The Characterization of electrochemical Surface

Rosa Arrigo

31.01.2014

Modern Methods in Heterogeneous Catalysis Research, Lecture Series Winter Semester 2013 / 2014

1) Classical electrochemistry: Kinetic study of the reaction occuring at

the interface. Tafel equation (1905) Use of inexpensive instrument (potentiostat_H. Gerischer invention 1957) and consist of measuring the e- flowing across the surface per unit time and area as function of potential, double layer constituents, T, P.

2) The development of electrochemical surface science: to study the interface structure The main concern is to characterize the structure of the electrochemical interface: Electrocatalytic effect. 3) New directions towards „Operando“ characterization of electrochemical surface.

Research lines:

Outline

• The structure of the interface solid/electrolyte

– Metal/electrolyte

• Concept of electrochemical potential

• Methods for the characterization of the electrochemical interface

• Voltammetric techniques

• Electrochemical surface science

Question:

Galvanic cell Electrolytic cell

Electrochemistry is concerned with chemical reactions which involve the transfer of electric charge at the interface between solids and liquids.

Introduction

At interfaces electron transfer proceeds, which requires the presence of electron donors or acceptors near the elctrode surface, the charge is transported through the solution by ion transfer.

The character of the conduction has a discontinuity at such an interface; the electrochemical reaction can be considered as a switch from electronic to electrolytic (ionic) conduction. Such a process constitutes a barrier for the transport of electricity.

Metal - electrolyte interface

.

When a metal is in contact with an electrolyte: 1. the electrode can polarize: a potential is set up

across the two phases. This structure _The double layer_is made up of electrons, solvent molecules, electroactive species (O2, H2) and ions differing in arrangment and local concentration as one proceeds from the metal to the solution.

Electrochemistry is the branch of chemistry concerned with the interrelation of electrical and chemical effect.

Metal/electrolyte interface

Bockris-Devanathan-Muller model

The structure of the electric double layer is based on the intefacial characteristic of the electrode, that is, the excess electronic charge at the metal surface, and on the charge compensating ions in the electrolyte. (additonally surface states due to the sudden truncation of the crystal contribute with specific interaction: chemisorption)

Distribution of the electronic density in the Jellium model

Surface Electrochemistry: A Molecular Level Approach, John O'M. Bockris , Shahad U.M. Khan

Capacitative and faradaic charge injection

Capacitive charge injection includes redistribution of charge in the electrode-electrolyte interface

Faradaic process includes transfer of electrons

Electrochemistry is the branch of chemistry concerned with the interrelation of electrical and chemical effects. When a metal is in contact with an electrolyte.......

0.1 A

ideally polarizable electrode the ideally non-polarizable electrode

Equivalent circuit

Ex. RHE Ex. supercapacitor

At potential such that the reaction is thermodinamically or kinetically unfavorable

Large change in potential upon The passage of infinitesimal current

no potential changes upon The passage of current: electrode of fixed potential

Electrochemical methods, A. J. Bard and L. R. Faulkner

Current-potential curve under steady-state conditions

𝜇i =𝜕𝐺

𝜕𝑛𝑖 𝑇, 𝑝, , 𝑛𝑗𝑖

µi= µi °+RT ln ai

Volta potential take into account the long-range order

columbic interaction

Surface potential short range order of adsorbed ion and

oriented solvent molecules in the double layer.

=Inner potential F=Faraday constant z=charge per ion

Chemical term Electric term

Potential concepts of electrochemistry

ϕ =χ+φ

Χ= surface potential Φ= Volta potential (outer potential)

𝜇i = 𝐹(𝐸𝑓 − 𝐾)

K=the energy to take one electrode from the RHE to vacuum

Electron transfer event/ reversible reaction

The electrochemical potential of electrons in a redox couple is given by the Nerst equation: Eredox=E°redox +

𝑅𝑇

𝑛𝐹ln 𝐶𝑜𝑥𝐶𝑟𝑒𝑑

The concentration of Ox and Red is given by the Nerst equation

The equilibrium controlling reaction is the exchange of electrons between electrode and redox couple

Reduction of water/oxidation of electrode

-

+

oxidation of water/ Reduction of electrode

Electron transfer event/oxidation reaction

e- tunneling

i= 𝑑𝑄

𝑑𝑡 (Columb/s)

𝑄

𝑛𝐹

𝐶𝑜𝑙𝑢𝑚𝑏

𝐶𝑜𝑙𝑢𝑚𝑏/𝑚𝑜𝑙= N (mol)

n= stoichiometric number of electrons consumed

v (𝑚𝑜𝑙 𝑠𝑒𝑐 )= 𝑑𝑁

𝑑𝑡=𝑖

𝑛𝐹

beside kinetic effect, the rate depends: a) on mass transport b) surface ads. and des. c) potential

Due to the heterogeneous character the rate is defined as current density

v (𝑚𝑜𝑙 𝑠𝑒𝑐 ∗ 𝑐𝑚2 )=𝑖

𝐴 𝑛𝐹 =

𝑗

𝑛𝐹

Kinetics of Faraidic reaction: the current as a measure of the rate of electron transfer

F=96485 C mol-1

This current which originates from chemical reaction, obeys Faraday´s law

Vf=kf.Co)=ic /nFA O + n e- R kf

kb

Vb=kb.CR =ia /nFA

i=ic-ia i=nFA(kf.Co-kb.CR)

Tafel equation

the rate of charge transfer depends on: potential, structure of the double layer, adsorption of reactants, intermediates, products, impurities, mass transport limitation.

Kinetics of Faradaic reaction

Butler-Volmer equation

ɳ=E-E0 overpotential

Tafel equation

For small ɳ both ic and ia contribute

For big ɳ only ic or ia contribute

where

Exchange current density when ic = ia

Tafel equation linearised ɳ = a + b ln i

Surface Electrochemistry: A Molecular Level Approach, Bockris and Khan, Plenum

1) Classic electrochemical methods

Surface Electrochemistry: A Molecular Level Approach, Bockris and Khan, Plenum

1) Classic electrochemical methods

Inorganic, physical, and biological chemists widely use electrochemical techniques for a variety of purposes, including fundamental studies of oxidation and reduction processes in various media, adsorption processes on surfaces, electron transfer and reaction mechanisms, kinetics of electron transfer processes, and transport, speciation, and thermodynamic properties of solvated species

The electrochemical cell, where the voltammetric experiment is carried out, consists of a working electrode, a reference electrode, and usually a counter (auxiliary) electrode.

RDE are particularly useful where mass transfer may be problematic: i.e. gas bubble

Chronoamperometry For the redox reaction O + ne- to R

A potential time profile is applied to the WE

At E1 no O red occurs and E2 O red occurs at diffusion controlled rate

At point 4 and 5 is limited by diffusion

i = nFACD½p-½t -½

Cottrell equation:

n = number of electrons transferred/molecule F = Faraday's constant (96,500 C mol-1) A = electrode area (cm2) D = diffusion coefficient (cm2 s-1) C = concentration (mol cm-3)

Plot of I vs t-1/2 is linear and it is a test of diffusion control regime

Diffusion layer increase with time

Cyclic voltammetry

Potential wave form for cyclic voltammetry

Potential sweep methods are the most

common electrochemical methods in use by

chemists today

• They provide an efficient and

straightforward assessment of the redox

behavior of molecular system.

As the electrode potential is constantly rising (or decreasing):

• at first capacitive current flows continuously due to the capacitive

charging of the working electrode's double layer;

• Faradaic current flow when the potential reaches values at which the

species in solution can undergo electrochemical conversions.

CV for Reversible Reaction (Nerstian)

• Reversibility requires that the electron transfer kinetics are fast enough to maintain the surface concentrations of O and R at the values required by the Nernst equation.

• If a redox system remains in equilibrium throughout the potential scan, the redox process is said to be reversible

ip = 2.69x105n3/2ACD1/2n1/2

For a reversible process, Eo is

given by the mean of the

peak potentials.

O + n e- R kf

kb

ic

ia

n = number of electrons transferred/molecule A = electrode surface area (cm2) C = concentration (mol cm-3) D = diffusion coefficient (cm2 s-1)

reversible electron transfer reactions

irreversible electron transfer reactions

http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l4html/cv.html

CV: effect of the scan rate

Cyclic voltammetry

D. M. Kolb, Surface Science 500 (2002) 722-740

I=? II= Sulphate ads. (+) and des. (-). III=H ads. (-) and des. (+) IV= HER

J. Electrochem. Soc. 1978 volume 125, issue 2, 348-349, Hydrogen Adsorption on Single Crystal Platinum

CV: H ads. Des. on Pt single crystal

Single crystal polycrystalline

Tafel plots for various HER catalysts

Chen Z., Nanoletter 10, 11 (2011)

Redox reaction involving H2/O2/H2O system

2) Electrochemical surface science Develop an atomistic picture of the solid/liquid interface to provide basis for mechanistic understanding of electrochemical processes.

EX-situ Approach

Surface Science : Single crystal and Ultrahigh vacuum (10-10 mbar)

Enable Electronic properties, adsorption sites, geometry of adsorbed species

STM and AFM

LEED XPS, AES,

UPS

Methods:

Microscopy: Diffraction: Electron Spectroscopy: Optical Spectroscopy:

ATR-SEIRAS, RAMAN

ellipsometry

In-situ methods

Electrochemical Surface Science: the presence of water limits the application of surface science methods to: optical methods (photon in – photon out) and local probe microscopy

Recent development: XANES, NAP-XPS

Ex-situ methods • The sample removed from the electrolyte is brought in vacuum without contact to air. Limitation: removal of the solvent by evaporation under vacuum , the potential change from that given by the potentiostat. This may change the double layer structure and what is left is strongly adsorbed species. However Hansen et al. demonstrated that it is possible to remove from the EC if • the transient current in the external circuit is negligibly small • The concentrations of the various species in the solution droplet which remains on the

electrode are equal to those of the initial solution.

Example Ex-situ methods: CV, XPS, LEED EC-UHV-XPS

Ex-situ surface preparation and analysis: Transfer between UHV and electrochemical cell H. E. Hoster and H. A. Gasteiger, Volume 2, Part 3, pp 236–265, John Wiley & Sons, Ltd, Chichester, 2003

16h

Example Ex-situ methods: XPS, CV, LEED

In-situ Ellipsometry Ellipsometry, since the 1960s, is a very sensitive technique for the study of the thickness of nanometer thick film of oxide on metal surface.

It is a non destructive method and can be used in-situ.

It is based on measuring the change of polarization as polarized light reflects or transmits from a material structure.

The polarization change is represented as an amplitude attenuation ratio, Ψ, and the phase difference, Δ, of the two orthogonal light waves (p) and (s). The result is an elliptically polarized light

http://jawoollam.com/tutorial_2.html

When two orthogonal light waves are in-phase, the resulting light will be linearly polarized

(s component)

(p component)

orthogonal waves of arbitrary amplitude and phase is elliptically polarised

R= amplitude reflected light /amplitude incident light

R

Oxide film formation

Experimental value of amplitude attenuation ratio, Ψ, and the phase difference, Δ are compared with theoretical value calculated for film of certain thickness and refractive index learn about nature of the film.

Powerful imaging tool, directly visualize electrochemical

processes in-situ and in real space at molecular or atomic levels.

Such interfacial electrochemical studies have been dramatically expanded over the past decade, covering areas in electrode surfaces, metal deposition, charge transfer, potential-dependent surface morphology, corrosion, batteries, semiconductors, and nanofabrication.

Events in the EC data correlate with changes in the topography of the sample surface.

In-situ Electrochemical STM (ECSTM): early 80’s

How to operate? Raster the tip across the surface, and

using the current as a feedback signal.

The tip-surface separation is controlled to be constant by keeping the tunneling current at a constant value.

The voltage necessary to keep the tip at a constant separation is used to produce a computer image of the surface.

What an STM measures? -local density of states -The wave functions of the electrons in the tip overlap those of the sample surface

Electrons tunnel from one surface to the other of lower potential.

STM

Example: potential-dependent surface structure in electrolyte solution Au(111) in 0.05 M H2SO4.

I: Au(111) (23x3) reconstruction (compressed first layer) II: lifting of reconstruction (Au ad-islands) III: ordered sulfate layer

Dretschkow, Wandlowski, in Wandelt, Thurgate (Eds.) Topics Appl. Phys. 85 (2003) 259.

The evolution of model catalytic systems: studies of structure, bonding and dynamics from single crystal metal surfaces to nanoparticles, and from low pressure (10-8mbar) to higher pressure (depending on the detection mode)

Aim: Covering the Material and Pressure gap

Recent development NAP-XPS

X-ray Photoemission Spectroscopy Photons in, electrons out

NAP-XPS

Limitation of electrons detection: Only several nm of thick layer P20 mbar (higher gas-phase pressure, produce e- scattering with attenuation of the signal)

Recent development toward Operando: XANES

X-ray Adsorption Near Edge Spectroscopy Photons in, Photon out

In Fluorescent yield mode, it is possible to investigate liquid phase reaction

Recent development toward Operando: XANES

In situ X-ray absorption spectroscopy investigation of a bifunctional manganese oxide catalyst with high activity for electrochemical water oxidation and oxygen reduction. Gorlin Y, Lassalle-Kaiser B, Benck JD, Gul S, Webb SM, Yachandra VK, Yano J, Jaramillo TF.

J Am Chem Soc. 2013 Jun 12;135(23):8525-34. doi: 10.1021/ja3104632. Epub 2013 Jun 3.

Recent development toward Operando: XANES

Literature

• Surface Electrochemistry: A Molecular Level Approach, John O'M. Bockris , Shahad U.M. Khan

• Electrochemical methods, A. J. Bard and L. R. Faulkner • An atomistic view of electrochemistry, D. M. Kolb, Surface science 500, 2002, 722-740 • Fuel Cell Catalysis a surface science approach, M. T. M. Koper, Wiley • Fuel Cell from fundamental to Applications, S. Srinivasan, Springer, chapter 2, available

on-line • Inorganic Electrochemistry: Theory, Practice and Application, Piero Zanello, RSC, Chapter

1 and 2. • Ex-situ surface preparation and analysis: Transfer between UHV and electrochemical

cell, H. E. Hoster and H. A. Gasteiger, Volume 2, Part 3, pp 236–265, John Wiley & Sons, Ltd, Chichester, 2003

Helmhlotz model (1853)

Gouy-chapman Model (1903)

Stern model (1924)

The double layer Excess of charge of opposite sign may accumulate on the two side of the interface acting like a plate condenser. Parallel plate condenser model: Two layers of charge of opposite sign separated by a fixed distance. The potential drop across the interface decrease linearly

Diffusion layer model: Marxwell-Boltzmann distribution of ions. It exclude ion-ion interaction important for concentrated solution.

Combination of the two above: ions are immobilised close to the electrode and also diffusely spread out in the solution. Not appliable to specifically adsorbed ions. Does not take into account solvent molecules around the ions and its impact on the structure of the double layer

The triple layer model

Angewandte Chemie International Edition Volume 40, Issue 7, pages 1162-1181, 27 MAR 2001

+

+

+

-

-

-

Esin and Markov, grahame and Devanathan Model

-

It takes into account that ions could be dehydrated in the direction of the metal and specifically adsorbed on the electrode. This is the inner layer (3A˚). This involves strong bond to the surface through partial electron transfer between the ion and the electrode (chemisorption). Na+ have a strong hydration shell so are minimally adsorbed; large anions like Cl- have less ion-solvent interaction then specific interaction may occur with charge transfer. The variation of the potential with the distance reveal a steep drop in the IHP and a small raise between IHP and OHP. After that the variation is like the diffusion layer.

Water dipole model: Bockris-Devanathan-Muller: strong interaction between solvent dipoles like water and charged electrode to form a oriented layer od water molecules contribure to the potential drop across the interafce.

Diffuse layer

Semiconductor/electrolyte interface

Semiconductor/electrolyte interface

Semiconductor/electrolyte interface

Imaging the structure of electrode surface: Example

STM images of the Au (100) electrode surface

(Left) Au (100) electrode in 0.1 M H2SO4 at -0.25 V vs. SCE, where potential-induced reconstruction proceeds. The initially unreconstructed surface is being gradually transformed into the reconstructed form.

(Right) The zoom shows a section of the surface, 3/4 of which has already been reconstructed; one single reconstruction row on the left hand side is seen to grow from bottom to the top of the image.

The electrolyte contains a redox couple, the equilibrium controlling reaction is the exchange of electrons according to the process: Az+ . solv + e- ↔ A(z-1)+ . solv at equilibrium The potential difference (Galvani potenial) across the S/E interface is

FΔS/E= µox - µred + Sµe- + RT ln 𝑎𝑜𝑥

𝑎𝑟𝑒𝑑

Changes in the chemical potential are compensated by changes in the Galvani potential at equilibrium

𝜇 Az+ + 𝜇 e − = 𝜇 A(z-1)+

Oxidized component

reduced component

Potential cannot be measured in absolute; the Galvani potential requires a relative scale versus the potential of a standard electrode in equilibrium with the electrolyte through a particular electrode reaction. Such electrode potentials are the difference in free energy between electron in the sample and the reference electrode.

Potential concepts of electrochemistry