Solar hydrogen generating devices can be classified into photoelectrochemical cells (PEC), buried photovoltaic devices (PV), photovoltaic-biased photoelectrochemical cells (PV-PEC), photocatalysts (PCs), and hybrids thereof.^1–3  Of these, PV devices achieve the highest efficiencies today, followed by PV-PEC devices, and photocatalytic systems (Figure 7.1).^4–6  Even though costs of PV have come down over the last decade, the technology is still not competitive with the low price of fossil fuels. PV costs are associated with expensive vacuum deposition techniques, high purity light absorbers, and noble metals for the electrolyzers. In PV-PEC devices, cost savings are realized by replacing the electrolyzer and part of the PV with photoelectrodes based on abundant metal compounds. However, efficiencies remain below those of PV and stability is not sufficient for industrial applications. These obstacles make it difficult to implement PEC and PV technology on a scale necessary to meet the global demand (13.5 TW in 2001) for renewable fuels.⁷