Comparison of Solar Electrochemical Thermal Hybrid Water Splitting

Solar electrochemical, solar thermal,1,2 and solar thermal/electrochemical hybrid3 hydrogen generation are introduced in this Section. Water electrolysis and electrolysis using solar concentrator technology were discussed in Chapters 3 and 4. The thermal and the hybrid processes will be discussed in depth in subsequent Sections of this chapter. At high temperatures (> 2000 °C), water chemically disproportionates to hydrogen and oxygen. Hence, in principle, by using solar energy to directly heat water to very high temperatures, hydrogen and oxygen gases can be generated. This is the basis for all direct thermochemical solar water splitting processes.1 However, catalysis, gas recombination, and containment materials limitations above 2000 °C have led to very low solar efficiencies for direct solar thermal hydrogen generation. In another approach, the utilization of a multi-step, indirect, solar thermal reaction processes to generate hydrogen at lower temperatures has been extensively studied, and a variety of pertinent reaction processes considered.2 These reactions are conducted in a cycle to regenerate and reuse the original reactions, ideally, with the only net reactant water, and the only net products hydrogen and oxygen. However, such cycles suffer from challenges often encountered in multi-step reactions.4 While these cycles can operate at lower temperatures than the direct thermal chemical generation of hydrogen, efficiency loses can occur at each of the steps in the multi-step sequence, resulting in low overall solar to hydrogen energy conversion efficiencies.

Electrochemical water splitting, generating H2 and O2 at separate electrodes, largely circumvents the gas recombination and high temperature limitations occurring in thermal hydrogen processes. Thus a hybrid of thermal dissociation and elec trolysis provides a pathway for efficient solar energy utilization. The hybrid method expands on existing solar electrochemical processes, which are therefore discussed briefly here. There has been significant, ongoing experimental5-15 and theoretical.5,16,17 interest in utilizing solar generated electrical charge to drive electrochemical water splitting (electrolysis) to generate hydrogen. In each of the above referenced studies, water electrolysis occurs at, or near, room temperature. Photoelectrochemi-cal models predict a maximum ~30% solar water splitting conversion efficiency by eliminating

• the linkage of photo to electrolysis surface area,

• non-ideal matching of photo and electrolysis potentials, and incorporating the effectiveness of contemporary

• electrolysis catalysts, and

• efficient multiple bandgap photoabsorbers (semiconductors).18

However, these models did not incorporate solar heat effects on the electrolysis energetics as elaborated below.

The UV and visible energy rich portion of the solar spectrum is transmitted through water (Chapters 1 and 2). Therefore a mediator for light absorption, such as a semiconductor, is required to drive the electrical charge for the water-splitting process. The PV (photovoltaic) process refers to a solar panel connected ex situ to electrochemically drive water splitting, e.g., an illuminated semiconductor-based photovoltaic device wired to an electrolyzer. On the other hand, the photoelectro-chemical process refers to in situ immersion of the illuminated semiconductor in a chemical solution (electrolyte) to electrochemically drive water splitting, as described in Chapter 7. The significant fundamental components of PV and photoelec-trochemical hydrogen generation are identical, but from a pragmatic viewpoint the PV process seems preferred, as it isolates the semiconductor from contact with and corrosion in the electrolyte. The UV and visible energy rich portion of the solar spectrum is transmitted through H2O. Semiconductors, such as TiO2, can split water, but their wide bandgap limits the photoresponse to a small fraction of the incident solar energy. Solar photoelectrochemical attempts to split water have utilized TiO2,20 InP,21 and also multiple bandgap semiconductors.19,22,23 Photoelectrochemical water splitting studies have generally focused on diminishing the high bandgap apparently required for solar water splitting, by tuning (decreasing) the bandgap of the semiconductor, Eg, to better match the water splitting potential, EH2O. Multiples of electrolyz-ers and photovoltaics can be combined to produce an efficient match of the generated and consumed power, as shown in Fig. 1. Also multiple bandgap semiconductors can be combined to generate a single photovoltage well-matched to the electrolysis cell, and over 18% conversion energy efficiency of solar to hydrogen was demonstrated, albeit at room temperature (still without the potential benefits of hybrid thermal hydrogen generation.)23

Unlike room temperature solar PV and photoelectrochemical electrolysis, the hybrid approach utilizes energy of the full solar spectrum, leading to substantially higher solar energy efficiencies. The IR radiation is energetically insufficient to drive conventional solar cells, and this solar radiation is normally discarded (by reflectance or as re-radiated heat.) On the other hand, in the hybrid approach, as seen in Fig. 2

Solar Water Electrolysis For Oxygen

Fig. 1. Alternate configurations varying the number of photo harvesting units and electrolysis units for solar water splitting.3 The photoconverter in the first system generates the requisite water electrolysis voltage and in the second system generates twice that voltage, while the photoconverter in the third and fourth units generate respectively only half or a third this voltage.

Solar Water Splitting

Fig. 2. Schematic representations of solar water electrolysis improvement through excess solar heat utilization.3

Fig. 2. Schematic representations of solar water electrolysis improvement through excess solar heat utilization.3

and as described in a latter Section of this chapter, the IR wavelengths are not discarded, but instead utilized to heat water. This in turn substantially decreases the necessary electrochemical potential to split the water, and substantially increases the solar hydrogen energy conversion efficiencies.

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