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by layer-by-layer deposition of WO 3 and BiVO 4 on a conducting glass. The
PEC studies showed that the photocurrent onsets of WO 3 and BiVO 4 /WO 3
were 0.25 and 0.05  V versus Ag/AgCl, indicating the flat-band potential
negatively shifted with BiVO 4 coating. The BiVO 4 /WO 3 composite electrodes
also show improved photocurrent densities compared to pristine WO 3 . The
results supported the notation that heterojunctions could improve the PEC
water oxidation of BiVO 4 .
Other ternary metal oxides, such as CuWO 4 (2.25  eV), InVO 4 (2.0  eV),
and FeVO 4 (2.7 eV), have also been synthesized and studied as photocatalysts
for water oxidation and pollutant degradation. Studies reported to date have
shown that ternary metal oxides hold promise as high performance photo-
electrode for solar water splitting.
3.3.4 Conclusion
With control over size, morphology, and element doping of semiconductors,
their performance for PEC hydrogen generation have been significantly
improved. While significant progress has been made, several challenging
issues still remain to be addressed for solar hydrogen generation. For example,
while element doping has been proved to be effective in enhancing the PEC
performance of metal oxides, the long-term stability of element-doped metal
oxides is a concern for practical applications. Also, there is still not a single
photoelectrode that can achieve highly efficient water splitting under nonbi-
ased condition. Although 12 % solar-to-hydrogen conversion efficiency has
been achieved in hybrid devices that integrated PEC cell with solar cell, the
cost and the electrochemical instability of the devices are limitations for large
scale production. To date, the solar to hydrogen conversion efficiency of PEC
systems are still low, especially compared with the conversion efficiency that
can be achieved in a solar cell-powered electrolysis process. It is highly
desirable and challenging to develop new low cost and high performance
photoelectrodes for solar hydrogen generation.
A number of semiconductor materials have been tested for photocatalytic
and photoelectrochemical hydrogen generation and have made significant
progress in enhancing the solar conversion efficiency. There are outstanding
challenges that need to be addressed. For example, most of the photocatalytic
systems require the addition of hole sacrificial reagents such as methanol
or sulfide ions, which is a major drawback for photocatalytic hydrogen
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