STCH Supernode

Supernode IDEA: Develop Atomistic Understanding of the Layered Perovskite Ba4CeMn3O12 and its Polytypes

Recipient Sandia National Laboratory/SNL (PI: Anthony McDaniel)

HydroGEN Node Experts

Lawrence Livermore National Laboratory/LLNL:

• Brandon Wood
• Tadashi Ogitsu

National Renewable Energy Laboratory/NREL:

• Robert Bell
• David Ginley
• Stephan Lany

Sandia National Laboratory/SNL:

• Eric Coker
• Josh Sugar

Water Splitting Technology STCH

Status Awarded

Abstract Develop a fundamental understanding of how the unique crystallographic structures induced by the Mn-O ligand bond found in layered perovskites influences properties critical to favorable thermochemical water splitting material behavior. This endeavor seeks to deliver a deeper understanding of the relationship between electronic structure and material redox performance through a carefully crafted project that uses select H2 AWSM resource nodes encompassing atomistic theory and advanced experimentation. Focused collaboration between these nodes will result in strengthened inter-lab relationships, enable labs to lead the research community, and provide detailed material data outside of the scope of current projects.

Next generation perovskite-oxide based materials for thermochemical water splitting (aka STCH) are key to developing a technology that efficiently converts solar energy to hydrogen. The challenge is to design optimal perovskites that achieve DOE performance targets, such as high efficiency operation at low thermal reduction temperature. Current research is mired in a heuristic approach to probing the enormous array of possible material compositions that manifest water splitting activity in perovskites. A comprehensive molecular-level picture of the factors that influence redox cycle behavior is missing.

Heretofore layered perovskites have not been considered for STCH applications. We have recently discovered a complex layered perovskite that is a B-site, Mn-based compound (Ba4CeMn3O12, or BCM) which outperforms all known perovskites tested for STCH functionality. BCM has several key differentiating attributes that make it a material worthy of deep exploration. Most notable are: 1) BCM is a line compound where Ce substitutes for Mn in a perfectly ordered intergrowth of cubic and hexagonal layers, 2) transition to a stoichiometric but more disordered polytype can be induced by exposure to high temperature and low oxygen partial pressure (i.e., thermal reduction), and 3) BCM reduces to a much greater extent than CeO2 (a state of the art fluorite) at 1350°C, and re-oxidizes to a much greater extent than SrxLa1-xMnyAl1-yO3 (a state of the art perovskite) in mixtures of steam and hydrogen. This last point is critical to achieving DOE performance targets and, while much better than SLMA, BCM still lags CeO2 in this metric meaning much can be learned from this model compound.

What is intriguing about BCM, and more broadly like-type Mn-based complex oxides, is how Mn behaves in these systems. The extent of covalency, or electron sharing, between Mn and the O ligand is highly malleable. And the notion of Mn having either a formal oxidation state or even a singular oxidation state in these complex oxides is false. In addition, the polytypes accessible to BCM are distinguished by the complexity of the Mn-O oligomer arrangements within the layered structures. Fundamentally we do not know to what extent any or all of the following play a role in our observation of enhanced water splitting thermodynamics and kinetics: 1) what conditions promote rare-earth (RE) substitutions on the B-site, 2) how various crystallographic arrangements affect thermal stability, defect equilibria, and transport kinetics, 3) whether or not both B-site cations (Mn, Ce, or any possible RE) participate in redox chemistry, and 4) do BCM’s polytype transitions and the non-stoichiometry therein participate in redox chemistry.

We believe that a comprehensive understanding of Mn’s local structure in BCM will lead to novel pathways for modifying and improving STCH perovskites. This Supernode project will examine BCM’s electronic structure under operating conditions during redox transitions and combine atomistic theory with advanced spectral microscopies to derive relationships between electronic structure and redox transitions. Our main deliverable will be holistic design rules that will inform high-throughput materials discovery efforts.