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Research Papers

# Using a Stack Shunt to Mitigate Catalyst Support Carbon Corrosion in Polymer Electrolyte Membrane Fuel Cell Stacks During Start-Stop Cycling

[+] Author and Article Information
Denis Bona

Department of Electronics
and Telecommunications,
Faculty of Engineering,
Politecnico di Torino,
Turin 10129, Italy;
Electro Power Systems SpA,
Via Livorno 60,
Turin 10144, Italy
e-mail: denis.bona@polito.it

Dennis E. Curtin

Consultant,
Fayetteville, NC 28304

Francesco Pedrazzo

Electro Power Systems SpA,
Via Livorno 60,
Turin 10144, Italy

Elena Maria Tresso

Department of Applied Science and Technology,
Faculty of Engineering,
Politecnico di Torino,
Turin 10129, Italy;
Italy Center for Space Human Robotics at Polito,
Istituto Italiano di Tecnologia,
Turin 10129, Italy

1Corresponding author.

Contributed by the Advanced Energy Systems Division of ASME for publication in the JOURNAL OF FUEL CELL SCIENCE AND TECHNOLOGY. Manuscript received July 21, 2013; final manuscript received August 13, 2013; published online November 8, 2013. Editor: Nigel M. Sammes.

J. Fuel Cell Sci. Technol 11(1), 011010 (Nov 08, 2013) (7 pages) Paper No: FC-13-1066; doi: 10.1115/1.4025535 History: Received July 21, 2013; Revised August 13, 2013

## Abstract

Carbon black based electrodes are generally recognized as state of the art for PEM fuel cell technology due to the high performance achieved with a relatively low Pt content. However, the catalyst carbon support is prone to carbon oxidation. This leads to a loss of the catalyst area and overall performance, along with a higher mass transport loss due to an increased flooding tendency. This phenomenon is particularly severe when the fuel cell experiences repetitive start-stop cycles. Therefore, specific countermeasures against catalyst layer carbon oxidation are required, especially for automotive and backup power applications, where the startup/shutdown rate is considerably high. The authors evaluated a basic design that uses a stack shunt. A properly modified control protocol, which includes the stack shunt, is able to avoid high cathode potential peaks, which are known to accelerate catalyst carbon support corrosion and its negative effects. During two separate durability tests, one adopting the shunt design and another using nonprotected shutdown, a 24-cell stack was subjected to continuous starts and stops for several months and its performance constantly monitored. The results show that when the shunt is used, there is a 37% reduction in the voltage degradation rate for each startup/shutdown cycle and a two-fold increase in the number of startup/shutdown cycles before an individual cell reached the specified “end of life” voltage criteria. Furthermore, ex situ FE-SEM analysis revealed cathode catalyst layer thinning, which is an indication that the emerging degradation mechanism is the catalyst support carbon corrosion, as expected. This provides further support that the constant voltage degradation rate typically experienced in PEMFCs can be primarily attributed to the catalyst support carbon corrosion rate. The proposed shunt protocol is very cost effective and does not require any substantial changes in the system. For this reason, its adoption is recommended as a viable method to decrease the catalyst support carbon corrosion rate and extend the operating life of the PEMFC stack.

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## Figures

Fig. 1

Schematic model of the reactions occurring in the cell when the anode is partially exposed to hydrogen and partially exposed to air

Fig. 2

Schematic of the test bench used for the experiment with unprotected startups and shutdowns

Fig. 3

Schematic of the test bench used for the experiment adopting the stack shunt protocol

Fig. 4

Voltage graph of the cells with unprotected startups and shutdowns

Fig. 5

Average and maximum cell voltage degradation rate with unprotected startups and shutdowns

Fig. 6

Stack polarization curve at the BOL and at the EOL

Fig. 7

(a) FE-SEM scan of an MEA at the BOL, and (b) FE-SEM scan of the MEA coming from cell no. 4 after the cycling test

Fig. 8

Voltage graph of the cells using the stack shunt protocol

Fig. 9

Average cell voltage and degradation rate using the stack shunt protocol

Fig. 10

Comparison of the degradation rate/cycle with unprotected startups and shutdowns and using the stack shunt protocol

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