Table of Contents

1. Research
2. Surface Phase Diagrams
3. Solar Thermochemical Hydrogen Production
4. Nanocrystal Synthesis
5. Computing Resources

Research

The Wexler Group develops computational materials chemistry methods for energy conversion and environmental applications. The projects described here concern catalyst surface reconstruction, solar thermochemical hydrogen production, and nanocrystal synthesis. Related work on CO₂ conversion, ferroelectric energy harvesting, and solar energy conversion is included on the Papers page.

Across these projects, we use statistical thermodynamics, first-principles quantum-mechanical calculations, Monte Carlo simulations, data science, and machine learning in collaboration with experimental groups. We use these methods to connect atomic-scale structures and energetics to thermodynamic observables and materials behavior during synthesis or under operating conditions. The current research questions are:

• How can catalyst surface structures be predicted as functions of temperature and chemical environment?
• How does perovskite composition affect redox thermodynamics and stability during solar thermochemical water splitting?
• How do precursors and ligands affect the crystal structure and phase of chalcogenide nanocrystals during synthesis?

Surface Phase Diagrams

Industrial chemical processes use heterogeneous catalysts. Their activity and selectivity depend on the atomic structure and composition of exposed surfaces under reaction conditions. Surface reconstruction changes surface periodicity, species coordination, composition, or thickness relative to the bulk material. Degradation decreases catalytic activity or selectivity over time under chemical or operational stresses. Temperature, pressure, and chemical composition affect both processes. Measurements under reactive conditions can be complicated by interactions with solutes and solvents, oxidation during sample transfer, and conditions that can damage experimental apparatus. We develop computational methods to predict equilibrium reconstructions of catalyst surfaces as functions of temperature and chemical environment. We use these equilibrium structures as reference states in subsequent studies of surfaces during catalytic turnover. As an initial demonstration, we applied nested sampling to Lennard-Jones gas particles adsorbed on low-index and vicinal Lennard-Jones solid surfaces. We used the sampled energies to construct a canonical partition function and calculate ensemble averages, including the constant-volume heat capacity and order parameters that characterize adsorbate phases.

Yang, M.; Pártay, L. B.; Wexler, R. B. Surface Phase Diagrams from Nested Sampling. Phys. Chem. Chem. Phys. 2024, 26 (18), 13862.

Chatbipho, T.; Yang, R.; Wexler, R. B.; Pártay, Livia B. Adsorbate Phase Transitions on Nanoclusters from Nested Sampling. arXiv 2025, 2506.01295.

Solar Thermochemical Hydrogen Production

Two-step solar thermochemical hydrogen production (STCH) cycles use redox-active metal oxides (MOx) to split water and produce hydrogen. During thermal reduction, concentrated solar radiation heats MOx above 1500 K at low oxygen partial pressure, producing the oxygen-deficient state MOx–δ. During reoxidation, superheated steam restores the initial oxide composition and produces hydrogen. Experiments on our (Ca, Ce)(Ti, Mn)O3–δ perovskite showed oxygen-vacancy formation and filling during cycling without a reported bulk phase transition under the conditions studied. We evaluate how composition and vacancy thermodynamics affect reaction kinetics, cycling stability, and durability. The project includes theoretical modeling, synthesis, characterization, material thermodynamics, reactor design and prototyping, system mass and energy flow analysis, and techno-economic analysis. Using prior HydroGEN results, we are developing an STCH cycle based on (Ca, Ce)(Ti, Mn)O3–δ perovskites and evaluating material cost, stability, and scalability.

Wexler, R. B.; Sai Gautam, G.; Bell, R. T.; Shulda, S.; Strange, N. A.; Trindell, J. A.; Sugar, J. D.; Nygren, E.; Sainio, S.; McDaniel, A. H.; Ginley, D.; Carter, E. A.; Stechel, E. B. Multiple and Nonlocal Cation Redox in Ca–Ce–Ti–Mn Oxide Perovskites for Solar Thermochemical Applications. Energy Environ. Sci. 2023, 16 (6), 2550.

Way, L.; Spataru, C. D.; Jones, R.; Trinkle, D. R.; Rowberg, A. J. E.; Varley, J. B.; Wexler, R. B.; Smyth, C. M.; Douglas, T. C.; Bishop, S. R.; Fuller, E.; McDaniel, A. H.; Lany, S.; Witman, M. D. Defect Diffusion Graph Neural Networks for Materials Discovery in High-Temperature, Clean Energy Applications. ChemRxiv 2024.

Nanocrystal Synthesis

We combine experimental and computational methods to determine how halides affect the crystal structure and phase of manganese chalcogenide nanocrystals during synthesis. We identify prenucleation species, measure the thermochemistry of reactions and surface-ligand interactions, and monitor nucleation and growth kinetics using in situ techniques. We use first-principles calculations to characterize atomic-scale interactions and mechanisms that affect crystal structure and phase. We use these results as inputs for kinetic and thermodynamic models of nanocrystal nucleation and growth. We also examine lanthanide chalcogenide nanocrystals, which have been studied less extensively than manganese chalcogenide nanocrystals. We seek chemical principles for synthesizing Mn and Ln chalcogenide nanocrystals and test whether those principles apply to other material classes.

Computing Resources

bear

Wexler Group

PowerEdge T550

Intel Xeon Gold 6338 Processor

2.00 GHz

64 cores

dragon

Wexler Group

Current Configuration: PowerEdge C6520

Intel Xeon Gold 6338 Processor

2.00 GHz

256 cores (4 nodes)

Theta

Argonne Leadership Computing Facility

Intel-Cray XC40

11.7 petaflops

One 64-core, 1.3-GHz Intel Xeon Phi 7230 processor per node

4,392 nodes and 281,088 cores

843 TB memory and 70 TB high-bandwidth memory

Aries network with Dragonfly topology