When repeatedly being used for 6 rounds, the biobattery had 35% of its initial current density after 12 days. The biobattery based on nonimmobilized enzymes and AQDS retained 60% of its initial current density after running for 2 h. When enzyme loading was increased by another 10-fold, the highest power density of biobattery was increased to 2.4 mW cmв€’2 at 37вЊ. The maximum power density of the biobattery based on nonimmobilized enzymes and AQDS was 1.1 mW cmв€’2 of anode at room temperature, 34-times of the biobattery based on immobilized enzymes and immobilized vitamin K3. At the same enzyme loading, power densities of biobatteries increased in an order from the case of immobilized enzymes and immobilized vitamin K3 to the case of nonimmobilized enzymes and immobilized vitamin K3 to the case of nonimmobilized enzymes and nonimmobilized AQDS. Here, we investigated an alternative solution вЂ“ the use of nonimmobilized thermostable enzymes (e.g., NAD-based glucose 6-phosphate dehydrogenase and flavin-containing diaphorase) and a nonimmobilized mediator (i.e., benzyl viologen or 9, 10-anthraquinone-2, 7-disulphonic acid, AQDS) for achieving high power densities from glucose 6-phosphate in closed EFCs (called biobatteries). AbstractThe immobilization of redox enzymes on the surface of electrodes is a typical practice for enzymatic fuel cells (EFCs) but enzyme immobilization usually results in low retaining enzymatic activities and low power densities.

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