Previous capability name: "Computational and Experimental Tools for Enhanced Thermochemical Hydrogen Production."

The creation of new functional classes of thermochemical water splitting materials depends on integration of several core capabilities: enabling the design, evaluation, and qualification of new materials against known standard materials. To achieve the required breakthroughs in (1) operating temperature, (2) efficiency, and (3) stability, which are needed to realize the real potential efficiencies of thermochemical water splitting, requires the ability to synthesize highly controlled new materials, as well as the ability to demonstrate standard materials to establish critical comparisons between laboratories. It also requires an understanding of the synthesis pathways for high temperature and the defect phase diagrams for operating materials. This will require a detailed understanding of the phase formation behavior as well as the defects and oxidation states both during processing and cycling. Thus, the overall capability is composed of a synthesis effort to make highly controlled materials and an analysis component to confirm the detailed crystal structure, defect distribution and oxidation states. These later measurements will be made in-situ in the synthesis and reaction modes.

NREL has a demonstrated ability to realize new very high-quality materials in bulk and thin film form. This includes those both known and computationally suggested materials. The overall goal is to take initial results from the computational and experimental efforts from the teams and produce very high-quality samples with known and defined properties to converge on new STCH redox systems worthy of further pilot scale investigation. Consequently, this node enables the growth of highly controlled crystalline and amorphous materials, with controlled critical properties of crystallinity, composition, defects, oxidation and topology at scale through the growth process or by subsequent annealing. This capability node includes:

Synthesis Tools for New Materials

Synthesis tools to rapidly explore new materials in a highly controlled way, with precise control of the synthesis conditions, including the temperature and redox environment (precise control of oxygen and other gas partial pressures from 10-20 to 100 torr). It is increasingly clear that a number of experimental parameters (many of which can now be computationally modeled) control the detailed synthesis, phase development, the defect concentrations and oxidation states which control the subsequent chemical activity. These parameters are defined by the synthesis and annealing approach, i.e., different parameters for hydrothermal vs. pulse laser deposition vs. ceramic processing. We have the ability to grow bulk and thin film redox oxides, conventional oxides, non-oxide systems, and very high-quality chalcogenides. To be effective, we will need to couple synthesis, detailed characterization and modeling.

• We have extensive experience in the synthetic development of highly controlled, single phase crystalline ceramics, in film and bulk form, as well as amorphous oxides, both at significant scale, up to hundreds of grams.
• Another advantage of this node is the ability to make very high-quality samples of known compositions, suitable for establishing lab-to-lab and project-to-project comparisons and providing well controlled model systems.

Experimental Synthesis and Characterization of Redox Oxides

• Growth - The ability to grow high quality film and bulk samples by a variety of growth techniques including sputtering, pulse laser deposition (PLD), evaporation, solution co-deposition, bulk ceramic processing, hydrothermal growth, and HIP (hot isostatic pressing) and hot-pressing processing can be done under an extremely broad range of temperature and oxygen partial pressures as needed and suggested by theory and experiment.
• Annealing - We have developed a high temperature annealing capability under controlled atmospheres that can anneal samples to control grain size and defect up to 1,800˚C. We have two annealing systems capable of these high-temperature anneals with sample sizes up to a kilogram or more.
• Characterization - The effectiveness of STCH materials is predicated on temperature and oxygen partial pressure dependent oxygen defect formation. As STCH moves towards perovskites and more complex oxides including phase changing materials with multiple oxygen sites available for vacancy formation during thermal cycling, intelligent material design requires fundamental knowledge of oxygen defects and cation reduction that is not currently available, even for the starting as synthesized systems. So, in addition to a very extensive suite of local oxide characterization tools, we will employ capabilities for in-situ X-ray Raman Scattering that enable direct measurements of oxygen vacancy concentrations and lattice positions, cation oxidation states, and phase. These investigations will be key for verifying the effectiveness of STCH materials and will directly inform computational predictions of next-generation STCH materials. This will allow us to look at these key materials properties ex-situ during cycles such as in Figure 1. Initial work will be ex-situ measurements using our existing tool set and coupling to other HydroGEN nodes. We will eventually target the application of cells such as those shown in Figure 2 to transition the capability to in-situ measurements.

Overall, these capabilities provide the needed infrastructure to rapidly develop and confirm the potential of new thermochemical water splitting materials. In addition, the understanding from the extensive computational capabilities in this area, coupled to the extensive experimental and characterization capabilities, can rapidly accelerate the optimization of materials for this application. In addition, it will be key in validating new materials performance against standard materials, which will be made available to HydroGEN. These capabilities are more broadly applicable to other HydroGEN materials as well.