ECS
Electrochemical methods for electrocatalysis
Aliaksandr Bandarenka
Physik-Department, Technische Universität München, James-Franck-Str. 1, 1 D-85748 Garching, Germany Outline ECS Outline
- Introduction
Introduction Potentiostatic - Potentiostatic techniques (DC) Potentiodynamic RDE - Potentiodynamic techniques (DC) Impedance Summary - Methods involving forced convection
- Electrochemical impedance spectroscopy
- Summary
2 Introduction ECS System investigation
Probing signal 2
Introduction • Response
Potentiostatic Probing signal 1 Properties Potentiodynamic Modeling of the “black box” RDE
Impedance
Summary
Probing signal 3
„Classical“ electrochemical techniques:
Change the electrode potential / apply a bias measure current
Change / apply the current measure the electrode potential 3 Introduction ECS An electrochemical system
- + Electrode (an electron conductor)
Introduction •
Potentiostatic
Potentiodynamic RDE ē ē Impedance
Summary
Electrolyte (an ionic conductor) Electrode (an electron conductor) electron (an Electrode
4 Introduction ECS
+ - H2O ↔ H + OH
Introduction •
Potentiostatic
Potentiodynamic + RDE 4H Impedance +4 ē - Summary 2OH 2H 2 -4ē
O2 + Response2H of+ the electrolyte
Response of Response of 5 the interface 1 the interface 2 Introduction ECS A reference electrode
- +
i Electrode (an electron conductor) Introduction • V Potentiostatic
Potentiodynamic RDE ē ē Impedance
Summary
Electrolyte (an ionic conductor) Electrode (an electron conductor) electron (an Electrode
4 Introduction ECS A reference electrode
Potentiostat (invented by Hickling, 1941)
Introduction • - + Potentiostatic i Potentiodynamic
V ElectrodeCounter RDE
Impedance Summary ē ē
Reference Electrode Working Electrode Working
4 Introduction ECS Modelling of the system in case of a 3-electrode setup
WERE CE Introduction • Working Counter electrode Potentiostatic electrode
Potentiodynamic Reference
RDE electrode
Impedance
Summary
Working Counter electrode Electrolyte electrode Interface Interface
8 Introduction ECS Working electrode. A general model
Introduction • Response of
Potentiostatic the electrolyte
Potentiodynamic
RDE
Impedance
Summary Response due to Response of Faradaic the EDL (electro- catalytic) reactions
9 Potentiostatic techniques (DC) ECS
Potentiostatic techniques
Introduction
Potentiostatic • Stationary electrodes
Potentiodynamic
RDE
Impedance
Summary
In these kind of techniques the electrode potential is often kept constant for relatively long time while the current is measured
10 Potentiostatic techniques (DC) ECS
0.1
Introduction A / Current
Potentiostatic •
Potentiodynamic Overpotential ( η) Overpotential ( η) RDE 0 Impedance Summary 1.23 Electrode potential / V vs RHE
Historically probably the first techniques to investigate one of the simplest electrocatalytic reaction, the -0.1 Hydrogen Evolution Reaction 11 Potentiostatic techniques (DC) ECS Measurement scheme
potential time
Introduction
Potentiostatic • Chronoamperometry Potentiodynamic
RDE
Impedance
Summary IFaradaic = const
„Activity“
0 Icapacitive = 0
12 Potentiostatic techniques (DC) ECS More complicated measurement scheme
Staircase “scan” ) Introduction (slow!) Potentiostatic •
Potentiodynamic Potential
RDE
Impedance time
Summary quasi-stationary Potential Current response Current ( Current
time
13 Potentiostatic techniques (DC) ECS Results for HER obtained by J. Tafel for different metal
electrodes in H2SO4 Introduction in 1905 Potentiostatic • (Zeitschrift fur physikalische
Potentiodynamic Chemie", Vol. 50, pp 641- 712, 1905 ) RDE
Impedance Electrode potentialElectrode (MMS) Summary
14 Stationary DC current Potentiostatic techniques (DC) ECS
Oxygen Evolution at RuO2 and a Perovskite Sample
Introduction
Potentiostatic • RuO 2 sample
Potentiodynamic
RDE
Impedance
Summary
Perovskite sample
15 Electrochemistry Communications 38 (2014) 142 Potentiostatic techniques (DC) ECS Possible comparison of different catalysts. Particular case of the oxygen evolution
Catalyst 1 ) Introduction
Potentiostatic • Some Catalyst 2 Potentiodynamic reference RDE current
Impedance
Summary quasi-stationary Reference potentials (overpotentials) Current (
Potential vs RHE The lower the overpotential 16 the better is the catalyst Potentiostatic techniques (DC) ECS Main advantages and disadvantages
Advantages: Probably the most straightforward techniques in Introduction sense of the response interpretation, which is Potentiostatic • important for the comparison of different catalysts Potentiodynamic
RDE Disadvantages:
Impedance
Summary Do not distinguish different Faradaic processes, which occur simultaneously
Relatively slow, rather stationary systems are required A lot of a priori known information should be taken into account: e.g. oxide growth on the surface of a metal 17 Potentiodynamic techniques (DC) ECS
Potentiodynamic techniques
Introduction Potentiostatic Stationary electrodes Potentiodynamic •
RDE
Impedance
Summary
In these kind of techniques the electrode potential is constantly changed while the current is measured
18 Potentiodynamic techniques (DC) ECS (Cyclic) voltammetry
The most widely used technique for acquiring qualitative and quantitative information about Introduction electrochemical (electrocatalytic) reactions Potentiostatic
Potentiodynamic •
RDE Provides information on redox processes, i.e. Impedance interfacial electron transfer including adsorption
Summary processes
Gives a quick overview on electrode potentials where electrochemical processes take place
19 Potentiodynamic techniques (DC) ECS Cyclic voltammetry. Measurement scheme
EMAX
Introduction
Potentiostatic Efinal
Potentiodynamic •
RDE Einitial Impedance
Summary
Electrode Electrode potential EMIN
Time - Sweep direction - Sweep rate 20 Potentiodynamic techniques (DC) ECS Cyclic voltammetry. Measurement scheme
Pt(111) in 0.1M HClO 4, Ar-saturated
Introduction
Potentiostatic
Potentiodynamic •
RDE
Impedance
Summary
21 Potentiodynamic techniques (DC) ECS Cyclic voltammogram
j = F( E)
Introduction
Potentiostatic
Potentiodynamic •
RDE
Impedance H-UPD OH ads Summary „EDL-region“
Pt(111) in 0.1M HClO , Ar-saturated 22 4 Potentiodynamic techniques (DC) ECS A voltammogram of CO-monolayer oxidation
0.1M HClO 4
Introduction Potentiostatic CO-monolayer Potentiodynamic • oxidation RDE
Impedance
Summary
CO-monolayer OH adsorption ads
23 Potentiodynamic techniques (DC) ECS Voltammograms of CO-monolayer oxidation
Scan rate = 50 mV/s
Introduction Potentiostatic More active surfaces Potentiodynamic •
RDE
Impedance
Summary
24 ACS Catalysis 7 (2017) 4355 Potentiodynamic techniques (DC) ECS Main advantages and disadvantages
Advantages: Non-stationary electrocatalytic systems can be Introduction investigated Potentiostatic Relatively fast and robust technique Potentiodynamic •
RDE
Impedance
Summary Disadvantages:
Does not distinguish different Faradaic processes, which occur simultaneously Often difficult to separate Faradaic and the double layer currents
25 Forced Convection ECS
Introduction
Potentiostatic
Potentiodynamic Methods involving forced RDE • convection Impedance
Summary
26 Forced Convection ECS There are three typical modes of ion transport in ionic conductors Electrode Migration - charged particles move to equalize potential gradients in the Introduction electrolyte. Potentiostatic
Potentiodynamic
RDE • Convection - material is moved by an Electrode Impedance external force such as flow, or rotation Summary of the electrode. Electrolyte
Diffusion - Movement of a species under the influence of a gradient of chemical potential (practically, a concentration gradient). 27 Forced Convection ECS Why hydrodynamic methods?
Mass transfer rates are larger than just by diffusion alone, therefore the relative contribution of mass transfer to electron transfer kinetics is relatively small Introduction
Potentiostatic Rather quick steady-state can be reached; the double- Potentiodynamic layer charging can be excluded at steady-state RDE • Insulating parts Impedance Summary Rotating disk electrode
One of few convective electrode systems for which the hydrodynamic equations and the convective-diffusion problem have been solved Disk electrodes analytically 28 Forced Convection ECS Why hydrodynamic methods?
Introduction
Potentiostatic
Potentiodynamic
RDE •
Impedance
Summary
29 Forced Convection ECS Rotating disk electrodes
Potentiostatic Electrode Introduction experiments
Potentiostatic Electrolyte Potentiodynamic
RDE •
Impedance
Summary Potentiodynamic experiments Examples: Oxygen reduction Hydrogen oxidation
Oxygen evolution Hydrogen evolution
30 Forced Convection ECS Rotating disk electrodes
Introduction
Potentiostatic Ar-saturated HClO 4
Potentiodynamic reduction “signature”Oxygen
RDE • O2-saturated HClO 4
Impedance
Summary -2 jlim / mA cm / mA j
31
Phys. Chem. Chem. Phys., 2013, 15 , 12998 Forced Convection ECS Rotating disk electrodes
1/2 Introduction jlim = BSelectrode C0ω Potentiostatic
Potentiodynamic
RDE •
Impedance
Summary 1 1 1 = + j jkinetic jlim
32 Forced Convection ECS Rotating disk electrodes. O2 reduction
Introduction Often important for Potentiostatic fuel cell research Potentiodynamic reference point RDE •
Impedance
Summary
33 Chemical Science 8 (2017) 2283 Forced Convection ECS Rotating disk electrodes.
O2 reduction activity ranking
Introduction
Potentiostatic
Potentiodynamic
RDE •
Impedance
Summary
34 Chemical Science 8 (2017) 2283 Forced Convection ECS Main advantages and disadvantages
Advantage:
Introduction Fast. Powerful tool to investigate kinetics of
Potentiostatic electrochemical reactions and test catalytic activities when mass transport plays an important Potentiodynamic role RDE •
Impedance
Summary Disadvantages:
Acquisition of the integral response. Difficult to distinguish between different Faradaic constituents
Problems with fast accumulation of impurities at the electrode surface during the experiments
35 Electrochemical Impedance Spectroscopy ECS
Introduction
Potentiostatic
Potentiodynamic Electrochemical Impedance RDE Spectroscopy Impedance •
Summary
36 Electrochemical Impedance Spectroscopy ECS Basics of EIS: data acquisition
Electrochemical system CE RE Introduction
Potentiostatic 10mV
Potentiodynamic
RDE WE Impedance • probing signal response
Summary E = E bias + E ac sin( ωit) I = I 0+Iac sin( ωit+ θ)
at different ac frequencies ωi
The “output” is:
1. Modulus of impedance |Z ω|, i.e. the ratio Eac / Iac
2. A value of the phase shift θω between the probing signal E and the current response I 37 Electrochemical Impedance Spectroscopy ECS Typical impedance spectra
Introduction
Potentiostatic
Potentiodynamic
RDE
Impedance •
Summary
ImZ =|Z|sin( θ) ReZ = |Z|cos( θ) 38 Electrochemical Impedance Spectroscopy ECS Linearity ( determines probing signal amplitude )
a resistor an electrochemical system i i Introduction
Potentiostatic
Potentiodynamic „Linear “ RDE
Impedance • E E
Summary
The amplitude of the ac probing signal in EIS experiments should be small in order to consider our system (quasi) linear.
Depending on the system, the amplitude of 1-10 mV is 39 acceptably small Electrochemical Impedance Spectroscopy ECS Quasi-stationarity
Complications:
Introduction
Potentiostatic
Potentiodynamic
RDE
Impedance •
Summary ≠ ≠ ≠
time Possible solution is to minimize the time for the measurements
40 Electrochemical Impedance Spectroscopy ECS Data analysis
Introduction
Potentiostatic
Potentiodynamic Electrolyte
RDE The double layer
Impedance •
Summary Electrochemical reactions
Dolin-Erschler-Randles approximation (1940-1947) 41 Electrochemical Impedance Spectroscopy ECS Equivalent electric circuit is a schematic representation of equations describing the electrochemical system
qM – excess surface charge density on the metal; f(E, v i….)
Introduction i – current due to electrochemical reaction f(E, C , v ….) Potentiostatic s i ρ – specific resistance of the electrolyte Potentiodynamic Cdl
RDE l – distance between electrodes Rel Impedance • sel – electrode surface area Rct Summary g – empirical constant depending on the cell geometry, positions of the electrodes etc.
Rct Rct Cdl Rel
42 Rct Cdl Rct Cdl Electrochemical Impedance Spectroscopy ECS Solving the inverse problem: Randles circuit
EIS spectrum
-1 Introduction Cdl =20 F Cdl = ( ωRct ) Potentiostatic Rel =10 Potentiodynamic R =100 ct Rct =100 RDE
Impedance •
Summary Rel =10
n- number of electrons; F = 96485 C/mol; Ci –surface concentrations of electroactive 43 species, ki- het. react. rate constants; Fi(θ)- some functions of the surface coverage (e.g. θ or 1- θ in case of Langmuir adsorption isotherm) Electrochemical Impedance Spectroscopy ECS Equivalent electric circuit elements are building blocks of EIS models
Introduction
Potentiostatic
Potentiodynamic
RDE
Impedance •
Summary
CPE (constant phase element) is a semi-empirical element. It can describe many phenomena, however the exact interpretation of its parameters n and Q is not straightforward and will depend 44 on system properties, models selected etc. Electrochemical Impedance Spectroscopy ECS
- Bad news about physical impedance models:
1. It is quite difficult to derive analytical equations for the physical EIS models. 2. Many of the parameters in these equations are complex Introduction functions of the electrode potential, e.g.
Potentiostatic
Potentiodynamic ZW RDE
Impedance • Surface concentrations CO and CR are normally kf = k0exp[-αnF( E-E0)/RT] not equal to the respective bulk concentrations Summary c and c . Both C and C are complex functions kb = k0exp[(1-α)nF( E-E0)/RT] O R O R of the electrode potential
- However the good news are:
1. For many of typical electrochemical reactions the analytical equations for the Faradaic impedance are known (someone else did this work for us). We only need to assemble the model (equivalent circuit) from the known components using some relatively simple rules.
45 2. There are many ways how to simplify the EIS analysis to extract valuable physico-chemical parameters. Electrochemical Impedance Spectroscopy ECS Picture from the past: “graphical analysis”
Introduction - Model postulation, e.g.
Potentiostatic
Potentiodynamic
RDE
Impedance •
Summary
- Model verification: just semi-quantitative 46 or visual inspection! Electrochemical Impedance Spectroscopy ECS The first software for the impedance data fitting using CNLS (1986): a new era of EIS
By Bernard A. Boukamp
Introduction Software: EQUIVCRT Potentiostatic
Potentiodynamic Solid State lonics , 18-19 (1986) 136
RDE Impedance • By James R. MacDonald Summary Software: LEVM
Solid State Ionics , 24(1) (1987) 61
Optimisation algorithm: Levenberg–Marquardt (iterative)
- Powerful algorithms for the check of data quality - Careful and more objective model verification 47 - More accurate estimation of parameters of the models Electrochemical Impedance Spectroscopy ECS Accurate analysis of EIS data is the key
Introduction
Potentiostatic
Potentiodynamic
RDE
Impedance •
Summary
48 Electrochemical Impedance Spectroscopy ECS Revealing phase transitions in adsorbate layers
−1 −φ Zi = C′DL (jω) , Introduction ′ Potentiostatic where C DL is the Potentiodynamic parameter, which is proportional to the double RDE layer capacitance, Impedance • φ ≤ 1 is the CPE-exponent, Summary which is directly related to the
49 Electrochemical Impedance Spectroscopy ECS Revealing phase transitions in adsorbate layers
O2-free 0.1M HClO 4
Introduction
Potentiostatic
Potentiodynamic
RDE 1/3ML OH* with H 2O Impedance •
Summary
1/3ML O*
O -sat 0.1M HClO 1/4ML O* 2 4 50 Langmuir 2011, 27(5), 2058 ECS Comparison between techniques Each technique provides information about only one or several aspects of interfacial processes
Techniques Target aspects Advantages Disadvantages
Acquisition of the Averaged Fast. Creates first integral response. Voltammetries, information about understanding of the Does not electrochemical system under static “distinguish” processes at the investigation. electrodes contributions from interface Investigation of non- stationary systems different processes
Averaged Fast. Powerful tool to Acquisition of the Voltammetries, information about investigate kinetics of integral response. electrochemical electrochemical Problems with rotating disc reactions under reactions, test accumulation of (ring) controllable mass catalytic activities and impurities at the electrodes transport of detect reaction electrode surface electroactive species intermediates during the experiments
Information about Relatively slow. different Powerful tool to reveal Frequent ambiguity constituents of physical models of the in the model Impedance simultaneously interface. Separates selection: one should spectroscopy running contributions from use a priori electrochemical different processes knowledge or processes. which occur additional Averaged for the simultaneously information from electrode surface other techniques 51 ECS
Where are the most active electrocatalytic centers?
52 ECS The Sabatier principle ( a qualitative concept ) (1911) The interactions between the catalyst and the reaction intermediates should be just right.
The catalyst surface should not bind them neither too strong nor too weak.
Paul Sabatier (1854-1941)
Imagination of a catalyst surface?
53 ECS The catalyst surface is not uniform?
“Most finely divided catalysts must have structures of great complexity. <…..> In general, we should look upon the surface as consisting of a checkerboard.” (1922)
Irving Langmuir (1881 –1957)
54 ECS The concept of active sites
“A catalyzed chemical reaction is e.g. not catalyzed over the entire solid surface of the catalyst but only at certain ‘active sites’ or centers” (1925)
Hugh Stott Taylor (1890-1974)
Namely the active sites at the surface should 55 bind the intermediates “just right” ECS Key initial procedures in (heterogeneous) electro-catalysis
Identification of the Optimization of the nature of the most electronic (adsorption) active sites properties of those active sites ? ?
? Maximisation of the density of those sites 56 at the surface ECS A big problem in heterogeneous catalysis: the nature of active sites is known only for few reactions and only few types of materials
I. Chorkendorff, J. W. Niemantsverdriet, Concepts of Modern Catalysis and Kinetics
57 ECS Identification of active sites using EC-STM?
58 J. Pfisterer, Y. Liang, O. Schneider, A.S. Bandarenka // Nature 549 (2017) 74 ECS Identification of active sites using electrochemical STM? Oxygen reduction reaction at a Pt(111) terrace pH=1
J. Pfisterer, Y. Liang, O. Schneider, A.S. Bandarenka // Nature 549 (2017) 74 59 ECS Hydrogen evolution at a Pt(111) terrace
pH=1
60 J. Pfisterer, Y. Liang, O. Schneider, A.S. Bandarenka // Nature 549 (2017) 74 ECS Oxygen reduction at a Pt(111)
pH=1
61 J. Pfisterer, Y. Liang, O. Schneider, A.S. Bandarenka // Nature 549 (2017) 74 ECS Hydrogen evolution at Au(111)/Pd(ML)
pH=1
62 J. Pfisterer, Y. Liang, O. Schneider, A.S. Bandarenka // Nature 549 (2017) 74 ECS Hydrogen evolution at Au(111)/Pt(ML) „OFF“ „ON“
Au Au
Pt Pt Au Au
63 Y. Liang, C. Csoklich, A.S. Bandarenka // in preparation ECS Oxygen electroreduction at Pt(111) in 0.1M LiOH
ORR ON
ORR OFF
ORR ON
ORR OFF
64 Y. Liang, D. McLaughlin, A.S. Bandarenka // in preparation ECS Oxygen electroreduction at Pt(111) in 0.1M LiOH
OFF
ON
65 Y. Liang, D. McLaughlin, A.S. Bandarenka // in preparation ECS Oxygen electroreduction at Pt(111) in 0.1M LiOH
66 Y. Liang, D. McLaughlin, A.S. Bandarenka // in preparation ECS Oxygen electroreduction at Pt(111) in 0.1M KOH
ORR ON
ORR OFF
ORR ON
67 Y. Liang, D. McLaughlin, A.S. Bandarenka // in preparation ECS Oxygen electroreduction at Pt(111) in 0.1M KOH
68 Y. Liang, D. McLaughlin, A.S. Bandarenka // in preparation ECS Oxygen electroreduction at Pt(111) in 0.1M CsOH
Compare with:
69 Y. Liang, D. McLaughlin, A.S. Bandarenka // in preparation Summary ECS Forward vs inverse problems We know the model We calculate the response
Forward problem:
Introduction
Potentiostatic Potentiodynamic We measure the response ??? RDE
Impedance Inverse problem: …
Summary • 2 3 y = a 0+a 1x+a 2x +a 3x +…???
The inverse problem consists of using the actual result of some measurements to infer the model and get the parameters that characterize the system.
While the forward problem has (in deterministic physics) a unique solution, the inverse problem does not
Learn more : Albert Tarantola . Inverse problem theory and methods for model parameter 70 estimation. Siam 2005, ISBN 0-89871-572-5 ECS
Thank you for your attention!
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