Semiconducting potassium tantalate electrodes. Photoassistance agents for the efficient electrolysis of water

Single crystals of the perovskites KTaO₃ and KTa_0.77Nb_0.23O₃ are shown to be efficient photoassistance agents for the electrolysis of water. These n-type semiconductors are photostable in concentrated alkaline solutions where they serve as the anode in the electrolysis reaction. Experiments with ¹⁸O-enriched water identify the electrolyte, not the crystal, as the source of the evolved oxygen. Stoichiometric data show the ratio of moles of electrons to moles of hydrogen to moles of oxygen produced to be nearly 4:2:1 as expected for the electrolysis reaction. Consistent with band gaps in the tantalates of ∼3.5 eV, light of wavelengths shorter than ∼390 nm produces photocurrent. Quantum yields for electron flow at 254 nm are as high as 0.3 to 0.5. The onset of anodic photocurrent is ∼-1.25 V vs. SCE in 8.6 M NaOH. These electrode photoassistance agents can operate with no external bias. Crystals of KTa_0.77Nb_0.23O₃ reduced with H₂ at various temperatures show modest variations in current-voltage properties. Optical to chemical energy conversion efficiencies of ∼6% for the KTaO₃ crystals and ∼4% for the KTa_0.77Nb_0.23O₃ crystals are possible. The tantalates examined compare favorably with other electrodes capable of photoassisting the electrolysis of water.