Use of nonimmobilized enzymes and mediators achieved high power densities in closed biobatteries

The immobilization of redox enzymes on the surface of electrodes is a typical practice for enzymatic fuel cells ( EFC s) but enzyme immobilization usually results in low retaining enzymatic activities and low power densities. Here, we investigated an alternative solution – 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 EFC s (called biobatteries). At the same enzyme loading, power densities of biobatteries increased in an order from the case of immobilized enzymes and immobilized vitamin K 3 to the case of nonimmobilized enzymes and immobilized vitamin K 3 to the case of nonimmobilized enzymes and nonimmobilized AQDS . The maximum power density of the biobattery based on nonimmobilized enzymes and AQDS was 1.1 mW cm −2 of anode at room temperature, 34‐times of the biobattery based on immobilized enzymes and immobilized vitamin K 3 . When enzyme loading was increased by another 10‐fold, the highest power density of biobattery was increased to 2.4 mW cm −2 at 37°C. The biobattery based on nonimmobilized enzymes and AQDS retained 60% of its initial current density after running for 2 h. When repeatedly being used for 6 rounds, the biobattery had 35% of its initial current density after 12 days. Our results suggest that closed biobatteries equipped with nonimmobilized thermostable enzymes and mediators feature simple system configuration and have high power densities.