Hydroxyl ion, mobility

Figures 18 to 20 show that the equilibrium cell voltage increases with the increase in fuel concentration. Although the cell performance increases initially but it does not increase proportionally with further increase in fuel concentration. This is because the increase in fuel concentration leads to the decrease in hydroxyl ion mobility. The hydrolysis reaction dominates with the increase in sodium borohydride concentration and thus the performance increases rather slowly. Further at higher concentration of NaBH4, viscosity of the fuel-electrolyte mixture increases leading to the rapid increase in concentration polarization at higher current densities and the performance decreases (Fig. 20). The maximum power density of 16.2 and 13.8 mW cm" were obtained for 3 M methanol and ethanol concentrations while 22.5 mW cm" for 2 M sodium borohydride. The fuel cell was operated at 25°C, 3 M KOH concentration and with 1 mg cm " of anode catalyst (Pt-black) loading catalyst and 3 mg cm" of cathode (Mn02) loading, respectively.

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