Electrocatalysis for Fuel Cells at Enzyme-Modified Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 602

are far superior to that observed for laccase adsorbed at an unmodified electrode,
indicating that the modified electrode both stabilizes and orientates laccase molecules
for effective electron transfer (Fig. 17.7 inset). This approach would also be suitable
for extension to high surface area forms of carbon.
17.2.2 Heme-Copper Oxidases
Reduction of O 2 directly to H 2 O in biology is also catalyzed by heme - copper oxidases
that carry out the last step in an aerobic respiratory chain, transferring electrons to O 2 (see
Fig. 17.2). Electrons are supplied sequentially by either cytochrome c or the quinol pool,
and, depending on the organism, these terminal oxidases are generally large membrane-
bound enzyme complexes [Richter and Ludwig, 2003; Ferguson-Miller and Babcock,
1996; Sch¨fer et al., 1999]. The overall turnover rate of heme - copper oxidases is in
the order of 1000 electrons s 21 , but for fuel cell applications their disadvantage relative
to multi-copper oxidases is the fairly large overpotential for O 2 reduction. This is associ-
ated with their physiological role in coupling electron transfer to proton pumping across
the membrane to establish a proton gradient that drives the formation of ATP (see
Fig. 17.2) [Wikstr¨m and Verkhovsky, 2006; Moser et al., 2006]. Thus, in a single-
compartment glucose/O 2 enzyme fuel cell with a cathode comprising cytochrome c
oxidase interacting with cytochrome c tethered on a gold electrode, the open circuit
voltage was less than 120 mV [Katz et al., 1999].
17.3 ENZYME-MODIFIED ANODES
17.3.1 Oxidation of Sugars and Other Alcohols
17.3.1.1 Enzymes for Alcohol Oxidation Several classes of enzymes cata-
lyze the oxidation of alcohols to aldehydes or ketones, and their favored substrates
range from simple primary alcohols to complex secondary alcohols, including
sugars [Kroutil et al., 2004; Ameyama, 1982]. Further oxidation to carboxylic acids
can be achieved by incorporating additional enzymes such as aldehyde dehydrogen-
ase. Enzymes that utilize O 2 as the electron acceptor are termed oxidases, while
those utilizing alternative oxidants are termed dehydrogenases.
Zinc-containing alcohol dehydrogenases take up two electrons and a proton from
alcohols in the form of a hydride. The hydride acceptor is usually NAD(P) þ (the oxi-
dized form of nicotinamide adenine dinucleotide (NADH) or its phosphorylated
derivative, NADPH). Several liver alcohol dehydrogenases have been structurally
characterized, and Fig. 17.8 shows the environment around the catalytic Zn center
and the bound NADH cofactor.
Alcohol dehydrogenases found in certain microorganisms utilize a pyrroloquino-
line quinone (PQQ) or flavin cofactor to pass electrons released upon oxidation of
alcohols to the heme electron-acceptor protein, cytochrome c. These membrane-
associated alcohol dehydrogenases form part of a respiratory chain, and the energy
from fuel oxidation therefore contributes to generation of a proton gradient across