Dynamic Electrochemistry - Cairo University 2020-05-31 · Electrochemistry Deals with studying interchange of chemical and electrical energy. studies chemical reactions involving

10/9/2018

Ahmad M. Mohammad 1

Ahmad Mahmoud Mohammad Alakraa, Ph.D.

Professor of Physical [email protected]

http://scholar.cu.edu.eg/?q=ammohammad/classes/

Dynamic ElectrochemistryChem 317 (Part II) ‐ Spring 2018

Office Hours: Mondays 12:00 –2:00 Wednesdays 12:00‐2:00

Chemistry New Building ‐ 1st Floor

References1‐ Chemistry: An Atoms First Approach, Steven S. Zumdahl

and Susan A. Zumdahl, 2012, Brooks Cole, a part ofCengage Learning.

2‐ Chemistry: The Central Science, Theodore L. Brown et al.,2012, Pearson Prentice Hall, USA

3‐ Chem 317 Note from the Department of Chemistry

CreditLevel One : First Semester

Code Subject Pre‐requisite

Practical (hr/wk)

Lecture (hr/wk)

Total cr

Contact hr

Chem317 Dynamic Electrochemistry

‐ 1 (1 × 1) 1 1

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Assessment

A 2h unseen written examination carries 60% of the totalmark

Midterm exam carrying 20% of the total mark. Two quizzes carrying 20 % of the total mark.

Passing Criteria 60% for the total course mark

Lectures attendance should exceed 70% in order to attend the final unseen examination

Electrochemistry Deals with studying interchange of chemical andelectrical energy.

studies chemical reactions involving electron transferin solutions at the interface of an electron conductor (ametal or a semiconductor) and an ionic conductor (theelectrolyte).

is primarily concerned with two opposite processesinvolving oxidation–reduction reactions

Applications Batteries , Corrosion, Electrolysis (aluminum, andCl2 production), Analysis (ion selective electrodes)

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Reversible electrochemical Cells Cells whose cell reactions can be get reversed when anexternal emf (Eext) greater than its capacity (Ecell) isapplied.

The current drawn from it is infinitesimally smallbecause the cell reactions remain virtually in a state ofequilibrium.

It obeys thermodynamic conditions of reversibility.If Eext = Ecell , current will not flow from/to the celland no chemical reactions will take place in the cell.If Eext < Ecell a little , very small current will flow fromthe cell and a small chemical change will occur.If Eext > Ecell a little , very small current will flow to(opposite direction) the cell and a small chemicalchange will occur.

Reversible electrochemical Cells

Metal/metal ion electrodes Metal/insoluble metal salt electrodes Gas electrodes Oxidation‐reduction electrodes (quinhydroneelectrode)

Examples

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Reversible Cells/ Example Daniel cell (E0= 1.1 V).

Daniel cell, E0= 1.1 V If an external emf less than 1.1 V is applied, currentwill flow from the cell according to

If an external emf of 1.1 V is applied, the cell reactionstops and no current flows (I = 0).

If an external emf higher than 1.1 V is applied, the cellreaction will be reversed and current will flow inopposite direction.

⟶ .

⇌ .

⟶ .

Spontaneous/Galvanic

Non‐Spontaneous/Electrolytic

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Eext < 1.1 V

e’s flow from Zn rod to Cu rod; hence, current flows

from Cu to Zn

Zn dissolves at anode and Cu deposits at cathode

No e’s flow and current flows

No chemical reactions

Eext = 1.1 V

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Ahmad M. Mohammad 6

Eext > 1.1 V

e’s flow from Cu rod to Zn rod; hence, current flows

from Zn to Cu

Zinc is deposited at the Zinc electrode and copper

dissolves at copper electrode

Irreversible electrochemical Cells Cells whose cell reactions can not be reversed when anexternal emf (Eext) greater than its capacity (Ecell) isapplied.

It does not obey thermodynamic conditions ofreversibility.

Example The Zn/H+/Ag cell

⟶ →

Anode

Cathode →

Or

If Eext > Ecell was applied, the reactions will not be reversed because H2 was escaped from the system.

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Galvanic/Electrolytic A galvanic (voltaic) process: generation of an electriccurrent from a spontaneous chemical reaction.

An electrolytic processes: uses a current to producechemical change.



⇌

The direction of electron flow in electrolytic cells is reversed and the sign, but not the magnitude, of the cell potential is reversed.

Galva

nic cells

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Energy released by spontaneous redox reaction is converted to electrical energy

Electrical energy is used to drive non‐spontaneous redox reaction

Oxidation half‐reaction:Y Y+ + e

Oxidation half‐reaction:Z Z + e

Reduction half‐reaction:Z+ + e Z

Reduction half‐reaction:Y+ + e Y

Overall cell reactionY + Z+ Y+ + Z (G <0)

Overall cell reactionY+ + Z Y + Z (G >0)

Galvanic Cell Electrolytic cellconverts chemical energy into

electrical energy.converts electrical energy into

chemical energy.Spontaneous redox reaction non‐spontaneous redox reaction

The two half‐cells are set up in different containers, being connected through the salt bridge or porous partition.

Both electrodes are placed in a same container in the solution of

molten electrolyte.

The anode is negative and cathode is the positive

electrode. The reaction at the anode is oxidation and that at

the cathode is reduction.

The anode is positive and cathode is the negative electrode. The

reaction at the anode is oxidation and that at the cathode is

reduction.Electrons are supplied by the species getting oxidized. They

move from anode to the cathode in the external circuit.

The external battery supplies the electrons. They enter through the cathode and come out through the

anode.

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Electrodes/Cell potential The electrode at which oxidation occurs is called the anode;

the electrode at which reduction occurs is called thecathode.

A galvanic cell consists of an oxidizing agent in onecompartment that pulls electrons through a wire from areducing agent in the other compartment.

The “pull,” or driving force, on the electrons is called the cellpotential (Ecell ), or the electromotive force (emf) of the cell.

The unit of electrical potential is the volt (abbreviated V),which is defined as 1 joule of work per coulomb of chargetransferred.

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Example

Potential when all solutes are at 1 M

and all gases are at 1 atm.

Standard electrode Potential

Example

Oxidation /Anode

Reduction/Cathode

. ⟶ .

⟶ . .

. ⟶ .

. ⟶ .

. ⟶ . .

⟶ ⟶

Employing red. pot.

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Electrolysis of molten

NaCl



⇌ (g)

Oxidn./Anode

Redn./Cath.⟶ .

⟶ .

⟶ .

Electrolysis Condition

The strongest reducing agent (Cl, having the highest reduction E0) will undergo oxidation. The strongest

oxidizing agent (Na+) will be reduced. If aqueous NaClwas used instead, H+ ions would undergo reduction

instead of Na+, because it is a stronger oxidizing agent.

⟶

⟶ .

in contrast to Galvanic cells

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Two‐electrode electrolytic cells

Salt bridge

For electrolysis, it is possible to have both electrodes separatedor not separated (if mixing products does not matter)

Electrodes’ PolarityThe electrodes in electrolytic and galvanic cells have opposite polarities. Nevertheless, in both, the anode is the electrode at which the oxidation occurs and the cathode is that at which the

reduction occurs.

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Origin of electrical double layer (DL)1‐ Ionization: results in excess es at the electrode surface

and hydrated +Ve ions that are attractedelectrostatically to the Ve charges at theelectrode surface forming the DL.

2‐ Specific adsorption: of non‐hydrated Vely chargedanions e.g., Cl , SCN (chemisorption)producing Ve charges at the electrode surfacethat attract +Vely charged hydrated cations toa distance of closest approach, hence, formingthe DL .

3‐ Adsorption of oriented dipoles: as water and that occursin the direction of the electric field.

Electrode

Electrolyte

Electrode

Electrolyte

Electrode

Electrolyte

IonizationOriented dipoles

Specific Adsorption

The distance of closest approach is 5 Ao. The charge separation inducts a potential drop acrossthe DL 1V. The potential field is therefore equal:∅

Very large

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Ei in two‐electrode electrolytic cells

Working Electrode The electrode under

consideration

Reference Electrode Its potential is practically

constant Electron transfer through its

interface is extremely fast

Polarizable/non‐polarizable electrodes Electrodes can be divided into two types: polarizableand non‐polarizable electrodes.

ideal polarizable electrode: Non‐Faradaic processes

is featured with no faraday current flow when theelectrode potential is varied.

usually can be used as Working or Counter electrodes. It has a very high polarization or charge transferresistance

Once the electrode potential is changed, the Faradaycurrent flows out.

It has almost a zero polarization resistance andinfinitely large exchange current density.

can be used as Reference electrodes.

ideal non‐polarizable electrode: Faradaic processes

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Polarizable electrodes‐ Equivalent circuits

An ideal polarizable electrode can berepresented by a capacitor(condenser) in equivalent circuit.

C

C

Rhi

In reality, an extremely weak currentflows at an actual polarizable electrode.Hence, a high resistance (Rhi) is connectedin parallel with the capacitor

Each electrode (Pt, Au, GC) has a potential window satisfying this feature

Polarizable electrodes‐ Equivalent circuits

CRhi

RF

If a redox species coexisted at the polarizable electrode,another redox reaction would occur and Faraday currentwould flow. In this case, a potential depending variableresistance (RF) should be added to the equivalent circuitin parallel. Furthermore, the effect of the speciesdiffusion should be included, and a Warburg Impedanceelement is connected to Faraday resistance (RF) inequivalent circuit.

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Ei in three‐electrode cells

WECE

RE

CE: counter (Auxiliary) electrode: completes thecircuit to measure the current without impeding itspassage, e.g., Pt sheet.

Electrode kineticsOne has to analyze all the steps involved in the wholeredox process to decide the slowest step that determinesthe speed of the process.

Polarization methodsIn polarization experiments potential–current curves arerecorded and then analyzed to obtain the kineticcharacteristics of the redox reaction. Mass transfer coefficient for reversible processes Heterogeneous rate constants for irreversible

processes. Both for quasi‐reversible processes

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Classic “Steady” method is suitable for irreversible and quasi‐reversible (i.e.,

slow charge transfer) processes.

Potential (current) is varied and the correspondingsteady current (potential) is recorded point by pointuntil the data for the polarization curve are collected.

Is simple and easy (not using sophisticatedinstrumentation) (advantages)

Is time consuming (disadvantage)

Relaxation method is suitable for all processes and time saving

(advantages). A potential (current) is applied and the instantaneous

current (potential) is recorded as a function of time. Analysis of E‐t or I‐t curves yields mass and charge

transfer characteristics. Polarization (E‐I) curves can be constructed

automatically by the simultaneous recording of thepotential of WE with respect to RE and instantaneouscurrent passing between WE and CE.

If E‐I curves were recorded slowly, results will besimilar to steady state measurements.

E.g., Amperometry, Coulometry, Cyclic voltammetry. It may also involves the application of AC signal to

with a DC signal to analyze impedance spectra.

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Number of electrons involved in a redox processFaraday’s law:

m: mass of the electroactive substance (g).i: current density passed (A)t time of the processes (s)Mw: molar mass of the electroactive species (g/mol)n: number of electrons involved (mol1 of electroactivesubstance)F: Faraday’s constant 96500 C/mol of esQ: charge or amount of Electricity (C)

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A blank experiment is necessary

Q’ : the charge consumed in presence of the supporting electrolyte and in absence of the electroactive species

Q : the charge consumed in presence of the supporting electrolyte and the electroactive species

To subtract the background and the effect of impurities

Conditions of reversibility

⟶ Anode

Cathode →

AgNO3

At equilibrium /Reversibility

i+

i

⇌

The potential (typically measured against a reference electrode) under a condition of reversibility (Inet = 0) is called the equilibrium (EI = 0) or reversible (Erev) potential

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Cathodically

polarized

Polarization (p): deviation from equilibrium

Anodically

polarized

Anodic: the electrode is connected to the +Ve pole of an electrical source

Cathodic: the electrode is connected to the Ve pole of an electrical source

Irreversible

Under

polarization

Anodic/Cathodic Polarization

≫≫

Anodic Polarization

≪≪

Cathodic Polarization

+ve current

ve current

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Polarization (p)/Overpotential ()

: is called also the open circuit potential

: is equilibrium potential calculated using Nernst equation

For a single redox process

Overpotential,

is the extra potential over the equilibrium potential that drives the process in a specific direction at a

given rate

Overvoltage,

Sign convention of Anodic polarization , a > 0

Cathodic polarization , c< 0

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Rate of electrochemical reaction

Is a measure for the rate of redox processes running at the anode and cathode of the

electrolytic cell.

Current density (A cm2):

→

Inspect the unit of

Rate of electrochemical reaction

can easily probe the rate of electrochemical reactions.

→

Current density (A cm2):

As electrochemical reactions concern with thematerial change and charge transfer at theElectrode/electrolyte interface, equations concernwith surface area of the electrode not the volumeof the electrolyte in the rate’s calculations.

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Steps of an electrochemical reaction→

Mass transfer (diffusion, migration, convection) ofreactants from the bulk solution to the double layer.

Possible chemical reaction (dissociation of a molecule,dehydration of an ion, structural change,crystallization of a metal or a salt, …etc).

Adsorption of reactants onto the electrode surface. Charge transfer (Electrode/reactants) across thedouble layer.

Desorption of the product off the electrode surface. Possible chemical reaction (hydration of an ion,association of a molecule, …etc).

Mass transfer (diffusion, migration, convection) ofProducts from the electrode surface to the bulksolution.

Electrode

Ox’adsne

Chemical Rx

e transfer

Red’ads

Ox’ OxsurfMass Transfer

Oxbulk

Red’Chemical Rx

Redsurf Redbulk

Surface region 0.5 1.0 nm

Diffusion layer 10 500 µm

Any of these steps may be limiting (rate determining)

generate

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Concentration (diffusion) polarization, c

Results when the concentrations of reactants/productsat the electrode surface are different under non‐equilibrium conditions from their values underequilibrium (bulk) values.

It grows and decays slowly on application andinterruption of current at a rate characteristic of thediffusion coefficients of the species involved.

Is the only form of µ affected by stirring and isunaffected by the nature of electrode surface.

Activation (charge transfer) polarization, ct

The polarization needed to promote the ECR in onedirection and retard it in the opposite direction.

It appears as a change in the value of potentialdifference across the electrical double layer betweennon‐equilibrium and equilibrium.

Ohmic (Resistance) polarization,

Results from the current flow in resistive electrolytes(Rsol) and/or electrodes (Re: oxide films or salts, gasbubbles, grease, dirt, ..etc).

It appears and disappears instantaneously when thepolarizing current is imposed or disconnected

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If only , c and ct are controlling

c

ct

The degree of reversibility in a given ECR depends on the rates of mass and charges transfers involved

Reversible IrreversibleQuasi‐reversible

Reversible processesRate of charge transfer >>> Rate of mass transfer

Reversible processes stays close to equilibrium (Nernst equation applies) regardless of the passing current.

ct = 0. In absence of ,

Mass transfer controlled

c

Irreversible processesRate of charge transfer <<< Rate of mass transfer

c = 0. In absence of ,

Charge transfer controlled

ct

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Quasi‐reversible processesRate of charge transfer is comparable to rate of

mass transfer

In absence of , c +

ct

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Dynamic Electrochemistry - Cairo University 2020-05-31 · Electrochemistry Deals with studying interchange of chemical and electrical energy. studies chemical reactions involving