Consumption of methanol and ethanol as a fuel in the passive direct fuel cells technologies is suitable and more useful for the portable application compared with hydrogen as a preliminary fuel due to the ease of management, including design of cell, transportation, and storage. However, the cost production of commercial membrane is still far from the acceptable commercialization stage. Based to our previous works, the low cost of cross-linked sodium alginate (SA) polymer electrolyte bio-membrane shown the virtuous chemical, mechanical, and thermal characterization as polymer electrolyte membrane in the direct methanol fuel cells (DMFCs). This study will further the investigation of cross-linked SA polymer electrolyte bio-membrane performance in the passive DMFCs and the passive direct ethanol fuel cells (DEFCs). The experimental study investigates the influence of the membrane thickness, loading of catalysts, temperature, type of fuel, and fuel concentration in order to achieve the optimal working operation performances. The passive DMFCs is improved from 1.45 up to 13.5 mW cm(-2) for the maximum peak of power density, which is obtained by using 0.16 mm as an optimum thick of SA bio-membrane that shown the highest selectivity 6.31 10(4) S s cm(-3), 4 mg cm(-2) of Pt-Ru as an optimum of anode catalyst loading, 2 mg cm(-2) of Pt at the cathode, 2M of methanol as an optimum fuel concentration, and an optimum temperature at 90 degrees C. Under the same conditions of cells, the passive DEFCs are shown to be 10.2 mW cm(-2) in the maximum peak of power density with 2M ethanol. Based on our knowledge, this is the first work that reports the optimization works of performance SA-based membrane in the passive DMFCs via experimental studies of single cells and the primary performance of passive DEFCs using the SA-based membrane as polymer electrolyte membrane.