Robert T. Bell

Thermochemical hydrogen production (TCH) is a promising approach to convert water into hydrogen by utilizing thermal energy from sources such as concentrated solar energy, as opposed to using electricity in electrochemical methods. This process consists of two main steps: first, a material undergoes thermal reduction at elevated temperatures (~ 1300 - 1500 °C) under low oxygen partial pressures (pO₂) of ~10^-3 -10 ^-5 atm, which results in the formation of oxygen vacancies. Next, the temperature of the system is reduced to ~800 - 1000 °C in the presence of steam, enabling water splitting through material oxidation. The commercial feasibility of TCH is dependent on identifying a material with an optimal reduction enthalpy that supports both the thermal reduction of redox-active cations and their re-oxidation, while also possessing high cycle stability. Perovskite metal oxides have received significant interest as potential TCH materials due to their ability to accommodate high concentrations of oxygen vacancies and their highly tunable thermodynamic properties. However, these materials often exhibit low hydrogen production due to re-oxidation and poor cycle stability challenges.

Electrochemical carbon deposition from molten carbonate salts using inert electrodes and no additional catalyst is systematically examined.

To identify promising metal oxides for thermochemical water-splitting, we propose a workflow involving defect machine learning model training, high-throughput computational screening, material down-selection, and experimental validation.

To meet expected 2045 LFP deployment, impurity characterization must improve. New, sufficiently pure, and low-cost iron precursors must be identified, and total phosphorous demand could consume 22% of current phosphate rock supply for fertilizers.

This study presents a high-efficiency electrochemical process for the mineralization of calcium carbonate (CaCO₃) from mildly acidic mine tailing supernatant water. Electrochemical pH control was used to promote carbonate speciation and precipitation from CO₂-saturated solutions, achieving >1 mol CaCO₃ precipitated per mol e^– at applied potentials between −1.4 and −1.6 V vs Ag/AgCl. Product morphology and polymorph selectivity were tunable via applied potential, yielding calcite and vaterite phases. Experiments using real mine tailings water confirmed selective calcite precipitation, despite the presence of sulfate and other trace elements. These results highlight a viable route for coupling CO₂ utilization with mine tailings valorization.

Mine tailings and other mining waste streams are produced at a gigaton scale each year, making them suitable for large-scale, thermodynamically downhill carbon mineralization processes. Highly alkaline tailings have received the majority of focus in mineralization because of their enhanced precipitation thermodynamics, but many tailing streams are neutral or mildly acidic. Additionally, the introduction of gaseous carbon dioxide, which promotes dissolved inorganic carbon formation, further acidifies the tailings streams. In this work, we investigate the electrochemical mineralization from mildly acidic conditions of calcium carbonates. We study the evolution over time in this system of the mass, morphology, and energy input using chronoamperometry in an electrochemical quartz crystal microbalance with in-situ imaging. These results illuminate a pathway towards mineralization of the large fraction of mine tailings composed of mildly acidic streams by revealing the mechanistic driving forces of mineralization under these conditions.

Thermochemical water-splitting (TCH) based on 2-step thermal redox cycles in metal oxides is a promising approach to generating H2, but state-of-the-art (SOTA) CeO2 has several practical limitations, which has motivated continued materials discovery efforts in this field. Here, we improve upon a SOTA defect graph neural network (dGNN) surrogate model's oxygen vacancy predictions and combine them with Materials Project phase diagrams to down-select and discover structurally diverse, experimentally known metal oxides whose TCH performance was previously unknown. Amongst twelve candidates selected based on our high-throughput screening and down-selection criteria, we achieved ~80% accuracy in identifying materials with stable redox cycling and hydrogen production in stagnation flow reactor water-splitting experiments. Closer to 100% accuracy can be achieved if higher-accuracy, hybrid DFT-predicted vacancy formation energies were computed and used in lieu of the most uncertain dGNN-based screening predictions, as they correct false positives to true negatives. Notably, two discovered candidates, Sr3PrMn2O6 and Ba2Fe2O5, display hydrogen yields greater than CeO2 under specific redox conditions. These results demonstrate our ability to computationally predict and experimentally validate promising candidate TCH materials that have the potential to compete with CeO2.

Molten salt electrochemical reduction (MSER) is the dominant production method for aluminum and titanium, and emerging technologies for producing other commodity metals with these highly electrified techniques have demonstrated promising results. However, the scale up of MSER of materials such as iron and silicon are limited by the challenging issues of materials compatibility and impurity control. This work describes an experimental MSER setup using both chloride and carbonate salts. This setup is designed for operations outside of a glovebox, which is practical from an industrial perspective, but creates unique challenges. This work serves as a practical guide to discuss some key operational difficulties such as temperature control, air and humidity, material compatibility, developing strong electrical measurements and techniques, and safety.

Cerium(III) carbonate is a precursor material for the synthesis of various Ce-containing compounds. In this work, a synchrotron powder X-ray diffraction study of commercially obtained ‘cerium(III) carbonate hydrate' indicates that multiple Ce-containing phases are present. The majority phase CeCO₃OH (52.49%_wt) was refined using an orthorhombic Pmcn structure model with a = 5.01019 (2) Å, b = 8.55011 (4) Å and c = 7.31940 (4) Å, which is based on a reported structure for the lanthanoid carbonate mineral ancylite. Additionally, a substantial portion of the precursor material is cubic cerium(IV) oxide (47.12%_wt).

To synthetically target a specific material with select performance, the underlying relationship between structure and function must be understood. For targeting magnetic properties, such understanding is underdeveloped for a relatively new class of layered hexagonal perovskites, the 12R-Ba4 MMn3O12 family. Here, we perform a detailed magnetostructural study of the layered hexagonal perovskite materials 12R-Ba4 MMn3O12, where M = diamagnetic Ce4+ or paramagnetic J eff approximate to 1/2 Pr4+. The material with M = Ce4+ is an antiferromagnet below T N approximate to 7.75 K, while the material with M = Pr4+ exhibits more complex behavior, with a net moment below 200 K and a sharp peak in the susceptibility at T N approximate to 12.15 K. Guided by the susceptibility data, we conduct variable-temperature powder neutron diffraction measurements to determine the magnetic structure of these two materials. The introduction of a magnetic interlayer cation cants the spins in the Mn3O12 trimers out of plane. We further characterize the crystal and electronic structures in these compounds using powder X-ray diffraction and X-ray absorption spectroscopy measurements coupled with first-principles theoretical calculations. The resulting detailed picture of the magnetic, crystal, and electronic structure will be useful for understanding the magnetism in similar 12R hexagonal perovskites and related materials.