Download PEM Fuel Cell with Dead-ended Operation

PEM Fuel Cell with Dead-ended Operation
Michael Pien, Marvin Warshay, Steven Lis, Radha Jalan
ElectroChem, Inc., 400 West Cummings Park
Woburn, Massachusetts 01801 USA
ElectroChem, Inc. has demonstrated a dead-ended
fuel cell operating at very low excess oxygen requirement
and maintaining very stable performance. The fuel cell not
only showed significantly higher voltages at high current
density but also demonstrated the gravity-independent
operation. The dead-ended operation significantly enhances
the fuel cell power density by creating self-humidification
and obtaining high efficiency of gas utilization.
Introduction
One of the issues with PEM fuel cell system stability is related to the formation of
condensed water in the fuel cell causing cell flooding condition which results in severe
operational problems, and even cell failure. For conventional H2/Air PEM power
systems, the problem is usually addressed by causing the air blower to create a significant
pressure difference between the inlet and outlet of each channel that is strong enough to
force out any water droplets that might be formed. This usually requires creating very
narrow channels and then operating with flow-through gas to create the needed flow rate
and pressure. In some other cases, fuel cell operators will "burp the stack" causing a
sudden release of gas in the hope of removing the water droplets.
In a H2/O2 PEM fuel cell, since pure O2 and H2 are used, a flow-through operation
becomes impractical in terms of the efficiency of gas utilization. While a H2/O2 fuel cell
can be operated by both hydrogen and oxygen dead-ended operation, it requires a very
different scenario of controlling the gas and removal of product water. The rapid
circulation of pure O2 has been used in H2/O2 PEM fuel cells but is not desirable because
of the potential fire hazards associated with fast moving mechanical components in a
pure oxygen atmosphere. ElectroChem has demonstrated a solution that makes the flow
channel into an open channel structure with wicking which automatically clears the liquid
water from blocking gas passages and moves the water toward the end of the cell.
Performance
A.
Fuel cell with open channel demonstrates higher performance than the
conventional flow channel cell.
As shown in Figure 1, the open channel cell produced a higher cell voltage than
did the baseline cell with the conventional flow channel. The impact of the design upon
cell voltage was most pronounced at the highest current densities.
1.1
1
H2, O2
30 psig, 75C
0.9
Cell Voltage, V
0.8
0.7
0.6
conventional channel
0.5
open channel
0.4
0.3
0.2
0.1
0
0
10
20
30
40
50
60
70
80
Current, A
25
H2, O2
30 psig, 75C
Power output, Watt
20
15
conventional channel
open channel
10
5
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Current density, A.cm2
Figure 1. The advantage of the cell in producing higher voltages
B.
The cell demonstrates Stable Operation at Extremely Low Excess Oxygen
Flow Rates—Enabling Passive Operation
The second important result was the demonstration of stable operation at
extremely low oxygen flow rates, i.e., at extremely low percentages of excess oxygen
(over stoichiometric value). The cell required a thirtieth of the excess oxygen of that the
conventional cell requires to maintain stable operation. The demonstration of the fuel
cell with stable operation at extremely low excess oxygen flow rates is indicated in the
test results seen in Figure 2. To reveal the effect of low oxygen excess flow rate on the
cell performance, the tests were taken with two consecutive runs with a difference in the
oxygen excess flow rate.
0.75
Cell voltage, V
0.74
0.73
20 sccm/HR@28C
10 sccm/HR@28C
0.72
0.71
0.7
0
50
100
150
200
250
Time, min
0.75
Cell voltage, V
0.74
0.73
10 sccm/HR@28C
5 sccm/HR@28C
0.72
0.71
0.7
0
50
100
150
200
250
300
350
Time, min
0.75
Cell voltage, V
0.74
0.73
5 sccm/HR@28C
2 sccm/HR@28C
0.72
0.71
0.7
0
50
100
150
Time, min
200
250
300
0.75
Cell voltage, V
0.74
0.73
2 sccm/HR@28C
1 sccm/HR@28C
0.72
0.71
0.7
0
50
100
150
200
250
300
Time. min
Figure 2. Effect of oxygen excess flow rates on the cell performance
C.
The Cell demonstrates no effects of Change in Orientation on
performance—Potential for “Zero g” Water Removal
The cell operated under four different orientations, 0º, 90º, 180º, and 270ºwith
respect to the location of the cell’s oxygen exit. At 180º, the oxygen exit is located at the
top of the cell in which the production water is required to move upward against the
gravity to leave the cell. As shown in Figure 3, there is no significant negative effect on
product water removal when the cell is at 180º orientation. These test results indicated
that “Zero gravity” product water removal is enabled for the cell.
180o Orientation
0.9
50 mA /cm 2
50 mA/cm 2
0.85
Cell voltage, V
100 mA/cm 2
100 mA /cm 2
0.8
200 mA/cm 2
200 mA/cm 2
0.75
0.7
400 mA/cm 2
0.65
0
100
200
300
400
500
Time, min
Cell temperature: 75oC
Cell pressure: 30 psig
O2 excess flow rate: 2 cc/min
H2 excess flow rate: 2 cc/min
Figure 3. Effect of orientation on the cell performance
600
Conclusion
The open channel results in increased exposure of the electrode to the reactant gas
which supports a higher mass transport at high current densities. The reduced need for
almost no excess oxygen flow and circulation results in operational benefits as well as
gas utilization advantages. This design significantly simplifies the PEM fuel cell
operation system while supporting high performance and high energy density
Acknowledgments
This work was funded by the National Aeronautics and Space Administration
under Contract NNJ06JD71C.