Single wall nanohorns as electrocatalyst support for vapour phase high temperature DMFC

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Single wall nanohorns as electrocatalyst support for vapour phase high temperature DMFC L. Branda˜o*, M. Boaventura, P. Ribeirinha LEPAE, Departamento de Engenharia Quı´mica, Faculdade de Engenharia, Universidade do Porto, R. Dr. Roberto Frias, 4200-465 Porto, Portugal

article info

abstract

Article history:

The use of single wall nanohorns (SWNH) as electrocatalyst support has proved to increase

Received 8 May 2012

the performance of polymer electrolyte membrane-based fuel cells. In order to investigate

Received in revised form

in more detail such behavior, the electrochemical characterization of SWNH based elec-

13 September 2012

trodes was performed. The use of SWNH in vapour phase high temperature direct meth-

Accepted 23 September 2012

anol fuel cells (HT-DMFC) was also addressed. Cyclic voltammetry experiments have

Available online 25 October 2012

indicated a higher electrochemical activity towards methanol electro-oxidation and a higher tolerance to carbonaceous species accumulation for a SWNH based electrode than

Keywords:

for carbon black and commercial corresponding ones. Carbon black electrode presented

SWNH

a better performance than SWNH one for oxygen reduction reaction at low current

DMFC

densities while, at higher overvoltages, SWNH electrode performed better. The exact role of

Cyclic voltammetry

the improved performance of SWNH based electrodes is yet not clear but may be related to

Vapour phase

a higher water vapour adsorption or electrode morphology. Vapour phase HT-DMFC

High temperature

operation showed the improved performance of the SWNH electrode in agreement with

Fuel cells

previous works and with the electrochemical characterization performed during this work; despite the higher ohmic resistance observed in comparison with the carbon black based electrode. Moreover, SWNH based electrode showed improved fuel cell stability during longer operation times. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Direct methanol fuel cells (DMFC) present advantages over hydrogen-fed polymeric electrolyte membrane fuel cells (PEMFC). Methanol has an energy power density per mass unit (20 MJ kg
1) approximately half of that of gasoline (ca. 44 MJ kg
1) and can be handled, stored, and transported easily. Despite methanol toxicity (it is rapidly absorbed by ingestion or inhalation), gasoline is considered to be more hazardous to health than methanol [1]. Additionally, if the cost is converted to VMJ
1, the price between methanol and gasoline is comparable

[2] (Rotterdam, prices of 2011). Also, the use of a liquid fuel should result in compact fuel cell devices showing high energy densities that are convenient for the increasing functionalities of portable devices. DMFC technology presents two major limitations: (i) poor oxidation kinetics of the fuel and (ii) the permeation of methanol into the cathode. This later forces the use of dilute methanol solutions at the anode (typically 0.5e1.0 M) requiring larger quantities of water that increases size and complexity of the system and it is particularly impractical for portable devices. Three main approaches have been proposed

* Corresponding author. E-mail address: [email protected] (L. Branda˜o). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.09.133

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to overcome these problems: (i) more active catalysts for methanol oxidation (ii) methanol-tolerant cathode catalysts, and (iii) membranes with a lower permeability towards methanol. One approach that, to some extent, addresses the three previous problems simultaneously is operating the fuel cell at higher temperature (above 120  C) and feeding the cell with a mixture of methanol and water in the vapour phase. Some of the advantages of vapour phase DMFC have been reported [3]: (i) no requirement for methanol dilution, making the fuel tank smaller and lighter despite the need for fuel exhaust recycling, (ii) the mass transfer and anode electrode kinetics are enhanced, (iii) higher energy efficiency when operating at higher temperatures, (iv) the absence of gas bubbles formation, (v) a higher cathode tolerance to methanol crossover, and (vi) methanol crossover in vapour phase decreases with temperature. There are very few works reporting DMFC at high temperatures (above 120  C) operating in vapour phase. Wang’s group [4] presented the first results for a PBI-based high temperature direct methanol fuel cell (HT-DMFC) system, in 1995. PBI presents high thermal stability, low methanol crossover and requires a very low hydration level. The peak power density of the cell was ca. 125 mW cm
2, at 200  C and atmospheric pressure, with 4 mgPtRu cm
2 and 4 mgPt cm
2 metal loadings. The authors used a commercial PBI powder (Celanese) and the membranes were casted from DMAc (dimethylacetamide). Later, the same research group published a more extensive study in which they analyzed the influence of several operating parameters in the PBI-based HT-DMFC; these included temperature, water/methanol ratio, catalyst loading, and oxygen partial pressure [5]. More recently, Lobato et al. [3] obtained a peak power density of ca. 139 mW cm
2 using an in-house synthesized PBI polymer; they tested the DMFC in vapour phase at 200  C and atmospheric pressure with 1 mgPtRu cm
2 and 1 mgPt cm
2 of metal loadings. In 2011, a study of a vapourfed HT-DMFC using a PBI system reported a fuel cell performance of 12e16 mW cm
2 obtained at 175  C, using 1.0 mgPtRu cm
2 and 0.5 mgPt cm
2. Besides the influence of temperature, methanol feed concentration, and oxygen pressure they also addressed a significant anode polarization due to the presence of phosphate in the catalyst layer and the influence of methanol crossover on the cathode performance [6]. Vapour-fed HT-DMFC can show higher energy efficiencies and power densities than low temperature DMFC, but they still show lower performances compared to H2 fuel cells (energy efficiencies up to 55% and current densities of 1 A cm
2). However, the vapour phase HT-DMFC operation opens the door for thermal integration of HT-PEMFC with a methanol steam reformer processor. Since methanol steam reforming (MSR) is endothermic and the fuel cell operation is exothermal, a synergetic effect can be achieved when the two types of cell reactors are stacked and operated at the same temperature. Moreover, fuel cells can be directly fed with the reformate stream. Despite low temperature MSR catalysts still present poor activity and low methanol conversion, this integration will create a hydrogen-enriched methanol feed that will improve the energy efficiency and the power density of the vapour phase HT-DMFC. Several strategies have been used to improve the kinetics of methanol oxidation by developing anode catalysts

supported in different types of carbon structures [7e9]. Among them, SWNH is a carbon material having a hornshaped tip at the top of the single walled nanotube. SWNH form aggregates with a large surface area where gas and liquid easily penetrates; additionally, they have excellent electronic properties. Previous reports by the authors show that the use of SWNH as Pt and/or PtRu supports in PEMFC, DMFC and HTPEMFC can bring several advantages compared to electrocatalysts supported on carbon back [10e13]. The use of SWNH showed catalytic activities 60% higher than using carbon black as the electrocatalyst support in DMFC and PEMFC [10e12]. Oxidized SWNH supports were also compared with carbon black in PEMFC electrodes [12]. The use of oxygen treated SWNH also showed catalytic activities 60% higher than using carbon black as the electrocatalyst support in PEMFC [12]. Electrochemical impedance spectroscopy (EIS) analysis indicated that the major improvement in performance is related to cathode kinetics in the SWNH electrode and to improvements in both anode and cathode electrodes for the oxidized SWNH sample [12]. SWNH and carbon black supports were also compared in HT-PEMFC [13] using MEAs made of phosphoric acid doped PBI, at 160  C. Similar peak power densities were obtained for both cases. However, electrochemical characterization showed a higher ohmic resistance for the Pt-SWNH compared to carbon black based MEA likely due to the different hydrophobic characters of the carbon supports. Furthermore, the PtSWNH anode presented lower charge transfer resistance while the Pt-SWNH cathode electrode presented a higher electrochemical surface area [13]. Following the same approach and targeting the possible integration of low temperature MSR with a vapour phase HTDMFC, in this work we report the study of the fuel cell performance when using SWNH as electrocatalyst support in a vapour phase HT-DMFC using a PBI/H3PO4 membrane synthesized by the solegel method [14]. Also, there are carried out electrochemical and physical characterizations (methanol electrochemical oxidation, oxygen reduction reaction and water vapour uptake) of the electrodes based on SWNH.

2.

Experimental section

2.1.

Materials and synthesis methods

2.1.1.

Electrocatalysts and electrodes preparation

Previous works present in detail the preparation of SWNH, electrocatalysts and electrode assembly [10,11,15]. Briefly, SWNH were produced by AC arc discharge in air. Ethylene glycol was used as reducing agent in order to dope the Pt and PtRu nanoparticles on the carbon supports. The concentration of Pt and PtRu (1:1 Pt: Ru) on the carbon supports was 10 wt. %. Catalytic active layers based on different carbon supports were spray dried on a commercial gas diffusion layer. For the vapour phase HT-DMFC tests, the load was 0.46  0.03 mgPtRu cm
2 and 0.30  0.04 mgPt cm
2 for the carbon black- and SWNH- based electrodes. For the cyclic voltammetry (CV) electrochemical characterization, the load was 0.15  0.01 mgPtRu cm
2 for both SWNH and carbon black based electrodes, while a load of 0.14  0.01 mgPt cm
2 and 0.28  0.02 gPt cm
2 for the SWNH

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and carbon black based electrodes was used, respectively. In this work, Nafion was not used in any of the prepared electrodes. Commercial electrodes used are Pt/C (0.5 mgPt cm
2, 10 wt.% Pt/Vulcan) and PtRu/C (1.0 mgPtRu cm
2, 15 wt. % PtRu (1:1 Pt: Ru)/Vulcan) both containing a proprietary amount of Nafion, from Electrochem, S.A.

2.1.2.

m-PBI membrane synthesis

Solegel m-PBI (poly[2,2-(m-phenylene)-5,5-bibenzimidazole]) type membranes were synthesized by solegel method using poly(phosphoric acid) (PPA) as solvent [14]. A 8 wt.% solution of the reagents in PPA was used. For that, 2.35 g of 3,30 ,4,40 tetraaminobiphenyl (Sigma) were mixed with 1.83 g of isophthalic acid (Sigma) (dried at 60  C under vacuum, overnight) in 47.28 g of PPA using a three neck round flask, at room temperature, overnight under a flux 1 L min
1 of N2. Following, the mixture was kept at a constant temperature of 140  C during 3 h and polymerization performed at 250  C during 20 h. Concentration was decreased to 5.3 wt.% with the help of 25 mL of phosphoric acid (85 wt. %) and stirred at 260  C during 1 h, followed by casting. The membrane was directly casted in a glass plate, thickness 0.5 mm, using a doctor blade. The hydrolysis of poly(phosphoric acid) was proceeded at room temperature and ambient relative humidity (RH) during 1 week (ca. 40% RH). Casted membrane was kept at a constant temperature of 110  C in an oven during 4 days prior to use. The doping level was ca. 25 molecules of phosphoric acid for 1 monomeric unit of m-PBI (based on titration with NaOH).

2.2.

Cyclic voltammetry electrochemical characterization

2.2.1.

Pt electrochemical surface area

1 cm2 electrodes were immersed in a 1.0 M H2SO4 oxygen free aqueous solution and tested at room temperature, using a SCE reference electrode and a Pt mesh as counter electrode. Mechanical stirring was used as well as inert gas bubbling through the electrolyte solution. IR drop was compensated. CV scans were performed at 75 mV s
1 from 0.2 V up to 1.2 V vs RHE by using a Zahner IM6e electrochemical workstation.

2.2.2.

Methanol electro-oxidation

1 cm2 electrodes were immersed in a 0.5 M H2SO4 and 1.0 M methanol oxygen free aqueous solution and tested at room temperature, using a SCE reference electrode and a Pt mesh as counter electrode. Mechanical stirring was used as well as inert gas bubbling through the electrolyte solution. IR drop was compensated. CV scans were performed at 100 mV s
1 from 0.2 V up to 1.1 V to 1.2 V vs RHE by using a Zahner IM6e electrochemical workstation.

2.2.3.

Oxygen reduction reaction

1 cm2 electrodes were immersed in a 0.5 M H2SO4 oxygen saturated aqueous solution and tested at room temperature, using a SCE reference electrode and a Dimensional Stable Anode [16] (DSA, composed by RuO2, IrO2 and TiO2) as counter electrode. Mechanical stirring was used and oxygen bubbling through the electrolyte solution. IR drop was compensated. CV scans were performed at 10 mV s
1 from OCV up to 0.2 V to 0.3 V vs RHE by using a Zahner IM6e electrochemical workstation.

2.3.

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Water vapour adsorption isotherms

Water vapour adsorption equilibrium data at 30  C on SWNH and carbon black samples were measured gravimetrically on a Rubotherm magnetic suspension balance having a 0.01 mg weighing resolution and a 0.02 mg reproducibility. This system, previously described [17], consisted of a fully computerized magnetic suspension balance, which automatically measures the weight of the sample as function of time, at constant temperature and pressure. A tank filled with water was used to control the vapour relative pressure on the environment surrounding the sample. A small amount of pure water was introduced in the tank, which was then evacuated for air removal, leaving a saturated water vapour atmosphere. The water tank was connected to the sample chamber through a valve, which allowed to control the relative humidity of the sample environment. The entire system was fully thermostated. Water vapour isotherms were obtained by setting pressure intervals relative to the saturation vapour pressure. Prior to the measurements, the samples were outgassed until constant weight was achieved at 0.001 bar and 70  C.

2.4.

Vapour phase HT-DMFC tests

The assembled carbon black and SWNH based MEAs, with an active area of 4 cm2, were placed in an Electrochem single cell and torque of 5 N m applied on each screw. MEAs were activated with hydrogen (0.15 L min
1) and air (0.05 L min
1) under dry conditions, atmospheric pressure, at 160  C and during 50 h. After activation, dry air (or oxygen) at 0.15 L min
1 was fed to the cathode and 2 mL min
1 of methanol aqueous solution (17.4 M, 1:1 M, methanol: water) was fed to the anode, during 6 h (thereafter referred as methanol cycle. Two serpentine tubes inside an oven conducted the reactants to the fuel cell allowing the heat up of air and the vaporization of the methanol solution. After each cycle, methanol was replaced by dry hydrogen (0.05 L min
1 of hydrogen) overnight, potentiostatically at 0.5 V; the total fuel cell testing time, including HT-PEMFC and vapour phase HT-DMFC, for all MEAs was from ca. 120 h up to 150 h. Each polarization curve was obtained in potentiostatic mode, starting at OCV and decreasing the potential. Impedance spectroscopy spectra were obtained in the frequency range from 100 kHz to 100 mHz with a perturbation amplitude of 5 mV using a Zahner IM6e electrochemical workstation coupled with a potentiostat PP-241.

3.

Results and discussion

3.1.

Electrocatalysts characterization

Fig. 1 shows the Pt metallic nanoparticles deposited on carbon black (Vulcan XC-72R, Cabot) and SWNH from previous characterization studies [11]. These characterization studies [11] showed that the metal nanoparticle size is very small in both cases with the nanoparticles uniformly distributed on the carbon supports and presenting a particle size of ca. 2e3 nm, for both supports and for both Pt and PtRu metal particles.

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Fig. 1 e TEM micrographs of Pt nanoparticles deposited on carbon black (Vulcan XC-72R) (left) and on SWNH (right), adapted from [11].

3.2.

Cyclic voltammetry electrochemical characterization

Fig. 2 shows the CV plots of Pt-based electrocatalysts supported on carbon black and SWNH. The difference in the area related to H-desorption, is mainly related to the different Pt loads (cf. Experimental section). However, the electrochemical surface area (ESA), which is normalized by the catalyst mass, is approximately 40% larger in the case of carbon black support than in the case of SWNH supportesee Fig. 2. This difference should be related to small differences in metal catalyst size distribution. The ESA of a commercial Pt/C electrode (0.5 mgPt cm
2, 10 wt.% Pt/Vulcan, Electrochem, S.A.) was also determined, 80 m2Pt g
1 Pt , data not shown. Fig. 3 shows CV scans obtained for methanol electrooxidation on SWNH and carbon black based PtRu electrodes, prepared in-house, and for a commercial PtRu electrode based on carbon black. Two major peaks are visible, one in the forward and the other in the backward scan for the three electrocatalysts tested, which reflect the electro-oxidation of methanol [18]. The peak current density obtained at the forward and backward scans are denoted as If and Ib,

Current

60 m

2

g

-1

100 m

2

g

-1

carbon black SWNH

20 mA

0.0

0.2

0.4

0.6 V vs RHE

0.8

1.0

1.2

Fig. 2 e Cyclic voltammetry scans obtained in 1 M sulfuric acid in the absence of methanol solution at ambient temperature (75 mV sL1) of the Pt -carbon black and eSWNH based electrocatalysts.

respectively. SWNH based electrode presents higher electrochemical activity than the other electrodes tested and also presents a higher If/Ib ratio. This indicates an advanced catalyst tolerance to carbonaceous species accumulation [18]. Actually, the higher current density observed for the SWNH electrode indicates a more complete oxidation of methanol to carbon dioxide during the anodic scan and less accumulation of carbonaceous residues on the catalyst surface. However, how SWNH affect the electrochemical activity and carbonaceous residues tolerance is not clear and further studies need to be performed. The commercial electrode showed the poorest performance and the lowest If/Ib ratio. Previous works [10,11] showed a better DMFC performance at 50  C when SWNH based electrodes were compared with carbon black based electrodes, as reprinted in Fig. 4. Additionally, this figure shows the DMFC characteristic and power density curves for commercial electrodes with similar catalyst loads. Noteworthy, both electrodes, anode and cathode, were prepared with the same type of support. Consequently, the performance improvement observed in the case of SWNH based electrodes can be related either to anode or cathode or both. The highest peak power density was obtained for the SWNH based electrodes while the lowest was observed for the commercial electrodes (Fig. 4), following the trend observed in the methanol electro-oxidation CV plots (Fig. 3). Nonetheless, the cathode support may still influence DMFC performance. Linear sweep voltammograms of oxygen reduction were obtained for the Pt-based electrocatalysts supported on carbon black and SWNH to assess their reduction activityeFig. 5. Carbon black based electrode exhibits a better performance than the SWNH based one, at low currents; at higher overvoltages (for voltages lower than ca. 0.65 V vs RHE), however, SWNH based electrode performs better. This is in accordance with a previous work by the authors, where SWNH electrodes inserted in a PEMFC showed an improved cathode kinetics [12]. ORR performance of the commercial electrode is also included for comparison; this electrode performs better than the in-house made electrodes, but for voltages lower than ca. 0.5 V vs RHE, SWNH presents the best activity. SWNH

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carbon black Current / A mg

Current / A mg PtRu-1

SWNH

If

If

0.05 A mg PtRu/CB commercial

0.2

0.4

0.6

0.8 V vs RHE

1.0

1.2

Electrode type carbon black SWNH commercial

-1

0.05 A mg PtRu 0.2

0.4

0.6

0.8

I f /I b 1.8 3.9 0.9

1.0

1.2

V vs RHE Fig. 3 e Cyclic voltammetry scans obtained for methanol electro-oxidation on the SWNH and carbon black based PtRu electrocatalysts (100 mV sL1); upper insert: scan for a commercial PtRu electrode (1.0 mgPtRu cmL2); table in the lower insert: ratio of If/Ib.

electrode allows better mass and/or charge transport for higher current densities. This might be related to SWNH electrode film structure or water management and should be further investigated. Moreover, an improved cathode kinetics in a dye sensitized solar cell (DSC) when using SWNH as Pt support was also observed when compared to a carbon black support [19]. Previous works [12,13] hypothesized that the charge transfer kinetics of the SWNH cathode at PEMFC and HTPEMFC, at high current densities, could be related to some

0.60

differences in the hydrophobic character of the SWNH compared to carbon black. To address this issue, water vapour adsorption measurements were performed at 30  C for both SWNH and carbon black supports (Fig. 6). The water vapour adsorption equilibrium isotherms indicate that SWNH adsorbs ca. 2 times more than carbon black, at 30  C. This feature could explain the higher ohmic resistance observed for a SWNH based MEA [13] compared to that of a carbon black based MEA in a HT-PEMFC test, probably because SWNH could be retaining more water at the electrodes that is not able to pass to the electrolyte membrane.

12

3.3.

10

This section compares the performance of a vapour phase HTDMFC when using different electrocatalyst carbon supports; SWNH and carbon black were used to support PtRu and Pt electrocatalysts. For assembling the MEAs, an in-house made PBI/H3PO4 electrolyte membrane was used. Figs. 7 and 8 show the performance observed for the carbon black and SWNH based electrodes, respectively, for three methanol cycles. For the carbon black based electrode, Fig. 7, performance decreased in the third methanol cycle. The SWNH based MEA presented a more stable performance during the three methanol cycles. A slightly better performance than the carbon black based MEA, mainly after the third cycle (ca. 30% more in the peak power density) was observed, although exhibiting more ca. 25% ohmic resistance (Fig. 9). This indicates that the improved maximum power density is probably related to the SWNH electrode better performance for both methanol oxidation and oxygen reduction as observed in the CV experiments described above. The better SWNH based electrode performance is also in

Vapour feed HT-DMFC performance

SWNH

0.50

Carbon Black

0.40

8

0.30

6

0.20

4

0.10

2 0

0.00 0

10

20 30 j / mA cm-2

40

50

Fig. 4 e Comparison of DMFC performances at 50  C; 2 M methanol solution at 2 mL minL1; 200 mLN minL1 O2 at atmospheric pressure and 30% relative humidity, Nafion 117. Anode loading: 0.9 mgPtRu cmL2, cathode loading: 0.5 mgPt cmL2, for the SWNH and carbon black carbon supports; and 1.0 mgPtRu cmL2 and 0.5 mgPt cmL2 for commercial electrodes, adapted from [10].

P / mW cm-2

E/ V

Commercial

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0.1

Log (Current / A mg Pt-1)

SWNH carbon black

0.1

Log (Current / A mg

)

0.01 0.01

Pt/C commercial 0.001

0.0001 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

V vs RHE

0.001 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

V vs RHE Fig. 5 e Oxygen reduction reaction scans for the three different electrodes in an oxygen saturated H2SO4 electrolyte; insert: commercial electrode (0.5 mgPt cmL2).

agreement with the DMFC performance at lower temperature (Fig. 4 [10,11],). A performance improvement due to higher operation temperature in DMFC (vapour feed conditions) is noted when compared with low temperature DMFC operation (50  C), Fig. 4. It should be emphasized that the conditions for Figs. 7 and 8 are more severe than for Fig. 4, which uses twice as much catalyst and is fed with oxygen instead of air. Figs. 10 and 11 show the EIS Nyquist plots obtained during the methanol cycling interruptions, i.e. under HT-PEMFC conditions. Fig. 10 shows the Nyquist plot for the carbon black based MEA; ohmic resistance increases after each methanol feeding cycle. After the first methanol feeding cycle, the performance of the electrodes under HT-PEMFC operation improved substantially when compared to the performance right after the activation. This could be related to a better distribution of the phosphoric acid from the PBI membrane into the electrodes; phosphoric acid probably migrates easily under stronger humidity conditions.

Fig. 7 e Vapour phase HT-DMFC performance cycles for the carbon black based electrodes at 160  C, 2 mL minL1 of methanol solution (17.4 M; 1:1 CH3OH:H2O molar ratio), 0.15 mL minL1 dry air; atmospheric pressure.

4.5

SWNH carbon black

q / mol kg-1

3.0

1.5

0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

P/ Po

Fig. 6 e Water vapour adsorption isotherms for carbon black and SWNH, at 30  C.

0.9

Fig. 8 e Vapour phase HT-DMFC performance cycles for the SWNH based electrodes at 160  C, 2 mL minL1 of methanol solution (17.4 M; 1:1 CH3OH:H2O molar ratio), 0.15 mL minL1 dry air; atmospheric pressure.

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Fig. 9 e Bode plots at 70 mA cmL2, 160  C and 1 bar, for Ptcarbon black and Pt-SWNH based electrodes under vapour phase HT-DMFC operation, after cycle 3.

Fig. 10 e Nyquist plots at 20 A cmL2, 160  C and 1 bar, for Pt-carbon back based electrodes under HT-PEMFC operation after each methanol cycle.

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Regarding the SWNH based MEA, Fig. 11 shows the Nyquist plot obtained under HT-PEMFC conditions and after each methanol feeding cycle. A higher ohmic resistance was obtained after the first methanol cycle, when compared to the value obtained after activation, remaining constant afterwards. Electrodes performance also increased after the first methanol cycle. Ohmic resistance is ca. 2 times higher for the SWNH based MEA than the corresponding carbon black one, in agreement with previous work by the authors [13], but also in agreement with the ohmic resistance observed under vapour feed HT-DMFC conditions for both MEAs (Fig. 9). This might be related to the different hydrophobic character between the two carbon supports as well as the electrodes morphology, as discussed before. A fourth methanol cycle was performed for the SWNH based electrode and no decrease in performance was observed (data not shown). Oxygen versus air feeding was also assessed; Fig. 12 shows the polarization curves and Bode plots obtained with a vapour feed HT-DMFC at 160  C equipped with SWNH based electrodes when air and oxygen streams are fed at atmospheric pressure. EIS analysis indicates that the limiting reaction under vapour phase HT-DMFC operating at 160  C is the anodic reaction e Fig. 12. Fig. 13 shows the performance of the vapour feed HTDMFC, at 160  C, equipped with the commercial electrodes and the in-house prepared m-PBI membrane [20]. The same MEA activation and methanol cycling procedures was accomplished for these electrodes. In the case of commercial electrodes the operating temperature effect is not so obvious, as depicted in Figs. 4 and 13. It is indeed observed a small maximum power density decrease for higher temperatures. This decrease should be related to the DMFC pure oxygen feed when operated at low temperature. Another feature is that power performance is ca. 3 times lower than the fuel cell equipped with in-house made electrodes both SWNH and carbon black based, although the catalyst load of the commercial electrodes is ca. 2 times higher. These results

Fig. 11 e Nyquist plots at 20 mA cmL2, 160  C and 1 bar, for Pt-SWNH based electrodes under HT-PEMFC operation after each methanol cycle.

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Fig. 12 e Vapour phase HT-DMFC performance for the SWNH based electrodes at 160  C, 2 mL minL1 of methanol solution (17.4 M; 1:1 CH3OH:H2O molar ratio), 0.15 mL minL1 dry air (or oxygen); atmospheric pressure. Insert: Bode plot at 10 mA cmL2, under the same conditions.

0.50

4.0

0.40

E/ V

0.30 2.0 Cycle 3

0.20

Cycle 2

Cycle 1

P / W cm-2

3.0

1.0

0.10

0.00

0.0 0

10

20

30 j / mA cm-2

40

50

60

Fig. 13 e Vapour feed HT-DMFC performance cycles for commercial electrodes at 160  C, 2 mL minL1 of methanol solution (17.4 M; 1:1 CH3OH:H2O molar ratio), 0.15 mL minL1 dry air; atmospheric pressure.

agree with the performance of the low temperature DMFC equipped with the three types of electrodes (Fig. 4). Nonetheless, the commercial electrodes assessed under HT-PEMFC conditions performed better than the in-house made electrodes either loaded with SWNH or carbon black based electrocatalysts e data not shown. This difference should be related to the higher metal loading of commercial electrodes.

4.

conversions; despite intensive research strategies this goal is still far. Regardless the lower methanol conversions, this integration will always produce hydrogen that will improve the energy efficiency of the vapour phase HT-DMFC and with lower heat consumption. This works address the performance of a vapour phase HT-DMFC performance for further integration with a methanol fuel processor by using SWNH as electrocatalyst support. SWNH support has shown previously better performance in DMFC and PEMFC as well as an increased stability in HT-PEMFC when compared to the corresponding carbon black electrocatalyst support. Electrochemical characterization of the electrodes accomplished in this work, using a three electrode configuration, have indicated that SWNH performs better on methanol electro-oxidation and oxygen reduction reactions than a carbon black support. Moreover, when tested in a vapour phase HT-DMFC environment, SWNH based electrodes presented an increased performance and a longer stability, confirming the advantages of using SWNH as electrocatalyst support in fuel cells.

Acknowledgments L. Branda˜o and M. Boaventura are grateful to the Portuguese Foundation for Science and Technology (FCT) for their postdoc grants (references SFRH/BPD/41233/2007 and SFRH/BPD/ 80599/2011, respectively). Financial support by FCT through the projects PTDC/CTM/108454/2008 and PTDC/EQU-EQU/ 104217/2008 is also acknowledged.

Conclusions

Vapour-fed HT-DMFC operation opens the door for thermal integration with a methanol steam reformer processor. A synergetic effect can be achieved because MSR is endothermic while the fuel cell operation is exothermal. For that, low temperature MSR catalysts should present high methanol

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