Carbon supported Pd-Ni and Pd-Ru-Ni nanocatalysts for the alkaline direct ethanol fuel cell (DEFC)

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Carbon supported Pd-Ni and Pd-Ru-Ni

nanocatalysts for the alkaline direct ethanol fuel cell (DEFC)

ASME2011, Washington, DC

Mkhulu K Mathe

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Outline

• Background and Introduction

• Synthesis of Electrocatalysts

• Characterization and Evaluation of the Electrocatalysts

• Performance measurement of the Electrocatalysts

• Concluding remarks

• Acknowledgements

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• DEFC anode studies

- ^synthesis

- Electrooxidation

- Performance

G.F. McLean et al. International Journal of Hydrogen Energy 27 (2002) 507 – 526

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4 Nanostructures Thin Film

catalytic active sites (active reaction area)

surface-to-volume ratio

transport of reactants and products

Synthesis of electrocatalysts

Electrocatalyst particles have to maintain electronic contact with support

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Metal salts/

precursors

5 M

₁⁺

El ectr ode / S ubst rate

Chemical Reduction / Precipitation

Chemical routes to

Nanoparticulate Multimetallic Electrocatalysts

M

_2(s)

Metal salts/

precursors M

₂⁺

M

_1(s)

Electrochemical deposition e

^-

M

₁

M

_2(s)

Transfer

To Electrode M

₁⁺

M

₂⁺

El ectr ode / S ubst rate

Reducing agent

V

e

^-

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6

Cu²⁺

(1) Clean substrate with blank electrolyte (BE);

Inject Cu²⁺solution at E >> E_Cu-Cu2+

S S S S S S S

(2) Potentiostatic electrodeposition at – E_dep> E_Cu-Cu2+(Underpotential

Deposition (UPD)) or E_dep< E_Cu-Cu2+

(small Overpotential Deposition (OPD) - to produce sacrificial Cu adlayer on active sites of the substrate; Rinse with BE Cu²⁺ Cu²⁺

S S S S S S S Cu Cu Cu Cu Cu

-2e

S S S S S S S Pt Cu Pt Cu Cu

(3) Inject H₂PtCl₆ solution and allow surface-limited redox-replacement (SLRR) of Cu by Pt at open circuit (OC)

Pt⁴⁺

S S S S S S S Pt Pt Pt Pt Pt

(4) Pt nanodeposit on substrate;

Rinse with BE and inject Cu²⁺

solution at E >> E_Cu-Cu2+

Pt⁴⁺

Cu²⁺

S S S S S S S Pt Pt Pt Pt Pt Cu Cu Cu Cu Cu

S S S S S S S Pt Pt Pt Pt Pt Ru Cu Ru Ru Cu

Cu²⁺ -2e Cu²⁺ Cu²⁺

(5) Potentiostatic electrodeposition at – E_dep to produce sacrificial Cu adlayer on active sites on Pt adlayers; Rinse with BE

Ru³⁺

Cu²⁺

S S S S S S S Pt Pt Pt Pt Pt Ru Ru Ru Ru Ru

(6) Inject RuCl₃ solution and allow surface-limited redox-replacement

(SLRR) of Cu by Ru at OC S S S S S S S

Pt Pt Pt Pt Pt Ru Ru Ru Ru Ru

Pt Pt Pt Pt Pt Ru Ru Ru Ru Ru Cu²⁺

(1) Clean substrate with blank electrolyte (BE);

Inject Cu²⁺solution at E >> E_Cu-Cu2+

S S S S S S S S S S S S S S

(2) Potentiostatic electrodeposition at – E_dep> E_Cu-Cu2+(Underpotential

Deposition (UPD)) or E_dep< E_Cu-Cu2+

(small Overpotential Deposition (OPD) - to produce sacrificial Cu adlayer on active sites of the substrate; Rinse with BE Cu²⁺ Cu²⁺

S S S S S S S Cu Cu Cu Cu Cu S S S S S S S S S S S S S S

Cu Cu Cu Cu Cu -2e

Pt Cu Pt Cu Cu

(3) Inject H₂PtCl₆ solution and allow surface-limited redox-replacement (SLRR) of Cu by Pt at open circuit (OC)

Pt⁴⁺

S S S S S S S Pt Pt Pt Pt Pt S S S S S S S S S S S S S S

Pt Pt Pt Pt Pt

(4) Pt nanodeposit on substrate;

Rinse with BE and inject Cu²⁺

solution at E >> E_Cu-Cu2+

Pt⁴⁺

Cu²⁺

Pt Pt Pt Pt Pt Cu Cu Cu Cu Cu

Pt Pt Pt Pt Pt Ru Cu Ru Ru Cu

Cu²⁺ -2e Cu²⁺ Cu²⁺

(5) Potentiostatic electrodeposition at – E_dep to produce sacrificial Cu adlayer on active sites on Pt adlayers; Rinse with BE

Ru³⁺

Cu²⁺

(6) Inject RuCl₃ solution and allow surface-limited redox-replacement

(SLRR) of Cu by Ru at OC SS SS SS SS SS SS SS Pt Pt Pt Pt Pt Ru Ru Ru Ru Ru

Sequential deposition coupled to Surface-limited Redox-

replacement reactions (SLRR): Synthesis of multilayered

bimetallic RuPt electrocatalyst

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Multi-stage electrodeposition

Time

Potential

Cu(s) or Pb(s) - UPD

a.Pd⁴⁺ at OC b.Redox-replacement

to form (Pd | Pt)/C a.Rinse

b.Cu²⁺

or Pb²⁺

Cu(s) or Pb(s) - UPD

a. Ruⁿ⁺at OC b.Redox-replacement to form (Ru | Pd | Pt)/C

Pt/C

a.Rinse b.Cu²⁺

or Pb²⁺

(Ru | Pd | Pt)/C

Noble-Metals studied = Pt, Ru, Au, Pd

Substrates = Carbon materials, Gold films

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Synthesis of Electrocatalysts

reducing agent: mixture NaBH

₄

and ethylene glycol

Low ethanol oxidation

performance vs Pd+Ru-Ni/C

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(a) a twinned single Pt2Ru3 nanoparticle of ca. 4.39/0.3 nm characteristic dimensions.

(b) distribution of Pt2Ru3 bimetallic nanoparticles on the high-surface area carbon support.

HRTEM and TEM micrographs

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Electrochemical characterization

-50 -40 -30 -20 -10 0 10 20 30

-1.1 -0.6 -0.1 0.4

Potential (V vs Ag/AgCl)

Current density (mA/cm2 )

RuNi/C PdRu/C PdRuNi/C

Pd oxides reduction region

-4 -3 -2 -1 0 1 2

-1.1 -0.6 -0.1 0.4

Potential (V vs Ag/AgCl) Current density (mA/cm2 )

Pd/C PdNi/C

Pd oxides reduction region

cyclic voltammograms in 0.5 M NaOH

Pd+ Ru-Ni/C

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cyclic voltammograms in ethanol

-30 -20 -10 0 10 20 30 40 50 60

-1.2 -0.7 -0.2 0.3

Potential (V vs Ag/AgCl)

Current density (mA/cm2 ) Pd/C

PdNi/C PdRuNi/C

\

-15 -10 -5 0 5 10 15 20

-1.2 -0.7 -0.2 0.3

Potential (V vs Ag/AgCl)

Current density (mA/cm2 )

RuNi/C PdRu/C

RuNi/C and PdRu/C:

no or low activity towards ethanol oxidation

Electrochemical characterization

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Concentration studies effect on current density

Effect of ethanol concentration

PdNi/C PdRuNi/C

-40 -20 0 20 40 60 80 100 120

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2

0.25M 0.5M 1.0M 2.0M 3.0M 4.0M C_{EtOH =}

-1.0 -0.8 -0.6

0.25M 0.5M 1.0M 2.0M 3.0M 4.0M CEtOH =

-40 -20 0 20 40 60 80 100 120

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2

0.25M 0.5M 1.0M 2.0M 3.0M 4.0M CEtOH =

-1.0 -0.8 -0.6

0.25M 0.5M 1.0M 2.0M 3.0M 4.0M CEtOH =

Raising ethanol concentration up to 3 M increased the coverage of the adsorbed ethoxy (CH

₃

COads) species on the nanocatalyst

surface, thus yielding an increase in current density

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Electro-catalyst performance: passive alkaline DEFC

Electro- catalyst

Open circuit voltage

(V)

Power/total loading (mW/mgPd)

PdCeO2/C (ACTA-SpA)

0.795 3.1736

PdNi/C 0.653 3.1916 PdRuNi/C 0.768 2.5798 PdRuSn/C 0.623 0.2386

Cathode: 0.1mg/cm2 FeCo (ACTA-SpA)

cell polarisation and current density curves loading vs. ocv

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0 5 10 15 20 25

Current density (mA/cm²)

Potential (V)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Power density (mW/cm2 ) PdRuNi/C

ACTA-Spa.

anode PdNi/C

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XRD micrographs of electrocatalysts

20 40 60 80 100

2 Theta (degree)

Int en si ty (a. u. )

C (002)

Ru (101)

Pd (200)

Pd (220)

PdRuNi/C (a)

RuNi/C

Pd (111)

20 40 60 80 100

2 Theta (degree)

Intensity (a.u.)

RuNi/C C (002)

Ru (101)

Ru/C

(insert XRD patterns of Ru/C and Ru-Ni/C)

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Conclusions

• Pd-Ni/C and Pd-Ru-Ni/C were prepared by chemical reduction method

• nanocatalysts (A&B) higher activities towards ethanol electro- oxidation

• effect of ethanol concentration variation – current density

• binary Pd-Ni/C performs better than ternary nanocatalyst -

current density

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16 Jacaranda trees, Main Campus, University of Pretoria

ACKNOWLEDGEMENTS

• Energy & Processes Team

• CSIR Executive

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Thank You

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References

- G.F. McLean et al. International Journal of Hydrogen Energy 27 (2002) 507 – 526

- A.V. Tripkovic´ et al. Electrochimica Acta 47 (2002) 3707/3714

- T.S. Mkwizu, et.al. 219

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Carbon supported Pd-Ni and Pd-Ru-Ni nanocatalysts for the alkaline direct ethanol fuel cell (DEFC)