METHODS OF MEASUREMENTS IN ELECTROCHEMICAL ENGINEERING

Dr. Manuel A. Rodrigo

Department of Chemical Engineering. Facultad de Ciencias Químicas. Universidad de Castilla La Mancha. Campus Universitario s/n. 13071

Ciudad Real. Spain.

Department of Chemical Engineering.

Universidad de Castilla La Mancha.

Spain

ESSEE 44th European Summer School on Electrochemical Engineering

Palić, Serbia and Montenegro17 – 22 September, 2006

CONTENTS

1. CURRENT DISTRIBUTION1.1 Importance of current distribution visualization1.2 Measurement of current distribution

1.2.1 TYPES OF MEASURING METHODS1.2.2 PARTIAL-CELL APPROACH1.2.3 SUBCELLS APPROACH1.2.4 SEGMENTED ELECTRODES1.2.5 RESISTORS NETWORK1.2.6 PRINTED CIRCUIT BOARD APPROACH1.2.7 TYPES OF MEASUREMENTS OF THE LOCAL CURRENT IN PASSIVE RESISTOR NETWORK1.2.8 MATHEMATICAL MODELLING1.2.9 MAGNETOTOMOGRAPHY

1.3. Some new applications: calculation of mass diffusion overpotential distribution in a PEMFC

2. MEASUREMENT OF MASS TRANSFER COEFFICIENTS BY ELECTROCHEMICAL TECHNIQUES2.1 Why?2.2 How?2.3 Typical setup for measuring average cell mass transfer coefficients2.4 Experimental procedure2.5 Calculation of the mass transfer coefficient

3. LOCAL MASS TRANSFER DISTRIBUTION3.1 Importance of local mass-transfer distribution visualization3.2 Limit current mapping3.3 Measurement of mass transfer by electrochemiluminiscence3.4 Mathematical modelling

4. WALL SHEAR STRESS4.1Importance of wall-shear stress distribution visualization4.2 Measurements of wall-shear stress4.3 Measurement of local shear in three-phase fluidized beds4.4 Wall shear stress in multiphase flow

1. CURRENT DISTRIBUTION

It is one of the more important parameters in the performance of an electrochemical cell, but unfortunately in the electrochemical industry and in the electrochemical literature, current distribution has not received the attention that it deserves

Uniform current distribution

Non-uniform current distribution

Through the wire flows the same current, but the current distribution on the electrode

surface is different

I

I

1.1 Importance of current distribution visualization

Electroplating: non-uniform current distribution can cause a local variation of the thickness of the deposited metal

Some examples of the importance of uniform current distribution

Electrolyses cell

Aluminium surface after an electro-dissolution process

Small contact-surface current feeder

Current efficiency 100% Current efficiency 83.3%

i 1/3i 2/3 i

Part of these electrons are consumed by an electrochemical side reaction because the desired reactant does not arrive to the anode surface at the required rate

(if reagents arrives to the electrode at the same rate that they are consumed)

non uniform corrosion of electrodes

Poor efficiency, changes in the products conversion ratio

Causes of non-uniform current distribution in FC during fuel cell operation:

inhomogeneities in the reactant concentration, contact pressure, temperature distribution, water management along the flow field etc.

Produce maximum power densitiesEnsure maximum lifetime for the cell components

PEM fuel cell

Uniform current distribution

Current scale

Examples of local current distribution in a circular-shape electrode

low

high

uniform

Factors affecting current distribution:

Geometry of the cell system. Current feeders or collectors

Conductivity of the electrolytes and the electrodes.

Activation overpotentials at the electrodes which depend on the electrode kinetic.

Concentration overpotentials which are mainly controlled by the mass transport processes.

Other factors

1.2 Measurement of current distribution

electrodes electrolyte

Load or power supply

b) Current distribution in one electrodea) Current distribution in the cell

anode cathode

Purpose of the measurement

Invasive methods

Partial approaches Subcells Segmented electrodes Passive resistor network

Non-invasive methods

Mathematical modelling

Magnetic measurements

Is the cell modified for the measurement? (is current distribution measurement associated with constructional modifications of the cell? )

no

yes

1.2.1 TYPES OF MEASURING METHODS

Portions or segments of the cell are tested independently by inactivating other portions.

1.2.2. PARTIAL-CELL APPROACH.

The inactivation can be carried out either by masking or by other procedure (e.g. in FC some parts of the MEA can be prepared without catalyst

electrodeelectrolyte

electrode

the specific performance is determined by difference.

subcell 3 inactive subcell 1 inactive

Subcell 1 Subcell 2 Subcell 3

To increase the accuracy more partial cells should be studied

Advantages: very simple, easy to manufactureDisadvantages: it can only be used as a first approach

CE

LL V

OLT

AG

EINTENSITY

whole cell

Subcell 1 inactive

Subcell 3 inactive

Several electrically isolated subcells are placed are conveniently placed at different locations in the cell

1.2.3. SUBCELLS APROACH

a section of the anode is punched out

a section of the cathode is punched out

Main cell

subcells

The step is repeated in several determined locations inside the cell

The former anodes and cathodes are replaced with smaller ones.

The resulting empty space is filled with a isolating gap

L ma

in c

ell

L 1L nL m

Subcell 1 Subcell n Subcell m Main cell

The subcells are separately controlled. To measure current distribution every subcell voltage has to be adjusted to fit approximately the mail cell voltage

AdvantagesGives more information on a much smaller scale about the localised current density than

the partial approach

Disadvantages Complex manufacture. Great care has to be taken to ensure proper alignment during assembly of the

cell

CE

LL V

OLT

AG

EINTENSITY

SUBCELL 5

MAIN CELL

SUBCELL 3

electrolyte

Segmented electrode or segmented BPP (in a FC)

Measurement circuits

isolation

1.2.4. SEGMENTED ELECTRODES

This approach allows a very accurate current distribution

mapping

Coverage of the whole electrode area

Good spatial resolution

Counter electrode

Piece of electrode

Volt-meterohmic resistor

To assume a high ratio between through-plane and in-plane conductivity segmented electrodes must be manufactured in a thin shape. This generates problems related to mechanical strength

Example of measuring device for each piece of electrode

Very invasive method. It can affect significantly to the current distribution. Big differences can exist between the measure and the actual current distribution

Passive resistor network

current Volt-meter

Buss plate

electrode

Coverage of the whole electrode area

Good spatial resolution

DrawbacksElectrical properties of the resistors depends on temperature

1.2.5 RESISTORS NETWORK

Main advantage: It does not require any modification of the electrodes (or of the BPP or MEA in FC)

It is less invasive

Main problem - appearing of lateral currents

Buss plate

electrode

Advantages

Improved mechanical strength

Resistor matrixIsolated wires

To assume a high ratio between through-plane and in-plane conductivity resistor matrix must be manufacture in a thin shape. This generates problems related to mechanical strength

Completely isolated resistors

Buss plate

electrode

Resistor matrixIsolated wires

Advantages

Less affected by in-plane current distribution

interconnected resistors

1.2.6 PRINTED CIRCUIT BOARDS APPROACH

Current collector

Through-holes

backside

frontside

current

Easy to manufacture

Possibility of multilayer manufactureEasy to add electrical components

Can be used as BPP in FC

1.2.7 TYPES OF MEASUREMENTS OF THE LOCAL CURRENT IN PASSIVE RESISTOR NETWORK

passively

Ohmic resistors

Hall-effect sensors

Current transformers

activelyMultichannel potentiostats

(only measure)

(Measure and manipulation)

Ohmic resistors

current Volt-meter

Very simple Frequently used Very invasive. It can affect the cell current distribution

The figure shows a thin sheet of semiconducting material (Hall element) through which a current is passed. The output connections are perpendicular to the direction of current. When no magnetic field is present, current distribution is uniform and no potential difference is seen across the output.

When a perpendicular magnetic field is present,a Lorentz force is exerted on the current. This force disturbs the current distribution, resulting in a potential difference (voltage) across the output. This voltage is the Hall voltage (VH). Its value is directly related to the magnetic field (B) and the current (I).

Hall-effect sensors

Hall effect sensors can be applied in many types of sensing devices. If the quantity (parameter) to

be sensed incorporates or can incorporate a magnetic field, a Hall sensor will perform the task

When a current-carrying conductor is placed into a magnetic field, a voltage will be generated perpendicular to both the

current and the field. This principle is known as the Hall effect.

Current follower circuit

To working electrode

To data acquisition card

-+

-+

Standard operational amplifier circuit for current-to-voltage conversion

For very low currents

Current distribution model

simulation

experiments

Experimental conditions

Modelled results

experimental results yes

no

Agreement?

e.g. product conversion

1.2.8 MATHEMATICAL MODELLING

e.g. New proposal

1.2.9 MAGNETOTOMOGRAPHY

xcellvision - Instrumentation for Fuel Cells and Fuel Cell System Simulation

Patented technologyNon invasive method

Sensor 1

Sensor 2

Sensors are used for magnetic field data acquisition as a function of the position. The experimental setup allows the sensor to measure the magnetic field strength (H) at different positions around the cell

xy

z

n

1n

2

1

nm1n

m111

n

1n

2

1

I

I

...

I

I

aa

...

aa

H

H

...

H

H

Hi

Ij

high

low

high

lowMap of the current intensity

2. MEASUREMENT OF MASS TRANSFER COEFFICIENTS BY ELECTROCHEMICAL TECHNIQUES

2.1 Why?

Electrode surface

currentConcentration of the electroactive species

high

low

Bulk solution

influence the current distribution

Affect to the product distributionAffect to the efficiency

e-

Electro

de

e-

Ssurface

R

F·nA·j

r melectroche

)SS(Akr surfacebulkmtransfermass

Sbulk

2.2 How?

46

36 )()( CNFeeCNFe

Typical concentration 5 mM of ferrocyanide and 20mM of ferricyanide to make sure a cathodic controlled electrochemical process

A large quantity of inert electrolyte (NaOH, Na2SO4, KSO4, …) has to be added as supporting electrolyte to minimize the migration effects (to make them negligible compared to diffusion and convection)

The area of the anode should be larger than that of cathode for a cathodic controlled-process

The method is based on a diffusion-controlled reaction at the electrode surface:

If the cathode is used as a

probe

2.3 Typical setup for measuring average cell mass transfer coefficients

The flow rate is measured by the

rotameter.

A

V

The reservoir contains the electrolyte

The electrical energy is applied with the power supply

connected to the electrodes

The pump propels the electrolyte through the electrochemical cell.

The heat exchanger keeps the electrolyte temperature at the

desired set point.

The electric measurement devices are used to obtain

high accuracy of voltage and current values, than those

provided by the power supply.

Oxygen and hydrogen generated in the electrochemical cell can be stripped with nitrogen.

The heterogeneous processes take place in the electrochemical cell, where mass transfer processes are studied.

Rotameter

Power SupplyElectrolyteReservoir

N2

Anode

Cell

Cathode

Pump

Heat Exchanger

RotameterRotameter

Power SupplyPower SupplyElectrolyteReservoir

N2N2

Anode

Cell

Cathode

Anode

Cell

Cathode

Cell

CathodeCathode

PumpPump

Heat Exchanger

Cur

rent

me

asu

red

Applied potential

Cb

0

V

I

Distance from the electrodeC

once

ntra

tion 0

0

a) No potential is applied to cell. No current

2.4 Experimental procedure

Cur

rent

me

asu

red

Applied potential

Cb

0

V

I

Distance from the electrode

Con

cent

ratio

n

0

0

a) Small potential is applied to cell. No current

Cur

rent

me

asu

red

Applied potential

Cb

0

V

I

Distance from the electrode

Con

cent

ratio

n

0

0

b) Potential scan begins

Cur

rent

me

asu

red

Applied potential

Cb

0

V

I

Distance from the electrode

Con

cent

ratio

n

0

0

Cur

rent

me

asu

red

Applied potential

Cb

0

V

I

Distance from the electrode

Con

cent

ratio

n

0

0

I limit

c) Current limit is reached

Cur

rent

me

asu

red

Applied potential

Cb

0

V

I

Distance from the electrode

Con

cent

ratio

n

0

0

I limit

d) Plateau zone

Cur

rent

me

asu

red

Applied potential

Cb

0

V

I

Distance from the electrode

Con

cent

ratio

n

0

0

I limit

Cur

rent

me

asu

red

Applied potential

Cb

0

V

I

Distance from the electrode

Con

cent

ratio

n

0

0

I limit

e) Other electrochemical processes (e.g. Electrolyte decomposition)

Cur

rent

me

asu

red

Applied potential

Cb

0

V

I

Distance from the electrode

Con

cent

ratio

n

0

0

I limit

e-

Electro

de

e-

Ssurface=0

R

F·nA·j

r limmelectroche

bulkmsurfacebulkmtransfermass S·A·k)SS(Akr

bulkm SFn

jk

limit

Sbulk

transfermassmelectroche rr

bulkmlim S·A·k

F·nA·j

2.5 Calculation of the mass transfer coefficient

3. LOCAL MASS-TRANSFER DISTRIBUTION

Why?

How?

By measuring the limit current at different positions on the electrodeBy using other techniques

Mass transfer greatly influence

current distribution

Mass transfer can be easily improved in a cell by using

turbulence promoters

Local mass transfer distribution can depend on a lot of factors:Design of the inlet

Design of the outletFlow characteristics

Turbulence promotersSmooth or uneven surfaces

…

Local mass transfer distribution can depend on a lot of factors:Design of the inlet

Design of the outletFlow characteristics

Turbulence promotersSmooth or uneven surfaces

…

3.1 Importance of mass-transfer distribution visualization

V voltmeter

A

A

A

ammeter

Push-buttonswitch

Power supply

cathode anode

3.2 Limit current mapping

Drawback many measuring sites

Arrays of microelectrodes

Corner plate

centre

Total current

Current of the main electrode

Current of microelectrodes

Measuring device

resistor

3.3 Measurement of mass transfer by electrochemiluminescence

Direct electrolyses

H2O2

+ N2 + light

Iridium tin dioxide electrode

Direct electrolyses

Very slow rate

H2O2

3.4 Mathematical modelling

Mass transfer distribution model

simulation

experiments

Experimental conditions

Modelled results

experimental results yes

no

Agreement?

e.g. product conversion

e.g. New proposal

4. WALL SHEAR STRESS

Theories of wall turbulence considers the existence and

interaction of turbulent bursts, ejections, sweeps and wall

streaks. A turbulent bursts is an ejection of fluid from the wall,

which also causes fluid to impige on the wall by simultaneous

formation of sweeps, or movement of fluids towards the

wall. Turbulent bursts and sweeps occur through the

formation of vortices and the lift-up of wall streaks.

4.1 Importance of wall-shear stress distribution visualization

JfnI ··limit In the diffusion regime Faraday’s law allows to link the mass flux to the wall of electroactive ions (J) to the limit current

)()( 'limitlimit limit

tIItI

Analyses of mass flux fluctuations

Statistical analyses of this parameter allows to obtain important information concerning the turbulent transfer characteristics within the viscous sublayer

Information about the wall turbulence in the

viscous sublayer

Traditional methodsLaser doppler anemommetryParticle imaging velocimetryThermoanemometry

Turbulent flow visualization

Electrochemical method

Main advantage

Schematic description of initiation of flow induced localized corrosion phenomena

metal

Oxide layer

y

u·

4.2 Measurements of wall shear stress

flow

anodecathode

Diffusion boundary layer

Viscous boundary layer

The electrochemical method is based on measurement of mass transfer coefficients. This coefficients are related to velocities in the proximity of the probes

A small dimension probe allows the measurement of only a local velocity gradient which can be related to local wall shear stress.

u(t)

microelectrodeThe time-dependent diffusion limited current density correlates with the time-dependent gradient of the streamwise flow velocity perpendicular to the wall which is proportional to the wall shear stress

N

H

This method can be applied with high resolution using microelectrodes or microelectrodes arrays incorporated flush and isolated into flat surfaces exposed to tangential flows

u(t)

3/1limit aI

S

cc

0

y

y

uS

microelectrode

N

H

For a newtonian fluid with dynamic viscosity the wall shear stress can be expressed

Local wall shear gradient

c, concentration of the electroactive species

3

12

·····8075.0

l

SDcAFnI

3

123

2

······8075.0 SDclLFnI

Levêque formula (valid for a circular electrode of area A)

Extension of the Levêque formula for a non circular electrode: L length of the electrode in the flow direction (m), l length of the electrode transverse to the flow direction (m)

D, diffusion coefficient (m2s-1), n number of electrons exchanged in the electrode reaction, F Faraday constant (96500 C/mol)

3

12

limit )( SDI

For a steady-state flow, the small electrode mounted flush with the insulating wall delivers a current I. This measured intensity increases with the applied potential between the two electrodes until the process becomes controlled by the diffusion of the reacting species to the surface of the working electrode. Then the value of the intensity is the limiting current. The probes behaves as a perfect mass sink

The single wall probe is applicable only for nonreversing conditions

If flow reversal occurs in the proximate wall flow region and additional information about the

flow direction is needed a “sandwich probe” should be used

The size of this probe should be equal or smaller to the typical size of the large flow structures to ensure homogeneity

The sandwich probe consists of two active

segments separated in the mean flow direction by a thin

insulating gap

Photolithography probes

To current followers

xz

100 m

i1

i2

X velocity component i1 + i2

Z velocity component i1 - i2

Counter electrode

Insulating gap

Plastic sphere

Support rigid tubeGold wire

4.3 Measurement of local shear in three-phase fluidized beds

Gas slug Gas slug

-1

-2

-5

i(A)

Liquid current limit

Gas current limit

Bubble flow Annular flowSlug flow

4.4 Wall shear stress in multiphase flow

Printed circuit board

Cooling channels

Shunt resistors

MEA +GDL

Anodic BPP

Cathodic BPP

Shunt resistors are integrated into the PCB using a multilayer design PCB can be easily manufactured in a way that guaranties the compatibility with the elements of the cellHigh flexibility to modular configuration (the same PCB can be used to study different configurations of the cell)The sense wires associated with the individual resistors can be integrated into the PCB and connected to the data acquisition system from the edge of the PCBThe invasive method does not affect to the fluid dynamic properties of the reactant gases and the electrical and thermal conductivity of the cell are not importantly modified.PCB can be introduced inside a BPP. This enable to measure current distribution in a stack

Shunt resistors are integrated into the PCB using a multilayer design PCB can be easily manufactured in a way that guaranties the compatibility with the elements of the cellHigh flexibility to modular configuration (the same PCB can be used to study different configurations of the cell)The sense wires associated with the individual resistors can be integrated into the PCB and connected to the data acquisition system from the edge of the PCBThe invasive method does not affect to the fluid dynamic properties of the reactant gases and the electrical and thermal conductivity of the cell are not importantly modified.PCB can be introduced inside a BPP. This enable to measure current distribution in a stack

Current collectors

Conductive layer (backside of the PCB)

load

1.3. Some new applications: calculation of mass diffusion overpotential distribution in a PEMFC

Direction of charge flux

V

Cel

l pot

entia

l

ElectrolyteANODE CATHODE

a +

diff

a + + reaction

UNIFORM OXYGEN CONCENTRATION

OF OXYGEN ON THE CATHODE BY FLOW

PULSE APROACH AND SEGMENTED-

ELECTRODE APROACH

CURRENT INTERRUPTION

METHOD

CURRENT DISTRIBUTION

MEASUREMENT WITH UNIFORM

OXYGEN CONCENTRATION

CELL RESISTANCE

MATHEMATICAL MODEL

MASS-DIFFUSION OVERPOTENTIAL

DISTRIBUTION

In PEMFC uneven current distribution are caused by non uniform

oxygen distribution inside the fuel cell

j,concjjj0 ir)iln(bEE

)iln(bEE c,0rev0

jhom,jjhom,0hom ir)iln(bEE

)ii(r)i

iln(bEE jjhom,j

j

jhom,homj,conc

To ensure that the oxygen concentration along the reaction surface is uniform, the flow pulse has to be strongly over stoichiometric and long enough to remove all excess water from the electrodes. At the same time the duration of the flow pulse must be short enough in order not to change the resistance of the proton conductive phases of the MEA

To ensure that the oxygen concentration along the reaction surface is uniform, the flow pulse has to be strongly over stoichiometric and long enough to remove all excess water from the electrodes. At the same time the duration of the flow pulse must be short enough in order not to change the resistance of the proton conductive phases of the MEA

CONDITIONSCell operated galvanostaticallyFor each current the cell was allowed to stabilize and then the current distribution was measuredA oxygen flow pulse of 10 s is introduced and the current distribution is measured again

CONDITIONSCell operated galvanostaticallyFor each current the cell was allowed to stabilize and then the current distribution was measuredA oxygen flow pulse of 10 s is introduced and the current distribution is measured again

MATHEMATICAL MODEL