Multi‐enzyme immobilized carbon‐felt electrodes are fabricated for application as a bioanode in biofuel cells to utilize both maltose and glucose as the fuel. The unique combination of three enzymes (maltase, mutarotase, and glucose oxidase) enables us to utilize maltose as the fuel for the bioanode. The new electrode based on carbon felt (CF) demonstrates a high oxidation current density of 6.5 mA cm−2 at 0.34 V vs. Ag/AgCl in a neutral phosphate buffer solution containing 0.025 mol dm−3 (≡M) maltose in a half‐cell configuration. Furthermore, we improve the bioanode performance by changing the surfactant from cetyltrimethylammonium bromide (CTAB) to Triton X‐100® (TX), which is used as a carbon nanotube (CNT) dispersant in the bioanode preparation process. A superior current density of 17 mA cm−2 at 0.34 V vs. Ag/AgCl is demonstrated with the multi‐enzyme bioanode by using TX as the CNT dispersant, owing to the good dispersion of CNTs attached to CF and the reduced deactivation and leakage of the enzymes. A maltose/O2 biofuel cell, composed of the multi‐enzyme immobilized bioanode and a biocathode based on bilirubin oxidase and mediator, delivers a maximum power density of 2.3 mW cm−2 and an open‐circuit voltage of 0.69 V in 0.2 M phosphate buffer solution containing 50 mM maltose.

Performance of bilirubin oxidase (BOD) modified gas diffusion biocathode has been highly improved by incorporating a hydrophilic polymer as binder to form effective Ketjen black (KB)-immobilized structure for the enzyme reaction on carbon paper electrode. An aqueous dispersion of styrene-butadiene rubber latex with sodium carboxymethylcellulose (SBR/CMC) was employed as hydrophilic binder. In direct electron transfer system, the biocathode with SBR/CMC binder exhibited larger O2 reduction current in phosphate buffer solution (PBS, pH 7.0) at room temperature, compared with that of a poly(vinylidine fluoride) binder electrode, which is a typical hydrophobic polymer. Performance of the biocathode was further improved by utilizing mediator electron transfer system, and the maximum current density of ∼27 mA cm−2 was achieved in pH 5.0 PBS. These improvements are believed to be due to hydrophilic property of the KB-immobilized electrode surface by the SBR/CMC binder, leading to high-dispersion of BOD on the surface and also stable interface formation of O2 gas / enzyme / electrolyte during the reaction. Furthermore, we fabricated a glucose/O2 cell composed of the biocathode and an opposite bioanode with glucose oxidase. The full-cell successfully achieved mW-class power density of 5.22 mW cm−2 with an open-circuit voltage of 0.61 V in pH 7.0 PBS.

Performance of gas-diffusion biocathodes to electrochemically reduce oxygen for biofuel cells was improved by introduction of a new water-soluble binder for carbon powder to form electrode layer on ozone-hydrophilized carbon paper (CP) electrode base. Four kinds of water-soluble binders for carbon powder were compared and sodium polyglutamate (PGluNa) was selected for use in combination with styrene-butadiene rubber (SBR) latex to form a hydrophilic binder. The biocathode with the binder revealed the largest cathodic current among the four biocathodes with different binders in cyclic voltammetry at 0 V vs. Ag/AgCl. Glutaraldehyde (GA) was adopted as a crosslinker of bilirubin oxidase (BOD) to improve the stability of the biocathodes, which stabilized the cathodic current as expected, however excessive GA caused denaturation of BOD to decrease the enzyme activity. Long-term stability test of the biocathode utilizing a binder of PGluNa by chronoamperometry revealed good stability of oxygen reduction current for 1 h.