Direct borohydride alkaline fuel cell

The application of gold as an electrocatalytic component within the fuel cell itself has to date been limited primarily to the historical use of a gold-platinum electrocatalyst for oxygen reduction in the Space Shuttle/Orbiter alkaline fuel cells (AFC)88 and the recent use of gold for borohydride oxidation in the direct borohydride alkaline fuel cell (DBAFC).89,90 Electrocatalysts with lower cost, improved carbon monoxide tolerance and higher... 

Figure 14.1 QinetiQ s tubular direct borohydride alkaline fuel cell (DBAFC), reproduced courtesy of QinetiQ Ltd.

Fig. 17 Current density-cell voltage characteristics for sodium borohydride in different electrolyte concentration in direct borohydride alkaline fuel cell at 25°C, Anode Pt-black Cathode Mn02.

Fig. 29 Current density-cell voltage characteristics at different temperatures in 2 M NaBH4/3 M KOH solution in direct borohydride alkaline fuel cell. Anode Pt-hlack Cathode Mn02.

Direct Alcohol and Borohydride Alkaline Fuel Cells... 

This chapter attempts to provide a critical review of the work carried out on alkaline fuel cell, which directly uses hydrogen rich liquid fuel and oxygen or air as an oxidant. The subjects covered are electrode materials, electrolyte, half-cell analysis and single cell performance in alkaline medium. Koscher et al. (2003) brought out elaborate review work on direct methanol alkaline fuel cell. Earlier Parsons et al. (1988) reviewed literature on anode electrode where, the oxidation of small organic molecules in acid as well as in alkaline conditions was considered. A review work on electro-oxidation of boron compounds was done by Morris et al. (1985). However, in this chapter use of three specific fuels, e.g., methanol, ethanol and sodium borohydride in alkaline fuel cell is discussed. 

Table 5. Different electro-catalyst used in direct methanol, ethanol or sodium borohydride alkaline fuel cell...

Fig. 14 (a) Schematic diagram of direct alcohol or borohydride alkaline fuel cell. 1. Fuel-electrolyte mixture storage 2. Exhausted-fuel-electrolyte mixture storage 3, 4. Peristaltic pump 5. Load 6. Anode terminal 7. Cathode terminal 8. Air 9. Cathode electrode 10. Anode electrode 11. Fuel and electrolyte mixture 12. Magnetic stirrer 13. Anode shield, (b) Experimental set-up for direct alchol or sodium borohydride alkaline fuel cell. 

The current density-cell voltage characteristics for methanol, ethanol and sodium borohydride fuels were shown in Figs. 27 to 29 at three different temperatures e.g., 25, 45, and 65°C. The cell performance increases with the increase in temperature because of decrease in activation polarization, concentration polarization and increase in ionic conductivity and mobility at higher temperature. The performance of direct sodium borohydride alkaline fuel cell does not increase appreciably with the increase in temperature and in fact shows decreasing trend at 65°C (Verma et al. 2005d). The reason for this decrease may be because of hydrogen gas liberation from sodium borohydride and loss of fuel at higher temperature. 

The lifetime test of direct alcohol and sodium borohydride alkaline fuel cell was conducted and the results pertaining to this is shown in Fig. 30. The useful operating lifetime of 380, 400, 510 h was found for methanol, ethanol and sodium borohydride fuels, respectively. The deterioration of performance of fuel cell may be because of the carbonate precipitate, oxide layer formation and adsorbed intermediate species on catalyst surface. The used up electrodes could be regenerated by treating the electrode with hydrochloric acid. The acid treatment might have removed the carbonates and other species from the electrode. The treated electrodes could regain more than 80% of the catalytic activity. The maximum power density with 2 M fuel and 3 M KOH obtained was 21.5 mW cm at 33 mA cm of current density for sodium borohydride at 60°C, whereas, methanol and ethanol produce 15 and 16 mW cm" of maximum power... 

Fig. 31 A two cell stack of direct sodium borohydride alkaline fuel ceU used for lighting a bulb.

Verma, A. and Basu, S. Direct use of alcohols and sodium borohydride as fuel in an alkaline fuel cell , J. Power Sources, 145 (2005b) 282-285. 

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