fuel cells measurement guide

8/10/2019 Fuel Cells Measurement Guide

1/16

Reference Guide to Electrical

Measurement of Fuel Cell TestingFuel Cell Measurement Basics and Tips from Measuring Instrument Experts


2/16

Introduction

In recent years, the increased demand of concern on the environment issues has accelerated the research and

development in the field of Fuel Cell technology.

The Fuel Cell technology has a boundary area in between the side of material/chemical engineering and

electric/electronic engineering on the other side. It is often heard that the methodology used in each specialized field

cannot be applied directly and the objectives of measurements are unclear in the research and development of fuel cells.

This Reference guide offers to make a contribution in response to those who has such opinion as "we are familiar

with the things that we must do to improve the fuel cell characteristics and the research issues, but we are having

difficulties for configuring an environment to perform such research."

We hope that this booklet will contribute a wider knowledge of electrical measurement of Fuel Cell Testing in

helping the reader with better understanding.

The target readers of this booklet are;

1. For Chemical, Electrical and Electrochemical engineers and researchers who evaluate fuel cells those are close to

being commercialization.2. For Electric or electronic engineers who operate and evaluate fuel cells.

i


3/16

Contents

Introduction i

Chapter 1 Measuring and Testing Fuel Cells 1

1.1 Fuel Cell Test System ..........................................................................................................................................1

1.1.1 Gas Supply System 1

1.1.2 Temperature Control 1

1.1.3 Humidity Control1

1.1.4 Cooling System1

1.1.5 Load System 1

1.2 Fuel Cell Operation..............................................................................................................................................1

Chapter 2 Static Tests (I-V Measurements) 2

2.1 Measuring System Configuration ........................................................................................................................2

2.1.1 Overview of the Electronic Load Unit2

2.1.2 Digital Voltmeter3

2.2 Before Running the Test System..........................................................................................................................3

2.2.1 Connecting to a Power Supply with Sufficient Capacity 3

2.2.2 Ground the System properly3

2.2.3 Isolation from Surrounding Noise Sources4

2.2.4 Load Cables 4

2.3 Fuel Cell Polarization Curve................................................................................................................................5

2.3.1 OCV (Open Circuit Voltage) 5

2.3.2 Internal Resistance (Ohmic Resistance) 6

2.3.3 Limiting the Current value 6

2.3.4 Estimation of Life Time 6

2.4 Notes Concerning the Electric Measurement System..........................................................................................6

2.4.1 Resulting from Load Cables6

2.4.2 Related to Sensing Wires6

2.4.3 Measuring the Current Using a Shunt6

2.5 Conditions of Fuel and Air for I-V Measurements ..............................................................................................7

Chapter 3 Dynamic Test (Impedance Measurement) 7

3.1 Method of Impedance Measurement ...................................................................................................................7

3.2 AC Impedance Method........................................................................................................................................7

3.2.1 Equipment Used for the measurement system8

3.2.2 Notes Concerning the Electric Measurement System 8

3.3 Current Interrupt Method.....................................................................................................................................9

3.3.1 Equipment Used9

ii


4/16

iii

3.3.2 Notes Concerning the Electric Measurement System 9

3.4 Impedance Measurement ...................................................................................................................................10

3.4.1 Impedance Measurement Using the AC Impedance Method 10

3.4.2 Impedance Measurement Using the Current Interrupt Method 11

3.5 Conditions of Fuel and Air for Impedance Measurements ................................................................................11

Acknowledgements 12


5/16

Chapter 1 Measuring andTesting Fuel Cells

Fuel cell test system is typically designed and manu-factured by companies that specialize in the test station

and deliver to the end users. Under the present circum-

stances, the measuring method of system varies depends

on the company. However, the most basic parameters

which are classified in the size of the load current, the

amount of reactive gas supplied, and the control of the

relative humidity and cell temperature.

1.1 Fuel Cell Test System

Fuel cell test system is to exclusively measure the

characteristics or performance of fuel cells. Fuel cell test

system typically consists of the system for a gas supply,temperature control, humidity control, cooling, load, and

the like.

1.1.1 Gas Supply System

The gas supply in fuel cell test system consists of

gas lines and their control units (mass flow meters, sole-

noid valves, etc.). The gas lines in the measurement sys-

tem typically include oxidation gas lines, fuel gas lines,

and measurement system purge gas lines. The oxidant is

typically oxygen or air. The fuel is typically hydrogen

gas, reformed gas, or methanol. Nitrogen gas is typically

used to purge the gas supply system. The gas supply sys-

tem has functions that control the pressure, flow rate, and

utilization rate of the fuel and oxidant.

1.1.2 Temperature Contro l

The temperature control uses thermocouples and

controller modules to control the fuel cell temperature,

gas inlet and outlet temperature, outlet temperature of the

fuel cell coolant, and the like.

1.1.3 Humidi ty Contro l

For PEMFCs, water management is critical to main-

tain the high conductivity of electrolytic membranes.

Therefore, humidification is applied to the electrolytic

membranes. There are various humidification methods

from the popular bubblers in small systems of a labora-

tory scale and application of high-pressure steams in

mid-size to large systems. The humidity control system

is used to control the relative humidity of fuel gas and

oxidation gas and ensures the stable operation of fuel

cells.

1.1.4 Cooling system

The temperature of fuel cells rises during operation

due to the heat generation. Therefore, it is necessary tokeep the temperature down of the fuel cells constantly

during operation. A typical cooling method is the

water-cooled system in which water is circulated through

the system. If the test system is large, a secondary

cooling system may be required.

1.1.5 Load System

Electric energy is generated during fuel cell opera-

tion. Normally, the energy that the fuel cell generates is

consumed using an Electronic load. The performance of

fuel cells is typically tested by varying the amount of

load current.

1.2 Fuel Cell Operation

A fuel cell is a device that generates a type of

chemical energy converted into electric energy. It is a

highly efficient power supply with environmental pollu-

tion free and it can be considered as a generator with fuel

and oxidant as inputs. For an efficient and a stable opera-

tion of fuel cells, it is necessary to control parameters

such as the temperature of the fuel cells under the opera-

tion and the humidity, flow rate, and utilization rate of

reactive gases. When the fuel cell operation is stopped, it

is necessary to stop the reaction completely and extract

the excess and absorbed gases to ensure safety. A typical

method is the nitrogen purge, which injects nitrogen gas

to draw out the excessive fuel and oxidant.

1


6/16

Chapter 2 Static Tests(I-V Measurements)

I-V tests measure the electromotive force by varying

the current over time.

2.1 Measuring System Configuration

Fig. 2.1 shows an outline of a measuring system

configuration. The main component of the electric sys-

tem is the electronic load unit. Digital voltmeters,

multi-channel A/D conversion boards, and scanners are

added if high accuracy is required for the electromotive

force or if electromotive force of each stack cell is to be

monitored.

PC

GasControl

SystemFuel

Cell

Electronic

Load

Digital

VoltMeter

Air or Oxygen

Hydrogen

Control

Control / Data Aqq.

Control /

Data Aqq.

Current

Fig. 2.1: Measurement system configuration

2.1.1 Overview of the Electronic Load Unit

The fundamentals of I-V tests are to arbitrary set

and run a load current from the fuel cells to be consumed

by a load and measure the electromotive force generated.

Variable resistors were used for this purpose before elec-

tronic load units were widely available. If the DUT (De-

vice Under Test) closely resembles a constant voltage

source, in other words the internal resistance is small and

the voltage is constant, there are not too may problems.

However, when measuring the DUT in which the internal

resistance changes such as in fuel cells, the controllabil-

ity to keep the current constant is poor when variableresistors are used. This is because the controlled parame-

ter is resistance. Moreover, because the allowable power

consumption varies depending on the resistance, it is

difficult to change the current over a wide range.

The electronic load unit is an application of an elec-

tronic circuit called constant-current circuit. Because

of the high level of controllability due to the fact that the

controlled parameter is current and not resistance and

because it is an electric circuit allowing automation, it

has become widely used today. Fig. 2.2 shows an outline

of the internal configuration of an electronic load unit.

Fig. 2.2: Outline of an electronic load unit

As an electronic load unit is based on a constant

current circuit, CC mode (Constant Current Mode), is its

basic operation mode. And its current direction performs

first-quadrant operation sunk only from the high poten-tial end. An electronic load unit has a minimum operat-

ing voltage that must be applied to achieve constant cur-

rent operation. This minimum operating voltage is an

important parameter when selecting an electronic load

unit. A load unit of 0 V input type 1 has a built-in power

supply unit for biasing that offsets the minimum operat-

ing voltage so that constant current operation is possible

even if the output voltage of the DUT is 0 V. For the case

of single-cell to several cell stacks, this type of electronic

load unit is convenient. On the other hand, because the

consumable power per volume decreases, you must se-

lect an appropriate electronic load unit by carefully

considering the actual voltage and current that will beused.

Electronic load units also equip with a function of

measuring the input DC voltage. This function should be

applicable, if high accuracy and precision are not so ma-

jor issue in the voltage measurement (such as monitoring

1 Our 0-V type products have the letter "A" attached to their model

name such as PLZ164WA and PLZ664WA.

2


7/16

aging properties).

The ratings of an electronic load unit are often indi-

cated as W, V, and A. This does not mean

that the electronic load unit is capable of flowing

A at V. The power mainly indicates its performance.When it is focused specifically in terms of accuracy and

precision for current measurement, it should be noted thatthe greater value of power does not serve the lesser value

of power. The actual operating range in which tests are to

be performed must be reviewed, and the appropriate elec-

tronic load unit must be selected.

2.1.2 Digi tal Vol tmeter

Because voltmeters are designed exclusively to

measure voltages, the accuracy of voltage measurement

is several grades higher than the function internal to an

electronic load unit. If you seek high accuracy in voltage

measurements, a digital voltmeter should be used. In

recent years, voltmeters even with 6 1/2-digit precisionhave become quite ordinary.

If the item being measured is a stack and voltage

measurement is to be performed on each cell simultane-

ously, a multi-channel A/D converter or scanner is

convenient. Our KFM2151 FC Scanner is designed for

such applications.

One parameter that we need to pay attention is the

input impedance. In some small fuel cells, the input

impedance of the instrument cannot be ignored in which

case we cannot assume the digital voltmeter to be an

open circuit electrochemically. Most digital voltmeters

have input impedance around 1 M (sufficient for gen-

eral applications), but the ability to switch a higher

impedance depending on the model. If you want to meas-

ure voltages at extremely high impedance, care must be

exercised in selecting the appropriate model. This should

not pose a problem for mid-size to large fuel cells.

2.2 Before Running the Test System

2.2.1 Connecting to a Power Supply with

Sufficient Capacity

The power required to operate a basic electronicload unit (not a 0 V input type) is primarily used for

internal control and the power used to drive the cooling

fan. This power is not that large. However, for a 0 V in-

put type, an additional power to drive the bias power

supply is necessary, and this power is quite large.

Additionally, we can expect that the whole fuel cell

evaluation system would consume considerable amount

of power by auxiliary systems that control the gas flow

and temperature as well as heater and electromagnetic

valves. If the power supply capacity is just enough to

supply the consumed power during steady-state opera-

tion, the possibility for a circuit breaker and the like to be

cut off increases if the consumed power increases tran-

siently. A power supply cutoff in a condition in which the

load current is flowing is undesirable from the load unit

point of view, because the risk of the unit breaking in-

creases. In addition, it is possible that the breaker will

not trip unless the rating is exceeded by a great amount.

Or, it is possible that the breaker will not trip during the

equipment startup or warm-up and will finally trip after

entering the test phase. It is essential to provide a power

supply with enough capacity to run the entire equipment.

When using a 200-240V line voltage, a basic elec-

tronic load unit can be operated from a standard outlet,

because the consumed current is relatively small and a

thick power input cable is not necessary. However, a

connection through an outlet leads possible accidentssuch as disconnecting cable by stepping over the cable.

If the power to the load is ON when such accidents occur,

it may cause to interrupt the test, moreover, it could dam-

age the fuel cell and/or the load unit. This is undesirable

in terms of safety. For a system configured using a 0 V

input type electronic load unit, a power input cable of

significant thickness is necessary, because the consumed

current from the built-in bias power supply is large.

Though the mobility of the equipment is sacrificed, we

recommend that a terminal be crimped to a three-wire

cab-tire cable and this terminal be screwed to a terminal

board. We recommend that this is the safe and practical

method.

2.2.2 Ground the System properly

An electronic load system is designed with the

premises to be securely grounded. If the system is not

grounded, electric shock may occur if you touch the case

or a spark may be generated when connecting a signal

line from another grounded instrument. Particularly for a

fuel cell evaluation system, this condition is extremely

dangerous because fuel gas may accumulate in the sur-

roundings and may trigger an explosion. Grounding the

electronic load system is a must for safety reasons.

Grounding is also important in terms of measuringpoint of view, because we can expect the tolerance of the

system to various types of noise to increase and the

reproducibility of measurements to improve.

3


8/16

2.2.3 Isolation from Surrounding Noise

Sources

The ON/OFF of a temperature-control heater, open-

ing and closing of electromagnetic valves, and the like

are electrical noise sources. These types of noise may

affect the system through the power line or through spaceas electromagnetic waves. It is important that the elec-

tronic load system be laid out and installed in a way that

minimizes the effects from noise. For example, the

following points should be exercised as much as possi-

ble.

Not bundle the power cable for the control equip-

ment that turns ON/OFF large amount of power and

the power cable for the electronic load system.

Use separate systems of input power lines.

Install them as far as possible.

The above should be also referred to load cables and

sensing wires.

2.2.4 Load Cables

It is often thought that load wiring is simply a matter

of connecting cables between the fuel cells (DUT) and

the input to the electronic load unit. This is indeed true

on the connection diagram. However, in an actual elec-

tronic load system, this connection is vital and forms the

basis for stable system operation and highly repeatable

measurements. This section describes the points to be

considered concerning load wiring.

The cables used to connect the fuel cell output to the

input terminal of an electronic load unit is quite large

compared to the cables you see in everyday life (exclud-

ing small-scale tests such as DMFC), because of the

large current that the cables can handle. A cable is

consisting of conductors that run current and covering

materials. Conductors are normally copper or tin wires.

If the cable wire is single-wire cables with a single

conductor, it is inconvenient because they cannot be bent

easily. Therefore, twisted wires consisting of multiple

thin wires (referred to as element wires) are typically

used. Covering is provided on the outside of the conduc-

tors for insulation. Various insulators are used for the

covering such as PVC, polyethylene, and synthetic rub-ber.

The capacity of the current that a cable can run var-

ies depending on the covering material, temperature in-

crease in the conductors, and the like. If a current

exceeding the allowable current capacity of the cable is

run and the temperature increases above the allowable

temperature, it can cause melting of the covering, smoke,

fire, carbonation, etc. The temperature increase in

conductors is mainly dependent on its resistance2, and

the conductor resistance is mainly determined by its

thickness. The thickness of conductors is expressed by

the cross-sectional area. If multiple thin wires are bun-

dled, it is the sum of the individual cross-sectional areas.

The cross-sectional area is typically indicated using mm2

or AWG (American Wire Gauge). Table 2.1 shows the

allowable currents by cross-sectional area for low volt-

age indoor wiring as given in the Technical Standard for

Electric Facilities in Japan.

Nominal

cross-sectional

area

AWG (Refer-

ence

cross-sectional

area) [mm2]

Allowable

current

[A]

Kikusui-

recommended

current

[A][mm2]

2 14 (2.08) 27 10

3.5 12 (3.31) 37 -

5.5 10 (5.26) 49 20

8 8 (8.27) 61 30

14 5 (13.3) 88 50

22 3 (21.15) 115 80

30 2 (33.62) 139 -

38 1 (42.41) 162 100

50 1/0 (53.49) 190 -

60 2/0 (67.43) 217 -

80 3/0 (85.01) 257 200

100 4/0 (107.2) 298 -

125 - (-) 344 -

150 - (-) 395 300

200 - (-) 469 -

250 - (-) 556 -

325 - (-) 650 -

Table 2.1: Nominal cross-sectional area of wires and

allowable currents

The Kikusui-recommended current specified in Ta-

ble 2.1 allows for margin taking into consideration how

the electronic load unit is used such as higher increases

in temperature when wires are twisted together.3 The

cross-sectional area of wires used in a typical power strip

is less than or equal to 2 mm2with an allowable current

of around 15 A. A wire with a cross-sectional area of

around 80 mm2 applies to the allowable current of 200A

and which diameter of wire is about 2 cm as the size of

thickness. Because technical standards for electric facili-

ties vary depending on the country, be sure to meet the

standards for the country in which the equipment is used.

In an actual wiring, there is a section where the end

of the cable makes contact with the electrode, and a con-

2 The resistance is obviously not zero.3 This is because the heat dissipation area is less than that of a single

wire.

4


9/16

tact resistance is present at that location. Like the

conductor resistance, the contact resistance cannot be

ignored if a large current is run through the cable. If the

contact resistance is large, the contact area heats up when

a current is applied and causes adverse effects on the

covering around the contact. As a typical, low-cost, and

reliable method, cables are usually connected to the fuel

cells or the input terminal of the electronic load unit us-

ing crimp terminals with screws or bolts. In this method,

the end of the cable with the covering stripped is inserted

into a terminal called a crimp terminal and compressed

together using a dedicated crimping tool. Then the crimp

terminal is fastened to the electrode or the like of the fuel

cells using a screw or bolt. For large currents, multiple

screws are used to secure the cable. There seems to be

cases in which Cannon connectors for large currents are

used, but they are not as typical as crimp terminals. In

addition, depending on the type of fuel cell, the tempera-

ture of the output terminal of fuel cells increases whenfuel cells are activated. To ensure safety, the cable must

be connected to a location where the output terminal

temperature is less than the allowable temperature of the

cable. The allowable temperature limit of a typical cable

is at around 105 C. If the specification of cable exceeds

its temperature limit, it may be a special design cable

which could be hardly acquired and managed for the

laying arrangement.

In addition to heat, thick cables are considerably

heavy. For example, 1 m of an 80-mm2 cable weighs

about 1 kg. If a 200-A current is wired over 5 m, 10 m of

80-mm2 cables are necessary, because two cables are

needed for positive and negative polarities. This meansthat the total weight of the cables is 10 kg. The cable can

no longer be moved as easily as a household power strip

and it could be heavier than the tested fuel cells them-

selves. If the cables are not fixed in place, it is possible

that the fuel cells may be pulled by the cables and fall

from the measurement bench or a gas pipe may become

loose. Therefore, consideration must also be given to

weight when laying and fixing the load cables.

Though it might not be a problem if the load current

is constant or changes slowly, however, the wiring also

exists inductance in addition to resistance which can

produce reactance. Reactance is like a resistor that takes

effect selectively in alternating current. Given frequencyf [Hz], inductance L [H], and reactance XL, their

relationship is expressed as XL = 2 fL []. Unlike

resistance, reactance does not consume power. However,

reactance causes the current phase to lag the voltage

phase, and large reactance can cause the electronic load

unit to oscillate. The cable inductance is mainly

determined by its shape and dimensions. The inductance

increases as the cable becomes longer or narrower and as

the loop formed by the cables is larger. Therefore, it is

desirable that the wiring be as short as possible. The

cable length should be kept less than 3 m (6 m total for

positive and negative polarities). Laying the cables

closely together or twisting the cables if it is possible

suppresses the inductance (reactance).

Below is a summary of this section.

Use a cable with a diameter suitable for the test cur-

rent.

Prepare the terminals appropriately and secure the

terminals with sufficient torque and enough screws,

bolts, or the like.

Take the weight of the cables into consideration and

lay and fix the cables in a way that avoids excessive

mechanical stress.

Use the shortest cables possible and lay the cables

closely together. If possible, twist the cables.

2.3 Fuel Cell Polarization Curve

Fig. 2.3 indicates a typical I-V curve.4

Fig. 2.3: A typical I-V curve

In general, we can determine the basic characteris-

tics of fuel cells from the polarization curve (I-V curve)

that is obtained through fuel cell tests.

2.3.1 OCV (Open Circui t Vol tage)

The voltage that is present when the load current is

0 A is called the OCV (Open Circuit Voltage). The volt-

age difference between this voltage and the logical value

of the electromotive force is called the activation over

voltage, and this is an indicator of whether the catalytic

agent is working effectively.

The actual OCV is closely related to the membrane

thickness and perforation.

4 It is also referred to as a Tafel plot.

5


10/16

In fact that power is consumed and heat is generated

means that a voltage is generated in the specified area ofthe fuel cell. This voltage is generated in the opposite

direction to the fuel cell voltage (electromotive force)

and results in the following condition.

2.3.2 Internal Resistance (Ohmic Resistance)

In Fig. 2.3, as the load current (density) is gradually

increased, the terminal voltage decreases drastically at

first, then eventually, the voltage start to decrease in

proportion to (linearly to) the current density. The V/I

in this section indicates the static internal resistance. Ascan be seen from the graph, the static internal resistance

is an ohmic resistance, which is a sum of the bulk

resistance of the substances standing between the anode

and cathode and the parasitic resistance such as contact

resistance.

Voltage at the load unit input terminal < voltage atthe fuel cell output terminal

Because the voltage of the fuel cell or the power sup-

ply appears as if it has dropped as viewed from the load

input terminal, this phenomenon is called a voltage drop

in the world of electronics. If this voltage drop is large,

the voltage at the load input terminal may fall below the

minimum operating voltage of the electronic load unit. If

we do not keep this phenomenon in mind, it is possible

to repeat the following actions.

2.3.3 Limi ting the Current value

As the current (density) is increased further, the

terminal voltage starts to decrease drastically again. This

region determines the limit value of practical current.

Stop the load current because something seems to be

wrong.2.3.4 Estimation of Life Time

Currently, there seems to be no unified indicator that

is widely accepted as a way to estimate the service life of

fuel cells. The terminal voltage gradually decreases when

CC measurements is performed of which the current

flows constantly for long period of time. A typical way to

estimate the service life of fuel cells is to measure the

voltage level that drops over a given number of hours,

calculate the drop in the electromotive force per unit

operating time, and estimate the time it takes for the

terminal voltage to drop to a predefined value.

The voltage drop disappears and the minimum opera-

tion voltage is cleared.

Restart the load current.

2.4 Notes Concerning the ElectronicMeasurement System

2.4.1 Resulting from Load Cables

Up to this point, we have stated that there are

following type of cables;

load cables have cable resistance (resistance in theconductor)

contact resistance

Those type of cables generate the heat when load

current flows through these resistances, and electronic

load units have a voltage called a minimum operating

voltage, and

The voltage at the load input terminal falls below the

minimum operating voltage once again.

It is important to pay attention and recognize that

cables are too thin or large contact resistance may result

in a large voltage drop and that the voltage at the loadinput terminal may fall below the minimum operating

voltage due to the voltage drop if tests are performed

using large currents on fuel cells operating very close to

the minimum operation voltage.

2.4.2 Related to Sensing Wires

As explained in the previous section, load cables

have resistance, and voltage drop occurs when current

flows through them. In other words, the voltage at the

fuel cell output terminal does not match the same value

of voltage at the input terminal of the electronic load unit.

Therefore, voltage sensing cables must be connected tothe fuel cell output terminal to measure the correct DC

voltage.

2.4.3 Measuring the Current Using a Shunt

The load current is measured on the electronic load

unit. However, if high accuracy is required, insert a

four-terminal resistor for measuring current called a

the performance of electronic load units is not war-ranted if the voltage at the load input terminal is less

than the minimum operating voltage.

6


11/16

7

shunt in the load current line. The voltage across the

shunt is measured and converted into current.

Because a shunt is a resistor, it generates the heat

while the current is drawn. If the current flow exceeds

the allowable limit through the shunt continuously, the

shunt will burn out. Even if it does not burn out, its value

of resistance will change permanently.

Shunts which handle for large currents are physi-

cally large, and it takes time to reach the temperature to

be stabilized. The resistive part of a shunt is made of

material with small temperature coefficient, but it is not

zero meaning that caution must be exercised when

measuring large currents. In addition, because the sense

voltage is 50 mV to 100 mV at full scale,5the sense volt-

age will be in the order of V for small currents. There-

fore, care must be exercised in the measurement.

A current transformer or a current sensor that uses a

current transformer is sometimes used to measure large

currents. In the past, the use of a current transformer wasavoided in current measurements requiring accuracy and

precision, because the linear characteristics of a current

transformer were often inferior to those of a shunt. How-

ever, some of the recent products can make measure-

ments at an equivalent level, and we believe that it will

gradually gain popularity in the future.

2.5 Conditions of Fuel and Air for I-VMeasurements

As explained in the last chapter, fuel cells must

supply fuel according to the load current. The amount of

supplied fuel must be varied according to the changes inthe load current. However, the speed of response of a

fuel cell control system is typically slower than the speed

for the variation of load current. Therefore, care must be

exercised in the speed in which the load current is varied

(swept).

5 The rated maximum current.

Chapter 3 Dynamic Test (Im-pedance Measurement)

If we refer to the resistance found from I-V

measurements as static impedance, impedance measure-

ment is to measure the dynamic impedance.

3.1 Method of Impedance Measure-ment

There are two methods of impedance measurement

that can be conducted today. They are the AC impedance

method and the current interrupt method.

The AC impedance method is to superimpose an AC

current component for measurement on the load current

and determines the impedance from this current and the

AC voltage that appears across the measured object.The current interrupt method is to interrupt the load

current at high speeds for a short interval and determines

the impedance from the voltage change that results.

Several other methods are available, but this booklet

will only cover these two most popular methods used

today.

3.2 AC Impedance Method

This section will explain the AC impedance method

in more detail.

The AC impedance method is to superimpose an AC

current component for measurement on the load current,it measures this superimposed AC current used to make

measurements and the AC voltage that appears across the

measured object, and performs signal processing to sepa-

rate the superimposed signal into a voltage amplitude

component of the same phase and a voltage amplitude

component of orthogonal phase, and calculates the resis-

tive component R and reactance component jX of the

impedance. From R and jX, the impedance magnitude|Z|

and phase angle are derived through ari thmetic.1 In

principle, it is possible to use CV operation2 that super-

imposes an AC current used to make measurements and

derives the impedance by measuring the resulting AC

current that flows. From an electrochemical point ofview, the CV measurement is desirable. However,

performing CV measurements on low impedance devices

such as fuel cells that are actually in operation is difficult

1 The difference between Cartesian coordinates and polar coordinates.2 Constant voltage operation.


12/16

in terms of electronic engineering.3 Therefore, CC meas-

urement is used in most cases for devices that measure

impedance while running a load current greater than 10

A.

3.2.1 Equipment used for the measurement

system

It is possible to construct a measurement system by

combining an electronic load unit for superimposing an

AC current signal used to make measurements and a

frequency response analyzer (FRA) with a DC electronic

load unit. However, it is assumed that it may be difficult

for the user to calibrate this system after the test system

is constructed.

Our test system, consisting of model KFM2150, FC

Impedance Meter, combined with model PLZ-4W Series,

Electronic Load, has been calibrated as a whole system

when it is delivered to customers, so it reduces a burdenof the user.

3.2.2 Notes Concerning the Electronic Meas-

urement System

Reduction of mutual induction

The coupling of the sensing loop and load current

loop becomes higher.

The AC current used to make measurements be-

comes larger.

The measurement frequency becomes higher.

Therefore, we must also make the sensing loop area as

small as possible. As for the practical exercise, it is

important to twist the wires close together to the measur-

ing point so that the wires do not come apart. In addition,

the coupling of the load current loop and sensing loop

becomes higher as the distance between the loops be-

comes smaller and as the surface of loop become more

parallel. In fact, separating the loops is not possible in

principle, it is a good idea to make the surface of loop

orthogonal to each other. Because the magnetic flux is

generated proportionally to the AC current used to make

measurements, the effects on measurements will be

smaller if the AC current used to make measurements iskept small but large enough so that the intended

electromotive force generated by the inductance

(measurement signal) is not buried in the noise. This

phenomenon is caused by the induced electromotive

force, and the induced electromotive force is actually the

reactance of the mutual inductance. Therefore, the higher

frequency makes higher the electromotive force and it

makes adverse effects on measurements.

When making impedance measurements, we must

take into consideration the mutual induction that results

from the inductance as described in the previous chapter.

Even if we minimize the area formed by the load cable

loop, we cannot make the size of area down to zero.

Therefore, there is always some inductance in the load

cables. If AC current flows through the cables, AC mag-

netic field is generated in the space around the load ca-

bles. If the loop formed by the sensing cables exists in

this space, the magnetic fluxes interact, and electromo-

tive force is generated in the sensing cable loop. This

electromotive force is generated independently from the

intended electromotive force that is generated by current

drawn through the objective impedance. If the

electromotive force caused by mutual induction is

significantly large with respect to the electromotive force

due to inductance, it will cause adverse effects onmeasurements. This undesirable electromotive force

becomes large as

Wiring for the Control Function related

If you are controlling the KFM2150 system using

your PC, the communication line connecting between the

KFM2150 system and your PC is important. If this line

is close to a noise source, the possibility of communica-

tion errors increases. The load cables (during the imped-

ance measurement) and wires controlling the electromag-

netic valves of the gas system and the like can become

noise sources. Therefore, bringing the communication

line in contact or close to these lines or twisting them

together should be avoided. Moreover, the communica-

tion line also becomes a noise source against the sensing

wires, so it is also a good idea to keep them apart and not

twist them together. As a result, it is recommended to

separate the load cables, communication and controllines, and sensing wires.

The KFM2150 system is equipped with I/O termi-

nals for contact signals. When making connection to

external devices using these terminals, the same

consideration must be given to wiring as with the

communication line. In addition, long wires connected

with the end opened become antennas for noise and may

cause errors in operation (such as activating an alarm to

turning off the load). Therefore, control wires that are not

The inductance of the sensing loop and load current

loop become larger.

3 CV operation means that the device must operate at impedance lower

than the DUT. However, it is difficult to produce a device that can

supply large current at such low output impedance because the fuel cell

impedance is extremely low..

8


13/16

used should be disconnected. system that a load unit is built in.

Level of AC current for measurement

It is defined that the term of Impedance measure-

ment has a meaning only if the DUT response is linear.However, electrochemical systems have high non-linear

characteristics. Therefore, it is advantageous to keep the

level of AC current used to make measurements low to

meet this requirement of linearity. On the other hand, it is

advantageous to use large AC currents (as long as the

circuit does not saturate) in terms of S/N4 for measuring

instruments. These two facts are in conflict. In reality,

the AC current should be large enough so that linearity is

not hindered but small enough in the range not to be af-

fected for S/N. For linearity, we must monitor the volt-

age response waveform to seek the appropriate current

level. For S/N, we can determine the appropriate current

level in relation to the fluctuation of the measured values.If the fluctuation is large even when the number of count

for averaging process is set to maximum, we can suspect

that the S/N is insufficient. According to reference [2],

the response can be assumed to be linear if voltage re-

sponse VAC is less than or equal to thermal voltage V T,

where VT is expressed by where R = 8.314 [J/mol-K]

where T is the temperature [K] and F is the Faraday con-

stant = 964851 [C].

The thermal voltage is 26 mV at 25 C and 32 mV at

100 C. It is stated that it is important for the amplitude

of the AC response signal be significantly smaller than

these values. The amplitude of a normal AC response

signal is 5 to 10 mV.

3.3 Current Interrupt Method

3.3.1 Equipment Used for the measurement

system

When using the current interrupt method, it is deter-

mined whether an electronic load unit can be used

depending on the level of interrupt speed required or the

level of current to be interrupted.

If the rise and fall times of the electronic load are

fast enough, the electronic load unit can be used to per-

form current interrupt. The KFM2150 also supports the

current interrupt method allowing you to make current

interrupt method measurements using only the KFM2150

To perform fast interrupts, a special interrupter

called a current interrupter that uses mercury relay or

the like is used in combination with a variable resistor

for power. However, it is not recommended for using an

electronic load unit with a current interrupter, becausethe load unit may be damaged from the surge voltage of

which the interrupter generates. In addition, there is a

way to observe and record of voltage change by using a

digital storage oscilloscope.

3.3.2 Notes Concerning the Electric Measure-

ment System

When making measurements using the current inter-

rupt method, we must pay attention to the reduction in

the inductance of the load cables even more than in the

case of using AC impedance method. If a current flowing

through an inductance component is varied, a reverseelectromotive force is generated according to the follow-

ing expression.

diV = Ldt

If this voltage is large, an arc is generated between

the electrodes, and the current cannot be interrupted. It is

extremely difficult to interrupt sharply a large direct cur-

rent. In addition, this electromotive force is superim-

posed on the voltage in response to the impedance of

current changes as a purpose of observation, and the

separation of the target signal from the electromotive

force lies in the hands of the observer. In a way, this

poses a problem of reproducibility, because the measure-

ment could be affected by the ability of observer.

RTVT= F

Furthermore, because the input terminals of an oscillo-

scope are typically connected to the chassis at the nega-

tive potential end,5 we may be faced with common-mode

noise problems depending on the observation conditions.

To perform impedance measurements using the current

interrupt method with high reproducibility, we must

design special equipment that minimizes the load and

sensing wiring to reduce the wiring inductance and

securely keep the residual inductance constant 6 and

minimize the coupling of the load cables and sensing

wires.

5 Called a single-ended input type and this term is used typically in

oscilloscopes. There are few oscilloscopes that are available used with

a type of balanced input.4 Signal-to-Noise Ratio. 6 Because the physical shape of the wiring is constant.

9


14/16

3.4 Impedance Measurement

I-V measurements are the most common method as

a simple measurement performed on fuel cells. I-V meas-

urements allow us to presume the internal resistance.

However, it is not possible that this method of measure-

ment to specify the process of complex reaction of the

inside fuel cells. The impedance measurement (plot)

method is a type of new electrochemical measurement

method and is widely applied in the characteristics meas-

urement of fuel cells. In the impedance measurement of

fuel cells, the changes in the induced voltage (or current)

indicates a response according to the fuel cell

characteristics when a given level of perturbation current

(or voltage) is applied to fuel cells in operation. We can

specify the process of complex reaction of the inside fuel

cells by performing the impedance measurement on fuel

cells in operation.

Fig. 3.1 shows the most typical equivalent circuit ofa fuel cell.

Rs

Rp

Cp

Fig. 3.1: The most typical equivalent circuit of a fuel cell

The Rs specified in the above figure is the internal

resistance or ohmic resistance of the fuel cell that comes

from electrolytic resistance and solution resistance. TheRp specified in the above figure is derived from the

charge-transfer resistance in the oxidation-reduction

reaction of the electrode surface. The Cp specified in the

above figure is derived from the electric double layer

resistance near the electrodes.

I-V tests allow us to determine the sum of Rs and

Rp. When performing the Impedance measurements, it

enable to separate the Rs and the Rp.

3.4.1 Impedance Measurement Using the AC

Impedance Method

In the case of the AC impedance method, measuring

at a high frequency at which the reactance of Cp is suffi-

ciently smaller than the resistance of Rp causes the Rp to

disappear and allows us to obtain data consisting of

mostly the Rs component. If measurement is performed

at a frequency at which the reactance of Cp is suffi-

ciently greater than Rp, we can obtain Rs+Rp that are

synonymous to the internal resistance in the I-V

measurement. Additionally, we can determine the electric

double layer capacitance. Because we can analyze the

characteristics in more detail than static internal resis-

tance by reviewing the complex impedance data trace

obtained by varying the frequency used in the impedance

measurement, this method is sometimes called imped-

ance spectroscopy.The examination of the plural number of impedance

measurement data obtained by varying the frequency

requires an appropriate representation method. For this,

Cole-Cole plot and Bode plot are mainly used in the field

of fuel cells. Fig. 3.2 and Fig. 3.3 are examples of a

Cole-Cole plot and Bode plot, respectively.

Fig. 3.2: Cole-Cole plot example

In a Cole-Cole plot, resistance R is plotted on the

horizontal axis, and reactance jX is plotted on the verti-

cal axis. In the field of fuel cells, it is common to plot

-jX in the first and second quadrants. Frequency

information is not expressed in the plot. In a Bode plot,

the logarithm of the frequency is plotted on the horizon-

tal axis, and the magnitude of impedance (logarithm) and

phase of the impedance are plotted on the vertical axis.

In the field of electricity, resistance and reactance are

often expressed as R and jX. In the field of fuel cells, R

(a component of real number of the impedance Z) is of-

ten expressed as Z' or Re Z, and jX (a component of

imaginary number of the impedance Z) is often ex-

pressed as Z'' or Im Z.

A Cole-Cole plot is preferred as a point of easy to

understand the relationship between the trace and the

status of the electrochemical reaction. However, the

drawback is that the frequency information is not ex-

pressed directly. On the other hand, Bode plot is

inconvenient in that it is difficult to see the detailed

10


15/16

11

3.4.2 Impedance Measurement Using the Cur-

rent Interrupt Method

changes because the impedance is plotted logarithmically.

But, it is preferred to the relaxation time that is easily

read from the phase change and frequency. Determining

which plot to be used should depend on the test and

analysis objectives.

It seems that the current interrupt method is typi-

cally used only to separate Rs. Consequently, the current

interrupt method is used as a function to determine Rs on

our KFM2150 System.In the world of electrochemistry, it is consideredgood practice to separate the anode and cathode to make

measurements using the three-electrode process that uses

a reference electrode when examining fuel cells. How-

ever, there is no reference electrode on an actual fuel cell.

Moreover, because no current must flow through the

reference electrode, high impedance is required on the

measuring instrument end. Because high impedance con-

flicts with low noise, high bandwidth, and high stability

in terms of electrical engineering, this is technically

difficult. Furthermore, it also requires measuring the

voltage on two channels, therefore, it is disadvantageous

in terms of cost when achieving a safe simultaneous

measurement. In the case of PEMFC or DMFC, it is aknown fact that the entire performance is virtually deter-

mined by the reaction at the anode. Therefore, if the

objective is to detect defects and deduce the cause rather

than an in-depth research and development of cells, the

two-electrode process will be sufficient. In addition, it is

a great advantage for the target object which is not re-

quired to be disassembled or damaged.

In principle, it is possible to examine the Rp-Cp

parallel section, but ensuring reproducibility is consid-

ered to be more difficult than with Rs.

Rp can be deduced by making a calculation with

Rs+Rp that is obtained from the I-V test result.

3.5 Conditions of Fuel and Air for Im-pedance Measurements

Fuel and air or oxygen is supplied in conjunction

with the load current basically in the same way as with

the I-V measurement. The gas control system does notneed to follow the measurement frequency. It is required

for the stability of time.

Fig. 3.3: Bode plot example


16/16

Acknowledgements

We are indebted to the member of our development for full cooperation in writing and editing this reference guide

booklet.

We also received advice on the contents concerning electrochemical and gas system control, which are outside of

our domain, from the researchers in the laboratory of fuel cells of Chinese Academy of Science, Dalian Institute of

Chemical Physics (DICP). We would like to take this opportunity to thank these people for their support.

Bibliography

[1]

Larminie, James and Dicks, Andrew. Fuel Cell Systems Explained, translated by Tsuchiya Haruki.

Ohmsha, 2004.

[2]

K. R. Cooper, Vijay Ramani, James M. Fenton, and H. Russell Kunz. Experimental Methods and Data Analyses for

Polymer Electrolyte Fuel Cells. Scribner Associates, Inc.

Reference Guide to Electrical Measurement of Fuel Cell Testing

Fuel Cell Measurement Basics and Tips from Measuring Instrument Experts

February 7, 2007 (1st Edition)

(Editor)

Kikusui Electronics Corporation

FC Project Promotion Office

1-1-3, Higashiyamata,Tsuzuki-ku, Yokohama, 224-0023, Japan

URL: http://www.kikusui.co.jp/en/index.html

(Distributor in USA)

Kikusui America, Inc.

1744 Rollins Road

Burlingame, CA 94010

Phone (650)259-5900

Fax (650)259-5904

URL: http://www.kikusui.us

12
http://www.kikusui.co.jp/en/index.htmlhttp://www.kikusui.co.jp/en/index.htmlhttp://www.kikusui.us/http://www.kikusui.us/http://www.kikusui.us/http://www.kikusui.co.jp/en/index.html

fuel cells measurement guide

Documents