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Technical Briefs

# Water Management Issues for Direct Borohydride/Peroxide Fuel Cells

[+] Author and Article Information
Scott Lux

U.S. Army ERDC-CERL, Champaign, IL 61822

Lifeng Gu, Grant Kopec, Robert Bernas, George Miley

Department of Nuclear, Plasma and Radiological Engineering, University of Illinois, Urbana, IL 61801

J. Fuel Cell Sci. Technol 7(2), 024501 (Jan 05, 2010) (5 pages) doi:10.1115/1.3176218 History: Received July 16, 2007; Revised April 02, 2009; Published January 05, 2010; Online January 05, 2010

## Abstract

This study evaluated water management strategies to lengthen the run time of a batch fueled direct sodium borohydride/peroxide $(NaBH4/H2O2)$ proton exchange membrane fuel cell. The term “batch fueled” refers specifically to a fuel tank containing a fixed volume of fuels for use in the run. The length of a run using a fixed fuel tank is strongly influenced by water dynamics. The water that reacts at the anode is produced at the cathode, and is transported through the membrane via drag and diffusion. Resulting concentration changes in the fuel of the $NaBH4/H2O2$ fuel cell were modeled to evaluate the run lifetime. The run time is defined as the amount of time required for $NaBH4$ or for $NaBO2$ (the byproduct compound) to reach either solubility limit or until the fuel is depleted, whichever occurs first. As part of the evaluation, an “effective” $H2O$ drag coefficient (net drag minus back diffusion) with Nafion® 112 was experimentally determined to be 1.14 and 4.36 at $25°C$ and $60°C$, respectively. The concentrations of the $NaBH4$ and $NaBO2$ solutions were calculated as a function of initial concentration, and for the case where $H2O$ was supplied to the anode compartment during operation. Several strategies to increase the run time by both passive and active water management were considered. It is found that the run time is increased from $10 W h$ to $57 W h$, with a decrease in the initial $NaBH4$ concentration from 30 wt % (typically employed in these cells) to 10 wt %. Adding 0.125 ml/min $H2O$ to the bulk anode solution increases the run time of a 10 wt % $NaBH4$ solution by a factor of 1.6. Adding 0.225 ml/min $H2O$ to 30 wt % $NaBH4$ bulk solution increases the run time by a factor of 4.4. While attractive for increasing run time, the practicality of water addition depends on its availability or requires incorporation of an added unit, designed to separate and recirculate water from the cathode solution.

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## Figures

Figure 1

Fuel cell operation creating water at the cathode and depleting water at the anode

Figure 2

Regression analysis for mass H2O2 versus time at 25°C

Figure 3

Increase in NaBH4 concentration as a function of energy produced at differing initial concentrations at 60°C (λ=4.36, I=5 A, V=0.5 V)

Figure 4

Weight percent of NaBH4 and NaBO2 as a function of time for a 10 wt % NaBH4 solution at 60°C (λ=4.36, I=5 A, V=0.5 V)

Figure 5

Weight percent at NaBH4 and NaBO2 as a function of energy for a 10 wt % NaBH4 solution with no added H2O and 0.125 ml/min−1 added H2O at 60°C (λ=4.36, I=5 A, V=0.5 V)

Figure 6

Remaining NaBH4 and energy produced as a function of added H2O for a 10 wt % NaBH4 solution at 60°C (λ=4.36, I=5 A, V=0.5 V)

Figure 7

Weight percent of NaBH4 as a function of energy for a 10 wt % NaBH4 solution for various water addition rates at 60°C (λ=4.36, I=5 A, V=0.5 V)

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