Electrochemical Microcalorimetry

Kai Etzel, Katrin Bickel and Rolf Schuster Physical Chemistry, Karlsruhe Institute of Technology, Germany research interests: -Surfaces in vacuum and electrochemical environment structure, phase transition, ordering processes 10 nm ‚electronic structure‘, scanning tunneling spectroscopy (electrochemical STM, XPS, …)

-Electrochemical microstructuring

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-Thermodynamics and kinetics of electrochemical reactions metal deposition, H-adsorption/evolution

(electrochemical STM, [mK] Temperature -0.3 0 0.1 0.2 microcalorimetry, surface plasmon resonance,…) Time [s]

1 Rolf Schuster Institute of Physical Chemistry, Physical Chemistry of Condensed Matter Electrochemical Microcalorimetry

Kai Etzel, Katrin Bickel and Rolf Schuster Physical 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 Cu-dissolution ⇒ decreasing temperature ⇒ increasing temperature

„electrochemical Peltier heats“ 2- SO4 Cu2+

Cu-plated

2 Rolf Schuster Institute of Physical Chemistry, Physical Chemistry of Condensed Matter What do we learn from electrochemical microcalorimetry?

In „conventional“ calorimetry: qm = Δ R H; p,T = const. In electrochemical calorimetry:

electrical work: wel,m = z ⋅ F ⋅φ = −Δ RG = −(Δ R H −TΔ R S); from the ‚chemical reaction‘ heat transfer from surrounding

⇒ qm = Δ RG − Δ R H = −TΔ R S; Ostwald (1903)

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

⇒ -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,...

3 Rolf Schuster Institute of Physical Chemistry, Physical Chemistry of Condensed Matter 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

reference counter electrode electrode potentiostat/ electrolyte galvanostat potential pulse Au-foil

metalized PVDF-foil

- temperature socket p≈ 100 mbar signal + charge amplifier

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.

4 Rolf Schuster Institute of Physical Chemistry, Physical Chemistry of Condensed Matter Cu dissolution from a Cu-layer (≈ 300 ML) on a 50 µm Au foil

(0.5 M CuSO / 5mM H SO ) 10 ms dissolution at η = 20 mV 4 2 4

-3 current set to zero after 10 ms 20x10

potential [V] 10 E 0 0.8 current 0.4 -6 2

[mA] 2.5·10 C/cm ≅ 0.04 ML Cu I 0.0

0.12 temperature 0.08 0.04 [arb. units] [arb.

T 0.00

Δ -3 0 20 40 60 80 100x10 t [s]

ΔT > 0 ⇒ ΔS < 0 entropy gain due to Cu-dissolution!?

No: entropy loss due to water bonding in the hydration shell s abs ≈−122 JK−−11 mol Cu2+ () aq

5 Rolf Schuster Institute of Physical Chemistry, Physical Chemistry of Condensed Matter Cu deposition/dissolution on Cu-bulk Ag deposition/dissolution on Ag-bulk

20mM Cu(ClO4)2 / 1M HClO4 50 µm Au-foil +Cu 20mM AgClO4 / 1M HClO4 50 µm Au-foil + Ag

-0.592 [V] [V] -0.172 E E -0.600 -0.180 0.0 0.0 [mA] [mA] -2.5 I I -1.0

0 1.0 0.8 -20 0.6 -40 0.4 -3 [arb. units] -60x10 [arb. units] 0.2 T T 0.0 Δ Δ 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.00 0.05 0.10 0.15 0.20 0.25 0.30 t [s] t [s] -0.582 -0.160 [V] [V] E E -0.590 -0.168

1.0 2.5 [mA] [mA] I I 0.0 0.0

-3 0.0 60x10 -0.2 40 -0.4 20 -0.6

[arb. units] -0.8 [arb. units] T 0 T -1.0 Δ 0.00 0.10 0.20 0.30 Δ 0.00 0.05 0.10 0.15 0.20 0.25 0.30 t [s] t [s]

Cu dissolution: ΔRS < 0 Ag dissolution: ΔRS > 0 abs −1 −1 abs −1 −1 s 2+ ≈ −122JK mol s + 51JK mol Cu (aq) Ag (aq) ≈ + dominated by water bonding dominated by production of ions in the hydration shell Cu bulk deposition vs. Cu underpotential deposition (UPD) Cu bulk Cu UPD

100

0 Polycrystalline Au in 10 mM CuSO / 0.1 M H SO -100 4 2 4 /(µA/cm²) j -200

0.0 0.1 0.2 0.3 0.4 0.5 E /V 0.0 0.4 /V /V 0.3 E E -0.2 0.2 0 0

-25 -25 /(mA/cm²) /(mA/cm²) j 80 j 60 0 40 /a. u. 20 /a. u. -50 T T Δ

0 Δ -100 0 20 40 60 80 100 t /ms Same net reaction Cu2+ + 2e- → Cu !?

7 Rolf Schuster Institute of Physical Chemistry, Physical Chemistry of Condensed Matter Microscopic processes ML of Cu(111) -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

0 heat, due to Cu UPD formation ) 2 -5 Cu2+ + 2e- → Cu

-10 heat, due to anion coadsorption: heat (µJ/cm -15 2+ 2- 0.3Cu + 0.3SO4 → 2+ 2- 0.3Cu ad + 0.3 SO4 ad -20

reversible (corrected for overpotential) Compatible with: S (Cu2+) ≈ -128 J/Kmol -25 abs 2- Sabs(SO4 ) ≈ 1 J/Kmol -400 -300 -200 -100 0 conversion /(µC/cm²) Cu depositon on Cu bulk

ΔRS helps in identifying reaction pathways and side reactions!

8 Rolf Schuster Institute of Physical Chemistry, Physical Chemistry of Condensed Matter First test experiments on charging/discharging LiCoO2

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

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

15 -2 10 5 0 1 2 3 LiCoO2 in -5 dimethyl-carbonate

-10 /ethylene-carbonate, LiPF6

current density / mA cm -15 scan rate: 5 mV/s

0.0 0.5 1.0 1.5 2.0 Potential vs. Pt / V

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

9 Rolf Schuster Institute of Physical Chemistry, Physical Chemistry of Condensed Matter Charging and discharging of slightly charged LiCoO2

0.30

[V] 0.34 [V]

E 0.28

0.32 E 0.26 0.15 0.00 -0.10 [mA] [mA] I 0.00 I -0.20 30 5 20 0 10 -5

[arb. units] [arb. -10 0 units] [arb. T T -15

Δ -3

Δ -3 0 20 40 60 80 100x10 0 20 40 60 80 100x10 t [s] t [s]

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

10 Rolf Schuster Institute of Physical Chemistry, Physical Chemistry of Condensed Matter 40

30 COLD Φ = 0.3V, slightly charged 20 Φ = 0.5V, moderately charged WARM 10 Φ = 1V, strongly charged 0

heat / conversion [kJ/eq] -10 WARM COLD

-0.2 -0.1 0.0 0.1 0.2 0.3 pulse amplitude [V]

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

11 Rolf Schuster Institute of Physical Chemistry, Physical Chemistry of Condensed Matter Future work on Li-ion batteries

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

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

- ΔRS for different states of charge of the electrode

- effect of charging and discharging cycles on ΔRS

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

12 Rolf Schuster Institute of Physical Chemistry, Physical Chemistry of Condensed Matter