Materials with the formula Sr2 CoNb1-x Tix O6-δ (x=1.00, 0.70; δ=number of oxygen vacancies) present a cubic perovskite-like structure. They are easily and reversibly reduced in N2 or Ar and re-oxidized in air upon heating. Oxidation by water (wet N2 ), involving splitting of water at a temperature as low as 700 °C, produces hydrogen. Both compounds displayed outstanding H2 production in the first thermochemical cycle, the Sr2 CoNb0.30 Ti0.70 O6-δ material retaining its outstanding performance upon cycling, whereas the hydrogen yield of the x=1 oxide showed a continuous decay. The retention of the materials' ability to promote water splitting correlated with their structural, chemical, and redox reversibility upon cycling. On reduction/oxidation, Co ions reversibly changed their oxidation state to compensate the release/recovery of oxygen in both compounds. However, in Sr2 CoTiO6-δ , two phases with different oxygen contents segregated, whereas in Sr2 CoNb0.30 Ti0.70 O6-δ this effect was not evident. Therefore, this latter material displayed a hydrogen production as high as 410 μmol g-1 perovskite after eight thermochemical cycles at 700 °C, which is among the highest ever reported, making this perovskite a promising candidate for thermosolar water splitting in real devices.
Keywords: hydrogen generation; perovskite phases; solar fuel; thermochemical cycle; water splitting.
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