Electro-catalysis

J. Rossmeisl

CAMD, DTU, Denmark

jr1

Slide 1

jr1 two talks about electro catalysisfirst issues, aproximations and fundementals Jan Rossmeisl; 26.04.2009

Andreas Züttel, Switzerland, 4/27/20092

Sustainable hydrogen

O2

O2

H2

H2

H2O

Dissociation

of water

Transport

Storage

Combustion

H2 O2

Sun

Photovoltaics

Wind

Hydropower ENERGY

ENERGY

A. Züttel, U. Friboug

e- e-

e- e-

HO

O2 is reducedH2 is oxidized

2H2 + O2 → 2H2OChemical energy → Heat

Electrochemistry

2H2 + O2 → 2H2O

Anode CathodeH2 → 2H++2e- 4H++4e-+O2 →2H2O

Uθ=0 Uθ=1.23V

Separate oxidation and reduction

The PEM Fuel cell

Cathode:

½O2+2H++2e- ���� H2O

½O2+H2 ���� H2O+electricity

Anode:

H2 ���� 2H++2e-

Electrochemical cell

e- e-

O2 + 2H+ + 2e-↔↔↔↔ H2OH2↔↔↔↔ 2H+ + 2e-

Electrolyte +

+

+

+

+

-

-

---

Electrochemistry

2H2 + O2 → 2H2OChemical energy → Heat

H2 → 2H++2e- 4H++4e-+O2 →2H2OChemical energy → Electrical energy

AnodeCathode

+

-

-

Electrolyte+

+

+

+-

-

-

φφφφcathode

φφφφelectrolyte

φφφφanode

Real electrochemical cell:

-

+

Challenges

• Chemical potential of H+ and e-

• Liquid solid interface

• Field effects

• Phase diagrams

• Charge transfer reactions

Ogasawara, Brena, Nordlund, Nyberg, Pelmenschikov, Petterson and Nilsson.PRL, 89, 2002, 276102

Henderson, Surf. Science Rep 46 (2002)

H2O-OH phases on Pt(111) (K. Bedürftiget al., J. Chem. Phys. 111 (1999), 11147).

STM image of layer on Pt(111), formed by adsorption of 2 L H2O on a (2x2)-O layer at 123 K and short annealing at 170 K. Tunneling parameters were -0.55 V, 0.17 nA, 220 x 220 Å2. Inset: Detail of a boundary between translational domains of the (3x3)/(r3xr3)R30°structure; -0.01 V, 27 nA, 22 x 25 Å2.

Details of areas with (a) the Pt(111) (r3xr3)R30° and (b) the Pt(111)-(3x3) structure from the measurement shown in left figure. -0.5 V, 0.17 nA, 34x25 Å2.

(a) (b)

Similar to Clay, Haq and Hodgon.PRL, 92, 2004, 46102

OH+H2O

∆∆∆∆Gw = -.33eV

In electrochemistry?

Water molecules are highly oriented in a ice like structure on Pt electrode. From a H-O stretching peak of 3200 cm-1 compared to 3400 cm-1 in liquid water.

Noguchi,Okada, Uosaki Faraday 140 (2008)

Including the water by-layer

Water by-layer

Ogasawara, Brena, Nordlund, Nyberg, Pelmenschikov, Petterson and Nilsson.PRL, 89, 2002, 276102

Similar to Clay, Haq and Hodgon.PRL, 92, 2004, 46102

OH+H2O

∆∆∆∆Gw = -.60eVOOH+H2O

∆∆∆∆Gw = -.22 eV

O+H2O

∆∆∆∆Gw = .0 eV

Challenges

• Chemical potential of H+ and e-

• Liquid solid interface

• Field effects

• Phase diagrams

• Charge transfer reactions

Field effect on binding energies

G.S. Karlberg, J. Rossmeisl, J.K. Nørskov, PCCP, 9 (2007) 5158-5161

Field effects

Rossmeisl, Nørskov, Taylor, Janik, Neurock. J. Phys. Chem. B 110, (2006), 21833-21839

Effect of water and field!!

G.S. Karlberg, T. Jaramillo, E.Skulason, J. Rossmeisl, T. Bligaard, J.K. Nørskov. PRL, 99, (2007), 126101

Challenges

• Chemical potential of H+ and e-

• Liquid solid interface

• Field effects

• Phase diagrams

• Charge transfer reactions

Theoretical Standard Hydrogen Electrode

Relating gas phase with electrochemistry

H2O� OH*+½H21. Get ∆∆∆∆E with DFT

2. Water surroundings: ∆∆∆∆Ew

3. Effects of local fields :∆∆∆∆Ew(U)

4. Zero point energy and entropy:

∆∆∆∆G0 =∆∆∆∆Ew+∆∆∆∆Ezpe-T∆∆∆∆S0

H2O � OH*+H++e-

1/2H2 ↔H++e-

1. SHE Convention:

∆∆∆∆G(U=0,cH+=1M ) = 0

2. Potential and pH:

∆∆∆∆G(U,cH+) = -eU-kTln(cH+)

Challenges

• Chemical potential of H+ and e-

• Liquid solid interface

• Field effects

• Phase diagrams

• Charge transfer reactions

U vs. SHE

H++e-+*↔ H*

How does the surface depends on potential?

µµµµ(H++e-) = -eU-kTln(aH+)

Hydrogen and potential

U=0

U=0.5V

H2(g)

H*

H+(aq)+e-

U=-0.5V

Water and potential

U=0

U=0.5V

H2O(l)

HO*+H++e-

O*+2(H++e-)

U=-0.5V

Phase-diagrams

Phase diagram 1

eU+Field effectOnly eU

Phase-diagram 2

Rossmeisl, Nørskov, Taylor, Janik, Neurock. J. Phys. Chem. B 101, (2006), 21833-21839

Only eU term

Charged surface

U vs. SHE

H++e-+*↔ H*

How does the surface depends on potential?

µµµµ(H++e-) = -eU-kTln(aH+)

Pourbaix Diagram• Bulk phase diagram with

potential and pH variables.

• In electro-catalysis the solid-liquid interface is of particular interest.

• The state of the surfacedepends on pH and potential.

• Limited experimental data on surface phase diagrams.

• Bulk electrochemical oxidation and reduction may be kinetically hindered.

M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, 1966

pH

U

Surface Pourbaix diagram

Assuming ideal gas activity. (Clearly wrong at extreme pressures).

Essentially obtaining the U-pO2 relation fromH2O (l) ↔ ½ O2 (g) + 2 e- + 2 H+

H.A. Hansen, J. Rossmeisl, J.K. Nørskov. PCCP. (2008) DOI: 10.1039/b803956a

U

pH

Ag

Cyclic voltammetry

t

i

Reversible but saturating surface reaction

i

t

Irreversible and saturating surface reaction

U

t

For instance, sweep rate 50 mV/s

Constant capacitancei = C dU/dt

i

t

CV for Pt(111) in different electrolytes [1]

OH related

[1] Markovic, N. M., Gasteiger, H. A., and Ross, P. N., J. Electrochem. Soc., 144, 1591–1597 (1997)

H2SO4related

H2 evolution

Cyclic voltammetry

Capacitance

Theoretical results

∆E versus coverage

Theoretical results

Low coverage free energy of adsorption

H-H

interaction

Markovic et al J. Phys. Chem. 101, 5405 (1997)

G.S. Karlberg, T. Jaramillo, E.Skulason, J. Rossmeisl, T. Bligaard, J.K. Nørskov. PRL, 99, (2007), 126101

Challenges

• Chemical potential of H+ and e-

• Liquid solid interface

• Field effects

• Phase diagrams

• Charge transfer reactions

Hydrogen Evolution Reaction

H+H2

VolmerH+ + e- -> Had

Tafel2Had -> H2

e-

V

H2H+

HeyrovskyHad + H

+ + e- -> H2

Pte-

HadHad

Overall reaction: 2(H+ + e-) -> H2

or

AnodeCathode

+

-

-

Electrolyte+

+

+

+-

-

-

φφφφcathode

φφφφelectrolyte

φφφφanode

Real electrochemical cell:

-

+

Adding hydrogen to the water

WF gives a relative U scale

Challenges

• Absolute scale of the potential – linking the vacuum level with the standard hydrogen electrode

• Keeping the potential constant during charge transfer reaction

U=0 vs. SHE

WF

Potential

?

Vacuum

Challenge 1: Vacuum vs. SHE

1/2H2(g)↔H+(aq)+e-

SHE Convention:

∆∆∆∆G(U=0) = 0

4.4 to 4.8eV

Hydrogen Evolution Reaction

H+H2

VolmerH+ + e- -> Had

Tafel2Had -> H2

e-

V

H2H+

HeyrovskyHad + H

+ + e- -> H2

Pte-

HadHad

Overall reaction: 2(H+ + e-) -> H2

or

Challenge 2: Constant potential

The potential is not the same in initial and final state

n=1

N=3

Θ=n/N=1/3

n=0

N=3

Θ=n/N=0

+ + +

- - -

n=3

N=9

Θ=n/N=3/9=1/3

n=2

N=9

Θ=n/N=2/9

- - -

+ + +

++

--

Challenges

• Absolute scale of the potential – linking the vacuum level with the standard hydrogen electrode

• Keeping the potential constant during charge transfer reaction

n=3

N=9

Θ=n/N=3/9=1/3

n=0

N=9

Θ=n/N=0

---

+ + +

The energy stored

Gint=(G(N,n)-G(N,0)-nµµµµH2/2)/N

=½C (U-Upzc)2

=½e2θθθθ2/C

(U-Upzc)=eθθθθ/C

µµµµH2/2*n/N ∝∝∝∝ θ ∝∝∝∝ U

Liniar in U :means

defines the minimum

C=26µµµµF/cm2

C=20µµµµF/cm2

T. Pajkossy, D.M. Kolb Electrochimica Acta 46 (2001) 3063

UPZC

WF

Vacuum

Vacuum vs. SHE

U=0 vs. SHE

Potential

Challenges

• Absolute scale of the potential – linking the vacuum level with the standard hydrogen electrode

• Keeping the potential constant during charge transfer reaction

n=1

N=9

Θ=n/N=1/9

n=0

N=9

∆∆∆∆Θ=1/9=1/N ∝∝∝∝ ∆∆∆∆U

+

-

Reacting one proton

Reacting one Proton∆∆∆∆Gcapacitor = N( Gint(N,n-1)- Gint(N,n))

=½e2N/C((θθθθ-1/N)2 - θθθθ2)

=-eU – ½e∆∆∆∆U

Potential energy diagram

Summary

• Electrochemical DFT is starting

• Linking electrochemistry and surfacescience

• Could play a part in electrocatalyst design-tomorrow!