investigation of dynamic driving cycle effect on performance degradation and micro-structure change...

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 2 3 6 9 – 2 3 7 6

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Investigation of dynamic driving cycle effect on performancedegradation and micro-structure change of PEM fuel cell

R. Lina,b,*, B. Lia,b, Y.P. Houa,b, J.M. Maa,b

aClean Energy Automotive Engineering Center, Tongji University, Shanghai 201804, ChinabSchool of Automotive Studies, Jiading Campus, Tongji University, 4800# Cao’an Road, Shanghai 201804, China

a r t i c l e i n f o

Article history:

Received 24 September 2008

Accepted 9 October 2008

Available online 4 February 2009

Keywords:

PEM fuel cell

Dynamic driving cycle

SEM

EIS

TEM

* Corresponding author. School of AutomotivE-mail address: [email protected] (R. L

0360-3199/$ – see front matter ª 2008 Publisdoi:10.1016/j.ijhydene.2008.10.054

a b s t r a c t

Degradation of PEMFC performance under dynamic loading cycling situation was studied

by simulating automotive working conditions. A protocol of driving cycle was set up. Each

driving cycle was consisting of six different periods. The performance of the fuel cell during

driving cycle periods was represented by time-dependent voltage profiles. Polarization

curves, measured in regular intervals, revealed a changed degradation rate in different

operation periods. A rapid degradation of performance could be observed after 280 h of

operation. In contrast to the starting conditions of the MEA, the degradation of membrane

and dissolution and aggregation of catalysts were clearly visible at the end of testing.

Furthermore, after 370 h of testing, Pt was not uniformly dispersed in the catalysts layer

and some also scattered in the membrane. Cracks and gaps appeared in the catalyst layer.

This change of micro-structure corresponded to the loss of electro-chemical active area of

the MEA and the increased membrane resistance and charge transfer resistance.

ª 2008 Published by Elsevier Ltd on behalf of International Association for Hydrogen

Energy.

1. Introduction example, 5000 h is the bottommost target of automotive lifetime

Proton exchange membrane fuel cell (PEMFC) is one promising

type of fuel cell which could be used as vehicular power source

to replace gasoline and diesel internal combustion engine.

However, some major technical issues still need to be solved for

the wide-spread marketing of fuel cell generators into the

transportation area [1]. The biggest challenge in the commer-

cialization of PEMFC is its cost and particularly durability.

The performance of fuelcell ispronetomaterial degradation.

A variety of tests running in stationary regime at roughly

nominal conditions, provide an overview of the effect of oper-

ating conditions, including the effect of reactant gas flows,

humidification levels and temperatures, on the fuel cell

performance and the corresponding morphological change of

single fuel cells or stacks [2,3]. But under steady-state operation

condition, testing times could be lengthy and costly. For

e Studies, Jiading Campuin).hed by Elsevier Ltd on be

and needs almost months running. It is generally acknowledged

that the performance of a fuel cell is impacted by the dynamic

load cycle much more strongly than by the constant load

condition [4–7]. This is because under dynamic driving cycle, the

variety of operation condition was much more dramatically

compared to the steady condition than under constant load

conditions. This may result to the oxidant starvation, local

‘‘hotspots’’ and physicaldegradation, sothe performance of fuel

cell would be much seriously degraded [6–8].

Setting up an accelerated testing was much more realistic

and necessary than the long-time steady-state testing. Many

efforts have been made to investigate the performance of PEM

fuel cell over dynamic driving cycles in the objective to

provide references for designing and manufacturing a robust

and durable air/hydrogen fuel cell for transportation applica-

tion [7–15].

s, Tongji University, Shanghai 201804, China. Fax:þ86-69589225.

half of International Association for Hydrogen Energy.

mailto:[email protected]

http://www.elsevier.com/locate/he

10

20

30

40

Cu

rren

t / A

one cycle


Many models had been set up to exploit the effect of

dynamic change of load on the durability of PEMFC under

different operation conditions. Effect of gas flows, temperature,

water management and humidity conditions under driving

cycles were studied. Furthermore, the degradation mechanism

was discussed after different electro-chemical and physical

characterizations of fuel cell key components [7,8].

Los Alamos laboratory had operated fuel cell stack utilizing

drive cycle protocol defined by DOE modified to include shut/

down cycles every 10 cycles. It was found the voltage loss is

much seriously in the shut-down cell than no shut-down cell

and more CO2 evolution at start-up compared to shut-down.

The accelerated testing affected the hydrophobicity of gas

diffusion layer obviously and OCV changed with time at

different humidification conditions. Also potential cycling was

used as an accelerated testing method to examine the oper-

ating conditions leading to loss of electro-catalyst surface [7,8].

The durability of a 100-W stack under specific dynamical

current solicitation was linked to a vehicle road cycle to study

the impact of transport environment and the operation

conditions and to prevent stack failure. The work found the

gas flow management in applying highly dynamical current

profiles was very important for the performance of PEMFC

[13]. Escribano et al used two types of cycling test procedures

differing in operation conditions to study the degradation of

MEA. The effects of temperature, oxidizer humidification and

membrane thickness on the durability had been investigated

under the cycling test conditions. The mechanical stress

induced during on/off operation by the successive hydrogen

and dehydration decreased the lifetime of the MEA [14].

Schulze et al [15] set the operation mode at the anode as ‘‘dead

end’’ (100% utilization), with various purging intervals to

understand the degradation mechanism in the MEA. In situ

and ex situ techniques were used to evaluate the operation

effect, especially the water balance for the electro-chemical

performance and characterize the degradation process of MEA

components. Two different degradation processes were

identified, which can be used to determine degradation

values.

However, lack of understanding of most degradation

mechanism and the difficulty of performing in situ, non-

destructive structural evolution of key components made this

topic difficult and required much effort. Considering during

real road driving of a vehicle, fuel cell should go through

dynamic automotive working conditions, the aim of this study

was to design a dynamic driving cycle, which consisted of cold

starting, idle running, full power running, and continuous

loading running and even overload running periods. The

effect of driving cycles on the performance and structure

degradation of fuel cell were studied in order to find the

degradation processes and to give instruction for fuel cell

application in the area of transportation.

0 200 400 600 800 1000 12000

Time / s

Fig. 1 – Driving cycle protocol in one cycle, evolution of

current with time during 20 min, capacity of overloading

was set at 35 A.

2. Experiment

2.1. Preparation of membrane electrode assembly (MEA)

The single fuel cell used in this study has been assembled of

MEA, gas diffusion layers and machined graphite flow

distribution plates. The MEA was fabricated via spraying

platinum catalyst ink on both sides of membrane (Nafion 112)

and followed by just physically placing gas diffusion layers

(GDLs, Toray 090) without hot-pressing process. The effective

electrode area was 50 cm2. The catalyst ink consists of 40% Pt/

C (JM Company), 5 wt.% solubilized Nafion (Dupont Company)

and isopropanol. The mass ratio of Pt/C catalyst to Nafion is

3:1. The loading of Pt/C catalysts for cathode and anode are

both at 0.4 mg cm�2.

2.2. Description of test conditions and dynamicdriving cycles

During the experiment, the fuel cell was regulated at 70 �C

with ambient back pressure. Anode and cathode sides were

100% humidified at a temperature of 85 �C, with a gas utili-

zation of 70% for H2 and 30% for air. The driving cycle exper-

iment consisted of operation of the fuel cell for about 370 h.

The dynamic driving cycles were designed by simulating real

internal engine vehicle with the consideration about cold

starting, idle running, constant load running, variable load

acceleration, full power running and overload running. Over-

load was pointed out as it normally existed when the vehicles

run on the road and plays a role of accelerating degradation. In

our experiment, every cycle consisted of six periods (by

simulating cold starting, idle running, full power running,

continuous loading running and even overload running

conditions) and lasted for 20 min. The capacity of overloading

was set at 35 A. Driving cycle protocol consisted of continu-

ously running the above cycles. The evolution of the current

related to driving cycle protocol in one cycle was presented in

Fig. 1.

2.3. Performance of fuel cell

The evolution of voltages related to the driving cycles at

different current densities was presented to show the tran-

sient response with change of dynamic load. But simply

acquiring the evolution of voltage with time will neither lead

0 50 100 150 200 250 300 350 400

0.4

0.6

0.8

1.0

-0.0023

-0.0015

-0.0013

-0.0011

-2.76E-4

-2.70E-4

-1.88E-4

-1.04E-4

Cell P

oten

tial / V

Time /h

OCV 200 500 700

Fig. 2 – Evolution of voltage versus time under driving

cycles, respectively, at 0, 200, 500 and 700 mA/cm2 current

densities.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 2 3 6 9 – 2 3 7 6 2371

to a comprehensive understanding of the degradation mech-

anism, nor provide knowledge of the performance degrada-

tion rates. Therefore, the data collected during driving cycles

will help determine and quantify the degradation mecha-

nisms that occurred over long periods [13]. Thus, polarization

curves were recorded before and at periodic time intervals of

driving cycles, after driving cycles for a certain time to,

respectively, establish initial baseline and subsequent

performance changes in galvanostatic state.

2.4. Characterization of fuel cell

Effects of dynamic driving cycles on morphology change of

membrane and catalysts, membrane resistance and active

reaction area before and after the test were characterized. The

initial characterization performance and images of fuel cell

were achieved before beginning the driving cycles.

Scanning electron microscopy (SEM) images of the MEA

were examined by a JSM-5600LV microscope to probe the

morphological changes of the electrodes and the connection

between catalyst layer and membrane. Energy dispersive

spectrometry (EDS) was used to investigate the distribution of

different elements over the MEA.

Transmission electron microscopy (TEM) images were

obtained with a Joel JEM 2010 microscope. TEM was used to

observe the particle size and distribution of Pt/C particles.

Catalyst was removed from the membrane by scraping the

catalyst layer and suspending the powder in ethanol solution.

Characterization of cyclic voltammetry (CV) was per-

formed using a VMP2/Z electro-chemical workstation. The

potential was scanned at a rate of 20 mV per second from

50 mV–1200 mV and vise versa by taking the anode electrode

as reversible hydrogen electrode (RHE). A series of CV

measurements at periodic time intervals was recorded to

quantify the active catalyst surface area of the MEA.

Once the polarization curves were recorded, an impedance

technique was used to characterize the dynamic behavior of

fuel cell. electro-chemical impedance spectroscopy (EIS)

measurements were carried out using a VMP2/Z workstation

by a PAR model at galvanostatic mode under a constant load

of 5 A with a frequency range between 10 kHz and 100 mHz.

The entire fuel cell impedance was obtained by feeding air

into the cathode and hydrogen into the anode side. All

impedance spectra reported herein were measured between

the fuel cell cathode and anode. A series of impedance

measurements at periodic time intervals was recorded during

the experiment applying a sine wave distortion (AC pertur-

bation) of 10 mA amplitude.

3. Results and discussion

3.1. Fuel cell performance degradation under dynamicdriving cycles

Fig. 2 presented the evolution of voltage versus time under

driving cycles, respectively, at 0, 200, 500 and 700 mA/cm2

current densities. It was noticed that the voltage decreased

with time whether under open circuit voltage condition or

under higher current density. After operated for 280 h, a much

more rapid decrease of voltage was observed comparing to the

period of 0–280 h. A trend associated with the evolution of fuel

cell voltage over time can be plotted, if one modulates the V–t

plot into a linear line. It could be found that during 0–280 h

operation, the evolution of voltage versus time decreased with

a rate of 0.274 mV/h, while during 280–350 h, a more rapid

degradation rate of 2.3 mV/h was found.

It was worth noting that the voltage decline trend was

linked to the impacts of the current densities. The slopes of V–

t plots increased with increasing current densities. Under

700 mA/cm2 current densities, they were almost twice as the

ones under OCV (loaded off) condition. It’s clear that degra-

dation of voltage profiles was influenced much more seriously

under higher current densities than under lower ones. It may

be due to the fact that running in higher current density fuel

cell need to output much power, which shows if the inner

structures of the fuel cell, like the connection of membrane

and catalyst, morphology of catalyst layer, particle size of

catalysts and so on, would be affected by degradation. The

structure degradation will be discussed in more detail in the

next part.

From plots of the evolution of voltage, it was found that

sometimes voltage oscillated though the trend of voltage

evolution was decreasing in time. This may be on the one

hand dueto the fluidic resistances and capacities of the gas

pipes and channels and on the other hand to the time

constants of the fuel cell gas diffusion layers.

Polarization curves of initial MEA and those measured in

time intervals (respectively, after 100, 200, 280 and 370 h

running) under driving cycles were presented in Fig. 3. It was

found that open circuit voltage (OCV) was about 0.96 V for the

fresh fuel cell. With time going on, OCV of the fuel cell

decreased gradually but a much more rapid decrease of

voltage was observed after 280 h of operation comparing to

the period of 0–280 h. It revealed that the performance of the

fuel cell decreased with driving cycles running on during 370 h

operation and the trend of degradation change happened just

after 280 h operation, which could be also found in Fig. 1.

0 200 400 600 800 10000.0

0.2

0.4

0.6

0.8

1.0

Cell P

oten

tial / V

Current density / mA.cm2

0 h 100 h200 h 280 h 370 h

Fig. 3 – Polarization curves of initial MEA and those

measured in time intervals (respectively, after 100, 200, 280

and 370 h running) under driving cycles.


3.2. SEM analysis of MEA after operated underdriving cycles

The fuel cell was dissembled to explore the degradation

mechanism by investigating alternations of membrane and

catalyst layer after driving cycles. Fig. 4 was the cross-section

Fig. 4 – Cross-section of MEA morphology and corresponding e

MEAs.

of MEA morphology and corresponding elements distribution,

respectively, for initial MEA and the one after driving cycle

operation. The colors shown in the scanning electron

microscopy (SEM) images were also clearly presented below. It

was the distribution of some elements composition of MEA.

From the axes of the spectra, it was found that the thickness of

initial MEA was about 70 mm. Anode side was located in the

range of 0–10 mm. In the middle, 10–60 mm, was the distribu-

tion of elements in membrane. Lastly, 60–70 mm was the

element distribution in cathode side. While after 370 h driving

cycles, MEA expanded and the thickness of it increased to

about 82 mm. The change of the thickness of anode was not so

clear and it was in the range of 0–10 mm. But the membrane

became distorted and expanded to about 58 mm (10–68 mm).

The cathode electrode twisted and expanded to about 14 mm

(68–82 mm).

From the spectra of elements’ distribution, it was found

that F element was uniformly dispersed in the membrane and

catalyst layer and Pt catalyst was only dispersed in the anode

and cathode side for the initial MEA. After 370 h driving cycle

operation, a visible degradation of F element band occurred in

the cathode side and in the interface of membrane and cata-

lyst layer. In the membrane, the band of Pt element appeared.

It was obviously that F element was depleted obviously,

especially in the cathode side and the interface of membrane

and catalyst layer. From these results, we thought that the

membrane started degradation at the electrode/membrane

lements distribution of (a) the initial and (b) the degraded

DegradedMEA

InitialMEA

Fig. 5 – Surface morphologies of the cathode electrode, (a) initial MEA; (b) the one after driving cycle operation.


interface of the anode and cathode and then continued

degradation from interface towards the center of the

membrane. Additionally, the scattering of Pt in the membrane

was maybe due to the dissolution of Pt under driving cycles [8].

Fig. 5 was the surface morphologies of the cathode elec-

trode, respectively, for the initial MEA and the one after

driving cycle operation. From Fig. 5, it can be seen for the

initial MEA, Pt/C catalysts were uniformly dispersed. While

after 370 h driving cycles, part of catalysts particles grew up

and some aggregated. Cracks appeared in most regions of the

catalyst layer after driving cycles. Corresponding EDS

mapping of Fig. 5 was presented in Fig. 6. It was found that Pt,

F and S elements were sparsely dispersed on the surface of

degraded MEA while those elements were thickly and evenly

dispersed on the surface of the initial one.

Considering the results of fuel cell performance under

driving cycles and defection of MEA structure, it was assumed

that a uniform dispersion of catalysts were beneficial to the

Fig. 6 – Corresponding ED

fuel cell performance, while degradation of membrane,

dissolution of catalyst, expanding of catalyst layer and cracks

and gaps in the catalyst layer were main reasons for fuel cell

performance degradation [8,15,16]. The degradation of MEA

after dynamic driving cycles was maybe due to the constant

switch on and off and overloading during dynamic change of

load.

3.3. Degradation of Pt/C catalysts after operateddriving cycles

As the catalyst layer is the most important region for electro-

chemical reaction, the change of catalyst morphology could

give evidence for the change of electro-chemical properties.

Fig. 7 was the TEM images of the initial and degraded catalyst.

For the initial MEA, Pt particles were uniformly dispersed on

the carbon support. After 370 h driving cycles, catalysts either

from anode or cathode electrodes obviously grew up. It was

S mapping of Fig. 4.

Fig. 7 – TEM micrographs of catalyst, (a) initial one, (b) the degraded catalyst from anode side, (c) degraded catalyst from

cathode side.


clearly seen that driving cycles affect catalyst layers, no

matter if anode or cathode, dramatically. As fuel cell operated

in 100% humidification condition, therefore, the growth

mechanism of Pt was maybe due to the Pt mobility (dissolu-

tion or re-precipitation) enhanced by water [8].

3.4. Cyclic voltammetry results after operation of manycycles/hours

Fig. 8 was cyclic voltammetry (CV) curves of the initial and

those measured in time intervals after many driving cycles. It

was found that the area of desorbed hydrogen (marked by

square, at about 0.09–0.366 V, versus RHE) decreased in

a moderate rate during 280 h driving cycles operation and

then followed by a much rapid degradation rate. As the area of

desorbed hydrogen is corresponding to the active catalyst

surface area, it means that loss of electro-catalyst surface

predominately happened with driving cycles, especially after

280 h of operation. Considering SEM and TEM results, it was

concluded that loss of electro-catalysts surface was due to the

dissolution and aggregation of Pt particles, which was caused

by the strong load on and off or even overload under dynamic

driving cycles.

0.0 0.2 0.4 0.6 0.8 1.0 1.2-300

-200

-100

0

100

200

300

400

I / m

A

Ewe / V

Prior to driving cycle100h200h280h370h

Fig. 8 – Cyclic voltammetry (CV) curves of the initial and

those measured in time intervals after many driving

cycles.

Furthermore, from degradation of fuel cell performance

and CV results, a strongly increased degradation rate

occurred, which was corresponding to the rate of active

electro-catalyst surface area loss. It was assumed that after

280 h of driving cycle operation the electro-chemical proper-

ties of MEA was defected seriously compared with the period

of 0–280 h operation.

As show in Fig. 8, it was further found that potentials of

oxygen reduction, at about 0.8 V, RHE, marked by the line,

were moving to lower values with increasing driving cycles,

while potentials of hydrogen reduction almost was main-

tained constant. The movement of the former one also sug-

gested the oxygen reduction activity of MEA will have

a considerable decrease with driving cycle running [17].

3.5. Impedances results under dynamic driving cycles

Electro-chemical impedance spectroscopy (EIS) has been

demonstrated to be a powerful technique to study the

fundamental processes in fuel cells [18]. The components of

MEA had been analyzed by EIS to further investigate the

mechanism for the rapid decay of fuel cell performance.

Impedance of the fuel cell operated under dynamic driving

cycle at 0, 100, 280 and 370 h operation was recorded in Fig. 9.

0.03 0.04 0.05 0.06 0.07

0.00

0.01

0.02

-Im

(Z

) / O

hm

Re(Z) / Ohm

Prior to driving cycle100h280h370h

Fig. 9 – Impedance of the fuel cell operated under dynamic

driving cycles.


At the beginning of the experiment, one recognized

a depressed semicircle from EIS plots. With increasing driving

cycles, the real as well as the imaginary parts of the imped-

ance plots increased at the same time. In addition, the inter-

cept of real part of impedance plots moved to high values with

driving cycles going on. Especially, after exceeding 280 h, the

intercept of real part after 370 h running greatly increased and

the diameter of the loop increased to double size. The differ-

ence for EIS plots between 280 h and 370 h operation was

much more distinctive than other points.

The intercept of impedance plots in high frequency was

generally taken as the resistance of the membrane. As the

fuel cell was in 100% humidity, the movement of intercept

in high frequency to positive direction wais mainly due to

the membrane degradation, which may make the conduc-

tivity of membrane decrease and was one of the crucial

issues for the degradation of fuel cell performance [19–22].

Diameter of Nyquist plot was mainly determined by the

charge transfer resistance of the interfacial oxygen reduc-

tion process. The diameter of the loops increased and

moved to positive direction reflecting an increased charge

transfer resistance within the catalyst layer with increasing

driving cycles.

Combined with results of SEM, degradation of fuel cell

performance and EIS plots, it was concluded that the defected

structure of the MEA (degradation of membrane and catalyst

layer) made resistance of fuel cell to greatly increase and

induced corresponding fuel cell performance degradation.

Distinctive difference could be found either for the EIS plots or

fuel cell performance after 280 h driving cycle operation.

Seriously increased charge resistance and membrane resis-

tance after 280 h operation was one of the reasons for the

abruptly degraded performance.

4. Conclusions

Dynamic driving cycles were designed to modulate the real

vehicle running as accelerated testing. The performance of

the fuel cell during driving cycles was represented by time-

dependent load profiles. A moderate degradation rate of the

voltage could be detected for about 280 h of operation which

was then followed by a much more rapid decrease of perfor-

mance. The decline in voltage over time suggested the

degradation of fuel cell performance.

Structural and electro-chemical changes within catalyst

layer and membrane were investigated to find the significant

failure for the cause of performance degradation. With driving

cycles running, the resistance of the membrane and charge

transfer increased. One of reasons was due to the degradation

of membrane, which affected the proton conductance and

resulted in corresponding fuel cell performance degradation.

Another reason was that part of the membrane and catalyst

departed away and the formation of cracks in catalyst layer

increased the charge transfer resistances of the electrodes.

Also the growth and aggregation of catalyst particles in the

catalyst layer resulted in a decreased active surface area for

chemical reaction.

It was clear that 280 h of driving cycles was a key point for

this experiment. At that point, performance of the fuel cell

was greatly decreased and corresponding electro-chemical

properties did also happen.

Our study will give valuable information for the application

of fuel cells in the area of transportation. In the next step, we

will detect the effect of different dynamic driving cycles on the

performance of the fuel cell and try to find the correlation

between them and give information to forecast the life of

a fuel cell.

Acknowledgments

The authors gratefully acknowledge the National Natural

Science Foundation (No.20703031), Young Teacher Research

Fund of the Doctoral Program of Higher Education

(No.1700279016) and Program for Young Excellent Talents in

Tongji University (No. 2006KJ022) for supporting this project.

Authors greatly thank Ms. Reibner Regine (DLR, Germany) for

the serious revision and kind references supply and Ms. Aip-

ing Jia for performing SEM measurements.

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