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.

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The total accumulated charge collected from test points b-i can be viewed as Supplemental Figure 1. As such, the charge per unit area in test point b represents only the charge collected at b; each successive test point contains the sum of charge collected at all previous test points in addition to the current test point.Supporting Figure 1: Charge Accumulation as a function of test point. Ex Situ Characterization of the SEI Layer: Sample PreparationUpon completion of in situ testing and associated radiation screening (approximately 60 days after NR experiments), cells were disassembled in a glovebox with atmospheric specifications as stated in the methods section. The working electrodes with SEI were rinsed with diethyl carbonate, DEC, and dried.

Neutron reflectometry analysis methods for under-determined, multi-layered structures are developed and used to determine the composition depth profile in cases where the structure is not known a priori. These methods, including statistical methods, sophisticated fitting routines, and coupling multiple data sets, are applied to hydrated and dehydrated Nafion nano-scaled films with thicknesses comparable to those found coating electrode particles in fuel cell catalyst layers. These results confirm the lamellar structure previously observed on hydrophilic substrates, and demonstrate that for hydrated films they can accurately be described as layers rich in both water and sulfonate groups alternating with water-poor layers containing an excess of fluorocarbon groups. The thickness of these layers increases slightly and the amplitude of the water volume fraction oscillation exponentially decreases away from the hydrophilic interface. For dehydrated films, the composition oscillations die out more rapidly. The Nafion-SiO2 substrate interface contains a partial monolayer of sulfonate groups bonded to the substrate and a large excess of water compared to that expected by the water-to-sulfonate ratio, λ, observed throughout the rest of the film. Films that were made thin enough to truncate this lamellar region showed a depth profile nearly identical to thicker films, indicating that there are no confinement or surface effects altering the structure. Comparing the SLD profile measured for films dried at 60 °C to modeled composition profiles derived by removing water from the hydrated lamellae suggests incomplete re-mixing of the polymer groups upon dehydration, indicated limited polymer mobility in these Nafion thin films.

Ambient pressure X-ray photoelectron spectroscopy (XPS) is used to measure near-surface oxidation states and local electric potentials of thin-film ceria electrodes operating in solid oxide electrochemical cells for H 2 O electrolysis and H 2 oxidation. Ceria electrodes which are 300 nm thick are deposited on YSZ electrolyte supports with porous Pt counter electrodes for single-chamber tests in H 2 /H 2 O mixtures. Between 635 and 740 °C, equilibrium (zero-bias) near-surface oxidation states between 70 and 85% Ce 3+ confirm increased surface reducibility relative to bulk ceria. Positive cell biases drive H 2 O electrolysis on ceria and further increase the percentage of Ce 3+ on the surface over 100 µm from an Au current collector, signifying broad regions of electrochemical activity due to mixed ionic-electronic conductivity of ceria. Negative biases to drive H 2 oxidation decrease the percentage of Ce 3+ from equilibrium values but with higher electrode impedances relative to H 2 O electrolysis. Additional tests indicate that increasing H 2 -to-H 2 O ratios enhances ceria activity for electrolysis.

Spatially resolved ambient pressure X-ray photoelectron spectroscopy has been used to measure and visualize regions of electrochemical activity, local surface potential losses, overpotentials, and oxidation state changes on single sided ceria/yttria-stabilized zirconia (YSZ)/Pt solid oxide electrochemical cells. When hydrogen electro-oxidation (negative applied bias) or water electrolysis (positive applied bias) is promoted on the ceria electrocatalyst, the Ce 3+ /Ce 4+ ratios shift away from equilibrium values and thereby demarcate electrochemically active regions on the ceria electrode. In addition to the ceria oxidation state shifts, inactive surface impurities with high photoelectron cross sections, such as Si, can provide local markers of activity through chemical and surface potential mappings under various electrochemical conditions. Localized removal of chemically active carbonaceous surface impurities also reveals regions of electrochemical oxidation activity on the ceria electrode. Finally, we show that electrochemical polarization of solid oxide electrochemical cells under different gas environments is used to control the ceria surface chemical state and oxygen vacancy density.

The lithium-ion batteries powering mass market electric vehicles must be capable of operating in a wide temperature range. Temperature variation has the potential to greatly affect the stability of the solid electrolyte interphase (SEI) responsible for mitigating capacity fade due to electrolyte decomposition in the lithium-ion battery. In this work, we investigate the solubility of the SEI on the silicon (Si) electrode, an alternative anode material to the conventional graphite electrode, at temperatures ranging from −10 to 50 °C. Through use of an electrochemical protocol with a high cathodic cutoff voltage, we measure the evolution of the SEI independently of competing Si mechanical stress. We correlate the electrochemical data with three-dimensional resistivity versus depth profiling as well as atomic force microscopy to show that SEI dissolution occurs at significantly faster rates at higher temperatures.

A one-dimensional button-cell model is developed and applied to explore the influence of anode microstructure on solid oxide fuel cell (SOFC) performance. The model couples porous-media gas transport and elementary electrochemical kinetics within a porous Ni-YSZ cermet anode, a dense YSZ electrolyte membrane and a composite LSM-YSZ cathode. In all cases the fuel is humidified H2 and air is the oxidizer. The effects of porosity, tortuosity, and other microstructural geometric factors are evaluated with respect to their overpotential contributions. The model is used to assist interpretation of the experimental results reported by Zhao and Virkar (J. Power Sources, 141:79-95, 2005). It is shown that there must be a physically reasonable positive correlation between porosity and tortuosity. The results also show that reducing anode porosity, which increases mass-transfer resistance, can significantly increase the thickness of the electrochemically active region.