Electrocatalysis: Experimental Techniques and Case Studies

Transcription

1 Electrocatalysis: Experimental Techniques and Case Studies 1) Introduction (what is electrochemistry?) Electric double layer Electrode potential 2) How to measure electrochemical reactions? Cyclic voltammetry Impedance spectroscopy 3) The metal/electrolyte interface Surface reconstruction Ion adsorption, ordered adsorption, SAMs 4) Electrode kinetics Butler-Volmer equation 5) Structure and reactivity Surface defects Surface strain Bimetallic surfaces

2 C = µfcm -2 E = 10 7 Vcm -1 Δq = 0.2 e - / surface atom E metal electrolyte

3 The Thermodynamic Equilibrium S ~ ~ μ = E μ i i S: Solution E: Electrode ~ μ = μ + i i zfϕ Me ~ μ ~ μ Me Me z ~ μ e Z Z + Z ze + z ~ μe = μ = μ Me Me Me = ~ μ μ z ~ μ Me Me e Z + ~ μe ~ μ e = = μ e F Fϕ ( E K) F K 4.5 ev for ~ μ e vs NHE The electrode potential is the electrochemical potential of the electrons in the metal

4 electrode potential metal E E solution pos. neg. E F Ox (Fe 3+ ) Red (Fe 2+ ) E F, redox D(E) D(E) The electronic equilibrium between metal electrode and redox electrolyte

5 7.0 Work Function (ev) Au Ag Pt 0.1 M HClO 4 Au Pt Ag Potential (V vs. SCE) Work functions of Au, Pt and Ag electrodes emersed from 0.1 M HClO 4 at different potentials. Arrows indicate work functions of clean surfaces. From R. Kötz

6 0.8 Current (µa) Cathodic Anodic E NHE / V Electrochemical spectrum of clean Pt electrode in 0.5 M H 2 SO 4 solution. From Conway et al.

7 In equilibrium or in steady state (E dc and I dc are constant in time) Stimulus: E(t)=E dc + E ac sin(ωt) Response: I(t)=I dc + I ac sin(ωt+ φ) Impedance: is a complex quantity, Z Z abs exp(iφ) = Z abs cos φ + i Z abs sin φ with Z abs E ac /I ac is a spectrum (typically 1 mhz<ω<1mhz ); is usually interpreted in terms of equivalent circuits containing R (resistance), C (capacitance) and other (e.g. W: diffusional impedance) elements.

8 Example No.1: charge transfer across a metal/electrolyte interface (an equivalent circuit is expected) 1: measurement of impedance spectrum 2: determination of equivalent circuit parameters 3: calculation of kinetic parameters from the equivalent circuit parameters δi δe 1 R ct = nfa c red,surf k E ox + c ox,surf k E red

9 Scanning electrochemical microscopy (SECM) From J. Heinze

10 Principal of SECM

11 Surface reconstruction of Au(100) in 0.1 M H 2 SO 4 10 nm x 10 nm 8 nm x 8 nm E SCE = 200 mv E SCE = 630 mv

12 Cyclic voltamogramme of Au(100) in 0.1 M H 2 SO 4 40 Au(100) : (hex) (1 x 1) j / µacm v = 50 mv / s E SCE / V

13 63 nm x 63 nm E SCE = V 84 nm x 84 nm +0.5 V 92 nm x 92 nm E SCE = V E SCE / V Potential-induced induced surface reconstruction of Au(100)

14 Ag(111) in 0.05 M H 2 SO mm CuSO mv vs. SCE 73 Å

15 "frizzy step on Ag(111) Literature: M. Dietterle et al., Surf. Sci. 327 (1995) L 495 M. Giesen et al., Surf. Sci. 468 (2000) 149

16 Ordered adsorption of sulfate on Au(111): 6 P 1 P nm x 10 nm V j / µacm P 1 ' Au(111) 0.1 M H 2 SO 4 10 mv/s P 2 ' V V E SCE / V E SCE = +0.8 V E SCE = V

17 Au(110) / 0.1 M H 2 SO mm [PdCl 4 ] 2-12 nm 40 nm E SCE = 0.55 V

18 Alkanethiol Self Assembled Monolayer

19 Butanthiol SAM on Au(100) in 0.1 M H 2 SO 4 15 x 15 mm 2 70 x 70 mm 2 E SCE = V E SCE = V

20 ( ) ( ) [ ] ( ) [ ] /RT F 1 exp c k F j /RT F exp c k F j F j density current c k rate reaction D e A F G G /RT G exp h T k / kt E exp k k constant rate D chem A chem A chem B a 0 ϕ α ϕ α ν ν ϕ α Δ = Δ = = = + Δ + Δ = Δ Δ = = Electrode Kinetics

21 At equilibrium : j + = j = j 0 (exchange current density) j = j + + j = 0 At an overpotential η = Δϕ Δϕ eq a net current j is flowing j = j 0 { exp[ ( 1 α ) Fη /RT] exp[ α Fη /RT]} Butler-Volmer equation

22 Die Butler-Volmer Volmer-Gleichung ( die Austauschstromdichte j 0 )

23 Hydrogen Evolution Reaction (HER) H O 3 H O + e e 1 2 H 1 2 H H O 2 + OH (acidic solution) (basic solution) Volmer Tafel mechanism I) II) H O 3 2H + ads + e H H 2 ads + H O 2 (Volmer reaction) (Tafel reaction) Volmer Heyrovsky mechanism I) III) H O H 3 ads + + e + H O 3 H + + e ads + H O H H O 2 (Volmer reaction) (Heyrovsky reaction)

24 I 0 /Acm -2 Volmer reaction 10-2 Heyrovsky reaction Tafel reaction M H bond strength/ kj mol Exchange current density for HER as a function of metal - hydrogen bond strength. From S. Trasatti

25 HER on Au single crystal surfaces crystallographic orientation j / µa cm Au(100) Au(111) Au(110) M H 2 SO 4 5 mv/s 20 C E / V vs. RHE

26 HER on Au single crystal surfaces trends for exchange current density j 0 / µa cm -2 Au(111) 0.12 thermally induced reconstruction 0.20 potential-induced reconstruction Au(100) 0.05 thermally induced reconstruction 0.10 potential-induced reconstruction 1-2 (1 x 1), unreconstructed α ~ 0.4

27 pseudomorphic overlayer massive metal

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29 0

30 H-adsorption on Pd monolayers Pd ML /Re(0001)

31 d-band model

32 H-adsorption on Pd(111) vs. d-band shift L.A. Kibler et al., Angew. Chem. Int. Ed. 44, (2005).

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34 HER on Pd monolayers 100 Pd*/PtRu Exchange Current Density (ma/cm 2 ) Pd*/Au Pd*/Pt calculated measured Pd*/Rh Pd*/Ir Pd*/Ru Pd*/Re Differential Free Energy of Hydrogen Adsorption (ev K) J Greeley, JK Norskov, LA Kibler, AM El-Aziz, DM Kolb, ChemPhysChem 7 (2006) 1032

35 HER on Pd / Au(111)

36 HER an Pd / Au(111) 1mV/s, 0.1M H 2 SO Au(111) j 0 / µa cm -2 j / macm Pd ML /Au(111)<1 Pd ML /Au(111) E / V vs. RHE extrapolation to 0 : j 0 Pd ML /Au(111) ~ 20 µa cm -2

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38 High activity for small Pd coverages 0-1 Au(111) j / macm Pd ML /Au(111)<1 Pd ML /Au(111) 6 <1% Pd E / V vs. RHE

39 Active sites for HER on Pd / Au(111) j 0 / µacm sum of 4 contributions: Pd-Au steps Pd terraces Pd-Pd steps Au(111)? high activity for small coverage Pd coverage / ML L.A. Kibler