Electrochemical Microcalorimetry - KIT

Institute of Physical Chemistry,

Physical Chemistry of Condensed Matter

1 Rolf Schuster

Electrochemical Microcalorimetry

Kai Etzel, Katrin Bickel and Rolf SchusterPhysical Chemistry, Karlsruhe Institute of Technology, Germany

-Thermodynamics and kinetics of electrochemical reactions

10 nm

research interests:

-Surfaces in vacuum and electrochemical environment structure, phase transition, ordering processes‚electronic structure‘, scanning tunneling spectroscopy

metal deposition, H-adsorption/evolution

-Electrochemical microstructuring

(electrochemical STM, XPS, …)

(electrochemical STM, microcalorimetry, surface plasmon resonance,…)

Tem

pera

ture

[mK

]

Time [s]

0

-0.30 0.1 0.2

Institute of Physical Chemistry,

Physical Chemistry of Condensed Matter

2 Rolf Schuster

Electrochemical Microcalorimetry

Kai Etzel, Katrin Bickel and Rolf SchusterPhysical Chemistry, Karlsruhe Institute of Technology, Germany

Historical: E. J. Mills, „On Electrostriction“, Proc. Roy. Soc. Lond. 26, 504 (1877)

E. Bouty, „Sur un phénomène analogue au phénomène de Peltier“,Comptes Rendus 89, 146 (1879)

Cu-deposition⇒ decreasing temperature

Cu-dissolution⇒ increasing temperature

Cu2+

SO42-

Cu-plated

„electrochemical Peltier heats“

Institute of Physical Chemistry,

Physical Chemistry of Condensed Matter

3 Rolf Schuster

What do we learn from electrochemical microcalorimetry?

In „conventional“ calorimetry:

;STHGq RRRm Δ−=Δ−Δ=⇒

;Hq Rm Δ= p,T = const.

In electrochemical calorimetry:electrical work: ( );, STHGFzw RRRmel Δ−Δ−=Δ−=⋅⋅= φ

from the ‚chemical reaction‘heat transfer from surrounding

We measure the reaction entropy, ΔRS, (if we are close to equilibrium).

Ostwald (1903)

-stoechiometry of the reaction, i.e. hints on elementary steps-entropies of hydration, i.e., involvement of solvent water -phase transitions and surface entropies

in addition: irreversible heat due to chemical reactions, i.e., complexation, crystallization,...

⇒

Institute of Physical Chemistry,

Physical Chemistry of Condensed Matter

4 Rolf Schuster

Can we achieve „monolayer sensitivity“?

Use thin electrode/sensor assembly with low heat capacity

Use pulsed electrochemical reactions:-fast enough to avoid heat loss into the electrolyte (and uptake of Joule heat

from the electrolyte)-slow enough to ensure thermal equilibration of the electrode/sensor assembly

potentiostat/galvanostat

+

-

charge amplifier

electrolyte

reference electrode

Au-foil

metalizedPVDF-foil

p 100 mbar≈

potentialpulse

temperaturesignalsocket

counter electrode

C. E. Borroni-Bird, and D. A. King, Rev. Sci. Instr. 62 (1991) 2177.J. T. Stuckless, N. A. Frei, and C. T. Campbell, Rev. Sci. Instr. 69 (1998) 2427.

Institute of Physical Chemistry,

Physical Chemistry of Condensed Matter

5 Rolf Schuster

20x10-3

100

E [V

]

100x10-3806040200t [s]

0.80.40.0I [

mA

]

0.120.080.040.00

ΔT [a

rb. u

nits

]

Cu dissolution from a Cu-layer (≈ 300 ML) on a 50 µm Au foil

(0.5 M CuSO4 / 5mM H2SO4)

potential

current

temperature

10 ms dissolution at η = 20 mV

current set to zero after 10 ms

2.5·10-6 C/cm2 ≅ 0.04 ML Cu

00 <Δ⇒>Δ ST entropy gain due to Cu-dissolution!?

No: entropy loss due to water bonding in the hydration shell

21 1

( )122 JK molabs

Cu aqs +

− −≈ −

Cu deposition/dissolution on Cu-bulk Ag deposition/dissolution on Ag-bulk20mM Cu(ClO4)2 / 1M HClO4 20mM AgClO4 / 1M HClO450 µm Au-foil +Cu 50 µm Au-foil + Ag

11)( molJK1222

−−−≈+abs

aqCus 11)( molJK51 −−+≈+

absaqAgs

-0.180-0.172E

[V]

0.300.250.200.150.100.050.00t [s]

-2.5

0.0

I [m

A]

1.00.80.60.40.20.0ΔT

[arb

. uni

ts]

-0.168-0.160

E [V

]

0.300.250.200.150.100.050.00t [s]

2.5

0.0I [m

A]

-1.0-0.8-0.6-0.4-0.20.0

ΔT [a

rb. u

nits

]

-0.600-0.592

E [V

]

0.300.250.200.150.100.050.00t [s]

-1.0

0.0I [

mA

]

-60x10-3-40-20

0

ΔT [a

rb. u

nits

]

-0.590-0.582

E [V

]

0.300.200.100.00t [s]

1.0

0.0I [m

A]

60x10-3

4020

0

ΔT

[arb

. uni

ts]

Cu dissolution: ΔRS < 0 Ag dissolution: ΔRS > 0

dominated by water bonding in the hydration shell

dominated by production of ions

Institute of Physical Chemistry,

Physical Chemistry of Condensed Matter

7 Rolf Schuster

Polycrystalline Au in 10 mM CuSO4 / 0.1 M H2SO4

-200

-100

0

100

j /(µ

A/c

m²)

0.50.40.30.20.10.0

E /V

Cu UPDCu bulk

Cu bulk deposition vs. Cu underpotential deposition (UPD)

0.40.30.2E

/V

-25

0

j /(m

A/c

m²)

-100

-50

0

ΔT

/a. u

.

-0.2

0.0

E /V

100806040200t /ms

-25

0

j /(m

A/c

m²)

80604020

0ΔT

/a. u

.

Same net reaction Cu2+ + 2e- → Cu !?

Institute of Physical Chemistry,

Physical Chemistry of Condensed Matter

8 Rolf Schuster

0.3Cu2+ + 0.3SO42- →

0.3Cu2+ad + 0.3 SO4

2-ad

Compatible with:Sabs(Cu2+) ≈ -128 J/KmolSabs(SO4

2-) ≈ 1 J/Kmol

Cu depositon on Cu bulk

Cu UPD formation

Microscopic processes

Cu2+ + 2e- → Cu

reve

rsib

lehe

at (µ

J/cm

2 )(c

orre

cted

for o

verp

oten

tial)

-25

-20

-15

-10

-5

0

-400 -300 -200 -100 0

conversion /(µC/cm²)

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0ML of Cu(111)

heat, due to

heat, due to anion coadsorption:

ΔRS helps in identifying reaction pathways and side reactions!

Institute of Physical Chemistry,

Physical Chemistry of Condensed Matter

9 Rolf Schuster

First test experiments on charging/discharging LiCoO2

in cooperation with Heino Sommer and Petr Novák, Paul Scherrer Institut

-15

-10

-5

0

5

10

15

curre

nt d

ensi

ty /

mA

cm

-2

2.01.51.00.50.0

Potential vs. Pt / V

scan rate: 5 mV/s

charging: LiCo(III)O2 → ‚Co(IV)O2‘ + Li+ + e-

discharging: ‚Co(IV)O2‘+ Li+ + e- → LiCo(III)O2

LiCoO2 in dimethyl-carbonate /ethylene-carbonate, LiPF6

1 2 3

Institute of Physical Chemistry,

Physical Chemistry of Condensed Matter

10 Rolf Schuster

0.300.280.26E

[V]

100x10-3806040200t [s]

-0.20-0.100.00

I [m

A]

-15-10-505

ΔT

[arb

. uni

ts]

0.340.32E

[V]

100x10-3806040200t [s]

0.15

0.00I [m

A]

302010

0

ΔT

[arb

. uni

ts]

Charging and discharging of slightly charged LiCoO2

We measure reversible heat effects, i.e., ΔRS

conversion ca. 2·1013 e-/cm2

(c.f., a Au(111) surface has 1.4·1015 atoms/cm2)

in cooperation with Heino Sommer and Petr Novák, Paul Scherrer Institut

Institute of Physical Chemistry,

Physical Chemistry of Condensed Matter

11 Rolf Schuster

40

30

20

10

0

-10heat

/ co

nver

sion

[kJ/

eq]

0.30.20.10.0-0.1-0.2

pulse amplitude [V]

COLD

COLD

WARM

WARM

Φ = 0.3V, slightly charged

Φ = 0.5V, moderately chargedΦ = 1V, strongly charged

in cooperation with Heino Sommer and Petr Novák, Paul Scherrer Institut

- We can measure heat effects and determine the reversible heat, i.e., ΔRS.- Charging, i.e., Li+ formation leads to warming, i.e., ΔRS < 0.- The heat per equivalent dependens on the state of charge of the electrode - ΔRS < 0 !? Explicable by:

stong solvation of Li+ in dimethyl-carbonate /ethylene-carbonate (?)or side reactions (decomposition of LiCoO2, coadsorption prosesses,…)

Institute of Physical Chemistry,

Physical Chemistry of Condensed Matter

12 Rolf Schuster

Future work on Li-ion batteries

- ΔRS for different states of charge of the electrode

- relyable calibration

- ΔRS for Li+ + e- → Li on Li-electrodes, dependence on the electrolyte

- effect of charging and discharging cycles on ΔRS

- ‚ideas‘ on elementary steps of the charging/discharging process

- ΔRS for Li+ + e- → Li upon intercalation of graphite / formation of the SEI