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Natural Degradation and Stimulated Recovery of Proton
Exchange Membrane Fuel Cell
Yuedong Zhana, *, Jianguo Zhub, Youguang Guob, and Li Lia
a
Department of Automation, Kunming University of Science and Technology, Kunming,
650093, China
b
Faculty of Engineering & Information Technology, University of Technology, Sydney,
NSW 2007, Australia
(Emails: [email protected]; [email protected]; [email protected])
*Corresponding author: Tel.: +86-871-5623806; Fax: +86-871-5916643, Email address:
[email protected] (Yuedong Zhan)
ABSTRACT
In this paper, the stimulated recovery rate for proton exchange membrane (PEM) fuel cells
after natural degradation has been investigated. Firstly, a 40,000 h lifetime test and the
analysis of the static and dynamical performance were conducted with a 63-cell PEM fuel
cell stack under certain storage conditions, such as the temperature of 24 °C and relative
humidity of 65%. In-situ measurement of the average degradation rate through the stack was
approximately 309 μVh-1 before using the stimulated recovery technique. Then, by using high
frequency pulse technology and control mitigation strategy to the PEM fuel cell, the
performance curves of each single cell and stack can be recovered partly, and the overall
stack average degradation rate after this stimulated recovery can be reduced to approximately
170 μVh-1. The studies indicate the existence of both recoverable and irrecoverable
degradations in the fuel cell. Furthermore, the equivalent circuit model and membrane
resistance were used to investigate the degradation mechanisms from the viewpoints of
modeling and control. The natural degradation of fuel cell is mainly caused by the increase of
the resistance, which is likely caused by membrane dehydration. The study in this paper
would provide deepen our understanding for the storage and degradation recovery of fuel
cells.
Key Words: Proton exchange membrane fuel cell; Durability; Natural degradation;
Stimulated recovery; Degradation rate
1. Introduction
There are two major motivations to develop the hydrogen technique for the operation of
different types of fuel cells. Firstly, the hydrogen can be made from diverse domestic
resources. Secondly, hydrogen can be employed to generate the power for stationaries,
vehicles, backup and portable power system with low or zero emissions. Among all kinds of
fuel cells, the proton exchange membrane (PEM) fuel cells are the most promising. They are
environmentally friendly, and have many advantages over conventional energy-conversion
devices, such as high efficiency and high power density, making them applicable across a
variety of power systems. Meanwhile, significant progress has been made over the past few
decades, especially in the fields of improved power density and effective material utilization.
However, the commercialization of this technology has been limited due to some technical
challenges, including the fuel cell system itself, as well as problems of on-board storage and
the need of an infrastructure for hydrogen fuel. According to the US Department of Energy
(DOE), the lifetime targets of PEM fuel cell in 2015 for transportation applications are 5,000
h for cars, 20,000 h for buses, 40,000 h for stationary systems, respectively, and the
degradation rate should be 2–10 μVh-1 [1]. At present, the lifetimes of fuel cell vehicles and
stationary systems developed are only around 1,700 h and 10,000 h, respectively, with
degradation rate of 15–30 μVh-1 [2]. Some reviews on the lifetime of fuel cells indicate that
the effect factors include the design and assembly of fuel cell, material degradation,
impurities and operational conditions (e.g. temperature, relative humidity, pressure, feed
starvation, load change, start-up and shut-down cycling, potential cycling, freezing or
thawing). Therefore, further research and developments interests are to address the issues
related to the degradation or durability, cost, performance and efficiency for PEM fuel cells in
order to achieve sustainable commercialization developments [3].
Studies on the storage-induced degradation and its stimulated recovery methods of the
components for the PEM fuel cells have not been reported. There are only some reviews in
other aspects.
PEM fuel cells are complex electrochemical devices, which consist of many components
such as catalysts, catalyst supports, gas diffusion layers (GDLs), membranes, bipolar plates,
sealings and gaskets. Due to the materials, design technologies, operation conditions,
contaminants, environments and other factors, each of these components can degrade or fail.
According to the review on PEMFC degradation, component degradation includes, but is not
limited to, the platinum catalyst degradation of Pt agglomeration and particle growth [4-7], Pt
elemental loss [8], Pt migration [9, 10], and Pt catalyst contamination [11], catalyst support
degradation [12-15], membrane degradation on chemical aspect [16-19] and mechanical
aspect [20-26], porous transport layer degradation on mechanical and thermal physics aspect
[27-30] and chemical and electrochemical aspect [31-35], degradation of graphite composite
bipolar plates [36-39] and metal bipolar plates [40], and degradation of other components
such as seals, endplates, and bus plates [41-43]. Some degradation mechanisms for a PEM
fuel cell, such as the carbon corrosion for a typical Pt/C catalyst, the Pt particle growth and
dissolution/precipitation [6], and the chain scission of perfluorosulfonic acid (PFSA)
membrane [18], can help deep understanding. In addition, the degradation processes of
different components are often related to materials, control and design of a fuel cell system.
Therefore, it is important to systematically understand the degradation mechanism of each
component so that novel component materials can be developed, and novel design, modeling
and control for single cell or stacks can be achieved to mitigate insufficient degradation of
fuel cell under different types of operation conditions.
On the other hand, to gain a detailed understanding of fuel cell degradation with respect to
microstructural change and performance improvement in the components and stack, some
long-term durability tests are often required to evaluate the degradation mechanisms.
However, it is generally impractical and costly to operate a fuel cell under its normal
conditions for several thousand hours, and hence accelerated stress tests (ASTs) and
durability test protocols are often employed [44, 45].
In this study, by using the natural degradation method instead of the AST, a 63-cell 300W
PEM fuel cell stack was bought in March 2008. After some experiments were conducted, the
stack had been stored in the Fuel Cell Lab until 17 September 2012at the University of
Technology Sydney (UTS), Australia. Based on the approximate 40,000 h of storage time, an
experimental research on the natural degradation of the PEM fuel cell was carried out, and
the static and dynamic performances were analyzed. To achieve the similar performance as
the original one back to 40,000 h ago, a stimulated recovery study was conducted using the
high frequency pulse technology and intelligent control methods on the stack. The stack
potential curve was measured continuously during experiments, and the relations between the
stack potential and other parameters, e.g., current, load change, recovery time and rate, are
investigated.
2. Experimental Study
2.1. Experimental setup
The experimental setup consists of an improved uninterrupted power supply (UPS) system
with stimulated recovery system (SRS), the PEM fuel cell (PEMFC) generating and test
system, supercapacitors (SCs), deep cycle lead-acid batteries and a data acquisition system.
The data acquisition system includes the analog voltage output devices NI6713, multifunction
I/O devices NI6036E, analog multiplexer with temperature sensor AMUX-64T and parallel
digital I/O interface PCI-6503. In the PEMFC generating and test system, the mass flow
controllers (type: F-201C-GAS-22V and F-112AC-GAS-22V, Bronkhorst) are used to
regulate the hydrogen and air. Because the PEMFC stack is self-humidified, the option to
humidify the hydrogen is not used in the generating system. The hydrotransmitter (type:
HD2008TV1, Delta OHM) and the pressure transmitter (type: AUS EX 1354X, Burkert)
between the inlets of cathode and anode are applied to measure the temperature and humidity
of air and hydrogen at inlet. The UPS system with backup fuel cell and battery/SC provides
the AC power source and connects with the linear loads (e.g. lamp box) and nonlinear loads
(e.g. PC). All physical parameters such as the currents and voltages of the UPS, PEMFC
stack and battery/SC, relative temperatures of hydrogen and air, pressure drop in the flow
fields, and gas mass flow of the reactants are recorded with the data acquisition system [46].
Fig. 1 illustrates schematically the structure of a single-phase high-frequency SRS system, a
backup PEMFC and battery power sources. Fig. 2 shows a photo of the experimental setup
and storage place of the PEMFC. Fig. 3 indicates the PEMFC generating system with
component details, consisting of a PEMFC stack, H2 humidifying and filtering, water-cooling
and air-cooling components, the control and monitoring of temperature, pressure and feed
mass flow rate. Hydrogen, nitrogen and air/oxygen are used in the system. A LabVIEW TM
software package designed by the authors is used to control the whole process.
2.2. PEM fuel cell parameters
A 63-cell Horizon® H-300 PEMFC stack, made in Horizon Fuel Cell Technologies,
Singapore, was utilized for the PEMFC performance testing and stimulated recovery research
under different operating conditions in March 2008. Because the PEMFC stack is a new type
(self-humidified) with air-breathing and air-cooling together, some parameters and
manufacturing technologies are confidential, such as the anode volume, catalyst and its
support, cathode volume, gas diffusion media, porous transport layer and flow field design.
The membrane electrode assembly (MEA) was assembled by sandwiching series 63 catalyst
coated membranes (CCMs) between the anode and cathode GDLs. The active area of
PEMFC is about 18 cm2 [47].
2.3. In-situ measurements and testing apparatus
The natural degradation and stimulated recovery testing under certain operating conditions
were carried out on the 300W PEMFC generating and test system, which was designed and
assemblied by the authors. During the testing, the 63-cell fuel cell stack was operated at
different currents of 0-8 A. The fuel cell stack temperature was kept at 24-55 °C and the
pressure of hydrogen was controlled at 5.8-6.5 PSI. The hydrogen flow rates were set at 3.9
and 1.2 standard liters per minute (SLPM) or at 1.2-1.5 stoichiometry of hydrogen. After the
operation from time to time, the potentials of single cell and stack were measured in-situ and
analyzed, with continuing degradation and stimulated recovery testing under these conditions.
On 3 March 2008, and from 17 September 2012 to 31 October 2012, the fuel cell
performance tests, such as the voltage of each single cell, polarization curve and current
interruption measurement under the conditions of different currents and temperatures, were
conducted. In order to prevent the fuel cell from further degradation, an intelligent
comprehensive control strategy was used when the PEMFC started up, shut down, or changed
load sharply. The membrane resistance or conductance was measured by using the current
interruption method, in which the real component of the resulting impedance represents the
Ohmic resistance of the stack. When the PEMFC was working, a stimulated recovery pulse
source from the SRS system was imposed on the stack, which is an AC source with a range of
high frequencies from 1 kHz to 100 kHz. During the durability recovery testing, the
polarization curve was measured and characterized on-line by a DS06034A oscilloscope
(Agilent Technologies, USA) and a PM3000A universal power analyzer (Voltech instruments
Ltd., UK) in periodic intervals. At the same time the possible degradation recovery process
was monitored.
3. Results and Discussion
3.1. Performance of natural degradation
The polarization curve of stack under natural durability testing and the average performance
of the 63 cells are depicted in Fig. 4. It passed nearly 40,000 h from 3 Mar. 2008 to 17 Sept.
2012, and the actual time is about 39,840 h (4 year×365 days+200 days)×24 hours. During
this time, the stack was placed in the Fuel Cell Laboratory, with a temperature of 24°C and
relative humidity of 65%, and there was no contamination source. According to Fig. 4, the
natural degradation rate is calculated as 309 µVh-1 (average value), by using the formula
below
Degradation rate 
Voltage before deg radation  Voltage after deg radation
Time(39840 h)
(1)
It can be seen that even if the PEMFC is not used, i.e. only stored in a place, its storage
conditions cannot meet the requirements as the natural degradation rate is over 10 μVh-1.
According to the lifetime targets of the US DOE for stationary systems (40,000 h) for the
year of 2015, the overall stack degradation rate should be 2–10 μVh-1 [2].
The voltage changes of each single cell with time under open-circuit (OC) conditions (I=0A)
are presented in Fig. 5. It can be seen that the voltage fluctuation of single cell is large. The
stack had not been used for a long time, so the components had experienced natural
degradation, because of different causes such as the membrane dehydration, membrane
natural dissolution, carbon support natural corrosion, natural loss of sulfonic acid groups in
the ionomer phase of the catalyst layer or in the membrane, film natural growth in the bipolar
plate surface, preferential alloy natural dissolution in the catalyst layer, hydrophilicity
changes and polytetrafluoroethylene (PTFE) natural decomposition in the catalyst layer
and/or GDL.
Fig. 6 shows the potential changes of each single cell when the stack current is about 3.16 A.
On 3 March 2008, the voltages of each single cell for the H-300 fuel cell were measured only
under the conditions of OC (I=0A), current I=3.16 A, and I =6.13 A. On 17 Sept. 2012, the
voltages of each single cell were measured under the conditions of the current 0-4.5A. If the
current value is over 4.5A, the PEMFC generating and test system would shut down.
Therefore, besides the external factors, such as the storage conditions (e.g., the temperature or
relative humidity), the natural durability of PEMFC is mainly affected by two internal
factors: the material properties, and the design and structure of the components and the stack.
That is, whether or not a PEMFC is used, the natural degradation is unavoidable because of
the change of membrane resistance ROhmic . And the degradation rate of the PEMFC depends
on the internal and external factors as mentioned above. Regardless the reasons for catalyst
support degradation, catalyst degradation, membrane degradation, porous transport layer
degradation, degradation of bipolar plates and degradation of other components, the change
of the electrical conductivity or resistivity and the capacitance of the electrochemical doublelayer charge inside a PEMFC will cause the static and dynamic performance degradation of
the fuel cell. The conclusion could be obtained according to the equivalent circuit model of a
PEMFC considering degradation effects, as shown in Fig. 7 [49].
In order to recover the PEMFC performance, a stimulated recovery method should also be
considered in the degradation investigation. Because the PEMFC is an electrochemical
energy conversion device, with a perspective of electrochemical reaction, its durability
involves the mechanical degradation, chemical or electrochemical degradation, thermal
degradation, and material degradation as mentioned in Section 1. However, according to the
electrochemical reversibility and thermodynamic reversibility related to the Gibbs free energy,
a process can be thermodynamically reversible if a reverse driving force is applied [48]; that
is to say that its performance after degradation could be recovered.
On the other hand, a current interruption method is employed to measure the membrane
resistance ROhmic , which can almost reflect the stack resistance RStack . The difference V
between the stack voltages before and after the current interruption, divided by the current, is
the stack resistance RStack , which is expressed by the following equation. From the membrane
resistance value, it is easy to evaluate the degradation rate of the PEMFC.
RStack  ROhmic 
V
I
(2)
According to the measurements of the stack resistance RStack , the value changed from the
initial 2.98 Ω measured on 3 Mar. 2008 to 6.44 Ω on 17 Sept. 2012. This has affected the
static performance of the fuel cell, such as the performance curves of each single cell and
stack, as depicted in Figs. 4-6.
Figs. 8(a) and 8(b) show the current and voltage dynamic performances of the stack measured
on 3 Mar. 2008 and 8 Oct. 2012, respectively. Obviously, the voltage degradation value has
been varied, but the dynamic response has no large change observed by using the DS06034A
oscilloscope. The double layers capacitor can be written as [50]
A
(3)
l
where  is the electrolyte’s electrical permittivity, A the effective surface area between
C 
electrolyte and electrode, and l the distance between the layers.
In the fuel cell, because of the porous structure of electrodes, A is large and l is very small
(in the order of nanometers), the resultant capacitance for the fuel cell is of a high value, and
hence there is little effect on the natural degradation for the fuel cell.
3.2. Performance of stimulated recovery
Let us look back at Figs. 4-6. Through the stimulated recovery experiments from 17
September
to 31 October 2012, the degradation of the PEMFC stack has been partly
recovered, and the stimulated recovery rate is approximately 170 µVh -1 (average value) from
computation value, as listed in Table 1 under the current I=0-7A. Therefore, a conclusion can
be drawn that the natural degradation and stimulated recovery of a PEMFC can be divided
into two parts: recoverable and irreversible.
To recover the fuel cell degradation, the reasons for the fuel cell voltage drop should be
deeply understood based on the electrochemical principles. Based on the DC/DC converter
designed for converting the DC voltage generated by the PEMFC into higher DC potential
supplying power for an SRS system, the RMS 2.6 A, peak-peak 4.38 A, 50 Hz currents with
1 kHz, 20 kHz or 50 kHz high frequency pulses have been exerted on the fuel cell,
respectively, as shown in Fig. 9. Fig. 10 shows that the degradation of the fuel cell has been
partly recovered. According to the measurements of the stack resistance RStack online,
degradation values have been recovered from the initial 6.44 Ω measured on 17 Sept. 2012 to
4.82 Ω and 31 Oct. 2012. Particularly, under low current conditions (I<1.0 A), the voltage
recovery is better, which shows that the natural degradation has smaller effect on the
activation loss.
Meanwhile, some auxiliary approaches have been presented to improve the performance of
PEMFC stack, in order to: (1) avoid the dehydration and drying of the membrane, fuel/gas
starvation of electrochemical reaction, the effects of load changing and start-stop cycles; (2)
prevent contaminant-induced degradation; (3) trace the output power of a UPS with backup
PEMFC and battery power sources; (4) make the stack run under the optimal conditions; and
(5) save the fuel. Therefore, an intelligent comprehensive control mitigation strategy is
investigated to control the stack temperature, the mass flows and pressure of the hydrogen
and air. Fig. 11 shows a block diagram of the intelligent comprehensive control mitigation
strategy.
For instance, the current interrupt control can help recover the performance of the stack. Fig.
12 indicates the power supply switching from PEMFC to battery when the current is
interrupted online. There are three functions for the current interruption: (1) measuring the
stack resistance online; (2) immediately removing the double layers charge of the fuel cell
and improving its performance; and (3) preventing the fuel cell from further degradation
caused by the loads’ cycle changing.
In our opinion, the stimulated recovery method is to cause a reverse direction of input current
ripples. To date, the current ripples on a fuel cell have already been studied recently by many
researchers. A conclusion is that the current ripples might affect the fuel cell performance and
decrease the overall system efficiency more than expected. In fact, the effects of the current
ripples on the fuel cell have not been completely understood yet. However, every coin has
two sides, in the recovery process of degradation for the fuel cell, the current ripples are
employed in the experiments [51].
As mentioned above, the natural degradation of a PEMFC depends on the materials, and the
design and manufacturing technologies from each single component to stack system level.
Therefore, the PEMFC must be optimized to enhance the durability while maintaining and
improving the performance and reducing the cost before its successful commercialization. On
the other hand, its performance can be recovered partly by employing the stimulated recovery
methods. Therefore, the irreversible degradation of fuel cell should be minimized in the
development of novel materials and optimization of geometrical structures of the cell
components, the manifold design and material and design capabilities among different
components, and the fuel/air conditioning, thermal management, and monitoring and
controlling of the operating conditions.
Based on the experimental data, the natural degradation mechanisms and stimulated recovery
strategies of PEMFC under certain operating conditions will be further explored. During the
testing, some in-situ electrochemical and chemical diagnostic tools will be employed,
including the linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical
impedance spectroscopy (EIS) and direct gas mass spectrometry (DGMS). Meanwhile, in
order to obtain the property change information of the components, such as catalyst layers,
GDLs and membranes during aging, some ex-situ diagnostic tools including scanning
electron microscopy (SEM), transmission electron microscopy (TEM), contact angle
goniometry and infrared camera will also be applied.
4. Conclusion
In this study, natural degradation and stimulated recovery tests were conducted with a 63-cell
PEM fuel cell stack under certain operating conditions. The experimental results show that
the PEM fuel cell, stored in natural conditions with temperature of 24°C and relative
humidity of 65%, experiences degradation even if it is not operated. The overall stack
degradation rate (average value of approximately 309 µVh-1) is much higher than 2-10 µVh-1,
which is the
US Department of Energy (DOE) lifetime target for 2015. In-situ
electrochemical diagnostic methods, including polarization curve, Ohmic resistance and
current interruption, and the equivalent circuit model of PEM fuel cell were employed during
the testing to explore the mechanisms of natural degradation and stimulated recovery. Then
the static and dynamic performances of the natural degradation were analyzed based on the
equivalent circuit model of PEM fuel cell. Through the stimulated recovery research for
degradation, using the high frequency pulse exerted on the fuel cell, the results show that the
stack performance can be recovered by the motivated action of a period of time, while the
stack degradation rate may recover to the average value of approximately 170 µVh -1 from
computation value. The natural degradation of a PEM fuel cell consists of the recovery part
and irreversible part, which may be mainly related to the material properties, design and
structure of a stack.
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Table 1: Degradation rate of a PEM fuel cell.
Stack current
(A)
Natural degradation rate
(µVh-1)
0
1
2
3
4
5
6
7
Average value
98
126
198
198
228
296
459
871
309
Degradation rate after
stimulation
(µVh-1)
68
48
110
123
168
193
228
419
170
Figure Captions:
1. Stimulated recovery system (SRS) with backup PEM fuel cell and battery.
2. Photo of the experimental setup of the PEM fuel cell system.
3. Schematic diagram of the PEM fuel cell generating and test control system.
4. Polarization curves of the fuel cell stack with natural degradation and stimulated
recovery test under current I=0-7A: temperature at 24-55°C, fuel pressure of
hydrogen at 5.8-6.5 PSI, stoichiometry of hydrogen at 1.5.
5. Polarization curves of stack with natural degradation and stimulated recovery test
under open-circuit conditions (I=0A): temperature at 24-55°C, fuel pressure of
hydrogen at 5.8-6.5 PSI, flow mass of hydrogen at 1.2 l/min.
6. Polarization curves of stack with natural degradation and stimulated recovery test:
temperature at 24-55°C, fuel pressure of hydrogen at 5.8-6.5 PSI, stoichiometry of
hydrogen at 1.5.
7. Equivalent circuit of a PEM fuel cell.
8. Current and voltage of PEM fuel cell when the UPS load changes:
(a) Measured on 3 Mar. 2008; (b) Measured on 11 Aug. 2012.
9. Stimulated recovery current (ch1) and voltage (ch2) waveforms.
10. Stimulated recovery processes of a PEM fuel cell. (Codes of measurement dates
indicated as follows: 1: on 3 Mar. 2008; 2: on 17 Sept. 2012 (am); 3: on 17 Sept. 2012
(pm); 4: on 18 Sept. 2012; 5: on 19 Sept. 2012; 6: on 24 Sept. 2012; 7: on 28 Sept.
2012; 8: on 18 Oct. 2012; 9: on 19 Oct. 2012; 10: on 22 Oct. 2012; 11: on 29 Oct.
2012; 12: on 31 Oct. 2012.
11. Configuration of intelligent comprehensive control mitigation strategy of a PEMFC
powered system for UPS applications.
12. Power supply switching from PEM fuel cell to battery when the current is interrupted.
Fig. 1. Stimulated recovery system (SRS) with backup PEM fuel cell and battery.
Fig. 2. Photo of the experimental setup of the PEM fuel cell system.
Fig. 3. Schematic diagram of the PEM fuel cell generating and test control system.
Fig. 4. Polarization curves of the fuel cell stack with natural degradation and stimulated
recovery test under current I=0-7A: temperature at 24-55°C, fuel pressure of hydrogen at 5.86.5 PSI, stoichiometry of hydrogen at 1.5.
Fig. 5. Polarization curves of stack with natural degradation and stimulated recovery test
under open-circuit conditions: temperature at 24-55°C, fuel pressure of hydrogen at 5.8-6.5
PSI, flow mass of hydrogen at 1.2 l/min.
Fig. 6. Polarization curves of stack with natural degradation and stimulated recovery test:
temperature at 24-55°C, fuel pressure of hydrogen at 5.8-6.5 PSI, stoichiometry of hydrogen
at 1.5.
Fig. 7. Equivalent circuit of a PEM fuel cell.
(a)
(b)
Fig. 8. Current and voltage of PEM fuel cell when the UPS load changes:
(a) Measured on 3 Mar. 2008; (b) Measured on 11 Aug. 2012.
Fig. 9. Stimulated recovery current (ch1) and voltage (ch2) waveforms.
Fig. 10. Stimulated recovery processes of a PEM fuel cell. (Codes of measurement dates
indicated as follows: 1: on 3 Mar. 2008; 2: on 17 Sept. 2012 (am); 3: on 17 Sept. 2012 (pm);
4: on 18 Sept. 2012; 5: on 19 Sept. 2012; 6: on 24 Sept. 2012;
7: on 28 Sept. 2012; 8: on 18 Oct. 2012; 9: on 19 Oct. 2012;
10: on 22 Oct. 2012; 11: on 29 Oct. 2012; 12: on 31 Oct. 2012.
Fig. 11. Configuration of intelligent comprehensive control mitigation strategy of a PEMFC
powered system for SRS applications.
Fig. 12. Power supply switching from PEM fuel cell to battery when the current is
interrupted.