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



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“



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,...

⇒



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.



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



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 !?



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!



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



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)




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


- 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,…)



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

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Electrochemical Microcalorimetry - KIT