The document experimentally studies the dynamic performance of a high-temperature PEM fuel cell under varying current loads. It finds that the cell exhibits hysteresis in polarization curves when current is swept from low to high and back. Under anodic flow-through operation, voltage undershoots and overshoots are less severe at high currents. The peak performance is higher under anodic dead-end operation, but decreases after purging and the shape of the dynamic voltage curve is similar under different purging intervals.

1. Dynamic performance of a high-temperature PEM fuel cell e An
experimental study
Caizhi Zhang a, b, *
, Zhitao Liu c
, Weijiang Zhou b
, Siew Hwa Chan a, b, **
, Youyi Wang d
a
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798, Singapore
b
Energy Research Institute at Nanyang Technological University, 50 Nanyang Avenue, Singapore, 637553, Singapore
c
The State Key Laboratory of Industrial Control Technology, Institute of Cyber-Systems and Control, Zhejiang University, Hangzhou, 310027, China
d
School of Electrical & Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798, Singapore
a r t i c l e i n f o
Article history:
Received 12 March 2015
Received in revised form
21 June 2015
Accepted 9 July 2015
Available online xxx
Keywords:
HT-PEMFC (high-temperature PEM fuel cell)
Flow-through mode
Dead-end mode
Dynamic voltage response
a b s t r a c t
The dynamic behaviour of a fuel cell under varying of load is important for control and optimization.
Experimental study on dynamic voltage of a HT-PEMFC (high-temperature PEM fuel cell) was conducted
under 5 different currents operating on an anodic flow-through mode, dead-end mode with fixed
purging intervals and dead-end mode with varying purging intervals after obtaining their forward and
backward polarization curves. The results revealed that the hydration/dehydration processes of phos-
phoric acid used in HT-PEMFC would affect the dynamic behaviour of the fuel cell, and can be concluded
by a few observations: (1) Hysteresis phenomenon was captured in the polarization study when cell
current is swept from low to high (forward sweeping), then high to low (backward sweeping); (2) Under
anodic flow-through mode, the magnitudes of voltage undershoots and overshoots were less severe at
high current than those at the low current operation; (3) The peak performance of HT-PEMFC under
anodic dead-end mode operation outperformed that under anodic flow-through mode. However, the
performance of HT-PEMFC reduced gradually after the purging and the shape of the dynamic voltage
curve under the longest purging interval overlapped with that under shorter purging intervals.
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
Fuel cell is an energy conversion device, which directly converts
the chemical energy of a fuel to the electricity electrochemically
[1,2]. It is a promising power source with high efficiency and
environmentally friendly [3,4]. Hydrogen is considered as an ideal
fuel for fuel cells. When a fuel cell operating on pure hydrogen, it
can achieve the best performance with zero pollution emissions [5].
However, the challenges of pure hydrogen production, storage and
distribution are obstacles to the commercialisation of the fuel cells.
The on-board fuel reforming for producing hydrogen-rich gases
from methanol, ethanol or other renewable hydrocarbons offers the
fuel flexibility solution to fuel cells applications [6e8]. The HT-
PEMFC (high-temperature PEM fuel cell) is considered to be an
alternative candidate for mobile and stationary power applications,
because it can operate on syngas (a reformed fuel) directly pro-
duced from the on-board fuel reforming [9e11].
Currently, the most extensive researches associated with HT-
PEMFC are related to the materials [9], such as the membranes
[12e14], catalysts [15e18] and GDE (gas diffusion layer) [19], and
heating strategies applied to efficiently and effectively increase the
temperature from its ambient temperature to the operating tem-
perature [20e26]. To prepare the HT-PEMFC for wide application,
understanding the dynamic behaviour of a fuel cell, is important for
the energy system design, integration, control and optimization
[3,7,27e29]. However, the study on dynamic behaviour of a HT-
PEMFC during load variation is scarce. Zenith et al. [28] investi-
gated the dynamics of a HT-PEMFC based on PBI (poly-
benzimidazole) membrane by applying the resistors as loads to the
cell from high to low and vice versa. A semi-empirical transient
model was then developed based on the collected data. Sousa et al.
[30] demonstrated a dynamic two-dimensional, non-isothermal
model to simulate the transient response in step changes of the cell
potential. Peng et al. [31] developed a transient three-dimensional
* Corresponding author. School of Mechanical and Aerospace Engineering,
Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798,
Singapore.
** Corresponding author. Energy Research Institute at Nanyang Technological
University, 50 Nanyang Avenue, Singapore, 637553, Singapore.
E-mail addresses: M110050@e.ntu.edu.sg (C. Zhang), mshchan@ntu.edu.sg
(S.H. Chan).
Contents lists available at ScienceDirect
Energy
journal homepage: www.elsevier.com/locate/energy
http://dx.doi.org/10.1016/j.energy.2015.07.026
0360-5442/© 2015 Elsevier Ltd. All rights reserved.
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2. model to predict the species concentration, current distribution
and local transient potential of a PBI membrane during the po-
tential variation. Generally, a fuel cell can operate on either a flow-
through mode or a dead-end mode at the anode side. The flow-
through mode of operation refers to free flow of hydrogen
through the anodic outlet of the cell, while the dead-end mode of
operation refers to periodically on-off control of a solenoid valve
mounted on the anodic outlet. According to our previous results in
Refs. [32,33], the HT-PEMFC exhibits highly dynamic especially
under dead end mode, hence more investigations are needed
especially varying the load under anodic dead-end mode of oper-
ation because the accumulated water vapour in the anode would
lead to significant degradation of the cell performance.
The objective of this study is to experimentally characterise the
dynamic responses of a HT-PEMFC during load variation under the
anodic flow-through and dead-end modes of operation. The rela-
tionship of the dynamic behaviours of the two modes of operation
would be established and discussed thoroughly.
2. Experimental setup and procedure
The hardware setup and the related schematic diagram of this
study are shown in Fig. 1 (a) and (b), respectively. The measure-
ments presented in this paper were carried out using a single cell
assembly of a HT-PEMFC (7) with PBI membrane, which was
housed in an oven (8, Memmert GmbH). The operating tempera-
ture of the cell was controlled by the digital oven with high accu-
racy (less than ±0.5 C) and low overshooting (less than 1 C). The
membrane-electrode-assembly (MEA, Celtec®
P1000) was
purchased from BASF Fuel Cell GmbH (Frankfurt, Germany) with an
active area of 45 cm2
. The phosphoric acid content in the obtained
membrane is more than 95 wt% or up to 70 phosphoric acid mol-
ecules per PBI repeat unit in the PBI matrix. The catalyst is the
Vulcan XC 72 supported Pt-alloy and the loading are 0.75 mgPt/cm2
and 1 mgPt/cm2
for cathode and anode, respectively. In addition,
the fabrication processes and more properties of the MEA has been
reported elsewhere [34,35]. The MEA was clamped by two bipolar
plates and two metal end plates. Two 5-pass serpentine flow field
embedded in the bipolar plates of anode and cathode attached
directly to the two metal end plates for current collection and for
improvement of the heat exchange with the oven. The operating
pressure of the fuel at the anode inlet was set at 50 mbar (gauge
pressure) via a pressure regulator (2), while the solenoid valve at
anode outlet (5) was used to switch the operation mode and
purging process. The solenoid valve was controlled by a micro-
controller unit (4, MCU (Microcontroller unit)). In the dead-end
mode of operation, the dwell time of valve closure and opening
were specified in each test. At the cathode, air as oxidant was
supplied via a mass flow controllers (10, MFC (Mass flow control-
lers), Alicat) in flow-through mode operation. The fuel cell testing
system (6, KIKUSUI) included impedance meter KFM2150, e-load
PLZ-664WA and FCTester software. The software was used to pro-
gramme the load to simulate different current load, while the data
(transient voltage) was record accordingly.
Firstly, the HT-PEMFC was heated and maintained at 160 C, and
then the anode and cathode were fed with dry hydrogen and dry
air, respectively. When the HT-PEMFC reached a steady-state per-
formance at current density of 0.2 A/cm2
, the polarization curves
were recorded before studying the transient voltage under the
flow-through mode of operation with an anodic inlet pressure of
0.05 bar. Subsequently, the transient voltage curves under fixed and
varying purging time were measured. The test conditions are
summarised in Table 1.
3. Results and discussion
3.1. Polarization characteristics
The polarization curves, as shown in Fig. 2, were measured after
the cell has reached the steady-state performance at given pressure
and temperature. The current load then increased from 0 to 26 A
(forward sweep) and back to 0 A (backward sweep) with 1 A per
step in every 10 s intervals. The hysteresis phenomenon was
observed.
In Nafion membrane based PEM fuel cell, hysteresis phenome-
non indicates the dynamic behaviour of the electrochemical device.
The reason of hysteresis is mainly related to the water flooding,
which is determined by the properties of materials and operating
conditions [36,37]. Hou [36] presented that the water storage ca-
pacity or water remove ability of GDL (gas diffusion layers) would
affect cell performance and location of cross point of polarization
curves (forward sweep curve and backward sweep curve) under
certain operating conditions, i.e. the cross points of polarization
curves appeared at the high current region when the SGL 10AA
carbon paper or E-TEK B1A carbon cloth used as GDL, because of the
GDL with large water storage capacity or easy water remove ability.
In this study, though the hysteresis phenomenon is quite similar
with the aforementioned characteristics of the cell with SGL 10AA
carbon paper and E-TEK B1A carbon cloth as GDL in Refs. [36], an
explanation of better performance during backward sweep
comparing with forward sweep is the variation of hydration level in
the membrane, because the humidity level in membrane directly
influences the conductivity of membrane. When the current in-
creases, more water vapours were generated and adsorbed by the
2
8
6
5
4
9
7
10
3
(a)
(9) Testing
software
(4) MCU
controller
+ -
(8) Oven
(7) HT-PEMFC
Exhaust
T
(6)
E-load
3
2
1
MFC
11
12
Air
5
H2
Exhaust
Anode
Cathode
(1, 12) compressed gas cylinder; (2, 11) pressure regulator;
(3, 5) solenoid valve; (10) mass flow controller
10
(b)
Fig. 1. Experimental setup (a) hardware setup, (b) schematic diagram.
C. Zhang et al. / Energy xxx (2015) 1e7
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3. phosphoric acid (PA, H3PO4) resulting in an increase of internal
conductivity. During backward swapping, the membrane was still
saturated or in higher humidity level since PA is a strong acid with
affinity of water adsorption, thus it exhibited an improved perfor-
mance [38,39]. In addition, the voltage difference at lower current
range is higher than that at higher current range. That is explained
as the increased water content in electrolyte results in increase of
the electrode kinetics. Increase of water content in PBI-H3PO4
system would increase (1) the exchange current density since the
additional protons are directly involved in the oxygen reduction
reactions, (2) the diffusion coefficient of oxygen since the viscosity
of the amorphous H3PO4 is decreased in the presence of water
leading to facilitate the oxygen transport within itself, and (3) the
solubility coefficient of oxygen because of reducing the polarization
in the electrolyte [16,39]. The increase of oxygen diffusion and
oxygen solubility in PBI-H3PO4 system facilitates the oxygen
permeability to the three phase boundary, thus improving the
utilization of catalyst particles.
3.2. Dynamic characteristics without anodic purging
Fig. 3 shows the dynamic response of voltage under the step-up
and step-down current load variation in the flow-through mode of
operation. The decrease and increase of cell voltage was associated
with the current load varying from 4 to 20 A then back to 4 A at 4 A
per step change. According to the indication of the horizontal dash
line, the maximum voltage under step-up current is lower than the
minimum voltage under step-down current. This is another hys-
teresis example of HT-PEMFC under dynamic operation. A com-
parison between the final voltages of each current step under flow
through mode and those in the polarization curves is shown in
Fig. 4. With increasing the current, the final voltage curve under
flow through mode deviated more obviously from the polarization
curve. It can be explained as the dry air and low water production
maintained a close humidity level in membrane at low current load
(e.g. 4 A) during the two types of testing. However, increasing of the
current results in more hydrated membrane, hence cell perfor-
mance under flow-through mode testing is better than polarization
testing at high current load (e.g. 20 A).
In Fig. 3, the undershooting and overshooting of voltage were
observed during current step-up and current step-down, respec-
tively. After the fast recovery of undershooting and overshooting,
the cell settled at a stable performance in several minutes. The
magnitudes of undershooting and overshooting of voltage, which is
voltage difference between the peak value and that settled at stable
performance, are shown in Fig. 5. As can be seen, the magnitudes of
undershooting and overshooting peaks are less severe at high
current than that at low current. In general, the magnitudes of
undershooting and overshooting are affected by the combinational
effects of equivalent capacitance, temperature variation, reactants
concentration (oxygen and hydrogen) and the membrane conduc-
tivity [40,41]. The behaviour of undershooting and overshooting
observed in this study are quite consistent with those observed in
LT-PEMFC (Low-temperature PEM fuel cell) reported by Tang et al.
[27]. They explained that the undershooting was attributed to the
Table 1
List of experimental conditions.
Parameter Detail
Test temperature 160
C
Anode fuel Dry H2
Cathode fuel Dry air
Anode operation Inlet pressure of 0.05 bar and dead-end
mode with purging
Cathode flowrate (Air) Flow-through mode with
mass flowrate of 487 sccm
Data sampling time 0.5 s
0 5 10 15 20 25 30
0.3
0.4
0.5
0.6
0.7
0.8
0.9
V-I, forward
V-I, backward
Power-I, forward
Power-I, backward
Voltage
(V)
Current (A)
0
2
4
6
8
10
12
Power
(W)
Fig. 2. Polarization curves.
0 40 80 120 160 200
0.45
0.50
0.55
0.60
0.65
0.70
0.75
Voltage
(V)
Time (min)
Voltage @ flow-through mode
Setting current
0
4
8
12
16
20
24
Current
(A)
Fig. 3. Dynamic response of the HT-PEMFC voltage under current load variation in
flow through mode.
0 5 10 15 20 25
0.3
0.4
0.5
0.6
0.7
0.8
0.9
I/V @ forward sweep
I/V @ backward sweep
Voltage under flow-through mode@ forward sweep
Voltage under flow-through mode @ backward sweep
Voltage
(V)
Current (A)
Fig. 4. Comparison of steady-state voltages under step-wise operation with those in
the polarization curves.
C. Zhang et al. / Energy xxx (2015) 1e7 3
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4. increase in membrane resistance caused by electro-osmotic drag at
the anode side, which took several seconds to re-wet the anode
side by back-diffused water, while the overshooting was dominated
by the double layer capacitive effects. In HT-PEMFC, the effect of
capacitance on the voltage transient is rather short (less than
0.05 s) [31] and the variation of temperature were not too critical
because the system was designed to operate at constant tempera-
ture. In addition, H2 and O2 were more than sufficient in the
operation because of high stoichiometric ratio used in flow-
through operation mode. On the other hand, water vapour pro-
duced at the cathode can be adsorbed and transported by PA in the
MEA (membrane-electrode-assembly). The adsorption and release
of water vapour process in the membrane and GDL (gas diffusion
layers) would influence the proton conduction. Thus, the conduc-
tivity of the HT-PEMFC would account for the magnitude of
undershooting or overshooting.
The PA in MEA was reported to start releasing water molecules
and form pyrophosphoric acid (H4P2O7) by acid dimerization at
around 130e140 C under dry conditions. Pyrophosphoric acid is
highly reversible, as it can restore to PA by adsorbing the moisture
in a humidified condition [32,42,43]. Thus, release and adsorption
of water vapour is in an equilibrium state of a given current as the
water vapour generated is proportional to the current. This
implicitly means that the internal resistance of the cell is also
constant at a given current. Momentarily when current is stepped
up from low to high, high internal resistance was expected at the
initial state and decreased gradually with increasing water vapour
adsorption at higher current. Contrarily, the internal resistance was
lower at initial high current load and remained at low resistance
when the current is stepped down. Finally, the internal resistance
would restore to its intended value when the fuel cell reached its
steady state as there would be less water generated at low current
load. In summary, the resistance of membrane decreases in the
forward sweeping but increases in the backward sweeping. How-
ever, the change in membrane resistance ðDRmÞ is reduced with
increasing current. Chen et al. [42] reported this similar trend in the
EIS (electrochemical impedance spectroscopy) result of HT-PEMFC,
of which the resistance of the membrane at 160 C decreased with
the increase of current, but the decreasing rate has been reduced.
Thus, according to the Ohm's law, the magnitudes of voltage un-
dershoots and overshoots are expressed as DV ¼ i DRm. DV rep-
resents the voltage variation and i is the operating current.
Accordingly, the peak voltages of undershoot and overshoot would
be decreased when the current was changed from 4 A to 20 A and
increased from 20 A to 4 A.
3.3. Dynamic characteristics with purging
Fig. 6 shows the dynamic voltage response (red-solid line (in the
web version)) against the same current steps in Fig. 3 when the HT-
PEMFC operated under dead-end mode. The purging duration is
fixed at 0.4 s with purging intervals of 3 min at purging pressure of
0.05 bar. The transient voltage under flow-through mode (black-
solid line) as shown in Fig. 3 is inserted in Fig. 6 for comparison
purpose. The dash line in Fig. 6 shows the peak voltage in the dead-
end mode. Comparing the peak voltage curve (dash line) with the
transient voltage curve under flow-through mode, the former has
shown better performance. It can be explained as improving the
conductivity of electrolyte by increasing the partial pressure of
water vapour in the anode side, because the water vapour trans-
ported from the cathode was accumulated in anode during anodic
dead-end mode of operation. Daletou et al. [44] reported that the
ionic conductivity of the membrane has increased by about 4.5
times with increasing steam partial pressure from 0.5 to 10 kPa at
170 C. Fig. 7 shows the difference between maximum peak and
minimum peak voltages under the dead-end mode (red-solid line
(in he web version) in Fig. 6) and difference between the maximum
peak voltage under the dead-end mode and that under the flow-
through mode. As can be seen, the voltage change was more
obvious when the current increased from 4 to 20 A since the
hydrogen is more diluted by the accumulated water vapour at
higher current. Though the voltage response under the dead-end
mode is more fluctuating than that under the flow-through
mode, the trend of the two curves (dash line and black-solid line)
shown in Fig. 6 is quite similar for all current steps, which means
that the purging process does not affect the transient voltage due to
the current load, hence the peak voltage of HT-PEMFC under dead-
end mode can be predicted from the voltage obtained at the flow-
through mode with suitable correction.
An investigation of dynamic behaviour of HT-PEMFC under
varying purging intervals were conducted with purging intervals
increased from 2 min to 6 min (i.e., 2, 3, 4, 5 and 6 min) and the
purging duration fixed at 0.4 s. The current variation step is same as
that in Figs. 3 and 6, but the testing period of each current step was
increased to 60 min. Each current step included 3 cycles of the 5
purging intervals (2, 3, 4, 5, 6 min). The transient voltage is shown
in Fig. 8(a). Fig. 8(b) shows a group of voltage curves abstracted
from Fig. 8(a), which is the second cycle of the 5 purging intervals in
each current step. As can be seen from Fig. 8(a) and (b), the cell
performance of each cycle is affected by the accumulated water
0 4 8 12 16 20 24
0.000
0.004
0.008
0.012
0.016
0.020
Voltage
(V)
Current (A)
Voltage of undershoot
Voltage of overshoot
Fig. 5. Magnitude of undershooting and overshooting voltage.
Fig. 6. Dynamic voltage response of HT-PEMFC under dead-end mode with fix purging
intervals.
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5. vapour and is aggravated under longer purging intervals and higher
current loads because of more residual water in the anode of the
fuel cell. The worse case occurred at the 5th purging intervals
(6 min) and at 20 A. It can be understood that the fuel dilution
would affect the cell performance leading to high overpotential in
two aspects, i.e., the Nernstian losses and the reaction losses. The
combined losses hconc can be expressed as [2]:
hconc ¼
RT
nF
1 þ
1
a
ln
jL
jL j
(1)
where the constants, R, T, n, F, a, represent universal gas constant,
temperature (K), number of moles of transferred electrons, Fara-
day's constant, transfer coefficient, respectively; j is operating
current density varied (0.089e0.444 A/cm2
and 0.444e0.089 A/cm2
in this study); jL is limiting current density and expressed as
jL ¼ nFDeff c0
R
d
(2)
Where Deff is effective diffusivity; d is the thickness of electrode; c0
R
represents the bulk reactant concentration. From Eqs. (1) and (2),
one can see that the limiting current density is proportional to the
concentration of the fuel. With fuel dilution, the limiting current
would decrease gradually leading to an increase in anode over-
potential, because on one hand the fuel has been consumed but on
the other hand water vapour accumulation increases due to the
water vapour gradient across the membrane [45]. Furthermore, the
higher current load would result in more water generated at the
cathode, thus higher water vapour gradient across the membrane
to facilitate the water vapour transported and accumulated in the
anode [32]. Thus, the worse case occurred at long purging intervals
and at high current load. Therefore, the purging intervals should be
controlled properly to prevent the performance degradation due to
excessive water vapour and to avoid purging too frequently causing
reduced fuel efficiency.
Fig. 8 (c) shows that the voltage curves abstracted from Fig. 8 (b)
in each purging intervals and are re-drawn on the same xey axis.
According to the result in Fig. 8 (c), it indicates that the shape of
dynamic voltage curve under the longest purging interval (6 min)
overlapped the dynamic voltage curve under shorter purging in-
tervals (i.e. 3 min), which means that the characteristics of dynamic
voltage can be described by the longest purging intervals and save
the effort to understand the dynamic behaviour under different
current and purging intervals. Hence, the implementation of con-
trol design and optimization can be carried out based on the
characteristics of different current loads at longest purging
intervals.
0 5 10 15 20 25
0.000
0.005
0.010
0.015
0.020
0.025
0.030
Purging duration @ 0.4S
Purging interval @ 3 min
Voltage
difference
(V)
Current (A)
Peak to peak voltage @ forward sweep
Peak to peak voltage @ backward sweep
Peak to flow-through @ forward sweep
Peak to flow-through @ backward sweep
Fig. 7. Voltage variation of peak to peak of the dynamic voltage under fixed purging
intervals.
Fig. 8. (a) Transient voltage response under varying purging intervals and current, (b)
the second cycle of the 5 purging intervals at each current step, (c) voltage curves at
the same current loads are abstracted from Fig. (b) at different purging intervals and
are re-drawn on the same time axis.
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6. 4. Conclusions
In this paper, the steady-state performance and dynamic per-
formance of a single HT-PEMFC based on PBI membrane has been
investigated experimentally. The hysteresis phenomenon has been
observed and analysed in the polarization behaviour. The dynamic
responses have been analysed in detail when the current load was
varied under anodic flow-through mode and dead-end mode of
operation. The performance of the cell under backward sweeping is
better than that of forward sweeping because of the aforemen-
tioned hysteresis effect. The typical transient phenomena, such as
undershoot voltage, overshoot voltage and purging effects on
voltage, were discussed and concluded as follows: (I) the under-
shoot and overshoot of voltages in current step-up and step-down
were mainly associated with the membrane hydration state. (II) The
magnitudes of the overshoot and undershoot voltage from each
peak to respective steady-state reduced at high current load state.
(III) The peak performance of the cell was improved when the cell
operated under anodic dead-end mode with purging introduced as
compared with that under flow-through mode. (IV) Though the
performance of cell exhibited more fluctuating in nature under
anodic dead end mode, the trend of the peak voltage was tally with
the performance under flow-through mode. (V) The purging in-
tervals and current load have significant effects on the cell perfor-
mance. The transient curves of voltage under different purging
intervals were quite repeatable and the transient voltage curve of
long purging interval would overlap the voltage curve of short
purging intervals, which suggest a simple way for one to under-
stand the dynamic behaviour under different current and purging
intervals. Hence, the control design and optimization can be carried
out based on the characteristics of the longest purging interval at
different current. These results are useful for mathematical model
development and validation as well as offering guidelines for
effectively dynamic control and water management of a HT-PEMFC.
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