295
Water Dissociation
11
Technologies for
Hydrogen
11.1 intrOdUCtiOn
The dissociation of water to produce hydrogen reversibly requires a supply of energy
as follows:
H 2 O H 2 + O 2
(11.1)
∆H
0
289 K = 241.93 kJ/mol, ∆G
0
228
0
298 K =
.71 kJ/mol, T∆S 298 K = 13.22 2 kJ/mol
This means that work = ∆G 0 and heat = T∆S are required to split the water at 25°C
and 1 atm. Here T is the temperature and ∆S is the change in entropy. ∆H and ∆G are
changes in heat of formation and free energy of formation. The superscript 0 denotes
standard conditions of 1 atm and 25°C. If the reaction does not proceed reversibly,
more work is required. The energy needed for this work can be provided in a number
of different ways, and these are evaluated in this chapter.
There are three major ways water can be dissociated to produce hydrogen. The
first method is electrolysis in which the water is dissociated electrochemically
using electrochemical cell. The cell can be operated in a number of different ways
(such as high temperature and high pressure), but all of them require significant
amount of energy to dissociate water. The second method is the use of photosynthesis and photocatalysis to dissociate water. This method also requires photonic
energy with or without a catalyst. The energy can, however, be provided using a
solar cell. The third method is thermal or thermochemical dissociation of water
in which water is dissociated either thermally or thermochemically. The latter
method uses a chemical substance (or substances) to carry out dissociation using a
series of chemical reactions. This method not only separates hydrogen and oxygen
upon dissociation, but also reduces the temperature required for the thermal dissociation. In the recent years, this method has been heavily explored. Besides these
three major methods, some miscellaneous methods such as chemical oxidation,
magmalysis, and radiolysis are also explored for water dissociation. All of these
are briefly discussed in this chapter.
As discussed in earlier chapter 4, the use of solar energy in steam gasification,
reforming, and solar cracking of fuels such as coal, biomass, and natural gas has
been gaining more acceptance. Similarly, three major technologies— electrochemical,
photochemical/photobiological, and thermochemical—for water dissociation can
Water Dissociation
11
Technologies for
Hydrogen
11.1 intrOdUCtiOn
The dissociation of water to produce hydrogen reversibly requires a supply of energy
as follows:
H 2 O H 2 + O 2
(11.1)
∆H
0
289 K = 241.93 kJ/mol, ∆G
0
228
0
298 K =
.71 kJ/mol, T∆S 298 K = 13.22 2 kJ/mol
This means that work = ∆G 0 and heat = T∆S are required to split the water at 25°C
and 1 atm. Here T is the temperature and ∆S is the change in entropy. ∆H and ∆G are
changes in heat of formation and free energy of formation. The superscript 0 denotes
standard conditions of 1 atm and 25°C. If the reaction does not proceed reversibly,
more work is required. The energy needed for this work can be provided in a number
of different ways, and these are evaluated in this chapter.
There are three major ways water can be dissociated to produce hydrogen. The
first method is electrolysis in which the water is dissociated electrochemically
using electrochemical cell. The cell can be operated in a number of different ways
(such as high temperature and high pressure), but all of them require significant
amount of energy to dissociate water. The second method is the use of photosynthesis and photocatalysis to dissociate water. This method also requires photonic
energy with or without a catalyst. The energy can, however, be provided using a
solar cell. The third method is thermal or thermochemical dissociation of water
in which water is dissociated either thermally or thermochemically. The latter
method uses a chemical substance (or substances) to carry out dissociation using a
series of chemical reactions. This method not only separates hydrogen and oxygen
upon dissociation, but also reduces the temperature required for the thermal dissociation. In the recent years, this method has been heavily explored. Besides these
three major methods, some miscellaneous methods such as chemical oxidation,
magmalysis, and radiolysis are also explored for water dissociation. All of these
are briefly discussed in this chapter.
As discussed in earlier chapter 4, the use of solar energy in steam gasification,
reforming, and solar cracking of fuels such as coal, biomass, and natural gas has
been gaining more acceptance. Similarly, three major technologies— electrochemical,
photochemical/photobiological, and thermochemical—for water dissociation can