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Physical & Interfacial Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons h B ld Main Chemistry Building Room 3.2 School of Chemistry Trinity College Dublin Trinity College Dublin [email protected] Module JS CH3304 Molecular Thermodynamics and Kinetics

Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

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Page 1: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Physical & Interfacial Physical & Interfacial Electrochemistry 2013

Professor Mike Lyons h B ld Main Chemistry Building Room 3.2

School of ChemistryTrinity College DublinTrinity College Dublin

[email protected]

Module JS CH3304 Molecular Thermodynamics and Kinetics

Page 2: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Fundamental Physical & Interfacial ElectrochemistryElectrochemistry.

• Lecture 1. • Review of basic concepts & survey of some EC systems.w f a c c nc pt & ur y f m E y t m .

• Lecture 2. • Ion-Solvent Interactions : Born Model of ionic solvation.

L t 3 • Lecture 3. • Ion-Ion interactions: Debye-Huckel (DH) Theory

• Lecture 4. • Equilibrium electrochemistry .

• Lecture 5.• The electrode/solution interface• The electrode/solution interface.

• Lecture 6/7.• Material transport in electrochemical systems.

• Diffusion, migration, convection.• Lecture 8/9.

– Electron transfer kinetics at electrode/solution interfacesElectron transfer kinetics at electrode/solution interfaces• Phenomenological approach• Microscopic QM approach (Marcus Theory).

Page 3: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Recommended Reading.g• Hibbert, Introduction to electrochemistry (Macmillan, 1993). Good

simple treatment. OK for this course.C t & B k U d t di V lt t 2nd diti I i l • Compton & Banks, Understanding Voltammetry, 2nd edition, Imperial College Press, 2011. Great book for theory of voltammetry.

• Brett & Brett, Electrochemistry : Principles, methods and li ti (OUP ) M d d t b t d l applications (OUP, ). More advanced account but good clear

explanations.• Rieger, Electrochemistry, 2nd edition. (Chapman &Hall, 1994). More

d d advanced account.• Bard and Faulkner, Electrochemical techniques : fundamentals &

applications, 2nd edition, (Wiley, 2001). The electrochemical bible for graduate study.

• Girault, Analytical and Physical Electrochemistry, Marcel Dekker/EPFL Press, 2004. Excellent and thorough presentation of essentials written g pwith characteristic French mathematical rigor.

• Bond, Broadening electrochemical Horizons, (OUP, 2002). ****Another excellent book explaining the principles of electrochemical methodology.p g p p gy

Page 4: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Physical & Interfacial Electrochemistry 2013y

Lecture 1Redox reactions at interfaces :

devices and applicationsdevices and applicationsin energy conversion & synthesis.

Page 5: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Definitions of Electrochemistry on the Web:

Ch i l h d d d b l t i t• Chemical changes and energy produced by electric currents.www.angelfire.com/pa/baconbacon/page4.html

• Chemical reactions driven by electrical energy.hi h d hill / it /0072480823/ t d t i 0/ l ht lhighered.mcgraw-hill.com/sites/0072480823/student_view0/glossary.html

• The study of chemical changes resulting from electrical action and electrical activity resulting from chemical changes.

bi /b k/bi l / i l i /l b l i d d t ti l htwww.bioon.com/book/biology/genomicglossaries/labels_sig_and_detection_gloss.asp.htm

• The branch of chemistry concerned with the electrical aspects of chemical reactions, particularly electrolysis and cells (batteries).

lt k h k/MSW b/S i W b/ h i t / l ht lwww.malton.n-yorks.sch.uk/MSWeb/ScienceWeb/chemistry/glossary.html

• Branch of chemistry that deals with the chemical action of electricity and the production of electricity by chemical reactions

d t p i t d /p l/ bwordnet.princeton.edu/perl/webwn

• Electrochemistry is the science of the reactions that can take place at the interface of an electronic conductor (the electrode, which can be a metal or a semiconductor including graphite) nd n i nic c nduct (th l ct l t ) and an ionic conductor (the electrolyte).

If a chemical reaction is caused by an external voltage, or if a voltage is caused by a chemical reaction, as in a Battery (electricity), it is an electrochemical reaction. In general, electrochemistry deals with situations where an oxidation and a reduction reaction is separated in space in space. en.wikipedia.org/wiki/Electrochemistry

Page 6: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Electrochemistry underlies much important technology :p gyMetal plating,Fuel Cells/Energy Conversion,Corrosion, Biosensors.

Page 7: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Physical Analytical & Materials Electrochemistry Center (PAME-TCD)School of Chemistry & CRANN TCD

Electrochemical Technology & Innovation (ETI) Group

School of Chemistry & CRANN, TCD.

PI: Professor Mike Lyons, [email protected]

Trinity Electrochemical Energy Conversion & El l i (TEECE) Electrocatalysis (TEECE) Group

Electroactive Materials Electrochemistry & Sensors (EMES) Group

A

Modeling & Simulation of -10

0

10

20

30

40

5

10

15

20

25

30

35

40

curr

ent /

µA

tion

peak

cur

rent

/ µA

Modeling & Simulation of Electrochemical Systems (MSES) Group

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6-20

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.00

potential / V (vs. Ag / AgCl)

oxid

at

concentration / mM

SWCNT modified Carbon for AA detection

Page 8: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

The electrode/electrolyte interface.

Electrode ElectrolyteIonically conductingmedium : electrolytem m ysolution, molten salt,solid electrolyte,polymericConduction polymericelectrolyte, etc.occurs via

migration ofelectrons .

ET

Solid statePhysics : energyband theory

Material transport occursvia migration, diffusion

Electronically conducting phase : metal semiconductor

band theory. gand convection

Electronically conducting phase : metal, semiconductor,Conducting polymer material etc.

Page 9: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Material transportthrough electrolyte

Heterogeneousinterfacial ETk l l through electrolyte

via diffusion, convection& migration.

kinetics : Butler-Volmer,Marcus Theory.

Ionics.Ion Ion interactionsIon-Ion interactions,Ion-solvent interactions.

Experimental Methodology:Electrochemical.SpectroscopySpectroscopy.Probe Microscopy.

Structure of ‘double layer’ Interface region.Diffusion layer approximation

Homogeneous chemicalkinetics.

Diffusion layer approximation.

The elements of Physical Electrochemistry

Page 10: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Bard et al. The electrode/solutioninterface- a status report.J Ph Ch 97 (1993) 7147J. Phys. Chem. 97 (1993) 7147-7173.

Page 11: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Electrode/solution interfaceI d b STM & AFMImaged by STM & AFM.

Page 12: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Interfacial electron transfer at electrode/solution interfaces:interfaces:

oxidation and reduction processes.

Electron sink electrode(Anode).

Electron source electrode(Cathode).

• Oxidation (electron loss) and reduction (electron gain) are

d ( d ) d

P A

termed (redox) processes and lie at the heart of electrochemistry.

ne-Q ne- B

• The rates of such processes are influenced by the

Oxidation or de-electronation. Reduction or electronation.

structure of the interface region located between an electrode and an l t l t l ti P = reductant (electron donor)

Q = ProductA = oxidant (electron acceptor)B = Product

electrolyte solution.

Page 13: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Local geography of electrode/solution

• Occurrence of ET reactionstend to make the compositionelectrode/solution

interface region.p

of the solution near theelectrode surface differentfrom that further away.

Electrode Diffusion Layer/reaction layer

yA Diffusion layer is generated. This is considerably thicker than the electrical double layer.y• Homogeneous chemicalreactions can also occur. Thesegenerate reaction layers which

i hi R i lare quite thin. Reaction layerthickness depends on magnitudeof chemical rate constant.

If l t d s t t d th • If electrodes are rotated the solution near the electrodesurface moves. This generatesa hydrodynamic layer A similar

InterfacialDouble Layerregion

Bulk solutiona hydrodynamic layer. A similarlayer is generated if theelectrolytic solution flows past or impinges on a stationaryregion or impinges on a stationaryelectrode.

Page 14: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Fundamentals of the electrochemical measurement.

Current is passed betweenworking and countergelectrodes. The potentialis measured betweenworking and reference

Referenceelectrode

electrodes/.

CounterWorkingelectrode

Counterelectrode

Page 15: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Negative electrode Positive electrodeNegative electrodeCathodeReduction

Positive electrodeAnodeOxidation

Electrolyte:I i d t

Electrode:Ionic conductorElectronic

Conductor

N t R tiNet Reaction

Page 16: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Potentiostat

P i l f d iPractical setup for dynamicElectrochemical measurements:Potentiostat and electrochemical cell.R f t JS P Ch m Expt C liRefer to JS P.Chem Expt. on CyclicVoltammetry.

Page 17: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Electrochemical cells

EC Cells are a tribute to the art of glassblowing.

Page 18: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Typical electrode configurations.yp f g

Page 19: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Typical range of electrode size.Small size = new chemistry and physics !Better space, time and energy resolution.

Standard electrodeStandard electrodesize

Much current research

Lots of newChemistry here!

Page 20: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Nano-sized carbon fibre electrodes.

Chen & Kucernak, Electrochem. Commun. 4 (2002) 80-85Chen & Kucernak, J. Phys. Chem.B. 106 (2002) 9396-9404

l h

nma 9.0

Electrophoreticpaint

nma 38

NanoelectrodeThis is the JS voltammetry expt.Using nanosize electrodes.

Page 21: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

The Ying / Yang of ElectrochemistryIsolated oxidation and reduction processes are not much good. These reactions must be coupled together in some way to perform a technologically useful function.An electrochemical cell is formed by coupling together individual oxidation and reduction processes in a specific configuration to form a useful device.There are two types of electrochemical cells based upon the general thermodynamic nature of the reaction (expressed as whether the change in Gibbs energy is positive or negative.gy p gOxidation and reduction reactions occurring at individual electrode/electrolyte interfaces can be coupled together either to produce an electrical voltage or to produce chemicals.

El l llg p

• Galvanic cell.– This is an electrochemical power

source

• Electrolytic cell.– This is an electrochemical

substance producer.The cell does work by absorbing source.

– The cell does work by releasing free energy from a spontaneous reaction to produce electricity.

B tt

– The cell does work by absorbing free energy from a source of electricity to drive a non-spontaneous reaction.

El t th i• Battery• Fuel cell

• Electrosynthesis.• Electroplating.

Page 22: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry
Page 23: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

ElectrolysisElectrolysis.Redox reactions in which the change in Gibbs energy • Redox reactions in which the change in Gibbs energy G is positive do not occur spontaneously.

• However they can be driven via application of either a y ppknown voltage or a known current.

• Electrolysis is the process of driving a reaction in a non spontaneous direction by using an electric non spontaneous direction by using an electric current.

• Hence an electrolytic or driven cell is an electrochemical device in which an electric current electrochemical device in which an electric current from an external source is used to drive a non spontaneous chemical reaction.El l d h b f l h • Electrolysis provides the basis of electrosynthesis and industrial electrochemistry.

Weinberg, J. Chem. Ed.,60 (1983) 268-271.

Page 24: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

The electrolysis of waterwater

Overall (cell) reaction2H2O(l) 2H2(g) + O2(g)

Oxidation half-reaction2H2O(l) 4H+(aq) + O2(g) + 4e-

Reduction half-reaction4H2O(l) + 4e- 2H2(g) + 4OH-(aq)

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Page 25: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Electrolysis cell: electrochemical substance producer

Electrical energy produces chemicals.

Overpotential losses increase net electrical energy net electrical energy needed asinput to drive reactions at electrodes.

Thermodynamics (Nernst Potential)

Kinetics: Cathode reactionOverpotential

Ohmic potential: Cell design

,e cell C AE i E IR

Cell designNeed to minimizeall overpotentiallosses to makeapplied potential ,

Kinetics: Anodic reaction overpotential

applied potentialas close to Nernstpotential as possible.

Page 26: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Burke-Lyons Duplex Layer ModelOxide/solution interface

HYDROUSHYDROUSOXIDE

METAL COMPACTOXIDE

= O = Fe = Na+(aq) = OH-

[Ni(III)Om(OH)n]p- + OH- → [Ni(III)Om+1(OH)n-1]p- + H2O + e-

[Ni(III)Om+1(OH)n-1]p- → [Ni(III)Om OOH(OH)n-2](p-1)- + e- …RDS[Ni(III)Om OOH(OH)n-2](p-1)- + 2OH- →

[Ni(III)O O (OH) ]p- + H O + e-

Lyons Brandon OERMechanism (2010) Oxidized Ni [Ni(III)Om O2 (OH)n-1]p + H2O + e

[Ni(III)Om OO(OH)n-1]p- + OH- →[Ni(III)Om(OH)n]p- + O2 + e-

Oxidized Ni.

(a) M.E.G. Lyons, M.P. Brandon Phys.Chem. Chem. Phys., 11(2008) 2203. (b) M.E.G. Lyons, M.P. Brandon, J. Electroanal. Chem., 631 (2009) 62. (c) M.E.G. Lyons, M.P. Brandon, J. Electroanal. Chem., 641 (2010) 119.

Metal oxide plays intrinsic role in OER.

Page 27: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Photoelectrochemical cells or PECs are solar cells which generate electrical

The PEC cell consists of a semiconductor photo anode which is irradiated with electromagnetic radiation The counter energy from light, including visible light.

Each cell consists of a semiconducting photoanode and a metal cathodeimmersed in an electrolyte

electromagnetic radiation. The counter electrode is a metal. The following processes take place in the cell when light is incident on the semiconductor electrode:immersed in an electrolyte.

Some photoelectrochemical cells simply produce electrical energy, while others p d h d n in p ss simil t

1. Photo generation of charge carriers (electron and hole pairs)

produce hydrogen in a process similar to the electrolysis of water. 2. Charge separation and migration of the

holes to the interface between the semiconductor and the electrolyte and of electrons to the counter electrode through electrons to the counter electrode through the external circuit. Now, holes are simply vacancies created in the valence band due to promotion of electrons from the valence band pto the conduction band. However, in the study of electronic behavior of materials, "holes" are considered to be independent entities, with their own masswith their own mass.

3. Electrode processes: oxidation of water to H+ and H2O by the holes at the photo anode

d d i f H i H b l PEC Cell :Fujishima & Honda 1973

and reduction of H+ ions to H2 by electrons at the cathode.

Page 28: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

The lower yellow band is the valence band of the n-type semiconductor, while the upper yellow band is the conduction band The yellow band is the conduction band. The energy difference between the top of valence band and the bottom of conduction band is termed as the band gap of semiconductor, Eg. g p , gPhotons having energy greater than Eg are absorbed by the semiconductor and free electrons are generated in the conduction band and free holes in the valence band band and free holes in the valence band.

2hν = 2e- + 2h+

The electrons and holes are separated due to The electrons and holes are separated due to the potential generated at the interface of the semiconductor-electrolyte due to band bending. The holes move to the interface and greact with water producing oxygen:

2h+ + H2O = 1/2 O2(gas) + 2H+(aq)

The electrons travel in the external circuit and arrive at the interface between the counter electrode and electrolyte. There, they reduce the H+ ions to H : they reduce the H+ ions to H2:

2e- + 2H+(aq) = H2(gas)

The complete reaction is absorption of photon The complete reaction is absorption of photon and splitting of water into hydrogen and oxygen.

Page 29: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

h f f h PEC Some other configurations of the PEC cell are also possible:

1 The semiconducting material may be a 1. The semiconducting material may be a p-type material. In this case, it will act as photo cathode, and reduction of H+

ions to H2 will take place at this ions to H2 will take place at this electrode. The counter electrode may me a metal in this case.

2. Both electrodes, the cathode and anode, are photo active semiconducting materials. In this case, the n-type l d ill d d id i electrode will act as anode and oxidation

of water to oxygen and H+ will take place at this electrode. The p-type electrode

ill t s th d h H+ i s ill b will act as cathode, where H+ ions will be reduced to H2.

http://www.nature.com/nature/journal/v414/n6861/pdf/414338a0.pdf

Page 30: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Industrial scale electrolysis: Chlor-alkali process.

We consider the electrolysis of brine We consider the electrolysis of brine (a concentrated NaCl solution). This electrolysis process converts an inexpensive salt to valuable chemicals inexpensive salt to valuable chemicals such as H2 , Cl2 and NaOH. In a modern electrolysis unit, a steelcathode and a titanium anode coatedcathode and a titanium anode coatedwith a conductive mixed metal oxidesuch as RuO2/TiO2 is generally used.The latter are termed dimensionallyThe latter are termed dimensionallystable anodes (DSA).Molecular hydrogen is evolved at theCathodeCathode.

2 22 2 2H O e H g OH

Recent developments in electrochemicalThe solution in the anodecompartment is usually acidified toH 3 4 i d h

pm mresearch have led to the development ofhigh surface area nickel or nickel alloycathodes which function more effectively.

pH 3-4 in order to suppress theevolution of molecular oxygen .Hence the net cell reaction is :y

The anode reaction is chlorine evolution. 22 2Cl Cl g e

2 2 22 2 2H O Cl H g Cl g OH

Page 31: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Now NaOH is produced directly in the cathodecompartment. This must be prevented frommixing with the anolyte solution becausehypocholorite and chlorate would be generated asside products if NaOH reacted with theside products if NaOH reacted with theelectrogenerated Cl2 .In the diaphragm cell an asbestos diaphragmseparates the anolyte and catholytecompartments . However only dilute NaOH isproduced using such a cell.In the yet more technologically advancedmembrane cell a cation selective ion exchangemembrane cell, a cation selective ion exchangemembrane is used instead.The latter allows Na+ ions to pass through fromthe anode compartment to the cathodecompartment, but there is no flow of solutionbetween the compartments. Concentrated NaOHmay be produced using this type of membranesystem There is currently much research effortsystem. There is currently much research effortconducted into the design of membrane materialswhich exhibit optimal performance under celloperating conditions.

Page 32: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Chlor-alkali electrolysis is a technique for the industrial production of chlorine and the alkali known as caustic soda production of chlorine and the alkali known as caustic soda (sodium hydroxide) from brine, a solution of common table salt (sodium chloride) in water. Three processes are in use: the diaphragm-cell process, p p g p ,the membrane-cell process, and the mercury-cell process.

Page 33: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Venkatesh & Tilak, J. Chem. Ed.,60 (1983) 276-278.

Page 34: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

In the diaphragm-cell process, a porous diaphragm divides the electrolytic cell, which contains brine, into an anode compartment y pand a cathode compartment. When an electric current passes through the brine, the salt’s chlorine ions and sodium ions move to the electrodes. Chlorine gas is produced at the anode, and sodium ions at the cathode react with the water, forming caustic soda. Some salt remains in the solution with the caustic soda and can be removed at a later stage.

In the membrane-cell process, the compartments are separated by a membrane rather than a diaphragm. Brine is pumped into the anode c mpartment and nly s dium i ns pass int the cath de c mpartment compartment, and only sodium ions pass into the cathode compartment, which contains pure water. Thus, the caustic soda produced has very little salt contamination.

In the mercury-cell process, mercury, which flows along the bottom of the electrolytic cell, serves as the cathode. When an electric current passes through the brine chlorine is produced at the anode current passes through the brine, chlorine is produced at the anode and sodium dissolves in the mercury, forming an amalgam of sodium and mercury. The amalgam is then poured into a separate vessel, where it decomposes into sodium and mercury. The sodium reacts with water in decomposes into sodium and mercury. The sodium reacts with water in the vessel, producing the purest caustic soda, while the mercury returns to the electrolytic cell.

Page 35: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

The Hydrogen Economy:Hydrogen as an energy carrierHydrogen as an energy carrier.

G.W. Crabtree, M.S. Dresselhaus, M.V. Buchanan, ‘The hydrogen Economy’ Physics Today, Dec.2004, pp.39-45.U B l ‘D h d k U. Bossel, ‘Does a hydrogen economy make sense?’ Proc. IEEE, 94 (10)(2006), pp.1826-1836.

http://www.foresight.gov.uk/Energy/hydrogen_and_fuel_cells_towards_a_sustainable_future.pdf

P.P. Edwards, V.L. Kuznetsov, W.I.F. David,N. Brandon. Energy Policy 36(2008) 4356-4362.

Page 36: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

• Fuel cells are devices which convert chemical energy directly to Fuel Cells : Introduction

Fu c ar c wh ch c n rt ch m ca n rgy r ct y t electrical energy. This is very different from conventional combustion based power plant which convert chemical energy to thermal energy, then thermal energy to kinetic energy, and only then kinetic energy to electrical energythen kinetic energy to electrical energy.

• The thermal to kinetic and kinetic to electrical conversion stages have efficiency losses associated with them which engineers have have efficiency losses associated with them which engineers have spent the last 150 years or so trying to reduce. The key loss however is in the combustion process (chemical to thermal stage). Due to the inherent thermodynamics of combustion there is an y fultimate efficiency which cannot be exceeded by any combustion engine - The Carnot Limit - a limit which does not apply to fuel cells.

• Using hydrogen as a fuel (which can be extacted from hydrocarbon fuels or renewable sources) a fuel cell electro-chemically oxidises the hydrogen using oxygen from the air generating electricity and the hydrogen using oxygen from the air generating electricity and some heat.

• The fuel cell makes more efficient use of the fuel and produces • The fuel cell makes more efficient use of the fuel and produces fewer pollutants e.g. reduced nitrogen oxides and carbon dioxide emissions, and no particulates.

Page 37: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Hydrogen/oxygen fuel cell

2H2(g) 4H+ + 4e-

Anode reaction

H2/O2 fuel2H2(g) 4H+ + 4e-

Anode reaction

H2/O2 fuel2H2(g) 4H+ + 4e-

Anode reaction

H2/O2 fuelH2/O2 fuelcell

H+ ionConducting

H2/O2 fuelcell

H+ ionConducting

H2/O2 fuelcell

H+ ionConductingConductingmembraneConductingmembraneConductingmembrane

Cathode reactionCathode reaction

O2(g) + 4e- +4H+ 2H2OO2(g) + 4e- +4H+ 2H2OO2(g) + 4e- +4H+ 2H2O

,eq cellG nFE Remember CH1101

, , ,eq cell eq C eq AE E E emember H

Electrochemistry:

Page 38: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Ballard PEM Fuel Cell.

Page 39: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

PEM Fuel Cell

Difficult to modeldue to complexstructuredinterphase regioninterphase region.

Page 40: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

,e cell Anode cathodeE i E IR

F ll l d f ll dFall in output potential as current is drawn from cell causedBy kinetic, transport and cell design factors.

Page 41: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Self-driving Fuel CellOverpotential losses reduce net voltage output.

iiEP

,e cell C AE i E IR

Page 42: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry
Page 43: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

SWCNT as catalytic platformsSWCNT as catalytic platforms

Page 44: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Kinetic and Mechanistic analysis SWCNT/Pt nanoparticle compositesnanoparticle composites

Page 45: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

This is where much of current Fuel Cellof current Fuel Cellresearch is at .

Simple version of MeOH oxidation.

Page 46: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Large number of ETgsteps implies largenumber of adsorbed reactionintermediates, hence havecomplex multistep reaction.

Methanol oxidation is trulycomplex !

Page 47: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Photoelectrochemical Cell Overall (cell) reaction2H2O(l) 2H2(g) + O2(g)

(PEC)Reduction half-reaction

2H2O(l) 2H2(g) O2(g)

e-

S TiO

Reduction half reaction2 H+(aq) + 2e- H2(g)

e-

e

H

SrTiO3

KTaO3

TiO

metal

O2H2e -TiO2

SnO2

Fe O

HERKinetics rapid

H2O H2Oh+

LiquidSC Metal OxideAnode

Fe2O3

OERKinetics slowR t li iti LiquidRate limiting

Oxidation half-reaction

Light is Converted to Electrical + Chemical Energy

4OH-(aq) O2(g) +2H2O + 4e-

Page 48: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

In recent years dye sensitized solar lls (DSSC) ‘G ät l’ lls h

DSSC Systems

cells (DSSC) or ‘Grätzel’ cells have attracted considerable interest worldwide due to their low production cost, excellent stability (ca.10 years , y ( yuseful lifetime in outside applications) and high efficiency to convert solar energy into electricity (typically 10%) electricity (typically 10%). Energy conversion in a DSSC device is based on the injection of an electron from a photoexcited state of the

*TiO S h TiO S

psensitizer dye (typically a bipyridine metal complex) into the conduction band of a nanocrystalline semiconductor (typically TiO2) 2 2

2 2* CB

TiO S h TiO S

TiO S TiO S e

TiO S e TiO S

(typically TiO2). These cells employ a liquid electrolyte (usually an iodide/triiodide I-/I3

- redox couple dissolved in an organic solvent)

2 2

2 2 33 2 1 2

1 2 3 2

CBTiO S e TiO S

TiO S I TiO S I

I e I

p g )to reduce the dye cation thereby regenerating the ground state of the dye. Regeneration of iodide ions which are oxidized in this reaction to tri iodide 3

3

1 2 3 2

2 3Pt

CB

I e I

I e I

are oxidized in this reaction to tri-iodide is achieved at a platinized counter electrode.

Page 49: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Light absorption is accomplished by a monolayer of photoactiveLight absorption is accomplished by a monolayer of photoactivedye (S) adsorbed chemically at the semiconductor surfaceand excited by interaction with an incident photon of light .After excitation (S*) the excited dye transfers an electronAfter excitation (S ) the excited dye transfers an electronto the semiconductor by an injection process.The efficiency will depend on the relative energy levels betweenthe semiconductor and the dye and on the kinetics of thethe semiconductor and the dye and on the kinetics of theelectron transfer process at the interface.For efficient cell operation the rate of electron injectionmust be faster than the rate of decay of S*. Furthermore themust be faster than the rate of decay of S . Furthermore therate of re-reduction of the oxidized sensitizer, the dye cation S+

by the electron donor (I-) in the electrolyte must be higher thanthe rate of back reaction of the injected electrons with the dyej ycations, as well as the rate of reaction of injected electronswith the electron acceptor (I3

-) in the electrolyte.Finally the kinetics of the reaction at the counter electrode mustalso guarantee the fast regeneration of the charge mediator (I-)or this reaction could also become rate limiting in the overall cellperformance.

Page 50: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

M Gratzel, Inorg. Ch m 44 (2005) 6841Chem., 44 (2005) 6841-6851.P.V. Kamar, J.Phys. Chem B 106 (2002) Chem.B., 106 (2002) 7729-7744.

Page 51: Physical & Interfacial Electrochemistry 2013 CH3035... · Physical & Interfacial Electrochemistry 2013 Professor Mike Lyons Main d h l BChemistry Building Room 3.2 School of Chemistry

Lecture SummaryLecture SummaryI thi l t h i t d d t ifi • In this lecture we have introduced two specific classes of electrochemical systems which have important device implications :mp mp– Galvanic Cells

• Spontaneous chemical reactions produce electrical voltageElectrolytic cells– Electrolytic cells

• Applied voltage causes non spontaneous chemical reaction• Both systems involve charge transfer at the y g

interface between an electrode and an electrolyte.• We also have provided a survey of aspects of

electrochemical energy converters: fuel cellselectrochemical energy converters: fuel cells.