Electrocatalysis in Alkaline Electrolytes - Research … HClO4 0.1M NaOH E 1/2 = 0.775V E 1/2 = 0.810V Specific Adsorption of Hydroxide Anion in 0.1M NaOH Water activation in 0.1M

Sanjeev Mukerjee

Nagappan Ramaswamy, Qinggang He, Daniel Abbott,

and Michael Bates

Department of Chemistry and Chemical Biology

Northeastern University, Boston, MA 02115

Electrocatalysis in Alkaline Electrolytes - Research Overview

AMFC Workshop Seminar – May 8, 2011

Acidic pH Alkaline pH

1e- + 1H+C

HH3C

Pt

OHOH2

CH3CH2OH

Pt

1e- + 1H+

Pt

C

OH3C

Pt

COH3C

H

OH2

Pt

O

H

Pt

O

C

OH3C

+ H

H

C

OH3C

O

Pt

1e- + 1H+

-H2 at low

coord Pt

Pt

CHx

Pt

C

O

CO2

Pt

2OH

111

sites

O

HxC

Pt

C

Acetyl

AA

O

HxC

Pt

C

di-

OVERVIEW

General Overview – Advantages of High pH

ORR Mechanistic Understanding

Question of Kinetic Facility in Alkaline Medium?

Inner- vs. Outer-sphere Electron Transfer

Non-noble Metal Macrocycle Electrocatalyst

Origin of ORR Activity

Redox Potential Tuning

Influence of Graphitic Defects

Smart Catalysts for Anodic Oxidation

Homogeneous Mediation using

Metal-organic Complexes

Alkaline Membrane Fuel Cell Studies

Interfacial Challenges and Future Prospects

Acidic Medium

Alkaline Medium

Direct: O2 + 4H+ + 4e- 2H

2O E

o = 1.230V

Series: O2 + 2H+ + 2e- H

2O

2 E

o = 0.695V

H2O

2 + 2H+ + 2e- 2H

2O E

o = 1.763V

Direct: O2 + 2H

2O + 4e- 4OH

- E

o = 0.401V

Series: O2 + H

2O + 2e- HO

2

- + OH

- E

o = -0.065V

HO2

- + H

2O + 2e- 3OH

- E

o = 0.867V

O2,aq

O2,ads

H2O

2,adsH

2O

H2O

2

k1 (4e-)

k2(2e-) k

3(2e-)

k4 k

5

O2,aq

O2,ads

HO-

2,adsOH

-

k1 (4e-)

k2(2e-) k

3(2e-)

k4 k

5

HO-

2

In alkaline medium: pH > 12 ˜ HO2

-

H+/H

2

O2/H

2O

O2/OH

-

0.000V

1.230V

0.401V

H2O/H

2-0.828V

SHE

ScaleO/R

O2/O

2

--0.301V

η=1.53V

η =0.70V

ORR - Acid vs. Alkaline Medium: Thermodynamic Advantages

1) Nernstian Potential Shift

2) Affects Adsorption Strengths of spectators,

and intermediate species

Tafel Plots

log ik [A/cm2geo]

1e-4 1e-3 1e-2P

ote

nti

al

[V V

s R

HE

]

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.1M HClO4

0.1M NaOH 25 mV

O2 Satd.

20 mV/s900 rpmRoom Temp.

How Facile is ORR in Alkaline Medium?

(c) Polarization Curves

Potential [V Vs RHE]

0.0 0.2 0.4 0.6 0.8 1.0

Cu

rren

t D

ensi

ty [

A/c

m2

geo

]

-5e-3

-4e-3

-3e-3

-2e-3

-1e-3

00.1M HClO4

0.1M NaOH

(e) Ring Current

Potential [V Vs RHE]

0.0 0.2 0.4 0.6 0.8 1.0

Rin

g C

urr

ent

[A]

0

1e-5

2e-5

3e-5

4e-5

0.1M HClO4

0.1M NaOH

900 rpm

20 mV/s

Potential [V Vs RHE]

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Cu

rren

t D

ensi

ty [

A/c

m2

geo]

-4e-4

-2e-4

0

2e-4

4e-4

0.1M HClO4

0.1M NaOH

E1/2 = 0.775V

E1/2 = 0.810V

Specific Adsorption of Hydroxide Anion

in 0.1M NaOH

Water activation

in 0.1M HClO4

30% Pt/C - 20 mV/s

Outer sphere electron transfer process

responsible for peroxide anion formation

in alkaline medium

ORR on Pt better in non-adsorbing acidic

electrolyte like 0.1M HClO4

Most claims of increased kinetic facility in alkaline

fuel cell are based on comparisons with phosphoric

acid fuel cell

Polariztion Curves

Disk Potential [V Vs RHE]

0.0 0.2 0.4 0.6 0.8 1.0

Cu

rren

t D

ensi

ty [

A/c

m2g

eo]

-5e-3

-4e-3

-3e-3

-2e-3

-1e-3

0

1e-3

0.1 M NaOH

1.0 M NaOH

ERing

= 1.1 V

Disk Potential [V Vs RHE]

0.0 0.2 0.4 0.6 0.8 1.0

Rin

g C

urr

ent

[A]

0.0

5.0e-6

1.0e-5

1.5e-5

2.0e-5

2.5e-5

3.0e-5

3.5e-5

0.1 M NaOH

1.0 M NaOH

HO2ˉ related to specifically adsorbed oxides on Pt

Pt-O(H) + O2 + H2O + 2e‾ ------> Pt-O(H) + HO2‾ + OHˉ

Effect of Specific Adsorption of Hydroxide Anions in Alkaline Medium

Outer sphere electron transfer process

responsible for peroxide anion formation

in alkaline medium

Gold Ring Electrode:

Au + HO2ˉ + OHˉ → Au + O2 + H2O + 2eˉ

Inner-sphere and Outer-sphere Electron Transfer Mechanisms

Desired 4eˉ transfer on oxide free Pt site Acidic pH

Pt + O2 + e-

+ H2O + OH-

+ e-

H2O

+ 2OH-

+ e-Pt + OH-

+ e-

Pt

O

O

Pt

O

O

Pt

O

O

H

Pt

O

O

H

Pt

O

O

H

Pt

O

O

H

Pt

O

H

Pt

O

H

Alkaline pH

Outer-Sphere Electron Transfer

[O2.(H2O)n]aq + eˉ → [O2•ˉ.(H2O)n]aq

[O2•ˉ.(H2O)n]aq → (O2

•ˉ)ads + nH2O

(O2•ˉ)ads + H2O → (HO2

•)ads + OHˉ

(HO2•)ads + eˉ → (HO2ˉ)ads

(HO2ˉ)ads → (HO2ˉ)aq

Specific adsorption of hydroxide anions

promote outer-sphere electron transfer

in alkaline medium

Inner-sphere and outer-sphere electron transfer compete in alkaline medium

Literature Survey on non-PGM Cathode Catalysts

Metal Porphyrin

Mukerjee et al

Cofacial Porphyrin

Anson et al

Pentlandite Co9Se8

Anderson et al

Metal Phthalocyanine

D. Chu et al - ARL

The starting complex is immaterial. Its important to know how the self-assembly

and distribution of the active site takes place!!!

Bottom-up approach needed in catalyst synthesis rather than a random mixing of

precursor materials and heat treatment.

Fe/N/C

Dodelet et al M-Polymer Composite

Zelenay et al M-Organic Framwork

D. Liu - ANL

Open Framework

Atanassov et al

ORR Activity of FeTPP/BPC Pyrolyzed at 800˚C

Pyrolyzed FeTPP exhibits

remarkably low H2O2 yield

due to its exotic structure as

explained in the next slides FeTPP on Black

Pearl Carbon

pyrolyzed at 800°C

ED [V Vs RHE]

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

i [A

/cm

2

geo

]

-5e-3

-4e-3

-3e-3

-2e-3

-1e-3

0

FeTPP

Pt

[c] Tafel Plots

log ik [A/cm2

geo]

10-5 10-4 10-3 10-2

E V

s R

HE

0.7

0.8

0.9

1.0

[a] Polarization Curves

[b] Ring Current

ED [V Vs RHE]

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

Rin

g C

urr

ent

[A]

2e-6

4e-6

6e-6

8e-6

1e-5

FeTPP

Pt

Square Wave Voltammetry - Evolution of Fe2+/Fe3+ Redox Potential

Potential [V Vs RHE]

1.0 1.2 1.4 1.6 1.8

Cu

rren

t [A

]

-1e-4

0

1e-4

2e-4

3e-4

4e-4

5e-4

Potential [V Vs RHE]

0.0 0.2 0.4 0.6 0.8 1.0

Cu

rren

t [A

]

-1e-4

0

1e-4

2e-4

3e-4

4e-4

5e-4

0300 C

0500 C

0600 C

0800 C

0900 C

300o

C 300o

C

500o

C

500o

C600o

C

800o

C

900o

C

600o

C800

oC

900o

C

Square Wave Voltammetry in 0.1M NaOH at 10Hz

Anodic shift in the Fe2+/Fe3+ redox potential

upon pyrolysis is observed to 1.25V

Similar mechanism observed in acidic medium

also

Pyrolysis Temperature [oC]

0 200 400 600 800 1000

EP

eak [

V]

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Two Fe2+/Fe3+

redox couples

Scan 5

Potential [V Vs RHE]

0.3 0.6 0.9 1.2 1.5 1.8

Cu

rren

t D

ensi

ty [

A/c

m2

geo

]

0

1e-3

2e-3

3e-3

4e-3Cyclic Voltammetry

E Vs RHE

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

0.1

mA

/cm

2

geo

0.314V Vs RHE

Fe(II)/Fe(III)

Ligand Oxidation

1.508V Vs RHE

Square Wave Voltammetry

FeTPPCl/BPCNon-pyrolyzed

Sqaure wave voltammetry removes all capacitive current contribution

Synchrotron Principles

Principles of X-ray Absorption Spectroscopy

XANES (< 50 eV)

•Absorber site symmetry

(e.g. Td, Oh, etc)

• Electronic configuration

• Geometric Binding Site

EXAFS (> 50 eV)

• Geometric information

• Bond length

• Coordination number

•BULK short range order

Synchrotron

Monochromator

Io

Spectro-electrochemicalCell

Fluorescence detector

ItIr

Referencefoil

300oC

R [Å]

0 1 2 3 4 5 6

| (R

)| [

Å-3

]

0.1V

Fit

800oC

R [Å]

0 1 2 3 4 5

| (R

)| [

Å-3

]

0.1V

Fit

Fe-N4

Fe-O

Fe-N4

Fe-O

Metallic Fe

Insitu Extended X-ray Absorption – Fe-Nx Short Range Structure

Fe-N

Coordination

Number

Fe-N

Bond

Length [Å]

300°C 4.00 1.996

800°C 4.00 1.976

In situ 0.1M NaOH electrolyte @ 0.1 V vs. RHE

Delta Mu – Identification of Active Site and Oxygen Adsorption Mode

Increasing defect sites is key to increasing active site density

Defective sites are regions of higher chemical potential and hence anodic shift in redox potential

ORR on FeTPP/C: Acid vs. Alkaline

(c) Peroxide Reduction

Potential [V Vs RHE]

0.2 0.4 0.6 0.8 1.0

i [A

/cm

2

geo

]

-2e-3

-1e-3

0

1e-3

2e-3

ORR

HRR

(b) Ring Current

Potential [V Vs RHE]

0.0 0.2 0.4 0.6 0.8 1.0

I R [

A]

0

1e-6

2e-6

3e-6

4e-6

5e-6

0.1M NaOH

0.1M HClO4

(a) ORR Polarization Curves

Potential [V Vs RHE]

0.0 0.2 0.4 0.6 0.8 1.0

i D [

A/c

m2

geo

]

-5e-3

-4e-3

-3e-3

-2e-3

-1e-3

0

0.1M NaOH

0.1M HClO4

0.1M HClO4

0.1M NaOH

Peroxide Intermediate is more stable on

FeTPP in alkaline medium

O2

H2O

e-

+ OH -

H2O

e-

e-e-

-OH -

FeII

Active Site

O

H

FeIII

Ferric-

Hydroperoxyl

O

OH

FeIII

Ferric

Hydroxyl

O

H

+ 2OH -

FeIII

Ferric

Superoxo

O

O

FeII

Adsorbed O2

O

O

FeII

Ferrous-

Hydroperoxyl

O

OH

Oxygen Reduction Mechanism on Fe-N4 Sites

In Alkaline Medium

Electrostatic Stabilization of peroxide intermediate

in alkaline medium is key to efficient 4eˉ Reduction Fe

II

Ferrous-

Hydroperoxyl

HO

OH

In Acidic Medium

Alkaline hydrogen oxidation – AEMFC Anodes

HOR/HER i0 much lower in alkaline vs. acid media

Need high loading non-PGM catalyst on AEMFC anode

Pmax of 50mW/cm2 obtained with: Cr-decorated Ni nanoparticles on anode & Ag nanoparticles on cathode

Gasteiger et al. JECS 2010 Lu et al. PNAS 2008

Literature Survey on Methanol/Ethanol Oxidation Anode Catalysts

EtOH binds to surface through Cα

Acetaldehyde/acetic acid pathways are

preferred

Selectivity is needed at electrode surface to

promote CO2 pathway

Adzic, R. R., Electrochim. Acta 2010, 55, 4331-4338

Markovic. N.M., Electrochim. Acta 2002, 47, 3707

1e- + 1H+C

HH3C

Pt

OHOH2

CH3CH2OH

Pt

1e- + 1H+

Pt

C

OH3C

Pt

COH3C

H

OH2

Pt

O

H

Pt

O

C

OH3C

+ H

H

C

OH3C

O

Pt

1e- + 1H+

-H2 at low

coord Pt

Pt

CHx

Pt

C

O

CO2

Pt

2OH

111

sites

O

HxC

Pt

C

Acetyl

AA

O

HxC

Pt

C

di-

Methanol Oxidation: Effect of pH, and Carbonates

Effect

of pH

Effect of

Carbonates

H+ OHˉ CO32-

Mobility

(cm2 V-1 s-1)

36.25 20.5 3.5

Infinite Diffusion

Coefficient (10-5

cm2/s)

9.3 5.3 0.9

Increasing pH or NaOH concentration increases MOR activity

Effect of carbonate is two-fold

Decreasing conductivity and increasing viscosity

Decreasing MOR activity by competitively

adsorbing with hydroxide anions

K. Scott et al, 2004, Electrochem. Acta, 49, 2443

K. Scott et al, 2003, J. Electroanal. Chem, 547, 17

Enhanced Electrocatalytic Processes: Polyvalent smart catalyst (TM) effect on C-C cleavage

Labeled Ethanol: C13 label study

0 2000 4000 6000

0.0

0.1

0.2

0.3

0.4

0.5

0 2000 4000 6000

time [s]

with co-catalyst

without cocatalyst

CO

2 f

ract

ion

0.67 V 0.77 V

time [s]

Clean Pt Pt-Ox

Grey = bulk

∆μ= μ(Pt-Ox) – μ(Pt-clean)

Atomic Level Picture of Electrocatalytic Pathways via

Determination of Specific Adsorption Sites

Proof of Pi-bonded ethylene pathway using synchrotron based in situ XANES on

Pt for EtOH oxidation in KOH solution with and without co-catalyst in solution

Presence of co-catalyst favors formation of pi-bonded Ethylene

type intermediate and hence enhances C-C Bond Cleavage

Experimental Signatures • Short dash (0.1 V) • Long dash (0.3 V) • Solid line (0.5V)

Comparison of theoretical and experimental signatures for oxidation of ethanol with and without co-catalyst for identifying species on the surface as a function of potential

pi-bonded ethylene is the mechanistically favored route to CO2

Acetyl species is mechanistically favored toward acetic acid route

Literature Survey on Alkaline Membrane Fuel Cell Performance

Pt-catalysts, H2/Atmospheric Air

Acta, 2010

Use of alcohol feed in the absence of KOH on the

anode leads to power densities of only ~10 mW/cm2.

Challenge lies at anode interface (vide infra)

Specific Adsorption of Quaternary Ammonium

Cations is the killer

i [A/cm2

geo]

0.0 0.2 0.4 0.6 0.8 1.0 1.2

EC

ell [

V]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

PD

[m

W/c

m2

geo

]

0

50

100

150

200

250

Pt/C

FeTPP/C

H2/O

2 - 50oC

Thicker Electrodes with

Non-PGM Cathodes

Better Ionomer Solutions

Needed

Anode: PtRu on Toray Paper (4mgPtRu/cm2 + 1mgAS4/cm2)

Cathode: BASF 30% Pt/C on GDL (1mgPt/cm2, 28.5% AS4 + 1mgAS4/cm2)

Membrane: Tokuyama A201

Steep cell voltage loss right in the activation overpotential region

Direct Ethanol AMFC – PtRu Anode

Polarization Curves

50oC - 100% RH

Current Density [mA/cm2

geo]

0 20 40 60 80 100

EC

ell [

V]

0.0

0.2

0.4

0.6

0.8

1.0

0.25M KOH + 1M EtOH

1M EtOH

PtRu Anode/A201/Pt CathodePossible Reasons

1) Methanol Crossover

2) Poor Ionic Conductivity

3) Carbonate Formation

4) Specific Adsorption of NR4+

1) Loss in Surface Area

2) Electrostatic Effects

Ethanol Oxidation on Pt Anode at 50oC

Potential [V Vs RHE]

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Curr

ent

Den

sity

[m

A/c

m2

geo

]

-20

0

20

40

60

80

100

0.25M KOH + 1M EtOH

1M EtOH

50oC

20 mV/s

Cathode: H2

Significant overpotential for EtOH oxidation in the absence of KOH

Understanding the origin of this overpotential is the key towards improving performance

Cell Temp: 50˚C, H2 Feed on Cathode used as reference electrode, Anode Feed: 8ml/min

Ethanol Oxidation on Pt and PtRu Anodes

Ionic

Resistance

[Ohm]

Charge

Transfer

Resistance

[Ohm]

0.25M KOH +

1M EtOH/O2

0.109 1.31

1M EtOH/O2 0.591 16.85

Z' [Ohm]

0 2 4 6 8 10 12 14 16 18 20

Z''

[O

hm

]

0

1

2

3

4

5

6

7

0.25M KOH + 1M EtOH

1M EtOH

Presence of KOH

Decreases the charge transfer resistance

Increases the ionic conductivity

Concentration of Contaminant, mM

0 20 40 60 80 100 120

% L

oss

in

Cu

rren

t D

ensi

ty

-100

-80

-60

-40

-20

0

(CH3)4N+OH-

(CH3CH2)4N+OH-

(CH3CH2CH2)4N+OH-

(C6H5CH2)(CH3)3N+OH-

30% Pt/C

0.1 M KOH + 0.5 M MeOH

0.6 V vs. RHE

Percent loss in MeOH oxidation current

in 0.1M KOH electrolyte in the presence

of various ammonium group based

contaminants

Specific Adsorption of Quaternary Ammonium Ions

Indicates strong specific adsorption of NR4+

and its effect on interfacial electron

transfer

1. Loss of surface area (blocking effect)

2. Electrostatic effects (may hinder OH- transport across IHP)

E [V vs. RHE]

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4C

urr

ent

[A]

-6e-3

-4e-3

-2e-3

0

2e-3

4e-3

6e-3

(w/ ionomer at anode) - No KOH

(w/ ionomer at anode) - w/ 0.1M KOH

(No ionomer at anode) - No KOH

(No ionomer at anode) - w/ 0.1M KOH

0.750 V

0.985 V

0.633 V

approx. -235 mV shift in

presence of ionomer

Experiments with charged redox couples

(φ2)

φ2(E1)

φ2(E2)

Specific Adsorption of Quaternary Ammonium Ions

Current Hypothesis

Potential drop across the

electrochemical double layer

Unfavorable Electrostatics

in the Inner-Helhmholtz Plane

Future work needed in this

direction

ORR - General Mechanistic Considerations

Inner- and Outer-sphere electron transfer steps compete in alkaline medium

Higher stability of peroxide intermediate is the primary kinetic advantage in

alkaline media

ORR - Pyrolyzed Non-noble Catalysts

Redox potential shift of Fe2+/Fe3+ metal center upon pyrolysis

Defect sites on graphitic plane hosts active sites

Ethanol Oxidation - Smart catalyst (TM) redox couple

Exhibits the highest reported enhancement of ethanol oxidation.

Selectivity to produce CO2

Delta-Mu indicates pi-bonded ethylene intermediate

Future Prospects - Interfacial Challenges

Results with H2/O2 are promising

Understanding specific adsorption of quaternary ammonium ions at the

anode interface is key

Conclusions

Important Considerations for Future Electrocatalysis Research

• Need to unravel Anode Electrode Issues: Electrostatics, Reaction Centers for Hydride formation and Oxidation

• Need for Increase in site density for Non PGM reaction centers

• Role of liquid electrolytes from an interfacial perspective

• Interaction of ionomer with active site: Specific adsorption etc.

• Durability issues need to be determined in conjunction with the operating conditions and the choice of ionomer.

Acknowledgements

Funding Agencies