. Hydrogen Energy 30 (2005) 1213.

15    Page 15 of 31

[26] T. Lopes, E. Antolini, F. Colmati, E.R. Gonzalez, Carbon supported Pt-Co (3:1) alloy as improved cathode electrocatalyst for direct ethanol fuel cells, J. Power Sources 164 (2007)

ip t

111. [27] N. Park, T. Shiraishi, K. Kamisugi, Y. Hara, K. Iizuka, T. Kado, S. Hayase, NiCoFe/C

A. Bonesi,

W.E. Triaca,

A. Di Blasi,

A. Stassi, V. Baglio,

us

[28] A.M. Castro Luna,

cr

cathode electrocatalysts for direct ethanol fuel cells, J. Appl. Electrochem. 38 (2008) 371.

V .Antonucci, A.S .Aricò, Investigation of a Pt-Fe/C catalyst for oxygen reduction reaction

an

in direct ethanol fuel cells, J. Nanopart. Res. 12 (2010) 357.

[29] F. Rosalbino, S. Delsante, G. Borzone, E. Angelini, Electrocatalytic behaviour of Co-Ni-R

M

(R=Rare earth metal) crystalline alloys as electrode materials for hydrogen evolution

d

reaction in alkaline medium, Int. J. Hydrogen Energy 33 (2008) 6696.

te

[30] M.M. Jaksic, Hypo-hyper-d-electronic interactive nature of synergism in catalysis and

Ac ce p

electrocatalysis for hydrogen reactions, Electrochim. Acta 45 (2000) 4085. [31] S.G. Neophytides, K. Murase, S. Zafeiratos, G.D. Papakonstantinou, F.E. Paloukis, N.V. Krstajic, M.M. Jaksic, Composite hypo-hyper-d-intermettallic and interionic phases as supported interactive electrocatalysts, J. Phys. Chem. B 110 (2006) 3030. [32] J.M. Jaksic, D. Labou, G.D. Papakonstantinou, A. Siokou, M.M. Jaksic, Novel spillover interrelating reversible electrocatalysts for oxygen and hydrogen electrode reactions,  J. Phys.Chem. C 114 (2010) 18298.

16    Page 16 of 31

[33] J.M. Jaksic, N.V. Krstajic, L.M. Vracar, S.G. Neophytides, D. Labou, P. Falaras, M.M. Jaksic, Spillover of primary oxides as a dynamic catalytic effect of interactive hypo-d-

ip t

oxide supports, Electrochim. Acta 53 (2007) 349. [34] M.M. Jaksic, Hypo-hyper-d-electronic interactive nature of interionic synergism in catalysis

cr

and electrocatalysis for hydrogen reactions, Int. J. Hydrogen Energy 26 (2001) 559.

us

[35] X. Wang, W. Xu, X. Zhou, T. Lu, W. Xing, C. Liu, J. Liao,  PtCeOx/C as a novel methanoltolerant electrocatalyst of oxygen reduction for direct methanol fuel cells, J. Solid State

an

Electrochem. 13 (2009) 1449.

[36] K.G. Nishanth, P. Sridhar, S. Pitchumani, Enhanced oxygen reduction reaction activity

M

through spillover effect by Pt-Y(OH)3/C catalyst in direct methanol fuel cells, Electrochem.

d

Commun. 13 (2011) 1465.

te

[37] G. Selvarani, S. Maheswari, P. Sridhar, S. Pitchumani, A.K. Shukla, Carbon-supported Pt-

Ac ce p

TiO2 as a methanol-tolerant oxygen-reduction catalyst for DMFCs, J. Electrochem. Soc. 156 (2009) B1354.

[38] L. Xiong, A. Manthiram, Synthesis and characterization of methanol tolarant Pt/TiOx/C nanocomposites for oxygen reduction in direct methanol fuel cells, Electrochim. Acta 49 (2004) 4163.

[39] S. Maheswari, S. Karthikeyan, P. Murugan, P. Sridhar, S. Pitchumani, Carbon-supported Pd-Co as cathode catalyst for APEMFCs and validation by DFT, Phys. Chem. Chem. Phys. 14 (2012) 9683.

17    Page 17 of 31

[40] G.F. A´lvarez, M. Mamlouk, K. Scott, An investigation of palladium oxygen reduction catalysts for the direct methanol fuel cell, Int. J. Electrochem. Sci. 2011 ( 2011) 684535.

ip t

[41] H.J. Kim, D.Y. Kim, H. Han, Y.G. Shul, PtRu/C-Au/TiO2 electrocatalyst for a direct

Ac ce p

te

d

M

an

us

cr

methanol fuel cell, J. Power Sources 159 (2006) 484.

18    Page 18 of 31

Figure captions Fig. 1. Powder XRD patterns for (a) Pt/C, (b) HT-Pt/C, (c) HT-Pt-TiO2(3:1)/C, (d) HT-Pt-

ip t

TiO2(2:1)/C and (e) HT-Pt-TiO2(1:1)/C catalysts. Fig. 2. Transmission electron micrographs for (a) Pt/C, (b) HT-Pt/C and (c) HT-Pt-TiO2(2:1)/C.

cr

Fig. 3. The histograms of particle-size distribution for (a) Pt/C, (b) HT-Pt/C and (c) HT-Pt-

us

TiO2(2:1)/C

an

Fig. 4. (a) SEM micrographs and (b) EDAX image for HT-Pt-TiO2(2:1)/C catalyst. Fig. 5. Cyclic voltammograms for Pt/C, HT-Pt/C and HT-Pt-TiO2/C catalysts containing Pt and

M

Ti in varying atomic ratios in N2-saturated aq. 0.5 M HClO4 at a scan rate of 50 mV s-1.

d

Fig. 6. Linear scan voltammograms of Pt/C and HT-Pt-TiO2/C catalysts containing Pt and Ti in

te

varying atomic ratios in aq. 0.5 M HClO4 saturated with pure oxygen at a scan rate of

Ac ce p

5 mV s-1 (Electrode rotation rate: 1600 rpm). Fig. 7. Linear scan voltammograms for Pt/C and HT-Pt-TiO2/C catalysts containing Pt and Ti in varying atomic ratios in aq. solution containing 0.5 M HClO4 and 1 M CH3CH2OH saturated with pure oxygen at a scan rate of 5 mV s-1 (Electrode rotation rate: 1600 rpm). Fig. 8. Steady-state performance data of DEFCs (CH3CH2OH and O2) containing HT-PtTiO2(2:1) /C cathode catalyst at varying temperature namely, 60, 70, 80 and 90 oC.

Fig. 9. Steady-state performance data of DEFCs (CH3CH2OH and O2) comprising Pt/C, HT-Pt/C and HT-Pt-TiO2(2:1)/C cathode catalysts at 70 oC.

19    Page 19 of 31

Ac ce p

te

d

M

an

us

cr

ip t

Fig. 10. Cell potential vs. Time for HT-Pt-TiO2(2:1)/C cathode catalyst at 70 oC.

20    Page 20 of 31

Table 1

Particle size by TEM (nm)

-

3-4

HT-Pt/C

>10

7-8

HT-Pt-TiO2(1:1)/C

6.8

-

HT-Pt-TiO2(2:1)/C

6.9

5-6

73

HT-Pt-TiO2(3:1)/C

8.7

-

68

us

22

06

36

Ac ce p

te

d

M

Pt/C

Electrochemical surface area (m2 g-1)

cr

Crystallite size (nm)

an

Catalysts

ip t

Physical and electrochemical parameters of the catalyst.

21    Page 21 of 31

Figure(s)

(a)

ip t

(111) (200)

cr

(220)

us

(200)

(220)

(b)

an

(111) (200)

(c)

M

(111)

(220)

(200)

(d)

(220)

d

(111)

te

(200)

(e)

Ac ce p

Intensity / a.u.

(111)

20

30

40

(220)

50

2 / deg.

60

70

80

Fig. 1.

1

Page 22 of 31

ip t cr us an M d te Ac ce p Fig. 2.

2

Page 23 of 31

ip t cr us an M d te Ac ce p

Fig. 3.

3

Page 24 of 31

ip t cr us an Ac ce p

te

d

M

(a)

(b) Fig. 4.

4

Page 25 of 31

HT-Pt-TiO2(3:1)/C

6

HT-Pt-TiO2(2:1)/C

4

us

2

cr

Pt/C HT-Pt/C

an

0 -2 -4

M

Current density / mA cm

-2

HT-Pt-TiO2(1:1)/C

ip t

8

-8 0.2

0.4

te

0.0

d

-6

0.6

0.8

1.0

Ac ce p

Potential / V vs. NHE

Fig. 5.

5

Page 26 of 31

ip t

-2

HT-Pt-TiO2(2:1)/C HT-Pt-TiO2(1:1)/C HT-Pt-TiO2(3:1)/C Pt/C

cr

-1

us

-3

an

-4

M

-5

-7 0.2

0.4

d

-6

0.6

0.8

1.0

te

Potential / V vs. NHE

Ac ce p

Current density / mA cm

-2

0

Fig. 6.

6

Page 27 of 31

20

(a)

10 5 0

15 10

0

0.2

0.4 0.6 0.8 1.0 Potential / V vs. NHE

1.2

(c)

M

0

Current density / mA cm

-2

1

-1

-3 -4

0.2

0.4 0.6 0.8 1.0 Potential / V vs. NHE

Ac ce p

-5 0.0

te

d

-2

1.2

0.2

0.4 0.6 0.8 1.0 Potential / V vs. NHE

3 2

HT-Pt-TiO2(1:1)/C

1.2

an

2 HT-Pt-TiO2(2:1)/C

0.0

1.4

us

0.0

-2

5

-5

-5

Current density / mA cm

ip t

Current density / mA cm

-2

Current density / mA cm

15

(b)

HT-Pt-TiO2(3:1)/C

-2

Pt/C

cr

20

1.4

(d)

1 0

-1 -2 -3 -4

1.4

-5 0.0

0.2

0.4 0.6 0.8 1.0 Potential / V vs. NHE

1.2

1.4

Fig. 7.

7

Page 28 of 31

0.8

60 o

at 60 C o

ip t

at 70 C o

o

at 90 C

cr

40

us

0.5

30

an

0.4 0.3

20

M

0.2

0

20

40

60

80

te

0.0

d

0.1

10

0 100

120

Current density / mA cm

Ac ce p

Cell voltage / V

0.6

-2

50

at 80 C

Power density / mW cm

0.7

140

160

180

-2

Fig. 8.

8

Page 29 of 31

0.8 Pt/C HT-Pt-T iO2(2:1)/C

40

cr

30

us

0.4

an

20

10

M

0.2

0.0 20

40

60

d

0

80

0 100

120

140

-2

te

Current density / mA cm

Ac ce p

Cell voltage / V

0.6

Power density / mW cm

-2

ip t

HT-Pt/C

Fig. 9.

9

Page 30 of 31

ip t us

cr

0.6

0.4

an

Cell voltage / V

0.8

M

0.2

0

2

4

d

0.0

HT- Pt-T iO2(2:1)/C 6

8

10

Ac ce p

te

Time / h

Fig. 10.

10

Page 31 of 31