Air Independent Fuel Cells Utilizing Borohydride and Hydrogen Peroxide
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1213-T01-09
Air Independent Fuel Cells Utilizing Borohydride and Hydrogen Peroxide R. Craig Urian Naval Undersea Warfare Center, 1176 Howell St., Newport, RI 02841, U.S.A. ABSTRACT The US Navy continues to pursue electrochemical power sources with high energy density for air-independent applications. The direct electro-oxidation and electro-reduction of sodium borohydride and hydrogen peroxide are one type of fuel cell being investigated to meet these goals. Electrode architecture, choice of catalyst, and membrane selection are three components that need to be considered in validating the forward development of a viable technology. INTRODUCTION The development of cost effective energy sources for undersea operations with a high energy density is a challenging task made only more complicated by constraints such as maintaining neutral buoyancy, air independence, variable operating environments, stringent safety requirements and rapid equipment turnaround time. The direct sodium borohydride-hydrogen peroxide fuel cell (DBHFC) is a promising technology capable of meeting many of these criteria. The standard potential for borohydride oxidation is -1.24V and yields 8 electrons as shown in equation 1. BH4- + 8OH- BO2- + 6H2O + 8e- -1.24V (1) This potential however, is typically not observed due to the hydrogen potential resulting from decomposition (hydrolysis) of the borohydride (shown in equation 2) which is discussed in more detail by Ponce de Leon et. al., [1]. BH4- + 2H2O BO2- + 4H2 (g) (2) In an alkaline environment the standard potential of hydrogen peroxide is 0.87V as shown in equation 3, while in an acidic environment with pH 0.0V, and achieve a fuel cell potential of >1.0V, the stored peroxide solution could be modified to contain a sufficient amount of acid to neutralize the hydroxide that is produced at the cathode. This approach although has a system level mass penalty that mitigates the voltage gain. Another approach to maintain a suitable catholyt condition in order to achieve a half cell potential > 0.0V and avoid the mass penalty of supporting acid may be to implement an anion exchange membrane. Initial efforts looking at anion exchange membranes have shown that it is possible to operate a DBHFC wherein the amount of supporting acid in the catholyte solution is significantly less than the amount required to neutralize the generated hydroxide and the density of the catholyte solution is still between 1.1 and 1.3 g/ml. Figure 3 is a constant current experiment (100mA/cm2) for one type of anion exchange membrane wherein the PdIr cathode demonstrated good stability at ~0.5V.
Figure 3 also depicts a PdIr anode half cell potential deteriorating overtime. The anode performance was recovered by the addition of hydroxide solution at 100 minutes to the circulating electrolyte. The addition of hydroxyl solution to re-establish the desired performance was an indication that sulfate ions were migrating across the membrane in addition to the hydroxyl ions. The hydroxyl ion concentration in the anolyte over time
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