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Ternary and quaternary electrocatalyst for PEMFC by ert554898

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									      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.br
1 – 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.br
1 – 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 dry
     The 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. Jouen, 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 H SO solution saturated with N . The evaluation of CO oxidation was
        2   4                           2
    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                Catalytic Layers                  Membrane                         MEA


     Weighting the            Spraying the ink in the    Cutting and Cleaning the    Cutting the Diffusion layers
     catalyst             1º side of the membrane          membrane in water



     Adding Water                                                                        Mounting the MEA
                                Drying at 100ºC         Treatment in water at 80ºC
                                                                -1 hour
   Ultrasonically – 30                                                               Hot pressing the Electrodes
      seconds             Spraying the ink in the 2º                                     with the membrane
                           side of the membrane         Treatment in Sulfuric Acid
                                                             at 80ºC -1 hour
 Adding Nafion Solution


                                Drying at 100ºC

   12 hours magnetic                                       Washing with water
      stirring
       Scanning Electron Microscopy
                  (SEM)
• Membrane Electrode Assembly (MEA).

                 Electrolyte:
                 Nafion
                 membrane
                    Catalytic
                    Layer
Diffusion Gas Electrode Structure
                   EDX Analysis
Catalyst           Pt molar%   Ru molar% Particle Size (nm)
                   from EDX    from EDX     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

                                         Pt (111)
                                                    Pt (200)
                                                                      Pt (220)
Relativ Intensity (cps)




                                                               PtUxOy/C




                                                               UxOy/C


                                                               VulcanXC72R

                          10   20   30       40         50       60         70   80

                                             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

                           4


                           3


                           2
Current Density (A g Pt)
-1




                           1


                           0


                           -1


                           -2                                                  -1
                                                                 Pt65U35/C 10mVs
                                                                               -1
                                                                 Pt65U35/C 20mVs
                           -3                                                        -1
                                                                 PtRu/C E-TEK 10mVs

                           -4
                             0,0    0,2      0,4        0,6          0,8            1,0
                                          Potential (V vs RHE)
                             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

                       6

                                    PtRu/C E-TEK 20% wt
                                    Pt65U35/C 20% wt  330mV
                       5

                                                                             733mV
) Pt




                       4
-1
Current Density (A g




                              5mV

                       3



                       2



                       1

                                                                    430mV
                       0
                        0,0           0,2        0,4          0,6           0,8      1,0
                                              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

                 1000
                                                             PEMFC - H2/O2
                 900                                            Pt65U35/C 20% wt
                                                                PtRu/C E-TEK 20% wt
                 800                                            PtRu/C 20% wt MEA E-TEK


                 700
Potential (mV)




                 600


                 500


                 400


                 300


                 200
                        0   50    100   150     200   250   300   350        400   450   500   550
                                                                        -2
                                              Current Density (mAcm )
                             PEMFC Operating with
                            Hydrogen+150ppm of CO

                 1000

                 900                                    H2+CO150ppm /O2

                 800                                          MEA PtRu/C 20% wt Etek
                                                              PtRu/C E-TEK 20% wt
                 700                                          Pt65U35/C 20 % wt
Potential (mV)




                 600

                 500

                 400

                 300

                 200

                 100

                   0
                        0    50   100      150    200     250        300    350        400
                                                                -2
                                        Current density (mAcm )
                           Direct Ethanol Fuel Cell

                 500

                 450
                                                                     -1
                                                     Ethanol 1molL /O2
                 400

                 350                                       PtRu/C E-tek 20 % wt
                                                           Pt65U35/C 20 % wt
                 300
Potential (mV)




                 250

                 200

                 150

                 100

                 50

                  0

                 -50
                       0    5   10      15     20     25        30        35      40
                                                           -2
                                     Current density (mAcm )
                 PtU Catalyst Behavior after using
                             Ethanol

                 1000

                                        Pt65U35/C 20 % wt H2/O2
                  900
                                        Pt65U35/C 20 % wt H2/CO 150 ppm
                                                                             -1
                  800                   Pt65U35/C 20 % wt ETHANOL 1.0 molL
                                                                                          -1
                                        Pt65U35/C 20 % wt H2/O2 after ETHANOL 1.0 mol L
                  700
Potential (mV)




                  600

                  500

                  400

                  300

                  200

                  100

                    0
                        0   100                200                  300                        400
                                                               -2
                                  Current density (mAcm )
               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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