Transformative Materials for High-Efficiency Thermochemical Production of Solar Fuels

Recipient Northwestern University/NWU (PI: Chris Wolverton)

Subs Northwestern University/NWU (PI: Sossina Haile)

Water Splitting Technology STCH

Status Awarded

Abstract Metal oxide-based two-step solar thermochemical (STC) H2O and CO2-splitting cycles are a promising route to convert solar thermal energy into fuels. The metal oxide materials are reduced at high temperatures (Step 1), and then at low (but still elevated) temperatures, the reduced oxide is used to split H2O or CO2 (Step 2). However, current applications of these cycles are limited by the efficiency of the metal oxide materials. A lower temperature for reduction is desirable, but that bring a concomitant reduction in the driving force for gas splitting. So, designing novel, high-efficiency materials is challenging. Here, we propose a joint computational-experimental project, combined with materials design strategies and high-throughput approaches to quickly discover and demonstrate novel thermochemical materials with superior properties.

Our approach is two-fold: In the first, we will use our previously-developed materials design map, combined with results from (already existing) high-throughput first-principles computation to experimentally study the properties of novel, predicted materials. In particular, we will experimentally explore a set of recently-predicted high-throughput perovskites. Study of these compositionally simple materials (which encompass only single elements on the A and B sites) will focus on obtaining high quality thermodynamic properties for validation of computational prediction of enthalpy and entropy of reduction. These thermodynamic quantities play a major role in designing materials with reduced temperatures of reduction but sufficient gas-splitting rates (a stated goal of this FOA).

The most promising of the ABO3 materials will be modified by A and B site substitutions, both computationally and experimentally, opening an enormous combinatorial space of materials, with the promise of being able to chemically “tune in” desired properties for STC. This vast composition space can only be reasonably explored using the high-throughput approaches, both computational and experimental, of this proposal. Computational screening will narrow the list of promising compounds, and experimental synthesis, stability, and redox measurements will further refine to the set of materials with optimal STC properties.

In the second area, we will use our combined computational/experimental approach to design a novel class of STC materials. Previous thermochemical materials have typically been chosen from one of two categories: (a) oxides that exhibit phase transformations during reduction, or (b) oxides that exhibit off-stoichiometry during reduction but do not undergo a phase transformation. The dichotomy between these two groups of materials has arisen due to the belief that off-stoichiometric materials exhibit better kinetics, due to the large quantities of oxygen vacancies in the material, whereas phase transforming materials exhibit a large degree of reduction, and hence a large resultant amount of fuel. Here, we propose to break this false dichotomy by designing a new set of materials and reactions: those which may exhibit a phase transformation between oxidized and reduced phase, but also exhibit specifically tuned off-stoichiometric composition in both phases. The former allows a large capacity of fuel, whereas the latter allows for fast kinetics. In depth, experimental and computational work will be used, in concert, to explore these materials, examine descriptors for their behavior, and ultimately to predict novel materials and reactions.

The proposed work will yield a new palette of demonstrated STC materials, specifically tailored to reduced enthalpy of reduction (reduce sufficiently <1400C) and oxidation under reasonable conditions stated as goals of the HydroGEN center.