210
5 Electrodynamics of Aqueous Media
chemical reaction of hydrogen and oxygen to electrical energy, and consists of the
electrodes, a proton or hydroxyl-conducting electrolyte, catalyst, and gas diffusion
layers. The fuel cell requires a source of hydrogen gas, uses oxygen from the air, and
produces water vapor as the product of the electrochemical reaction. Note that fuel
cells are very efficient, but have the lowest power density among all electrochemical
energy systems, thus, they should be used with supercapacitors, which together provide all the required parameters needed, for example, for electric vehicles. Thus, fuel
cells are perspective competitors of the Li-ion batteries in the nearest future. In addition, a PEMC supplying an average household with electricity would also provide
them with drinking water [82], which makes this type of device very ecologically
friendly and suitable for indoor applications.
Finally, hydrogen, which is needed for the hydrogen fuel cell, is an intermediate
product that stores electric energy to be converted into electric power on demand.
The easiest and most efficient way to produce the hydrogen of very high purity (up to
99.999%) is the electrolysis of water. The composition of the modern electrolyzer is
similar to that shown in Fig. 5.20, with some additional features, such as stirrers and
ultrasonic generators, which increase the efficiency. Alkaline electrolyzers operate
via the transport of hydroxide ions (OH
− ) through the electrolyte from the cathode to
the anode with hydrogen being generated on the cathode side. Hydrogen gas released
in this way can be used as hydrogen fuel, or remixed with the oxygen to create oxyhydrogen gas, which is used in welding and other applications. Sometimes called water
splitting, electrolysis requires a minimum potential difference of 1.23 V. However,
as discussed in Sect. 5.2, the properties of water can be significantly changed near
the interfaces, which can reduce the threshold of the electrolysis. Moreover, recent
studies of the nuclear quantum effects in protic systems and their role in polarization
effects [83] show that the mechanism of autodissociation in water needs to be revised,
taking into account short (sub-femtosecond) processes and the spatial heterogeneity
of water (see Chaps. 3 and 4).
Note that for all the systems described above, the most important parameter of
water is the electric conductivity, or ability to transfer the electric charge. Figure 5.21
shows the temperature dependencies of different ionic conductors. As one can see
water and ice in their bulk states shows negligibly small electrical conductivity in
comparison with the main superionic conductors. However, as demonstrated in [41],
and discussed in Sect. 5.2, interfacial water has an electric conductivity five orders of
magnitude higher than that in bulk water, whose absolute value exceeds the conductivity of other electrolytes in an extended frequency range around room temperature.
The remarkable properties of water at the nanoscale pave the way for the development
of proton exchange membranes and aqueous electrolytes with better characteristics
than are currently used. The increase of the proton-transfer rate will proportionally increase the power density of the corresponding electrochemical devices, which
should, in particular, increase the efficiency and reduce the cost of electricity.
players in this market still use PEMC, however, there are many reasons to expect that SOFC will
oust other fuel cell types in the near future.
5 Electrodynamics of Aqueous Media
chemical reaction of hydrogen and oxygen to electrical energy, and consists of the
electrodes, a proton or hydroxyl-conducting electrolyte, catalyst, and gas diffusion
layers. The fuel cell requires a source of hydrogen gas, uses oxygen from the air, and
produces water vapor as the product of the electrochemical reaction. Note that fuel
cells are very efficient, but have the lowest power density among all electrochemical
energy systems, thus, they should be used with supercapacitors, which together provide all the required parameters needed, for example, for electric vehicles. Thus, fuel
cells are perspective competitors of the Li-ion batteries in the nearest future. In addition, a PEMC supplying an average household with electricity would also provide
them with drinking water [82], which makes this type of device very ecologically
friendly and suitable for indoor applications.
Finally, hydrogen, which is needed for the hydrogen fuel cell, is an intermediate
product that stores electric energy to be converted into electric power on demand.
The easiest and most efficient way to produce the hydrogen of very high purity (up to
99.999%) is the electrolysis of water. The composition of the modern electrolyzer is
similar to that shown in Fig. 5.20, with some additional features, such as stirrers and
ultrasonic generators, which increase the efficiency. Alkaline electrolyzers operate
via the transport of hydroxide ions (OH
− ) through the electrolyte from the cathode to
the anode with hydrogen being generated on the cathode side. Hydrogen gas released
in this way can be used as hydrogen fuel, or remixed with the oxygen to create oxyhydrogen gas, which is used in welding and other applications. Sometimes called water
splitting, electrolysis requires a minimum potential difference of 1.23 V. However,
as discussed in Sect. 5.2, the properties of water can be significantly changed near
the interfaces, which can reduce the threshold of the electrolysis. Moreover, recent
studies of the nuclear quantum effects in protic systems and their role in polarization
effects [83] show that the mechanism of autodissociation in water needs to be revised,
taking into account short (sub-femtosecond) processes and the spatial heterogeneity
of water (see Chaps. 3 and 4).
Note that for all the systems described above, the most important parameter of
water is the electric conductivity, or ability to transfer the electric charge. Figure 5.21
shows the temperature dependencies of different ionic conductors. As one can see
water and ice in their bulk states shows negligibly small electrical conductivity in
comparison with the main superionic conductors. However, as demonstrated in [41],
and discussed in Sect. 5.2, interfacial water has an electric conductivity five orders of
magnitude higher than that in bulk water, whose absolute value exceeds the conductivity of other electrolytes in an extended frequency range around room temperature.
The remarkable properties of water at the nanoscale pave the way for the development
of proton exchange membranes and aqueous electrolytes with better characteristics
than are currently used. The increase of the proton-transfer rate will proportionally increase the power density of the corresponding electrochemical devices, which
should, in particular, increase the efficiency and reduce the cost of electricity.
players in this market still use PEMC, however, there are many reasons to expect that SOFC will
oust other fuel cell types in the near future.