Electro-catalytic supports for noble metals for exploitation as electrodes for fuel cells Ph.D., Seminar - II Sep. 20, 2006 T. Maiyalagan CYD01006

Outline Introduction Principle of DMFC 1 Introduction Principle of DMFC Role of supports in electrocatalyst Nitrogen containing carbon nanotubes Metal oxide nanomaterials (TiO2 nanotubes & WO3 nanorods) Conducting polymer nanotubules and polymer -metal oxide nanocomposite (PEDOT–V2O5 ) Conclusion

What are fuel cells ? Direct Energy Conversion Vs Indirect Technology C. K. Dyer, J. Power. Sources, 106 (2002) 245

Various types of fuel cells Research area Methanol Fuel Cells Direct methanol fuel cell Indirect methanol fuel cell Fuel : Hydrogen (Reformed from methanol) Fuel reformer makes the system bulky Fuel : Methanol

Direct Methanol Fuel Cell (DMFC) DMFC advantages More environmentally friendly High efficiency Low temperature (60-130 0 C) Long membrane lifetime Methanol is safe to store and distribute than hydrogen Low emission future power sources Catalyst poisoning Pt-CO Anode: CH3OH + H2O  CO2 + 6H+ + 6e- E = 0.016 V (RHE)Cathode: 3/2O2 + 6H+ + 6e-  3 H2O E = 1.229 V (RHE) Overall : CH3OH + 3/2O2  CO2 + 2H2O E = 1.213 V (RHE) Heinzel et al, J. Power Sources 105 (2002) 250

REDUCTION OF PLATINUM LOADINGS Objective  To reduce the amount of noble metal loading in the electrodes  Increase metal dispersion on suitable electronically conducting supports REDUCTION OF PLATINUM LOADINGS Dispersion Utilization  Stability

Why supported catalyst? High Temperature Unsupported catalyst High Temperature Supported catalyst Role of support ?

Role of electro-catalyst supports High surface-to-volume ratios High dispersion of noble metal particles to reduce noble metal loadings Avoid the agglomeration of the metal particles in operation, Good stability of electrocatalysts Improved the activity of electro-catalysts through the interaction between metal and support Lower the resistance of mass transportation Always superior to respective unsupported and conventional supported systems Shorten the time of DMFC commercialization

Increase in metal dispersion, Strategy Metal catalysts Electrocatalyst supports Unsupported Conducting polymers Metal oxides Carbon nanotubes Conducting polymer nanotubes, Polymer- metal oxide nanocomposite TiO2, WO3 nanomaterials Nitrogen containing carbon nanotubes Poor performance, not economical Stability Increase in metal dispersion, Economical  Interactions between the noble metal and the support may lead to increase catalyst performance

Role of nitrogen on the carbon nanotube supported anodes for methanol oxidation Why carbon materials are used as an electrocatalyst support ? Electrochemical properties - wide electrochemical potential window Chemical properties - Good corrosion resistance Electrical properties - Good conductivity Mechanical properties - Dimensional & mechanical stability - Light weight & adequate strength Carbon nanotube as an electrode support in fuel cells Mesoporosity (2-50nm) Improved mass transfer Better dispersion Surface bound groups High accessible surface area High purity → avoids self-poisoning Good electronic conductivity

Nitrogen functionalization in carbon support Pt bound strongly to nitrogen sites so higher dispersion, avoids sintering Addition of nitrogen increases the conductivity of the material by raising the Fermi level towards the conduction band The presence of nitrogen generates catalytic site and this catalytic site is responsible for increased activity of methanol oxidation Electrode Methanol oxidation potential at +50 mA/cm2 (V) Untreated (U) 604 Nitrogen functionalized (N) 554 Sulfur functionalized (S) 633 Current – potential curve for sulfur functionalized (S), nitrogen functionalized (N) and un-functionalized (U) carbon supports S.C. Roy et al., J. Electrochem. Soc., 144 (1997) 2323

Nitrogen induces catalytic activity + ê + ê + ê + ê + ê + ê carbon oxygen hydrogen Pt Pt e- e- e- e-

Synthesis of Nitrogen containing carbon nanotubes Present work NITROGEN CONTAINING POLYMERS PPP N= 0% PVP N=12.9% PPY N=21.2% PVI N=33.0%

Synthesis of CNT from poly(paraphenylene) AlCl3(0.5M),CuCl2 Benzene (1M) Alumina membrane Polymerization, RT , 3h PPP/alumina Carbonization, Ar atm CNT/alumina 48% HF, 24 h CNTPPP

Electrochemical studies Cyclic Voltammograms in 1 M H2SO4 aqueous solution at (a) Glassy carbon coated carbon nanotube electrode (b) Glassy carbon coated Vulcan electrode (c) Glassy carbon.

Loading of catalyst inside nanotubes 73mM H2PtCl6 12 h H2 atm, 823 K, 3 h 48% HF,24 h

Electrocatalytic activity of the catalyst Cyclic Voltammograms of (c) GC/CNTppp-Pt--Nafion Nafion in 1 M H2SO4/1 M CH3OH run at 50 mV/s

Synthesis of N-CNTs from Poly(vinylpyrolidone) in dichloromethane AFM Alumina membrane Carbonization, Ar atm PVP/alumina TEM 48% HF, 24 h CNTPVP SEM

XPS T. Maiyalagan, B.Viswanathan, Mater.Chem.Phys , 93 (2005) 291

Cyclic Voltammograms of (a) GC/CNTppp-Pt-Nafion Nafion in 1 M H2SO4/1 MCH3OH run at 50 mV/s T. Maiyalagan, B. Viswanathan, U.V. Varadaraju, Electrochem. Commun. 7 (2005) 905

Synthesis of N-CNTs from Poly(pyrrole) FeCl36H2O(0.2M) Pyrrole (0.1M) PTSA (0.2 M) Alumina membrane SEM: N-CNT Polymerization, RT , 3h PPY/alumina Carbonization, Ar atm TEM: Pt/N-CNT 48% HF, 24 h CNTPPY

Synthesis of N-CNTs from Poly(vinylimidazole) Benzene Vinyl imidazole (0.1m) AIBN Alumina membrane Polymerization , 68o C, 10 h PVI/alumina Carbonization, Ar atm CNT/alumina 48% HF, 24 h CNTPVI

Elemental analysis (CHN) Calculated from polymer Calculated from carbon nanotubes SAMPLE % C % N % H PPP-CNT 93.0 0.0 4.9 92.3 1.8 PVP-CNT 64.8 12.6 8.2 87.0 6.6 0.8 PPY-CNT 66.7 21.2 6.1 78.2 10.3 0.6 PVI-CNT 63.8 29.8 6.4 75.5 18.9

Electrocatalytic activity Electrode Nitrogen content Activity Ip (mA/cm2) Pt - 0.076 GC/ETek 20% Pt/C Naf 1.3 GC/CNT-Pt PPP-Naf 0.0 12.4 GC/CNT-Pt PVP -Naf 6.63 16.2 GC/CNT-Pt PPY -Naf 10.5 21.4 GC/CNT-Pt PVI-Naf 16.7  18.6 Electrocatalytic activity of the N-CNTs electrodes in comparison with commercial catalysts for methanol oxidation Specific activity for methanol oxidation versus nitrogen content of the catalysts

Stability of the electrodes

Salient Features Higher surface area of the CNT - better utilization of the catalytic particles Tubular morphology & nitrogen presence - better dispersion of Pt E-TEK < Pt PPP-CNT < Pt PVP-CNT < Pt PVI-CNT < Pt PPY-CNT Nitrogen presence – dispersion & also in increasing the hydrophilic nature of the catalyst Correlation between the catalytic activity and the nitrogen concentration (at%)

Methanol oxidation catalysts Electro-oxidation of methanol on Pt/TiO2 nanotube catalysts Methanol oxidation catalysts Pt alloys Intrinsic mechanism (bifunctional mechanism) Pt-Ru Pt with metal oxides Promoted mechanism (hydrogen spill over and oxygen spill over) TiO2, WO3 O H C O P t Ru

Synthesis of Pt /TiO2 nanotube catalyst Alumina Membrane Titanium isopropoxide in isopropanol Impregnation Calcined at 873 K, 2 h TiO2 Anatase /Alumina Immersion in 73mM H2PtCl6, 12 h 823 K, H2 atm, 3h 3 M NaOH Pt-TiO2 Nanotube

UV and Raman spectra Raman spectrum UV Spectrum

SEM & TEM images of TiO2 nanotubes Pt/TiO2 nanotube

X- Ray diffraction pattern A - Anatase R - Rutile Pt- Platinum (111)

Electrode fabrication 10 mg Pt/TiO2 nanotubes/ 100 l water Ultrasonicated for 30 min Dispersion (10 l) / Glassy Carbon (0.07 cm2) Dried in air 5 l Nafion (binder) Solvent evaporated ELECTRODE Counter electrode-platinum foil Reference electrode-Ag/AgCl/KCl(saturated)

Electrocatalytic activity of the catalyst Electrocatalyst Anodic scan peak potential (V) vs Ag/AgCl Anodic peak current density (mA cm-2) Bulk Pt 0.16 20 % Pt/C 0.762 1.3 Pt/TiO2 nanotube 0.680 13.2 Cyclic voltammograms in 0.5 M H2SO4 and 1 M CH3OH Scan rate : 50mV/s

Promoting effect of TiO2 nanotube support Strong metal support interaction (SMSI) OH adsorption on TiO2 facilitates CO oxidation Pt-CO + OH(ads) → Pt∙∙∙CO2 + H+ + e- OH(ads) forms on TiO2 surface and oxidizes Pt-CO Model presentation of M-OH transfer and spillover upon metallic part of electrocatalyst. T. Maiyalagan, B. Viswanathan and U. V. Varadaruju J. Nanosci. Nanotech 6 (2006) 2067

Salient Features  TiO2 nanotubes have been explored as support for Pt  High platinum dispersion was obtained on these TiO2 nanotube supports by the reduction of the Pt ions with H2 at 873 K  The resulting electrodes were tested for methanol oxidation reaction and shows higher catalytic activity than the commercial catalyst  Strong metal support interaction (SMSI)  COads on the Pt sites reacts with surface hydroxyl species (OHads), at the adjacent TiO2 sites facilitates CO oxidation  TiO2 nanotube morphology was also assumed to be responsible for the higher specific activity of methanol oxidation

Electro-oxidation of methanol on Pt/WO3 nanorods catalysts Metal oxides - promising CO tolerance Metal oxides supports to Pt must fulfill the requirement of forming O-containing surface species at low potentials Pt + H2O PtO + 2 H+ + 2 e- Eo = 0.99V Mo + 3H2O MoO3 +6 H+ + 6 e- Eo = 0.25V W + 2H2O WO2 + 4H+ + 4 e- Eo = -0.05V W + 3H2O WO3 + 6H+ + 6 e- Eo = 0.09V Lower values of Eo for the Mo/MoOx, and W/WOx than that for Pt /PtO enhance the ability of OH adsorption Metal oxides forms hydrogen bronzes provides H2 spill over and oxygen spill over mechanism Ability to act as redox centers Ionic and Electronic conductivity M.S. Antelman, Encyclopedia of chemical electrode potentials, Plenum, New York, 1982

Electro-oxidation of methanol on Pt/WO3 catalysts P. K. Shen et al., Catalysis Today 38 (1997) 439

Template Synthesis of WO3 nanorods 10 gm Phosphotungstic acid + 30 ml methanol Impregnation H3PW12O40/Alumina membrane Calcined at 873 K 3h WO3/Alumina membrane HF 73 mM H2PtCl6 Evaporated to dryness H2 atm, 623 K, 4h Pt/WO3 nanorods (dark powder) WO3 nanorods (light blue powder)

FTIR spectra 990 cm-1 is for as(W-Od) 1080 cm-1 is for  as (X-Oa) 894 cm-1 is for as(W-Ob-W) 817 cm-1 is for as(W-Oc-W) The strong absorption between 500 and 1000 cm-1 can be associated with the W-O-W stretching modes ( characteristic peak of tungsten oxide )

UV and Raman spectrum UV spectrum Raman spectrum 717.6 and 808 cm-1 : O-W-O stretching modes

X- Ray diffraction pattern XRD pattern of tungsten oxide nanorods

SEM/EDX images Mag:3000K

TEM/EDX images 200 nm 100 nm

Electrochemical studies Bulk WO3 WO3 nanorods xH+ + xe- + WO3 ---------------> HxWO3 (formation of hydrogen tungsten bronze)

Hydrogen spill-over effect on Pt/WO3 nanorods Pt + H+ + e- ---------- > xPt WO3 + xPt-H ---------- >HxWO3 + xPt-H HxWO3 --------------->xH+ + xe- + WO3 overlap between reoxidation of hydrogen tungsten bronze and desorption of hydrogen on Pt, difficult to estimate the active surface area of Pt

Electrocatalytic activity of the catalyst Specific activity mA/cm2 Pt loading g/cm2 Specific activity mA/cm2 Pt/Vulcan 20 23 Pt/WO3 nanorods 60

Salient Features Metal oxide nanorods as supports for Pt electrodes in the electrooxidation of methanol in acid electrolyte has been investigated WO3 was demonstrated to promote electrocatalytic oxidation of methanol by interacting with Pt via hydrogen spillover or through the formation of highly conductive tungsten bronzes Improvement of performance may be due to the presence of OHads groups on the oxide surface, that should facilitate oxidation of poisoning CO intermediates The promotional activity of WO3 nanorod is related to the W(VI)/W(IV) redox couple acting as a surface mediator for the oxidation of surface methanolic residues The nanorods morphology of the base-metal oxide is promoting the activity for platinum

Why Conducting polymers ? Electro-oxidation of methanol on electrodeposited platinum in Poly(o-phenylenediamine) nanotubule electrodes Why Conducting polymers ?  Easy Fabrication  Better stability  Good electronic /ionic conductivity  Three-dimensional structures and high porosity of the polymeric matrices leads to high dispersion Minimum transport limitations  High dispersion of metallic particles inside these polymers gives electrodes with higher surface areas and enhanced electrocatalytic activity

Polymers matrices used Polyaniline, Polypyrrole Polythiophene Poly (3,4, ethylenedioxythiophene) Poly (2 hydroxy 3-amino phenazine) Poly (o-aminophenol) Poly (o-phenylenediamine) - high aromaticity and high thermal stability - catalyst for electrochemical reduction of dioxygen - sensor for many chemical species, ladder polymer

Template assisted electrochemical synthesis of conducting polymer nanotubes Matrix = Alumina membrane Graphite electrode (Nafion coating) Polymer nanostructure Electrochemical Dissolution polymerization Alumina membrane Conducting composite Nanotubes 0.6 µm-thickness 200 nm pore diameter

Experimental setup AE WE RE Schematic view of an electrochemical cell for the formation of nanostructured materials. RE, reference electrode; AE, auxiliary electrode; WE, working electrode (template membrane with a deposited Nafion contact layer).

Template assisted electrochemical synthesis of Pt/PoPD nanotubules Graphite 1 cm2 Coating of 5 wt% Nafion, Al2O3 Membrane hot pressed on Graphite at 393 K, 3 min. Graphite /Naf/Al2O3   0.5 M OPD / 0.5 M H2SO4 Scanned between -0.2 V to 1.2 V vs Ag/AgCl at 50 mV/s GR/Naf/Al2O3/PoPD Electro reduction in 0.01 M H2PtCl6 0.8 V to -0.2 V GR/Naf/Al2O3-PoPD-Pt Cyclic voltammograms during the electropolymerization of o-phenylenediamine in 0.5M oPD + 0.5M H2SO4 solution (v = 50mVs−1) Dissolution of Al2O3 in 0.2M NaOH, Followed by immersion in 1% HBF4 (10 min) GR/Naf/PoPDtemp-Pt

FT-IR and Raman spectrum N-H stretching C=Stretching 812 cm−1 1,4-disubstitude ring These bands are assigned to stretching of Aminobenzene units of azo groups

UV spectrum

Electron microscopic images of Pt/PoPD nanotubules AFM Graphite/PoPD nanotube TEM image of Pt/PoPD Graphite/Pt-PoPD nanotube

Electrocatalytic activity of the catalyst Cyclic Voltammograms of GR/Naf/PoPD Temp in 0.5 M H2SO4 ( after the dissolution of the template)

Plot of anodic peak current density as a function of Platinum loading on Nanotubule and conventional electrodes. (Current densities were evaluated from CV run in 0.5 M H2SO4 / 1M CH3OH at 50 mV/s )

Electrocatalytic activity of the catalyst S.No Electrode Onset Potential (V) Activity* Forward sweep Reverse sweep I ( mA cm-2) E 1 2 GR/Naf/PoPD Temp –Pt GR/Naf/PoPD Conv –Pt +0.1 +0.2 84 13.3a 0.82 0.70 -- 6. a --- 0.45 * Activity evaluated from cyclic voltammogram run in 0.5 M H2SO4 / 1M CH3OH scanned between -0.2 and 1 V vs Ag/AgCl a peak current density (mA cm-2)

Stability of the electrodes GR/Naf/PoPD Temp –Pt GC/ 20 % Pt/C (E-TEK) GR/Naf/PoPD Conv –Pt

Salient Features Template synthesis of conducting poly (o-phenylenediamine) yielded cylindrical nanotubules with outer diameter matches with pore diameter of the template used Polymer nanotubules have adequate conductivity and allows both the incorporation of metal particles and an easy accessibility of the methanol to the catalytic sites.  Polymer nanotubules electrode have higher electrocatalytic activity than the conventional polymer film electrode.

Nanocomposites with catalytic activity Electro-oxidation of methanol on Pt supported PEDOT/ V2O5 nanocomposites electrodes Nanocomposites Organic-inorganic hybrid materials: “Compounds which contain both inorganic and organic moieties within the same micro structure” Organic moiety - flexibility Inorganic moiety - thermal stability and structural rigidity Active moiety- catalysis Nanocomposites with catalytic activity PPY-SiO2 and PANI-SiO2 hybrid nanocomposites offer large surface area - Exploited as host for entrapping large amount of metal particles with desirable catalytic activity

Poly (3,4, ethylenedioxythiophene) (PEDOT) Has wide potential range Low band gap Electroactive Enhanced stability compared to polyaniline & polypyrrole Excellent environmental stability High electrical conductivity Transparency in thin oxidized film and is always in the p-doped state, i.e. conductive Applications solid electrolytic capacitors, antielectrostatic agents, transparent electrodes in light emitting diodes, and underlayers for the metallization of printed circuit boards Vanadium(V) oxide xerogel (VXG) Exhibiting high proton conductivity, as well as electronic conductivity arising from the presence of V(IV) ions

V2O5 catalysts for methanol oxidation E0/V +0.5 VO2+/V3+ CH3OH/HCHO HCHO/HCOOH CH3OH/CO2 Vanadium potentials and steps of methanol oxidation HCOOH/CO2 V3+/V2+ -0.5 Comparison of the potentials at which methanol is usually oxidized electrochemically and the normal redox couples. The normal potentials of the probable intermediates in the methanol oxidation are also included B. FolKesson et al., J. Electroanal. Chem., 267 (1989) 149

PEDOT is meant to substitute the carbon usually mixed with the inorganic oxide-based electrodes to improve their electronic conductivity; the PEDOT thus functions as electronic conductor 13.4 A0 + This PEDOT-V2O5 based organic inorganic nanocomposites could be a good Catalyst support for methanol oxidation

Synthesis of poly(3,4-ethylenedioxythiophene)/ V2O5 nanocomposites V2O5 Powder (1 g, or 5.5 10-3 mol) 100 ml aqueous solution of hydrogen peroxide (10%) V2O5.nH2O gels 0.75ml EDOT Ammonium persulphate Dried at 60 0 C PEDOT/V2O5 nanocomposite

Preparation of Pt /PEDOT/ V2O5 nanocomposite 5 %H2Pt Cl6 (35ml) Stirring at 800C for 30 min to allow dispersion 0.1 N NaOH added to bring the pH 10-11 20% Formaldehyde Stirred at 70C for 1 hour Cooled Filtered and washed with distilled water Dried in air oven at 125  C for 4h 20% Pt-PEDOT/V2O5nanocomposite

FTIR spectra After intercalation Extra peaks --C-O-C OH After intercalation Extra peaks --C-O-C stretching vibrations FT-IR spectra of (a) V2O5 (b) V2O5 Xerogel and (C) PEDOT/V2 O5 nanocomposites

X- Ray diffraction pattern After intercalation Incorporation of PEDOT in V2O5 increases the interlayer spacing to 13.4 A0 X-Ray diffraction patterns of (a) V2O5 and (b) PEDOT-V2O5 nanocomposite

Thermal studies first one step 8 % weight loss below 120 C----- due to water second one step16.2 % weight loss up to 420 C ----combustion of organic polymer mass gain 2.6 % up to 650 C ---- formation of orthorhombic V2O5 Thermogravimetric curves of (a) V2O5 (b) V2O5 xerogel and (c) PEDOT-V2O5 nanocomposite

SEM images of PEDOT/ V2O5 nanocomposite

SEM images of PEDOT/ V2O5 nanocomposite Layered structure

SEM/EDX images of Pt-PEDOT/ V2O5 nanocomposite EDX mapping of Pt on nanocomposite

XRD and TEM images of Pt-PEDOT/ V2O5 nanocomposite Average particle size of 4.52 nm

Electrochemical studies V5+/VO2+ V5+ Cyclic voltammograms in 0.5 M H2SO4 Scan rate : 50mV/s

Electrocatalytic activity of the catalyst Cyclic voltammograms in 0.5 M H2SO4 and 1 M CH3OH Scan rate : 50mV/s

Electrocatalytic activity of the catalyst Stability of the electrodes Methanol oxidation activity of Pt/ PEDOT-V2O5 catalyst prepared by formaldehyde reduction method Catalyst Pt loading g/cm2 Methanol oxidation activity mA/cm2 Pt/(C6H4O2 S)0.4V2O5.0.5H2O 10 28.4 Pt/Vulcan 15 Nearly two times higher activity for the same Pt loading

Salient Features The Pt loaded on PEDOT-V2O5 nanocomposite has been evaluated as the electrode for methanol oxidation in acid medium EDX mapping and TEM images shows fine dispersion of Pt particles in the nanocomposite leading to a better utilisation of the noble metal catalyst Electrocatalytic activity of methanol oxidation reaction for Pt/PEDOT-V2O5 was higher than Pt/C

Conclusions The salient points of the present investigation are: The supports for noble metal electro-catalyst have a definite role in the development of electrodes for fuel cells The functionalization of the support carbon materials is a mean to increase dispersion and also intrinsic activity of the noble metal sites Oxide supports, though may lead to net loss of energy, can favour the removal of otherwise poisons for the noble metal electro-catalytic sites Conducting polymers and their composites can be conceived as alternate supports for noble metal electrodes for effective dispersion and intrinsic activity