Sodium borohydride direct fuel cell

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. 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. 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. 

The product of the decomposition is, like in the Millennium Cell system, sodium metaborate. The system shows the same problems for the regeneration of NaBH4 from an aqueous solution of metaborate as described below. One advantage of the direct borohydride fuel cell systems is that platinum as catalyst is not needed. Unfortunately, depending on the temperature of the solution, some hydrogen gas is produced in a side reaction. However, this hydrogen can be piped out or can be used as additional fuel in a subsequent PEM fuel cell. 

Fuel cells are usually open systems, and their energy density and specific energy is largely based on the storage of fuel (and oxidant in the case of air-independent systems). Of the fuel cell types considered here, hydrogen PEMs, PAFCs and hydrogen AFCs use hydrogen as fuel, direct methanol fuel cells (DMFC) use aqueous methanol solution or pure methanol, and (alkaline) direct borohydride fuel cells (DBFC) use sodium borohydride solution as the liquid fuel. 

The price of sodium borohydride is currently too high by far for a practical application. Compared with hydrogen production from natural gas its price is 130 times higher. However, the idea of applying sodium borohydride as a fuel has the prerequisite of recycling the sodium borate product, and this could lower the price on the basis of mass production [98]. An alternative to hydrogen generation from sodium borohydride is the direct borohydride fuel cell, which is not within the scope of this book, so will not be discussed. 

Wee J.-H., A comparison of sodium borohydride as a fuel for proton exchange fuel cells and for direct borohydride cells, J. Power Sources, 155, 329 (2006). 

An interesting type of dissolved fuel fuel cell that has received renewed interest in recent times is based on sodium borohydride (sodium telrahydridoborate). This is being considered as a hydrogen carrier (see Chapter 8), bnt its use as a direct fuel is actually slightly better thermodynamically. The principle has been known and demonstrated for many years (Indig and Snyder, 1962 and Williams, 1966), but interest has recently been revived (Amendola et al., 1999). The fuel (NaBKj) is dissolved in the electrolyte, and the fuel anode reaction is... 

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. 

Fig. 30 Lifetime of the direct alcohol and sodium borohydride fuel cell at constant load.

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