Water Splitting and Photosynthesis
The decomposition of liquid water to form gaseous hydrogen and oxygen:
H20(i H2(g)+ i02(g) (1)
is a highly endothermic and endergonic process with AH° = 285.9 kJ/mol and AG° = 237.2 kJ/mol. This reaction may be driven either electrochemically or thermally via the use of solar energy.
The standard potential AE° for Reaction 1 corresponding to the transfer of two electrons is given by:
In Eq. 2, F is the Faraday constant (96485 C mol-1) and the negative sign denotes the thermodynamically non-spontaneous nature of the water splitting process. The actual voltage required for electrolysis will depend on the fugacities of the gaseous products in Reaction 1 as well as on the electrode reaction kinetics (overpotentials)
along with the Ohmic resistance losses in the cell. In practice, steady-state electrolysis of water at 298 K requires the application of ~1.50 V.
Figure 6 contains a Pourbaix diagram for water;31 the zones in this diagram are labeled by the formulas for the predominant species at the electrode potential and pH indicated on the axes. Thus the threshold (thermodynamic) potentials for the decomposition of water via:
4 OH—(aq 4e-+ O2(g) + 2H2O(i) (2b)
2H2O(i ) + 2e—^ H2(g) + 2OH—(aq) (3b)
clearly depend on solution pH and they vary at a Nernstian rate of —0.059 V/pH at 298 K.
Optimizing the rates of the electrochemical processes (Reactions 2 and 3) constitute much of the R&D focus in electrochemical or photoelectrochemical splitting of water. Two-compartment cells are also employed to spatially separate the evolved gases with special attention being paid to the proton transport membranes (e.g., Na-fionR). Chapter 3 provides a summary of the progress made in water electrolyzer technologies.
Water is transparent to the wavelengths constituting the solar spectrum. Therefore, photocatalytic or photoelectrochemical splitting of water requires an agent (semiconductor, dye, or chromophore) capable of first absorbing sunlight and generating electron-hole pairs. Molecular approaches are discussed in Chapter 6 and semiconductor-based approaches are described in Chapter 7.
Thermochemical splitting of water involves heating water to a high temperature and separating the hydrogen from the equilibrium mixture. Unfortunately the decomposition of water does not proceed until temperatures around 2500 K are reached. This and other thermal routes are discussed in Chapter 5. Solar thermal processes are handicapped by the Carnot efficiency limits. On the other hand, solar photonic processes are limited by fundamental considerations associated with bandgap excitation; these have been reviewed in Refs.32 and 33.
The water splitting reaction, Eq. 1, have been stated here as the Holy Grail21 of hydrogen generation using solar energy. However other chemical reactions have been investigated and include, for example:34,35
2HBr hu > H2 + Br2 (4)
2H2O + 2Cl--^ 2 OH- + Cl2 + H2 (5)
However, these alternative schemes are fraught with problems associated with the generation and handling of toxic or hazardous by-products such as Br2 and Cl2.
Turning to photobiological schemes for producing H2 (Chapter 8), a complex reaction scheme uses solar energy to convert H2O into O2 and reducing equivalents which appear as NADPH. In photosystem 1, the reducing equivalents in NADPH are used to reduce CO2 to carbohydrates:
6H2O + 6CO2 + 48 hu ^ C8Hi2O6 + 6O2 (6)
or in bacteria, used directly as a reductive energy source.36,37 In artificial photosynthesis, the goal is to harness solar energy to drive high-energy, small-molecule reactions such as water splitting (Reaction 1) or CO2 reduction, Reaction 7:38
2H2O + 2CO2 + 4 hu ^ 2HCOOH + O2 (7)
Photobiological processes for H2 production are considered in Chapter 8.
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