Sr- and Co-doped LaGaO3−δ with high O2 and H2 yields in solar thermochemical water splitting

Abstract

Solar thermochemical CO₂/H₂O splitting cycles based on redox oxides have emerged as a potential strategy to store dilute and intermittent solar energy. It is desirable to explore other materials that can convert solar energy into chemical energy with high efficiency. Herein, a thermochemical water splitting (TCWS) cycle based on Sr- and Co-doped LaGaO_3−δ (La_1−xSr_xGa_1−yCo_yO_3−δ) was successfully developed. Compared with inert LaGaO_3−δ, the oxygen vacancy concentration (δ) is gradually increased as the contents of Sr- and/or Co-dopants increased. The δ value of LaGa_0.4Co_0.6O_3−δ (LGC60) reaches 0.16 after reduction for 40 min at 1350 °C, which is 5 and 4 times that of Ce_0.85Zr_0.15O_2−δ and LSMA6464, respectively. The H₂ yield of LGC60 (478 μmol H₂ g⁻¹_perovskite) is 15 times that of the current state-of-the-art material, CeO_2−δ (32 μmol H₂ g⁻¹_material), when reduced at 1350 °C and re-oxidized at optimized 800 or 1000 °C. 1.0 atom% IrO_x as a catalyst doped into the structure of LGC50, named LGC50-Ir(1.0), can increase the H₂ release rate by 1.3-fold. XPS results reveal that the H₂O splitting reaction is driven by the Co²⁺/Co³⁺ redox pair. DFT calculations predict that the oxygen vacancy formation energies (E_V) of La_1−xSr_xGa_1−yCo_yO₃ span from nearly 0.7 to about 5.3 eV under vacuum conditions at 1623 K, depending on the contents of Sr- and Co-dopants. Thus, doping Co at the B-site is responsible for the introduction of redox activity, and adjusting Sr- and Co-contents is effective in tuning the δ and E_V values of La_1−xSr_xGa_1−yCo_yO_3−δ. It should be pointed out that, at present, the steam to hydrogen conversion ratios for LaGa_1−yCo_yO_3−δ under the used evaluation conditions are still very low (<1%) and far away from the practical demand, which is a common tradeoff in this field, and more attention should be paid in future material screening work. This discovery provides a useful strategy to design perovskite oxides with enhanced TCWS activity and it will promote the development of solar thermochemical fuel conversion science.