Water Dissociation Technologies for Hydrogen
301
[27,28] (Rajeshwar, 2012, pers. comm.). This method is in general not preferred
because not only it uses light as a source for hydrogen generation, but the overall
process also requires electrical components.
11.2.6 PhoTovolTAiC eleCTrolySiS
One way to avoid corrosion problem in photoelecrolysis is to use the concept of
photovoltaic cell working in air and electrolyzing a distant electrolyzer [11,28]. Here,
semiconductors are not in direct contact with the solution so that corrosion problems
cease to exist. This device can effectively be used with solar energy-generated electricity. The device contains two cells.
The best setup for efficiency is, however, recorded by Murphy–Bockris cell [28]
using n-on-p gallium arsenide coated with ruthenium oxide and p-on-n gallium
arsenide coated with platinum. Such a cell gave about 8% conversion of light to
hydrogen production at current density in the range of 10 mA cm −2 . The cell life
was also at least as good as that of the photovoltaics (PV) in air. Two advantages of
Murphy–Bockris cell are that (1) cell is in solution so that the concentration of light
upon the electrode that gives high temperatures can be used to provide household
heat and (2) only one device is needed compared to two that are needed in PV cell
in air. There are numerous ways to use PV cell in conjunction with electrolysis. One
common method is described in Section 11.2.7.
11.2.7 SolAr eleCTrolySiS
The process of solar electrolysis involves generation of solar electricity via PV or
concentrating solar power (CSP) followed by electrolysis of water [1,2,6,8,9,12]. This
process is considered to be a benchmark for other thermochemical solar processes
for water splitting that offers potential for energy-efficient large-scale production of
H 2 (Figure 11.1). For solar electricity generated from PV cell and assuming solar
thermal efficiencies at 15% or 20% and electrolyzer efficiency at 80%, the overall
solar-to-hydrogen conversion efficiency will range from 12% to 16% [1,2,6,8,9,12].
If we assume solar thermal electricity cost of $0.08/kWh, the projected cost of H 2
will range from $0.15 to $0.20/kWh, that is, from $6 to $8/kg H 2 [1,2,6,8,9,12]. For
PV electricity, costs are expected to be twice as high. HTE process can significantly
reduce electricity demand if it is operated at around 800°C–1000°C via SOEC.
The high-temperature heat required for such a process can be supplied by the CSP
system [1].
11.3 PhOtOChemiCal and its deriVatiVe teChnOlOGies
The dissociation of water can be assisted by photocatalysts that are directly suspended in water [29,30]. As shown below, a number of photocatalysts are possible.
Early work by Gray et al. [31–34], Whitten et al. [35,36], and Maverick and Gray
[37] showed that polynuclear inorganic complexes, excited metal complexes, and
surfactant ruthenium complexes can help photochemical decomposition of water
to produce hydrogen. Kiwi et al. [38] presented a review of homogeneous and
301
[27,28] (Rajeshwar, 2012, pers. comm.). This method is in general not preferred
because not only it uses light as a source for hydrogen generation, but the overall
process also requires electrical components.
11.2.6 PhoTovolTAiC eleCTrolySiS
One way to avoid corrosion problem in photoelecrolysis is to use the concept of
photovoltaic cell working in air and electrolyzing a distant electrolyzer [11,28]. Here,
semiconductors are not in direct contact with the solution so that corrosion problems
cease to exist. This device can effectively be used with solar energy-generated electricity. The device contains two cells.
The best setup for efficiency is, however, recorded by Murphy–Bockris cell [28]
using n-on-p gallium arsenide coated with ruthenium oxide and p-on-n gallium
arsenide coated with platinum. Such a cell gave about 8% conversion of light to
hydrogen production at current density in the range of 10 mA cm −2 . The cell life
was also at least as good as that of the photovoltaics (PV) in air. Two advantages of
Murphy–Bockris cell are that (1) cell is in solution so that the concentration of light
upon the electrode that gives high temperatures can be used to provide household
heat and (2) only one device is needed compared to two that are needed in PV cell
in air. There are numerous ways to use PV cell in conjunction with electrolysis. One
common method is described in Section 11.2.7.
11.2.7 SolAr eleCTrolySiS
The process of solar electrolysis involves generation of solar electricity via PV or
concentrating solar power (CSP) followed by electrolysis of water [1,2,6,8,9,12]. This
process is considered to be a benchmark for other thermochemical solar processes
for water splitting that offers potential for energy-efficient large-scale production of
H 2 (Figure 11.1). For solar electricity generated from PV cell and assuming solar
thermal efficiencies at 15% or 20% and electrolyzer efficiency at 80%, the overall
solar-to-hydrogen conversion efficiency will range from 12% to 16% [1,2,6,8,9,12].
If we assume solar thermal electricity cost of $0.08/kWh, the projected cost of H 2
will range from $0.15 to $0.20/kWh, that is, from $6 to $8/kg H 2 [1,2,6,8,9,12]. For
PV electricity, costs are expected to be twice as high. HTE process can significantly
reduce electricity demand if it is operated at around 800°C–1000°C via SOEC.
The high-temperature heat required for such a process can be supplied by the CSP
system [1].
11.3 PhOtOChemiCal and its deriVatiVe teChnOlOGies
The dissociation of water can be assisted by photocatalysts that are directly suspended in water [29,30]. As shown below, a number of photocatalysts are possible.
Early work by Gray et al. [31–34], Whitten et al. [35,36], and Maverick and Gray
[37] showed that polynuclear inorganic complexes, excited metal complexes, and
surfactant ruthenium complexes can help photochemical decomposition of water
to produce hydrogen. Kiwi et al. [38] presented a review of homogeneous and