PtU/C Electrocatalysts for Ethanol, H2, H2-CO and CO Electrooxidation for DEFC and PEMFC applications

Carmo, M.2; Linardi,M.2;Seo, E.M. 2;Andreoli, M. 2; Dantas Filho, P.L.1; Burani,G.F.1; Franco, E.G.1,2*.

*egberto@iee.usp.br1 – Instituto de Eletrotécnica e Energia-IEE-USP

2 – Instituto de Pesquisas Energéticas e Nucleares-IPEN/CNEN-SP

PtU/C Electrocatalyseur pour Oxydation D’Ethanol, H2, H2-CO et CO pour utilisation de Pile Direct à

Ethanol (DEFC)

Carmo, M.2; Linardi,M.2;Seo, E.M. 2;Andreoli, M. 2; Dantas Filho, P.L.1; Burani,G.F.1; Franco, E.G.1,2*.

*egberto@iee.usp.br1 – Instituto de Eletrotécnica e Energia-IEE-USP

2 – Instituto de Pesquisas Energéticas e Nucleares-IPEN/CNEN-SP

Abstract

• This work presents results with noble metal catalysts, PtU and PtRu E-TEK supported on Carbon Vulcan XC-72R. The nanoparticles were synthesized following the Colloid Method. X-ray Diffraction (XRD), Energy Dispersive Analysis by X-rays (EDX) experiments were carried out to characterize the nanoparticles obtained. Cyclic voltammograms (CV) using the Porous Thin Layer Electrode Technique were obtained for the catalysts surface evaluation and to check the electrocatalytic behavior of the nanocatalysts for CO oxidation. The electrocataysts were evaluated for H2/O2 H2+CO/O2 in PEMFC and for Ethanol oxidation in DEFC.

Résume

• Cette étude présent résultats avec l’utilisation de catalyseurs des métaux nobles, PtU et PtRu E-TEK soutenu par CarbonVulcan XC-72R. Les nanoparticules ont été synthétisées par la méthode colloïdal. Les characthéristiques des nanoparticules ont été déterminées par diffraction de rayons X (XRD) et analyse d’énergie dispersive de rayons X (EDX). Pour évaluer la surface de le catalyseur et la performance de le nanocatalyseur pour l’oxydation de CO ont été obtenues par cycliques voltampérométries. Les électrocatalyseurs ont été évalués pour H2/O2, H2+CO/O2 dans pile à membrane polymère (PEMFC) et pour oxydation d’éthanol en pile direct à éthanol (DEFC).

Introduction

• Proton exchange membrane fuel cells (PEMFC) are suitable for portable, mobile and stationary applications, due to their inherent advantages, such as high-power density, reduced system weight, simple construction and quick start-up, even at low operating temperatures, producing low (or no) emissions [1-3]. For such applications, in order to provide the best performance, platinum metallic is the most important electrocatalysts material employed for several kinds of electrochemical reaction; in particular of those occurring in PEMFC gas diffusion electrodes. Alternatively, the hydrogen may be obtained from the reforming of methanol, ethanol or natural gas.

Ethanol

• However, the hydrogen obtained by this method contains small amounts of carbon monoxide, which strongly adsorbs on Pt, blocking the adsorption and oxidation of hydrogen. Studies also exist operating the PEMFC with ethanol [4,5], what would be a strategically alternative for Brazil, which dominates the production (got from sugar cane), and considering also the existing logistic of distribution for moving cars using this renewable fuel. In despite of this, there are still some disadvantages by using methanol [6] or ethanol as fuel, for example, the fact that the electrochemical oxidation reaction of methanol or ethanol has a process less efficient if compared with the reaction of hydrogen oxidation. In the case of the direct ethanol use, there is another difficulty: the presence of one strong carbon-carbon binding which needs to be catalytically broken, beyond the formation of others intermediates as aldehydes, acetic acid and CO, all these diminishing the efficiency of the total oxidation reaction. In this way, to make the technology applicable, the development of materials that are able to oxidize CO at low overpotentials is necessary [7-13].

Experimental Procedure

Attainment of the UO2Cl2 dryThe experimental procedure was divided in two stages; the first one consisted of

producing the Tri-Hydrated Uranium Chloride (UO2Cl2.3H2O), and after preceding the dehydration and getting Uranium chloride dry.

Attainment of the tri-hydrated UO2Cl2

To 7.000g of Uranium Nitrate hex-hydrated was added 20 mL of concentrated HCl to eliminate nitrate in the nitrous oxide form. After that, was proceeded the heating until it get dry. This operation was repeated for eight times, and in each time 20 mL of acid was added. After these stages, the desired product was obtained, as the equation:

UO2(NO3)2.6H2O + HClconc. UO2Cl2.3H2O + 2NOxgás + (6-x)H2Ogas + HClgas

Attainment of the dry UO2Cl2

The Colloid method needs to be operated in dry conditions and in inert atmosphere, and for this requirement Uranium Chloride were obtained. The tri-hydrated uranium chloride was heated at 250º for 30 minutes using a dry gaseous mixture of HCl and nitrogen, to perform the complete water removal. After this stage the dry Uranium chloride was used as precursor for the PtU/C synthesis.

UO2Cl2.3H2O + HClgas.UO2Cl2 +HClgas + 3H2Ogas

Synthesis of the Catalyst

• Reducing agent:N(oct)4Br + KHB(et)3 N(oct)4HB(et) 3 + KBr ¯ (1)

THF

• Colloid:MeXn + N(oct)4HB(et)3 Me*[N(oct)4]+ + nB(et)3 + n/2 H2 + nX- (2)

THF colloid

H. Bönnemann, W. Brijoux, R. Brinkmann, E. Dinjus, T. Jouen, B. Korall, Angew. Chem. Int. Ed. Eng. 30 (1991), p. 1312.

Catalyst

• Scanning Electron Microscopy (SEM) picture:

Physical Characterization

• The Pt:Ru and Pt:U atomic ratios of the electrocatalysts were obtained using a scanning electron microscope Philips XL30 with a 20 keV electron beam and equipped with EDAX DX-4 microanalyses. The XRD analyses were performed using a STOE STADI-P diffractometer with germanium monochromized Cu Ka radiation and a position-sensitive detector with 40 apertures in transmission mode.

• The catalysts were also examined by X-ray diffraction techniques (XDR) using a URD-6 Carl Zeiss-Jena diffractometer. The X-ray diffraftograms were obtained with a scan rate of 1º min-1 and an incident wavelength of 1.5406 Å (KαCu). The XDR data were used to estimate the Pt lattice parameter and, using Scherrer´s equation [21,22], the average crystallite size.

Electrochemical Characterization

• Electrochemical studies of the electrocatalysts were carried out using the thin porous layer technique [5, 21, 23]. An amount of 20mg of the electrocatalysts was added to 20 g of water. The mixture was submitted to an ultrasound bath for 5 min, where drops of a PTFE (Polytetrafluorethylen) suspension were added. Again, the mixture was submitted to an ultrasound bath for 5 min, filtered and transferred to the cavity (0.30mm deep and area with 0.36 cm2) of the working electrode. The quantity of the electrocatalysts in the working electrode was determined with a precision of 0.0001g.

• By the cyclic voltammetry experiments, the current values (I) were expressed in amperes and were normalized per gram of platinum. The reference electrode was RHE (Reversible Hydrogen Electrode) and the counter electrode was a platinized Pt net with 4 cm2. Cyclic voltammetry was performed in a 0.5 molL-1 H2SO4 solution saturated with N2. The evaluation of CO oxidation was performed in this way; the potential of the working electrode was fixed in 50mV vs. RHE and CO gas was boiled in the electrochemical cell solution for 2 hour. After that, N2 was boiled in the electrochemical cell solution to remove the CO dissolved in the solution. The anodic sweeping was carried at 10mVs-1 in H2SO4 0.5 molL-1.

Electrochemical Characterization

• For comparative purposes a commercial carbon supported PtRu catalysts from E-TEK (20%wt.%; Pt:Ru molar ratio 1:1) was used. Electrochemical measurements were made using a Microquimica (model MQPG01, Brazil) potentiostat/galvanostat coupled to a computer using the Microquimica Software.

• The process MEA (membrane electrode assemble) preparation were initiated with the treatment of the Nafion® membrane in hydrogen peroxide (3%) at 80 °C for one hour, after that transferred to Milli-Q water at 80° and followed by sulphuric acid 1,0 molL-1 at 80°C. After that, the membranes were washed in Milli-Q water. For the electrodes preparation, the catalyst powder was added to water with a desired amount of the conducting polymer solution (Nafion®). The system was ultrasonically for 2 minutes and magnetic stirred overnight and sprayed in the membrane. The catalytic load was controlled during the spray process. The MEA were hot pressed at 127°C with 5 ton of pressure. After that, were carried a thermal treatment at 135°C for 30 minutes [24]. The MEA were placed in the unit cell and the operation was started at 600 mV for two hours, and after this time, initiated the register of the polarization curve. In Figure 2 are presented the flowchart for the MEA manufacture process. The MEA´s were tested in unit cells (25 cm2) from Electrochem®(USA), the gas from White Martins and the Ethanol from Casa Americana®.

MEA Fabrication

Catalytic Ink

Weighting the catalyst

Adding Water

Ultrasonically – 30 seconds

Adding Nafion Solution

12 hours magnetic stirring

Spraying the ink in the 1º side of the membrane

Drying at 100ºC

Spraying the ink in the 2º side of the membrane

Cutting and Cleaning the membrane in water

Drying at 100ºC

Treatment in water at 80ºC -1 hour

Treatment in Sulfuric Acid at 80ºC -1 hour

Washing with water

Cutting the Diffusion layers

Mounting the MEA

Hot pressing the Electrodes with the membrane

MEAMembraneCatalytic Layers

Scanning Electron Microscopy (SEM)

• Membrane Electrode Assembly (MEA).

Catalytic Layer

Electrolyte: Nafion membrane

Diffusion Gas Electrode Structure

EDX Analysis

Catalyst Pt molar% from EDX

Ru molar% from EDX

Particle Size (nm) From XRD

PtRu/C 20wt% E-TEK 54.4 45.6 1.48 Pt65U35/C 20% 65.8 34.2 4.13

EDX analysis shows the results for molar perceptual composition obtained by the EDX analysis. These EDX analyses showed small discrepancies when compared with the nominal compositions used to calculate the precursor’s quantities in the catalysts preparation. The results showed that the colloid method had a very good performance in the catalysts preparation in the desired quantities.

XRD Analysis

• The diversity of the oxidation states that Uranium presents, beyond the probable reduced size of the nanoparticle, becomes the identification by DRX analysis of the existing phases difficult, and only with the using of spectroscopic techniques, the study about the Uranium oxidation states can be developed.

The crystallite size of the catalysts was estimated from the 220 XRD peak using the Scherrer´s equation [21, 22] and the results are presented in the table 1. The PtRu/C catalysts from E-TEK presented a smaller particle size compared with the Pt65U35/C catalyst. Probably the particles of Platinum were attracted by the particles of Uranium, because of its size and because of its large oxidation states, but even in this case the particle size of Pt65U35/C catalysts, can be compared with the size of another alloys founded in the literature. Spinace et al. reported particles size about 5.5nm for Pt3Ru1/C prepared by alcohol reduction method [5]; T. J. Schmidt et al. showed results for PtNi/C about 4 nm and for Pt3Co1/C about 3 nm [25]; Colmati et al. [26] reported results for Pt67Sn33/C catalysts using the formic acid method, with the particle size of 4,9 nm. In this sense, the results presented by the catalysts Pt65U35/C is similar with the another alloys prepared in the literature.

XRD Analysis

10 20 30 40 50 60 70 80

UxO

y/C

VulcanXC72R

Pt (220)Pt (200)

Pt (111)

PtUxO

y/C

Rel

ativ

Inte

nsi

ty (

cps)

2 Theta (º)

Cyclic Voltamograms

• The cyclic voltamograms (CV) for PtRu and Pt65U35/C in H2SO4 0.5molL-1. This study using the Thin Porous Layer Technique is considered a very good tool for the catalysts surface evaluation and the catalysts behavior evaluation before the tests in the fuel cell [5, 21, 23, 27]. The CV presents characteristics for each type of catalysts, which depend on the metal [28]. The PtRu/C catalyst show a defined Hydrogen upd (under potential deposition) region (between 0-0.4V) as observed for Pt65U35/C, these peaks is less defined because the adsorption/desorption hydrogen peaks are not developed in Ruthenium, and the CV shows that this behavior can be applied for the catalysts Pt65U35/C. The double layer region (between 0.4-0.6V) for the PtRu/C catalysts is larger than for Pt65U35/C, this indicate the presence of more oxygenated species in the PtRu/C in this potential region, and this indicates too that the PtRu/C have a smaller particle size resulting in a larger surface area, as showed by the DRX analysis.

Cyclic Voltamograms

0,0 0,2 0,4 0,6 0,8 1,0-4

-3

-2

-1

0

1

2

3

4

C

urr

en

t De

nsi

ty (

A g

-1

Pt)

Potential (V vs RHE)

Pt65

U35

/C 10mVs-1

Pt65

U35

/C 20mVs-1

PtRu/C E-TEK 10mVs-1

CO Striping

• The shape of CO stripping peak depends of the catalysts nature, in the case of pure platinum, the adsorbed CO monolayer occur with a maximum at 0.750 V vs. RHE. The CO stripping voltamograms of the prepared PtRu/C and Pt65U35/C catalysts recorded at room temperature and at 10 mVs-1. The curves show a surprising result, for the Pt65U35/C catalysts the starting potential for CO oxidation is 5mV vs. RHE, and the maximum peak for CO oxidation is 330 mV vs. RHE. For PtRu/C the starting oxidation potential is 430mV vs. RHE and the maximum oxidation peak is 733mV vs. RHE.

• As discussed in the introduction of this work, the development of catalysts that are able to oxidize CO at low overpotentials is necessary [7-9], and meanly because the working potential that gives for the DMFC and or DEFC the maximum current and consequently the maximum power is between 200 and 400 mV [26].

• The alternative to favor the oxidation of the intermediate, as the CO, is the use of a second metal, forming catalysts that decrease the potentials that are able to form oxygenated species on platinum, facilitating the oxidation of the COads, the bifunctional mechanism [10].

CO Striping

• But the state of the art in the literature is that the PtRu/C is the catalyst that has presented the best results for the use in direct methanol fuel cells [11], as the best CO-tolerant material. Gasteiger et al. [29] reported results using PtRu and pure Ru, the maximum peak for CO oxidation was around 0.590V vs. RHE for pure Ru and 0.5V vs. RHE for Pt1Ru1. Schmidt et al. [30] showed results using PdAu/C for CO oxidation around 0.7V vs. RHE, and Colmati et al. [26] reported results using Pt90Sn10/C, and the maximum peak for CO oxidation was about 0.650V vs. RHE. Comparing with these data, the results presented by the catalysts Pt65U35/C shows that it is the best catalysts for CO oxidation at low potential, (maximum peak: 330mV vs. RHE).

• This fact probably is due to the various Uranium oxidation states that makes easy the water adsorption at lower potential and consequently make possible the CO oxidation at low potentials. It is interesting to note the presence of a shoulder tendency at 0.1 V vs. RHE for the Pt65U35/C, which suggest that the catalysts contains two or more types of reaction sites and/or crystallites with different activities.

CO Striping

0,0 0,2 0,4 0,6 0,8 1,00

1

2

3

4

5

6

733mV

430mV

5mV

PtRu/C E-TEK 20% wt Pt

65U

35/C 20% wt 330mV

C

urr

en

t De

nsi

ty (

A g

-1

Pt)

Potential (V vs RHE)

Fuel Cell Tests

• Finally, the electrodes and consequently the membrane electrode assembly (MEA) were prepared using the hot spray pressing method [24], and the polarization responses were plotted showing polarization curves fed with H2/O2, H2-100ppmCO/O2 and Ethanol 1molL-1 respectively for the prepared materials. The anodes were prepared using the materials Pt1Ru1/C E-TEK 20% and Pt65U35/C 20% with a loading of 0.4mgPtcm-2 and the cathodes were prepared using Pt/C E-TEK 20% with a loading of 0.6mgPtcm-2. For comparison were tested a MEA Pt1Ru1/C E-TEK with the loading of 0.4mgPtcm-2, anode and cathode.

PEMFC Operating with Hydrogen

0 50 100 150 200 250 300 350 400 450 500 550200

300

400

500

600

700

800

900

1000

PEMFC - H2/O2

Pt65U35/C 20% wt PtRu/C E-TEK 20% wt PtRu/C 20% wt MEA E-TEK

Pot

entia

l (m

V)

Current Density (mAcm-2)

PEMFC Operating with Hydrogen+150ppm of CO

0 50 100 150 200 250 300 350 4000

100

200

300

400

500

600

700

800

900

1000

H2+CO150ppm /O2

MEA PtRu/C 20% wt Etek PtRu/C E-TEK 20% wt Pt65U35/C 20 % wt

Pot

entia

l (m

V)

Current density (mAcm-2)

Direct Ethanol Fuel Cell

0 5 10 15 20 25 30 35 40-50

0

50

100

150

200

250

300

350

400

450

500

Ethanol 1molL-1/O2

PtRu/C E-tek 20 % wt Pt65U35/C 20 % wt

Po

tent

ial (

mV

)

Current density (mAcm-2)

PtU Catalyst Behavior after using Ethanol

0 100 200 300 4000

100

200

300

400

500

600

700

800

900

1000

Pt65

U35

/C 20 % wt H2/O

2

Pt65U35/C 20 % wt H2/CO 150 ppm

Pt65

U35

/C 20 % wt ETHANOL 1.0 molL-1

Pt65

U35

/C 20 % wt H2/O

2 after ETHANOL 1.0 mol L

-1

Pot

entia

l (m

V)

Current density (mAcm-2)

Fuel Cell Tests - Results

• In summary, these results shows that the materials from E-TEK presented electrocatalytic performance superior to that obtained for Pt65U35/C, but the results showed for Pt65U35/C is similar to the alloy PtSn published in previous works [31-33]. But the results obtained and showed in the figure 9 for the material Pt65U35/C are at least interesting. It is well known in the literature that a catalyst or a MEA that works first with H2/O2 and than with H2-CO/O2 or with Methanol/Ethanol/O2 in this sequence, with the time, it occurs a catalysts or MEA degradation, and the first performance that presented the MEA working with H2/O2 is unreachable. This occurs meanly because of the CO poisoning of the catalysts, the methanol crossover into the cathode side, poisoning the cathode and causing mixed potentials, the degradation of the membrane; and only cycling the potential with cyclic voltametric technique is possible to “clean” the catalysts and to recover the first performance. But in this study, the results showed for the Pt65U35/C catalysts presents a different behavior.

Fuel Cell Tests - Results

• After this sequence, H2/O2 – H2-CO/O2 – Ethanol/O2 and finally working again with H2/O2, the catalysts presented the same first performance. The overall results shows that the metal Uranium presents an important behavior in this reaction, probably in an intensive oxygenated species formation at low potentials, causing the oxidation of the CO species adsorbed in the catalysts to CO2. These results suggest that the using of a catalysts PtxRuyUz/C alloy with a small Uranium quantities could be a promising catalysts for using in these applications, Uranium would act as a oxygenated species donator. Further experiments have to be conducted in order to clearly evaluate the best composition for this catalysts PtxUy, PtxRuyUz or PtxSnyUz to get better results in the polarization curves and to make clear the behavior of the Uranium in this reactions.

Conclusions• The main conclusions of this study can be summarized as follows:

– The colloid method had a good performance in the catalysts preparation, but other studies have to be developed to make changes in the method procedures in order to produce catalysts with fewer impurities or using some chemical or heat treatment.

– The energy dispersive analysis (EDX) results of the catalysts are in good agreement with the method utilized, showing that the colloid method was successful in the way to produce the catalysts with the desired compositions.

– DRX analysis showed particle size results for Pt65U35/C in according with the alloys previous published.

– The results of anodic sweep voltametry for CO oxidation showed an unpublished result, PtU/C was able to start the CO oxidation at 5mV vs. RHE and with a maximum current peak for CO oxidation at 330 mV vs. RHE.

– Polarization curves indicate that Pt65U35/C did not show degradation after CO poisoning by ethanol, showing that Uranium could be an important material for CO removing and avoid CO poisoning at long time operation.

Acknowledgements

• The authors thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), the Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP) and the Insituto de Pesquisas Tecnológicas do Estado de São Paulo for financial assistance to the project.

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