7.2 Biofuel Cell
123
that can complete oxidation of alcohol was reported in 1998, in which three NADdependent enzymes: alcohol dehydrogenase, aldehyde dehydrogenase, and formate
dehydrogenase, were utilized to fully oxidize methanol to carbon dioxide (CO 2 )
and water [40]. A glucose/O 2 biofuel cell utilizing NAD-dependent gluconate 5dehydrogenase as well as NAD-dependent GDH in the bioanode for a 4-electron
oxidation of glucose was reported to exhibit a P max of 10 mW cm
−2 [41]. The value
was two times larger than that of a glucose/O 2 biofuel cell utilizing only GDH
for the bioanode [30]. An in vitro 15-enzyme pathway that can complete oxidize
co-utilize glucose, sucrose and fructose in biofuel cell was designed, achieving a
faraday efficiency of approximately 95 % for these three sugars and yielding a P max
of 1.08 mW cm
−2 [42]. Such enzymatic cascades in biofuel cells is well reviewed
[43].
Secondly, low power output of biofuel cells is a major issue that constrains their
potential use to applications. The power density of biofuel cells reported so far was
at most in the level of several mW cm
−2 and is still much lower than that of the
conventional fuel cells. The principal reason for this matter is pure electrochemical
connection between enzymes and electrodes. In a DET-type system, the electron
transfer efficiency relies on the distance between the redox-active sites of enzymes
and electrode surfaces. From this viewpoint, deep understanding of the interactions
between enzymes and electrodes is essential to realize a rapid interfacial electron
transfer based on oriented-orientation of enzymes at functionalized electrode surfaces
[44, 45]. In particular, recent researches have shown that several electrodes with
mesoporous structures significantly improve the interfacial electron transfer kinetics
between enzymes and the electrodes in DET-type bioelectrocatalysis [46, 47]. While
in the MET-type system, suitable mediators with high affinity to enzymes and high
turnover efficiency in electrode are demanded [10, 48].
Thirdly, in typical enzyme-catalyzed systems, biofuel cells often suffer from poor
operational stability, resulting in short lifetimes, and higher costs. Although relatively
stable proton exchange membrane fuel cells, or microbial fuel cells catalyzed by selfreproduced microorganisms can be reused for months, most biofuel cells can operate
only for hours or days. Instability arises not just with the enzymes, but with the use
of soluble cofactors such as NAD, ATP, and coenzyme A, which are essential to
drive several redox enzyme reactions, and of other components including mediators.
A glucose/O 2 biofuel cells combined with pH reactivation of laccase biocathode
illustrated one-year stability [17]. Besides, the complexity of the biological systems
may frequently lead to negative effects on the stability of biofuel cells, such as an issue
of the biofouling of the electrode for implantable biofuel cells, or of the inhibition
ascribed to O 2 and positive electrode potentials as in H 2 /O 2 biofuel cells.
For almost all biofuel cells reported to date, the voltage at which usable power
can be extracted is below the minimal requirement to power commercially available
electronic devices. This drawback is inevitable as, from a thermodynamic point of
view, the maximum gap in the formal potentials between two bioelectrodes is ~1.2 V
(e.g. for H 2 /O 2 biofuel cells), and it is much less than that of lithium ion batteries
(e.g. ~4.2 V). In many cases, the involvement of mediators leads to an additional
decrease in the voltage output of biofuel cell. But, sophisticated optimization of
123
that can complete oxidation of alcohol was reported in 1998, in which three NADdependent enzymes: alcohol dehydrogenase, aldehyde dehydrogenase, and formate
dehydrogenase, were utilized to fully oxidize methanol to carbon dioxide (CO 2 )
and water [40]. A glucose/O 2 biofuel cell utilizing NAD-dependent gluconate 5dehydrogenase as well as NAD-dependent GDH in the bioanode for a 4-electron
oxidation of glucose was reported to exhibit a P max of 10 mW cm
−2 [41]. The value
was two times larger than that of a glucose/O 2 biofuel cell utilizing only GDH
for the bioanode [30]. An in vitro 15-enzyme pathway that can complete oxidize
co-utilize glucose, sucrose and fructose in biofuel cell was designed, achieving a
faraday efficiency of approximately 95 % for these three sugars and yielding a P max
of 1.08 mW cm
−2 [42]. Such enzymatic cascades in biofuel cells is well reviewed
[43].
Secondly, low power output of biofuel cells is a major issue that constrains their
potential use to applications. The power density of biofuel cells reported so far was
at most in the level of several mW cm
−2 and is still much lower than that of the
conventional fuel cells. The principal reason for this matter is pure electrochemical
connection between enzymes and electrodes. In a DET-type system, the electron
transfer efficiency relies on the distance between the redox-active sites of enzymes
and electrode surfaces. From this viewpoint, deep understanding of the interactions
between enzymes and electrodes is essential to realize a rapid interfacial electron
transfer based on oriented-orientation of enzymes at functionalized electrode surfaces
[44, 45]. In particular, recent researches have shown that several electrodes with
mesoporous structures significantly improve the interfacial electron transfer kinetics
between enzymes and the electrodes in DET-type bioelectrocatalysis [46, 47]. While
in the MET-type system, suitable mediators with high affinity to enzymes and high
turnover efficiency in electrode are demanded [10, 48].
Thirdly, in typical enzyme-catalyzed systems, biofuel cells often suffer from poor
operational stability, resulting in short lifetimes, and higher costs. Although relatively
stable proton exchange membrane fuel cells, or microbial fuel cells catalyzed by selfreproduced microorganisms can be reused for months, most biofuel cells can operate
only for hours or days. Instability arises not just with the enzymes, but with the use
of soluble cofactors such as NAD, ATP, and coenzyme A, which are essential to
drive several redox enzyme reactions, and of other components including mediators.
A glucose/O 2 biofuel cells combined with pH reactivation of laccase biocathode
illustrated one-year stability [17]. Besides, the complexity of the biological systems
may frequently lead to negative effects on the stability of biofuel cells, such as an issue
of the biofouling of the electrode for implantable biofuel cells, or of the inhibition
ascribed to O 2 and positive electrode potentials as in H 2 /O 2 biofuel cells.
For almost all biofuel cells reported to date, the voltage at which usable power
can be extracted is below the minimal requirement to power commercially available
electronic devices. This drawback is inevitable as, from a thermodynamic point of
view, the maximum gap in the formal potentials between two bioelectrodes is ~1.2 V
(e.g. for H 2 /O 2 biofuel cells), and it is much less than that of lithium ion batteries
(e.g. ~4.2 V). In many cases, the involvement of mediators leads to an additional
decrease in the voltage output of biofuel cell. But, sophisticated optimization of