Topics in Current Chemistry (2019) 377:11
1 3
1 Introduction
Easy storage and handling, high energy density, and wide availability are features
that make alcohols attractive fuel cell liquid combustibles and the most promising
alternative power sources for transportation, portable electronics, and stationary
applications. However, many obstacles have restrained the more rapid development
of direct alcohol fuel cells. The major problem in oxidation of any alcohol other
than methanol is particularly difficult breaking of the C–C bond, which is a necessary step for a complete oxidation to CO 2 . The complete oxidation of any primary
monohydroxy alcohol can be written as:
Due to the complex oxidation and sets of parallel reactions, the predominant
pathway of any alcohol leads to incomplete oxidation products with the transfer of
less than 6n electrons [1–4].
The mechanism of the simplest alcohol (methanol) oxidation on Pt has been studied for several decades as the only alcohol that does not need the C–C bond cleavage
for the oxidation to CO 2 . However, ethanol has the intrinsic advantage in terms of
doubling the number of electrons exchanged. Furthermore, its low toxicity, comparable electrochemical activity, high theoretical mass/energy density (8.0 kWh/kg),
and easy production by fermentation from sugar-containing materials makes it an
attractive power source. Unlike methanol, ethanol is a renewable fuel requiring simple logistics for its applications. These features make it an attractive fuel for fuel
cells and favorable alternative power source. Unfortunately, the slow and incomplete
ethanol oxidation reaction (EOR) occurring at the anode even on the best available
catalysts known to-date is the main impediment to practical application and commercialization of direct ethanol fuel cells (DEFCs). Additional factors hindering
development of DEFCs include low activity, complex reaction mechanism, ethanol crossover from the anode to the cathode and CO poisoning, as well as the high
costs of precious metals of Pt-based catalysts [1–7]. Although details of the reaction mechanism are still debated, good agreement exists about reaction pathways
that form acetaldehyde (CH 3 CHO) and acetic acid (CH 3 COOH) as the main products of the oxidation of ethanol at a Pt electrode in acidic solution (Fig. 1). Thus,
at moderately positive potentials of interest for fuel cell applications (up to 0.6 V),
the main oxidation products have the C–C bond intact and the reaction yields two
or four electrons [1–4]. Further oxidation involving cleavage of the C–C bond on
C n H 2n+1 OH + (2n − 1)H 2 O → nCO 2 + 6nH
+ + 6ne
− .
Fig. 1 Schematic representations of the parallel pathways during EOR [2]
Reprinted from the journal
2
1 3
1 Introduction
Easy storage and handling, high energy density, and wide availability are features
that make alcohols attractive fuel cell liquid combustibles and the most promising
alternative power sources for transportation, portable electronics, and stationary
applications. However, many obstacles have restrained the more rapid development
of direct alcohol fuel cells. The major problem in oxidation of any alcohol other
than methanol is particularly difficult breaking of the C–C bond, which is a necessary step for a complete oxidation to CO 2 . The complete oxidation of any primary
monohydroxy alcohol can be written as:
Due to the complex oxidation and sets of parallel reactions, the predominant
pathway of any alcohol leads to incomplete oxidation products with the transfer of
less than 6n electrons [1–4].
The mechanism of the simplest alcohol (methanol) oxidation on Pt has been studied for several decades as the only alcohol that does not need the C–C bond cleavage
for the oxidation to CO 2 . However, ethanol has the intrinsic advantage in terms of
doubling the number of electrons exchanged. Furthermore, its low toxicity, comparable electrochemical activity, high theoretical mass/energy density (8.0 kWh/kg),
and easy production by fermentation from sugar-containing materials makes it an
attractive power source. Unlike methanol, ethanol is a renewable fuel requiring simple logistics for its applications. These features make it an attractive fuel for fuel
cells and favorable alternative power source. Unfortunately, the slow and incomplete
ethanol oxidation reaction (EOR) occurring at the anode even on the best available
catalysts known to-date is the main impediment to practical application and commercialization of direct ethanol fuel cells (DEFCs). Additional factors hindering
development of DEFCs include low activity, complex reaction mechanism, ethanol crossover from the anode to the cathode and CO poisoning, as well as the high
costs of precious metals of Pt-based catalysts [1–7]. Although details of the reaction mechanism are still debated, good agreement exists about reaction pathways
that form acetaldehyde (CH 3 CHO) and acetic acid (CH 3 COOH) as the main products of the oxidation of ethanol at a Pt electrode in acidic solution (Fig. 1). Thus,
at moderately positive potentials of interest for fuel cell applications (up to 0.6 V),
the main oxidation products have the C–C bond intact and the reaction yields two
or four electrons [1–4]. Further oxidation involving cleavage of the C–C bond on
C n H 2n+1 OH + (2n − 1)H 2 O → nCO 2 + 6nH
+ + 6ne
− .
Fig. 1 Schematic representations of the parallel pathways during EOR [2]
Reprinted from the journal
2