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The use of PEM unitised regenerative fuel cells in solar- hydrogen systems for remote area power supply
Doddathimmaiah and Andrews, 2006
The use of PEM unitised regenerative fuel cells in solar- hydrogen systems for
remote area power supply
Arun K. Doddathimmaiah and John Andrews*
School of Aerospace, Mechanical and Manufacturing Engineering
RMIT University
Bundoora, Melbourne 3083
AUSTRALIA
*Author for correspondence
E-mail: [email protected]
ABSTRACT
Remote area power supply (RAPS) is a potential early market for renewable energy – hydrogen
systems because of the relatively high costs of conventional energy sources in remote regions. Solarhydrogen RAPS systems commonly employ photovoltaic panels, a Proton Exchange Membrane
(PEM) electrolyser, a storage for hydrogen gas, and a PEM fuel cell. Currently such systems are more
costly than conventional RAPS systems employing diesel generator back up or battery storage.
Unitised regenerative fuel cells (URFCs) have the potential to lower the costs of solar hydrogen RAPS
systems since a URFC employs the same hardware for both the electrolyser and fuel cell functions.
The need to buy a separate electrolyser and a separate fuel cell, both expensive items, is thus
avoided. URFCs are in principle particularly suited for use in RAPS applications since the electrolyser
function and fuel cell function are never required simultaneously. The present paper reports
experimental findings on the performance of a URFC compared to that of a dedicated PEM
electrolyser and a dedicated fuel cell. A design for a single-cell PEM URFC for use in experiments is
described. The experimental data give a good quantitative description of the performance
characteristics of all the devices. It is found that the performance of the URFC in the electrolyser mode
is closely similar to that of the stand-alone electrolyser. In the fuel cell mode the URFC performance is,
however, lower than that of the stand-alone fuel cell. The wider implications of these findings for the
economics of future solar-hydrogen RAPS systems are discussed, and a design target of URFCs for
renewable-energy RAPS applications proposed.
KEY WORDS: Solar hydrogen, remote area power supply, unitised regenerative fuel cell, proton
exchange membrane
1
INTRODUCTION
Renewable energy - hydrogen systems for remote area power supply (RAPS) applications are a
promising early market for hydrogen production, storage and utilisation technologies, because of the
high cost of both conventional RAPS systems relying on diesel or petrol generators, and solar
photovoltaic or wind systems employing batteries for storage. Critical limitations of the latter are that
batteries can only store electrical energy for a relatively short period, typically only a few days, in the
absence of recharging, and have a short overall lifetime. Hydrogen storage systems have the potential
on the other hand to provide low-loss storage on a season-to-season basis (that is, for six months or
more) and should be able to be designed with a lifetime much longer than batteries.
There is a growing body of experience with both solar and wind hydrogen systems for RAPS
applications. Some experimental and demonstration solar-hydrogen systems in the power range 1.5
kW to 250 kW have been built and operated [1]. Photovoltaic cells have generally been used as the
renewable energy generation technology and Proton Exchange Membrane (PEM) fuel cells to convert
stored hydrogen back to electricity. Increasingly PEM electrolysers are also being employed in RAPS
applications [2]. However, there is need for considerable further cost reductions for solar/wind –
hydrogen systems to become economically competitive, and design development and system testing
before commercial products can be made and sold [3].
World Hydrogen Energy Conference,
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The use of PEM unitised regenerative fuel cells in solar- hydrogen systems for remote area power supply
Doddathimmaiah and Andrews
The present paper explores a promising opportunity for reducing the cost of the hydrogen subsystem
used in a RAPS application, namely the use of recently-developed Unitised Regenerative Fuel Cell
(URFC) systems based on PEM technology. A PEM URFC combines the electrolyser and fuel cell
functions in a single unit, and the concept has attracted considerable research interest [4]. The URFC
concepts is ideally suited for a solar or wind – hydrogen RAPS system since in this application the
electrolyser function is never needed at the same time as the fuel cell function. If the capital cost of the
URFC can be kept much lower than the combined cost of a separate electrolyser and fuel cell, and its
efficiencies in both modes are close to those in separate units, then its use in solar-hydrogen RAPS
systems could lead to overall cost savings [5, 6].
2
UNITISED REGENERATIVE FUEL CELLS
2.1
Principles of a URFC
In a conventional PEM electrolyser, electrical energy plus some heat drawn from the environment is
used to split water into hydrogen and oxygen:
H2O + electricity + heat → H2 + ½ O2
In a PEM fuel cell the reverse reaction takes place as hydrogen is recombined with oxygen, the latter
usually coming from air to avoid having to store oxygen as well as hydrogen, with the production of
electricity, water and heat.
H2 + ½ O2 → H2O + electricity + heat.
In the usual hydrogen production and storage system for RAPS or other applications, a stack of PEM
cells is used for the electrolysis function, and a separate stack of PEM cells for the fuel cell function.
The innovative feature of the URFC concept is to use a specially-designed single stack of PEM cells
for both the electrolysis and fuel cell functions [4]. Since the basic structure of a dedicated PEM
electrolyser and a dedicated PEM fuel cell stack is the same, the use of a single stack to perform both
functions offers the prospect of substantial cost reductions.
A schematic of a URFC is shown in Figure 1, and the various electrochemical reactions that take place
in each mode are presented in Figure 2. In the electrolysis mode (lower half of Figure 2) water is
introduced at the anode where it is split by the electric field in combination with the catalyst into
oxygen, protons, and electrons. The oxygen evolves as gaseous O2 at the surface of the electrode,
while the protons are driven through the membrane and the electrons move through the external
circuit. At the cathode, the protons combine with the electrons to evolve gaseous hydrogen. In the fuel
cell mode, the hydrogen and oxygen are supplied to the respective electrodes, and electricity is
generated producing water once again. In principle this water can be recycled for use once again in
electrolysis.
Electrolysis mode
Fuel cell mode
Water
Oxygen from air
Water
Electricity
URFC
stack
Electricity
H2
H2
Heat
Oxygen
Hydrogen storage
Figure 1: A schematic of a URFC system for storing electrical energy as hydrogen and then reusing this
hydrogen for producing electricity.
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Doddathimmaiah and Andrews
Fuel cell
Mode (Discharging)
–
4e
(–)
(+)
4H+
Oxygen
O2 + 4H+ + 4e−
Proton exchange membrane
Hydrogen
electrodes
Oxygen
electrodes
4H+
Oxygen
+
−
4H 4e + O2
Process water
4H+ + 4e–
2H2
2H2O
Product water
2H2O
Hydrogen
Hydrogen
+
4H + 4e
(+)
−
2H2
(–)
–
4e
Electrolyser cell
mode (Charging)
Figure 2: Electrochemical reactions taking place in the electrolysis (lower) and fuel cell modes (upper) of
a URFC. Note that the entire cell area is used in both modes.
The URFC in combination with a storage system for the hydrogen thus serves as a store for electrical
energy in RAPS or a range of other applications. In a renewable-energy RAPS system, any surplus
solar or wind energy input over the load to be supplied can be fed into the URFC in electrolysis mode
(or ‘E-mode’) to produce hydrogen gas for storage. When the solar or wind input is insufficient to meet
the load, the deficit can be met by reversing the URFC to fuel cell mode (‘FC-mode’) and drawing on
stored hydrogen and oxygen from the air as the input fuels. In such an application, simultaneous
operation in E-mode and FC mode is never required, so the URFC is ideally suitable. The overall
process is silent, relatively efficient, zero greenhouse emissions, and with proper design is very safe.
2.2
Work on URFCs to date
Mitlitsky et al. pointed out that regenerative fuel cells have a wide range of potential applications
including energy storage devices coupled to RE sources, power plants for automobiles and propulsion
systems for satellites [7]. URFCs are also being considered for use in NASA missions [8].
A number of URFCs employing PEM membrane electrode assemblies capable of working reversibly in
either the electrolyser or fuel cell modes have been developed [9]. For example, Proton Energy
Systems, in association with ATK Thiokol and General Dynamics, have developed a form of URFC
system called ‘Unigen’ primarily for use in backup electricity supplies in spacecraft [10]. However, this
system is arguably not a fully-fledged URFC since it still incorporates separate hardware for the
electrolyser and fuel cell functions. Lynntech Inc. USA, commercially offers Membrane Electrode
Assemblies (MEAs) for URFCs with varying catalytic loading and membrane area [11].
Cisar et al. reported that mixing/alloying platinum black with a mix of iridium oxide and ruthenium oxide
is preferred because it reduces the overall cost of MEAs while minimising the over potential for oxygen
evolution [11]. Under fuel cell conditions when oxygen reduction is occurring, the platinum component
is active while the ruthenium-iridium oxide component is inactive. Under electrolyser conditions and
oxygen evolution, both platinum and ruthenium-iridium oxide components are active.
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The use of PEM unitised regenerative fuel cells in solar- hydrogen systems for remote area power supply
Doddathimmaiah and Andrews
Dhar [12] reported that PEM URFC energy storage systems can achieve comparable performance to
individual electrolyser and fuel cell units. Ongoing research work has focused on bifunctional
electrodes and MEA development, for example, the research programs at AIST in Japan and the
Dalian Institute of Chemical Physics in China [13]. A project to develop reversible fuel cells to the point
of commercial applications in stand-alone PV systems and uninterruptible power supplies, called
Revcell, is in progress in The Netherlands [14].
2.3
The power ratio for a URFC
A number of relevant features of URFC operation can be gleaned from a consideration of typical
polarisation curves for E-mode and FC-mode functioning of a PEM cell per unit effective membrane
area. By normalising the curves in the two modes according to unit area we are in effect representing
the URFC situation in which a cell with a given effective membrane area is operating reversibly in the
two modes. Figure 3 thus shows indicative polarisation curves (voltage versus current density) for a
dedicated PEM electrolyser single cell and a dedicated PEM single fuel cell, as well as the
corresponding curves for a PEM URFC.
Voltage (V)
A´
A
URFC electrolyser
URFC fuel cell
Voc
Dedicated electrolyser
Vrev = 1.23
B´
B
Dedicated fuel cell
Current density (A/cm2)
ifc,max
Figure 3: Polarisation curves in electrolyser and fuel cell modes in dedicated electrolyser, fuel cells and
URFC. Points A and A´ and B and B´, are the maximum power points in the E and FC modes respectively.
The corresponding power density versus current density curves are shown in Figure 4.
Power density
A´
A
URFC electrolyser
URFC fuel cell
B´
Dedicated electrolyser
Dedicated fuel cell
B
2
Current density (A/cm )
Figure 4: Power density versus current density curves corresponding to the URFC in E-mode and FC
mode, and a dedicated PEM electrolyser and fuel cell, whose polarization curves are shown in Figure 3.
Points A and A´, and B and B´, are the maximum power points in the E and FC modes respectively.
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The use of PEM unitised regenerative fuel cells in solar- hydrogen systems for remote area power supply
Doddathimmaiah and Andrews
The energy efficiency of an electrolyser (ηe, defined as energy content of hydrogen produced, based
on HHV divided by the electrical energy input) is given by:
ηe = µF (1.23/ Vcell)
where ηF is the Faraday efficiency reflecting any loss of hydrogen produced within the cell system [15].
Hence the performance of an electrolyser improves as the excess of Vcutin over the reversible potential
of 1.23 V for the water decomposition and recombination reaction decreases, and as the positive
slope of the V-I curve decreases. Generally a URFC in E-mode will have energy efficiency a little lower
than that of a comparable dedicated electrolyser, and this feature is reflected in Figure 3 by both the
higher cut-in voltage and greater slope for the URFC’s E-mode curve.
The energy efficiency of a fuel cell (ηfc, defined as electrical energy output divided by the energy
content of hydrogen fuel consumed, again based on HHV for consistency) is given by:
ηfc = µfuel (Vcell/1.23)
where ηfuel is the fuel utilisation coefficient reflecting any loss of input hydrogen within the cell system
[15]. The performance of a fuel cell therefore increases with the open circuit voltage, and decreases
as the modulus of the negative slope of its V-I curve increases (that is, with the rate of decline of cell
voltage with current density. The curve for the URFC in FC-mode is thus shown as just below that of
the dedicated fuel cell curve in Figure 3, and with a slightly more negative slope.
The different signs of the slopes in E-mode and FC-mode can be explained as follows. In FC-mode,
the cell is a source of electromotive force that drives current around the external circuit. As the current
produced increases, the potential difference across the cell reduces. In effect the current flow acts to
reduce the charge build up (net negative on the hydrogen electrode and net positive on the oxygen
electrode) produced by the electrochemical reactions at each electrode. In E-mode, the electromotive
force driving the effective current in the external circuit and through the cell is provided by the external
power source, and the current flows in the reverse direction to that in FC-mode. Now as the voltage is
increased above the cut-in voltage, current steadily increases. The positive slope reflects the resistive
and other irreversible energy losses in the cell. The greater these losses, the greater the positive slope
will be.
The power input to the cell in E-mode increases approximately quadratically with increasing current
density, and the maximum power input occurs when the maximum allowable current density is
reached (point A for the dedicated electrolyser and point A’ for the URFC in Figures 3 and 4). The
power output in FC-mode increases to a maximum and then falls to zero at the maximum current
density (short-circuit condition). The maximum power points are shown by the points B and B’ in
Figures 3 and 4 for the dedicated electrolyser and URFC respectively.
It can be deduced from this analysis that the maximum power input to a URFC will always be several
times the maximum power output. Drawing on typical polarisation curves for a URFC in its two modes
as reported in this paper and elsewhere [12, 16] the ratio of power in/power out for a URFC is at best
likely to be just over three and in practice four or more. Hence if we want a URFC with a 1 kW peak
electrical power output for a RAPS system, its maximum power input is likely to be around 4 kW. This
power-ratio feature is clearly important in the design of URFC-based solar-hydrogen systems for
RAPS applications.
2.4
Design aims for URFCs
Drawing upon this theoretical analysis, and work on URFCs done to date, a number of key design
aims for URFCs suitable for RAPS applications can be proposed.
Firstly the performance of a URFC in E-mode and FC-mode in terms of energy efficiency needs to be
equal to or only slightly lower than that of a comparable dedicated PEM electrolyser and dedicated
PEM fuel cell. To achieve this goal, a major technical challenge is to develop a stable and highly
active bifunctional oxygen electrode. The choice of electrocatalysts to use is critical in this endeavour
+
[17]. Platinum in its reduced form is the best electrocatalyst for oxygen reduction (reaction with H ions
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The use of PEM unitised regenerative fuel cells in solar- hydrogen systems for remote area power supply
Doddathimmaiah and Andrews
and electrons to form water) but is not is not the best catalyst for water splitting (to form oxygen gas,
+
H ions and electrons).
Swette et al. showed that iridium oxide evolves oxygen at a far lower over potential than platinum and
would be a good candidate for oxygen evolution even though a relatively poor candidate for oxygen
reduction [18].
Zhigang, Baolin and Ming used Pt black and IrO2 as oxygen electrode catalysts and have
2
demonstrated that 50 wt% Pt + 50 wt% IrO2 with a catalyst loading of just 0.4 mg/cm performed well
in both electrolyser and fuel cell modes [13]. Liu et al [16] have reported the preparation of composite
oxygen electrodes with Pt-black and iridium oxide (50 wt% Pt + 50 wt% IrO2) that functioned in both Emode and FC-mode with fairly constant performance over 25 cycles between modes.
Besides the development of reversible electrodes, proper and reliable reactant management is very
important to achieve better URFC performance [7].The management of reactants inside the URFC cell
is greatly dependent on the material and construction of the cell/stack. Maintaining sufficient water
flow to the catalyst layer in the oxygen electrode during electrolysis mode, while avoiding flooding of
gas diffusion channels in the backing layer of this electrode on switching to electrolysis mode, is vital
for reliable functioning of URFC. Avoidance of flooding requires keeping the quantity of water in and
around the oxygen electrode to the minimum that is needed to allow electrolysis to take place
unhindered by water shortage, and effective removal of water (liquid and vapour) that is produced in
fuel cell operation. This is usually accomplished by the presence of a diffusion backing/layer which has
a mixture of hydrophilic and hydrophobic regions. The hydrophilic regions ensure sufficient delivery of
water to the catalyst so that the membrane is hydrated during electrolyser operation. The hydrophobic
regions ensure sufficient delivery of oxygen to the electrocatalyst for oxygen reduction reaction to take
place during fuel cell reaction.
In conventional PEM fuel cells, the gas diffusion backing layer is generally made of a porous carbon
material such as carbon cloth or paper. These are not suitable for a URFC because oxidation of
material surfaces in the oxygen electrode and backing layers due to the combined presence of water
and oxygen must be avoided. Ioroi et al. [19] investigated a variety of titanium gas-diffusion backings
(GDBs) coated with different amounts of polytetrafluoroethylene (PTFE) loadings for the oxygen
electrode of a URFC. It was found that the URFC performance significantly depended on the amount
of PTFE loading in the backing layer on the oxygen electrode, with increasing loadings beyond a
certain amount leading to poorer performance. But URFCs with no PTFE-coated backing layer
performed very poorly. However, the PTFE coating on the GDB of the H2 electrode did not affect the
cell performance.
A second key design aim is to achieve a total system cost for a URFC that is well below the total cost
for a system comprising a comparable separate PEM electrolyser and PEM fuel cell. At this stage we
are seeking to develop a URFC for a solar/wind hydrogen RAPS system with a delivered power of 1 -2
kW that is 40% cheaper than a hydrogen subsystem employing a dedicated electrolyser and fuel cell.
This may be accomplished by keeping the costs of materials and construction of a URFC close to
those for a PEM electrolyser of the same membrane area and current density. It will also be essential
to keep the balance of system costs for the URFC - that is, for water management, thermal regulation,
and gas supply and extraction - to approximately the same level as or lower than the balance of
system costs for a separate PEM electrolyser and fuel cell with equivalent performance.
If it was possible to use a URFC in place of a separate electrolyser and fuel cell and reduce the
combine capital cost of these components by 40% without significant adverse effect on their
performance, the unit cost of the power from a solar-hydrogen system over its lifetime could be cut by
about 10% [20]. There could be an added reduction of cost if there are savings in balance of system
costs too.
3
DESIGN OF A SINGLE-CELL URFC
A single-cell proton exchange membrane URFC has been designed and is planned to be constructed
to compare its performance with a dedicated electrolyser and PEM fuel cell that we have constructed
at RMIT University. The design of this URFC seeks to realise optimal performance in both electrolyser
and fuel cell modes (Figures 5 and 6). The design is simple and provides for easy assembly and
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The use of PEM unitised regenerative fuel cells in solar- hydrogen systems for remote area power supply
Doddathimmaiah and Andrews
disassembly. The Membrane Electrode Assembly (MEA) used was purchased from Lynntech, Inc.,
®
USA. and employs a membrane made from Nafion - 115 with an active area of 3.175 cm x 3.175 cm
and a total membrane area of 5.08 cm x 5.08 cm. The catalyst loadings are as follows:
2
Anode catalyst: Pt Black and IrRuOx, 2.0 mg/cm loading
2
Cathode catalyst: Pt Black, 4.0 mg/cm loading.
A key feature of the design is the provision of a single water reservoir in the lowest horizontal channel
in the end plate in contact with the oxygen electrode. The water level in this reservoir is maintained at
a constant level throughout E-mode and FC-mode operation by adding or removing water from an
external water supply. The rate of water consumption in E-mode is estimated to be at maximum only
0.48 ml/hour, and the rate of water production in FC mode will be considerably less than this. The flow
rate of the water supply and removal is thus very low in both modes. The water wets a strip of the
porous backing layer along its lower edge and continually rises through this layer, which is hydrophilic,
by capillary action and diffusion through polymer material. Hence a continuous supply of water to the
catalyst sites during electrolysis is provided. The membrane itself is hydrated by using humidified
hydrogen and oxygen gas inputs from storage.
Design features to be investigated experimentally are:
•
•
•
•
The level of the water in contact with oxygen electrode
The optimal material and structure for the backing layer on the oxygen electrode to maintain
sufficient water supply during electrolysis and water removal during fuel cell operation
Catalyst loadings on the membrane electrode assembly, particularly on the oxygen electrode.
The configuration of the gas flow channels on both oxygen and hydrogen sides.
Hard stop
O2 from gas
storage
Tension bolts
Silicone rubber seal
O2 to gas
storage
H2 to gas
storage
O2 electrode
Membrane electrode
assembly
Catalyst layer
H2 electrode
Water to wet
gas diffusion
backing
Catalyst layer
H2 from gas
storage
Water level
maintenance
at level D
Tension bolts
Metallic end
plates and gas
flow channels
Hard stop
Figure 5: A design for a single-cell unitised regenerative fuel cell for use in experimental comparisons
with dedicated PEM electrolysers and fuel cells.
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The use of PEM unitised regenerative fuel cells in solar- hydrogen systems for remote area power supply
Doddathimmaiah and Andrews
Hole for gas
exit/entry
Channels cut
into metal
Hole for
water exit
Hole for
water supply
Figure 6: The gas flow channel configuration in the oxygen-side end plate in the URFC design (Figure 5).
The hydrogen-side end plate is similar except for the use of the hole at the bottom left for gas supply and
absence of the hole at the bottom right.
4
A URFC VERSUS A DEDICATED ELECTROLYSER AND FUEL CELL: EXPERIMENTAL
COMPARISONS
4.1
Experimental set-up
Experiments have been conducted to compare the performance of a dedicated PEM fuel cell and
dedicated PEM electrolyser with that of a purchased single-cell URFC obtained from Fuel Cell Store.
2
2
The URFC used had an active membrane area of 9 cm and total membrane area of 16 cm . The
2
anode and cathode electrodes are catalysed with 3 mg Pt/cm . There is no gas diffusion layer in the
MEA of this URFC.
In fuel cell mode, the polarisation curve of each test cell was measured using the experimental set-up
displayed in Figure 7. Gas supply for the fuel cell operation was controlled via valves. Hydrogen was
supplied through a storage tank initially filled by electrolyser operation. During cell operation,
provisions were made to humidify the incoming hydrogen gas. At each potential, a period of two
minutes was allowed for the current and hydrogen usage readings to stabilise. The cell was operated
at a room temperature of 18°C and atmospheric pressure.
+
FC
—
Figure 6:
V
Range
20V DC
A
Range
10 or 20A DC
Circuit for determining energy efficiency of fuel cell
During electrolysis, at the beginning 20 minutes were allowed for the temperature to equilibrate. The
cell was polarised to a constant voltage with a D.C power supply. At each potential, two minutes were
allowed for the current to stabilise. In most instances, a fairly stable current was obtained, but in a few
cases the current continued to drift downward with time. The reason for this drift may have been
improper gas removal resulting in a buildup of ohmic resistance at the electrode-membrane interface.
The current decay was observed throughout the experiment and was less during measurement of the
initial polarisation curve in electrolyser mode.
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Doddathimmaiah and Andrews
Energy efficiency was evaluated as the ratio of energy content of hydrogen produced based on the
High Heating Value (HHV) to the energy input to the electrolyser. Faraday efficiency was calculated as
the ratio of measured rate of hydrogen production to the theoretical maximum rate calculated from
current flow. The volumes of hydrogen produced in both URFC electrolyser mode and dedicated PEM
electrolyser were measured using the difference in water level of the storage tank. The variation of
current drawn with the input voltage was plotted.
Energy efficiency of the fuel cell was calculated as the ratio of energy supplied by the fuel cell to the
energy content (based on HHV) of hydrogen consumed. The fuel utilisation coefficient was calculated
as the ratio of measured cell voltage to the voltage calculated using the HHV and assuming all input
hydrogen is consumed. Hydrogen consumption was determined by the volume of water displaced
during fuel cell operation.
4.2
Results
The measured polarisation curves in electrolyser and fuel cell modes of the URFC are compared to
the corresponding curves for the dedicated PEM electrolyser and fuel cell in Figure 8. Power versus
current density curves for the various cells and modes are plotted in Figure 9, and hydrogen
production versus current density for the electrolyser modes in Figure 10. The associated variations in
energy efficiency with current density are presented in Figure 11.
2.1
Voltage (V)
1.8
1.5
URFC electrolyser
1.2
0.9
URFC fuel cell
0.6
Dedicated electrolyser
Dedicated fuel cell
0.3
0.0
0
50
100
150
200
250
Current density (mA/cm2)
Figure 7: Polarisation curves for the URFC in E and FC modes, and the dedicated PEM electrolyser and
fuel cell.
Power (Output/Input), W
2.0
1.6
URFC electrolyser
1.2
URFC fuel cell
0.8
Dedicated electrolyser
Dedicated fuel cell
0.4
0.0
30
50
70
90
110
130
150
170
190
210
230
250
Current density (mA/cm2)
Figure 8: Power versus current density curves for the URFC in FC mode and the dedicated PEM fuel
cell
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Current density (mA/cm 2)
200
160
120
80
URFC electrolyser
40
Dedicated electrolyser
0
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
Volume of hydrogen produced (cm 3/sec)
Figure 9: Current density versus volume of hydrogen produced
Energy efficiency
100
80
URFC electrolyser
60
URFC fuel cell
40
Dedicated electrolyser
20
Dedicated fuel cell
0
30
50
70
90
110
130
150
170
190
210
230
250
Current density (mA/cm2)
Figure 10: Energy efficiencies of the URFC in E and FC modes compared to those for the dedicated
electrolyser and fuel cell.
4.3
Discussion
The shapes of the experimentally-determined polarization curves for the dedicated PEM electrolyser
and PEM fuel cell (Figure 8) follow closely the forms expected from the general analysis in section 2
(Figure 3), as do the power versus current density curves (Figure 9 compared to Figure 4). From
Figure 9 it can be seen that the power ratio (maximum power in divided by maximum power out) is
about 4 for this URFC.
The hydrogen production rate of the dedicated PEM electrolyser was slightly greater than that of the
2
URFC in E-mode for a given current density. At a current density of 140 mA/cm , the hydrogen
3
production rate per unit cell area of the URFC was about 0.05 cm /min, while that of the dedicated
3
PEM electrolyser was 0.06 cm /min.
The performance of the URFC in fuel-cell mode was also slightly lower than that of the dedicated PEM
2
fuel cell. For example, at a current density of 60 mA/cm , the power output of the URFC was 0.12
2
2
W/cm compared 0.16 W/cm for the dedicated fuel cell.
The energy efficiency of the URFC in E-mode was only slightly lower than that of the dedicated PEM
electrolyser, being no more than 5 % points less than the latter over the full range of current densities.
The energy efficiency of the URFC peaks at about 90% and falls to just under 80% at the highest
current density. The energy efficiencies of the dedicated PEM fuel cell and that of the URFC in FC-
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Doddathimmaiah and Andrews
mode were much lower, the former peaking at 50% and then falling to below 30% at a current density
2
of 130 mA/cm . This could be because of open ended fuel cell operation which might have resulted in
low fuel cell utilisation co-efficient. The use of the HHV (rather than the Low Heating Value) in
calculating energy efficiencies also tends to accentuate the differences in the efficiencies obtained in
the two modes.
The energy efficiency of the URFC in FC mode was significantly lower than that of the dedicated PEM
fuel cell over the whole range of current densities, indicating that this particular URFC is far from
optimal for reversible operation. The low energy efficiency in URFC FC-mode was possibly due to the
oxygen electrode retaining too much water after operation in E-mode so that the diffusion of oxygen
gas to catalyst sites was inhibited.
The observations here that the performance of the URFC in electrolyser mode is very close that of a
dedicated PEM electrolyser bode well for the future application of URFCs in solar-hydrogen RAPS
systems. However, it clear that that performance of the URFC in FC-mode needs to be improved so
that it is much closer to that of a dedicated is only slightly less than that of a comparable dedicated
PEM fuel cell.
5
CONCLUSIONS
An analysis of typical voltage-current characteristic curves of a PEM cell with a given active membrane
electrode assembly active area indicates that the electrical power output of a URFC in FC mode is
likely to be less than a third of the maximum electrical power input in E-mode, and may be less than a
quarter in practice.
For URFCs to be economically preferable to a separate electrolyser and fuel cell in a hydrogen energy
storage system for renewable-energy RAPS applications, the following key design goals are
proposed:
• The performance of a URFC in E-mode and FC-mode in terms of energy efficiency that is
equal to or only slightly lower than that of a comparable dedicated PEM electrolyser and
dedicated PEM fuel cell.
• Capital costs of the URFC-based storage system that are 40% lower than those for a
comparable system employing a dedicated electrolyser and fuel cell.
An experimental comparison of a small single-cell URFC with a comparable dedicated PEM fuel cell
has been performed. The energy efficiency of the URFC in E-mode was only slightly lower than that of
the dedicated PEM electrolyser, being no more than 5 % points less than the latter over the full range
of current densities. In FC mode the energy efficiency of the URFC was significantly lower than that of
the dedicated PEM fuel cell, indicating that the particular URFC tested was far from optimisation for
reversible operation.
A design for a single-cell URFC using a commercially-available reversible membrane electrode
assembly is described. Experiments to compare the performance of this URFC with comparable
dedicated PEM electrolysers and fuel cells are in progress.
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