96
used fuel source, due to its high elemental abundance, the clean-burning capability
of the fuel, and the high specific energy (143 MJ/kg). Pure hydrogen gas itself is not
abundant, but methane is rich in hydrogen atoms and is becoming increasingly
available. Combining steam with methane to produce carbon dioxide and hydrogen
gas is known as reforming. Although the reformation process involves the formation
of carbon dioxide (albeit much less than for gasoline when compared to each unit of
heat released), a greenhouse gas, this can be carried out in a different location such
that the fuel cell will not produce this gas in cities or closed locations. It is also
notable that hydrogen fuel cells are at least twice as efficient (nearly 85%) as a
hydrogen-burning internal combustion engine, as limitations imposed via Carnot
efficiencies in a cycled engine utilizing gas expansion and compression no longer
apply here.
4.1 Proton Transport
Proton transport is of great importance for life; in one of the most fundamental processes, protons are pumped across a membrane in photosynthesis. Further, it was
discovered that the best naturally occurring proton conductor known to date exists
in the jelly found inside a shark’s head; In particular, it is found within the “ampullae of Lorenzini” (AoL)—an array of electro-sensory organs that can be found in
cartilaginous fish. The conductivity was measured to be ~2 × 10
−3
S cm
−1
, a remarkable value when compared to the synthetic, state-of-the-art ion conducting polymer
Nafion
®
, which is only about 40 times larger, according to the authors [27]. The
keratin sulfate found in the jelly is suggested to play an important role. It may be
that biology continues to inspire new discoveries in this area, as one biomimetic
MOF was designed already for lithium ion transport [28]. One may even consider a
most fundamental measurement—pH—that relies on proton transport.
In the archetypical proton conducting hydrogen fuel cell, an electron is removed
from hydrogen atoms at the anode, leaving behind a proton. The membrane must
allow for movement of the protons. At the cathode, the protons combine with oxygen to form water. Additionally, these membranes should be impermeable to the gas
such that there is less wasted fuel via crossover. The technology operates under
inherently acidic conditions with protons in excess, which limits the types of materials that can withstand these conditions, including the electrodes and catalyst.
Therefore, platinum is the typical metal used in these cases, but is susceptible to
carbon monoxide poisoning over time (note CO is an intermediate in the reforming
process), and is cost-prohibitive.
As depicted in Fig. 2, two different mechanisms are often used to describe the
transport of ions in fuel cells, which are distinguishable by their activation energy.
In well-defined hydrogen-bonded networks, a proton can jump between neighboring molecules, which is accompanied by an additional reorganization of the proton
solvating environment, which then enables the next jump. For reference, Nafion
®
,
the current standard for proton transport via the Grotthuss mechanism, exhibits an
C. A. Bauer
used fuel source, due to its high elemental abundance, the clean-burning capability
of the fuel, and the high specific energy (143 MJ/kg). Pure hydrogen gas itself is not
abundant, but methane is rich in hydrogen atoms and is becoming increasingly
available. Combining steam with methane to produce carbon dioxide and hydrogen
gas is known as reforming. Although the reformation process involves the formation
of carbon dioxide (albeit much less than for gasoline when compared to each unit of
heat released), a greenhouse gas, this can be carried out in a different location such
that the fuel cell will not produce this gas in cities or closed locations. It is also
notable that hydrogen fuel cells are at least twice as efficient (nearly 85%) as a
hydrogen-burning internal combustion engine, as limitations imposed via Carnot
efficiencies in a cycled engine utilizing gas expansion and compression no longer
apply here.
4.1 Proton Transport
Proton transport is of great importance for life; in one of the most fundamental processes, protons are pumped across a membrane in photosynthesis. Further, it was
discovered that the best naturally occurring proton conductor known to date exists
in the jelly found inside a shark’s head; In particular, it is found within the “ampullae of Lorenzini” (AoL)—an array of electro-sensory organs that can be found in
cartilaginous fish. The conductivity was measured to be ~2 × 10
−3
S cm
−1
, a remarkable value when compared to the synthetic, state-of-the-art ion conducting polymer
Nafion
®
, which is only about 40 times larger, according to the authors [27]. The
keratin sulfate found in the jelly is suggested to play an important role. It may be
that biology continues to inspire new discoveries in this area, as one biomimetic
MOF was designed already for lithium ion transport [28]. One may even consider a
most fundamental measurement—pH—that relies on proton transport.
In the archetypical proton conducting hydrogen fuel cell, an electron is removed
from hydrogen atoms at the anode, leaving behind a proton. The membrane must
allow for movement of the protons. At the cathode, the protons combine with oxygen to form water. Additionally, these membranes should be impermeable to the gas
such that there is less wasted fuel via crossover. The technology operates under
inherently acidic conditions with protons in excess, which limits the types of materials that can withstand these conditions, including the electrodes and catalyst.
Therefore, platinum is the typical metal used in these cases, but is susceptible to
carbon monoxide poisoning over time (note CO is an intermediate in the reforming
process), and is cost-prohibitive.
As depicted in Fig. 2, two different mechanisms are often used to describe the
transport of ions in fuel cells, which are distinguishable by their activation energy.
In well-defined hydrogen-bonded networks, a proton can jump between neighboring molecules, which is accompanied by an additional reorganization of the proton
solvating environment, which then enables the next jump. For reference, Nafion
®
,
the current standard for proton transport via the Grotthuss mechanism, exhibits an
C. A. Bauer