2011 Workshop Singapore Catalysis Society

Catalysts for PEM Fuel Cells

Lin JianyiH t C t l iHeterogeneous Catalysis

Institute of Chemical and Engineering SciencesA*STAR

Fuel Cell likes a chemical plantReference: Fuel Cell Handbook (DOE/NETL‐2002/1179)

• If Oxidation and reduction at same place & time2H2 + O2 ––> 2H2O + 286 kJ/mol (heat)

• A fuel Cell likes an entire chemical plant

NASA uses hydrogen fuel to launch the space shuttles.

• A fuel Cell likes an entire chemical plant where oxidation and reduction are physically separated at two electrodes,physically separated at two electrodes,

W = ΔG = ΔH – TΔS= (-286 kJ/mol) – (-49 kJ/mol) = -237 kJ/mol

How does fuel cell work? (animation)http://www humboldt edu/~serc/animation htmlhttp://www.humboldt.edu/ serc/animation.html

1.Hydrogen molecules enter the fuel cell. Each hydrogen atom is split into a proton (the hydrogen ion) and an electron.

2.The protons pass through an "electrolyte" membrane.

3. The electrons are sent through the wire to power the vehicle's electric motor. Then they return to the fuel cell.

4. Finally the hydrogen ions, the electrons and oxygen combine to form water. Water is continuously removed from the fuel cell as the ions and electrons keep flowing through the cell.

Polymer electrolyte membrane Fuel Cellor Proton Exchange Membrane FC (PEMFC)or Proton Exchange Membrane FC (PEMFC)

• Membrane electrode assembly (MEA)Anode(catalyst)/electrolyte/Cathode(catalyst)Anode(catalyst)/electrolyte/Cathode(catalyst)

• An ion-conducting electrolyte is needed to transport the ions (H+)

• Pt/C catalysts are needed both for cathode and anode

• Pt is the best catalyst since 1842

Pt/XC72

Why Pt?

Pt is the best catalyst for H2 oxidation at anodey 2

Small Over-potentialH 2H+ + 2H2 2H+ + 2e

• Oxygen Reduction Reaction (ORR) is a complex process • Pt is the best catalyst

(Charles C Liang and Andre L Juliard J Electroanal Chem 9 (1965) 390)

• Path A (K1) – Four-electron reduction

(Charles C. Liang and Andre L. Juliard, J. Electroanal. Chem., 9 (1965) 390)

T ORR P h

O 2+ 4 H+ + 4 e 2 H2O ; E = 1.229 V

Two ORR Pathways

• Path B – indirect pathway, involves two-electron reduction followed by further two-electron reductionO + 2 H+ + 2 e H O ;O 2+ 2 H + 2 e H2O2; H2O2+ 2 H+ + 2 e 2 H2O ;

• A two-electron reduction of oxygen not only reduces the efficiency of the t th t id ti th t i i t d ith th d d h dsystem, the strong oxidation that is associated with the produced hydrogen

peroxide can degrade the catalytic activity of catalysts and membrane, resulting in significant fuel cell degradation, or even failure.

O2 bond weakening can improve ORR kinetics 

Pt electronic structure: d band center, d orbital vacancy

Surface geometric structure:

Pt-Pt spacing

Pt : the best ORR catalyst

M HO + Pt M-OH + PtOM HO2 + Pt M-OH + PtO

Volcano typeVolcano-type behavior

• Suitable O binding energy • Low d-band center• Particle size and shape

(730)>>(110) > (100) > (111)

Comparison of the oxygen reduction activity of Pt nanoparticles of different sizes

Pt Particle size Onset potential ORR activity Mass Catalyst

Ptmetal loading

(wt%)

Particle size (nm)

Onset potential for oxygen reduction

(mV vs. NHE)

ORR activity at

+0.7 V vs. NHE

(mA cm-2)

Mass activity at +0.7 V vs.

NHE (A g-1)

Pt1C

Pt2C

19.3

19.5

1.7-1.9

2.3-2.6

+935

+930

1.9

2.5

34

44

Pt3C 19.3 3.2-3.4 +930 4.3 76

Pt4C

Pt5C

19.1

19.3

3.8-4.0

4.6-4.9

+920

+890

3.4

2.8

60

50

Pt6C

Commercial Pt/C (E-TEK)

19.6

19.8

5.7-6.1

3.5-3.9

+880

+920

2.2

3.1

39

55

9Optimum size of Pt for oxygen reduction is 3 - 4 nm

Fuel Cells: Trend and Challenges Fuel cells manufactured and sales have steadily increased recently.y y Portable PEMFC exhibits stronger growth than other application sectors. The market continues to be dominated by PEMFC

High energy density (up to 10x),

Horizon miniPAK Ultra Cell U25

• High manufacturing cost: Pt cost and availability • Short durabilityShort durability

(38% l )

• Lowering the Pt-loading for anode electrodes from today’s 0.2–0.4 mgPt/cm2

down to 0 05 mgPt/cm2 is straightforward due to the large activity of Pt toward

(38% total system cost)

down to 0.05 mgPt/cm is straightforward due to the large activity of Pt toward the H2-oxidation reaction.

• Lowering of the cathode loadings of ca. 0.4 mgPt/cm2 is limited by the poor g g g y pactivity of Pt for the oxygen reduction reaction (ORR)

• Cell potential E is the difference between the cathode potential (+) Ecand the anode potential (‐) Ea.          E = Ec – Ea

• Reversible cell potential at 1 atm and 25oC

O2 + 4 H+ + 4 e– = 2 H2O      1.23 V   Ec2 H+ + 2 e– H 0 00 V E2 H+ + 2 e– = H2 0.00 V   Ea

• For H2/O2 fuel cell: Eo = Eco – Eao = 1.23 V

• In practice a single fuel cell produces only about 0 7 volts• In practice a single fuel cell produces only about 0.7 volts.

• ~0.4 V over-potential loss (60% of PEMFC’ overall efficiency loss) is due to slow ORR electrode kinetics.

Main objective of PEMFC researches:

Develop oxygen reduction reaction (ORR) catalysts, alternative to pure platinum, capable of fulfilling cost,alternative to pure platinum, capable of fulfilling cost, performance and durability requirements. 

h l f l h b d l dThree classes of ORR catalysts have been developed:

• Pt‐based catalysts with lower Pt contentsyPt‐M (alloys or intermetallic) catalysts (M= Co, Ni, Cr, Fe, Mo, Bi)Monolayer Pt/M core‐shell catalysts (M= Pd, Au etc)Pt/Metal oxide promoter/CPt/Metal oxide promoter/C

• New‐generation chalcogenides (Ru‐Mo sulfide, selenides)  • Non‐precious metal/heteroatomic polymer nanocomposites

(Fe, CoN4 macrocyclic compounds (e.g. Fe/Co porphyrins, phthalocyanines)

PtM Alloy Catalysts

• Calculations and experiments show that PtM alloys > Pure Pt( C C ) 3 f• PtM (M=Ni,Co,Cr or Mn) > Pt 3-5 folds in kinetics

• PtTi, PtFe, PtMn enhance 20-40 mV at practical current density range• PtCrCu with CuO or CrOx enhance the activity by 6x vs. pure Pt

Pt d band vacancy/center electronic effect for O O scissionPt d-band vacancy/center electronic effect for O-O scissionPt-Pt distance ensemble and structure effectM promote redox process inhibit Pt-OH formation

PtM CatalystsS. Mukerjee, et al, J. Electrochem. Soc., 142 (1995) 1409j , , , ( )

Catalyst Pt-Pt distance (Å)

Pt d-band vacancies

E kJ/moldistance (Å) vacancies

PtPt53Ni47

Pt F

2.772.662 69

0.3700.3780 390

kJ/mol

75.9

Pt51Fe49

Pt49Co51

Pt50Cr50

2.692.712.73

0.3900.3900.401

57.027.523.2

• Pt/Cr, Pt/Co, and Pt/Ni contracts the Pt-Pt bond distances. This would enhance a dual site mechanismThis would enhance a dual site mechanism(Equation 3 Pt-HO2 + Pt Pt-OH + PtO).

• The Pt-Pt bond distance in the alloy is related to the strength

• The alloying elements (Ni,Co) have a positive charge with

of the Pt-OH bond, i.e. the intermediate formed in the rate-determining step of molecular dioxygen reduction.

y g ( , ) p grespect to Pt atoms, i.e. oxidized. OHad on Pt sites surrounded by “oxide”-covered Ni and Co atoms may be reduced significantly due to lateral repulsive interactionssignificantly due to lateral repulsive interactions.

• Surface roughening of the Pt alloy due to leaching of the moreeasily oxidizable base-metal

• Alloying of Pt with transition metal increases the Pt d-band vacancy,

• Catalyst sintering or particle size growth is slower in PtM/CCatalyst sintering or particle size growth is slower in PtM/Cthan in Pt/C.

T. Toda, et al, J. Electrochem. Soc., 146 (1999) 3750

PtM Core‐Shell CatalystsJ. Bell http://www.ncsa.uiuc.edu/CoverStories/pt‐monolayer

• Pt/Pd is the best, improving the overall efficiency of the ORR by 33%.

• Reduce the Pt load by 10x

Nanocomposite with little or no PtPt/MO; Pt/WC; Pt/LaB6

• MnO2/C nanoparticles exhibited good ORR activity. g y

• M2+‐MnOx/C  (M: Ni, Mg or Ca) exhibited ORR activity close to that of a Pt/Vulcan XC72 from E‐Tek.• Nano Au–Pd/WC catalysts > Pt/C. 

fWC itself has the catalytic activity to enhance the catalytic activity of the metal. 

• Ag–WC/C show essentially equal performance compared to Pt/C

• Reduce Pt loading in fuel cell cathodes by a factor of >20 (Pt/Fe2O3)

performance compared to Pt/C.

• Pt/LaB6 reduce O‐O better due to low work function

Non‐Pt CatalystsFeN : increase activity by ligationFeN4: increase activity by ligation

31%Fe3d + 69% N2p

Metal macrocyclic or porphyrins MPcSquare planar complexes with the central metal atom symmetrically surrounded by four nitrogen atoms; Delocalization of ‘’ electrons high conductivity

The RuFeNx/C catalyst showed the onset potential for ORR as highas 0 9 V(NHE) which is comparable toas 0.9 V(NHE) which is comparable to that of the Pt/C catalyst.

Lefevre et al Science 324 (2009) 71

Jose H. Zagal, Coord. Chem. Rev., 119 (1992) 89

• Transition metal chalcogenides e.g. Ru2Se8 or RuxSycan catalyze ORR as well.

Catalyst Elemental composition by

EDX

Se/Ru atomic ratio

Crystallite size from XRD (nm)

Onset potential (mV) for oxygen

reduction

ORR activity at +0.65 V vs. NHE

(mA/cm2)

y

Ru/CDX975Ru1Se0.2/CDX975Ru1Se0.4/CDX975Ru1Se0.6/CDX975Ru1Se0.8/CDX975

100:-87.7:12.376.6:23.468.5:31.562.2:37.8

0.00.180.380.590.78

3.03.03.13.13.1

+850+875+890+905+885

1.32.13.04.21.61 0.8

Ru1Se1/CDX975Pt/C (E-TEK)

56.2:43.8-

1.00-

3.1 +870+930

1.44.0

ORR activity exhibits a maximum for the Ru1Se0.6/CDX975 catalyst

ORR activity of Ru1Se0.6/CDX975 was comparable with that of Pt/C (E-TEK) catalyst

21

Se/Ru atomic ratio vs. ORR current densityof as-synthesized RuxSey/CDX975 catalysts

M. Bron, et al, J. Electroanal. Chem., 500 (2001) 510

CO‐tolerant PtM anode Catalysts

Preferential CO Oxidation to CO<10ppm

Equation 8 demands similarity between the promoter M–O q y pand the Pt–C bond energy of ~590 kJ/mol. A plot of M–O binary bond energies in proximity to the Pt–C bond energy suggests a component library that includes Mo Ru Os Snsuggests a component library that includes Mo, Ru, Os, Sn, and Re.

Intermetallic compound?

Both PtBi and PtBi2 are known: ordered structures (not alloys) that are metallic conductors. Experiments with PtBi showed a dramatic improvementExperiments with PtBi showed a dramatic improvementover Pt.

High performance Pt/C catalysts for self-humidifying fuel cells

(functionalized Carbon Black as Pt Support)( pp )

H2-Metal Hydride Tank

Pressure Regulator Valve Micro

MrMr Poh Chee Kok, Dr Tian Zhiqun, Dr Lim San HuaPoh Chee Kok, Dr Tian Zhiqun, Dr Lim San Hua

V+ C+

Hydride Tank Regulator Valvepurging valve

Purging circuit

• ICES catalyst showed 30% higher max output power density

Vout

GND

C-

DC/DC converter

• ICES catalyst showed 30% higher max. output power density

27

As result of carbon functionalization: Better Pt Dispersion

Pt/XC72 (functional)Pt/XC72

More Electrochemical Active Pt Sites (EAS)

T.P.Oxidation of Pt/C Cyclic Voltammogram

Better wettability improves water management, particularly suitable for air-breathing self-humidifying PEMFCs g y g

• When thin NRE211 is used as the electrolyte,Pt/CA-CB and Pt/C-Com show little difference.

• When thicker NRE212 is used

Better Wettability

When thicker NRE212 is used Pt/CA-CB-NRE212 increases power to 102mWcm−2, ~25% higher than commercial one (75 mWcm-2).

• Particularly useful when it applies to a air-breathing FCs.

N211 N211 25N212 N212 50

H2O

• Water management is crucial • Back diffusion

Electro-osmotic dragH2O

Electro osmotic drag

• For N212 water back diffusion is poor and better wettability helps

f f l / l /

Fuctionalized‐Carbon‐Blacks supported PtRu fuel cell catalyst is more CO tolerant, particularly for self‐humidifying cells

Performance of commercial PtRu/C catalyst vs PtRu/CA‐CB with    (a) fully humidified H2/O2

(b) dry (H2+10ppm CO)/O2

© dry H2/O2 after removal of CO

• The PtRu on functionalized-carbon-black also have a 25% higher performance than commercial PtRu/C catalyst using dryhigher performance than commercial PtRu/C catalyst using dry H2 with CO concentration of 11.4ppm.

• Less difference under humidified condition -

ads2 eHOH-PtOHPt

-2adsads eHCOOH-PtCO-Pt

Thank YouThank You for Your Attention