Defect graph neural networks for materials discovery in high-temperature clean-energy applications.

We present a graph neural network approach that fully automates the prediction of defect formation enthalpies for any crystallographic site from the ideal crystal structure, without the need to create defected atomic structure models as input. Here we used density functional theory reference data for vacancy defects in oxides, to train a defect graph neural network (dGNN) model that replaces the density functional theory supercell relaxations otherwise required for each symmetrically unique crystal site. Interfaced with thermodynamic calculations of reduction entropies and associated free energies, the dGNN model is applied to the screening of oxides in the Materials Project database, connecting the zero-kelvin defect enthalpies to high-temperature process conditions relevant for solar thermochemical hydrogen production and other energy applications. The dGNN approach is applicable to arbitrary structures with an accuracy limited principally by the amount and diversity of the training data, and it is generalizable to other defect types and advanced graph convolution architectures. It will help to tackle future materials discovery problems in clean energy and beyond.

Origin and Tunability of Unusually Large Surface Capacitance in Doped Cerium Oxide Studied by Ambient-Pressure X-Ray Photoelectron Spectroscopy.

The volumetric redox (chemical) capacitance of the surface of CeO2-δ films is quantified in situ to be 100-fold larger than the bulk values under catalytically relevant conditions. Sm addition slightly lowers the surface oxygen nonstoichiometry, but effects a 10-fold enhancement in surface chemical capacitance by mitigating defect interactions, highlighting the importance of differential nonstoichiometry for catalysis.

Oxidation stages of Ni electrodes in solid oxide fuel cell environments.

Nickel is the most commonly used anode for solid-oxide fuel cells (SOFC) due to its fast kinetics and low price. A leading cause of degradation in Ni electrodes is oxidation. Here we use operando ambient-pressure X-ray photoelectron spectroscopy (XPS) to chemically characterize the Ni electrode of a fuel cell anode during oxidation in a H2/H2O atmosphere. We find three different stages of Ni oxidation in the model SOFC. In the first two stages, the Ni exposed to the gas remains metallic but the Ni at the interface with the zirconia electrolyte is oxidized. In the third oxidation stage, we find that Ni transforms to NiOOH, a phase not previously considered in the SOFC literature. We show that the transformation between Ni and NiOOH is reversible and is initiated at the Ni/gas interface. In addition we find that NiOOH stores charge, as evidenced by the stable discharge plateau (voltage) measured as this oxyhydroxide phase reduces to metallic Ni.

Intercalation pathway in many-particle LiFePO4 electrode revealed by nanoscale state-of-charge mapping.

The intercalation pathway of lithium iron phosphate (LFP) in the positive electrode of a lithium-ion battery was probed at the ∼40 nm length scale using oxidation-state-sensitive X-ray microscopy. Combined with morphological observations of the same exact locations using transmission electron microscopy, we quantified the local state-of-charge of approximately 450 individual LFP particles over nearly the entire thickness of the porous electrode. With the electrode charged to 50% state-of-charge in 0.5 h, we observed that the overwhelming majority of particles were either almost completely delithiated or lithiated. Specifically, only ∼2% of individual particles were at an intermediate state-of-charge. From this small fraction of particles that were actively undergoing delithiation, we conclude that the time needed to charge a particle is ∼1/50 the time needed to charge the entire particle ensemble. Surprisingly, we observed a very weak correlation between the sequence of delithiation and the particle size, contrary to the common expectation that smaller particles delithiate before larger ones. Our quantitative results unambiguously confirm the mosaic (particle-by-particle) pathway of intercalation and suggest that the rate-limiting process of charging is initiating the phase transformation by, for example, a nucleation-like event. Therefore, strategies for further enhancing the performance of LFP electrodes should not focus on increasing the phase-boundary velocity but on the rate of phase-transformation initiation.

Electrochemical intermediate species and reaction pathway in H2 oxidation on solid electrolytes.

We use spatially resolved photoelectron spectroscopy performed in operando to identify the reaction intermediates of the hydrogen electro-oxidation reaction on yttria-stabilized zirconia (YSZ) electrolytes with Pt electrodes. We find that hydroxyl on the zirconia electrolyte is a reaction intermediate in the hydrogen oxidation reaction and that it participates in the rate-determining step. In contrast to the general wisdom, the limiting step does not involve the transfer of charge. These results allow us to propose the detailed reaction pathway, which provides direct insight into how to accelerate the kinetics.

Note: Fixture for characterizing electrochemical devices in-operando in traditional vacuum systems.

We describe a fixture that allows electrochemical devices to be studied under electrical bias in the type of vacuum systems commonly used in surface science. Three spring-loaded probes provide independent contacts for device operation and the characterization in vacuum or under in situ conditions with reactive gases. We document the robustness of the electrical contacts over large temperature changes and their reliability for conventional electrochemical measurements such as impedance spectroscopy. The optical access provided to the device enables the analysis by many techniques, as we demonstrate using x-ray photoelectron spectroscopy to measure local electrical potentials on a solid-oxide electrolyte device operating at high temperature in near-ambient pressure.

Measuring fundamental properties in operating solid oxide electrochemical cells by using in situ X-ray photoelectron spectroscopy.

Photoelectron spectroscopic measurements have the potential to provide detailed mechanistic insight by resolving chemical states, electrochemically active regions and local potentials or potential losses in operating solid oxide electrochemical cells (SOCs), such as fuel cells. However, high-vacuum requirements have limited X-ray photoelectron spectroscopy (XPS) analysis of electrochemical cells to ex situ investigations. Using a combination of ambient-pressure XPS and CeO(2-x)/YSZ/Pt single-chamber cells, we carry out in situ spectroscopy to probe oxidation states of all exposed surfaces in operational SOCs at 750 °C in 1 mbar reactant gases H(2) and H(2)O. Kinetic energy shifts of core-level photoelectron spectra provide a direct measure of the local surface potentials and a basis for calculating local overpotentials across exposed interfaces. The mixed ionic/electronic conducting CeO(2-x) electrodes undergo Ce(3+)/Ce(4+) oxidation-reduction changes with applied bias. The simultaneous measurements of local surface Ce oxidation states and electric potentials reveal the active ceria regions during H(2) electro-oxidation and H(2)O electrolysis. The active regions extend ~150 µm from the current collectors and are not limited by the three-phase-boundary interfaces associated with other SOC materials. The persistence of the Ce(3+)/Ce(4+) shifts in the ~150 µm active region suggests that the surface reaction kinetics and lateral electron transport on the thin ceria electrodes are co-limiting processes.

Measuring individual overpotentials in an operating solid-oxide electrochemical cell.

We use photo-electrons as a non-contact probe to measure local electrical potentials in a solid-oxide electrochemical cell. We characterize the cell in operando at near-ambient pressure using spatially-resolved X-ray photoemission spectroscopy. The overpotentials at the interfaces between the Ni and Pt electrodes and the yttria-stabilized zirconia (YSZ) electrolyte are directly measured. The method is validated using electrochemical impedance spectroscopy. Using the overpotentials, which characterize the cell's inefficiencies, we compare without ambiguity the electro-catalytic efficiencies of Ni and Pt, finding that on Ni H(2)O splitting proceeds more rapidly than H(2) oxidation, while on Pt, H(2) oxidation proceeds more rapidly than H(2)O splitting.