High Entropy Perovskites for Green Hydrogen Generation/Utilization
LIU X. 1
1 West Virginia University, Morgantown, United States
In recent years, high-entropy oxides started to gain momentum due to their exciting features over the traditional oxides, such as flexibility in electronic/thermal expansion properties, lattice distortion-enabled suppression of cation migration, synergic effects between A- and B-site elements for exceptional electrode performance, etc. In this presentation, we will discuss our recent work on developing high entropy perovskites (HEPs) for two important clean hydrogen generation/utilization pathways: solid oxide cells/electrolysis cells (SOCs) & solar thermal-chemical hydrogen generation (STCH):
SOCs: A new series of HEPs are investigated as oxygen electrode materials for SOCs. Multiple rare-earth, alkaline-earth, and high-order transition metal elements are used for the A-site of this ABO3 structure. Due to the retaining of alkaline-earth elements Sr and/or Ba, the electrical conductivities of these HEPs are in the order of 100 S/cm at 550−700 °C, a value that can practically eliminate the electronic resistance of the porous cathode. Three out of eight candidates show similar or better performance than the (La0.6Sr0.4)(Co0.2Fe0.8)O3−δ (LSCF) benchmark. It is found that A-site elements can cast a substantial influence on the overall performance even with a change as small as 10% of the total cations. It seems that each element has its individual “phenomenal activity” that can be transferred from one candidate to the other in the general setting of the perovskite structure, leading to the best candidate by using the three most active elements simultaneously at the A-site. Excellent Cr tolerance has been observed on the (La0.2Sr0.2Pr0.2Y0.2Ba0.2)Co0.2Fe0.8O3−δ sample, showing degradation of only 0.25%/kh
during a 41-day operation in the presence of Cr, while LSCF increases by 100% within the first day in the
same condition. X-ray photoelectron spectroscopy discovers no Sr segregation as LSCF is found in this HEP; rather, the active element Y takes more A-sites on the outermost layer after long-term operation.
STCH: We have developed two families of HEPs as redox oxides for two-step STCH. B-site HEPs (La0.8Sr0.2)(Mn(1-x)/3Fe(1-x)/3CoxAl(1-x)/3)O3−δ with tunable Co contents were investigated. The thermodynamic properties (Δδ) are enhanced with the increasing Co content, whereas the intrinsic kinetic properties (oxygen surface exchange coefficient) are decreased with the growing Co content. (La0.8Sr0.2)(Mn0.2Fe0.2Co0.4Al0.2)O3) showed the best balance between the thermodynamics and kinetics properties, rendering a high hydrogen production of 89.97 mmol moloxide−1 within a 1-h duration at an optimized STCH condition. It exhibited high phase stability and moderate cycling durability even after 51 cycles under very harsh interrupted cycling conditions involving startup heating and shutdown cooling. Monte Carlo (MC) sampling based on density functional theory (DFT) demonstrated that oxygen vacancies prefer to form on the Co octahedron position among all B-site metal octahedron positions and the valence change of Co is the most obvious. Another class of A-site HEP (La1/6Pr1/6Nd1/6Gd1/6Ba1/6Sr1/6)MnO3−δ was developed. The H2 production is positively correlated with the number of A-site metals (i.e., configuration entropy). (La1/6Pr1/6Nd1/6Gd1/6Ba1/6Sr1/6)MnO3−δ delivered the highest H2 production of 98.24 mmol moloxide−1 and excellent stability for 50 cycles.