Novel perovskite materials for thermal water splitting at moderate temperature

Abstract

Materials with the formula Sr_2CoNb_1-xTi_xO_(6-delta) (x=1.00, 0.70; delta=number of oxygen vacancies) present a cubic perovskite-like structure. They are easily and reversibly reduced in N_2 or Ar and re-oxidized in air upon heating. Oxidation by water (wet N_2), involving splitting of water at a temperature as low as 700 ºC, produces hydrogen. Both compounds displayed outstanding H_2 production in the first thermochemical cycle, the Sr_2CoNb_(0.30)Ti_(0.70)O_(6-delta) 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 Sr_2CoTiO_(6-delta), two phases with different oxygen contents segregated, whereas in Sr_2CoNb_(0.30)Ti_(0.70)O_(6-delta) this effect was not evident. Therefore, this latter material displayed a hydrogen production as high as 410 mu molH_2/g_(perovskite) after eight thermochemical cycles at 700 ºC, which is among the highest ever reported, making this perovskite a promising candidate for thermosolar water splitting in real devices.