Environmental Engineering Reference
In-Depth Information
electronic contact with an electrode surface is probed electrochemically under turn-
over or nonturnover conditions, is increasingly providing important information
about enzyme reactions, and leads directly into technological applications of redox
enzymes [L´ger et al., 2003; Vincent et al., 2007]. In many cases, electron transfer
between the adsorbed enzyme and the electrode surface is very fast, meaning that
the electrocatalytic current response reports directly on the kinetics of processes
within the enzyme. In other cases, strategies for “wiring” redox centers in the protein
to the electrode are employed. The use of an electrode modified with a conducting
polymer in which redox-active groups mediate transfer of electrons allows multiple
layers of enzyme molecules to be addressed, increasing the current response. This
chapter describes methods for attachment of enzymes to electrodes, examples of
enzyme electrocatalysis of reactions that are relevant to fuel cells, and the ways in
which enzymes have been utilized in energy cycling devices.
17.1.1 Relationship to Energy Harnessing in Biology
In the ubiquitous bacterium Escherichia coli, oxidation of fuels (including succinate,
formate, glycerol-3-phosphate, lactate, and H
2
) catalyzed by a series of enzymes
facing the periplasm is coupled to reduction of a range of electron acceptors in the
cytoplasm (e.g., fumarate, nitrate, nitrite, or O
2
) via a pool of electron-carrier quinols
that are soluble in the lipid bilayer of the cytoplasmic membrane. In E. coli,H
2
oxi-
dation occurs only under anaerobic growth conditions, but in certain strictly aerobic
bacteria such as Ralstonia species, oxidation of H
2
is coupled to O
2
reduction (similar
to the situation in an H
2
/O
2
fuel cell; Fig. 17.2) [Burgdorf et al., 2005]. The energy
released during the biological reactions is stored in the form of a proton gradient
across the membrane, generated by uptake of H
þ
from the cytoplasmic side of the
membrane and H
þ
release on the periplasmic side. (This is equivalent to using a
fuel cell to charge a capacitor.) Controlled diffusion of protons back through the
large membrane-spanning enzyme ATP synthase drives the phosphorylation of adeno-
sine diphosphate (ADP) to generate adenosine triphosphate (ATP), and hydrolysis of
ATP can drive chemical reactions elsewhere in the cell [Nicholls and Ferguson, 2002].
Microbes grow on a wide range of energy sources (e.g., starch, glucose, lactate,
methanol, glycerol, and H
2
), meaning that bioderived catalysts for oxidation of a
range of fuels (including biomass and sewerage waste) are potentially available.
Here, we focus mainly on biological catalysts for oxidation of H
2
, sugars, and other
alcohols, and for reduction of O
2
(simply provided from air), since these reactions
are directly relevant to fuel cell electrocatalysis. Enzymes usually catalyze very
specific reactions; for example, laccase cleanly catalyzes the four-electron reduction
of O
2
to H
2
O without release of peroxide or superoxide. Methanol is oxidized only
to formate by alcohol dehydrogenase, but, with a series of enzymes operating
in series, it is possible to carry out a cascade of oxidation reactions to form CO
2
[Yue and Lowther, 1986; Palmore et al., 1998].
17.1.2 Enzymes in Fuel Cell Electrocatalysis
Motivations for exploring enzymes in fuel cell catalysis are both intellectual and
applied. Using enzymes as electrocatalysts, there is scope for creation of fuel cells
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