physics e19 interfaces and energy conversion zae bayern bavarian centre of applied energy research...

Physics E19Interfaces andEnergy Conversion

ZAE BAYERN

Bavarian Centre of Applied Energy Research

Division 1: Technology for Energy Systems and Renewable Energies

Study of the Ethanol and Ethylen Glycol Electrooxidation

by Fuel Cell Differential Electrochemical Mass Spectroscopy (FC-DEMS)

V. Rao1, C. Cremers2, U. Stimming1,2

1Technische Universität München, Physik-Department E19James-Frank-Str., 85748 Garching

2ZAE Bayern, Abteilung 1Walther-Meißner-Str. 6, 85748 Garching

DPG Frühjahrstagung 2006, Arbeitskreis Energie, München, 20. März 2006


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Motivation for studying Ethanol Oxidation Reaction (EOR)

•Much less toxic than methanol: ``as safe as beer´´

•High energy density (10 kWh/kg)

•Easily available from renewable resources

Ethanol Oxidation Reaction (EOR) using FC-DEMS†

• To study the completeness of EOR

• To understand the differences between fuel cell and model electrode conditions

• To study the mechanism of EOR for different catalysts such as Pt, PtSn, PtRu, PtRh

† FC-DEMS = Differential Electrochemical Mass Spectroscopy at Fuel Cells


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• CO2 current efficiency for EtOH and EG oxidation as a function of potential, temperature, fuel concentration

• Electro-oxidation of the probable intermediates acetic acid and acetaldehyde

• Dependency of CO2 current efficiency on the kind of catalyst: experiments with Pt, PtSn and PtRu based catalysts

• Dependency of CO2 current efficiency on the catalyst loading and thus catalyst layer thickness: concept of residence time and active area

Outline


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Commonly Accepted Ethanol Oxidation Reaction Scheme

CH3--CH2OH

CH3--CHO

CH3--COOHad H3C--COOC2H5

CO2

.CHad .COad

C2H5OH

The dominant pathway is dictated by exp. conditions

CH4


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methanoltank

watertank

exittank

DEMSsensor

anode flowfield

gasexit

DEMS sensor for fuel cell gas diffusion electrodes

DEMS set-up

0 200 400 600 800 1000 1200

0,28

0 200 400 600 800 1000 1200

0

4

8

0 200 400 600 800 1000 1200

0

10

200 200 400 600 800 1000 1200

-0,8

-0,4

0 200 400 600 800 1000 1200

050

100150200

m/z = 61

I i / p

A

potential(mV/RHE)

m/z = 15

I i / p

Am/z = 29

I i / p

AI i /

pA

m/z = 22

I F /

mA

0 200 400 600 800 1000 1200

3,6

4,2

4,80 200 400 600 800 1000 1200

7,2

7,4

0 200 400 600 800 1000 1200

-3,926

-3,925

0 200 400 600 800 1000 1200

0

50

m/z= 15

I i / p

Apotential(mV/RHE)

m/z= 29

I i / p

AI i /

nA

I F /

mA

m/z= 22

DEMS measurement of the oxidation of ethanol using 1.0 M (left) or 0.1M (right) solution of ethanol as fuel

vacuumto MS

anodeoutlet

teflon discwith holes

detectioncylinder

microporousmembrane

o-ring


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CO2 current efficiency as a function of potential and temperature

CO2 current efficiency

- Increases significantly with increasing temperature

- decreases for anode potentials > 0.5 – 0.6V

The first observation indicates a temperature activated process

The second observation may be explained by increasing coverage of Pt with oxygen species

Catalysts 40% Pt/C (E-Tek) with a loading of 5 mg/cm2;Anode feed 0.1 M Ethanol at 5 ml/min;The approximate error limit is 10%.


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Apparent activation energies

Arrhenius plots for - the Faradic current (upper)- the CO2 m/z = 22 MS-signal (lower)

Catalysts 40% Pt/C, 5 mg/cm2Anode feed 0.1 M Ethanol, 5 ml/minAnode potential 0.6 V (RHE)

The apparent activation energy for the CO2 formation is much higher than that found for the oxidation of adsorbed CO.

CO oxidation does not appear to be the rate determining step.

0,0027 0,0028 0,0029 0,0030 0,0031 0,0032 0,0033-8,0

-7,5

-7,0

-6,5

-6,0

-5,5

-5,0

-4,5

-4,00,0027 0,0028 0,0029 0,0030 0,0031 0,0032 0,0033

2,5

3,0

3,5

4,0

4,5

5,00.1M EtOH

Ea= 53 kJ/Mol

ln(I

m22

)(m

/z=2

2)

1/T(K)

Ea= 31 kJ/Mol

ln(I

F)

molkJCOE oxada /20)(


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CO2 current efficiency as a function of concentration

The maximum CO2 current efficiency decreases with increasing concentration of ethanolPossible explanation:Intermediate products face higher competition for re-adsorption and thus further oxidation

0,01 0,1 1

0,008

0,012

0,016

0,02

0,01 0,1 1

1E-13

2E-13

I F / A

concentration (M/L)

0.45

0.40

I i(m2

2/A

)

Reaction orders calculated from first two points are depicted on the figure; temperature is fixed at 30 oC; anode potential is set to 0.6 V vs RHE


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Electro-oxidation of intermediates: Acetaldehyde

0 200 400 600 800 1000 1200-200

0

200

400

600

800

1000

1200

0 200 400 600 800 1000 1200

-3.93E-009

-3.93E-009

-3.93E-009

-3.93E-009

-3.93E-009

-3.93E-009

-3.93E-009

-3.93E-009

curr

ent [

x10

mA

]

curr0.1M acetaldehyde

90 oC,5ml/minute5mV/s

40%Pt/C,8mg/cm2 Pt

I MS

_22

potential(mV)

m22

Electro-oxidation of acetaldehyde yields • high CO2 current efficiencies• fairly high faradic currents


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Electro-oxidation of intermediates: Acetic acid

Acetic acid is highly resistant to oxidation at Pt and quite resistant to oxidation at PtSn/C

This rules out acetic acid as a major pathway for CO2 formation

0 200 400 600 800 1000 1200-8

-6

-4

-2

0

2

4

6

Pt-unsupported 4.3mg/cm2 ,70 oC,5mV/s,0.1M Acetic Acid

curr

ent [

mA

]

potential [mV vs RHE]

with 0.1M AA without AA

0 200 400 600 800 1000

-15

-10

-5

0

5

10

15 20% PtSn/C(2mg/cm2),90 oC5mV/s

curr

ent(m

A)

potential(mV)

without AA with 0.1M AA


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Updated reaction pathway scheme

CH3--CH2OH

CH3--CHO

CH3--COOHad H3C--COOC2H5

CO2

.CHad .COad

C2H5OH

CH4

X


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Dependency of the CO2 current efficiency on the kind of catalyst used

Faradic currents for ethanol oxidationare similar at PtSn/C and PtRu/C

At PtRu/C practically no CO2 is formed!


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Effect of catalyst loading / catalyst layer thickness

0,4 0,5 0,6 0,7 0,8 0,90,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

CO2 efficiency at 90 oC

0.1 M EtOH, 5 ml/min flow rate40%Pt/C

CO

2 c

urr

en

t effi

cie

ncy

potential / RHE

0.20mg/cm2

0.25mg/cm2

0.80mg/cm2

2.45mg/cm2

4.20mg/cm2

8.00 mg/cm2

CO2 current efficiency increases withincreasing Pt loading.

This corresponds to:

- Increased active surface area;

- Increased electrode thickness and thus increased residence time.

0 1 2 3 4 5 6 7 8 90,0

0,2

0,4

0,6

0,8

40% Pt/C

Unsupported Pt

20%PtSn/vulcan

CO2 current efficiency

at 90 oC, 0.1MEtOH, 0.6V/RHE

CO

2 c

urr

en

t effi

cie

ncy

Platinum loading(mg/cm2)


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Effect of surface area with different types of catalyst

0 2 4 6 8

0

100

200

300

400

500

600

700

CO

str

ipp

ing

ch

arg

e(m

C)

Platinum loading(mg/cm2)

40% Pt/C

Unsupported Pt

20%PtSn/vulcan

0 100 200 300 400 500 600 7000,0

0,2

0,4

0,6

0,8

1,0 40% Pt/C

Unsupported Pt

CO

2 c

urr

en

t effi

cie

ncy

CO stripping charge(mC)

20%PtSn/vulcan

For the supported catalysts Pt/C and PtSn/C,CO2 current efficiency appeared to be correlated with the CO stripping charge in the same way.

The unsupported catalyst behaves differently.Perhaps due to the much thinner catalyst layer.


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First tests on the ethylene glycol electro-oxidation

0,4 0,5 0,6 0,7 0,8 0,90,2

0,4

0,6

0,8

1,0

CO

2 c

urre

nt e

ffici

ency

Potential/ RHE

30 oC

50 oC

70 oC

90 oC

0.1M Ethyelene Glycol

The anode feed is 0.1 M EG at 5 ml / minute. The approximate error limit is: ±10 %. 5 mg / cm2 metal loading using 40 % Pt / C. Arrhenius plots

Anode potential set to 0.6V(RHE)

0,0026 0,0028 0,0030 0,0032 0,0034

-7

-6

-5

-4

-30,0026 0,0028 0,0030 0,0032 0,0034

-4,0

-3,8

-3,6

-3,4

-3,2

-3,0

-2,8

-2,6

-2,4

-2,2

-2,0

-1,8

Ea= 45kJ/mol

ln(I

MS(m

/z =

44

))

1/T(K)

ln(I

F/m

A)

Ea= 25kJ/mol


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Conclusions

1) CO2 current efficiencies for the EOR depend strongly on the potential, temperature and concentration;

2) CO2 current efficiencies for EGOR do not depend on the potential, unlike EOR;

3) Catalyst layer thickness and electrochemical active area also affect the CO2 current efficiencies strongly;

4) The kind of catalyst used is important: PtRu(1:1) exhibits very low CO2 formation;

5) Fuel cell behaves like a chemical reactor: residence time dependence.

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