Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)
40Development of Compact & High Efficiency Polymer
Electrolyte Fuel Cell System for Enclosed Spaces

TOSHIHIRO TANI*1

MITSUYOSHI IWATA*2

TAKUYA MORIGA*3

HIDEKI ITO*4

KEIICHI NAKAGAWA*4

KOKI SUGIHARA*5

In recent years, the need for undersea equipment for the utilization of ocean resources has
been increasing. Responding to the advancement of devices and the lengthening of exploration
periods, fuel cells are attracting attention as a power source capable of supplying electric powerover an extended period of time, in contrast to conventional storage batteries, which don’t havehigh power capacity. For use in undersea equipment, Mitsubishi Heavy Industries, Ltd. (MHI) has
developed a new compact high-efficiency fuel cell system that employs a new structure in its gas
circulation system and new fuel cells. This paper provides an overview of the new fuel cell systemand presents MHI’s development efforts.

|1. Introduction
In 2000, MHI delivered the “Urashima” unmanned autonomous underwater vehicle to theJapan Marine Science and Technology Center (the present Japan Agency for Marine-Earth Scienceand Technology). Thereafter, the power source of the Urashima was replaced with a polymer
electrolyte fuel cell system and the Urashima attained a continuous underwater cruise of 317 km,
the world record for distance (Figure 1).1 In this way, the capability of fuel cell systems for theextension of cruise range has already been demonstrated. In recent years, however, further
improvement of efficiency, compactness, durability, and reliability of fuel cell systems is required
due to the demand for high capacity power sources for undersea equipment responding to theadvancement of devices such as sensors and the lengthening of exploration periods. To meet theseneeds, MHI has developed a new fuel cell system based on examinations of both fuel cell stacks
and the entire system.

Figure 1 External view of "Urashima" unmanned autonomous
underwater vehicle1*1

Chief Staff Manager, Submarine Department, Naval Ship Division, Integrated Defense & Space Systems*2

Fuel Cell Business Department, Power Systems Division*3

Chief Staff Manager, Technology Planning Department, Technology & Innovation Headquarters*4

Hiroshima Research & Development Center, Technology & Innovation Headquarters*5

Submarine Department, Naval Ship Division, Integrated Defense & Space Systems

Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)41|2. Overview of new fuel cell system
2.1Problems with fuel cell systems for enclosed spaces

To supply electric power in enclosed spaces for a long time, a fuel cell is required to attainnot only efficient use of the fuel, hydrogen and oxygen, as well as improvement of the power
generation efficiency of the entire system, but also power reduction of auxiliary devices installed in
the system and downsizing of the system itself. The fuel cell system installed in the Urashimaneeded a gas circulation device that recycled unreacted gas and a humidifier device that humidified
the gas, resulting in problems such as a reduction of system efficiency due to the high power
consumption of auxiliary devices and the impossibility of downsizing of the system.
However, both the gas circulation device and the humidifier device could not be eliminatedbecause the former was necessary for the generation of gas flow for discharging water formed in
the fuel cell system from the power generation portion, and the latter was required for performanceimprovement and durability retention of the catalyst and electrolyte membrane of the fuel cell.
In addition, a trace amount of hydrogen leakage was an issue for the use of the system in an
enclosed space for a long time.

2.2

Operation of new fuel cell system

Figure 2 shows a gas system schematic diagram of the new fuel cell system developed for theelimination of the gas circulation device and the humidifier device. In this system, gas passes
through one of the two fuel cell stacks (the upstream stack) and then flows into the other one(downstream stack). The functions of the upstream and downstream stacks are switched at setintervals by a valve.
In this operation, the gas needed for consumption in both the upstream and downstream stacks
flows into the upstream stack. Water generated in the upstream stack is discharged by the flow of thegas because the gas needed for consumption in the downstream stack passes through the outlet of theupstream stack.
On the other hand, water generated in the downstream stack is accumulated because there is
no gas flow during normal operation, but it is discharged by the gas flow that occurs when the valve
is switched and the downstream stack turns into upstream stack. 1
In this way, the switching of the valve serves in a manner similar to the conventional gas
circulation device. In addition, water formed by the downstream stack is effective for humidification
for the two stacks due to their periodical switching operation, and serves in a manner similar to thehumidifier device. Therefore this system enables the establishment of a fuel cell system that has
functions approximating gas circulation and humidifier devices, even when they are eliminated.

Figure 2 Gas system of new fuel cell system1

Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)42

2.3

Fuel cell stack for realization of new fuel cell system

MHI has also been proceeding with the development of fuel cell stacks suitable for the newfuel cell system. Specifically, the channel shape of the gas distributing separator is optimized forstable power generation even at a low flow rate, and a sealing structure is employed for the overall
stack in order to significantly reduce hydrogen leakage. In addition, the structure and material of
the catalyst layer is optimized for the improvement of power generation efficiency. MHI is also
working on improving the durability of the cell.

2.4“Multi-Less” fuel cell system

As described above, the new fuel cell system enables high-efficiency, through theelimination of the gas circulation and humidifier devices (Blower-Less and Humidifier-Less) and asignificant reduction of hydrogen leakage (Leak-Less). Accordingly, the system is known as theHigh Efficiency Multi-Less Fuel Cell System.

|3. Development of elements for new fuel cell system
3.1

Improvement of performance and durability in fuel cell

Figure 3 shows the relationship between the electric current and the cell voltage to indicate
the power generating performance of a fuel cell stack using cells in which the structure andmaterial of the catalyst layer are optimized. Compared with the cell installed on the Urashima, thenew system attains the improvement of power generating efficiency over the entire operation range(0 to 20 A) due to improvements such as the use of a high-activity catalyst. 1
It is known that the polymer membrane of the fuel cell deteriorates significantly when thefuel cell is operated under low humidity. MHI considers the primary cause of polymer membrane
deterioration to be hydroxyl radicals derived from hydrogen peroxide by-produced in electrodecatalyst layers, and has developed unique long-life cells with radical scavenger layers that scavenge
or stabilize hydroxyl radicals before they reach the polymer membrane (Figure 4).2

Figure 3 Power generation performance of cell (HGC-1) for
new fuel cell system1

Figure 4 Schematic diagram of cell with radical collection layers2

Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)43

Figure 5 compares the Accelerated durability test results of a standard cell and a cell withradical scavenger layers using rare earth carbonate. The horizontal and vertical axes of Figure 5 are
the operation hours and cell voltage, respectively. In the test for the standard cell, failure occurredand continuous operation was made impossible after 16 starts/stops and 324 operation hours. The
OCV (open circuit voltage) at that time was 0.85 V and the hydrogen concentration in the oxygen
electrode outlet gas was 2.0 vol%. From this, it is presumed that the breakage of the polymer
membranes led to hydrogen leakage resulting in the impossibility of continuous operation. In thetest for the cell with radical scavenger collection layers, on the other hand, the power generation
voltage drop from the initial state was relatively low, and operation could continue successfullyeven after 230 starts/stops and 5,000 operation hours. In addition, the high OCV value was retained
and no increase of hydrogen concentration in the oxygen electrode outlet gas occurred. In this way,
the suppression of polymer membrane deterioration could be confirmed.3

Figure 5 Accelerated durability test results of cell with radical scavengerlayers3

3.2

Development of elements for new fuel cell system

Figure 6 shows the external view of the testing device for the element test of the new fuelcell system and the fuel cell stack.

Figure 6 External view of testing device for element test of new fuel cellsystem and fuel cell stack1

Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)44

Figure 7 shows the power generation of stack A in the testing device for the element test of
the new fuel cell system. In Figure 7, the horizontal axis represents the operation hours and thevertical axis represents cell voltage, and the rectangular wave represents the switching of the valve.
The cell voltage becomes a little bit higher when stack A is the upstream stack and a little bit lower
when stack A is the downstream stack, but maintains a steady state on average. This demonstrates
that power generation can be performed with each cell voltage maintained roughly equal.

Figure 7 Power generation of stack A in testing device for element
test of new fuel cell system1

Figure 8 shows the relationship between operation pressure, operation load, and power
generation efficiency. When the operation pressure is increased from 200 kPaA to 400 kPaA, thepower generation efficiency also increases from 53% (higher heating value, HHV) to 56% (HHV)
at 100% load and from 61% (HHV) to 63% (HHV) at 20% load. This demonstrates that stable andhigh-efficiency power generation can be performed under various operational conditions.1

Figure 8 Relationship between operation pressure, operation loadand power generation efficiency in testing device for element
test of new fuel cell system1

In this way, the short-term performance of the new fuel cell system has been verified. Thestability verification of long-term power generation was then performed. Figure 9 shows the test
results under the condition of 100% load and operation pressure of 400 kPaA. In this figure, thehorizontal axis represents operation hours and the vertical axis represents power generation voltage.
These test results demonstrate that stable power generation can be maintained for 1,000 hours. 1

Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)45

Figure 9 Verification result of long-term power generation stability
in testing device for element test of new fuel cell system1|4. Progress of development and fabrication of new fuel cell systemprototype
MHI has designed and fabricated a prototype of the new fuel cell system based on thedevelopment results of the elements. Table 1 and Figure 10 show target specifications and theexternal view of the prototype system, respectively. 4 In this prototype, 70% of the space for fuelcell power generation is occupied by reaction-related fuel cell components, and electric devices areinstalled in the remaining space. MHI has completed fabrication of the prototype and obtained datafor the evaluation of its power generation efficiency, reliability, operability, durability and otherfactors, in land and underwater tests, and is proceeding with discovering the effectiveness andchallenges of this system.5
Table 1 Specifications of new fuel cell system prototype4
Item
Dimensions
WeightPower generation
outputElectric voltage
Operation
StartSpecifications
Φ600 x 900 mm(Inside dimensions except for fuel tank)
200 kg or lighter
300 W (representative point)
24 V (equipped with DC-DC converter)
Capable of stand-alone operation(system monitoring is needed)
Nitrogen purge during stopping is unnecessary
Quick start with a single touch
Data courtesy of JAMSTEC

Photo courtesy of JAMSTEC

Figure 10

External view of new fuel cell system prototype4

Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)46|5. Conclusion
This paper has presented the features and development progress of MHI’s new fuel cellsystem. MHI aims to contribute to improvement in efficiency and the accuracy of exploration
through the realization of a small, highly reliable and high-efficiency fuel cell system for enclosed
spaces that enables longer-term operation of deep-seabed exploration equipment and underwatervehicles. This system is also expected to significantly reduce operational costs. The system can also
be used as a power source not only under the sea, but also in enclosed environments such as
aerospace applications, clean rooms, shelters.6 MHI will make efforts for the establishment of eveneasier to use fuel cell systems in the future.
The technologies described in this paper were developed through contract research andcollaborative research. The authors would like to express their gratitude to the members of
JAMSTEC and the New Energy and Industrial Technology Development Organization forallowing the use of their various precious technical documents and data.

References
1. Yoshida H. et al., A compact High Efficiency PEFC System for Underwater Platforms, ECS Transactions,26(1) 67-76 (2010)
2. Gengo, T. et al., Highly Efficient Polymer Electrolyte Fuel Cell (PEFC) Contributing to the Environment，Mitsubishi Heavy Industries Technical Review Vol. 45 No. 1 (2008) pp. 31-33
3. Itou et al., Prevention of Degradation of a Polymer Electrolyte Fuel Cell, Journal of The Society ofChemical Engineers, Japan Vol. 35 No. 3 (2009), pp. 304-311
4. Yoshida H. et al., A Prototype of the High Efficiency Three-Less (HETL) Fuel Cell System for Underwater
Platforms, ECS Transaction, 30(1) 295-304 (2011)
5. Hyakudome T. et al., Development of Fuel Cell System for Underwater Power Source (130117-046),
OCEANS’13 MTS/IEEE
6. Press release of Japan Agency for Marine-Earth Science and Technology (2013)
http://www.jamstec.go.jp/j/about/press_release/20131113/