Abstract
The growing demand for energy due to the increasing world population and industrialization of countries has caused pollution and climate change concerns. A global transition from fossil fuel energy to sustainable energy sources, in particular solar energy, calls for breakthroughs in energy storage and H2 fuel production. Two-step solar thermochemical hydrogen (STCH) production has emerged as an attractive route to produce clean H2 fuel from concentrated solar thermal energy. The two-step chemical looping is readily achieved using nonstoichiometric oxides such as ceria and perovskite oxides because those materials can retain their structural integrity throughout the cycling process. A new class of redox oxides, high-entropy perovskite oxides (HEPOs), are proposed to show great promises in the STCH application These novel HEPO materials have an unprecedented vast compositional space, large tunability, and unique thermodynamic and kinetic behaviors. Herein, we present that a HEPO family of La0.8Sr0.2(Mn[1-x]/3Fe[1-x]/3CoxAl[1-x]/3)O3-δ (x = 0.16, 0.2, 0.25, or 0.4) on the basis of an established STCH active material La0.6Sr0.4Mn0.6Al0.4O3-δ, was developed using high-energy ball milling (HEBM) and sintering. We comprehensively characterized the synthesized HEPOs in terms of phase purity, morphology, homogeneous elemental distribution, identification of redox active elements, reducibility extent (Δδ) and cycling structure stability and explored their STCH production performances. The optimal sample (La0.8Sr0.2(Mn0.2Fe0.2Co0.4Al0.2)O3-δ) showed a large Δδ ≥ 0.15 under the conditions of reduction at 1320 ℃ and oxidation at 870 ℃. All HEPO samples demonstrated R3-c phase stability during the cycling. Elemental mapping showed homogeneous distribution. Characterization of the electrical conductivity relaxation of the materials showed intrinsic material kinetic properties for oxygen surface exchange coefficient k ≥ 7.5 x 10-4 cm/s at 700 ℃, indicating the high kinetics for the HEPO reduction. The STCH production testing in our prototype setup showed that several promising samples had a high H2 yield of more than 300 μmol/g within less than 45 minutes upon exposure to 40 vol % steam and good cycling stability. This work opens up an avenue for the rational design of HEPOs for the solar thermochemical hydrogen production.
Figure 1